Direct simulation of turbulent Rayleigh-Benard convection in liquid sodium
International Nuclear Information System (INIS)
Woerner, M.
1994-11-01
The numerical results are analysed to investigate both the structures and mechanisms of convection and the statistical features of turbulence in natural convection of liquid metals. The simulations are performed with the finite volume code TURBIT which is extended by a semi-implicit time integration scheme for the energy equation. Due to the implicit treatment of thermal diffusion the computational time for simulation of natural convection in liquid metals is reduced by about one order of magnitude, as compared to the original fully explicit code version. Results for Rayleigh-Benard convection in liquid sodium with Prandtl number Pr=0.006 are given for four different Rayleigh numbers: Ra=3 000, Ra=6 000, Ra=12 000, and Ra=24 000. At the Rayleigh number Ra=3 000 the inertial convection is identified. It is characterized by large two-dimensional vortices, which rotate like a solid body. These vortices are also observed in the simulations for Ra=6 000, Ra=12 000 and Ra=24 000, but, they only exist in certain regions and for short time intervals. The appearance of these two-dimensional structures in three-dimensional, time-dependent and turbulent convection is explained by the relative importance of the non-linear terms in the momentum and energy equation, which is totally different in both equations, and by the coupling of these equations by the buoyancy and the convective term. In order to improve and validate statistical turbulence model for application to natural convection in liquid metals, budgets of turbulence kinetic energy, turbulent heat flux and temperature variance are calculated from the numerical results. For several unknown correlations closure assumptions used in standard turbulence models are analyzed and model coefficients are determined. (orig./HP) [de
Advection diffusion model for particles deposition in Rayleigh-Benard turbulent flows
International Nuclear Information System (INIS)
Oresta, P.; Lippolis, A.; Verzicco, R.; Soldati, A.
2005-01-01
In this paper, Direct Numerical Simulation (DNS) and Lagrangian Particle Tracking are used to precisely investigate the turbulent thermally driven flow and particles dispersion in a closed, slender cylindrical domain. The numerical simulations are carried out for Rayleigh (Ra) and Prandtl numbers (Pr) equal to Ra = 2X10 8 and Pr = 0.7, considering three sets of particles with Stokes numbers, based on Kolmogorov scale, equal to St k 1.3, St k 0.65 and St k = 0.13. This data are used to calculate a priori the drift velocity and the turbulent diffusion coefficient for the Advection Diffusion model. These quantities are function of the Stokes, Froude, Rayleigh and Prandtl numbers only. One dimensional, time dependent, Advection- Diffusion Equation (ADE) is presented to predict particles deposition in Rayleigh-Benard flow in the cylindrical domain. This archetype configuration models flow and aerosol dynamics, produced in case of accident in the passive containment cooling system (PCCS) of a nuclear reactor. ADE results show a good agreement with DNS data for all the sets of particles investigated. (author)
Reduced-Order Modeling of 3D Rayleigh-Benard Turbulent Convection
Hassanzadeh, Pedram; Grover, Piyush; Nabi, Saleh
2017-11-01
Accurate Reduced-Order Models (ROMs) of turbulent geophysical flows have broad applications in science and engineering; for example, to study the climate system or to perform real-time flow control/optimization in energy systems. Here we focus on 3D Rayleigh-Benard turbulent convection at the Rayleigh number of 106 as a prototype for turbulent geophysical flows, which are dominantly buoyancy driven. The purpose of the study is to evaluate and improve the performance of different model reduction techniques using this setting. One-dimensional ROMs for horizontally averaged temperature are calculated using several methods. Specifically, the Linear Response Function (LRF) of the system is calculated from a large DNS dataset using Dynamic Mode Decomposition (DMD) and Fluctuation-Dissipation Theorem (FDT). The LRF is also calculated using the Green's function method of Hassanzadeh and Kuang (2016, J. Atmos. Sci.), which is based on using numerous forced DNS runs. The performance of these LRFs in estimating the system's response to weak external forcings or controlling the time-mean flow are compared and contrasted. The spectral properties of the LRFs and the scaling of the accuracy with the length of the dataset (for the data-driven methods) are also discussed.
The dynamics of droplets in moist Rayleigh-Benard turbulence
Chandrakar, Kamal Kant; van der Voort, Dennis; Kinney, Greg; Cantrell, Will; Shaw, Raymond
2017-11-01
Clouds are an intricate part of the climate, and strongly influence atmospheric dynamics and radiative balances. While properties such as cloud albedo and precipitation rate are large scale effects, these properties are determined by dynamics on the microscale, such droplet sizes, liquid water content, etc. The growth of droplets from condensation is dependent on a multitude of parameters, such as aerosol concentration (nucleation sites) and turbulence (scalar fluctuations and coalescence). However, the precise mechanism behind droplet growth and clustering in a cloud environment is still unclear. In this investigation we use a facility called the Pi Chamber to generate a (miniature) cloud in a laboratory setting with known boundary conditions, such as aerosol concentration, temperature, and humidity. Through the use of particle imaging velocimetry (PIV) on the droplets generated in the cloud, we can investigate the dynamics of these cloud droplets in the convective (Rayleigh-Benard) turbulence generated through an induced temperature gradient. We show the influence of the temperature gradient and Froude number (gravity forces) on the changing turbulence anisotropy, large scale circulation, and small-scale dissipation rates. This work was supported by National Science Foundation Grant AGS-1623429.
Temperature boundary layer profiles in turbulent Rayleigh-Benard convection
Ching, Emily S. C.; Emran, Mohammad S.; Horn, Susanne; Shishkina, Olga
2017-11-01
Classical boundary-layer theory for steady flows cannot adequately describe the boundary layer profiles in turbulent Rayleigh-Benard convection. We have developed a thermal boundary layer equation which takes into account fluctuations in terms of an eddy thermal diffusivity. Based on Prandtl's mixing length ideas, we relate the eddy thermal diffusivity to the stream function. With this proposed relation, we can solve the thermal boundary layer equation and obtain a closed-form expression for the dimensionless mean temperature profile in terms of two independent parameters: θ(ξ) =1/b∫0b ξ [ 1 +3a3/b3(η - arctan(η)) ] - c dη , where ξ is the similarity variable and the parameters a, b, and c are related by the condition θ(∞) = 1 . With a proper choice of the parameters, our predictions of the temperature profile are in excellent agreement with the results of our direct numerical simulations for a wide range of Prandtl numbers (Pr), from Pr=0.01 to Pr=2547.9. OS, ME and SH acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG) under Grants Sh405/4-2 (Heisenberg fellowship), Sh405/3-2 and Ho 5890/1-1, respectively.
Large-scale patterns in Rayleigh-Benard convection
International Nuclear Information System (INIS)
Hardenberg, J. von; Parodi, A.; Passoni, G.; Provenzale, A.; Spiegel, E.A.
2008-01-01
Rayleigh-Benard convection at large Rayleigh number is characterized by the presence of intense, vertically moving plumes. Both laboratory and numerical experiments reveal that the rising and descending plumes aggregate into separate clusters so as to produce large-scale updrafts and downdrafts. The horizontal scales of the aggregates reported so far have been comparable to the horizontal extent of the containers, but it has not been clear whether that represents a limitation imposed by domain size. In this work, we present numerical simulations of convection at sufficiently large aspect ratio to ascertain whether there is an intrinsic saturation scale for the clustering process when that ratio is large enough. From a series of simulations of Rayleigh-Benard convection with Rayleigh numbers between 10 5 and 10 8 and with aspect ratios up to 12π, we conclude that the clustering process has a finite horizontal saturation scale with at most a weak dependence on Rayleigh number in the range studied
Three caveats for linear stability theory: Rayleigh-Benard convection
International Nuclear Information System (INIS)
Greenside, H.S.
1984-06-01
Recent theories and experiments challenge the applicability of linear stability theory near the onset of buoyancy-driven (Rayleigh-Benard) convection. This stability theory, based on small perturbations of infinite parallel rolls, is found to miss several important features of the convective flow. The reason is that the lateral boundaries have a profound influence on the possible wave numbers and flow patterns even for the largest cells studied. Also, the nonlinear growth of incoherent unstable modes distorts the rolls, leading to a spatially disordered and sometimes temporally nonperiodic flow. Finally, the relation of the skewed varicose instability to the onset of turbulence (nonperiodic time dependence) is examined. Linear stability theory may not suffice to predict the onset of time dependence in large cells close to threshold
An Experimental Study on Rayleigh-Benard Natural Convection
International Nuclear Information System (INIS)
Moon, Je Young; Chung, Bum Jin
2012-01-01
Core melt in a severe accident condition, forms a molten pool in the reactor vessel lower head. The molten pool is divided by a metallic pool (top) and an oxide pool (bottom) by the density difference. Due to the decay heat generated in oxide pool, Rayleigh- Benard natural convection heated from below and cooled from above occurs in the metallic pool. Experiments were performed to investigate Rayleigh- Benard natural convection as a preparatory study before an in-depth severe accident study. The natural convection heat transfers were measured varying the plate separation distance and the area of plate with and without the side wall. Using the analogy concept, heat transfer experiments were replaced by mass transfer experiments. A cupric acid.copper sulfate (H 2 SO 4 -CuSO 4 ) electroplating system was adopted as the mass transfer system and the electric currents were measured rather than the heat
Rayleigh-Benard Natural Convection Cell Formation and Nusselt number
International Nuclear Information System (INIS)
Moon, Je Young; Chung, Bum Jin
2013-01-01
The experimental results lie within the predictions of the existing heat transfer correlations for the Rayleigh-Benard natural convections even though the material properties were different. For shorter separation distances, the heat transfers enhance due to the active interaction between heated and cooled plumes. For a step temperature difference, the time dependent Nusselt number variations were investigated. Both experimental and numerical results showed that with time the Nusselt number decreases monotonically to a minimum point presenting the onset of convection. As the hot and cold plumes increase and convey the heat to the other plates, the Nusselt number increases to the local maximum point, presenting the vertical movements of the plumes. Then, the Nusselt number fluctuates with the formation of square cells and larger vortices. This also predicted by the mass transfer experiment. The experiments and calculations show similar trend but the timings were different. These discrepancies are caused by the disturbances inherent in both systems. The molten pool is formed in a hypothetical severe accident condition at the lower head of reactor vessel and is stratified into two layers by the density difference: an upper metallic layer and a lower oxide pool. Rayleigh-Benard natural convection occurs in the metallic layer of relocated molten pool. This study aimed at the investigation of the time-dependent cell formation and Nusselt number variation in Rayleigh-Benard natural convection. Time dependent variation of Nusselt number was also measured experimentally and analyzed numerically to investigate the relationship between the cell formation and Nusselt number. Based on the analogy, heat transfer experiments were replaced by mass transfer experiments using a sulfuric acid-copper sulfate (H 2 SO 4 -CuSO 4 ) electroplating system. Numerical analysis using the commercial CFD program FLUENT 6.3 were carried out with the same material properties and heating conditions
Czech Academy of Sciences Publication Activity Database
Musilová, Věra; Králík, Tomáš; La Mantia, M.; Macek, Michal; Urban, Pavel; Skrbek, L.
2017-01-01
Roč. 832, OCT 26 (2017), s. 721-744 ISSN 0022-1120 R&D Projects: GA ČR(CZ) GA17-03572S; GA MŠk(CZ) LO1212 Institutional support: RVO:68081731 Keywords : Benard convection * turbulent convection * turbulent flows Subject RIV: BK - Fluid Dynamics OBOR OECD: Fluids and plasma physics (including surface physics) Impact factor: 2.821, year: 2016
Energy Technology Data Exchange (ETDEWEB)
Martinand, D
2003-01-15
This analytical study deals with the spatio-temporal evolution of linear thermo-convective instabilities in a horizontal fluid layer heated from below (the Rayleigh--Benard system) and subject to a horizontal pressure gradient (Poiseuille flow). The novelty consists of a spatially inhomogeneous temperature, in the form of a two-dimensional bump imposed on the lower plate, while the upper plate is kept at a constant temperature. The inhomogeneous boundary temperature and the mean flow of the Rayleigh--Benard--Poiseuille system break the symmetries of the classical Rayleigh--Benard system. The instabilities of interest are therefore spatially localised packets of convection rolls. If a mode of this type is synchronized, it is called a global mode. Assuming that the characteristic scale of the spatial variation of the lower plate temperature is large compared to the wavelength of the rolls, global modes are sought in the form of Eigenmodes in the confined vertical direction, modulated by a two-dimensional WKBJ expansion in the slowly-varying horizontal directions. Such an expansion breaks down at points where the group velocity of the instability vanishes, i.e. at WKBJ turning points. In the neighbourhood of one such point, located at the top of the temperature bump, the boundedness of the solution imposes a selection criterion for the global modes which provides the growth rate (or equivalently the critical threshold), the frequency and the wave vector of the most amplified global mode. This study thus generalizes to two-dimensional cases the methods used and the results obtained for one-dimensional inhomogeneities. The analysis is first applied to a simplified governing equation obtained by an envelope formalism and the analytical results are compared with numerical solutions of the amplitude equation. The formalism is finally applied to the Rayleigh--Benard--Poiseuille system described by the Navier--Stokes equations with the Boussinesq approximation. (author)
Laser speckle velocimetry applied to Rayleigh-Benard convection
International Nuclear Information System (INIS)
Arroyo, M.P.; Yonte, T.; Quintanilla, M.; Saviron, J.M.
1986-01-01
An application of speckle velocimetry technique to Rayleigh-Benard convection is presented. A 5-mW He-Ne laser allows precise determination of the two-dimensional velocity flow field, up to several mm/sec. The digital techniques used to analyze automatically the multiexposed photographs and to generate velocity and vorticity fields are described. The obtained results are in good agreement with previously reported data. The ability of the technique to cover other experimental conditions is discussed. 14 references
Universality in quasiperiodic Rayleigh-Benard convection
International Nuclear Information System (INIS)
Ecke, R.E.; Mainieri, R.; Sullivan, T.S.
1991-01-01
We study universal scaling properties of quasiperiodic Rayleigh-Benard convection in a 3 He--superfluid- 4 He mixture. The critical line is located in a parameter space of Rayleigh and Prandtl numbers using a transient-Poincare-section technique to identify transitions from nodal periodic points to spiral periodic points within resonance horns. We measure the radial and angular contraction rates and extract the linear-stability eigenvalues (Flouquet multipliers) of the periodic point. At the crossings of the critical line with the lines of fixed golden-mean-tail winding number we determine the universality class of our experimental dynamics using f(α) and trajectory-scaling-function analyses. A technique is used to obtain a robust five-scale approximation to the universal trajectory scaling function. Different methods of multifractal analysis are employed and an understanding of statistical and systematic errors in these procedures is developed. The power law of the inflection point of the map, determined for three golden-mean-tail winding numbers, is 2.9±0.3, corresponding to the universality class of the sine map
Rayleigh-Benard convection as a Nambu-metriplectic problem
International Nuclear Information System (INIS)
Bihlo, A
2008-01-01
The traditional Hamiltonian structure of the equations governing conservative Rayleigh-Benard convection (RBC) is singular, i.e., its Poisson bracket possesses nontrivial Casimir functionals. We show that a special form of one of these Casimirs can be used to extend the bilinear Poisson bracket to a trilinear generalized Nambu bracket. It is further shown that the equations governing dissipative RBC can be written as the superposition of the conservative Nambu bracket with a dissipative symmetric bracket. This leads to a Nambu-metriplectic system, which completes the geometrical picture of RBC. (fast track communication)
Homogeneous purely buoyancy driven turbulent flow
Arakeri, Jaywant; Cholemari, Murali; Pawar, Shashikant
2010-11-01
An unstable density difference across a long vertical tube open at both ends leads to convection that is axially homogeneous with a linear density gradient. We report results from such tube convection experiments, with driving density caused by salt concentration difference or temperature difference. At high enough Rayleigh numbers (Ra) the convection is turbulent with zero mean flow and zero mean Reynolds shear stresses; thus turbulent production is purely by buoyancy. We observe different regimes of turbulent convection. At very high Ra the Nusselt number scales as the square root of the Rayleigh number, giving the so-called "ultimate regime" of convection predicted for Rayleigh-Benard convection in limit of infinite Ra. Turbulent convection at intermediate Ra, the Nusselt number scales as Ra^0.3. In both regimes, the flux and the Taylor scale Reynolds number are more than order of magnitude larger than those obtained in Rayleigh-Benard convection. Absence of a mean flow makes this an ideal flow to study shear free turbulence near a wall.
Sterl, S.H.; Li, H.M.; Zhong, J.Q.
2016-01-01
In this paper, we present results from an experimental study into turbulent Rayleigh-Bénard convection forced externally by periodically modulated unidirectional rotation rates. We find that the azimuthal rotation velocity θ(t) and thermal amplitude δ(t) of the large-scale circulation (LSC) are
International Nuclear Information System (INIS)
Gellert, M; Beltrame, P; Egbers, C
2005-01-01
Spherical Rayleigh-Benard convection under the influence of an artificial central force field produced by the so-called dielectrophoretic effect is studied as a simplified model of the flow in the outer earth core. The fluid motion there is most probably driving the earth's dynamo and the energy source for the earth's magnetic field. Studying convective flows in earth-like geometry could lead to a deeper understanding of the basics of these processes. This research is a preparatory study for the experiments on the International Space Station (ISS). A bifurcation-theoretical approach shows the existence of heteroclinic cycles between spherical modes (l, l + 1) for the non-rotating system. This behavior depends strong on the radius ratio of the spheres and will be hard to detect in the experiment. For slow rotations interactions of the azimuthal modes (m, m + 1) found in numerical simulations for supercritical states are supposed to be experimentally observable
Quasiperiodicity, mode-locking, and universal scaling in Rayleigh-Benard convection
International Nuclear Information System (INIS)
Ecke, R.E.
1990-01-01
This major review paper describes research on a model nonlinear dynamical system of small-aspect-ratio Rayleigh-Benard convection in 3 He - 4 He mixtures. The nonlinear effects of mode locking and quasiperiodic behavior are described. Analysis techniques for characterizing the state of the dynamical system include Fourier transforms, Poincare sections, phase differences, transients, multifractal f(∝) spectra and scaling function dynamics. Theoretical results such as the fractal staircase of mode-locked intervals and the Arnold tongues are reproduced in experimental data. New techniques for analyzing scaling dynamics are developed and discussed. This is a tutorial article that introduces the major important concepts in nonlinear dynamics and focuses on experimental problems and techniques. 77 refs
Vortex statistics in turbulent rotating convection
Kunnen, R.P.J.; Clercx, H.J.H.; Geurts, B.J.
2010-01-01
The vortices emerging in rotating turbulent Rayleigh-Bénard convection in water at Rayleigh number Ra=6.0×108 are investigated using stereoscopic particle image velocimetry and by direct numerical simulation. The so-called Q criterion is used to detect the vortices from velocity fields. This
Introductory analysis of Benard-Marangoni convection
International Nuclear Information System (INIS)
Maroto, J A; Perez-Munuzuri, V; Romero-Cano, M S
2007-01-01
We describe experiments on Benard-Marangoni convection which permit a useful understanding of the main concepts involved in this phenomenon such as, for example, Benard cells, aspect ratio, Rayleigh and Marangoni numbers, Crispation number and critical conditions. In spite of the complexity of convection theory, we carry out a simple and introductory analysis which has the additional advantage of providing very suggestive experiments. As a consequence, we recommend our device for use as a laboratory experiment for undergraduate students of the thermodynamics of nonlinear and fluid physics
Introductory analysis of Benard-Marangoni convection
Energy Technology Data Exchange (ETDEWEB)
Maroto, J A [Group of Physics and Chemistry of Linares, Escuela Politecnica Superior, St Alfonso X El Sabio, 28, University of Jaen, E-23700 Linares, Jaen (Spain); Perez-Munuzuri, V [Group of Nonlinear Physics, University of Santiago de Compostela, E-15782 Santiago de Compostela (Spain); Romero-Cano, M S [Group of Complex Fluids Physics, Department of Applied Physics, University of Almeria, E-04120 Almeria (Spain)
2007-03-15
We describe experiments on Benard-Marangoni convection which permit a useful understanding of the main concepts involved in this phenomenon such as, for example, Benard cells, aspect ratio, Rayleigh and Marangoni numbers, Crispation number and critical conditions. In spite of the complexity of convection theory, we carry out a simple and introductory analysis which has the additional advantage of providing very suggestive experiments. As a consequence, we recommend our device for use as a laboratory experiment for undergraduate students of the thermodynamics of nonlinear and fluid physics.
Rotating Rayleigh-Bénard convection at low Prandtl number
Aguirre Guzman, Andres; Ostilla-Monico, Rodolfo; Clercx, Herman; Kunnen, Rudie
2017-11-01
Most geo- and astrophysical convective flows are too remote or too complex for direct measurements of the physical quantities involved, and thus a reduced framework with the main physical constituents is beneficial. This approach is given by the problem of rotating Rayleigh-Bénard convection (RRBC). For large-scale systems, the governing parameters of RRBC take extreme values, leading to the geostrophic turbulent regime. We perform Direct Numerical Simulations to investigate the transition to this regime at low Prandtl number (Pr). In low- Pr fluids, thermal diffusivity dominates over momentum diffusivity; we use Pr = 0.1 , relevant to liquid metals. In particular, we study the convective heat transfer (Nusselt number Nu) as a function of rotation (assessed by the Ekman number Ek). The strength of the buoyant forcing (Rayleigh number Ra) is Ra = 1 ×1010 to ensure turbulent convection. Varying Ek , we observe a change of the power-law scaling Nu Ekβ that suggests a transition to geostrophic turbulence, which is likely to occur at Ek = 9 ×10-7 . The thermal boundary layer thickness, however, may suggest a transition at lower Ekman numbers, indicating that perhaps not all statistical quantities show a transitional behaviour at the same Ek .
Breakdown of large-scale circulation in turbulent rotating convection
Kunnen, R.P.J.; Clercx, H.J.H.; Geurts, Bernardus J.
2008-01-01
Turbulent rotating convection in a cylinder is investigated both numerically and experimentally at Rayleigh number Ra = $10^9$ and Prandtl number $\\sigma$ = 6.4. In this Letter we discuss two topics: the breakdown under rotation of the domain-filling large-scale circulation (LSC) typical for
Breakdown of the large-scale circulation in $\\Gamma = 1/2$ rotating Rayleigh-Bénard flow
Stevens, Richard Johannes Antonius Maria; Clercx, H.J.H.; Lohse, Detlef
2012-01-01
Experiments and simulations of rotating Rayleigh-Bénard convection in cylindrical samples have revealed an increase in heat transport with increasing rotation rate. This heat transport enhancement is intimately related to a transition in the turbulent flow structure from a regime dominated by a
New phenomena in variable-density Rayleigh-Taylor turbulence
Energy Technology Data Exchange (ETDEWEB)
Livescu, D; Ristorcelli, J R; Petersen, M R; Gore, R A, E-mail: livescu@lanl.gov [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)
2010-12-15
This paper presents several issues related to mixing and turbulence structure in buoyancy-driven turbulence at low to moderate Atwood numbers, A, found from direct numerical simulations in two configurations: classical Rayleigh-Taylor instability and an idealized triply periodic Rayleigh-Taylor flow. Simulations at A up to 0.5 are used to examine the turbulence characteristics and contrast them with those obtained close to the Boussinesq approximation. The data sets used represent the largest simulations to date in each configuration. One of the more remarkable issues explored, first reported in (Livescu and Ristorcelli 2008 J. Fluid Mech. 605 145-80), is the marked difference in mixing between different density fluids as opposed to the mixing that occurs between fluids of commensurate densities, corresponding to the Boussinesq approximation. Thus, in the triply periodic configuration and the non-Boussinesq case, an initially symmetric density probability density function becomes skewed, showing that the mixing is asymmetric, with pure heavy fluid mixing more slowly than pure light fluid. A mechanism producing the mixing asymmetry is proposed and the consequences for the classical Rayleigh-Taylor configuration are discussed. In addition, it is shown that anomalous small-scale anisotropy found in the homogeneous configuration (Livescu and Ristorcelli 2008 J. Fluid Mech. 605 145-80) and Rayleigh-Taylor turbulence at A=0.5 (Livescu et al 2008 J. Turbul. 10 1-32) also occurs near the Boussinesq limit. Results pertaining to the moment closure modelling of Rayleigh-Taylor turbulence are also presented. Although the Rayleigh-Taylor mixing layer width reaches self-similar growth relatively fast, the lower-order terms in the self-similar expressions for turbulence moments have long-lasting effects and derived quantities, such as the turbulent Reynolds number, are slow to follow the self-similar predictions. Since eddy diffusivity in the popular gradient transport hypothesis
Numerical simulation of Rayleigh-Taylor turbulent mixing layers
International Nuclear Information System (INIS)
Poujade, O.; Lardjane, N.; Peybernes, M.; Boulet, M.
2009-01-01
Accelerations in actual Rayleigh-Taylor instabilities are often variable. This article focuses on a particular class of variable accelerations where g(t) ∝ t n . A reference database is built from high resolution hydrodynamic numerical simulations. The successful comparison with a simple OD analytical model and the statistical 2SFK (2-Structure, 2-Fluid, 2-Turbulence) turbulence model is provided. Moreover, we show the difference between the mechanism at play in the Rayleigh-Taylor turbulent mixing zone and Kolmogorov's in the self similar developed turbulent regime. (authors)
Boundary Layer Control of Rotating Convection Systems
King, E. M.; Stellmach, S.; Noir, J.; Hansen, U.; Aurnou, J. M.
2008-12-01
Rotating convection is ubiquitous in the natural universe, and is likely responsible for planetary processes such magnetic field generation. Rapidly rotating convection is typically organized by the Coriolis force into tall, thin, coherent convection columns which are aligned with the axis of rotation. This organizational effect of rotation is thought to be responsible for the strength and structure of magnetic fields generated by convecting planetary interiors. As thermal forcing is increased, the relative influence of rotation weakens, and fully three-dimensional convection can exist. It has long been assumed that rotational effects will dominate convection dynamics when the ratio of buoyancy to the Coriolis force, the convective Rossby number, Roc, is less than unity. We investigate the influence of rotation on turbulent Rayleigh-Benard convection via a suite of coupled laboratory and numerical experiments over a broad parameter range: Rayleigh number, 10310; Ekman number, 10-6≤ E ≤ ∞; and Prandtl number, 1≤ Pr ≤ 100. In particular, we measure heat transfer (as characterized by the Nusselt number, Nu) as a function of the Rayleigh number for several different Ekman and Prandtl numbers. Two distinct heat transfer scaling regimes are identified: non-rotating style heat transfer, Nu ~ Ra2/7, and quasigeostrophic style heat transfer, Nu~ Ra6/5. The transition between the non-rotating regime and the rotationally dominant regime is described as a function of the Ekman number, E. We show that the regime transition depends not on the global force balance Roc, but on the relative thicknesses of the thermal and Ekman boundary layers. The transition scaling provides a predictive criterion for the applicability of convection models to natural systems such as Earth's core.
Rotating thermal convection at very large Rayleigh numbers
Weiss, Stephan; van Gils, Dennis; Ahlers, Guenter; Bodenschatz, Eberhard
2016-11-01
The large scale thermal convection systems in geo- and astrophysics are usually influenced by Coriolis forces caused by the rotation of their celestial bodies. To better understand the influence of rotation on the convective flow field and the heat transport at these conditions, we study Rayleigh-Bénard convection, using pressurized sulfur hexaflouride (SF6) at up to 19 bars in a cylinder of diameter D=1.12 m and a height of L=2.24 m. The gas is heated from below and cooled from above and the convection cell sits on a rotating table inside a large pressure vessel (the "Uboot of Göttingen"). With this setup Rayleigh numbers of up to Ra =1015 can be reached, while Ekman numbers as low as Ek =10-8 are possible. The Prandtl number in these experiment is kept constant at Pr = 0 . 8 . We report on heat flux measurements (expressed by the Nusselt number Nu) as well as measurements from more than 150 temperature probes inside the flow. We thank the Deutsche Forschungsgemeinschaft (DFG) for financial support through SFB963: "Astrophysical Flow Instabilities and Turbulence". The work of GA was supported in part by the US National Science Foundation through Grant DMR11-58514.
Turbulent convection in liquid metal with and without rotation.
King, Eric M; Aurnou, Jonathan M
2013-04-23
The magnetic fields of Earth and other planets are generated by turbulent, rotating convection in liquid metal. Liquid metals are peculiar in that they diffuse heat more readily than momentum, quantified by their small Prandtl numbers, Pr rotating Rayleigh-Bénard convection experiments in the liquid metal gallium (Pr = 0.025) over a range of nondimensional buoyancy forcing (Ra) and rotation periods (E). Our primary diagnostic is the efficiency of convective heat transfer (Nu). In general, we find that the convective behavior of liquid metal differs substantially from that of moderate Pr fluids, such as water. In particular, a transition between rotationally constrained and weakly rotating turbulent states is identified, and this transition differs substantially from that observed in moderate Pr fluids. This difference, we hypothesize, may explain the different classes of magnetic fields observed on the Gas and Ice Giant planets, whose dynamo regions consist of Pr 1 fluids, respectively.
Turbulent boundary layer in high Rayleigh number convection in air.
du Puits, Ronald; Li, Ling; Resagk, Christian; Thess, André; Willert, Christian
2014-03-28
Flow visualizations and particle image velocimetry measurements in the boundary layer of a Rayleigh-Bénard experiment are presented for the Rayleigh number Ra=1.4×1010. Our visualizations indicate that the appearance of the flow structures is similar to ordinary (isothermal) turbulent boundary layers. Our particle image velocimetry measurements show that vorticity with both positive and negative sign is generated and that the smallest flow structures are 1 order of magnitude smaller than the boundary layer thickness. Additional local measurements using laser Doppler velocimetry yield turbulence intensities up to I=0.4 as in turbulent atmospheric boundary layers. From our observations, we conclude that the convective boundary layer becomes turbulent locally and temporarily although its Reynolds number Re≈200 is considerably smaller than the value 420 underlying existing phenomenological theories. We think that, in turbulent Rayleigh-Bénard convection, the transition of the boundary layer towards turbulence depends on subtle details of the flow field and is therefore not universal.
Rotating turbulent Rayleigh-Bénard convection subject to harmonically forced flow reversals
Geurts, B.J.; Kunnen, R.P.J.
2014-01-01
The characteristics of turbulent flow in a cylindrical Rayleigh–B´enard convection cell which can be modified considerably in case rotation is included in the dynamics. By incorporating the additional effects of an Euler force, i.e., effects induced by nonconstant rotation rates, a remarkably strong
Turbulence statistics and energy budget in rotating Rayleigh-Bénard convection
Kunnen, R.P.J.; Geurts, Bernardus J.; Clercx, H.J.H.
The strongly-modified turbulence statistics of Rayleigh–Bénard convection subject to various rotation rates is addressed by numerical investigations. The flow is simulated in a domain with periodic boundary conditions in the horizontal directions, and confined vertically by parallel no-slip
A priori study of subgrid-scale features in turbulent Rayleigh-Bénard convection
Dabbagh, F.; Trias, F. X.; Gorobets, A.; Oliva, A.
2017-10-01
At the crossroad between flow topology analysis and turbulence modeling, a priori studies are a reliable tool to understand the underlying physics of the subgrid-scale (SGS) motions in turbulent flows. In this paper, properties of the SGS features in the framework of a large-eddy simulation are studied for a turbulent Rayleigh-Bénard convection (RBC). To do so, data from direct numerical simulation (DNS) of a turbulent air-filled RBC in a rectangular cavity of aspect ratio unity and π spanwise open-ended distance are used at two Rayleigh numbers R a ∈{1 08,1 010 } [Dabbagh et al., "On the evolution of flow topology in turbulent Rayleigh-Bénard convection," Phys. Fluids 28, 115105 (2016)]. First, DNS at Ra = 108 is used to assess the performance of eddy-viscosity models such as QR, Wall-Adapting Local Eddy-viscosity (WALE), and the recent S3PQR-models proposed by Trias et al. ["Building proper invariants for eddy-viscosity subgrid-scale models," Phys. Fluids 27, 065103 (2015)]. The outcomes imply that the eddy-viscosity modeling smoothes the coarse-grained viscous straining and retrieves fairly well the effect of the kinetic unfiltered scales in order to reproduce the coherent large scales. However, these models fail to approach the exact evolution of the SGS heat flux and are incapable to reproduce well the further dominant rotational enstrophy pertaining to the buoyant production. Afterwards, the key ingredients of eddy-viscosity, νt, and eddy-diffusivity, κt, are calculated a priori and revealed positive prevalent values to maintain a turbulent wind essentially driven by the mean buoyant force at the sidewalls. The topological analysis suggests that the effective turbulent diffusion paradigm and the hypothesis of a constant turbulent Prandtl number are only applicable in the large-scale strain-dominated areas in the bulk. It is shown that the bulk-dominated rotational structures of vortex-stretching (and its synchronous viscous dissipative structures) hold
Dijkstra, Henk A.
1992-01-01
Multiple steady flow patterns occur in surface-tension/buoyancy-driven convection in a liquid layer heated from below (Rayleigh-Benard-Marangoni flows). Techniques of numerical bifurcation theory are used to study the multiplicity and stability of two-dimensional steady flow patterns (rolls) in rectangular small-aspect-ratio containers as the aspect ratio is varied. For pure Marangoni flows at moderate Biot and Prandtl number, the transitions occurring when paths of codimension 1 singularities intersect determine to a large extent the multiplicity of stable patterns. These transitions also lead, for example, to Hopf bifurcations and stable periodic flows for a small range in aspect ratio. The influence of the type of lateral walls on the multiplicity of steady states is considered. 'No-slip' lateral walls lead to hysteresis effects and typically restrict the number of stable flow patterns (with respect to 'slippery' sidewalls) through the occurrence of saddle node bifurcations. In this way 'no-slip' sidewalls induce a selection of certain patterns, which typically have the largest Nusselt number, through secondary bifurcation.
Stochastic model of Rayleigh-Taylor turbulent mixing
International Nuclear Information System (INIS)
Abarzhi, S.I.; Cadjan, M.; Fedotov, S.
2007-01-01
We propose a stochastic model to describe the random character of the dissipation process in Rayleigh-Taylor turbulent mixing. The parameter alpha, used conventionally to characterize the mixing growth-rate, is not a universal constant and is very sensitive to the statistical properties of the dissipation. The ratio between the rates of momentum loss and momentum gain is the statistic invariant and a robust parameter to diagnose with or without turbulent diffusion accounted for
Periodic Boundary Motion in Thermal Turbulence
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Zhang, Jun; Libchaber, Albert
2000-01-01
A free-floating plate is introduced in a Benard convection cell with an open surface. It partially covers the cell and distorts the local heat flux, inducing a coherent flow that in turn moves the plate. Remarkably, the plate can be driven to a periodic motion even under the action of a turbulent fluid. The period of the oscillation depends on the coverage ratio, and on the Rayleigh number of the convective system. The plate oscillatory behavior observed in this experiment may be related to a geological model, in which continents drift in a quasiperiodic fashion. (c) 2000 The American Physical Society
Rayleigh Waves in a Rotating Orthotropic Micropolar Elastic Solid Half-Space
Directory of Open Access Journals (Sweden)
Baljeet Singh
2013-01-01
Full Text Available A problem on Rayleigh wave in a rotating half-space of an orthotropic micropolar material is considered. The governing equations are solved for surface wave solutions in the half space of the material. These solutions satisfy the boundary conditions at free surface of the half-space to obtain the frequency equation of the Rayleigh wave. For numerical purpose, the frequency equation is approximated. The nondimensional speed of Rayleigh wave is computed and shown graphically versus nondimensional frequency and rotation-frequency ratio for both orthotropic micropolar elastic and isotropic micropolar elastic cases. The numerical results show the effects of rotation, orthotropy, and nondimensional frequency on the nondimensional speed of the Rayleigh wave.
Sterl, Sebastian; Li, Hui-Min; Zhong, Jin-Qiang
2016-12-01
In this paper, we present results from an experimental study into turbulent Rayleigh-Bénard convection forced externally by periodically modulated unidirectional rotation rates. We find that the azimuthal rotation velocity θ ˙(t ) and thermal amplitude δ (t ) of the large-scale circulation (LSC) are modulated by the forcing, exhibiting a variety of dynamics including increasing phase delays and a resonant peak in the amplitude of θ ˙(t ) . We also focus on the influence of modulated rotation rates on the frequency of occurrence η of stochastic cessation or reorientation events, and on the interplay between such events and the periodically modulated response of θ ˙(t ) . Here we identify a mechanism by which η can be amplified by the modulated response, and these normally stochastic events can occur with high regularity. We provide a modeling framework that explains the observed amplitude and phase responses, and we extend this approach to make predictions for the occurrence of cessation events and the probability distributions of θ ˙(t ) and δ (t ) during different phases of a modulation cycle, based on an adiabatic approach that treats each phase separately. Last, we show that such periodic forcing has consequences beyond influencing LSC dynamics, by investigating how it can modify the heat transport even under conditions where the Ekman pumping effect is predominant and strong enhancement of heat transport occurs. We identify phase and amplitude responses of the heat transport, and we show how increased modulations influence the average Nusselt number.
Non-Oberbeck-Boussinesq effects in two-dimensional Rayleigh-Bénard convection in glycerol
Sugiyama, K.; Calzavarini, E.; Grossmann, S.; Lohse, Detlef
2007-01-01
We numerically analyze Non-Oberbeck-Boussinesq (NOB) effects in two-dimensional Rayleigh-Benard flow in glycerol, which shows a dramatic change in the viscosity with temperature. The results are presented both as functions of the Rayleigh number Ra up to 108 (for fixed temperature difference Î�
Heat transfer and large scale dynamics in turbulent Rayleigh-Bénard convection
Ahlers, Günter; Grossmann, Siegfried; Lohse, Detlef
2009-01-01
The progress in our understanding of several aspects of turbulent Rayleigh-Bénard convection is reviewed. The focus is on the question of how the Nusselt number and the Reynolds number depend on the Rayleigh number Ra and the Prandtl number Pr, and on how the thicknesses of the thermal and the
Turbulent thermal superstructures in Rayleigh-Bénard convection
Stevens, Richard J. A. M.; Blass, Alexander; Zhu, Xiaojue; Verzicco, Roberto; Lohse, Detlef
2018-04-01
We report the observation of superstructures, i.e., very large-scale and long living coherent structures in highly turbulent Rayleigh-Bénard convection up to Rayleigh Ra=109 . We perform direct numerical simulations in horizontally periodic domains with aspect ratios up to Γ =128 . In the considered Ra number regime the thermal superstructures have a horizontal extend of six to seven times the height of the domain and their size is independent of Ra. Many laboratory experiments and numerical simulations have focused on small aspect ratio cells in order to achieve the highest possible Ra. However, here we show that for very high Ra integral quantities such as the Nusselt number and volume averaged Reynolds number only converge to the large aspect ratio limit around Γ ≈4 , while horizontally averaged statistics such as standard deviation and kurtosis converge around Γ ≈8 , the integral scale converges around Γ ≈32 , and the peak position of the temperature variance and turbulent kinetic energy spectra only converge around Γ ≈64 .
International Nuclear Information System (INIS)
Pomeau, Y.
1981-07-01
In this work it is reviewed a few known types of transition to turbulence, as the cascade of period doubling and the intermittent transition. This happens in dynamical systems with a few degrees of freedom, as modelled by the iteration of non linear maps. Then it is presented specific transitions for systems with many degrees of freedom. It is condidered first the occurence of a low frequency broadband noise in large cells at the onset of Rayleigh-Benard convection; then the transition by intermittent bursts in parallel flows. In this last case, one is concerned with localized and finite amplitude perturbations. Simple geometric arguments show that these fluctuations, when they are isolated and with a well definite relative speed, exist for a single value of the Reynolds number only [fr
Energy Technology Data Exchange (ETDEWEB)
Rollin, Bertrand [Los Alamos National Laboratory; Andrews, Malcolm J [Los Alamos National Laboratory
2010-01-01
We present our progress toward setting initial conditions in variable density turbulence models. In particular, we concentrate our efforts on the BHR turbulence model for turbulent Rayleigh-Taylor instability. Our approach is to predict profiles of relevant parameters before the fully turbulent regime and use them as initial conditions for the turbulence model. We use an idealized model of the mixing between two interpenetrating fluids to define the initial profiles for the turbulence model parameters. Velocities and volume fractions used in the idealized mixing model are obtained respectively from a set of ordinary differential equations modeling the growth of the Rayleigh-Taylor instability and from an idealization of the density profile in the mixing layer. A comparison between predicted initial profiles for the turbulence model parameters and initial profiles of the parameters obtained from low Atwood number three dimensional simulations show reasonable agreement.
Heat Transport Enhancement of Turbulent Thermal Convection by Inserted Channels
Xia, Ke-Qing; Zhang, Lu
2017-11-01
We report an experimental study on the heat transport properties of turbulent Rayleigh Benard Convection (RBC) in a rectangular cell with two types of 3D-printed structures inserted inside. The first one splits the original rectangular cell into 60 identical sub cells whose aspect ratio is 1:1:10 (length, width, height). The second one splits the cell into 30 sub cells, each with a 1:2:10 aspect ratio and a baffle in the center. We find that for large Rayleigh numbers (Ra), the Nusselt numbers (Nu) of both structures increase compared with that of the empty rectangular cell. An enhancement in Nu as much as 20% is found for the second type of insertion at Rayleigh number 2 ×109 . Moreover, the Nu-Ra scaling shows a transition with both geometries. The particle image velocimetry (PIV) measurement within a single sub unit indicates that the transition may be related to the laminar to turbulent transition in flow field. Direct numerical simulations (DNS) confirm the experimental results. Our results demonstrate the potential in using insertions to enhance passive heat transfer. This work was supported by the Research Grants Council (RGC) of HKSAR (Nos. CUHK404513 and CUHK14301115).
Turbulence modeling of natural convection in enclosures: A review
International Nuclear Information System (INIS)
Choi, Seok Ki; Kim, Seong O
2012-01-01
In this paper a review of recent developments of turbulence models for natural convection in enclosures is presented. The emphasis is placed on the effect of the treatments of Reynolds stress and turbulent heat flux on the stability and accuracy of the solution for natural convection in enclosures. The turbulence models considered in the preset study are the two-layer k -ε model, the shear stress transport (SST) model, the elliptic-relaxation (V2-f) model and the elliptic-blending second-moment closure (EBM). Three different treatments of the turbulent heat flux are the generalized gradient diffusion hypothesis (GGDH), the algebraic flux model (AFM) and the differential flux model (DFM). The mathematical formulation of the above turbulence models and their solution method are presented. Evaluation of turbulence models are performed for turbulent natural convection in a 1:5 rectangular cavity ( Ra = 4.3x10 10 ) and in a square cavity with conducting top and bottom walls ( Ra =1.58x10 9 ) and the Rayleigh-Benard convection ( Ra = 2x10 6 ∼ Ra =10 9 ). The relative performances of turbulence models are examined and their successes and shortcomings are addressed
Boundary layers in turbulent convection for air, liquid gallium and liquid sodium
Scheel, Janet; Schumacher, Joerg
2017-11-01
The scaling of physical quantities that characterize the shape and dynamics of the viscous and thermal boundary layers with respect to the Rayleigh number will be presented for three series of three-dimensional high-resolution direct numerical simulations of Rayleigh-Benard convection (RBC) in a closed cylindrical cell of aspect ratio one. The simulations have been conducted for convection in air at a Prandtl number Pr = 0.7, in liquid gallium at Pr = 0.021 and in liquid sodium at Pr = 0.005. Then we discuss three statistical analysis methods which have been developed to predict the transition of turbulent RBC into the ultimate regime. The methods are based on the large-scale properties of the velocity profile. All three methods indicate that the range of critical Rayleigh numbers is shifted to smaller magnitudes as the Prandtl number becomes smaller. This work is supported by the Priority Programme SPP 1881 of the Deutsche Forschungsgemeinschaft.
Energy Technology Data Exchange (ETDEWEB)
Rollin, Bertrand [Los Alamos National Laboratory; Andrews, Malcolm J [Los Alamos National Laboratory
2010-01-01
We present our progress toward setting initial conditions in variable density turbulence models. In particular, we concentrate our efforts on the BHR turbulence model for turbulent Rayleigh-Taylor instability. Our approach is to predict profiles of relevant variables before fully turbulent regime and use them as initial conditions for the turbulence model. We use an idealized model of mixing between two interpenetrating fluids to define the initial profiles for the turbulence model variables. Velocities and volume fractions used in the idealized mixing model are obtained respectively from a set of ordinary differential equations modeling the growth of the Rayleigh-Taylor instability and from an idealization of the density profile in the mixing layer. A comparison between predicted profiles for the turbulence model variables and profiles of the variables obtained from low Atwood number three dimensional simulations show reasonable agreement.
Experimental investigation of turbulent mixing by Rayleigh-Taylor instability
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Youngs, D.L.
1992-01-01
A key feature of compressible turbulent mixing is the generation of vorticity via the ∇px ∇(1/ρ) term. This source of vorticity is also present in incompressible flows involving the mixing of fluids of different density, for example Rayleigh-Taylor unstable flows. This paper gives a summary of an experimental investigation of turbulent mixing at a plane boundary between two fluids, of densities ρ 1 , and ρ 2 . (ρ 1 > ρ 2 ) due to Rayleigh-Taylor instability. The two fluids are near incompressible and mixing occurs when an approximately constant acceleration, g, is applied normal to the interface with direction from fluid 2 to fluid 1. Full details of the experimental programme are given in a set of three reports. Some of the earlier experiments are also described by Read. Previous experimental work and much of the theoretical research has concentrated on studying the growth of the instability from a single wavelength perturbation rather than turbulent mixing. Notable exceptions are published in the Russian literature. A related process, turbulent mixing induced by the passage of shock waves though an interface between fluids of different density is described by Andronov et al. The major purpose of the experiments described here was to study the evolution of the instability from small random perturbations where it is found that large and larger structures appear as time proceeds. A novel technique was used to provide the desired acceleration. The two fluids were enclosed in a rectangular tank, the lighter fluid 2 initially resting on top of the denser fluid 1. One or more rocket motors were then used to drive the tank vertically downwards. The aim of the experimental programme is to provide data for the calibration of a turbulence model used to predict mixing in real situations
Effect of magnetic field on the Rayleigh Taylor instability of rotating and stratified plasma
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Sharma, PK; Tiwari, Anita; Argal, Shraddha
2017-01-01
In the present study the effect of magnetic field and rotation have been carried out on the Rayleigh Taylor instability of conducting and rotating plasma, which is assumed to be incompressible and confined between two rigid planes z = 0 and z = h. The dispersion relation of the problem is obtained by solving the basic MHD equations of the problem with the help normal mode technique and appropriate boundary conditions. The dispersion relation of the medium is analysed and the effect of magnetic field and angular velocity (rotation effect) have been examined on the growth rate of Rayleigh Taylor instability. It is found that the magnetic field and angular velocity (rotation effect) have stabilizing influence on the Rayleigh Taylor instability. (paper)
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Sarazin, Y
2004-03-01
This document gathers the lectures made in the framework of a Ph.D level physics class dedicated to plasma physics. This course is made up of 3 parts : 1) collisions and transport, 2) transport and turbulence, and 3) study of a few exchange instabilities. More precisely the first part deals with the following issues: thermonuclear fusion, Coulomb collisions, particles trajectories in a tokamak, neo-classical transport in tokamaks, the bootstrap current, and ware pinch. The second part involves: particle transport in tokamaks, quasi-linear transport, resonance islands, resonance in tokamaks, from quasi to non-linear transport, and non-linear saturation of turbulence. The third part deals with: shift velocities in fluid theory, a model for inter-change instabilities, Rayleigh-Benard instability, Hasegawa-Wakatani model, and Hasegawa-Mima model. This document ends with a series of appendices dealing with: particle-wave interaction, determination of the curvature parameter G, Rossby waves.
Turbulent Superstructures in Rayleigh-Bénard convection at different Prandtl number
Schumacher, Jörg; Pandey, Ambrish; Ender, Martin; Westermann, Rüdiger; Scheel, Janet D.
2017-11-01
Large-scale patterns of the temperature and velocity field in horizontally extended cells can be considered as turbulent superstructures in Rayleigh-Bénard convection (RBC). These structures are obtained once the turbulent fluctuations are removed by a finite-time average. Their existence has been reported for example in Bailon-Cuba et al.. This large-scale order obeys a strong similarity with the well-studied patterns from the weakly nonlinear regime at lower Rayleigh number in RBC. In the present work we analyze the superstructures of RBC at different Prandtl number for Prandtl values between Pr = 0.005 for liquid sodium and 7 for water. The characteristic evolution time scales, the typical spatial extension of the rolls and the properties of the defects of the resulting superstructure patterns are analyzed. Data are obtained from well-resolved spectral element direct numerical simulations. The work is supported by the Priority Programme SPP 1881 of the Deutsche Forschungsgemeinschaft.
Cell structures caused by settling particles in turbulent Rayleigh-Bénard convection
Lee, Changhoon; Park, Sangro
2016-11-01
Turbulent thermal convection is an important phenomenon frequently found in nature and industrial processes, often with laden particles. In the last several decades, the vast majority of studies have addressed single phase convective flow with focus on the scaling relation of flow parameters associated with heat transfer. Particle-laden Rayleigh-Bénard convection, however, has not been sufficiently studied. In this study, modulation of cell structures by settling particles in turbulent Rayleigh-Bénard convection in a doubly periodic square channel is investigated using direct numerical simulation with a point particle approach. Flow parameters are fixed at Rayleigh number=106, Prandtl number=0.7, the aspect ratio=6, and Froude number=0.19. We report from the simulations that settling heavy particles modulate irregular large-scale thermal plume structures into organized polygonal cell structures. Different shapes of flow structures are obtained for different particle diameters and mass loadings. We found that polygonal cell structures arise due to asymmetric feedback force exerted by particles onto hot and cold plumes. Increasing the number of particles augments the asymmetry and the polygonal cell structures become smaller, eventually going to the hexagonal structures.
Tentative detection of clear-air turbulence using a ground-based Rayleigh lidar.
Hauchecorne, Alain; Cot, Charles; Dalaudier, Francis; Porteneuve, Jacques; Gaudo, Thierry; Wilson, Richard; Cénac, Claire; Laqui, Christian; Keckhut, Philippe; Perrin, Jean-Marie; Dolfi, Agnès; Cézard, Nicolas; Lombard, Laurent; Besson, Claudine
2016-05-01
Atmospheric gravity waves and turbulence generate small-scale fluctuations of wind, pressure, density, and temperature in the atmosphere. These fluctuations represent a real hazard for commercial aircraft and are known by the generic name of clear-air turbulence (CAT). Numerical weather prediction models do not resolve CAT and therefore provide only a probability of occurrence. A ground-based Rayleigh lidar was designed and implemented to remotely detect and characterize the atmospheric variability induced by turbulence in vertical scales between 40 m and a few hundred meters. Field measurements were performed at Observatoire de Haute-Provence (OHP, France) on 8 December 2008 and 23 June 2009. The estimate of the mean squared amplitude of bidimensional fluctuations of lidar signal showed excess compared to the estimated contribution of the instrumental noise. This excess can be attributed to atmospheric turbulence with a 95% confidence level. During the first night, data from collocated stratosphere-troposphere (ST) radar were available. Altitudes of the turbulent layers detected by the lidar were roughly consistent with those of layers with enhanced radar echo. The derived values of turbulence parameters Cn2 or CT2 were in the range of those published in the literature using ST radar data. However, the detection was at the limit of the instrumental noise and additional measurement campaigns are highly desirable to confirm these initial results. This is to our knowledge the first successful attempt to detect CAT in the free troposphere using an incoherent Rayleigh lidar system. The built lidar device may serve as a test bed for the definition of embarked CAT detection lidar systems aboard airliners.
Benard convection in gaps and cavities
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Mueller, U.
1981-04-01
The article contains two parts. In the first part a condensed review of the most striking phenomena in Benard convection in laterally confined fluid layers is given. In the second part recent experimental and theoretical work on Benard convection in gaps is presented an analysed. (orig.) [de
GRAVITATIONALLY UNSTABLE FLAMES: RAYLEIGH-TAYLOR STRETCHING VERSUS TURBULENT WRINKLING
International Nuclear Information System (INIS)
Hicks, E. P.; Rosner, R.
2013-01-01
In this paper, we provide support for the Rayleigh-Taylor-(RT)-based subgrid model used in full-star simulations of deflagrations in Type Ia supernovae explosions. We use the results of a parameter study of two-dimensional direct numerical simulations of an RT unstable model flame to distinguish between the two main types of subgrid models (RT or turbulence dominated) in the flamelet regime. First, we give scalings for the turbulent flame speed, the Reynolds number, the viscous scale, and the size of the burning region as the non-dimensional gravity (G) is varied. The flame speed is well predicted by an RT-based flame speed model. Next, the above scalings are used to calculate the Karlovitz number (Ka) and to discuss appropriate combustion regimes. No transition to thin reaction zones is seen at Ka = 1, although such a transition is expected by turbulence-dominated subgrid models. Finally, we confirm a basic physical premise of the RT subgrid model, namely, that the flame is fractal, and thus self-similar. By modeling the turbulent flame speed, we demonstrate that it is affected more by large-scale RT stretching than by small-scale turbulent wrinkling. In this way, the RT instability controls the flame directly from the large scales. Overall, these results support the RT subgrid model.
The effect of centrifugal buoyancy on the heat transport in rotating Rayleigh-Bénard convection
Horn, Susanne; Aurnou, Jonathan
2017-11-01
In a rapidly rotating and differentially heated fluid, the centrifugal acceleration can play a similar role to that of gravity in generating convective motion. However, in the paradigm system of rotating Rayleigh-Bénard convection, centrifugal buoyancy is typically not considered in theoretical studies and, thus, usually undesired in laboratory experiments, despite being unavoidable. How centrifugal buoyancy affects the turbulent flow, including the heat transport, is still largely unknown, in particular, when it can be considered negligible. We study this problem by means of direct numerical simulations. Unlike in experiments, we are able to systematically vary the Froude number Fr (ratio of centrifugal to gravitational acceleration) and the Rossby number Ro (dimensionless rotation rate) independently, and even set each to zero exactly. We show that the centrifugal acceleration simultaneously leads to contending phenomena, e.g. reflected by an increase and a decrease of the center temperature, or a suppression and an enhancement of the heat transfer efficiency. Which one prevails as net effect strongly depends on the combination of Fr and Ro. Furthermore, we discuss implications for experiments of rapidly rotating convection. SH acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG) under Grant HO 5890/1-1, JA by the NSF Geophysics Program.
Pattern selection in single-component systems coupling Benard convection and solidification
International Nuclear Information System (INIS)
Davis, S.H.; Mueller, U.; Dietsche, C.
1983-12-01
A horizontal layer is heated from below and cooled from above so that the enclosed single-component liquid is frozen in the upper part of the layer. When the imposed temperature difference is such that the Rayleigh number across the liquid is supercritical, there is Benard convection coupled with the dynamics of the solidification interface. An experiment is presented which shows that the interfacial corrugations that result are two-dimensional when this ''ice'' is thin but hexagonal when the ''ice'' is thick. A weakly-nonlinear convective instability theory is presented which explains this behavior, and isolates the mechanism of the pattern selection. Jump behavior is seen in the liquid-layer thickness at the onset of hexagonal convection. (orig.) [de
Weiss, Stephan; Wei, Ping; Ahlers, Guenter
2015-11-01
Turbulent thermal convection under rotation shows a remarkable variety of different flow states. The Nusselt number (Nu) at slow rotation rates (expressed as the dimensionless inverse Rossby number 1/Ro), for example, is not a monotonic function of 1/Ro. Different 1/Ro-ranges can be observed with different slopes ∂Nu / ∂ (1 / Ro) . Some of these ranges are connected by sharp transitions where ∂Nu / ∂ (1 / Ro) changes discontinuously. We investigate different regimes in cylindrical samples of aspect ratio Γ = 1 by measuring temperatures at the sidewall of the sample for various Prandtl numbers in the range 3 Deutsche Forschungsgemeinschaft.
International Nuclear Information System (INIS)
Garcia Velarde, M.
1977-01-01
Thermoconvective instabilities in horizontal fluid layers are discussed with emphasis on the Rayleigh-Benard model problem. Steady solutions and time-dependent phenomena (relaxation oscillations and transition to turbulence) are studied within the nonlinear Boussinesq-Oberbeck approximation. Homogeneous steady solutions, limit cycles, and inhomogeneous (ordered) spatial structures are also studied in simple reaction-diffusion systems. Lastly, the non-periodic attractor that appears at large Rayleigh numbers in the truncated Boussinesq-Oberbeck model of Lorenz, is constructed, and a discussion of turbulent behavior is given. (author) [es
Numerical simulation of turbulent buoyant flows in horizontal channels
International Nuclear Information System (INIS)
Seiter, C.
1995-09-01
A numerical method is presented, to calculate the three-dimensional, time-dependent large scale structure of turbulent buoyant flows. The subject of the study is the Rayleigh-Benard-convection with air (Pr=0.71, Ra=2.5 10 6 , 10 7 ) and sodium (Pr=0.006, Ra=8.4 10 4 , 2.5 10 5 , 10 6 , 10 7 ) and a fluid layer with water and an internal heat source (Pr=7.0, Ra I =1.5 10 10 ) at moderate and high Rayleigh-numbers. The goal of the work is both, the analysis of structures of instantaneous as well as the statistical analysis of spatially and/or time averaged data, to give a contribution to the investigation of the characteristics of turbulent natural convection mainly in fluids with small Prandtl-numbers. The large eddy simulation of natural convection requires the development of appropriate momentum and heat subgrid scale models and the formulation of new boundary conditions. The used energy-length-models in the computer code TURBIT are extended methodically by modification of the characteristic length scales of the sub scale turbulence. The reduction or the increase of the sub scale turbulence correlations, caused by the influence of solid boundaries or the stratification, is considered. In the same way the new boundary conditions for the diffusive terms of the conservation equations are seen to be necessary, when the thermal or in the case of liquid metals the more critical hydrodynamic boundary layer is resolved insufficiently or not at all. The extended and new methods, models and boundary conditions, which enabled the realization of the planned simulations, are presented. (orig.)
Nusselt number and bulk temperature in turbulent Rayleigh-Bénard convection
Bodenschatz, Eberhard; Weiss, Stephan; Shishkina, Olga; International CollaborationTurbulence Research Collaboration
2017-11-01
We present an algorithm to calculate the Nusselt number (Nu) in measurements of the heat transport in turbulent Rayleigh-Bénard convection under general non-Oberbeck-Boussinesq (NOB) conditions. We further critically analyze the different ways to evaluate the dependences of Nu over the Rayleigh number (Ra) and show the sensitivity of these dependences to the reference temperatures in the bulk, top and bottom boundary layers (BLs). Finally we propose a method to predict the bulk temperature and a way to calculate the reference temperatures of the top and bottom BLs and validate them against the Göttingen measurements. The work is supported by the Max Planck Society and the Deutsche Forschungsgemeinschaft (DFG) under the Grant Sh 405/4 - Heisenberg fellowship.
Quantum effects on the Rayleigh-Taylor instability in a horizontal inhomogeneous rotating plasma
International Nuclear Information System (INIS)
Hoshoudy, G. A.
2009-01-01
The Rayleigh-Taylor instability is studied analytically in inhomogeneous plasma rotating uniformly in an external transverse magnetic field. The influence of the quantum mechanism is considered. For a stratified layer the linear growth rate is obtained. Some special cases that isolate the effect of various parameters on the growth rate of the Rayleigh-Taylor instability are discussed. It is shown that for some cases, the presence of the external transverse magnetic field beside the quantum effect will bring about more stability on the Rayleigh-Taylor instability.
Direct numerical simulation of homogeneous stratified rotating turbulence
Energy Technology Data Exchange (ETDEWEB)
Iida, O.; Tsujimura, S.; Nagano, Y. [Nagoya Institute of Technology, Department of Mech. Eng., Nagoya (Japan)
2005-12-01
The effects of the Prandtl number on stratified rotating turbulence have been studied in homogeneous turbulence by using direct numerical simulations and a rapid distortion theory. Fluctuations under strong stable-density stratification can be theoretically divided into the WAVE and the potential vorticity (PV) modes. In low-Prandtl-number fluids, the WAVE mode deteriorates, while the PV mode remains. Imposing rotation on a low-Prandtl-number fluid makes turbulence two-dimensional as well as geostrophic; it is found from the instantaneous turbulent structure that the vortices merge to form a few vertically-elongated vortex columns. During the period toward two-dimensionalization, the vertical vortices become asymmetric in the sense of rotation. (orig.)
Effects of modulation on Rayleigh-Benard convection. Part I
Directory of Open Access Journals (Sweden)
B. S. Bhadauria
2004-01-01
Full Text Available The linear stability of a horizontal layer of fluid heated from below and above is considered. In addition to a steady temperature difference between the walls of the fluid layer, a time-dependent periodic perturbation is applied to the wall temperatures. Only infinitesimal disturbances are considered. Numerical results for the critical Rayleigh number are obtained at various Prandtl numbers and for various values of the frequency. Some comparisons have been made with the known results.
Modeling of the thermal boundary layer in turbulent Rayleigh-Bénard convection
Emran, Mohammad; Shishkina, Olga
2016-11-01
We report modeling of the thermal boundary layer in turbulent Rayleigh-Bénard convection (RBC), which incorporates the effect of turbulent fluctuations. The study is based on the thermal boundary layer equation from Shishkina et al., and new Direct Numerical Simulations (DNS) of RBC in a cylindrical cell of the aspect ratio 1, for the Prandtl number variation of several orders of magnitude. Our modeled temperature profiles are found to agree with the DNS much better than those obtained with the classical Prandtl-Blasius or Falkner-Skan approaches. The work is supported by the Deutsche Forschungsgemeinschaft (DFG) under the Grant Sh405/4 - Heisenberg fellowship and SFB963, Project A06.
Direct numerical simulation and statistical analysis of turbulent convection in lead-bismuth
Energy Technology Data Exchange (ETDEWEB)
Otic, I.; Grotzbach, G. [Forschungszentrum Karlsruhe GmbH, Institut fuer Kern-und Energietechnik (Germany)
2003-07-01
Improved turbulent heat flux models are required to develop and analyze the reactor concept of an lead-bismuth cooled Accelerator-Driven-System. Because of specific properties of many liquid metals we have still no sensors for accurate measurements of the high frequency velocity fluctuations. So, the development of the turbulent heat transfer models which are required in our CFD (computational fluid dynamics) tools needs also data from direct numerical simulations of turbulent flows. We use new simulation results for the model problem of Rayleigh-Benard convection to show some peculiarities of the turbulent natural convection in lead-bismuth (Pr = 0.025). Simulations for this flow at sufficiently large turbulence levels became only recently feasible because this flow requires the resolution of very small velocity scales with the need for recording long-wave structures for the slow changes in the convective temperature field. The results are analyzed regarding the principle convection and heat transfer features. They are also used to perform statistical analysis to show that the currently available modeling is indeed not adequate for these fluids. Basing on the knowledge of the details of the statistical features of turbulence in this convection type and using the two-point correlation technique, a proposal for an improved statistical turbulence model is developed which is expected to account better for the peculiarities of the heat transfer in the turbulent convection in low Prandtl number fluids. (authors)
Magneto-Rayleigh-Taylor instability driven by a rotating magnetic field
Duan, Shuchao; Xie, Weiping; Cao, Jintao; Li, Ding
2018-04-01
In this paper, we analyze theoretically the magneto-Rayleigh-Taylor instability driven by a rotating magnetic field. Slab configurations of finite thickness are treated both with and without using the Wenzel-Kramers-Brillouin approximation. Regardless of the slab thickness, the directional rotation of the driving magnetic field contributes to suppressing these instabilities. The two factors of the finite thickness and directional rotation of the magnetic field cooperate to enhance suppression, with the finite thickness playing a role only when the orientation of the magnetic field is time varying. The suppression becomes stronger as the driving magnetic field rotates faster, and all modes are suppressed, in contrast to the case of a non-rotating magnetic field, for which the vertical mode cannot be suppressed. This implies that the dynamically alternate configuration of a Theta-pinch and a Z-pinch may be applicable to the concept of Theta-Z liner inertial fusion.
The structure of sidewall boundary layers in conned rotating Rayleigh-Bénard convection
Kunnen, R.P.J.; Clercx, H.J.H.; van Heijst, G.J.F.
2013-01-01
Turbulent rotating convection is usually studied in a cylindrical geometry, as this is its most convenient experimental realization. In our previous work (Kunnen et al., J. Fluid Mech., vol. 688, 2011, pp. 422–442) we studied turbulent rotating convection in a cylinder with the emphasis on the
Mean-field theory of differential rotation in density stratified turbulent convection
Rogachevskii, I.
2018-04-01
A mean-field theory of differential rotation in a density stratified turbulent convection has been developed. This theory is based on the combined effects of the turbulent heat flux and anisotropy of turbulent convection on the Reynolds stress. A coupled system of dynamical budget equations consisting in the equations for the Reynolds stress, the entropy fluctuations and the turbulent heat flux has been solved. To close the system of these equations, the spectral approach, which is valid for large Reynolds and Péclet numbers, has been applied. The adopted model of the background turbulent convection takes into account an increase of the turbulence anisotropy and a decrease of the turbulent correlation time with the rotation rate. This theory yields the radial profile of the differential rotation which is in agreement with that for the solar differential rotation.
Effect of plumes on measuring the large scale circulation in turbulent Rayleigh-Bénard convection
Stevens, Richard Johannes Antonius Maria; Clercx, H.J.H.; Lohse, Detlef
2011-01-01
We studied the properties of the large-scale circulation (LSC) in turbulent Rayleigh-Bénard (RB) convection by using results from direct numerical simulations in which we placed a large number of numerical probes close to the sidewall. The LSC orientation is determined by either a cosine or a
Breakdown of the large-scale circulation in Γ=1/2 rotating Rayleigh-Bénard flow.
Stevens, Richard J A M; Clercx, Herman J H; Lohse, Detlef
2012-11-01
Experiments and simulations of rotating Rayleigh-Bénard convection in cylindrical samples have revealed an increase in heat transport with increasing rotation rate. This heat transport enhancement is intimately related to a transition in the turbulent flow structure from a regime dominated by a large-scale circulation (LSC), consisting of a single convection roll, at no or weak rotation to a regime dominated by vertically aligned vortices at strong rotation. For a sample with an aspect ratio Γ=D/L=1 (D is the sample diameter and L is its height) the transition between the two regimes is indicated by a strong decrease in the LSC strength. In contrast, for Γ=1/2, Weiss and Ahlers [J. Fluid Mech. 688, 461 (2011)] revealed the presence of a LSC-like sidewall temperature signature beyond the critical rotation rate. They suggested that this might be due to the formation of a two-vortex state, in which one vortex extends vertically from the bottom into the sample interior and brings up warm fluid while another vortex brings down cold fluid from the top; this flow field would yield a sidewall temperature signature similar to that of the LSC. Here we show by direct numerical simulations for Γ=1/2 and parameters that allow direct comparison with experiment that the spatial organization of the vertically aligned vortical structures in the convection cell do indeed yield (for the time average) a sinusoidal variation of the temperature near the sidewall, as found in the experiment. This is also the essential and nontrivial difference with the Γ=1 sample, where the vertically aligned vortices are distributed randomly.
LES of turbulent flow in a concentric annulus with rotating outer wall
International Nuclear Information System (INIS)
Hadžiabdić, M.; Hanjalić, K.; Mullyadzhanov, R.
2013-01-01
Highlights: • High rotation up to N = 2 dampens progressively the turbulence near the rotating outer wall. • At 2 2.8, while tending to laminarize, the flow exhibits distinct Taylor-Couette vortical rolls. -- Abstract: Fully-developed turbulent flow in a concentric annulus, r 1 /r 2 = 0.5, Re h = 12,500, with the outer wall rotating at a range of rotation rates N = U θ,wall /U b from 0.5 up to 4 is studied by large-eddy simulations. The focus is on the effects of moderate to very high rotation rates on the mean flow, turbulence statistics and eddy structure. For N up to ∼2, an increase in the rotation rate dampens progressively the turbulence near the rotating outer wall, while affecting only mildly the inner-wall region. At higher rotation rates this trend is reversed: for N = 2.8 close to the inner wall turbulence is dramatically reduced while the outer wall region remains turbulent with discernible helical vortices as the dominant turbulent structure. The turbulence parameters and eddy structures differ significantly for N = 2 and 2.8. This switch is attributed to the centrifuged turbulence (generated near the inner wall) prevailing over the axial inertial force as well as over the counteracting laminarizing effects of the rotating outer wall. At still higher rotation, N = 4, the flow gets laminarized but with distinct spiralling vortices akin to the Taylor–Couette rolls found between the two counter-rotating cylinders without axial flow, which is the limiting case when N approaches to infinity. The ratio of the centrifugal to axial inertial forces, Ta/Re 2 ∝ N 2 (where Ta is the Taylor number) is considered as a possible criterion for defining the conditions for the above regime change
The effect of acceleration on turbulent entrainment
International Nuclear Information System (INIS)
Breidenthal, Robert E
2008-01-01
A new class of self-similar turbulent flows is proposed, which exhibits dramatically reduced entrainment rates. Under strong acceleration, the rotation period of the large-scale vortices is forced to decrease linearly in time. In ordinary unforced turbulence, the rotation period always increases linearly with time, at least in the mean. However, by imposing an exponential acceleration on the flow, the vortex rotation period is forced to become the e-folding timescale of the acceleration. If the e-folding timescale itself decreases linearly in time, the forcing is 'super-exponential', characterized by an acceleration parameter α. Based on dimensional and heuristic arguments, a model suggests that the dissipation rate is an exponential function of α and the dimensions of the conserved quantity of the flow. Acceleration decreases the dissipation and entrainment rates in all canonical laboratory flows except for Rayleigh-Taylor. Experiments of exponential jets and super-exponential transverse jets are in accord with the model. As noted by Johari, acceleration is the only known means of affecting the entrainment rate of the far-field jet. Numerical simulations of Rayleigh-Taylor flow by Cook and Greenough are also consistent. In the limit of large acceleration, vortices do not move far before their rotation period changes substantially. In this sense, extreme acceleration corresponds to stationary vortices.
A model for near-wall dynamics in turbulent Rayleigh Bénard convection
Theerthan, S. Ananda; Arakeri, Jaywant H.
1998-10-01
Experiments indicate that turbulent free convection over a horizontal surface (e.g. Rayleigh Bénard convection) consists of essentially line plumes near the walls, at least for moderately high Rayleigh numbers. Based on this evidence, we propose here a two-dimensional model for near-wall dynamics in Rayleigh Bénard convection and in general for convection over heated horizontal surfaces. The model proposes a periodic array of steady laminar two-dimensional plumes. A plume is fed on either side by boundary layers on the wall. The results from the model are obtained in two ways. One of the methods uses the similarity solution of Rotem & Classen (1969) for the boundary layer and the similarity solution of Fuji (1963) for the plume. We have derived expressions for mean temperature and temperature and velocity fluctuations near the wall. In the second approach, we compute the two-dimensional flow field in a two-dimensional rectangular open cavity. The number of plumes in the cavity depends on the length of the cavity. The plume spacing is determined from the critical length at which the number of plumes increases by one. The results for average plume spacing and the distribution of r.m.s. temperature and velocity fluctuations are shown to be in acceptable agreement with experimental results.
Rayleigh-Benard convection in a Hele-Shaw cell - a numerical study
International Nuclear Information System (INIS)
Guenther, C.; Mueller, U.
1987-05-01
Free convection in narrow vertical gaps heated from below gives rise to several different flow patterns as has been demonstrated by previous experimental investigations. A numerical study is presented aimed at simulating the observed flow phenomena in Hele-Shaw cells of small lateral extend. The numerical study is based on the assumption that the flow is essentially two-dimensional. This allows an approach using a one-term Galerkin approximation with respect to the direction perpendicular to the gap and a finite difference scheme with regard to the coordinates in the plane of the gap. The calculations result in realistic values of the critical Rayleigh numbers for the onset of steady and oscillatory convection. Most of the observed unsteady flow patterns can be simulated numerically. It is shown that five different stable flow patterns can occur at one particular Rayleigh number. The different stable flow patterns are coupled by a variety of complex transitions. Moreover the calculations show that a realistic description of the observed flow phenomena can not be obtained by a simplified model using the Darcy law in the momentum equation and implying slip flow at the small confining boundaries. (orig.) [de
Rayleigh-Taylor and Richtmyer-Meshkov instability induced flow, turbulence, and mixing. I
Zhou, Ye
2017-12-01
Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities play an important role in a wide range of engineering, geophysical, and astrophysical flows. They represent a triggering event that, in many cases, leads to large-scale turbulent mixing. Much effort has been expended over the past 140 years, beginning with the seminal work of Lord Rayleigh, to predict the evolution of the instabilities and of the instability-induced mixing layers. The objective of Part I of this review is to provide the basic properties of the flow, turbulence, and mixing induced by RT, RM, and Kelvin-Helmholtz (KH) instabilities. Historical efforts to study these instabilities are briefly reviewed, and the significance of these instabilities is discussed for a variety of flows, particularly for astrophysical flows and for the case of inertial confinement fusion. Early experimental efforts are described, and analytical attempts to model the linear, and nonlinear regimes of these mixing layers are examined. These analytical efforts include models for both single-mode and multi-mode initial conditions, as well as multi-scale models to describe the evolution. Comparisons of these models and theories to experimental and simulation studies are then presented. Next, attention is paid to the issue of the influence of stabilizing mechanisms (e.g., viscosity, surface tension, and diffuse interface) on the evolution of these instabilities, as well as the limitations and successes of numerical methods. Efforts to study these instabilities and mixing layers using group-theoretic ideas, as well as more formal notions of turbulence cascade processes during the later stages of the induced mixing layers, are inspected. A key element of the review is the discussion of the late-time self-similar scaling for the RT and RM growth factors, α and θ. These parameters are influenced by the initial conditions and much of the observed variation can be explained by this. In some cases, these instabilities
Efficiency of Heat Transfer in Turbulent Rayleigh-Benard Convection
Czech Academy of Sciences Publication Activity Database
Urban, Pavel; Musilová, Věra; Skrbek, L.
2011-01-01
Roč. 107, č. 1 (2011), 014302:1-4 ISSN 0031-9007 R&D Projects: GA AV ČR KJB200650902 Institutional research plan: CEZ:AV0Z20650511 Keywords : natural convection * thermal convection Subject RIV: BK - Fluid Dynamics Impact factor: 7.370, year: 2011
Wang, Yin; Xu, Wei; He, Xiao-Zhou; Yik, Hiu-Fai; Wang, Xiao-Ping; Schumacher, Jorg; Tong, Penger
2017-11-01
We report a combined experimental and numerical study of the scaling properties of the temperature variance profile η(z) along the central z axis of turbulent Rayleigh-Bénard convection in a thin disk cell and an upright cylinder of aspect ratio unity. In the mixing zone outside the thermal boundary layer region, the measured η(z) is found to scale with the cell height H in both cells and obey a power law, η(z) (z/H)ɛ, with the obtained values of ɛ being very close to -1. Based on the experimental and numerical findings, we derive a new equation for η(z) in the mixing zone, which has a power-law solution in good agreement with the experimental and numerical results. Our work thus provides a common framework for understanding the effect of boundary layer fluctuations on the scaling properties of the temperature variance profile in turbulent Rayleigh-Bénard convection. This work was supported in part by Hong Kong Research Grants Council.
International Nuclear Information System (INIS)
Sun, Z.; Schubert, G.
1995-01-01
In this study, we carry out numerical simulations of thermal convection in a rapidly rotating spherical fluid shell at high Taylor number Ta and Rayleigh number R with a nonlinear, three-dimensional, time-dependent, spectral-transform code. The parameters used in the simulations are chosen to be in a range which allows us to study two different types of convection, i.e., single column and multi-layered types, and the transition between them. Numerical solutions feature highly time-dependent north--south open columnar convective cells. The cells occur irregularly in longitude, are quasi-layered in cylindrical radius, and maintain alternating bands of mean zonal flow. The complex convective structure and the banded mean zonal flow are results of the high Taylor and Rayleigh numbers. The transition between the two types of convection appears to occur gradually with increasing Rayleigh and Taylor numbers. At a Taylor number of 10 7 the differential rotation pattern consists of an inner cylindrical region of subrotation and an outer cylindrical shell of superrotation manifest at the outer boundary as an equatorial superrotation and a high latitude subrotation. The differential rotation pattern is similar at Ta=10 8 and low Rayleigh number. Cylindrical shells of alternately directed mean zonal flow begin to develop at Ta=10 8 and R=50R c and at Ta=10 9 and R=25R c . This pattern is seen on the outer surface as a latitudinally-banded zonal flow consisting of an equatorial superrotation, a middle and high latitude subrotation, and a polar superrotation. At Ta=10 9 and R=50R c the differential rotation appears at the surface as a broad eastward flow in the equatorial region with alternating bands of westward and eastward flow at high latitudes. copyright 1995 American Institute of Physics
Kinetic thermal structure in turbulent Rayleigh-Bénard convection
Chen, Jun; Yin, Ze-Xia; She, Zhen-Su; Bao, Yun
2017-11-01
Plumes are believed to be the most important heat carrier in turbulent Rayleigh-Bénard convection (RBC). However, a physically sound and clear definition of plume is still absent. We report here the investigation of a definition of plume called kinetic thermal structure (KTS), based on the analysis of vertical velocity gradient (Λ = ∂w / ∂z), using direct numerical simulation (DNS) data of the three-dimensional RBC in a rectangular cell for Pr = 0.7 and Ra = 1 ×108 5 ×109 . It is shown that the conditional average of temperature on Λ exhibits such a behavior that when Λ is larger than a threshold, the volume carries a constant temperature of fluid, hence defines an unambiguous thermal structure, KTS. The DNS show that the KTS behaves in a sheet-like shape near the conducting plate, and becomes slender and smaller with increasing Ra . The heat flux carried by KTS displays a scaling law, with an exponent larger than the global- Nu - Ra scaling, indicating stronger heat transport than the turbulent background. An advantage of the KTS is its connection to the balance equation allowing, for the first time, a prediction of the Ra -dependence of its vertical velocity and the characteristic Λ threshold, validated by DNS. Supported by NSFC (11172006, 11221062, 11452002), and by MOST (China) 973 project (2009CB724100).
Subgrid-scale models for large-eddy simulation of rotating turbulent channel flows
Silvis, Maurits H.; Bae, Hyunji Jane; Trias, F. Xavier; Abkar, Mahdi; Moin, Parviz; Verstappen, Roel
2017-11-01
We aim to design subgrid-scale models for large-eddy simulation of rotating turbulent flows. Rotating turbulent flows form a challenging test case for large-eddy simulation due to the presence of the Coriolis force. The Coriolis force conserves the total kinetic energy while transporting it from small to large scales of motion, leading to the formation of large-scale anisotropic flow structures. The Coriolis force may also cause partial flow laminarization and the occurrence of turbulent bursts. Many subgrid-scale models for large-eddy simulation are, however, primarily designed to parametrize the dissipative nature of turbulent flows, ignoring the specific characteristics of transport processes. We, therefore, propose a new subgrid-scale model that, in addition to the usual dissipative eddy viscosity term, contains a nondissipative nonlinear model term designed to capture transport processes, such as those due to rotation. We show that the addition of this nonlinear model term leads to improved predictions of the energy spectra of rotating homogeneous isotropic turbulence as well as of the Reynolds stress anisotropy in spanwise-rotating plane-channel flows. This work is financed by the Netherlands Organisation for Scientific Research (NWO) under Project Number 613.001.212.
Near-Wall Turbulence Modelling of Rotating and Curved Shear Flows
Energy Technology Data Exchange (ETDEWEB)
Pettersson, Bjoern Anders
1997-12-31
This thesis deals with verification and refinement of turbulence models within the framework of the Reynolds-averaged approach. It pays special attention to modelling the near-wall region, where the turbulence is strongly non-homogeneous and anisotropic. It also studies in detail the effects associated with an imposed rotation of the reference frame or streamline curvature. The objective with near-wall turbulence closure modelling is to formulate a set of equations governing single point turbulence statistics, which can be solved in the region of the flow which extends to the wall. This is in contrast to the commonly adopted wall-function approach in which the wall-boundary conditions are replaced by matching conditions in the logarithmic region. The near-wall models allow more flexibility by not requiring any such universal behaviour. Assessment of the novel elliptic relaxation approach to model the proximity of a solid boundary reveals an encouraging potential used in conjunction with second-moment and eddy-viscosity closures. The most natural level of closure modelling to predict flows affected by streamline curvatures or an imposed rotation of the reference frame is at the second-moment closure (SMC) level. Although SMCs naturally accounts for the effects of system rotation, the usual application of a scalar dissipation rate equation is shown to require ad hoc corrections in some cases in order to give good results. The elliptic relaxation approach is also used in conjunction with non-linear pressure-strain models and very encouraging results are obtained for rotating flows. Rotational induced secondary motions are vital to predicting the effects of system rotation. Some severe weaknesses of non-linear pressure-strain models are also indicated. Finally, a modelling methodology for anisotropic dissipation in nearly homogeneous turbulence are proposed. 84 refs., 56 figs., 16 tabs.
Compressibility and rotation effects on transport suppression in magnetohydrodynamic turbulence
International Nuclear Information System (INIS)
Yoshizawa, A.
1996-01-01
Compressibility and rotation effects on turbulent transports in magnetohydrodynamic (MHD) flows under arbitrary mean field are investigated using a Markovianized two-scale statistical approach. Some new aspects of MHD turbulence are pointed out in close relation to plasma compressibility. Special attention is paid to the turbulent electromotive force, which plays a central role in the generation of magnetic and velocity fluctuations. In addition to plasma rotation, the interaction between compressibility and magnetic fields is shown to bring a few factors suppressing MHD fluctuations and, eventually, density and temperature transports, even in the presence of steep mean density and temperature gradients. This finding is discussed in the context of the turbulence-suppression mechanism in the tokamak close-quote s high-confinement modes. copyright 1996 American Institute of Physics
Direct numerical simulation of the Rayleigh-Taylor instability with the spectral element method
International Nuclear Information System (INIS)
Zhang Xu; Tan Duowang
2009-01-01
A novel method is proposed to simulate Rayleigh-Taylor instabilities using a specially-developed unsteady three-dimensional high-order spectral element method code. The numerical model used consists of Navier-Stokes equations and a transport-diffusive equation. The code is first validated with the results of linear stability perturbation theory. Then several characteristics of the Rayleigh-Taylor instabilities are studied using this three-dimensional unsteady code, including instantaneous turbulent structures and statistical turbulent mixing heights under different initial wave numbers. These results indicate that turbulent structures of Rayleigh-Taylor instabilities are strongly dependent on the initial conditions. The results also suggest that a high-order numerical method should provide the capability of simulating small scale fluctuations of Rayleigh-Taylor instabilities of turbulent flows. (authors)
Tokamak turbulence in self-regulated differentially rotating flow and L-H transition dynamics
International Nuclear Information System (INIS)
Terry, P.W.; Carreras, B.A.; Sidikman, K.
1992-01-01
An analytical study of turbulence in the presence of turbulently generated differentially rotating flow is presented as a paradigm for fluctuation dynamics in L- and H-mode plasmas. Using a drift wave model, the role of both flow shear and flow curvature (second radial derivative of the poloidal ExB flow) is detailed in linear and saturated turbulence phases. In the strong turbulence saturated state, finite amplitude-induced modification of the fluctuation structure near low order rational surfaces strongly inhibits flow shear suppression. Suppression by curvature is not diminished, but it occurs through a frequency shift. A description of L-H mode transition dynamics based on the self-consistent linking of turbulence suppression by differentially rotating flow and generation of flow by turbulent momentum transport is presented. In this model, rising edge temperature triggers a transition characterized by spontaneous generation of differentially rotating flow and decreasing turbulence intensity
International Nuclear Information System (INIS)
Zhou Ye; Remington, B.A.; Robey, H.F.; Cook, A.W.; Glendinning, S.G.; Dimits, A.; Buckingham, A.C.; Zimmerman, G.B.; Burke, E.W.; Peyser, T.A.; Cabot, W.; Eliason, D.
2003-01-01
Turbulent hydrodynamic mixing induced by the Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities occurs in settings as varied as exploding stars (supernovae), inertial confinement fusion (ICF) capsule implosions, and macroscopic flows in fluid dynamics facilities such as shock tubes. Turbulence theory and modeling have been applied to RT and RM induced flows and developed into a quantitative description of turbulence from the onset to the asymptotic end-state. The treatment, based on a combined approach of theory, direct numerical simulation (DNS), and experimental data analysis, has broad generality. Three areas of progress will be reported. First, a robust, easy to apply criteria will be reported for the mixing transition in a time-dependent flow. This allows an assessment of whether flows, be they from supernova explosions or ICF experiments, should be mixed down to the molecular scale or not. Second, through DNS, the structure, scaling, and spectral evolution of the RT instability induced flow will be inspected. Finally, using these new physical insights, a two-scale, dynamic mix model has been developed that can be applied to simulations of ICF experiments and astrophysics situations alike
Thermal turbulent convection: thermal plumes and fluctuations
International Nuclear Information System (INIS)
Gibert, M.
2007-10-01
In this study we investigate the phenomenon of thermal turbulent convection in new and unprecedented ways. The first system we studied experimentally is an infinite vertical channel, where a constant vertical mean gradient of temperature exists. Inside this channel the average mass flux is null. The results obtained from our measurements reveal that the flow is mainly inertial; indeed the dissipative coefficients (here the viscosity) play a role only to define a coherence length L. This length is the distance over which the thermal plumes can be considered as 'free falling' objects. The horizontal transport, of heat and momentum, is entirely due to fluctuations. The associated 'mixing length' is small compared to the channel width. In the other hand, the vertical heat transport is due to coherent structures: the heat plumes. Those objects were also investigated in a Lagrangian study of the flow in the bulk of a Rayleigh-Benard cell. The probe, which has the same density as the fluid used in this experiment, is a sphere of 2 cm in diameter with embarked thermometers and radio-emitter. The heat plumes transport it, which allows a statistical study of such objects. (author)
The structure and dynamics of patterns of Benard convection cells
International Nuclear Information System (INIS)
Rivier, N.; Imperial Coll. of Science and Technology, London; Lausanne Univ.
1990-08-01
Benard-Marangoni convection, in containers with large aspect ratio, exhibits space-filling cellular structures, highly deformable, but crystallized. They contain dislocations and grain boundaries generated and moved by elementary topological transformations, and are subjected to a weak shear stress due to the earth's rotation. The cellular structure and its fluctuations are analyzed from a crystallographic viewpoint, by using two complementary approaches. One is a global analysis of cellular structures in cylindrical symmetry. Their structural stability and defect pattern are obtained as topological mode-locking of a continuous structural parameter. The other, a local, molecular dynamics of the cells, gives a realistic parametrization of the forces and the transformations by generalizing the Voronoi cell construction in one extra dimension. 23 refs., 8 figs
Rayleigh-Bénard turbulence modified by two-way coupled inertial, nonisothermal particles
Park, Hyungwon John; O'Keefe, Kevin; Richter, David H.
2018-03-01
Direct numerical simulation (DNS) combined with the Lagrangian point particle model is used to study Rayleigh-Bénard convection in order to understand modifications due to the interaction of inertial, nonisothermal particles with buoyancy-driven turbulence. In this system, turbulence can be altered through direct momentum coupling, as well as through buoyancy modification via thermal coupling between phases. We quantify the effect of the dispersed phase by changes to the total integrated turbulent kinetic energy (TKE) and Nusselt number (Nu). The dispersed particles experience gravitational settling and are introduced at the lower wall so that turbulence must overcome the settling velocity for the particles to vertically distribute throughout the domain. We focus primarily on particle inertia, settling velocity, mass fraction, and the ratio of the particle to fluid specific heat. Furthermore, individual contributions by the momentum coupling and thermal coupling are studied to see which most significantly changes Nu and TKE. Our results show that particles with Stokes number of order unity maximize Nu, corresponding to a peak of clustering and attenuation of TKE. Increased mass fractions lead to a linear increase of Nu and decrease of TKE. With varying specific heat ratio, Nu and TKE exhibit monotonic behaviors, where in the high limit particles become isothermal and depend upon the initialized particle temperature. It is also shown that particles two-way coupled only through momentum attenuate Nu and weaken TKE, while thermal-only coupling also weakens TKE but enhances Nu. When both couplings are present, however, thermal coupling overwhelms the momentum coupling attenuation, and the net result is an enhancement of Nu.
Nootz, Gero; Matt, Silvia; Kanaev, Andrey; Judd, Kyle P; Hou, Weilin
2017-08-01
The propagation of a laser beam through Rayleigh-Bénard (RB) turbulence is investigated experimentally and by way of numerical simulation. For the experimental part, a focused laser beam transversed a 5 m×0.5 m×0.5 m water filled tank lengthwise. The tank is heated from the bottom and cooled from the top to produce convective RB turbulence. The effect of the turbulence on the beam is recorded on the exit of the beam from the tank. From the centroid motion of the beam, the index of refraction structure constant Cn2 is determined. For the numerical efforts RB turbulence is simulated for a tank of the same geometry. The simulated temperature fields are converted to the index of refraction distributions, and Cn2 is extracted from the index of refraction structure functions, as well as from the simulated beam wander. To model the effect on beam propagation, the simulated index of refraction fields are converted to discrete index of refraction phase screens. These phase screens are then used in a split-step beam propagation method to investigate the effect of the turbulence on a laser beam. The beam wander as well as the index of refraction structure parameter Cn2 determined from the experiment and simulation are compared and found to be in good agreement.
Formation of protostars in collapsing, rotating, turbulent clouds
International Nuclear Information System (INIS)
Regev, O.; Shaviv, G.
1981-01-01
Collapse and star formation processes in rotating turbulent interstellar gas clouds have been studied. For this purpose numerical collapse calculations have been performed for a number of representative cases. These calculations have been carried out by a two-dimensional hydrodynamical computer code, which solves the equations of hydrodynamics explicitly, coupled to the Poisson equation. The computer code has been written especially for this work and has been thoroughly tested. The calculations in this work have been performed with an effort to obtain physically reliable results (by repeating the same calculations with different numerical spatial resolutions). A physical mechanism for angular momentum transport by turbulent viscosity has been proposed and incorporated in new collapse calculations. The main results can be summerized as follows: When there is no physical mechanism for angular momentum transport, the result of the collaps is a ringlike structure. The turbulent viscosity affects the nature of the collaps. For the two cases studied, the mass of the central object is a major fraction (30%) of the total mass of the system. The exact form of the central object and its ultimate fate depend on the parameters, especially rotational energy/gravitational energy and Re. The present calculations cannot predict the future evolution of the central object. In the new theoretical model proposed, a central protostar forms as a result of the collaps of a protostellar rotating cloud
Centrifugally Driven Rayleigh-Taylor Instability
Scase, Matthew; Hill, Richard
2017-11-01
The instability that develops at the interface between two fluids of differing density due to the rapid rotation of the system may be considered as a limit of high-rotation rate Rayleigh-Taylor instability. Previously the authors have considered the effect of rotation on a gravitationally dominated Rayleigh-Taylor instability and have shown that some growth modes of instability may be suppressed completely by the stabilizing effect of rotation (Phys. Rev. Fluids 2:024801, Sci. Rep. 5:11706). Here we consider the case of very high rotation rates and a negligible gravitational field. The initial condition is of a dense inner cylinder of fluid surrounded by a lighter layer of fluid. As the system is rotated about the generating axis of the cylinder, the dense inner fluid moves away from the axis and the familiar bubbles and spikes of Rayleigh-Taylor instability develop at the interface. The system may be thought of as a ``fluid-fluid centrifuge''. By developing a model based on an Orr-Sommerfeld equation, we consider the effects of viscosity, surface tension and interface diffusion on the growth rate and modes of instability. We show that under particular circumstances some modes may be stabilized. School of Mathematical Sciences.
Sudjai, W.; Juntasaro, V.; Juttijudata, V.
2018-01-01
The accuracy of predicting turbulence induced secondary flows is crucially important in many industrial applications such as turbine blade internal cooling passages in a gas turbine and fuel rod bundles in a nuclear reactor. A straight square duct is popularly used to reveal the characteristic of turbulence induced secondary flows which consists of two counter rotating vortices distributed in each corner of the duct. For a rotating duct, the flow can be divided into the pressure side and the suction side. The turbulence induced secondary flows are converted to the Coriolis force driven two large circulations with a pair of additional vortices on the pressure wall due to the rotational effect. In this paper, the Large Eddy Simulation (LES) of turbulence induced secondary flows in a straight square duct is performed using the ANSYS FLUENT CFD software. A dynamic kinetic energy subgrid-scale model is used to describe the three-dimensional incompressible turbulent flows in the stationary and the rotating straight square ducts. The Reynolds number based on the friction velocity and the hydraulic diameter is 300 with the various rotation numbers for the rotating cases. The flow is assumed fully developed by imposing the constant pressure gradient in the streamwise direction. For the rotating cases, the rotational axis is placed perpendicular to the streamwise direction. The simulation results on the secondary flows and the turbulent statistics are found to be in good agreement with the available Direct Numerical Simulation (DNS) data. Finally, the details of the Coriolis effects are discussed.
Rough horizontal plates: heat transfer and hysteresis
Energy Technology Data Exchange (ETDEWEB)
Tisserand, J-C; Gasteuil, Y; Pabiou, H; Castaing, B; Chilla, F [Universite de Lyon, ENS Lyon, CNRS, 46 Allee d' ltalie, 69364 Lyon Cedex 7 (France); Creyssels, M [LMFA, CNRS, Ecole Centrale Lyon, 69134 Ecully Cedex (France); Gibert, M, E-mail: mathieu.creyssels@ec-lyon.fr [Also at MPI-DS (LFPN) Gottingen (Germany)
2011-12-22
To investigate the influence of a rough-wall boundary layer on turbulent heat transport, an experiment of high-Rayleigh convection in water is carried out in a Rayleigh-Benard cell with a rough lower plate and a smooth upper plate. A transition in the heat transport is observed when the thermal boundary layer thickness becomes comparable to or smaller than the roughness height. Besides, at larger Rayleigh numbers than the threshold value, heat transport is found to be increased up to 60%. This enhancement cannot be explained simply by an increase in the contact area of the rough surface since the contact area is increased only by a factor of 40%. Finally, a simple model is proposed to explain the enhanced heat transport.
On the phase lag of turbulent dissipation in rotating tidal flows
Zhang, Qianjiang; Wu, Jiaxue
2018-03-01
Field observations of rotating tidal flows in a shallow tidally swept sea reveal that a notable phase lag of both shear production and turbulent dissipation increases with height above the seafloor. These vertical delays of turbulent quantities are approximately equivalent in magnitude to that of squared mean shear. The shear production approximately equals turbulent dissipation over the phase-lag column, and thus a main mechanism of phase lag of dissipation is mean shear, rather than vertical diffusion of turbulent kinetic energy. By relating the phase lag of dissipation to that of the mean shear, a simple formulation with constant eddy viscosity is developed to describe the phase lag in rotating tidal flows. An analytical solution indicates that the phase lag increases linearly with height subjected to a combined effect of tidal frequency, Coriolis parameter and eddy viscosity. The vertical diffusion of momentum associated with eddy viscosity produces the phase lag of squared mean shear, and resultant delay of turbulent quantities. Its magnitude is inhibited by Earth's rotation. Furthermore, a theoretical formulation of the phase lag with a parabolic eddy viscosity profile can be constructed. A first-order approximation of this formulation is still a linear function of height, and its magnitude is approximately 0.8 times that with constant viscosity. Finally, the theoretical solutions of phase lag with realistic viscosity can be satisfactorily justified by realistic phase lags of dissipation.
Transitions in turbulent rotating convection: A Lagrangian perspective : A Lagrangian perspective
Rajaei, H.; Joshi, P.R.; Alards, K.M.J.; Kunnen, R.P.J.; Toschi, F.; Clercx, H.J.H.
2016-01-01
Using measurements of Lagrangian acceleration in turbulent rotating convection and accompanying direct numerical simulations, we show that the transition between turbulent states reported earlier [e.g., S. Weiss et al., Phys. Rev. Lett. 105, 224501 (2010)] is a boundary-layer transition between the
Pawar, Shashikant S.; Arakeri, Jaywant H.
2016-06-01
Kinetic energy and scalar spectra from the measurements in high Rayleigh number axially homogeneous buoyancy driven turbulent flow are presented. Kinetic energy and concentration (scalar) spectra are obtained from the experiments wherein density difference is created using brine and fresh water and temperature spectra are obtained from the experiments in which heat is used. Scaling of the frequency spectra of lateral and longitudinal velocity near the tube axis is closer to the Kolmogorov-Obukhov scaling, while the scalar spectra show some evidence of dual scaling, Bolgiano-Obukhov scaling followed by Obukhov-Corrsin scaling. These scalings are also observed in the corresponding second order spatial structure functions of velocity and concentration fluctuations.
Propagation of rotational Risley-prism-array-based Gaussian beams in turbulent atmosphere
Chen, Feng; Ma, Haotong; Dong, Li; Ren, Ge; Qi, Bo; Tan, Yufeng
2018-03-01
Limited by the size and weight of prism and optical assembling, Rotational Risley-prism-array system is a simple but effective way to realize high power and superior beam quality of deflecting laser output. In this paper, the propagation of the rotational Risley-prism-array-based Gaussian beam array in atmospheric turbulence is studied in detail. An analytical expression for the average intensity distribution at the receiving plane is derived based on nonparaxial ray tracing method and extended Huygens-Fresnel principle. Power in the diffraction-limited bucket is chosen to evaluate beam quality. The effect of deviation angle, propagation distance and intensity of turbulence on beam quality is studied in detail by quantitative simulation. It reveals that with the propagation distance increasing, the intensity distribution gradually evolves from multiple-petal-like shape into the pattern that contains one main-lobe in the center with multiple side-lobes in weak turbulence. The beam quality of rotational Risley-prism-array-based Gaussian beam array with lower deviation angle is better than its counterpart with higher deviation angle when propagating in weak and medium turbulent (i.e. Cn2 beam quality of higher deviation angle arrays degrades faster as the intensity of turbulence gets stronger. In the case of propagating in strong turbulence, the long propagation distance (i.e. z > 10km ) and deviation angle have no influence on beam quality.
Sustained turbulence and magnetic energy in non-rotating shear flows
DEFF Research Database (Denmark)
Nauman, Farrukh; Blackman, Eric G.
2017-01-01
From numerical simulations, we show that non-rotating magnetohydrodynamic shear flows are unstable to finite amplitude velocity perturbations and become turbulent, leading to the growth and sustenance of magnetic energy, including large scale fields. This supports the concept that sustained...... magnetic energy from turbulence is independent of the driving mechanism for large enough magnetic Reynolds numbers....
Kleeorin, N.
2018-06-01
We discuss a mean-field theory of the generation of large-scale vorticity in a rotating density stratified developed turbulence with inhomogeneous kinetic helicity. We show that the large-scale non-uniform flow is produced due to either a combined action of a density stratified rotating turbulence and uniform kinetic helicity or a combined effect of a rotating incompressible turbulence and inhomogeneous kinetic helicity. These effects result in the formation of a large-scale shear, and in turn its interaction with the small-scale turbulence causes an excitation of the large-scale instability (known as a vorticity dynamo) due to a combined effect of the large-scale shear and Reynolds stress-induced generation of the mean vorticity. The latter is due to the effect of large-scale shear on the Reynolds stress. A fast rotation suppresses this large-scale instability.
THE FORMATION OF ROTATIONAL DISCONTINUITIES IN COMPRESSIVE THREE-DIMENSIONAL MHD TURBULENCE
Energy Technology Data Exchange (ETDEWEB)
Yang, Liping; Feng, Xueshang [SIGMA Weather Group, State Key Laboratory for Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, 100190, Beijing (China); Zhang, Lei; He, Jiansen; Tu, Chuanyi; Wang, Linghua; Wang, Xin [School of Earth and Space Sciences, Peking University, 100871 Beijing (China); Marsch, Eckart [Institute for Experimental and Applied Physics, Christian Albrechts University at Kiel, D-24118 Kiel (Germany); Zhang, Shaohua, E-mail: jshept@gmail.com [Center of Spacecraft Assembly Integration and Test, China Academy of Space Technology, Beijing 100094 (China)
2015-08-20
Measurements of solar wind turbulence reveal the ubiquity of discontinuities. In this study we investigate how the discontinuities, especially rotational discontinuities (RDs), are formed in MHD turbulence. In a simulation of the decaying compressive three-dimensional (3D) MHD turbulence with an imposed uniform background magnetic field, we detect RDs with sharp field rotations and little variations of magnetic field intensity, as well as mass density. At the same time, in the de Hoffman–Teller frame, the plasma velocity is nearly in agreement with the Alfvén speed, and is field-aligned on both sides of the discontinuity. We take one of the identified RDs to analyze its 3D structure and temporal evolution in detail. By checking the magnetic field and plasma parameters, we find that the identified RD evolves from the steepening of the Alfvén wave with moderate amplitude, and that steepening is caused by the nonuniformity of the Alfvén speed in the ambient turbulence.
Direct numerical simulation of turbulent Rayleigh-Bénard convection in a vertical thin disk
Xu, Wei; Wang, Yin; He, Xiao-Zhou; Yik, Hiu-Fai; Wang, Xiao-Ping; Schumacher, Jorg; Tong, Penger
2017-11-01
We report a direct numerical simulation (DNS) of turbulent Rayleigh-Bénard convection in a thin vertical disk with a high-order spectral element method code NEK5000. An unstructured mesh is used to adapt the turbulent flow in the thin disk and to ensure that the mesh sizes satisfy the refined Groetzbach criterion and a new criterion for thin boundary layers proposed by Shishkina et al. The DNS results for the mean and variance temperature profiles in the thermal boundary layer region are found to be in good agreement with the predictions of the new boundary layer models proposed by Shishkina et al. and Wang et al.. Furthermore, we numerically calculate the five budget terms in the boundary layer equation, which are difficult to measure in experiment. The DNS results agree well with the theoretical predictions by Wang et al. Our numerical work thus provides a strong support for the development of a common framework for understanding the effect of boundary layer fluctuations. This work was supported in part by Hong Kong Research Grants Council.
Surfactant-Enhanced Benard Convection on an Evaporating Drop
Nguyen, Van X.; Stebe, Kathleen J.
2001-11-01
Surfactant effects on an evaporating drop are studied experimentally. Using a fluorescent probe, the distribution and surface phase of the surfactant is directly imaged throughout the evaporation process. From these experiments, we identify conditions in which surfactants promote surface tension-driven Benard instabilities in aqueous systems. The drops under study contain finely divided particles, which act as tracers in the flow, and form well-defined patterns after the drop evaporates. Two flow fields have been reported in this system. The first occurs because the contact line becomes pinned by solid particles at the contact line region. In order for the contact line to remain fixed, an outward flow toward the ring results, driving further accumulation at the contact ring. A ‘coffee ring’ of particles is left as residue after the drop evaporates[1]. The second flow is Benard convection, driven by surface tension gradients on the drop[2,3]. In our experiments, an insoluble monolayer of pentadecanoic acid is spread at the interface of a pendant drop. The surface tension is recorded, and the drop is deposited on a well-defined solid substrate. Fluorescent images of the surface phase of the surfactant are recorded as the drop evaporates. The surfactant monolayer assumes a variety of surface states as a function of the area per molecule at the interface: surface gaseous, surface liquid expanded, and surface liquid condensed phases[4]. Depending upon the surface state of the surfactant as the drop evaporates, transitions of residue patterns left by the particles occur, from the coffee ring pattern to Benard cells to irregular patterns, suggesting a strong resistance to outward flow are observed. The occurrence of Benard cells on a surfactant-rich interface occurs when the interface is in LE-LC coexistence. Prior research concerning surfactant effects on this instability predict that surfactants are strongly stabilizing[5]. The mechanisms for this change in behavior
Large eddy simulation of rotating turbulent flows and heat transfer by the lattice Boltzmann method
Liou, Tong-Miin; Wang, Chun-Sheng
2018-01-01
Due to its advantage in parallel efficiency and wall treatment over conventional Navier-Stokes equation-based methods, the lattice Boltzmann method (LBM) has emerged as an efficient tool in simulating turbulent heat and fluid flows. To properly simulate the rotating turbulent flow and heat transfer, which plays a pivotal role in tremendous engineering devices such as gas turbines, wind turbines, centrifugal compressors, and rotary machines, the lattice Boltzmann equations must be reformulated in a rotating coordinate. In this study, a single-rotating reference frame (SRF) formulation of the Boltzmann equations is newly proposed combined with a subgrid scale model for the large eddy simulation of rotating turbulent flows and heat transfer. The subgrid scale closure is modeled by a shear-improved Smagorinsky model. Since the strain rates are also locally determined by the non-equilibrium part of the distribution function, the calculation process is entirely local. The pressure-driven turbulent channel flow with spanwise rotation and heat transfer is used for validating the approach. The Reynolds number characterized by the friction velocity and channel half height is fixed at 194, whereas the rotation number in terms of the friction velocity and channel height ranges from 0 to 3.0. A working fluid of air is chosen, which corresponds to a Prandtl number of 0.71. Calculated results are demonstrated in terms of mean velocity, Reynolds stress, root mean square (RMS) velocity fluctuations, mean temperature, RMS temperature fluctuations, and turbulent heat flux. Good agreement is found between the present LBM predictions and previous direct numerical simulation data obtained by solving the conventional Navier-Stokes equations, which confirms the capability of the proposed SRF LBM and subgrid scale relaxation time formulation for the computation of rotating turbulent flows and heat transfer.
Study of Rayleigh-Love coupling from Spatial Gradient Observation
Lin, C. J.; Hosseini, K.; Donner, S.; Vernon, F.; Wassermann, J. M.; Igel, H.
2017-12-01
We present a new method to study Rayleigh-Love coupling. Instead of using seismograms solely, where ground motion is recorded as function of time, we incorporate with rotation and strain, also called spatial gradient where ground is represented as function of distance. Seismic rotation and strain are intrinsic different observable wavefield so are helpful to indentify wave type and wave propagation. A Mw 7.5 earthquake on 29 March 2015 occurred in Kokopo, Papua New Guinea recorded by a dense seismic array at PFO, California are used to obtaint seismic spatial gradient. We firstly estimate time series of azimuthal direction and phase velocity of SH wave and Rayleigh wave by analyzing collocated seismograms and rotations. This result also compares with frequency wavenumber methods using a nearby ANZA seismic array. We find the direction of Rayleigh wave fits well with great-circle back azimuth during wave propagation, while the direction of Love wave deviates from that, especially when main energy of Rayleigh wave arrives. From the analysis of cross-correlation between areal strain and vertical rotation, it reveals that high coherence, either positive or negative, happens at the same time when Love wave deparate from great-circle path. We also find the observed azimuth of Love wave and polarized particle motion of Rayleigh wave fits well with the fast direction of Rayleigh wave, for the period of 50 secs. We conclude the cause of deviated azimuth of Love wave is due to Rayleigh-Love coupling, as surface wave propagates through the area with anisotropic structure.
International Nuclear Information System (INIS)
Brandão, P V; Alves, L S de B; Barletta, A
2014-01-01
The present paper investigates the transition from convective to absolute instability induced by viscous dissipation. As far as the authors are aware, this is the first time such a study is reported in the literature. Its framework is provided by the Poiseuille-Darcy-Benard convection of a Newtonian fluid. We found the same behaviour observed in the absence of viscous dissipation whenever the Gebhart number is smaller than Ge < 0.95, which is the stabilising effect of the cross flow. When 0.95 < Ge < 4.31, weak cross flows still stabilise the onset of absolute instability but stronger cross flows destabilise it. For a stronger viscous dissipation, i.e. Ge > 4.31, the cross flow always destabilises this onset. The latter two conditions create a scenario where viscous dissipation is capable of inducing a transition to absolute instability in the absence of wall heating, i.e. with a zero Rayleigh number
Turbulent convection in liquid metal with and without rotation
King, Eric M.; Aurnou, Jonathan M.
2013-01-01
The magnetic fields of Earth and other planets are generated by turbulent, rotating convection in liquid metal. Liquid metals are peculiar in that they diffuse heat more readily than momentum, quantified by their small Prandtl numbers, . Most analog models of planetary dynamos, however, use moderate fluids, and the systematic influence of reducing is not well understood. We perform rotating Rayleigh–Bénard convection experiments in the liquid metal gallium over a range of nondimensional bu...
Rayleigh-Bénard convection instability in the presence of temperature variation at the lower wall
Directory of Open Access Journals (Sweden)
Jovanović Miloš M.
2012-01-01
Full Text Available This paper analyzes the two-dimensional viscous fluid flow between two parallel plates, where the lower plate is heated and the upper one is cooled. The temperature difference between the plates is gradually increased during a certain time period, and afterwards it is temporarily constant. The temperature distribution on the lower plate is not constant in x-direction, and there is longitudinal sinusoidal temperature variation imposed on the mean temperature. We investigate the wave number and amplitude influence of this variation on the stability of Rayleigh-Benard convective cells, by direct numerical simulation of 2-D Navier-Stokes and energy equation.
Energy transfer in turbulence under rotation
Buzzicotti, Michele; Aluie, Hussein; Biferale, Luca; Linkmann, Moritz
2018-03-01
It is known that rapidly rotating turbulent flows are characterized by the emergence of simultaneous upscale and downscale energy transfer. Indeed, both numerics and experiments show the formation of large-scale anisotropic vortices together with the development of small-scale dissipative structures. However the organization of interactions leading to this complex dynamics remains unclear. Two different mechanisms are known to be able to transfer energy upscale in a turbulent flow. The first is characterized by two-dimensional interactions among triads lying on the two-dimensional, three-component (2D3C)/slow manifold, namely on the Fourier plane perpendicular to the rotation axis. The second mechanism is three-dimensional and consists of interactions between triads with the same sign of helicity (homochiral). Here, we present a detailed numerical study of rotating flows using a suite of high-Reynolds-number direct numerical simulations (DNS) within different parameter regimes to analyze both upscale and downscale cascade ranges. We find that the upscale cascade at wave numbers close to the forcing scale is generated by increasingly dominant homochiral interactions which couple the three-dimensional bulk and the 2D3C plane. This coupling produces an accumulation of energy in the 2D3C plane, which then transfers energy to smaller wave numbers thanks to the two-dimensional mechanism. In the forward cascade range, we find that the energy transfer is dominated by heterochiral triads and is dominated primarily by interaction within the fast manifold where kz≠0 . We further analyze the energy transfer in different regions in the real-space domain. In particular, we distinguish high-strain from high-vorticity regions and we uncover that while the mean transfer is produced inside regions of strain, the rare but extreme events of energy transfer occur primarily inside the large-scale column vortices.
Energy Technology Data Exchange (ETDEWEB)
Gibert, M
2007-10-15
In this study we investigate the phenomenon of thermal turbulent convection in new and unprecedented ways. The first system we studied experimentally is an infinite vertical channel, where a constant vertical mean gradient of temperature exists. Inside this channel the average mass flux is null. The results obtained from our measurements reveal that the flow is mainly inertial; indeed the dissipative coefficients (here the viscosity) play a role only to define a coherence length L. This length is the distance over which the thermal plumes can be considered as 'free falling' objects. The horizontal transport, of heat and momentum, is entirely due to fluctuations. The associated 'mixing length' is small compared to the channel width. In the other hand, the vertical heat transport is due to coherent structures: the heat plumes. Those objects were also investigated in a Lagrangian study of the flow in the bulk of a Rayleigh-Benard cell. The probe, which has the same density as the fluid used in this experiment, is a sphere of 2 cm in diameter with embarked thermometers and radio-emitter. The heat plumes transport it, which allows a statistical study of such objects. (author)
Trapped Electron Mode Turbulence Driven Intrinsic Rotation in Tokamak Plasmas
International Nuclear Information System (INIS)
Wang, W.X.; Hahm, T.S.; Ethier, S.; Zakharov, L.E.
2011-01-01
Recent progress from global gyrokinetic simulations in understanding the origin of intrinsic rotation in toroidal plasmas is reported with emphasis on electron thermal transport dominated regimes. The turbulence driven intrinsic torque associated with nonlinear residual stress generation by the fluctuation intensity and the intensity gradient in the presence of zonal flow shear induced asymmetry in the parallel wavenumber spectrum is shown to scale close to linearly with plasma gradients and the inverse of the plasma current. These results qualitatively reproduce empirical scalings of intrinsic rotation observed in various experiments. The origin of current scaling is found to be due to enhanced kll symmetry breaking induced by the increased radial variation of the safety factor as the current decreases. The physics origin for the linear dependence of intrinsic torque on pressure gradient is that both turbulence intensity and the zonal flow shear, which are two key ingredients for driving residual stress, increase with the strength of turbulence drive, which is R0/LTe and R0/Lne for the trapped electron mode.
International Nuclear Information System (INIS)
Liu, Y.; Ecke, R.E.
1999-01-01
We present experimental measurements of a sidewall traveling wave in rotating Rayleigh-Bacute enard convection. The fluid, water with Prandtl number about 6.3, was confined in a 1-cm-high cylindrical cell with radius-to-height ratio Γ=5. We used simultaneous optical-shadowgraph, heat-transport, and local temperature measurements to determine the stability and characteristics of the traveling-wave state for dimensionless rotation rates 60<Ω<420. The state is well described by the one-dimensional complex Ginzburg-Landau (CGL) equation for which the linear and nonlinear coefficients were determined for Ω=274. The Eckhaus-Benjamin-Feir-stability boundary was established and the phase-diffusion coefficient and nonlinear group velocity were determined in the stable regime. Higher-order corrections to the CGL equation were also investigated. copyright 1999 The American Physical Society
On specification of initial conditions in turbulence models
Energy Technology Data Exchange (ETDEWEB)
Rollin, Bertrand [Los Alamos National Laboratory; Andrews, Malcolm J [Los Alamos National Laboratory
2010-12-01
Recent research has shown that initial conditions have a significant influence on the evolution of a flow towards turbulence. This important finding offers a unique opportunity for turbulence control, but also raises the question of how to properly specify initial conditions in turbulence models. We study this problem in the context of the Rayleigh-Taylor instability. The Rayleigh-Taylor instability is an interfacial fluid instability that leads to turbulence and turbulent mixing. It occurs when a light fluid is accelerated in to a heavy fluid because of misalignment between density and pressure gradients. The Rayleigh-Taylor instability plays a key role in a wide variety of natural and man-made flows ranging from supernovae to the implosion phase of Inertial Confinement Fusion (ICF). Our approach consists of providing the turbulence models with a predicted profile of its key variables at the appropriate time in accordance to the initial conditions of the problem.
Rotating turbulent Rayleigh–Bénard convection subject to harmonically forced flow reversals
Geurts, Bernardus J.; Kunnen, Rudie P.J.
2014-01-01
The characteristics of turbulent flow in a cylindrical Rayleigh–Bénard convection cell which can be modified considerably in case rotation is included in the dynamics. By incorporating the additional effects of an Euler force, i.e., effects induced by non-constant rotation rates, a remarkably strong
Instabilities responsible for magnetic turbulence in laboratory rotating plasma
International Nuclear Information System (INIS)
Mikhailovskii, A.B.; Lominadze, J.G.; Churikov, A.P.; Erokhin, N.N.; Pustovitov, V.D.; Konovalov, S.V.
2008-01-01
Instabilities responsible for magnetic turbulence in laboratory rotating plasma are investigated. It is shown that the plasma compressibility gives a new driving mechanism in addition to the known Velikhov effect due to the negative rotation frequency gradient. This new mechanism is related to the perpendicular plasma pressure gradient, while the density gradient gives an additional drive depending also on the pressure gradient. It is shown that these new effects can manifest themselves even in the absence of the equilibrium magnetic field, which corresponds to nonmagnetic instabilities
Moore, Joan G.; Moore, John
1992-01-01
The flow in the NASA Low-Speed Impeller is affected by both curvature and rotation. The flow curves due to the following: (1) geometric curvature, e.g. the curvature of the hub and shroud profiles in the meridional plane and the curvature of the backswept impeller blades; and (2) secondary flow vortices, e.g. the tip leakage vortex. Changes in the turbulence and effective turbulent viscosity in the impeller are investigated. The effects of these changes on three-dimensional flow development are discussed. Two predictions of the flow in the impeller, one with, and one without modification to the turbulent viscosity due to rotation and curvature, are compared. Some experimental and theoretical background for the modified mixing length model of turbulent viscosity will also be presented.
Kinetic theory of instabilities responsible for magnetic turbulence in laboratory rotating plasma
International Nuclear Information System (INIS)
Mikhailovskii, A.B.; Lominadze, J.G.; Churikov, A.P.; Pustovitov, V.D.; Erokhin, N.N.; Konovalov, S.V.
2008-01-01
The problem of instabilities responsible for magnetic turbulence in collisionless laboratory rotating plasma is investigated. It is shown that the standard mechanism of driving the magnetorotational instability (MRI), due to negative rotation frequency gradient, disappears in such a plasma. Instead of it, a new driving mechanism due to plasma pressure gradient is predicted
Transitional boundary layer in low-Prandtl-number convection at high Rayleigh number
Schumacher, Joerg; Bandaru, Vinodh; Pandey, Ambrish; Scheel, Janet
2016-11-01
The boundary layer structure of the velocity and temperature fields in turbulent Rayleigh-Bénard flows in closed cylindrical cells of unit aspect ratio is revisited from a transitional and turbulent viscous boundary layer perspective. When the Rayleigh number is large enough the boundary layer dynamics at the bottom and top plates can be separated into an impact region of downwelling plumes, an ejection region of upwelling plumes and an interior region (away from side walls) that is dominated by a shear flow of varying orientation. This interior plate region is compared here to classical wall-bounded shear flows. The working fluid is liquid mercury or liquid gallium at a Prandtl number of Pr = 0 . 021 for a range of Rayleigh numbers of 3 ×105 Deutsche Forschungsgemeinschaft.
International Nuclear Information System (INIS)
Jacobs, Jeffrey W.
2006-01-01
The objective of this three-year research program is to study the development of turbulence in Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities. Incompressible RT and RM instabilities are studied in an apparatus in which a box containing two unequal density liquids is accelerated on a linear rail system either impulsively (by bouncing it off of a spring) to produce RM instability, or at a constant downward rate (using a weight and pulley system) to produce RT instability. These experiments are distinguished from others in the field in that they are initialized with well defined, measurable initial perturbations and are well visualized utilizing planar laser induced fluorescence imaging. New experiments are proposed aimed at generating fully turbulent RM and RT instabilities and quantifying the turbulent development once fully turbulent flows are achieved. The proposed experiments focus on the development and the subsequent application of techniques to accelerate the production of fully turbulent instabilities and the quantification of the turbulent instabilities once they are achieved. The proposed tasks include: the development of RM and RT experiments utilizing fluid combinations having larger density ratios than those previously used; the development of RM experiments with larger acceleration impulse than that previously used; and the investigation of the multi-mode and three-dimensional instabilities by the development of new techniques for generating short wavelength initial perturbations. Progress towards fulfilling these goals is currently well on track. Recent results have been obtained on experiments that utilize Faraday resonance for the production of a nearly single-mode three-dimensional perturbation with a short enough wavelength to yield a self-similar instability at late-times. Last year we reported that we can reliably generate Faraday internal waves on the interface in our experimental apparatus by oscillating the tank containing the
Numerical simulation of helical-vortex effects in Rayleigh-Bénard convection
Directory of Open Access Journals (Sweden)
G. V. Levina
2006-01-01
Full Text Available A numerical approach is substantiated for searching for the large-scale alpha-like instability in thermoconvective turbulence. The main idea of the search strategy is the application of a forcing function which can have a physical interpretation. The forcing simulates the influence of small-scale helical turbulence generated in a rotating fluid with internal heat sources and is applied to naturally induced fully developed convective flows. The strategy is tested using the Rayleigh-Bénard convection in an extended horizontal layer of incompressible fluid heated from below. The most important finding is an enlargement of the typical horizontal scale of the forming helical convective structures accompanied by a cells merging, an essential increase in the kinetic energy of flows and intensification of heat transfer. The results of modeling allow explaining how the helical feedback can work providing the non-zero mean helicity generation and the mutual intensification of horizontal and vertical circulation, and demonstrate how the energy of the additional helical source can be effectively converted into the energy of intensive large-scale vortex flow.
Turbulent convection experiment at high Rayleigh number to support CAP1400 IVR strategy
Energy Technology Data Exchange (ETDEWEB)
Ma, Li, E-mail: mali@snptrd.com [State Nuclear Hua Qing(Beijing) Nuclear Power Technology R& D Centre Co., Ltd, Building A, State Nuclear Power Research Institute, Future Science & Technology Park, Changping Dist., Beijing 102209 (China); Li, Jing, E-mail: lijing@snptrd.com [State Nuclear Hua Qing(Beijing) Nuclear Power Technology R& D Centre Co., Ltd, Building A, State Nuclear Power Research Institute, Future Science & Technology Park, Changping Dist., Beijing 102209 (China); Ji, Shui, E-mail: jishui@snptrd.com [State Nuclear Hua Qing(Beijing) Nuclear Power Technology R& D Centre Co., Ltd, Building A, State Nuclear Power Research Institute, Future Science & Technology Park, Changping Dist., Beijing 102209 (China); Chang, Huajian, E-mail: changhuajian@snptrd.com [State Nuclear Hua Qing(Beijing) Nuclear Power Technology R& D Centre Co., Ltd, Building A, State Nuclear Power Research Institute, Future Science & Technology Park, Changping Dist., Beijing 102209 (China); Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084 (China)
2015-10-15
Highlights: • The facility reached high Ra number at 10{sup 12} of CAP1400 working condition. • The fitting formula Nu = 0.085 × Ra{sup 0.315} was established to calculate the heat flux in the metal layer at high Ra for the CAP1400. • The coupling method can accurately and safely predict the heat flow distribution of metal layer in high Ra number conditions. • The experiment results will predict the relationship between axial and radial heat transfer well. - Abstract: The characteristics of the heat transfer and the calculation of heat flux in metal layer are both the critical problems for in-vessel retention (IVR) strategy. Turbulent convection occurs in the metal layer when the Rayleigh number (Ra) becomes sufficient high. The Globe–Dropkin (G–D) correlation (Globe and Dropkin, 1959) and Chu–Churchill (C–C) correlation (Churchill and Chu, 1975) have been widely used to calculate the heat flux in the metal layer, where the valid range of the Ra is from 1.5 × 10{sup 5} to 6.8 × 10{sup 8} in G–D correlation and less than 10{sup 12} in C–C correlation. However, with the increase of reactor power, both the Rayleigh number and the rate of heat transfer below the bottom of metal layer of the molten pool will increase, and in this case the Rayleigh number even can reach 10{sup 11} for the China Advanced Passive Plant CAP1400. Accordingly, the G–D correlation is not suitable for the CAP1400. Therefore, our experiment purposes are to establish the appropriate correlation at high Ra for the CAP1400 and predict the axial and radial distribution of the heat transfer in the metal layer with the heat transfer behavior of metal layer experiment (HELM) facility. The experiments are divided into two parts. Each part concerns 39 runs and 47 experimental conditions. Its corresponding results are obtained at middle Prandtl number (Pr = 7 for water) and the Nusselt number is found to be proportional to Ra{sup 0.315} in the range 3.93 × 10{sup 8} < Ra < 3.57
Turbulent convection experiment at high Rayleigh number to support CAP1400 IVR strategy
International Nuclear Information System (INIS)
Ma, Li; Li, Jing; Ji, Shui; Chang, Huajian
2015-01-01
Highlights: • The facility reached high Ra number at 10 12 of CAP1400 working condition. • The fitting formula Nu = 0.085 × Ra 0.315 was established to calculate the heat flux in the metal layer at high Ra for the CAP1400. • The coupling method can accurately and safely predict the heat flow distribution of metal layer in high Ra number conditions. • The experiment results will predict the relationship between axial and radial heat transfer well. - Abstract: The characteristics of the heat transfer and the calculation of heat flux in metal layer are both the critical problems for in-vessel retention (IVR) strategy. Turbulent convection occurs in the metal layer when the Rayleigh number (Ra) becomes sufficient high. The Globe–Dropkin (G–D) correlation (Globe and Dropkin, 1959) and Chu–Churchill (C–C) correlation (Churchill and Chu, 1975) have been widely used to calculate the heat flux in the metal layer, where the valid range of the Ra is from 1.5 × 10 5 to 6.8 × 10 8 in G–D correlation and less than 10 12 in C–C correlation. However, with the increase of reactor power, both the Rayleigh number and the rate of heat transfer below the bottom of metal layer of the molten pool will increase, and in this case the Rayleigh number even can reach 10 11 for the China Advanced Passive Plant CAP1400. Accordingly, the G–D correlation is not suitable for the CAP1400. Therefore, our experiment purposes are to establish the appropriate correlation at high Ra for the CAP1400 and predict the axial and radial distribution of the heat transfer in the metal layer with the heat transfer behavior of metal layer experiment (HELM) facility. The experiments are divided into two parts. Each part concerns 39 runs and 47 experimental conditions. Its corresponding results are obtained at middle Prandtl number (Pr = 7 for water) and the Nusselt number is found to be proportional to Ra 0.315 in the range 3.93 × 10 8 < Ra < 3.57 × 10 12 . Furthermore, the experiment
International Nuclear Information System (INIS)
McKee, G.; Gohil, P.; Schlossberg, D.; Boedo, J.; Burrell, K.; deGrassie, J.; Groebner, R.; Makowski, M.; Moyer, R.; Petty, C.; Rhodes, T.; Schmitz, L.; Shafer, M.; Solomon, W.; Umansky, M.; Wang, G.; White, A.; Xu, X.
2008-01-01
The injected power required to induce a transition from L-mode to H-mode plasmas is found to depend strongly on the injected neutral beam torque and consequent plasma toroidal rotation. Edge turbulence and flows, measured near the outboard midplane of the plasma (0.85 < r/a < 1.0) on DIII-D with the high-sensitivity 2D beam emission spectroscopy (BES) system, likewise vary with rotation and suggest a causative connection. The L-H power threshold in plasmas with the ion (del)B drift away from the X-point decreases from 4-6 MW with co-current beam injection, to 2-3 MW with near zero net injected torque, and to <2 MW with counter injection. Plasmas with the ion (del)B drift towards the X-point exhibit a qualitatively similar though less pronounced power threshold dependence on rotation. 2D edge turbulence measurements with BES show an increasing poloidal flow shear as the L-H transition is approached in all conditions. At low rotation, the poloidal flow of turbulent eddies near the edge reverses prior to the L-H transition, generating a significant poloidal flow shear that exceeds the measured turbulence decorrelation rate. This increased poloidal turbulence velocity shear may facilitate the L-H transition. No such reversal is observed in high rotation plasmas. The poloidal turbulence velocity spectrum exhibits a transition from a Geodesic Acoustic Mode zonal flow to a higher-power, lower frequency, zero-mean-frequency zonal flow as rotation varies from co-current to balanced during a torque scan at constant injected neutral beam power, perhaps also facilitating the L-H transition. This reduced power threshold at lower toroidal rotation may benefit inherently low-rotation plasmas such as ITER
Bursting and large-scale intermittency in turbulent convection with differential rotation
DEFF Research Database (Denmark)
Garcia, O.E.; Bian, N.H.
2003-01-01
The tilting mechanism, which generates differential rotation in two-dimensional turbulent convection, is shown to produce relaxation oscillations in the mean flow energy integral and bursts in the global fluctuation level, akin to Lotka-Volterra oscillations. The basic reason for such behavior...
Application of the Proper Orthogonal Decomposition to Turbulent Czochralski Convective Flows
International Nuclear Information System (INIS)
Rahal, S; Cerisier, P; Azuma, H
2007-01-01
The aim of this work is to study the general aspects of the convective flow instabilities in a simulated Czochralski system. We considered the influence of the buoyancy and crystal rotation. Velocity fields, obtained by an ultrasonic technique, the corresponding 2D Fourier spectra and a correlation function, have been used. Steady, quasi-periodic and turbulent flows, are successively recognized, as the Reynolds number was increased, for a fixed Rayleigh number. The orthogonal decomposition method was applied and the numbers of modes, involved in the dynamics of turbulent flows, calculated. As far as we know, this method has been used for the first time to study the Czochralski convective flows. This method provides also information on the most important modes and allows simple theoretical models to be established. The large rotation rates of the crystal were found to stabilize the flow, and conversely the temperature gradients destabilize the flow. Indeed, the increase of the rotation effects reduces the number of involved modes and oscillations, and conversely, as expected, the increase of the buoyancy effects induces more modes to be involved in the dynamics. Thus, the flow oscillations can be reduced either by increasing the crystal rotation rate to the adequate value, as shown in this study or by imposing a magnetic field
The turbulent mixing of non-Newtonian fluids
Demianov, A. Yu; Doludenko, A. N.; Inogamov, N. A.; Son, E. E.
2013-07-01
The turbulence caused by the Rayleigh-Taylor instability represents a complicated phenomenon. It is usually related to the major hydrodynamic activities, the tangling of the media contact boundary, merging, separation and intermixing of originally smoothed initial structures. An important role in the theory of the Rayleigh-Taylor instability is played by the discontinuity of density on a contact interface between two homogeneous (in terms of density) fluids. A numerical modeling of the intermixing of two fluids with different rheology whose densities differ twice as a result of the Rayleigh-Taylor instability has been carried out. The coefficients of turbulent intermixing in a multimode statement of the problem for the Bingham, dilatant and pseudo-plastic fluids have been obtained.
Validation of the kinetic-turbulent-neoclassical theory for edge intrinsic rotation in DIII-D
Ashourvan, Arash; Grierson, B. A.; Battaglia, D. J.; Haskey, S. R.; Stoltzfus-Dueck, T.
2018-05-01
In a recent kinetic model of edge main-ion (deuterium) toroidal velocity, intrinsic rotation results from neoclassical orbits in an inhomogeneous turbulent field [T. Stoltzfus-Dueck, Phys. Rev. Lett. 108, 065002 (2012)]. This model predicts a value for the toroidal velocity that is co-current for a typical inboard X-point plasma at the core-edge boundary (ρ ˜ 0.9). Using this model, the velocity prediction is tested on the DIII-D tokamak for a database of L-mode and H-mode plasmas with nominally low neutral beam torque, including both signs of plasma current. Values for the flux-surface-averaged main-ion rotation velocity in the database are obtained from the impurity carbon rotation by analytically calculating the main-ion—impurity neoclassical offset. The deuterium rotation obtained in this manner has been validated by direct main-ion measurements for a limited number of cases. Key theoretical parameters of ion temperature and turbulent scale length are varied across a wide range in an experimental database of discharges. Using a characteristic electron temperature scale length as a proxy for a turbulent scale length, the predicted main-ion rotation velocity has a general agreement with the experimental measurements for neutral beam injection (NBI) powers in the range PNBI balanced—but high powered—NBI, the net injected torque through the edge can exceed 1 Nm in the counter-current direction. The theory model has been extended to compute the rotation degradation from this counter-current NBI torque by solving a reduced momentum evolution equation for the edge and found the revised velocity prediction to be in agreement with experiment. Using the theory modeled—and now tested—velocity to predict the bulk plasma rotation opens up a path to more confidently projecting the confinement and stability in ITER.
Random forcing of geostrophic motion in rotating stratified turbulence
Waite, Michael L.
2017-12-01
Random forcing of geostrophic motion is a common approach in idealized simulations of rotating stratified turbulence. Such forcing represents the injection of energy into large-scale balanced motion, and the resulting breakdown of quasi-geostrophic turbulence into inertia-gravity waves and stratified turbulence can shed light on the turbulent cascade processes of the atmospheric mesoscale. White noise forcing is commonly employed, which excites all frequencies equally, including frequencies much higher than the natural frequencies of large-scale vortices. In this paper, the effects of these high frequencies in the forcing are investigated. Geostrophic motion is randomly forced with red noise over a range of decorrelation time scales τ, from a few time steps to twice the large-scale vortex time scale. It is found that short τ (i.e., nearly white noise) results in about 46% more gravity wave energy than longer τ, despite the fact that waves are not directly forced. We argue that this effect is due to wave-vortex interactions, through which the high frequencies in the forcing are able to excite waves at their natural frequencies. It is concluded that white noise forcing should be avoided, even if it is only applied to the geostrophic motion, when a careful investigation of spontaneous wave generation is needed.
Turbulent structures in cylindrical density currents in a rotating frame of reference
Salinas, Jorge S.; Cantero, Mariano I.; Dari, Enzo A.; Bonometti, Thomas
2018-06-01
Gravity currents are flows generated by the action of gravity on fluids with different densities. In some geophysical applications, modeling such flows makes it necessary to account for rotating effects, modifying the dynamics of the flow. While previous works on rotating stratified flows focused on currents of large Coriolis number, the present work focuses on flows with small Coriolis numbers (i.e. moderate-to-large Rossby numbers). In this work, cylindrical rotating gravity currents are investigated by means of highly resolved simulations. A brief analysis of the mean flow evolution to the final state is presented to provide a complete picture of the flow dynamics. The numerical results, showing the well-known oscillatory behavior of the flow (inertial waves) and a final state lens shape (geostrophic adjustment), are in good agreement with experimental observations and theoretical models. The turbulent structures in the flow are visualized and described using, among others, a stereoscopic visualization and videos as supplementary material. In particular, the structure of the lobes and clefts at the front of the current is presented in association to local turbulent structures. In rotating gravity currents, the vortices observed at the lobes front are not of hairpin type but are rather of Kelvin-Helmholtz type.
Two-dimensional turbulent convection
Mazzino, Andrea
2017-11-01
We present an overview of the most relevant, and sometimes contrasting, theoretical approaches to Rayleigh-Taylor and mean-gradient-forced Rayleigh-Bénard two-dimensional turbulence together with numerical and experimental evidences for their support. The main aim of this overview is to emphasize that, despite the different character of these two systems, especially in relation to their steadiness/unsteadiness, turbulent fluctuations are well described by the same scaling relationships originated from the Bolgiano balance. The latter states that inertial terms and buoyancy terms balance at small scales giving rise to an inverse kinetic energy cascade. The main difference with respect to the inverse energy cascade in hydrodynamic turbulence [R. H. Kraichnan, "Inertial ranges in two-dimensional turbulence," Phys. Fluids 10, 1417 (1967)] is that the rate of cascade of kinetic energy here is not constant along the inertial range of scales. Thanks to the absence of physical boundaries, the two systems here investigated turned out to be a natural physical realization of the Kraichnan scaling regime hitherto associated with the elusive "ultimate state of thermal convection" [R. H. Kraichnan, "Turbulent thermal convection at arbitrary Prandtl number," Phys. Fluids 5, 1374-1389 (1962)].
Directory of Open Access Journals (Sweden)
M. Alpers
2004-01-01
Full Text Available For the first time, three different temperature lidar methods are combined to obtain time-resolved complete temperature profiles with high altitude resolution over an altitude range from the planetary boundary layer up to the lower thermosphere (about 1–105 km. The Leibniz-Institute of Atmospheric Physics (IAP at Kühlungsborn, Germany (54° N, 12° E operates two lidar instruments, using three different temperature measurement methods, optimized for three altitude ranges: (1 Probing the spectral Doppler broadening of the potassium D1 resonance lines with a tunable narrow-band laser allows atmospheric temperature profiles to be determined at metal layer altitudes (80–105 km. (2 Between about 20 and 90 km, temperatures were calculated from Rayleigh backscattering by air molecules, where the upper start values for the calculation algorithm were taken from the potassium lidar results. Correction methods have been applied to account for, e.g. Rayleigh extinction or Mie scattering of aerosols below about 32 km. (3 At altitudes below about 25 km, backscattering in the Rotational Raman lines is strong enough to obtain temperatures by measuring the temperature dependent spectral shape of the Rotational Raman spectrum. This method works well down to about 1 km. The instrumental configurations of the IAP lidars were optimized for a 3–6 km overlap of the temperature profiles at the method transition altitudes. We present two night-long measurements with clear wave structures propagating from the lower stratosphere up to the lower thermosphere.
Plume structure in high-Rayleigh-number convection
Puthenveettil, Baburaj A.; Arakeri, Jaywant H.
2005-10-01
Near-wall structures in turbulent natural convection at Rayleigh numbers of 10^{10} to 10^{11} at A Schmidt number of 602 are visualized by a new method of driving the convection across a fine membrane using concentration differences of sodium chloride. The visualizations show the near-wall flow to consist of sheet plumes. A wide variety of large-scale flow cells, scaling with the cross-section dimension, are observed. Multiple large-scale flow cells are seen at aspect ratio (AR)= 0.65, while only a single circulation cell is detected at AR= 0.435. The cells (or the mean wind) are driven by plumes coming together to form columns of rising lighter fluid. The wind in turn aligns the sheet plumes along the direction of shear. the mean wind direction is seen to change with time. The near-wall dynamics show plumes initiated at points, which elongate to form sheets and then merge. Increase in rayleigh number results in a larger number of closely and regularly spaced plumes. The plume spacings show a common log normal probability distribution function, independent of the rayleigh number and the aspect ratio. We propose that the near-wall structure is made of laminar natural-convection boundary layers, which become unstable to give rise to sheet plumes, and show that the predictions of a model constructed on this hypothesis match the experiments. Based on these findings, we conclude that in the presence of a mean wind, the local near-wall boundary layers associated with each sheet plume in high-rayleigh-number turbulent natural convection are likely to be laminar mixed convection type.
Testing neoclassical and turbulent effects on poloidal rotation in the core of DIII-D
Energy Technology Data Exchange (ETDEWEB)
Chrystal, C. [University of California San Diego, 9500 Gilman Dr., La Jolla, California 92093-0417 (United States); Burrell, K. H.; Staebler, G. M.; Kinsey, J. E.; Lao, L. L.; Grassie, J. S. de [General Atomics, P.O. Box 85608, San Diego, California 92186-5608 (United States); Grierson, B. A.; Solomon, W. M.; Wang, W. X. [Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, New Jersey 08543-0451 (United States); Rhodes, T. L.; Schmitz, L. [University of California Los Angeles, P.O. Box 957099, Los Angeles, California 90095-7099 (United States); Mordijck, S. [College of William and Mary, P.O. Box 8795, Williamsburg, Virginia 23187-8795 (United States); Meneghini, O. [Oak Ridge Associated Universities, 1299 Bethel Valley Rd, Bldg SC-200, Oak Ridge, Tennessee 37830 (United States)
2014-07-15
Experimental tests of ion poloidal rotation theories have been performed on DIII-D using a novel impurity poloidal rotation diagnostic. These tests show significant disagreements with theoretical predictions in various conditions, including L-mode plasmas with internal transport barriers (ITB), H-mode plasmas, and QH-mode plasmas. The theories tested include standard neoclassical theory, turbulence driven Reynolds stress, and fast-ion friction on the thermal ions. Poloidal rotation is observed to spin up at the formation of an ITB and makes a significant contribution to the measurement of the E{sup →}×B{sup →} shear that forms the ITB. In ITB cases, neoclassical theory agrees quantitatively with the experimental measurements only in the steep gradient region. Significant quantitative disagreement with neoclassical predictions is seen in the cores of ITB, QH-, and H-mode plasmas, demonstrating that neoclassical theory is an incomplete description of poloidal rotation. The addition of turbulence driven Reynolds stress does not remedy this disagreement; linear stability calculations and Doppler backscattering measurements show that disagreement increases as turbulence levels decline. Furthermore, the effect of fast-ion friction, by itself, does not lead to improved agreement; in QH-mode plasmas, neoclassical predictions are closest to experimental results in plasmas with the largest fast ion friction. Predictions from a new model that combines all three effects show somewhat better agreement in the H-mode case, but discrepancies well outside the experimental error bars remain.
Testing neoclassical and turbulent effects on poloidal rotation in the core of DIII-D
International Nuclear Information System (INIS)
Chrystal, C.; Burrell, K. H.; Staebler, G. M.; Kinsey, J. E.; Lao, L. L.; Grassie, J. S. de; Grierson, B. A.; Solomon, W. M.; Wang, W. X.; Rhodes, T. L.; Schmitz, L.; Mordijck, S.; Meneghini, O.
2014-01-01
Experimental tests of ion poloidal rotation theories have been performed on DIII-D using a novel impurity poloidal rotation diagnostic. These tests show significant disagreements with theoretical predictions in various conditions, including L-mode plasmas with internal transport barriers (ITB), H-mode plasmas, and QH-mode plasmas. The theories tested include standard neoclassical theory, turbulence driven Reynolds stress, and fast-ion friction on the thermal ions. Poloidal rotation is observed to spin up at the formation of an ITB and makes a significant contribution to the measurement of the E → ×B → shear that forms the ITB. In ITB cases, neoclassical theory agrees quantitatively with the experimental measurements only in the steep gradient region. Significant quantitative disagreement with neoclassical predictions is seen in the cores of ITB, QH-, and H-mode plasmas, demonstrating that neoclassical theory is an incomplete description of poloidal rotation. The addition of turbulence driven Reynolds stress does not remedy this disagreement; linear stability calculations and Doppler backscattering measurements show that disagreement increases as turbulence levels decline. Furthermore, the effect of fast-ion friction, by itself, does not lead to improved agreement; in QH-mode plasmas, neoclassical predictions are closest to experimental results in plasmas with the largest fast ion friction. Predictions from a new model that combines all three effects show somewhat better agreement in the H-mode case, but discrepancies well outside the experimental error bars remain
Nonlinear dynamics and anisotropic structure of rotating sheared turbulence.
Salhi, A; Jacobitz, F G; Schneider, K; Cambon, C
2014-01-01
Homogeneous turbulence in rotating shear flows is studied by means of pseudospectral direct numerical simulation and analytical spectral linear theory (SLT). The ratio of the Coriolis parameter to shear rate is varied over a wide range by changing the rotation strength, while a constant moderate shear rate is used to enable significant contributions to the nonlinear interscale energy transfer and to the nonlinear intercomponental redistribution terms. In the destabilized and neutral cases, in the sense of kinetic energy evolution, nonlinearity cannot saturate the growth of the largest scales. It permits the smallest scale to stabilize by a scale-by-scale quasibalance between the nonlinear energy transfer and the dissipation spectrum. In the stabilized cases, the role of rotation is mainly nonlinear, and interacting inertial waves can affect almost all scales as in purely rotating flows. In order to isolate the nonlinear effect of rotation, the two-dimensional manifold with vanishing spanwise wave number is revisited and both two-component spectra and single-point two-dimensional energy components exhibit an important effect of rotation, whereas the SLT as well as the purely two-dimensional nonlinear analysis are unaffected by rotation as stated by the Proudman theorem. The other two-dimensional manifold with vanishing streamwise wave number is analyzed with similar tools because it is essential for any shear flow. Finally, the spectral approach is used to disentangle, in an analytical way, the linear and nonlinear terms in the dynamical equations.
Key issues review: numerical studies of turbulence in stars
Arnett, W. David; Meakin, Casey
2016-10-01
Three major problems of single-star astrophysics are convection, magnetic fields and rotation. Numerical simulations of convection in stars now have sufficient resolution to be truly turbulent, with effective Reynolds numbers of \\text{Re}>{{10}4} , and some turbulent boundary layers have been resolved. Implications of these developments are discussed for stellar structure, evolution and explosion as supernovae. Methods for three-dimensional (3D) simulations of stars are compared and discussed for 3D atmospheres, solar rotation, core-collapse and stellar boundary layers. Reynolds-averaged Navier-Stokes (RANS) analysis of the numerical simulations has been shown to provide a novel and quantitative estimate of resolution errors. Present treatments of stellar boundaries require revision, even for early burning stages (e.g. for mixing regions during He-burning). As stellar core-collapse is approached, asymmetry and fluctuations grow, rendering spherically symmetric models of progenitors more unrealistic. Numerical resolution of several different types of three-dimensional (3D) stellar simulations are compared; it is suggested that core-collapse simulations may be under-resolved. The Rayleigh-Taylor instability in explosions has a deep connection to convection, for which the abundance structure in supernova remnants may provide evidence.
Turbulent heat transfer studies in annulus with inner cylinder rotation
International Nuclear Information System (INIS)
Kuzay, T.M.; Scott, C.J.
1977-01-01
Experimental investigations of turbulent heat transfer are made in a large-gap annulus with both rotating and nonrotating inner cylinder. The vertical annular channel has an electrically heated outer wall; the inner wall i thermally and electrically insulated. The axial air flow is allowed to develop before rotation and heating are imparted. The resulting temperature fields are investigated using thermocouple probes located near the channel exit. The wall heat flux, wall axial temperature development, and radial temperature profiles are measured. For each axial Reynolds number, three heat flux rates are used. Excellent correlation is established between rotational and nonrotational Nusselt number. The proper correlation parameter is a physical quantity characterizing the flow helix. This parameter is the inverse of the ratio of axial travel of the flow helix in terms of hydraulic diameter, per half revolution of the spinning wall
DEFF Research Database (Denmark)
Diamond, P.H.; McDevitt, C.J.; Güran, Ö.D.
2009-01-01
Recent results in the theory of turbulent momentum transport and the origins of intrinsic rotation are summarized. Special attention is focused on aspects of momentum transport critical to intrinsic rotation, namely the residual stress and the edge toroidal flow velocity pinch. Novel results...
Energy Technology Data Exchange (ETDEWEB)
Herbert, John M. [Kansas State Univ., Manhattan, KS (United States). Dept. of Chemistry
1997-01-01
Rayleigh-Schroedinger perturbation theory is an effective and popular tool for describing low-lying vibrational and rotational states of molecules. This method, in conjunction with ab initio techniques for computation of electronic potential energy surfaces, can be used to calculate first-principles molecular vibrational-rotational energies to successive orders of approximation. Because of mathematical complexities, however, such perturbation calculations are rarely extended beyond the second order of approximation, although recent work by Herbert has provided a formula for the nth-order energy correction. This report extends that work and furnishes the remaining theoretical details (including a general formula for the Rayleigh-Schroedinger expansion coefficients) necessary for calculation of energy corrections to arbitrary order. The commercial computer algebra software Mathematica is employed to perform the prohibitively tedious symbolic manipulations necessary for derivation of generalized energy formulae in terms of universal constants, molecular constants, and quantum numbers. As a pedagogical example, a Hamiltonian operator tailored specifically to diatomic molecules is derived, and the perturbation formulae obtained from this Hamiltonian are evaluated for a number of such molecules. This work provides a foundation for future analyses of polyatomic molecules, since it demonstrates that arbitrary-order perturbation theory can successfully be applied with the aid of commercially available computer algebra software.
Suppression of turbulent resistivity in turbulent Couette flow
Si, Jiahe; Colgate, Stirling A.; Sonnenfeld, Richard G.; Nornberg, Mark D.; Li, Hui; Colgate, Arthur S.; Westpfahl, David J.; Romero, Van D.; Martinic, Joe
2015-07-01
Turbulent transport in rapidly rotating shear flow very efficiently transports angular momentum, a critical feature of instabilities responsible both for the dynamics of accretion disks and the turbulent power dissipation in a centrifuge. Turbulent mixing can efficiently transport other quantities like heat and even magnetic flux by enhanced diffusion. This enhancement is particularly evident in homogeneous, isotropic turbulent flows of liquid metals. In the New Mexico dynamo experiment, the effective resistivity is measured using both differential rotation and pulsed magnetic field decay to demonstrate that at very high Reynolds number rotating shear flow can be described entirely by mean flow induction with very little contribution from correlated velocity fluctuations.
Suppression of turbulent resistivity in turbulent Couette flow
Energy Technology Data Exchange (ETDEWEB)
Si, Jiahe, E-mail: jsi@nmt.edu; Sonnenfeld, Richard G.; Colgate, Arthur S.; Westpfahl, David J.; Romero, Van D.; Martinic, Joe [New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 (United States); Colgate, Stirling A.; Li, Hui [Los Alamos National Laboratory, Los Alamos, New Mexico 87544 (United States); Nornberg, Mark D. [University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
2015-07-15
Turbulent transport in rapidly rotating shear flow very efficiently transports angular momentum, a critical feature of instabilities responsible both for the dynamics of accretion disks and the turbulent power dissipation in a centrifuge. Turbulent mixing can efficiently transport other quantities like heat and even magnetic flux by enhanced diffusion. This enhancement is particularly evident in homogeneous, isotropic turbulent flows of liquid metals. In the New Mexico dynamo experiment, the effective resistivity is measured using both differential rotation and pulsed magnetic field decay to demonstrate that at very high Reynolds number rotating shear flow can be described entirely by mean flow induction with very little contribution from correlated velocity fluctuations.
Suppression of turbulent resistivity in turbulent Couette flow
International Nuclear Information System (INIS)
Si, Jiahe; Sonnenfeld, Richard G.; Colgate, Arthur S.; Westpfahl, David J.; Romero, Van D.; Martinic, Joe; Colgate, Stirling A.; Li, Hui; Nornberg, Mark D.
2015-01-01
Turbulent transport in rapidly rotating shear flow very efficiently transports angular momentum, a critical feature of instabilities responsible both for the dynamics of accretion disks and the turbulent power dissipation in a centrifuge. Turbulent mixing can efficiently transport other quantities like heat and even magnetic flux by enhanced diffusion. This enhancement is particularly evident in homogeneous, isotropic turbulent flows of liquid metals. In the New Mexico dynamo experiment, the effective resistivity is measured using both differential rotation and pulsed magnetic field decay to demonstrate that at very high Reynolds number rotating shear flow can be described entirely by mean flow induction with very little contribution from correlated velocity fluctuations
Thermal Rayleigh-Marangoni convection in a three-layer liquid-metal-battery model
Köllner, Thomas; Boeck, Thomas; Schumacher, Jörg
2017-05-01
The combined effects of buoyancy-driven Rayleigh-Bénard convection (RC) and surface tension-driven Marangoni convection (MC) are studied in a triple-layer configuration which serves as a simplified model for a liquid metal battery (LMB). The three-layer model consists of a liquid metal alloy cathode, a molten salt separation layer, and a liquid metal anode at the top. Convection is triggered by the temperature gradient between the hot electrolyte and the colder electrodes, which is a consequence of the release of resistive heat during operation. We present a linear stability analysis of the state of pure thermal conduction in combination with three-dimensional direct numerical simulations of the nonlinear turbulent evolution on the basis of a pseudospectral method. Five different modes of convection are identified in the configuration, which are partly coupled to each other: RC in the upper electrode, RC with internal heating in the molten salt layer, and MC at both interfaces between molten salt and electrode as well as anticonvection in the middle layer and lower electrode. The linear stability analysis confirms that the additional Marangoni effect in the present setup increases the growth rates of the linearly unstable modes, i.e., Marangoni and Rayleigh-Bénard instability act together in the molten salt layer. The critical Grashof and Marangoni numbers decrease with increasing middle layer thickness. The calculated thresholds for the onset of convection are found for realistic current densities of laboratory-sized LMBs. The global turbulent heat transfer follows scaling predictions for internally heated RC. The global turbulent momentum transfer is comparable with turbulent convection in the classical Rayleigh-Bénard case. In summary, our studies show that incorporating Marangoni effects generates smaller flow structures, alters the velocity magnitudes, and enhances the turbulent heat transfer across the triple-layer configuration.
Non-Oberbeck-Boussinesq Effects in Gaseous Rayleigh-Bénard Convection
Ahlers, Günter; Fontenele Araujo Junior, F.; Funfschilling, Denis; Grossmann, Siegfried; Lohse, Detlef
2007-01-01
Non-Oberbeck-Boussinesq (NOB) effects are measured experimentally and calculated theoretically for strongly turbulent Rayleigh-Be´nard convection of ethane gas under pressure where the material properties strongly depend on the temperature. Relative to the Oberbeck-Boussinesq case we find a decrease
Self-similarity in high Atwood number Rayleigh-Taylor experiments
Mikhaeil, Mark; Suchandra, Prasoon; Pathikonda, Gokul; Ranjan, Devesh
2017-11-01
Self-similarity is a critical concept in turbulent and mixing flows. In the Rayleigh-Taylor instability, theory and simulations have shown that the flow exhibits properties of self-similarity as the mixing Reynolds number exceeds 20000 and the flow enters the turbulent regime. Here, we present results from the first large Atwood number (0.7) Rayleigh-Taylor experimental campaign for mixing Reynolds number beyond 20000 in an effort to characterize the self-similar nature of the instability. Experiments are performed in a statistically steady gas tunnel facility, allowing for the evaluation of turbulence statistics. A visualization diagnostic is used to study the evolution of the mixing width as the instability grows. This allows for computation of the instability growth rate. For the first time in such a facility, stereoscopic particle image velocimetry is used to resolve three-component velocity information in a plane. Velocity means, fluctuations, and correlations are considered as well as their appropriate scaling. Probability density functions of velocity fields, energy spectra, and higher-order statistics are also presented. The energy budget of the flow is described, including the ratio of the kinetic energy to the released potential energy. This work was supported by the DOE-NNSA SSAA Grant DE-NA0002922.
On soft stability loss in rotating turbulent MHD flows
International Nuclear Information System (INIS)
Kapusta, Arkady; Mikhailovich, Boris
2014-01-01
The problem of the stability of turbulent flows of liquid metal in a cylindrical cavity against small velocity disturbances under the action of a rotating magnetic field (RMF) has been studied. The flow is considered in the induction-free approximation using the ‘external’ friction model. A system of dimensionless equations is examined in cylindrical coordinates. The results of computations performed on the basis of this mathematical model using the exchange of stabilities principle have shown a good consistency between the critical values of computed and experimental Reynolds numbers. (paper)
GPU Implementation of High Rayleigh Number Three-Dimensional Mantle Convection
Sanchez, D. A.; Yuen, D. A.; Wright, G. B.; Barnett, G. A.
2010-12-01
Although we have entered the age of petascale computing, many factors are still prohibiting high-performance computing (HPC) from infiltrating all suitable scientific disciplines. For this reason and others, application of GPU to HPC is gaining traction in the scientific world. With its low price point, high performance potential, and competitive scalability, GPU has been an option well worth considering for the last few years. Moreover with the advent of NVIDIA's Fermi architecture, which brings ECC memory, better double-precision performance, and more RAM to GPU, there is a strong message of corporate support for GPU in HPC. However many doubts linger concerning the practicality of using GPU for scientific computing. In particular, GPU has a reputation for being difficult to program and suitable for only a small subset of problems. Although inroads have been made in addressing these concerns, for many scientists GPU still has hurdles to clear before becoming an acceptable choice. We explore the applicability of GPU to geophysics by implementing a three-dimensional, second-order finite-difference model of Rayleigh-Benard thermal convection on an NVIDIA GPU using C for CUDA. Our code reaches sufficient resolution, on the order of 500x500x250 evenly-spaced finite-difference gridpoints, on a single GPU. We make extensive use of highly optimized CUBLAS routines, allowing us to achieve performance on the order of O( 0.1 ) µs per timestep*gridpoint at this resolution. This performance has allowed us to study high Rayleigh number simulations, on the order of 2x10^7, on a single GPU.
Thresholds for the onset of fluid and magnetofluid turbulence
International Nuclear Information System (INIS)
Montgomery, D.
1982-01-01
Hydrodynamic stability theory has focussed on a few simple test cases to obtain the sharpest possible confrontations between theory and experiment. Six of these are briefly reviewed: plane Poiseuille and Couette flow, pipe flow, rotating Couette flow, thermally-driven Benard convection, and the Blasius laminar boundary layer. Linear perturbation theory seems inadequate in the first three cases, and satisfactory in the last three. Insufficient information, experimental or theoretical, exists in magnetohydrodynamics to make any comparably decisive tests
Turbulent transport stabilization by ICRH minority fast ions in low rotating JET ILW L-mode plasmas
Bonanomi, N.; Mantica, P.; Di Siena, A.; Delabie, E.; Giroud, C.; Johnson, T.; Lerche, E.; Menmuir, S.; Tsalas, M.; Van Eester, D.; Contributors, JET
2018-05-01
The first experimental demonstration that fast ion induced stabilization of thermal turbulent transport takes place also at low values of plasma toroidal rotation has been obtained in JET ILW (ITER-like wall) L-mode plasmas with high (3He)-D ICRH (ion cyclotron resonance heating) power. A reduction of the gyro-Bohm normalized ion heat flux and higher values of the normalized ion temperature gradient have been observed at high ICRH power and low NBI (neutral beam injection) power and plasma rotation. Gyrokinetic simulations indicate that ITG (ion temperature gradient) turbulence stabilization induced by the presence of high-energetic 3He ions is the key mechanism in order to explain the experimental observations. Two main mechanisms have been identified to be responsible for the turbulence stabilization: a linear electrostatic wave-fast particle resonance mechanism and a nonlinear electromagnetic mechanism. The dependence of the stabilization on the 3He distribution function has also been studied.
Jiang, Zhou; Xia, Zhenhua; Shi, Yipeng; Chen, Shiyi
2018-04-01
A fully developed spanwise rotating turbulent channel flow has been numerically investigated utilizing large-eddy simulation. Our focus is to assess the performances of the dynamic variants of eddy viscosity models, including dynamic Vreman's model (DVM), dynamic wall adapting local eddy viscosity (DWALE) model, dynamic σ (Dσ ) model, and the dynamic volumetric strain-stretching (DVSS) model, in this canonical flow. The results with dynamic Smagorinsky model (DSM) and direct numerical simulations (DNS) are used as references. Our results show that the DVM has a wrong asymptotic behavior in the near wall region, while the other three models can correctly predict it. In the high rotation case, the DWALE can get reliable mean velocity profile, but the turbulence intensities in the wall-normal and spanwise directions show clear deviations from DNS data. DVSS exhibits poor predictions on both the mean velocity profile and turbulence intensities. In all three cases, Dσ performs the best.
He, Xiaozhou; van Gils, Dennis P M; Bodenschatz, Eberhard; Ahlers, Guenter
2014-05-02
We report measurements of the temperature variance σ(2)(z,r) and frequency power spectrum P(f,z,r) (z is the distance from the sample bottom and r the radial coordinate) in turbulent Rayleigh-Bénard convection (RBC) for Rayleigh numbers Ra = 1.6 × 10(13) and 1.1 × 10(15) and for a Prandtl number Pr ≃ 0.8 for a sample with a height L = 224 cm and aspect ratio D/L=0.50 (D is the diameter). For z/L ≲ 0.1 σ(2)(z,r) was consistent with a logarithmic dependence on z, and there was a universal (independent of Ra, r, and z) normalized spectrum which, for 0.02 ≲ fτ(0) ≲ 0.2, had the form P(fτ(0)) = P(0)(fτ(0))(-1) with P(0) = 0.208 ± 0.008 a universal constant. Here τ(0) = sqrt[2R] where R is the radius of curvature of the temperature autocorrelation function C(τ) at τ = 0. For z/L ≃ 0.5 the measurements yielded P(fτ(0))∼(fτ(0))(-α) with α in the range from 3/2 to 5/3. All the results are similar to those for velocity fluctuations in shear flows at sufficiently large Reynolds numbers, suggesting the possibility of an analogy between the flows that is yet to be determined in detail.
Energy Technology Data Exchange (ETDEWEB)
Abd-Alla, A. M.; Abo-Dahab, S. M. [Taif University, Taif (Egypt); Khan, Aftab [COMSATS, Chakshahzad (Pakistan)
2015-10-15
In this paper, we investigated the propagation of surface waves in a rotating fibre-reinforced viscoelastic anisotropic media of a higher order and fraction orders of nth order including time rate of strain with voids. The general surface wave speed is derived to study the effect of rotation and voids on surface waves. Particular cases for Stoneley, Love and Rayleigh waves are also discussed. The results obtained in this investigation are more general in the sense that some earlier published results are obtained from our result as special cases. In order zero our results are well agreeing with classical results. Also by neglecting the reinforced elastic parameters and voids the results reduce to well known isotropic medium. Comparison was made with the results obtained in the presence and absence of rotation and parameters for fibre-reinforced of the material medium. It is observed that Love wave remains unaffected with respect to rotation and voids. It is also observed that, surface waves cannot propagate in a fast rotating medium. Numerical results are given and illustrated graphically.
International Nuclear Information System (INIS)
Abd-Alla, A. M.; Abo-Dahab, S. M.; Khan, Aftab
2015-01-01
In this paper, we investigated the propagation of surface waves in a rotating fibre-reinforced viscoelastic anisotropic media of a higher order and fraction orders of nth order including time rate of strain with voids. The general surface wave speed is derived to study the effect of rotation and voids on surface waves. Particular cases for Stoneley, Love and Rayleigh waves are also discussed. The results obtained in this investigation are more general in the sense that some earlier published results are obtained from our result as special cases. In order zero our results are well agreeing with classical results. Also by neglecting the reinforced elastic parameters and voids the results reduce to well known isotropic medium. Comparison was made with the results obtained in the presence and absence of rotation and parameters for fibre-reinforced of the material medium. It is observed that Love wave remains unaffected with respect to rotation and voids. It is also observed that, surface waves cannot propagate in a fast rotating medium. Numerical results are given and illustrated graphically.
Cantrell, W. H., II; Chang, K.; Ciochetto, D.; Niedermeier, D.; Bench, J.; Shaw, R. A.
2014-12-01
A detailed understanding of gas-aerosol-cloud interaction within the turbulent atmosphere is of prime importance for an accurate understanding of Earth's climate system. As one example: While every cloud droplet began as an aerosol particle, not every aerosol particle becomes a cloud droplet. The particle to droplet transformation requires that the particle be exposed to some critical concentration of water vapor, which differs for different combinations of particle size and chemical composition. Similarly, the formation of ice particles in mixed phase clouds is also catalyzed by aerosol particles. Even in the simplest scenarios it is challenging to gain a full understanding of the aerosol activation and ice nucleation processes. At least two other factors contribute significantly to the complexity observed in the atmosphere. First, aerosols and cloud particles are not static entities, but are continuously interacting with their chemical environment, and therefore changing in their properties. Second, clouds are ubiquitously turbulent, so thermodynamic and compositional variables, such as water vapor or other trace gas concentrations, fluctuate in space and time. Indeed, the coupling between turbulence and microphysical processes is one of the major research challenges in cloud physics. We have developed a multiphase, turbulent reaction chamber, (dubbed the Π Chamber, after the internal volume of 3.14 cubic meters) designed to address the problems outlined above. It is capable of pressures ranging from sea level to ~ 100 mbar, and can sustain temperatures of +40 to -55 ºC. We can independently control the temperatures on the surfaces of three heat transfer zones. This allows us to establish a temperature gradient between the floor and ceiling inducing Rayleigh-Benard convection and inducing a turbulent environment. Interior surfaces are electropolished stainless steel to facilitate cleaning before and after chemistry experiments. At present, supporting
Preferential states of rotating turbulent flows in a square container with a step topography
Tenreiro, M.; Trieling, R.R.; Zavala Sansón, L.; Heijst, van G.J.F.
2013-01-01
The self-organization of confined, quasi-two-dimensional turbulent flows in a rotating square container with a step-like topography is investigated by means of laboratory experiments and numerical simulations based on a rigid lid, shallow-water formulation. The domain is divided by a bottom
Benard convection in liquid sodium layers
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Kek, V.
1989-08-01
In a sodium layer heated from below and cooled from above, the integral Nusselt numbers are determined in a range of Rayleigh numbers 1.5x10 3 5 . The experiments are performed in containers with dimensions of 500 mm in diameter and 15 mm and 45 mm in height. The relevant quantities are evaluated from measured temperature and heating power data. The experiments show that the heat transfer across the layer is determined mainly by heat conduction up to Rayleigh number Ra ≅ 10 4 . Beyond this value a significant increase of the convective heat transport is observed. At a Rayleigh number of 4x10 4 the Nusselt number achieves the value Nu = 1.7. This result differs from values given by Nusselt-Rayleigh number correlations reported in the literature for liquids with higher Prandtl number. A regression analysis of the experimental data results empirical correlations for the Nusselt number. A time series analysis of the time dependent temperature signals shows that the measured temperature fluctuations exhibit predominantly stochastic features. However, in the lower range of Rayleigh numbers 1.5x10 3 4 certain regular frequencies can be identified from peaks in broadband power density spectra. These frequencies correspond to fluctuations of a period of 80 to 200 seconds. These regular frequencies are explained by instabilities of the cellular pattern in the convection layer reported in the literature. (orig./HP) [de
Generation of a rotating liquid liner by tangential injection
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Burton, R.L.; Turchi, P.J.; Jenkins, D.J.; Lanham, R.E.; Cameron, J.; Cooper, A.L.
1979-01-01
Efficient compression of low mass-density payloads by the implosion of higher mass-density liquid cylinders or liners, as in the NRL LINUS concept for controlled thermonuclear fusion, requires rotation of the liner material to avoid Rayleigh--Taylor instabilities at the liner-payload interface. Experimentally, such implosions have been demonstrated with liners formed within rotating implosion chambers. The present work uses a scale-model experimental apparatus to investigate the possibility of creating liner rotation by tangential injection of the liquid liner material. Different modes of behavior are obtained depending on the fluid exhaust procedures. Right-circular, cylindrical free surfaces are achieved with axial exhaust of fluid at radii interior to the injection nozzles, for which the liner exhibits a combination of solid-body and free vortex flows in different regions. Measurements allow estimates of power losses to viscous shear, turbulence, etc. A simple model based on open-channel flow is then derived, which is in good agreement with experiment, and is used to extrapolate results to the scale of a possible LINUS fusion reactor
Bursting and large-scale intermittency in turbulent convection with differential rotation
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Garcia, O.E.; Bian, N.H.
2003-01-01
The tilting mechanism, which generates differential rotation in two-dimensional turbulent convection, is shown to produce relaxation oscillations in the mean flow energy integral and bursts in the global fluctuation level, akin to Lotka-Volterra oscillations. The basic reason for such behavior is the unidirectional and conservative transfer of kinetic energy from the fluctuating motions to the mean component of the flows, and its dissipation at large scales. Results from numerical simulations further demonstrate the intimate relation between these low-frequency modulations and the large-scale intermittency of convective turbulence, as manifested by exponential tails in single-point probability distribution functions. Moreover, the spatio-temporal evolution of convective structures illustrates the mechanism triggering avalanche events in the transport process. The latter involves the overlap of delocalized mixing regions when the barrier to transport, produced by the mean component of the flow, transiently disappears
Lappa, Marcello
2011-09-01
Rayleigh-Bénard convection in finite-size enclosures exhibits really intricate features when turbulent states are reached and thermal plumes play a crucial role in a number of them. This complex mechanism may be regarded as a "machine" containing many different working parts: boundary layers, mixing zones, jets, and a relatively free and isothermal central region. These parts are generally regarded as the constitutive "ingredients" whose interplay leads to the emergence of a macroscopic pattern with well-defined properties. Like the Lorenz model (but with the due differences) such a complex structure has a prevailing two-dimensional nature and can be oriented clockwise or anticlockwise (both configurations are equally likely to occur and the flow can exhibit occasional and irregular "reversals" from one to the other without a change in magnitude). It is usually referred to in the literature as "wind of turbulence" or "flywheel". The present article provides insights into the possible origin of such dynamics and related patterning behavior (supported by "ad hoc" novel numerical simulations carried out for Pr=15 and O(10)⩽Ra⩽O(10)) together with a short exposition of existing theories, also illustrating open points and future directions of research.
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Li, Manman; Yan, Shaohui; Yao, Baoli; Liang, Yansheng; Lei, Ming; Yang, Yanlong
2016-01-01
Optical vortex beams carry optical orbital angular momentum (OAM) and can induce an orbital motion of trapped particles in optical trapping. We show that the state of polarization (SOP) of vortex beams will affect the details of this optically induced orbital motion to some extent. Numerical results demonstrate that focusing the vortex beams with circular, radial or azimuthal polarizations can induce a uniform orbital motion on a trapped Rayleigh particle, while in the focal field of the vortex beam with linear polarization the particle experiences a non-uniform orbital motion. Among the formers, the vortex beam with circular polarization induces a maximum optical torque on the particle. Furthermore, by varying the topological charge of the vortex beams, the vortex beam with circular polarization gives rise to an optimum torque superior to those given by the other three vortex beams. These facts suggest that the circularly polarized vortex beam is more suitable for rotating particles. - Highlights: • States of polarization of vortex beams affect the optically induced orbital motion of particles. • The dependences of the force and orbital torque on the topological charge, the size and the absorptivity of particles were calculated. • Focused vortex beams with circular, radial or azimuthal polarizations induce a uniform orbital motion on particles. • Particles experience a non-uniform orbital motion in the focused linearly polarized vortex beam. • The circularly polarized vortex beam is a superior candidate for rotating particles.
Cabra, R.; Chen, J. Y.; Dibble, R. W.; Myhrvold, T.; Karpetis, A. N.; Barlow, R. S.
2002-01-01
An experiment and numerical investigation is presented of a lifted turbulent H2/N2 jet flame in a coflow of hot, vitiated gases. The vitiated coflow burner emulates the coupling of turbulent mixing and chemical kinetics exemplary of the reacting flow in the recirculation region of advanced combustors. It also simplifies numerical investigation of this coupled problem by removing the complexity of recirculating flow. Scalar measurements are reported for a lifted turbulent jet flame of H2/N2 (Re = 23,600, H/d = 10) in a coflow of hot combustion products from a lean H2/Air flame ((empty set) = 0.25, T = 1,045 K). The combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence is used to obtain simultaneous measurements of temperature and concentrations of the major species, OH, and NO. The data attest to the success of the experimental design in providing a uniform vitiated coflow throughout the entire test region. Two combustion models (PDF: joint scalar Probability Density Function and EDC: Eddy Dissipation Concept) are used in conjunction with various turbulence models to predict the lift-off height (H(sub PDF)/d = 7,H(sub EDC)/d = 8.5). Kalghatgi's classic phenomenological theory, which is based on scaling arguments, yields a reasonably accurate prediction (H(sub K)/d = 11.4) of the lift-off height for the present flame. The vitiated coflow admits the possibility of auto-ignition of mixed fluid, and the success of the present parabolic implementation of the PDF model in predicting a stable lifted flame is attributable to such ignition. The measurements indicate a thickened turbulent reaction zone at the flame base. Experimental results and numerical investigations support the plausibility of turbulent premixed flame propagation by small scale (on the order of the flame thickness) recirculation and mixing of hot products into reactants and subsequent rapid ignition of the mixture.
Transition to the Ultimate Regime in Two-Dimensional Rayleigh-Bénard Convection
Zhu, Xiaojue; Mathai, Varghese; Stevens, Richard J. A. M.; Verzicco, Roberto; Lohse, Detlef
2018-04-01
The possible transition to the so-called ultimate regime, wherein both the bulk and the boundary layers are turbulent, has been an outstanding issue in thermal convection, since the seminal work by Kraichnan [Phys. Fluids 5, 1374 (1962), 10.1063/1.1706533]. Yet, when this transition takes place and how the local flow induces it is not fully understood. Here, by performing two-dimensional simulations of Rayleigh-Bénard turbulence covering six decades in Rayleigh number Ra up to 1 014 for Prandtl number Pr =1 , for the first time in numerical simulations we find the transition to the ultimate regime, namely, at Ra*=1013 . We reveal how the emission of thermal plumes enhances the global heat transport, leading to a steeper increase of the Nusselt number than the classical Malkus scaling Nu ˜Ra1 /3 [Proc. R. Soc. A 225, 196 (1954), 10.1098/rspa.1954.0197]. Beyond the transition, the mean velocity profiles are logarithmic throughout, indicating turbulent boundary layers. In contrast, the temperature profiles are only locally logarithmic, namely, within the regions where plumes are emitted, and where the local Nusselt number has an effective scaling Nu ˜Ra0.38 , corresponding to the effective scaling in the ultimate regime.
Directory of Open Access Journals (Sweden)
Liou Tong-Miin
2005-01-01
Full Text Available The local turbulent fluid flow and heat transfer in a rotating two-pass square duct with 19 pairs of in-line 90 ∘ ribs have been investigated computationally. A Reynolds-averaged Navier-Stokes equation (RANS with a two-layer k − ϵ turbulence model was solved. The in-line 90 ∘ ribs were arranged on the leading and trailing walls with rib height-to-hydraulic diameter ratio and pitch-to-height ratio of 0.136 and 10, respectively. The Reynolds number, based on duct hydraulic diameter and bulk mean velocity, was fixed at 1.0 × 10 4 whereas the rotational number varied from 0 to 0.2 . Results are validated with previous measured velocity field and heat transfer coefficient distributions. The validation shows that the effect of rotation on the passage-averaged Nusselt number ratio can be predicted reasonably well; nevertheless, the transverse mean velocity and, in turn, the distribution of regional-averaged Nusselt number ratio are markedly underpredicted in the regions toward which the Coriolis force is directed. Further CFD studies are needed.
International Nuclear Information System (INIS)
Ihle, Christian F.; Nino, Yarko
2011-01-01
Stability conditions of a quiescent, horizontally infinite fluid layer with adiabatic bottom subject to sudden cooling from above are studied. Here, at difference from Rayleigh-Benard convection, the temperature base state is never steady. Instability limits are studied using linear analysis while stability is analyzed using the energy method. Critical stability curves in terms of Rayleigh numbers and convection onset times were obtained for several kinematic boundary conditions. Stability curves resulting from energy and linear approaches exhibit the same temporal growth rate for large values of time, suggesting a bound for the temporal asymptotic behavior of the energy method. - Highlights: → Non-penetrative convection appears after a time-evolving temperature base state. → Global stability and instability limits were analyzed. → Critical Rayleigh numbers were computed for different kinematic boundary conditions. → Adiabatic, bottom boundary was found to have a de-stabilizing effect. → System is less stable than in Benard convection.
Front propagation in Rayleigh-Taylor systems with reaction
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Scagliarini, A; Biferale, L; Sbragaglia, M; Mantovani, F; Pivanti, M; Schifano, S F; Tripiccione, R; Pozzati, F; Toschi, F
2011-01-01
A special feature of Rayleigh-Taylor systems with chemical reactions is the competition between turbulent mixing and the 'burning processes', which leads to a highly non-trivial dynamics. We studied the problem performing high resolution numerical simulations of a 2d system, using a thermal lattice Boltzmann (LB) model. We spanned the various regimes emerging at changing the relative chemical/turbulent time scales, from slow to fast reaction; in the former case we found numerical evidence of an enhancement of the front propagation speed (with respect to the laminar case), providing a phenomenological argument to explain the observed behaviour. When the reaction is very fast, instead, the formation of sharp fronts separating patches of pure phases, leads to an increase of intermittency in the small scale statistics of the temperature field.
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Drew, D.A.; Flaherty, J.E.
1986-01-01
The mathematical analysis of fluid mechanics and stability and its applications in science and engineering are examined in reviews and reports. Topics addressed include Taylor-vortex flow, isothermal fluid-film lubrication theories, the morphology of spiral galaxies, rotating-fluid problems in ballistics, coupled Lorenz oscillators, the connection between chaos and turbulence, plane-front alloy solidification versus free-surface Benard convection, and the nonlinear stability of spiral flow between rotating cylinders with a small gap. Consideration is given to resonance conditions for forced two-dimensional channel flows, the secondary bifurcation of standing surface waves in rectangular basins, instability in the flow of granular materials, and the supercritical dynamics of baroclinic disturbances
Turbulence-chemistry interactions in reacting flows
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Barlow, R.S.; Carter, C.D. [Sandia National Laboratories, Livermore, CA (United States)
1993-12-01
Interactions between turbulence and chemistry in nonpremixed flames are investigated through multiscalar measurements. Simultaneous point measurements of major species, NO, OH, temperature, and mixture fraction are obtained by combining spontaneous Raman scattering, Rayleigh scattering, and laser-induced fluorescence (LIF). NO and OH fluorescence signals are converted to quantitative concentrations by applying shot-to-shot corrections for local variations of the Boltzmann fraction and collisional quenching rate. These measurements of instantaneous thermochemical states in turbulent flames provide insights into the fundamental nature of turbulence-chemistry interactions. The measurements also constitute a unique data base for evaluation and refinement of turbulent combustion models. Experimental work during the past year has focused on three areas: (1) investigation of the effects of differential molecular diffusion in turbulent combustion: (2) experiments on the effects of Halon CF{sub 3}Br, a fire retardant, on the structure of turbulent flames of CH{sub 4} and CO/H{sub 2}/N{sub 2}; and (3) experiments on NO formation in turbulent hydrogen jet flames.
Generation and evolution of turbulence in an annulus between two concentric rotating cylinders
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Kataoka, K.; Deguchi, T.
1987-01-01
The objective of the present work is to observe the generation and spectral evolution of time-dependent wavy disturbances in the Taylor-Couette flow. It is well known that as the Reynolds number Re = R/sub i/Ω d/ν, based on the rotation speed (Ω: angular velocity) of the inner cylinder, is gradually increased, the following five dynamical transitions occur stepwise in sequence: laminar Couette flow → laminar Taylor vortex flow → wavy vortex flow → quasi-periodic wavy vortex flow → weakly turbulent wavy vortex flow → turbulent vortex flow. Time-dependent wavy disturbances appear when the transition to wavy vortex flow occurs as a result of instability of the laminar Taylor vortex flow. The disturbances are regularly periodic because it results from the azimuthally traveling waves. The next transition to the quasi-periodic wavy vortex flow is accompanied by the amplitude modulation of the wave motion. The first fundamental frequency f/sub 1/ comes from the passing frequency of the azimuthally traveling waves and the second fundamental frequency f/sub 2/ from the modulation frequency. When the transition to the weakly turbulent wavy vortex flow occurs, chaotic turbulence first appears, A spectral analysis is made to analyze the temporal variation in the local velocity gradient measured on both the inner and outer cylinder walls by using an electrochemical technique
The multifractal nature of plume structure in high-Rayleigh-number convection
Puthenveettil, Baburaj A.; Ananthakrishna, G.; Arakeri, Jaywant H.
2005-03-01
The geometrically different planforms of near-wall plume structure in turbulent natural convection, visualized by driving the convection using concentration differences across a membrane, are shown to have a common multifractal spectrum of singularities for Rayleigh numbers in the range 1010-1011 at Schmidt number of 602. The scaling is seen for a length scale range of 25 and is independent of the Rayleigh number, the flux, the strength and nature of the large-scale flow, and the aspect ratio. Similar scaling is observed for the plume structures obtained in the presence of a weak flow across the membrane. This common non-trivial spatial scaling is proposed to be due to the same underlying generating process for the near-wall plume structures.
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Yoshimura, H.; Wang, Z.; Wu, F.
1984-01-01
Differential rotation dependence of the selection mechanism for magnetic parity of solar and stellar cycles is studied by assuming various differential rotation profiles inn the dynamo equation. The parity selection depends on propagation direction of oscillating magnetic fields in the form of dynamo waves which propagate along isorotation surfaces. When there is any radial gradient in the differential rotation, dynamo waves propagate either equatorward or poleward. In the former case, field systems of the two hemispheres approach each other and collide at the equator. Then, odd parity is selected. In the latter case, field systems of the two hemispheres recede from each other and do not collide at the equator, an even parity is selected. Thus the equatorial migration of wings of the butterfly iagram of the solar cycle and its odd parity are intrinsically related. In the case of purely latitudibnal differential rotation, dynamo waves propagate purely radially and growth rates of odd and even modes are nearly the same even when dynamo strength is weak when the parity selection mechanism should work most efficiently. In this case, anisotropy of turbulent diffusivity is a decisive factor to separate odd and even modes. Unlike in the case of radial-gradient-dominated differential rotation in which any difference between diffusivities for poloidal and toroidal fields enhancess the parity selection without changing the parity, the parity selection in the case of latitudinal-gradient-dominated differential rotation depends on the difference of diffusivities for poloidal and toroidal fields. When diffusivity for poloidal fields iss larger than that for toroidal fields, odd parity is selected; and when diffusivity for toroidal fields is larger, even parity is selected
Physics of Intrinsic Rotation in Flux-Driven ITG Turbulence
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Ku, S.; Abiteboul, J.; Dimond, P.H.; Dif-Pradalier, G.; Kwon, J.M.; Sarazin, Y.; Hahm, T.S.; Garbet, X.; Chang, C.S.; Latu, G.; Yoon, E.S.; Ghendrih, Ph.; Yi, S.; Strugarek, A.; Solomon, W.; Grandgirard, V.
2012-01-01
Global, heat flux-driven ITG gyrokinetic simulations which manifest the formation of macroscopic, mean toroidal flow profiles with peak thermal Mach number 0.05, are reported. Both a particle-in-cell (XGC1p) and a semi-Lagrangian (GYSELA) approach are utilized without a priori assumptions of scale-separation between turbulence and mean fields. Flux-driven ITG simulations with different edge flow boundary conditions show in both approaches the development of net unidirectional intrinsic rotation in the co-current direction. Intrinsic torque is shown to scale approximately linearly with the inverse scale length of the ion temperature gradient. External momentum input is shown to effectively cancel the intrinsic rotation profile, thus confirming the existence of a local residual stress and intrinsic torque. Fluctuation intensity, intrinsic torque and mean flow are demonstrated to develop inwards from the boundary. The measured correlations between residual stress and two fluctuation spectrum symmetry breakers, namely E x B shear and intensity gradient, are similar. Avalanches of (positive) heat flux, which propagate either outwards or inwards, are correlated with avalanches of (negative) parallel momentum flux, so that outward transport of heat and inward transport of parallel momentum are correlated and mediated by avalanches. The probability distribution functions of the outward heat flux and the inward momentum flux show strong structural similarity
Wang, Zhan-zhi; Xiong, Ying
2013-04-01
A growing interest has been devoted to the contra-rotating propellers (CRPs) due to their high propulsive efficiency, torque balance, low fuel consumption, low cavitations, low noise performance and low hull vibration. Compared with the single-screw system, it is more difficult for the open water performance prediction because forward and aft propellers interact with each other and generate a more complicated flow field around the CRPs system. The current work focuses on the open water performance prediction of contra-rotating propellers by RANS and sliding mesh method considering the effect of computational time step size and turbulence model. The validation study has been performed on two sets of contra-rotating propellers developed by David W Taylor Naval Ship R & D center. Compared with the experimental data, it shows that RANS with sliding mesh method and SST k-ω turbulence model has a good precision in the open water performance prediction of contra-rotating propellers, and small time step size can improve the level of accuracy for CRPs with the same blade number of forward and aft propellers, while a relatively large time step size is a better choice for CRPs with different blade numbers.
Rayleigh-Taylor mixing with time-dependent acceleration
Abarzhi, Snezhana
2016-10-01
We extend the momentum model to describe Rayleigh-Taylor (RT) mixing driven by a time-dependent acceleration. The acceleration is a power-law function of time, similarly to astrophysical and plasma fusion applications. In RT flow the dynamics of a fluid parcel is driven by a balance per unit mass of the rates of momentum gain and loss. We find analytical solutions in the cases of balanced and imbalanced gains and losses, and identify their dependence on the acceleration exponent. The existence is shown of two typical regimes of self-similar RT mixing-acceleration-driven Rayleigh-Taylor-type and dissipation-driven Richtymer-Meshkov-type with the latter being in general non-universal. Possible scenarios are proposed for transitions from the balanced dynamics to the imbalanced self-similar dynamics. Scaling and correlations properties of RT mixing are studied on the basis of dimensional analysis. Departures are outlined of RT dynamics with time-dependent acceleration from canonical cases of homogeneous turbulence as well as blast waves with first and second kind self-similarity. The work is supported by the US National Science Foundation.
Territorial characteristics of low frequency electrostatic fluctuations in a simple magnetized torus
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Kaur, R.; Singh, R.; Sarada Sree, A.; Mattoo, S. K.; Singh, A. K.
2011-01-01
This paper presents an experimental investigation of turbulence in simple toroidal plasma devices without rotational transform. It is argued that Rayleigh-Taylor (flute interchange) mode may be one of the source mechanisms for the observed turbulence but is not sufficient to explain its observed global characteristics. Taking BETA device as an example, we show that pure Rayleigh-Taylor mode cannot explain (i) the observation of mode maximum at the location other than where density scale length is minimum, (ii) the comparable value of amplitude level of fluctuations in good curvature region, and (iii) the decrease in the mode amplitude with increasing magnetic field. Investigations have revealed that there exists not only poloidal plasma flow but also that it is sheared. Including this effect explains the first observation. However, modification brought about by velocity shear in the Rayleigh-Taylor mode still does not explain our second and third observations. We have taken an approach that since Rayleigh-Taylor is not excited in a good curvature region, it cannot be the source of turbulence there. Nor is it defensible to say that turbulence born in a bad curvature region is carried over through ExB rotation to the good curvature region. Consequently, we have invoked cross-field Simon-Hoh instability for this region. Experimental evidence supporting our proposal is presented. This paper concludes that toroidal devices have simultaneous existence of different self-consistent sources of turbulence in different regions of the device.
Zhou, Quan; Sugiyama, K.; Stevens, Richard Johannes Antonius Maria; Grossmann, Siegfried; Lohse, Detlef; Xia, K.
2011-01-01
We investigate the structures of the near-plate velocity and temperature profiles at different horizontal positions along the conducting bottom (and top) plate of a Rayleigh-Bénard convection cell, using two-dimensional (2D) numerical data obtained at the Rayleigh number Ra = 108 and the Prandtl
Theoretical and experimental research on nonlinear hydrodynamic stability and transition is presented. Bifurcations, amplitude equations, pattern in experiments, and shear flows are considered. Particular attention is given to bifurcations of plane viscous fluid flow and transition to turbulence, chaotic traveling wave covection, chaotic behavior of parametrically excited surface waves in square geometry, amplitude analysis of the Swift-Hohenberg equation, traveling wave convection in finite containers, focus instability in axisymmetric Rayleigh-Benard convection, scaling and pattern formation in flowing sand, dynamical behavior of instabilities in spherical gap flows, and nonlinear short-wavelength Taylor vortices. Also discussed are stability of a flow past a two-dimensional grid, inertia wave breakdown in a precessing fluid, flow-induced instabilities in directional solidification, structure and dynamical properties of convection in binary fluid mixtures, and instability competition for convecting superfluid mixtures.
The structure concept in the description of mixing turbulence: the 2SFK model
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Llor, A.; Poujade, O.; Lardjane, N.
2009-01-01
To meet our modelling needs on turbulent flows produced by gravitational instabilities (of Rayleigh-Taylor or Richtmyer-Meshkov type), we have developed an original approach, designated as 2SFK for '2-structure, 2-fluid, 2-turbulent'. We provide the physical elements, theoretical, experimental, and numerical, which support this choice. A full description being out of question here, we give the principles of the model derivation, which hinges around an averaging conditioned by presence functions of the large structures in the flow, and discuss its distinctive properties compared to usual 'single-fluid' models. Numerical 1-dimension results on elementary flows illustrate the satisfactory behaviour of the model. All along this article, emphasis is given on the peculiar characteristics of turbulence in the Rayleigh-Taylor flow (possibly under variable acceleration): energy balance, characteristic size of large eddies, directed transport, enhanced diffusion, etc. (authors)
Oscillatory Convection in Rotating Liquid Metals
Bertin, Vincent; Grannan, Alex; Aurnou, Jonathan
2016-11-01
We have performed laboratory experiments in a aspect ratio Γ = 2 cylinder using liquid gallium (Pr = 0 . 023) as the working fluid. The Ekman number varies from E = 4 ×10-5 to 4 ×10-6 and the Rayleigh number varies from Ra = 3 ×105 to 2 ×107 . Using heat transfer and temperature measurements within the fluid, we characterize the different styles of low Pr rotating convective flow. The convection threshold is first overcome in the form of a container scale inertial oscillatory mode. At stronger forcing, wall-localized modes develop, coexisting with the inertial oscillatory modes in the bulk. When the strength of the buoyancy increases further, the bulk flow becomes turbulent while the wall modes remain. Our results imply that rotating convective flows in liquid metals do not develop in the form of quasi-steady columns, as in Pr = 1 planetary and stellar dynamo models, but in the form of oscillatory motions. Therefore, convection driven dynamo action in low Pr fluids can differ substantively than that occurring in typical Pr = 1 numerical models. Our results also suggest that low wavenumber, wall modes may be dynamically and observationally important in liquid metal dynamo systems. We thank the NSF Geophysics Program for support of this project.
Study on the Instability of Two-Phase Flow in the Heat-Absorbing Tube of Trough Solar Collector
Directory of Open Access Journals (Sweden)
Ying Zhang
2017-01-01
Full Text Available The Marangoni effect and Rayleigh-Benard effect in the two-phase region of solar trough heat-absorbing tube are simulated by FTM (front tracking method. Considering the Marangoni effect alone, although surface tension gradient and surface tension affect the interface wave, the two effects have different characteristics. The surface tension gradient caused by the temperature gradient is one of the factors that swing the interface. The amplitude attenuation of the interface wave decreases with the increase of the Marangoni number (Ma. In general, the surface tension gradient enhances the convection opposite to the temperature gradient. Under the gravity field, the Rayleigh-Benard effect influences the development of the vortex structure in the flow field, which in turn affects the velocity gradient near the interface to influence the evolution of the interface fluctuation. In a small Rayleigh number (Ra, the buoyancy convection reduces the velocity gradient, thus suppressing the evolution of the interfacial wave. In the range of Ra 4.0E4, the situation is just the opposite. The larger the Ra is, the stronger the promoting effect is.
Equipartition and transport in two-dimensional electrostatic turbulence
DEFF Research Database (Denmark)
Naulin, V.; Nycander, J.; Juul Rasmussen, J.
1998-01-01
Turbulent equipartition is investigated for the nonlinear evolution of pressure driven flute modes of a plasma in an inhomogeneous magnetic field. The Rayleigh-Taylor instability is recovered by linear stability analysis, and occurs when the pressure profile is more peaked than the profile of the...
On the relative rotational motion between rigid fibers and fluid in turbulent channel flow
Energy Technology Data Exchange (ETDEWEB)
Marchioli, C. [Department of Electrical, Management and Mechanical Engineering, University of Udine, 33100 Udine (Italy); Zhao, L., E-mail: lihao.zhao@ntnu.no [Department of Energy and Process Engineering, Norwegian University of Science and Technology, 7491 Trondheim (Norway); Andersson, H. I. [Department of Electrical, Management and Mechanical Engineering, University of Udine, 33100 Udine (Italy); Department of Energy and Process Engineering, Norwegian University of Science and Technology, 7491 Trondheim (Norway)
2016-01-15
In this study, the rotation of small rigid fibers relative to the surrounding fluid in wall-bounded turbulence is examined by means of direct numerical simulations coupled with Lagrangian tracking. Statistics of the relative (fiber-to-fluid) angular velocity, referred to as slip spin in the present study, are evaluated by modelling fibers as prolate spheroidal particles with Stokes number, St, ranging from 1 to 100 and aspect ratio, λ, ranging from 3 to 50. Results are compared one-to-one with those obtained for spherical particles (λ = 1) to highlight effects due to fiber length. The statistical moments of the slip spin show that differences in the rotation rate of fibers and fluid are influenced by inertia, but depend strongly also on fiber length: Departures from the spherical shape, even when small, are associated with an increase of rotational inertia and prevent fibers from passively following the surrounding fluid. An increase of fiber length, in addition, decouples the rotational dynamics of a fiber from its translational dynamics suggesting that the two motions can be modelled independently only for long enough fibers (e.g., for aspect ratios of order ten or higher in the present simulations)
Xi, Heng-Dong; Chen, Xin; Xia, Ke-Qing
2017-11-01
We report an experimental study of the temperature oscillation and the sloshing motion of the large-scale circulation (LSC) in turbulent Rayleigh-Bénard convection in water. Temperature measurements were made in aspect ratio one cylindrical cell by probes put in fluid and embedded in the sidewall simultaneously, and located at the 1/4, 1/2 and 3/4 heights of the convection cell. The results show that the temperature measured in fluid contains information of both the LSC and the signature of the hot and cold plumes, while the temperature measured in sidewall only contains information of the LSC. It is found that the sloshing motion of the LSC can be measured by both the temperatures in fluid and in sidewall. We also studies the effect of cell tilting on the temperature oscillation and sloshing motion of the LSC. It is found that both the amplitude and the frequency of the temperature oscillation (and the sloshing motion) increase when the tilt angle increases, while the off-center distance of the sloshing motion of the LSC remains unchanged. This work is supported by the NSFC of China (Grant Nos. 11472094 and U1613227), the RGC of Hong Kong SAR (Grant No. 403712) and the 111 project of China (Grant No. B17037).
Ground, Cody R.; Gopal, Vijay; Maddalena, Luca
2018-04-01
By introducing large-scale streamwise vortices into a supersonic flow it is possible to enhance the rate of mixing between two fluid streams. However, increased vorticity content alone does not explicitly serve as a predictor of mixing enhancement. Additional factors, particularly the mutual interactions occurring between neighboring vortical structures, affect the underlying fundamental physics that influence the rate at which the fluids mix. As part of a larger systematic study on supersonic streamwise vortex interactions, this work experimentally quantifies the average rate of mixing of helium and air in the presence of two separate modes of vortex interaction, the merging and non-merging of a pair of co-rotating vortices. In these experiments vortex-generating expansion ramps are placed on a strut injector. The freestream Mach number is set at 2.5 and helium is injected as a passive scalar. Average injectant mole fractions at selected flow planes downstream of the injector are measured utilizing the filtered Rayleigh scattering technique. The filtered Rayleigh scattering measurements reveal that, in the domain surveyed, the merging vortex interaction strongly displaces the plume from its initial horizontal orientation while the non-merging vortex interaction more rapidly mixes the helium and air. The results of the current experiments are consistent with associated knowledge derived from previous analyses of the two studied configurations which have included the detailed experimental characterization of entrainment, turbulent kinetic energy, and vorticity of both modes of vortex interaction.
Dynamics of a secondary instability in Benard-Marangoni convection with unidimensional heating
International Nuclear Information System (INIS)
Burguete, J.; Mancini, H.L.; Perez-Garcia, C.
1993-01-01
The dynamics of Benard-Marangoni convection with unidimensional heating in a pure fluid is studied experimentally. Convection begins with rolls parallel to the heater. The characteristics of these primary rolls have been determined. When the temperature difference across the liquid layer is increased beyond a critical value a secondary instability appears. Motions transverse to the heater with a definite wavelength can be seen. Moreover, for small angles between the heater and the fluid surface, the pattern drifts along the heater with a velocity that depends almost linearly on the inclination. A phenomenological phase equation is proposed to interpret this observation. (orig.)
Rayleigh-Taylor mixing with space-dependent acceleration
Abarzhi, Snezhana
2016-11-01
We extend the momentum model to describe Rayleigh-Taylor (RT) mixing driven by a space-dependent acceleration. The acceleration is a power-law function of space coordinate, similarly to astrophysical and plasma fusion applications. In RT flow the dynamics of a fluid parcel is driven by a balance per unit mass of the rates of momentum gain and loss. We find analytical solutions in the cases of balanced and imbalanced gains and losses, and identify their dependence on the acceleration exponent. The existence is shown of two typical sub-regimes of self-similar RT mixing - the acceleration-driven Rayleigh-Taylor-type mixing and dissipation-driven Richtymer-Meshkov-type mixing with the latter being in general non-universal. Possible scenarios are proposed for transitions from the balanced dynamics to the imbalanced self-similar dynamics. Scaling and correlations properties of RT mixing are studied on the basis of dimensional analysis. Departures are outlined of RT dynamics with space-dependent acceleration from canonical cases of homogeneous turbulence as well as blast waves with first and second kind self-similarity. The work is supported by the US National Science Foundation.
Basal melting driven by turbulent thermal convection
Rabbanipour Esfahani, Babak; Hirata, Silvia C.; Berti, Stefano; Calzavarini, Enrico
2018-05-01
Melting and, conversely, solidification processes in the presence of convection are key to many geophysical problems. An essential question related to these phenomena concerns the estimation of the (time-evolving) melting rate, which is tightly connected to the turbulent convective dynamics in the bulk of the melt fluid and the heat transfer at the liquid-solid interface. In this work, we consider a convective-melting model, constructed as a generalization of the Rayleigh-Bénard system, accounting for the basal melting of a solid. As the change of phase proceeds, a fluid layer grows at the heated bottom of the system and eventually reaches a turbulent convection state. By means of extensive lattice-Boltzmann numerical simulations employing an enthalpy formulation of the governing equations, we explore the model dynamics in two- and three-dimensional configurations. The focus of the analysis is on the scaling of global quantities like the heat flux and the kinetic energy with the Rayleigh number, as well as on the interface morphology and the effects of space dimensionality. Independently of dimensionality, we find that the convective-melting system behavior shares strong resemblances with that of the Rayleigh-Bénard one, and that the heat flux is only weakly enhanced with respect to that case. Such similarities are understood, at least to some extent, considering the resulting slow motion of the melting front (with respect to the turbulent fluid velocity fluctuations) and its generally little roughness (compared to the height of the fluid layer). Varying the Stefan number, accounting for the thermodynamical properties of the material, also seems to have only a mild effect, which implies the possibility of extrapolating results in numerically delicate low-Stefan setups from more convenient high-Stefan ones. Finally, we discuss the implications of our findings for the geophysically relevant problem of modeling Arctic ice melt ponds.
Feedback control and heat transfer measurements in a Rayleigh-Bénard convection cell
Vial, M.; Hernández, R. H.
2017-07-01
We report experimental results on the heat transfer and instability onset of a Rayleigh-Bénard convection cell of aspect ratios 6:3:1 filled with a high Prandtl aqueous solution of glycerol under feedback control. We investigate the transient and stationary response of both local temperature readings and heat transfer fluxes on the Rayleigh Bénard cell in both conductive and convective states when we perform two independent feedback control actions on both hot and cold walls. We evaluate the performance of both controllers to maintain a temperature gradient independently if the system is below or above the convection threshold. As the convection cell can be rotated at 180° about the shorter axis of the cell, it was possible to perform transitions between thermal conduction and convection regimes and vice versa under a constant temperature difference maintained by both independent controllers. The experimental setup provided an accurate measurement of the critical Rayleigh number and the evolution of the Nusselt number as a function of the Rayleigh number in the moderately supercritical regime (R a cellular convection pattern formed by 6 transverse rolls throughout the range of Rayleigh numbers.
International Nuclear Information System (INIS)
Kawazura, Y.; Yoshida, Z.
2012-01-01
Two different types of self-organizing and sustaining ordered motion in fluids or plasmas--one is a Benard convection (or streamer) and the other is a zonal flow--have been compared by introducing a thermodynamic phenomenological model and evaluating the corresponding entropy production rates (EP). These two systems have different topologies in their equivalent circuits: the Benard convection is modeled by parallel connection of linear and nonlinear conductances, while the zonal flow is modeled by series connection. The ''power supply'' that drives the systems is also a determinant of operating modes. When the energy flux is a control parameter (as in usual plasma experiments), the driver is modeled by a constant-current power supply, and when the temperature difference between two separate boundaries is controlled (as in usual computational studies), the driver is modeled by a constant-voltage power supply. The parallel (series)-connection system tends to minimize (maximize) the total EP when a constant-current power supply drives the system. This minimum/maximum relation flips when a constant-voltage power supply is connected.
International Nuclear Information System (INIS)
Li Linghuai; Sofia, Sabatino; Basu, Sarbani; Demarque, Pierre; Ventura, Paolo; Penza, Valentina; Bi Shaolan
2009-01-01
In the second paper of this series we pursue two objectives. First, in order to make the code more sensitive to small effects, we remove many approximations made in Paper I. Second, we include turbulence and rotation in the two-dimensional framework. The stellar equilibrium is described by means of a set of five differential equations, with the introduction of a new dependent variable, namely the perturbation to the radial gravity, that is found when the nonradial effects are considered in the solution of the Poisson equation. Following the scheme of the first paper, we write the equations in such a way that the two-dimensional effects can be easily disentangled. The key concept introduced in this series is the equipotential surface. We use the underlying cause-effect relation to develop a recurrence relation to calculate the equipotential surface functions for uniform rotation, differential rotation, rotation-like toroidal magnetic fields, and turbulence. We also develop a more precise code to numerically solve the two-dimensional stellar structure and evolution equations based on the equipotential surface calculations. We have shown that with this formulation we can achieve the precision required by observations by appropriately selecting the convergence criterion. Several examples are presented to show that the method works well. Since we are interested in modeling the effects of a dynamo-type field on the detailed envelope structure and global properties of the Sun, the code has been optimized for short timescales phenomena (down to 1 yr). The time dependence of the code has so far been tested exclusively to address such problems.
Richardson effects in turbulent buoyant flows
Biggi, Renaud; Blanquart, Guillaume
2010-11-01
Rayleigh Taylor instabilities are found in a wide range of scientific fields from supernova explosions to underwater hot plumes. The turbulent flow is affected by the presence of buoyancy forces and may not follow the Kolmogorov theory anymore. The objective of the present work is to analyze the complex interactions between turbulence and buoyancy. Towards that goal, simulations have been performed with a high order, conservative, low Mach number code [Desjardins et. al. JCP 2010]. The configuration corresponds to a cubic box initially filled with homogeneous isotropic turbulence with heavy fluid on top and light gas at the bottom. The initial turbulent field was forced using linear forcing up to a Reynolds number of Reλ=55 [Meneveau & Rosales, POF 2005]. The Richardson number based on the rms velocity and the integral length scale was varied from 0.1 to 10 to investigate cases with weak and strong buoyancy. Cases with gravity as a stabilizer of turbulence (gravity pointing up) were also considered. The evolution of the turbulent kinetic energy and the total kinetic energy was analyzed and a simple phenomenological model was proposed. Finally, the energy spectra and the isotropy of the flow were also investigated.
Energy Technology Data Exchange (ETDEWEB)
Santos-Lima, R.; De Gouveia Dal Pino, E. M. [Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Universidade de Sao Paulo, R. do Matao, 1226, Sao Paulo, SP 05508-090 (Brazil); Lazarian, A. [Department of Astronomy, University of Wisconsin, Madison, WI 53706 (United States)
2012-03-01
The formation of protostellar disks out of molecular cloud cores is still not fully understood. Under ideal MHD conditions, the removal of angular momentum from the disk progenitor by the typically embedded magnetic field may prevent the formation of a rotationally supported disk during the main protostellar accretion phase of low-mass stars. This has been known as the magnetic braking problem and the most investigated mechanism to alleviate this problem and help remove the excess of magnetic flux during the star formation process, the so-called ambipolar diffusion (AD), has been shown to be not sufficient to weaken the magnetic braking at least at this stage of the disk formation. In this work, motivated by recent progress in the understanding of magnetic reconnection in turbulent environments, we appeal to the diffusion of magnetic field mediated by magnetic reconnection as an alternative mechanism for removing magnetic flux. We investigate numerically this mechanism during the later phases of the protostellar disk formation and show its high efficiency. By means of fully three-dimensional MHD simulations, we show that the diffusivity arising from turbulent magnetic reconnection is able to transport magnetic flux to the outskirts of the disk progenitor at timescales compatible with the collapse, allowing the formation of a rotationally supported disk around the protostar of dimensions {approx}100 AU, with a nearly Keplerian profile in the early accretion phase. Since MHD turbulence is expected to be present in protostellar disks, this is a natural mechanism for removing magnetic flux excess and allowing the formation of these disks. This mechanism dismisses the necessity of postulating a hypothetical increase of the ohmic resistivity as discussed in the literature. Together with our earlier work which showed that magnetic flux removal from molecular cloud cores is very efficient, this work calls for reconsidering the relative role of AD in the processes of star
Energy Technology Data Exchange (ETDEWEB)
Seiler, J.M.; Bonnet, J.M.; Bernaz, L. [CEA Grenoble (France)
2001-07-01
Extensive studies have been performed to investigate the heat transfer within a molten corium pool (homogeneous, stratified and with miscibility gap): Synthesis of heat transfer correlations in molten pool (homogeneous and stratified), Focusing effect in stratified metal layer, DNS analysis of Rayleigh Benard instabilities at the top boundary; interpretation of the different convection regimes and exponents affecting the Rayleigh number in the heat transfer correlations, Molten pool model for corium presenting a miscibility gap. Condition for de-stratification. (authors)
International Nuclear Information System (INIS)
Seiler, J.M.; Bonnet, J.M.; Bernaz, L.
2001-01-01
Extensive studies have been performed to investigate the heat transfer within a molten corium pool (homogeneous, stratified and with miscibility gap): Synthesis of heat transfer correlations in molten pool (homogeneous and stratified), Focusing effect in stratified metal layer, DNS analysis of Rayleigh Benard instabilities at the top boundary; interpretation of the different convection regimes and exponents affecting the Rayleigh number in the heat transfer correlations, Molten pool model for corium presenting a miscibility gap. Condition for de-stratification. (authors)
Subcritical thermal convection of liquid metals in a rapidly rotating sphere
Cardin, P.; Schaeffer, N.; Guervilly, C.; Kaplan, E.
2017-12-01
Planetary cores consist of liquid metals (low Prandtl number Pr) that convect as the core cools. Here we study nonlinear convection in a rotating (low Ekman number Ek) planetary core using a fully 3D direct (down to Ek=10-7) and a quasi geostrophic (down to Ek=10-10) numerical simulations. Near the critical thermal forcing (Rayleigh number Ra), convection onsets as thermal Rossby waves, but as Ra increases, this state is superceded by one dominated by advection. At moderate rotation, these states (here called the weak branch and strong branch, respectively) are continuously connected. As the planetary core rotates faster, the continuous transition is replaced by hysteresis cycles and subcriticality until the weak branch disappears entirely and the strong branch onsets in a turbulent state at Ekforcing decreases well below the linear onset of convection (Ra 0.4Racrit in this study for Ek=10-10 and Pr=0.01). We highlight the importance of the Reynolds stress, which is required for convection to persist below the linear onset. We further note the presence of a strong zonal flow that is nonetheless unimportant to the convective subcritical state. Our study suggests that, in the asymptotic regime of rapid rotation relevant for planetary interiors, thermal convection of liquid metals in a sphere onsets and shuts down through a subcritical bifurcation. This scenario may be relevant to explain the lunar and martian dynamo extinctions.
Heat Transfer and Flows of Thermal Convection in a Fluid-Saturated Rotating Porous Medium
Directory of Open Access Journals (Sweden)
Jianhong Kang
2015-01-01
Full Text Available Thermal convection at the steady state for high Rayleigh number in a rotating porous half space is investigated. Taking into account the effect of rotation, Darcy equation is extended to incorporate the Coriolis force term in a rotating reference frame. The velocity and temperature fields of thermal convection are obtained by using the homotopy analysis method. The influences of Taylor number and Rayleigh number on the Nusselt number, velocity profile, and temperature distribution are discussed in detail. It is found that the Nusselt number decreases rapidly with the increase of Taylor number but tends to have an asymptotic value. Besides, the rotation can give rise to downward flow in contrast with the upward thermal convection.
Nowotarski, C. J.
2017-12-01
Though most strong to violent tornadoes are associated with supercell thunderstorms, quasi-linear convective systems (QLCSs) pose a risk of tornadoes, often at times and locations where supercell tornadoes are less common. Because QLCS low-level mesocyclones and tornado signatures tend to be less coherent, forecasting such tornadoes remains particularly difficult. The majority of simulations of such storms rely on horizontally homogeneous base states lacking resolved boundary layer turbulence and surface fluxes. Previous work has suggested that heterogeneities associated with boundary layer turbulence in the form of horizontal convective rolls can influence the evolution and characteristics of low-level mesocyclones in supercell thunderstorms. This study extends methods for generating boundary layer convection to idealized simulations of QLCSs. QLCS simulations with resolved boundary layer turbulence will be compared against a control simulation with a laminar boundary layer. Effects of turbulence, the resultant heterogeneity in the near-storm environment, and surface friction on bulk storm characteristics and the intensity, morphology, and evolution of low-level rotation will be presented. Although maximum surface vertical vorticity values are similar, when boundary layer turbulence is included, a greater number of miso- and meso-scale vortices develop along the QLCS gust front. The source of this vorticity is analyzed using Eulerian decomposition of vorticity tendency terms and trajectory analysis to delineate the relative importance of surface friction and baroclinicity in generating QLCS vortices. The role of anvil shading in suppressing boundary layer turbulence in the near-storm environment and subsequent effects on QLCS vortices will also be presented. Finally, implications of the results regarding inclusion of more realistic boundary layers in future idealized simulations of deep convection will be discussed.
Mathematical, physical and numerical principles essential for models of turbulent mixing
Energy Technology Data Exchange (ETDEWEB)
Sharp, David Howland [Los Alamos National Laboratory; Lim, Hyunkyung [STONY BROOK UNIV; Yu, Yan [STONY BROOK UNIV; Glimm, James G [STONY BROOK UNIV
2009-01-01
We propose mathematical, physical and numerical principles which are important for the modeling of turbulent mixing, especially the classical and well studied Rayleigh-Taylor and Richtmyer-Meshkov instabilities which involve acceleration driven mixing of a fluid discontinuity layer, by a steady accerleration or an impulsive force.
Homogeneous turbulence dynamics
Sagaut, Pierre
2018-01-01
This book provides state-of-the-art results and theories in homogeneous turbulence, including anisotropy and compressibility effects with extension to quantum turbulence, magneto-hydodynamic turbulence and turbulence in non-newtonian fluids. Each chapter is devoted to a given type of interaction (strain, rotation, shear, etc.), and presents and compares experimental data, numerical results, analysis of the Reynolds stress budget equations and advanced multipoint spectral theories. The role of both linear and non-linear mechanisms is emphasized. The link between the statistical properties and the dynamics of coherent structures is also addressed. Despite its restriction to homogeneous turbulence, the book is of interest to all people working in turbulence, since the basic physical mechanisms which are present in all turbulent flows are explained. The reader will find a unified presentation of the results and a clear presentation of existing controversies. Special attention is given to bridge the results obta...
Indian Academy of Sciences (India)
Volume 4 Issue 6 June 1999 pp 82-90 Classroom. Rayleigh-Benard Convection - Physics of a Widespread Phenomenon · Arunn Narasimhan · More Details Fulltext PDF. Volume 13 Issue 7 July 2008 pp 638-647 General Article. Why do Elephants have Big Ear Flaps? Arunn Narasimhan · More Details Fulltext PDF ...
Flow Topology Transition via Global Bifurcation in Thermally Driven Turbulence
Xie, Yi-Chao; Ding, Guang-Yu; Xia, Ke-Qing
2018-05-01
We report an experimental observation of a flow topology transition via global bifurcation in a turbulent Rayleigh-Bénard convection. This transition corresponds to a spontaneous symmetry breaking with the flow becomes more turbulent. Simultaneous measurements of the large-scale flow (LSF) structure and the heat transport show that the LSF bifurcates from a high heat transport efficiency quadrupole state to a less symmetric dipole state with a lower heat transport efficiency. In the transition zone, the system switches spontaneously and stochastically between the two long-lived metastable states.
International Nuclear Information System (INIS)
Chebbi, Besma; Bouzaiane, Mounir; Lili, Taieb
2009-01-01
In this work, effects of rotation on the evolution of kinematic and thermal fields in homogeneous sheared turbulence are investigated using second order closure modeling. The Launder-Reece-Ro di models, the Speziale-Sarkar-Gatski model and the Shih-Lumley models are retained for pressure-strain correlation and pressure-temperature correlation. Whereas classic models are retained for time evolution equations of kinematic and thermal dissipation rates. The fourth order Runge-Kutta method is used to resolve three non linear differential systems obtained after modeling. The numerical integration is carried out separately for several values of the dimensionless rotation number R equal to 0, 0.25 and 0.5. The obtained results are compared to the recent results of Direct Numerical Simulations of G.Brethouwer. The results have confirmed the asymptotic equilibrium behaviors of kinematic and thermal dimensionless parameters. Furthermore they have shown that rotation affects not only kinematic field but also thermal field. The coupling between the Speziale-Sarkar-Gatski model and the Launder-Reece-Rodi model is of a big contribution on the prediction of kinematic and thermal fields
Generation of rotation and shear flow in an imploding liner
Energy Technology Data Exchange (ETDEWEB)
Hammer, J H; Ryutov, D D [Lawrence Livermore National Lab., Livermore, CA (United States)
1997-12-31
There exist several techniques that can set the liner into rotation and/or excite an embedded shear flow at any desired depth of the liner material. A common element of all these techniques is the use of properly used left-right asymmetric structures, situated either on the liner surface or embedded in the shell. Both rotation and shear flow get enhanced in the course of the liner implosion because of the angular momentum conservation. While fast enough rotation should stabilize the Rayleigh-Taylor instability near the turn-around point, the shear flow can also have a stabilizing effect on the interface. The specific model presented in the paper shows that a strong enough shear causes stabilization of a broad class of Rayleigh-Taylor perturbations. Thus, the use of left-right asymmetric structure for generation of rotation and shear flow is an interesting new option for improvement of the quality of the liner implosions. (J.U.). 4 figs., 12 refs.
Magnetic Helicities and Dynamo Action in Magneto-rotational Turbulence
Energy Technology Data Exchange (ETDEWEB)
Bodo, G.; Rossi, P. [INAF/Osservatorio Astrofisico di Torino, Strada Osservatorio 20, I-10025 Pino Torinese (Italy); Cattaneo, F. [Department of Astronomy and Astrophysics, The University of Chicago, 5640 S. Ellis Avenue, Chicago IL 60637 (United States); Mignone, A., E-mail: bodo@oato.inaf.it [Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino (Italy)
2017-07-10
We examine the relationship between magnetic flux generation, taken as an indicator of large-scale dynamo action, and magnetic helicity, computed as an integral over the dynamo volume, in a simple dynamo. We consider dynamo action driven by magneto-rotational turbulence (MRT) within the shearing-box approximation. We consider magnetically open boundary conditions that allow a flux of helicity in or out of the computational domain. We circumvent the problem of the lack of gauge invariance in open domains by choosing a particular gauge—the winding gauge—that provides a natural interpretation in terms of the average winding number of pairwise field lines. We use this gauge precisely to define and measure the helicity and the helicity flux for several realizations of dynamo action. We find in these cases that the system as a whole does not break reflectional symmetry and that the total helicity remains small even in cases when substantial magnetic flux is generated. We find no particular connection between the generation of magnetic flux and the helicity or the helicity flux through the boundaries. We suggest that this result may be due to the essentially nonlinear nature of the dynamo processes in MRT.
Rayleigh/Raman/LIF measurements in a turbulent lean premixed combustor
Energy Technology Data Exchange (ETDEWEB)
Nandula, S.P.; Pitz, R.W. [Vanderbilt Univ., Nashville, TN (United States); Barlow, R.S. [Sandia National Labs., Livermore, CA (United States)] [and others
1995-10-01
Much of the industrial electrical generation capability being added worldwide is gas-turbine engine based and is fueled by natural gas. These gas-turbine engines use lean premixed (LP) combustion to meet the strict NO{sub x} emission standards, while maintaining acceptable levels of CO. In conventional, diffusion flame gas turbine combustors, large amount of NO{sub x} forms in the hot stoichiometric zones via the Zeldovich (thermal) mechanism. Hence, lean premixed combustors are rapidly becoming the norm, since they are specifically designed to avoid these hot stoichiometric zones and the associated thermal NO, However, considerable research and development are still required to reduce the NO{sub x} levels (25-40 ppmvd adjusted to 15% O{sub 2} with the current technology), to the projected goal of under 10 ppmvd by the turn of the century. Achieving this objective would require extensive experiments in LP natural gas (or CH{sub 4}) flames for understanding the combustion phenomena underlying the formation of the exhaust pollutants. Although LP combustion is an effective way to control NO{sub x}, the downside is that it increases the CO emissions. The formation and destruction of the pollutants (NO{sub x} and CO) are strongly affected by the fluid mechanics, the finite-rate chemistry, and their (turbulence-chemistry) interactions. Hence, a thorough understanding of these interactions is vital for controlling and reducing the pollutant emissions. The present research is contributing to this goal by providing a detailed nonintrusive laser based data set with good spatial and temporal resolutions of the pollutants (NO and CO) along with the major species, temperature, and OH. The measurements reported in this work, along with the existing velocity data on a turbulent LP combustor burning CH{sub 4}, would provide insight into the turbulence-chemistry interactions and their effect on pollutant formation.
Griffiths, Ross W; Gayen, Bishakhdatta
2015-11-13
A large-scale circulation, a turbulent boundary layer, and a turbulent plume are noted features of convection at large Rayleigh numbers under differential heating on a single horizontal boundary. These might be attributed to the forcing, which in all studies has been limited to a unidirectional gradient over the domain scale. We instead apply forcing on a length scale smaller than the domain, and with variation in both horizontal directions. Direct numerical simulations show turbulence throughout the domain, a regime transition to a dominant domain-scale circulation, and a region of logarithmic velocity in the boundary layer, despite zero net heat flux. The results show significant similarities to Rayleigh-Bénard convection, demonstrate the significance of plume merging, support the hypothesis that the key driver of convection is the production of available potential energy without necessarily supplying total potential energy, and imply that contributions to domain-scale circulation in the oceans need not be solely from the large-scale gradients of forcing.
The Effects of Radiation on the Linear Stability of a horizontal layer ...
African Journals Online (AJOL)
The effect of radiation on the onset of Rayleigh-Benard convection is studied in the case of a radiating Newtonian fluid in a fluid-saturated horizontal porous layer heated from below. The radiative heat transfer is treated using the differential approximation for optically thin limiting case. The linear stability theory is employed ...
Indian Academy of Sciences (India)
First page Back Continue Last page Overview Graphics. Flux scaling: Ultimate regime. With the Nusselt number and the mixing length scales, we get the Nusselt number and Reynolds number (w'd/ν) scalings: and or. and. scaling expected to occur at extremely high Ra Rayleigh-Benard convection. Get the ultimate regime ...
Effect of periodic bottom plate heating on large scale flow in turbulent Rayleigh-Bénard convection
Czech Academy of Sciences Publication Activity Database
Kuqali, M.; Babuin, Simone; Niemela, J.J.
2015-01-01
Roč. 8, č. 3 (2015), 483-489 ISSN 1735-3572 Institutional support: RVO:68378271 Keywords : Rayleigh-Bénard convection * Reynolds number Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 0.888, year: 2015
Study of heat transfer at the upper boundary of a fluid bath with voluminal power dissipation
International Nuclear Information System (INIS)
Bernaz, L.
1998-01-01
In order to examine the possibilities of external cooling of reactor vessels in the case of core meltdown accident in PWR reactors, it is necessary to determine the distribution of heat fluxes at the boundaries of the hemispherical bath. This distribution is controlled by the natural convection movements inside the bath. The first part of the document introduces the problem. The second part is a bibliographic analysis of previous works performed on natural convection inside a liquid bath with an internal heat source. The modeling of heat transfer at the upper boundary is assimilated to the Rayleigh-Benard turbulent convection. An analytical model of weak turbulence regime is developed in part 3. Different laws about the relationship between thermal plume characteristics and the parameters of the system are defined. Part 4 presents the experimental devices and scale models (MARABEC and BALI) used for the validation of the theoretical model and for the obtention of a database of reactor parameters. The laser induced fluorescence visualisation technique is used for the parametric study of between plumes spacing. The fifth part is devoted to the presentation and interpretation of results. (J.S.)
The Oscillatory Nature of Rotating Convection in Liquid Metal
Aurnou, J. M.; Bertin, V. L.; Grannan, A. M.
2016-12-01
Earth's magnetic field is assumed to be generated by fluid motions in its liquid metal core. In this fluid, the heat diffuses significantly more than momentum and thus, the ratio of these two diffusivities, the Prandtl number Pr=ν/Κ, is well below unity. The convective flow dynamics of liquid metal is very different from Pr ≈ 1 fluids like water and those used in current dynamo simulations. In order to characterize rapidly rotating thermal convection in low Pr number fluids, we have performed laboratory experiments in a cylinder using liquid gallium (Pr ≈ 0.023) as the working fluid. The Ekman number, which characterizes the effect of rotation, varies from E = 4 10-5 to 4 10-6 and the dimensionless buoyancy forcing (Rayleigh number, Ra) varies from Ra =3 105 to 2 107. Using heat transfer measurements (Nusselt number, Nu) as well as temperature measurements within the fluid, we characterize the different styles of low Pr rotating convective flow. The convection threshold is first overcome in the form of a container scale inertial oscillatory mode. At stronger forcing, wall-localized modes are identified for the first time in liquid metal laboratory experiments. These wall modes coexist with the bulk inertial oscillatory modes. When the strengh of the buoyancy increases, the bulk flow becomes turbulent while the wall modes remain. Our results imply that rotating convective flows in liquid metals do not develop in the form of quasi-steady columns, as in Pr ≈ 1 dynamo models, but in the form of oscillatory motions. Therefore, the flows that drive thermally-driven dynamo action in low Pr geophysical and astrophysical fluids can differ substantively than those occuring in current-day Pr ≈ 1 numerical models. In addition, our results suggest that relatively low wavenumber, wall-attached modes may be dynamically important in rapidly-rotating convection in liquid metals.
Reynolds-Stress and Triple-Product Models Applied to a Flow with Rotation and Curvature
Olsen, Michael E.
2016-01-01
Turbulence models, with increasing complexity, up to triple product terms, are applied to the flow in a rotating pipe. The rotating pipe is a challenging case for turbulence models as it contains significant rotational and curvature effects. The flow field starts with the classic fully developed pipe flow, with a stationary pipe wall. This well defined condition is then subjected to a section of pipe with a rotating wall. The rotating wall introduces a second velocity scale, and creates Reynolds shear stresses in the radial-circumferential and circumferential-axial planes. Furthermore, the wall rotation introduces a flow stabilization, and actually reduces the turbulent kinetic energy as the flow moves along the rotating wall section. It is shown in the present work that the Reynolds stress models are capable of predicting significant reduction in the turbulent kinetic energy, but triple product improves the predictions of the centerline turbulent kinetic energy, which is governed by convection, dissipation and transport terms, as the production terms vanish on the pipe axis.
Drift wave turbulence in low-β plasmas
DEFF Research Database (Denmark)
Mikkelsen, Torben; Larsen, Søren Ejling; Pécseli, Hans
1983-01-01
Experimental investigations of strong turbulence associated with the radial density gradient of a rotating magnetized plasma column are reported. The experiment is designed to make Taylor's hypothesis effective, in order to allow a simple interpretation of measured frequency spectra in terms of w...... spectrum is demonstrated. Some aspects of the relative diffusion of a test-cloud of charged particles released in the turbulent field are discussed.......Experimental investigations of strong turbulence associated with the radial density gradient of a rotating magnetized plasma column are reported. The experiment is designed to make Taylor's hypothesis effective, in order to allow a simple interpretation of measured frequency spectra in terms...... of wavenumber spectra. The spectral index of the turbulent potential fluctuations is determined and the variation of the spectral intensity is investigated for varying magnetic fields. The results compare favourably with theoretical predictions. The importance of distinguishing subranges in the turbulent...
Study of two-dimensional interchange turbulence
International Nuclear Information System (INIS)
Sugama, Hideo; Wakatani, Masahiro.
1990-04-01
An eddy viscosity model describing enstrophy transfer in two-dimensional turbulence is presented. This model is similar to that of Canuto et al. and provides an equation for the energy spectral function F(k) as a function of the energy input rate to the system per unit wavenumber, γ s (k). In the enstrophy-transfer inertial range, F(k)∝ k -3 is predicted by the model. The eddy viscosity model is applied to the interchange turbulence of a plasma in shearless magnetic field. Numerical simulation of the two-dimensional interchange turbulence demonstrates that the energy spectrum in the high wavenumber region is well described by this model. The turbulent transport driven by the interchange turbulence is expressed in terms of the Nusselt number Nu, the Rayleigh number Ra and Prantl number Pr in the same manner as that of thermal convection problem. When we use the linear growth rate for γ s (k), our theoretical model predicts that Nu ∝ (Ra·Pr) 1/2 for a constant background pressure gradient and Nu ∝ (Ra·Pr) 1/3 for a self-consistent background pressure profile with the stress-free slip boundary conditions. The latter agrees with our numerical result showing Nu ∝ Ra 1/3 . (author)
Large eddy simulations of an airfoil in turbulent inflow
DEFF Research Database (Denmark)
Gilling, Lasse; Sørensen, Niels N.
2008-01-01
Wind turbines operate in the turbulent boundary layer of the atmosphere and due to the rotational sampling effect the blades experience a high level of turbulence [1]. In this project the effect of turbulence is investigated by large eddy simulations of the turbulent flow past a NACA 0015 airfoil...
Non-Oberbeck-Boussinesq effects in strongly turbulent Rayleigh-Bénard convection
Ahlers, Günter; Brown, Eric; Fontenele Araujo Junior, F.; Funfschilling, Denis; Grossmann, Siegfried; Lohse, Detlef
2006-01-01
Non-Oberbeck–Boussinesq (NOB) effects on the Nusselt number $Nu$ and Reynolds number $\\hbox{\\it Re}$ in strongly turbulent Rayleigh–Bénard (RB) convection in liquids were investigated both experimentally and theoretically. In the experiments the heat current, the temperature difference, and the
3-D simulations to investigate initial condition effects on the growth of Rayleigh-Taylor mixing
Energy Technology Data Exchange (ETDEWEB)
Andrews, Malcolm J [Los Alamos National Laboratory
2008-01-01
The effect of initial conditions on the growth rate of turbulent Rayleigh-Taylor (RT) mixing has been studied using carefully formulated numerical simulations. An integrated large-eddy simulation (ILES) that uses a finite-volume technique was employed to solve the three-dimensional incompressible Euler equations with numerical dissipation. The initial conditions were chosen to test the dependence of the RT growth parameters ({alpha}{sub b}, {alpha}{sub s}) on variations in (a) the spectral bandwidth, (b) the spectral shape, and (c) discrete banded spectra. Our findings support the notion that the overall growth of the RT mixing is strongly dependent on initial conditions. Variation in spectral shapes and bandwidths are found to have a complex effect of the late time development of the RT mixing layer, and raise the question of whether we can design RT transition and turbulence based on our choice of initial conditions. In addition, our results provide a useful database for the initialization and development of closures describing RT transition and turbulence.
Jacobitz, Frank G; Schneider, Kai; Bos, Wouter J T; Farge, Marie
2016-01-01
The acceleration statistics of sheared and rotating homogeneous turbulence are studied using direct numerical simulation results. The statistical properties of Lagrangian and Eulerian accelerations are considered together with the influence of the rotation to shear ratio, as well as the scale dependence of their statistics. The probability density functions (pdfs) of both Lagrangian and Eulerian accelerations show a strong and similar dependence on the rotation to shear ratio. The variance and flatness of both accelerations are analyzed and the extreme values of the Eulerian acceleration are observed to be above those of the Lagrangian acceleration. For strong rotation it is observed that flatness yields values close to three, corresponding to Gaussian-like behavior, and for moderate and vanishing rotation the flatness increases. Furthermore, the Lagrangian and Eulerian accelerations are shown to be strongly correlated for strong rotation due to a reduced nonlinear term in this case. A wavelet-based scale-dependent analysis shows that the flatness of both Eulerian and Lagrangian accelerations increases as scale decreases, which provides evidence for intermittent behavior. For strong rotation the Eulerian acceleration is even more intermittent than the Lagrangian acceleration, while the opposite result is obtained for moderate rotation. Moreover, the dynamics of a passive scalar with gradient production in the direction of the mean velocity gradient is analyzed and the influence of the rotation to shear ratio is studied. Concerning the concentration of a passive scalar spread by the flow, the pdf of its Eulerian time rate of change presents higher extreme values than those of its Lagrangian time rate of change. This suggests that the Eulerian time rate of change of scalar concentration is mainly due to advection, while its Lagrangian counterpart is only due to gradient production and viscous dissipation.
Turbulence and particle acceleration
International Nuclear Information System (INIS)
Scott, J.S.
1975-01-01
A model for the production of high energy particles in the supernova remnant Cas A is considered. The ordered expansion of the fast moving knots produce turbulent cells in the ambient interstellar medium. The turbulent cells act as magnetic scattering centers and charged particles are accelerated to large energies by the second order Fermi mechanism. Model predictions are shown to be consistent with the observed shape and time dependence of the radio spectrum, and with the scale size of magnetic field irregularities. Assuming a galactic supernova rate at 1/50 yr -1 , this mechanism is capable of producing the observed galactic cosmic ray flux and spectrum below 10 16 eV/nucleon. Several observed features of galactic cosmic rays are shown to be consistent with model predictions. A model for the objects known as radio tall galaxies is also presented. Independent blobs of magnetized plasma emerging from an active radio galaxy into an intracluster medium become turbulent due to Rayleigh--Taylor and Kelvin--Helmholz instabilities. The turbulence produces both in situ betatron and 2nd order Fermi accelerations. Predictions of the dependence of spectral index and flux on distance along the tail match observations well. Fitting provides values of physical parameters in the blobs. The relevance of this method of particle acceleration for the problem of the origin of x-ray emission in clusters of galaxies is discussed
The Rayleigh-Taylor instability in a self-gravitating two-layer fluid sphere
International Nuclear Information System (INIS)
Ida, Shigeru; Nakagawa, Yoshitsugu; Nakazawa, Kiyoshi
1989-01-01
The Rayleigh-Taylor instability is studied in a self-gravitating two-layer fluid sphere: an inner sphere and an outer layer. The density and the viscosity are assumed to be constant in each region. Analytic expressions of the dispersion relations are obtained in inviscid and viscid cases. This examination aims at the investigation of the Earth's core formation. The fluid sphere corresponds to the proto-Earth in the accretion stage. The instability is examined without rotation of the fluid sphere, while the proto-Earth is rotating. However, it is shown that the Coriolis force does not influence the conclusion in the Earth's core formation problem. 5 refs.; 10 figs
Planform structure and heat transfer in turbulent free convection over horizontal surfaces
Theerthan, S. Ananda; Arakeri, Jaywant H.
2000-04-01
This paper deals with turbulent free convection in a horizontal fluid layer above a heated surface. Experiments have been carried out on a heated surface to obtain and analyze the planform structure and the heat transfer under different conditions. Water is the working fluid and the range of flux Rayleigh numbers (Ra) covered is 3×107-2×1010. The different conditions correspond to Rayleigh-Bénard convection, convection with either the top water surface open to atmosphere or covered with an insulating plate, and with an imposed external flow on the heated boundary. Without the external flow the planform is one of randomly oriented line plumes. At large Rayleigh number Ra and small aspect ratio (AR), these line plumes seem to align along the diagonal, presumably due to a large scale flow. The side views show inclined dyelines, again indicating a large scale flow. When the external flow is imposed, the line plumes clearly align in the direction of external flow. The nondimensional average plume spacing, Raλ1/3, varies between 40 and 90. The heat transfer rate, for all the experiments conducted, represented as RaδT-1/3, where δT is the conduction layer thickness, varies only between 0.1-0.2, showing that in turbulent convection the heat transfer rates are similar under the different conditions.
Density-ratio effects on buoyancy-driven variable-density turbulent mixing
Aslangil, Denis; Livescu, Daniel; Banerjee, Arindam
2017-11-01
Density-ratio effects on the turbulent mixing of two incompressible, miscible fluids with different densities subject to constant acceleration are studied by means of high-resolution Direct Numerical Simulations. In a triply periodic domain, turbulence is generated by stirring in response to the differential buoyancy forces within the flow. Later, as the fluids become molecularly mixed, dissipation starts to overcome turbulence generation by bouyancy. Thus, the flow evolution includes both turbulence growth and decay, and it displays features present in the core region of the mixing layer of the Rayleigh-Taylor as well as Richtmyer-Meshkov instabilities. We extend the previous studies by investigating a broad range of density-ratio, from 1-14.4:1, corresponding to Atwood numbers of 0.05-0.87. Here, we focus on the Atwood number dependence of mixing-efficiency, that is defined based on the energy-conversion ratios from potential energy to total and turbulent kinetic energies, the decay characteristics of buoyancy-assisted variable-density homogeneous turbulence, and the effects of high density-ratios on the turbulence structure and mixing process. Authors acknowledge financial support from DOE-SSAA (DE-NA0003195) and NSF CAREER (#1453056) awards.
Self-channeling of high-power laser pulses through strong atmospheric turbulence
Peñano, J.; Palastro, J. P.; Hafizi, B.; Helle, M. H.; DiComo, G. P.
2017-07-01
We present an unusual example of truly long-range propagation of high-power laser pulses through strong atmospheric turbulence. A form of nonlinear self-channeling is achieved when the laser power is close to the self-focusing power of air and the transverse dimensions of the pulse are smaller than the coherence diameter of turbulence. In this mode, nonlinear self-focusing counteracts diffraction, and turbulence-induced spreading is greatly reduced. Furthermore, the laser intensity is below the ionization threshold so that multiphoton absorption and plasma defocusing are avoided. Simulations show that the pulse can propagate many Rayleigh lengths (several kilometers) while maintaining a high intensity. In the presence of aerosols, or other extinction mechanisms that deplete laser energy, the pulse can be chirped to maintain the channeling.
Li, Chunggang; Tsubokura, Makoto; Wang, Weihsiang
2017-11-01
The automatic dissipation adjustment (ADA) model based on truncated Navier-Stokes equations is utilized to investigate the feasibility of using implicit large eddy simulation (ILES) with ADA model on the transition in natural convection. Due to the high Rayleigh number coming from the larger temperature difference (300K), Roe scheme modified for low Mach numbers coordinating ADA model is used to resolve the complicated flow field. Based on the qualitative agreement of the comparisons with DNS and experimental results and the capability of numerically predicating a -3 decay law for the temporal power spectrum of the temperature fluctuation, this study thus validates the feasibility of ILES with ADA model on turbulent natural convection. With the advantages of ease of implementation because no explicit modeling terms are needed and nearly free of tuning parameters, ADA model offers to become a promising tool for turbulent thermal convection. Part of the results is obtained using the K computer at the RIKEN Advanced Institute for Computational Science (Proposal number hp160232).
Flow Visualization in Supersonic Turbulent Boundary Layers.
Smith, Michael Wayne
This thesis is a collection of novel flow visualizations of two different flat-plate, zero pressure gradient, supersonic, turbulent boundary layers (M = 2.8, Re _theta ~ 82,000, and M = 2.5, Re_ theta ~ 25,000, respectively). The physics of supersonic shear flows has recently drawn increasing attention with the renewed interest in flight at super and hypersonic speeds. This work was driven by the belief that the study of organized, Reynolds -stress producing turbulence structures will lead to improved techniques for the modelling and control of high-speed boundary layers. Although flow-visualization is often thought of as a tool for providing qualitative information about complex flow fields, in this thesis an emphasis is placed on deriving quantitative results from image data whenever possible. Three visualization techniques were applied--'selective cut-off' schlieren, droplet seeding, and Rayleigh scattering. Two experiments employed 'selective cut-off' schlieren. In the first, high-speed movies (40,000 fps) were made of strong density gradient fronts leaning downstream at between 30^circ and 60^ circ and travelling at about 0.9U _infty. In the second experiment, the same fronts were detected with hot-wires and imaged in real time, thus allowing the examination of the density gradient fronts and their associated single-point mass -flux signals. Two experiments employed droplet seeding. In both experiments, the boundary layer was seeded by injecting a stream of acetone through a single point in the wall. The acetone is atomized by the high shear at the wall into a 'fog' of tiny (~3.5mu m) droplets. In the first droplet experiment, the fog was illuminated with copper-vapor laser sheets of various orientations. The copper vapor laser pulses 'froze' the fog motion, revealing a variety of organized turbulence structures, some with characteristic downstream inclinations, others with large-scale roll-up on the scale of delta. In the second droplet experiment, high
Homogeneous wave turbulence driven by tidal flows
Favier, B.; Le Reun, T.; Barker, A.; Le Bars, M.
2017-12-01
When a moon orbits around a planet, the rotation of the induced tidal bulge drives a homogeneous, periodic, large-scale flow. The combination of such an excitation with the rotating motion of the planet has been shown to drive parametric resonance of a pair of inertial waves in a mechanism called the elliptical instability. Geophysical fluid layers can also be stratified: this is the case for instance of the Earth's oceans and, as suggested by several studies, of the upper part of the Earth's liquid Outer Core. We thus investigate the stability of a rotating and stratified layer undergoing tidal distortion in the limit where either rotation or stratification is dominant. We show that the periodic tidal flow drives a parametric subharmonic resonance of inertial (resp. internal) waves in the rotating (resp. stratified) case. The instability saturates into a wave turbulence pervading the whole fluid layer. In such a state, the instability mechanism conveys the tidal energy from the large scale tidal flow to the resonant modes, which then feed a succession of triadic resonances also generating small spatial scales. In the rotating case, we observe a kinetic energy spectrum with a k-2 slope for which the Coriolis force is dominant at all spatial scales. In the stratified case, where the timescale separation is increased between the tidal excitation and the Brunt-Väisälä frequencies, the temporal spectrum decays with a ω-2 power law up to the cut-off frequency beyond which waves do not exist. This result is reminiscent of the Garrett and Munk spectrum measured in the oceans and theoretically described as a manifestation of internal wave turbulence. In addition to revealing an instability driving homogeneous turbulence in geophysical fluid layers, our approach is also an efficient numerical tool to investigate the possibly universal properties of wave turbulence in a geophysical context.
Molecular Rayleigh Scattering Diagnostic for Dynamic Temperature, Velocity, and Density Measurements
Mielke, Amy R.; Elam, Kristie A.; Sung, Chi-Jen
2006-01-01
A molecular Rayleigh scattering technique is developed to measure dynamic gas temperature, velocity, and density in unseeded turbulent flows at sampling rates up to 16 kHz. A high power CW laser beam is focused at a point in an air jet plume and Rayleigh scattered light is collected and spectrally resolved. The spectrum of the light, which contains information about the temperature and velocity of the flow, is analyzed using a Fabry-Perot interferometer. The circular interference fringe pattern is divided into four concentric regions and sampled at 1 and 16 kHz using photon counting electronics. Monitoring the relative change in intensity within each region allows for measurement of gas temperature and velocity. Independently monitoring the total scattered light intensity provides a measure of gas density. A low speed heated jet is used to validate the measurement of temperature fluctuations and an acoustically excited nozzle flow is studied to validate velocity fluctuation measurements. Power spectral density calculations of the property fluctuations, as well as mean and fluctuating quantities are presented. Temperature fluctuation results are compared with constant current anemometry measurements and velocity fluctuation results are compared with constant temperature anemometry measurements at the same locations.
Canuto, V. M.
1994-01-01
The Reynolds numbers that characterize geophysical and astrophysical turbulence (Re approximately equals 10(exp 8) for the planetary boundary layer and Re approximately equals 10(exp 14) for the Sun's interior) are too large to allow a direct numerical simulation (DNS) of the fundamental Navier-Stokes and temperature equations. In fact, the spatial number of grid points N approximately Re(exp 9/4) exceeds the computational capability of today's supercomputers. Alternative treatments are the ensemble-time average approach, and/or the volume average approach. Since the first method (Reynolds stress approach) is largely analytical, the resulting turbulence equations entail manageable computational requirements and can thus be linked to a stellar evolutionary code or, in the geophysical case, to general circulation models. In the volume average approach, one carries out a large eddy simulation (LES) which resolves numerically the largest scales, while the unresolved scales must be treated theoretically with a subgrid scale model (SGS). Contrary to the ensemble average approach, the LES+SGS approach has considerable computational requirements. Even if this prevents (for the time being) a LES+SGS model to be linked to stellar or geophysical codes, it is still of the greatest relevance as an 'experimental tool' to be used, inter alia, to improve the parameterizations needed in the ensemble average approach. Such a methodology has been successfully adopted in studies of the convective planetary boundary layer. Experienc e with the LES+SGS approach from different fields has shown that its reliability depends on the healthiness of the SGS model for numerical stability as well as for physical completeness. At present, the most widely used SGS model, the Smagorinsky model, accounts for the effect of the shear induced by the large resolved scales on the unresolved scales but does not account for the effects of buoyancy, anisotropy, rotation, and stable stratification. The
Ground Boundary Conditions for Thermal Convection Over Horizontal Surfaces at High Rayleigh Numbers
Hanjalić, K.; Hrebtov, M.
2016-07-01
We present "wall functions" for treating the ground boundary conditions in the computation of thermal convection over horizontal surfaces at high Rayleigh numbers using coarse numerical grids. The functions are formulated for an algebraic-flux model closed by transport equations for the turbulence kinetic energy, its dissipation rate and scalar variance, but could also be applied to other turbulence models. The three-equation algebraic-flux model, solved in a T-RANS mode ("Transient" Reynolds-averaged Navier-Stokes, based on triple decomposition), was shown earlier to reproduce well a number of generic buoyancy-driven flows over heated surfaces, albeit by integrating equations up to the wall. Here we show that by using a set of wall functions satisfactory results are found for the ensemble-averaged properties even on a very coarse computational grid. This is illustrated by the computations of the time evolution of a penetrative mixed layer and Rayleigh-Bénard (open-ended, 4:4:1 domain) convection, using 10 × 10 × 100 and 10 × 10 × 20 grids, compared also with finer grids (e.g. 60 × 60 × 100), as well as with one-dimensional treatment using 1 × 1 × 100 and 1 × 1 × 20 nodes. The approach is deemed functional for simulations of a convective boundary layer and mesoscale atmospheric flows, and pollutant transport over realistic complex hilly terrain with heat islands, urban and natural canopies, for diurnal cycles, or subjected to other time and space variations in ground conditions and stratification.
Up-down symmetry of the turbulent transport of toroidal angular momentum in tokamaks
International Nuclear Information System (INIS)
Parra, Felix I.; Barnes, Michael; Peeters, Arthur G.
2011-01-01
Two symmetries of the local nonlinear δf gyrokinetic system of equations in tokamaks in the high flow regime are presented. The turbulent transport of toroidal angular momentum changes sign under an up-down reflection of the tokamak and a sign change of both the rotation and the rotation shear. Thus, the turbulent transport of toroidal angular momentum must vanish for up-down symmetric tokamaks in the absence of both rotation and rotation shear. This has important implications for the modeling of spontaneous rotation.
Energy Technology Data Exchange (ETDEWEB)
Mueschke, N J; Andrews, M J; Schilling, O
2005-09-26
The initial multi-mode interfacial velocity and density perturbations present at the onset of a small Atwood number, incompressible, miscible, Rayleigh-Taylor instability-driven mixing layer have been quantified using a combination of experimental techniques. The streamwise interfacial and spanwise interfacial perturbations were measured using high-resolution thermocouples and planar laser-induced fluorescence (PLIF), respectively. The initial multi-mode streamwise velocity perturbations at the two-fluid density interface were measured using particle-image velocimetry (PIV). It was found that the measured initial conditions describe an initially anisotropic state, in which the perturbations in the streamwise and spanwise directions are independent of one another. The evolution of various fluctuating velocity and density statistics, together with velocity and density variance spectra, were measured using PIV and high-resolution thermocouple data. The evolution of the velocity and density statistics is used to investigate the early-time evolution and the onset of strongly-nonlinear, transitional dynamics within the mixing layer. The early-time evolution of the density and vertical velocity variance spectra indicate that velocity fluctuations are the dominant mechanism driving the instability development. The implications of the present experimental measurements on the initialization of Reynolds-averaged turbulent transport and mixing models and of direct and large-eddy simulations of Rayleigh-Taylor instability-induced turbulence are discussed.
Xia, Haiyun; Dou, Xiankang; Shangguan, Mingjia; Zhao, Ruocan; Sun, Dongsong; Wang, Chong; Qiu, Jiawei; Shu, Zhifeng; Xue, Xianghui; Han, Yuli; Han, Yan
2014-09-08
Temperature detection remains challenging in the low stratosphere, where the Rayleigh integration lidar is perturbed by aerosol contamination and ozone absorption while the rotational Raman lidar is suffered from its low scattering cross section. To correct the impacts of temperature on the Rayleigh Doppler lidar, a high spectral resolution lidar (HSRL) based on cavity scanning Fabry-Perot Interferometer (FPI) is developed. By considering the effect of the laser spectral width, Doppler broadening of the molecular backscatter, divergence of the light beam and mirror defects of the FPI, a well-behaved transmission function is proved to show the principle of HSRL in detail. Analysis of the statistical error of the HSRL is carried out in the data processing. A temperature lidar using both HSRL and Rayleigh integration techniques is incorporated into the Rayleigh Doppler wind lidar. Simultaneous wind and temperature detection is carried out based on the combined system at Delhi (37.371°N, 97.374°E; 2850 m above the sea level) in Qinghai province, China. Lower Stratosphere temperature has been measured using HSRL between 18 and 50 km with temporal resolution of 2000 seconds. The statistical error of the derived temperatures is between 0.2 and 9.2 K. The temperature profile retrieved from the HSRL and wind profile from the Rayleigh Doppler lidar show good agreement with the radiosonde data. Specifically, the max temperature deviation between the HSRL and radiosonde is 4.7 K from 18 km to 36 km, and it is 2.7 K between the HSRL and Rayleigh integration lidar from 27 km to 34 km.
Modulated convection at high frequencies and large modulation amplitudes
International Nuclear Information System (INIS)
Swift, J.B.; Hohenberg, P.C.
1987-01-01
Modulated Rayleigh-Benard convection is analyzed for high frequencies and large modulation amplitudes. The linear theory of Gershuni and Zhukhovitskii is generalized to the nonlinear domain, and a subcritical bifurcation to convection is found in agreement with the experiments of Niemela and Donnelly. The crossover between the high-frequency (''Stokes layer'') regime and the low-frequency regime studied previously is analyzed
On the prediction of turbulent secondary flows
Speziale, C. G.; So, R. M. C.; Younis, B. A.
1992-01-01
The prediction of turbulent secondary flows, with Reynolds stress models, in circular pipes and non-circular ducts is reviewed. Turbulence-driven secondary flows in straight non-circular ducts are considered along with turbulent secondary flows in pipes and ducts that arise from curvature or a system rotation. The physical mechanisms that generate these different kinds of secondary flows are outlined and the level of turbulence closure required to properly compute each type is discussed in detail. Illustrative computations of a variety of different secondary flows obtained from two-equation turbulence models and second-order closures are provided to amplify these points.
Seismic signature of turbulence during the 2017 Oroville Dam spillway erosion crisis
Goodling, Phillip J.; Lekic, Vedran; Prestegaard, Karen
2018-05-01
Knowing the location of large-scale turbulent eddies during catastrophic flooding events improves predictions of erosive scour. The erosion damage to the Oroville Dam flood control spillway in early 2017 is an example of the erosive power of turbulent flow. During this event, a defect in the simple concrete channel quickly eroded into a 47 m deep chasm. Erosion by turbulent flow is difficult to evaluate in real time, but near-channel seismic monitoring provides a tool to evaluate flow dynamics from a safe distance. Previous studies have had limited ability to identify source location or the type of surface wave (i.e., Love or Rayleigh wave) excited by different river processes. Here we use a single three-component seismometer method (frequency-dependent polarization analysis) to characterize the dominant seismic source location and seismic surface waves produced by the Oroville Dam flood control spillway, using the abrupt change in spillway geometry as a natural experiment. We find that the scaling exponent between seismic power and release discharge is greater following damage to the spillway, suggesting additional sources of turbulent energy dissipation excite more seismic energy. The mean azimuth in the 5-10 Hz frequency band was used to resolve the location of spillway damage. Observed polarization attributes deviate from those expected for a Rayleigh wave, though numerical modeling indicates these deviations may be explained by propagation up the uneven hillside topography. Our results suggest frequency-dependent polarization analysis is a promising approach for locating areas of increased flow turbulence. This method could be applied to other erosion problems near engineered structures as well as to understanding energy dissipation, erosion, and channel morphology development in natural rivers, particularly at high discharges.
Seismic signature of turbulence during the 2017 Oroville Dam spillway erosion crisis
Directory of Open Access Journals (Sweden)
P. J. Goodling
2018-05-01
Full Text Available Knowing the location of large-scale turbulent eddies during catastrophic flooding events improves predictions of erosive scour. The erosion damage to the Oroville Dam flood control spillway in early 2017 is an example of the erosive power of turbulent flow. During this event, a defect in the simple concrete channel quickly eroded into a 47 m deep chasm. Erosion by turbulent flow is difficult to evaluate in real time, but near-channel seismic monitoring provides a tool to evaluate flow dynamics from a safe distance. Previous studies have had limited ability to identify source location or the type of surface wave (i.e., Love or Rayleigh wave excited by different river processes. Here we use a single three-component seismometer method (frequency-dependent polarization analysis to characterize the dominant seismic source location and seismic surface waves produced by the Oroville Dam flood control spillway, using the abrupt change in spillway geometry as a natural experiment. We find that the scaling exponent between seismic power and release discharge is greater following damage to the spillway, suggesting additional sources of turbulent energy dissipation excite more seismic energy. The mean azimuth in the 5–10 Hz frequency band was used to resolve the location of spillway damage. Observed polarization attributes deviate from those expected for a Rayleigh wave, though numerical modeling indicates these deviations may be explained by propagation up the uneven hillside topography. Our results suggest frequency-dependent polarization analysis is a promising approach for locating areas of increased flow turbulence. This method could be applied to other erosion problems near engineered structures as well as to understanding energy dissipation, erosion, and channel morphology development in natural rivers, particularly at high discharges.
Energy Technology Data Exchange (ETDEWEB)
T.S. Hahm; Z. Lin; P.H. Diamond; G. Rewoldt; W.X. Wang; S. Ethier; O. Gurcan; W.W. Lee; W.M. Tang
2004-12-21
An integrated program of gyrokinetic particle simulation and theory has been developed to investigate several outstanding issues in both turbulence and neoclassical physics. Gyrokinetic particle simulations of toroidal ion temperature gradient (ITG) turbulence spreading using the GTC code and its related dynamical model have been extended to the case with radially increasing ion temperature gradient, to study the inward spreading of edge turbulence toward the core. Due to turbulence spreading from the edge, the turbulence intensity in the core region is significantly enhanced over the value obtained from simulations of the core region only. Even when the core gradient is within the Dimits shift regime (i.e., self-generated zonal flows reduce the transport to a negligible value), a significant level of turbulence and transport is observed in the core due to spreading from the edge. The scaling of the turbulent front propagation speed is closer to the prediction from our nonlinear diffusion model than one based on linear toroidal coupling. A calculation of ion poloidal rotation in the presence of sharp density and toroidal angular rotation frequency gradients from the GTC-Neo particle simulation code shows that the results are significantly different from the conventional neoclassical theory predictions. An energy conserving set of a fully electromagnetic nonlinear gyrokinetic Vlasov equation and Maxwell's equations, which is applicable to edge turbulence, is being derived via the phase-space action variational Lie perturbation method. Our generalized ordering takes the ion poloidal gyroradius to be on the order of the radial electric field gradient length.
International Nuclear Information System (INIS)
Hahm, T.S.; Lin, Z.; Diamond, P.H.; Gurcan, O.; Rewoldt, G.; Wang, W.X.; Ethier, S.; Lee, W.W.; Lewandowski, J.L.V.; Tang, W.M.
2005-01-01
An integrated program of gyrokinetic particle simulation and theory has been developed to investigate several outstanding issues in both turbulence and neoclassical physics. Gyrokinetic particle simulations of toroidal ion temperature gradient (ITG) turbulence spreading using the GTC code and its related dynamical model have been extended to the case with radially increasing ion temperature gradient, to study the inward spreading of edge turbulence toward the core. Due to turbulence spreading from the edge, the turbulence intensity in the core region is significantly enhanced over the value obtained from simulations of the core region only. Even when the core gradient is within the Dimits shift regime (i.e., self-generated zonal flows reduce the transport to a negligible value), a significant level of turbulence and transport is observed in the core due to spreading from the edge. The scaling of the turbulent front propagation speed is closer to the prediction from our nonlinear diffusion model than one based on linear toroidal coupling. A calculation of ion poloidal rotation in the presence of sharp density and toroidal angular rotation frequency gradients from the GTC-Neo particle simulation code shows that the results are significantly different from the conventional neoclassical theory predictions. An energy conserving set of a fully electromagnetic nonlinear gyrokinetic Vlasov equation and Maxwell's equations, which is applicable to edge turbulence, is being derived via the phase-space action variational Lie perturbation method. Our generalized ordering takes the ion poloidal gyroradius to be on the order of the radial electric field gradient length. (author)
Modeling of Turbulent Swirling Flows
Shih, Tsan-Hsing; Zhu, Jiang; Liou, William; Chen, Kuo-Huey; Liu, Nan-Suey; Lumley, John L.
1997-01-01
Aircraft engine combustors generally involve turbulent swirling flows in order to enhance fuel-air mixing and flame stabilization. It has long been recognized that eddy viscosity turbulence models are unable to appropriately model swirling flows. Therefore, it has been suggested that, for the modeling of these flows, a second order closure scheme should be considered because of its ability in the modeling of rotational and curvature effects. However, this scheme will require solution of many complicated second moment transport equations (six Reynolds stresses plus other scalar fluxes and variances), which is a difficult task for any CFD implementations. Also, this scheme will require a large amount of computer resources for a general combustor swirling flow. This report is devoted to the development of a cubic Reynolds stress-strain model for turbulent swirling flows, and was inspired by the work of Launder's group at UMIST. Using this type of model, one only needs to solve two turbulence equations, one for the turbulent kinetic energy k and the other for the dissipation rate epsilon. The cubic model developed in this report is based on a general Reynolds stress-strain relationship. Two flows have been chosen for model evaluation. One is a fully developed rotating pipe flow, and the other is a more complex flow with swirl and recirculation.
Turbulence induced lift experienced by large particles in a turbulent flow
International Nuclear Information System (INIS)
Zimmermann, Robert; Gasteuil, Yoann; Volk, Romain; Pumir, Alain; Pinton, Jean-François; Bourgoin, Mickaël
2011-01-01
The translation and rotation of a large, neutrally buoyant, particle, freely advected by a turbulent flow is determined experimentally. We observe that, both, the orientation the angular velocity with respect to the trajectory and the translational acceleration conditioned on the spinning velocity provides evidence of a lift force, F lift ∝ ω × ν rel , acting on the particle. New results of the dynamics of the coupling between the particle's rotation and its translation are presented.
Ahlers, Günter; Calzavarini, E.; Fontenele Araujo Junior, F.; Funfschilling, Denis; Grossmann, Siegfried; Lohse, Detlef; Sugiyama, K.
2008-01-01
As shown in earlier work [Ahlers et al., J. Fluid Mech. 569, 409 (2006)], non-Oberbeck-Boussinesq (NOB) corrections to the center temperature in turbulent Rayleigh-Bénard convection in water and also in glycerol are governed by the temperature dependences of the kinematic viscosity and the thermal
Transitions in rapidly rotating convection dynamos
Tilgner, A.
2013-12-01
It is commonly assumed that buoyancy in the fluid core powers the geodynamo. We study here the minimal model of a convection driven dynamo, which is a horizontal plane layer in a gravity field, filled with electrically conducting fluid, heated from below and cooled from above, and rotating about a vertical axis. Such a plane layer may be viewed as a local approximation to the geophysically more relevant spherical geometry. The numerical simulations have been run on graphics processing units with at least 960 cores. If the convection is driven stronger and stronger at fixed rotation rate, the flow behaves at some point as if it was not rotating. This transition shows in the scaling of the heat transport which can be used to distinguish slow from rapid rotation. One expects dynamos to behave differently in these two flow regimes. But even within the convection flows which are rapidly rotating according to this criterion, it will be shown that different types of dynamos exist. In one state, the magnetic field strength obeys a scaling indicative of a magnetostrophic balance, in which the Lorentz force is in equilibrium with the Coriolis force. The flow in this case is helical. A different state exists at higher magnetic Reynolds numbers, in which the magnetic energy obeys a different scaling law and the helicity of the flow is much reduced. As one increases the Rayleigh number, all other parameters kept constant, one may find both types of dynamos separated by an interval of Rayleigh numbers in which there are no dynamos at all. The effect of these transitions on energy dissipation and mean field generation have also been studied.
International Nuclear Information System (INIS)
Sawyer, Jordan; Wu, Yue; Zhang, Zhili; Adams, Steven F.
2013-01-01
Nonintrusive spatially resolved rotational temperature measurements in an atmospheric air microdischarge are presented. The measurements were based on coherent microwave Rayleigh scattering (Radar) from resonance-enhanced multiphoton ionization of molecular oxygen. The open air DC microdischarge source operated in a stable “normal-glow” mode and pin-to-pin electrodes spaced 1.3 mm apart. The second harmonic of a tunable dye laser beam was focused between the two electrodes and scanned between 286 and 288 nm. Coherent microwave Rayleigh scattering was used to collect the two-photon rotational spectra of O 2 at C 3 Π(v = 2)←X 3 Σ(v′ = 0) transitions. The Boltzmann plots from analyses of the O 2 rotational lines determined local rotational temperatures at various axial locations between the electrodes. The molecular oxygen rotational temperature varied from ∼1150 K to ∼1350 K within the discharge area. The measurements had an accuracy of ∼±50 K.
Sawyer, Jordan; Wu, Yue; Zhang, Zhili; Adams, Steven F.
2013-06-01
Nonintrusive spatially resolved rotational temperature measurements in an atmospheric air microdischarge are presented. The measurements were based on coherent microwave Rayleigh scattering (Radar) from resonance-enhanced multiphoton ionization of molecular oxygen. The open air DC microdischarge source operated in a stable "normal-glow" mode and pin-to-pin electrodes spaced 1.3 mm apart. The second harmonic of a tunable dye laser beam was focused between the two electrodes and scanned between 286 and 288 nm. Coherent microwave Rayleigh scattering was used to collect the two-photon rotational spectra of O2 at C3Π(v = 2)←X3Σ(v' = 0) transitions. The Boltzmann plots from analyses of the O2 rotational lines determined local rotational temperatures at various axial locations between the electrodes. The molecular oxygen rotational temperature varied from ˜1150 K to ˜1350 K within the discharge area. The measurements had an accuracy of ˜±50 K.
The Rayleigh-Taylor instability in inertial fusion, astrophysical plasma and flames
International Nuclear Information System (INIS)
Bychkov, V; Modestov, M; Akkerman, V; Eriksson, L-E
2007-01-01
Previous results are reviewed and new results are presented on the Rayleigh-Taylor instability in inertial confined fusion, flames and supernovae including gravitational and thermonuclear explosion mechanisms. The instability couples micro-scale plasma effects to large-scale hydrodynamic phenomena. In inertial fusion the instability reduces target compression. In supernovae the instability produces large-scale convection, which determines the fate of the star. The instability is often accompanied by mass flux through the unstable interface, which may have either a stabilizing or a destabilizing influence. Destabilization happens due to the Darrieus-Landau instability of a deflagration front. Still, it is unclear whether the instabilities lead to well-organized large-scale structures (bubbles) or to relatively isotropic turbulence (mixing layer)
Turbulent and neoclassical toroidal momentum transport in tokamak plasmas
International Nuclear Information System (INIS)
Abiteboul, J.
2012-10-01
The goal of magnetic confinement devices such as tokamaks is to produce energy from nuclear fusion reactions in plasmas at low densities and high temperatures. Experimentally, toroidal flows have been found to significantly improve the energy confinement, and therefore the performance of the machine. As extrinsic momentum sources will be limited in future fusion devices such as ITER, an understanding of the physics of toroidal momentum transport and the generation of intrinsic toroidal rotation in tokamaks would be an important step in order to predict the rotation profile in experiments. Among the mechanisms expected to contribute to the generation of toroidal rotation is the transport of momentum by electrostatic turbulence, which governs heat transport in tokamaks. Due to the low collisionality of the plasma, kinetic modeling is mandatory for the study of tokamak turbulence. In principle, this implies the modeling of a six-dimensional distribution function representing the density of particles in position and velocity phase-space, which can be reduced to five dimensions when considering only frequencies below the particle cyclotron frequency. This approximation, relevant for the study of turbulence in tokamaks, leads to the so-called gyrokinetic model and brings the computational cost of the model within the presently available numerical resources. In this work, we study the transport of toroidal momentum in tokamaks in the framework of the gyrokinetic model. First, we show that this reduced model is indeed capable of accurately modeling momentum transport by deriving a local conservation equation of toroidal momentum, and verifying it numerically with the gyrokinetic code GYSELA. Secondly, we show how electrostatic turbulence can break the axisymmetry and generate toroidal rotation, while a strong link between turbulent heat and momentum transport is identified, as both exhibit the same large-scale avalanche-like events. The dynamics of turbulent transport are
Heat-flux enhancement by vapour-bubble nucleation in Rayleigh-Bénard turbulence
Narezo Guzman, Daniela; Xie, Yanbo; Chen, S.; Fernandez Rivas, David; Sun, Chao; Lohse, Detlef; Ahlers, Günter
2016-01-01
We report on the enhancement of turbulent convective heat transport due to vapour-bubble nucleation at the bottom plate of a cylindrical Rayleigh–Bénard sample (aspect ratio 1.00, diameter 8.8 cm) filled with liquid. Microcavities acted as nucleation sites, allowing for well-controlled bubble
Quantifying near-wall coherent structures in turbulent convection
Gunasegarane, G. S.; A Puthenveettil, Baburaj; K Agrawal, Yogesh; Schmeling, Daniel; Bosbach, Johannes; Arakeri, Jaywant; IIT Madras-DLR-IISc Collaboration
2011-11-01
We present planforms of line plumes formed on horizontal surfaces in turbulent convection, along with the length of near- wall line plumes measured from these planforms, in a six decade range of Rayleigh numbers (105 < Ra <1011) and at three Prandtl numbers (Pr = 0 . 7 , 6 , 602). Using geometric constraints on the relations for the mean plume spacings, we obtain expressions for the total length of these near-wall plumes in turbulent convection. The plume length per unit area (Lp / A), made dimensionless by the near-wall length scale in turbulent convection (Zw) remains a constant for a given fluid. The Nusselt number is shown to be directly proportional to Lp H / A for a given fluid layer of height H. Increase in Pr has a weak influence in decreasing Lp / A . These expressions match the measurements, thereby showing that the assumption of laminar natural convection boundary layers in turbulent convection is consistent with the observed total length of line plumes. We then show that similar relationships are obtained based on the assumption that the line plumes are the outcome of the instability of laminar natural convection boundary layers on the horizontal surfaces.
Supercritical Quasi-Conduction States in Stochastic Rayleigh-Benard Convection
2011-09-15
is 10 (see table 1). The sensitivity (in the sense of Sobol [39]) of the integrated Nusselt number with respect to the amplitude of the boundary...using a multi-element quadrature formula [32]. Following Sobol [39], we shall define global sensitivity indices as the ratio between the variance of...39] I. M. Sobol , Global sensitivity indices for nonlinear mathematical models and their monte carlo estimates, Math. Comput. Simul. 55 (2001) 271
What is the contribution of scattering to the Love-to-Rayleigh ratio in ambient microseismic noise?
Ziane, D.; Hadziioannou, C.
2015-12-01
Several observations show the existence of both Rayleigh and Love waves in the secondary microseism. While the Rayleigh wave excitation is well described by Longuet-Higgins, the process responsible for Love wave generation still needs further investigation. Several different mechanisms could excite Love waves in this frequency band: broadly speaking, we can differentiate between source effects, like pressure variations on the oblique sea floor, or internal effects in the medium along the propagation path, such as scattering and conversions. Here we will focus on the internal effects. We perform single scattering tests in 2D and 3D to gain a better understanding of the scattering radiation pattern and the conversion between P, S, Rayleigh and Love waves. Furthermore, we use random media with continuous variations of the elastic parameters to create a scattering regime similar to the Earths interior, e.g. Gaussian or von Karmann correlation functions. The aim is to explore the contribution of scattering along the propagation path to the observed Love to Rayleigh wave energy ratios, assuming a purely vertical force source mechanism. We use finite different solvers to calculate the synthetic seismograms, and to separate the different wave types we measure the rotational and divergent components of the wave field.
Spectral analysis of the turbulent mixing of two fluids
Energy Technology Data Exchange (ETDEWEB)
Steinkamp, M.J.
1996-02-01
The authors describe a spectral approach to the investigation of fluid instability, generalized turbulence, and the interpenetration of fluids across an interface. The technique also applies to a single fluid with large variations in density. Departures of fluctuating velocity components from the local mean are far subsonic, but the mean Mach number can be large. Validity of the description is demonstrated by comparisons with experiments on turbulent mixing due to the late stages of Rayleigh-Taylor instability, when the dynamics become approximately self-similar in response to a constant body force. Generic forms for anisotropic spectral structure are described and used as a basis for deriving spectrally integrated moment equations that can be incorporated into computer codes for scientific and engineering analyses.
Study of scattering from turbulence structure generated by propeller with FLUENT
Luo, Gen
2017-07-01
In this article, the turbulence structure generated by a propeller is simulated with the computational fluid dynamics (CFD) software FLUENT. With the method of moments, the backscattering radar cross sections (RCS) of the turbulence structure are calculated. The scattering results can reflect the turbulent intensity of the wave profiles. For the wake turbulence with low rotating speed, the scattering intensity of HH polarization is much smaller than VV polarization at large incident angles. When the turbulence becomes stronger with high rotating speed, the scattering intensity of HH polarization also becomes stronger at large incident angles, which is almost the same with VV polarization. And also, the bistatic scattering of the turbulence structure has the similar situation. These scattering results indicate that the turbulence structure can also give rise to an anomaly compared with traditional sea surface. The study of electromagnetic (EM) scattering from turbulence structure generated by the propeller can help in better understanding of the scattering from different kinds of waves and provide more bases to explain the anomalies of EM scattering from sea surfaces.
Energy Technology Data Exchange (ETDEWEB)
Boughanem, H.
1998-03-24
The assumption of gradient transport for the mean reaction progress variable has a limited domain of validity in premixed turbulent combustion. The existence of two turbulent transport regimes, gradient and counter-gradient, is demonstrated in the present work using Direct Numerical Simulations (DNS) of plane flame configurations. The DNS data base describes the influence of the heat release factor, of the turbulence-to-flame velocity ratio, and of an external pressure gradient. The simulations reveal a strong correlation between the regime of turbulent transport and the turbulent flame speed and turbulent flame thickness. These effects re not well described by current turbulent combustion models. A conditional approach `fresh gases / burnt gases` is proposed to overcome these difficulties. Furthermore, he development of flame instabilities in turbulent configurations is also observed in the simulations. A criterion is derived that determines the domain of occurrence of these instabilities (Darrieus- Landau instabilities, Rayleigh- Taylor instabilities, thermo-diffusive instabilities). This criterion suggests that the domain of occurrence of flame instabilities is not limited to small Reynolds numbers. (author) 98 refs.
International Nuclear Information System (INIS)
Brochard, F.; Gravier, E.; Bonhomme, G.
2006-01-01
The spatiotemporal transition scenario of flute instabilities from a regular to a turbulent state is experimentally investigated in the low-β plasma column of a thermionic discharge. The same transition scenario, i.e., the Ruelle-Takens route to turbulence, is found for both the Kelvin-Helmholtz and the Rayleigh-Taylor instabilities. It is demonstrated that the transition can be more or less smooth, according to the discharge mode. In both cases, a strong radial dependence is observed, which is linked to the velocity shear layer in the case of the Kelvin-Helmholtz instability
PDF turbulence modeling and DNS
Hsu, A. T.
1992-01-01
The problem of time discontinuity (or jump condition) in the coalescence/dispersion (C/D) mixing model is addressed in probability density function (pdf). A C/D mixing model continuous in time is introduced. With the continuous mixing model, the process of chemical reaction can be fully coupled with mixing. In the case of homogeneous turbulence decay, the new model predicts a pdf very close to a Gaussian distribution, with finite higher moments also close to that of a Gaussian distribution. Results from the continuous mixing model are compared with both experimental data and numerical results from conventional C/D models. The effect of Coriolis forces on compressible homogeneous turbulence is studied using direct numerical simulation (DNS). The numerical method used in this study is an eight order compact difference scheme. Contrary to the conclusions reached by previous DNS studies on incompressible isotropic turbulence, the present results show that the Coriolis force increases the dissipation rate of turbulent kinetic energy, and that anisotropy develops as the Coriolis force increases. The Taylor-Proudman theory does apply since the derivatives in the direction of the rotation axis vanishes rapidly. A closer analysis reveals that the dissipation rate of the incompressible component of the turbulent kinetic energy indeed decreases with a higher rotation rate, consistent with incompressible flow simulations (Bardina), while the dissipation rate of the compressible part increases; the net gain is positive. Inertial waves are observed in the simulation results.
Ivers, D. J.; Phillips, C. G.
2018-03-01
We re-consider the plate-like model of turbulence in the Earth's core, proposed by Braginsky and Meytlis (1990), and show that it is plausible for core parameters not only in polar regions but extends to mid- and low-latitudes where rotation and gravity are not parallel, except in a very thin equatorial layer. In this model the turbulence is highly anisotropic with preferred directions imposed by the Earth's rotation and the magnetic field. Current geodynamo computations effectively model sub-grid scale turbulence by using isotropic viscous and thermal diffusion values significantly greater than the molecular values of the Earth's core. We consider a local turbulent dynamo model for the Earth's core in which the mean magnetic field, velocity and temperature satisfy the Boussinesq induction, momentum and heat equations with an isotropic turbulent Ekman number and Roberts number. The anisotropy is modelled only in the thermal diffusion tensor with the Earth's rotation and magnetic field as preferred directions. Nonlocal organising effects of gravity and rotation (but not aspect ratio in the Earth's core) such as an inverse cascade and nonlocal transport are assumed to occur at longer length scales, which computations may accurately capture with sufficient resolution. To investigate the implications of this anisotropy for the proposed turbulent dynamo model we investigate the linear instability of turbulent magnetoconvection on length scales longer than the background turbulence in a rotating sphere with electrically insulating exterior for no-slip and isothermal boundary conditions. The equations are linearised about an axisymmetric basic state with a conductive temperature, azimuthal magnetic field and differential rotation. The basic state temperature is a function of the anisotropy and the spherical radius. Elsasser numbers in the range 1-20 and turbulent Roberts numbers 0.01-1 are considered for both equatorial symmetries of the magnetic basic state. It is found
Rayleigh-Taylor mixing in supernova experiments
International Nuclear Information System (INIS)
Swisher, N. C.; Abarzhi, S. I.; Kuranz, C. C.; Arnett, D.; Hurricane, O.; Remington, B. A.; Robey, H. F.
2015-01-01
We report a scrupulous analysis of data in supernova experiments that are conducted at high power laser facilities in order to study core-collapse supernova SN1987A. Parameters of the experimental system are properly scaled to investigate the interaction of a blast-wave with helium-hydrogen interface, and the induced Rayleigh-Taylor instability and Rayleigh-Taylor mixing of the denser and lighter fluids with time-dependent acceleration. We analyze all available experimental images of the Rayleigh-Taylor flow in supernova experiments and measure delicate features of the interfacial dynamics. A new scaling is identified for calibration of experimental data to enable their accurate analysis and comparisons. By properly accounting for the imprint of the experimental conditions, the data set size and statistics are substantially increased. New theoretical solutions are reported to describe asymptotic dynamics of Rayleigh-Taylor flow with time-dependent acceleration by applying theoretical analysis that considers symmetries and momentum transport. Good qualitative and quantitative agreement is achieved of the experimental data with the theory and simulations. Our study indicates that in supernova experiments Rayleigh-Taylor flow is in the mixing regime, the interface amplitude contributes substantially to the characteristic length scale for energy dissipation; Rayleigh-Taylor mixing keeps order
Nonlinear Flow Generation By Electrostatic Turbulence In Tokamaks
International Nuclear Information System (INIS)
Wang, W.X.; Diamond, P.H.; Hahm, T.S.; Ethier, S.; Rewoldt, G.; Tang, W.M.
2010-01-01
Global gyrokinetic simulations have revealed an important nonlinear flow generation process due to the residual stress produced by electrostatic turbulence of ion temperature gradient (ITG) modes and trapped electron modes (TEM). In collisionless TEM (CTEM) turbulence, nonlinear residual stress generation by both the fluctuation intensity and the intensity gradient in the presence of broken symmetry in the parallel wave number spectrum is identified for the first time. Concerning the origin of the symmetry breaking, turbulence self-generated low frequency zonal flow shear has been identified to be a key, universal mechanism in various turbulence regimes. Simulations reported here also indicate the existence of other mechanisms beyond E - B shear. The ITG turbulence driven 'intrinsic' torque associated with residual stress is shown to increase close to linearly with the ion temperature gradient, in qualitative agreement with experimental observations in various devices. In CTEM dominated regimes, a net toroidal rotation is driven in the cocurrent direction by 'intrinsic' torque, consistent with the experimental trend of observed intrinsic rotation. The finding of a 'flow pinch' in CTEM turbulence may offer an interesting new insight into the underlying dynamics governing the radial penetration of modulated flows in perturbation experiments. Finally, simulations also reveal highly distinct phase space structures between CTEM and ITG turbulence driven momentum, energy and particle fluxes, elucidating the roles of resonant and non-resonant particles.
Plume dynamics in quasi-2D turbulent convection
International Nuclear Information System (INIS)
Bizon, C.; Werne, J.; Predtechensky, A.A.; Julien, K.; McCormick, W.D.; Swift, J.B.; Swinney, H.L.
1997-01-01
We have studied turbulent convection in a vertical thin (Hele-Shaw) cell at very high Rayleigh numbers (up to 7x10 4 times the value for convective onset) through experiment, simulation, and analysis. Experimentally, convection is driven by an imposed concentration gradient in an isothermal cell. Model equations treat the fields in two dimensions, with the reduced dimension exerting its influence through a linear wall friction. Linear stability analysis of these equations demonstrates that as the thickness of the cell tends to zero, the critical Rayleigh number and wave number for convective onset do not depend on the velocity conditions at the top and bottom boundaries (i.e., no-slip or stress-free). At finite cell thickness δ, however, solutions with different boundary conditions behave differently. We simulate the model equations numerically for both types of boundary conditions. Time sequences of the full concentration fields from experiment and simulation display a large number of solutal plumes that are born in thin concentration boundary layers, merge to form vertical channels, and sometimes split at their tips via a Rayleigh-Taylor instability. Power spectra of the concentration field reveal scaling regions with slopes that depend on the Rayleigh number. We examine the scaling of nondimensional heat flux (the Nusselt number, Nu) and rms vertical velocity (the Pacute eclet number, Pe) with the Rayleigh number (Ra * ) for the simulations. Both no-slip and stress-free solutions exhibit the scaling NuRa * ∼Pe 2 that we develop from simple arguments involving dynamics in the interior, away from cell boundaries. In addition, for stress-free solutions a second relation, Nu∼√(nPe), is dictated by stagnation-point flows occurring at the horizontal boundaries; n is the number of plumes per unit length. (Abstract Truncated)
Energy spectrum of buoyancy-driven turbulence
Kumar, Abhishek
2014-08-25
Using high-resolution direct numerical simulation and arguments based on the kinetic energy flux Πu, we demonstrate that, for stably stratified flows, the kinetic energy spectrum Eu(k)∼k-11/5, the potential energy spectrum Eθ(k)∼k-7/5, and Πu(k)∼k-4/5 are consistent with the Bolgiano-Obukhov scaling. This scaling arises due to the conversion of kinetic energy to the potential energy by buoyancy. For weaker buoyancy, this conversion is weak, hence Eu(k) follows Kolmogorov\\'s spectrum with a constant energy flux. For Rayleigh-Bénard convection, we show that the energy supply rate by buoyancy is positive, which leads to an increasing Πu(k) with k, thus ruling out Bolgiano-Obukhov scaling for the convective turbulence. Our numerical results show that convective turbulence for unit Prandt number exhibits a constant Πu(k) and Eu(k)∼k-5/3 for a narrow band of wave numbers. © 2014 American Physical Society.
Importance sampling the Rayleigh phase function
DEFF Research Database (Denmark)
Frisvad, Jeppe Revall
2011-01-01
Rayleigh scattering is used frequently in Monte Carlo simulation of multiple scattering. The Rayleigh phase function is quite simple, and one might expect that it should be simple to importance sample it efficiently. However, there seems to be no one good way of sampling it in the literature....... This paper provides the details of several different techniques for importance sampling the Rayleigh phase function, and it includes a comparison of their performance as well as hints toward efficient implementation....
Computational simulation of turbulent natural convection in a volumetrically heated square cavity
International Nuclear Information System (INIS)
Vieira, Camila Braga; Su, Jian; Niceno, Bojan
2012-01-01
This work aims to analyze the turbulent natural convection in a volumetrically heated fluid with similar characteristics of an oxide layer of a molten core in the lower head of the pressure vessel. The simulations were carried out in a square cavity with isothermal walls, for Rayleigh numbers (Ra) ranging from 10 9 to 10 11 . Different turbulence models based on Reynolds Averaged Navier-Stokes equations were studied, such as the standard k - ε, low-Reynolds-k - ε, and Shear Stress Transport (SST), using the open-source Computational Fluid Dynamics (CFD) code - Open FOAM (Open Field Operation and Manipulation). The results of the three turbulence models were compared versus the results of experimental correlations and other authors’ simulations, and the conclusion was that the most promising model proves to be the SST, due to its accuracy and robustness. (author)
Magneto-rotational instability in differentially rotating liquid metals
International Nuclear Information System (INIS)
Velikhov, E.P.; Ivanov, A.A.; Lakhin, V.P.; Serebrennikov, K.S.
2006-01-01
We study the stability of Couette flow between two cylinders in the presence of axial magnetic field in local WKB approximation. We find the analytical expression of the critical angular velocity minimized over the wave number and the imposed magnetic field as a function of the measure of deviation of the rotation law from the Rayleigh line. The result found is in a good agreement with the previously known numerical results based on the global analysis. We perform a minimization of the critical Reynolds number over the wave number at fixed magnetic field both analytically and numerically. We show that a compromise between resistive suppression of magneto-rotational instability at weak magnetic field and the increase of the critical Reynolds number with the increase of magnetic field is possible. It takes place at moderate values of magnetic field of order 3x10 2 gauss giving the critical Reynolds number of order 4x10 4
International Nuclear Information System (INIS)
Boudjemadi, R.
1996-03-01
The main objectives of this thesis are the direct numerical simulation of natural convection in a vertical differentially heated slot and the improvements of second-order turbulence modelling. A three-dimensional direct numerical simulation code has been developed in order to gain a better understanding of turbulence properties in natural convection flows. This code has been validated in several physical configurations: non-stratified natural convection flows (conduction solution), stratified natural convection flows (double boundary layer solution), transitional and turbulent Poiseuille flows. For the conduction solution, the turbulent regime was reached at a Rayleigh number of 1*10 5 and 5.4*10 5 . A detailed analysis of these results has revealed the principal qualities of the available models but has also pointed our their shortcomings. This data base has been used in order to improve the triple correlations transport models and to select the turbulent time scales suitable for such flows. (author). 122 refs., figs., tabs., 4 appends
Sbaa, Sarah; Hollender, Fabrice; Perron, Vincent; Imtiaz, Afifa; Bard, Pierre-Yves; Mariscal, Armand; Cochard, Alain; Dujardin, Alain
2017-09-01
Although rotational seismology has progressed in recent decades, the links between rotational ground motion and site soil conditions are poorly documented. New experiments were performed on Kefalonia Island (Greece) following two large earthquakes ( M W = 6.0, M W = 5.9) in early 2014 on two well-characterized sites (soft soil, V S30 250 m/s; rock, V S30 830 m/s, V S30 being harmonic average shear-wave velocity between 0 and 30 m depth). These earthquakes led to large six-component (three translations and three rotations) datasets of hundreds of well-recorded events. The relationship between peak translational acceleration versus peak rotational velocity is found sensitive to the site conditions mainly for the rotation around the vertical axis (torsion; dominated by Love waves): the stiffer the soil, the lower the torsion, for a given level of translational acceleration. For rotation around the horizontal axes (rocking; dominated by Rayleigh waves), this acceleration/rotation relationship exhibits much weaker differences between soft and rock sites. Using only the rotation sensor, an estimate of the Love-to-Rayleigh energy ratios could be carried out and provided the same results as previous studies that have analyzed the Love- and Rayleigh-wave energy proportions using data from translational arrays deployed at the same two sites. The coupling of translational and rotational measurements appears to be useful, not only for direct applications of engineering seismology, but also to investigate the composition of the wavefield, while avoiding deployment of dense arrays. The availability of new, low-noise rotation sensors that are easy to deploy in the field is of great interest and should extend the use of rotation sensors and expand their possible applications.[Figure not available: see fulltext.
Shishkina, Olga; Wagner, Sebastian; Horn, Susanne
2014-03-01
We derive the asymptotes for the ratio of the thermal to viscous boundary layer thicknesses for infinite and infinitesimal Prandtl numbers Pr as functions of the angle β between the large-scale circulation and an isothermal heated or cooled surface for the case of turbulent thermal convection with laminar-like boundary layers. For this purpose, we apply the Falkner-Skan ansatz, which is a generalization of the Prandtl-Blasius one to a nonhorizontal free-stream flow above the viscous boundary layer. Based on our direct numerical simulations (DNS) of turbulent Rayleigh-Bénard convection for Pr=0.1, 1, and 10 and moderate Rayleigh numbers up to 108 we evaluate the value of β that is found to be around 0.7π for all investigated cases. Our theoretical predictions for the boundary layer thicknesses for this β and the considered Pr are in good agreement with the DNS results.
Investigation of intrinsic toroidal rotation scaling in KSTAR
Yoo, J. W.; Lee, S. G.; Ko, S. H.; Seol, J.; Lee, H. H.; Kim, J. H.
2017-07-01
The behaviors of an intrinsic toroidal rotation without any external momentum sources are investigated in KSTAR. In these experiments, pure ohmic discharges with a wide range of plasma parameters are carefully selected and analyzed to speculate an unrevealed origin of toroidal rotation excluding any unnecessary heating sources, magnetic perturbations, and strong magneto-hydrodynamic activities. The measured core toroidal rotation in KSTAR is mostly in the counter-current direction and its magnitude strongly depends on the ion temperature divided by plasma current (Ti/IP). Especially the core toroidal rotation in the steady-state is well fitted by Ti/IP scaling with a slope of ˜-23, and the possible explanation of the scaling is compared with various candidates. As a result, the calculated offset rotation could not explain the measured core toroidal rotation since KSTAR has an extremely low intrinsic error field. For the stability conditions for ion and electron turbulences, it is hard to determine a dominant turbulence mode in this study. In addition, the intrinsic toroidal rotation level in ITER is estimated based on the KSTAR scaling since the intrinsic rotation plays an important role in stabilizing resistive wall modes for future reference.
Mixed convection between horizontal plates and consequences for chemical vapor deposition flows
International Nuclear Information System (INIS)
Chiu, K.C.
1986-01-01
To simulate the fluid dynamics of VD systems, mixed convection between horizontal plates (AR = width/height = 10) heated from below was studied by laser Doppler anemometry in a range 1368 < Ra < 8300 and 15 < R3 < 170. The entrance effects were characterized by two lengths: one for the onset of bouyancy-driven instability, and one for the full development of longitudinal convection rolls. Explicit expressions for both entrance lengths are given in terms of Ra and Re. In addition, unsteady longitudinal convection rolls were observed. These are discussed in terms of the admixture of transverse convection rolls and/or contributions from upstream turbulence. For the fully developed region it is shown analytically that the transverse velocities of the longitudinal convection rolls, v and w, are independent of the forced flow and are identical to those of the two-dimensional Rayleigh-Benard convection rolls. These fundamental results serve as a base for the discussion of horizontal CVD flows. The entrance and sidewall effects are found to have pronounced influences on the flow patterns observed in CVD (AR = 2) reactors
Magnetostrophic Rotating Magnetoconvection
King, Eric; Aurnou, Jonathan
2016-11-01
Planetary magnetic fields are generated by turbulent convection within their vast interior liquid metal cores. Although direct observation is not possible, this liquid metal circulation is thought to be dominated by the controlling influences of Coriolis and Lorentz forces. Theory famously predicts that local-scale convection naturally settles into the so-called magnetostrophic state, where the Coriolis and Lorentz forces partially cancel, and convection is optimally efficient. To date, no laboratory experiments have reached the magnetostrophic regime in turbulent liquid metal convection. Furthermore, computational dynamo simulations have as yet failed to produce a globally magnetostrophic dynamo, which has led some to question the existence of the magnetostrophic state. Here, we present results from the first turbulent magnetostrophic rotating magnetoconvection experiments using the liquid metal gallium. We find that turbulent convection in the magnetostrophic regime is, in fact, maximally efficient. The experimental results clarify these previously disparate results, suggesting that the fluid dynamics saturate in magnetostrophic balance within turbulent liquid metal, planetary cores. The authors thank the NSF Geophysics Program for financial support.
RADIO POLARIMETRY SIGNATURES OF STRONG MAGNETIC TURBULENCE IN SUPERNOVA REMNANTS
International Nuclear Information System (INIS)
Stroman, Wendy; Pohl, Martin
2009-01-01
We discuss the emission and transport of polarized radio-band synchrotron radiation near the forward shocks of young shell-type supernova remnants, for which X-ray data indicate a strong amplification of turbulent magnetic field. Modeling the magnetic turbulence through the superposition of waves, we calculate the degree of polarization and the magnetic polarization direction which is at 90 deg. to the conventional electric polarization direction. We find that isotropic strong turbulence will produce weakly polarized radio emission even in the absence of internal Faraday rotation. If anisotropy is imposed on the magnetic-field structure, the degree of polarization can be significantly increased, provided internal Faraday rotation is inefficient. Both for shock compression and a mixture with a homogeneous field, the increase in polarization degree goes along with a fairly precise alignment of the magnetic-polarization angle with the direction of the dominant magnetic-field component, implying tangential magnetic polarization at the rims in the case of shock compression. We compare our model with high-resolution radio polarimetry data of Tycho's remnant. Using the absence of internal Faraday rotation we find a soft limit for the amplitude of magnetic turbulence, δB ∼ 0 . An alternative viable scenario involves anisotropic turbulence with stronger amplitudes in the radial direction, as was observed in recent Magnetohydrodynamics simulations of shocks propagating through a medium with significant density fluctuations.
CISM Course on Rotating Fluids
1992-01-01
The volume presents a comprehensive overview of rotation effects on fluid behavior, emphasizing non-linear processes. The subject is introduced by giving a range of examples of rotating fluids encountered in geophysics and engineering. This is then followed by a discussion of the relevant scales and parameters of rotating flow, and an introduction to geostrophic balance and vorticity concepts. There are few books on rotating fluids and this volume is, therefore, a welcome addition. It is the first volume which contains a unified view of turbulence in rotating fluids, instability and vortex dynamics. Some aspects of wave motions covered here are not found elsewhere.
Turbulent wind at the equatorial segment of an operating Darrieus wind turbine blade
Connell, J. R.; Morris, V. R.
1989-09-01
Six turbulent wind time series, measured at equally spaced equator-height locations on a circle 3 m outside a 34-m Darrieus rotor, are analyzed to approximate the wind fluctuations experienced by the rotor. The flatwise lower root-bending stress of one blade was concurrently recorded. The wind data are analyzed in three ways: wind components that are radial and tangential to the rotation of a blade were rotationally sampled; induction and wake effects of the rotor were estimated from the six Eulerian time series; and turbulence spectra of both the measured wind and the modeled wind from the PNL theory of rotationally sampled turbulence. The wind and the rotor response are related by computing the spectral response function of the flatwise lower root-bending stress. Two bands of resonant response that surround the first and second flatwise modal frequencies shift with the rotor rotation rate.
Recent results in Rayleigh scattering
International Nuclear Information System (INIS)
Kahane, S.; Shahal, O.; Moreh, R.; Ben-Gurion Univ. of the Negev, Beer-Sheva
1997-01-01
New measurements of Rayleigh scattering, employing neutron capture γ rays are presented. Experimental conditions are achieved such that the Rayleigh contribution is dominant and much larger than other competing coherent process. A detailed comparison with the modified relativistic form factor approximation (MRFF) is made. It is found that MRFF overestimates the true cross sections by 3-4%. (author)
The comparative analysis of the different mechanisms of toroidal rotation in tokamaks
Energy Technology Data Exchange (ETDEWEB)
Sabot, R [Association Euratom-CEA, Centre d` Etudes Nucleaires de Cadarache, 13 - Saint-Paul-lez-Durance (France). Dept. de Recherches sur la Fusion Controlee; Parail, V [Kurchatov Institute, Moscow (Russian Federation)
1994-07-01
The toroidal plasma rotation appears as one the possible mechanism for suppression of plasma turbulence. Several mechanisms are believed to contribute to the toroidal plasma rotation. The results of numerical simulation of the toroidal rotation on JET are presented, where are taken into consideration the following effects: the neoclassical viscosity due to banana and ripple trapped particles, the anomalous viscosity due to plasma turbulence, the momentum input by NBI (neutron beam injection) and ion momentum loss near the separatrix due to prompt ion losses. The NBI appeared to be the principal source of toroidal plasma rotation. 6 refs., 2 figs.
Directory of Open Access Journals (Sweden)
Xingtuan Yang
2015-01-01
Full Text Available This study investigates the anisotropic characteristics of turbulent energy dissipation rate in a rotating jet flow via direct numerical simulation. The turbulent energy dissipation tensor, including its eigenvalues in the swirling flows with different rotating velocities, is analyzed to investigate the anisotropic characteristics of turbulence and dissipation. In addition, the probability density function of the eigenvalues of turbulence dissipation tensor is presented. The isotropic subrange of PDF always exists in swirling flows relevant to small-scale vortex structure. Thus, with remarkable large-scale vortex breakdown, the isotropic subrange of PDF is reduced in strongly swirling flows, and anisotropic energy dissipation is proven to exist in the core region of the vortex breakdown. More specifically, strong anisotropic turbulence dissipation occurs concentratively in the vortex breakdown region, whereas nearly isotropic turbulence dissipation occurs dispersively in the peripheral region of the strong swirling flows.
Short Rayleigh Length Free Electron Lasers
Crooker, P P; Armstead, R L; Blau, J
2004-01-01
Conventional free electron laser (FEL) oscillators minimize the optical mode volume around the electron beam in the undulator by making the resonator Rayleigh length about one third of the undulator length. This maximizes gain and beam-mode coupling. In compact configurations of high-power infrared FELs or moderate power UV FELs, the resulting optical intensity can damage the resonator mirrors. To increase the spot size and thereby reduce the optical intensity at the mirrors below the damage threshold, a shorter Rayleigh length can be used, but the FEL interaction is significantly altered. A new FEL interaction is described and analyzed with a Rayleigh length that is only one tenth the undulator length, or less. The effect of mirror vibration and positioning are more critical in the short Rayleigh length design, but we find that they are still within normal design tolerances.
International Nuclear Information System (INIS)
Puragliesi, R.; Dehbi, A.; Leriche, E.; Soldati, A.; Deville, M.O.
2011-01-01
Highlights: → 2D study of micro-size particle depletion driven by chaotic natural convective flows in square domains. → Description of velocity and temperature first and second moments with changing in the Rayleigh number. → Strong decoupling between the turbulent kinetic energy and the dissipation rate. → Particle recirculation sustained by the vertical hot boundary layer. → Deposition mostly induced by gravity, thermophoretic and lift forces are negligible. - Abstract: In this work we investigate numerically particle deposition in the buoyancy driven flow of the differentially heated cavity (DHC). We consider two values of the Rayleigh number (Ra = 10 9 , 10 10 ) and three values of the particle diameter (d p = 15, 25, 35 [μm]). We consider the cavity filled with air and particles with the same density of water ρ w = 1000 [kg/m 3 ] (aerosol). We use direct numerical simulations (DNS) for the continuous phase, and we solve transient Navier-Stokes and energy transport equations written in an Eulerian framework, under the Boussinesq approximation, for the viscous incompressible Newtonian fluid with constant Prandtl number (Pr = 0.71). First- and second-order statistics are presented for the continuous phase as well as important quantities like turbulent kinetic energy (TKE) and temperature variance with the associated production and dissipation fields. The TKE production shows different behaviour at the two Rayleigh numbers. The Lagrangian approach has been chosen for the dispersed phase description. The forces taken into account are drag, gravity, buoyancy, lift and thermophoresis. A first incursion in the sedimentation mechanisms is presented. Current results indicate that the largest contribution to particle deposition is caused by gravitational settling, but a strong recirculating zone, which liftoffs and segregates particles, contributes to decrease settling. Deposition takes place mostly at the bottom wall. The influence of lift and thermophoretic
Energy Technology Data Exchange (ETDEWEB)
Puragliesi, R., E-mail: riccardo.puragliesi@psi.ch [Nuclear Energy and Safety Research Department, Paul Scherrer Institut, CH-5232 Villigen PSI (Switzerland); Laboratoire d' Ingenierie Numerique, Ecole Polytechnique Federale de Lausanne, Station 9, CH-1015 Lausanne (Switzerland); Dehbi, A., E-mail: abdel.dehbi@psi.ch [Nuclear Energy and Safety Research Department, Paul Scherrer Institut, CH-5232 Villigen PSI (Switzerland); Leriche, E., E-mail: emmanuel.leriche@univ-st-etienne.fr [Universite de Lyon, F-42023 Saint-Etienne, LMFA-UJM St-Etienne, CNRS UMR 5509 Universite de St-Etienne, 23 rue Docteur Paul Michelon, F-42023 Saint-Etienne (France); Soldati, A., E-mail: soldati@uniud.it [Dipartimento di Energetica e Macchine, Universita di Udine, Via delle Scienze 208, IT-33100 Udine (Italy); Deville, M.O., E-mail: michel.deville@epfl.ch [Laboratoire d' Ingenierie Numerique, Ecole Polytechnique Federale de Lausanne, Station 9, CH-1015 Lausanne (Switzerland)
2011-10-15
Highlights: > 2D study of micro-size particle depletion driven by chaotic natural convective flows in square domains. > Description of velocity and temperature first and second moments with changing in the Rayleigh number. > Strong decoupling between the turbulent kinetic energy and the dissipation rate. > Particle recirculation sustained by the vertical hot boundary layer. > Deposition mostly induced by gravity, thermophoretic and lift forces are negligible. - Abstract: In this work we investigate numerically particle deposition in the buoyancy driven flow of the differentially heated cavity (DHC). We consider two values of the Rayleigh number (Ra = 10{sup 9}, 10{sup 10}) and three values of the particle diameter (d{sub p} = 15, 25, 35 [{mu}m]). We consider the cavity filled with air and particles with the same density of water {rho}{sub w} = 1000 [kg/m{sup 3}] (aerosol). We use direct numerical simulations (DNS) for the continuous phase, and we solve transient Navier-Stokes and energy transport equations written in an Eulerian framework, under the Boussinesq approximation, for the viscous incompressible Newtonian fluid with constant Prandtl number (Pr = 0.71). First- and second-order statistics are presented for the continuous phase as well as important quantities like turbulent kinetic energy (TKE) and temperature variance with the associated production and dissipation fields. The TKE production shows different behaviour at the two Rayleigh numbers. The Lagrangian approach has been chosen for the dispersed phase description. The forces taken into account are drag, gravity, buoyancy, lift and thermophoresis. A first incursion in the sedimentation mechanisms is presented. Current results indicate that the largest contribution to particle deposition is caused by gravitational settling, but a strong recirculating zone, which liftoffs and segregates particles, contributes to decrease settling. Deposition takes place mostly at the bottom wall. The influence of lift
On the role of turbulence on momentum redistribution in fusion devices
International Nuclear Information System (INIS)
Hidalgo, C.
2005-01-01
The mechanisms underlying the generation of plasma flows play a crucial role in understanding key issues on transport in magnetically confined plasmas. It is well known the importance of driving shear in plasma rotation in the development of transport barriers. Rotation can be driven by external forces such as momentum from Neutral Beam Injection (NBI). However, in large scale devices like ITER (where the available NBI power is limited and the energy of injected neutrals must be high to reach the core plasma region) the NBI driven rotation will be limited. Then, it is important to study the possible role of other mechanisms which can drive plasma rotation. The amplitude of parallel flow measured in the scrape-off layer (SOL) is significantly larger than those resulting from simulations. Recent experiments have pointed out the possible influence of turbulence in explaining a component of the anomalous flows observed in the plasma boundary region. In the plasma core region, evidence of anomalous toroidal momentum transport has been reported. Different mechanisms have been proposed to explain these results, including neoclassical effects, turbulence driven models and fast particle effects. The response of toroidal rotation to near-perpendicular NB injection on JT-60U has been interpreted on the basis of the influence of loss of high-energy particles. The flow reversal observed in the CHS stellarator can be explained by the spontaneous flow driven by large radial electric fields. Neoclassical effects can also play an import role. Recent experiments in the TJ-II stellarator have shown that the generation of spontaneous perpendicular sheared flows requires a minimum plasma density. Near this critical density, the level of edge turbulent transport and the turbulent kinetic energy significantly increases in the plasma edge. Experimental results also show significant turbulent parallel forces at plasma densities above the threshold value to trigger perpendicular ExB sheared
TURBULENT DISKS ARE NEVER STABLE: FRAGMENTATION AND TURBULENCE-PROMOTED PLANET FORMATION
Energy Technology Data Exchange (ETDEWEB)
Hopkins, Philip F. [TAPIR, Mailcode 350-17, California Institute of Technology, Pasadena, CA 91125 (United States); Christiansen, Jessie L., E-mail: phopkins@caltech.edu [SETI Institute/NASA Ames Research Center, M/S 244-30, Moffett Field, CA 94035 (United States)
2013-10-10
A fundamental assumption in our understanding of disks is that when the Toomre Q >> 1, the disk is stable against fragmentation into self-gravitating objects (and so cannot form planets via direct collapse). But if disks are turbulent, this neglects a spectrum of stochastic density fluctuations that can produce rare, high-density mass concentrations. Here, we use a recently developed analytic framework to predict the statistics of these fluctuations, i.e., the rate of fragmentation and mass spectrum of fragments formed in a turbulent Keplerian disk. Turbulent disks are never completely stable: we calculate the (always finite) probability of forming self-gravitating structures via stochastic turbulent density fluctuations in such disks. Modest sub-sonic turbulence above Mach number M∼0.1 can produce a few stochastic fragmentation or 'direct collapse' events over ∼Myr timescales, even if Q >> 1 and cooling is slow (t{sub cool} >> t{sub orbit}). In transsonic turbulence this extends to Q ∼ 100. We derive the true Q-criterion needed to suppress such events, which scales exponentially with Mach number. We specify to turbulence driven by magneto-rotational instability, convection, or spiral waves and derive equivalent criteria in terms of Q and the cooling time. Cooling times ∼> 50 t{sub dyn} may be required to completely suppress fragmentation. These gravo-turbulent events produce mass spectra peaked near ∼(Q M{sub disk}/M{sub *}){sup 2} M{sub disk} (rocky-to-giant planet masses, increasing with distance from the star). We apply this to protoplanetary disk models and show that even minimum-mass solar nebulae could experience stochastic collapse events, provided a source of turbulence.
TURBULENT DISKS ARE NEVER STABLE: FRAGMENTATION AND TURBULENCE-PROMOTED PLANET FORMATION
International Nuclear Information System (INIS)
Hopkins, Philip F.; Christiansen, Jessie L.
2013-01-01
A fundamental assumption in our understanding of disks is that when the Toomre Q >> 1, the disk is stable against fragmentation into self-gravitating objects (and so cannot form planets via direct collapse). But if disks are turbulent, this neglects a spectrum of stochastic density fluctuations that can produce rare, high-density mass concentrations. Here, we use a recently developed analytic framework to predict the statistics of these fluctuations, i.e., the rate of fragmentation and mass spectrum of fragments formed in a turbulent Keplerian disk. Turbulent disks are never completely stable: we calculate the (always finite) probability of forming self-gravitating structures via stochastic turbulent density fluctuations in such disks. Modest sub-sonic turbulence above Mach number M∼0.1 can produce a few stochastic fragmentation or 'direct collapse' events over ∼Myr timescales, even if Q >> 1 and cooling is slow (t cool >> t orbit ). In transsonic turbulence this extends to Q ∼ 100. We derive the true Q-criterion needed to suppress such events, which scales exponentially with Mach number. We specify to turbulence driven by magneto-rotational instability, convection, or spiral waves and derive equivalent criteria in terms of Q and the cooling time. Cooling times ∼> 50 t dyn may be required to completely suppress fragmentation. These gravo-turbulent events produce mass spectra peaked near ∼(Q M disk /M * ) 2 M disk (rocky-to-giant planet masses, increasing with distance from the star). We apply this to protoplanetary disk models and show that even minimum-mass solar nebulae could experience stochastic collapse events, provided a source of turbulence
Grannan, Alexander Michael
2017-08-01
The energy for driving turbulent flows in planetary fluid layers comes from a combination of thermocompositional sources and the motion of the boundary in contact with the fluid through mechanisms like precessional, tidal, and librational forcing. Characterizing the resulting turbulent fluid motions are necessary for understanding many aspects of the planet's dynamics and evolution including the generation of magnetic fields in the electrically conducting fluid layers and dissipation in the oceans. Although such flows are strongly inertial they are also strongly influenced by the Coriolis force whose source is in the rotation of the body and tends to constrain the inertial effects and provide support for fluid instabilities that might in-turn generate turbulence. Furthermore, the magnetic fields generated by the electrically conducting fluids act back on the fluid through the Lorentz force that also tends to constrain the flow. The goal of this dissertation is to investigate the characteristics of turbulent flows under the influence of mechanical, convective, rotational and magnetic forcing. In order to investigate the response of the fluid to mechanical forcing, I have modified a unique set of laboratory experiments that allows me to quantify the generation of turbulence driven by the periodic oscillations of the fluid containing boundary through tides and libration. These laboratory experiments replicate the fundamental ingredients found in planetary environments and are necessary for the excitation of instabilities that drive the turbulent fluid motions. For librational forcing, a rigid ellipsoidal container and ellipsoidal shell of isothermal unstratified fluid is made to rotate with a superimposed oscillation while, for tidal forcing, an elastic ellipsoidal container of isothermal unstratified fluid is made to rotate while an independently rotating perturbance also flexes the elastic container. By varying the strength and frequencies of these oscillations the
Energy Technology Data Exchange (ETDEWEB)
Eltayeb, I A; Elbashir, T B A, E-mail: ieltayeb@squ.edu.om, E-mail: elbashir@squ.edu.om [Department of Mathematics and Statistics, College of Science, Sultan Qaboos University, Muscat 123 (Oman)
2017-08-15
The linear and nonlinear stabilities of second sound waves in a rotating porous Darcy–Brinkman layer in local thermal non-equilibrium are studied when the heat flux in the solid obeys the Cattaneo law. The simultaneous action of the Brinkman effect (effective viscosity) and rotation is shown to destabilise the layer, as compared to either of them acting alone, for both stationary and overstable modes. The effective viscosity tends to favour overstable modes while rotation tends to favour stationary convection. Rapid rotation invokes a negative viscosity effect that suppresses the stabilising effect of porosity so that the stability characteristics resemble those of the classical rotating Benard layer. A formal weakly nonlinear analysis yields evolution equations of the Landau–Stuart type governing the slow time development of the amplitudes of the unstable waves. The equilibrium points of the evolution equations are analysed and the overall development of the amplitudes is examined. Both overstable and stationary modes can exhibit supercritical stability; supercritical instability, subcritical instability and stability are not possible. The dependence of the supercritical stability on the relative values of the six dimensionless parameters representing thermal non-equilibrium, rotation, porosity, relaxation time, thermal diffusivities and Brinkman effect is illustrated as regions in regime diagrams in the parameter space. The dependence of the heat transfer and the mean heat flux on the parameters of the problem is also discussed. (paper)
Dufresne, Nathaniel P.
An experimental investigation of the axial and azimuthal (swirl) velocity field in the wake of a single 3-bladed wind turbine with rotor diameter of 0.91m was conducted, up to 20 diameters downstream. The turbine was positioned in the free stream, near the entrance of the 6m x 2.7m cross section of the University of New Hampshire (UNH) Flow Physics Facility. Velocity measurements were conducted at different rotor loading conditions with blade tip-speed ratios from 2.0 to 2.8. A Pitot-static tube and constant temperature hot-wire anemometer with a multi-wire sensor were used to measure velocity fields. An equilibrium similarity theory for the turbulent axisymmetric wake with rotation was outlined, and first evidence for a new scaling function for the mean swirling velocity component, Wmax ∝ x-1 ∝ U3/2o a was found; where W represents swirl, x represents downstream distance, and Uo, represents the centerline velocity deficit in the wake.
Instantaneous structure of a boundary layer subjected to free-stream turbulence
Hearst, R. Jason; de Silva, Charitha; Dogan, Eda; Ganapathisubramani, Bharathram
2017-11-01
A canonical turbulent boundary layer (TBL) has a distinct turbulent/non-turbulent interface (TNTI) separating the rotational wall-bounded fluid from the irrotational free-stream. If an intermittency profile is constructed separating the flow above and below the TNTI, this profile can be described by an error-function. Within the turbulent region, the flow is separated by interfaces that demarcate uniform momentum zones (UMZs). We observe that these characteristics of a TBL change if there is free-stream turbulence (FST). First, the entire flow is rotational, and thus a distinct TNTI does not exist. Nonetheless, it is possible to identify an interface that approximately separates the flow with mean zero vorticity from the distinctly wall-signed vorticity. This turbulent/turbulent interface is shown to be closer to the wall than the traditional TNTI, and the resulting intermittency profile is not an error-function. Also, UMZs appear to be masked by the free-stream perturbations. Despite these differences, a velocity field of a TBL with homogeneous, isotropic turbulence superimposed and weighted with the empirical intermittency profile, qualitatively reproduces the 1st and 2nd-order statistics. These findings suggest that a TBL subjected to FST may be described by a simple model. EPSRC, ERC, NSERC, Zonta International.
Spatial extent of quantum turbulence in non-rotating superfluid 3He-B
International Nuclear Information System (INIS)
Bradley, D.I.; Fisher, S.N.; Guenault, A.M.; Lowe, M.R.; Pickett, G.R.; Rahm, A.
2003-01-01
Quantum turbulence has been shown to reflect a beam of quasiparticles in the B-phase of superfluid 3 He by Andreev processes. We have investigated the evolution of the turbulence generated by a vibrating wire resonator driven at high velocities and temperatures down to ∼0.1T c . The vibrating wire produces vorticity together with the expected quasiparticle beam whenever the wire velocity exceeds the critical pair breaking velocity. By using an array of detector wires we are able to investigate the development of the turbulence both in space and time. We observe that the turbulence propagates preferentially along the direction of the quasiparticle beam and drops off in a roughly exponential manner with a decay length of the order of 2 mm
Reynolds Stress Closure for Inertial Frames and Rotating Frames
Petty, Charles; Benard, Andre
2017-11-01
In a rotating frame-of-reference, the Coriolis acceleration and the mean vorticity field have a profound impact on the redistribution of kinetic energy among the three components of the fluctuating velocity. Consequently, the normalized Reynolds (NR) stress is not objective. Furthermore, because the Reynolds stress is defined as an ensemble average of a product of fluctuating velocity vector fields, its eigenvalues must be non-negative for all turbulent flows. These fundamental properties (realizability and non-objectivity) of the NR-stress cannot be compromised in computational fluid dynamic (CFD) simulations of turbulent flows in either inertial frames or in rotating frames. The recently developed universal realizable anisotropic prestress (URAPS) closure for the NR-stress depends explicitly on the local mean velocity gradient and the Coriolis operator. The URAPS-closure is a significant paradigm shift from turbulent closure models that assume that dyadic-valued operators associated with turbulent fluctuations are objective.
International Nuclear Information System (INIS)
Nuzhnyj, A.S.; Rozanov, V.B.; Stepanov, R.V.; Shumskij, S.A.
2005-01-01
Stability of target compression in the laser thermonuclear synthesis is discussed. The process is determined by developing the Rayleigh-Taylor instability (RNI). A program unit for description of the RNI evolution by its initial distributions is developed. The results of statistical analysis of the RT mixing calculations are given. The analysis is carried out by means of learning base system and is substantiated on the generalization of great number of data, fulfilled by means of the neural network methods [ru
The generation of resonant turbulence for a premixed burner
Verbeek, Antonie Alex; Pos, R.C.; Stoffels, Genie G.M.; Geurts, Bernardus J.; van der Meer, Theodorus H.
2012-01-01
Is it possible to optimize the turbulent combustion of a low swirl burner by using resonance in turbulence? To that end an active grid is constructed that consists of two perforated disks of which one is rotating, creating a system of pulsating jets, which in the end can be used as a central
Rayleigh wave effects in an elastic half-space.
Aggarwal, H. R.
1972-01-01
Consideration of Rayleigh wave effects in a homogeneous isotropic linearly elastic half-space subject to an impulsive uniform disk pressure loading. An approximate formula is obtained for the Rayleigh wave effects. It is shown that the Rayleigh waves near the center of loading arise from the portion of the dilatational and shear waves moving toward the axis, after they originate at the edge of the load disk. A study is made of the vertical displacement due to Rayleigh waves at points on the axis near the surface of the elastic half-space.
Turbulent Helicity in the Atmospheric Boundary Layer
Chkhetiani, Otto G.; Kurgansky, Michael V.; Vazaeva, Natalia V.
2018-05-01
We consider the assumption postulated by Deusebio and Lindborg (J Fluid Mech 755:654-671, 2014) that the helicity injected into the Ekman boundary layer undergoes a cascade, with preservation of its sign (right- or alternatively left-handedness), which is a signature of the system rotation, from large to small scales, down to the Kolmogorov microscale of turbulence. At the same time, recent direct field measurements of turbulent helicity in the steppe region of southern Russia near Tsimlyansk Reservoir show the opposite sign of helicity from that expected. A possible explanation for this phenomenon may be the joint action of different scales of atmospheric flows within the boundary layer, including the sea-breeze circulation over the test site. In this regard, we consider a superposition of the classic Ekman spiral solution and Prandtl's jet-like slope-wind profile to describe the planetary boundary-layer wind structure. The latter solution mimics a hydrostatic shallow breeze circulation over a non-uniformly heated surface. A 180°-wide sector on the hodograph plane exists, within which the relative orientation of the Ekman and Prandtl velocity profiles favours the left rotation with height of the resulting wind velocity vector in the lowermost part of the boundary layer. This explains the negative (left-handed) helicity cascade toward small-scale turbulent motions, which agrees with the direct field measurements of turbulent helicity in Tsimlyansk. A simple turbulent relaxation model is proposed that explains the measured positive values of the relatively minor contribution to turbulent helicity from the vertical components of velocity and vorticity.
Turbulent momentum transport due to neoclassical flows
International Nuclear Information System (INIS)
Lee, Jungpyo; Barnes, Michael; Parra, Felix I; Belli, Emily; Candy, Jeff
2015-01-01
Intrinsic toroidal rotation in a tokamak can be driven by turbulent momentum transport due to neoclassical flow effects breaking a symmetry of turbulence. In this paper we categorize the contributions due to neoclassical effects to the turbulent momentum transport, and evaluate each contribution using gyrokinetic simulations. We find that the relative importance of each contribution changes with collisionality. For low collisionality, the dominant contributions come from neoclassical particle and parallel flows. For moderate collisionality, there are non-negligible contributions due to neoclassical poloidal electric field and poloidal gradients of density and temperature, which are not important for low collisionality. (paper)
Rayleigh scattering in few-mode optical fibers.
Wang, Zhen; Wu, Hao; Hu, Xiaolong; Zhao, Ningbo; Mo, Qi; Li, Guifang
2016-10-24
The extremely low loss of silica fibers has enabled the telecommunication revolution, but single-mode fiber-optic communication systems have been driven to their capacity limits. As a means to overcome this capacity crunch, space-division multiplexing (SDM) using few-mode fibers (FMF) has been proposed and demonstrated. In single-mode optical fibers, Rayleigh scattering serves as the dominant mechanism for optical loss. However, to date, the role of Rayleigh scattering in FMFs remains elusive. Here we establish and experimentally validate a general model for Rayleigh scattering in FMFs. Rayleigh backscattering not only sets the intrinsic loss limit for FMFs but also provides the theoretical foundation for few-mode optical time-domain reflectometry, which can be used to probe perturbation-induced mode-coupling dynamics in FMFs. We also show that forward inter-modal Rayleigh scattering ultimately sets a fundamental limit on inter-modal-crosstalk for FMFs. Therefore, this work not only has implications specifically for SDM systems but also broadly for few-mode fiber optics and its applications in amplifiers, lasers, and sensors in which inter-modal crosstalk imposes a fundamental performance limitation.
Bivariate Rayleigh Distribution and its Properties
Directory of Open Access Journals (Sweden)
Ahmad Saeed Akhter
2007-01-01
Full Text Available Rayleigh (1880 observed that the sea waves follow no law because of the complexities of the sea, but it has been seen that the probability distributions of wave heights, wave length, wave induce pitch, wave and heave motions of the ships follow the Rayleigh distribution. At present, several different quantities are in use for describing the state of the sea; for example, the mean height of the waves, the root mean square height, the height of the “significant waves” (the mean height of the highest one-third of all the waves the maximum height over a given interval of the time, and so on. At present, the ship building industry knows less than any other construction industry about the service conditions under which it must operate. Only small efforts have been made to establish the stresses and motions and to incorporate the result of such studies in to design. This is due to the complexity of the problem caused by the extensive variability of the sea and the corresponding response of the ships. Although the problem appears feasible, yet it is possible to predict service conditions for ships in an orderly and relatively simple manner Rayleigh (1980 derived it from the amplitude of sound resulting from many independent sources. This distribution is also connected with one or two dimensions and is sometimes referred to as “random walk” frequency distribution. The Rayleigh distribution can be derived from the bivariate normal distribution when the variate are independent and random with equal variances. We try to construct bivariate Rayleigh distribution with marginal Rayleigh distribution function and discuss its fundamental properties.
A μp based automation system for Raman and Rayleigh spectrometers
International Nuclear Information System (INIS)
Kesavamoorthy, R.; Arora, A.K.; Vasumathi, D.
1988-01-01
μp based data acquisition cum automation system for Raman and Rayleigh Spectrometers is described. The experiments require simultaneous acquisition of different digital data in two separate counters, their storage and rotation of grating through stepper motor in a repetitive cycle. Various modes of operation are selected through a function keyboard. The current status of the experiment is also displayed using 7 segment 12 element display unit. The input parameters are fed through a hexadecimal keyboard before the start of the experiment. The stored data can be send to a printer/terminal or to a PC through a serial port after the completion of the experiment. (author)
Rotation, Stability and Transport
Energy Technology Data Exchange (ETDEWEB)
Connor, J. W.
2007-07-01
Tokamak plasmas can frequently exhibit high levels of rotation and rotation shear. This can usually be attributed to various sources: injection of momentum, e.g. through neutral beams, flows driven by plasma gradients or torques resulting from non-ambipolar particle loss; however, the source sometimes remains a mystery, such as the spontaneous rotation observed in Ohmic plasmas. The equilibrium rotation profile is given by the balance of these sources with transport and other losses; the edge boundary conditions can play an important role in determining this profile . Such plasma rotation, particularly sheared rotation, is predicted theoretically to have a significant influence on plasma behaviour. In the first place, sonic flows can significantly affect tokamak equilibria and neoclassical transport losses. However, the influence of rotation on plasma stability and turbulence is more profound. At the macroscopic level it affects the behaviour of the gross MHD modes that influence plasma operational limits. This includes sawteeth, the seeding of neoclassical tearing modes, resistive wall modes and the onset of disruptions through error fields, mode locking and reconnection. At the microscopic level it has a major effect on the stability of ballooning modes, both ideal MHD and drift wave instabilities such as ion temperature gradient (ITG) modes. In the non-linear state, as unstable drift waves evolve into turbulent structures, sheared rotation also tears apart eddies, thereby reducing the resulting transport. There is considerable experimental evidence for these effects on both MHD stability and plasma confinement. In particular, the appearance of improved confinement modes with transport barriers, such as edge H-mode barriers and internal transport barriers (ITBs) appears to correlate well with the presence of sheared plasma rotation. This talk will describe the theory underlying some of these phenomena involving plasma rotation, on both macroscopic and microscopic
Turbulence and turbulent drag reduction in swirling flow: Inertial versus viscous forcing.
Burnishev, Yuri; Steinberg, Victor
2015-08-01
We report unexpected results of a drastic difference in the transition to fully developed turbulent and turbulent drag reduction (TDR) regimes and in their properties in a von Karman swirling flow with counter-rotating disks of water-based polymer solutions for viscous (by smooth disks) as well as inertial (by bladed disks) forcing and by tracking just torque Γ(t) and pressure p(t) . For the viscous forcing, just a single TDR regime is found with the transition values of the Reynolds number (Re) Re turb c =Re TDR c ≃(4.8±0.2)×10(5) independent of ϕ , whereas for the inertial forcing two turbulent regimes are revealed. The first transition is to fully developed turbulence, and the second one is to the TDR regime with both Re turb c and Re TDR c depending on polymer concentration ϕ . Both regimes differ by the values of C f and C p , by the scaling exponents of the fundamental turbulent characteristics, by the nonmonotonic dependencies of skewness and flatness of the pressure PDFs on Re, and by the different frequency power spectra of p with the different dependencies of the main vortex peak frequency in the p power spectra on ϕ and Re. Thus our experimental results show the transition to the TDR regime in a von Karman swirling flow for the viscous and inertial forcings in a sharp contrast to the recent experiments [Phys. Fluids 10, 426 (1998); Phys. Rev. E 47, R28(R) (1993); and J. Phys.: Condens. Matter 17, S1195 (2005)] where the transition to TDR is observed in the same swirling flow with counter-rotating disks only for the viscous forcing. The latter result has led its authors to the wrong conclusion that TDR is a solely boundary effect contrary to the inertial forcing associated with the bulk effect, and this conception is currently rather widely accepted in literature.
2D turbulence structure observed by a fast framing camera system in linear magnetized device PANTA
International Nuclear Information System (INIS)
Ohdachi, Satoshi; Inagaki, S.; Kobayashi, T.; Goto, M.
2015-01-01
Mesoscale structure, such as the zonal flow and the streamer plays important role in the drift-wave turbulence. The interaction of the mesoscale structure and the turbulence is not only interesting phenomena but also a key to understand the turbulence driven transport in the magnetically confined plasmas. In the cylindrical magnetized device, PANTA, the interaction of the streamer and the drift wave has been found by the bi-spectrum analysis of the turbulence. In order to study the mesoscale physics directly, the 2D turbulence is studied by a fast-framing visible camera system view from a window located at the end plate of the device. The parameters of the plasma is the following; Te∼3eV, n ∼ 1x10 19 m -3 , Ti∼0.3eV, B=900G, Neutral pressure P n =0.8 mTorr, a∼ 6cm, L=4m, Helicon source (7MHz, 3kW). Fluctuating component of the visible image is decomposed by the Fourier-Bessel expansion method. Several rotating mode is observed simultaneously. From the images, m = 1 (f∼0.7 kHz) and m = 2, 3 (f∼-3.4 kHz) components which rotate in the opposite direction can be easily distinguished. Though the modes rotate constantly in most time, there appear periods where the radially complicated node structure is formed (for example, m=3 component, t = 142.5∼6 in the figure) and coherent mode structures are disturbed. Then, a new rotating period is started again with different phase of the initial rotation until the next event happens. The typical time interval of the event is 0.5 to 1.0 times of the one rotation of the slow m = 1 mode. The wave-wave interaction might be interrupted occasionally. Detailed analysis of the turbulence using imaging technique will be discussed. (author)
Rayleigh scattering and depolarization ratio in linear alkylbenzene
International Nuclear Information System (INIS)
Liu, Qian; Zhou, Xiang; Huang, Wenqian; Zhang, Yuning; Wu, Wenjie; Luo, Wentai; Yu, Miao; Zheng, Yangheng; Zhou, Li; Cao, Jun; Wang, Yifang
2015-01-01
It is planned to use linear alkylbenzene (LAB) as the organic solvent for the Jiangmen Underground Neutrino Observatory (JUNO) liquid scintillator detectors, due to its ultra-transparency. However, the current Rayleigh scattering length calculation for LAB disagrees with the experimental measurement. This paper reports for the first time that the Rayleigh scattering of LAB is anisotropic, with a depolarization ratio of 0.31±0.01(stat.)±0.01(sys.). We use an indirect method for Rayleigh scattering measurement with the Einstein–Smoluchowski–Cabannes formula, and the Rayleigh scattering length of LAB is determined to be 28.2±1.0 m at 430 nm
The Magnetic Rayleigh-Taylor Instability in Astrophysical Discs
Contopoulos, I.; Kazanas, D.; Papadopoulos, D. B.
2016-01-01
This is our first study of the magnetic Rayleigh-Taylor instability at the inner edge of an astrophysical disc around a central back hole. We derive the equations governing small-amplitude oscillations in general relativistic ideal magnetodydrodynamics and obtain a criterion for the onset of the instability. We suggest that static disc configurations where magnetic field is held by the disc material are unstable around a Schwarzschild black hole. On the other hand, we find that such configurations are stabilized by the space-time rotation around a Kerr black hole. We obtain a crude estimate of the maximum amount of poloidal magnetic flux that can be accumulated around the centre, and suggest that it is proportional to the black hole spin. Finally, we discuss the astrophysical implications of our result for the theoretical and observational estimations of the black hole jet power.
Energy Technology Data Exchange (ETDEWEB)
Abo-Dahab, S. M. [Taif University, Taif (Saudi Arabia); Abd-Alla, A. M. [SVU, Qena (Egypt); Khan, Aftab [Sohag University, Sohag (Egypt)
2015-08-15
The aim of this paper is to study the propagation of surface waves in a rotating fibre-reinforced viscoelastic media of higher order under the influence of magnetic field. The general surface wave speeds derived to study the effects of rotation and magnetic field on surface waves. Particular cases for Stoneley, Love and Rayleigh waves are also discussed and dispersion relation for the waves has been deduced. The results obtained in this investigation are more general in the sense that some earlier published results are obtained from our result as special cases. For order zero our results are well agreement to fibre-reinforced materials. Also by neglecting the reinforced elastic parameters, the results reduce to well known isotropic medium. It is observed that in a rotating medium the surface waves are dispersive. Also magnetic effects play a significant roll. It is observed that Love wave remain unaffected in a rotating medium but remain under the influence of magnetic field. Rayleigh waves are affected by rotation and magnetic field whereas Stoneley waves are independent of Maxwell stresses. It is also observed that, surface waves cannot propagate in a fast rotating medium or in the presence of magnetic field of high intensity. Numerical results for particular materials are given and illustrated graphically. The results indicate that the effect of rotation and magnetic field are very pronounced.
International Nuclear Information System (INIS)
Abo-Dahab, S. M.; Abd-Alla, A. M.; Khan, Aftab
2015-01-01
The aim of this paper is to study the propagation of surface waves in a rotating fibre-reinforced viscoelastic media of higher order under the influence of magnetic field. The general surface wave speeds derived to study the effects of rotation and magnetic field on surface waves. Particular cases for Stoneley, Love and Rayleigh waves are also discussed and dispersion relation for the waves has been deduced. The results obtained in this investigation are more general in the sense that some earlier published results are obtained from our result as special cases. For order zero our results are well agreement to fibre-reinforced materials. Also by neglecting the reinforced elastic parameters, the results reduce to well known isotropic medium. It is observed that in a rotating medium the surface waves are dispersive. Also magnetic effects play a significant roll. It is observed that Love wave remain unaffected in a rotating medium but remain under the influence of magnetic field. Rayleigh waves are affected by rotation and magnetic field whereas Stoneley waves are independent of Maxwell stresses. It is also observed that, surface waves cannot propagate in a fast rotating medium or in the presence of magnetic field of high intensity. Numerical results for particular materials are given and illustrated graphically. The results indicate that the effect of rotation and magnetic field are very pronounced.
Diffusive separation of particles by diffusion in swirled turbulent flows
International Nuclear Information System (INIS)
Arbuzov, V.N.; Shiliaev, M.I.
1984-01-01
An analysis of the dynamics of turbulent flow and diffusive separation of solid particles in a centrifugal air separator (consisting of two flat disks rotating at the same angular velocity) is presented. A closed set of balances for all the components of the tensor of turbulent stresses, extended to the entire flow region, is employed in the numerical analysis of transition and turbulent air flows between the rotating disks. The analytical relationships obtained for the case of the mixed flow for the various components of the average velocity, energy of fluctuations, and turbulence level in the circumferential direction agreed well with the theoretical and experimental distributions of Bakke, et al. (1973). It is shown that at high Reynolds numbers the flow is isotropic, the dependence of the circumferential component of the average velocity obeys a power law, and the generation of the radial component is controlled by the local centrifugal field. The sharpness of particle separation was calculated by the eddy diffusion equation and was found to depend on the geometry and the operating conditions. 8 references
Table-top rotating turbulence : an experimental insight through Particle Tracking
Castello, Del L.
2010-01-01
The influence of the Earth background rotation on oceanic and atmospheric currents, as well as the effects of a rapid rotation on the flow inside industrial machineries like mixers, turbines, and compressors, are only the most typical examples of fluid flows affected by rotation. Despite the
RAYLEIGH SCATTERING MODELS WITH CORRELATION INTEGRAL
Directory of Open Access Journals (Sweden)
S. F. Kolomiets
2014-01-01
Full Text Available This article offers one of possible approaches to the use of the classical correlation concept in Rayleigh scattering models. Classical correlation in contrast to three types of correlations corresponding to stochastic point flows opens the door to the efficient explanation of the interaction between periodical structure of incident radiation and discreet stochastic structure of distributed scatters typical for Rayleigh problems.
Chromo-Rayleigh interactions of dark matter
International Nuclear Information System (INIS)
Bai, Yang; Osborne, James
2015-01-01
For a wide range of models, dark matter can interact with QCD gluons via chromo-Rayleigh interactions. We point out that the Large Hadron Collider (LHC), as a gluon machine, provides a superb probe of such interactions. In this paper, we introduce simplified models to UV-complete two effective dark matter chromo-Rayleigh interactions and identify the corresponding collider signatures, including four jets or a pair of di-jet resonances plus missing transverse energy. After performing collider studies for both the 8 TeV and 14 TeV LHC, we find that the LHC can be more sensitive to dark matter chromo-Rayleigh interactions than direct detection experiments and thus provides the best opportunity for future discovery of this class of models.
Effects of rotation on flow in an asymmetric rib-roughened duct: LES study
International Nuclear Information System (INIS)
Borello, D.; Salvagni, A.; Hanjalić, K.
2015-01-01
Highlights: • Ribbed duct reproduces most of the phenomena occurring in internal cooling channels of blade turbines (rotor and stator). • LES analysis of the flow in a ribbed duct was carried out aiming at detecting the influence of rotation on the turbulence. • In destabilizing conditions, rotation enhances turbulence close to the ribbed duct thus enhancing removal of fluid from the wall and improving mixing. • In stabilizing conditions, turbulence is suppressed by rotation close to the ribbed wall. - Abstract: We report on large-eddy simulations (LES) of fully-developed asymmetric flow in a duct of a rectangular cross-section in which square-sectioned, equally-spaced ribs oriented perpendicular to the flow direction, were mounted on one of the walls. The configuration mimics a passage of internal cooling of a gas-turbine blade. The duct flow at a Reynolds number Re = 15,000 (based on hydraulic diameter D_h and bulk flow velocity U_0) was subjected to clock-wise (stabilising) and anti-clock-wise (destabilising) orthogonal rotation at a moderate rotational number Ro = ΩD_h/U_0 = 0.3, where Ω is the angular velocity. The LES results reproduced well the available experimental results of Coletti et al. (2011) (in the mid-plane adjacent to the ribbed wall) and provided insight into the whole duct complementing the reference PIV measurement. We analyzed the effects of stabilising and destabilising rotation on the flow, vortical structures and turbulence statistics by comparison with the non-rotating case. The analysis includes the identification of depth of penetration of the rib-effects into the bulk flow, influence of flow three-dimensionality and the role of secondary motions, all shown to be strongly affected by the rotation and its direction.
Finite element analysis of rotating beams physics based interpolation
Ganguli, Ranjan
2017-01-01
This book addresses the solution of rotating beam free-vibration problems using the finite element method. It provides an introduction to the governing equation of a rotating beam, before outlining the solution procedures using Rayleigh-Ritz, Galerkin and finite element methods. The possibility of improving the convergence of finite element methods through a judicious selection of interpolation functions, which are closer to the problem physics, is also addressed. The book offers a valuable guide for students and researchers working on rotating beam problems – important engineering structures used in helicopter rotors, wind turbines, gas turbines, steam turbines and propellers – and their applications. It can also be used as a textbook for specialized graduate and professional courses on advanced applications of finite element analysis.
On two-dimensionalization of three-dimensional turbulence in shell models
DEFF Research Database (Denmark)
Chakraborty, Sagar; Jensen, Mogens Høgh; Sarkar, A.
2010-01-01
Applying a modified version of the Gledzer-Ohkitani-Yamada (GOY) shell model, the signatures of so-called two-dimensionalization effect of three-dimensional incompressible, homogeneous, isotropic fully developed unforced turbulence have been studied and reproduced. Within the framework of shell m......-similar PDFs for longitudinal velocity differences are also presented for the rotating 3D turbulence case....
Energy Technology Data Exchange (ETDEWEB)
Bau, H.H. [Univ. of Pennsylvania, Philadelphia, PA (United States)
1995-12-31
Using stability theory, numerical simulations, and in some instances experiments, it is demonstrated that the critical Rayleigh number for the bifurcation (1) from the no-motion (conduction) state to the motion state and (2) from time-independent convection to time-dependent, oscillatory convection in the thermal convection loop and Rayleigh-Benard problems can be significantly increased or decreased. This is accomplished through the use of a feedback controller effectuating small perturbations in the boundary data. The controller consists of sensors which detect deviations in the fluid`s temperature from the motionless, conductive values and then direct actuators to respond to these deviations in such a way as to suppress the naturally occurring flow instabilities. Actuators which modify the boundary`s temperature/heat flux are considered. The feedback controller can also be used to control flow patterns and generate complex dynamic behavior at relatively low Rayleigh numbers.
DEFF Research Database (Denmark)
Zhu, Jiajian; Gao, Jinlong; Ehn, Andreas
2014-01-01
and vibrational temperatures of a gliding arc generated at atmospheric pressure air are investigated. Translational temperatures (about 1100 K) were measured by laser-induced Rayleigh scattering, and two-dimensional temperature imaging was performed. Rotational and vibrational temperatures (about 3600 K and 6700...
On the interaction of Rayleigh surface waves with structures
International Nuclear Information System (INIS)
Simpson, I.C.
1976-12-01
A two-dimensional soil-structure interaction analysis is carried out for transient Rayleigh surface waves that are incident on a structure. The structure is modelled by a three-degree of freedom rigid basemat to which is attached a flexible superstructure, modelled by a single mass-spring system. The structural responses to a given Rayleigh wave train are compared with those that would have been obtained if the free-field acceleration-time history had been applied as a normally incident body wave. The results clearly exhibit the 'frequency filtering' effects of the rigid basemat on the incident Rayleigh waves. It is shown that, if seismic excitation of a structure is, in fact, due to Rayleigh surface waves, then an analysis assuming normally incident body waves can considerably over-estimate structural response, both at basemat level for horizontal and vertical oscillations of the superstructure. However, in the examples considered here, relatively large rocking effects were induced by the Rayleigh waves, thus giving maximum horizontal accelerations in the superstructure that were of comparable magnitude for Rayleigh and normally incident body waves. (author)
Otugen, M. Volkan; Popovic, Svetozar
1996-01-01
Ongoing research in Rayleigh scattering diagnostics for variable density low speed flow applications and for supersonic flow measurements are described. During the past several years, the focus has been on the development and use of a Nd:YAG-based Rayleigh scattering system with improved signal-to-noise characteristics and with applicability to complex, confined flows. This activity serves other research projects in the Aerodynamics Laboratory which require the non-contact, accurate, time-frozen measurement of gas density, pressure, and temperature (each separately), in a fairly wide dynamic range of each parameter. Recently, with the acquisition of a new seed-injected Nd:YAG laser, effort also has been directed to the development of a high-speed velocity probe based on a spectrally resolved Rayleigh scattering technique.
Meniscus Stability in Rotating Systems
Reichel, Yvonne; Dreyer, Michael
2013-11-01
In this study, the stability of free surfaces of fluid between two rotating coaxial, circular disks is examined. Radially mounted baffles are used to form menisci of equal size. To the center of the upper disk, a tube is connected in which a separate meniscus is formed. Assuming solid-body rotation and ignoring dynamic effects, it is observed that the free surfaces between the disks fail to remain stable once the rotation speed exceeds a critical value. In other words, Rayleigh-Taylor instability ensues when the capillary forces fail to balance centrifugal forces. Dimensionless critical rotation speeds are studied by means of the Surface Evolver via SE-FIT for varied number of baffles, the normalized distance between the disks, and the normalized central tube radius. Drop tower tests are performed to confirm some of the numerical results. The computation also reveals that there are different modes of instability as a function of the relevant parameters. This study was funded by the space agency of the German Aerospace Center with resources of the Federal Ministry of Economics and Technology on the basis of a resolution of the German Bundestag under grant number 50 RL 1320.
Czech Academy of Sciences Publication Activity Database
Urban, Pavel; Hanzelka, Pavel; Musilová, Věra; Králík, Tomáš; La Mantia, M.; Srnka, Aleš; Skrbek, L.
2014-01-01
Roč. 16, č. 5 (2014), 053042: 1-40 ISSN 1367-2630 R&D Projects: GA ČR GPP203/12/P897 Institutional support: RVO:68081731 Keywords : Rayleigh-Bénard convection * heat transfer efficiency * cryogenic helium Subject RIV: BK - Fluid Dynamics Impact factor: 3.558, year: 2014
Rayleigh scattering in an emitter-nanofiber-coupling system
Tang, Shui-Jing; Gao, Fei; Xu, Da; Li, Yan; Gong, Qihuang; Xiao, Yun-Feng
2017-04-01
Scattering is a general process in both fundamental and applied physics. In this paper, we investigate Rayleigh scattering of a solid-state-emitter coupled to a nanofiber, by S -matrix-like theory in k -space description. Under this model, both Rayleigh scattering and dipole interaction are studied between a two-level artificial atom embedded in a nanocrystal and fiber modes (guided and radiation modes). It is found that Rayleigh scattering plays a critical role in the transport properties and quantum statistics of photons. On the one hand, Rayleigh scattering produces the transparency in the optical transmitted field of the nanofiber, accompanied by the change of atomic phase, population, and frequency shift. On the other hand, the interference between two kinds of scattering fields by Rayleigh scattering and dipole transition modifies the photon statistics (second-order autocorrelation function) of output fields, showing a strong wavelength dependence. This study provides guidance for the solid-state emitter acting as a single-photon source and can be extended to explore the scattering effect in many-body physics.
Cascade of kinetic energy and scalar variance in DC electrokinetic turbulence
Zhao, Wei; Wang, Guiren
2017-11-01
Turbulent flow can be generated by DC electrokinetic (EK) force based on the electric conductivity and permittivity variations in fluids, as have been demonstrated by Varshney et al (2016), where a -1.4 slope of velocity power spectrum is observed. Here, we theoretically found the scaling exponents of velocity and scalar structures in the electric-body-force (EBF) dominant subregion of DC EK turbulence were 2/5 (equivalent to the -7/5 slope of velocity power spectrum) and 4/5 respectively. The theory perfectly explains the experimental results of Varshney et al. (2016). Based on Kármán-Howarth equation with forcing terms, the energy cascade process of DC EK turbulence was also investigated. Depending on the electric Rayleigh number (Rae) , two different energy cascade processes may happen. When Rae is small, the kinetic energy cascades along inertial subregion and EBF dominant subregion in sequence, before it is dissipated by fluid viscosity. When Rae is sufficiently large, the inertial subregion may be absent with EBF dominant subregion left. This investigation is very important on understand EK turbulence, which could be widely existed in nature and applied in engineerings. The work was supported by NSFC (11672229), and NSF (CAREER CBET-0954977 and MRI CBET-1040227).
Density Effects on Post-shock Turbulence Structure
Tian, Yifeng; Jaberi, Farhad; Livescu, Daniel; Li, Zhaorui; Michigan State University Collaboration; Los Alamos National Laboratory Collaboration; Texas A&M University-Corpus Christi Collaboration
2017-11-01
The effects of density variations due to mixture composition on post-shock turbulence structure are studied using turbulence-resolving shock-capturing simulations. This work extends the canonical Shock-Turbulence Interaction (STI) problem to involve significant variable density effects. The numerical method has been verified using a series of grid and LIA convergence tests, and is used to generate accurate post-shock turbulence data for a detailed flow study. Density effects on post-shock turbulent statistics are shown to be significant, leading to an increased amplification of turbulent kinetic energy (TKE). Eulerian and Lagrangian analyses show that the increase in the post-shock correlation between rotation and strain is weakened in the case with significant density variations (referred to as the ``multi-fluid'' case). Similar to previous single-fluid results and LIA predictions, the shock wave significantly changes the topology of the turbulent structures, exhibiting a symmetrization of the joint PDF of second and third invariant of the deviatoric part of velocity gradient tensor. In the multi-fluid case, this trend is more significant and mainly manifested in the heavy fluid regions. Lagrangian data are also used to study the evolution of turbulence structure away from the shock wave and assess the accuracy of Lagrangian dynamical models.
Turbulence closure for mixing length theories
Jermyn, Adam S.; Lesaffre, Pierre; Tout, Christopher A.; Chitre, Shashikumar M.
2018-05-01
We present an approach to turbulence closure based on mixing length theory with three-dimensional fluctuations against a two-dimensional background. This model is intended to be rapidly computable for implementation in stellar evolution software and to capture a wide range of relevant phenomena with just a single free parameter, namely the mixing length. We incorporate magnetic, rotational, baroclinic, and buoyancy effects exactly within the formalism of linear growth theories with non-linear decay. We treat differential rotation effects perturbatively in the corotating frame using a novel controlled approximation, which matches the time evolution of the reference frame to arbitrary order. We then implement this model in an efficient open source code and discuss the resulting turbulent stresses and transport coefficients. We demonstrate that this model exhibits convective, baroclinic, and shear instabilities as well as the magnetorotational instability. It also exhibits non-linear saturation behaviour, and we use this to extract the asymptotic scaling of various transport coefficients in physically interesting limits.
Scrape-off layer tokamak plasma turbulence
Bisai, N.; Singh, R.; Kaw, P. K.
2012-05-01
Two-dimensional (2D) interchange turbulence in the scrape-off layer of tokamak plasmas and their subsequent contribution to anomalous plasma transport has been studied in recent years using electron continuity, current balance, and electron energy equations. In this paper, numerically it is demonstrated that the inclusion of ion energy equation in the simulation changes the nature of plasma turbulence. Finite ion temperature reduces floating potential by about 15% compared with the cold ion temperature approximation and also reduces the radial electric field. Rotation of plasma blobs at an angular velocity about 1.5×105 rad/s has been observed. It is found that blob rotation keeps plasma blob charge separation at an angular position with respect to the vertical direction that gives a generation of radial electric field. Plasma blobs with high electron temperature gradients can align the charge separation almost in the radial direction. Influence of high ion temperature and its gradient has been presented.
PREFACE: Turbulent Mixing and Beyond Turbulent Mixing and Beyond
Abarzhi, Snezhana I.; Gauthier, Serge; Rosner, Robert
2008-10-01
presentations were published in the Book of Abstracts, International Conference `Turbulent Mixing and Beyond', August 18-26, 2007, Copyright 2007 Abdus Salam International Centre for Theoretical Physics, Trieste, Italy, ISBN 92-95003-36-5. This Topical Issue consists of nearly 60 articles accepted for publication in the Conference Proceedings and reflects a substantial part of the Conference contributions. The articles cover a broad variety of TMB-2007 themes and are sorted alphabetically by the last name of the first author within each of the following topics: Canonical Turbulence and Turbulent Mixing (invariant, scaling, spectral properties, scalar transports) Wall-bounded Flows (structure and fundamentals, unsteady boundary layers, super-sonic flows, shock - boundary layer interaction) Interfacial Dynamics (Rayleigh-Taylor, Richtmyer-Meshkov and Kelvin-Helmholtz instabilities) Unsteady Turbulent Processes (turbulence and turbulent mixing in unsteady, multiphase and anisotropic flows) High Energy Density Physics (laser-material interaction, Z-pinches, laser-driven, heavy-ion and magnetic fusion) Astrophysics (supernovae, interstellar medium, star formation, stellar interiors, early Universe, cosmic micro-wave background) Magneto-hydrodynamics (magneto-convection, magneto-rotational instability, accretion disks, dynamo) Plasmas in Ionosphere (coupled plasmas, anomalous resistance, ionosphere) Physics of Atmosphere (environmental fluid dynamics, forecasting, data analysis, error estimate) Geophysics (turbulent convection in stratified, rotating and active flows) Combustion (dynamics of flames, fires, blast waves and explosions) Mathematical Aspects of Multi-Scale Dynamics (vortex dynamics, singularities, discontinuities, asymptotic dynamics, weak solutions, well- and ill-posedness) Statistical Approaches, Stochastic Processes and Probabilistic Description (uncertainty quantification, anomalous diffusion, long-tail distributions, wavelets) Advanced Numerical Simulations
Wall roughness induces asymptotic ultimate turbulence
Zhu, Xiaojue; Verschoof, Ruben A.; Bakhuis, Dennis; Huisman, Sander G.; Verzicco, Roberto; Sun, Chao; Lohse, Detlef
2018-04-01
Turbulence governs the transport of heat, mass and momentum on multiple scales. In real-world applications, wall-bounded turbulence typically involves surfaces that are rough; however, characterizing and understanding the effects of wall roughness on turbulence remains a challenge. Here, by combining extensive experiments and numerical simulations, we examine the paradigmatic Taylor-Couette system, which describes the closed flow between two independently rotating coaxial cylinders. We show how wall roughness greatly enhances the overall transport properties and the corresponding scaling exponents associated with wall-bounded turbulence. We reveal that if only one of the walls is rough, the bulk velocity is slaved to the rough side, due to the much stronger coupling to that wall by the detaching flow structures. If both walls are rough, the viscosity dependence is eliminated, giving rise to asymptotic ultimate turbulence—the upper limit of transport—the existence of which was predicted more than 50 years ago. In this limit, the scaling laws can be extrapolated to arbitrarily large Reynolds numbers.
Laser diagnostics of high vibrational and rotational H2-states
International Nuclear Information System (INIS)
Mosbach, Th.; Schulz-von der Gathen, V.; Doebele, H.F.
2002-01-01
We report on measurements of vibrational and rotational excited electronic-ground-state hydrogen molecules in a magnetic multipole plasma source by LIF with VUV radiation. The measurements are taken after rapid shut-off of the discharge current. Absolute level populations are obtained using Rayleigh scattering calibration with Krypton. The theoretically predicted suprathermal population of the vibrational distribution is clearly identified. We found also non-Boltzmann rotational distributions for the high vibrational states. The addition of noble gases (Argon and Xenon) to hydrogen leads to a decrease of the vibrational population. (Abstract Copyright [2002], Wiley Periodicals, Inc.)
Regimes of Axisymmetric Flow and Scaling Laws in a Rotating Annulus with Local Convective Forcing
Directory of Open Access Journals (Sweden)
Susie Wright
2017-07-01
Full Text Available We present a numerical study of axisymmetric flow in a rotating annulus in which local thermal forcing, via a heated annular ring on the outside of the base and a cooled circular disk in the centre of the top surface, drives convection. This new configuration is a variant of the classical thermally-driven annulus, where uniform heating and cooling are applied through the outer and inner sidewalls respectively. The annulus provides an analogue to a planetary circulation and the new configuration, with its more relaxed vertical thermal boundary conditions, is expected to better emulate vigorous convection in the tropics and polar regions as well as baroclinic instability in the mid-latitude baroclinic zone. Using the Met Office/Oxford Rotating Annulus Laboratory (MORALS code, we have investigated a series of equilibrated, two dimensional axisymmetric flows across a large region of parameter space. These are characterized in terms of their velocity and temperature fields. When rotation is applied several distinct flow regimes may be identified for different rotation rates and strengths of differential heating. These regimes are defined as a function of the ratio of the horizontal Ekman layer thickness to the non-rotating thermal boundary layer thickness and are found to be similar to those identified in previous annulus experiments. Convection without rotation is also considered and the scaling of the heat transport with Rayleigh number is calculated. This is then compared with existing work on the classical annulus as well as horizontal and Rayleigh-Bénard convection. As with previous studies on both rotating and non-rotating convection the system’s behaviour is found to be aspect ratio dependent. This dependence is seen in the scaling of the non-rotating Nusselt number and in transitions between regimes in the rotating case although further investigation is required to fully explain these observations.
Lattice BGK simulation of natural convection
International Nuclear Information System (INIS)
Chen, Yu; Ohashi, Hirotada; Akiyama, Mamoru
1995-01-01
Recently a new thermal lattice Bhatnagar-Gross-Krook fluid model was suggested by the authors. In this study, this new model was applied into the numerical simulation of natural convection, namely the Rayleigh Benard flow. The critical number for the onset of convective phenomenon was numerically measured and compared with that of theoretical prediction. A gravity dependent deviation was found in the numerical simulation, which is explained as an unavoidable consequence of the incorporation of gravity force in the lattice BGK system. (author)
International Nuclear Information System (INIS)
Bairi, A.
2008-01-01
Natural convection in air filled 2D tilted square cavities is experimentally and numerically studied. The hot and cold walls of the cavity are maintained isothermal at temperatures T h and T c , respectively, and the channel of the cavity is adiabatic. Measurements and simulations are performed for various geometrical and thermal configurations. Different values of the Rayleigh number Ra and the tilt angle α of the cavity are considered. The range of Ra covered in our work extends from 10 to 10 10 while α varies from 0 to 360 deg. This permits the analysis of several significant situations corresponding to vertical active walls (α 0 deg.), hot wall at the bottom (α = 90 deg.; Rayleigh-Benard convection) and hot wall at the top, pure conductive mode (α = 270 deg.). A computational 2D model based on the finite volume method is used for solving the mass, momentum and energy transfer governing equations. The simulation provides thermal and dynamic maps of the fluid for all configurations treated. The influence of Ra and α on the flow pattern and on the convective heat transfer are analysed and discussed. The thermal boundary distribution and the convective heat transfer calculated are, for most treated cases, close to those obtained experimentally by means of a simple bench specifically designed for this purpose. The maximum discrepancy between the simulations and measurements is relatively small, corresponding to the expected uncertainty of the model and measurements. The convective heat transfer is determined while calculating the radiative and conductive contributions to the global exchange. Radiation is determined by the radiosity method associated with the measured field of temperatures as well as to the global IR emissivities of all the internal elements of the cavity. The average Nusselt number Nu-bar is used to quantify the calculated convective contribution of the heat exchange within the cavity, and is compared with the measured value Nu-bar m . We
Turbulence spectra, transport, and E × B flows in helical plasmas
International Nuclear Information System (INIS)
Watanabe, T.-H.; Nunami, M.; Sugama, H.; Satake, S.; Matsuoka, S.; Ishizawa, A.; Tanaka, K.; Maeyama, Shinya
2012-11-01
Gyrokinetic simulation of ion temperature gradient turbulence and zonal flows for helical plasmas has been validated against the Large Helical Device experiments with high ion temperature, where a reduced modeling of ion heat transport is also considered. It is confirmed by the entropy transfer analysis that the turbulence spectrum elongated in the radial wavenumber space is associated with successive interactions with zonal flows. A novel multi-scale simulation for turbulence and zonal flows in poloidally-rotating helical plasmas has demonstrated strong zonal flow generation by turbulence, which implies that turbulent transport processes in non-axisymmetric systems are coupled to neoclassical transport through the macroscopic E × B flows determined by the ambipolarty condition for neoclassical particle fluxes. (author)
Understanding and Predicting Profile Structure and Parametric Scaling of Intrinsic Rotation
Wang, Weixing
2016-10-01
It is shown for the first time that turbulence-driven residual Reynolds stress can account for both the shape and magnitude of the observed intrinsic toroidal rotation profile. Nonlinear, global gyrokinetic simulations using GTS of DIII-D ECH plasmas indicate a substantial ITG fluctuation-induced non-diffusive momentum flux generated around a mid-radius-peaked intrinsic toroidal rotation profile. The non-diffusive momentum flux is dominated by the residual stress with a negligible contribution from the momentum pinch. The residual stress profile shows a robust anti-gradient, dipole structure in a set of ECH discharges with varying ECH power. Such interesting features of non-diffusive momentum fluxes, in connection with edge momentum sources and sinks, are found to be critical to drive the non-monotonic core rotation profiles in the experiments. Both turbulence intensity gradient and zonal flow ExB shear are identified as major contributors to the generation of the k∥-asymmetry needed for the residual stress generation. By balancing the residual stress and the momentum diffusion, a self-organized, steady-state rotation profile is calculated. The predicted core rotation profiles agree well with the experimentally measured main-ion toroidal rotation. The validated model is further used to investigate the characteristic dependence of global rotation profile structure in the multi-dimensional parametric space covering turbulence type, q-profile structure and collisionality with the goal of developing physics understanding needed for rotation profile control and optimization. Interesting results obtained include intrinsic rotation reversal induced by ITG-TEM transition in flat-q profile regime and by change in q-profile from weak to normal shear.. Fluctuation-generated poloidal Reynolds stress is also shown to significantly modify the neoclassical poloidal rotation in a way consistent with experimental observations. Finally, the first-principles-based model is applied
Morphing continuum analysis of energy transfer in compressible turbulence
Cheikh, Mohamad Ibrahim; Wonnell, Louis B.; Chen, James
2018-02-01
A shock-preserving finite volume solver with the generalized Lax-Friedrichs splitting flux for morphing continuum theory (MCT) is presented and verified. The numerical MCT solver is showcased in a supersonic turbulent flow with Mach 2.93 over an 8∘ compression ramp. The simulation results validated MCT with experiments as an alternative for modeling compressible turbulence. The required size of the smallest mesh cell for the MCT simulation is shown to be almost an order larger than that in a similar direct numerical simulation study. The comparison shows MCT is a much more computationally friendly theory than the classical Navier-Stokes equations. The dynamics of energy cascade at the length scale of individual eddies is illuminated through the subscale rotation introduced by MCT. In this regard, MCT provides a statistical averaging procedure for capturing energy transfer in compressible turbulence, not found in classical fluid theories. Analysis of the MCT results show the existence of a statistical coupling of the internal and translational kinetic energy fluctuations with the corresponding eddy rotational energy fluctuations, indicating a multiscale transfer of energy. In conclusion, MCT gives a new characterization of the energy cascade within compressible turbulence without the use of excessive computational resources.
Theory of inertial waves in rotating fluids
Gelash, Andrey; L'vov, Victor; Zakharov, Vladimir
2017-04-01
The inertial waves emerge in the geophysical and astrophysical flows as a result of Earth rotation [1]. The linear theory of inertial waves is known well [2] while the influence of nonlinear effects of wave interactions are subject of many recent theoretical and experimental studies. The three-wave interactions which are allowed by inertial waves dispersion law (frequency is proportional to cosine of the angle between wave direction and axes of rotation) play an exceptional role. The recent studies on similar type of waves - internal waves, have demonstrated the possibility of formation of natural wave attractors in the ocean (see [3] and references herein). This wave focusing leads to the emergence of strong three-wave interactions and subsequent flows mixing. We believe that similar phenomena can take place for inertial waves in rotating flows. In this work we present theoretical study of three-wave and four-wave interactions for inertial waves. As the main theoretical tool we suggest the complete Hamiltonian formalism for inertial waves in rotating incompressible fluids [4]. We study three-wave decay instability and then present statistical description of inertial waves in the frame of Hamiltonian formalism. We obtain kinetic equation, anisotropic wave turbulence spectra and study the problem of parametric wave turbulence. These spectra were previously found in [5] by helicity decomposition method. Taking this into account we discuss the advantages of suggested Hamiltonian formalism and its future applications. Andrey Gelash thanks support of the RFBR (Grant No.16-31-60086 mol_a_dk) and Dr. E. Ermanyuk, Dr. I. Sibgatullin for the fruitful discussions. [1] Le Gal, P. Waves and instabilities in rotating and stratified flows, Fluid Dynamics in Physics, Engineering and Environmental Applications. Springer Berlin Heidelberg, 25-40, 2013. [2] Greenspan, H. P. The theory of rotating fluids. CUP Archive, 1968. [3] Brouzet, C., Sibgatullin, I. N., Scolan, H., Ermanyuk, E
Directory of Open Access Journals (Sweden)
Ran Tao
2014-01-01
Full Text Available RANS simulation is widely used in the flow prediction of centrifugal pumps. Influenced by impeller rotation and streamline curvature, the eddy viscosity models with turbulence isotropy assumption are not accurate enough. In this study, Spalart-Shur rotation/curvature correction was applied on the SST k-ω turbulence model. The comparative assessment of the correction was proceeded in the simulations of a centrifugal pump impeller. CFD results were compared with existing PIV and LDV data under the design and low flow rate off-design conditions. Results show the improvements of the simulation especially in the situation that turbulence strongly produced due to undesirable flow structures. Under the design condition, more reasonable turbulence kinetic energy contour was captured after correction. Under the low flow rate off-design condition, the prediction of turbulence kinetic energy and velocity distributions became much more accurate when using the corrected model. So, the rotation/curvature correction was proved effective in this study. And, it is also proved acceptable and recommended to use in the engineering simulations of centrifugal pump impellers.
Computational simulation of turbulent natural convection in a corium pool
International Nuclear Information System (INIS)
Vieira, Camila B.; Su, Jian; Niceno, Bojan
2013-01-01
After a severe accident in a nuclear power plant, the total thermal loading on the vessel of a nuclear reactor is controlled by the convective heat transfer. Taking that fact into account, this work aimed to analyze the turbulent natural convection inside a representative lower head cavity. By means of an open-source CFD code, OpenFOAM (Open Field Operation and Manipulation), numerical simulations were performed to investigate a volumetrically heated fluid (Pr = 7.0) at internal Rayleigh (Ra) numbers ranging from 10 8 to 10 15 . Bearing in mind that severe accident scenario and the physical-chemical effects are many and complex, the fluid analyzed was considered Newtonian, with constant physical properties, homogeneous and single phase. Even working with that simplifications, the modeling of turbulent natural convection has posed a considerable challenge for the Reynolds Averaged Navier-Stokes (RANS) equations based models, not only because of the complete unsteadiness of the flow and the strong turbulence effects in the near wall regions, but also because of the correct treatment of the turbulent heat fluxes (θu i ). So, this work outlined three approaches for treating the turbulent heat fluxes: the Simple Gradient Diffusion Hypothesis (SGDH), the Generalized Gradient Diffusion Hypothesis (GGDH) and the Algebraic Flux Model (AFM). Simulations performed at BALI test based geometry with a four equations model, k-ε-v 2 -f (commonly called as v 2 -f and V2-f), showed that despite of AFM and GGDH have provided reasonable agreement with experimental data for turbulent natural convection in a differentially heated cavity, they proved to be very unstable for buoyancy-driven flows with internal source in comparison to SGDH model. (author)
Reynolds-Stress and Triple-Product Models Applied to Flows with Rotation and Curvature
Olsen, Michael E.
2016-01-01
Predictions for Reynolds-stress and triple product turbulence models are compared for flows with significant rotational effects. Driver spinning cylinder flowfield and Zaets rotating pipe case are to be investigated at a minimum.
Subgrid Modeling of AGN-driven Turbulence in Galaxy Clusters
Scannapieco, Evan; Brüggen, Marcus
2008-10-01
Hot, underdense bubbles powered by active galactic nuclei (AGNs) are likely to play a key role in halting catastrophic cooling in the centers of cool-core galaxy clusters. We present three-dimensional simulations that capture the evolution of such bubbles, using an adaptive mesh hydrodynamic code, FLASH3, to which we have added a subgrid model of turbulence and mixing. While pure hydro simulations indicate that AGN bubbles are disrupted into resolution-dependent pockets of underdense gas, proper modeling of subgrid turbulence indicates that this is a poor approximation to a turbulent cascade that continues far beyond the resolution limit. Instead, Rayleigh-Taylor instabilities act to effectively mix the heated region with its surroundings, while at the same time preserving it as a coherent structure, consistent with observations. Thus, bubbles are transformed into hot clouds of mixed material as they move outward in the hydrostatic intracluster medium (ICM), much as large airbursts lead to a distinctive "mushroom cloud" structure as they rise in the hydrostatic atmosphere of Earth. Properly capturing the evolution of such clouds has important implications for many ICM properties. In particular, it significantly changes the impact of AGNs on the distribution of entropy and metals in cool-core clusters such as Perseus.
International Nuclear Information System (INIS)
Rozanov, Vladimir V.; Vountas, Marco
2014-01-01
Rotational Raman scattering of solar light in Earth's atmosphere leads to the filling-in of Fraunhofer and telluric lines observed in the reflected spectrum. The phenomenological derivation of the inelastic radiative transfer equation including rotational Raman scattering is presented. The different forms of the approximate radiative transfer equation with first-order rotational Raman scattering terms are obtained employing the Cabannes, Rayleigh, and Cabannes–Rayleigh scattering models. The solution of these equations is considered in the framework of the discrete-ordinates method using rigorous and approximate approaches to derive particular integrals. An alternative forward-adjoint technique is suggested as well. A detailed description of the model including the exact spectral matching and a binning scheme that significantly speeds up the calculations is given. The considered solution techniques are implemented in the radiative transfer software package SCIATRAN and a specified benchmark setup is presented to enable readers to compare with own results transparently. -- Highlights: • We derived the radiative transfer equation accounting for rotational Raman scattering. • Different approximate radiative transfer approaches with first order scattering were used. • Rigorous and approximate approaches are shown to derive particular integrals. • An alternative forward-adjoint technique is suggested as well. • An additional spectral binning scheme which speeds up the calculations is presented
Description of group-theoretical model of developed turbulence
International Nuclear Information System (INIS)
Saveliev, V L; Gorokhovski, M A
2008-01-01
We propose to associate the phenomenon of stationary turbulence with the special self-similar solutions of the Euler equations. These solutions represent the linear superposition of eigenfields of spatial symmetry subgroup generators and imply their dependence on time through the parameter of the symmetry transformation only. From this model, it follows that for developed turbulent process, changing the scale of averaging (filtering) of the velocity field is equivalent to composition of scaling, translation and rotation transformations. We call this property a renormalization-group invariance of filtered turbulent fields. The renormalization group invariance provides an opportunity to transform the averaged Navier-Stokes equation over a small scale (inner threshold of the turbulence) to larger scales by simple scaling. From the methodological point of view, it is significant to note that the turbulent viscosity term appeared not as a result of averaging of the nonlinear term in the Navier-Stokes equation, but from the molecular viscosity term with the help of renormalization group transformation.
A self-consistent turbulence generated scenario for L-H transition
International Nuclear Information System (INIS)
Zhang, Y.Z.; Mahajan, S.M.
1992-10-01
The turbulence-induced ion banana polarization current associated with steep ion temperature gradients is explored as a possible mechanism for generating poloidal momentum at the tokamak edge. In the light of a recently developed two-dimensional turbulence theory, one can obtain a simple closed expression relating this current (determined by turbulence levels) to the derivatives of the poloidal rotation speed. A self-consistent system, then, emerges, if we balance the turbulence-induced poloidal momentum with that dissipated by viscosity. Under suitable conditions this system may show a bifurcation controlled by a parameter dependent on temperature gradients. Both the bifurcation point, and the shear layer width are predicted for a prescribed flow in terms of a scale characterizing the nonlinearity of viscosity. The crucial relevance of the flow parity with the turbulence scenario is analyzed
Gyrokinetic Simulation of Global Turbulent Transport Properties in Tokamak Experiments
Energy Technology Data Exchange (ETDEWEB)
Wang, W.X.; Lin, Z.; Tang, W.M.; Lee, W.W.; Ethier, S.; Lewandowski, J.L.V.; Rewoldt, G.; Hahm, T.S.; Manickam, J.
2006-01-01
A general geometry gyro-kinetic model for particle simulation of plasma turbulence in tokamak experiments is described. It incorporates the comprehensive influence of noncircular cross section, realistic plasma profiles, plasma rotation, neoclassical (equilibrium) electric fields, and Coulomb collisions. An interesting result of global turbulence development in a shaped tokamak plasma is presented with regard to nonlinear turbulence spreading into the linearly stable region. The mutual interaction between turbulence and zonal flows in collisionless plasmas is studied with a focus on identifying possible nonlinear saturation mechanisms for zonal flows. A bursting temporal behavior with a period longer than the geodesic acoustic oscillation period is observed even in a collisionless system. Our simulation results suggest that the zonal flows can drive turbulence. However, this process is too weak to be an effective zonal flow saturation mechanism.
Effects of shock waves on Rayleigh-Taylor instability
International Nuclear Information System (INIS)
Zhang Yongtao; Shu Chiwang; Zhou Ye
2006-01-01
A numerical simulation of two-dimensional compressible Navier-Stokes equations using a high-order weighted essentially nonoscillatory finite difference shock capturing scheme is carried out in this paper, to study the effect of shock waves on the development of Rayleigh-Taylor instability. Shocks with different Mach numbers are introduced ahead or behind the Rayleigh-Taylor interface, and their effect on the transition to instability is demonstrated and compared. It is observed that shock waves can speed up the transition to instability for the Rayleigh-Taylor interface significantly. Stronger shocks are more effective in this speed-up process
From Newton's bucket to rotating polygons
DEFF Research Database (Denmark)
Bach, B.; Linnartz, E. C.; Vested, Malene Louise Hovgaard
2014-01-01
We present an experimental study of 'polygons' forming on the free surface of a swirling water flow in a partially filled cylindrical container. In our set-up, we rotate the bottom plate and the cylinder wall with separate motors. We thereby vary rotation rate and shear strength independently...... and move from a rigidly rotating 'Newton's bucket' flow to one where bottom and cylinder wall are rotating oppositely and the surface is strongly turbulent but flat on average. Between those two extremes, we find polygonal states for which the rotational symmetry is spontaneously broken. We investigate...... the phase diagram spanned by the two rotational frequencies at a given water filling height and find polygons in a regime, where the two frequencies are sufficiently different and, predominantly, when they have opposite signs. In addition to the extension of the family of polygons found with the stationary...
QUADRO: A SUPERVISED DIMENSION REDUCTION METHOD VIA RAYLEIGH QUOTIENT OPTIMIZATION.
Fan, Jianqing; Ke, Zheng Tracy; Liu, Han; Xia, Lucy
We propose a novel Rayleigh quotient based sparse quadratic dimension reduction method-named QUADRO (Quadratic Dimension Reduction via Rayleigh Optimization)-for analyzing high-dimensional data. Unlike in the linear setting where Rayleigh quotient optimization coincides with classification, these two problems are very different under nonlinear settings. In this paper, we clarify this difference and show that Rayleigh quotient optimization may be of independent scientific interests. One major challenge of Rayleigh quotient optimization is that the variance of quadratic statistics involves all fourth cross-moments of predictors, which are infeasible to compute for high-dimensional applications and may accumulate too many stochastic errors. This issue is resolved by considering a family of elliptical models. Moreover, for heavy-tail distributions, robust estimates of mean vectors and covariance matrices are employed to guarantee uniform convergence in estimating non-polynomially many parameters, even though only the fourth moments are assumed. Methodologically, QUADRO is based on elliptical models which allow us to formulate the Rayleigh quotient maximization as a convex optimization problem. Computationally, we propose an efficient linearized augmented Lagrangian method to solve the constrained optimization problem. Theoretically, we provide explicit rates of convergence in terms of Rayleigh quotient under both Gaussian and general elliptical models. Thorough numerical results on both synthetic and real datasets are also provided to back up our theoretical results.
Anisotropic turbulence and zonal jets in rotating flows with a β-effect
Directory of Open Access Journals (Sweden)
B. Galperin
2006-01-01
Full Text Available Numerical studies of small-scale forced, two-dimensional turbulent flows on the surface of a rotating sphere have revealed strong large-scale anisotropization that culminates in the emergence of quasi-steady sets of alternating zonal jets, or zonation. The kinetic energy spectrum of such flows also becomes strongly anisotropic. For the zonal modes, a steep spectral distribution, E(n=CZ (Ω/R2 n-5, is established, where CZ=O(1 is a non-dimensional coefficient, Ω is the angular velocity, and R is the radius of the sphere, respectively. For other, non-zonal modes, the classical, Kolmogorov-Batchelor-Kraichnan, spectral law is preserved. This flow regime, referred to as a zonostrophic regime, appears to have wide applicability to large-scale planetary and terrestrial circulations as long as those are characterized by strong rotation, vertically stable stratification and small Burger numbers. The well-known manifestations of this regime are the banded disks of the outer planets of our Solar System. Relatively less known examples are systems of narrow, subsurface, alternating zonal jets throughout all major oceans discovered in state-of-the-art, eddy-permitting simulations of the general oceanic circulation. Furthermore, laboratory experiments recently conducted using the Coriolis turntable have basically confirmed that the lateral gradient of ''planetary vorticity'' (emulated via the topographic β-effect is the primary cause of the zonation and that the latter is entwined with the development of the strongly anisotropic kinetic energy spectrum that tends to attain the same zonal and non-zonal distributions, −5 and , respectively, in both the slope and the magnitude, as the corresponding spectra in other environmental conditions. The non-dimensional coefficient CZ in the −5 spectral law appears to be invariant, , in a variety of simulated and natural flows. This paper provides a brief review of the zonostrophic regime. The review includes the
Rayleigh scattering from ions near threshold
International Nuclear Information System (INIS)
Roy, S.C.; Gupta, S.K.S.; Kissel, L.; Pratt, R.H.
1988-01-01
Theoretical studies of Rayleigh scattering of photons from neon atoms with different degrees of ionization, for energies both below and above the K-edges of the ions, are presented. Some unexpected structures both in Rayleigh scattering and in photoionization from neutral and weakly ionized atoms, very close to threshold, have been reported. It has recently been realized that some of the predicted structures may have a nonphysical origin and are due to the limitation of the independent-particle model and also to the use of a Coulombic Latter tail. Use of a K-shell vacancy potential - in which an electron is assumed to be removed from the K-shell - in calculating K-shell Rayleigh scattering amplitudes removes some of the structure effects near threshold. We present in this work a discussion of scattering angular distributions and total cross sections, obtained utilizing vacancy potentials, and compare these predictions with those previously obtained in other potential model. (author) [pt
Turbulent Liquid Metal Dynamo Experiments
International Nuclear Information System (INIS)
Forest, Cary
2007-01-01
The self-generation of magnetic fields in planets and stars--the dynamo effect--is a long-standing problem of magnetohydrodynamics and plasma physics. Until recently, research on the self-excitation process has been primarily theoretical. In this talk, I will begin with a tutorial on how magnetic fields are generated in planets and stars, describing the 'Standard Model' of self-excitation known as the alpha-omega dynamo. In this model, axisymmetric differential rotation can produce the majority of the magnetic field, but some non-axisymmetric, turbulence driven currents are also necessary. Understanding the conversion of turbulent kinetic energy in the fluid motion into electrical currents and thus magnetic fields, is a major challenge for both experiments and theory at this time. I will then report on recent results from a 1 meter diameter, spherical, liquid sodium dynamo experiment at the University of Wisconsin, in which the first clear evidence for these turbulence driven currents has been observed.
Goodson, Matthew D.; Heitsch, Fabian; Eklund, Karl; Williams, Virginia A.
2017-07-01
Turbulence models attempt to account for unresolved dynamics and diffusion in hydrodynamical simulations. We develop a common framework for two-equation Reynolds-averaged Navier-Stokes turbulence models, and we implement six models in the athena code. We verify each implementation with the standard subsonic mixing layer, although the level of agreement depends on the definition of the mixing layer width. We then test the validity of each model into the supersonic regime, showing that compressibility corrections can improve agreement with experiment. For models with buoyancy effects, we also verify our implementation via the growth of the Rayleigh-Taylor instability in a stratified medium. The models are then applied to the ubiquitous astrophysical shock-cloud interaction in three dimensions. We focus on the mixing of shock and cloud material, comparing results from turbulence models to high-resolution simulations (up to 200 cells per cloud radius) and ensemble-averaged simulations. We find that the turbulence models lead to increased spreading and mixing of the cloud, although no two models predict the same result. Increased mixing is also observed in inviscid simulations at resolutions greater than 100 cells per radius, which suggests that the turbulent mixing begins to be resolved.
Predicting chaotic time series
International Nuclear Information System (INIS)
Farmer, J.D.; Sidorowich, J.J.
1987-01-01
We present a forecasting technique for chaotic data. After embedding a time series in a state space using delay coordinates, we ''learn'' the induced nonlinear mapping using local approximation. This allows us to make short-term predictions of the future behavior of a time series, using information based only on past values. We present an error estimate for this technique, and demonstrate its effectiveness by applying it to several examples, including data from the Mackey-Glass delay differential equation, Rayleigh-Benard convection, and Taylor-Couette flow
Simple lecture demonstrations of instability and self-organization
International Nuclear Information System (INIS)
Mayer, V V; Varaksina, E I; Saranin, V A
2014-01-01
A dielectric liquid layer with an electric field created inside it is proposed as a means for demonstrating the phenomenon of self-organization. The field is produced by the distributed charge transferred by a corona discharge from the tip to the liquid surface. The theory of the phenomenon is presented. An analogy with the Rayleigh – Taylor instability is drawn and a comparison with the Benard instability is given. The practicality of the method for both natural sciences and the humanities is discussed. (methodological notes)
Zamora, Blas; Kaiser, Antonio S.
2012-01-01
The effects of the air variable properties (density, viscosity and thermal conductivity) on the buoyancy-driven flows established in open square cavities are investigated, as well as the influence of the stated boundary conditions at open edges and the employed differencing scheme. Two-dimensional, laminar, transitional and turbulent simulations are obtained, considering both uniform wall temperature and uniform heat flux heating conditions. In transitional and turbulent cases, the low-Reynolds k - ω turbulence model is employed. The average Nusselt number and the dimensionless mass-flow rate have been obtained for a wide and not yet covered range of the Rayleigh number varying from 103 to 1016. The results obtained taking into account variable properties effects are compared with those calculated assuming constant properties and the Boussinesq approximation. For uniform heat flux heating, a correlation for the critical heating parameter above which the burnout phenomenon can be obtained is presented, not reported in previous works. The effects of variable properties on the flow patterns are analyzed.
Hairpin vortices in turbulent boundary layers
International Nuclear Information System (INIS)
Eitel-Amor, G; Schlatter, P; Flores, O
2014-01-01
The present work addresses the question whether hairpin vortices are a dominant feature of near-wall turbulence and which role they play during transition. First, the parent-offspring mechanism is investigated in temporal simulations of a single hairpin vortex introduced in a mean shear flow corresponding to turbulent channels and boundary layers up to Re τ = 590. Using an eddy viscosity computed from resolved simulations, the effect of a turbulent background is also considered. Tracking the vortical structure downstream, it is found that secondary hairpins are created shortly after initialization. Thereafter, all rotational structures decay, whereas this effect is enforced in the presence of an eddy viscosity. In a second approach, a laminar boundary layer is tripped to transition by insertion of a regular pattern of hairpins by means of defined volumetric forces representing an ejection event. The idea is to create a synthetic turbulent boundary layer dominated by hairpin-like vortices. The flow for Re τ < 250 is analysed with respect to the lifetime of individual hairpin-like vortices. Both the temporal and spatial simulations demonstrate that the regeneration process is rather short-lived and may not sustain once a turbulent background has formed. From the transitional flow simulations, it is conjectured that the forest of hairpins reported in former DNS studies is an outer layer phenomenon not being connected to the onset of near-wall turbulence.
Rayleigh scattering under light-atom coherent interaction
Takamizawa, Akifumi; Shimoda, Koichi
2012-01-01
Semi-classical calculation of an oscillating dipole induced in a two-level atom indicates that spherical radiation from the dipole under coherent interaction, i.e., Rayleigh scattering, has a power level comparable to that of spontaneous emission resulting from an incoherent process. Whereas spontaneous emission is nearly isotropic and has random polarization generally, Rayleigh scattering is strongly anisotropic and polarized in association with incident light. In the case where Rabi frequen...
High-frequency Rayleigh-wave method
Xia, J.; Miller, R.D.; Xu, Y.; Luo, Y.; Chen, C.; Liu, J.; Ivanov, J.; Zeng, C.
2009-01-01
High-frequency (???2 Hz) Rayleigh-wave data acquired with a multichannel recording system have been utilized to determine shear (S)-wave velocities in near-surface geophysics since the early 1980s. This overview article discusses the main research results of high-frequency surface-wave techniques achieved by research groups at the Kansas Geological Survey and China University of Geosciences in the last 15 years. The multichannel analysis of surface wave (MASW) method is a non-invasive acoustic approach to estimate near-surface S-wave velocity. The differences between MASW results and direct borehole measurements are approximately 15% or less and random. Studies show that simultaneous inversion with higher modes and the fundamental mode can increase model resolution and an investigation depth. The other important seismic property, quality factor (Q), can also be estimated with the MASW method by inverting attenuation coefficients of Rayleigh waves. An inverted model (S-wave velocity or Q) obtained using a damped least-squares method can be assessed by an optimal damping vector in a vicinity of the inverted model determined by an objective function, which is the trace of a weighted sum of model-resolution and model-covariance matrices. Current developments include modeling high-frequency Rayleigh-waves in near-surface media, which builds a foundation for shallow seismic or Rayleigh-wave inversion in the time-offset domain; imaging dispersive energy with high resolution in the frequency-velocity domain and possibly with data in an arbitrary acquisition geometry, which opens a door for 3D surface-wave techniques; and successfully separating surface-wave modes, which provides a valuable tool to perform S-wave velocity profiling with high-horizontal resolution. ?? China University of Geosciences (Wuhan) and Springer-Verlag GmbH 2009.
Computational simulation of turbulent natural convection in a corium pool
Energy Technology Data Exchange (ETDEWEB)
Vieira, Camila B.; Su, Jian, E-mail: camila@lasme.coppe.ufrj.br, E-mail: sujian@lasme.coppe.ufrj.br [Coordenacao dos Cursos de Pos-Graduacao em Engenharia (COPPE/UFRJ), Rio de Janeiro, RJ (Brazil). Programa de Engenharia Nuclear; Niceno, Bojan, E-mail: bojan.niceno@psi.ch [Paul Scherrer Institut (PSI), Villigen (Switzerland). Nuclear Energy and Safety
2013-07-01
After a severe accident in a nuclear power plant, the total thermal loading on the vessel of a nuclear reactor is controlled by the convective heat transfer. Taking that fact into account, this work aimed to analyze the turbulent natural convection inside a representative lower head cavity. By means of an open-source CFD code, OpenFOAM (Open Field Operation and Manipulation), numerical simulations were performed to investigate a volumetrically heated fluid (Pr = 7.0) at internal Rayleigh (Ra) numbers ranging from 10{sup 8} to 10{sup 15}. Bearing in mind that severe accident scenario and the physical-chemical effects are many and complex, the fluid analyzed was considered Newtonian, with constant physical properties, homogeneous and single phase. Even working with that simplifications, the modeling of turbulent natural convection has posed a considerable challenge for the Reynolds Averaged Navier-Stokes (RANS) equations based models, not only because of the complete unsteadiness of the flow and the strong turbulence effects in the near wall regions, but also because of the correct treatment of the turbulent heat fluxes (θu{sub i}). So, this work outlined three approaches for treating the turbulent heat fluxes: the Simple Gradient Diffusion Hypothesis (SGDH), the Generalized Gradient Diffusion Hypothesis (GGDH) and the Algebraic Flux Model (AFM). Simulations performed at BALI test based geometry with a four equations model, k-ε-v{sup 2} -f (commonly called as v{sup 2}-f and V2-f), showed that despite of AFM and GGDH have provided reasonable agreement with experimental data for turbulent natural convection in a differentially heated cavity, they proved to be very unstable for buoyancy-driven flows with internal source in comparison to SGDH model. (author)
Inertial modes and their transition to turbulence in a differentially rotating spherical gap flow
Hoff, Michael; Harlander, Uwe; Andrés Triana, Santiago; Egbers, Christoph
2016-04-01
dominant mode (l,m,ˆω) = (3,2,˜ 0.71) is increasing with increasing |Ro| until a critical Rossby number Rocrit. Accompanying with this is an increase of the zonal mean flow outside the tangent cylinder, leading to enhanced angular momentum transport. At the particular Rocrit, the wave mode, and the entire flow, breaks up into smaller-scale turbulence [6], together with a strong increase of the zonal mean flow inside the tangent cylinder. We found that the critical Rossby number scales approximately with E1/5. References [1] Aldridge, K. D.; Lumb, L. I. (1987): Inertial waves identified in the Earth's fluid outer core. Nature 325 (6103), S. 421-423. DOI: 10.1038/325421a0. [2] Greenspan, H. P. (1968): The theory of rotating fluids. London: Cambridge U.P. (Cambridge monographs on mechanics and applied mathematics). [3] Kelley, D. H.; Triana, S. A.; Zimmerman, D. S.; Lathrop, D. P. (2010): Selection of inertial modes in spherical Couette flow. Phys. Rev. E 81 (2), 26311. DOI: 10.1103/PhysRevE.81.026311. [4] Rieutord, M.; Triana, S. A.; Zimmerman, D. S.; Lathrop, D. P. (2012): Excitation of inertial modes in an experimental spherical Couette flow. Phys. Rev. E 86 (2), 026304. DOI: 10.1103/PhysRevE.86.026304. [5] Hoff, M., Harlander, U., Egbers, C. (2016): Experimental survey of linear and nonlinear inertial waves and wave instabilities in a spherical shell. J. Fluid Mech., (in print) [6] Kerswell, R. R. (1999): Secondary instabilities in rapidly rotating fluids: inertial wave breakdown. Journal of Fluid Mechanics 382, S. 283-306. DOI: 10.1017/S0022112098003954.
Rayleigh-Brillouin scattering in SF6 in the kinetic regime
Wang, Yuanqing; Yu, Yin; Liang, Kun; Marques, Wilson; van de Water, Willem; Ubachs, Wim
2017-02-01
Rayleigh-Brillouin spectral profiles are measured with a laser-based scatterometry setup for a 90° scattering angle at a high signal-to-noise ratio (r.m.s. noise below 0.15% w.r.t. peak intensity) in sulfur-hexafluoride gas for pressures in the range 0.2-5 bar and for a wavelength of λ = 403.0 nm. The high quality data are compared to a number of light scattering models in order to address the effects of rotational and vibrational relaxation. While the vibrational relaxation rate is so slow that vibration degrees of freedom remain frozen, rotations relax on time scales comparable to those of the density fluctuations. Therefore, the heat capacity, the thermal conductivity and the bulk viscosity are all frequency-dependent transport coefficients. This is relevant for the Tenti model that depends on the values chosen for these transport coefficients. This is not the case for the other two models considered: a kinetic model based on rough-sphere interactions, and a model based on fluctuating hydrodynamics. The deviations with the experiment are similar between the three different models, except for the hydrodynamic model at pressures p≲ 2bar . As all models are in line with the ideal gas law, we hypothesize the presence of real gas effects in the measured spectra.
Radiative effects on turbulent buoyancy-driven air flow in open square cavities
International Nuclear Information System (INIS)
Zamora, B.; Kaiser, A.S.
2016-01-01
The effects of the radiative effects and the air variable properties (density, viscosity and thermal conductivity) on the buoyancy-driven flows established in open square cavities are investigated. Two-dimensional, laminar, transitional and turbulent simulations are obtained, considering both uniform wall temperature and uniform heat flux heating conditions. In transitional and turbulent cases, the low- Reynolds k-ω turbulence model is employed. The average Nusselt number and the dimensionless mass-flow rate have been obtained for a wide range of the Rayleigh number varying from 10 3 to 10 16 . The results obtained taking into account the variable thermophysical properties of air are compared to those calculated assuming constant properties and the Boussinesq approximation. In addition, the influence of considering surface radiative effects on the differences reached for the Nusselt number and the mass flow rate obtained with several intensities of heating is studied; specifically, the effects of thermal radiation on the appearance of the burnout phenomenon is analyzed. The changes produced in the flow patterns into the cavity when the radiative heat transfer and the effects of variation of properties are relevant, are also shown. (authors)
International Nuclear Information System (INIS)
Ma Jun; Wang Chunni; Li Yanlong; Pu Zhongsheng; Jin Wuyin
2008-01-01
This paper proposes a scheme of parameter perturbation to suppress the stable rotating spiral wave, meandering spiral wave and turbulence in the excitable media, which is described by the modified Fitzhugh–Nagumo (MFHN) model. The controllable parameter in the MFHN model is perturbed with a weak pulse and the pulse period is decided by the rotating period of the spiral wave approximatively. It is confirmed that the spiral wave and spiral turbulence can be suppressed greatly. Drift and instability of spiral wave can be observed in the numerical simulation tests before the whole media become homogeneous finally. (general)
Translational, rotational, vibrational and electron temperatures of a gliding arc discharge
DEFF Research Database (Denmark)
Zhu, Jiajian; Ehn, Andreas; Gao, Jinlong
2017-01-01
, 0) band was used to simulate the rotational temperature (Tr) of the gliding arc discharge whereas the NO A–X (1, 0) and (0, 1) bands were used to determine its vibrational temperature (Tv). The instantaneous reduced electric field strength E/N was obtained by simultaneously measuring......Translational, rotational, vibrational and electron temperatures of a gliding arc discharge in atmospheric pressure air were experimentally investigated using in situ, non-intrusive optical diagnostic techniques. The gliding arc discharge was driven by a 35 kHz alternating current (AC) power source...... and operated in a glow-type regime. The two-dimensional distribution of the translational temperature (Tt) of the gliding arc discharge was determined using planar laser-induced Rayleigh scattering. The rotational and vibrational temperatures were obtained by simulating the experimental spectra. The OH A–X (0...
The Intrinsic Variability in the Water Vapor Saturation Ratio due to Turbulence
Anderson, J. C.; Cantrell, W. H.; Chandrakar, K. K.; Kostinski, A. B.; Niedermeier, D.; Shaw, R. A.
2017-12-01
In the atmosphere, the concentration of water vapor plays an important role in Earth's weather and climate. The mean concentration of water vapor is key to its efficiency as a greenhouse gas; the fluctuations about the mean are important for heat fluxes near the surface of earth. In boundary layer clouds, fluctuations in the water vapor concentration are linked to turbulence. Conditions representative of boundary layer clouds are simulated in Michigan Tech's multiphase, turbulent reaction chamber, the ∏ chamber, where the boundary conditions are controlled and repeatable. Measurements for temperature and water vapor concentration were recorded under forced Rayleigh-Bénard convection. As expected, the distributions for temperature and water vapor concentration broaden as the turbulence becomes more vigorous. From these two measurements the saturation ratio can be calculated. The fluctuations in the water vapor concentration are more important to the variability in the saturation ratio than fluctuations in temperature. In a cloud, these fluctuations in the saturation ratio can result in some cloud droplets experiencing much higher supersaturations. Those "lucky" droplets grow by condensation at a faster rate than other cloud droplets. The difference in the droplet growth rate could contribute to a broadened droplet distribution, which leads to the onset of collision-coalescence. With more intense turbulence these effect will become more pronounced as the fluctuations about the mean saturation ratio become more pronounced.
International Nuclear Information System (INIS)
Anon.
1984-01-01
A hearing on the nomination of Benard C. Rusche to the new position of Director of the Office of Civilian Radioactive Waste Management in DOE began with an introduction and endorsement by Senator Thurmond of South Carolina. Testimony by the nominee emphasized the need to move ahead on radioactive waste disposal and storage arrangements that will provide protection for the environment and the public. His key points included the need for a common framework and an integrated program that would include opportunities for public discussion of all affected parties. He expressed a commitment to responsible and accountable procedures and cooperation with other regulatory agencies
Laminar-Turbulent transition on Wind Turbines
DEFF Research Database (Denmark)
Martinez Hernandez, Gabriel Gerardo
The present thesis deals with the study of the rotational effects on the laminar-turbulent transition on wind turbine blades. Linear stability theory is used to formulate the stability equations that include the effect of rotation. The mean flow required as an input to stability computations...... parametrized and adapted to an wind turbine rotor geometry. The blade is resolved in radial sections along which calculations are performed. The obtained mean flow is classified according to the parameters used on the rotating configuration, geometry and operational conditions. The stability diagrams have been...... to define the resultant wave magnitude and direction. The propagation of disturbances in the boundary layers in three dimensional flows is relatively a complicated phenomena. The report discusses the available methods and techniques used to predict the transition location. Some common wind turbine airfoils...
Transition from Spirals to Defect-Mediated Turbulence Driven by a Doppler Instability
International Nuclear Information System (INIS)
Ouyang, Qi; Swinney, Harry L.; Li, Ge
2000-01-01
A transition from rotating chemical spirals to turbulence is observed in experiments on the Belousov-Zhabotinsky reaction. The transition occurs when the waves near the spiral tip spontaneously break, generating defects. Measurements reveal that this defect-mediated turbulence is caused by the Doppler effect on the traveling waves. The observations are in good accord with numerical simulations and theory. (c) 2000 The American Physical Society
Universal equations and constants of turbulent motion
International Nuclear Information System (INIS)
Baumert, H Z
2013-01-01
This paper presents a parameter-free theory of shear-generated turbulence at asymptotically high Reynolds numbers in incompressible fluids. It is based on a two-fluids concept. Both components are materially identical and inviscid. The first component is an ensemble of quasi-rigid dipole-vortex tubes (vortex filaments, excitations) as quasi-particles in chaotic motion. The second is a superfluid performing evasive motions between the tubes. The local dipole motions follow Helmholtz' law. The vortex radii scale with the energy-containing length scale. Collisions between quasi-particles lead either to annihilation (likewise rotation, turbulent dissipation) or to scattering (counterrotation, turbulent diffusion). There are analogies with birth and death processes of population dynamics and their master equations and with Landau's two-fluid theory of liquid helium. For free homogeneous decay the theory predicts the turbulent kinetic energy to follow t −1 . With an adiabatic wall condition it predicts the logarithmic law with von Kármán's constant as 1/√(2 π)= 0.399. Likewise rotating couples form localized dissipative patches almost at rest (→ intermittency) wherein under local quasi-steady conditions the spectrum evolves into an ‘Apollonian gear’ as discussed first by Herrmann (1990 Correlation and Connectivity (Dordrecht: Kluwer) pp 108–20). Dissipation happens exclusively at scale zero and at finite scales this system is frictionless and reminds of Prigogine's (1947 Etude Thermodynamique des Phenomenes Irreversibles (Liege: Desoer) p 143) law of minimum (here: zero) entropy production. The theory predicts further the prefactor of the 3D-wavenumber spectrum (a Kolmogorov constant) as 1/3 (4 π) 2/3 =1.802, well within the scatter range of observational, experimental and direct numerical simulation results. (paper)
Universal equations and constants of turbulent motion
Baumert, H. Z.
2013-07-01
This paper presents a parameter-free theory of shear-generated turbulence at asymptotically high Reynolds numbers in incompressible fluids. It is based on a two-fluids concept. Both components are materially identical and inviscid. The first component is an ensemble of quasi-rigid dipole-vortex tubes (vortex filaments, excitations) as quasi-particles in chaotic motion. The second is a superfluid performing evasive motions between the tubes. The local dipole motions follow Helmholtz' law. The vortex radii scale with the energy-containing length scale. Collisions between quasi-particles lead either to annihilation (likewise rotation, turbulent dissipation) or to scattering (counterrotation, turbulent diffusion). There are analogies with birth and death processes of population dynamics and their master equations and with Landau's two-fluid theory of liquid helium. For free homogeneous decay the theory predicts the turbulent kinetic energy to follow t-1. With an adiabatic wall condition it predicts the logarithmic law with von Kármán's constant as 1/\\sqrt {2\\,\\pi }= 0.399 . Likewise rotating couples form localized dissipative patches almost at rest (→ intermittency) wherein under local quasi-steady conditions the spectrum evolves into an ‘Apollonian gear’ as discussed first by Herrmann (1990 Correlation and Connectivity (Dordrecht: Kluwer) pp 108-20). Dissipation happens exclusively at scale zero and at finite scales this system is frictionless and reminds of Prigogine's (1947 Etude Thermodynamique des Phenomenes Irreversibles (Liege: Desoer) p 143) law of minimum (here: zero) entropy production. The theory predicts further the prefactor of the 3D-wavenumber spectrum (a Kolmogorov constant) as \\frac {1}{3}(4\\,\\pi )^{2/3}=1.802 , well within the scatter range of observational, experimental and direct numerical simulation results.
Banerjee, Supratik; Kritsuk, Alexei G.
2018-02-01
Three-dimensional, compressible, magnetohydrodynamic turbulence of an isothermal, self-gravitating fluid is analyzed using two-point statistics in the asymptotic limit of large Reynolds numbers (both kinetic and magnetic). Following an alternative formulation proposed by Banerjee and Galtier [Phys. Rev. E 93, 033120 (2016), 10.1103/PhysRevE.93.033120; J. Phys. A: Math. Theor. 50, 015501 (2017), 10.1088/1751-8113/50/1/015501], an exact relation has been derived for the total energy transfer. This approach results in a simpler relation expressed entirely in terms of mixed second-order structure functions. The kinetic, thermodynamic, magnetic, and gravitational contributions to the energy transfer rate can be easily separated in the present form. By construction, the new formalism includes such additional effects as global rotation, the Hall term in the induction equation, etc. The analysis shows that solid-body rotation cannot alter the energy flux rate of compressible turbulence. However, the contribution of a uniform background magnetic field to the flux is shown to be nontrivial unlike in the incompressible case. Finally, the compressible, turbulent energy flux rate does not vanish completely due to simple alignments, which leads to a zero turbulent energy flux rate in the incompressible case.
Intrinsic Ambipolarity and Rotation in Stellarators
International Nuclear Information System (INIS)
Helander, P.; Simakov, A. N.
2008-01-01
It is shown that collisional plasma transport is intrinsically ambipolar only in quasiaxisymmetric or quasihelically symmetric magnetic configurations. Only in such fields can the plasma rotate freely, and then only in the direction of quasisymmetry. In a non-quasi-symmetric magnetic field, the average radial electric field is determined by parallel viscosity, which in turn is usually governed by collisional processes. Locally, the radial electric field may be affected by turbulent Reynolds stress producing zonal flows, but on a radial average taken over several ion gyroradii, it is determined by parallel viscosity, at least if the turbulence is electrostatic and obeys the conventional gyrokinetic orderings. This differs from the situation in a tokamak, where there is no flow damping by parallel viscosity in the symmetry direction and the turbulent Reynolds stress may affect the global radial electric field
Hartland, Tucker; Schilling, Oleg
2017-11-01
Analytical self-similar solutions to several families of single- and two-scale, eddy viscosity and Reynolds stress turbulence models are presented for Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholtz instability-induced turbulent mixing. The use of algebraic relationships between model coefficients and physical observables (e.g., experimental growth rates) following from the self-similar solutions to calibrate a member of a given family of turbulence models is shown. It is demonstrated numerically that the algebraic relations accurately predict the value and variation of physical outputs of a Reynolds-averaged simulation in flow regimes that are consistent with the simplifying assumptions used to derive the solutions. The use of experimental and numerical simulation data on Reynolds stress anisotropy ratios to calibrate a Reynolds stress model is briefly illustrated. The implications of the analytical solutions for future Reynolds-averaged modeling of hydrodynamic instability-induced mixing are briefly discussed. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Turbulence assessment at potential turbine sites
Energy Technology Data Exchange (ETDEWEB)
Daniels, A. [Univ. of Hawaii, Honolulu, HI (United States)
1996-12-31
As opposed to a fixed anemometer, the Tala kite is free to move in the air. The motion of the kite is not random, it moves with or against the speed gradient towards the center of passing turbulence events of higher or lower speeds thus allowing the kite to measure event maximum or minimum speed rather than the speed at some unknown distance from the event center like a fixed anemometer. This behavior is confirmed both by a theoretical aerodynamics analysis of the kite motion and by data from a field study where kite and hot film anemometer (HFA) events, defined by the rain flow count method, were compared with flap events on a rotating turbine blade. The HFAs simulated too few events lasting too long while the kites reproduced both the number of events and event periods remarkably close. It is concluded that the kite is the optimal tool for measuring turbulence at potential turbine sites. Kite turbulence can form the bases for economic return estimates and an example is given where less windy sites could be more economical than other more turbulent higher speed sites. 13 refs., 8 figs.
Precession of a rapidly rotating cylinder flow: traverse through resonance
Lopez, Juan; Marques, Francisco
2014-11-01
The flow in a rapidly rotating cylinder that is titled and also rotating around another axis can undergo sudden transitions to turbulence. Experimental observations of this have been associated with triadic resonances. The experimental and theoretical results are well-established in the literature, but there remains a lack of understanding of the physical mechanisms at play in the sudden transition from laminar to turbulent flow with very small variations in the governing parameters. Here, we present direct numerical simulations of a traverse in parameter space through an isolated resonance, and describe in detail the bifurcations involved in the sudden transition. U.S. National Science Foundation Grant CBET-1336410 and Spanish Ministry of Education and Science Grant (with FEDER funds) FIS2013-40880.
On the mechanism of elasto-inertial turbulence.
Dubief, Yves; Terrapon, Vincent E; Soria, Julio
2013-11-01
Elasto-inertial turbulence (EIT) is a new state of turbulence found in inertial flows with polymer additives. The dynamics of turbulence generated and controlled by such additives is investigated from the perspective of the coupling between polymer dynamics and flow structures. Direct numerical simulations of channel flow with Reynolds numbers ranging from 1000 to 6000 (based on the bulk and the channel height) are used to study the formation and dynamics of elastic instabilities and their effects on the flow. The flow topology of EIT is found to differ significantly from Newtonian wall-turbulence. Structures identified by positive (rotational flow topology) and negative (extensional/compressional flow topology) second invariant Q a isosurfaces of the velocity gradient are cylindrical and aligned in the spanwise direction. Polymers are significantly stretched in sheet-like regions that extend in the streamwise direction with a small upward tilt. The Q a cylindrical structures emerge from the sheets of high polymer extension, in a mechanism of energy transfer from the fluctuations of the polymer stress work to the turbulent kinetic energy. At subcritical Reynolds numbers, EIT is observed at modest Weissenberg number ( Wi , ratio polymer relaxation time to viscous time scale). For supercritical Reynolds numbers, flows approach EIT at large Wi . EIT provides new insights on the nature of the asymptotic state of polymer drag reduction (maximum drag reduction), and explains the phenomenon of early turbulence, or onset of turbulence at lower Reynolds numbers than for Newtonian flows observed in some polymeric flows.
Strange attractors in weakly turbulent Couette-Taylor flow
Brandstater, A.; Swinney, Harry L.
1987-01-01
An experiment is conducted on the transition from quasi-periodic to weakly turbulent flow of a fluid contained between concentric cylinders with the inner cylinder rotating and the outer cylinder at rest. Power spectra, phase-space portraits, and circle maps obtained from velocity time-series data indicate that the nonperiodic behavior observed is deterministic, that is, it is described by strange attractors. Various problems that arise in computing the dimension of strange attractors constructed from experimental data are discussed and it is shown that these problems impose severe requirements on the quantity and accuracy of data necessary for determining dimensions greater than about 5. In the present experiment the attractor dimension increases from 2 at the onset of turbulence to about 4 at a Reynolds number 50-percent above the onset of turbulence.
The residual zonal dynamics in a toroidally rotating tokamak
International Nuclear Information System (INIS)
Zhou Deng
2015-01-01
Zonal flows, initially driven by ion-temperature-gradient turbulence, may evolve due to the neoclassic polarization in a collisionless tokamak plasma. In this presentation, the form of the residual zonal flow is presented for tokamak plasmas rotating toroidally at arbitrary velocity. The gyro-kinetic equation is analytically solved to give the expression of residual zonal flows with arbitrary rotating velocity. The zonal flow level decreases as the rotating velocity increases. The numerical evaluation is in good agreement with the previous simulation result for high aspect ratio tokamaks. (author)
Sensible Heat Flux Related to Variations in Atmospheric Turbulence Kinetic Energy on a Sandy Beach
2017-06-01
production, turbulent transport by pressure fluctuations, dissipation and flux divergence . The TKE budget as explained by Srivastava and Sarthi (2002...generation of turbulence. Term 3 is flux divergence , which describes the differential transport of TKE by turbulent eddies. Term 4, dissipation, is a sink...the time series data to align all signals to the same time base. Winds were rotated into a shore-normal frame of reference. All data outside of T
Energy Technology Data Exchange (ETDEWEB)
Ravelet, F
2005-09-15
We report experimental studies of the turbulent von Karman flow, inertially stirred between counter-rotating impellers. We first study the flow and its transition from laminar to turbulent regime. We highlight the role of slowly varying large scales, due to the presence of an azimuthal mixing layer. The large scales of this flow can be unstable in turbulent regime. We study the statistics of the transitions between the different mean states. The second part is dedicated to an experiment in liquid sodium, called VKS2. We optimize the time-averaged flow in order to allow kinematic dynamo action. We report the very first results of the experiment, and discuss the role of the large scales temporal non-stationariness. (author)
Directory of Open Access Journals (Sweden)
Semen I. Sherman
2015-01-01
Full Text Available The paper presents the first tectonophysical reconstruction of initial divisibility of the protolithosphere as a result of convection in the cooling primitive mantle. Initial division of the protolithosphere into separate masses, i.e. prototypes of the blocks, and their size are predetermined by the emerging Rayleigh-Benard convection cells. In studies of geology and geodynamics, the Rayleigh-Benard convection cells were first referred to as a factor to explain the formation of initial continental cores. Considering the Rayleigh-Benard cells and their structural relics can help clarify initial divisibility of the protolithosphere and the origin of the major lithospheric plates, i.e. prototypes of continents. In our opinion, the initial mega-scale block structure of the protolithosphere and the emerging lithosphere were predetermined by the Rayleigh-Benard cells as they were preserved in the emerging lithosphere and their lower boundaries corresponded to the core-mantle boundary, i.e. one of the major discontinuities of the planet. Our theoretical estimations are in good agreement with the number and sizes of the Earth's theorized first supercontinents, Vaalbara and Ur. In our tectonophysical discussion of the formation of the lithospheric block structure, we analyze in detail the map of modern lithospheric plates [Bird, 2003] in combination with the materials from [Sherman et al., 2000]. In the hierarchy of the blocks comprising the contemporary lithosphere, which sizes are widely variable, two groups of blocks are clearly distinguished. The first group includes megablocks with the average geometric size above 6500 km. Their formation is related to convection in the Earth mantle at the present stage of the geodynamic evolution of the Earth, as well as at all the previous stages, including the earliest one, when the protolithosphere emerged. The second group includes medium-sized blocks with the average geometric size of less than 4500 km and
Chaotic cold accretion on to black holes in rotating atmospheres
Gaspari, M.; Brighenti, F.; Temi, P.
2015-07-01
The fueling of black holes is one key problem in the evolution of baryons in the universe. Chaotic cold accretion (CCA) profoundly differs from classic accretion models, as Bondi and thin disc theories. Using 3D high-resolution hydrodynamic simulations, we now probe the impact of rotation on the hot and cold accretion flow in a typical massive galaxy. In the hot mode, with or without turbulence, the pressure-dominated flow forms a geometrically thick rotational barrier, suppressing the black hole accretion rate to ~1/3 of the spherical case value. When radiative cooling is dominant, the gas loses pressure support and quickly circularizes in a cold thin disk; the accretion rate is decoupled from the cooling rate, although it is higher than that of the hot mode. In the more common state of a turbulent and heated atmosphere, CCA drives the dynamics if the gas velocity dispersion exceeds the rotational velocity, i.e., turbulent Taylor number Tat 1), the broadening of the distribution and the efficiency of collisions diminish, damping the accretion rate ∝ Tat-1, until the cold disk drives the dynamics. This is exacerbated by the increased difficulty to grow TI in a rotating halo. The simulated sub-Eddington accretion rates cover the range inferred from AGN cavity observations. CCA predicts inner flat X-ray temperature and r-1 density profiles, as recently discovered in M 87 and NGC 3115. The synthetic Hα images reproduce the main features of cold gas observations in massive ellipticals, as the line fluxes and the filaments versus disk morphology. Such dichotomy is key for the long-term AGN feedback cycle. As gas cools, filamentary CCA develops and boosts AGN heating; the cold mode is thus reduced and the rotating disk remains the sole cold structure. Its consumption leaves the atmosphere in hot mode with suppressed accretion and feedback, reloading the cycle.
International Nuclear Information System (INIS)
Choi, Hang Seok; Park, Tae Seon
2013-01-01
Highlights: ► With changing aspect ratio, the effect of secondary flows on the turbulent heat transfer is scrutinized by a LES. ► The conditional sampling technique of instantaneous near-wall streamwise vortices is developed. ► Clockwise and counter-clockwise rotating streamwise vortices are sampled and discussed with the wall heat transfer. ► The hot-sweep motions of CW and CCW vortices clearly appear with increasing aspect ratio. -- Abstract: The effect of aspect ratio of rectangular duct on the turbulent flow and heat transfer is very important for its engineering applications. But the turbulent thermal fields have not been fundamentally scrutinized in spite of its engineering significance especially for cooling device. Hence, in the present study, large eddy simulation is applied to the turbulent flow and heat transfer in rectangular ducts with varying aspect ratio. The turbulent statistics of the flow and thermal quantities are calculated and the characteristics of wall Nusselt number are investigated for each rectangular duct. Especially, to scrutinize near-wall streamwise vortices, a conditional sampling technique is developed and adopted. Clockwise and counter-clockwise rotating streamwise vortices are sampled and the probability density function of the vortex circulation Reynolds number and wall Nusselt number are calculated. From the results, the time-averaged secondary flow caused by instantaneous vortical motions has a great effect on the heat and momentum transport of the flow in the rectangular ducts. Hence, the wall Nusselt number is enhanced near the downwash flow region of the secondary flow. However, with increasing the aspect ratio, the effects of the hot-sweep flow of the clockwise and counter-clockwise rotating vortices become equally dominant near the wall normal bisector of the ducts. During time averaging process, these two counter-rotating vortices are canceled out each other diminishing a secondary flow but they still enhance the
The onset of nonpenetrative convection in a suddenly cooled layer of fluid
Energy Technology Data Exchange (ETDEWEB)
Ihle, Christian F. [Program in Fluid Dynamics, Facultad de Ciencias Fisicas y Matematicas, Universidad de Chile, Blanco Encalada 2002 Of. 327, Santiago (Chile); Nino, Yarko [Departamento de Ingenieria Civil, Division de Recursos Hidricos y Medio Ambiente, Universidad de Chile, Av. Blanco Encalada 2002, Santiago (Chile)
2006-04-15
Conditions for the onset of nonpenetrative convection in a horizontal Boussinesq fluid layer subject to a step change in temperature are studied using propagation theory. A wide range of Prandtl numbers and two different kinematic boundary conditions are considered. It is shown that for high Rayleigh numbers, critical conditions for the onset of convective motion reproduce exactly those for the unsteady Rayleigh-Benard instability. Present results extend those of previous research and show a tendency of the rigid-rigid and free-rigid critical curves to converge for low Prandtl numbers. Comparison between present and previously reported results on critical conditions for the onset of instabilities and onset time using different methods yields good agreement on a middle to high Prandtl number range. A ratio of 10 between experimentally measured and theoretically predicted onset times is suggested for stress-free bounded systems. (author)
Angular momentum transfer in primordial discs and the rotation of the first stars
Hirano, Shingo; Bromm, Volker
2018-05-01
We investigate the rotation velocity of the first stars by modelling the angular momentum transfer in the primordial accretion disc. Assessing the impact of magnetic braking, we consider the transition in angular momentum transport mode at the Alfvén radius, from the dynamically dominated free-fall accretion to the magnetically dominated solid-body one. The accreting protostar at the centre of the primordial star-forming cloud rotates with close to breakup speed in the case without magnetic fields. Considering a physically motivated model for small-scale turbulent dynamo amplification, we find that stellar rotation speed quickly declines if a large fraction of the initial turbulent energy is converted to magnetic energy (≳ 0.14). Alternatively, if the dynamo process were inefficient, for amplification due to flux freezing, stars would become slow rotators if the pre-galactic magnetic field strength is above a critical value, ≃10-8.2 G, evaluated at a scale of nH = 1 cm-3, which is significantly higher than plausible cosmological seed values (˜10-15 G). Because of the rapid decline of the stellar rotational speed over a narrow range in model parameters, the first stars encounter a bimodal fate: rapid rotation at almost the breakup level, or the near absence of any rotation.
Toroidal rotation studies in KSTAR
Lee, S. G.; Lee, H. H.; Yoo, J. W.; Kim, Y. S.; Ko, W. H.; Terzolo, L.; Bitter, M.; Hill, K.; KSTAR Team
2014-10-01
Investigation of the toroidal rotation is one of the most important topics for the magnetically confined fusion plasma researches since it is essential for the stabilization of resistive wall modes and its shear plays an important role to improve plasma confinement by suppressing turbulent transport. The most advantage of KSTAR tokamak for toroidal rotation studies is that it equips two main diagnostics including the high-resolution X-ray imaging crystal spectrometer (XICS) and charge exchange spectroscopy (CES). Simultaneous core toroidal rotation and ion temperature measurements of different impurity species from the XICS and CES have shown in reasonable agreement with various plasma discharges in KSTAR. It has been observed that the toroidal rotation in KSTAR is faster than that of other tokamak devices with similar machine size and momentum input. This may due to an intrinsically low toroidal field ripple and error field of the KSTAR device. A strong braking of the toroidal rotation by the n = 1 non-resonant magnetic perturbations (NRMPs) also indicates these low toroidal field ripple and error field. Recently, it has been found that n = 2 NRMPs can also damp the toroidal rotation in KSTAR. The detail toroidal rotation studies will be presented. Work supported by the Korea Ministry of Science, ICT and Future Planning under the KSTAR project.
Turbulence Nature and the Inverse Problem
Pyatnitsky, L. N
2009-01-01
Hydrodynamic equations well describe averaged parameters of turbulent steady flows, at least in pipes where boundary conditions can be estimated. The equations might outline the parameters fluctuations as well, if entry conditions at current boundaries were known. This raises, in addition, the more comprehensive problem of the primary perturbation nature, noted by H.A. Lorentz, which still remains unsolved. Generally, any flow steadiness should be supported by pressure waves emitted by some external source, e.g. a piston or a receiver. The wave plane front in channels quickly takes convex configuration owing to Rayleigh's law of diffraction divergence. The Schlieren technique and pressure wave registration were employed to investigate the wave interaction with boundary layer, while reflecting from the channel wall. The reflection induces boundary-layer local separation and following pressure rapid increase within the perturbation zone. It propagates as an acoustic wave packet of spherical shape, bearing oscil...
International Nuclear Information System (INIS)
Goto, R.; Hatori, T.; Miura, H.; Ito, A.; Sato, M.
2015-01-01
Two-fluid and the finite Larmor effects on linear and nonlinear growth of the Rayleigh-Taylor instability in a two-dimensional slab are studied numerically with special attention to high-wave-number dynamics and nonlinear structure formation at a low β-value. The two effects stabilize the unstable high wave number modes for a certain range of the β-value. In nonlinear simulations, the absence of the high wave number modes in the linear stage leads to the formation of the density field structure much larger than that in the single-fluid magnetohydrodynamic simulation, together with a sharp density gradient as well as a large velocity difference. The formation of the sharp velocity difference leads to a subsequent Kelvin-Helmholtz-type instability only when both the two-fluid and finite Larmor radius terms are incorporated, whereas it is not observed otherwise. It is shown that the emergence of the secondary instability can modify the outline of the turbulent structures associated with the primary Rayleigh-Taylor instability
Rotationally induced fragmentation in the prestellar core L1544
Energy Technology Data Exchange (ETDEWEB)
Klapp, Jaime; Zavala, Miguel [Departamento de Física, Instituto Nacional de Investigaciones Nucleares (ININ), Km. 36.5, Carretera México-Toluca, La Marquesa 52750, Estado de México (Mexico); Sigalotti, Leonardo Di G.; Peña-Polo, Franklin; Troconis, Jorge [Centro de Física, Instituto Venezolano de Investigaciones Científicas (IVIC), Apartado Postal 20632, Caracas 1020A (Venezuela, Bolivarian Republic of)
2014-01-10
Recent observations indicate that there is no correlation between the level of turbulence and fragmentation in detected protostellar cores, suggesting that turbulence works mainly before gravitationally bound prestellar cores form and that their inner parts are likely to be velocity coherent. Based on this evidence, we simulate the collapse and fragmentation of an isolated, initially centrally condensed, uniformly rotating core of total mass M = 5.4 M {sub ☉}, using the smoothed particle hydrodynamics code GADGET-2 modified with the inclusion of sink particles, in order to compare the statistical properties of the resulting stellar ensembles with previous gravoturbulent fragmentation models. The initial conditions are intended to fit the observed properties of the prestellar core L1544. We find that for ratios of the rotational to the gravitational energy β ≥ 0.05, a massive disk is formed at the core center from which a central primary condenses after ∼50 kyr. Soon thereafter the disk fragments into secondary protostars, consistent with an intermediate mode of star formation in which groups of 10-100 stars form from a single core. The models predict peak accretion rates between ∼10{sup –5} and 10{sup –4} M {sub ☉} yr{sup –1} for all stars and reproduce many of the statistical properties predicted from gravoturbulent fragmentation, suggesting that on the small scales of low-mass, dense cores these are independent of whether the contracting gas is turbulent or purely rotating.
Preliminary study of Rayleigh-Taylor instability in wire-array Z-pinch
International Nuclear Information System (INIS)
He Kaihui; Feng Kaiming; Li Qiang; Gao Chunming
2000-01-01
It is important to research into the MHD Rayleigh-Taylor instability developed in Z-pinch implosion. A snowplough model of the single wire Z-pinch is presented. The perturbation amplitude of Rayleigh-Taylor instability in the wire-array Z-pinch is analyzed quantitatively. Sheared axial flow is put forward to mitigate and reduce the Rayleigh-Taylor instability. And other approaches used to mitigate MHD instability in such a super-fast process are explored
Radiative heat transfer in a heat generating and turbulently convecting fluid layer
International Nuclear Information System (INIS)
Cheung, F.B.; Chan, S.H.; Chawla, T.C.; Cho, D.H.
1980-01-01
The coupled problem of radiative transport and turbulent natural convection in a volumetrically heated, horizontal gray fluid medium, bounded from above by a rigid, isothermal wall and below by a rigid, adiabatic wall, is investigated analytically. An approximate method based upon the boundary layer approach is employed to obtain the dependence of heat transfer at the upper wall on the principal parameters of the problem, which, for moderate Prandtl number, are the Rayleigh number, Ra, the optical thickness, KL, and the conduction-radiation coupling parameter, N. Also obtained in this study is the behaviour of the thermal boundary layer at the upper wall. At large kL, the contribution of thermal radiation to heat transfer in the layer is found to be negligible for N > 10, moderate for N approximately 1, and overwhelming for N < 0.1. However, at small kL, thermal radiation is found to be important only for N < 0.01. While a higher level of turbulence results in a thinner boundary layer, a larger effect of radiation is found to result in a thicker one. Thus, in the presence of strong thermal radiation, a much larger value of Ra is required for the boundary layer approach to remain valid. Under severe radiation conditions, no boundary layer flow regime is found to exist even at very high Rayleigh numbers. Accordingly, the ranges of applicability of the present results are determined and the approximate method justified. In particular, the validity of the present analysis is tested in three limiting cases, ie those of kL → infinity, N → infinity, and Ra → infinity, and is further confirmed by comparison with the numerical solution (author)
Attractors of the periodically forced Rayleigh system
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Petre Bazavan
2011-07-01
Full Text Available The autonomous second order nonlinear ordinary differential equation(ODE introduced in 1883 by Lord Rayleigh, is the equation whichappears to be the closest to the ODE of the harmonic oscillator withdumping.In this paper we present a numerical study of the periodic andchaotic attractors in the dynamical system associated with the generalized Rayleigh equation. Transition between periodic and quasiperiodic motion is also studied. Numerical results describe the system dynamics changes (in particular bifurcations, when the forcing frequency is varied and thus, periodic, quasiperiodic or chaotic behaviour regions are predicted.
Befrui, Bizhan A.
1995-01-01
This viewgraph presentation discusses the following: STAR-CD computational features; STAR-CD turbulence models; common features of industrial complex flows; industry-specific CFD development requirements; applications and experiences of industrial complex flows, including flow in rotating disc cavities, diffusion hole film cooling, internal blade cooling, and external car aerodynamics; and conclusions on turbulence modeling needs.
Nonlinear saturation of the Rayleigh endash Taylor instability
International Nuclear Information System (INIS)
Das, A.; Mahajan, S.; Kaw, P.; Sen, A.; Benkadda, S.; Verga, A.
1997-01-01
A detailed numerical simulation of the nonlinear state of the Rayleigh endash Taylor instability has been carried out. There are three distinct phases of evolution where it is governed by the (i) linear effects, (ii) effects arising from the conventional nonlinear terms and (iii) subtle nonlinear effects arising through the coupling terms. During the third phase of evolution, there is a self-consistent generation of shear flow which saturates the Rayleigh endash Taylor instability even in situations (with periodic boundaries) where, in principle, an infinite amount of gravitational energy can be tapped. The Galerkin approximation is presented to provide an understanding of our numerical findings. Last, there is an attempt to provide a comprehensive understanding of the nonlinear state of the Rayleigh endash Taylor instability by comparing and contrasting this work with earlier studies. copyright 1997 American Institute of Physics
Helicity--vorticity turbulent pumping of magnetic fields in the solar dynamo
Pipin, V. V.
2012-01-01
The interaction of helical convective motions and differential rotation in the solar convection zone results in turbulent drift of a large-scale magnetic field. We discuss the pumping mechanism and its impact on the solar dynamo.
Secondary Flow Phenomena in Rotating Radial Straight Pipes
Cheng, K. C.; Wang, Liqiu
1995-01-01
Flow visualization results for secondary flow phenomena near the exit of a rotating radial-axis straight pipe (length ࡁ = 82 cm, inside diameter d = 3.81 cm, ࡁ/d 21.52) are presented to study the stabilizing (relaminarization) and destabilizing (early transition from laminar to turbulent flow) effects of Coriolis forces for Reynolds numbers Re = 500 ∼ 4,500 and rotating speeds n = 0 ∼ 200 rpm. The flow visualization was realised by smoke injection method. The main features of the trans...
Introduction to turbulent dynamical systems in complex systems
Majda, Andrew J
2016-01-01
This volume is a research expository article on the applied mathematics of turbulent dynamical systems through the paradigm of modern applied mathematics. It involves the blending of rigorous mathematical theory, qualitative and quantitative modeling, and novel numerical procedures driven by the goal of understanding physical phenomena which are of central importance to the field. The contents cover general framework, concrete examples, and instructive qualitative models. Accessible open problems are mentioned throughout. Topics covered include: · Geophysical flows with rotation, topography, deterministic and random forcing · New statistical energy principles for general turbulent dynamical systems, with applications · Linear statistical response theory combined with information theory to cope with model errors · Reduced low order models · Recent mathematical strategies for online data assimilation of turbulent dynamical systems as well as rigorous results for finite ensemble Kalman filters The volume wi...
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Xingtuan Yang
2015-05-01
Full Text Available A direct numerical simulation study of the characteristics of macroscopic and microscopic rotating motions in swirling jets confined in a rectangular flow domain is carried out. The different structures of vortex cores for different swirl levels are illustrated. It is found that the vortex cores of low swirl flows are of regular cylindrical-helix patterns, whereas those of the high swirl flows are characterized by the formation of the bubble-type vortex breakdown followed by the radiant processing vortex cores. The results of mean velocity fields show the general procedures of vortex origination. Moreover, the effects of macroscopic and microscopic rotating motions with respect to the mean and fluctuation fields of the swirling flows are evaluated. The microscopic rotating effects, especially the effects with respect to the turbulent fluctuation motion, are increasingly intermittent with the increase in the swirl levels. In contrast, the maximum value of the probability density functions with respect to the macroscopic rotating effects of the fluctuation motion occurs at moderate swirl levels since the macroscopic rotating effects are attenuated by the formation of the bubble vortex breakdown with a region of stagnant fluids at supercritical swirl levels.
Anomalous diffusion in geophysical and laboratory turbulence
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A. Tsinober
1994-01-01
Full Text Available We present an overview and some new results on anomalous diffusion of passive scalar in turbulent flows (including those used by Richardson in his famous paper in 1926. The obtained results are based on the analysis of the properties of invariant quantities (energy, enstrophy, dissipation, enstrophy generation, helicity density, etc. - i.e. independent of the choice of the system of reference as the most appropriate to describe physical processes - in three different turbulent laboratory flows (grid-flow, jet and boundary layer, see Tsinober et al. (1992 and Kit et al. (1993. The emphasis is made on the relations between the asymptotic properties of the intermittency exponents of higher order moments of different turbulent fields (energy, dissipation, helicity, spontaneous breaking of isotropy and reflexional symmetry and the variability of turbulent diffusion in the atmospheric boundary layer, in the troposphere and in the stratosphere. It is argued that local spontaneous breaking of isotropy of turbulent flow results in anomalous scaling laws for turbulent diffusion (as compared to the scaling law of Richardson which are observed, as a rule, in different atmospheric layers from the atmospheric boundary layer (ABL to the stratosphere. Breaking of rotational symmetry is important in the ABL, whereas reflexional symmetry breaking is dominating in the troposphere locally and in the stratosphere globally. The results are of speculative nature and further analysis is necessary to validate or disprove the claims made, since the correspondence with the experimental results may occur for the wrong reasons as happens from time to time in the field of turbulence.
Laser-Driven Hydrodynamic Experiments in the Turbulent Plasma Regime: from OMEGA to NIF
International Nuclear Information System (INIS)
Robey, H F; Miles, A R; Hansen, J F; Blue, B E; Drake, R P
2003-01-01
There is a great deal of interest in studying the evolution of hydrodynamic phenomena in high energy density plasmas that have transitioned beyond the initial phases of instability into an Ely developed turbulent state. Motivation for this study arises both in fusion plasmas as well as in numerous astrophysical applications where the understanding of turbulent mixing is essential. Double-shell ignition targets, for example, are subject to large growth of short wavelength perturbations on both surfaces of the high-Z inner shell. These perturbations, initiated by Richtmyer-Meshkov and Rayleigh-Taylor instabilities, can transition to a turbulent state and will lead to deleterious mixing of the cooler shell material with the hot burning fuel. In astrophysical plasmas, due to the extremely large scale, turbulent hydrodynamic mixing is also of wide-spread interest. The radial mixing that occurs in the explosion phase of core-collapse supernovae is an example that has received much attention in recent years and yet remains only poorly understood. In all of these cases, numerical simulation of the flow field is very difficult due to the large Reynolds number and corresponding wide range of spatial scales characterizing the plasma. Laboratory experiments on high energy density facilities that can access this regime are therefore of great interest. Experiments exploring the transition to turbulence that are currently being conducted on the Omega laser will be described. We will also discuss experiments being planned for the initial commissioning phases of the NIF as well as the enhanced experimental parameter space that will become available, as additional quads are made operational
Extended Rayleigh Damping Model
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Naohiro Nakamura
2016-07-01
Full Text Available In dynamic analysis, frequency domain analysis can be used if the entire structure is linear. However, time history analysis is generally used if nonlinear elements are present. Rayleigh damping has been widely used in time history response analysis. Many articles have reported the problems associated with this damping and suggested remedies. A basic problem is that the frequency area across which the damping ratio is almost constant is too narrow. If the area could be expanded while incurring only a small increase in computational cost, this would provide an appropriate remedy for this problem. In this study, a novel damping model capable of expanding the constant frequency area by more than five times was proposed based on the study of a causal damping model. This model was constructed by adding two terms to the Rayleigh damping model and can be applied to the linear elements in the time history analysis of a nonlinear structure. The accuracy and efficiency of the model were confirmed using example analyses.
Kaspi, Yohai
This thesis studies the dynamics of a rotating compressible gas sphere, driven by internal convection, as a model for the dynamics on the giant planets. We develop a new general circulation model for the Jovian atmosphere, based on the MITgcm dynamical core augmenting the nonhydrostatic model. The grid extends deep into the planet's interior allowing the model to compute the dynamics of a whole sphere of gas rather than a spherical shell (including the strong variations in gravity and the equation of state). Different from most previous 3D convection models, this model is anelastic rather than Boussinesq and thereby incorporates the full density variation of the planet. We show that the density gradients caused by convection drive the system away from an isentropic and therefore barotropic state as previously assumed, leading to significant baroclinic shear. This shear is concentrated mainly in the upper levels and associated with baroclinic compressibility effects. The interior flow organizes in large cyclonically rotating columnar eddies parallel to the rotation axis, which drive upgradient angular momentum eddy fluxes, generating the observed equatorial superrotation. Heat fluxes align with the axis of rotation, contributing to the observed flat meridional emission. We show the transition from weak convection cases with symmetric spiraling columnar modes similar to those found in previous analytic linear theory, to more turbulent cases which exhibit similar, though less regular and solely cyclonic, convection columns which manifest on the surface in the form of waves embedded within the superrotation. We develop a mechanical understanding of this system and scaling laws by studying simpler configurations and the dependence on physical properties such as the rotation period, bottom boundary location and forcing structure. These columnar cyclonic structures propagate eastward, driven by dynamics similar to that of a Rossby wave except that the restoring planetary
NONLINEAR DYNAMO IN A ROTATING ELECTRICALLY CONDUCTING FLUID
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M. I. Kopp
2017-05-01
Full Text Available We found a new large-scale instability, which arises in the rotating conductive fluid with small-scale turbulence. Turbulence is generated by small-scale external force with a low Reynolds number. The theory is built simply by the method of multiscale asymptotic expansions. Nonlinear equations for vortex and magnetic perturbations obtained in the third order for small Reynolds number. It is shown that the combined effects of the Coriolis force and the small external forces in a rotating conducting fluid possible large-scale instability. The large-scale increments of the instability, correspond to generation as the vortex and magnetic disturbances. This type of instability is classified as hydrodynamic and MHD alpha-effect. We studied the stationary regimes of nonlinear equations of magneto-vortex dynamo. In the limit of weakly conducting fluid found stationary solutions in the form of helical kinks. In the limit of high conductivity fluid was obtained stationary solutions in the form of nonlinear periodic waves and kinks.
International Nuclear Information System (INIS)
Capdevila, R; Trias, F X; Pérez-Segarra, C D; Lehmkuhl, O; Colomer, G
2011-01-01
In the present work, turbulent natural convection in a tall differentially heated cavity of aspect ratio 5:1, filled with air (Pr = 0.7) under a Rayleigh number based on the height of 4.5 · 10 10 , is studied numerically. Two different situations have been analysed. In the first one, the cavity is filled with a transparent medium. In the second one, the cavity contains a grey participating gas. The turbulent flow is described by means of Large Eddy Simulation (LES) using symmetry-preserving discretizations. Simulations are compared with experimental data available in the literature and with Direct Numerical Simulations (DNS). Surface and gas radiation have been simulated using the Discrete Ordinates Method (DOM). The influence of radiation on fluid flow behaviour has also been analysed.
The Rayleigh-Taylor instability in the spherical pinch
International Nuclear Information System (INIS)
Chen, H.B.; Hilko, B.; Panarella, E.
1994-01-01
The spherical pinch (SP) concept is an outgrowth of the inertial confinement model (ICF). Unlike the ICF where instabilities, especially the Rayleigh-Taylor instability, have been studied extensively, the instability study of the spherical pinch has just begun. The Raleigh-Taylor instability is investigated for the first time in the SP in the present work. By using the simple condition for the Rayleigh-Taylor instability ∇p · ∇p < O (density and pressure gradients have opposite direction), we have qualitatively identified the regions for development of instabilities in the SP. It is found that the explosion phase (central discharge) is stable and instabilities take place in the imploding phase. However, the growth rate for the instability is not in exponential form, and the appearance of the Rayleigh-Taylor instability does not prevent the main shock wave from converging to the center of the sphere
Transitional-turbulent spots and turbulent-turbulent spots in boundary layers.
Wu, Xiaohua; Moin, Parviz; Wallace, James M; Skarda, Jinhie; Lozano-Durán, Adrián; Hickey, Jean-Pierre
2017-07-03
Two observations drawn from a thoroughly validated direct numerical simulation of the canonical spatially developing, zero-pressure gradient, smooth, flat-plate boundary layer are presented here. The first is that, for bypass transition in the narrow sense defined herein, we found that the transitional-turbulent spot inception mechanism is analogous to the secondary instability of boundary-layer natural transition, namely a spanwise vortex filament becomes a [Formula: see text] vortex and then, a hairpin packet. Long streak meandering does occur but usually when a streak is infected by a nearby existing transitional-turbulent spot. Streak waviness and breakdown are, therefore, not the mechanisms for the inception of transitional-turbulent spots found here. Rather, they only facilitate the growth and spreading of existing transitional-turbulent spots. The second observation is the discovery, in the inner layer of the developed turbulent boundary layer, of what we call turbulent-turbulent spots. These turbulent-turbulent spots are dense concentrations of small-scale vortices with high swirling strength originating from hairpin packets. Although structurally quite similar to the transitional-turbulent spots, these turbulent-turbulent spots are generated locally in the fully turbulent environment, and they are persistent with a systematic variation of detection threshold level. They exert indentation, segmentation, and termination on the viscous sublayer streaks, and they coincide with local concentrations of high levels of Reynolds shear stress, enstrophy, and temperature fluctuations. The sublayer streaks seem to be passive and are often simply the rims of the indentation pockets arising from the turbulent-turbulent spots.
Spanwise homogeneous buoyancy-drag model for Rayleigh-Taylor mixing and experimental evaluation
International Nuclear Information System (INIS)
Dimonte, Guy
2000-01-01
A buoyancy-drag model for Rayleigh-Taylor (RT) mixing is developed on the premise that the bubble and spike regions behave as distinct and spanwise homogeneous fluids. Then, mass conservation is applied accross the mixing zone to obtain their average mixture densities dynamically. These are used to explicitly calculate the inertia and buoyancy terms in the evolutionary equation. The only unknown parameter in the model is the Newtonian drag constant C∼2.5±0.6, which is determined from turbulent RT experiments over various Atwood numbers A and acceleration histories g(t). The bubble (i=2) and spike (i=1) amplitudes are found to obey the familiar h i =α i Agt 2 for a constant g and h i ∼t θ i for an impulsive g. For bubbles, both α 2 and θ 2 are insensitive to A. For the spikes, both α 1 and θ 1 increase as a power law with the density ratio. However, θ 1 is not universal because it depends on the initial value of h 1 /h 2 . (c) 2000 American Institute of Physics
Rayleigh reciprocity relations: Applications
International Nuclear Information System (INIS)
Lin Ju; Li Xiao-Lei; Wang Ning
2016-01-01
Classical reciprocity relations have wide applications in acoustics, from field representation to generalized optical theorem. In this paper we introduce our recent results on the applications and generalization of classical Rayleigh reciprocity relation: higher derivative reciprocity relations as a generalization of the classical one and a theoretical proof on the Green’s function retrieval from volume noises. (special topic)
The time scale for the transition to turbulence in a high Reynolds number, accelerated flow
International Nuclear Information System (INIS)
Robey, H.F.; Zhou Ye; Buckingham, A.C.; Keiter, P.; Remington, B.A.; Drake, R.P.
2003-01-01
An experiment is described in which an interface between materials of different density is subjected to an acceleration history consisting of a strong shock followed by a period of deceleration. The resulting flow at this interface, initiated by the deposition of strong laser radiation into the initially well characterized solid materials, is unstable to both the Richtmyer-Meshkov (RM) and Rayleigh-Taylor (RT) instabilities. These experiments are of importance in their ability to access a difficult experimental regime characterized by very high energy density (high temperature and pressure) as well as large Reynolds number and Mach number. Such conditions are of interest, for example, in the study of the RM/RT induced mixing that occurs during the explosion of a core-collapse supernova. Under these experimental conditions, the flow is in the plasma state and given enough time will transition to turbulence. By analysis of the experimental data and a corresponding one-dimensional numerical simulation of the experiment, it is shown that the Reynolds number is sufficiently large (Re>10 5 ) to support a turbulent flow. An estimate of three key turbulence length scales (the Taylor and Kolmogorov microscales and a viscous diffusion scale), however, shows that the temporal duration of the present flow is insufficient to allow for the development of a turbulent inertial subrange. A methodology is described for estimating the time required under these conditions for the development of a fully turbulent flow
A turbulence-induced switch in phytoplankton swimming behavior
Carrara, Francesco; Sengupta, Anupam; Stocker, Roman
2015-11-01
Phytoplankton, unicellular photosynthetic organisms that form the basis of life in aquatic environments, are frequently exposed to turbulence, which has long been known to affect phytoplankton fitness and species succession. Yet, mechanisms by which phytoplankton may adapt to turbulence have remained unknown. Here we present a striking behavioral response of a motile species - the red-tide-producing raphidophyte Heterosigma akashiwo - to hydrodynamic cues mimicking those experienced in ocean turbulence. In the absence of turbulence, H. akashiwo exhibits preferential upwards swimming (`negative gravitaxis'), observable as a strong accumulation of cells at the top of an experimental container. When cells were exposed to overturning in an automated chamber - representing a minimum experimental model of rotation by Kolmogorov-scale turbulent eddies - the population robustly split in two nearly equi-abundant subpopulations, one swimming upward and one swimming downward. Microscopic observations at the single-cell level showed that the behavioral switch was accompanied by a rapid morphological change. A mechanistic model that takes into account cell shape confirms that modulation of morphology can alter the hydrodynamic stress distribution over the cell body, which, in turn, triggers the observed switch in phytoplankton migration direction. This active response to fluid flow, whereby microscale morphological changes influence ocean-scale migration dynamics, could be part of a bet-hedging strategy to maximize the chances of at least a fraction of the population evading high-turbulence microzones.
Numerical Investigation of Turbulence Models for a Superlaminar Journal Bearing
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Aoshuang Ding
2018-01-01
Full Text Available With rotating machineries working at high speeds, oil flow in bearings becomes superlaminar. Under superlaminar conditions, flow exhibits between laminar and fully developed turbulence. In this study, superlaminar oil flow in an oil-lubricated tilting-pad journal bearing is analyzed through computational fluid dynamics (CFD. A three-dimensional bearing model is established. CFD results from the laminar model and 14 turbulence models are compared with experimental findings. The laminar simulation results of pad-side pressure are inconsistent with the experimental data. Thus, the turbulence effects on superlaminar flow should be considered. The simulated temperature and pressure distributions from the classical fully developed turbulence models cannot correctly fit the experimental data. As such, turbulence models should be corrected for superlaminar flow. However, several corrections, such as transition correction, are unsuitable. Among all the flow models, the SST model with low-Re correction exhibits the best pressure distribution and turbulence viscosity ratio. Velocity profile analysis confirms that a buffer layer plays an important role in the superlaminar boundary layer. Classical fully developed turbulence models cannot accurately predict the buffer layer, but this problem can be resolved by initiating an appropriate low-Re correction. Therefore, the SST model with low-Re correction yields suitable results for superlaminar flows in bearings.
Intrinsic non-inductive current driven by ETG turbulence in tokamaks
Singh, Rameswar; Kaw, P. K.; Singh, R.; Gürcan, Ã.-. D.
2017-10-01
Motivated by observations and physics understanding of the phenomenon of intrinsic rotation, it is suggested that similar considerations for electron dynamics may result in intrinsic current in tokamaks. We have investigated the possibility of intrinsic non-inductive current in the turbulent plasma of tokamaks. Ohm's law is generalized to include the effect of turbulent fluctuations in the mean field approach. This clearly leads to the identification of sources and the mechanisms of non-inductive current drive by electron temperature gradient turbulence. It is found that a mean parallel electro-motive force and hence a mean parallel current can be generated by (1) the divergence of residual current flux density and (2) a non-flux like turbulent source from the density and parallel electric field correlations. Both residual flux and the non-flux source require parallel wave-number k∥ symmetry breaking for their survival which can be supplied by various means like mean E × B shear, turbulence intensity gradient, etc. Estimates of turbulence driven current are compared with the background bootstrap current in the pedestal region. It is found that turbulence driven current is nearly 10% of the bootstrap current and hence can have a significant influence on the equilibrium current density profiles and current shear driven modes.
International Nuclear Information System (INIS)
Casner, A.; Masse, L.; Delorme, B.; Jacquet, L.; Liberatore, S.; Smalyuk, V.; Martinez, D.; Seugling, R.; Park, H.S.; Remington, B.A.; Moore, A.; Igumenshev, I.; Chicanne, C.
2013-01-01
In the context of National Ignition Facility Basic Science program we propose to study on the NIF ablative Rayleigh-Taylor (RT) instability in transition from weakly nonlinear to highly nonlinear regimes. Based on the analogy between flame front and ablation front, highly nonlinear RT instability measurements at the ablation front can provide important insights into the initial deflagration stage of thermonuclear supernovae of type Ia. NIF provides a unique platform to study the rich physics of nonlinear and turbulent mixing flows in High Energy Density plasmas because it can accelerate targets over much larger distances and longer time periods than previously achieved on the NOVA and OMEGA lasers. In one shot, growth of RT modulations can be measured from the weakly nonlinear stage near nonlinear saturation levels to the highly nonlinear bubble-competition, bubble-merger regimes and perhaps into a turbulent-like regime. The role of ablation on highly-nonlinear RT instability evolution will be comprehensively studied by varying ablation velocity using indirect and direct-drive platforms. We present a detailed hydro-code design of the indirect-drive platform and discuss the implementation plan for these experiments which only use NIF diagnostics already qualified. (authors)
Faiez, Reza; Rezaei, Yazdan
2017-10-01
Time-dependent, finite volume method calculations of momentum and heat transfer were carried out to investigate the correlation between oscillatory convection and the crystallization front dynamics during the Czochralski (Cz) growth of an oxide material. The present modeling allows us to illustrate the modification of the interface shape during the time period of oscillation of the flow manifesting as the formation of a cold plume beneath the phase boundary. It was shown that the instability mechanism is associated with an irreversible dramatic change in the interface shape, which occurs at a critical Reynolds number significantly lower than that is predicted by the quasi-stationary global model analysis of the Cz growth system. The baroclinic term which appears in the vorticity equation in a rotating stratified fluid is used to describe the numerical results of the model. The properties of the thermal waves were studied in the monitoring points located nearby the interface. The waves are regular but not in fact vertically correlated as observed in the case of baroclinic waves. The Rayleigh-Benard dynamics is suggested to be the predominant mechanism even though the instability is primarily baroclinic.
Airflows and turbulent flux measurements in mountainous terrain: Part 1. Canopy and local effects
Turnipseed, Andrew A.; Anderson, Dean E.; Blanken, Peter D.; Baugh, William M.; Monson, Russell K.
2003-01-01
We have studied the effects of local topography and canopy structure on turbulent flux measurements at a site located in mountainous terrain within a subalpine, coniferous forest. Our primary aim was to determine whether the complex terrain of the site affects the accuracy of eddy flux measurements from a practical perspective. We observed displacement heights, roughness lengths, spectral peaks, turbulent length scales, and profiles of turbulent intensities that were comparable in magnitude and pattern to those reported for forest canopies in simpler terrain. We conclude that in many of these statistical measures, the local canopy exerts considerably more influence than does topographical complexity. Lack of vertical flux divergence and modeling suggests that the flux footprints for the site are within the standards acceptable for the application of flux statistics. We investigated three different methods of coordinate rotation: double rotation (DR), triple rotation (TR), and planar-fit rotation (PF). Significant variability in rotation angles at low wind speeds was encountered with the commonly used DR and TR methods, as opposed to the PF method, causing some overestimation of the fluxes. However, these differences in fluxes were small when applied to large datasets involving sensible heat and CO2 fluxes. We observed evidence of frequent drainage flows near the ground during stable, stratified conditions at night. Concurrent with the appearance of these flows, we observed a positive bias in the mean vertical wind speed, presumably due to subtle topographic variations inducing a flow convergence below the measurement sensors. In the presence of such drainage flows, advection of scalars and non-zero bias in the mean vertical wind speed can complicate closure of the mass conservation budget at the site.
Rayleigh wave ellipticity across the Iberian Peninsula and Morocco
Gómez García, Clara; Villaseñor, Antonio
2015-04-01
Spectral amplitude ratios between horizontal and vertical components (H/V ratios) from seismic records are useful to evaluate site effects, predict ground motion and invert for S velocity in the top several hundred meters. These spectral ratios can be obtained from both ambient noise and earthquakes. H/V ratios from ambient noise depend on the content and predominant wave types: body waves, Rayleigh waves, a mixture of different waves, etc. The H/V ratio computed in this way is assumed to measure Rayleigh wave ellipticity since ambient vibrations are dominated by Rayleigh waves. H/V ratios from earthquakes are able to determine the local crustal structure at the vicinity of the recording station. These ratios obtained from earthquakes are based on surface wave ellipticity measurements. Although long period (>20 seconds) Rayleigh H/V ratio is not currently used because of large scatter has been reported and uncertainly about whether these measurements are compatible with traditional phase and group velocity measurements, we will investigate whether it is possible to obtain stable estimates after collecting statistics for many earthquakes. We will use teleseismic events from shallow earthquakes (depth ≤ 40 km) between 2007 January 1 and 2012 December 31 with M ≥ 6 and we will compute H/V ratios for more than 400 stations from several seismic networks across the Iberian Peninsula and Morocco for periods between 20 and 100 seconds. Also H/V ratios from cross-correlations of ambient noise in different components for each station pair will be computed. Shorter period H/V ratio measurements based on ambient noise cross-correlations are strongly sensitive to near-surface structure, rather than longer period earthquake Rayleigh waves. The combination of ellipticity measurements based on earthquakes and ambient noise will allow us to perform a joint inversion with Rayleigh wave phase velocity. Upper crustal structure is better constrained by the joint inversion compared
Surface roughness effects on turbulent Couette flow
Lee, Young Mo; Lee, Jae Hwa
2017-11-01
Direct numerical simulation of a turbulent Couette flow with two-dimensional (2-D) rod roughness is performed to examine the effects of the surface roughness. The Reynolds number based on the channel centerline laminar velocity (Uco) and channel half height (h) is Re =7200. The 2-D rods are periodically arranged with a streamwise pitch of λ = 8 k on the bottom wall, and the roughness height is k = 0.12 h. It is shown that the wall-normal extent for the logarithmic layer is significantly shortened in the rough-wall turbulent Couette flow, compared to a turbulent Couette flow with smooth wall. Although the Reynolds stresses are increased in a turbulent channel flow with surface roughness in the outer layer due to large-scale ejection motions produced by the 2-D rods, those of the rough-wall Couette flow are decreased. Isosurfaces of the u-structures averaged in time suggest that the decrease of the turbulent activity near the centerline is associated with weakened large-scale counter-rotating roll modes by the surface roughness. This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1A09000537) and the Ministry of Science, ICT & Future Planning (NRF-2017R1A5A1015311).
Energy Technology Data Exchange (ETDEWEB)
Boudjemadi, R.
1996-03-01
The main objectives of this thesis are the direct numerical simulation of natural convection in a vertical differentially heated slot and the improvements of second-order turbulence modelling. A three-dimensional direct numerical simulation code has been developed in order to gain a better understanding of turbulence properties in natural convection flows. This code has been validated in several physical configurations: non-stratified natural convection flows (conduction solution), stratified natural convection flows (double boundary layer solution), transitional and turbulent Poiseuille flows. For the conduction solution, the turbulent regime was reached at a Rayleigh number of 1*10{sup 5} and 5.4*10{sup 5}. A detailed analysis of these results has revealed the principal qualities of the available models but has also pointed our their shortcomings. This data base has been used in order to improve the triple correlations transport models and to select the turbulent time scales suitable for such flows. (author). 122 refs., figs., tabs., 4 appends.
Turbulence modeling of the Von Karman flow: Viscous and inertial stirrings
International Nuclear Information System (INIS)
Poncet, Sebastien; Schiestel, Roland; Monchaux, Romain
2008-01-01
The present work considers the turbulent Von Karman flow generated by two counter-rotating smooth flat (viscous stirring) or bladed (inertial stirring) disks. Numerical predictions based on one-point statistical modeling using a low-Reynolds number second-order full stress transport closure (RSM model) are compared to velocity measurements performed at CEA (Commissariat a l'Energie Atomique, France). The main and significant novelty of this paper is the use of a drag force in the momentum equations to reproduce the effects of inertial stirring instead of modeling the blades themselves. The influences of the rotational Reynolds number, the aspect ratio of the cavity, the rotating disk speed ratio and of the presence or not of impellers are investigated to get a precise knowledge of both the dynamics and the turbulence properties in the Von Karman configuration. In particular, we highlighted the transition between the merged and separated boundary layer regimes and the one between the Batchelor [Batchelor, G.K., 1951. Note on a class of solutions of the Navier-Stokes equations representing steady rotationally-symmetric flow. Quat. J. Mech. Appl. Math. 4 (1), 29-41] and the Stewartson [Stewartson, K., 1953. On the flow between two rotating coaxial disks. Proc. Camb. Philos. Soc. 49, 333-341] flow structures in the smooth disk case. We determined also the transition between the one cell and the two cell regimes for both viscous and inertial stirrings
Numerical study of natural turbulent convection of nanofluids in a tall cavity heated from below
Directory of Open Access Journals (Sweden)
Mebrouk Ridha
2016-01-01
Full Text Available In the present paper a numerical study of natural turbulent convection in a tall cavity filled with nanofluids. The cavity has a heat source embedded on its bottom wall, while the left, right and top walls of the cavity are maintained at a relatively low temperature. The working fluid is a water based nanofluid having three nanoparticle types: alumina, copper and copper oxid. The influence of pertinent parameters such as Rayleigh number, the type of nanofluid and solid volume fraction of nanoparticles on the cooling performance is studied. Steady forms of twodimensional Reynolds-Averaged-Navier-Stokes equations and conservation equations of mass and energy, coupled with the Boussinesq approximation, are solved by the control volume based discretisation method employing the SIMPLE algorithm for pressure-velocity coupling. Turbulence is modeled using the standard k-ε model. The Rayleigh number, Ra, is varied from 2.491009 to 2.491011. The volume fractions of nanoparticles were varied in the interval 0≤φ≤ 6% . Stream lines, isotherms, velocity profiles and Temperature profiles are presented for various combinations of Ra, the type of nanofluid and solid volume fraction of nanoparticles. The results are reported in the form of average Nusselt number on the heated wall. It is shown that for all values of Ra, the average heat transfer rate from the heat source increases almost linearly and monotonically as the solid volume fraction increases. Finally the average heat transfer rate takes on values that decrease according to the ordering Cu, CuO and Al2O3.
Magnetostrophic balance as the optimal state for turbulent magnetoconvection.
King, Eric M; Aurnou, Jonathan M
2015-01-27
The magnetic fields of Earth and other planets are generated by turbulent convection in the vast oceans of liquid metal within them. Although direct observation is not possible, this liquid metal circulation is thought to be dominated by the controlling influences of planetary rotation and magnetic fields through the Coriolis and Lorentz forces. Theory famously predicts that planetary dynamo systems naturally settle into the so-called magnetostrophic state, where the Coriolis and Lorentz forces partially cancel, and convection is optimally efficient. Although this magnetostrophic theory correctly predicts the strength of Earth's magnetic field, no laboratory experiments have reached the magnetostrophic regime in turbulent liquid metal convection. Furthermore, computational dynamo simulations have as yet failed to produce a magnetostrophic dynamo, which has led some to question the existence of the magnetostrophic state. Here, we present results from the first, to our knowledge, turbulent, magnetostrophic convection experiments using the liquid metal gallium. We find that turbulent convection in the magnetostrophic regime is, in fact, maximally efficient. The experimental results clarify these previously disparate results, suggesting that the dynamically optimal magnetostrophic state is the natural expression of turbulent planetary dynamo systems.
Wang, Menghua
2016-05-30
To understand and assess the effect of the sensor spectral response function (SRF) on the accuracy of the top of the atmosphere (TOA) Rayleigh-scattering radiance computation, new TOA Rayleigh radiance lookup tables (LUTs) over global oceans and inland waters have been generated. The new Rayleigh LUTs include spectral coverage of 335-2555 nm, all possible solar-sensor geometries, and surface wind speeds of 0-30 m/s. Using the new Rayleigh LUTs, the sensor SRF effect on the accuracy of the TOA Rayleigh radiance computation has been evaluated for spectral bands of the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi National Polar-orbiting Partnership (SNPP) satellite and the Joint Polar Satellite System (JPSS)-1, showing some important uncertainties for VIIRS-SNPP particularly for large solar- and/or sensor-zenith angles as well as for large Rayleigh optical thicknesses (i.e., short wavelengths) and bands with broad spectral bandwidths. To accurately account for the sensor SRF effect, a new correction algorithm has been developed for VIIRS spectral bands, which improves the TOA Rayleigh radiance accuracy to ~0.01% even for the large solar-zenith angles of 70°-80°, compared with the error of ~0.7% without applying the correction for the VIIRS-SNPP 410 nm band. The same methodology that accounts for the sensor SRF effect on the Rayleigh radiance computation can be used for other satellite sensors. In addition, with the new Rayleigh LUTs, the effect of surface atmospheric pressure variation on the TOA Rayleigh radiance computation can be calculated precisely, and no specific atmospheric pressure correction algorithm is needed. There are some other important applications and advantages to using the new Rayleigh LUTs for satellite remote sensing, including an efficient and accurate TOA Rayleigh radiance computation for hyperspectral satellite remote sensing, detector-based TOA Rayleigh radiance computation, Rayleigh radiance calculations for high altitude
International Nuclear Information System (INIS)
Bakosi, Jozsef; Ristorcelli, Raymond J.
2010-01-01
Probability density function (PDF) methods are extended to variable-density pressure-gradient-driven turbulence. We apply the new method to compute the joint PDF of density and velocity in a non-premixed binary mixture of different-density molecularly mixing fluids under gravity. The full time-evolution of the joint PDF is captured in the highly non-equilibrium flow: starting from a quiescent state, transitioning to fully developed turbulence and finally dissipated by molecular diffusion. High-Atwood-number effects (as distinguished from the Boussinesq case) are accounted for: both hydrodynamic turbulence and material mixing are treated at arbitrary density ratios, with the specific volume, mass flux and all their correlations in closed form. An extension of the generalized Langevin model, originally developed for the Lagrangian fluid particle velocity in constant-density shear-driven turbulence, is constructed for variable-density pressure-gradient-driven flows. The persistent small-scale anisotropy, a fundamentally 'non-Kolmogorovian' feature of flows under external acceleration forces, is captured by a tensorial diffusion term based on the external body force. The material mixing model for the fluid density, an active scalar, is developed based on the beta distribution. The beta-PDF is shown to be capable of capturing the mixing asymmetry and that it can accurately represent the density through transition, in fully developed turbulence and in the decay process. The joint model for hydrodynamics and active material mixing yields a time-accurate evolution of the turbulent kinetic energy and Reynolds stress anisotropy without resorting to gradient diffusion hypotheses, and represents the mixing state by the density PDF itself, eliminating the need for dubious mixing measures. Direct numerical simulations of the homogeneous Rayleigh-Taylor instability are used for model validation.
Turbulence model for melt pool natural convection heat transfer
International Nuclear Information System (INIS)
Kelkar, K.M.; Patankar, S.V.
1994-01-01
Under severe reactor accident scenarios, pools of molten core material may form in the reactor core or in the hemispherically shaped lower plenum of the reactor vessel. Such molten pools are internally heated due to the radioactive decay heat that gives rise to buoyant flows in the molten pool. The flow in such pools is strongly influenced by the turbulent mixing because the expected Rayleigh numbers under accidents scenarios are very high. The variation of the local heat flux over the boundaries of the molten pools are important in determining the subsequent melt progression behavior. This study reports results of an ongoing effort towards providing a well validated mathematical model for the prediction of buoyant flow and heat transfer in internally heated pool under conditions expected in severe accident scenarios
Multiscale gyrokinetics for rotating tokamak plasmas: fluctuations, transport and energy flows.
Abel, I G; Plunk, G G; Wang, E; Barnes, M; Cowley, S C; Dorland, W; Schekochihin, A A
2013-11-01
This paper presents a complete theoretical framework for studying turbulence and transport in rapidly rotating tokamak plasmas. The fundamental scale separations present in plasma turbulence are codified as an asymptotic expansion in the ratio ε = ρi/α of the gyroradius to the equilibrium scale length. Proceeding order by order in this expansion, a set of coupled multiscale equations is developed. They describe an instantaneous equilibrium, the fluctuations driven by gradients in the equilibrium quantities, and the transport-timescale evolution of mean profiles of these quantities driven by the interplay between the equilibrium and the fluctuations. The equilibrium distribution functions are local Maxwellians with each flux surface rotating toroidally as a rigid body. The magnetic equilibrium is obtained from the generalized Grad-Shafranov equation for a rotating plasma, determining the magnetic flux function from the mean pressure and velocity profiles of the plasma. The slow (resistive-timescale) evolution of the magnetic field is given by an evolution equation for the safety factor q. Large-scale deviations of the distribution function from a Maxwellian are given by neoclassical theory. The fluctuations are determined by the 'high-flow' gyrokinetic equation, from which we derive the governing principle for gyrokinetic turbulence in tokamaks: the conservation and local (in space) cascade of the free energy of the fluctuations (i.e. there is no turbulence spreading). Transport equations for the evolution of the mean density, temperature and flow velocity profiles are derived. These transport equations show how the neoclassical and fluctuating corrections to the equilibrium Maxwellian act back upon the mean profiles through fluxes and heating. The energy and entropy conservation laws for the mean profiles are derived from the transport equations. Total energy, thermal, kinetic and magnetic, is conserved and there is no net turbulent heating. Entropy is produced
The convection patterns in microemulsions
International Nuclear Information System (INIS)
Korneta, W.; Lopez Quintela, M.A.; Fernandez Novoa, A.
1991-07-01
The Rayleigh-Benard convection in the microemulsion consisting of water (7.5%), cyclohexan (oil-61.7%) and diethylenglycolmonobutylether (surfactant-30.8%) is studied from the onset of convection to the phase separation. The five classes of convection patterns are observed and recorded on the video: localized travelling waves, travelling waves, travelling waves and localized steady rolls, steady rolls and steady polygons. The Fourier transforms and histograms of these patterns are presented. The origin of any pattern is discussed. The intermittent behaviour close to the phase separation was observed. Possible applications of the obtained results are suggested. (author). 6 refs, 4 figs
Rayleigh waves in elastic medium with double porosity
Directory of Open Access Journals (Sweden)
Rajneesh KUMAR
2018-03-01
Full Text Available The present paper deals with the propagation of Rayleigh waves in isotropic homogeneous elastic half-space with double porosity whose surface is subjected to stress-free boundary conditions. The compact secular equations for elastic solid half-space with voids are deduced as special cases from the present analysis. In order to illustrate the analytical developments, the secular equations have been solved numerically. The computer simulated results for copper materials in respect of Rayleigh wave velocity and attenuation coe¢ cient have been presented graphically.
Behaviour of turbulence models near a turbulent/non-turbulent interface revisited
International Nuclear Information System (INIS)
Ferrey, P.; Aupoix, B.
2006-01-01
The behaviour of turbulence models near a turbulent/non-turbulent interface is investigated. The analysis holds as well for two-equation as for Reynolds stress turbulence models using Daly and Harlow diffusion model. The behaviour near the interface is shown not to be a power law, as usually considered, but a more complex parametric solution. Why previous works seemed to numerically confirm the power law solution is explained. Constraints for turbulence modelling, i.e., for ensuring that models have a good behaviour near a turbulent/non-turbulent interface so that the solution is not sensitive to small turbulence levels imposed in the irrotational flow, are drawn
DEFF Research Database (Denmark)
Elm, Jonas; Norman, Patrick; Bilde, Merete
2014-01-01
The Rayleigh and hyper Rayleigh scattering properties of the binary (H 2SO4)(H2O)n and ternary (H 2SO4)(NH3)(H2O)n clusters are investigated using a quantum mechanical response theory approach. The molecular Rayleigh scattering intensities are expressed using the dipole polarizability α...... and hyperpolarizability β tensors. Using density functional theory, we elucidate the effect of cluster morphology on the scattering properties using a combinatorial sampling approach. We find that the Rayleigh scattering intensity depends quadratically on the number of water molecules in the cluster and that a single...... ammonia molecule is able to induce a high anisotropy, which further increases the scattering intensity. The hyper Rayleigh scattering activities are found to be extremely low. This study presents the first attempt to map the scattering of atmospheric molecular clusters using a bottom-up approach...
International Nuclear Information System (INIS)
Hillier, Andrew; Isobe, Hiroaki; Shibata, Kazunari; Berger, Thomas
2012-01-01
The launch of the Hinode satellite led to the discovery of rising plumes, dark in chromospheric lines, that propagate from large (∼10 Mm) bubbles that form at the base of quiescent prominences. The plumes move through a height of approximately 10 Mm while developing highly turbulent profiles. The magnetic Rayleigh-Taylor instability was hypothesized to be the mechanism that drives these flows. In this study, using three-dimensional (3D) MHD simulations, we investigate the nonlinear stability of the Kippenhahn-Schlüter prominence model for the interchange mode of the magnetic Rayleigh-Taylor instability. The model simulates the rise of a buoyant tube inside the quiescent prominence model, where the interchange of magnetic field lines becomes possible at the boundary between the buoyant tube and the prominence. Hillier et al. presented the initial results of this study, where upflows of constant velocity (maximum found 6 km s –1 ) and a maximum plume width ≈1.5 Mm which propagate through a height of approximately 6 Mm were found. Nonlinear interaction between plumes was found to be important for determining the plume dynamics. In this paper, using the results of ideal MHD simulations, we determine how the initial parameters for the model and buoyant tube affect the evolution of instability. We find that the 3D mode of the magnetic Rayleigh-Taylor instability grows, creating upflows aligned with the magnetic field of constant velocity (maximum found 7.3 km s –1 ). The width of the upflows is dependent on the initial conditions, with a range of 0.5-4 Mm which propagate through heights of 3-6 Mm. These results are in general agreement with the observations of the rising plumes.
Directory of Open Access Journals (Sweden)
Kareema Abed Al-Kadim
2017-12-01
Full Text Available In this paper Rayleigh Pareto distribution have introduced denote by( R_PD. We stated some useful functions. Therefor we give some of its properties like the entropy function, mean, mode, median , variance , the r-th moment about the mean, the rth moment about the origin, reliability, hazard functions, coefficients of variation, of sekeness and of kurtosis. Finally, we estimate the parameters so the aim of this search is to introduce a new distribution
Boundary layers and scaling relations in natural thermal convection
Shishkina, Olga; Lohse, Detlef; Grossmann, Siegfried
2017-11-01
We analyse the boundary layer (BL) equations in natural thermal convection, which includes vertical convection (VC), where the fluid is confined between two differently heated vertical walls, horizontal convection (HC), where the fluid is heated at one part of the bottom plate and cooled at some other part, and Rayleigh-Benard convection (RBC). For BL dominated regimes we derive the scaling relations of the Nusselt and Reynolds numbers (Nu, Re) with the Rayleigh and Prandtl numbers (Ra, Pr). For VC the scaling relations are obtained directly from the BL equations, while for HC they are derived by applying the Grossmann-Lohse theory to the case of VC. In particular, for RBC with large Pr we derive Nu Pr0Ra1/3 and Re Pr-1Ra2/3. The work is supported by the Deutsche Forschungsgemeinschaft (DFG) under the Grant Sh 405/4 - Heisenberg fellowship.
Momentum transport in gyrokinetic turbulence
Energy Technology Data Exchange (ETDEWEB)
Buchholz, Rico
2016-07-01
In this thesis, the gyrokinetic-Vlasov code GKW is used to study turbulent transport, with a focus on radial transport of toroidal momentum. To support the studies on turbulent transport an eigenvalue solver has been implemented into GKW. This allows to find, not only the most unstable mode, but also subdominant modes. Furthermore it is possible to follow the modes in parameter scans. Furthermore, two fundamental mechanisms that can generate an intrinsic rotation have been investigated: profile shearing and the velocity nonlinearity. The study of toroidal momentum transport in a tokamak due to profile shearing reveals that the momentum flux can not be accurately described by the gradient in the turbulent intensity. Consequently, a description using the profile variation is used. A linear model has been developed that is able to reproduce the variations in the momentum flux as the profiles of density and temperature vary, reasonably well. It uses, not only the gradient length of density and temperature profile, but also their derivative, i.e. the second derivative of the logarithm of the temperature and the density profile. It is shown that both first as well as second derivatives contribute to the generation of a momentum flux. A difference between the linear and nonlinear simulations has been found with respect to the behaviour of the momentum flux. In linear simulations the momentum flux is independent of the normalized Larmor radius ρ{sub *}, whereas it is linear in ρ{sub *} for nonlinear simulations, provided ρ{sub *} is small enough (≤4.10{sup -3}). Nonlinear simulations reveal that the profile shearing can generate an intrinsic rotation comparable to that of current experiments. Under reactor conditions, however, the intrinsic rotation from the profile shearing is expected to be small due to the small normalized Larmor radius ρ{sub *}
Boundary Layer Transition Detection on a Rotor Blade Using Rotating Mirror Thermography
Heineck, James T.; Schuelein, Erich; Raffel, Markus
2014-01-01
Laminar-to-turbulent transition on a rotor blade in hover has been imaged using an area-scan infrared camera. A new method for tracking a blade using a rotating mirror was employed. The mirror axis of rotation roughly corresponded to the rotor axis of rotation and the mirror rotational frequency is 1/2 that of the rotor. This permitted the use of cameras whose integration time was too long to prevent image blur due to the motion of the blade. This article will show the use of this method for a rotor blade at different collective pitch angles.
Stability of an expanding cylindrical plasma envelope: Rayleigh--Taylor instability
International Nuclear Information System (INIS)
Han, S.J.
1982-01-01
The stability of a cylindrically symmetric plasma envelope driven outward by blast waves is considered. The plasma fluid is assumed to be a compressible, isentropic gas describable as an ideal gas ( p = arho/sup γ/, γ>1). The stability problem of such an envelope undergoing self-similar motion is solved by considering the initial-value problem. It is shown that in the early phase of an expansion, the envelope is unstable to Rayleigh--Taylor modes which develop at the inner surface. In the later phase of the expansion, the Rayleigh--Taylor modes are weakened due to the geometrical divergence effect. The implications of the time-dependent behavior of the Rayleigh--Taylor instability for plasma switches are discussed
Effect of turbulence on the disintegration rate of flushable consumer products.
Karadagli, Fatih; Rittmann, Bruce E; McAvoy, Drew C; Richardson, John E
2012-05-01
A previously developed model for the physical disintegration of flushable consumer products is expanded by investigating the effects of turbulence on the rate of physical disintegration. Disintegration experiments were conducted with cardboard tampon applicators at 100, 150, and 200 rotations per minute, corresponding to Reynold's numbers of 25,900, 39,400, and 52,900, respectively, which were estimated by using computational fluid dynamics modeling. The experiments were simulated with the disintegration model to obtain best-fit values of the kinetic and distribution parameters. Computed rate coefficients (ki) for all solid sizes (i.e., greater than 8, 4 to 8, 2 to 4, and 1 to 2 mm) increased strongly with Reynold's number or rotational speed. Thus, turbulence strongly affected the disintegration rate of flushable products, and the relationship of the ki values to Reynold's number can be included in mathematical representations of physical disintegration.
PLANETESIMAL AND PROTOPLANET DYNAMICS IN A TURBULENT PROTOPLANETARY DISK: IDEAL STRATIFIED DISKS
International Nuclear Information System (INIS)
Yang, Chao-Chin; Mac Low, Mordecai-Mark; Menou, Kristen
2012-01-01
Due to the gravitational influence of density fluctuations driven by magneto-rotational instability in the gas disk, planetesimals and protoplanets undergo diffusive radial migration as well as changes in other orbital properties. The magnitude of the effect on particle orbits can have important consequences for planet formation scenarios. We use the local-shearing-box approximation to simulate an ideal, isothermal, magnetized gas disk with vertical density stratification and simultaneously evolve numerous massless particles moving under the gravitational field of the gas and the host star. We measure the evolution of the particle orbital properties, including mean radius, eccentricity, inclination, and velocity dispersion, and its dependence on the disk properties and the particle initial conditions. Although the results converge with resolution for fixed box dimensions, we find the response of the particles to the gravity of the turbulent gas correlates with the horizontal box size, up to 16 disk scale heights. This correlation indicates that caution should be exercised when interpreting local-shearing-box models involving gravitational physics of magneto-rotational turbulence. Based on heuristic arguments, nevertheless, the criterion L h /R ∼ O(1), where L h is the horizontal box size and R is the distance to the host star, is proposed to possibly circumvent this conundrum. If this criterion holds, we can still conclude that magneto-rotational turbulence seems likely to be ineffective at driving either diffusive migration or collisional erosion under most circumstances.
Nonlinear vortex structures and Rayleigh instability condition in shear flow plasmas
International Nuclear Information System (INIS)
Haque, Q.; Saleem, H.; Mirza, A.M.
2009-01-01
Full text: It is shown that the shear flow produced by externally applied electric field can unstable the drift waves. Due to shear flow, the Rayleigh instability condition is modified, which is obtained for both electron-ion and electron-positron-ion plasmas. These shear flow driven drift waves can be responsible for large amplitude electrostatic fluctuations in tokamak edges. In the nonlinear regime, the stationary structures may appear in electron-positron-ion plasmas similar to electron-ion plasmas. The nonlinear vortex structures like counter rotating dipole vortices and vortex chains can be formed with the aid of special type of shear flows. The positrons can be used as a probe in laboratory plasmas, which make it a multi-component plasma. The presence of positrons in electron-ion plasma system can affect the speed and amplitude of the nonlinear vortex structures. This investigation can have application in both laboratory and astrophysical plasmas. (author)
International Nuclear Information System (INIS)
Viecelli, J.A.
1993-01-01
The Hamiltonian flow of a set of point vortices of like sign and strength has a low-temperature phase consisting of a rotating triangular lattice of vortices, and a normal temperature turbulent phase consisting of random clusters of vorticity that orbit about a common center along random tracks. The mean-field flow in the normal temperature phase has similarities with turbulent quasi-two-dimensional rotating laboratory and geophysical flows, whereas the low-temperature phase displays effects associated with quantum fluids. In the normal temperature phase the vortices follow power-law clustering distributions, while in the time domain random interval modulation of the vortex orbit radii fluctuations produces singular fractional exponent power-law low-frequency spectra corresponding to time autocorrelation functions with fractional exponent power-law tails. Enhanced diffusion is present in the turbulent state, whereas in the solid-body rotation state vortices thermally diffuse across the lattice. Over the entire temperature range the interaction energy of a single vortex in the field of the rest of the vortices follows positive temperature Fermi--Dirac statistics, with the zero temperature limit corresponding to the rotating crystal phase, and the infinite temperature limit corresponding to a Maxwellian distribution. Analyses of weather records dependent on the large-scale quasi-two-dimensional atmospheric circulation suggest the presence of singular fractional exponent power-law spectra and fractional exponent power-law autocorrelation tails, consistent with the theory
Life stages of wall-bounded decay of Taylor-Couette turbulence
Ostilla-Mónico, Rodolfo; Zhu, Xiaojue; Arza, Vamsi Spandan; Verzicco, Roberto; Lohse, Detlef
2017-01-01
The decay of Taylor-Couette turbulence, i.e., the flow between two coaxial and independently rotating cylinders, is numerically studied by instantaneously stopping the forcing from an initially statistically stationary flow field at a Reynolds number of Re=3.5×104. The effect of wall friction is
Strained spiral vortex model for turbulent fine structure
Lundgren, T. S.
1982-01-01
A model for the intermittent fine structure of high Reynolds number turbulence is proposed. The model consists of slender axially strained spiral vortex solutions of the Navier-Stokes equation. The tightening of the spiral turns by the differential rotation of the induced swirling velocity produces a cascade of velocity fluctuations to smaller scale. The Kolmogorov energy spectrum is a result of this model.
Gyrokinetic Particle Simulation of Turbulent Transport in Burning Plasmas
Energy Technology Data Exchange (ETDEWEB)
Diamond, P.H.; Lin, Z.; Wang, W.; Horton, W.; Klasky, S.; Decyk, V.; Ma, K.-L.; Chames, J.; Adams, M.
2011-09-21
The three-year project GPS-TTBP resulted in over 152 publications and 135 presentations. This summary focuses on the scientific progress made by the project team. A major focus of the project was on the physics intrinsic rotation in tokamaks. Progress included the first ever flux driven study of net intrinsic spin-up, mediated by boundary effects (in collaboration with CPES), detailed studies of the microphysics origins of the Rice scaling, comparative studies of symmetry breaking mechanisms, a pioneering study of intrinsic torque driven by trapped electron modes, and studies of intrinsic rotation generation as a thermodynamic engine. Validation studies were performed with C-Mod, DIII-D and CSDX. This work resulted in very successful completion of the FY2010 Theory Milestone Activity for OFES, and several prominent papers of the 2008 and 2010 IAEA Conferences. A second major focus was on the relation between zonal flow formation and transport non-locality. This culminated in the discovery of the ExB staircase - a conceptually new phenomenon. This also makes useful interdisciplinary contact with the physics of the PV staircase, well-known in oceans and atmospheres. A third topic where progress was made was in the simulation and theory of turbulence spreading. This work, now well cited, is important for understanding the dynamics of non-locality in turbulent transport. Progress was made in studies of conjectured non-diffusive transport in trapped electron turbulence. Pioneering studies of ITB formation, coupling to intrinsic rotation and hysteresis were completed. These results may be especially significant for future ITER operation. All told, the physics per dollar performance of this project was quite good. The intense focus was beneficial and SciDAC resources were essential to its success.
Gyrokinetic Particle Simulation of Turbulent Transport in Burning Plasmas
International Nuclear Information System (INIS)
Diamond, P.H.; Lin, Z.; Wang, W.; Horton, W.; Klasky, S.; Decyk, V.; Ma, K.-L.; Chames, J.; Adams, M.
2011-01-01
The three-year project GPS-TTBP resulted in over 152 publications and 135 presentations. This summary focuses on the scientific progress made by the project team. A major focus of the project was on the physics intrinsic rotation in tokamaks. Progress included the first ever flux driven study of net intrinsic spin-up, mediated by boundary effects (in collaboration with CPES), detailed studies of the microphysics origins of the Rice scaling, comparative studies of symmetry breaking mechanisms, a pioneering study of intrinsic torque driven by trapped electron modes, and studies of intrinsic rotation generation as a thermodynamic engine. Validation studies were performed with C-Mod, DIII-D and CSDX. This work resulted in very successful completion of the FY2010 Theory Milestone Activity for OFES, and several prominent papers of the 2008 and 2010 IAEA Conferences. A second major focus was on the relation between zonal flow formation and transport non-locality. This culminated in the discovery of the ExB staircase - a conceptually new phenomenon. This also makes useful interdisciplinary contact with the physics of the PV staircase, well-known in oceans and atmospheres. A third topic where progress was made was in the simulation and theory of turbulence spreading. This work, now well cited, is important for understanding the dynamics of non-locality in turbulent transport. Progress was made in studies of conjectured non-diffusive transport in trapped electron turbulence. Pioneering studies of ITB formation, coupling to intrinsic rotation and hysteresis were completed. These results may be especially significant for future ITER operation. All told, the physics per dollar performance of this project was quite good. The intense focus was beneficial and SciDAC resources were essential to its success.
Faraday rotation signatures of fluctuation dynamos in young galaxies
Sur, Sharanya; Bhat, Pallavi; Subramanian, Kandaswamy
2018-03-01
Observations of Faraday rotation through high-redshift galaxies have revealed that they host coherent magnetic fields that are of comparable strengths to those observed in nearby galaxies. These fields could be generated by fluctuation dynamos. We use idealized numerical simulations of such dynamos in forced compressible turbulence up to rms Mach number of 2.4 to probe the resulting rotation measure (RM) and the degree of coherence of the magnetic field. We obtain rms values of RM at dynamo saturation of the order of 45-55 per cent of the value expected in a model where fields are assumed to be coherent on the forcing scale of turbulence. We show that the dominant contribution to the RM in subsonic and transonic cases comes from the general sea of volume filling fields, rather than from the rarer structures. However, in the supersonic case, strong field regions as well as moderately overdense regions contribute significantly. Our results can account for the observed RMs in young galaxies.
Behaviors and transitions along the path to magnetostrophic convection
Grannan, A. M.; Vogt, T.; Horn, S.; Hawkins, E. K.; Aggarwal, A.; Aurnou, J. M.
2017-12-01
The generation of magnetic fields in planetary and stellar interiors are believed to be controlled primarily by turbulent convection constrained by Coriolis and Lorentz forces in their electrically conducting fluid layers. Yet relatively few laboratory experiments are capable of investigating the different regimes of turbulent magnetohydrodynamic convection. In this work, we perform one laboratory experiment in a cylinder at a fixed heat flux using the liquid metal gallium in order to investigate, sequentially: Rayleigh-Bènard convection without any imposed constraints, magnetoconvection with a Lorentz constraint imposed by vertical magnetic field, rotating convection with a Coriolis constraint imposed by rotation, and finally the magnetostrophic convective regime where both Coriolis and Lorentz are imposed and equal. Using an array of internal and external temperature probes, we show that each regime along the path to magnetostrophic convection is unique. The behaviors and transitions in the dominant modes of convection as well as their fundamental frequencies and wavenumbers are investigated.
The generation of resonant turbulence for a premixed burner
Verbeek, Antonie Alex; Pos, R.C.; Stoffels, Genie G.M.; Geurts, Bernardus J.; van der Meer, Th.H.
Is it possible to optimize the turbulent combustion of a low swirl burner by using resonance in turbu- lence? To that end an active grid is constructed that consists of two perforated disks of which one is rotat- ing, creating a system of pulsating jets, which in the end can be used as a central
Energy Technology Data Exchange (ETDEWEB)
Capdevila, R; Trias, F X; Perez-Segarra, C D [Centre Tecnologic de Transferencia de Calor (CTTC), Lab.Termotecnia i Energetica, Universitat Politecnica de Catalunya (UPC), C/Colom, 11, E08222 Terrassa, Barcelona (Spain); Lehmkuhl, O; Colomer, G, E-mail: cttc@cttc.upc.edu, E-mail: termofluids@termofluids.com [Termofluids, S. L., Magi Colet 8, E08204 Sabadell, Barcelona (Spain)
2011-12-22
In the present work, turbulent natural convection in a tall differentially heated cavity of aspect ratio 5:1, filled with air (Pr = 0.7) under a Rayleigh number based on the height of 4.5 {center_dot} 10{sup 10}, is studied numerically. Two different situations have been analysed. In the first one, the cavity is filled with a transparent medium. In the second one, the cavity contains a grey participating gas. The turbulent flow is described by means of Large Eddy Simulation (LES) using symmetry-preserving discretizations. Simulations are compared with experimental data available in the literature and with Direct Numerical Simulations (DNS). Surface and gas radiation have been simulated using the Discrete Ordinates Method (DOM). The influence of radiation on fluid flow behaviour has also been analysed.
Hydrodynamical and magnetohydrodynamic global bifurcations in a highly turbulent von Karman flow
International Nuclear Information System (INIS)
Ravelet, F.
2005-09-01
We report experimental studies of the turbulent von Karman flow, inertially stirred between counter-rotating impellers. We first study the flow and its transition from laminar to turbulent regime. We highlight the role of slowly varying large scales, due to the presence of an azimuthal mixing layer. The large scales of this flow can be unstable in turbulent regime. We study the statistics of the transitions between the different mean states. The second part is dedicated to an experiment in liquid sodium, called VKS2. We optimize the time-averaged flow in order to allow kinematic dynamo action. We report the very first results of the experiment, and discuss the role of the large scales temporal non-stationariness. (author)
Measurements of turbulent premixed flame dynamics using cinema stereoscopic PIV
Energy Technology Data Exchange (ETDEWEB)
Steinberg, Adam M.; Driscoll, James F. [University of Michigan, Department of Aerospace Engineering, Ann Arbor, MI (United States); Ceccio, Steven L. [University of Michigan, Department of Mechanical Engineering, Ann Arbor, MI (United States)
2008-06-15
A new experimental method is described that provides high-speed movies of turbulent premixed flame wrinkling dynamics and the associated vorticity fields. This method employs cinema stereoscopic particle image velocimetry and has been applied to a turbulent slot Bunsen flame. Three-component velocity fields were measured with high temporal and spatial resolutions of 0.9 ms and 140{mu}m, respectively. The flame-front location was determined using a new multi-step method based on particle image gradients, which is described. Comparisons are made between flame fronts found with this method and simultaneous CH-PLIF images. These show that the flame contour determined corresponds well to the true location of maximum gas density gradient. Time histories of typical eddy-flame interactions are reported and several important phenomena identified. Outwardly rotating eddy pairs wrinkle the flame and are attenuated at they pass through the flamelet. Significant flame-generated vorticity is produced downstream of the wrinkled tip. Similar wrinkles are caused by larger groups of outwardly rotating eddies. Inwardly rotating pairs cause significant convex wrinkles that grow as the flame propagates. These wrinkles encounter other eddies that alter their behavior. The effects of the hydrodynamic and diffusive instabilities are observed and found to be significant contributors to the formation and propagation of wrinkles. (orig.)
Energy Technology Data Exchange (ETDEWEB)
Yamamoto, H; Mizutani, K; Saito, t [Iwate University, Iwate (Japan). Faculty of Engineering
1997-10-22
Discussions were given on the possibility of estimating Rayleigh-wave spectral ratio utilizing phase difference between horizontal movements and vertical movements by using a three-component single-station seismograph. The test has selected as an observation point a location in the city of Kushiro where a pulp and paper mill generating microtremors is the focal point, and the underground structure at that point has been estimated by using the vertical array observation method. The observation system has used three components of a velocity type seismograph having a natural period of one second, an amplifier and an analog data recorder. As a result of the discussions, the following matters were made clear: the spectral ratio with a phase difference of 90 degrees agrees with the frequency at a peak trough of the theoretical Rayleigh-wave spectral ratio; the values of the spectral ratio at the phase difference of 90 degrees and the values of the theoretical Rayleigh-wave spectral ratio correspond well excepting in frequency bands of the peak trough; and these results suggest that the Rayleigh-wave spectral ratio may be estimated by utilizing the phase difference between horizontal movements and vertical movements. Estimation of the underground structure by using the inverse analysis of this Rayleigh-wave spectral ratio is expected in the future. 6 refs., 5 figs., tab.
International Nuclear Information System (INIS)
Amati, G.; Koal, K.; Massaioli, F.; Sreenivasan, K.R.; Verzicco, R.
2006-12-01
The results from direct numerical simulations of turbulent Boussinesq convection are briefly presented. The flow is computed for a cylindrical cell of aspect ratio 1/2 in order to compare with the results from recent experiments. The results span eight decades of Ra from 2x10 6 to 2x10 14 and form the baseline data for a strictly Boussinesq fluid of constant Prandtl number (Pr=0.7). A conclusion is that the Nusselt number varies nearly as the 1/3 power of Ra for about four decades towards the upper end of the Ra range covered. (author)
The hydrodynamics of accretion discs. Pt. 1,2. Stability. Turbulent models
Energy Technology Data Exchange (ETDEWEB)
Stewart, J
1975-08-01
The disc is idealized to be a stationary axisymmetric toroidal flow of a compressible fluid. The stability against linearized short wavelength perturbations is discussed. When the Rayleigh and Schwarzschild criteria are satisfied, the flow is stable against axisymmetric perturbations. However, almost all non-axisymmetric perturbations are not secularly stable, and examples of dynamically unstable modes are given. For turbulent models, two new points are made. The crucial problem for the horizontal structure of the disc, the prescription of the Reynolds stress gradient, is resolved by a direct calculation from first principles. A preliminary attempt is also made to describe the vertical structure, leading to a sandwich model. The predictions of this theory are shown to be consistent with the fine scale structure of Cyg X-1. (DE)
Rayleigh-Taylor instability and mixing in SN 1987A
International Nuclear Information System (INIS)
Ebisuzaki, T.; Shigeyama, T.; Nomoto, K.
1989-01-01
The stability of the supernova ejecta is compared with the Rayleigh-Taylor instability for a realistic model of SN 1987A. A linear analysis indicates that the layers around the composition interface between the hydrogen-rich and helium zones, and become Rayleigh-Taylor unstable between the helium and metal zones. In these layers, the pressure increases outward because of deceleration due to the reverse shock which forms when the blast shock hits the massive hydrogen-rich envelope. On the contrary, the density steeply decreases outward because of the preexisting nuclear burning shell. Then, these layers undergo the Raleigh-Taylor instability because of the opposite signs of the pressure and density gradients. The estimated growth rate is larger than the expansion rate of the supernova. The Rayleigh-Taylor instability near the composition interface is likely to induce mixing, which has been strongly suggested from observations of SN 1987A. 25 refs
Rayleigh-Taylor instability of cylindrical jets with radial motion
Energy Technology Data Exchange (ETDEWEB)
Chen, Xiang M. [GE Nuclear, Wilmington, NC (United States); Schrock, V.E.; Peterson, P.F. [Univ. of California, Berkeley, CA (United States)
1995-09-01
Rayleigh-Taylor instability of an interface between fluids with different densities subjected to accelleration normal to itself has interested researchers for almost a century. The classic analyses of a flat interface by Rayleigh and Taylor have shown that this type of instability depends on the direction of acceleration and the density differences of the two fluids. Plesset later analyzed the stability of a spherically symmetric flows (and a spherical interface) and concluded that the instability also depends on the velocity of the interface as well as the direction and magnitude of radial acceleration. The instability induced by radial motion in cylindrical systems seems to have been neglected by previous researchers. This paper analyzes the Rayleigh-Taylor type of the spherical case, the radial velocity also plays an important role. As an application, the example of a liquid jet surface in an Inertial Confinement Fusion (ICF) reactor design is analyzed.
A simple analytic approximation to the Rayleigh-Bénard stability threshold
Prosperetti, Andrea
2011-01-01
The Rayleigh-Bénard linear stability problem is solved by means of a Fourier series expansion. It is found that truncating the series to just the first term gives an excellent explicit approximation to the marginal stability relation between the Rayleigh number and the wave number of the
Transport modelling including radial electric field and plasma rotation
International Nuclear Information System (INIS)
Fukuyama, A.; Fuji, Y.; Itoh, S.-I.
1994-01-01
Using a simple turbulent transport model with a constant diffusion coefficient and a fixed temperature profile, the density profile in a steady state and the transient behaviour during the co and counter neutral beam injection are studied. More consistent analysis has been initiated with a turbulent transport model based on the current diffusive high-n ballooning mode. The enhancement of the radial electric field due to ion orbit losses and the reduction of the transport due to the poloidal rotation shear are demonstrated. The preliminary calculation indicates a sensitive temperature dependence of the density profile. (author)
Dipping-interface mapping using mode-separated Rayleigh waves
Luo, Y.; Xia, J.; Xu, Y.; Zeng, C.; Miller, R.D.; Liu, Q.
2009-01-01
Multichannel analysis of surface waves (MASW) method is a non-invasive geophysical technique that uses the dispersive characteristic of Rayleigh waves to estimate a vertical shear (S)-wave velocity profile. A pseudo-2D S-wave velocity section is constructed by aligning 1D S-wave velocity profiles at the midpoint of each receiver spread that are contoured using a spatial interpolation scheme. The horizontal resolution of the section is therefore most influenced by the receiver spread length and the source interval. Based on the assumption that a dipping-layer model can be regarded as stepped flat layers, high-resolution linear Radon transform (LRT) has been proposed to image Rayleigh-wave dispersive energy and separate modes of Rayleigh waves from a multichannel record. With the mode-separation technique, therefore, a dispersion curve that possesses satisfactory accuracy can be calculated using a pair of consecutive traces within a mode-separated shot gather. In this study, using synthetic models containing a dipping layer with a slope of 5, 10, 15, 20, or 30 degrees and a real-world example, we assess the ability of using high-resolution LRT to image and separate fundamental-mode Rayleigh waves from raw surface-wave data and accuracy of dispersion curves generated by a pair of consecutive traces within a mode-separated shot gather. Results of synthetic and real-world examples demonstrate that a dipping interface with a slope smaller than 15 degrees can be successfully mapped by separated fundamental waves using high-resolution LRT. ?? Birkh??user Verlag, Basel 2009.
Kozaka, Orçun E.; Özkan, Gökhan; Özdemir, Bedii I.
2004-01-01
Turbulent structure of flow behind a model car is investigated with local velocity measurements with emphasis on large structures and their relevance to aerodynamic forces. Results show that two counter-rotating helical vortices, which are formed within the inner wake region, play a key role in determining the flux of kinetic energy. The turbulence is generated within the outermost shear layers due to the instabilities, which also seem to be the basic drive for these relatively organized structures. The measured terms of the turbulent kinetic energy production, which are only part of the full expression, indicate that vortex centres act similar to the manifolds draining the energy in the streamwise direction. As the approach velocity increases, the streamwise convection becomes the dominant means of turbulent transport and, thus, the acquisition of turbulence by relatively non-turbulent flow around the wake region is suppressed.
A Numerical Model for Prediction of Residual Stress Using Rayleigh Waves
International Nuclear Information System (INIS)
Yuan, Mao Dan; Kang, To; Kim, Hak Joon; Song, Sung Jin
2011-01-01
In this work, a numerical model is proposed for the relation between the magnitudes and the depth residual stress with the velocity of Rayleigh wave. Three cases, stress-free, uniform stress and layered stress, are investigated for the change tendency of the Rayleigh wave speed. Using the simulated signal with variation of residual stress magnitude and depth, investigation of the parameters for fitting residual stress and velocity change are performed. The speed change of Rayleigh wave shows a linear relation with the magnitude and an exponential relation with the depth of residual stress. The combination of these two effects could be used for the depth profile evaluation of the residual stress
Turbulence evaluation at PSI-2 by fast visible imaging
Energy Technology Data Exchange (ETDEWEB)
Hubeny, Michael; Reinhart, Michael; Huber, Alexander; Unterberg, Bernhard [Institute for Energy and Climate Research - Plasma Physics, Forschungszentrum Juelich GmbH, EURATOM Association (Germany)
2014-07-01
Turbulent transport in the plasma edge poses a critical challenge for fusion reactors due to the high heat and particle fluxes on plasma facing components. Various chemical and physical processes lead to a deterioration of the surface morphology as well as the inner structure of wall materials. These processes depend strongly on the temperature and density of particles in their onset-threshold region. The turbulent, intermittent structures found in the edge of toroidal machines are also present in linear plasma devices, which running steady state, makes them ideal for studying properties of turbulence. A fast CMOS camera with a typical time resolution of several 100.000 fps can resolve short turbulent events (blobs) in the linear plasma device PSI-2. Movies of the plasma were taken perpendicular from a side port and axial through the hollow plasma source. Properties of the intermittent transport are evaluated by conditional averaging and other statistical methods to investigate the dependence of turbulence on discharge parameters and working gases. Intermittent transport events show a strong correlation to the global plasma rotation, thus they might be triggered by an m=1 instability, which appears as a 4 and 8 kHz Fourier component. The profile of the skewness supports the presence of blobs just outside of the density maximum.
Chemical Applications of Second Harmonic Rayleigh Scattering ...
Indian Academy of Sciences (India)
Chemical Applications of Second Harmonic Rayleigh Scattering Puspendu Kumar Das Department of Inorganic and Physical Chemistry Indian Institute of Science Bangalore 560012, India pkdas@ipc.iisc.ernet.in.
Simulations of Tokamak Edge Turbulence Including Self-Consistent Zonal Flows
Cohen, Bruce; Umansky, Maxim
2013-10-01
Progress on simulations of electromagnetic drift-resistive ballooning turbulence in the tokamak edge is summarized in this mini-conference talk. A more detailed report on this work is presented in a poster at this conference. This work extends our previous work to include self-consistent zonal flows and their effects. The previous work addressed the simulation of L-mode tokamak edge turbulence using the turbulence code BOUT. The calculations used realistic single-null geometry and plasma parameters of the DIII-D tokamak and produced fluctuation amplitudes, fluctuation spectra, and particle and thermal fluxes that compare favorably to experimental data. In the effect of sheared ExB poloidal rotation is included with an imposed static radial electric field fitted to experimental data. In the new work here we include the radial electric field self-consistently driven by the microturbulence, which contributes to the sheared ExB poloidal rotation (zonal flow generation). We present simulations with/without zonal flows for both cylindrical geometry, as in the UCLA Large Plasma Device, and for the DIII-D tokamak L-mode cases in to quantify the influence of self-consistent zonal flows on the microturbulence and the concomitant transport. This work was performed under the auspices of the US Department of Energy under contract DE-AC52-07NA27344 at the Lawrence Livermore National Laboratory.
Grain size measurements by ultrasonic Rayleigh surface waves
International Nuclear Information System (INIS)
Palanichamy, P.; Jayakumar, T.
1996-01-01
The use of Rayleigh surface waves to determine average grain size nondestructively in an austenitic stainless steel AISI type 316 stainless is discussed. Two commercial type 4MHz frequency surface wave transducers, one as transmitter and the other as receiver were employed for the measurement of surface wave amplitudes. Relative amplitudes of the Rayleigh surface waves were correlated with the metallographically obtained grain sizes. Results indicate that surface/sub-surface average grain sizes of AISI type 316 austenitic stainless steel can be estimated with a confidence level of more than 80% in the grain size range 30-170 μm. (author)
Rayleigh-Schrödinger series and Birkhoff decomposition
Novelli, Jean-Christophe; Paul, Thierry; Sauzin, David; Thibon, Jean-Yves
2018-01-01
We derive new expressions for the Rayleigh-Schrödinger series describing the perturbation of eigenvalues of quantum Hamiltonians. The method, somehow close to the so-called dimensional renormalization in quantum field theory, involves the Birkhoff decomposition of some Laurent series built up out of explicit fully non-resonant terms present in the usual expression of the Rayleigh-Schrödinger series. Our results provide new combinatorial formulae and a new way of deriving perturbation series in quantum mechanics. More generally we prove that such a decomposition provides solutions of general normal form problems in Lie algebras.
Mode cross coupling observations with a rotation sensor
Nader-Nieto, M. F.; Igel, H.; Ferreira, A. M.; Al-Attar, D.
2013-12-01
The Earth's free oscillations induced by large earthquakes have been one of the most important ways to measure the Earth's internal structure and processes. They provide important large scale constraints on a variety of elastic parameters, attenuation and density of the Earth's deep interior. The potential of rotational seismic records for long period seismology was proven useful as a complement to traditional measurements in the study of the Earth's free oscillations. Thanks to the high resolution of the G-ring laser located at Geodetic Observatory Wettzell, Germany, we are now able to study the spectral energy generated by rotations in the low frequency range. On a SNREI Earth, a vertical component rotational sensor is primarily excited by horizontally polarised shear motions (SH waves, Love waves) with theoretically no sensitivity to compressional waves and conversions (P-SV) and Rayleigh waves. Consequently, in the context of the Earth's normal modes, this instrument detects mostly toroidal modes. Here, we present observations of spectral energy of both toroidal and spheroidal normal modes in the G-ring Laser records of one of the largest magnitude events recently recorded: Tohoku-Oki, Japan, 2011. In an attempt to determine the mechanisms responsible for spheroidal energy in the vertical axes rotational spectra, we first rule out instrumental effects as well as the effect of local heterogeneity. Second, we carry out a simulation of an ideal rotational sensor taking into account the effects of the Earth's daily rotation, its hydrostatic ellipticity and structural heterogeneity, finding a good fit to the data. Simulations considering each effect separately are performed in order to evaluate the sensitivity of rotational motions to global effects with respect to traditional translation measurements.
Depolarization Rayleigh scattering as a means of molecular concentration determination in plasmas
Meulenbroeks, R.F.G.; Schram, D.C.; Jaegers, L.J.M.; Sanden, van de M.C.M.
1992-01-01
The difference in polarization for Rayleigh scattered radiation on spherically and nonspherically symmetric scattering objects has been used to obtain molecular species concentrations in plasmas of simple composition. Using a Rayleigh scattering diagnostic, the depolarized component of the scattered
PLANETESIMAL AND PROTOPLANET DYNAMICS IN A TURBULENT PROTOPLANETARY DISK: IDEAL UNSTRATIFIED DISKS
International Nuclear Information System (INIS)
Yang, Chao-Chin; Mac Low, Mordecai-Mark; Menou, Kristen
2009-01-01
The dynamics of planetesimals and planetary cores may be strongly influenced by density perturbations driven by magneto-rotational turbulence in their natal protoplanetary gas disks. Using the local shearing box approximation, we perform numerical simulations of planetesimals moving as massless particles in a turbulent, magnetized, unstratified gas disk. Our fiducial disk model shows turbulent accretion characterized by a Shakura-Sunyaev viscosity parameter of α ∼ 10 -2 , with rms density perturbations of ∼10%. We measure the statistical evolution of particle orbital properties in our simulations including mean radius, eccentricity, and velocity dispersion. We confirm random walk growth in time of all three properties, the first time that this has been done with direct orbital integration in a local model. We find that the growth rate increases with the box size used at least up to boxes of eight scale heights in horizontal size. However, even our largest boxes show velocity dispersions sufficiently low that collisional destruction of planetesimals should be unimportant in the inner disk throughout its lifetime. Our direct integrations agree with earlier torque measurements showing that type I migration dominates over diffusive migration by stochastic torques for most objects in the planetary core and terrestrial planet mass range. Diffusive migration remains important for objects in the mass range of kilometer-sized planetesimals. Discrepancies in the derived magnitude of turbulence between local and global simulations of magneto-rotationally unstable disks remains an open issue, with important consequences for planet formation scenarios.
Energy Technology Data Exchange (ETDEWEB)
Leger, J J [Commissariat a l' Energie Atomique, Saclay (France). Centre d' Etudes Nucleaires
1969-07-01
The aim of this study is to develop a rapid and accurate measurement, with the Rayleigh interferometer, of the free diffusion coefficient of translation of macromolecules in solution. After having explained the choice of a diffusion cell with laminar lateral flow, and explained the principle of the Rayleigh interferometer, a semi-automatic technique of free diffusion are then introduced. Solutions are proposed for systems composed of two or three components, such as biopolymers. The paper ends by drafting the possible treatment of recorded experimental data by means of electronic computer. (author) [French] Cette etude a ete entreprise pour mettre au point une methode precise et rapide de mesure, par interferometre de Rayleigh, du coefficient de diffusion libre de translation des macromolecules en solution. Apres avoir justifie le choix d'une cellule de diffusion a ecoulement laminaire lateral et explique le principe de l'interferometre de Rayleigh, l'auteur decrit une technique semi-automatique d'enregistrement des cliches d'interference. Il introduit ensuite les equations differentielles de diffusion libre et propose des solutions pour les systemes a deux et trois composants applicables aux biopolymeres. L'article se termine par une esquisse concernant le traitement des donnees experimentales enregistrees au moyen du calcul electronique. (auteur)
International Nuclear Information System (INIS)
Anon.
1989-01-01
Papers on turbulent shear flows are presented, covering topics such as the structure of pressure fluctuations, fossil two-dimensional turbulence in the ocean, turbulence production and eddy structure in wall turbulence, bypass transition in a heated boundary layer, a turbulent spot in plane Poiseuille flow, the evolution of an axisymmetric jet, plane mixing layer development, vortex models of a pseudoturbulent shear flow, numerical techniques for turbulence studies, Reynolds stress in the wall region of turbulent pipe flow, the turbulent structure of a momentumless wake, the near field of the transverse jet. Additional topics include a turbulent boundary layer disturbed by a cylinder, evolving mixing layers, flow analysis in a vortex flowmeter, ejections and bursts in pulsatile turbulent wall flow measurements, a flat plate oscillating in pitch, turbulent buoyant flows, isothermal lobed mixer flows, flow distortion on a turbulent scalar field, two phase flows. In addition, papers on the applications of turbulent shear flow studies are given, including air pollutant deposition, closures, oceanography, instrumentation, heat transfer, rotating flows, combustion, coherent structures, turbulence control, and scalar transport modeling
Energy Technology Data Exchange (ETDEWEB)
Jacques, R.; Le Quere, P.; Daube, O. [Centre National de la Recherche Scientifique (CNRS), 91 - Orsay (France)
1997-12-31
Turbulent flows between a fixed disc and a rotating disc are encountered in various applications like turbo-machineries or torque converters of automatic gear boxes. These flows are characterised by particular physical phenomena mainly due to the effects of rotation (Coriolis and inertia forces) and thus, classical k-{epsilon}-type modeling gives approximative results. The aim of this work is to study these flows using direct numerical simulation in order to provide precise information about the statistical turbulent quantities and to improve the k-{epsilon} modeling in the industrial MATHILDA code of the ONERA and used by SNECMA company (aerospace industry). The results presented are restricted to the comparison between results obtained with direct simulation and results obtained with the MATHILDA code in the same configuration. (J.S.) 8 refs.
Rayleigh-Taylor instability of cylindrical jets with radial motion
International Nuclear Information System (INIS)
Chen, X.M.; Schrock, V.E.; Peterson, P.F.
1997-01-01
Rayleigh-Taylor instability of an interface between fluids with different densities subjected to acceleration normal to itself has interested researchers for almost a century. The classic analyses of a flat interface by Rayleigh and Taylor have shown that this type of instability depends on the direction of acceleration and the density differences of the two fluids. Plesset later analyzed the stability of a spherically symmetric flows (and a spherical interface) and concluded that the instability also depends on the velocity of the interface as well as the direction and magnitude of radial acceleration. The instability induced by radial motion in cylindrical systems seems to have been neglected by previous researchers. This paper analyzes the Rayleigh-Taylor type of instability for a cylindrical surface with radial motions. The results of the analysis show that, like the spherical case, the radial velocity also plays an important role. As an application, the example of a liquid jet surface in an Inertial Confinement Fusion (ICF) reactor design is analyzed. (orig.)
Dynamic stabilization of the imploding-shell Rayleigh-Taylor instability
International Nuclear Information System (INIS)
Boris, J.P.
1977-01-01
A method for dynamic stabilization of the Rayleigh-Taylor (R-T) instability on the surface of an imploding fusion pellet is discussed. The driving laser beams are modulated in intensity so the ablation layer is subject to a rapidly and strongly oscillating acceleration. A substantial band of the Rayleigh-Taylor instability spectrum can be stabilized by this oscillation even though the time average acceleration vector lies in the destabilizing direction. By adjusting the frequency, structure, and amplitude of the modulation, the band of dynamically stabilized modes can be made to include the most unstable and dangerous modes. Thus considerably higher aspect ratio shells (i.e., thinner shells) could implode successfully than had been previously considered stable enough. Both theory and numerical simulations support this conclusion for the case of laser-driven pellet implosions. Similar modulation via transverse beam oscillations or parallel bunching should also work to stabilize the most dangerous surface Rayleigh-Taylor modes in relativistic electron-, ion- and heavy ion-pellet fusion schemes. (U.K.)
The influence of turbulence on the aero-elastic instability of wind turbines
DEFF Research Database (Denmark)
Zhang, Zili; Nielsen, Søren R.K.
2014-01-01
Modern multi-megawatt wind turbines are designed with longer and slender blades using new composite materials and advanced fabrication methods. The trend towards lighter and more flexible blades may lead to aeroelastic instability of wind turbines under certain circumstances, thus resulting...... calibrated to the NREL 5 MW baseline wind turbine. Aeroelastic stability of the wind turbine system has been evaluated for various values of the rated generator torque, the rated rotational speed of the rotor, the mean wind speed and the turbulence intensity. Critical turbulence intensity, at which the wind...
Structure measurements in a synthetic turbulent boundary layer
Arakeri, Jaywant H.
1987-09-01
Extensive hot-wire measurements have been made to determine the structure of the large eddy in a synthejc turbulent boundary layer on a flat-plate model. The experiments were carried out in a wind tunnel at a nominal free-stream velocity of 12 m/s. The synthetic turbulent boundary layer had a hexagonal pattern of eddies and a ratio of streamwise scale to spanwise scale of 3.2:1. The measured celerity of the large eddy was 84.2 percent of the free-stream velocity. There was some loss of coherence, but very little distortion, as the eddies moved downstream. Several mean properties of the synthetic boundary layer were found to agree quite well with the mean properties of a natural turbulent boundary layer at the same Reynolds number. The large eddy is composed of a pair of primary counter-rotating vortices about five [...] long in the streamwise direction and about one [...] apart in the spanwise direction, where [...] is the mean boundary-layer thickness. The sense of the primary pair is such as to pump fluid away from the wall in the region between the vortices. A secondary pair of counter-rotating streamwise vortices, having a sense opposite to that of the primary pair, is observed outside of and slightly downstream from the primary vortices. Both pairs of vortices extend across the full thickness of the boundary layer and are inclined at a shallow angle to the surface of the flat plate. The data show that the mean vorticity vectors are not tangential to the large-eddy vortices. In fact, the streamwise and normal vorticity components that signal the presence of the eddy are of the same order of magnitude. Definite signatures are obtained in terms of the mean skin-friction coefficient and the mean wake parameter averaged at constant phase. Velocities induced by the vortices are partly responsible for entrainment of irrotational fluid, for transport of momentum, for generation of Reynolds stresses, and for maintenance of streamwise and normal vorticity in the outer
Turbulence characteristics of the Bödewadt layer in a large enclosed rotor-stator system
Randriamampianina, Anthony; Poncet, Sébastien
2006-05-01
A three-dimensional (3D) direct numerical simulation is combined with a laboratory study to describe the turbulent flow in an enclosed annular rotor-stator cavity characterized by a large aspect ratio G =(b-a)/h=18.32 and a small radius ratio a /b=0.152, where a and b are the inner and outer radii of the rotating disk and h is the interdisk spacing. The rotation rate Ω under consideration is equivalent to the rotational Reynolds number Re =Ωb2/ν=9.5×104, where ν is the kinematic viscosity of the fluid. This corresponds to a value at which an experiment carried out at the laboratory has shown that the stator boundary layer is turbulent, whereas the rotor boundary layer is still laminar. Comparisons of the 3D computed solution with velocity measurements have given good agreement for the mean and turbulent fields. The results enhance evidence of weak turbulence at this Reynolds number, by comparing the turbulence properties with available data in the literature [M. Lygren and H. I. Andersson, J. Fluid Mech. 426, 297 (2001)]. An approximately self-similar boundary layer behavior is observed along the stator side. The reduction of the structural parameter a1 under the typical value 0.15 and the variation in the wall-normal direction of the different characteristic angles show that this boundary layer is three-dimensional. A quadrant analysis [H. S. Kang, H. Choi, and J. Y. Yoo, Phys. Fluids 10, 2315 (1998)] of conditionally averaged velocities is performed to identify the contributions of different events (ejections and sweeps) on the Reynolds shear stress producing vortical structures. The asymmetries observed in the conditionally averaged quadrant analysis are dominated by Reynolds stress-producing events in this Bödewadt layer. Moreover, case 1 vortices (with a positive wall induced velocity) are found to be the major source of generation of special strong events, in agreement with the conclusions of Lygren and Andersson [J. Fluid Mech. 426, 297 (2001)].
Jhang, Hogun; Kim, S. S.; Kwon, J. M.; Terzolo, L.; Kim, J. Y.; Diamond, P. H.
2010-11-01
It is accepted that the intrinsic rotation is generated via the residual stress, which is non-diffusive components of the turbulent Reynolds stress, without external momentum input. The physics leading to the onset of intrinsic rotation in L- and H- mode plasmas have been elucidated elsewhere. However, the physics responsible for the generation and transport of the intrinsic rotation and its relationship to the formation of internal transport barriers (ITBs) in reversed shear (RS) plasmas have not been explored in detail, which is the main subject in the present work. The revised version of the global gyrofluid code TRB is used for this study. It is found that the large intrinsic rotation (˜10-30% of the ion sound speed depending on ITB characteristics) is generated near the ITB region and propagates into the core. The intrinsic rotation increases linearly as the temperature gradient at ITB position increases, albeit not indefinitely. Key parameters related to the symmetry breaking, such as turbulent intensity and its gradient, the flux surface averaged parallel wavenumber are evaluated dynamically during the ITB formation. In particular, the role of reversed shear and the q-profile curvature is presented in relation to the symmetry breaking in RS plasmas.
Mathematical modeling of turbulent stratified flows. Application of liquid metal fast breeders
Energy Technology Data Exchange (ETDEWEB)
Villand, M; Grand, D [CEA-Service des Transferts Thermiques, Grenoble (France)
1983-07-01
Mathematical model of turbulent stratified flow was proposed under the following assumptions: Newtonian fluid; incompressible fluid; coupling between temperature and momentum fields according to Boussinesq approximation; two-dimensional invariance for translation or rotation; coordinates cartesian or curvilinear. Solutions obtained by the proposed method are presented.
Rayleigh Taylor instability of two superposed compressible fluids in un-magnetized plasma
International Nuclear Information System (INIS)
Sharma, P K; Tiwari, A; Argal, S; Chhajlani, R K
2014-01-01
The linear Rayleigh Taylor instability of two superposed compressible Newtonian fluids is discussed with the effect of surface tension which can play important roles in space plasma. As in both the superposed Newtonian fluids, the system is stable for potentially stable case and unstable for potentially unstable case in the present problem also. The equations of the problem are solved by normal mode method and a dispersion relation is obtained for such a system. The behaviour of growth rate is examined in the presence of surface tension and it is found that the surface tension has stabilizing influence on the Rayleigh Taylor instability of two superposed compressible fluids. Numerical analysis is performed to show the effect of sound velocity and surface tension on the growth rate of Rayleigh Taylor instability. It is found that both parameters have stabilizing influence on the growth rate of Rayleigh Taylor instability.
Hydromagnetic waves, turbulence, and collisionless processes in the interplanetary medium
International Nuclear Information System (INIS)
Barnes, A.
1983-01-01
The solar wind does not flow quietly. It seethes and undulates, fluctuating on time scales that range from the solar rotation period down to fractions of milliseconds. Most of the power in interplanetary waves and turbulence lies at hydromagnetic scales. These fluctuations are normally of large amplitude, containing enough energy to affect solar and galactic cosmic rays, and may be the remnants of a coronal turbulence field powerful enough to play a major role in accelerating the solar wind itself. The origin and evolution of interplanetary hydromagnetic waves and turbulence, and their influence on the large-scale dynamics of the solar wind are among the most fundamental questions of solar-terrestrial physics. First hydrodynamic waves and turbulences in the interplanetary medium are discussed in two sections, respectively. Because the length and time scales for hydromagnetic fluctuations are very much smaller than the corresponding Coulomb collision scales of the plasma ions and electrons, the interplanetary variations are modelled as fluctuations in a magnetohydrodynamic fluid. In the last section, collisionless phenomena are discussed. They are of qualitative significance. (Auth.)
Passive retrieval of Rayleigh waves in disordered elastic media
International Nuclear Information System (INIS)
Larose, Eric; Derode, Arnaud; Clorennec, Dominique; Margerin, Ludovic; Campillo, Michel
2005-01-01
When averaged over sources or disorder, cross correlation of diffuse fields yields the Green's function between two passive sensors. This technique is applied to elastic ultrasonic waves in an open scattering slab mimicking seismic waves in the Earth's crust. It appears that the Rayleigh wave reconstruction depends on the scattering properties of the elastic slab. Special attention is paid to the specific role of bulk to Rayleigh wave coupling, which may result in unexpected phenomena, such as a persistent time asymmetry in the diffuse regime
In situ nanoparticle diagnostics by multi-wavelength Rayleigh-Mie scattering ellipsometry
Gebauer, G
2003-01-01
We present and discuss the method of multiple-wavelength Rayleigh-Mie scattering ellipsometry for the in situ analysis of nanoparticles. It is applied to the problem of nanoparticles suspended in low-pressure plasmas. We discuss experimental results demonstrating that the size distribution and the complex refractive index can be determined with high accuracy and present a study on the in situ analysis of etching of melamine-formaldehyde nanoparticles suspended in an oxygen plasma. It is also shown that particles with a shell structure (core plus mantle) can be analysed by Rayleigh-Mie scattering ellipsometry. Rayleigh-Mie scattering ellipsometry is also applicable to in situ analysis of nanoparticles under high gas pressures and in liquids.
Investigation of particle lift off in a turbulent boundary layer
Barros, Diogo; Tee, Yi Hui; Morse, Nicholas; Hiltbrand, Ben; Longmire, Ellen
2017-11-01
Entrainment and suspension of particles within turbulent flows occur widely in environmental and industrial processes. Three-dimensional particle tracking experiments are thus conducted in a water channel to understand the interaction of finite-size particles with a turbulent boundary layer. A neutrally buoyant sphere made of wax and iron oxide is first held in place on the bounding surface by a magnet before being released and tracked. The sphere is marked with dots to monitor rotation as well as translation. By setting up two pairs of cameras in a stereoscopic configuration, the trajectories of the sphere are reconstructed and tracked over a distance of 4 to 6 δ. Sphere diameters ranging from 40 to 130 wall units, initial particle Reynolds numbers of 600 to 2000 and friction Reynolds numbers of 500 to 1800 are considered. For this parameter set, the particle typically lifts off from the wall after release before falling back toward the wall. Aspects of both particle rotation and translation will be discussed. Supported by NSF (CBET-1510154).
Parameter identification in a generalized time-harmonic Rayleigh damping model for elastography.
Directory of Open Access Journals (Sweden)
Elijah E W Van Houten
Full Text Available The identifiability of the two damping components of a Generalized Rayleigh Damping model is investigated through analysis of the continuum equilibrium equations as well as a simple spring-mass system. Generalized Rayleigh Damping provides a more diversified attenuation model than pure Viscoelasticity, with two parameters to describe attenuation effects and account for the complex damping behavior found in biological tissue. For heterogeneous Rayleigh Damped materials, there is no equivalent Viscoelastic system to describe the observed motions. For homogeneous systems, the inverse problem to determine the two Rayleigh Damping components is seen to be uniquely posed, in the sense that the inverse matrix for parameter identification is full rank, with certain conditions: when either multi-frequency data is available or when both shear and dilatational wave propagation is taken into account. For the multi-frequency case, the frequency dependency of the elastic parameters adds a level of complexity to the reconstruction problem that must be addressed for reasonable solutions. For the dilatational wave case, the accuracy of compressional wave measurement in fluid saturated soft tissues becomes an issue for qualitative parameter identification. These issues can be addressed with reasonable assumptions on the negligible damping levels of dilatational waves in soft tissue. In general, the parameters of a Generalized Rayleigh Damping model are identifiable for the elastography inverse problem, although with more complex conditions than the simpler Viscoelastic damping model. The value of this approach is the additional structural information provided by the Generalized Rayleigh Damping model, which can be linked to tissue composition as well as rheological interpretations.
International Nuclear Information System (INIS)
Sasaki, Kazuki; Suzuki, Naoya; Saito, Hiroki; Akamatsu, Daisuke
2009-01-01
The Rayleigh-Taylor instability at the interface in an immiscible two-component Bose-Einstein condensate is investigated using the mean field and Bogoliubov theories. Rayleigh-Taylor fingers are found to grow from the interface and mushroom patterns are formed. Quantized vortex rings and vortex lines are then generated around the mushrooms. The Rayleigh-Taylor instability and mushroom-pattern formation can be observed in a trapped system.
Thermal convection at low Rayleigh number from concentrated sources in porous media
International Nuclear Information System (INIS)
Hickox, C.E.
1980-01-01
A simple mathematical theory is proposed for the analysis of natural convective motion, at low Rayleigh number, from a concentrated source of heat in a fluid-saturated porous medium. The theory consists of retaining only the leading terms of series expansions of the dependent variables in terms of the Rayleigh number, is thus linear, and is valid only in the limit of small Rayleigh number. Based on fundamental results for a variety of isolated sources, superposition is used to provide solutions for situations of practical interest. Special emphasis is given to the analysis of sub-seabed disposal of nuclear waste. 8 figures
On Amplify-and-Forward Relaying Over Hyper-Rayleigh Fading Channels
Directory of Open Access Journals (Sweden)
S. H. Alvi
2014-12-01
Full Text Available Relayed transmission holds promise for the next generation of wireless communication systems due to the performance gains it can provide over non-cooperative systems. Recently hyper-Rayleigh fading, which represents fading conditions more severe than Rayleigh fading, has received attention in the context of many practical communication scenarios. Though power allocation for Amplify-and-Forward (AF relaying networks has been studied in the literature, a theoretical analysis of the power allocation problem for hyper-Rayleigh fading channels is a novel contribution of this work. We develop an optimal power allocation (OPA strategy for a dual-hop AF relaying network in which the relay-destination link experiences hyper-Rayleigh fading. A new closed-form expression for the average signal-to-noise ratio (SNR at destination is derived and it is shown to provide a new upper-bound on the average SNR at destination, which outperforms a previously proposed upper-bound based on the well-known harmonic-geometric mean inequality. An OPA across the source and relay nodes, subject to a sum-power constraint, is proposed and it is shown to provide measurable performance gains in average SNR and SNR outage at the destination relative to the case of equal power allocation.
DEFF Research Database (Denmark)
Birkedal, Dan; Shah, Jagdeep; Shchegrov, Andrei V.
2000-01-01
Resonant Rayleigh scattering from quantum well excitons is investigated using ultrafast spectral interferometry. We isolate the coherent Rayleigh scattering from incoherent luminescence in a single speckle. Averaging the resonant Rayleigh intensity over several speckles allows us to identify...... features in support of quantum corrections to the classical description of the underlying scattering process....
Some Recent Developments in Turbulence Closure Modeling
Durbin, Paul A.
2018-01-01
Turbulence closure models are central to a good deal of applied computational fluid dynamical analysis. Closure modeling endures as a productive area of research. This review covers recent developments in elliptic relaxation and elliptic blending models, unified rotation and curvature corrections, transition prediction, hybrid simulation, and data-driven methods. The focus is on closure models in which transport equations are solved for scalar variables, such as the turbulent kinetic energy, a timescale, or a measure of anisotropy. Algebraic constitutive representations are reviewed for their role in relating scalar closures to the Reynolds stress tensor. Seamless and nonzonal methods, which invoke a single closure model, are reviewed, especially detached eddy simulation (DES) and adaptive DES. Other topics surveyed include data-driven modeling and intermittency and laminar fluctuation models for transition prediction. The review concludes with an outlook.
The effect of pressure on spontaneous Rayleigh-Brillouin scattering spectrum in nitrogen
Yang, Chuanyin; Wu, Tao; Shang, Jingcheng; Zhang, Xinyi; Hu, Rongjing; He, XingDao
2018-05-01
In order to study the effect of gas pressure on spontaneous Rayleigh-Brillouin scattering spectrum and verify the validity of Tenti S6 model at pressures up to 8 atm, the spontaneous Rayleigh-Brillouin scattering experiment in nitrogen was performed for a wavelength of 532 nm at the constant room temperature of 296 K and a 90° scattering angle. By comparing the experimental spectrum with the theoretical spectrum, the normalized root mean square deviation was calculated and found less than 2.2%. It is verified that Tenti S6 model can be applied to the spontaneous Rayleigh-Brillion scattering of nitrogen under higher pressures. The results of the experimental data analysis demonstrate that pressure has more effect on Brillouin peak intensity and has negligible effect on Brillouin frequency shift, and pressure retrieval based on spontaneous Rayleigh-Brillouin scattering profile is a promising method for remote of pressure, such as harsh environment applications. Some factors that caused experiment deviations are also discussed.
Rayleigh-Taylor/gravitational instability in dense magnetoplasmas
Energy Technology Data Exchange (ETDEWEB)
Ali, S., E-mail: shahid.ali@ncp.edu.p [National Centre for Physics, Quaid-i-Azam University Campus, Islamabad (Pakistan); IPFN, Instituto Superior Tecnico, Av. Rovisco Pais, 1049-001 Lisboa (Portugal); Ahmed, Z. [COMSATS Institute of Information Technology, Department of Physics, Wah Campus (Pakistan); Mirza, Arshad M. [Theoretical Plasma Physics Group, Physics Department, Quaid-i-Azam University, Islamabad 45320 (Pakistan); Ahmad, I. [COMSATS Institute of Information Technology, Department of Physics, Islamabad Campus (Pakistan)
2009-08-10
The Rayleigh-Taylor instability is investigated in a nonuniform dense quantum magnetoplasma. For this purpose, a quantum hydrodynamical model is used for the electrons whereas the ions are assumed to be cold and classical. The dispersion relation for the Rayleigh-Taylor instability becomes modified with the quantum corrections associated with the Fermi pressure law and the quantum Bohm potential force. Numerically, it is found that the quantum speed and density gradient significantly modify the growth rate of RT instability. In a dense quantum magnetoplasma case, the linear growth rate of RT instability becomes significantly higher than its classical value and the modes are found to be highly localized. The present investigation should be useful in the studies of dense astrophysical magnetoplasmas as well as in laser-produced plasmas.
Rayleigh-Taylor/gravitational instability in dense magnetoplasmas
International Nuclear Information System (INIS)
Ali, S.; Ahmed, Z.; Mirza, Arshad M.; Ahmad, I.
2009-01-01
The Rayleigh-Taylor instability is investigated in a nonuniform dense quantum magnetoplasma. For this purpose, a quantum hydrodynamical model is used for the electrons whereas the ions are assumed to be cold and classical. The dispersion relation for the Rayleigh-Taylor instability becomes modified with the quantum corrections associated with the Fermi pressure law and the quantum Bohm potential force. Numerically, it is found that the quantum speed and density gradient significantly modify the growth rate of RT instability. In a dense quantum magnetoplasma case, the linear growth rate of RT instability becomes significantly higher than its classical value and the modes are found to be highly localized. The present investigation should be useful in the studies of dense astrophysical magnetoplasmas as well as in laser-produced plasmas.
Turbulence modelling for incompressible flows
International Nuclear Information System (INIS)
Rodi, W.
1985-12-01
EUROMECH colloquium 180 was held at Karlsruhe from 4-6 July, 1984, with the aim of bringing together specialists working in the area of turbulence modelling and of reviewing the state-of-the-art in this field. 44 scientists from 12 countries participated and 28 papers were presented. The meeting started with a review of the performance of two-equation turbulence models employing transport equations for both the velocity and the length scale of turbulence. These models are now generally well established, but it was found that their application to certain flow situations remains problematic. The modelling assumptions involved in Reynolds stress-equation models were reviewed next, and new assumptions were proposed. It was generally agreed that, as computing power increases, these more complex models will become more popular also for practical applications. The increase in computing power also allows more and more to resolve the viscous sublayer with low Reynolds numbers models, and the capabilities and problems of these models were discussed. In this connection, special aspects of boundary layer calculations were also discussed, namely those associated with 3D boundary layers, converging and diverging flow and slightly detached boundary layers. The complex physical phenomena prevalent in situations under the influence of buoyancy and rotation were reviewed, and several papers were presented on models for simulating these effects. (orig./HP) [de
Experimental investigation of small scale geometries in a turbulent round jet
Energy Technology Data Exchange (ETDEWEB)
Gampert, Markus; Schaefer, Philip; Peters, Norbert, E-mail: mgampert@itv.rwth-aachen.de [Institute for Combustion Technology, RWTH Aachen Templergraben 64, 52056 Aachen (Germany)
2011-12-22
In the present work, we present a method to gather highly accurate three-dimensional measurements of a scalar field in order to experimentally validate the theory of dissipation elements as developped by Wang and Peters (2006, 2008). Combining a two-dimensional high-speed Rayleigh scattering technique with Taylor's hypothesis allows to resolve the concentration field of gaseous propane discharging into ambient air from a turbulent round jet at a Reynolds number (based on nozzle diameter and exit velocity) of 2,800 down to the Kolmogorov scale in every spatial direction. Based on the acquired data, the normalized probability density function of the length of dissipation elements P-tilde (l-tilde) is investigated at various downstream positions x/d = 15 - 40 and an excellent agreement with the theoretically derived model equation is obtained.
Asymmetry of edge plasma turbulence in biasing experiments on tokamak TF-2
International Nuclear Information System (INIS)
Budaev, V.P.
1994-01-01
It was observed in tokamaks the suppression of edge turbulence causes by setting a radial electric field at the plasma edge. The poloidal plasma rotation governed by this electric field is likely to result in changes in edge convention and poloidal asymmetry, however there is no experimental evidence about that of the experimental database concerning the biasing and conditions of edge plasma electrostatic turbulence excitation is not still complete. Also a relation between macroscopic convection and small-scale electrostatic turbulence have not yet revealed both in biasing and non biasing plasmas. In this paper results from biasing experiments carried on on ohmically heated tokamak TF-2 are presented. Changes in both equilibrium and fluctuated edge plasma parameters also convection and turbulence driven particle flux were demonstrated in probe measurements with biasing of electrode immersed within Last Closed Flux Surface (LCFS). Poloidal edge plasma structure and charge in asymmetry have demonstrated in the biasing experiments. (author). 6 refs, 4 figs
''Turbulent Equipartition'' Theory of Toroidal Momentum Pinch
International Nuclear Information System (INIS)
Hahm, T.S.; Diamond, P.H.; Gurcan, O.D.; Rewaldt, G.
2008-01-01
The mode-independent part of magnetic curvature driven turbulent convective (TuroCo) pinch of the angular momentum density (Hahm et al., Phys. Plasmas 14,072302 (2007)) which was originally derived from the gyrokinetic equation, can be interpreted in terms of the turbulent equipartition (TEP) theory. It is shown that the previous results can be obtained from the local conservation of 'magnetically weighted angular momentum density', nm i U # parallel# R/B 2 , and its homogenization due to turbulent flows. It is also demonstrated that the magnetic curvature modification of the parallel acceleration in the nonlinear gyrokinetic equation in the laboratory frame, which was shown to be responsible for the TEP part of the TurCo pinch of angular momentum density in the previous work, is closely related to the Coriolis drift coupling to the perturbed electric field. In addition, the origin of the diffusive flux in the rotating frame is highlighted. Finally, it is illustrated that there should be a difference in scalings between the momentum pinch originated from inherently toroidal effects and that coming from other mechanisms which exist in a simpler geometry.
Powerful Swirl Generation of Flow-driven Rotating Mixing Vane for Enhancing CHF
International Nuclear Information System (INIS)
Seo, Han; Seo, Seok Bin; Heo, Hyo; Bang, In Cheol
2014-01-01
Mixing vanes are utilized to improve CHF and heat transfer performance in the rod bundle during normal operation. Experimental measurement of the swirling flow from a split vane pair was conducted using particle image velocimetry (PIV) and boroscope. The lateral velocity fields show that the swirling flow was initially centered in the subchannel and the computational fluid dynamics (CFD) analysis was performed based on the experiment. To visualize flow patterns in the 5Χ5 subchannel using PIV, matching the refraction between the working fluid and the structure was considered and the experiment aimed to develop the experimental data for providing fundamental information of the CFD analysis. The fixed split vane is the main mixing inducer in the fuel assembly. In a heat exchanger research, propeller type swirl generates at several pitch ratios and different blades angles were used to enhance heat transfer rate. Significant improvements of the heat transfer rate using the propellers were confirmed due to creation of tangential flow. In the present study, the mixing effect of rotation vane which has a shape of propeller was studied using PIV. A split vane was considered in the experiment to show the effect of rotation vane. Vertical and horizontal flow analyses were conducted to show the possible use of rotation vane in a subchannel. In the present work, the study of flow visualization using three types of vanes is conducted to show the mixing effect. The vertical flow and the horizontal flow distributions were analyzed in the two experimental facilities. For the vertical flow facility, flow distributions, flow profiles, and the turbulence kinetic energy are analyzed at the centerline of the channel. The results show that the rotation vane has the highest flow and turbulence kinetic intensity at the centerline of the channel. For the horizontal flow facility, the results indicate that lateral flow of the rotation vane is generated and maintained along with the flow
Effects of thermal conduction and compressibility on Rayleigh-Taylor instability
International Nuclear Information System (INIS)
Takabe, Hideaki; Mima, Kunioki.
1980-01-01
In order to study the stability of the ablation front in laser driven implosion, the thermal conduction and compressibility effects on the Rayleigh-Taylor instability are considered. It is found that the thermal conduction effect cannot stabilize the Rayleigh-Taylor mode, but reduce the growth rate in the short wavelength case. But, the growth rate is found not to differ from the classical value √gk in the long wavelength limit, where the compressibility is essential. (author)
Turbulence simulations of blob formation and radial propagation in toroidally magnetized plasmas
DEFF Research Database (Denmark)
Garcia, O.E.; Naulin, V.; Nielsen, A.H.
2006-01-01
the presence of long- range correlations in the particle density fluctuations. Finally, conditional statistics of the particle flux demonstrates the intermittency of the turbulent plasma transport and the quasi-periodic apparency of blob structures due to bursting in the global turbulence level....... of particles and heat, which is coupled to a scrape-off layer with linear damping terms for all dependent variables corresponding to transport along open magnetic field lines. The formation of blob structures is related to profile variations caused by bursting in the global turbulence level, which is due...... to a dynamical regulation by self- sustained differential rotation of the plasma layer. Radial propagation of the blob structures follows from a vertical charge polarization due to magnetic guiding centre drifts in the toroidally magnetized plasma. Statistical analysis of the particle density, radial electric...
An application of the unifying theory of thermal convection in vertical natural convection
Ng, Chong Shen; Ooi, Andrew; Lohse, Detlef; Chung, Daniel
2014-11-01
Using direct numerical simulations of vertical natural convection (VNC) at Rayleigh numbers 1 . 0 ×105 - 1 . 0 ×109 and Prandtl number 0 . 709 , we provide support for a generalised applicability of the Grossmann-Lohse (GL) theory, originally developed for horizontal natural (Rayleigh-Bénard) convection. In accordance with the theory, the boundary-layer thicknesses of the velocity and temperature fields in VNC obey laminar-like scaling, whereas away from the walls, the dissipation of the turbulent fluctuations obey the scaling for fully developed turbulence. In contrast to Rayleigh-Bénard convection, the direction of gravity in VNC is parallel to the mean flow. Thus, there no longer exists an exact relation linking the normalised global dissipations to the Nusselt, Rayleigh and Prandtl numbers. Nevertheless, we show that the unclosed term, namely the global-averaged buoyancy flux, also exhibits laminar and turbulent scaling, consistent with the GL theory. The findings suggest that, similar to Rayleigh-Bénard convection, a pure power-law relationship between the Nusselt, Rayleigh and Prandtl numbers is not the best description for VNC and existing empirical power-law relationships should be recalibrated to better reflect the underlying physics.
Intermittency and emergence of coherent structures in wave turbulence of a vibrating plate
Mordant, Nicolas; Miquel, Benjamin
2017-10-01
We report numerical investigations of wave turbulence in a vibrating plate. The possibility to implement advanced measurement techniques and long-time numerical simulations makes this system extremely valuable for wave turbulence studies. The purely 2D character of dynamics of the elastic plate makes it much simpler to handle compared to much more complex 3D physical systems that are typical of geo- and astrophysical issues (ocean surface or internal waves, magnetized plasmas or strongly rotating and/or stratified flows). When the forcing is small the observed wave turbulence is consistent with the predictions of the weak turbulent theory. Here we focus on the case of stronger forcing for which coherent structures can be observed. These structures look similar to the folds and D-cones that are commonly observed for strongly deformed static thin elastic sheets (crumpled paper) except that they evolve dynamically in our forced system. We describe their evolution and show that their emergence is associated with statistical intermittency (lack of self similarity) of strongly nonlinear wave turbulence. This behavior is reminiscent of intermittency in Navier-Stokes turbulence. Experimental data show hints of the weak to strong turbulence transition. However, due to technical limitations and dissipation, the strong nonlinear regime remains out of reach of experiments and therefore has been explored numerically.
Development of Pre-set Counter-rotating Streamwise Vortices in Wavy Channel
Budiman, A.C.
2015-10-23
Development of counter-rotating streamwise vortices in a rectangular channel with one-sided wavy surface has been experimentally quantified using hot-wire anemometry. The wavy surface has fixed amplitude of 3.75 mm. The counter-rotating vortices are pre-set by means of a sawtooth pattern cut at the leading edge of the wavy surface. Variations of the central streamwise velocity Uc with a channel gap H = 35 mm and 50 mm (corresponding to a Reynolds number from 1600 to 4400) change the instability of the flow which can be distinguished from the velocity contours at a certain spanwise plane. The streamwise velocity contours and turbulence intensity for Reynolds number Re = 3100 and H = 35 mm show the disappearance of the mushroom-like vortices prior to turbulence near the second peak of the wavy surface, while for higher Re, this phenomenon occurs earlier. Under certain conditions, for example, for Re = 4400 and H = 50 mm, the splitting of the vortices can also be observed.
Development of Pre-set Counter-rotating Streamwise Vortices in Wavy Channel
Budiman, A.C.; Mitsudharmadi, Hatsari; Bouremel, Y.; Winoto, S.H.; Low, H.T.
2015-01-01
Development of counter-rotating streamwise vortices in a rectangular channel with one-sided wavy surface has been experimentally quantified using hot-wire anemometry. The wavy surface has fixed amplitude of 3.75 mm. The counter-rotating vortices are pre-set by means of a sawtooth pattern cut at the leading edge of the wavy surface. Variations of the central streamwise velocity Uc with a channel gap H = 35 mm and 50 mm (corresponding to a Reynolds number from 1600 to 4400) change the instability of the flow which can be distinguished from the velocity contours at a certain spanwise plane. The streamwise velocity contours and turbulence intensity for Reynolds number Re = 3100 and H = 35 mm show the disappearance of the mushroom-like vortices prior to turbulence near the second peak of the wavy surface, while for higher Re, this phenomenon occurs earlier. Under certain conditions, for example, for Re = 4400 and H = 50 mm, the splitting of the vortices can also be observed.
Czech Academy of Sciences Publication Activity Database
Brokešová, J.; Málek, Jiří
2016-01-01
Roč. 256, JUL (2016), s. 1-12 ISSN 0031-9201 R&D Projects: GA ČR GA15-02363S Institutional support: RVO:67985891 Keywords : seismic rotation * true backazimuth * apparent seismic phase velocity * Rayleigh-wave dispersion Subject RIV: DC - Siesmology, Volcanology, Earth Structure Impact factor: 2.075, year: 2016
International Nuclear Information System (INIS)
Gerhauser, H.
1980-02-01
Two superimposed miscible liquids are separated by a diffuse boundary layer providing a steady transition of density. If the heavy fluid is on top of the light one, Rayleigh-Taylor-instabilities develop and cause a rapid interchange and eventually an intermixing. This process can be subjected to dynamic stabilization by enforcing vertical oscillations upon the whole system. However, since only part of the unstable mode spectrum is completely stabilized, the remaining weakly unstable modes lead to turbulent transport processes through the boundary layer ('anomalous diffusion'), so that only a quasistationary equilibrium is achieved. In the present paper, previous experimental results on the dynamic stabilization of water superimposed by an aqueous ZnJ-solution are theoretically interpreted, and the observed spatial structure as well as the time development of the density profiles are explained. There exists an analogy between these phenomena and turbulent transport processes in tokamak discharges such as the sawtooth oscillations of internal disruptions. (orig.) [de
Enstrophy generation in a shock-dominated turbulence
International Nuclear Information System (INIS)
Miura, Hideaki.
1995-09-01
A mechanism of enstrophy generation is investigated numerically in a shock-dominated turbulence driven by a random external force which has only the compressible component. Enstrophy is generated, especially on collision of shock, as a pair of vortex tube of opposite sense of rotation behind curved shocks. The roles of various terms in enstrophy equation are clarified in enstrophy generation process. Generation of enstrophy is enhanced by strong alignment of each term of the enstrophy equation with the vorticity vector. (author)
Simulations of Turbulence in Tokamak Edge and Effects of Self-Consistent Zonal Flows
Cohen, Bruce; Umansky, Maxim
2013-10-01
Progress is reported on simulations of electromagnetic drift-resistive ballooning turbulence in the tokamak edge. This extends previous work to include self-consistent zonal flows and their effects. The previous work addressed simulation of L-mode tokamak edge turbulence using the turbulence code BOUT that solves Braginskii-based plasma fluid equations in tokamak edge domain. The calculations use realistic single-null geometry and plasma parameters of the DIII-D tokamak and produce fluctuation amplitudes, fluctuation spectra, and particle and thermal fluxes that compare favorably to experimental data. In the effect of sheared ExB poloidal rotation is included with an imposed static radial electric field fitted to experimental data. In the new work here we include the radial electric field self-consistently driven by the microturbulence, which contributes to the sheared ExB poloidal rotation (zonal flow generation). We present simulations with/without zonal flows for both cylindrical geometry, as in the UCLA Large Plasma Device, and for the DIII-D tokamak L-mode cases in to quantify the influence of self-consistent zonal flows on the microturbulence and the concomitant transport. This work was performed under the auspices of the U.S. Department of Energy under contract DE-AC52-07NA27344 at the Lawrence Livermore National Laboratory.
International Nuclear Information System (INIS)
Cottier, Pierre
2013-01-01
The magnetic confinement in tokamaks is for now the most advanced way towards energy production by nuclear fusion. Both theoretical and experimental studies showed that rotation generation can increase its performance by reducing the turbulent transport in tokamak plasmas. The rotation influence on the heat and particle fluxes is studied along with the angular momentum transport with the quasi-linear gyro-kinetic eigenvalue code QuaLiKiz. For this purpose, the QuaLiKiz code is modified in order to take the plasma rotation into account and compute the angular momentum flux. It is shown that QuaLiKiz framework is able to correctly predict the angular momentum flux including the E*B shear induced residual stress as well as the influence of rotation on the heat and particle fluxes. The major approximations of QuaLiKiz formalisms are reviewed, in particular the ballooning representation at its lowest order and the eigenfunctions calculated in the hydrodynamic limit. The construction of the quasi-linear fluxes is also reviewed in details and the quasi-linear angular momentum flux is derived. The different contributions to the turbulent momentum flux are studied and successfully compared both against non-linear gyro-kinetic simulations and experimental data. (author) [fr
Finite plate thickness effects on the Rayleigh-Taylor instability in elastic-plastic materials
Polavarapu, Rinosh; Banerjee, Arindam
2017-11-01
The majority of theoretical studies have tackled the Rayleigh-Taylor instability (RTI) problem in solids using an infinitely thick plate. Recent theoretical studies by Piriz et al. (PRE 95, 053108, 2017) have explored finite thickness effects. We seek to validate this recent theoretical estimate experimentally using our rotating wheel RTI experiment in an accelerated elastic-plastic material. The test section consists of a container filled with air and mayonnaise (a non-Newtonian emulsion) with an initial perturbation between two materials. The plate thickness effects are studied by varying the depth of the soft-solid. A set of experiments is run by employing different initial conditions with different container dimensions. Additionally, the effect of acceleration rate (driving pressure rise time) on the instability threshold with reference to the finite thickness will also be inspected. Furthermore, the experimental results are compared to the analytical strength models related to finite thickness effects on RTI. Authors acknowledge financial support from DOE-SSAA Grant # DE-NA0003195 and LANL subcontract #370333.