Analytical solution of conjugate turbulent forced convection boundary layer flow over plates
Directory of Open Access Journals (Sweden)
Joneydi Shariatzadeh Omid
2016-01-01
Full Text Available A conjugate (coupled forced convection heat transfer from a heated conducting plate under turbulent boundary layer flow is considered. A heated plate of finite thickness is cooled under turbulent forced convection boundary layer flow. Because the conduction and convection boundary layer flow is coupled (conjugated in the problem, a semi-analytical solution based on Differential Transform Method (DTM is presented for solving the non-linear integro-differential equation occurring in the problem. The main conclusion is that in the conjugate heat transfer case the temperature distribution of the plate is flatter than the one in the non-conjugate case. This feature is more pronounced under turbulent flow when compared with the laminar flow.
Rotating turbulent Rayleigh–Bénard convection subject to harmonically forced flow reversals
Geurts, Bernard 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
Directory of Open Access Journals (Sweden)
Yu Ji
2017-03-01
Full Text Available The entropy generation analysis of fully turbulent convective heat transfer to nanofluids in a circular tube is investigated numerically using the Reynolds Averaged Navier–Stokes (RANS model. The nanofluids with particle concentration of 0%, 1%, 2%, 4% and 6% are treated as single phases of effective properties. The uniform heat flux is enforced at the tube wall. To confirm the validity of the numerical approach, the results have been compared with empirical correlations and analytical formula. The self-similarity profiles of local entropy generation are also studied, in which the peak values of entropy generation by direct dissipation, turbulent dissipation, mean temperature gradients and fluctuating temperature gradients for different Reynolds number as well as different particle concentration are observed. In addition, the effects of Reynolds number, volume fraction of nanoparticles and heat flux on total entropy generation and Bejan number are discussed. In the results, the intersection points of total entropy generation for water and four nanofluids are observed, when the entropy generation decrease before the intersection and increase after the intersection as the particle concentration increases. Finally, by definition of Ep, which combines the first law and second law of thermodynamics and attributed to evaluate the real performance of heat transfer processes, the optimal Reynolds number Reop corresponding to the best performance and the advisable Reynolds number Read providing the appropriate Reynolds number range for nanofluids in convective heat transfer can be determined.
Bouchenafa, Rachid; Saim, Rachid; Abboudi, Said
2015-09-01
Forced convection is a phenomenon associated with the heat transfer fluid flows. The presence of convection affects simultaneously the thermal and hydrodynamic fields, the problem is thus coupled. This form of heat transfer inside ducts occurs in many practical applications such as solar collectors, heat exchangers, cooling of electronic components as well as chemical and nuclear. In this work, we are interested primarily for a numerical study of thermo-hydraulic performances of an incompressible turbulent flow of air through a heat sink composed of several rows of bars of square section. Profiles and the axial velocity fields, as well as profiles and the distribution of the Nusselt number are plotted for all the geometry considered and chosen for different sections. The effects of geometrical parameters of the model and the operating parameters on the dynamic and thermal behavior of the air are analyzed.
Park, Hae-Kyun; Chung, Bum-Jin
2016-12-01
The turbulent forced convection heat transfer of rectangular fins in a duct was investigated by varying the tip clearance and Pr. Mass transfer experiments using a H2SO4-CuSO4 electroplating system were performed based on the analogy between heat and mass transfers. FLUENT 6.3 was used for calculations. Turbulent models were tested and the Reynolds Stress Model was chosen, which showed a 1.15 % discrepancy with the existing correlation for a simple tube flow when Pr = 2, but 13 % when Pr = 2014. For a more complex fin channel, the discrepancy increased up to 30 %. The optimal tip clearances, corresponding to maximum heat transfer rates, did not vary with Pr, which is explained using the temperature contours. The results were also compared with the laminar case where Pr influenced the optimal tip clearance.
Sterl, Sebastian; Zhong, Jin-Qiang
2016-01-01
In this paper, we present results from an experimental study into turbulent Rayleigh-Benard convection forced externally by periodically modulated unidirectional rotation rates. We find that the azimuthal rotation velocity $\\dot{\\theta}$(t) and thermal amplitude $\\delta$(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 $\\dot{\\theta}$(t). We also focus on the influence of modulated rotation rates on the frequency of occurrence $\\eta$ of stochastic cessation/reorientation events, and on the interplay between such events and the periodically modulated response of $\\dot{\\theta}$(t). Here we identify a mechanism by which $\\eta$ can be amplfied 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 extend this approach to make predictions for the occurrence ...
Probing the energy cascade of convective turbulence.
Kunnen, R P J; Clercx, H J H
2014-12-01
The existence of a buoyancy-dominated scaling range in convective turbulence is a longstanding open question. We investigate this issue by considering the scale-by-scale energy budget in direct numerical simulations of Rayleigh-Bénard convection. We try to minimize the so-called Bolgiano length scale, the length scale at which buoyancy becomes dominant for scaling. Therefore, we deliberately choose modest Rayleigh numbers Ra=2.5×10(6) and 2.5×10(7). The budget reveals that buoyant forcing, turbulent energy transfer, and dissipation are contributing significantly over a wide range of scales. Thereby neither Kolmogorov-like (balance of turbulent transfer and dissipation) nor Bolgiano-Obukhov-like scaling (balance of turbulent transfer and buoyancy) is expected in the structure functions, which indeed reveal inconclusive scaling behavior. Furthermore, we consider the calculation of the Bolgiano length scale. To account for correlations between the dissipation rates of kinetic energy and thermal variance we propose to average the Bolgiano length scale directly. This gives an estimate, which is one order of magnitude larger than the previous estimate, and actually larger than the domain itself. Rather than studying the scaling of structure functions, we propose that the use of scale-by-scale energy budgets resolving anisotropic contributions is appropriate to consider the energy cascade mechanisms in turbulent convection.
Ranjan, Pritanshu; Dewan, Anupam
2015-11-01
The effect of wall proximity on flow and heat transfer around a square cylinder placed inside a channel is numerically investigated. This flow configuration is a fundamental problem and is widely encountered in several engineering applications. The presence of wall close to the cylinder can alter the shedding process and this in turn can affect the thermal transport in the wake region. Many researchers have studied this phenomenon experimentally but the heat transfer characteristics around a square cylinder placed inside a channel still remain an open question. We present here an insight into this problem. The simulations were carried out for a Reynolds number of 37,000 (based on cylinder diameter, D) and as a function of gap height, G/D, at different blockage ratios. A variable resolution modelling approach (PANS SST k- ω model) was used to study turbulence structures. The results are presented in terms of pressure coefficient, drag coefficient, thermal fluctuations and local and average Nusselt number (Nu). The results obtained showed that, for G / D < 0 . 5 very weak shedding process at random time intervals occurs suggesting the suppression of vortex shedding due to wall. Thus, the local and average Nu decrease as the cylinder is moved towards wall at all blockage ratios.
Transitions in turbulent rotating convection
Rajaei, Hadi; Alards, Kim; Kunnen, Rudie; Toschi, Federico; Clercx, Herman; Fluid Dynamics Lab Team
2015-11-01
This study aims to explore the flow transition from one state to the other in rotating Rayleigh-Bènard convection using Lagrangian acceleration statistics. 3D particle tracking velocimetry (3D-PTV) is employed in a water-filled cylindrical tank of equal height and diameter. The measurements are performed at the center and close to the top plate at a Rayleigh number Ra = 1.28e9 and Prandtl number Pr = 6.7 for different rotation rates. In parallel, direct numerical simulation (DNS) has been performed to provide detailed information on the boundary layers. We report the acceleration pdfs for different rotation rates and show how the transition from weakly to strongly rotating Rayleigh-Bènard affects the acceleration pdfs in the bulk and boundary layers. We observe that the shapes of the acceleration PDFs as well as the isotropy in the cell center are largely unaffected while crossing the transition point. However, acceleration pdfs at the top show a clear change at the transition point. Using acceleration pdfs and DNS data, we show that the transition between turbulent states is actually a boundary layer transition between Prandtl-Blasius type (typical of non-rotating convection) and Ekman type.
Scaling and universality in turbulent convection.
Celani, Antonio; Matsumoto, Takeshi; Mazzino, Andrea; Vergassola, Massimo
2002-02-01
Anomalous correlation functions of the temperature field in two-dimensional turbulent convection are shown to be universal with respect to the choice of external sources. Moreover, they are equal to the anomalous correlations of the concentration field of a passive tracer advected by the convective flow itself. The statistics of velocity differences is found to be universal, self-similar, and close to Gaussian. These results point to the conclusion that temperature intermittency in two-dimensional turbulent convection may be traced back to the existence of statistically preserved structures, as it is in passive scalar turbulence.
A numerical study of momentum and forced convection heat transfer ...
African Journals Online (AJOL)
temperature fields, axial velocity profiles, local and average Nusselt numbers, and skin frictions were ... Key words: Finite volume method - Turbulent flow - Forced convection - Waved baffles. .... numerical simulations are conducted in a two-.
Simard, Corinne; Charbonneau, Paul; Dubé, Caroline
2016-10-01
We perform a mean-field analysis of the EULAG-MHD millenium simulation of global magnetohydrodynamical convection presented in Passos and Charbonneau (2014). The turbulent electromotive force (emf) operating in the simulation is assumed to be linearly related to the cyclic axisymmetric mean magnetic field and its first spatial derivatives. At every grid point in the simulation's meridional plane, this assumed relationship involves 27 independent tensorial coefficients. Expanding on Racine et al. (2011), we extract these coefficients from the simulation data through a least-squares minimization procedure based on singular value decomposition. The reconstructed α -tensor shows good agreement with that obtained by Racine et al. (2011), who did not include derivatives of the mean-field in their fit, as well as with the α -tensor extracted by Augustson et al. (2015) from a distinct ASH MHD simulation. The isotropic part of the turbulent magnetic diffusivity tensor β is positive definite and reaches values of 5.0 ×107 m2 s-1 in the middle of the convecting fluid layers. The spatial variations of both αϕϕ and βϕϕ component are well reproduced by expressions obtained under the Second Order Correlation Approximation, with a good matching of amplitude requiring a turbulent correlation time about five times smaller than the estimated turnover time of the small-scale turbulent flow. By segmenting the simulation data into epochs of magnetic cycle minima and maxima, we also measure α - and β -quenching. We find the magnetic quenching of the α -effect to be driven primarily by a reduction of the small-scale flow's kinetic helicity, with variations of the current helicity playing a lesser role in most locations in the simulation domain. Our measurements of turbulent diffusivity quenching are restricted to the βϕϕ component, but indicate a weaker quenching, by a factor of ≃ 1.36, than of the α -effect, which in our simulation drops by a factor of three between
Simard, C; Dube, C
2016-01-01
We perform a mean-field analysis of the EULAG-MHD millenium simulation of global magnetohydrodynamical convection presented in Passos et al. 2014. The turbulent electromotive force operating in the simulation is assumed to be linearly related to the cyclic axisymmetric mean magnetic field and its first spatial derivatives. At every grid point in the simulation's meridional plane, this assumed relationship involves 27 independent tensorial coefficients. Expanding on Racine et al. 2011, we extract these coefficients from the simulation data through a least-squares minimization procedure based on singular value decomposition. The reconstructed alpha-tensor shows good agreement with that obtained by Racine et al. 2011, who did not include derivatives of the mean-field in their fit, as well as with the alpha-tensor extracted by Augustson et al. 2015 from a distinct ASH MHD simulation. The isotropic part of the turbulent magnetic diffusivity tensor beta is positive definite and reaches values of 5.0x10^7 m2s-1 in t...
Turbulent Convection in Stellar Interiors. II. The Velocity Field
Arnett, David; Young, P A
2008-01-01
We analyze stellar convection with the aid of 3D hydrodynamic simulations, introducing the turbulent cascade into our theoretical analysis. We devise closures of the Reynolds-decomposed mean field equations by simple physical modeling of the simulations (we relate temperature and density fluctuations via coefficients); the procedure (CABS, Convection Algorithms Based on Simulations) is terrestrially testable and is amenable to systematic improvement. We develop a turbulent kinetic energy equation which contains both nonlocal and time dependent terms, and is appropriate if the convective transit time is shorter than the evolutionary time scale. The interpretation of mixing-length theory (MLT) as generally used in astrophysics is incorrect; MLT forces the mixing length to be an imposed constant. Direct tests show that the damping associated with the flow is that suggested by Kolmogorov. The eddy size is approximately the depth of the convection zone, and this dissipation length corresponds to the "mixing length...
Turbulent Convection in the Classical Variable Stars
Kollath, Z
1999-01-01
We give a status report of convective Cepheid and RR Lyrae model pulsations. Some striking successes can be reported, despite the use of a rather simple treatment of turbulent convection with a 1D time-dependent diffusion equation for the turbulent energy. It is now possible to obtain stable double-mode (beat) pulsations in both Cepheid and RR Lyrae models with astrophysical parameters, i.e. periods and amplitude ratios, that are in agreement with observations. The turbulent convective models, however, have difficulties giving global agreement with the observations. In particular, the Magellanic Cloud Cepheids, that have been observed in connection with the microlensing projects have imposed novel observational constraints because of the low metallicity of the MCs.
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 criter
Mixed convection in turbulent channels with unstable stratification
Pirozzoli, Sergio; Verzicco, Roberto; Orlandi, Paolo
2016-01-01
We study turbulent flows in planar channels with unstable thermal stratification, using direct numerical simulations in a wide range of Reynolds and Rayleigh numbers and reaching flow conditions which are representative of asymptotic developed turbulence. The combined effect of forced and free convection produces a peculiar pattern of quasi--streamwise rollers occupying the full channel thickness with aspect--ratio considerably higher than unity; it has been observed that they have an important redistributing effect on temperature and momentum. The mean values and the variances of the flow variables do not appear to follow Prandtl's scaling in the flow regime near free convection, except for the temperature and vertical velocity fluctuations, which are more affected by turbulent plumes. Nevertheless, we find that the Monin--Obukhov theory still yields a useful representation of the main flow features. In particular, the widely used Businger--Dyer relationships provide a convenient way of accounting for the bu...
Turbulent convection model in the overshooting region: II. Theoretical analysis
Zhang, S Q
2012-01-01
Turbulent convection models are thought to be good tools to deal with the convective overshooting in the stellar interior. However, they are too complex to be applied in calculations of stellar structure and evolution. In order to understand the physical processes of the convective overshooting and to simplify the application of turbulent convection models, a semi-analytic solution is necessary. We obtain the approximate solution and asymptotic solution of the turbulent convection model in the overshooting region, and find some important properties of the convective overshooting: I. The overshooting region can be partitioned into three parts: a thin region just outside the convective boundary with high efficiency of turbulent heat transfer, a power law dissipation region of turbulent kinetic energy in the middle, and a thermal dissipation area with rapidly decreasing turbulent kinetic energy. The decaying indices of the turbulent correlations $k$, $\\bar{u_{r}'T'}$, and $\\bar{T'T'}$ are only determined by the ...
Near isotropic behaviour of turbulent thermal convection
Nath, Dinesh; Kumar, Abhishek; Verma, Mahendra K
2016-01-01
We investigate the anisotropy in turbulent convection in a 3D box using direct numerical simulation. We compute the anisotropic parameter $A = u_\\perp^{2}/(2u_{\\parallel}^{2})$, where $u_{\\perp}$ and $u_{\\parallel}$ are the components of velocity perpendicular and parallel to the buoyancy direction, the shell and ring spectra, and shell-to-shell energy transfers. We observe that the flow is nearly isotropic for the Prandtl number $\\mathrm{Pr} \\approx 1$, but the anisotropy increases with the Prandtl number. For $\\mathrm{Pr}=\\infty$, $A \\approx 0.3$, thus anisotropy is not very significant even in extreme cases. We also observe that $u_{\\parallel}$ feeds energy to $u_{\\perp}$ via pressure. The computation of shell-to-shell energy transfers show that the energy transfer in turbulent convection is local and forward, similar to fluid turbulence. These results are consistent with the Kolmogorov's spectrum observed by Kumar et al.~[Phys. Rev. E {\\bf 90}, 023016 (2014)] for turbulent convection.
Internal Gravity Wave Excitation by Turbulent Convection
Lecoanet, Daniel
2012-01-01
We calculate the flux of internal gravity waves (IGWs) generated by turbulent convection in stars. We solve for the IGW eigenfunctions analytically near the radiative-convective interface in a local, Boussinesq, and cartesian domain. We consider both discontinuous and smooth transitions between the radiative and convective regions and derive Green's functions to solve for the IGWs in the radiative region. We find that if the radiative-convective transition is smooth, the IGW flux ~ F_conv (d/H), where F_conv is the flux carried by the convective motions, d is the width of the transition region, and H is the pressure scale height. This can be much larger than the standard result in the literature for a discontinuous radiative-convective transition, which gives a wave flux ~ F_conv M, where M is the convective Mach number. However, in the smooth transition case, the most efficiently excited perturbations will break immediately when they enter the radiative region. The flux of IGWs which do not break and are abl...
Directory of Open Access Journals (Sweden)
Withada Jedsadaratanachai
2017-01-01
Full Text Available Performance assessments on heat transfer, pressure loss, and thermal enhancement factor in the circular tube heat exchanger inserted with the V-orifices are investigated numerically. The influences of the blockage ratio, gap spacing ratio, and orifice arrangement are reported for turbulent regime, Re=3000–10,000. The finite volume method and SIMPLE algorithm are selected to solve the present problem. The mechanisms on flow and heat transfer characteristics are described. The periodic concepts on flow and heat transfer are also studied. The numerical results show that the gap spacing ratio is main reason for the changes of the flow and heat transfer topologies. The gap distance helps to adjust the optimum point of the thermal performance, especially at high flow blockage ratio. In addition, the optimum thermal performance of the present system is around 2.25 at the lowest Reynolds number, Re=3000.
Numerical simulation of the turbulent convective buoyant flow of sodium over a backward- facing step
Schumm, T.; Frohnapfel, B.; Marocco, L.
2016-09-01
A forced convective and a buoyancy-aided turbulent liquid sodium flow over a backward-facing step with a constant heat flux applied on the indented wall is simulated. Linear eddy viscosity models are used for the Reynolds stresses. Turbulent heat fluxes are modelled with a single gradient diffusion hypotheses with two different approaches to evaluate the turbulent Prandtl number. Moreover, the influence of turbulence on heat transfer to sodium is also assessed through simulations with zero turbulent thermal diffusivity. The results are compared with DNS data from literature. The velocity and turbulent kinetic energy profiles predicted by all models are in good agreement with the DNS data. The local Nusselt number trend is qualitatively well captured, however, its magnitude is underestimated by all models for the mixed convection case. For forced convection, the heat transfer is overestimated by all heat flux models. The simulation with neglected turbulent heat transfer shows the best overall agreement for the forced convection case. For the mixed convection best agreement is obtained using a correlation to locally evaluate the turbulent thermal diffusivity.
Magnetic flux concentrations from turbulent stratified convection
Käpylä, P J; Kleeorin, N; Käpylä, M J; Rogachevskii, I
2015-01-01
(abridged) Context: The mechanisms that cause the formation of sunspots are still unclear. Aims: We study the self-organisation of initially uniform sub-equipartition magnetic fields by highly stratified turbulent convection. Methods: We perform simulations of magnetoconvection in Cartesian domains that are $8.5$-$24$ Mm deep and $34$-$96$ Mm wide. We impose either a vertical or a horizontal uniform magnetic field in a convection-driven turbulent flow. Results: We find that super-equipartition magnetic flux concentrations are formed near the surface with domain depths of $12.5$ and $24$ Mm. The size of the concentrations increases as the box size increases and the largest structures ($20$ Mm horizontally) are obtained in the 24 Mm deep models. The field strength in the concentrations is in the range of $3$-$5$ kG. The concentrations grow approximately linearly in time. The effective magnetic pressure measured in the simulations is positive near the surface and negative in the bulk of the convection zone. Its ...
INTERMITTENCY AND SCALING IN TURBULENT CONVECTION
Institute of Scientific and Technical Information of China (English)
Emily S. C. CHING
2003-01-01
Both the velocity and temperature measurements taken in turbulent Rayleigh-B'enard convection experiments have been analyzed. It is found that both the velocity and temperature fluctuations are intermittent and can be well-described by the She-Leveque hierarchical structure. A positive correlation between the vertical velocity and the temperature differences is found both at the center,near the sidewall and near the bottom of the convection cell, supporting that buoyancy is significant in the Bolgiano regime. Moreover, the intermittent nature of the temperature fluctuations in the Bolgiano regime can be attributed to the variations in the temperature dissipation rate. However, the relations between the velocity and temperature structure functions and their correlations implied by the Bolgiano-Obukhov scaling are not supported by experimental measurements.
Electromagnetically driven dwarf tornados in turbulent convection
Kenjeres, S.
2011-01-01
Motivated by the concept of interdependency of turbulent flow and electromagnetic fields inside the spiraling galaxies, we explored the possibilities of generating a localized Lorentz force that will produce a three-dimensional swirling flow in weakly conductive fluids. Multiple vortical flow patter
Electromagnetically driven dwarf tornados in turbulent convection
Kenjeres, S.
2011-01-01
Motivated by the concept of interdependency of turbulent flow and electromagnetic fields inside the spiraling galaxies, we explored the possibilities of generating a localized Lorentz force that will produce a three-dimensional swirling flow in weakly conductive fluids. Multiple vortical flow patter
Large Eddy Simulations of Severe Convection Induced Turbulence
Ahmad, Nash'at; Proctor, Fred
2011-01-01
Convective storms can pose a serious risk to aviation operations since they are often accompanied by turbulence, heavy rain, hail, icing, lightning, strong winds, and poor visibility. They can cause major delays in air traffic due to the re-routing of flights, and by disrupting operations at the airports in the vicinity of the storm system. In this study, the Terminal Area Simulation System is used to simulate five different convective events ranging from a mesoscale convective complex to isolated storms. The occurrence of convection induced turbulence is analyzed from these simulations. The validation of model results with the radar data and other observations is reported and an aircraft-centric turbulence hazard metric calculated for each case is discussed. The turbulence analysis showed that large pockets of significant turbulence hazard can be found in regions of low radar reflectivity. Moderate and severe turbulence was often found in building cumulus turrets and overshooting tops.
Testing turbulent closure models with convection simulations
Snellman, J E; Mantere, M J; Rheinhardt, M; Dintrans, B
2012-01-01
Aims: To compare simple analytical closure models of turbulent Boussinesq convection for stellar applications with direct three-dimensional simulations both in homogeneous and inhomogeneous (bounded) setups. Methods: We use simple analytical closure models to compute the fluxes of angular momentum and heat as a function of rotation rate measured by the Taylor number. We also investigate cases with varying angles between the angular velocity and gravity vectors, corresponding to locating the computational domain at different latitudes ranging from the pole to the equator of the star. We perform three-dimensional numerical simulations in the same parameter regimes for comparison. The free parameters appearing in the closure models are calibrated by two fit methods using simulation data. Unique determination of the closure parameters is possible only in the non-rotating case and when the system is placed at the pole. In the other cases the fit procedures yield somewhat differing results. The quality of the closu...
Convective Induced Turbulence (CIT) Detection via Total Lightning Sensing Project
National Aeronautics and Space Administration — We proposes to build a prototype Convective-Induced Turbulence (CIT) hazard detection system based on total lightning sensing as an indicator of the location and...
Turbulent natural and mixed convection along a vertical plate
Energy Technology Data Exchange (ETDEWEB)
Abu-Mulaweh, H.I.; Armaly, B.F.; Chen, T.S.; Zhao, J.Z.
1997-07-01
Measurements of turbulent boundary-layer air flow in natural and mixed convection adjacent to an isothermal vertical flat plate are reported. Laser-Doppler velocimeter and cold wire anemometer were used, respectively, to measure simultaneously the mean turbulent velocity and temperature distributions were measured for a temperature difference, {Delta}T, of 30 C between the heated wall and the free stream air at a fixed location x = 3 m (with a corresponding Grashof number Gr{sub x} = 8.55 x 10{sup 10}), and for a range of free stream velocities 0 m/s {le} U{sub {infinity} } {le} 0.41 m/s. The effect of small free stream velocity on the turbulent natural convection is examined. These results reveal that the introduction of small free stream velocity on turbulent natural convection flow suppresses turbulence and decreases the heat transfer rate from the heated wall.
Boundary layer structure in turbulent Rayleigh-Benard convection
Shi, Nan; Schumacher, Joerg
2012-01-01
The structure of the boundary layers in turbulent Rayleigh-Benard convection is studied by means of three-dimensional direct numerical simulations. We consider convection in a cylindrical cell at an aspect ratio one for Rayleigh numbers of Ra=3e+9 and 3e+10 at fixed Prandtl number Pr=0.7. Similar to the experimental results in the same setup and for the same Prandtl number, the structure of the laminar boundary layers of the velocity and temperature fields is found to deviate from the prediction of the Prandtl-Blasius-Pohlhausen theory. Deviations decrease when a dynamical rescaling of the data with an instantaneously defined boundary layer thickness is performed and the analysis plane is aligned with the instantaneous direction of the large-scale circulation in the closed cell. Our numerical results demonstrate that important assumptions which enter existing classical laminar boundary layer theories for forced and natural convection are violated, such as the strict two-dimensionality of the dynamics or the s...
Guo, Ning; Finnerman, Oskar; Ström, Henrik
2016-06-01
The effect of turbulent velocity fluctuations on the convective heat transfer to single droplets in a turbulent channel flow are investigated numerically. It is found that for properties relevant to typical liquid spray applications, the convective heat transfer is enhanced with increasing droplet size and bulk Reynolds number. The combined effect of convective heat transfer enhancement and increased driving forces for heat and mass transfer due to droplet dispersion is thereafter investigated for a commercial spray application. The probability distribution functions of droplet properties in the spray are found to be significantly affected by the presence of turbulent velocity fluctuations in the carrier phase.
Limitations of estimating turbulent convection velocities from PIV
de Kat, Roeland; Dawson, James R; Ganapathisubramani, Bharathram
2013-01-01
This paper deals with determination of turbulent convection velocities from particle image velocimetry (PIV). Turbulent convection velocities are of interest because they can be used to map temporal information into space. Convection velocity can be defined in several different ways. One approach is to use the phase-spectrum of two signals with a time-separation. Obtaining convection velocity per wavenumber involves determining a spatial spectrum. PIV data is limited in spatial resolution and sample length. The influence of truncation of both spatial resolution and frequency resolution is investigated, as well as the influences of spatial filtering and measurement noise. These issues are investigated by using a synthetic data set obtained by creating velocity-time data with an imposed spectrum. Results from the validation show that, when applying a Hamming window before determining the phase spectrum, there is a usable range of wavenumbers for which convection velocities can be determined. Simulation of flow ...
Investigation of a turbulent convective buoyant flow of sodium over a backward- facing step
Schumm, Tobias; Frohnapfel, Bettina; Marocco, Luca
2017-08-01
The influence of buoyancy-aided mixed convection on the heat transfer downstream of a sudden expansion of a plane channel is investigated by means of the steady-state Reynolds averaged Navier Stokes (RANS) simulation. A linear eddy viscosity model is used to compute the Reynolds stresses. The turbulent heat fluxes are modelled with a single gradient diffusion hypotheses using a local correlation to evaluate the turbulent Prandtl number. The velocity, turbulent kinetic energy and Reynolds shear stress profiles predicted are in good agreement with available DNS data. The local Stanton number trend is qualitatively well captured, however, its magnitude is underestimated for the mixed convection cases. Compared to the case of forced convection, the flow field and heat transfer change significantly. An influence of buoyancy is reported at very low Richardson numbers (Ri). A steady increase in heat transfer with rising influence of buoyancy is observed.
Nagata, Kouji; Sakai, Yasuhiko; Komori, Satoru
2011-01-01
Effects of weak, small-scale freestream turbulence on turbulent boundary layers with and without thermal convection are experimentally investigated using a wind tunnel. Two experiments are carried out: the first is isothermal boundary layers with and without grid turbulence, and the second is non-isothermal boundary layers with and without grid turbulence. Both boundary layers develop under a small favorable pressure gradient. For the latter case, the bottom wall of the test section is heated...
Turbulent Compressible Convection with Rotation. 2; Mean Flows and Differential Rotation
Brummell, Nicholas H.; Hurlburt, Neal E.; Toomre, Juri
1998-01-01
The effects of rotation on turbulent, compressible convection within stellar envelopes are studied through three-dimensional numerical simulations conducted within a local f-plane model. This work seeks to understand the types of differential rotation that can be established in convective envelopes of stars like the Sun, for which recent helioseismic observations suggest an angular velocity profile with depth and latitude at variance with many theoretical predictions. This paper analyzes the mechanisms that are responsible for the mean (horizontally averaged) zonal and meridional flows that are produced by convection influenced by Coriolis forces. The compressible convection is considered for a range of Rayleigh, Taylor, and Prandtl (and thus Rossby) numbers encompassing both laminar and turbulent flow conditions under weak and strong rotational constraints. When the nonlinearities are moderate, the effects of rotation on the resulting laminar cellular convection leads to distinctive tilts of the cell boundaries away from the vertical. These yield correlations between vertical and horizontal motions that generate Reynolds stresses that can drive mean flows, interpretable as differential rotation and meridional circulations. Under more vigorous forcing, the resulting turbulent convection involves complicated and contorted fluid particle trajectories, with few clear correlations between vertical and horizontal motions, punctuated by an evolving and intricate downflow network that can extend over much of the depth of the layer. Within such networks are some coherent structures of vortical downflow that tend to align with the rotation axis. These yield a novel turbulent alignment mechanism, distinct from the laminar tilting of cellular boundaries, that can provide the principal correlated motions and thus Reynolds stresses and subsequently mean flows. The emergence of such coherent structures that can persist amidst more random motions is a characteristic of turbulence
Turbulent Compressible Convection with Rotation. Part 1; Flow Structure and Evolution
Brummell, Nicholas H.; Hurlburt, Neal E.; Toomre, Juri
1996-01-01
The effects of Coriolis forces on compressible convection are studied using three-dimensional numerical simulations carried out within a local modified f-plane model. The physics is simplified by considering a perfect gas occupying a rectilinear domain placed tangentially to a rotating sphere at various latitudes, through which a destabilizing heat flux is driven. The resulting convection is considered for a range of Rayleigh, Taylor, and Prandtl (and thus Rossby) numbers, evaluating conditions where the influence of rotation is both weak and strong. Given the computational demands of these high-resolution simulations, the parameter space is explored sparsely to ascertain the differences between laminar and turbulent rotating convection. The first paper in this series examines the effects of rotation on the flow structure within the convection, its evolution, and some consequences for mixing. Subsequent papers consider the large-scale mean shear flows that are generated by the convection, and the effects of rotation on the convective energetics and transport properties. It is found here that the structure of rotating turbulent convection is similar to earlier nonrotating studies, with a laminar, cellular surface network disguising a fully turbulent interior punctuated by vertically coherent structures. However, the temporal signature of the surface flows is modified by inertial motions to yield new cellular evolution patterns and an overall increase in the mobility of the network. The turbulent convection contains vortex tubes of many scales, including large-scale coherent structures spanning the full vertical extent of the domain involving multiple density scale heights. Remarkably, such structures align with the rotation vector via the influence of Coriolis forces on turbulent motions, in contrast with the zonal tilting of streamlines found in laminar flows. Such novel turbulent mechanisms alter the correlations which drive mean shearing flows and affect the
Formation of Large-Scale Semi-Organized Structures in Turbulent Convection
Elperin, T; Rogachevskii, I; Zilitinkevich, S
2002-01-01
A new mean-field theory of turbulent convection is developed. This theory predicts the convective wind instability in a shear-free turbulent convection which causes formation of large-scale semi-organized fluid motions in the form of cells or rolls. Spatial characteristics of these motions, such as the minimum size of the growing perturbations and the size of perturbations with the maximum growth rate, are determined. This study predicts also the existence of the convective shear instability in a sheared turbulent convection which results in generation of convective shear waves with a nonzero hydrodynamic helicity. Increase of shear promotes excitation of the convective shear instability. Applications of the obtained results to the atmospheric turbulent convection and the laboratory experiments on turbulent convection are discussed. This theory can be applied also for the describing a mesogranular turbulent convection in astrophysics.
Gokoglu, Suleyman A.; Rosner, Daniel E.
1986-01-01
A formulation previously developed to predict and correlate the thermophoretically-augmented submicron particle mass transfer rate to cold surfaces is found to account for the thermophoretically reduced particle mass transfer rate to overheated surfaces such that thermophoresis brings about a 10-decade reduction below the convective mass transfer rate expected by pure Brownian diffusion and convection alone. Thermophoretic blowing is shown to produce effects on particle concentration boundary-layer (BL) structure and wall mass transfer rates similar to those produced by real blowing through a porous wall. The applicability of the correlations to developing BL-situations is demonstrated by a numerical example relevant to wet-steam technology.
Internal Wave Generation by Turbulent Convection
Lecoanet, D.; Le Bars, M.; Burns, K. J.; Vasil, G. M.; Quataert, E.; Brown, B. P.; Oishi, J.
2015-12-01
Recent measurements suggest that a portion of the Earth's core may be stably stratified. If this is the case, then the Earth's core joins the many planetary and stellar objects which have a stably stratified region adjacent to a convective region. The stably stratified region admits internal gravity waves which can transport angular momentum, energy, and affect magnetic field generation. We describe experiments & simulations of convective excitation of internal waves in water, exploiting its density maximum at 4C. The simulations show that waves are excited within the bulk of the convection zone, opposed to at the interface between the convective and stably stratified regions. We will also present 3D simulations using a compressible fluid. These simulations provide greater freedom in choosing the thermal equilibrium of the system, and are run at higher Rayleigh number.
Single phase channel flow forced convection heat transfer
Energy Technology Data Exchange (ETDEWEB)
Hartnett, J.P.
1999-04-01
A review of the current knowledge of single phase forced convection channel flow of liquids (Pr > 5) is presented. Two basic channel geometries are considered, the circular tube and the rectangular duct. Both laminar flow and turbulent flow are covered. The review begins with a brief overview of the heat transfer behavior of Newtonian fluids followed by a more detailed presentation of the behavior of purely viscous and viscoelastic Non-Newtonian fluids. Recent developments dealing with aqueous solutions of high molecular weight polymers and aqueous solutions of surfactants are discussed. The review concludes by citing a number of challenging research opportunities.
Forced Convection Heat Transfer in Circular Pipes
Tosun, Ismail
2007-01-01
One of the pitfalls of engineering education is to lose the physical insight of the problem while tackling the mathematical part. Forced convection heat transfer (the Graetz-Nusselt problem) certainly falls into this category. The equation of energy together with the equation of motion leads to a partial differential equation subject to various…
Turbulent Chemical Diffusion in Convectively Bounded Carbon Flames
Lecoanet, Daniel; Quataert, Eliot; Bildsten, Lars; Timmes, F X; Burns, Keaton J; Vasil, Geoffrey M; Oishi, Jeffrey S; Brown, Benjamin P
2016-01-01
It has been proposed that mixing induced by convective overshoot can disrupt the inward propagation of carbon deflagrations in super-asymptotic giant branch stars. To test this theory, we study an idealized model of convectively bounded carbon flames with 3D hydrodynamic simulations of the Boussinesq equations using the pseudospectral code Dedalus. Because the flame propagation timescale is $\\sim 10^5$ times longer than the convection timescale, we approximate the flame as fixed in space, and only consider its effects on the buoyancy of the fluid. By evolving a passive scalar field, we derive a turbulent chemical diffusivity produced by the convection as a function of height, $D_t(z)$. Convection can stall a flame if the chemical mixing timescale, set by the turbulent chemical diffusivity, $D_t$, is shorter than the flame propagation timescale, set by the thermal diffusivity, $\\kappa$, i.e., when $D_t>\\kappa$. However, we find $D_t<\\kappa$ for most of the flame because convective plumes are not dense enoug...
Nagata, Kouji; Sakai, Yasuhiko; Komori, Satoru
2011-06-01
Effects of weak, small-scale freestream turbulence on turbulent boundary layers with and without thermal convection are experimentally investigated using a wind tunnel. Two experiments are carried out: the first is isothermal boundary layers with and without grid turbulence, and the second is non-isothermal boundary layers with and without grid turbulence. Both boundary layers develop under a small favorable pressure gradient. For the latter case, the bottom wall of the test section is heated at a constant wall temperature to investigate the effects of thermal convection under the effects of freestream turbulence. For both cases, the turbulence intensity in the freestream is Tu = 1.3% ˜ 2.4%, and the integral length scale of freestream turbulence, L∞, is much smaller than the boundary layer thickness δ, i.e., L∞/δ ≪1. The Reynolds numbers Reθ based on the momentum thickness and freestream speed U∞ are Reθ = 560, 1100, 1310, and 2330 in isothermal boundary layers without grid turbulence. Instantaneous velocities, U and V, and instantaneous temperature T are simultaneously measured using a hot-wire anemometry and a constant-current resistance thermometer. The results show that the rms velocities and Reynolds shear stress normalized by the friction velocity are strongly suppressed by the freestream turbulence throughout the boundary layer in both isothermal and non-isothermal boundary layers. In the non-isothermal boundary layers, the normalized rms temperature and vertical turbulent heat flux are also strongly suppressed by the freestream turbulence. Turbulent momentum and heat transfer at the wall are enhanced by the freestream turbulence and the enhancement is notable in unstable stratification. The power spectra of u, v, and θ and their cospectra show that motions of almost all scales are suppressed by the freestream turbulence in both the isothermal and non-isothermal boundary layers.
Entropy Production and Fluctuation Relation in Turbulent Convection
Chibbaro, Sergio; Zonta, Francesco
2016-11-01
We report on a numerical experiment performed to analyze fluctuations of the entropy production in turbulent thermal convection. Using Direct Numerical Simulations (DNS), we estimate the entropy production from instantaneous measurements of the local temperature and velocity fields sampled along the trajectory of a large number of point-wise Lagrangian tracers. Entropy production is related to the work made by buoyancy force. The entropy production is characterized by large fluctuations and becomes often negative. This represents a sort of "finite-time" violation of the second principle of thermodynamics, since the direction of the energy flux is opposite to that prescribed by the external gradient. We provide a physical-sound definition of energy-scale characterizing the sytem, based upon Kolmogorov theory. Then, we link our results with recent theory of statistical mechanics of nonequilibrium systems, notably the results obtained by Evans, Cohen, Morris and Gallavotti for generic reversible dynamical systems. We show that the fluctuations of entropy production observed in the present system verify neatly the Fluctuation Relation (FR), cornerstone of that theory, even though the system is time-irreversible.
Confinement and dynamical regulation in two-dimensional convective turbulence
DEFF Research Database (Denmark)
Bian, N.H.; Garcia, O.E.
2003-01-01
In this work the nature of confinement improvement implied by the self-consistent generation of mean flows in two-dimensional convective turbulence is studied. The confinement variations are linked to two distinct regulation mechanisms which are also shown to be at the origin of low-frequency bur......In this work the nature of confinement improvement implied by the self-consistent generation of mean flows in two-dimensional convective turbulence is studied. The confinement variations are linked to two distinct regulation mechanisms which are also shown to be at the origin of low......-frequency bursting in the fluctuation level and the convective heat flux integral, both resulting in a state of large-scale intermittency. The first one involves the control of convective transport by sheared mean flows. This regulation relies on the conservative transfer of kinetic energy from tilted fluctuations...... to the mean component of the flow. Bursting can also result from the quasi-linear modification of the linear instability drive which is the mean pressure gradient. For each bursting process the relevant zero-dimensional model equations are given. These are finally coupled in a minimal model of convection...
Universality of energy spectrum in turbulent Rayleigh-Benard convection
Bai, Kunlun; Hoeller, Judith; Brown, Eric
2016-11-01
We present study of energy spectrum in turbulent Rayleigh-Benard convection, in both cylindrical and cubic containers, tilting and non-tilting conditions, and with Rayleigh number ranging from 0 . 5 ×109 to 1 ×1010 . For these different conditions of geometry, tilt, and Rayleigh number, the temperature spectra measured on the system side walls are significantly different from each other. Even for the same condition, the spectrum varies depending on whether the sensors locate in the path of large-scale circulations. However, quite interestingly, once the signals of large-scale circulations are subtracted from the raw temperature, all spectra display a universal shape, regardless of system geometry, tilt, Rayleigh number, and location of sensors. It suggests that one could model the large-scale circulations and small-scale fluctuations separately in turbulent Rayleigh-Benard convection.
Turbulent convection in the Sun: modeling in unstructured meshes
Olshevsky, Vyacheslav; Ham, Frank
2014-01-01
We adopted an unstructured hydrodynamical solver CharLES to the problem of global convection in the Sun. With the aim to investigate the properties of solar turbulent convection and reproduce differential rotation pattern. We performed simulations in two spherical shells, with 1.3 and 10 million cells. In the first, coarse mesh, the solution does not reproduce realistic convection, and is dominated by numerical effects. In the second mesh, thermal conduction leads to cooling of bottom layers, that could not be compensated by solar irradiance. More simulations in the 10M cells mesh should be performed to investigate the influence of transport coefficients and numerical effects. Our estimate of the code performance suggests, that realistic simulations in even finer grids could be performed for reasonable computational cost.
Efficient Turbulent Compressible Convection in the Deep Stellar Atmosphere
Tian, Chun-Lin; Chan, Kwing-Lam; Xiong, Da-Run
2008-01-01
This paper reports an application of gas-kinetic BGK scheme to the computation of turbulent compressible convection in the stellar interior. After incorporating the Sub-grid Scale (SGS) turbulence model into the BGK scheme, we tested the effects of numerical parameters on the quantitative relationships among the thermodynamic variables, their fluctuations and correlations in a very deep, initially gravity-stratified stellar atmosphere. Comparison indicates that the thermal properties and dynamic properties are dominated by different aspects of numerical models separately. An adjustable Deardorff constant in the SGS model $c_\\mu=0.25$ and an amplitude of artificial viscosity in the gas-kinetic BGK scheme $C_2=0$ are appropriate for current study. We also calculated the density-weighted auto- and cross-correlation functions in Xiong's (\\cite{xiong77}) turbulent stellar convection theories based on which the gradient type of models of the non-local transport and the anisotropy of the turbulence are preliminarily...
Efficient turbulent compressible convection in the deep stellar atmosphere
Institute of Scientific and Technical Information of China (English)
Chun-Lin Tian; Li-Cai Deng; Kwing-Lam Chan; Da-Run Xiong
2009-01-01
We report on an application of gas-kinetic BGK scheme to the computation of turbulent compressible convection in the stellar interior. After incorporating the Sub-grid Scale (SGS) turbulence model into the BGK scheme, we tested the effects of numerical parameters on the quantitative relationships among the thermodynamic variables, their fluctuations and correlations in a very deep, initially gravity-stratified stellar atmosphere. Comparison indicates that the thermal properties and dynamic properties are dominated by different aspects of numerical models separately. An adjustable Deardorff constant in the SGS model cμ = 0.25 and an amplitude of artificial viscosity in the gas-kinetic BGK scheme C2 = 0 are appropriate for the current study. We also calculated the density-weighted auto-and cross-correlation functions in Xiong's turbulent stellar convection theory based on which the gradient type of models of the non-local transport and the anisotropy of the turbulence were preliminarily studied. No universal relations or con-stant parameters were found for these models.
Modelling turbulent stellar convection zones: sub-grid scales effects
Strugarek, A; Brun, A S; Charbonneau, P; Mathis, S; Smolarkiewicz, P K
2016-01-01
The impressive development of global numerical simulations of turbulent stellar interiors unveiled a variety of possible differential rotation (solar or anti-solar), meridional circulation (single or multi-cellular), and dynamo states (stable large scale toroidal field or periodically reversing magnetic fields). Various numerical schemes, based on the so-called anelastic set of equations, were used to obtain these results. It appears today mandatory to assess their robustness with respect to the details of the numerics, and in particular to the treatment of turbulent sub-grid scales. We report on an ongoing comparison between two global models, the ASH and EULAG codes. In EULAG the sub-grid scales are treated implicitly by the numerical scheme, while in ASH their effect is generally modelled by using enhanced dissipation coefficients. We characterize the sub-grid scales effect in a turbulent convection simulation with EULAG. We assess their effect at each resolved scale with a detailed energy budget. We deriv...
Heat transfer in a gray tube with forced convection, internal radiation and axial wall conduction
Chung, B. T. F.; Thompson, J. E.
1983-01-01
A method of successive approximations is employed to solve the problem of heat transfer to a transparent gas flowing through a radiating-conducting tube with turbulent forced convection between the tube wall and the gas, and with energy generation in the wall. Emphasis is given to the effect of emissivity of the wall to the tube and gas temperature profiles.
Thornton, E. A.
1979-01-01
Three practical problems in conduction/forced convection heat transfer are analyzed using a simplified engineering formulation of convective finite elements. Upwind and conventional finite element solutions are compared for steady-state and transient applications.
Numerical Simulation of Water/Al2O3 Nanofluid Turbulent Convection
Directory of Open Access Journals (Sweden)
Vincenzo Bianco
2010-01-01
Full Text Available Turbulent forced convection flow of a water-Al2O3 nanofluid in a circular tube subjected to a constant and uniform temperature at the wall is numerically analyzed. The two-phase mixture model is employed to simulate the nanofluid convection, taking into account appropriate thermophysical properties. Particles are assumed spherical with a diameter equal to 38 nm. It is found that convective heat transfer coefficient for nanofluids is greater than that of the base liquid. Heat transfer enhancement is increasing with the particle volume concentration and Reynolds number. Comparisons with correlations present in the literature are accomplished and a very good agreement is found with Pak and Cho (1998. As for the friction factor, it shows a good agreement with the classical correlation used for normal fluid, such as Blasius formula.
Transition between free, mixed and forced convection
Jaeger, W.; Trimborn, F.; Niemann, M.; Saini, V.; Hering, W.; Stieglitz, R.; Pritz, B.; Fröhlich, J.; Gabi, M.
2017-07-01
In this contribution, numerical methods are discussed to predict the heat transfer to liquid metal flowing in rectangular flow channels. A correct representation of the thermo-hydraulic behaviour is necessary, because these numerical methods are used to perform design and safety studies of components with rectangular channels. Hence, it must be proven that simulation results are an adequate representation of the real conditions. Up to now, the majority of simulations are related to forced convection of liquid metals flowing in circular pipes or rod bundle, because these geometries represent most of the components in process engineering (e.g. piping, heat exchanger). Open questions related to liquid metal heat transfer, among others, is the behaviour during the transition of the heat transfer regimes. Therefore, this contribution aims to provide useful information related to the transition from forced to mixed and free convection, with the focus on a rectangular flow channel. The assessment of the thermo-hydraulic behaviour under transitional heat transfer regimes is pursued by means of system code simulations, RANS CFD simulations, LES and DNS, and experimental investigations. Thereby, each of the results will compared to the others. The comparison of external experimental data, DNS data, RANS data and system code simulation results shows that the global heat transfer can be consistently represented for forced convection in rectangular flow channels by these means. Furthermore, LES data is in agreement with RANS CFD results for different Richardson numbers with respect to temperature and velocity distribution. The agreement of the simulation results among each other and the hopefully successful validation by means of experimental data will fosters the confidence in the predicting capabilities of numerical methods, which can be applied to engineering application.
Effect of heat flux on differential rotation in turbulent convection
Kleeorin, N
2006-01-01
We studied the effect of the turbulent heat flux on the Reynolds stresses in a rotating turbulent convection. To this end we solved a coupled system of dynamical equations which includes the equations for the Reynolds stresses, the entropy fluctuations and the turbulent heat flux. We used a spectral $\\tau$ approximation in order to close the system of dynamical equations. We found that the ratio of the contributions to the Reynolds stresses caused by the turbulent heat flux and the anisotropic eddy viscosity is of the order of $\\sim 10 (L_\\rho / l_0)^2$, where $l_{0}$ is the maximum scale of turbulent motions and $L_\\rho$ is the fluid density variation scale. This effect is crucial for the formation of the differential rotation and should be taken into account in the theories of the differential rotation of the Sun, stars and planets. In particular, we demonstrated that this effect may cause the differential rotation which is comparable with the typical solar differential rotation.
Small Scale Forcing in a Turbulent Boundary Layer
Lorkowski, Thomas; Rathnasingham, Ruben; Breuer, Kenneth S.
1996-11-01
In order to understand the effect of small scale forcing on turbulent flows and its implications on control, an experimental investigation is made into the forcing of the inertial scales in the wall region of a turbulent boundary layer. A wall-mounted resonant actuator is used to produce a local vortical structure in the streamwise direction which is convected downstream by the boundary layer flow. The frequency associated with this structure is governed by the resonant frequency of the device and falls in the range of the inertial scales at the Reynolds number of the experiment (Re_θ = 2000). Hot-wire anemometry is used to map the velocity field at several stations downstream of the actuator. The signals are also conditioned to identify the effect of the actuator on different scales in the flow. Amplitude and modulation effects are also discussed. ^*Supported by ONR Grant N00014-92-J-1910.
Statistics and scaling in turbulent Rayleigh-Bénard convection
Ching, Emily SC
2013-01-01
This Brief addresses two issues of interest of turbulent Rayleigh-Bénard convection. The first issue is the characterization and understanding of the statistics of the velocity and temperature fluctuations in the system. The second issue is the revelation and understanding of the nature of the scaling behavior of the velocity temperature structure functions. The problem under the Oberbeck-Boussinesq approximation is formulated. The statistical tools, including probability density functions (PDF) and conditional statistics, for studying fluctuations are introduced, and implicit PDF formulae for
Supersonic Turbulent Convection and the Origin of the Planets
Prentice, A. J. R.; Dyt, C. P.
2000-10-01
We report a new set of calculations which support the view that supersonic turbulent convection played a major role in the formation of the solar system. A flux-corrected transport scheme (Zalesak, J. Comp. Phys.} 31 335 1979) is used to numerically simulate thermal convection in a 2D ideal gas layer that is heated from below and is stratified gravitationally across many scale heights. The temperature T0 at the top boundary and the temperature gradient (∂ T/∂ z)1 at the lower boundary are kept constant during the computation. The initial atmosphere is superadiabatic with polytropic index m = 1, specific heats ratio γ = 1.4 and temperature contrast T1}/T{0 = 11. This layer mimics a section of the outer layer of the proto-solar cloud (Dyt & Prentice, MNRAS 296 56 1998). Because the Reynolds number of the real atmosphere is so large, motions whose scale is less than the computational grid size are represented with a Smagorinsky sub-grid scale turbulence approximation (Chan et al, Ap.J.} 263 935 1982). That is, a velocity-dependent turbulent viscosity ν t and thermal diffusivity κ t are chosen so that the high wavenumber kinetic energy spectrum follows Kolmogorov's -5/3 law. The flow soon evolves to a configuration consisting of a network of giant convective cells. At cell boundaries, the downflows are spatially concentrated and rapid. Turbulent pressures t range up to 3 times the local gas pressure pgas. The convection eliminates nearly all of the superadiabaticity in the lower 90% of the atmosphere. In the top 10%, ∂ T/∂ z increases sharply and a steep density inversion occurs, with ρ increasing by a factor of 3-4. This result gives new credibility to the modern Laplacian theory of solar system origin (Moon & Planets} 19 341 1978; ibid 73 237 1996; Phys. Lett. A} 213 253 1996). Even so, we need t ≈ 10 pgas if the proto-solar cloud is to shed discrete gas rings whose orbits match the mean planetary spacings and whose chemical condensates match the
The eddy heat-flux in rotating turbulent convection
Rüdiger, G; Kitchatinov, L L; Küker, M
2004-01-01
The three components of the heat-flux vector $F =\\rho C_p are numerically computed for a stratified rotating turbulent convection using the NIRVANA code in a flat box. The latitudinal component $F_\\theta$ proves to be negative (positive) in the northern (southern) hemisphere so that the heat always flows towards the poles. As a surprise, the radial heat-flux $F_r$ peaks at the equator rather than at the poles (Taylor numbers O(10^6)). The same behavior is observed for the radial turbulence intensity $$ which for \\emph{free} turbulence is also believed to peak at the poles (see Eq. (19) below). As we can show, however, the consequences of this unexpected result (also obtained by Kaepylae, Korpi and Tuominen 2004) for the theory of differential rotation are small as mainly the $F_\\theta$ is responsible to solve the `Taylor number puzzle'. In all our simulations the azimuthal component $F_\\phi$ proves to be negative so that the rotating turbulence produces an westwards directed azimuthal heat-flux which should ...
Flow structure in turbulent rotating Rayleigh-Bénard convection
Kunnen, Rudie; Corre, Yoann; Clercx, Herman
2012-11-01
Turbulent Rayleigh-Bénard convection is usually studied in an upright cylinder. The addition of axial rotation has profound effects on the flow structuring. The well-known large-scale circulation (LSC) of the non-rotating case is still found at low rotation rates but is replaced by an irregular array of vertically aligned vortical plumes at higher rotation rates. We report PIV measurements of turbulent rotating convection in a cylindrical cell of diameter-to-height aspect ratio Γ = 1 / 2 at Rayleigh number Ra = 4 . 5 ×109 and at many rotation rates covering both the LSC and the vortical-plume regime. We focus on: (i) the azimuthal precession of the LSC, (ii) collective motions of the vortical plumes, and (iii) the sidewall boundary layers. With these results we can clarify remarkable differences between the Γ = 1 and Γ = 1 / 2 cases reported recently in the literature. Traineeship project carried out in Eindhoven as part of Master's Degree at Université Paris-Sud, France.
Heat transport measurements in turbulent rotating Rayleigh-Benard convection
Energy Technology Data Exchange (ETDEWEB)
Ecke, Robert E [Los Alamos National Laboratory; Liu, Yuanming [Los Alamos National Laboratory
2008-01-01
We present experimental heat transport measurements of turbulent Rayleigh-Benard convection with rotation about a vertical axis. The fluid, water with Prandtl number ({sigma}) about 6, was confined in a cell which had a square cross section of 7.3 cm x 7.3 cm and a height of 9.4 cm. Heat transport was measured for Rayleigh numbers 2 x 10{sup 5} < Ra < 5 x 10{sup 8} and Taylor numbers 0 < Ta < 5 x 10{sup 9}. We show the variation of normalized heat transport, the Nusselt number, at fixed dimensional rotation rate {Omega}{sub D}, at fixed Ra varying Ta, at fixed Ta varying Ra, and at fixed Rossby number Ro. The scaling of heat transport in the range 10{sup 7} to about 10{sup 9} is roughly 0.29 with a Ro dependent coefficient or equivalently is also well fit by a combination of power laws of the form a Ra{sup 1/5} + b Ra{sup 1/3} . The range of Ra is not sufficient to differentiate single power law or combined power law scaling. The overall impact of rotation on heat transport in turbulent convection is assessed.
Development of a mechanistic model for forced convection subcooled boiling
Shaver, Dillon R.
The focus of this work is on the formulation, implementation, and testing of a mechanistic model of subcooled boiling. Subcooled boiling is the process of vapor generation on a heated wall when the bulk liquid temperature is still below saturation. This is part of a larger effort by the US DoE's CASL project to apply advanced computational tools to the simulation of light water reactors. To support this effort, the formulation of the dispersed field model is described and a complete model of interfacial forces is formulated. The model has been implemented in the NPHASE-CMFD computer code with a K-epsilon model of turbulence. The interfacial force models are built on extensive work by other authors, and include novel formulations of the turbulent dispersion and lift forces. The complete model of interfacial forces is compared to experiments for adiabatic bubbly flows, including both steady-state and unsteady conditions. The same model is then applied to a transient gas/liquid flow in a complex geometry of fuel channels in a sodium fast reactor. Building on the foundation of the interfacial force model, a mechanistic model of forced-convection subcooled boiling is proposed. This model uses the heat flux partitioning concept and accounts for condensation of bubbles attached to the wall. This allows the model to capture the enhanced heat transfer associated with boiling before the point of net generation of vapor, a phenomenon consistent with existing experimental observations. The model is compared to four different experiments encompassing flows of light water, heavy water, and R12 at different pressures, in cylindrical channels, an internally heated annulus, and a rectangular channel. The experimental data includes axial and radial profiles of both liquid temperature and vapor volume fraction, and the agreement can be considered quite good. The complete model is then applied to simulations of subcooled boiling in nuclear reactor subchannels consistent with the
Some Properties of the Kinetic Energy Flux and Dissipation in Turbulent Stellar Convection Zones
Meakin, Casey
2010-01-01
We investigate simulated turbulent flow within thermally driven stellar convection zones. Different driving sources are studied, including cooling at the top of the convectively unstable region, as occurs in surface convection zones; and heating at the base by nuclear burning. The transport of enthalpy and kinetic energy, and the distribution of turbulent kinetic energy dissipation are studied. We emphasize the importance of global constraints on shaping the quasi-steady flow characteristics, and present an analysis of turbulent convection which is posed as a boundary value problem that can be easily incorporated into standard stellar evolution codes for deep, efficient convection. Direct comparison is made between the theoretical analysis and the simulated flow and very good agreement is found. Some common assumptions traditionally used to treat quasi-steady turbulent flow in stellar models are briefly discussed. The importance and proper treatment of convective boundaries are indicated.
Directory of Open Access Journals (Sweden)
Hanafi Abdalla S.
2008-01-01
Full Text Available This paper presents experimental and numerical studies for the case of turbulent forced and mixed convection flow of water through narrow vertical rectangular channel. The channel is composed of two parallel plates which are heated at a uniform heat flux, whereas, the other two sides of the channel are thermally insulated. The plates are of 64 mm in width, 800 mm in height, and separated from each other at a narrow gap of 2.7 mm. The Nusselt number distribution along the flow direction normalized by the Nusselt number for the case of turbulent forced convection flow is obtained experimentally with a comparison with the numerical results obtained from a commercial computer code. The quantitative determination of the nor- malized Nusselt number with respect to the dimension-less number Z = (Gr/Re21/8Pr0.5 is presented with a comparison with previous experimental results. Qualitative results are presented for the normalized temperature and velocity profiles in the transverse direction with a comparison between the forced and mixed convection flow for both the cases of upward and downward flow directions. The effect of the axial locations and the parameter Gr/Re on the variation of the normalized temperature profiles in the transverse direction for both the regions of forced and mixed convection and for both of the upward and downward flow directions are obtained. The normalized velocity profiles in the transverse directions are also determined at different inlet velocity and heat fluxes for the previous cases. It is found that the normalized Nusselt number is greater than one in the mixed convection region for both the cases of upward and downward flow and correlated well with the dimension-less parameter Z for both of the forced and mixed convection regions. The temperature profiles increase with increasing the axial location along the flow direction or the parameter Gr/Re for both of the forced and mixed convection regions, but this increase is
Turbulent mixed convection in asymmetrically heated vertical channel
Directory of Open Access Journals (Sweden)
Mokni Ameni
2012-01-01
Full Text Available In this paper an investigation of mixed convection from vertical heated channel is undertaken. The aim is to explore the heat transfer obtained by adding a forced flow, issued from a flat nozzle located in the entry section of a channel, to the up-going fluid along its walls. Forced and free convection are combined studied in order to increase the cooling requirements. The study deals with both symmetrically and asymmetrically heated channel. The Reynolds number based on the nozzle width and the jet velocity is assumed to be 3 103 and 2.104; whereas, the Rayleigh number based on the channel length and the wall temperature difference varies from 2.57 1010 to 5.15 1012. The heating asymmetry effect on the flow development including the mean velocity and temperature the local Nusselt number, the mass flow rate and heat transfer are examined.
Dynamics of Turbulent Convection and Convective Overshoot in a Moderate Mass Star
Kitiashvili, Irina N; Mansour, Nagi N; Wray, Alan A
2015-01-01
Continued progress in observational stellar astrophysics requires a deep understanding of the underlying convection dynamics. We present results of realistic 3D radiative hydrodynamic simulations of the outer layers of a moderate mass star (1.47 Msun), including the full convection zone, the overshoot region, and the top layers of the radiative zone. The simulation results show that the surface granulation has a broad range of scales, from 2 to 12 Mm, and that large granules are organized in well-defined clusters, consisting of several granules. Comparison of the mean structure profiles from 3D simulations with the corresponding 1D standard stellar model shows an increase of the stellar radius by ~800 km, as well as significant changes in the thermodynamic structure and turbulent properties of the ionization zones. Convective downdrafts in the intergranular lanes between granulation clusters reach speeds of more than 20 km/s, penetrate through the whole convection zone, hit the radiative zone, and form a 8 Mm...
Modeling turbulent stellar convection zones: Sub-grid scales effects
Strugarek, A.; Beaudoin, P.; Brun, A. S.; Charbonneau, P.; Mathis, S.; Smolarkiewicz, P. K.
2016-10-01
The impressive development of global numerical simulations of turbulent stellar interiors unveiled a variety of possible differential rotation (solar or anti-solar), meridional circulation (single or multi-cellular), and dynamo states (stable large scale toroidal field or periodically reversing magnetic fields). Various numerical schemes, based on the so-called anelastic set of equations, were used to obtain these results. It appears today mandatory to assess their robustness with respect to the details of the numerics, and in particular to the treatment of turbulent sub-grid scales. We report on an ongoing comparison between two global models, the ASH and EULAG codes. In EULAG the sub-grid scales are treated implicitly by the numerical scheme, while in ASH their effect is generally modeled by using enhanced dissipation coefficients. We characterize the sub-grid scales effect in a turbulent convection simulation with EULAG. We assess their effect at each resolved scale with a detailed energy budget. We derive equivalent eddy-diffusion coefficients and use the derived diffusivities in twin ASH numerical simulations. We find a good agreement between the large-scale flows developing in the two codes in the hydrodynamic regime, which encourages further investigation in the magnetohydrodynamic regime for various dynamo solutions.
A novel combined model of discrete and mixture phases for nanoparticles in convective turbulent flow
Mahdavi, Mostafa; Sharifpur, Mohsen; Meyer, Josua P.
2017-08-01
In this study, a new combined model is presented to study the flow and discrete phase features of nano-size particles for turbulent convection in a horizontal tube. Due to the complexity and many phenomena involved in particle-liquid turbulent flows, the conventional models are not able to properly predict some hidden aspects of the flow. Therefore, a new form of Brownian force is implemented in the discrete phase model to predict the migration of the particles as well as energy equation has modified for particles. Then, the final results are exported to the mixture equations of the flow. The effects of the mass diffusion due to thermophoresis, Brownian motion, and turbulent dispersion are implemented as source terms in equations. The results are compared with the experimental measurements from the literature and are adequately validated. The accuracy of predicted heat transfer and friction coefficients is also discussed versus measurements. The migration of the particles toward the centre of the tube is properly captured. The results show the non-uniform distribution of particles in the turbulent flow due to strong turbulent dispersion. The proposed combined model can open new viewpoints of particle-fluid interaction flows.
A criterion to detect line plumes from velocity fields in turbulent convection
Koothur, Vipin
2015-01-01
We present a simple, new criterion to extract line plumes from the velocity fields, without using the temperature field, in a horizontal plane close to the plate in turbulent convection. The existing coherent structure detection criteria from velocity fields, proposed for shear driven wall turbulence, are first shown to be inadequate for turbulent convection. Based on physical arguments, we then propose that the negative values of $\\overline{\
DYNAMICS OF TURBULENT CONVECTION AND CONVECTIVE OVERSHOOT IN A MODERATE-MASS STAR
Energy Technology Data Exchange (ETDEWEB)
Kitiashvili, I. N.; Mansour, N. N.; Wray, A. A. [NASA Ames Research Center, Moffett Field, CA 94035 (United States); Kosovichev, A. G., E-mail: irina.n.kitiashvili@nasa.gov [New Jersey Institute of Technology, Newark, NJ 07102 (United States)
2016-04-10
We present results of realistic three-dimensional (3D) radiative hydrodynamic simulations of the outer layers of a moderate-mass star (1.47 M {sub ⊙}), including the full convection zone, the overshoot region, and the top layers of the radiative zone. The simulation results show that the surface granulation has a broad range of scales, from 2 to 12 Mm, and that large granules are organized in well-defined clusters, consisting of several granules. Comparison of the mean structure profiles from 3D simulations with the corresponding one-dimensional (1D) standard stellar model shows an increase of the stellar radius by ∼800 km, as well as significant changes in the thermodynamic structure and turbulent properties of the ionization zones. Convective downdrafts in the intergranular lanes between granulation clusters reach speeds of more than 20 km s{sup −1}, penetrate through the whole convection zone, hit the radiative zone, and form an 8 Mm thick overshoot layer. Contrary to semi-empirical overshooting models, our results show that the 3D dynamic overshoot region consists of two layers: a nearly adiabatic extension of the convection zone and a deeper layer of enhanced subadiabatic stratification. This layer is formed because of heating caused by the braking of the overshooting convective plumes. This effect has to be taken into account in stellar modeling and the interpretation of asteroseismology data. In particular, we demonstrate that the deviations of the mean structure of the 3D model from the 1D standard model of the same mass and composition are qualitatively similar to the deviations for the Sun found by helioseismology.
Turbulence convective heat transfer for cooling the photovoltaic cells
Arianmehr, Iman
Solar PV (photovoltaic) is a rapidly advancing renewable energy technology which converts sunlight directly into electricity. One of the outstanding challenges of the current PV technology is the reduction in its conversion efficiency with increasing PV panel temperature, which is closely associated with the increase in solar intensity and the ambient temperature surrounding the PV panels. To more effectively capture the available energy when the sun is most intense, significant efforts have been invested in active and passive cooling research over the last few years. While integrated cooling systems can lead to the highest total efficiencies, they are usually neither the most feasible nor the most cost effective solutions. This work examines some simple passive means of manipulating the prevailing wind turbulence to enhance convective heat transfer over a heated plate in a wind tunnel.
Simultaneous temperature and velocity Lagrangian measurements in turbulent thermal convection
Liot, O; Zonta, F; Chibbaro, S; Coudarchet, T; Gasteuil, Y; Pinton, J -F; Salort, J; Chillà, F
2015-01-01
We report joint Lagrangian velocity and temperature measurements in turbulent thermal convection. Measurements are performed using an improved version (extended autonomy) of the neutrally-buoyant instrumented particle that was used by to performed experiments in a parallelepipedic Rayleigh-Benard cell. The temperature signal is obtained from a RFtransmitter. Simultaneously, we determine particle's position and velocity with one camera, which grants access to the Lagrangian heat flux. Due to the extended autonomy of the present particle, we obtain well converged temperature and velocity statistics, as well as pseudo-eulerian maps of velocity and heat flux. Present experimental results have also been compared with the results obtained by a corresponding campaign of Direct Numerical Simulations and Lagrangian Tracking of massless tracers. The comparison between experimental and numerical results show the accuracy and reliability of our experimental measurements. Finally, the analysis of lagrangian velocity and t...
Transitions in turbulent rotating Rayleigh-B\\'enard convection
Schmitz, S
2010-01-01
Numerical simulations of rotating Rayleigh-B\\'enard convection are presented for both no slip and free slip boundaries. The goal is to find a criterion distinguishing convective flows dominated by the Coriolis force from those nearly unaffected by rotation. If one uses heat transport as an indicator of which regime the flow is in, one finds that the transition between the flow regimes always occurs at the same value of a certain combination of Reynolds, Prandtl and Ekman numbers for both boundary conditions. If on the other hand one uses the helicity of the velocity field to identify flows nearly independent of rotation, one finds the transition at a different location in parameter space.
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)
Scaling in large Prandtl number turbulent thermal convection
Dubrulle, B
2011-01-01
We study the scaling properties of heat transfer $Nu$ in turbulent thermal convection at large Prandtl number $Pr$ using a quasi-linear theory. We show that two regimes arise, depending on the Reynolds number $Re$. At low Reynolds number, $Nu Pr^{-1/2}$ and $Re$ are a function of $Ra Pr^{-3/2}$. At large Reynolds number $Nu Pr^{1/3}$ and $Re Pr$ are function only of $Ra Pr^{2/3}$ (within logarithmic corrections). In practice, since $Nu$ is always close to $Ra^{1/3}$, this corresponds to a much weaker dependence of the heat transfer in the Prandtl number at low Reynolds number than at large Reynolds number. This difference may solve an existing controversy between measurements in SF6 (large $Re$) and in alcohol/water (lower $Re$). We link these regimes with a possible global bifurcation in the turbulent mean flow. We further show how a scaling theory could be used to describe these two regimes through a single universal function. This function presents a bimodal character for intermediate range of Reynolds num...
Mccolgan, C. J.; Larson, R. S.
1978-01-01
The effect of light on the mean flow and turbulence properties of a 0.056 m circular jet were determined in a free jet wind tunnel. The nozzle exit velocity was 122 m/sec, and the wind tunnel velocity was set at 0, 12, 37, and 61 m/sec. Measurements of flow properties including mean velocity, turbulence intensity and spectra, and eddy convection velocity were carried out using two linearized hot wire anemometers. Normalization factors were determined for the mean velocity and turbulence convection velocity.
Zhong, Jin-Qiang; Wang, Xue-ying
2016-01-01
We present measurements of the azimuthal orientation {\\theta}(t) and thermal amplitude {\\delta}(t) of the large-scale circulation (LSC) of turbulent rotating convection within an unprecedented large Rossby number range 170. We identify the mechanism through which the mean retrograde rotation speed can be enhanced by stochastic cessations in the presence of weak Coriolis force, and show that a low-dimensional, stochastic model provides predictions of the observed large-scale flow dynamics and interprets its retrograde rotation.
Plume dynamics in quasi-2D turbulent convection.
Bizon, C.; Werne, J.; Predtechensky, A. A.; Julien, K.; McCormick, W. D.; Swift, J. B.; Swinney, Harry L.
1997-03-01
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 delta, 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 Peclet number, Pe) with the Rayleigh number (Ra(*)) for the simulations. Both no-slip and stress-free solutions exhibit the scaling NuRa(*) approximately 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 approximately nPe, is dictated by stagnation-point flows occurring at the horizontal boundaries; n is the number of plumes per unit length. No-slip solutions exhibit no such organization of the boundary flow and the results appear to agree with Priestley's prediction of Nu
Stochastic excitation of the solar oscillations by turbulent convection
Willette, Gregory Thomas
1994-01-01
The thesis topic is the stochastic excitation of the solar p-modes by turbulent convection, and the work consists of four parts: three theoretical sections and one observational. In the first section of the thesis, an explicit calculation of the acoustic radiation of a buoyant oscillating bubble is presented as a model for the excitation of the solar p-modes. The central scientific issue addressed in this work is the cancellation of monopole and dipole radiation fields in an anisotropic medium, first pointed out by Goldreich and Kumar (1990). When the bubble oscillation frequency is small compared to the acoustic cutoff, the monopole and dipole disturbances cancel to the quadrupole order in the far field. The second section deals with the role of convective structures in a wide number of problems, including the creation of acoustic disturbances, the transport of heat and magnetic fields, and the penetration of flows into stable layers of the atmosphere (overshoot). A model of plume convection is developed to discuss these issues. It is argued that the scaleheight-sized flows (the only energetically significant ones) are properly characterized as coherent, entropy-preserving plumes, in contradistinction to the picture of amorphous parcels of fluid suggested by the Mixing Length Theory, and in spite of the large Reynolds numbers typical in astrophysical convection. The third section of the thesis is an analysis of high-resolution surface velocity data taken with a magneto-optical filter on the 10 inch telescope at Big Bear Solar Observatory. Estimates are obtained for the frequencies and amplitudes of the solar oscillations of high spherical harmonic degree (l approximately less than 2000). The observed mode energies follow a Boltzmann distribution (P(E) varies as exp(-E/(bar-E)), as is predicted in the stochastic excitation model. In the final section of the thesis, a derivation of the variational principle for an incompressible fluid is presented. The Lagrange and
Energy Technology Data Exchange (ETDEWEB)
Yi, S.; Kwon, J. M.; Rhee, T. [National Fusion Research Institute, Eoeun-dong, Yuseong-gu, Daejeon 305-333 (Korea, Republic of); Diamond, P. H. [National Fusion Research Institute, Eoeun-dong, Yuseong-gu, Daejeon 305-333 (Korea, Republic of); Center for Astrophysics and Space Sciences and Department of Physics, University of California San Diego, La Jolla, California 92093-0429 (United States)
2012-11-15
This paper reports the results of gyrokinetic simulation studies of ion temperature gradient driven turbulence which investigate the role of non-resonant modes in turbulence spreading, turbulence regulation, and self-generated plasma rotation. Non-resonant modes, which are those without a rational surface within the simulation domain, are identified as nonlinearly driven, radially extended convective cells. Even though the amplitudes of such convective cells are much smaller than that of the resonant, localized turbulence eddies, we find from bicoherence analysis that the mode-mode interactions in the presence of such convective cells increase the efficiency of turbulence spreading associated with nonlocality phenomena. Artificial suppression of the convective cells shows that turbulence spreading is reduced, and that the turbulence intensity profile is more localized. The more localized turbulence intensity profile produces stronger Reynolds stress and E Multiplication-Sign B shear flows, which in turn results in more effective turbulence self-regulation. This suggests that models without non-resonant modes may significantly underestimate turbulent fluctuation levels and transport.
Turbulence properties in the solar convection envelope: properties of the overshooting regions
Institute of Scientific and Technical Information of China (English)
Qian-Sheng Zhang; Yan Li
2009-01-01
We apply the turbulent convection model (TCM) to investigate properties of tur-bulence in the solar convective envelope, especially in overshooting regions. The results show TCM gives negative turbulent heat flux uγ′T′in overshooting regions, which is sim-ilar to other nonlocal turbulent convection theories. The turbulent temperature fluctuation T′T′shows peaks in overshooting regions. Most important, we find that the downward overshooting region below the base of the solar convection zone is a thin cellular layer filled with roll-shaped convective cells. The overshooting length for the temperature gradi-ent is much shorter than that for element mixing because turbulent heat flux of downward and upward moving convective cells counteract each other in this cellular overshooting region. Comparing the models' sound speed with observations, we find that raking the convective overshooting into account helps to improve the sound speed profile of our nonlocal solar models. Comparing the p-mode oscillation frequencies with observations,we validated that increasing the diffusion parameters and decreasing the dissipation pa-rameters of TCM make the p-mode oscillation frequencies of the solar model be in betteragreement with observations.
Sid, Samir; Terrapon, Vincent; Dubief, Yves
2015-11-01
Results of direct numerical simulation of turbulent channel flows under unstable stratification are reported. Two Reynolds number are considered: Reτ = 180 , 395 and the Rayleigh number ranges between Ra = [106 -109 ] . The Prandtl number is set to 1. The channel is periodic in both streamwise and spanwise directions and non-slip/isothermal boundary conditions are imposed at the walls. The temperature difference between the walls is set so that the stratification is unstable and the coupling between temperature and momentum is achieved using the Boussinesq approximation. The dependency of the typical large scale convective structures on both Reynolds and Rayleigh numbers are investigated through cross flow sectional statistics and instantaneous flow field visualizations. Moreover, the effects of the natural convection on the coherent structures associated to the cycle of wall-bounded turbulence (Jimenez, et al. JFM 1999), namely velocity streaks and streamwise vortices, are examined. Finally, macroscopic quantities such as friction coefficient and Nusselt number are reported as a function of the Rayleigh number and are compared for both Reynolds numbers. The Belgian Team acknowledges computational resources from CÉCI (F.R.S.-FNRS grant No.2.5020.11) and the PRACE infrastructure. YD acknowledges the support of NSF and DOE under grant NSF/DOE 1258697.
The nature of near-wall convection velocity in turbulent channel flow
Institute of Scientific and Technical Information of China (English)
Yuhui Cao; Jun Chen; Zhensu She
2008-01-01
A novel notion of turbulent structure-the local cascade structure-is introduced to study the convection phenomenon in a turbulent channel flow. A space-time cross-correlation method is used to calculate the convection velo-city. It is found that there are two characteristic convection speeds near the wall, one associated with small-scale streaks of a lower speed and another with streamwise vortices and hairpin vortices of a higher speed. The new concept of tur-bulent structure is powerful to illustrate the dominant role of coherent structures in the near-wall convection, and to reveal also the nature of the convection-the propagation of patterns of velocity fluctuations-which is scale-dependent.
Subgrid-scale heat flux modeling for large eddy simulation of turbulent mixed convection
Morar, Dejan
2014-01-01
In the present work, new subgrid-scale (SGS) heat flux model for large eddy simulation (LES) of turbulent mixed convection is developed. The new model explicitly includes the buoyancy production term. It is based on the algebraic equations and dynamic procedure is applied to calculate model coefficients. An experiment on turbulent mixed convection to water in a vertical duct is used for validation of the model.
Observation of dendritic growth under the influence of forced convection
Roshchupkina, O.; Shevchenko, N.; Eckert, S.
2015-06-01
The directional solidification of Ga-25wt%In alloys within a Hele-Shaw cell was visualized by X-ray radioscopy. The investigations are focused on the impact of melt convection on the dendritic growth. Natural convection occurs during a bottom up solidification because lighter solute is rejected during crystallization. Forced convection was produced by a specific electromagnetic pump. The direction of forced melt flow is almost horizontal at the solidification front. Melt flow induces various effects on grain morphology primarily caused by convective transport of solute, such as a facilitation of the growth of primary trunks or lateral branches, dendrite remelting, fragmentation or freckle formation depending on the dendrite orientation, the flow direction and intensity. Forced flow eliminates solutal plumes and damps local fluctuations of solute. A preferential growth of the secondary arms occurs at the upstream side of the dendrites, whereas high solute concentration at the downstream side inhibits the formation of secondary branches.
Turbulent eddy-time-correlation in the solar convective zone
Belkacem, K; Goupil, M J; Baudin, F; Salabert, D; Appourchaux, T
2010-01-01
Theoretical modeling of the driving processes of solar-like oscillations is a powerful way of understanding the properties of the convective zones of solar-type stars. In this framework, the description of the temporal correlation between turbulent eddies is an essential ingredient to model mode amplitudes. However, there is a debate between a Gaussian or Lorentzian description of the eddy-time correlation function (Samadi et al. 2003, Chaplin et al. 2005). Indeed, a Gaussian description reproduces the low-frequency shape of the mode amplitude for the Sun, but is unsatisfactory from a theoretical point of view (Houdek, 2009) and leads to other disagreements with observations (Samadi et al., 2007). These are solved by using a Lorentzian description, but there the low-frequency shape of the solar observations is not correctly reproduced. We reconcile the two descriptions by adopting the sweeping approximation, which consists in assuming that the eddy-time-correlation function is dominated by the advection of ed...
Strong turbulent convection: distributed chaos and large-scale circulation
Bershadskii, A
2016-01-01
Two types of spontaneous breaking of the space translational symmetry in distributed chaos have been considered for turbulent thermal convection at large values of Rayleigh number. First type is related to boundaries and second type is related to appearance of inertial range of scales. The first type is dominated by vorticity correlation integral: $\\int_{V} \\langle {\\boldsymbol \\omega} ({\\bf x},t) \\cdot {\\boldsymbol \\omega} ({\\bf x} + {\\bf r},t) \\rangle_{V} d{\\bf r}$ and is characterized by stretched exponential spectrum $\\exp-(k/k_{\\beta})^{\\beta }$ with $\\beta =1/2$. The second type is dominated by energy correlation integral: $\\int_{V} \\langle {\\bf u}^2 ({\\bf x},t) ~ {\\bf u}^2({\\bf x} + {\\bf r},t) \\rangle_{V} d{\\bf r}$ and is characterized by $\\beta =3/5$. Good agreement has been established with laboratory experimental data at large values of Rayleigh number $Ra \\sim 10^{11}-10^{14}$ (the range relevant to solar photosphere). Taylor hypothesis transforms the wavenumber spectrum to frequency spectrum $\\exp...
Meneguz, Elena; Turp, Debi; Wells, Helen
2015-04-01
It is well known that encounters with moderate or severe turbulence can lead to passenger injuries and incur high costs for airlines from compensation and litigation. As one of two World Area Forecast Centres (WAFCs), the Met Office has responsibility for forecasting en-route weather hazards worldwide for aviation above a height of 10,000 ft. Observations from commercial aircraft provide a basis for gaining a better understanding of turbulence and for improving turbulence forecasts through verification. However there is currently a lack of information regarding the possible cause of the observed turbulence, or whether the turbulence occurred within cloud. Such information would be invaluable for the development of forecasting techniques for particular types of turbulence and for forecast verification. Of all the possible sources of turbulence, convective activity is believed to be a major cause of turbulence. Its relative importance over the Europe and North Atlantic area has not been yet quantified in a systematic way: in this study, a new approach is developed to automate identification of turbulent encounters in the proximity of convective clouds. Observations of convection are provided from two independent sources: a surface based lightning network and satellite imagery. Lightning observations are taken from the Met Office Arrival Time Detections network (ATDnet). ATDnet has been designed to identify cloud-to-ground flashes over Europe but also detects (a smaller fraction of) strikes over the North Atlantic. Meteosat Second Generation (MSG) satellite products are used to identify convective clouds by applying a brightness temperature filtering technique. The morphological features of cold cloud tops are also investigated. The system is run for all in situ turbulence reports received from airlines for a total of 12 months during summer 2013 and 2014 for the domain of interest. Results of this preliminary short term climatological study show significant intra
On the onset of turbulence in natural convection on inclined plates
Rodríguez Sevillano, Angel; Pérez Grande, María Isabel; Meseguer Ruiz, José
2011-01-01
The problem of determination of the turbulence onset in natural convection on heated inclined plates in an air environment has been experimentally revisited. The transition has been detected by using hot wire velocity measurements. The onset of turbulence has been considered to take place where velocity fluctuations (measured through turbulence intensity) start to grow. Experiments have shown that the onset depends not only on the Grashof number defined in terms of the temperature difference ...
Predicting Turbulent Convective Heat Transfer in Fully Developed Duct Flows
Rokni, Masoud; Gatski, Thomas B.
2001-01-01
The performance of an explicit algebraic stress model (EASM) is assessed in predicting the turbulent flow and forced heat transfer in both straight and wavy ducts, with rectangular, trapezoidal and triangular cross-sections, under fully developed conditions. A comparison of secondary flow patterns. including velocity vectors and velocity and temperature contours, are shown in order to study the effect of waviness on flow dynamics, and comparisons between the hydraulic parameters. Fanning friction factor and Nusselt number, are also presented. In all cases. isothermal conditions are imposed on the duct walls, and the turbulent heat fluxes are modeled using gradient-diffusion type models. The formulation is valid for Reynolds numbers up to 10(exp 5) and this minimizes the need for wall functions that have been used with mixed success in previous studies of complex duct flows. In addition, the present formulation imposes minimal demand on the number of grid points without any convergence or stability problems. Criteria in terms of heat transfer and friction factor needed to choose the optimal wavy duct cross-section for industrial applications among the ones considered are discussed.
Minder, J. R.; Smith, R. B.; Nugent, A. D.; Kirshbaum, D. J.
2011-12-01
Shallow convection is a pervasive feature of orographic precipitation, but its detailed role remains poorly understood. The mountainous Caribbean island of Dominica is a natural laboratory for isolating the role of shallow convection in orographic rainfall. It lies in a region of persistent easterly trade wind flow, and receives much of its rainfall from shallow convection that is forced mechanically by ascent of easterly flow over the Dominican terrain. The Dominica Experiment (DOMEX) has focused on convective orographic precipitation over the island from 2007-2011. The first phase of the project developed a climatology of rainfall and theories to explain the observed enhancement over the terrain. The second phase of the project (Apr-May 2011) has provided a detailed view of 20 individual rainfall events with data from: surface gauges, time-lapse photography, operational radar scans, a mountaintop weather station, and both in situ and remote observations from the University of Wyoming King Air research aircraft. Focusing on ascent--forced convection during DOMEX has revealed a number of the key processes that control the rainfall. Upwind of the island, clouds and water vapor anomalies exist that appear to play a crucial role in seeding the convection over the terrain and determining its vigor. Over the windward slopes the air is readily lifted with little flow deflection. Strong convective cells rapidly develop to produce large rainfall rates. Over the lee slopes of the terrain there is an abrupt transition to a deep and turbulent plunging flow that quickly eliminates convective clouds, but allows for the spillover of rainfall. The air that passes over the island is transformed such that low-levels are dried, warmed and decelerated, and the downwind wake becomes less hospitable to trade wind cumuli.
Pratt, J; Müller, W -C; Chapman, S C; Watkins, N W
2016-01-01
We investigate the utility of the convex hull to analyze physical questions related to the dispersion of a group of much more than four Lagrangian tracer particles in a turbulent flow. Validation of standard dispersion behaviors is a necessary preliminary step for use of the convex hull to describe turbulent flows. In simulations of statistically homogeneous and stationary Navier-Stokes turbulence, neutral fluid Boussinesq convection, and MHD Boussinesq convection we show that the convex hull can be used to reasonably capture the dispersive behavior of a large group of tracer particles. We validate dispersion results produced with convex hull analysis against scalings for Lagrangian particle pair dispersion. In addition to this basic validation study, we show that convex hull analysis provides information that particle pair dispersion does not, in the form of a extreme value statistics, surface area, and volume for a cluster of particles. We use the convex hull surface area and volume to examine the degree of...
Imbalanced Relativistic Force-Free Magnetohydrodynamic Turbulence
Cho, Jungyeon
2013-01-01
When magnetic energy density is much larger than that of matter, as in pulsar/black hole magnetospheres, the medium becomes force-free and we need relativity to describe it. As in non-relativistic magnetohydrodynamics (MHD), Alfv\\'enic MHD turbulence in the relativistic limit can be described by interactions of counter-traveling wave packets. In this paper we numerically study strong imbalanced MHD turbulence in such environments. Here, imbalanced turbulence means the waves traveling in one direction (dominant waves) have higher amplitudes than the opposite-traveling waves (sub-dominant waves). We find that (1) spectrum of the dominant waves is steeper than that of sub-dominant waves, (2) the anisotropy of the dominant waves is weaker than that of sub-dominant waves, and (3) the dependence of the ratio of magnetic energy densities of dominant and sub-dominant waves on the ratio of energy injection rates is steeper than quadratic (i.e., \\$b_+^2/b_-^2 \\propto (\\epsilon_+/\\epsilon_-)^n \\$ with n>2). These result...
Electromagnetic fields and electrical currents in deep turbulent convective clouds
Benmoshe, Nir; Khain, Alexander
2013-04-01
Charge separation and lightning formation in a thunderstorm is explicitly simulated using spectral bin microphysics the Hebrew University Cloud Model (HUCM) with resolution of 50 m. The model microphysics is based on solving equations for eight size distribution functions for aerosols, drops, three types of ice crystals, aggregates, graupel and hail. Each size distribution is defined on a mass grid containing 43 bins. The model describes the processes of nucleation of cloud particles, diffusion growth, collisions between all types of hydrometeors, differential sedimentation, freezing, melting, breakup of droplets and aggregates, etc' using the equations basing on the first principles, without any parameterization assumptions. Turbulence effects on droplet collisions are taken into account. Charge separation is calculated by collisions between graupel, hail and ice crystals in the presence of liquid water. The charge obtained by particles as a result of collisions depends on the particle size, the temperature, the presence of liquid water, following laboratory results by Takahashi. These charges are transported by convective motions and differential sedimentation depending on mass and type of particles air density. The charges are redistributed between different hydrometeors in course of particle collisions, as well as during freezing, melting and breakup. These charge transformations create time dependent electricity field. The field of electrical potential is determined by solving the Poisson equation. The recursive procedure similar to that developed by Mansell (2002) is used to calculate the lightning path with connects zones where the potential gradients exceeded the breakdown threshold. The electric currents in the clouds are being calculated. The magnetic field near and inside the clouds are shown. The relationship between lightning intensity and cloud microstructure is investigated. It is shown, for instance, that increase in aerosol concentration leads to
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)
Forced Gravity Waves and the Tropospheric Response to Convection
Halliday, Oliver; Parker, Doug; Griffiths, Stephen; Stirling, Alison
2017-04-01
It has been known for some time that gravity waves facilitate atmospheric adjustment to convective heating. Further, convectively forced gravity waves condition the neighbouring atmosphere for the initiation and / or suppression of convection. Despite this, the radiation of gravity waves in macro-scale models (which are typically forced at the grid-scale, by existing parameterization schemes) is not well understood. We present here theoretical and numerical work directed toward improving our understanding of convectively forced gravity wave effects at the mesoscale. Using the linear hydrostatic equations of motion for an incompressible (but non-Boussinesq) fluid with vertically varying buoyancy frequency, we find a radiating solution to prescribed sensible heating. We then interrogate the spatial and temporal sensitivity of the vertical velocity and potential temperature response to different heating functions, considering the remote and near-field forced response both to steady and pulsed heating. We find that the meso-scale tropospheric response to convection is significantly dependent on the upward radiation characteristics of the gravity waves, which are in turn dependent upon the temporal and spatial structure of the source, and stratification of the domain. Moving from a trapped to upwardly-radiating solution there is a 50% reduction in tropospherically averaged vertical velocity, but significant perturbations persist for up to 4 hours in the far-field. Furthermore, we find the tropospheric adjustment to be sensitive to the horizontal length scale of the heating, observing a 20% reduction in vertical velocity when comparing the response from a 10 km to a 100 km heat source. We assess the implications for parameterization of convection in coarse-grained models in the light of these findings and argue that an idealized 'full-physics' nonlinear simulation of deep convection in the MetUM is qualitatively described by the linear solution: departures are quantified
A Laboratory Study of the Turbulent Velocity Characteristics in the Convective Boundary Layer
Institute of Scientific and Technical Information of China (English)
无
2005-01-01
Based on the measurement of the velocity field in the convective boundary layer (CBL) in a convection water tank with the particle image velocimetry (PIV) technique, this paper studies the characteristics of the CBL turbulent velocity in a modified convection tank. The experiment results show that the velocity distribution in the mixed layer clearly possesses the characteristics of the CBL thermals, and the turbulent eddies can be seen obviously. The comparison of the vertical distribution of the turbulent velocity variables indicates that the modeling in the new tank is better than in the old one. The experiment data show that the thermal's motion in the entrainment zone sometimes fluctuates obviously due to the intermittence of turbulence. Analyses show that this fluctuation can influence the agreement of the measurement data with the parameterization scheme, in which the convective Richardson number is used to characterize the entrainment zone depth. The normalized square velocity w2i/w2* at the top of the mixed layer seems to be time-dependent, and has a decreasing trend during the experiments. This implies that the vertical turbulent velocity at the top of the mixed layer may not be proportional to the convective velocity (w*).
Large-scale instabilities in a non-rotating turbulent convection
Elperin, T; Kleeorin, N; Rogachevskii, I
2006-01-01
Formation of large-scale coherent structures in a turbulent convection via excitation of large-scale instability is studied. The redistribution of the turbulent heat flux due to non-uniform large-scale motions plays a crucial role in the formation of the coherent large-scale structures in the turbulent convection. The modification of the turbulent heat flux results in strong reduction of the critical Rayleigh number (based on the eddy viscosity and turbulent temperature diffusivity) required for the excitation of the large-scale instability. The mean-field equations which describe the large-scale instability, are solved numerically. We determine the key parameters that affect formation of the large-scale coherent structures in the turbulent convection. In particular, the degree of thermal anisotropy and the lateral background heat flux strongly modify the growth rates of the large-scale instability, the frequencies of the generated convective-shear waves and change the thresholds required for the excitation o...
Institute of Scientific and Technical Information of China (English)
Xiang-Jun Lai; Yan Li
2011-01-01
Based on the turbulent convection model (TCM),we investigate chemical mixing in the bottom overshooting region of the convective envelope of intermediatemass stars,focusing on its influence on the formation and extension of blue loops in the Hertzsprung-Russell (HR) diagram.A diffusive mixing model is adopted during the Red Giant Branch (RGB) phase.The properties of the blue loop are changed by modification of the element profiles above the H-burning shell,which results from the incomplete mixing in the bottom overshooting region when the stellar model evolves up along the RGB.Such modification of the element profiles will lead to an increase of opacity in the region just above the H-burning shell and a decrease of opacity in the outer homogeneous convection zone,which will result in a quick decrease of the H-shell nuclear luminosity LH when the stellar model evolves from the RGB tip to its bottom and,finally,a much weaker and smaller convection zone will be obtained in the stellar envelope.This helps to form a longer blue loop.The extension of the blue loop is very sensitive to the parameters (Cx and αTCM ) of the diffusive mixing model and of the TCM.The results mainly show that:1) comparing the results of the classical model with the mixing-length theory,the lengths of the obtained blue loops with different combinations of the values of Cx and αTCM are all increased and the length of the blue loop increases with the values of parameters CX and αTCM; 2) the diffusive mixing model can significantly extend the time of stellar models lingering on the blue side of the HR diagram,even though the length of the blue loop for the 7M(O) star has a less prominent difference between the classical and diffusive mixing model;3) both the observations referring to the location of the Cepheid instability strip and the number ratio NB/NR of blue to red evolved stars in the Galactic open clusters can confine the two parameters in a range of 0.5 ≤ αTCM ≤ 0.9 and 10-5 ≤ CX
Effects of very high turbulence on convective heat transfer
Moffat, R. J.; Maciejewski, P. K.
1984-01-01
The effects of high-intensity, large-scale turbulence on turbulent boundary-layer heat transfer are studied. Flow fields were produced with turbulence intensities up to 40% and length scales up to several times the boundary layer thickness. In addition, three different types of turbulence will be compared to see whether they have the same effect on the boundary layer. The three are: the far field of a free jet, flow downstream of a grid, and flow downstream of a simulated gas turbine combustor. Each turbulence field will be characterized by several measures: intensity (by component), scale, and spectrum. Heat transfer will be measured on a 2.5 m long, 0.5 m wide flat plate using the energy-balance technique. The same plate will be used in each of the four flow fields; a low-turbulence tunnel for baseline data, and the three flow situations mentioned.
Moist turbulent Rayleigh-Benard convection with Neumann and Dirichlet boundary conditions
Weidauer, Thomas
2012-01-01
Turbulent Rayleigh-Benard convection with phase changes in an extended layer between two parallel impermeable planes is studied by means of three-dimensional direct numerical simulations for Rayleigh numbers between 10^4 and 1.5\\times 10^7 and for Prandtl number Pr=0.7. Two different sets of boundary conditions of temperature and total water content are compared: imposed constant amplitudes which translate into Dirichlet boundary conditions for the scalar field fluctuations about the quiescent diffusive equilibrium and constant imposed flux boundary conditions that result in Neumann boundary conditions. Moist turbulent convection is in the conditionally unstable regime throughout this study for which unsaturated air parcels are stably and saturated air parcels unstably stratified. A direct comparison of both sets of boundary conditions with the same parameters requires to start the turbulence simulations out of differently saturated equilibrium states. Similar to dry Rayleigh-Benard convection the differences...
Directory of Open Access Journals (Sweden)
M Safaei
2016-09-01
Full Text Available In the present study, first the turbulent natural convection and then laminar mixed convection of air flow was solved in a room and the calculated outcomes are compared with results of other scientists and after showing validation of calculations, aforementioned flow is solved as a turbulent mixed convection flow, using the valid turbulence models Standard k-ε, RNG k-ε and RSM. To solve governing differential equations for this flow, finite volume method was used. This method is a specific case of residual weighting method. The results show that at high Richardson Numbers, the flow is rather stationary at the center of the enclosure. Moreover, it is distinguished that when Richardson Number increases the maximum of local Nusselt decreases. Therefore, it can be said that less number of Richardson Number, more rate of heat transfer.
Pollack, David; Gueremy, Jean-Francois; Beau, Isabelle
2010-05-01
The aim of this work is to analyse the behaviour of turbulence and convective parameterizations included in the Météo-France ALADIN-CLIMAT Limited Area Model in the frame of a 48 hour simulation of an AMMA case study, in comparison to observations and to a CRM (Méso-NH, with a 5 km horizontal grid-mesh) simulation carried out under the same boundary forcings. This framework provides an intermediate step of parameterization validation between the Single Column Model and Global Climate Model simulation studies. The chosen case study is the 26-27th July 2006 over a 43° x 40° region centred over Burkina Faso, in continuation to a previous work done with a HAPEX-Sahel case-study. During this 2 day period, two successive mesoscale convective systems are located ahead and in phase with the trough of an African easterly Wave (AEW). Both LAM and CRM simulations have been performed over the same considered domain, using the same ECMWF boundary forcings. Sensitivity tests to resolution (both horizontal and vertical) have been first carried out with ALADIN-CLIMAT. Second, the two different convection schemes used in ALADIN-CLIMAT show two kinds of response mainly due to their different formulations of triggering (no constraints in the dry layer under the convective cloud versus continuous treatment of convection including this dry layer) and closure (moisture convergence versus CAPE). Third, the impact of convective downdrafts will be shown. Fourth, the impact of different boundary forcing fields will also be presented.
Pollack, D.; Gueremy, J. F.; Beau, I.
2009-04-01
The aim of this work is to analyse the behaviour of turbulence and convective parameterizations included in the Météo-France ALADIN-CLIMAT Limited Area Model in the frame of a 24 hour simulation of a HAPEX-Sahel case study, in comparison to observations and to a CRM (Méso-NH, with a 5 km horizontal grid-mesh) simulation carried out under the same boundary forcings. This framework provides an intermediate step of parameterization validation between the Single Column Model and Global Climate Model simulation studies. The chosen case study is the 21st August 1992 over a 12° x 12° region centred over SW Niger. It is characterised by the life cycle of a westward propagating convective system associated to an African Easterly Wave. Both LAM and CRM simulations have been performed over the same considered domain, using the same ERA40 boundary forcings. Sensitivity tests to resolution (both horizontal and vertical) have been first carried out with ALADIN-CLIMAT. Second, the two different convection schemes used in ALADIN-CLIMAT show two kinds of response mainly due to their different formulations of triggering (no constraints in the dry layer under the convective cloud versus continuous treatment of convection including this dry layer) and closure (moisture convergence versus CAPE). Third, the impact of entrainment at the top of the PBL (included in the turbulence scheme) and of convective downdrafts will be shown. Fourth, the impact of different boundary forcing fields will also be presented.
Rosenblum, Erica; Traxler, Adrienne; Stellmach, Stephan
2010-01-01
Double-diffusive convection, often referred to as semi-convection in astrophysics, occurs in thermally and compositionally stratified systems which are stable according to the Ledoux-criterion but unstable according to the Schwarzchild criterion. This process has been given relatively little attention so far, and its properties remain poorly constrained. In this paper, we present and analyze a set of three-dimensional simulations of this phenomenon in a Cartesian domain under the Boussinesq approximation. We find that in some cases the double-diffusive convection saturates into a state of homogeneous turbulence, but with turbulent fluxes several orders of magnitude smaller than those expected from direct overturning convection. In other cases the system rapidly and spontaneously develops closely-packed thermo-compositional layers, which later successively merge until a single layer is left. We compare the output of our simulations with an existing theory of layer formation in the oceanographic context, and fi...
Joule-Thomson effect and internal convection heat transfer in turbulent He II flow
Walstrom, P. L.
1988-01-01
The temperature rise in highly turbulent He II flowing in tubing was measured in the temperature range 1.6-2.1 K. The effect of internal convection heat transport on the predicted temperature profiles is calculated from the two-fluid model with mutual friction. The model predictions are in good agreement with the measurements, provided that the pressure gradient term is retained in the expression for internal convection heat flow.
Joule-Thomson effect and internal convection heat transfer in turbulent He II flow
Walstrom, P. L.
1988-03-01
The temperature rise in highly turbulent He II flowing in tubing was measured in the temperature range 1.6-2.1 K. The effect of internal convection heat transport on the predicted temperature profiles is calculated from the two-fluid model with mutual friction. The model predictions are in good agreement with the measurements, provided that the pressure gradient term is retained in the expression for internal convection heat flow.
Global and local statistics in turbulent convection at low Prandtl numbers
Scheel, Janet D
2016-01-01
Statistical properties of turbulent Rayleigh-Benard convection at low Prandtl numbers (Pr), which are typical for liquid metals such as mercury, gallium or liquid sodium, are investigated in high-resolution three-dimensional spectral element simulations in a closed cylindrical cell with an aspect ratio of one and are compared to previous turbulent convection simulations in air. We compare the scaling of global momentum and heat transfer. The scaling exponents are found to be in agreement with experiments. Mean profiles of the root-mean-square velocity as well as the thermal and kinetic energy dissipation rates have growing amplitudes with decreasing Prandtl number which underlies a more vigorous bulk turbulence in the low-Pr regime. The skin-friction coefficient displays a Reynolds-number dependence that is close to that of an isothermal, intermittently turbulent velocity boundary layer. The thermal boundary layer thicknesses are larger as Pr decreases and conversely the velocity boundary layer thicknesses be...
Nonlocal model for the turbulent fluxes due to thermal convection in rectilinear shearing flow
Smolec, R; Gough, D O
2011-01-01
We revisit a phenomenological description of turbulent thermal convection along the lines proposed by Gough (1977) in which eddies grow solely by extracting energy from the unstably stratified mean state and are subsequently destroyed by internal shear instability. This work is part of an ongoing investigation for finding a procedure to calculate the turbulent fluxes of heat and momentum in the presence of a shearing background flow in stars.
Modelling turbulent fluxes due to thermal convection in rectilinear shearing flow
Smolec, R; Gough, D O
2010-01-01
We revisit a phenomenological description of turbulent thermal convection along the lines proposed originally by Gough (1965) in which eddies grow solely by extracting energy from the unstably stratified mean state and are subsequently destroyed by internal shear instability. This work is part of an ongoing investigation for finding a procedure to calculate the turbulent fluxes of heat and momentum in the presence of a shearing background flow in stars.
NANOFLUID PROPERTIES FOR FORCED CONVECTION HEAT TRANSFER: AN OVERVIEW
Directory of Open Access Journals (Sweden)
W.H.Azmi
2013-06-01
Full Text Available Nanoﬂuids offer a significant advantage over conventional heat transfer ﬂuids and consequently, they have attracted much attention in recent years. The engineered suspension of nano-sized particles in a base liquid alters the properties of these nanofluids. Many researchers have measured and modeled the thermal conductivity and viscosity of nanofluids. The estimation of forced convective heat transfer coefficients is done through experiments with either metal or nonmetal solid particles dispersed in water. Regression equations are developed for the determination of the thermal conductivity and viscosity of nanofluids. The parameters influencing the decrease in convection heat transfer, observed by certain investigators, is explained.
Wen, Mao-Yu; Yeh, Cheng-Hsiung
2016-12-01
This paper presents a numerical simulation of the heat transfer performance under forced convection for two different types of circular pin fin heat sinks with (Type A) and without (Type B) a hollow in the heated base. COMSOL Multiphysics, which is used for the thermal hydraulic analyses, has proven to be a powerful finite-element-based simulation tool for solving multiple physics-based systems of partial and ordinary differential equations. The standard κ- ɛ two-equations turbulence model is employed to describe the turbulent structure and behavior. The numerical results are validated with the experimental results, and are shown to be in good agreement. The effects of the Reynolds number, height of the fin, finning factor and the perforated base plate on the heat-transfer coefficient are investigated and evaluated. The present study strongly recommends the use of a small hollow ( (Dh /Db ) sink.
Wen, Mao-Yu; Yeh, Cheng-Hsiung
2017-06-01
This paper presents a numerical simulation of the heat transfer performance under forced convection for two different types of circular pin fin heat sinks with (Type A) and without (Type B) a hollow in the heated base. COMSOL Multiphysics, which is used for the thermal hydraulic analyses, has proven to be a powerful finite-element-based simulation tool for solving multiple physics-based systems of partial and ordinary differential equations. The standard κ - \\varepsilon two-equations turbulence model is employed to describe the turbulent structure and behavior. The numerical results are validated with the experimental results, and are shown to be in good agreement. The effects of the Reynolds number, height of the fin, finning factor and the perforated base plate on the heat-transfer coefficient are investigated and evaluated. The present study strongly recommends the use of a small hollow ( (Dh /Db ) heat sink.
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
Convective Heat Transfer Analysis in Fluid Flow with Turbulence Promoters with Heat Pipes
Directory of Open Access Journals (Sweden)
Theodor Mateescu
2007-01-01
Full Text Available The present paper proposes the analysis and the simulation of the convection heat transfer into the fluid flow with turbulence promoters utilizing heat pipes. The study is based on the necesity of the unconventional energy forms capitalization, increasing of the energy efficiency and leads to the energy consumtion decrease in concordance with the sustainable development concept.
Effects of shear in the convective boundary layer: analysis of the turbulent kinetic energy budget
Pino, D.; Vilà-Guerau de Arellano, J.
2008-01-01
Effects of convective and mechanical turbulence at the entrainment zone are studied through the use of systematic Large-Eddy Simulation (LES) experiments. Five LES experiments with different shear characteristics in the quasi-steady barotropic boundary layer were conducted by increasing the value of
Prime modes of fluid circulation in large-aspect-ratio turbulent Rayleigh-Bénard convection
Verdoold, J.; Tummers, M.J.; Hanjalić, K.
2009-01-01
Based on a detailed experimental investigation in an aspect-ratio-4 rectangular cell in the range 3.7×107≤Ra≤3.7×109, we present evidence of possible scenarios of the long-term dynamics of large-scale circulations (LSC) in bounded large-aspect-ratio turbulent Rayleigh-Bénard convection. Karhunen-Loè
Experimental techniques for turbulent Taylor–Couette flow and Rayleigh–Bénard convection
Sun, Chao; Zhou, Quan
2014-01-01
Taylor–Couette (TC) flow and Rayleigh–B´enard (RB) convection are two systems in hydrodynamics, which have been widely used to investigate the primary instabilities, pattern formation, and transitions from laminar to turbulent flow. These two systems are known to have an elegant mathematical similar
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...
Heat transfer and large scale dynamics in turbulent Rayleigh-Bénard convection
Ahlers, Guenter; 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 kinet
Scalings of field correlations and heat transport in turbulent convection.
Verma, Mahendra K; Mishra, Pankaj K; Pandey, Ambrish; Paul, Supriyo
2012-01-01
Using direct numerical simulations of Rayleigh-Bénard convection under free-slip boundary condition, we show that the normalized correlation function between the vertical velocity field and the temperature field, as well as the normalized viscous dissipation rate, scales as Ra-0.22 for moderately large Rayleigh number Ra. This scaling accounts for the Nusselt number Nu exponent of approximately 0.3, as observed in experiments. Numerical simulations also reveal that the aforementioned normalized correlation functions are constants for the convection simulation under periodic boundary conditions.
Plates of the dinosaur stegosaurus: forced convection heat loss fins?
Farlow, J O; Thompson, C V; Rosner, D E
1976-06-11
It is suggested that the plates along the arched back and tail of Stegosaurus served an important thermoregulatory function as forced convection "fins." Wind tunnel experiments on finned models, internal heat conduction calculations, and direct observations of the morphology and internal structure of stegosaur plates support this hypothesis, demonstrating the comparative effectiveness of the plates as heat dissipaters, controllable through input blood flow rate, temperature, and body orientation (with respect to wind).
Sudden relaminarisation and lifetimes in forced isotropic turbulence
Linkmann, Moritz
2015-01-01
We demonstrate an unexpected connection between isotropic turbulence and wall-bounded shear flows. We perform direct numerical simulations of isotropic turbulence forced at large scales at moderate Reynolds numbers and observe sudden transitions from chaotic dynamics to a spatially simple flow, analogous to the laminar state in wall-bounded shear flows. We find that the survival probabilities of turbulence are exponential and the typical lifetimes increase super-exponentially with the Reynolds number. Our results suggest that both isotropic turbulence and wall-bounded shear flows share the same phase-space dynamics.
From convection rolls to finger convection in double-diffusive turbulence
Yang, Yantao; Lohse, Detlef
2015-01-01
Double diffusive convection (DDC), which is the buoyancy driven flow with fluid density depending on two scalar components, is ubiquitous in many natural and engineering enviroments. Of great interests are scalers transfer rate and flow structures. Here we systematically investigate DDC flow between two horizontal plates, driven by an unstable salinity gradient and stabilized by a temperature gradient. Counterintuitively, when increasing the stabilizing temperature gradient, the salinity flux first increases, even though the velocity monotonically decreases, before it finally breaks down to the purely diffusive value. The enhanced salinity transport is traced back to a transition in the overall flow pattern, namely from large scale convection rolls to well-organised vertically-oriented salt fingers. We also show and explain that the unifying theory of thermal convection originally developed by Grossmann and Lohse for Rayleigh-B\\'{e}nard convection can be directly applied to DDC flow for a wide range of contro...
Dynamo action in dissipative, forced, rotating MHD turbulence
Shebalin, John V.
2016-06-01
Magnetohydrodynamic (MHD) turbulence is an inherent feature of large-scale, energetic astrophysical and geophysical magnetofluids. In general, these are rotating and are energized through buoyancy and shear, while viscosity and resistivity provide a means of dissipation of kinetic and magnetic energy. Studies of unforced, rotating, ideal (i.e., non-dissipative) MHD turbulence have produced interesting results, but it is important to determine how these results are affected by dissipation and forcing. Here, we extend our previous work and examine dissipative, forced, and rotating MHD turbulence. Incompressibility is assumed, and finite Fourier series represent turbulent velocity and magnetic field on a 643 grid. Forcing occurs at an intermediate wave number by a method that keeps total energy relatively constant and allows for injection of kinetic and magnetic helicity. We find that 3-D energy spectra are asymmetric when forcing is present. We also find that dynamo action occurs when forcing has either kinetic or magnetic helicity, with magnetic helicity injection being more important. In forced, dissipative MHD turbulence, the dynamo manifests itself as a large-scale coherent structure that is similar to that seen in the ideal case. These results imply that MHD turbulence, per se, may play a fundamental role in the creation and maintenance of large-scale (i.e., dipolar) stellar and planetary magnetic fields.
Effects of anisotropy of turbulent convection in mean-field solar dynamo models
Pipin, V V
2013-01-01
We study how anisotropy of turbulent convection affects diffusion of large-scale magnetic fields and the dynamo process on the Sun. The effect of anisotropy is calculated in a mean-field magneto-hydrodynamics framework using the minimal $\\tau$-approximation. We examine two types of mean-field dynamo models: the well-known benchmark flux-transport model, and a distributed-dynamo model with the subsurface rotational shear layer. For both models we investigate effects of the double-cell meridional circulation, recently suggested by helioseismology. We introduce a parameter of anisotropy as a ratio of the radial and horizontal intensity of turbulent mixing, to characterize the anisotropy effects. It is found that the anisotropy of turbulent convection affects the distribution of magnetic fields inside the convection zone. The concentration of the magnetic flux near the bottom and top boundaries of the convection zone is greater when the anisotropy is stronger. It is shown that the critical dynamo number and the d...
Multiscale Turbulence Models Based on Convected Fluid Microstructure
Holm, Darryl D
2012-01-01
The Euler-Poincar\\'e approach to complex fluids is used to derive multiscale equations for computationally modelling Euler flows as a basis for modelling turbulence. The model is based on a \\emph{kinematic sweeping ansatz} (KSA) which assumes that the mean fluid flow serves as a Lagrangian frame of motion for the fluctuation dynamics. Thus, we regard the motion of a fluid parcel on the computationally resolvable length scales as a moving Lagrange coordinate for the fluctuating (zero-mean) motion of fluid parcels at the unresolved scales. Even in the simplest 2-scale version on which we concentrate here, the contributions of the fluctuating motion under the KSA to the mean motion yields a system of equations that extends known results and appears to be suitable for modelling nonlinear backscatter (energy transfer from smaller to larger scales) in turbulence using multiscale methods.
Transition to turbulence in strongly heated vertical natural convection boundary layers
De Larochelambert, Thierry
2008-01-01
The mechanisms governing the transition to turbulence in natural convection boundary layers along strongly heated vertical walls remain neither very clear nor well understood, because of the lack of experiments and the difficulties of physical modelling. Our measurements bring experimental data focusing on this transition in quiescent air along radiating and conducting plates in the whole range of 2000 to 8000 W/m\\^2 heating rate. The analysis of the time series obtained by sliding window cross-correlation thermoanemometry leads us to point out coherent turbulent structures on short heights throughout the thin boundary layer, which seem to be governed by heat transfer and time-microscales of turbulence through the inner sublayer. Physical interpretations are given to relate to the observed heat transfer correlation and these turbulence transition structures along with radiation and conduction.
Estimation of turbulent diffusivity with direct numerical simulation of stellar convection
Hotta, H; Yokoyama, T
2012-01-01
We investigate the value of horizontal turbulent diffusivity {\\eta} by numerical calculation of thermal convection. In this study, we introduce a new method whereby the turbulent diffusivity is estimated by monitoring the time devel- opment of the passive scalar, which is initially distributed in a given Gaussian function with a spatial scale d0. Our conclusions are as follows: (1) Assuming the relation {\\eta} = Lcvrms/3 where vrms is the RMS velocity, the characteristic length Lc is restricted by the shortest one among the pressure (density) scale height and the region depth. (2) The value of turbulent diffusivity becomes greater with the larger initial distribution scale d0. (3) The approximation of turbulent diffusion holds better when the ratio of the initial distribution scale d0 to the characteristic length Lc is larger.
The Application of Statistical Turbulence Theory to Convective Instabilities.
1986-11-01
1985]. Here, we shall discuss the applications of statistical turbulence theory to determine the quasi-steady-state spectral density function (SDF...follow Sudan’s prescription for deriving the form of one- dimensional spectral - density function . The main difference is that we have attempted to relax...the conservation property of the basic equations is ’V preserved in the DIA or WCA equations for the spectral density function itself. This property is
Various aspects of magnetic field influence on forced convection
Directory of Open Access Journals (Sweden)
Pleskacz Lukasz
2016-01-01
Full Text Available Flows in the channels of various geometry can be found everywhere in industrial or daily life applications. They are used to deliver media to certain locations or they are the place where heat may be exchanged. For Authors both points of view are interesting. The enhancement methods for heat transfer during the forced convection are demanded due to a technological development and tendency to miniaturization. At the same time it is also worth to find mechanisms that would help to avoid negative effects like pressure losses or sedimentation in the channel flows. This paper shows and discuss various aspects of magnetic field influence on forced convection. A mathematical model consisted of the mass, momentum and energy conservation equations. In the momentum conservation equation magnetic force term was included. In order to calculate this magnetic force Biot-Savart’s law was utilized. Numerical analysis was performed with the usage of commonly applied software. However, userdefined functions were implemented. The results revealed that both temperature and velocity fields were influenced by the strong magnetic field.
Energy Technology Data Exchange (ETDEWEB)
Castillo, Victor Manuel [Univ. of California, Davis, CA (United States)
1999-01-01
A collocation method using cubic splines is developed and applied to simulate steady and time-dependent, including turbulent, thermally convecting flows for two-dimensional compressible fluids. The state variables and the fluxes of the conserved quantities are approximated by cubic splines in both space direction. This method is shown to be numerically conservative and to have a local truncation error proportional to the fourth power of the grid spacing. A ''dual-staggered'' Cartesian grid, where energy and momentum are updated on one grid and mass density on the other, is used to discretize the flux form of the compressible Navier-Stokes equations. Each grid-line is staggered so that the fluxes, in each direction, are calculated at the grid midpoints. This numerical method is validated by simulating thermally convecting flows, from steady to turbulent, reproducing known results. Once validated, the method is used to investigate many aspects of thermal convection with high numerical accuracy. Simulations demonstrate that multiple steady solutions can coexist at the same Rayleigh number for compressible convection. As a system is driven further from equilibrium, a drop in the time-averaged dimensionless heat flux (and the dimensionless internal entropy production rate) occurs at the transition from laminar-periodic to chaotic flow. This observation is consistent with experiments of real convecting fluids. Near this transition, both harmonic and chaotic solutions may exist for the same Rayleigh number. The chaotic flow loses phase-space information at a greater rate, while the periodic flow transports heat (produces entropy) more effectively. A linear sum of the dimensionless forms of these rates connects the two flow morphologies over the entire range for which they coexist. For simulations of systems with higher Rayleigh numbers, a scaling relation exists relating the dimensionless heat flux to the two-seventh's power of the Rayleigh number
Energy Technology Data Exchange (ETDEWEB)
Castillo, V M
2005-01-12
A collocation method using cubic splines is developed and applied to simulate steady and time-dependent, including turbulent, thermally convecting flows for two-dimensional compressible fluids. The state variables and the fluxes of the conserved quantities are approximated by cubic splines in both space direction. This method is shown to be numerically conservative and to have a local truncation error proportional to the fourth power of the grid spacing. A ''dual-staggered'' Cartesian grid, where energy and momentum are updated on one grid and mass density on the other, is used to discretize the flux form of the compressible Navier-Stokes equations. Each grid-line is staggered so that the fluxes, in each direction, are calculated at the grid midpoints. This numerical method is validated by simulating thermally convecting flows, from steady to turbulent, reproducing known results. Once validated, the method is used to investigate many aspects of thermal convection with high numerical accuracy. Simulations demonstrate that multiple steady solutions can coexist at the same Rayleigh number for compressible convection. As a system is driven further from equilibrium, a drop in the time-averaged dimensionless heat flux (and the dimensionless internal entropy production rate) occurs at the transition from laminar-periodic to chaotic flow. This observation is consistent with experiments of real convecting fluids. Near this transition, both harmonic and chaotic solutions may exist for the same Rayleigh number. The chaotic flow loses phase-space information at a greater rate, while the periodic flow transports heat (produces entropy) more effectively. A linear sum of the dimensionless forms of these rates connects the two flow morphologies over the entire range for which they coexist. For simulations of systems with higher Rayleigh numbers, a scaling relation exists relating the dimensionless heat flux to the two-seventh's power of the Rayleigh number
S-process nucleosynthesis in AGB stars with the full spectrum of turbulence scheme for convection.
Yagüe, A.; García-Hernández, D. A.; Ventura, P.; Lugaro, M.
2016-07-01
The chemical evolution of asymptotic giant branch (AGB) stars models depends greatly on the input physics (e.g. convective model, mass loss recipe). Variations of hot bottom burning (HBB) strength, or third dredge-up (TDU) efficiency are among the main consequences of adopting different input physics in the AGB models. The ATON evolutionary code stands apart from others in that it uses the Full Spectrum of Turbulence convective model. Here we present the first results of a newly developed s-process nucleosynthesis module for ATON AGB models. Our results are compared also with observations and theoretical predictions of present AGB nucleosynthesis models using different input physics.
Spiral plume structures in turbulent natural convection between two vertical walls
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
By means of direct numerical simulation, coherent structures are investigated in turbulent natural convection between two vertical differentially heated walls. It is observed that large-scale spanwise vortices and spiral plume structures exist together in the flow. Spiral plume structures appear at the positions with relatively large helicity, large normal vorticity and high fluctuating temperature. In this note, the shape, the characteristics and formation of the spiral structures are studied and compared with those in Rayleigh-Bénard convection. The conditional sampling analysis indicates the main properties of the spiral structures.
Freche, John C; Schum, Eugene F
1951-01-01
Blade-to-coolant convective heat-transfer coefficients were obtained on a forced-convection water-cooled single-stage turbine over a large laminar flow range and over a portion of the transition range between laminar and turbulent flow. The convective coefficients were correlated by the general relation for forced-convection heat transfer with laminar flow. Natural-convection heat transfer was negligible for this turbine over the Grashof number range investigated. Comparison of turbine data with stationary tube data for the laminar flow of heated liquids showed good agreement. Calculated average midspan blade temperatures using theoretical gas-to-blade coefficients and blade-to-coolant coefficients from stationary-tube data resulted in close agreement with experimental data.
The spectrum of a turbulent passive scalar in the viscous-convective range
Qian, J.
1990-08-01
The closed equations of isotropic turbulence, obtained by the method of nonequilibrium statistical mechanics and a perturbation-variation approach (Qian 1983, 1985, 1988), are applied to the study of the spectrum dynamics of a turbulent passive scalar in the viscous-convective range. Batchelor's 1/k spectrum is further confirmed. Moreover, the effective average value of the least principal rate of strain gamma in Batchelor's spectrum function is theoretically evaluated, and the results are in agreement with experimental data reported by Grant et al. (1968) and Williams and Paulson (1977).
Angioni, C.; Bilato, R.; Casson, F. J.; Fable, E.; Mantica, P.; Odstrcil, T.; Valisa, M.; ASDEX Upgrade Team; Contributors, JET
2017-02-01
In tokamaks, the role of turbulent transport of heavy impurities, relative to that of neoclassical transport, increases with increasing size of the plasma, as clarified by means of general scalings, which use the ITER standard scenario parameters as reference, and by actual results from a selection of discharges from ASDEX Upgrade and JET. This motivates the theoretical investigation of the properties of the turbulent convection of heavy impurities by nonlinear gyrokinetic simulations in the experimentally relevant conditions of comparable ion and electron heat fluxes. These conditions also correspond to an intermediate regime between dominant ion temperature gradient turbulence and trapped electron mode turbulence. At moderate plasma toroidal rotation, the turbulent convection of heavy impurities, computed with nonlinear gyrokinetic simulations, is found to be directed outward, in contrast to that obtained by quasi-linear calculations based on the most unstable linear mode, which is directed inward. In this mixed turbulence regime, with comparable electron and ion heat fluxes, the nonlinear results of the impurity transport can be explained by the coexistence of both ion temperature gradient and trapped electron modes in the turbulent state, both contributing to the turbulent convection and diffusion of the impurity. The impact of toroidal rotation on the turbulent convection is also clarified.
Thermal interaction between free convection and forced convection along a vertical conducting wall
Shu, Jian-Jun
2015-01-01
A theoretical study is presented in this paper to investigate the conjugate heat transfer across a vertical finite wall separating two forced and free convection flows at different temperatures. The heat conduction in the wall is in the transversal direction and countercurrent boundary layers are formed on the both sides of the wall. The governing equations of this problem and their corresponding boundary conditions are all cast into a dimensionless form by using a non-similarity transformation. These resultant equations with multiple singular points are solved numerically using a very efficient singular perturbation method. The effects of the resistance parameters and Prandtl numbers on heat transfer characteristics are investigated.
Sudden relaminarisation and lifetimes in forced isotropic turbulence
Linkmann, Moritz; Morozov, Alexander
2015-11-01
We demonstrate an unexpected connection between isotropic turbulence and wall-bounded shear flows. We perform direct numerical simulations of isotropic turbulence forced at large scales at moderate Reynolds numbers and observe sudden transitions from chaotic dynamics to a spatially simple flow, analogous to the laminar state in wall bounded shear flows. We find that the survival probabilities of turbulence are exponential and the typical lifetimes increase super-exponentially with the Reynolds number, similar to results on relaminarisation of localised turbulence in pipe and plane Couette flow. Results from simulations subjecting the observed large-scale flow to random perturbations of variable amplitude demonstrate that it is a linearly stable simple exact solution that can be destabilised by a finite-amplitude perturbation, like the Hagen-Poiseuille profile in pipe flow. Our results suggest that both isotropic turbulence and wall-bounded shear flows qualitatively share the same phase-space dynamics.
Convection Heat Transfer in Three-Dimensional Turbulent Separated/Reattached Flow
Energy Technology Data Exchange (ETDEWEB)
Bassem F. Armaly
2007-10-31
The measurements and the simulation of convective heat transfer in separated flow have been a challenge to researchers for many years. Measurements have been limited to two-dimensional flow and simulations failed to predict accurately turbulent heat transfer in the separated and reattached flow region (prediction are higher than measurements by more than 50%). A coordinated experimental and numerical effort has been initiated under this grant for examining the momentum and thermal transport in three-dimensional separated and reattached flow in an effort to provide new measurements that can be used for benchmarking and for improving the simulation capabilities of 3-D convection in separated/reattached flow regime. High-resolution and non-invasive measurements techniques are developed and employed in this study to quantify the magnitude and the behavior of the three velocity components and the resulting convective heat transfer. In addition, simulation capabilities are developed and employed for improving the simulation of 3-D convective separated/reattached flow. Such basic measurements and simulation capabilities are needed for improving the design and performance evaluation of complex (3-D) heat exchanging equipment. Three-dimensional (3-D) convective air flow adjacent to backward-facing step in rectangular channel is selected for the experimental component of this study. This geometry is simple but it exhibits all the complexities that appear in any other separated/reattached flow, thus making the results generated in this study applicable to any other separated and reattached flow. Boundary conditions, inflow, outflow, and wall thermal treatment in this geometry can be well measured and controlled. The geometry can be constructed with optical access for non-intrusive measurements of the flow and thermal fields. A three-component laser Doppler velocimeter (LDV) is employed to measure simultaneously the three-velocity components and their turbulent fluctuations
Effects of Radiative Diffusion on Thin Flux Tubes in Turbulent Solar-like Convection
Weber, Maria A
2015-01-01
We study the combined effects of convection and radiative diffusion on the evolution of thin magnetic flux tubes in the solar interior. Radiative diffusion is the primary supplier of heat to convective motions in the lower convection zone, and it results in a heat input per unit volume of magnetic flux tubes that has been ignored by many previous thin flux tube studies. We use a thin flux tube model subject to convection taken from a rotating spherical shell of turbulent, solar-like convection as described by Weber, Fan, and Miesch (2011, Astrophys. J., 741, 11; 2013, Solar Phys., 287, 239), now taking into account the influence of radiative heating on flux tubes of large-scale active regions. Our simulations show that flux tubes of less than or equal to 60 kG subject to solar-like convective flows do not anchor in the overshoot region, but rather drift upward due to the increased buoyancy of the flux tube earlier in its evolution as a result of the inclusion of radiative diffusion. Flux tubes of magnetic fie...
Full spectrum of turbulence convective mixing. II. Lithium production in AGB stars
Mazzitelli, I.; D'Antona, F.; Ventura, P.
1999-08-01
We present results from new, detailed computations of lithium production by hot bottom burning (HBB) in asymptotic giant branch (AGB) stars of intermediate mass (3.5 mass, metallicity, mass loss rate, convection and overshooting are discussed. In particular, nuclear burning, turbulent mixing and convective overshooting (if any) are self-consistently coupled by a diffusive algorithm, and the Full Spectrum of Turbulence (FST) model of convection is adopted, with test comparisons to Mixing Length Theory (MLT) stellar models. All the evolutions are followed from pre-main sequence down to late AGB, when stars do not appear any longer lithium rich. A ``reference mass" of 6 M_sun has been chosen since, although relatively close to the upper limit for which degenerate (12) C ignition occurs, all the studied mechanisms show up more clearly. HBB is always found above ~ log L/L_sun = 4.4, but the range of (initial) masses reaching HBB is largely dependent on convection model, overshooting and metallicity. For solar chemistry, masses >= 4M_sun evolve through HBB in the FST case and including core overshooting whereas, with solarly tuned MLT models and no overshooting, only masses >= 6M_sun can reach HBB. These constraints can give feedbacks about the more correct convection model and/or the extent of overshooting, thanks to the signatures of HBB in AGB stars in clusters of known turnoff masses and metallicity. Overshooting (when included) is addressed as an exponentially decreasing diffusion above formally convective regions. It makes convective cores during the main sequence to grow larger, and also starting masses and luminosities in AGB are then larger. However, also preliminary results obtained when allowing displacement of convective elements below convective regions in AGB are shown. In the ``reference" case (6M_sun), we find that overshooting from below the convective envelope totally suppresses thermal pulses and ultimately leads to the formation of massive ( ~ 1M
The impact of rotation on turbulent tidal friction in stellar and planetary convective regions
Mathis, Stéphane; Guenel, Mathieu; Gallet, Florian; Poncin-Lafitte, Christophe Le
2016-01-01
Turbulent friction in convective regions in stars and planets is one of the key physical mechanisms that drive the dissipation of the kinetic energy of tidal flows in their interiors and the evolution of their systems. This friction acts both on the equilibrium/non-wave like tide and on tidal inertial waves in these layers. It is thus necessary to obtain a robust prescription for this friction. In the current state-of-the-art, it is modeled by a turbulent eddy-viscosity coefficient, based on mixing-length theory, applied on velocities of tides. However, none of the current prescriptions take into account the action of rotation that can strongly affects turbulent convection. Therefore, we use theoretical scaling laws for convective velocities and characteristic lengthscales in rotating stars and planets that have been recently confirmed by 3-D high-resolution nonlinear Cartesian numerical simulations to derive a new prescription. A corresponding local model of tidal waves is used to understand the consequences...
Velocity fields and optical turbulence near the boundary in a strongly convective laboratory flow
Matt, Silvia; Hou, Weilin; Goode, Wesley; Hellman, Samuel
2016-05-01
Boundary layers around moving underwater vehicles or other platforms can be a limiting factor for optical communication. Turbulence in the boundary layer of a body moving through a stratified medium can lead to small variations in the index of refraction, which impede optical signals. As a first step towards investigating this boundary layer effect on underwater optics, we study the flow near the boundary in the Rayleigh-Bénard laboratory tank at the Naval Research Laboratory Stennis Space Center. The tank is set up to generate temperature-driven, i.e., convective turbulence, and allows control of the turbulence intensity. This controlled turbulence environment is complemented by computational fluid dynamics simulations to visualize and quantify multi-scale flow patterns. The boundary layer dynamics in the laboratory tank are quantified using a state-of-the-art Particle Image Velocimetry (PIV) system to examine the boundary layer velocities and turbulence parameters. The velocity fields and flow dynamics from the PIV are compared to the numerical model and show the model to accurately reproduce the velocity range and flow dynamics. The temperature variations and thus optical turbulence effects can then be inferred from the model temperature data. Optical turbulence is also visible in the raw data from the PIV system. The newly collected data are consistent with previously reported measurements from high-resolution Acoustic Doppler Velocimeter profilers (Nortek Vectrino), as well as fast thermistor probes and novel next-generation fiber-optics temperature sensors. This multi-level approach to studying optical turbulence near a boundary, combining in-situ measurements, optical techniques, and numerical simulations, can provide new insight and aid in mitigating turbulence impacts on underwater optical signal transmission.
Guzik, Joyce A; Nelson, N J; Lovekin, C; Kosak, K; Kitiashvili, I N; Mansour, N N; Kosovichev, A
2016-01-01
We present multidimensional modeling of convection and oscillations in main-sequence stars somewhat more massive than the Sun, using three separate approaches: 1) Using the 3-D planar StellarBox radiation hydrodynamics code to model the envelope convection zone and part of the radiative zone. Our goals are to examine the interaction of stellar pulsations with turbulent convection in the envelope, excitation of acoustic modes, and the role of convective overshooting; 2) Applying the spherical 3-D MHD ASH (Anelastic Spherical Harmonics) code to simulate the core convection and radiative zone. Our goal is to determine whether core convection can excite low-frequency gravity modes, and thereby explain the presence of low frequencies for some hybrid gamma Doradus/delta Scuti variables for which the envelope convection zone is too shallow for the convective blocking mechanism to drive gravity modes; 3) Applying the ROTORC 2-D stellar evolution and dynamics code to calculate evolution with a variety of initial rotat...
Magnus Force of Common Projectile Bodies with Turbulent Layers
Institute of Scientific and Technical Information of China (English)
CHEN Jun
2005-01-01
Calculating formulae of Magnus force on common projectile bodies (cone-shaped and parabola-shaped) with turbulent layers were built based on Magnus theory. The effects of temperature exponential were considered, and curve-fitting approaches were adopted in the research that could give more exact result data. Both flow layer constants and shape constants are presented in Magnus force formulae, which are useful to evaluate Magnus force in different states.
Puhales, Franciano Scremin; Rizza, Umberto; Degrazia, Gervásio Annes; Acevedo, Otávio Costa
2013-02-01
In this work a parametrization for the transport terms of the turbulent kinetic energy (TKE) budget equation, valid for a convective boundary layer (CBL) is presented. This is a hard task to accomplish from experimental data, especially because of the difficulty associated to the measurements of pressure turbulent fluctuations, which are necessary to determine the pressure correlation TKE transport term. Thus, employing a large eddy simulation (LES) a full diurnal planetary boundary layer (PBL) cycle was simulated. In this simulation a forcing obtained from experimental data is used, so that the numerical experiment represents a more realistic case than a stationary PBL. For this study all terms of the TKE budget equation were determined for a CBL. From these data, polynomials that describe the TKE transport terms’ vertical profiles were adjusted. The polynomials found are a good description of the LES data, and from them it is shown that a simple formulation that directly relates the transport terms to the TKE magnitude has advantages on other parameterizations commonly used in CBL numerical models. Furthermore, the present study shows that the TKE turbulent transport term dominates over the TKE transport by pressure perturbations and that for most of the CBL these two terms have opposite signs.
Brasseur, James; Lavely, Adam; Nandi, Tarak
2016-11-01
Whereas the primary function of a wind turbine (WT) is the generation of electricity, wind farm profitability is decreased both by integrated losses in power and increases in premature failures of drivetrain components resulting from energetic nonsteady aerodynamic forcings of WT rotors by atmospheric and wake turbulence. Here we contrast the physics underlying dominant nonsteady atmospheric turbulence forcings of the bending moments in the WT rotor plane (torque/power) vs. the out-of-plane bending moments (OPBM) that underlie premature drivetrain component failure. Using an advanced actuator line model of the 5 MW NREL and the 1.5 MW GE wind turbine rotors embedded within a high-fidelity spectral LES of a typical daytime convective atmospheric boundary layer, we show that (1) the physics underlying large torque vs. OBPM fluctuations are associated with fundamentally different turbulence eddy characteristics and (2) nonsteady response centers on 4 characteristic time scales associated advection of eddies and load response of blades cutting through internal turbulence eddy structure. Supported by DOE. Computer resources by NSF/XSEDE.
Numerical study of forced convective heat transfer around airships
Dai, Qiumin; Fang, Xiande
2016-02-01
Forced convective heat transfer is an important factor that affects the thermal characteristics of airships. In this paper, the steady state forced convective heat transfer around an ellipsoid is numerically investigated. The numerical simulation is carried out by commercial computational fluid dynamic (CFD) software over the extended Re range from 20 to 108 and the aspect ratio from 2 to 4. Based on the regression and optimization with software, a new piecewise correlation of the Nusselt number at constant wall temperature for ellipsoid is proposed, which is suitable for applications to airships and other ellipse shaped bodies such as elliptical balloons. The thermal characteristics of a stratospheric airship in midsummer located in the north hemisphere are numerical studied. The helium temperature predicated using the new correlation is compared to those predicted by correlations applicable for spheres and flat plates. The results show that the helium temperature obtained using the new correlation at noon is about 5.4 K lower than that using the correlation of spheres and about 2.1 K higher than that of flat plates.
Effects of polymer additives in the bulk of turbulent thermal convection
Xie, Yi-Chao; Funfschilling, Denis; Li, Xiao-Ming; Ni, Rui; Xia, Ke-Qing
2015-01-01
We present experimental evidence that a minute amount of polymer additives can significantly enhance heat transport in the bulk region of turbulent thermal convection. The effects of polymer additives are found to be the \\textit{suppression} of turbulent background fluctuations that give rise to incoherent heat fluxes that make no net contribution to heat transport, and at the same time to \\textit{increase} the coherency of temperature and velocity fields. The suppression of small-scale turbulent fluctuations leads to more coherent thermal plumes that result in the heat transport enhancement. The fact that polymer additives can increase the coherency of thermal plumes is supported by the measurements of a number of local quantities, such as the extracted plume amplitude and width, the velocity autocorrelation functions and the velocity-temperature cross-correlation coefficient. The results from local measurements also suggest the existence of a threshold value for the polymer concentration, only above which c...
Energy Technology Data Exchange (ETDEWEB)
Bucci, M., E-mail: matteo.bucci@cea.fr [Univ. of Pisa, DIMNP, Pisa (Italy); CEA Saclay, CEA/DEN/DANS/DM2S/SFME/LETR (France); Ambrosini, W.; Forgione, N. [Univ. of Pisa, DIMNP, Pisa (Italy); Lioce, D [Univ. of Pisa, DIMNP, Pisa (Italy); Westinghouse Electric Belguim, Nivelles (Belgium)
2011-07-01
This paper discusses the results obtained from recent experimental investigations devoted to the study of steam condensation in the presence of air and a light noncondensable gas. The experiments are intended to provide data for the validation of engineering models and CFD codes. The original experimental data herein discussed focus on forced convection turbulent boundary layer conditions and involve atmospheric pressure, different conditions for mixture velocity (from 1.5 to 3.5 m/s), mixture composition (form 0 to 75 per cent of the light species in the overall amount of noncondensable gases) and two nominal electrical power supply of the steam generator. The experimental data are qualified against correlations based on the heat and mass transfer analogy and to the predictions obtained by an in house condensation model implemented in a commercial CFD code. (author)
Geostrophic convective turbulence: The effect of boundary layers
Ostilla-Mónico, Rodolfo; Kunnen, Rudie P J; Verzicco, Roberto; Lohse, Detlef
2014-01-01
This Letter presents results of the first direct numerical simulations of rotating Rayleigh--B\\'enard convection in the so-called geostrophic regime, (hence very small Ekman numbers $\\mathcal{O}(10^{-7})$ and high Rayleigh numbers~$Ra=10^{10}$ and~$5\\cdot 10^{10}$), employing the \\emph{full} Navier--Stokes equations. In the geostrophic regime the criteria of very strong rotation and large supercriticality are met simultaneously, which is true for many geophysical and astrophysical flows. Until now, numerical approaches of this regime have been based on \\emph{reduced} versions of the Navier--Stokes equations (cf. Sprague \\emph{et al.} J. Fluid Mech., \\textbf{551}, 141 (2006)), omitting the effect of the viscous (Ekman) boundary layers. By using different velocity boundary conditions at the plates, we study the effect of these Ekman layers. We find that the formation of large-scale structures (Rubio \\emph{et al.} (Phys. Rev. Lett. \\textbf{112} (2014)), which indicates the presence of an inverse energy cascade, ...
Directory of Open Access Journals (Sweden)
Bianco Vincenzo
2011-01-01
Full Text Available Abstract In this article, developing turbulent forced convection flow of a water-Al2O3 nanofluid in a square tube, subjected to constant and uniform wall heat flux, is numerically investigated. The mixture model is employed to simulate the nanofluid flow and the investigation is accomplished for particles size equal to 38 nm. An entropy generation analysis is also proposed in order to find the optimal working condition for the given geometry under given boundary conditions. A simple analytical procedure is proposed to evaluate the entropy generation and its results are compared with the numerical calculations, showing a very good agreement. A comparison of the resulting Nusselt numbers with experimental correlations available in literature is accomplished. To minimize entropy generation, the optimal Reynolds number is determined.
Bianco, Vincenzo; Nardini, Sergio; Manca, Oronzio
2011-03-24
In this article, developing turbulent forced convection flow of a water-Al2O3 nanofluid in a square tube, subjected to constant and uniform wall heat flux, is numerically investigated. The mixture model is employed to simulate the nanofluid flow and the investigation is accomplished for particles size equal to 38 nm.An entropy generation analysis is also proposed in order to find the optimal working condition for the given geometry under given boundary conditions. A simple analytical procedure is proposed to evaluate the entropy generation and its results are compared with the numerical calculations, showing a very good agreement.A comparison of the resulting Nusselt numbers with experimental correlations available in literature is accomplished. To minimize entropy generation, the optimal Reynolds number is determined.
Surface waves propagation on a turbulent flow forced electromagnetically
Gutiérrez, Pablo
2015-01-01
We study the propagation of monochromatic surface waves on a turbulent flow. The flow is generated in a layer of liquid metal by an electromagnetic forcing. This forcing creates a quasi two-dimensional (2D) turbulence with strong vertical vorticity. The turbulent flow contains much more energy than the surface waves. In order to focus on the surface wave, the deformations induced by the turbulent flow are removed. This is done by performing a coherent phase averaging. For wavelengths smaller than the forcing lengthscale, we observe a significant increase of the wavelength of the propagating wave that has not been reported before. We suggest that it can be explained by the random deflection of the wave induced by the velocity gradient of the turbulent flow. Under this assumption, the wavelength shift is an estimate of the fluctuations of deflection angle. The local measurements of the wave frequency far from the wavemaker do not reveal such systematic behavior, although a small shift is measured. Finally we qu...
Petenko, Igor; Argentini, Stefania; Casasanta, Giampietro; Kallistratova, Margarita; Sozzi, Roberto; Viola, Angelo
2016-11-01
In the period January-February 2014, observations were made at the Concordia station, Dome C, Antarctica to study atmospheric turbulence in the boundary layer using a high-resolution sodar. The turbulence structure was observed beginning from the lowest height of about 2 m, with a vertical resolution of less than 2 m. Typical patterns of the diurnal evolution of the spatio-temporal structure of turbulence detected by the sodar are analyzed. Here, we focus on the wavelike processes observed within the transition period from stable to unstable stratification occurring in the morning hours. Thanks to the high-resolution sodar measurements during the development of the convection near the surface, clear undulations were detected in the overlying turbulent layer for a significant part of the time. The wavelike pattern exhibits a regular braid structure, with undulations associated with internal gravity waves attributed to Kelvin-Helmholtz shear instability. The main spatial and temporal scales of the wavelike structures were determined, with predominant periodicity of the observed wavy patterns estimated to be 40-50 s. The horizontal scales roughly estimated using Taylor's frozen turbulence hypothesis are about 250-350 m.
Dynamics and flow-coupling in two-layer turbulent thermal convection
Xie, Yi-Chao
2015-01-01
We present an experimental investigation of the dynamics and flow-coupling of convective turbulent flows in a cylindrical Rayleigh-Benard convection cell with two immiscible fluids, water and fluorinert FC-77 electronic liquid (FC77). It is found that one large-scale circulation (LSC) roll exists in each of the fluid layers, and that their circulation planes have two preferred azimuthal orientations separated by $\\sim\\pi$. A surprising finding of the study is that cessations/reversals of the LSC in FC77 of the two-layer system occur much more frequently than they do in single-layer turbulent RBC, and that a cessation is most likely to result in a flow reversal of the LSC, which is in sharp contrast with the uniform distribution of the orientational angular change of the LSC before and after cessations in single-layer turbulent RBC. This implies that the dynamics governing cessations and reversals in the two systems are very different. Two coupling modes, thermal coupling (flow directions of the two LSCs are o...
Evaporation of a binary liquid film by forced convection
Directory of Open Access Journals (Sweden)
Nasr Abdelaziz
2011-01-01
Full Text Available This paper deals with a numerical analysis of the evaporation of a thin binary liquid film by forced convection inside a channel constituted by two parallel plates. The first plate is externally insulated and wetted by a thin water ethylene glycol film while the second is dry and isothermal. The liquid mixture consists of water (the more volatile component and ethylene glycol while the gas mixture has three components: dry air, water vapour and ethylene-glycol vapour. The set of non linear and coupled equations expressing the conservation of mass, momentum, energy and species in the liquid and gas mixtures is solved numerically using a finite difference method. Results concerns with the effects of inlet ambience conditions and the inlet liquid concentration of ethylene glycol on the distribution of the temperature, concentrations profiles and the axial variation of the evaporation rate of species i.
The coupling of conduction with forced convection in Graetz problems
Energy Technology Data Exchange (ETDEWEB)
Pozzi, A.; Lupo, M. (Univ. of Naples (Italy))
1990-05-01
This paper presents an analytical solution of the energy equation for a coupled conduction-forced convection heat transfer problem in ducts. In order to compare the thermal field in plane and circular ducts, the solution is obtained in a form describing both flows. The method is based on an asymptotic expansion of the Laplace transform of the temperature and on an application of stationary-phase method; it enabled the authors to write the solution, for any value of the coupling parameter p, in terms of confluent hypergeometric functions. Some simple and accurate expressions of the interface temperature, temperature at the axis, bulk temperature, and Nusselt number are given for small and high values of p. The accuracy of the results is proved by a comparison with those obtained through an expansion in terms of 120 eigenfunctions.
Analysis of flow instabilities in forced-convection steam generator
Institute of Scientific and Technical Information of China (English)
无
2006-01-01
Because of the practical importance of two-phase instabilities, substantial efforts have been made to date to understand the physical phenomena governing such instabilities and to develop computational tools to model the dynamics. The purpose of this study is to present a numerical model for the analysis of flow-induced instabilities in forced-convection steam generator. The model is based on the assumption of homogeneous two-phase flow and thermodynamic equilibrium of the phases. The thermal capacity of the heater wall has been included in the analysis. The model is used to analyze the flow instabilities in the steam generator and to study the effects of system pressure, mass flux, inlet temperature and inlet/outlet restriction, gap size, the ratio of do /di, and the ratio of qi/qo on the system behavior.
Ergodicity in randomly forced Rayleigh-Bénard convection
Földes, J.; Glatt-Holtz, N. E.; Richards, G.; Whitehead, J. P.
2016-11-01
We consider the Boussinesq approximation for Rayleigh-Bénard convection perturbed by an additive noise and with boundary conditions corresponding to heating from below. In two space dimensions, with sufficient stochastic forcing in the temperature component and large Prandtl number Pr > 0, we establish the existence of a unique ergodic invariant measure. In three space dimensions, we prove the existence of a statistically invariant state, and establish unique ergodicity for the infinite Prandtl Boussinesq system. Throughout this work we provide streamlined proofs of unique ergodicity which invoke an asymptotic coupling argument, a delicate usage of the maximum principle, and exponential martingale inequalities. Lastly, we show that the background method of Constantin and Doering (1996 Nonlinearity 9 1049-60) can be applied in our stochastic setting, and prove bounds on the Nusselt number relative to the unique invariant measure.
Mechanistic Multidimensional Modeling of Forced Convection Boiling Heat Transfer
Directory of Open Access Journals (Sweden)
Michael Z. Podowski
2009-01-01
Full Text Available Due to the importance of boiling heat transfer in general, and boiling crisis in particular, for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems, extensive efforts have been made in the past to develop a variety of methods and tools to evaluate the boiling heat transfer coefficient and to assess the onset of temperature excursion and critical heat flux (CHF at various operating conditions of boiling channels. The objective of this paper is to present mathematical modeling concepts behind the development of mechanistic multidimensional models of low-quality forced convection boiling, including the mechanisms leading to temperature excursion and the onset of CHF.
Soybean drying characteristics in microwave rotary dryer with forced convection
Institute of Scientific and Technical Information of China (English)
Ruifang WANG; Zhanyong LI; Yanhua LI; Jingsheng YE
2009-01-01
A new hybrid drying technique by combining microwave and forced convection drying within a rotary drum, i.e., microwave rotary drying, was developed with the purpose to improve the uniformity of microwave drying. In a laboratory microwave rotary dryer, rewetted soybean was utilized as experimental material to study the effects of drum rotating speed, ventilation flow rate, and specific microwave power on the drying kinetics and cracking ratio of soybean. It was found that, with rotation, the cracking ratio can be lowered but without distinct improvement in the drying rate. Increasing ventilation flow rate and specific microwave power can improve the drying rate, but the cracking ratio also increases as a negative result. The cracking ratio lower than 10% can be attained for ventilation flow rate lower than 2.0 m3·h-1 or specific microwave energy lower than 0.4 kW·kg-1 in the present experiments.
Stationarity of linearly forced turbulence in finite domains.
Gravanis, E; Akylas, E
2011-10-01
A simple scheme of forcing turbulence away from decay was introduced by Lundgren some time ago, the "linear forcing," which amounts to a force term that is linear in the velocity field with a constant coefficient. The evolution of linearly forced turbulence toward a stationary final state, as indicated by direct numerical simulations (DNS), is examined from a theoretical point of view based on symmetry arguments. In order to follow closely the DNS, the flow is assumed to live in a cubic domain with periodic boundary conditions. The simplicity of the linear forcing scheme allows one to rewrite the problem as one of decaying turbulence with a decreasing viscosity. Its late-time behavior can then be studied by scaling symmetry considerations. The evolution of the system in the description of "decaying" turbulence can be understood as the gradual symmetry breaking of a larger approximate symmetry to a smaller symmetry that is exact at late times. The latter symmetry implies a stationary state: In the original description all correlators are constant in time, while, in the "decaying" turbulence description, that state possesses constant Reynolds number and integral length scale. The finiteness of the domain is intimately related to the evolution of the system to a stationary state at late times: In linear forcing there is no other large scale than the domain size, therefore, it is the only scale available to set the magnitude of the necessarily constant integral length scale in the stationary state. A high degree of local isotropy is implied by the late-time exact symmetry, the symmetries of the domain itself, and the solenoidal nature of the velocity field. The fluctuations observed in the DNS for all quantities in the stationary state can be associated with deviations from isotropy that is necessarily broken at the large scale by the finiteness of the domain. Indeed, to strengthen this conclusion somewhat, self-preserving isotropic turbulence models are used to study
Free and forced convective-diffusion solutions by finite element methods
Energy Technology Data Exchange (ETDEWEB)
Gartling, D.K.; Nickell, R.E.
1976-01-01
Several free and forced convective-diffusion examples are solved and compared to either laboratory experiment or closed-form analysis. The problems solved illustrate the application of finite element methods to both strongly-coupled and weakly-coupled velocity and temperature fields governed by the steady-state momentum and energy equations. Special attention is given to internal forced convection with temperature-dependent viscosity and free convection within an enclosure.
Skin-friction drag analysis from the forced convection modeling in simplified underwater swimming.
Polidori, G; Taïar, R; Fohanno, S; Mai, T H; Lodini, A
2006-01-01
This study deals with skin-friction drag analysis in underwater swimming. Although lower than profile drag, skin-friction drag remains significant and is the second and only other contribution to total drag in the case of underwater swimming. The question arises whether varying the thermal gradient between the underwater swimmer and the pool water may modify the surface shear stress distribution and the resulting skin-friction drag acting on a swimmer's body. As far as the authors are aware, such a question has not previously been addressed. Therefore, the purpose of this study was to quantify the effect of this thermal gradient by using the integral formalism applied to the forced convection theory. From a simplified model in a range of pool temperatures (20-30 degrees C) it was demonstrated that, whatever the swimming speeds, a 5.3% reduction in the skin-friction drag would occur with increasing average boundary-layer temperature provided that the flow remained laminar. However, as the majority of the flow is actually turbulent, a turbulent flow analysis leads to the major conclusion that friction drag is a function of underwater speed, leading to a possible 1.5% reduction for fast swimming speeds above 1m/s. Furthermore, simple correlations between the surface shear stress and resulting skin-friction drag are derived in terms of the boundary-layer temperature, which may be readily used in underwater swimming situations.
Horizontal velocity field near the hot plate in turbulent natural convection
Koothur, Vipin
2014-01-01
We study the velocity field in a horizontal (x-y) plane 1.5 mm above the hot plate in turbulent natural convection using PIV at a Rayleigh number Raw=106 and Prandtl number Pr=5.2. The plane of measurement is inside the velocity boundary layer estimated from the natural convection boundary layer equations[7] as well as inside the velocity boundary layer due to the large scale flow[2, 5].The boundary layer comprises of line plumes with sinking fluid between them. The instantaneous velocity variation from the center of the sinking fluid to the line plumes is found to deviate with the classical Prandtl-Blasius laminar boundary layer profile, which is assumed to be the nature of boundary layer by the GL theory [2, 5]. Our results agree well with the natural convection boundary layer profile. The time averaged mean velocity variation deviates from both natural convection and Blasius type profiles as expected as it depends on the orientation of the line plumes. Our measurement result is a proof to the theory of the...
Idealised hydrodynamic simulations of turbulent oxygen-burning shell convection in 4{\\pi} geometry
Jones, Samuel; Sandalski, Stou; Davis, Austin; Woodward, Paul; Herwig, Falk
2016-01-01
This work investigates the properties of convection in stars with particular emphasis on entrainment across the upper convective boundary (CB). Idealised simulations of turbulent convection in the O-burning shell of a massive star are performed in $4\\pi$ geometry on $768^3$ and $1536^3$ grids, driven by a representative heating rate. A heating series is also performed on the $768^3$ grid. The $1536^3$ simulation exhibits an entrainment rate at the upper CB of $1.33\\times10^{-6}~M_\\odot~\\mathrm{s}^{-1}$. The $768^3$ simulation with the same heating rate agrees within 17%. The entrainment rate at the upper convective boundary is found to scale linearly with the driving luminosity and with the cube of the shear velocity at the upper boundary, while the radial RMS fluid velocity scales with the cube root of the driving luminosity, as expected. The mixing is analysed in a 1D diffusion framework, resulting in a simple model for CB mixing. The analysis confirms previous findings that limiting the MLT mixing length t...
Further improvements of a new model for turbulent convection in stars
Canuto, V. M.; Mazzitelli, I.
1992-01-01
The effects of including a variable molecular weight and of using the newest opacities of Rogers and Iglesias (1991) as inputs to a recent model by Canuto and Mazzitelli (1991) for stellar turbulent convection are studied. Solar evolutionary tracks are used to conclude that the the original model for turbulence with mixing length Lambda = z, Giuli's variable Q unequal to 1 and the new opacities yields a fit to solar T(eff) within 0.5 percent. A formulation of Lambda is proposed that extends the purely nonlocal Lambda = z expression to include local effects. A new expression for Lambda is obtained which generalizes both the mixing length theory (MLT) phenomenological expression for Lambda as well as the model Lambda = z. It is argued that the MLT should now be abandoned.
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.
Spectra and probability distributions of thermal flux in turbulent Rayleigh-B\\'{e}nard convection
Pharasi, Hirdesh K; Kumar, Krishna; Bhattacharjee, Jayanta K
2016-01-01
The spectra of turbulent heat flux $\\mathrm{H}(k)$ in Rayleigh-B\\'{e}nard convection with and without uniform rotation are presented. The spectrum $\\mathrm{H}(k)$ scales with wave number $k$ as $\\sim k^{-2}$. The scaling exponent is almost independent of the Taylor number $\\mathrm{Ta}$ and Prandtl number $\\mathrm{Pr}$ for higher values of the reduced Rayleigh number $r$ ($ > 10^3$). The exponent, however, depends on $\\mathrm{Ta}$ and $\\mathrm{Pr}$ for smaller values of $r$ ($<10^3$). The probability distribution functions of the local heat fluxes are non-Gaussian and have exponential tails.
Boundary layers in turbulent Rayleigh-B\\'enard convection in air
Puits, Ronald du; Resagk, Christian; Thess, André
2012-01-01
The boundary layer flow in a Rayleigh-B\\'enard convection cell of rectangular shape has been visualized in this fluid dynamics video. The experiment has been undertaken in air at a Rayleigh number $Ra=1.3\\times 10^{10}$ and a Prandtl number $Pr=0.7$. Various sequences captured at selected positions of the heating plate show that the boundary layer is a very transient flow region characterized by coherent structures that permanently evolve. It becomes fully turbulent in the areas where the large-scale circulation impinge or leave the bottom plate.
Test of steady-state fluctuation theorem in turbulent Rayleigh-Bénard convection
Shang, X.-D.; Tong, P.; Xia, K.-Q.
2005-07-01
The local entropy production rate σ(r,t) in turbulent thermal convection is obtained from simultaneous velocity and temperature measurements in an aspect-ratio-one cell filled with water. The statistical properties of the time-averaged σ(r,t) are analyzed and the results are compared with the predictions of the steady-state fluctuation theorem (SSFT) of Gallavotti and Cohen. The experiment reveals that the SSFT can indeed be extended to the local variables, but further development is needed in order to incorporate the common dynamic complexities of far-from-equilibrium systems into the theory.
Energetic dynamics of a rotating horizontal convection model of an ocean basin with wind forcing
Zemskova, Varvara; White, Brian; Scotti, Alberto
2016-11-01
We analyze the energetic dynamics in a rotating horizontal convection model, where flow is driven by a differential buoyancy forcing along a horizontal surface. This model is used to quantify the influence of surface heating and cooling and surface wind stress on the Meridional Overturning Circulation. We study a model of the Southern Ocean in a rectangular basin with surface cooling on one end (the South pole) and surface warming on the other end (mid-latitudes). Free-slip boundary conditions are imposed in the closed box, while zonally periodic boundary conditions are enforced in the reentrant channel. Wind stress and differential buoyancy forcing are applied at the top boundary. The problem is solved numerically using a 3D DNS model based on a finite-volume AMR solver for the Boussinesq Navier-Stokes equations with rotation. The overall dynamics, including large-scale overturning, baroclinic eddying, turbulent mixing, and resulting energy cascades are investigated using the local Available Potential Energy framework introduced in. We study the relative contributions of surface buoyancy and wind forcing along with the effects of bottom topography to the energetic balance of this dynamic model. This research is part of the Blue Waters sustained-petascale computing project, supported by the NSF (awards OCI-0725070, ACI-1238993 and ACI-14-44747) and the state of Illinois.
MacRorie, Michael
1995-01-01
The interaction between convecting spanwise vortices and a flat plate turbulent boundary layer was studied experimentally. The results are relevant to the flow downstream of unsteady airfoils or spoilers. Vortices were generated with a rapidly pitched airfoil upstream of a test plate leading edge in a low-speed wind tunnel. By varying the height of the vortex generator the degree to which the vortices interacted with the test plate was controlled. Dynamic stall vortices of both positive and negative circulation were studied with Reynolds numbers (Gamma/upsilon ) of 9300 and 7400 respectively. The free-stream velocity was 5.9 m/s for all cases and the boundary layer momentum thickness Reynolds number was 480 at the primary measurement station. The measurement techniques were hot -wire anemometry (single and cross wire) and smoke-wire visualization. The results focus on two distinct aspects of the flow, first is the decay and diffusion rates of the vortices. Only in the case where a negative circulation vortex impinges directly on the leading edge does surface interaction significantly increase the vortex decay/diffusion rate. The second aspect is the response of the turbulent boundary layer to the convecting vortices. Wall shear stress measurements show that the passage of a positive circulation vortex results in an increase in wall shear after a delay-time, while the negative circulation vortices result in a decrease in wall shear. An application of log-law scaling to the ensemble-averaged mean flow was found to produce a velocity scale which resembles one based on measured wall shear stress but is offset by a phase lag. The ratio of turbulent shear stress to the two-dimensional turbulent kinetic energy was generally not constant, although it did show a constant value across the boundary layer at different phases of the interaction.
Correlation of Cloud Droplet Growth with the Scalar Fluctuations in a Turbulent Moist Convection
Chandrakar, Kamal Kant; Cantrell, Will; Chang, Kelken; Ciochetto, David; Niedermeier, Dennis; Ovchinnikov, Mikhail; Shaw, Raymond; Yang, Fan
2016-11-01
Cloud droplet growth in a turbulent environment is studied by creating turbulent moist Rayleigh-Bénard convection in the Michigan Tech Pi Chamber. Cloud formation is achieved by injecting aerosols into the water-supersaturated environment created by the isobaric mixing of saturated air at different temperatures. A range of steady-state cloud droplet number concentration is achieved by supplying aerosols at different rates. As steady-state droplet number concentration is decreased the mean droplet size increases as expected, but also the width of the size distribution increases. This increase in the width is associated with larger supersaturation fluctuations due to the slow droplet microphysical response (sink of the water vapor) compared to the fast turbulent mixing (source of the water vapor). The observed standard deviation of the squared droplet radius is a linear function of the combined time scale of the system τs- 1 =τc- 1 +τt- 1 ; here, τc is the phase relaxation time and τt is the turbulence correlation time. A stochastic differential equation approach for supersaturation also predicts the same linear response. This finding has significance for cloud-radiation budgets and precipitation formation. This work was supported by the National Science Foundation, Grant AGS-1623429.
Turbulent kinetic energy generation in the convective boundary layer derived from thermodynamics
Slameršak, Aljoša; Renner, Maik; Ganzeveld, Laurens; Hartogensis, Oscar; Kolle, Olaf; Kleidon, Axel
2016-04-01
Turbulent heat fluxes facilitate the bulk of heat transfer between the surface and lower atmosphere, which results in the diurnal growth of convective boundary layer (CBL) and turbulent kinetic energy generation (TKE). Here we postulate the hypothesis that TKE generation in the CBL occurs as a result of heat transfer in a "Carnot-like" heat engine with temporal changes in the internal energy of the boundary layer. We used the Tennekes energy-balance model of CBL and extended it with the analysis of the entropy balance to derive the estimates of TKE generation in the CBL. These TKE generation estimates were compared to the turbulent dissipation from a simple dissipation model from Moeng and Sullivan, to test the validity of our heat engine hypothesis. In addition, to evaluate the performance of the dissipation model, this was independently validated by a comparison of its estimates with the turbulent dissipation calculations based on spectral analysis of eddy covariance wind measurements at a German field station. Our analysis demonstrates how a consistent application of thermodynamics can be used to obtain an independent physical constraint on the diurnal boundary layer evolution. Furthermore, our analysis suggests that the CBL operates at the thermodynamic limit, thus imposing a thermodynamic constraint on surface-atmosphere exchange.
Directory of Open Access Journals (Sweden)
Kovalenko A. V.
2015-01-01
Full Text Available In the article, we have suggested a general mathematical model of non-stationary and non-isothermal process of a binary electrolyte transfer in dilute solutions in an electro-membrane system (EMS, taking into account the joint action of gravitational convection, forced convection and electro convection in potential dynamic mode. This model is a boundary problem for a system of two-dimensional quasi-linear Navier-Stokes equation and Nernst-Planck-Poisson in partial derivatives equation. We have developed a theory of similarity of the process of heat and mass transfer in electro-membrane systems, specifically, in a desalting channel of electro dialysis apparatus, taking into account joint actions of concentration polarization, space charge, gravity convection, forced convection and electro convection. It is shown that the criterion of electro convection does not directly depend on the initial concentration, and, therefore, electro convection occurs at any initial concentration. At the same time, the criterion of concentration convection linearly dependents on the initial concentration, and, therefore, at high concentrations, concentration convection prevails, while at lower concentrations, the role of gravitational convection begins to fall whereas the role of electro convection increases. The theory of similarity of the process of heat and mass transfer in the desalting channel of electro dialysis apparatus built in this work taking into account the joint action of concentration polarization, space charge, gravity convection, forced convection and electro convection is important for engineering calculations, for scaling the results of experiments in an electro-membrane cell for industrial electro dialysis water desalting apparatus
Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence.
Squire, J; Bhattacharjee, A
2015-11-01
This article presents a calculation of the mean electromotive force arising from general small-scale magnetohydrodynamical turbulence, within the framework of the second-order correlation approximation. With the goal of improving understanding of the accretion disk dynamo, effects arising through small-scale magnetic fluctuations, velocity gradients, density and turbulence stratification, and rotation, are included. The primary result, which supplements numerical findings, is that an off-diagonal turbulent resistivity due to magnetic fluctuations can produce large-scale dynamo action-the magnetic analog of the "shear-current" effect. In addition, consideration of α effects in the stratified regions of disks gives the puzzling result that there is no strong prediction for a sign of α, since the effects due to kinetic and magnetic fluctuations, as well as those due to shear and rotation, are each of opposing signs and tend to cancel each other.
FREELY DECAYING TURBULENCE IN FORCE-FREE ELECTRODYNAMICS
Energy Technology Data Exchange (ETDEWEB)
Zrake, Jonathan; East, William E. [Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, SLAC National Accelerator Laboratory, Menlo Park, CA 94025 (United States)
2016-02-01
Freely decaying, relativistic force-free turbulence is studied for the first time. We initiate the magnetic field at a short wavelength and simulate its relaxation toward equilibrium on two- and three-dimensional periodic domains in both helical and nonhelical settings. Force-free turbulent relaxation is found to exhibit an inverse cascade in all settings and in three dimensions to have a magnetic energy spectrum consistent with the Kolmogorov 5/3 power law. Three-dimensional relaxations also obey the Taylor hypothesis; they settle promptly into the lowest-energy configuration allowed by conservation of the total magnetic helicity. However, in two dimensions, the relaxed state is a force-free equilibrium whose energy greatly exceeds the Taylor minimum and that contains persistent force-free current layers and isolated flux tubes. We explain this behavior in terms of additional topological invariants that exist only in two dimensions, namely the helicity enclosed within each level surface of the magnetic potential function. The speed and completeness of turbulent magnetic free-energy discharge could help account for rapidly variable gamma-ray emission from the Crab Nebula, gamma-ray bursts, blazars, and radio galaxies.
Extreme dissipation event due to plume collision in a turbulent convection cell
Schumacher, Jörg; Scheel, Janet D.
2016-10-01
An extreme dissipation event in the bulk of a closed three-dimensional turbulent convection cell is found to be correlated with a strong reduction of the large-scale circulation flow in the system that happens at the same time as a plume emission event from the bottom plate. The reduction in the large-scale circulation opens the possibility for a nearly frontal collision of down- and upwelling plumes and the generation of a high-amplitude thermal dissipation layer in the bulk. This collision is locally connected to a subsequent high-amplitude energy dissipation event in the form of a strong shear layer. Our analysis illustrates the impact of transitions in the large-scale structures on extreme events at the smallest scales of the turbulence, a direct link that is observed in a flow with boundary layers. We also show that detection of extreme dissipation events which determine the far-tail statistics of the dissipation fields in the bulk requires long-time integrations of the equations of motion over at least a hundred convective time units.
Directory of Open Access Journals (Sweden)
K Rahmani
2013-01-01
Full Text Available This work concerns the study of heat transfer by means of natural convection with fluids circulating in enclosures. These topics are largely studied both experimentally and numerically due to their wide industrial application in various fields such as nuclear energy, the heating and cooling of buildings, solar collectors, etc. A great deal of relevant research work consists in numerical simulations of natural convection mechanisms with laminar flows in closed cavities. In this context, the present study comes as a contribution in numerical form investigating the turbulent natural convection in vertical enclosure which presents sinusoidal protuberances on one of its vertical walls. Both the top and bottom of the enclosure are open to allow the fluid flow. The horizontal walls are supposed adiabatic. We are interested in determining for various amplitudes and periods. The influence of geometry on several factors such as: temperature, the number of local Nusselt, the turbulent kinetic energy k and its dissipationï¥. Based on the Navier-Stokes equations and Boussinesq approximation, the equations were solved by the CFD technique using the Finite Volume Method In the case of enclosures having the form ratio equal to 0.6 (A=0.6. Given the steady conditions of heat flow on the vertical walls and the pressures at the entry and exit of the cavity, the results show that when we gradually increase the amplitudes of the protuberance wall (say a=0.005 m, a=0.010, a=0.015, a= 0.02, and a=0.025, the maximal temperature increases with the increase of amplitude. This is due to the rise of the heat transfer surface of the modified wall. Regarding heat transfer parameters, the results show that the number of local Nusselt varies relatively with the amplitudes. This explains that the modified wall is affected locally by a pure conduction. The results obtained in this study are in agreement with recent works of several authors.
Turbulent convection driven by internal radiative heating of melt ponds on sea ice
Wells, Andrew; Langton, Tom; Rees Jones, David; Moon, Woosok
2016-11-01
The melting of Arctic sea ice is strongly influenced by heat transfer through melt ponds which form on the ice surface. Melt ponds are internally heated by the absorption of incoming radiation and cooled by surface heat fluxes, resulting in vigorous buoyancy-driven convection in the pond interior. Motivated by this setting, we conduct two-dimensional direct-numerical simulations of the turbulent convective flow of a Boussinesq fluid between two horizontal boundaries, with internal heating predicted from a two-stream radiation model. A linearised thermal boundary condition describes heat exchange with the overlying atmosphere, whilst the lower boundary is isothermal. Vertically asymmetric convective flow modifies the upper surface temperature, and hence controls the partitioning of the incoming heat flux between emission at the upper and lower boundaries. We determine how the downward heat flux into the ice varies with a Rayleigh number based on the internal heating rate, the flux ratio of background surface cooling compared to internal heating, and a Biot number characterising the sensitivity of surface fluxes to surface temperature. Thus we elucidate the physical controls on heat transfer through Arctic melt ponds which determine the fate of sea ice in the summer.
Starspots due to large-scale vortices in rotating turbulent convection
Käpylä, P J; Hackman, T
2011-01-01
We study the generation of large-scale vortices in rotating turbulent convection by means of Cartesian direct numerical simulations. We find that for sufficiently rapid rotation, cyclonic structures on a scale large in comparison to that of the convective eddies, emerge, provided that the fluid Reynolds number exceeds a critical value. For slower rotation, cold cyclonic vortices are preferred, whereas for rapid rotation, warm anti-cyclonic vortices are favoured. In some runs in the intermediate regime both types of cyclones co-exist for thousands of convective turnover times. The temperature contrast between the vortices and the surrounding atmosphere is of the order of five per cent. We relate the simulation results to observations of rapidly rotating late-type stars that are known to exhibit large high-latitude spots from Doppler imaging. In many cases, cool spots are accompanied with spotted regions with temperatures higher than the average. In this paper, we investigate a scenario according to which the s...
A new framework for simulating forced homogeneous buoyant turbulent flows
Carroll, Phares L.; Blanquart, Guillaume
2015-06-01
This work proposes a new simulation methodology to study variable density turbulent buoyant flows. The mathematical framework, referred to as homogeneous buoyant turbulence, relies on a triply periodic domain and incorporates numerical forcing methods commonly used in simulation studies of homogeneous, isotropic flows. In order to separate the effects due to buoyancy from those due to large-scale gradients, the linear scalar forcing technique is used to maintain the scalar variance at a constant value. Two sources of kinetic energy production are considered in the momentum equation, namely shear via an isotropic forcing term and buoyancy via the gravity term. The simulation framework is designed such that the four dimensionless parameters of importance in buoyant mixing, namely the Reynolds, Richardson, Atwood, and Schmidt numbers, can be independently varied and controlled. The framework is used to interrogate fully non-buoyant, fully buoyant, and partially buoyant turbulent flows. The results show that the statistics of the scalar fields (mixture fraction and density) are not influenced by the energy production mechanism (shear vs. buoyancy). On the other hand, the velocity field exhibits anisotropy, namely a larger variance in the direction of gravity which is associated with a statistical dependence of the velocity component on the local fluid density.
Lagrangian statistics and flow topology in forced 2-D turbulence
Energy Technology Data Exchange (ETDEWEB)
Kadoch, B. [Universite d' Aix-Marseille; Del-Castillo-Negrete, Diego B [ORNL; Bos, W.J.T. [CNRS, Ecole Centrale de Lyon, Universite Claude Bernard Lyon; Schneider, Kai [Universite d' Aix-Marseille
2011-01-01
A study of the relationship between Lagrangian statistics and flow topology in fluid turbulence is presented. The topology is characterized using the Weiss criterion, which provides a conceptually simple tool to partition the flow into topologically different regions: elliptic (vortex dominated), hyperbolic (deformation dominated), and intermediate (turbulent background). The flow corresponds to forced two-dimensional Navier-Stokes turbulence in doubly periodic and circular bounded domains, the latter with no-slip boundary conditions. In the double periodic domain, the probability density function (pdf) of the Weiss field exhibits a negative skewness consistent with the fact that in periodic domains the flow is dominated by coherent vortex structures. On the other hand, in the circular domain, the elliptic and hyperbolic regions seem to be statistically similar. We follow a Lagrangian approach and obtain the statistics by tracking large ensembles of passively advected tracers. The pdfs of residence time in the topologically different regions are computed introducing the Lagrangian Weiss field, i.e., the Weiss field computed along the particles' trajectories. In elliptic and hyperbolic regions, the pdfs of the residence time have self-similar algebraic decaying tails. In contrast, in the intermediate regions the pdf has exponential decaying tails. The conditional pdfs (with respect to the flow topology) of the Lagrangian velocity exhibit Gaussian-like behavior in the periodic and in the bounded domains. In contrast to the freely decaying turbulence case, the conditional pdfs of the Lagrangian acceleration in forced turbulence show a comparable level of intermittency in both the periodic and the bounded domains. The conditional pdfs of the Lagrangian curvature are characterized, in all cases, by self-similar power-law behavior with a decay exponent of order - 2.
Hot Bottom Burning in Asymptotic Giant Branch Stars and the Turbulent Convection Model
D'Antona, Francesca; Mazzitelli, Italo
1996-10-01
We investigate the effect of two different local turbulent convection models on the structure of intermediate-mass stars (IMSs, 3.5 Msun ≤ M ≤7 Msun) in the asymptotic giant branch (AGB) phase where, according to observations, they should experience hot bottom burning (HBB). Evolutionary models adopting either the mixing length theory (MLT) or the Canuto & Mazzitelli (CM) description of stellar convection are discussed. It is found that, while the MLT structures require some degree of tuning to achieve, at the bottom of the convective envelope, the large temperatures required for HBB, the CM structures spontaneously achieve these conditions. Since the observational evidence for HBB (existence of a class of very luminous, lithium-rich AGB stars in the Magellanic Clouds showing low 12C/13C ratios) is quite compelling, the above result provides a further, successful test for the CM convective model, in stellar conditions far from solar. With the aid of the CM model, we then explore a number of problems related to the late evolution of this class of objects, and give first results for (1) the luminosity evolution of IMSs in the AGB phase (core mass-luminosity relation and luminosity range in which HBB occurs) for Population I and Population II structures, (2) the minimum core mass for semidegenerate carbon ignition (˜1.05 Msun), (3) the relation between initial mass and final white dwarf (WD) mass (also based on some observational evidences about the upper AGB stars), and (4) the expected mass function of massive WDs. Confirmation of the theoretical framework could arise from an observational test: the luminosity function of AGB stars is expected to show a gap at Mbol ˜ -6, which would distinguish between the low-luminosity regime, in which AGBs become carbon stars, and the upper luminosities, at which they undergo HBB, have very low 12C/13C ratios, and become lithium rich.
Natrajan, V. K.; Christensen, K. T.
2011-08-01
The thermal-transport characteristics of transitional and turbulent flow through smooth- and rough-wall rectangular microchannels (Dh = 600 μm) under constant-heat-flux conditions at three of the four walls were investigated by performing non-intrusive and spatially resolved measurements of fluid temperature via two-color fluorescent thermometry. These measurements, along with bulk pressure-drop measurements, were performed over the Reynolds-number range 1300\\le {\\emph {Re}} \\le 5200. The pressure-drop results revealed the onset of transition above {\\emph {Re}_{cr}}\\cong 1800 for the smooth-wall case, consistent with the onset of transition at the macroscale. However, with increasing surface roughness, deviation from laminar behavior was noted at progressively lower Re which indicates that {\\emph {Re}_{cr}} is a function of roughness. Mean temperature profiles calculated from data sets acquired in the transitional regime for the smooth- and rough-wall cases illustrated deviation from fully developed laminar behavior for {\\emph {Re} \\gt \\emph {Re}_{cr}}. Nevertheless, these profiles still suggest similarities in the transitional pathway of the thermal-transport behavior for the smooth and rough cases save for a relative shift due to the onset of transition at lower Re with increasing surface roughness. Estimates of the bulk Nusselt number indicated enhancement in thermal transport over the smooth-wall case with increasing surface roughness in both the transitional and turbulent regimes, though the smooth-wall data agreed well with macroscale predictions over the range of turbulent Re considered. While the shift in the transitional pathway of the thermal transport behavior toward lower Re accounts for a portion of this enhancement, an increase in turbulent convection with increasing surface roughness might also contribute in this regard.
Zhong, J.Q.; Sterl, S.H.; Li, H.M.
2015-01-01
We present measurements of the azimuthal rotation velocity $\\dot{{\\it\\theta}}(t)$θ˙(t) and thermal amplitude ${\\it\\delta}(t)$δ(t) of the large-scale circulation in turbulent Rayleigh–Bénard convection with modulated rotation. Both $\\dot{{\\it\\theta}}(t)$θ˙(t) and ${\\it\\delta}(t)$δ(t) exhibit clear os
Freely decaying turbulence in force-free electrodynamics
Zrake, Jonathan
2015-01-01
Freely decaying relativistic force-free turbulence is studied for the first time. We initiate the magnetic field at a short wavelength and simulate its relaxation toward equilibrium on two and three dimensional periodic domains, in both helical and non-helical settings. Force-free turbulent relaxation is found to exhibit an inverse cascade in all settings, and in 3D to have a magnetic energy spectrum consistent with the Kolmogorov $5/3$ power law. 3D relaxations also obey the Taylor hypothesis; they settle promptly into the lowest energy configuration allowed by conservation of the total magnetic helicity. But in 2D, the relaxed state is a force-free equilibrium whose energy greatly exceeds the Taylor minimum, and which contains persistent force-free current layers and isolated flux tubes. We explain this behavior in terms of additional topological invariants that exist only in two dimensions, namely the helicity enclosed within each level surface of the magnetic potential function. The speed and completeness...
Pérez Grande, Isabel; Rodriguez Sevillano, Angel; Meseguer, Jos
In June, 8th, 2009 the balloon-borne solar telescope SUNRISE was launched from the Swedish Space Corporation balloon facility Esrange. A telescope with a mirror of 1 m in diameter ob-served the Sun during six days until the mission was terminated in Canada. The design process of SUNRISE and of any optical telescope requires the analysis of the effect of surrounding air on the quality of images. The turbulence encountered in the local telescope environment de-grades its optical performance. This phenomenon called `seeing' consists of optical aberrations produced by density non-homogeneities in the air along the optical path. The refraction index of air changes due to thermal non-uniformities so that the wavefront incident on the mirror is randomly distorted, and therefore, images are altered. When telescope mirrors are heated, as it happens in solar telescopes, and therefore they are at a temperature different from the environment's, natural convection occurs. It is then crucial to know whether the flow in front of the mirror is laminar or turbulent. After reviewing the literature, it was found that the scattering of results about the onset of the transition gives only rough orders of magnitude of the values of the critical Grashof numbers. Aiming to obtain more information about it, the problem of determination of the turbulence onset in natural convection on heated inclined plates in air environment was experimentally revisited. The transition has been determined from hot wire velocity measurements. The onset of turbulence has been considered to take place where velocity perturbations start to grow. Experiments have shown that the onset depends not only on the Grashof number, but also on other parameters as the temperature difference between the heated plate and the surrounding air. A correlation between dimensionless Grashof and Reynolds numbers has been obtained, fitting extraordinarily well the experimental data. The results are obtained in terms of non
Measured Instantaneous Viscous Boundary Layer in Turbulent Rayleigh-B\\'{e}nard Convection
Zhou, Quan
2009-01-01
We report measurements of the instantaneous viscous boundary layer (BL) thickness $\\delta_v(t)$ in turbulent Rayleigh-B\\'{e}nard convection. It is found that $\\delta_v(t)$ obtained from the measured instantaneous two-dimensional velocity field exhibits intermittent fluctuations. For small values, $\\delta_v(t)$ obeys a lognormal distribution, whereas for large values the distribution of $\\delta_v(t)$ exhibits an exponential tail. The variation of $\\delta_v(t)$ with time is found to be driven by the fluctuations of the large-scale mean flow velocity, as expected, and the local horizontal velocities close to the plate can be used as an instant measure of this variation. It is further found that the mean velocity profile measured in the laboratory frame can now be brought into coincidence with the theoretical Blasius laminar BL profile, if it is resampled relative to the time-dependent frame of $\\delta_v(t)$.
Heat transfer enhancement induced by wall inclination in turbulent thermal convection.
Kenjereš, Saša
2015-11-01
We present a series of numerical simulations of turbulent thermal convection of air in an intermediate range or Rayleigh numbers (10(6)≤Ra≤10(9)) with different configurations of a thermally active lower surface. The geometry of the lower surface is designed in such a way that it represents a simplified version of a mountain slope with different inclinations (i.e., "Λ"- and "V"-shaped geometry). We find that different wall inclinations significantly affect the local heat transfer by imposing local clustering of instantaneous thermal plumes along the inclination peaks. The present results reveal that significant enhancement of the integral heat transfer can be obtained (up to 32%) when compared to a standard Rayleigh-Bénard configuration with flat horizontal walls. This is achieved through combined effects of the enlargement of the heated surface and reorganization of the large-scale flow structures.
Weiss, Stephan; Zhong, Jin-Qiang; Clercx, Herman J H; Lohse, Detlef; Ahlers, Guenter; 10.1103/PhysRevLett.105.224501
2011-01-01
In turbulent thermal convection in cylindrical samples of aspect ratio \\Gamma = D/L (D is the diameter and L the height) the Nusselt number Nu is enhanced when the sample is rotated about its vertical axis, because of the formation of Ekman vortices that extract additional fluid out of thermal boundary layers at the top and bottom. We show from experiments and direct numerical simulations that the enhancement occurs only above a bifurcation point at a critical inverse Rossby number $1/\\Ro_c$, with $1/\\Ro_c \\propto 1/\\Gamma$. We present a Ginzburg-Landau like model that explains the existence of a bifurcation at finite $1/\\Ro_c$ as a finite-size effect. The model yields the proportionality between $1/\\Ro_c$ and $1/\\Gamma$ and is consistent with several other measured or computed system properties.
Test of the steady-state fluctuation theorem in turbulent Rayleigh-B'enard convection
Tong, Penger; Shang, Xiaodong
2005-11-01
Local entropy production rate σ(r,t) in turbulent thermal convection is obtained from simultaneous velocity and temperature measurements in an aspect-ratio-one cell filled with water. The statistical properties of the time-averaged σ(r,t) are analyzed and the results are compared with the predictions of the steady state fluctuation theorem (SSFT) of Gallavotti and Cohen. The experiment reveals that the SSFT can indeed be extended to the local variables, but further development is needed in order to incorporate the common dynamic complexities of far-from-equilibrium systems into the theory. *Work supported by the Research Grants Council of Hong Kong SAR under Grant Nos. HKUST603504 (P.T.) and CUHK403003 (K.Q.X.).
Sustaining processes from recurrent flows in body-forced turbulence
Lucas, Dan
2016-01-01
By extracting unstable invariant solutions directly from body-forced three-dimensional turbulence, we study the dynamical processes at play when the forcing is large scale and either unidirectional in the momentum or the vorticity equations. In the former case, the dynamical processes familiar from recent work on linearly-stable shear flows - variously called the Self-Sustaining Process (Waleffe 1997) or Vortex-Wave Interaction (Hall & Smith 1991; Hall & Sherwin 2010) - are important even when the base flow is linearly unstable. In the latter case, where the forcing drives Taylor-Green vortices, a number of mechanisms are observed from the various types of periodic orbits isolated. In particular, two different transient growth mechanisms are discussed to explain the more complex states found.
Rincon, F; Schekochihin, A A; Rieutord, M
2016-01-01
The Sun provides us with the only spatially well-resolved astrophysical example of turbulent thermal convection. While various aspects of solar photospheric turbulence, such as granulation (one-Megameter horizontal scale), are well understood, the questions of the physical origin and dynamical organization of larger-scale flows, such as the 30-Megameters supergranulation and flows deep in the solar convection zone, remain largely open in spite of their importance for solar dynamics and magnetism. Here, we present a new critical global observational characterization of multiscale photospheric flows and subsequently formulate an anisotropic extension of the Bolgiano-Obukhov scaling theory of hydrodynamic stratified turbulence that may explain several of their distinctive dynamical properties. Our combined analysis suggests that photospheric flows in the horizontal range of scales between supergranulation and granulation have a typical vertical correlation scale of 2.5 to 4 Megameters and operate in a strongly a...
Yaglom, A. M.
1994-02-01
Most of the existing theoretical models for statistical characteristics of turbulence in convective boundary layers are based on the similarity theory by Monin and Obukhov [Trudy Geofiz. Inst. Akad. Nauk SSSR 24(151), 163 (1954)], and its further refinements. A number of such models was recently reconsidered and partially compared with available data by Kader and Yaglom [J. Fluid Mech. 212, 637 (1990); Turbulence and Coherent Structures (Kluwer, Dordrecht, 1991), p. 387]. However, in these papers the data related to variances =σ2u and =σ2v of horizontal velocity components were not considered at all, and the data on horizontal velocity spectra Eu(k) and Ev(k) were used only for a restricted range of not too small wave numbers k. This is connected with findings by Kaimal et al. [Q. J. R. Meteorol. Soc. 98, 563 (1972)] and Panofsky et al. [Boundary-Layer Meteorol. 11, 355 (1977)], who showed that the Monin-Obukhov theory cannot be applied to velocity variance σ2u and σ2v and to spectra Eu(k) and Ev(k) in energy ranges of wave numbers. It is shown in this paper that a simple generalization of the traditional similarity theory, which takes into account the influence of large-scale organized structures, leads to new models of horizontal velocity variances and spectra, which describe the observed deviations of these characteristics from the predictions based on the Monin-Obukhov theory, and agree satisfactorily with the available data. The application of the same approach to the temperature spectrum and variance explains why the observed deviations of temperature spectrum in convective boundary layers from the Monin-Obukhov similarity does not lead to marked violations of the same similarity as applied to temperature variance =σ2t.
Sunset decay of the convective turbulence with Large-Eddy Simulation under realistic conditions
Rizza, U.; Miglietta, M. M.; Degrazia, G. A.; Acevedo, O. C.; Marques Filho, E. P.
2013-10-01
Large-Eddy Simulation is performed for a single day from the Cooperative Atmosphere-Surface Exchange Study (CASES-99) field program. This study investigates an observed case of evening transition boundary layer over land. Parameters of the ambient atmosphere in the LES-decay studies conducted so far were typically prescribed in an idealized form. To provide suitable data under the wide range of the PBL weather conditions, the LES should be able to adequately reproduce the PBL turbulence dynamics including-if possible-baroclinicity, radiation, large scale advection and not only be related to a decreasing surface heating. In addition LES-decay studies usually assume that the sensible heat flux decreases instantaneously or with a very short time scale. The main purpose of this investigation is to study the decay of boundary-layer average turbulent kinetic energy at sunset with Large-Eddy Simulation that is forced with realistic environment conditions. This allows investigating the Turbulent Kinetic Energy decay over the realistic time scale that is observed in the atmosphere. During the intermediate and last stage of decay of the boundary-layer average Turbulent Kinetic Energy the exponents of the decay power law t go from 2 to 6, as evidenced by experimental results and recent analytical modeling in the surface layer.
Zhou, Quan
2010-01-01
We present a systematic experimental study of geometric and statistical properties of thermal plumes in turbulent Rayleigh-B\\'{e}nard convection using the thermochromic-liquid-crystal (TLC) technique. The experiments were performed in three water-filled cylindrical convection cells with aspect ratios 2, 1, and 0.5 and over the Rayleigh-number range $5\\times10^7 \\leq Ra \\leq 10^{11}$. TLC thermal images of horizontal plane cuts at various depths below the top plate were acquired. Three-dimensional images of thermal plumes were then reconstructed from the two-dimensional slices of the temperature field. The results show that the often-called sheetlike plumes are really one-dimensional structures and may be called rodlike plumes. We find that the number densities for both sheetlike/rodlike and mushroomlike plumes have power-law dependence on $Ra$ with scaling exponents of $\\sim 0.3$, which is close to that between the Nusselt number $Nu$ and $Ra$. This result suggests that it is the plume number that primarily d...
Variable turbulent convection as the cause of the Blazhko effect - testing the Stothers model
Smolec, R; Kolenberg, K; Bryson, S; Cote, M T; Morris, R L
2011-01-01
The amplitude and phase modulation observed in a significant fraction of the RR Lyrae variables - the Blazhko effect - represents a long-standing enigma in stellar pulsation theory. No satisfactory explanation for the Blazhko effect has been proposed so far. In this paper we focus on the Stothers (2006) idea, in which modulation is caused by changes in the structure of the outer convective zone, caused by a quasi-periodically changing magnetic field. However, up to this date no quantitative estimates were made to investigate whether such a mechanism can be operational and whether it is capable of reproducing the light variation we observe in Blazhko variables. We address the latter problem. We use a simplified model, in which the variation of turbulent convection is introduced into the non-linear hydrodynamic models in an ad hoc way, neglecting interaction with the magnetic field. We study the light curve variation through the modulation cycle and properties of the resulting frequency spectra. Our results are...
Rai, Raj K.; Berg, Larry K.; Kosović, Branko; Mirocha, Jeffrey D.; Pekour, Mikhail S.; Shaw, William J.
2016-11-01
The Weather Research and Forecasting (WRF) model can be used to simulate atmospheric processes ranging from quasi-global to tens of m in scale. Here we employ large-eddy simulation (LES) using the WRF model, with the LES-domain nested within a mesoscale WRF model domain with grid spacing decreasing from 12.15 km (mesoscale) to 0.03 km (LES). We simulate real-world conditions in the convective planetary boundary layer over an area of complex terrain. The WRF-LES model results are evaluated against observations collected during the US Department of Energy-supported Columbia Basin Wind Energy Study. Comparison of the first- and second-order moments, turbulence spectrum, and probability density function of wind speed shows good agreement between the simulations and observations. One key result is to demonstrate that a systematic methodology needs to be applied to select the grid spacing and refinement ratio used between domains, to avoid having a grid resolution that falls in the grey zone and to minimize artefacts in the WRF-LES model solutions. Furthermore, the WRF-LES model variables show large variability in space and time caused by the complex topography in the LES domain. Analyses of WRF-LES model results show that the flow structures, such as roll vortices and convective cells, vary depending on both the location and time of day as well as the distance from the inflow boundaries.
Energy Technology Data Exchange (ETDEWEB)
Rai, Raj K.; Berg, Larry K.; Kosović, Branko; Mirocha, Jeffrey D.; Pekour, Mikhail S.; Shaw, William J.
2016-11-25
High resolution numerical simulation can provide insight into important physical processes that occur within the planetary boundary layer (PBL). The present work employs large eddy simulation (LES) using the Weather Forecasting and Research (WRF) model, with the LES domain nested within mesoscale simulation, to simulate real conditions in the convective PBL over an area of complex terrain. A multiple nesting approach has been used to downsize the grid spacing from 12.15 km (mesoscale) to 0.03 km (LES). A careful selection of grid spacing in the WRF Meso domain has been conducted to minimize artifacts in the WRF-LES solutions. The WRF-LES results have been evaluated with in situ and remote sensing observations collected during the US Department of Energy-supported Columbia BasinWind Energy Study (CBWES). Comparison of the first- and second-order moments, turbulence spectrum, and probability density function (PDF) of wind speed shows good agreement between the simulations and data. Furthermore, the WRF-LES variables show a great deal of variability in space and time caused by the complex topography in the LES domain. The WRF-LES results show that the flow structures, such as roll vortices and convective cells, vary depending on both the location and time of day. In addition to basic studies related to boundary-layer meteorology, results from these simulations can be used in other applications, such as studying wind energy resources, atmospheric dispersion, fire weather etc.
FULLY CONVECTIVE MAGNETO-ROTATIONAL TURBULENCE IN LARGE ASPECT-RATIO SHEARING BOXES
Energy Technology Data Exchange (ETDEWEB)
Bodo, G.; Rossi, P. [INAF, Osservatorio Astronomico di Torino, Strada Osservatorio 20, I-10025 Pino Torinese (Italy); Cattaneo, F. [The Computation Institute, The University of Chicago, 5735 South Ellis Avenue, Chicago, IL 60637 (United States); Mignone, A., E-mail: bodo@oato.inaf.it [Dipartimento di Fisica, Univesità di Torino, via Pietro Giuria 1, I-10125 Torino (Italy)
2015-01-20
We present a numerical study of turbulence and dynamo action in stratified shearing boxes with both finite and zero net magnetic flux. We assume that the fluid obeys the perfect gas law and has finite thermal diffusivity. The latter is chosen to be small enough so that vigorous convective states develop. The properties of these convective solutions are analyzed as the aspect ratio of the computational domain is varied and as the value of the mean field is increased. For the cases with zero net flux, we find that a well-defined converged state is obtained for large enough aspect ratios. In the converged state, the dynamo can be extremely efficient and can generate substantial toroidal flux. We identify solutions in which the toroidal field is mostly symmetric about the mid-plane and solutions in which it is mostly anti-symmetric. The symmetric solutions are found to be more efficient at transporting angular momentum and can give rise to a luminosity that is up to an order of magnitude larger than the corresponding value for the anti-symmetric states. In the cases with a finite net flux, the system appears to spend most of the time in the symmetric states.
De Bruin, Henk; Hartogensis, Oscar
2015-04-01
In this study we will investigate the assumption that in the atmospheric surface layer the outer scale (L0) is proportional to the height above the surface, under dry convective conditions. For this purpose we analyzed raw sonic anemometers data collected at 3.5 m and at 9 m in a field campaign at the Santa Cruz Flats (32040.3190'N, 111032.641'W, 526 m of elevation) near Eloy, Arizona. For simplicity, we define the L0 as that separation distance at which the spatial correlation coefficient of air temperature at two points in the surface layer is 0.5. Then, according to the 2/3-Kolmogorov scaling law in the inertial sub-range, L0 is determined by the variance and the structure parameter of T . It is found that L0 does not scale with height. Possible reasons for this negative result will be discussed, by considering the methodology to determine structure parameters, Taylor's frozen turbulence hypothesis, effects of intermittency and Monin-Obukhov flux relationships for variance and structure parameter for T . The question is asked whether the concept of surface constant flux layer still holds under strong convective condition.
Rai, Raj K.; Berg, Larry K.; Kosović, Branko; Mirocha, Jeffrey D.; Pekour, Mikhail S.; Shaw, William J.
2017-04-01
The Weather Research and Forecasting (WRF) model can be used to simulate atmospheric processes ranging from quasi-global to tens of m in scale. Here we employ large-eddy simulation (LES) using the WRF model, with the LES-domain nested within a mesoscale WRF model domain with grid spacing decreasing from 12.15 km (mesoscale) to 0.03 km (LES). We simulate real-world conditions in the convective planetary boundary layer over an area of complex terrain. The WRF-LES model results are evaluated against observations collected during the US Department of Energy-supported Columbia Basin Wind Energy Study. Comparison of the first- and second-order moments, turbulence spectrum, and probability density function of wind speed shows good agreement between the simulations and observations. One key result is to demonstrate that a systematic methodology needs to be applied to select the grid spacing and refinement ratio used between domains, to avoid having a grid resolution that falls in the grey zone and to minimize artefacts in the WRF-LES model solutions. Furthermore, the WRF-LES model variables show large variability in space and time caused by the complex topography in the LES domain. Analyses of WRF-LES model results show that the flow structures, such as roll vortices and convective cells, vary depending on both the location and time of day as well as the distance from the inflow boundaries.
Forces on stationary particles in near-bed turbulent flows
Schmeeckle, M.W.; Nelson, J.M.; Shreve, R.L.
2007-01-01
In natural flows, bed sediment particles are entrained and moved by the fluctuating forces, such as lift and drag, exerted by the overlying flow on the particles. To develop a better understanding of these forces and the relation of the forces to the local flow, the downstream and vertical components of force on near-bed fixed particles and of fluid velocity above or in front of them were measured synchronously at turbulence-resolving frequencies (200 or 500 Hz) in a laboratory flume. Measurements were made for a spherical test particle fixed at various heights above a smooth bed, above a smooth bed downstream of a downstream-facing step, and in a gravel bed of similarly sized particles as well as for a cubical test particle and 7 natural particles above a smooth bed. Horizontal force was well correlated with downstream velocity and not correlated with vertical velocity or vertical momentum flux. The standard drag formula worked well to predict the horizontal force, but the required value of the drag coefficient was significantly higher than generally used to model bed load motion. For the spheres, cubes, and natural particles, average drag coefficients were found to be 0.76, 1.36, and 0.91, respectively. For comparison, the drag coefficient for a sphere settling in still water at similar particle Reynolds numbers is only about 0.4. The variability of the horizontal force relative to its mean was strongly increased by the presence of the step and the gravel bed. Peak deviations were about 30% of the mean force for the sphere over the smooth bed, about twice the mean with the step, and 4 times it for the sphere protruding roughly half its diameter above the gravel bed. Vertical force correlated poorly with downstream velocity, vertical velocity, and vertical momentum flux whether measured over or ahead of the test particle. Typical formulas for shear-induced lift based on Bernoulli's principle poorly predict the vertical forces on near-bed particles. The
Idealized hydrodynamic simulations of turbulent oxygen-burning shell convection in 4π geometry
Jones, S.; Andrassy, R.; Sandalski, S.; Davis, A.; Woodward, P.; Herwig, F.
2017-03-01
This work investigates the properties of convection in stars with particular emphasis on entrainment across the upper convective boundary (CB). Idealized simulations of turbulent convection in the O-burning shell of a massive star are performed in 4π geometry on 7683 and 15363 grids, driven by a representative heating rate. A heating series is also performed on the 7683 grid. The 15363 simulation exhibits an entrainment rate at the upper CB of 1.33 × 10-6 M⊙ s-1. The 7683 simulation with the same heating rate agrees within 17 per cent. The entrainment rate at the upper CB is found to scale linearly with the driving luminosity and with the cube of the shear velocity at the upper boundary, while the radial rms fluid velocity scales with the cube root of the driving luminosity, as expected. The mixing is analysed in a 1D diffusion framework, resulting in a simple model for CB mixing. The analysis confirms that limiting the MLT mixing length to the distance to the CB in 1D simulations better represents the spherically averaged radial velocity profiles from the 3D simulations and provide an improved determination of the reference diffusion coefficient D0 for the exponential diffusion CB mixing model in 1D. From the 3D simulation data, we adopt as the CB the location of the maximum gradient in the horizontal velocity component which has 2σ spatial fluctuations of ≈0.17HP. The exponentially decaying diffusion CB mixing model with f = 0.03 reproduces the spherically averaged 3D abundance profiles.
Aspect ratio dependence of heat transfer and large-scale flow in turbulent convection
Bailon-Cuba, Jorge; Schumacher, Joerg
2010-01-01
The heat transport and corresponding changes in the large-scale circulation (LSC) in turbulent Rayleigh-B\\'{e}nard convection are studied by means of three-dimensional direct numerical simulations as a function of the aspect ratio $\\Gamma$ of a closed cylindrical cell and the Rayleigh number $Ra$. For small and moderate aspect ratios, the global heat transfer law $Nu=A\\times Ra^{\\beta}$ shows a power law dependence of both fit coefficients $A$ and $\\beta$ on the aspect ratio. A minimum Nusselt number coincides with the point where the LSC undergoes a transition from a single-roll to a double-roll pattern. With increasing aspect ratio, we detect complex multi-roll LSC configurations. The aspect ratio dependence of the turbulent heat transfer for small and moderate $\\Gamma$ is in line with a varying amount of energy contained in the LSC, as quantified by the Proper Orthogonal Decomposition analysis. For $\\Gamma\\gtrsim 8$ the heat transfer becomes independent of the aspect ratio.
Extreme dissipation event due to plume collision in a turbulent convection cell
Schumacher, Joerg
2016-01-01
An extreme dissipation event in the bulk of a closed three-dimensional turbulent convection cell is found to be correlated with a strong reduction of the large-scale circulation flow in the system that happens at the same time as a plume emission event from the bottom plate. The reduction in the large-scale circulation opens the possibility for a nearly frontal collision of down- and upwelling plumes and the generation of a high-amplitude thermal dissipation layer in the bulk. This collision is locally connected to a subsequent high-amplitude energy dissipation event in the form of a strong shear layer. Our analysis illustrates the impact of transitions in the large-scale structures on extreme events at the smallest scales of the turbulence, a direct link that is observed in a flow with boundary layers. We also show that detection of extreme dissipation events which determine the far-tail statistics of the dissipation fields in the bulk requires long-time integrations of the equations of motion over at least ...
The Göttingen rotating turbulent Rayleigh-Bénard convection facility
Bodenschatz, Eberhard; van Gils, Dennis; He, Xiaozhou; Ahlers, Guenter; International CollaborationTurbulence Research, EuHIT Collaboration
2015-11-01
This presentation will focus on the newly commissioned rotating RBC facility at the Max Planck Institute for Dynamics and Self-Organization (MPIDS). The MPIDS has a pressure vessel, called the Uboot of Göttingen, which can house different RBC cells. By pressurizing the Uboot with sulfur hexafluoride, nitrogen, or helium up to 19 bars one can obtain Rayleigh numbers spanning 109 Uboot, on top of which the current RBC cell of aspect ratio 0.50 can be installed. The accessible parameter space is 0 . 02 < Ro-1 < 20 for the inverse Rossby number and 10-8 < Ek <10-3 for the Ekman number. At strong rotation (small Ek) but still turbulently convective (large Ra) one enters the geostrophic turbulent regime. Recent experiments involve measuring in and near this regime of which preliminary results will be shown and discussed. We thank the Max Planck Society, the German Science Foundation SFB 963, the NSF grant DMR11-58514, and EuHIT for generous support.
Mccolgan, C. J.; Larson, R. S.
1977-01-01
The effect of flight on the mean flow and turbulence properties of a 0.056m circular jet were determined in a free jet wind tunnel. The nozzle exit velocity was 122 m/sec, and the wind tunnel velocity was set at 0, 12, 37, and 61 m/sec. Measurements of flow properties including mean velocity, turbulence intensity and spectra, and eddy convection velocity were carried out using two linearized hot wire anemometers. This report contains the raw data and graphical presentations. The final technical report includes a description of the test facilities, test hardware, along with significant test results and conclusions.
On the evolution of flow topology in turbulent Rayleigh-Bénard convection
Dabbagh, F.; Trias, F. X.; Gorobets, A.; Oliva, A.
2016-11-01
Small-scale dynamics is the spirit of turbulence physics. It implicates many attributes of flow topology evolution, coherent structures, hairpin vorticity dynamics, and mechanism of the kinetic energy cascade. In this work, several dynamical aspects of the small-scale motions have been numerically studied in a framework of Rayleigh-Bénard convection (RBC). To do so, direct numerical simulations have been carried out at two Rayleigh numbers Ra = 108 and 1010, inside an air-filled rectangular cell of aspect ratio unity and π span-wise open-ended distance. As a main feature, the average rate of the invariants of the velocity gradient tensor (QG, RG) has displayed the so-called "teardrop" spiraling shape through the bulk region. Therein, the mean trajectories are swirling inwards revealing a periodic spin around the converging origin of a constant period that is found to be proportional to the plumes lifetime. This suggests that the thermal plumes participate in the coherent large-scale circulation and the turbulent wind created in the bulk. Particularly, it happens when the plumes elongate substantially to contribute to the large-scale eddies at the lower turbulent state. Supplementary small-scale properties, which are widely common in many turbulent flows have been observed in RBC. For example, the strong preferential alignment of vorticity with the intermediate eigenstrain vector, and the asymmetric alignment between vorticity and the vortex-stretching vector. It has been deduced that in a hard turbulent flow regime, local self-amplifications of straining regions aid in contracting the vorticity worms, and enhance the local interactions vorticity/strain to support the linear vortex-stretching contributions. On the other hand, the evolution of invariants pertained to the traceless part of velocity-times-temperature gradient tensor has also been considered in order to determine the role of thermals in the fine-scale dynamics. These new invariants show an
Predicting Turbulent Convective Heat Transfer in Three-Dimensional Duct Flows
Rokni, M.; Gatski, T. B.
1999-01-01
The performance of an explicit algebraic stress model is assessed in predicting the turbulent flow and forced heat transfer in straight ducts, with square, rectangular, trapezoidal and triangular cross-sections, under fully developed conditions over a range of Reynolds numbers. Iso-thermal conditions are imposed on the duct walls and the turbulent heat fluxes are modeled by gradient-diffusion type models. At high Reynolds numbers (>/= 10(exp 5)), wall functions are used for the velocity and temperature fields; while at low Reynolds numbers damping functions are introduced into the models. Hydraulic parameters such as friction factor and Nusselt number are well predicted even when damping functions are used, and the present formulation imposes minimal demand on the number of grid points without any convergence or stability problems. Comparison between the models is presented in terms of the hydraulic parameters, friction factor and Nusselt number, as well as in terms of the secondary flow patterns occurring within the ducts.
Convective Heat Transfer Coefficients of the Human Body under Forced Convection from Ceiling
DEFF Research Database (Denmark)
Kurazumi, Yoshihito; Rezgals, Lauris; Melikov, Arsen Krikor
2014-01-01
The average convective heat transfer coefficient for a seated human body exposed to downward flow from above was determined. Thermal manikin with complex body shape and size of an average Scandinavian female was used. The surface temperature distribution of the manikin’s body was as the skin...... of the convective heat transfer coefficient of the whole body (hc [W/(m2•K)]) was proposed: hc=4.088+6.592V1.715 for a seated naked body at 20ºC and hc=2.874+7.427V1.345 for a seated naked body at 26ºC. Differences in the convective heat transfer coefficient of the whole body in low air velocity range, V
Csaki, Andras; Piazza, Marie; Truhetz, Heimo; Machulskaya, Ekaterina
2016-04-01
One of the biggest challenge in regional climate models is the proper use of parameterizations. Especially when resolutions increase some of the physical processes become (partially) resolved by the model dynamics and hence parameterizations can be switched off. One of the goal of this study is to investigate the relation between deep and shallow convection and the role of turbulence in COSMO-CLM. In the framework of the project Non-Hydrostatic Climate Modeling II (NHCM-2; www.nhcm-2.eu), funded by the Austrian Science Fund (FWF; project number P24758-N29), sensitivity experiments with the COSMO-CLM version 5.0, using the usual TKE and TKESV (Turbulent Kinetic Energy Scalar Variances) scheme, recently developed at the German Weather Service; DWD) turbulence scheme and the Tiedke scheme with shallow-convection on/off, are performed. These experiments, driven by analysis fields of the Integrated Forecast System (IFS) of the ECMWF, are run with 3 km horizontal grid spacing and 60 vertical levels ("convection permitting" resolution) over the greater Alpine region. The model output is compared to analysis fields (near surface) of the Austrian nowcasting system INCA (1 km grid spacing) and analysis fields (2D and 3D) of the Swiss forecast model COSMO-7 (7 km grid spacing). In addition to a basic evaluation (temperature and precipitation), cloud over, total radiation, moisture, and vertical wind speed are also considered. First results indicate that using the Tiedke scheme in this resolution leads to a lack of the precipitation in summer afternoon, because of unrealistic high cloud production. Shallow convection avoids this and leads to a more realistic diurnal cycle in summer precipitation. Further results (including TKSVE) will be shown.
Finite-Element Analysis of Forced Convection and Conduction
Wieting, A. R.
1982-01-01
TAP2 thermal-analysis program was developed as part of research on finite element methodology for thermal analysis of convectively cooled structures, such as scramjet engines and hypersonic aircraft. Program simplifies computations when both structural and thermal analyses are required and is suited for thermal analysis of nuclear reactors and solar-panel heating systems.
Indian Academy of Sciences (India)
ASHOK K BARIK; PRASANTA K SATAPATHY; SUDHANSU S SAHOO
2016-07-01
In this paper, the forced convective heat transfer enhancement with nanofluids in a 90° pipe bend has been presented. Numerical investigation is carried out for the turbulent flow through the pipe employing finite volume method. The governing differential equations are discretized using hexahedral cells, and theresulting algebraic equations are solved using Commercial solver Fluent 6.3. In order to close the time averaged Navier–Stokes equations, the two-equation k–e turbulence model with a standard wall function have been used.The duct Reynolds number is varied in the range of 2,500–6,000. It is observed that the heat transfer is enhanced significantly by varying the volume fraction of the nanofluid. It is also found that the heat transfer is increased with Reynolds number. A strong secondary flow is observed due to the presence of the wall. Turbulent kinetic energy near outer wall is found to be higher than the inner wall of the bend. A comparative assessment for the heat transfer enhancement with different types of nanofluids is also presented. The computed results of areaweighted average Nusselt numbers are validated with some of the existing literature
A Study of Nucleate Boiling with Forced Convection in Microgravity
Merte, Herman, Jr.
1999-01-01
The ultimate objective of basic studies of flow boiling in microgravity is to improve the understanding of the processes involved, as manifested by the ability to predict its behavior. This is not yet the case for boiling heat transfer even in earth gravity, despite the considerable research activity over the past 30 years. The elements that constitute the nucleate boiling process - nucleation, growth, motion, and collapse of the vapor bubbles (if the bulk liquid is subcooled) - are common to both pool and flow boiling. It is well known that the imposition of bulk liquid motion affects the vapor bubble behavior relative to pool boiling, but does not appear to significantly influence the heat transfer. Indeed, it has been recommended in the past that empirical correlations or experimental data of pool boiling be used for design purposes with forced convection nucleate boiling. It is anticipated that such will most certainly not be possible for boiling in microgravity, based on observations made with pool boiling in microgravity. In earth gravity buoyancy will act to remove the vapor bubbles from the vicinity of the heater surface regardless of how much the imposed bulk velocity is reduced, depending, of course, on the geometry of the system. Vapor bubbles have been observed to dramatically increase in size in pool boiling in microgravity, and the heat flux at which dryout took place was reduced considerably below what is generally termed the critical heat flux (CHF) in earth gravity, depending on the bulk liquid subcooling. However, at heat flux levels below dryout, the nucleate pool boiling process was enhanced considerably over that in earth gravity, in spite of the large vapor bubbles formed in microgravity and perhaps as a consequence. These large vapor bubbles tended to remain in the vicinity of the heater surface, and the enhanced heat transfer appeared to be associated with the presence of what variously has been referred to as a liquid microlayer between the
Evaluation of finite element formulations for transient conduction forced-convection analysis
Thornton, E. A.; Wieting, A. R.
1979-01-01
Numerical studies clarifying the advantages and disavantages of conventional versus upwind convective finite elements are presented along with lumped versus consistent formulations for practical conduction forced-convection analysis. A finite-element procedure for treatment of negligible capacitance fluid nodes is presented. The procedure is based on procedures used in finite-element structural dynamics to treat nodes with negligible structural mass. Two finite-element programs and a finite-difference lumped-parameter program used in the studies are discussed. Evaluation studies utilizing three convection and two combined conduction-convection problems are then presented and discussed. Additionally, the computational time saving offered by the finite element procedure is considered for a practical combined conduction-convection problem.
Forced Convection Heat Transfer in Plate Channels Filled with Packed Beds or Sintered Porous Media
Institute of Scientific and Technical Information of China (English)
姜培学; 李勐; 任泽霈
2002-01-01
In the present work, forced convection heat transfer in plate channels filled with metallic or non-metallic particles (packed beds) or sintered porous media is simulated numerically using a thermal non-equilibrium model. The numerical simulation results are compared with experimental data. The difference between convection heat transfer in packed beds and in sintered porous media and the effects of the boundary condition assumptions are investigated. The results show that the numerical simulation of convection heat transfer of air or water in packed beds using the local thermal non-equilibrium model and the variable porosity model agrees well with the experimental data. The convection heat transfer coefficient in sintered porous media is much higher than that in packed beds. In the numerical simulation of convection heat transfer in sintered porous media, the boundary conditions on the wall should be that the particle temperatures are equal to the fluid temperature.
Numerical turbulence forced through localized random expansion waves
Mee, A J; Mee, Antony J.; Brandenburg, Axel
2006-01-01
In an attempt to determine the outer scale of turbulence driven by localized sources, such as supernova explosions in the interstellar medium, we consider a forcing function given by the gradient of gaussian profiles localized at random positions. Different coherence times of the forcing function are considered. In order to isolate the effects specific to the nature of the forcing function we consider the case of an isothermal equation of state and restrict ourselves to forcing amplitudes such that the flow remains subsonic. When the coherence time is short, the outer scale agrees with the scale of the gaussian. Longer coherence times can cause extra power at large scales, but this would not yield power law behavior at scales larger than that of the expansion waves. At scales smaller than the scale of the expansion waves the spectrum is close to power law with a spectral exponent of -2. The resulting flow is virtually free of vorticity. Viscous driving of vorticity turns out to be weak and self-amplification ...
Heat transfer in forced convection boiling of oil-non-azeotropic binary refrigerant mixtures
Energy Technology Data Exchange (ETDEWEB)
Sami, S.M.; Tulej, P.; Fang, L. (Moncton Univ., NB (Canada))
1993-12-01
In this paper, the results of the heat transfer, forced convection, boiling characteristics of non-azeotropic refrigerant mixtures and oil are presented. This includes heat transfer coefficients for pure and binary mixtures under boiling conditions outside enhanced surface tubing. Local convective heat transfer coefficients have been determined using a modified Wilson-plot technique. Heat transfer correlations were established as a function of the binary mixture mass flow rate, and oil concentration, as well as key flow parameters. (author)
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.
Kwon, Younghwan; Lee, Kwangho; Park, Minchan; Koo, Kyoungmin; Lee, Jaekeun; Doh, Youngjin; Lee, Soowon; Kim, Doohyun; Jung, Yoongho
2013-12-01
An experimental investigation on the characteristics of the convective heat transfer in the fully developing region of a circular straight tube with a constant heat flux was carried out with Al2O3 nanofluids. Stable nanofluids, which were water-based suspensions of Al2O3 nanoparticles, were prepared by two-step method. The effects of the thermal conductivity, viscosity, and heat capacity of the nanofluids on convective heat transfer were investigated. The result showed that the coefficient enhancement of the convective heat transfer in the Al2O3 nanofluids was increased with increasing fluid temperature compared to that of water at a volume fraction of 3.0% in the turbulent flow region. Thermal conductivity was increased from 8% to 20%, and the increment of convective heat transfer coefficient was enhanced from 14% to 30% with fluid temperature from 22 degrees C to 75 degrees C, respectively. We observed that the increment of convective heat transfer coefficient in nanofluids was much higher than that of the thermal conductivity at a given temperature condition. The enhancement of Brownian motion due to the decreasing kinematic viscosity led to a higher convective heat transfer coefficient at a higher temperature condition.
Maki, Syou; Tanaka, Keito; Morimoto, Shotaro
2017-02-01
We examined, by three-dimensional numerical computations, the magnetothermal convection of air (a paramagnetic substance) enclosed in a cylindrical vessel with a Rayleigh-Benard model under the application of an axisymmetric magnetic force at the center of a solenoidal superconducting magnet. Axisymmetric steady convective flows were induced when the magnitude of the radial component of the magnetic force (fmR) was 1.0 and 5.0 times that of the gravitational force at the vessel sidewall; e.g., the hot air was concentrated at the vessel center and the cold air was driven to the vicinity of the vessel sidewall. This flow pattern was similar to the case of water (a diamagnetic substance), although the axisymmetric arrangements of hot and cold water were the reverse of the present convection of air. When fmR was 0.5 times that of the gravitational force, the axisymmetric flows appeared only in the vicinity of the vessel sidewall. Unsteady convective rolls simultaneously occurred in the vessel center, and they repeatedly combined and separated from each other. When fmR was 0.1 times that of the gravitational force, there were barely any axisymmetric flows in the close vicinity of the vessel sidewall, while the initial convective flows remained in most other parts of the vessel. Thus, we varied the magnitude of fmR and clarified the transitional processes of isothermal and velocity distributions of magnetothermal convection. We discuss those convective flows with the magnitude and direction of fmR.
Nonlocal modulation of the energy cascade in broadband-forced turbulence
Kuczaj, Arkadiusz K.; Geurts, Bernardus J.; McComb, W. David
2006-01-01
Classically, large-scale forced turbulence is characterized by a transfer of energy from large to small scales via nonlinear interactions. We have investigated the changes in this energy transfer process in broadband forced turbulence where an additional perturbation of flow at smaller scales is int
Temperature variance profiles of turbulent thermal convection at high Rayleigh numbers
He, Xiaozhou; Bodenschatz, Eberhard; Ahlers, Guenter
2016-11-01
We present measurements of the Nusselt number Nu , and of the temperature variance σ2 as a function of vertical position z, in turbulent Rayleigh-Bénard convection of two cylindrical samples with aspect ratios (diameter D/height L) Γ = 0 . 50 and 0 . 33 . Both samples had D = 1 . 12 m but different L. We used compressed SF6 gas at pressures up to 19 bars as the fluid. The measurements covered the Rayleigh-number range 1013 < Ra < 5 ×1015 at a Prandtl number Pr = 0 . 80 . Near the side wall we found that σ2 is independent of Ra when plotted as a function of z / λ where λ ≡ L / (2 Nu) is a thermal boundary-layer thickness. The profiles σ2 (z / λ) for the two Γ values overlapped and followed a logarithmic function for 20 z / λ 120 . With the observed "-1"-scaling of the temperature power spectra and on the basis of the Perry-Townsend similarity hypothesis, we derived a fitting function σ2 =p1 ln (z / λ) +p2 +p3(z / λ) - 0 . 5 which describes the σ2 data up to z / λ = 1500 . Supported by the Max Planck Society, the Volkswagenstiftung, the DFD Sonderforschungsbereich SFB963, and NSF Grant DMR11-58514.
Effect of Plumes on Measuring the Large Scale Circulation in Turbulent Rayleigh-B\\'enard Convection
Stevens, Richard J A M; Lohse, Detlef
2011-01-01
We studied the properties of the large-scale circulation (LSC) in turbulent Rayleigh-B\\'enard (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 polynomial fit to the azimuthal temperature or azimuthal vertical velocity profile measured with the probes. We study the LSC in \\Gamma=D/L=1/2 and \\Gamma=1 samples, where D is the diameter and L the height. For Pr=6.4 in an aspect ratio \\Gamma=1 sample at $Ra=1\\times10^8$ and $5\\times10^8$ the obtained LSC orientation is the same, irrespective of whether the data of only 8 or all 64 probes per horizontal plane are considered. In a \\Gamma=1/2 sample with $Pr=0.7$ at $Ra=1\\times10^8$ the influence of plumes on the azimuthal temperature and azimuthal vertical velocity profiles is stronger. Due to passing plumes and/or the corner flow the apparent LSC orientation obtained using a cosine fit can result in a misinterpretati...
Turbulent Rayleigh-Bénard convection with polymers: Understanding how heat flux is modified
Benzi, Roberto; Ching, Emily S. C.; De Angelis, Elisabetta
2016-12-01
We study how polymers affect the heat flux in turbulent Rayleigh-Bénard convection at moderate Rayleigh numbers using direct numerical simulations with polymers of different relaxation times. We find that heat flux is enhanced by polymers and the amount of heat enhancement first increases and then decreases with the Weissenberg number, which is the ratio of the polymer relaxation time to the typical time scale of the flow. We show that this nonmonotonic behavior of the heat flux enhancement is the combined effect of the decrease in the viscous energy dissipation rate due to the viscosity of the Newtonian fluid and the increase in the energy dissipation rate due to polymers when Weissenberg number is increased. We explain why the viscous energy dissipation rate decreases with the Weissenberg number. Then by carrying out a generalized boundary layer analysis supplemented by a space-dependent effective viscosity from the numerical simulations, we provide a theoretical understanding of the change of the heat flux when the viscous energy dissipation rate is held constant. Our analysis thus provides a physical way to understand the numerical results.
Breaking the boundary layer symmetry in turbulent convection using wall geometry
Toppaladoddi, Srikanth; Wettlaufer, John S
2014-01-01
We systematically probe the interaction of the boundary layer with the core flow during two-dimensional turbulent Rayleigh-B\\'{e}nard convection using numerical simulations and scaling theory. The boundary layer/core flow interaction is manipulated by configuring the top plate with a sinusoidal geometry and breaking the symmetry between the top and bottom thermal boundary layers. At long wavelength the planar results are recovered. However, at intermediate wavelengths, and for Rayleigh numbers ($Ra$) such that the amplitude of the roughness elements is larger than the boundary layer thickness, there is enhanced cold plume production at the tips of the elements. It is found that, while the interior of the flow is well mixed as in the classical theory of Malkus, the mean temperature is lower than that in the planar case. For a Prandtl number of unity and $Ra = 10^6$ to $2.5 \\times 10^9$ we find a Nusselt number ($Nu$) scaling law of $Nu = 0.052 \\times Ra^{0.34}$, in good agreement with recent experiments. The c...
Search for the ``ultimate state" in turbulent Rayleigh-B'enard convection
Ahlers, Guenter; Funfschilling, Denis; Bodenschatz, Eberhard
2009-11-01
Measurements of the Nusselt number Nu will be reported for turbulent Rayleigh-B'enard convection of a cylindrical sample. They cover the Rayleigh-number range 10^11 Ra 2x10^15 using N2 (Pr = 0.72) and SF6 (Pr = 0.79 to 0.84) at pressures up to 19 bars and near-ambient temperatures. The sample had a height L=2.24m and diameter D = 1.12m and utilized the high-pressure vessel known as the ``Uboot of G"ottingen" at the Max Planck Institute for Dynamics and Self-Organization in G"ottingen, Germany. For Ra 4x10^13 the data yielded Nu Ra^γeff with γeff= 0.308 and did not show the transition near Ra = 10^11 to an ``ultimate regime" that was reported by Chavanne et al. At Ra = 4x10^13 there is a well defined but continuous transition to a regime where γeff is smaller than 0.30.
Rincon, F.; Roudier, T.; Schekochihin, A. A.; Rieutord, M.
2017-03-01
The Sun provides us with the only spatially well-resolved astrophysical example of turbulent thermal convection. While various aspects of solar photospheric turbulence, such as granulation (one-Megameter horizontal scale), are well understood, the questions of the physical origin and dynamical organization of larger-scale flows, such as the 30-Megameters supergranulation and flows deep in the solar convection zone, remain largely open in spite of their importance for solar dynamics and magnetism. Here, we present a new critical global observational characterization of multiscale photospheric flows and subsequently formulate an anisotropic extension of the Bolgiano-Obukhov theory of hydrodynamic stratified turbulence that may explain several of their distinctive dynamical properties. Our combined analysis suggests that photospheric flows in the horizontal range of scales between supergranulation and granulation have a typical vertical correlation scale of 2.5 to 4 Megameters and operate in a strongly anisotropic, self-similar, nonlinear, buoyant dynamical regime. While the theory remains speculative at this stage, it lends itself to quantitative comparisons with future high-resolution acoustic tomography of subsurface layers and advanced numerical models. Such a validation exercise may also lead to new insights into the asymptotic dynamical regimes in which other, unresolved turbulent anisotropic astrophysical fluid systems supporting waves or instabilities operate.
Multi-crystalline silicon solidification under controlled forced convection
Cablea, M.; Zaidat, K.; Gagnoud, A.; Nouri, A.; Chichignoud, G.; Delannoy, Y.
2015-05-01
Multi-crystalline silicon wafers have a lower production cost compared to mono-crystalline wafers. This comes at the price of reduced quality in terms of electrical properties and as a result the solar cells made from such materials have a reduced efficiency. The presence of different impurities in the bulk material plays an important role during the solidification process. The impurities are related to different defects (dislocations, grain boundaries) encountered in multi-crystalline wafers. Applying an alternative magnetic field during the solidification process has various benefits. Impurities concentration in the final ingot could be reduced, especially metallic species, due to a convective term added in the liquid that reduces the concentration of impurities in the solute boundary layer. Another aspect is the solidification interface shape that is influenced by the electromagnetic stirring. A vertical Bridgman type furnace was used in order to study the solidification process of Si under the influence of a travelling magnetic field able to induce a convective flow in the liquid. The furnace was equipped with a Bitter type three-phase electromagnet that provides the required magnetic field. A numerical model of the furnace was developed in ANSYS Fluent commercial software. This paper presents experimental and numerical results of this approach, where interface markings were performed.
Flow Visualization of Forced and Natural Convection in Internal Cavities
Energy Technology Data Exchange (ETDEWEB)
John Crepeau; Hugh M. Mcllroy,Jr.; Donald M. McEligot; Keith G. Condie; Glenn McCreery; Randy Clarsean; Robert S. Brodkey; Yann G. Guezennec
2002-01-31
The report descries innovative flow visualization techniques, fluid mechanics measurements and computational models of flows in a spent nuclear fuel canister. The flow visualization methods used a fluid that reacted with a metal plate to show how a local reaction affects the surrounding flow. A matched index of refraction facility was used to take mean flow and turbulence measurements within a generic spent nuclear fuel canister. Computational models were also made of the flow in the canister. It was determined that the flow field in the canister was very complex, and modifications may need to be made to ensure that the spent fuel elements are completely passivated.
Uddin, Md Jashim; Khan, Waqar A; Ismail, A I Md
2013-01-01
A two-dimensional steady forced convective flow of a Newtonian fluid past a convectively heated permeable vertically moving plate in the presence of a variable magnetic field and radiation effect has been investigated numerically. The plate moves either in assisting or opposing direction to the free stream. The plate and free stream velocities are considered to be proportional to x(m) whilst the magnetic field and mass transfer velocity are taken to be proportional to x((m-1)/2) where x is the distance along the plate from the leading edge of the plate. Instead of using existing similarity transformations, we use a linear group of transformations to transform the governing equations into similarity equations with relevant boundary conditions. Numerical solutions of the similarity equations are presented to show the effects of the controlling parameters on the dimensionless velocity, temperature and concentration profiles as well as on the friction factor, rate of heat and mass transfer. It is found that the rate of heat transfer elevates with the mass transfer velocity, convective heat transfer, Prandtl number, velocity ratio and the magnetic field parameters. It is also found that the rate of mass transfer enhances with the mass transfer velocity, velocity ratio, power law index and the Schmidt number, whilst it suppresses with the magnetic field parameter. Our results are compared with the results existing in the open literature. The comparisons are satisfactory.
Convection forced by a descending dry layer and low-level moist convergence
Russell, Andrew; Vaughan, Geraint; Norton, Emily G.; Ricketts, Hugo M. A.; Morcrette, Cyril J.; Hewison, Tim J.; Browning, Keith. A.; Blyth, Alan M.
2009-03-01
A narrow line of convective showers was observed over southern England on 18 July 2005 during the Convective Storm Initiation Project (CSIP). The showers formed behind a cold front (CF), beneath two apparently descending dry layers (i.e. sloping so that they descended relative to the instruments observing them). The lowermost dry layer was associated with a tropopause fold from a depression, which formed 2 d earlier from a breaking Rossby wave, located northwest of the UK. The uppermost dry layer had fragmented from the original streamer due to rotation around the depression (This rotation was also responsible for the observations of apparent descent-ascent would otherwise be seen behind a CF). The lowermost dry layer descended over the UK and overran higher θw air beneath it, resulting in potential instability. Combined with a surface convergence line (which triggered the convection but had less impact on the convective available potential energy than the potential instability), convection was forced up to 5.5 km where the uppermost dry layer capped it. The period when convection was possible was very short, thus explaining the narrowness of the shower band. Convective Storm Initiation Project observations and model data are presented to illustrate the unique processes in this case.
Study on Forced Convective Heat Transfer of Non-Newtonian Nanofluids
Institute of Scientific and Technical Information of China (English)
Yurong He; Yubin Men; Xing Liu; Huilin Lu; Haisheng Chen; Yulong Ding
2009-01-01
This paper is concerned with the forced convective heat transfer of dilute liquid suspensions of nanoparticles (nanofluids) flowing through a straight pipe under laminar conditions. Stable nanofluids are formulated by using the high shear mixing and ultrasonication methods. They are then characterised for their size, surface charge, thermal and rheological properties and tested for their convective heat transfer behaviour. Mathematical model-ling is performed to simulate the convective heat transfer of nanofluids using a single phase flow model and con-sidering nanofluids as both Newtonian and non-Newtonian fluid. Both experiments and mathematical modelling show that nanofluids can substantially enhance the convective heat transfer. Analyses of the results suggest that the non-Newtonian character of nanofluids influences the overall enhancement, especially for nanofluids with an obvious non-Newtonian character.
Analysis of the Taylor dissipation surrogate in forced isotropic turbulence
McComb, W David; Yoffe, Samuel R
2013-01-01
From the energy balance in wavenumber space expressed by the Lin equation, we derive a new form for the local Karman-Howarth equation for forced isotropic turbulence in real space. This equation is then cast into a dimensionless form, from which a combined analytical and numerical study leads us to deduce a new model for the scale-independent nondimensional dissipation rate $\\Ceps$, which takes the form $\\Ceps = \\Cinf + C_L/R_L$, where the asymptotic value $\\Cinf$ can be evaluated from the third-order structure function. This is found to fit the numerical data with $\\Cinf = 0.47 \\pm 0.01$ and $C_L= 18.5 \\pm 1.3$. By considering $\\Ceps - \\Cinf$ on logarithmic scales, we show that $R_L^{-1}$ is indeed the correct Reynolds number behaviour. The model is compared to previous attempts in the literature, with encouraging agreement. The effects of the scale-dependence of the inertial and viscous terms due to finite forcing are then considered and shown to compensate one another, such that the model equation is appli...
The dependence of optical turbulence on thermal and mechanical forces over the sea
van Eijk, Alexander M. J.; Sprung, Detlev; Sucher, Erik; Eisele, Christian; Seiffer, Dirk; Stein, Karin
2016-09-01
Optical turbulence for over-water conditions was investigated in a long-term experiment over False Bay near Cape Town, South Africa. A sonic anemometer and two boundary-layer scintillometers were deployed to access in-situ turbulence as well as the integrated turbulence over two 1.8 and 8.7 km paths. Statistical analysis reveals spatial temporal variations of the turbulence conditions over False Bay, which might be related to differences in the atmospheric conditions and/or the surface (water) temperatures. An analysis in terms of mechanical and thermal forcing reveals that the latter factor is more dominant in determining the turbulence strength.
Simulation of Effects of the Saffman Force and the Magnus Force on Sand Saltation in Turbulent Flow
Energy Technology Data Exchange (ETDEWEB)
Zheng, Zhongquan C.; Zou, Xueyong; Yang, Xiaofan; Cheng, Hong
2011-12-11
The effects of both the Saffman force and Magnus force on sand saltation are investigated. Turbulent flows in a channel and over a barchans dune are considered with sand particles injected into the flow. The results show that both of the forces increase the height and skipping distance of sand saltation, with the Magnus force giving more significant effect on the height. These forces can also increase the sand settling at the lee side of the barchans dune.
Energy Technology Data Exchange (ETDEWEB)
Pal, Sandip, E-mail: sup252@PSU.EDU
2016-06-01
The convective boundary layer (CBL) turbulence is the key process for exchanging heat, momentum, moisture and trace gases between the earth's surface and the lower part of the troposphere. The turbulence parameterization of the CBL is a challenging but important component in numerical models. In particular, correct estimation of CBL turbulence features, parameterization, and the determination of the contribution of eddy diffusivity are important for simulating convection initiation, and the dispersion of health hazardous air pollutants and Greenhouse gases. In general, measurements of higher-order moments of water vapor mixing ratio (q) variability yield unique estimates of turbulence in the CBL. Using the high-resolution lidar-derived profiles of q variance, third-order moment, and skewness and analyzing concurrent profiles of vertical velocity, potential temperature, horizontal wind and time series of near-surface measurements of surface flux and meteorological parameters, a conceptual framework based on bottom up approach is proposed here for the first time for a robust characterization of the turbulent structure of CBL over land so that our understanding on the processes governing CBL q turbulence could be improved. Finally, principal component analyses will be applied on the lidar-derived long-term data sets of q turbulence statistics to identify the meteorological factors and the dominant physical mechanisms governing the CBL turbulence features. - Highlights: • Lidar based study for CBL turbulence features • Water vapor and aerosol turbulence profiles • Processes governing boundary layer turbulence profiles using lidars.
Directory of Open Access Journals (Sweden)
S. Galmarini
2007-01-01
Full Text Available The combined effect of turbulent transport and radioactive decay on the distribution of 222Rn and its progeny in convective atmospheric boundary layers (CBL is investigated. Large eddy simulation is used to simulate their dispersion in steady state CBL and in unsteady conditions represented by the growth of a CBL within a pre-existing reservoir layer. The exact decomposition of the concentration and flux budget equations under steady state conditions allowed us to determine which processes are responsible for the vertical distribution of 222Rn and its progeny. Their mean concentrations are directly correlated with their half-life, e.g. 222Rn and 210Pb are the most abundant whereas 218Po show the lowest concentrations. 222Rn flux decreases linearly with height and its flux budget is similar to the one of inert emitted scalar, i.e., a balance between on the one hand the gradient and the buoyancy production terms, and on the other hand the pressure and dissipation at smaller scales which tends to destroy the fluxes. While 222Rn exhibits the typical bottom-up behavior, the maximum flux location of the daughters is moving upwards while their rank in the 222Rn progeny is increasing leading to a typical top-down behavior for 210Pb. We also found that the relevant radioactive decaying contributions of 222Rn short-lived daughters (218Po and 214Pb act as flux sources leading to deviations from the linear flux shape. In addition, while analyzing the vertical distribution of the radioactive decay contributions to the concentrations, e.g. the decaying zone, we found a variation in height of 222Rn daughters' radioactive transformations. Under unsteady conditions, the same behaviors reported under steady state conditions are found: deviation of the fluxes from the linear shape for 218Po, enhanced discrepancy in height of the radioactive transformation contributions for all the daughters. In addition, 222Rn and its progeny concentrations decrease due to the
Zhou, Bowen; Xue, Ming; Zhu, Kefeng
2017-04-01
Compared to the representation of vertical turbulent mixing through various PBL schemes, the treatment of horizontal turbulence mixing in the boundary layer within mesoscale models, with O(10) km horizontal grid spacing, has received much less attention. In mesoscale models, subgrid-scale horizontal fluxes most often adopt the gradient-diffusion assumption. The horizontal mixing coefficients are usually set to a constant, or through the 2D Smagorinsky formulation, or in some cases based on the 1.5-order turbulence kinetic energy (TKE) closure. In this work, horizontal turbulent mixing parameterizations using physically based characteristic velocity and length scales are proposed for the convective boundary layer based on analysis of a well-resolved, wide-domain large-eddy simulation (LES). The proposed schemes involve different levels of sophistication. The first two schemes can be used together with first-order PBL schemes, while the third uses TKE to define its characteristic velocity scale and can be used together with TKE-based higher-order PBL schemes. The current horizontal mixing formulations are also assessed a priori through the filtered LES results to illustrate their limitations. The proposed parameterizations are tested a posteriori in idealized simulations of turbulent dispersion of a passive scalar. Comparisons show improved horizontal dispersion by the proposed schemes, and further demonstrate the weakness of the current schemes.
Analytical Solution of Forced-Convective Boundary-Layer Flow over a Flat Plate
DEFF Research Database (Denmark)
Mirgolbabaei, H.; Barari, Amin; Ibsen, Lars Bo
2010-01-01
In this letter, the problem of forced convection heat transfer over a horizontal flat plate is investigated by employing the Adomian Decomposition Method (ADM). The series solution of the nonlinear differential equations governing on the problem is developed. Comparison between results obtained...
Analytical Solution of Forced-Convective Boundary-Layer Flow over a Flat Plate
DEFF Research Database (Denmark)
Mirgolbabaei, H.; Barari, Amin; Ibsen, Lars Bo;
2010-01-01
In this letter, the problem of forced convection heat transfer over a horizontal flat plate is investigated by employing the Adomian Decomposition Method (ADM). The series solution of the nonlinear differential equations governing on the problem is developed. Comparison between results obtained...
Laboratory grey cast iron continuous casting line with electromagnetic forced convection support
2010-01-01
The article describes the construction of a 20 mm diameter grey cast iron ingots continuous casting laboratory line. This line is made ofthree main units: melting unit (induction furnace), casting unit and the pulling unit. In order to improve the homogeneity of themicrostructure of ingots (by applying forced convection of liquid metal during the crystallization process) in this case a crystallizer system generating the forced movement of liquid metal based on a system of electrical power win...
Mixed time integration schemes for transient conduction forced-convection analysis
Liu, W. K.; Lin, J. I.
1983-01-01
A partition procedure for forced-convection conduction transient problems is presented. Mixed time partitions are defined wherein coupled conduction force-matrix equations are discretized using an implicit integration method, followed by derivation of a mixed time integration technique. Explicit-implicit and explicit-explicit partitions are performed for a stability analysis for transient conditions, e.g., those found in an actively air-cooled engine and airframe structure.
Forced Convective Heat Transfer in a Porous Plate Channel
Institute of Scientific and Technical Information of China (English)
PeixueJiang; ZhanWang; 等
1997-01-01
Fored convective heat transfer in a plate channel filled with metallic spherical particales was investigated experimentally and numerically.The test section ,58mm×80mm×50mm in size,was heated by a 0.4mm thick plate electrical heater,The coolant water flow rate ranged from 0.015 to 0.833 kg/s.The local wall temperature distribution was measured along with the inlet and outlet fliud temperatures and pressures.The results illustrate the heat transfer augmentation and increased pressure drop caused by the porous medium.The heat transfer coefficient was increased 5-12 times by the porous media although the hydraulic resistance was increased even more.The Nusselt number and the heat transfer coefficient increased with decreasing particle diameter,while the pressure drop decreased as the particle diameter increased.It was found that,for the conditions studied(metallic packed bed),the effect of thermal dispersion did not need to be considered in the physical model,as opposed to a non-metallic packed bed,where thermal dispersion is important.
Zhou, Quan; Lu, Zhi-Ming; Liu, Yu-Lu
2010-01-01
We report an experimental investigation of the longitudinal space-time cross-correlation function of the velocity field, $C(r,\\tau)$, in a cylindrical turbulent Rayleigh-B\\'{e}nard convection cell using the particle image velocimetry (PIV) technique. We show that while the Taylor's frozen-flow hypothesis does not hold in turbulent thermal convection, the recent elliptic model advanced for turbulent shear flows [He & Zhang, \\emph{Phys. Rev. E} \\textbf{73}, 055303(R) (2006)] is valid for the present velocity field for all over the cell, i.e., the isocorrelation contours of the measured $C(r,\\tau)$ have a shape of elliptical curves and hence $C(r,\\tau)$ can be related to $C(r_E,0)$ via $r_E^2=(r-\\beta\\tau)^2+\\gamma^2\\tau^2$ with $\\beta$ and $\\gamma$ being two characteristic velocities. We further show that the fitted $\\beta$ is proportional to the mean velocity of the flow, but the values of $\\gamma$ are larger than the theoretical predictions. Specifically, we focus on two representative regions in the cell...
Dag, Yusuf
Forced convection over traditional surfaces such as flat plate, cylinder and sphere have been well researched and documented. Data on forced convection over airfoil surfaces, however, remain very scanty in literature. High altitude vehicles that employ airfoils as lifting surfaces often suffer leading edge ice accretions which have tremendous negative consequences on the lifting capabilities and stability of the vehicle. One of the ways of mitigating the effect of ice accretion involves judicious leading edge convective cooling technique which in turn depends on the accuracy of convective heat transfer coefficient used in the analysis. In this study empirical investigation of convective heat transfer measurements on asymmetric airfoil is presented at different angle of attacks ranging from 0° to 20° under subsonic flow regime. The top and bottom surface temperatures are measured at given points using Senflex hot film sensors (Tao System Inc.) and used to determine heat transfer characteristics of the airfoils. The model surfaces are subjected to constant heat fluxes using KP Kapton flexible heating pads. The monitored temperature data are then utilized to determine the heat convection coefficients modelled empirically as the Nusselt Number on the surface of the airfoil. The experimental work is conducted in an open circuit-Eiffel type wind tunnel, powered by a 37 kW electrical motor that is able to generate subsonic air velocities up to around 41 m/s in the 24 square-inch test section. The heat transfer experiments have been carried out under constant heat flux supply to the asymmetric airfoil. The convective heat transfer coefficients are determined from measured surface temperature and free stream temperature and investigated in the form of Nusselt number. The variation of Nusselt number is shown with Reynolds number at various angles of attacks. It is concluded that Nusselt number increases with increasing Reynolds number and increase in angle of attack from 0
Experimental study of temperature fluctuations in forced stably stratified turbulent flows
Eidelman, A; Gluzman, Y; Kleeorin, N; Rogachevskii, I
2013-01-01
We study experimentally temperature fluctuations in stably stratified forced turbulence in air flow. In the experiments with an imposed vertical temperature gradient, the turbulence is produced by two oscillating grids located nearby the side walls of the chamber. Particle Image Velocimetry is used to determine the turbulent and mean velocity fields, and a specially designed temperature probe with sensitive thermocouples is employed to measure the temperature field. We found that the ratio [(\\ell_x \
Measurement of turbulent kinetic energy spectrum - Part 2: Convection record measurements
DEFF Research Database (Denmark)
Velte, Clara Marika; Buchhave, Preben; Hodzic, Azur
2017-01-01
A novel exact temporal to spatial mapping for point measurements in turbulence has been applied to various flow conditions existing in a round turbulent jet. The conditions range between equilibrium and non-equilibrium as well as mid to high turbulence intensities. The exact mapping applies to al...
Measurement of turbulent kinetic energy spectrum - Part 2: Convection record measurements
DEFF Research Database (Denmark)
Velte, Clara Marika; Buchhave, Preben; Hodzic, Azur
2016-01-01
A novel exact temporal to spatial mapping for point measurements in turbulence has been applied to various flow conditions existing in a round turbulent jet. The conditions range between equilibrium and non-equilibrium as well as mid to high turbulence intensities. The exact mapping applies to al...
Torres, Olivier; Braconnot, Pascale; Gainusa-Bogdan, Alina; Hourdin, Frédéric; Marti, Olivier; Pelletier, Charles
2016-04-01
The turbulent fluxes across the ocean/atmosphere interface represent one of the principal driving forces of the global atmospheric and oceanic circulation and are also responsible for various phenomena like the water supply to the atmospheric column, which itself is extremely important for atmospheric convection. Although the representation of these fluxes has been the subject of major studies, it still remains a very challenging problem. Our aim is to better understand the role of these fluxes in climate change experiments and in the equator-pole redistribution of heat and water by the oceanic and atmospheric circulation. For this, we are developing a methodology starting from idealized 1D experiments and going all the way up to fully coupled ocean-atmosphere simulations of past and future climates. The poster will propose a synthesis of different simulations we have performed with a 1D version of the LMDz atmosphere model towards a first objective of understanding how different parameterizations of the turbulent fluxes affect the moisture content of the atmosphere and the feedback with the atmospheric boundary layer and convection schemes. Air-sea fluxes are not directly resolved by the models because they are subgrid-scale phenomena and are therefore represented by parametrizations. We investigate the differences between several 1D simulations of the TOGA-COARE campaign (1992-1993, Pacific warm pool region), for which 1D boundary conditions and observations are available to test the results of atmospheric models. Each simulation considers a different version of the LMDz model in terms of bulk formula (four) used to compute the turbulent fluxes. We also consider how the representation of gustiness in these parameterizations affects the results. The use of this LMDz test case (very constrained within an idealized framework) allows us to determine how the response of surface fluxes helps to reinforce or damp the atmospheric water vapor content or cloud feedbacks
Vertical Convection in Turbulent Accretion Disks and Light Curves of the A0620-00 1975 Outburst
Malanchev, Konstantin
2015-01-01
We present a model of the non-stationary $\\alpha$-disk with account for the irradiation and the vertical convection in the outer accretion disk where hydrogen is partially ionized. We include the viscous energy generation in the mix-length convection equations in accretion disks. The optical and X-ray light curves of X-ray nova A0620-00 are investigated in terms of this model. The turbulent viscosity parameter of the accretion disk is estimated, $\\alpha = 0.5 \\div 0.6$, which is necessary to explain the luminosity decay rate on the descending branch of the X-ray light curve for the A0620-00 1975 outburst. The secondary luminosity maximum on the light curves is explained by assuming an additional injection of matter into the accretion disk from the optical companion.
Mocák, Miroslav; Viallet, Maxime; Arnett, David
2014-01-01
We present a statistical analysis of turbulent convection in stars within our Reynolds-Averaged Navier Stokes (RANS) framework in spherical geometry which we derived from first principles. The primary results reported in this document include: (1) an extensive set of mean-field equations for compressible, multi-species hydrodynamics, and (2) corresponding mean-field data computed from various simulation models. Some supplementary scale analysis data is also presented. The simulation data which is presented includes: (1) shell convection during oxygen burning in a 23 solar mass supernova progenitor, (2) envelope convection in a 5 solar mass red giant, (3) shell convection during the helium flash, and (4) a hydrogen injection flash in a 1.25 solar mass star. These simulations have been partially described previously in Meakin [2006], Meakin and Arnett [2007a,b, 2010], Arnett et al. [2009, 2010], Viallet et al. [2011, 2013a,b] and Mocak et al. [2009, 2011]. New data is also included in this document with several...
Scherf, A.; Roth, R.
1996-12-01
During the field campaign of EFEDA II several aircraft measurements were performed in order to evaluate area mean values of turbulent energy fluxes over a relatively flat terrain in a desertification threatened area in Spain. Since earlier field experiments indicated differences between airborne measurements and surface observations, we tried to close the gap by carefully analysing the turbulence measurements. In order to evaluate the influence of the temporal variation of the convective boundary layer, the rise of the inversion, derived from simultaneously performed radiosonde ascents, was taken into account. By estimating the linear approximated fields of the meteorological parameters, it was possible to calculate the mean values of these quantities as well as the temporal and spatial derivatives, which are necessary for the evaluation of the advective terms of the energy budget. In this way is possible to examine the terms of the conservation equations in a supplementary way.
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.
Numerical analysis of laminar forced convection in a spherical annulus
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Tuft, D.B.
1980-07-21
Calculations of steady laminar incompressible fluid-flow and heat transfer in a spherical annulus are presented. Steady pressures, temperatures, velocities, and heat transfer coefficients are calculated for an insulated outer sphere and a 0/sup 0/C isothermal inner sphere with 50/sup 0/C heated water flowing in the annulus. The inner sphere radius is 13.97 cm, the outer sphere radius is 16.83 cm and the radius ratio is 1.2. The transient axisymmetric equations of heat, mass, and momentum conservation are solved numerically in spherical coordinates. The transient solution is carried out in time until steady state is achieved. A variable mesh is used to improve resolution near the inner sphere where temperature and velocity gradients are steep. It is believed that this is the first fully two-dimensional analysis of forced flow in a spherical annulus. Local and bulk Nusselt numbers are presented for Reynolds numbers from 4.4 to 440. Computed bulk Nusselt numbers ranged from 2 to 50 and are compared to experimental results from the literature. Inlet flow jetting off the inner sphere and flow separation are predicted by the analysis. The location of wall jet separation was found to be a function of Reynolds number, indicating the location of separation depends upon the ratio of inertia to viscous forces. Wall jet separation has a pronounced effect on the distribution of local heat flux. The area between inlet and separation was found to be the most significant area for heat transfer. Radial distributions of azimuthal velocity and temperature are presented for various angles beginning at the inlet. Inner sphere pressure distribution is presented and the effect on flow separation is discussed.
Heat-transport enhancement in rotating turbulent Rayleigh-Bénard convection
Weiss, Stephan; Wei, Ping; Ahlers, Guenter
2016-04-01
We present new Nusselt-number (Nu) measurements for slowly rotating turbulent thermal convection in cylindrical samples with aspect ratio Γ =1.00 and provide a comprehensive correlation of all available data for that Γ . In the experiment compressed gasses (nitrogen and sulfur hexafluride) as well as the fluorocarbon C6F14 (3M Fluorinert FC72) and isopropanol were used as the convecting fluids. The data span the Prandtl-number (Pr) range 0.74
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
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H. Pietersen
2014-07-01
Full Text Available We study the disturbances of CBL dynamics due to large-scale atmospheric contributions for a representative day observed during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST campaign. We first reproduce the observed boundary-layer dynamics by combining the Dutch Atmospheric Large-Eddy Simulation (DALES model with a mixed-layer theory based model. We find that by only taking surface and entrainment fluxes into account, the boundary-layer height is overestimated by 70%. If we constrain our numerical experiments with the BLLAST comprehensive data set, we are able to quantify the contributions of advection of heat and moisture, and subsidence. We find that subsidence has a clear diurnal pattern. Supported by the presence of a nearby mountain range, this pattern suggests that not only synoptic scales exert their influence on the boundary layer, but also mesoscale circulations. Finally, we study whether the vertical and temporal evolution of turbulent variables are influenced by these large-scale forcings. Our model results show good correspondence of the vertical structure of turbulent variables with observations. Our findings further indicate that when large-scale advection and subsidence are applied, the values for turbulent kinetic are lower than without these large-scale forcings. We conclude that the prototypical CBL can still be used as a valid representation of the boundary-layer dynamics near regions characterized by complex topography and small-scale surface heterogeneity, provided that surface- and large-scale forcings are well characterized.
Convectively Forced Gravity Waves and their Sensitivity to Heating Profile and Atmospheric Structure
Halliday, Oliver; Parker, Douglas; Griffiths, Stephen; Vosper, Simon; Stirling, Alison
2016-04-01
It has been known for some time that convective heating is communicated to its environment by gravity waves. Despite this, the radiation of gravity waves in macro-scale models, which are typically forced at the grid-scale by meso-scale parameterization schemes, is not well understood. We present here theoretical work directed toward improving our fundamental understanding of convectively forced gravity wave effects at the meso-scale, in order to begin to address this problem. Starting with the hydrostatic, non-rotating, 2D, Boussinesq equations in a slab geometry, we find a radiating, analytical solution to prescribed sensible heat forcing for both the vertical velocity and potential temperature response. Both Steady and pulsed heating with adjustable horizontal structure is considered. From these solutions we construct a simple model capable of interrogating the spatial and temporal sensitivity to chosen heating functions of the remote forced response in particular. By varying the assumed buoyancy frequency, the influence of the model stratosphere on the upward radiation of gravity waves, and in turn, on the tropospheric response can be understood. Further, we find that the macro-scale response to convection is highly dependent on the radiation characteristics of gravity waves, which are in turn dependent upon the temporal and spatial structure of the source, and upper boundary condition of the domain.
Uhlig, Ralf; Frantz, Cathy; Fritsch, Andreas
2016-05-01
External receiver configurations are directly exposed to ambient wind. Therefore, a precise determination of the convective losses is a key factor in the prediction and evaluation of the efficiency of the solar absorbers. Based on several studies, the forced convective losses of external receivers are modeled using correlations for a roughened cylinder in a cross-flow of air. However at high wind velocities, the thermal efficiency measured during the Solar Two experiment was considerably lower than the efficiency predicted by these correlations. A detailed review of the available literature on the convective losses of external receivers has been made. Three CFD models of different level of detail have been developed to analyze the influence of the actual shape of the receiver and tower configuration, of the receiver shape and of the absorber panels on the forced convective heat transfer coefficients. The heat transfer coefficients deduced from the correlations have been compared to the results of the CFD simulations. In a final step the influence of both modeling approaches on the thermal efficiency of an external tubular receiver has been studied in a thermal FE model of the Solar Two receiver.
Natural versus forced convection in laminar starting plumes
Rogers, Michael C
2009-01-01
A starting plume or jet has a well-defined, evolving head that is driven through the surrounding quiescent fluid by a localized flux of either buoyancy or momentum, or both. We studied the scaling and morphology of starting plumes produced by a constant flux of buoyant fluid from a small, submerged outlet. The plumes were laminar and spanned a wide range of plume Richardson numbers Ri. Ri is the dimensionless ratio of the buoyancy forces to inertial effects, and is thus our measurements crossed over the transition between buoyancy-driven plumes and momentum-driven jets. We found that the ascent velocity of the plume, nondimensionalized by Ri, exhibits a power law relationship with Re, the Reynolds number of the injected fluid in the outlet pipe. We also found that as the threshold between buoyancy-driven and momentum-driven flow was crossed, two distinct types of plume head mophologies existed: confined heads, produced in the Ri > 1 regime, and dispersed heads, which are found in the Ri < 1 regime. Head di...
Effect of electromagnetic force on turbulent flow of molten metal in aluminum electrolysis cells
Institute of Scientific and Technical Information of China (English)
周萍; 梅炽; 周乃君; 姜昌伟
2004-01-01
The standard k-ε model was adopted to simulate the flow field of molten metal in three aluminum electrolysis cells with different anode risers. The Hartman number, Reynolds number and the turbulent Reynolds number of molten metal were calculated quantitatively. The turbulent Reynolds number is in the order of 103 , and Reynolds number is in the order of 104 if taking the depth of molten metal as the characteristic length. The results show that the molten metal flow is the turbulence of high Reynolds number, the turbulent Reynolds number is more appropriate than Reynolds number to be used to describe the turbulent characteristic of molten metal, and Hartman number displays very well that electromagnetic force inhibits turbulent motion of molten metal.
Verdoold, J.
2010-01-01
This dissertation focuses on turbulent thermal convection, which occurs in a wide range of (geo)physical situations, like in the atmosphere, the oceans, the interior of stars or planets, and engineering applications, like metal casting or crystal growth processes. In this work, a special type of th
Wang, J. X.; Jia, P. Y.; Wang, Y. S.; Jiang, L.
2010-03-01
In this article, using Gibson-Ashby constitutive model, we suggest a new method for numerical investigation of forced convection heat transfer in porous foam metal, and try to consolidate the study for mechanical property and that for thermal characteristic. By available experimental data, we simulated to two cases, namely as the transfer in porous media for diameter is 0.6 mm and porosity is 0.402, and for diameter is 1.6 mm and porosity is 0.462. The result, from our constitutive model for single forced convection heat transfer, corresponds well with the experimental data. As for pressure drop prediction in porous is in good agreement with experiment, and the error is only 5% to 10%, but for transfer is less accurate, the error is about 20%, which is acceptable in practice. So it is done that constitutive model is used to simulate the transfer property.
Effect of Selected Factors on Drying Process of Tomato in Forced Convection Solar Energy Dryer
U.S. Muhammed; A.M.I. El-Okene; Isiaka, M
2012-01-01
The effect of air velocity, slice thickness and grazing materials in drying process of tomato in forced convection solar energy dryer was evaluated. The result is to serve as an input for solar energy development for drying of vegetable and fruit products in North West Ecological zone of Nigeria. In order to evaluate the effects of the above factors in drying operation, a split-split-plot experimental design was used. Differences among the treatments and their interactions were tested with or...
Cao, Y.; Faghri, A.
1991-01-01
The performance of a thermal energy storage module is simulated numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid with low Prandtl numbers are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. The numerical results show that module geometry is crucial to the design of a space-based thermal energy storage system.
Directory of Open Access Journals (Sweden)
Prasenjit Dey
2016-01-01
Full Text Available An unsteady two-dimensional forced convection over a square cylinder with sharp and rounded corner edge is numerically analyzed for the low Reynolds number laminar flow regime. In this study, the analysis is carried out for Reynolds number (Re in the range of 80 to 180 with Prandtl number (Pr variation from 0.01 to 1000 for various corner radius (r=0.50, 0.51, 0.54, 0.59, 0.64 and 0.71. The lateral sides of the computational domain are kept constant to maintain the blockage as 5%. Heat transfer due to unsteady forced convection has been predicted by Artificial Neural network (ANN. The present ANN is trained by the input and output data which has been acquired from the numerical simulation, performed in finite volume based Computational Fluid Dynamics (CFD commercial software FLUENT. The heat transfer characteristics over the sharp and rounded corner square cylinder are evaluated by analyzing the local Nusselt number (Nulocal, average Nusselt number (Nuavg at various Reynolds number, Prandtl numbers and for various corner radii. It is found that the heat transfer rate of a circular cylinder can be enhanced by 12% when Re is varying and 14% when Prandtl number is varying by introducing a new cylinder geometry of corner radius r=0.51. It is found that the unsteady forced convection heat transfer over a cylinder can be predicted appropriately by ANN. It is also observed that the back propagation ANN can predict the heat transfer characteristics of forced convection very quickly compared to a standard CFD method.
Stationary two-dimensional turbulence statistics using a Markovian forcing scheme
San, Omer; 10.1016/j.compfluid.2012.10.002
2012-01-01
In this study we investigate the statistics of two-dimensional stationary turbulence using a Markovian forcing scheme, which correlates the forcing process in the current time step to the previous time step according to a defined memory coefficient. In addition to the Markovian forcing mechanism, the hyperviscous dissipation mechanism for small scales and the Ekman friction type of linear damping mechanism for the large scales are included in the model. We examine the effects of various dissipation and forcing parameters on the turbulence statistics in both wave space and physical space. Our analysis includes the effects of the effective forcing scale, the bandwidth of the forcing, the memory correlation coefficient, and the forcing amplitude, along with the large scale friction and small scale dissipation coefficients. Scaling exponents of structure functions and energy spectra are calculated, and the role of the parameters associated with the Markovian forcing is discussed. We found that the scaling exponen...
Analysis of streamwise conduction in forced convection of microchannels using fin approach
Institute of Scientific and Technical Information of China (English)
Suhandran MUNIANDY; Yew Mun HUNG
2011-01-01
The effects induced by streamwise conduction on the thermal characteristics of forced convection for single-phase liquid flow in rectangular microchannel heat sinks under imposed constant wall temperature have been studied.By employing the fin approach in the first law of analysis,models with and without streamwise conduction term in the energy equation were developed for hydrodynamically and thermally fully-developed flow under local thermal non-equilibrium for the solid and fluid phases.These two models were solved to obtain closed form analytical solutions for the fluid and solid temperature distributions and the analysis emphasized details of the variations induced by the streamwise conduction on the fluid temperature distributions.The effects of the Peclet number,aspect ratio,and thermal conductivity ratio on the thermal characteristics of forced convection in microchannel heat sinks were analyzed and discussed.This study reveals the conditions under which the effect of streamwise conduction is significant and should not be neglected in the forced convective heat transfer analysis ofmicrochannel heat sinks.
Pal, Sandip
2016-06-01
The convective boundary layer (CBL) turbulence is the key process for exchanging heat, momentum, moisture and trace gases between the earth's surface and the lower part of the troposphere. The turbulence parameterization of the CBL is a challenging but important component in numerical models. In particular, correct estimation of CBL turbulence features, parameterization, and the determination of the contribution of eddy diffusivity are important for simulating convection initiation, and the dispersion of health hazardous air pollutants and Greenhouse gases. In general, measurements of higher-order moments of water vapor mixing ratio (q) variability yield unique estimates of turbulence in the CBL. Using the high-resolution lidar-derived profiles of q variance, third-order moment, and skewness and analyzing concurrent profiles of vertical velocity, potential temperature, horizontal wind and time series of near-surface measurements of surface flux and meteorological parameters, a conceptual framework based on bottom up approach is proposed here for the first time for a robust characterization of the turbulent structure of CBL over land so that our understanding on the processes governing CBL q turbulence could be improved. Finally, principal component analyses will be applied on the lidar-derived long-term data sets of q turbulence statistics to identify the meteorological factors and the dominant physical mechanisms governing the CBL turbulence features.
Laboratory grey cast iron continuous casting line with electromagnetic forced convection support
Directory of Open Access Journals (Sweden)
J. Szajnar
2010-07-01
Full Text Available The article describes the construction of a 20 mm diameter grey cast iron ingots continuous casting laboratory line. This line is made ofthree main units: melting unit (induction furnace, casting unit and the pulling unit. In order to improve the homogeneity of themicrostructure of ingots (by applying forced convection of liquid metal during the crystallization process in this case a crystallizer system generating the forced movement of liquid metal based on a system of electrical power windings of the AC specific frequency. Thissolution allowed to obtain a homogeneous microstructure of the continuous casting of cast iron EN-GJL-200 species.
Thermal evidence for Taylor columns in turbulent rotating Rayleigh-Bénard convection.
King, Eric M; Aurnou, Jonathan M
2012-01-01
We investigate flow structures in rotating Rayleigh-Bénard convection experiments in water using thermal measurements. We focus on correlations between time series measurements of temperature in the top and bottom boundaries. Distinct anticorrelations are observed for rapidly rotating convection, which are argued to attest to heat transport by convective Taylor columns. In support of this argument, these quasigeostrophic flow structures are directly observed in flow visualizations, and their thermal signature is qualitatively reproduced by a simple model of heat transport by columnar flow. Weakly rotating and nonrotating convection produces positively correlated temperature changes across the layer, indicative of heat transport by large-scale circulation. We separate these regimes using a transition parameter that depends on the Rayleigh and Ekman numbers, RaE3/2.
Static magnetic fields enhance turbulence
Pothérat, Alban
2015-01-01
More often than not, turbulence occurs under the influence of external fields, mostly rotation and magnetic fields generated either by planets, stellar objects or by an industrial environment. Their effect on the anisotropy and the dissipative behaviour of turbulence is recognised but complex, and it is still difficult to even tell whether they enhance or dampen turbulence. For example, externally imposed magnetic fields suppress free turbulence in electrically conducting fluids (Moffatt 1967), and make it two-dimensional (2D) (Sommeria & Moreau 1982); but their effect on the intensity of forced turbulence, as in pipes, convective flows or otherwise, is not clear. We shall prove that since two-dimensionalisation preferentially affects larger scales, these undergo much less dissipation and sustain intense turbulent fluctuations. When higher magnetic fields are imposed, quasi-2D structures retain more kinetic energy, so that rather than suppressing forced turbulence, external magnetic fields indirectly enha...
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Salimpour, Mohammad Reza; Golmohammadi, Kia; Sedaghat, Ahmad [Isfahan University of Technology, Isfahan (Iran, Islamic Republic of); Campo, Antonio [The University of Texas at San Antonio, San Antonio (United States)
2015-09-15
The convective heat transfer for the turbulent flow of water/TiO{sub 2} nanofluid inside helically horizontal corrugated tubes is investigated in this paper using experimental techniques. The tube boundary condition is a uniform wall temperature. The test apparatus was designed and assembled with a test section containing 93 cm copper tubes with internal and external diameters of 7.71 mm and 9.52 mm, respectively. First, the heat transfer characteristics of the distilled water turbulent flow in a plain copper tube were measured preliminarily. Second, various test runs were performed for nanofluids with two nanoparticle concentrations (0.1% and 0.5%), two corrugation depth to diameter ratios (0.0648 and 0.103), two corrugation pitch to diameter ratios (0.917 and 1.297), and two corrugation width to diameter ratios (0.363 and 0.492) that were all within the range of turbulent Reynolds numbers (3000 < Re < 15000). The experimental results reveal that the Nusselt number augments the dual increments in corrugation depth and width and with the decrements in corrugation pitch, particularly for high Reynolds numbers. The nanoparticles have a stronger effect on the heat transfer in helically corrugated tubes with higher corrugation depths and widths as well as lower corrugation pitches. A correlation for the Nusselt number in terms of the helically corrugated tubes is introduced based on the linear regression analysis of the experimental data.
Zavala-Guillén, I.; Xamán, J.; Álvarez, G.; Arce, J.; Hernández-Pérez, I.; Gijón-Rivera, M.
2016-03-01
This study reports the modeling of the turbulent natural convection in a double air-channel solar chimney (SC-DC) and its comparison with a single air-channel solar chimney (SC-C). Prediction of the mass flow and the thermal behavior of the SC-DC were obtained under three different climates of Mexico during one summer day. The climates correspond to: tropical savannah (Mérida), arid desert (Hermosillo) and temperate with warm summer (Mexico City). A code based on the Finite Volume Method was developed and a k-ω turbulence model has been used to model air turbulence in the solar chimney (SC). The code was validated against experimental data. The results indicate that during the day the SC-DC extracts about 50% more mass flow than the SC-C. When the SC-DC is located in Mérida, Hermosillo and Mexico City, the air-changes extracted along the day were 60, 63 and 52, respectively. The air temperature at the outlet of the chimney increased up to 33%, 38% and 61% with respect to the temperature it has at the inlet for Mérida, Hermosillo and Mexico City, respectively.
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KEARNEY,SEAN P.; REYES,FELIPE V.
2000-12-13
In this paper, an acetone planar laser-induced fluorescence (PLIF) technique for nonintrusive, temperature imaging is demonstrated in gas-phase (Pr = 0.72) turbulent Rayleigh-Benard convection at Rayleigh number, Ra = 1.3 x 10{sup 5}. The PLIF technique provides quantitative, spatially correlated temperature data without the flow intrusion or time lag associated with physical probes and without the significant path averaging that plagues most optical heat-transfer diagnostic tools, such as the Mach-Zehnder interferometer, thus making PLIF an attractive choice for quantitative thermal imaging in easily perturbed, complex three-dimensional flow fields. The instantaneous (20-ns integration time) thermal images presented have a spatial resolution of 176 x 176 x 500 {micro}m and a single-pulse temperature measurement precision of {+-}5.5 K, or 5.4 % of the total temperature difference. These images represent a 2-D slice through a complex, 3-D flow allowing for the thermal structure of the turbulence to be quantified. Statistics such as the horizontally averaged temperature profile, rms temperature fluctuation, two-point spatial correlations, and conditionally averaged plume structures are computed from an ensemble of 100 temperature images. The profiles of the mean temperature and rms temperature fluctuation are in good agreement with previously published data, and the results obtained from the two-point spatial correlations and conditionally averaged temperature fields show the importance of large-scale coherent structures in this turbulent flow.
The effect of natural and forced melt convection on dendritic solidification in Ga-In alloys
Shevchenko, N.; Roshchupkina, O.; Sokolova, O.; Eckert, S.
2015-05-01
The directional solidification of Ga-25 wt%In alloys within a Hele-Shaw cell was visualized by means of X-ray radioscopy. The experimental investigations are especially focused on the impact of melt convection on the dendritic growth. Natural convection occurs during a bottom up solidification because lighter solute is rejected at the solid-liquid interface leading to an unstable density stratification. Forced convection was produced by a rotating wheel with two parallel disks containing at their inner sides a set of permanent NdFeB magnets with alternating polarization. The direction of forced melt flow is almost horizontal at the solidification front whereas local flow velocities in the range between 0.1 and 1.0 mm/s were achieved by controlling the rotation speed of the magnetic wheel. Melt flow induces various effects on the grain morphology primarily caused by the convective transport of solute. Our observations show a facilitation of the growth of primary trunks or lateral branches, suppression of side branching, dendrite remelting and fragmentation. The manifestation of all phenomena depends on the dendrite orientation, local direction and intensity of the flow. The forced flow eliminates the solutal plumes and damps the local fluctuations of solute concentration. It provokes a preferential growth of the secondary arms at the upstream side of the primary dendrite arms, whereas the high solute concentration at the downstream side of the dendrites can inhibit the formation of secondary branches completely. Moreover, the flow changes the inclination angle of the dendrites and the angle between primary trunks and secondary arms.
Modeling Rotating Turbulent Flows with the Body Force Potential Model.
Bhattacharya, Amitabh; Perot, Blair
2000-11-01
Like a Reynolds Stress Transport equation model, the turbulent potential model has an explicit Coriolis acceleration term that appears in the model that accounts for rotation effects. In this work the additional secondary effects that system rotation has on the dissipation rate, return-to-isotropy, and fast pressure strain terms are also included in the model. The resulting model is tested in the context of rotating isotropic turbulence, rotating homogeneous shear flow, rotating channel flow, and swirling pipe flow. Many of the model changes are applicable to Reynolds stress transport equation models. All model modifications are frame indifferent.
Turbulence effects on warm-rain formation in precipitating shallow convection revisited
Seifert, Axel; ONISHI, RYO
2016-01-01
Two different collection kernels which include turbulence effects on the collision rate of liquid droplets are used as a basis to develop a parameterization of the warm-rain processes autoconversion, accretion, and self-collection. The new parameterization is tested and validated with the help of a 1-D bin microphysics model. Large-eddy simulations of the rain formation in shallow cumulus clouds confirm previous results that turbulence effects can significantly enhance the d...
Turbulence effects on warm rain formation in precipitating shallow convection revisited
Seifert, Axel; ONISHI, RYO
2016-01-01
Two different collection kernels which include turbulence effects on the collision rate of liquid droplets are used as a basis to develop a parameterization of the warm rain processes autoconversion, accretion and selfcollection. The new parameterization is tested and validated with help of a 1D bin microphysics model. Large-eddy simulations of the rain formation in shallow cumulus clouds confirm previous results that turbulence effects can significantly enhance ...
Solar drying of whole mint plant under natural and forced convection
Directory of Open Access Journals (Sweden)
Y.I. Sallam
2015-03-01
Full Text Available Two identical prototype solar dryers (direct and indirect having the same dimensions were used to dry whole mint. Both prototypes were operated under natural and forced convection modes. In the case of the later one the ambient air was entered the dryer with the velocity of 4.2 m s−1. The effect of flow mode and the type of solar dryers on the drying kinetics of whole mint were investigated. Ten empirical models were used to fit the drying curves; nine of them represented well the solar drying behavior of mint. The results indicated that drying of mint under different operating conditions occurred in the falling rate period, where no constant rate period of drying was observed. Also, the obtained data revealed that the drying rate of mint under forced convection was higher than that of mint under natural convection, especially during first hours of drying (first day. The values of the effective diffusivity coefficient for the mint drying ranged between 1.2 × 10−11 and 1.33 × 10−11 m2 s−1.
Solar drying of whole mint plant under natural and forced convection.
Sallam, Y I; Aly, M H; Nassar, A F; Mohamed, E A
2015-03-01
Two identical prototype solar dryers (direct and indirect) having the same dimensions were used to dry whole mint. Both prototypes were operated under natural and forced convection modes. In the case of the later one the ambient air was entered the dryer with the velocity of 4.2 m s(-1). The effect of flow mode and the type of solar dryers on the drying kinetics of whole mint were investigated. Ten empirical models were used to fit the drying curves; nine of them represented well the solar drying behavior of mint. The results indicated that drying of mint under different operating conditions occurred in the falling rate period, where no constant rate period of drying was observed. Also, the obtained data revealed that the drying rate of mint under forced convection was higher than that of mint under natural convection, especially during first hours of drying (first day). The values of the effective diffusivity coefficient for the mint drying ranged between 1.2 × 10(-11) and 1.33 × 10(-11) m(2) s(-1).
Energy Technology Data Exchange (ETDEWEB)
Wenji, Song [Guangzhou Institute of Energy Conversion, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China); Key Laboratory of Renewable Energy and Gas Hydrate, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China); Graduate School of Chinese Academy of Sciences, Beijing 100039 (China); Rui, Xiao; Chong, Huang; Shihui, He; Kaijun, Dong; Ziping, Feng [Guangzhou Institute of Energy Conversion, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China); Key Laboratory of Renewable Energy and Gas Hydrate, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China)
2009-11-15
Tetra-n-butyl-ammonium bromide (TBAB) clathrate hydrate slurry (CHS) is one kind of secondary refrigerants, which is promising to be applied into air-conditioning or latent-heat transportation systems as a thermal storage or cold carrying medium for energy saving. It is a solid-liquid two phase mixture which is easy to produce and has high latent heat and good fluidity. In this paper, the heat transfer characteristics of TBAB slurry were investigated in a horizontal stainless steel tube under different solid mass fractions and flow velocities with constant heat flux. One velocity region of weakened heat transfer was found. Moreover, TBAB CHS was treated as a kind of Bingham fluids, and the influences of the solid particles, flow velocity and types of flow on the forced convective heat transfer coefficients of TBAB CHS were investigated. At last, criterial correlations of Nusselt number for laminar and turbulent flows in the form of power function were summarized, and the error with experimental results was within {+-}20%. (author)
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.
Analytical Studies of Two-Dimensional Channel Turbulent Flow Subjected to Coriolis Force
鬼頭, 修己; 中林, 功一; キトウ, オサミ; Kito, Osami
1992-01-01
Coriolis effects on fully developed turbulent flow in a two-dimensional channel rotating about an axis perpendicular to its axis are considered. The Coriolis force has stabilizing/destabilizing effects on turbulence, and the mean velocity distribution changes accordingly. Experimental and numerical studies on the velocity characteristics have already been conducted by other researchers for various conditions. However, we cannot assemble the overall picture of the Coriolis effect on the veloci...
Forcing-dependent dynamics and emergence of helicity in rotating turbulence
Dallas, Vassilios
2016-01-01
The effects of large scale mechanical forcing on the dynamics of rotating turbulent flows are studied by means of numerical simulations, varying systematically the nature of the mechanical force in time. We demonstrate that the statistically stationary solutions of these flows depend on the nature of the forcing mechanism. Rapidly enough rotating flows with a forcing that has a persistent direction relatively to the axis of rotation bifurcate from a non-helical state to a helical state despite the fact that the forcing is non-helical. We find that the nature of the mechanical force in time and the emergence of helicity have direct implications on the cascade dynamics of these flows, determining the anisotropy in the flow, the energy condensation at large scales and the power-law energy spectra that are consistent with previous findings and phenomenologies under strong and weak-wave turbulent conditions.
A model for dispersion from area sources in convective turbulence. [for air pollution
Crane, G.; Panofsky, H. A.; Zeman, O.
1977-01-01
Four independent estimates of the vertical distribution of the eddy coefficient for dispersion of a passive contaminant from an extensive area source in a convective layer have been presented. The estimates were based on the following methods: (1) a second-order closure prediction, (2) field data of pollutant concentrations over Los Angeles, (3) lab measurements of particle dispersion, and (4) assumption of equality between momentum and mass transfer coefficients in the free convective limit. It is suggested that K-values estimated both from second-order closure theory and from Los Angeles measurements are systematically underestimated.
Heimpel, Moritz; Aurnou, Jonathan
2007-04-01
The origin of zonal jets on the jovian planets has long been a topic of scientific debate. In this paper we show that deep convection in a spherical shell can generate zonal flow comparable to that observed on Jupiter and Saturn, including a broad prograde equatorial jet and multiple alternating jets at higher latitudes. We present fully turbulent, 3D spherical numerical simulations of rapidly rotating convection with different spherical shell geometries. The resulting global flow fields tend to be segregated into three regions (north, equatorial, and south), bounded by the tangent cylinder that circumscribes the inner boundary equator. In all of our simulations a strong prograde equatorial jet forms outside the tangent cylinder, whereas multiple jets form in the northern and southern hemispheres, inside the tangent cylinder. The jet scaling of our numerical models and of Jupiter and Saturn is consistent with the theory of geostrophic turbulence, which we extend to include the effect of spherical shell geometry. Zonal flow in a spherical shell is distinguished from that in a full sphere or a shallow layer by the effect of the tangent cylinder, which marks a reversal in the sign of the planetary β-parameter and a jump in the Rhines length. This jump is manifest in the numerical simulations as a sharp equatorward increase in jet widths—a transition that is also observed on Jupiter and Saturn. The location of this transition gives an estimate of the depth of zonal flow, which seems to be consistent with current models of the jovian and saturnian interiors.
Turbulent convection and dissolution under sloping Ice-shelves in Saline water
Mondal, Mainak; Gayen, Bishakhdatta; Griffiths, Ross; Kerr, Ross
2016-11-01
We have carried out numerical experiments with geophysicallyrelevant slope angles (5º-20º) over domains of O(1 m) -O(10 m) bracketingAntarctic conditions. Contact of the ice with cold and saltywater drives a turbulentbuoyant flow predominantly due to freshening from melting ice-interface in the upslope direction. The dissolution rate decreases with shallower angles and is predicted by our theory. Thickness of the thermal and salinity boundary layer increases with decreasing slope angle whereas the interface conditions remains insensitive to the inclination. The dissolution rate is independent of slope length when boundary layer is turbulent. For steeper angles turbulent kinetic energy is mainly produced by the buoyancy flux but it's contribution rapidly decreases with shallower angles. For shallower slopes turbulence is sustained by shear production due to decrease of velocity boundary layer. Our results can be used to explain many dynamical processes under inclined ice shelves around Antarctica.
Kitiashvili, I N; Goode, P R; Kosovichev, A G; Lele, S K; Mansour, N N; Wray, A A; Yurchyshyn, V B
2012-01-01
Turbulent properties of the quiet Sun represent the basic state of surface conditions, and a background for various processes of solar activity. Therefore understanding of properties and dynamics of this `basic' state is important for investigation of more complex phenomena, formation and development of observed phenomena in the photosphere and atmosphere. For characterization of the turbulent properties we compare kinetic energy spectra on granular and sub-granular scales obtained from infrared TiO observations with the New Solar Telescope (Big Bear Solar Observatory) and from 3D radiative MHD numerical simulations ('SolarBox' code). We find that the numerical simulations require a high spatial resolution with 10 - 25 km grid-step in order to reproduce the inertial (Kolmogorov) turbulence range. The observational data require an averaging procedure to remove noise and potential instrumental artifacts. The resulting kinetic energy spectra show a good agreement between the simulations and observations, opening...
Disentangling inertial waves from eddy turbulence in a forced rotating turbulence experiment
Campagne, Antoine; Moisy, Frédéric; Cortet, Pierre-Philippe
2015-01-01
We present a spatio-temporal analysis of a statistically stationary rotating turbulence experiment, aiming to extract a signature of inertial waves, and to determine the scales and frequencies at which they can be detected. The analysis uses two-point spatial correlations of the temporal Fourier transform of velocity fields obtained from time-resolved stereoscopic particle image velocimetry measurements in the rotating frame. We quantify the degree of anisotropy of turbulence as a function of frequency and spatial scale. We show that this space-time-dependent anisotropy is well described by the dispersion relation of linear inertial waves at large scale, while smaller scales are dominated by the sweeping of the waves by fluid motion at larger scales. This sweeping effect is mostly due to the low-frequency quasi-two-dimensional component of the turbulent flow, a prominent feature of our experiment which is not accounted for by wave turbulence theory. These results question the relevance of this theory for rota...
Scaling and renormalization for the Kolmogorov-Petrovskii-Piskunov equationwith turbulent convection
Fedotov, Sergei
1997-03-01
The problem of determining the upper bounds for the ensemble-averaged reaction front position and speed in a fully developed three-dimensional turbulent flow has been examined, in which the reaction is of Kolmogorov-Petrovskii-Piskunov type and turbulent velocity is a Gaussian random field exhibiting long-range correlations and infrared divergence in the limit of large Reynolds number. An asymptotic method has been developed that gives the general formalism for determining the upper bounds for reaction front in the long-time, large-distance limit. Two anomalous scaling regimes and corresponding scaling functions have been determined by the use of exact renormalization procedure.
A theoretical study of the spheroidal droplet evaporation in forced convection
Li, Jie; Zhang, Jian
2014-11-01
In many applications, the shape of a droplet may be assumed to be an oblate spheroid. A theoretical study is conducted on the evaporation of an oblate spheroidal droplet under forced convection conditions. Closed-form analytical expressions of the mass evaporation rate for an oblate spheroid are derived, in the regime of controlled mass-transfer and heat-transfer, respectively. The variation of droplet size during the evaporation process is presented in the regime of shrinking dynamic model. Comparing with the droplets having the same surface area, an increase in the aspect ratio enhances the mass evaporation rate and prolongs the burnout time.
Sabin, C. M.; Poppendiek, H. F.
1971-01-01
A number of heat transfer and fluid flow mechanisms that control once-through, forced convection potassium boiling are studied analytically. The topics discussed are: (1) flow through tubes containing helical wire inserts, (2) motion of droplets entrained in vapor flow, (3) liquid phase distribution in boilers, (4) temperature distributions in boiler tube walls, (5) mechanisms of heat transfer regime change, and (6) heat transfer in boiler tubes. Whenever possible, comparisons of predicted and actual performances are made. The model work presented aids in the prediction of operating characteristics of actual boilers.
Numerical simulation of forced convection in a duct subjected to microwave heating
Energy Technology Data Exchange (ETDEWEB)
Zhu, J.; Kuznetsov, A.V. [North Carolina State University, Department of Mechanical and Aerospace Engineering, Campus Box 7910, Raleigh, NC (United States); Sandeep, K.P. [North Carolina State University, Department of Food Science, Raleigh, NC (United States)
2007-01-15
In this paper, forced convection in a rectangular duct subjected to microwave heating is investigated. Three types of non-Newtonian liquids flowing through the duct are considered, specifically, apple sauce, skim milk, and tomato sauce. A finite difference time domain method is used to solve Maxwell's equations simulating the electromagnetic field. The three-dimensional temperature field is determined by solving the coupled momentum, energy, and Maxwell's equations. Numerical results show that the heating pattern strongly depends on the dielectric properties of the fluid in the duct and the geometry of the microwave heating system. (orig.)
Forced Convective Air Cooling from Electronic Component Arrays in a Parallel Plate Channel
Institute of Scientific and Technical Information of China (English)
D.Y.Cai; Y.P.Gan; 等
1994-01-01
This paper discusses air forced convection heat transfer from inline protruding elements arranged in eight rows.The streamwise and spanwise spacings between elements were varied using a splitter plate that can be positioned at three different modular configurations.A set of empirical formulas was presented to correlate the experimental data for the design of air cooling systems.Arrays of componets with one odd-size module have been tested also.Experimental results show that blocks near the entrance and behind the odd-size module have improved performance compared with uniform arrangements.Accordingly,temperature sensitive components are suggested to be arranged in these locations.
Critical Heat Flux in forced Convective Boiling with a Wall Jet
Institute of Scientific and Technical Information of China (English)
王迅
2004-01-01
The critical heat flux (CHF) in the forced convective boiling with a wall jet has been investigated.The experiments of CHF with a wall jet have been performed over a wide range of ρl/ρg=6.6-1 603 and ΔTsub=0-60 K.The mechanism on CHF is discussed and a CHF model based on heat balance in sublayer can provide a good clue for analyzing and deriving CHF.Finally,a generalized correlation is presented, which can predict CHF for saturated and subcooled conditions.
Institute of Scientific and Technical Information of China (English)
无
2008-01-01
Entropy generation for thermally developing forced convection in a porous medium bounded by two isothermal parallel plates is investigated analytically on the basis of the Darcy flow model where the viscous dissipation effects had also been taken into account.A parametric study showed that decreasing the group parameter and the Péclet number increases the entropy generation while for the Brinkman number the converse is true.Heatline visualization technique is applied with an emphasis on the Br ＜ 0 case where there is somewhere that heat transfer changes direction at some streamwise location to the wall instead of its original direction,i.e.,from the wall.
Effects of turbulence on the drag force on a golf ball
Cross, Rod
2016-09-01
Measurements are presented of the drag force on a golf ball dropped vertically into a tank of water. As observed previously in air, the drag coefficient drops sharply when the flow becomes turbulent. The experiment would be suitable for undergraduate students since it can be undertaken at low ball speeds and since the effects of turbulence are easily observed on video film. A modified golf ball was used to show how a ball with a smooth and a rough side, such as a cricket ball, is subject to a side force when the ball surface itself is asymmetrical in the transverse direction.
Kleinman, Leonid S.; Red, X. B., Jr.
1995-01-01
An algorithm has been developed for time-dependent forced convective diffusion-reaction having convection by a recirculating flow field within the drop that is hydrodynamically coupled at the interface with a convective external flow field that at infinity becomes a uniform free-streaming flow. The concentration field inside the droplet is likewise coupled with that outside by boundary conditions at the interface. A chemical reaction can take place either inside or outside the droplet, or reactions can take place in both phases. The algorithm has been implemented, and for comparison results are shown here for the case of no reaction in either phase and for the case of an external first order reaction, both for unsteady behavior. For pure interphase mass transfer, concentration isocontours, local and average Sherwood numbers, and average droplet concentrations have been obtained as a function of the physical properties and external flow field. For mass transfer enhanced by an external reaction, in addition to the above forms of results, we present the enhancement factor, with the results now also depending upon the (dimensionless) rate of reaction.
Boundary layer control of rotating convection systems.
King, Eric M; Stellmach, Stephan; Noir, Jerome; Hansen, Ulrich; Aurnou, Jonathan M
2009-01-15
Turbulent rotating convection controls many observed features of stars and planets, such as magnetic fields, atmospheric jets and emitted heat flux patterns. It has long been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force. Here, however, we present results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. We formulate a predictive description of the transition between the two regimes on the basis of the competition between these two boundary layers. This transition scaling theory unifies the disparate results of an extensive array of previous experiments, and is broadly applicable to natural convection systems.
Cheng, Jian-Ping; Zhang, Hong-Na; Cai, Wei-Hua; Li, Si-Ning; Li, Feng-Chen
2017-07-01
The present paper presents direct numerical simulations of Rayleigh-Bénard convection (RBC) in an enclosed cell filled with the polymer solution in order to investigate the viscoelastic effect on the characteristics of heat transport and large-scale circulation (LSC) of RBC. To overcome the difficulties in numerically solving a high Weissenberg number (Wi) problem of viscoelastic fluid flow with strong elastic effect, the log-conformation reformulation method was implemented. Numerical results showed that the addition of polymers reduced the heat flux and the amount of heat transfer reduction (HTR) behaves nonmonotonically, which firstly increases but then decreases with Wi. The maximum HTR reaches around 8.7 % at the critical Wi. The nonmonotonic behavior of HTR as a function of Wi was then corroborated with the modifications of the period of LSC and turbulent energy as well as viscous boundary layer thickness. Finally, a standard turbulent kinetic energy (TKE) budget analysis was done for the whole domain, the boundary layer region, and the bulk region. It showed that the role change of elastic stress contributions to TKE is mainly responsible for this nonmonotonic behavior of HTR.
Jouve, Laurene
2009-01-01
We present the first 3D MHD study in spherical geometry of the non-linear dynamical evolution of magnetic flux tubes in a turbulent rotating convection zone. We study numerically the rise of magnetic toroidal flux ropes from the base of a modelled convection zone up to the top of our computational domain where bipolar patches are formed. We compare the dynamical behaviour of flux tubes in a fully convective shell possessing self-consistently generated mean flows such as meridional circulation and differential rotation, with reference calculations done in a quiet isentropic zone. We find that two parameters influence the tubes during their rise through the convection zone: the initial field strength and amount of twist, thus confirming previous findings in Cartesian geometry. Further, when the tube is sufficiently strong with respect to the equipartition field, it rises almost radially independently of the initial latitude (either low or high). By contrast, weaker field cases indicate that downflows and upflow...
Modeling H2 formation in the turbulent ISM: Solenoidal versus compressive turbulent forcing
Milosavljevic, Milica; Federrath, Christoph; Klessen, Ralf S
2011-01-01
We present results from high-resolution three-dimensional simulations of the turbulent interstellar medium that study the influence of the nature of the turbulence on the formation of molecular hydrogen. We have examined both solenoidal (divergence-free) and compressive (curl-free) turbulent driving, and show that compressive driving leads to faster H2 formation, owing to the higher peak densities produced in the gas. The difference in the H2 formation rate can be as much as an order of magnitude at early times, but declines at later times as the highest density regions become fully molecular and stop contributing to the total H2 formation rate. We have also used our results to test a simple prescription suggested by Gnedin et al. (2009) for modeling the influence of unresolved density fluctuations on the H2 formation rate in large-scale simulations of the ISM. We find that this approach works well when the H2 fraction is small, but breaks down once the highest density gas becomes fully molecular.
He, Xiaozhou; Ahlers, Guenter
2016-01-01
We present measurements of the orientation $\\theta_0$ and temperature amplitude $\\delta$ of the large-scale circulation in a cylindrical sample of turbulent Rayleigh-Benard convection (RBC) with aspect ratio $\\Gamma \\equiv D/L = 1.00$ ($D$ and $L$ are the diameter and height respectively) and for the Prandtl number $Pr \\simeq 0.8$. Results for $\\theta_0$ revealed a preferred orientation with upflow in the West, consistent with a broken azimuthal invariance due to Earth's Coriolis force [see \\cite{BA06b}]. They yielded the azimuthal diffusivity $D_\\theta$ and a corresponding Reynolds number $Re_{\\theta}$ for Rayleigh numbers over the range $2\\times 10^{12} < Ra < 1.5\\times 10^{14}$. In the classical state ($Ra < 2\\times 10^{13}$) the results were consistent with the measurements by \\cite{BA06a} for $Ra < 10^{11}$ and $Pr = 4.38$ which gave $Re_{\\theta} \\propto Ra^{0.28}$, and with the Prandtl-number dependence $Re_{\\theta} \\propto Pr^{-1.2}$ as found previously also for the velocity-fluctuation Rey...
Gousseau, P; Blocken, B; van Heijst, G J F
2011-10-30
Computational Fluid Dynamics (CFD) is increasingly used to predict wind flow and pollutant dispersion around buildings. The two most frequently used approaches are solving the Reynolds-averaged Navier-Stokes (RANS) equations and Large-Eddy Simulation (LES). In the present study, we compare the convective and turbulent mass fluxes predicted by these two approaches for two configurations of isolated buildings with distinctive features. We use this analysis to clarify the role of these two components of mass transport on the prediction accuracy of RANS and LES in terms of mean concentration. It is shown that the proper simulation of the convective fluxes is essential to predict an accurate concentration field. In addition, appropriate parameterization of the turbulent fluxes is needed with RANS models, while only the subgrid-scale effects are modeled with LES. Therefore, when the source is located outside of recirculation regions (case 1), both RANS and LES can provide accurate results. When the influence of the building is higher (case 2), RANS models predict erroneous convective fluxes and are largely outperformed by LES in terms of prediction accuracy of mean concentration. These conclusions suggest that the choice of the appropriate turbulence model depends on the configuration of the dispersion problem under study. It is also shown that for both cases LES predicts a counter-gradient mechanism of the streamwise turbulent mass transport, which is not reproduced by the gradient-diffusion hypothesis that is generally used with RANS models.
Sundarraj, Pradeepkumar; Taylor, Robert A.; Banerjee, Debosmita; Maity, Dipak; Sinha Roy, Susanta
2017-01-01
Hybrid solar thermoelectric generators (HSTEGs) have garnered significant research attention recently due to their potential ability to cogenerate heat and electricity. In this paper, theoretical and experimental investigations of the electrical and thermal performance of a HSTEG system are reported. In order to validate the theoretical model, a laboratory scale HSTEG system (based on forced convection cooling) is developed. The HSTEG consists of six thermoelectric generator modules, an electrical heater, and a stainless steel cooling block. Our experimental analysis shows that the HSTEG is capable of producing a maximum electrical power output of 4.7 W, an electrical efficiency of 1.2% and thermal efficiency of 61% for an average temperature difference of 92 °C across the TEG modules with a heater power input of 382 W. These experimental results of the HSTEG system are found to be in good agreement with the theoretical prediction. This experimental/theoretical analysis can also serve as a guide for evaluating the performance of the HSTEG system with forced convection cooling.
Influence of an external magnetic field on forced turbulence in a swirling flow of liquid metal
Gallet, Basile; Mordant, Nicolas
2009-01-01
We report an experimental investigation on the influence of an external magnetic field on forced 3D turbulence of liquid gallium in a closed vessel. We observe an exponential damping of the turbulent velocity fluctuations as a function of the interaction parameter N (ratio of Lorentz force over inertial terms of the Navier-Stokes equation). The flow structures develop some anisotropy but do not become bidimensional. From a dynamical viewpoint, the damping first occurs homogeneously over the whole spectrum of frequencies. For larger values of N, a very strong additional damping occurs at the highest frequencies. However, the injected mechanical power remains independent of the applied magnetic field. The simultaneous measurement of induced magnetic field and electrical potential differences shows a very weak correlation between magnetic field and velocity fluctuations. The observed reduction of the fluctuations is in agreement with a previously proposed mechanism for the saturation of turbulent dynamos and wit...
Hoefnagels, Paul B. J.; Wei, Ping; Narezo Guzman, Daniela; Sun, Chao; Lohse, Detlef; Ahlers, Guenter
2017-07-01
We report on an experimental study of the large-scale flow (LSF) and Reynolds numbers in turbulent convection in a cylindrical sample with height equal to its diameter and heated locally around the center of its bottom plate (locally heated convection). The sample size and shape are the same as those of Narezo Guzman et al. [D. Narezo Guzman et al., J. Fluid Mech. 787, 331 (2015), 10.1017/jfm.2015.701; D. Narezo Guzman et al., J. Fluid Mech. 795, 60 (2016), 10.1017/jfm.2016.178]. Measurements are made at a nearly constant Rayleigh number as a function of the mean temperature, both in the presence of controlled boiling (two-phase flow) and for the superheated fluid (one-phase flow). Superheat values Tb-To n up to about 11 K (Tb is the bottom-plate temperature and To n is the lowest Tb at which boiling is observed) are used. The LSF is less organized than it is in (uniformly heated) Rayleigh-Bénard convection (RBC), where it takes the form of a single convection roll. Large-scale-flow-induced sinusoidal azimuthal temperature variations (like those found for RBC) could be detected only in the lower portion of the sample, indicating a less organized flow in the upper portions. Reynolds numbers are determined using the elliptic model (EM) of He and Zhang [G.-W. He and J.-B. Zhang, Phys. Rev. E 73, 055303(R) (2006), 10.1103/PhysRevE.73.055303]. We found that for our system the EM is applicable over a wide range of space and time displacements, as long as these displacements are within the inertial range of the temporal and spatial spectrum. At three locations in the sample the results show that the vertical mean-flow velocity component is reduced while the fluctuation velocity is enhanced by the bubbles of the two-phase flow. Enhancements of velocity fluctuations up to about 60% are found at the largest superheat values. Local temperature measurements within the sample reveal temperature oscillations that also used to determine a Reynolds number. These results are
Institute of Scientific and Technical Information of China (English)
Tao Zhi; Cheng Zeyuan; Zhu Jianqin; Li Haiwang
2016-01-01
A variety of turbulence models were used to perform numerical simulations of heat transfer for hydrocarbon fuel flowing upward and downward through uniformly heated vertical pipes at supercritical pressure. Inlet temperatures varied from 373 K to 663 K, with heat flux rang-ing from 300 kW/m2 to 550 kW/m2. Comparative analyses between predicted and experimental results were used to evaluate the ability of turbulence models to respond to variable thermophys-ical properties of hydrocarbon fuel at supercritical pressure. It was found that the prediction per-formance of turbulence models is mainly determined by the damping function, which enables them to respond differently to local flow conditions. Although prediction accuracy for experimental results varied from condition to condition, the shear stress transport (SST) and launder and sharma models performed better than all other models used in the study. For very small buoyancy-influenced runs, the thermal-induced acceleration due to variations in density lead to the impairment of heat transfer occurring in the vicinity of pseudo-critical points, and heat transfer was enhanced at higher temperatures through the combined action of four thermophysical properties: density, viscosity, thermal conductivity and specific heat. For very large buoyancy-influenced runs, the thermal-induced acceleration effect was over predicted by the LS and AB models.
Directory of Open Access Journals (Sweden)
Tao Zhi
2016-10-01
Full Text Available A variety of turbulence models were used to perform numerical simulations of heat transfer for hydrocarbon fuel flowing upward and downward through uniformly heated vertical pipes at supercritical pressure. Inlet temperatures varied from 373 K to 663 K, with heat flux ranging from 300 kW/m2 to 550 kW/m2. Comparative analyses between predicted and experimental results were used to evaluate the ability of turbulence models to respond to variable thermophysical properties of hydrocarbon fuel at supercritical pressure. It was found that the prediction performance of turbulence models is mainly determined by the damping function, which enables them to respond differently to local flow conditions. Although prediction accuracy for experimental results varied from condition to condition, the shear stress transport (SST and launder and sharma models performed better than all other models used in the study. For very small buoyancy-influenced runs, the thermal-induced acceleration due to variations in density lead to the impairment of heat transfer occurring in the vicinity of pseudo-critical points, and heat transfer was enhanced at higher temperatures through the combined action of four thermophysical properties: density, viscosity, thermal conductivity and specific heat. For very large buoyancy-influenced runs, the thermal-induced acceleration effect was over predicted by the LS and AB models.
Bailly, Christophe
2015-01-01
This book covers the major problems of turbulence and turbulent processes, including physical phenomena, their modeling and their simulation. After a general introduction in Chapter 1 illustrating many aspects dealing with turbulent flows, averaged equations and kinetic energy budgets are provided in Chapter 2. The concept of turbulent viscosity as a closure of the Reynolds stress is also introduced. Wall-bounded flows are presented in Chapter 3, and aspects specific to boundary layers and channel or pipe flows are also pointed out. Free shear flows, namely free jets and wakes, are considered in Chapter 4. Chapter 5 deals with vortex dynamics. Homogeneous turbulence, isotropy, and dynamics of isotropic turbulence are presented in Chapters 6 and 7. Turbulence is then described both in the physical space and in the wave number space. Time dependent numerical simulations are presented in Chapter 8, where an introduction to large eddy simulation is offered. The last three chapters of the book summarize remarka...
Forcing homogeneous turbulence in DNS of particulate flow with interface resolution and gravity
Chouippe, Agathe
2015-01-01
We consider the case of finite-size spherical particles which are settling under gravity in a homogeneous turbulent background flow. Turbulence is forced with the aid of the random forcing method of Eswaran and Pope [Comput. Fluids, 16(3):257-278, 1988], while the solid particles are represented with an immersed-boundary method. The forcing scheme is used to generate isotropic turbulence in vertically elongated boxes in order to warrant better decorrelation of the Lagrangian signals in the direction of gravity. Since only a limited number of Fourier modes are forced, it is possible to evaluate the forcing field directly in physical space, thereby avoiding full-size transforms. The budget of box-averaged kinetic energy is derived from the forced momentum equations. Medium-sized simulations for dilute suspensions at low Taylor-scale Reynolds number $Re_\\lambda=65$, small density ratio $\\rho_p/\\rho_f=1.5$ and for two Galileo numbers $Ga=0$ and 120 are carried out over long time intervals in order to exclude the ...
Nie, Ji; Sobel, Adam H
2016-01-01
Extratropical extreme precipitation events are usually associated with large-scale flow disturbances, strong ascent and large latent heat release. The causal relationships between these factors are often not obvious, however, and the roles of different physical processes in producing the extreme precipitation event can be difficult to disentangle. Here, we examine the large-scale forcings and convective heating feedback in the precipitation events which caused the 2010 Pakistan flood within the Column Quasi-Geostrophic framework. A cloud-revolving model (CRM) is forced with the large-scale forcings (other than large-scale vertical motion) computed from the quasi-geostrophic omega equation with input data from a reanalysis data set, and the large-scale vertical motion is diagnosed interactively with the simulated convection. Numerical results show that the positive feedback of convective heating to large-scale dynamics is essential in amplifying the precipitation intensity to the observed values. Orographic li...
Energy Technology Data Exchange (ETDEWEB)
Francisco Valentin; Narbeh Artoun; Masahiro Kawaji; Donald M. McEligot
2015-08-01
Fundamental high pressure/high temperature forced convection experiments have been conducted in support of the development of a Very High Temperature Reactor (VHTR) with a prismatic core. The experiments utilize a high temperature/high pressure gas flow test facility constructed for forced convection and natural circulation experiments. The test section has a single 16.8 mm ID flow channel in a 2.7 m long, 108 mm OD graphite column with four 2.3kW electric heater rods placed symmetrically around the flow channel. This experimental study presents the role of buoyancy forces in enhancing or reducing convection heat transfer for helium at high pressures up to 70 bar and high temperatures up to 873 degrees K. Wall temperatures have been compared among 10 cases covering the inlet Re numbers ranging from 500 to 3,000. Downward flows display higher and lower wall temperatures in the upstream and downstream regions, respectively, than the upward flow cases due to the influence of buoyancy forces. In the entrance region, convection heat transfer is reduced due to buoyancy leading to higher wall temperatures, while in the downstream region, buoyancyinduced mixing causes higher convection heat transfer and lower wall temperatures. However, their influences are reduced as the Reynolds number increases. This experimental study is of specific interest to VHTR design and validation of safety analysis codes.
Heat transport in boiling turbulent Rayleigh-B\\'{e}nard convection
Lakkaraju, Rajaram; Oresta, Paolo; Verzicco, Roberto; Lohse, Detlef; Prosperetti, Andrea
2014-01-01
Boiling is an extremely effective way to promote heat transfer from a hot surface to a liquid due to several mechanisms many of which are not understood in quantitative detail. An important component of the overall process is that the buoyancy of the bubbles compounds with that of the liquid to give rise to a much enhanced natural convection. In this paper we focus specifically on this enhancement and present a numerical study of the resulting two-phase Rayleigh-B\\'enard convection process. We make no attempt to model other aspects of the boiling process such as bubble nucleation and detachment. We consider a cylindrical cell with a diameter equal to its height. The cell base and top are held at temperatures above and below the boiling point of the liquid, respectively. By keeping the temperature difference constant and changing the liquid pressure we study the effect of the liquid superheat in a Rayleigh number range that, in the absence of boiling, would be between $2\\times10^6$ and $5\\times10^9$. We find a...
Study and Application of Forced Convection in Road Bus Heating System
Directory of Open Access Journals (Sweden)
Scheila Sandi Biazus
2015-03-01
Full Text Available This work deals with a replacing the heating system of intercity buses, made by long heat exchangers distributed on the sides of the passenger saloon by small and compact ones that use forced convection to heat dissipation. At the beginning the calculation was made of the heat capacity of a heat exchanger with the parameters defined by the supplier, and then conducted an analysis of change in these parameters in order to improve its efficiency. The method for examining the heat exchanger is based on the correlations available in the convection flow inside and outside to determine the respective convective coefficients, and therefore the overall coefficient of heat transfer. Following, the heat exchanger in its original form, was tested on bench. Thus, from the thermal load of the bus model studied, the amount of exchangers required has been defined to satisfy the thermal comfort of passengers. Field tests of the heating systems with new heat exchangers were performed to obtain actual data of the proposed system. The original factory system showed to be efficient in meeting the needs thermal load and economically viable, such that the critical thermal load can be met with ten heaters installed along the body. Furthermore, this system offers the possibility of varying the parameters to best fit to the project, where the flow variation of the fans or the spacing between the fins of the heat exchanger decreases the amount of heaters required. At the same time, the system showed the need for further study to assess its distribution along the passenger saloon in order to meet a best uniformity temperature distribution.
The Turbulent Convective Plume at Ice Shelf Fronts and the Sides of Tabular Icebergs
Kerr, R. C.; McConnochie, C. D.
2015-12-01
We present laboratory experiments and theoretical analysis that quantify the turbulent buoyant plume formed by the dissolution of a vertical ice face in homogeneous salt water. In our experiments, we vary the temperature and salinity of the salt water and measure the dissolution rate of the ice, the temperature of the ice-water interface, the maximum vertical velocity of the buoyant plume, and the rate at which the laboratory tank becomes stratified with buoyant fluid. Using this experimental information, we then construct a theoretical model of the turbulent buoyant plume as a function of height. The plume has a top-hat entrainment coefficient of 0.048 ± 0.006, and is found to have substantial drag. The plume model is used to calculate a plume width, velocity, buoyancy and Reynolds number for typical dissolving icebergs and ice shelf fronts. Our laboratory experiments also examine the effect of a linear salinity gradient on the dissolution of a vertical ice face. As the stratification is increased, the dissolution rate, the interface temperature and the maximum vertical plume velocity all decrease, and their dependence on height changes. We also outline a method of scaling the effects of stratification from our laboratory experiments to the much larger vertical scales of ice shelves and icebergs.
Directory of Open Access Journals (Sweden)
S.K Ghosh
2013-01-01
Full Text Available A theoretical analysis is presented for transient, fully-developed magnetohydrodynamic free and forced convection flow of a viscous, incompressible, Newtonian fluid in a rotating horizontal parallel-plate channel subjected to a uniform strength, static, oblique magnetic field acting at an angle to the positive direction of the axis of rotation. A constant pressure gradient is imposed along the longitudinal axis of the channel. Magnetic Reynolds number is sufficiently small to negate the effects of magnetic induction. The channel plates are electrically non-conducting. The conservation equations are formulated in an (x,y,z coordinate system and normalized using appropriate transformations. The resulting non-dimensional coupled ordinary differential equations for primary and secondary velocity components and transformed boundary conditions are found to be reciprocal of the Ekman number ( 2 K =1/Ek, non-dimensional pressure gradient parameter (Px, Hartmann number ( 2 M , Grashof number (Gr, magnetic field inclination ( and oscillation frequency (. Complex variables are employed to solve the two-point boundary value problem. A steady state resonance of the velocity field is identified for 4 4 4 1/ 2 16 2 1 K M Sin . Furthermore the solutions indicate that the condition 1/2 1 4 4 4 cos 16 2 T K M Sin signifies an oscillatory turbulent dynamo mechanism. A critical Grashof number (Grcx is also evaluated for which primary flow reversal arises at the upper channel plate. A similar criterion for Grashof number (Grcy is established for the onset of secondary flow reversal at the upper plate. A detailed assessment of the influence of the control parameters on primary and secondary velocity evolution in the channel is also conducted. The model finds applications in MHD (Magneto Hydro Dynamic energy generators and also magnetic materials processing systems.
Single layer solar drying behaviour of Citrus aurantium leaves under forced convection
Energy Technology Data Exchange (ETDEWEB)
Ait Mohamed, L.; Lahsasni, S. [Ecole Normale Superieure, Marrakech (Morocco). Laboratoire d' Energie Solaire et des Plantes Aromatiques et Medicinales; Unite de Chimie Agroalimentaire, Marrakech (Morocco). Faculte des Sciences Semlalia; Kouhila, M.; Jamali, A. [Ecole Normale Superieure, Marrakech (Morocco). Laboratoire d' Energie Solaire et des Plantes Aromatiques et Medicinales; Kechaou, N. [Ecole Nationale d' Ingenieurs de Sfax (Tunisia); Mahrouz, M. [Unite de Chimie Agroalimentaire, Marrakech (Morocco). Faculte des Sciences Semlalia
2005-06-01
Convective solar drying experiments in thin layers of Citrus aurantium leaves grown in Marrakech, morocco, were conducted. An indirect forced convection solar dryer consisting of a solar air collector, an auxiliary heater, a circulation fan and a drying cabinet is used for the experiments. The air temperature was varied from 50 to 60{sup o}C; the relative humidity from 41% to 53%; and the drying air flow rate from 0.0277 to 0.0833 m{sup 3}/s. Thirteen statistical models, which are semi-theoretical and/or empirical, were tested for fitting the experimental data. A nonlinear regression analysis using a statistical computer program was used to evaluate the constants of the models. The Midilli-Kucuk drying model was found to be the most suitable for describing the solar drying curves of Citrus aurantium leaves with a correlation coefficient (r) of 0.99998, chi-square ({chi}{sup 2}) of 4.664 x 10{sup -6} and MBE of 4.8381 x 10{sup -4}. (author)
Kleinman, Leonid S.; Reed, X. B., Jr.
1995-01-01
An algorithm has been developed for the forced convective diffusion-reaction problem for convection inside and outside a droplet by a recirculating flow field hydrodynamically coupled at the droplet interface with an external flow field that at infinity becomes a uniform streaming flow. The concentration field inside the droplet is likewise coupled with that outside by boundary conditions at the interface. A chemical reaction can take place either inside or outside the droplet or reactions can take place in both phases. The algorithm has been implemented and results are shown here for the case of no reaction and for the case of an external first order reaction, both for unsteady behavior. For pure interphase mass transfer, concentration isocontours, local and average Sherwood numbers, and average droplet concentrations have been obtained as a function of the physical properties and external flow field. For mass transfer enhanced by an external reaction, in addition to the above forms of results, we present the enhancement factor, with the results now also depending upon the (dimensionless) rate of reaction.
Transient-forced convection film boiling on an isothermal flat plate.
Nagendra, H. R.
1972-01-01
Development of a new approach for the solution of transient-forced convection film boiling on an isothermal flat plate using the boundary layer model. The similarity variables are used to convert the governing partial differential equations to ordinary ones. The results of numerical solutions of these ordinary equations indicate that the transient process can be classified as one-dimensional conduction, intermediate, and the steady-state regions. The time required for the one-dimensional conduction and the time necessary to attain a steady-state condition are obtained. The use of local similarity approximations for the intermediate regime facilitates prediction of complete boundary layer growth. Using the ratio of time at any instant to the steady-state time as abscissa, the curves representing the boundary layer growth can be merged into a single mean curve within 5%. Further, the analysis shows that the average rate of heat transfer during transient is 50 to 100% higher than those at steady state. The average rate of vapor convected away is 10 to 15% lower than at steady state while the average rate of accumulation to form the vapor layer is 1 to 14 times larger.
Turbulent convection in a horizontal duct with strong axial magnetic field
Zhang, Xuan; Zikanov, Oleg
2014-11-01
Convection in a horizontal duct with one heated wall is studied computationally. The work is motivated by the concept of a blanket for fusion reactors, according to which liquid metal slowly flows in toroidal ducts aligned with the main component of the magnetic field. We first assume that the magnetic field is sufficiently strong for the flow to be purely two-dimensional and analyze chaotic flow regimes at very high Grashof numbers. Furthermore, three-dimensional perturbations are considered and the relation between the length of the duct and the critical Hartmann number, below which the flow becomes three-dimensional, is determined. Financial support was provided by the US NSF (Grant CBET 1232851).
Laboratory study of forced rotating shallow water turbulence
Espa, Stefania; Di Nitto, Gabriella; Cenedese, Antonio
2011-12-01
During the last three decades several authors have studied the appearance of multiple zonal jets in planetary atmospheres and in the Earths oceans. The appearance of zonal jets has been recovered in numerical simulations (Yoden & Yamada, 1993), laboratory experiments (Afanasyev & Wells, 2005; Espa et al., 2008, 2010) and in field measurements of the atmosphere of giant planets (Galperin et al., 2001). Recent studies have revealed the presence of zonation also in the Earths oceans, in fact zonal jets have been found in the outputs of Oceanic General Circulation Models-GCMs (Nakano & Hasumi, 2005) and from the analysis of satellite altimetry observations (Maximenko et al., 2005). In previous works (Espa et al., 2008, 2010) we have investigated the impact of the variation of the rotation rate and of the fluid depth on jets organization in decaying and forced regimes. In this work we show results from experiments performed in a bigger domain in which the fluid is forced continuously. The experimental set-up consists of a rotating tank (1m in diameter) where the initial distribution of vorticity has been generated via the Lorentz force in an electromagnetic cell. The latitudinal variation of the Coriolis parameter has been simulated by the parabolic profile assumed by the free surface of the rotating fluid. Flow measurements have been performed using an image analysis technique. Experiments have been performed changing the tank rotation rate and the fluid thickness. We have investigated the flow in terms of zonal and radial flow pattern, flow variability and jet scales.
Grannan, Alex; Favier, Benjamin; Bills, Bruce; Lemasquerier, Daphne; Le Bars, Michael; Aurnou, Jonathan
2016-10-01
The turbulent fluid motions generated in the liquid metal cores and oceans of planetary bodies can have profound effects on energy dissipation and magnetic field generation. An important driver of such fluid motions is mechanical forcing from precession, libration, and tidal forcing. On Earth, the dissipation of energy through tidal forcing occurs primarily in the oceans and may be due, in part, to nonlinear tidally forced resonances. However, the role that such nonlinear resonances play are not generally considered for other planetary bodies also possessing oceans and liquid metal cores.Recent laboratory experimental and numerical studies of Grannan et al. 2014 and Favier et al. 2015 have shown that nonlinear fluid resonances generated by sufficiently strong librational forcing can drive turbulent flows in ellipsoidal containers that mimic gravitational deformations. In Grannan et al. 2016, similar results were found for strong tidal forcing. Thus, a generalized scaling law for the turbulent r.m.s. velocity is derived, U~ɛβE-α, where ɛ is the dimensionless amplitude of the tidal or librational forcing, β is the dimensionless tidal deformation of the body, E is the dimensionless Ekman number characterizing the ratio of viscous to Coriolis forces, and α is a varying exponent.Using planetary values for tidal and librational forcing parameters, the turbulent dissipation is estimated for multiple bodies. For the subsurface oceans of Europa and Enceladus, the amount of nonlinear dissipation is comparable to the dissipation generated from linear resonances of the fluid layer and from upper bounding estimates of the tidal dissipation in the solid icy shell. In addition, our estimates of this turbulent dissipation provide bounds for the stratification in these subsurface oceans. Finally we find that dissipation from these nonlinear resonances in the liquid metal cores of the the early and present Earth, Io, and several exoplanets may help drive the dynamos in these
Okken, A; Blijham, C; Franz, W; Bohn, E
1982-07-01
To assess the effect of forced convection of heated air exchange in preterm infants in conventional incubators, we measured insensible water loss and total heat loss in preterm infants in a conventional forced convection incubator (air velocity 15 to 25 cm/second) and in a specially constructed still-air incubator (air velocity 0 to 2 cm/second) at equal operative temperature and humidity. Under the forced conditions, insensible water loss in the preterm infants increased by a mean 52% from 1.04 +/- 0.24 (mean +/- SD) to 1.58 +/- 0.51 ml/kg/hour (P less than 0.001). The ensuing increase in evaporative heat loss was partly reflected in the small but significant increase in total heat loss from 1.65 +/- 0.47 to 1.80 +/- 0.44 kcal/kg/hour (P less than 0.02). In the forced convection incubator, the increased evaporative heat loss in preterm infants was apparently partly compensated by a decreased nonevaporative heat loss. If reduction of insensible water loss is required, preterm infants should not be subjected to forced convection in incubators.
Experiments on forced convection form a horizontal heated plate in a packed bed of glass spheres
Energy Technology Data Exchange (ETDEWEB)
Renken, K.J. (Univ. of Wisconsin, Milwaukee (USA)); Poulikakos, D. (Univ. of Illinois, Chicago (USA))
1989-02-01
This paper presents an experimental investigation of boundary-layer forced convective heat transfer from a flat isothermal plate in a packed bed of spheres. Extensive experimental results are reported for the thermal boundary-layer thickness, the temperature field, and the local wall heat flux (represented by the local Nusselt number). Theoretical findings of previous investigations using the Darcy flow model as well as a general model for themomentum equation accouting for flow inertia and macroscopic shear wtih and without variable porosity are used to evaluate the theoretical models. Several trends are revealed regarding the conditions of validity of these flow models. Overall the general flow model including variable porosity appears to perform better, even through the need for serious improvements in modeling becomes apparent.
Experimental-theoretical analysis of laminar internal forced convection with nanofluids
Energy Technology Data Exchange (ETDEWEB)
Cerqueira, Ivana G.; Cotta, Renato M. [Lab. of Transmission and Technology of Heat-LTTC. Mechanical Eng. Dept. - POLI and COPPE/UFRJ, Rio de Janeiro, RJ (Brazil)], E-mail: cotta@mecanica.coppe.ufrj.br; Mota, Carlos Alberto A. [Conselho Nacional de Pesquisas - CNPq, Brasilia, DF (Brazil)], e-mail: carlosal@cnpq.br; Nunes, Jeziel S. [INPI, Rio de Janeiro, RJ (Brazil)], e-mail: jeziel@inpi.gov.br
2010-07-01
This work reports fundamental experimental-theoretical research related to heat transfer enhancement in laminar channel flow with nanofluids, which are essentially modifications of the base fluid with the dispersion of metal oxide nanoparticles. The theoretical work was performed by making use of mixed symbolic-numerical computation (Mathematica 7.0 platform) and a hybrid numerical-analytical methodology (Generalized Integral Transform Technique - GITT) in accurately handling the governing partial differential equations for the heat and fluid flow problem formulation with temperature dependency in all the thermophysical properties. Experimental work was also undertaken based on a thermohydraulic circuit built for this purpose, and sample results are presented to verify the proposed model. The aim is to illustrate detailed modeling and robust simulation attempting to reach an explanation of the controversial heat transfer enhancement observed in laminar forced convection with nanofluids. (author)
Institute of Scientific and Technical Information of China (English)
李志锦; 丑纪范
1994-01-01
According to the theory of the nonlinear dynamical systems with forcing and dissipation,theconceptual ideas are proposed that the intense convective system is an attractor of the meso-scale dynamicalsystem,in which the large-scale fields serve as the bifurcation parameters that evolve slowly.The intenseconvective systems occur explosively when the intensity of the large-scale system passes a bifurcation point.Thus,its prediction is just to predict the evolution of the attractor,independent of the mesoscale initial con-ditions.A primitive spectral limited-area model has been developed and two realistic cases,which were cap-tured by the Eastern China area mesoscale experiment,have been simulated by using this model.The resultshave proved the theoretical points.
Institute of Scientific and Technical Information of China (English)
YANG Xiao; LIU Xuemei
2007-01-01
Based on the Darcy fluid model, by considering the effects of viscous dissipation due to the interaction between solid skeleton and pore fluid flow and thermal conduction in the direction of the fluid flow, the thermally developing forced convection of the local thermal equili- brium and the corresponding thermal stresses in a semi- infmite saturated porous plate channel are investigated in this paper. The expressions of temperature, local Nusselt number and corresponding thermal stresses are obtained by means of the Fourier series, and the distributions of the same are also shown. Furthermore, influences of the Péclet number (Pe) and Brinkman number (Br) on temperature, Nusselt number (Nu) and thermal stress are revealed numerically.
The effects of magnetic field on forced and free convection flow
Abdel-Khalek, M M
2003-01-01
The effects of magnetic field and permeability of the porous medium on unsteady forced and free convection flow past an infinite vertical porous plate have been studied when the temperature of the plate is oscillating with time about a constant nonzero mean value and in the presence of temperature dependent heat source. The governing equations for the hydromagnetic fluid flow and the heat transfer are solved subject to the relevant boundary conditions. The perturbation technique is used to obtain expressions for velocity field, skin friction and Nusselt number. Also, the effects of magnetic parameter, heat source parameter, suction parameter and permeability of porous medium on velocity field, skin friction and Nusselt number are discussed.
Forced convection of power-law fluids flow over a rotating nonisothermal body
Kim, H. W.; Essemyi, A. J.
1993-10-01
Presented is an analysis of steady laminar flow of power-law fluids past a rotating body with nonisothermal surfaces. A coordinate transformation combined with the Merk-type series expansion is employed to transform the governing momentum equations into a set of coupled ordinary differential equations. The equations are numerically integrated to obtain the axial and tangential velocity gradients for determining the friction coefficient. For forced convection, a generalized coordinate transformation is used to analyze the temperature field of the power-law flow. Solutions to the transformed energy equations are obtained in the form of universal functions. The heat transfer coefficients in terms of NuRe(sup 1/(n + 1)) are presented for a rotating sphere. The effects of power-law index, rotation sphere, Prandtl number, and the location of step discontinuity in surface temperature on the local Nusselt number are fully investigated and demonstrated.
Effect of Heated Perimeter on Forced Convection Heat Transfer of he i at a Supercritical Pressure
Doi, D.; Shiotsu, M.; Shirai, Y.; Hama, K.
2008-03-01
The forced convection heat transfer coefficients were measured on two pairs of test plates all 6.0 mm in width and located face to face on inner walls of a rectangular duct. Each pair having length of 20 mm and 80 mm, respectively, was connected in series electrically. The rectangular duct was 420 mm in length and 5 mm×6 mm in inner cross section. The experiments were performed for inlet temperatures from 2.2 to 6.5 K, flow velocities from 0.1 to 5.6 m/s, and at a supercritical pressure of 2.8 atm. Comparison of the obtained Nusselt numbers with the former results with a single test plate showed the clear effect of a heated perimeter. Non-dimensional heat transfer equation including the effect of heated perimeter is presented.
Combined effect of free and forced convection on MHD flow in a rotating porous channel
Directory of Open Access Journals (Sweden)
D. R. V. Prasada Rao
1982-01-01
Full Text Available This paper gives a steady linear theory of the combined effect of the free and forced convection in rotating hydromagnetic viscous fluid flows in a porous channel under the action of a uniform magnetic field. The flow is governed by the Grashof number G, the Hartmann number H, the Ekman number E, and the suction Reynolds number S. The solutions for the velocity field, temperature distribution, magnetic field, mass rate of flow and the shear stresses on the channel boundaries are obtained using a perturbation method with the small parameter S. The nature of the associated boundary layers is investigated for various values of the governing flow parameters. The velocity, the temperature, and the shear stresses are discussed numerically by drawing profiles with reference to the variations in the flow parameters.
Forced convection analysis for generalized Burgers nanofluid flow over a stretching sheet
Khan, Masood; Khan, Waqar Azeem
2015-10-01
This article reports the two-dimensional forced convective flow of a generalized Burgers fluid over a linearly stretched sheet under the impacts of nano-sized material particles. Utilizing appropriate similarity transformations the coupled nonlinear partial differential equations are converted into a set of coupled nonlinear ordinary differential equations. The analytic results are carried out through the homotopy analysis method (HAM) to investigate the impact of various pertinent parameters for the velocity, temperature and concentration fields. The obtained results are presented in tabular form as well as graphically and discussed in detail. The presented results show that the rate of heat transfer at the wall and rate of nanoparticle volume fraction diminish with each increment of the thermophoresis parameter. While incremented values of the Brownian motion parameter lead to a quite opposite effect on the rates of heat transfer and nanoparticle volume fraction at the wall.
CFD Analysis of nanofluid forced convection heat transport in laminar flow through a compact pipe
Yu, Kitae; Park, Cheol; Kim, Sedon; Song, Heegun; Jeong, Hyomin
2017-08-01
In the present paper, developing laminar forced convection flows were numerically investigated by using water-Al2O3 nano-fluid through a circular compact pipe which has 4.5mm diameter. Each model has a steady state and uniform heat flux (UHF) at the wall. The whole numerical experiments were processed under the Re = 1050 and the nano-fluid models were made by the Alumina volume fraction. A single-phase fluid models were defined through nano-fluid physical and thermal properties calculations, Two-phase model(mixture granular model) were processed in 100nm diameter. The results show that Nusselt number and heat transfer rate are improved as the Al2O3 volume fraction increased. All of the numerical flow simulations are processed by the FLUENT. The results show the increment of thermal transfer from the volume fraction concentration.
Directory of Open Access Journals (Sweden)
Stefano Guarino
2017-08-01
Full Text Available In this paper, the heat transfer performances of aluminum metal foams, placed on horizontal plane surface, was evaluated in forced convection conditions. Three different types of contacts between the sample and the heated base plate have been investigated: simple contact, brazed contact and grease paste contact. First, in order to perform the study, an ad hoc experimental set-up was built. Second, the value of thermal contact resistance was estimated. The results show that both the use of a conductive paste and the brazing contact, realized by means of a copper electro-deposition, allows a great reduction of the global thermal resistance, increasing de facto the global heat transfer coefficient of almost 80%, compared to the simple contact case. Finally, it was shown that, while the contribution of thermal resistance is negligible for the cases of brazed and grease paste contact, it is significantly high for the case of simple contact.
A theoretical study of the spheroidal droplet evaporation in forced convection
Energy Technology Data Exchange (ETDEWEB)
Li, Jie, E-mail: leejay1986@163.com; Zhang, Jian
2014-11-07
In many applications, the shape of a droplet may be assumed to be an oblate spheroid. A theoretical study is conducted on the evaporation of an oblate spheroidal droplet under forced convection conditions. Closed-form analytical expressions of the mass evaporation rate for an oblate spheroid are derived, in the regime of controlled mass-transfer and heat-transfer, respectively. The variation of droplet size during the evaporation process is presented in the regime of shrinking dynamic model. Comparing with the droplets having the same surface area, an increase in the aspect ratio enhances the mass evaporation rate and prolongs the burnout time. - Highlights: • Fully algebraic solutions for the spheroidal droplet evaporation rate is obtained. • We examine the effect of aspect ratio on the droplet evaporation. • We propose a calculation method of Nusselt number for spheroidal droplet.
Laminar forced convection slip-flow in a micro-annulus between two concentric cylinders
Energy Technology Data Exchange (ETDEWEB)
Avci, Mete; Aydin, Orhan [Karadeniz Technical University, Department of Mechanical Engineering, 61080 Trabzon (Turkey)
2008-07-01
Forced convection heat transfer in hydrodynamically and thermally fully developed flows of viscous dissipating gases in annular microducts between two concentric micro cylinders is analyzed analytically. The viscous dissipation effect, the velocity slip and the temperature jump at the wall are taken into consideration. Two different cases of the thermal boundary conditions are considered: uniform heat flux at the outer wall and adiabatic inner wall (Case A) and uniform heat flux at the inner wall and adiabatic outer wall (Case B). Solutions for the velocity and temperature distributions and the Nusselt number are obtained for different values of the aspect ratio, the Knudsen number and the Brinkman number. The analytical results obtained are compared with those available in the literature and an excellent agreement is observed. (author)
The effects of outward forced convective flow on inward diffusion in human dentine in vitro.
Pashley, D H; Matthews, W G
1993-07-01
In vitro experiments were conducted to evaluate the influence of outward forced convective flow on the inward diffusion of radioactive iodide. When the smear layer was present, application of 15 cmH2O (1.47 kPa) outward-directed filtration pressure reduced the inward flux of iodide by about 10-20% depending upon the hydraulic conductance of each specimen. When the smear layer was removed by acid etching, the same 1.47 kPa pressure lowered the inward iodide flux by as much as 60%, depending on the hydraulic conductance. The results demonstrate the importance of the balance between inward diffusion and outward bulk-fluid movement on the rate of permeation of exogenous solutes.
Thermal boundary layer profiles in turbulent Rayleigh-B\\'enard convection in a cylindrical sample
Stevens, Richard J A M; Grossmann, Siegfried; Verzicco, Roberto; Xia, Ke-Qing; Lohse, Detlef
2011-01-01
We numerically investigate the structures of the near-plate temperature profiles close to the bottom and top plates of turbulent Rayleigh-B\\'{e}nard flow in a cylindrical sample at Rayleigh numbers $Ra=10^8$ to $Ra=2\\times10^{12}$ and Prandtl numbers Pr=6.4 and Pr=0.7 thus extending previous results for quasi-2-dimensional systems to 3D systems for the first time. The results show that the instantaneous temperature profiles scaled by the dynamical frame method [Q. Zhou and K.-Q. Xia, Phys. Rev. Lett. 104, 104301 (2010)] agree well with the classical Prandtl-Blasius laminar boundary layer (BL) profiles, especially for low Ra and high Pr. The agreement is slightly less, but still good, for lower Pr, where the thermal BL is more exposed to the bulk fluctuations due to the thinner kinetic BL, and higher Ra, where more plumes are passing the measurement location.
Assessment of Edge Turbulence and Convective Transport through Velocity Field Analysis
Energy Technology Data Exchange (ETDEWEB)
Munsat, Tobin [Univ. of Colorado, Boulder, CO (United States). Center for Integrated Plasma Studies
2015-03-14
Over the course of this grant period, we have conducted three major studies, each of which has resulted in a primary publication (described below): First, we investigated the flow and shear behavior of the edge plasma and scrape-off layer (SOL) in NSTX using the GPI diagnostic. Calculation of local, time resolved velocity maps using the hybrid optical flow and pattern matching velocimetry code enabled analysis of turbulent flow and shear behavior in these plasmas. Second, we used GPI measurements made during RF heated H-mode operation, to identify intermittent periodic edge intensity fluctuations which precede ELMs and ELM-induced back transitions from H-mode to Lmode. These edge oscillations have a well-defined mode structure and are visible up to 100-200 μs preceding the ELM events. Finally, we performed an in-depth investigation between the fluctuation measurements made by the GPI and BES diagnostics on NSTX.
Enhancing filling of interconnect deep trenches using forced convection magneto-electroplating
Said, R. A.
2006-01-01
Filling deep trenches and cavities is currently accomplished by copper electro-less plating technology utilizing super-conformal deposition methods. Unlike typical electrolyses processes, where an electric potential is applied between the anodes to activate the plating reaction, electro-less plating relies on chemical agents to activate deposition. To achieve super-conformal deposition, special electrolytic paths must be used. This poses a challenge to the fabrication of narrower trenches, and thus requires the development of other deposition schemes. This work proposes an alternative solution to the filling of deep trenches that avoids the difficulties outlined above, using a forced convection magneto-electroplating method. The technique operates as in typical electrolysis processes, however, with forcing the flow of the plating electrolyte, by hydro-dynamic means, in the presence of an externally applied magnetic field. This arrangement introduces a Lorentz type of force that enhances the transport of deposit species toward desired locations, such as deep regions in interconnect trenches. The proposed method is demonstrated by filling interconnect trenches with aspect ratio as high as 3:1. Quality of samples filled using the proposed magneto-electroplating method is compared with the quality of samples filled by typical electroplating method.
Performance of a Forced-Convection Greenhouse Dryer for Fish Drying
Directory of Open Access Journals (Sweden)
Martunis Martunis
2013-04-01
Full Text Available ABSTRACT. This research present experimental performance of a forced convection greenhouse dryer for drying of fish. The greenhouse dryer was installed at Aceh province, Indonesia. It has a concrete floor with the area of 6×4 m2. The roof of dryer is built in semi-cylindrical shape and covered with transparent polycarbonate sheets. Two axial flow fans powered by a 50-watt solar cell module was used to generate forced convection for ventilating the dryer.To investigate its performance, the dryer was used to dry two batches of fish. The Results showed that to dry 50 kg fish with initial moisture 68 % required 11 hours. Incontrast, to dry the same amount of fish using sun drying take a time about 2 days. The air temperature inside greenhouse dryer at noon in the clear day was 45-55°C. Kinerja Pengering Rumah Kaca Dengan Metode Konveksi Paksa Untuk Pengeringan Ikan ABSTRAK. Penelitian ini memperlihatkan hasil percobaan terhadap kinerja pengering rumah kaca dengan metode konveksi udara secara paksa pada pengeringan ikan. Penelitian pengering rumah kaca ini dilakukan di Propinsi Aceh, Indonesia. Pengering ini berlantaikan beton dengan luas sebesar 6x4 m2. .Atapnya dibuat berbentuk semi-selinder dan ditutup dengan lembaran plastik transparan berbahan polikarbonat. Dua buah kipas dengan aliran udara secara aksial dipasang dengan sumber daya berasal dari panel surya sebesar 50 Watt dan dipakai untuk menghasilkan konveksi udara paksa pada ventilasi pengering. Untuk menginvestigasi kinerja rumah kaca ini, pengering ini digunakan untuk mengeringkan dua tumpukan ikan. Hasilnya menunjukkan bahwa untuk mengeringkan sebanyak 50 kg ikan dengan kadar air awalnya sebesar 68% membutuhkan waktu selama 11 jam. Sebaliknya, dengan menggunakan sinar matahari secara langsung, untuk mengeringkan ikan dengan jumlah yang sama, maka waktu yang dibutuhkan lebih lama yaitu sekitar 2 hari. Suhu udara di dalam rumah pengering tepat pada siang hari yang cerah berkisar antara 45
Batubara, Fatimah; Misran, Erni; Dina, Sari Farah; Heppy
2017-06-01
Research on potato drying using the indirect solar dryer with flat-plate finned collector and forced convection has been done. The research was conducted at the outdoor field of Laboratory of Institute for Research and Standardization of Industry on June 14th-23rd, 2016 from 9:00 am to 4:00 pm. This research aims to obtain the drying kinetics model of potato (Solanumtuberosum L.) using an indirect solar dryer's (ISD) with flat plate-finned collector and forced convection. The result will be compared to the open sun drying (OSD) method. Weather conditions during the drying process took place as follows; surrounding air temperature was in the range 27 to 34.7 °C, relative humidity (RH) 29.5 to 61.0% and the intensity of solar radiation 105.6 to 863.1 Watt/m2. The dried potato thicknesses were 1.0 cm, 1.5 cm and 2.0 cm, with the average initial water content of 76.46%. The average temperature in the collector chamber ranged from 42.2 to 57.4 °C and the drying chamber was at 46.2 °C. The best drying result was obtained from a sample size of 1 cm thickness using the IDS method with an average drying rate of 0.018 kg H2O per kg dry-weight.hour and the water content was constant at 5.02% in 21 hours of drying time. The most suitable kinetics model is Page model, equation MR = exp (-0.049 t1,336) for 1.0 cm thickness, exp (-0.066 t1,222) for 1.5 cm thickness and exp (-0.049 t1,221) for 2.0 cm thickness. The quality of potato drying using ISD method is better than using OSD which can be seen from the color produced.
Low-dimensional model of turbulent Rayleigh-Benard convection in a Cartesian cell with square domain
Bailon-Cuba, Jorge
2011-01-01
A low-dimensional model (LDM) for turbulent Rayleigh-Benard convection in a Cartesian cell with square domain, based on the Galerkin projection of the Boussinesq equations onto a finite set of empirical eigenfunctions, is presented. The empirical eigenfunctions are obtained from a joint Proper Orthogonal Decomposition (POD) of the velocity and temperature fields using the Snapshot Method on the basis of a direct numerical simulation (DNS). The resulting LDM is a quadratic inhomogeneous system of coupled ordinary differential equations which we use to describe the long-time temporal evolution of the large-scale mode amplitudes for a Rayleigh number of 1e5 and a Prandtl number of 0.7. The truncation to a finite number of degrees of freedom, that does not exceed a number of 310 for the present, requires the additional implementation of an eddy viscosity-diffusivity to capture the missing dissipation of the small-scale modes. The magnitude of this additional dissipation mechanism is determined by requiring statis...
Feltz, Wayne
2016-04-01
A suite of aviation related decision support products have been in development to meet GOES-R science requirements since 2008 and are being evaluated to assess meteorological hazards to aircraft in flight derived from the current generation of European Spinning Enhanced Visible and Infrared Imager (SEVIRI) imager data. This presentation will focus on GOES-R Advanced Baseline Imager (ABI) measurement requirements relating to satellite-based aviation convective, turbulence, and volcanic ash/SO2 products that can be applied globally on next generation geostationary imagers including the Japanese Himawari, South Korean COMS (AMI), and European Metop-SG imagers. These new methodologies have relevance on current generation GOES and SEVIRI imagers, and overview will include discussion on how product utility has been improved through satellite GOES-R/JPSS Proving Ground NOAA testbed activities. Satellite-based decision support for aviation context toward improvement of future air transportation route planning and warning for the general public with emphasis on successfully bridging research to operations will also be discussed with anticipated October 2016 launch of GOES-R.
Khoo, B. C.; Chew, Y. T.; Teo, C. J.
This work continues the studies of Khoo et al. (Exp. Fluids 29: 448-460, 2001), where experiments were performed in turbulent-channel and flat-plate boundary-layer flows using near-wall hot-wire probes. The probability density function (pdf) of the wall-shear stress and streamwise velocity fluctuations in the viscous sublayer, buffer region and beyond were compared and analyzed. The convective velocity Uc of the streamwise velocity fluctuations in the very near-wall region was obtained using a two-point correlation technique. It was found that in the viscous sublayer, Uc is approximately constant at 13uτ and 15uτ, respectively, for the channel and boundary-layer flows. Spectra data for the viscous sublayer are presented for the first time, and the normalized spectral plots for different flow conditions collapse at high frequencies or wavenumbers, thus indicating the possible presence of small-scale universality at different Reynolds numbers. The integral time scale corresponding to the streamwise velocity fluctuations in the viscous sublayer is also presented.
Institute of Scientific and Technical Information of China (English)
Z. Lin; R.E. Waltz
2007-01-01
@@ Turbulent transport driven by plasma pressure gradients [Tangl978] is one of the most important scientific challenges in burning plasma experiments since the balance between turbulent transport and the self-heating by the fusion products (a-particles) determines the performance of a fusion reactor like ITER.
Phenomenology of two-dimensional stably stratified turbulence under large-scale forcing
Kumar, Abhishek; Sukhatmae, Jai
2016-01-01
In this paper we characterize the scaling of energy spectra, and the interscale transfer of energy and enstrophy, for strongly, moderately and weakly stably stratified two-dimensional (2D) turbulence under large-scale random forcing. In the strongly stratified case, a large-scale vertically sheared horizontal flow (VSHF) co-exists with small scale turbulence. The VSHF consists of internal gravity waves and the turbulent flow has a kinetic energy (KE) spectrum that follows an approximate $k^{-3}$ scaling with zero KE flux and a robust positive enstrophy flux. The spectrum of the turbulent potential energy (PE) also approximately follows a $k^{-3}$ power-law and its flux is directed to small scales. For moderate stratification, there is no VSHF and the KE of the turbulent flow exhibits Bolgiano-Obukhov scaling that transitions from a shallow $k^{-11/5}$ form at large scales, to a steeper approximate $k^{-3}$ scaling at small scales. The entire range of scales shows a strong forward enstrophy flux, and interesti...
The role of Stewartson and Ekman layers in turbulent rotating Rayleigh-B\\'enard convection
Kunnen, Rudie P J; Overkamp, Jim; Sun, Chao; van Heijst, GertJan F; Clercx, Herman J H
2011-01-01
When the classical Rayleigh-B\\'enard (RB) system is rotated about its vertical axis roughly three regimes can be identified. In regime I (weak rotation) the large scale circulation (LSC) is the dominant feature of the flow. In regime II (moderate rotation) the LSC is replaced by vertically aligned vortices. Regime III (strong rotation) is characterized by suppression of the vertical velocity fluctuations. Using results from experiments and direct numerical simulations of RB convection for a cell with a diameter-to-height aspect ratio equal to one at $Ra \\sim 10^8-10^9$ ($Pr=4-6$) and $0 \\lesssim 1/Ro \\lesssim 25$ we identified the characteristics of the azimuthal temperature profiles at the sidewall in the different regimes. In regime I the azimuthal wall temperature profile shows a cosine shape and a vertical temperature gradient due to plumes that travel with the LSC close to the sidewall. In regime II and III this cosine profile disappears, but the vertical wall temperature gradient is still observed. It t...
Traugott, Hadar
2015-01-01
Turbulent resuspension is the process of lifting solid particles from the bottom by turbulent flow, ubiquitous in natural and industrial problems. The process is a sequence of events that start with an incipient motion when the particle is dislodged from its place, continue as sliding or rolling along the surface, and ending with the detachment of the particle from the surface and lifting it up into the flow. In this study we measure in details the motion of freely moving solid spherical particles along the bottom smooth wall under an oscillating grid turbulence and track them through the lift-off events. We measure simultaneously the Lagrangian trajectories of the particles and the flow tracers around them. We estimate the local flow parameters and extract the different force terms that act on a particle. For the particles of the diameter comparable with the Kolmogorov length scale, either sliding or rolling along the smooth wall under a zero-mean turbulent flow, we find that: i) the lift force is a dominant...
Numerical simulation of turbulent flow around a forced moving circular cylinder on cut cells
Institute of Scientific and Technical Information of China (English)
BAI Wei
2013-01-01
Fixed and forced moving circular cylinders in turbulent flows are studied by using the Large Eddy Simulation (LES) and two-equation based Detached Eddy Simulation (DES) turbulence models. The Cartesian cut cell approach is adopted to track the body surface across a stationary background grid covering the whole computational domain. A cell-centered finite volume method of second-order accuracy in both time and space is developed to solve the flow field in fluid cells, which is also modified accordingly in cut cells and merged cells. In order to compare different turbulence models, the current flow past a fixed circular cylinder at a mode- rate Reynolds number,Re=3 900, is tested first. The model is also applied to the simulation of a forced oscillating circular cylinder in the turbulent flow, and the influences of different oscillation amplitudes, frequencies and free stream velocities are discussed. The numerical results indicate that the present numerical model based on the Cartesian cut cell approach is capable of solving the turbu- lent flow around a body undergoing motions, which is a foundation for the possible future study on wake induced oscillation and vor- tex induced vibration.
Kolmogorov turbulence in a random-force-driven Burgers equation
Chekhlov, A; Chekhlov, Alexei; Yakhot, Victor
1995-01-01
The dynamics of velocity fluctuations, governed by the one-dimensional Burgers equation, driven by a white-in-time random force with the spatial spectrum \\overline{|f(k)|^2}\\proptok^{-1}, is considered. High-resolution numerical experiments conducted in this work give the energy spectrum E(k)\\propto k^{-\\beta} with \\beta =5/3\\pm 0.02. The observed two-point correlation function C(k,\\omega) reveals \\omega\\propto k^z with the "dynamical exponent" z\\approx 2/3. High-order moments of velocity differences show strong intermittency and are dominated by powerful large-scale shocks. The results are compared with predictions of the one-loop renormalized perturbation expansion.
Vanapalli, Srinivas; ter Brake, Hermanus J.M.
2013-01-01
Nanofluids are considered for improving the heat exchange in forced convective flow. In literature, the benefit of nanofluids compared to the corresponding base fluid is represented by several figures-of-merit in which the heat transfer benefit and the cost of pumping the fluid are considered. These
Vanapalli, S.; Brake, ter H.J.M.
2013-01-01
Nanofluids are considered for improving the heat exchange in forced convective flow. In literature, the benefit of nanofluids compared to the corresponding base fluid is represented by several figures-of-merit in which the heat transfer benefit and the cost of pumping the fluid are considered. These
Institute of Scientific and Technical Information of China (English)
WeizhongLi; WeichengWang; 等
1995-01-01
Physical and mathematical models are developed to describe the forced convection condensation heat transfer of saturated vapor flowing axially outside a horizontal tube.The numerical solution of the models indicates the effects of vapor velocity on the liquid film thickness.The result verifies the enhancement of condensation heat transfer caused by such flow.
Influence of Reynolds number and forcing type in a turbulent von K\\'arm\\'an flow
Saint-Michel, Brice; Marié, Louis; Ravelet, Florent; Daviaud, François
2014-01-01
We present a detailed study of of a global bifurcation occuring in a turbulent von K\\'arm\\'an swirling flow. In this system, the statistically steady states progressively display hysteretic behaviour when the Reynolds number is increased above the transition to turbulence. We examine in detail this hysteresis using asymmetric forcing conditions --- rotating the impellers at different speeds. For very high Reynolds numbers, we study the sensitivity of the hysteresis cycle --- using complementary Particle Image Velocimetry (PIV) and global mechanical measurements --- to the forcing nature, imposing either the torque or the speed of the impellers. New mean states, displaying multiple quasi-steady states and negative differential responses, are experimentally observed in torque control. A simple analogy with electrical circuits is performed to understand the link between multi-stability and negative responses. The system is compared to other, similar "bulk" systems, to understand some relevant ingredients of nega...
Cheng, Anning; Xu, Kuan-Man
2006-01-01
The abilities of a cloud-resolving model (CRM) with double-Gaussian based and quasi-Gaussian based third-order closures (TOCs) to simulate shallow cumuli and their transition to deep convective clouds are compared in this study. The quasi-Gaussian based TOC is fully prognostic (FP), while the double-Gaussian based TOC is intermediately prognostic (IP). The latter only predicts three important third-order moments while the former predicts all the third-order moments. A shallow cumulus case is simulated by single-column versions of the FP and IP TOC models and a large-eddy simulation (LES). The IP TOC improves the simulation of shallow cumulus greatly over the FP TOC by producing more realistic cloud structures, when both are compared to LES. Large differences of the second- and third-order moments between the FP and IP TOC simulations appear in the cloud layer, because the cloud top height is underestimated by the FP TOC simulation. Sensitivity experiments and analysis of probability density functions (pdfs) used in the TOCs show that both the turbulence-scale condensation and higher-order moments are important to realistic simulations of the boundary-layer shallow cumuli.A shallow to deep convective cloud transition case is also simulated by the two-dimensional (2-D) version of the CRM with FP and IP TOCs. Both simulations can capture the transition from the shallow cumuli to deep convective clouds. The IP simulations produce more and deeper shallow cumuli than the FP simulations, but the FP simulations produce larger and wider convective clouds than the IP simulations. The temporal evolutions of cloud and precipitation are closely related to the turbulent transport, the cold pool and the cloud-scale circulation. The large amount of turbulent mixing associated with the shallow cumuli slows down the increase of the convective available potential energy and inhibits the early transition to deep convective clouds in the IP simulation. When the deep convective clouds
Nie, Ji; Shaevitz, Daniel A.; Sobel, Adam H.
2016-09-01
Extratropical extreme precipitation events are usually associated with large-scale flow disturbances, strong ascent, and large latent heat release. The causal relationships between these factors are often not obvious, however, the roles of different physical processes in producing the extreme precipitation event can be difficult to disentangle. Here we examine the large-scale forcings and convective heating feedback in the precipitation events, which caused the 2010 Pakistan flood within the Column Quasi-Geostrophic framework. A cloud-revolving model (CRM) is forced with large-scale forcings (other than large-scale vertical motion) computed from the quasi-geostrophic omega equation using input data from a reanalysis data set, and the large-scale vertical motion is diagnosed interactively with the simulated convection. Numerical results show that the positive feedback of convective heating to large-scale dynamics is essential in amplifying the precipitation intensity to the observed values. Orographic lifting is the most important dynamic forcing in both events, while differential potential vorticity advection also contributes to the triggering of the first event. Horizontal moisture advection modulates the extreme events mainly by setting the environmental humidity, which modulates the amplitude of the convection's response to the dynamic forcings. When the CRM is replaced by either a single-column model (SCM) with parameterized convection or a dry model with a reduced effective static stability, the model results show substantial discrepancies compared with reanalysis data. The reasons for these discrepancies are examined, and the implications for global models and theoretical models are discussed.
Influence of Forced Convection on the Solidification of Metal in Cast-Iron and Ceramic Ingot Molds
Kalashnikova, O. A.; Dremov, V. V.
2016-09-01
A nonstationary problem of solidification of an ingot in molds with walls of varying thermal conductivity has been solved by the variational method with account taken of forced convection in the liquid phase of the metal. A formula for the temperature distribution in the liquid phase and the dependence of the time of advance of the solidification front on its coordinates have been obtained. Numerical calculations of the time of solidification of the ingot in castiron and ceramic molds have been done at different convection rates.
Anomalously Weak Solar Convection
Hanasoge, Shravan M.; Duvall, Thomas L.; Sreenivasan, Katepalli R.
2012-01-01
Convection in the solar interior is thought to comprise structures on a spectrum of scales. This conclusion emerges from phenomenological studies and numerical simulations, though neither covers the proper range of dynamical parameters of solar convection. Here, we analyze observations of the wavefield in the solar photosphere using techniques of time-distance helioseismology to image flows in the solar interior. We downsample and synthesize 900 billion wavefield observations to produce 3 billion cross-correlations, which we average and fit, measuring 5 million wave travel times. Using these travel times, we deduce the underlying flow systems and study their statistics to bound convective velocity magnitudes in the solar interior, as a function of depth and spherical- harmonic degree l..Within the wavenumber band l convective velocities are 20-100 times weaker than current theoretical estimates. This constraint suggests the prevalence of a different paradigm of turbulence from that predicted by existing models, prompting the question: what mechanism transports the heat flux of a solar luminosity outwards? Advection is dominated by Coriolis forces for wavenumbers l convection may be quasi-geostrophic. The fact that isorotation contours in the Sun are not coaligned with the axis of rotation suggests the presence of a latitudinal entropy gradient.
The Mechanical Greenhouse: Burial of Heat by Turbulence in Hot Jupiter Atmospheres
Youdin, Andrew N
2010-01-01
The intense irradiation received by hot Jupiters suppresses convection in the outer layers of their atmospheres and lowers their cooling rates. "Inflated" hot Jupiters, i.e., those with anomalously large transit radii, require additional sources of heat or suppressed cooling. We consider the effect of forced turbulent mixing in the radiative layer, which could be driven by atmospheric circulation or by another mechanism. Due to stable stratification in the atmosphere, forced turbulence drives a downward flux of heat. Weak turbulent mixing slows the cooling rate by this process, as if the planet was irradiated more intensely. Stronger turbulent mixing buries heat into the convective interior, provided the turbulence extends to the radiative-convective boundary. This inflates the planet until a balance is reached between the heat buried into and radiated from the interior. We also include the direct injection of heat due to the dissipation of turbulence or other effects. Such heating is already known to slow pl...
Stochastic Convection Parameterizations
Teixeira, Joao; Reynolds, Carolyn; Suselj, Kay; Matheou, Georgios
2012-01-01
computational fluid dynamics, radiation, clouds, turbulence, convection, gravity waves, surface interaction, radiation interaction, cloud and aerosol microphysics, complexity (vegetation, biogeochemistry, radiation versus turbulence/convection stochastic approach, non-linearities, Monte Carlo, high resolutions, large-Eddy Simulations, cloud structure, plumes, saturation in tropics, forecasting, parameterizations, stochastic, radiation-clod interaction, hurricane forecasts
Directory of Open Access Journals (Sweden)
M. Goodarzi
2014-01-01
Full Text Available The effect of radiation on laminar and turbulent mixed convection heat transfer of a semitransparent medium in a square enclosure was studied numerically using the Finite Volume Method. A structured mesh and the SIMPLE algorithm were utilized to model the governing equations. Turbulence and radiation were modeled with the RNG k-ε model and Discrete Ordinates (DO model, respectively. For Richardson numbers ranging from 0.1 to 10, simulations were performed for Rayleigh numbers in laminar flow (104 and turbulent flow (108. The model predictions were validated against previous numerical studies and good agreement was observed. The simulated results indicate that for laminar and turbulent motion states, computing the radiation heat transfer significantly enhanced the Nusselt number (Nu as well as the heat transfer coefficient. Higher Richardson numbers did not noticeably affect the average Nusselt number and corresponding heat transfer rate. Besides, as expected, the heat transfer rate for the turbulent flow regime surpassed that in the laminar regime. The simulations additionally demonstrated that for a constant Richardson number, computing the radiation heat transfer majorly affected the heat transfer structure in the enclosure; however, its impact on the fluid flow structure was negligible.
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.
Lee, Hyunho; Baik, Jong-Jin; Han, Ji-Young
2014-12-01
The effects of turbulence-induced collision enhancement (TICE) on mixed-phase deep convective clouds are numerically investigated using a 2-D cloud model with bin microphysics for uniform and sheared basic-state wind profiles and different aerosol concentrations. Graupel particles account for the most of the cloud mass in all simulation cases. In the uniform basic-state wind cases, graupel particles with moderate sizes account for some of the total graupel mass in the cases with TICE, whereas graupel particles with large sizes account for almost all the total graupel mass in the cases without TICE. This is because the growth of ice crystals into small graupel particles is enhanced due to TICE. The changes in the size distributions of graupel particles due to TICE result in a decrease in the mass-averaged mean terminal velocity of graupel particles. Therefore, the downward flux of graupel mass, and thus the melting of graupel particles, is reduced due to TICE, leading to a decrease in the amount of surface precipitation. Moreover, under the low aerosol concentration, TICE increases the sublimation of ice particles, consequently playing a partial role in reducing the amount of surface precipitation. The effects of TICE are less pronounced in the sheared basic-state wind cases than in the uniform basic-state wind cases because the number of ice crystals is much smaller in the sheared basic-state wind cases than in the uniform basic-state wind cases. Thus, the size distributions of graupel particles in the cases with and without TICE show little difference.
Theoretical and Computational Study of Forced-Convection Heat Transfer at Supercritical Pressures
Zhong, Jianguo
In the simulation of turbulent fluid flow and heat transfer at supercritical pressures, substantial difficulties have been encountered in the modeling of turbulence and bounda-ry layer. This is due to significant fluid property variations with respect to the local temperature and pressure, especially in the near-wall region of a heated wall, where large temperature differences occur. The classical turbulence models available in literature were typically developed for constant-property fluids, where an empirical wall function in the high-Re k-epsilon model, and a damping function in the low-Re k-epsilon model were derived based on the constant-property data to solve the boundary layer. As it can be found in the existing literature, large differences have been observed between the experimental and numerical simulation results of the heat transfer coefficient predictions in the en-hanced and deteriorated heat transfer situations for supercritical fluids. In this thesis, a novel near-wall treatment method is proposed to treat large property variations in the thermal and velocity sub-layers. In the near-wall region, the supercritical fluids can be considered thermal-conductive and viscous forces dominated. The thick-ness of the viscous sub-layer (VSL) and the conduction sub-layer (CSL) can be related to the wall shear stress and local Prandtl number information by using computational CFD models, such as that implemented in the NPHASE-CMFD code. The fluids' bulk and wall temperature information has been obtained from the literature review of experi-mental measurements. The wall temperature and heat transfer coefficient calculated from the k-epsilon model with the proposed wall treatment method have been found to be in good agreement with experimental data for both heat transfer enhancement and deterioration cases for two most widely used fluids: CO2 and water. The proposed model has been applied in the reactor-scale thermal-hydraulic analysis of different flow path
Forced heat convection in a reciprocating duct fitted with 45 degree ribs
Energy Technology Data Exchange (ETDEWEB)
Shyy Woei, Chang [National Kaohsiung Institute of Marine Technology, Dept. of Marine Engineering, Taiwan (China)
2002-03-01
This experimental study investigated heat-transfer physics of forced convection in a reciprocating square duct fitted with 45 deg C crossed ribs on two opposite walls. The parametric conditions involved several Reynolds, pulsating and buoyancy numbers, respectively, in the ranges of 600-10 000, 0-10, and 0-0.14 with five different reciprocating frequencies tested, namely, 0.67, 1, 1.33, 1.67 and 2 Hz. The rib-induced flows in the static duct produced an augmentation of heat transfer in the range of 260-300% compared to the smooth-walled situation. The reciprocating heat-transfer data reconfirmed the appearance of large-scale wavy-like axial heat transfer distribution that differed significantly from the stationary results. The manner in which the pulsating force and convective inertia, with and without buoyancy interaction, interactively affected the local heat transfer along the rib-roughened surface was illustrated using a number of experimentally based observations and extrapolations. The buoyancy interaction in the reciprocating duct reduced heat transfer, which effect was enhanced by increasing the pulsating number, but appeared to be a weak function of Reynolds number. When the Reynolds and pulsating numbers were relatively low, a range of heat transfer impediments, that could lead the spatial-time averaged heat-transfer to levels about 71 % of non-reciprocating values, was observed. A further increase of pulsating number resulted in a subsequent heat-transfer recovery, which tendency could lead to heat-transfer improvement from the non-reciprocating level. An empirical correlation to evaluate the spatial-time averaged heat transfer over the reciprocating ribbed duct was developed to assist the design activity. The possibility to further enhance heat-transfer via the use of angled ribs in a reciprocating duct is confirmed, but it is important to ensure that the range of reciprocating flow parameters produced does not create heat-transfer impediment in order
Conceptual Design of Forced Convection Molten Salt Heat Transfer Testing Loop
Energy Technology Data Exchange (ETDEWEB)
Manohar S. Sohal; Piyush Sabharwall; Pattrick Calderoni; Alan K. Wertsching; S. Brandon Grover
2010-09-01
This report develops a proposal to design and construct a forced convection test loop. A detailed test plan will then be conducted to obtain data on heat transfer, thermodynamic, and corrosion characteristics of the molten salts and fluid-solid interaction. In particular, this report outlines an experimental research and development test plan. The most important initial requirement for heat transfer test of molten salt systems is the establishment of reference coolant materials to use in the experiments. An earlier report produced within the same project highlighted how thermophysical properties of the materials that directly impact the heat transfer behavior are strongly correlated to the composition and impurities concentration of the melt. It is therefore essential to establish laboratory techniques that can measure the melt composition, and to develop purification methods that would allow the production of large quantities of coolant with the desired purity. A companion report describes the options available to reach such objectives. In particular, that report outlines an experimental research and development test plan that would include following steps: •Molten Salts: The candidate molten salts for investigation will be selected. •Materials of Construction: Materials of construction for the test loop, heat exchangers, and fluid-solid corrosion tests in the test loop will also be selected. •Scaling Analysis: Scaling analysis to design the test loop will be performed. •Test Plan: A comprehensive test plan to include all the tests that are being planned in the short and long term time frame will be developed. •Design the Test Loop: The forced convection test loop will be designed including extensive mechanical design, instrument selection, data acquisition system, safety requirements, and related precautionary measures. •Fabricate the Test Loop. •Perform the Tests. •Uncertainty Analysis: As a part of the data collection, uncertainty analysis will
Directory of Open Access Journals (Sweden)
C. Wang
2009-10-01
Full Text Available Previous works have suggested that the direct radiative forcing (DRF of black carbon (BC aerosols are able to force a significant change in tropical convective precipitation ranging from the Pacific and Indian Ocean to the Atlantic Ocean. In this in-depth analysis, the sensitivity of this modeled effect of BC on tropical convective precipitation to the emissions of BC from 5 major regions of the world has been examined. In a zonal mean base, the effect of BC on tropical convective precipitation is a result of a displacement of ITCZ toward the forcing (warming hemisphere. However, a substantial difference exists in this effect associated with BC over different continents. The BC effect on convective precipitation over the tropical Pacific Ocean is found to be most sensitive to the emissions from Central and North America due to a persistent presence of BC aerosols from these two regions in the lowermost troposphere over the Eastern Pacific. The BC effect over the tropical Indian and Atlantic Ocean is most sensitive to the emissions from South as well as East Asia and Africa, respectively. Interestingly, the summation of these individual effects associated with emissions from various regions mostly exceeds their actual combined effect as shown in the model run driven by the global BC emissions, so that they must offset each other in certain locations and a nonlinearity of this type of effect is thus defined. It is known that anthropogenic aerosols contain many scattering-dominant constituents that might exert an effect opposite to that of absorbing BC. The combined aerosol forcing is thus likely differing from the BC-only one. Nevertheless, this study along with others of its kind that isolates the DRF of BC from other forcings provides an insight of the potentially important climate response to anthropogenic forcings particularly related to the unique particulate solar absorption.
Directory of Open Access Journals (Sweden)
Wei Cai
2014-06-01
Full Text Available The convective drying kinetics of porous medium was investigated numerically. A mathematical model for forced convective drying was established to estimate the evolution of moisture content and temperature inside multilayered porous medium. The set of coupled partial differential equations with the specified boundary and initial conditions were solved numerically using a MATLAB code. An experimental setup of convective drying had been constructed and validated the theoretical model. The temperature and moisture content of the potato samples were dynamically measured and recorded during the drying process. Results indicate that thermal diffusion coefficient has significant positive impact on temperature distribution and mass diffusion coefficient might directly affect the moisture content distribution. Soret effect has a significant impact on heat flux and temperature distribution in the presence of large temperature gradient.
Energy Technology Data Exchange (ETDEWEB)
Brasil, Cristiana S.; Valle, Ramon M.; Cortez, Marcio F.B.; Ferreira, Andre G. [Minas Gerais Univ., Belo Horizonte, MG (Brazil). Dept. de Engenharia Mecanica]. E-mail: tite@demec.ufmg.br; ramon@demec.ufmg.br; fonteboa@demec.ufmg.br; ferreira@demec.ufmg.br
2000-07-01
This paper presents a theoretical analysis of the turbulent natural convection in a solar chimney operating in steady flow, with prescribed conditions of temperature in the ground. The solar radiation heats the air under the cover, which flows to the tower without artificial pumping. The hot air produced may be used to dry several agricultural products. The numerical analysis of the natural convection in this kind of dryer has fundamental importance on the design and building of this device. The mathematical model includes the conservation laws for mass, momentum and thermal energy and the transport equations for the turbulence model variables (k and e ). The k- e model of turbulence with wall functions was used. A computational code using the Finite Volume Method in Generalized Coordinates was developed to solve the system of equations that describes thermal and hydro dynamically the flow. The velocity and temperature fields are shown to the flow in the solar chimney. With geometrical alterations on the device, one can obtain a detailed description of the flow, which allow the guideline for a suitable configuration to build an experimental prototype. (author)
Forced magnetohydrodynamic turbulence in three dimensions using Taylor-Green symmetries.
Krstulovic, G; Brachet, M E; Pouquet, A
2014-04-01
We examine the scaling laws of magnetohydrodynamic (MHD) turbulence for three different types of forcing functions and imposing at all times the fourfold symmetries of the Taylor-Green (TG) vortex generalized to MHD; no uniform magnetic field is present and the magnetic Prandtl number is equal to unity. We also include pumping in the induction equation, and we take the three configurations studied in the decaying case in Lee et al. [Phys. Rev. E 81, 016318 (2010)]. To that effect, we employ direct numerical simulations up to an equivalent resolution of 20483 grid points. We find that, similarly to the case when the forcing is absent, different spectral indices for the total energy spectrum emerge, corresponding to either a Kolmogorov law, an Iroshnikov-Kraichnan law that arises from the interactions of turbulent eddies and Alfvén waves, or to weak turbulence when the large-scale magnetic field is strong. We also examine the inertial range dynamics in terms of the ratios of kinetic to magnetic energy, and of the turnover time to the Alfvén time, and analyze the temporal variations of these quasiequilibria.
The emergence of zonal jets in forced rotating shallow water turbulence: A laboratory study
Espa, S.; Di Nitto, G.; Cenedese, A.
2010-11-01
The emergence of a sequence of alternating intense and elongated eastward-westward bands i.e. zonal jets in the atmosphere of the giant planets and in Earth's oceans have been widely investigated. Nevertheless jets formation and role as material barriers remain still unclear. Jets are generated in a quasi-2D turbulent flow due to the latitudinal variation of the Coriolis parameter (the so-called β-effect) which modifies the inverse cascade process channeling energy towards zonal modes. In previous experiments we have investigated the impact of the variation of the rotation rate, of the domain geometry and of the initial spectra on jets organization in a decaying regime. In this work we investigate the formation of jets in a continuously forced flow, we characterize the observed regime and also we attempt to verify the existence of an universal regime corresponding to the so-called zonostrophic turbulence. The experimental set-up consists of a rotating tank where turbulence is generated by electromagnetically forcing a shallow layer of an electrolyte solution, and the variation of the Coriolis parameter has been simulated by the parabolic profile assumed by the free surface of the fluid under rotation. Flow measurements have been performed using image analysis.
Experimental StudyHigh Altitude Forced Convective Cooling of Electromechanical Actuation Systems
2016-01-01
within industry, so as to keep its applicability to what might be used to cool EMAS. Therefore, a parallel plate sink was chosen. This type of heat ...cooling 3 Approved for public release; distribution unlimited. o Convection, liquid to air heat exchanger o Convection, air cooling with fan...surface or heat exchanger to remove the thermal energy away from critical components and convect it to an airstream. The most common source of the air
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azzedine ABDEDOU
2015-01-01
Full Text Available Two criteria are used and compared to investigate the local thermal equilibrium assumption in a forced convection through a porous channel. The first criterion is based on the maximum local temperature difference between the solid and fluid phases, while the second is based on the average of the local differences between the temperature of the solid phase and the fluid phase. For this purpose, the momentum and energy equations based on the Darcy-Brinkman-Forchheimer and the local thermal non equilibrium models are solved numerically using the finite volume method. The analysis focused on searching thermophysical parameters ranges which validate local thermal equilibrium hypothesis. Thus, by using the two criteria, the obtained results mainly revealed that this local thermal equilibrium assumption is verified for low thermal conductivity ratio and Reynolds number values and for high interstitial Biot number and porosity, while it is unfavorably affected by the high values of Prandtl number. However, it is also found that the parameters ranges corresponding to the local equilibrium validity depends on the selected local thermal non equilibrium criterion.
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Jamradloedluk, Jindaporn; Wiriyaumpaiwong, Songchai [Mahasarakham Univ. Khamriang, Kantarawichai, Mahasarakham (Thailand)
2008-07-01
Solar energy, a form of sustainable energy, has a great potential for a wide variety of applications because it is abundant and accessible, especially for countries located in the tropical region. Drying process is one of the prominent techniques for utilization of solar energy. This research work proposes a forced convection solar drying of osmotically pretreated fruits viz. mango, guava, and pineapple. The fruit cubes with a dimension of 1cm x 1cm x 1cm were immersed in 35% w./w. sucrose solution prior to the drying process. Drying kinetics, color and hardness of the final products obtained from solar drying were investigated and compared with those obtained from open air-sun drying. Desorption isotherms of the osmosed fruits were also examined and five mathematical models were used to fit the desorption curves. Experimental results revealed that solar drying provided higher drying rate than natural sun drying. Color of glace fruit processed by solar drying was more intense, indicated by lower value of lightness and higher value of yellowness, than that processed by sun drying. Hardness of the products dehydrated by both drying methods, however, was not significantly different (p>0.05). Validation of the mathematical models developed showed that the GAB model was most effective for describing desorption isotherms of osmotically pretreated mango and pineapple whereas Peleg's model was most effective for describing desorption isotherms of osmotically pretreated guava. (orig.)
Munir, Asif; Shahzad, Azeem; Khan, Masood
2014-01-01
The major focus of this article is to analyze the forced convective heat transfer in a steady boundary layer flow of Sisko fluid over a nonlinear stretching sheet. Two cases are studied, namely (i) the sheet with variable temperature (PST case) and (ii) the sheet with variable heat flux (PHF case). The heat transfer aspects are investigated for both integer and non-integer values of the power-law index. The governing partial differential equations are reduced to a system of nonlinear ordinary differential equations using appropriate similarity variables and solved numerically. The numerical results are obtained by the shooting method using adaptive Runge Kutta method with Broyden's method in the domain[Formula: see text]. The numerical results for the temperature field are found to be strongly dependent upon the power-law index, stretching parameter, wall temperature parameter, material parameter of the Sisko fluid and Prandtl number. In addition, the local Nusselt number versus wall temperature parameter is also graphed and tabulated for different values of pertaining parameters. Further, numerical results are validated by comparison with exact solutions as well as previously published results in the literature.
Munir, Asif; Shahzad, Azeem; Khan, Masood
2014-01-01
The major focus of this article is to analyze the forced convective heat transfer in a steady boundary layer flow of Sisko fluid over a nonlinear stretching sheet. Two cases are studied, namely (i) the sheet with variable temperature (PST case) and (ii) the sheet with variable heat flux (PHF case). The heat transfer aspects are investigated for both integer and non-integer values of the power-law index. The governing partial differential equations are reduced to a system of nonlinear ordinary differential equations using appropriate similarity variables and solved numerically. The numerical results are obtained by the shooting method using adaptive Runge Kutta method with Broyden’s method in the domain. The numerical results for the temperature field are found to be strongly dependent upon the power-law index, stretching parameter, wall temperature parameter, material parameter of the Sisko fluid and Prandtl number. In addition, the local Nusselt number versus wall temperature parameter is also graphed and tabulated for different values of pertaining parameters. Further, numerical results are validated by comparison with exact solutions as well as previously published results in the literature. PMID:24949738
Institute of Scientific and Technical Information of China (English)
LU Wenqiang; BAI Fengwu
2004-01-01
In this paper, a new model to analyze laminar forced convective enhanced heat transfer in latent functionally thermal fluid is developed. The main characteristics of the model are: I) a new formula of the specific heat at constant pressure is used; ii) a real heat transfer process is considered; that is, heat transfer processes occur not only between working fluid and microcapsules, but also between the mixture and tube wall; iii) the new method, which combines the newly developed axisymmetrical dual reciprocity boundary element method (DRBEM) with finite difference method (FDM), is used to solve the control equations of this problem. The new model is validated by experimental data.Some new physical results on the variational characteristics of the specific heat at constant pressure with space and time during phase-change process, the time-marching history of the phase-change interfaces and so on are obtained. Several main physical factors that affect enhanced heat transfer in latent functionally thermal fluid are numerically analyzed.Some new understandings for the mechanism of enhanced heat transfer in the functionally fluid are obtained.
López, R.; Vaca, M.; Terres, H.; Lizardi, A.; Morales, J.; Flores, J.; Chávez, S.
2015-01-01
The sunflower is an annual plant native to the Americas. It possesses a large inflorescence (flowering head), and its name is derived from the flower's shape and image, which is often used to capture the sun. The plant has a rough, broad, hairy stem, coarsely toothed, with rough leaves, and circular flower heads. The sunflower seeds are appreciated for their oil, which has become a widespread cooking ingredient. Leaves of the sunflower can be used as cattle feed, while the stems contain a fiber that may be used in paper production. Recently this flower has been used in phytoremediation of soils, contaminated with heavy metals. Sunflower has been probed as an efficient phytoextractor of chromium, lead, aluminum, zinc, cadmium from soil. In this work we present the experimental results of the drying of the sunflower stem, cut in 100 mm longitudinal sections, with diameters in the range of 11-18 mm. The aim was to obtain a dry and easy-to-handle final product, since these plants were originally cultivated in order to extract heavy metals from a polluted soil. The dried stems could then be easily confined or sent to recycle premises to concentrate the metals. The drying process was done in forced convection within a hot air tunnel. The used temperature was 60 °C, the velocity of air was 3 m/s and the required times were 8 hours. The initial average wet mass was 28 g and the final value was 5 g, resulting in the aimed product.
Transient forced convection in the entrance region of a porous tube
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Al-Nimr, M.A.; Aldoss, T.; Naji, M.I. (Jordan Univ. of Science and Technology, Irbid (Jordan))
1994-04-01
Study of the flow of energy-carrying fluids through porous media is a rapidly growing branch of fluid mechanics and heat transfer since its applications include thermal insulation, direct contact heat exchangers, nuclear waste repositories, and the petroleum and geothermal industries. The problem of transient forced convection in the entrance region of a cylindrical porous medium with developing thermal boundary layer is investigated. The hydrodynamic behavior of the flow is assumed to be steady and fully developed, and both Darcian and non-Darcian effects on the flow are considered. The numerical solution uses finite-difference discretization. Thermal transients are created by a step change in the temperature of the tube wall. The effect of different fluid and solid-matrix parameters on Nusselt number, thermal entrance length, total heat absorbed and mixing cup temperature are described. Numerical solutions are obtained for the velocity and temperature fields. It is found that increasing the inertia parameter increases the heat absorbed by the fluid. 15 refs., 11 figs.
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Ledezma, G.A. [Duke Univ., Durham, NC (United States). Mechanical Engineering Dept.; Campo, A. [Idaho State Univ., Pocatello, ID (United States). Coll. of Engineering
1999-04-01
The utilization of internal longitudinal finned tubes has received unparallel attention in the heat transfer literature over the years as a result of its imminent application in high performance compact heat exchangers to enhance the heat transfer between laminar streams of viscous fluids and tube walls. Here, the central goal of this paper is to report a simple approximate way for the prediction of the two asymptotes for the local Nusselt number in laminar forced convection flows inside internal longitudinal finned tubes. The computational attributes of the Method Of Lines (MOL) are propitious for the determination of asymptotic temperature solutions and corresponding heat transfer rates (one for Z {r_arrow} 0 and the other for z {r_arrow} {infinity}). The two local Nusselt number sub-distributions, namely Nu{sub z{r_arrow}0} and Nu{sub z{r_arrow}{infinity}}, blend themselves into an approximate Nusselt number distribution that covers the entire z-domain in a natural way.
Effect of Selected Factors on Drying Process of Tomato in Forced Convection Solar Energy Dryer
Directory of Open Access Journals (Sweden)
U.S. Muhammed
2012-09-01
Full Text Available The effect of air velocity, slice thickness and grazing materials in drying process of tomato in forced convection solar energy dryer was evaluated. The result is to serve as an input for solar energy development for drying of vegetable and fruit products in North West Ecological zone of Nigeria. In order to evaluate the effects of the above factors in drying operation, a split-split-plot experimental design was used. Differences among the treatments and their interactions were tested with orthogonal contrast test to access their significance while further analysis were done to compare all possible pairs of treatment means using Duncan’s Multiple Range Test (DMRT. The results showed that there is no significant difference, at 95% probability level, between the means of the three glazing materials used. However, the variations in mean slice thickness and in mean air flow rate are highly significant at 99% probability level. The results further revealed that drying rate increasing with decrease in slice thickness and increase in air flow rate. Drying of 15, 20 and 25 mm slice thickness of tomato was achieved in time range of 21-24, 27-29 and 30-50 h, respectively.
Arquiza, J. M. R. Apollo; Morrow, Robert; Remiker, Ross; Hunter, Jean B.
2017-09-01
During long-term space missions, astronauts generate wet trash, including food containers with uneaten portions, moist hygiene wipes and wet paper towels. This waste produces two problems: the loss of water and the generation of odors and health hazards by microbial growth. These problems are solved by a closed-loop, forced-convection, heat-pump drying system which stops microbial activity by both pasteurization and desiccation, and recovers water in a gravity-independent porous media condensing heat exchanger. A transient, pseudo-homogeneous continuum model for the drying of wet ersatz trash was formulated for this system. The model is based on the conservation equations for energy and moisture applied to the air and solid phases and includes the unique trash characteristic of having both dry and wet solids. Experimentally determined heat and mass transfer coefficients, together with the moisture sorption equilibrium relationship for the wet material are used in the model. The resulting system of differential equations is solved by the finite-volume method as implemented by the commercial software COMSOL. Model simulations agreed well with experimental data under certain conditions. The validated model will be used in the optimization of the entire closed-loop system consisting of fan, air heater, dryer vessel, heat-pump condenser, and heat-recovery modules.
Experimental study of an upward sub-cooled forced convection in a rectangular channel
Kouidri, A.; Madani, B.; Roubi, B.; Hamadouche, A.
2016-07-01
The upward sub-cooled forced convection in a rectangular channel is investigated experimentally. The aim of the present work is the studying of the local heat transfer phenomena. Concerning the experimentation: the n-pentane is used as a working fluid, the independent variables are: the velocity in the range from 0.04 to 0.086 m/s and heat flux density with values between 1.8 and 7.36 W/cm2. The results show that the local Nusselt number distribution is not uniform along the channel; however, uniformity is observed in the mean Nusselt number for Reynolds under 1600. On the other hand, a new correlation to predict the local fluid temperature is established as a function of local wall temperature. The wall's heat is dissipated under the common effect of the sub-cooled regime; therefore, the local heat transfer coefficient is increased. The study of the thermal equilibrium showed that for Reynolds less than 1500; almost all of the heat flux generated by the heater cartridges is absorbed by the fluid.
Directory of Open Access Journals (Sweden)
AR. Umayal Sundari
2013-06-01
Full Text Available A forced convection solar drier is designed with evacuated tube collector and a blower. The performance of the designed drier is evaluated by carrying drying experiments at Thanjavur District, Tamilnadu, India with amla. Solar drying of amla is carried at different air velocity flow rates – 4m/s, 4.25m/s and 4.5m/s and is compared with natural sun drying. The temperature of the dryingchamber ranges from 53ºC to 82ºC while the ambient temperature ranges from 29ºC to 32ºC. The efficiency of the designed drier varies from 38.61% to 43.7% where as the efficiency of sun drying varies from 12.5% to 14.15%. It is observed that the efficiency of the drier increases with increase in air velocity flow rates. Initial moisture content of amla ranges between 83.6% and 84.3% and the equilibrium moisture content ranges between 0.1% and 0.6%. Solar drying takes 5 to 7 hours to reach safe moisture content where as sun drying takes 13 to15 hours. Also the quality of solar dried amla is better in terms of colour, odour, flavour and appearance than the sun dried amla. The observed result of the present work shows that the proposed solar drier is good for drying amla in this region.
Nanofluid flow and forced convection heat transfer over a stretching surface considering heat source
Mohammadpour, M.; Valipour, P.; Shambooli, M.; Ayani, M.; Mirparizi, M.
2015-07-01
In this paper, magnetic field effects on the forced convection flow of a nanofluid over a stretching surface in the presence of heat generation/absorption are studied. The equations of continuity, momentum and energy are transformed into ordinary differential equations and solved numerically using the fourth-order Runge-Kutta integration scheme featuring the shooting technique. Different types of nanoparticles as copper (Cu), silver (Ag), alumina (Al2O3) and titania (TiO2) with water as their base fluid has been considered. The influence of significant parameters, such as magnetic parameter, volume fraction of the nanoparticles, heat generation/absorption parameter, velocity ratio parameter and temperature index parameter on the flow and heat transfer characteristics are discussed. The results show that the values of temperature profiles increase with increasing heat generation/absorption and volume fraction of the nanoparticles but they decrease with increasing velocity ratio parameter and temperature index parameter. Also, it can be found that selecting silver as nanoparticle leads to the highest heat transfer enhancement.
Ilday, Serim; Akguc, Gursoy B.; Tokel, Onur; Makey, Ghaith; Yavuz, Ozgun; Yavuz, Koray; Pavlov, Ihor; Ilday, F. Omer; Gulseren, Oguz
We report a new dynamical self-assembly mechanism, where judicious use of convective and strong Brownian forces enables effective patterning of colloidal nanoparticles that are almost two orders of magnitude smaller than the laser beam. Optical trapping or tweezing effects are not involved, but the laser is used to create steep thermal gradients through multi-photon absorption, and thereby guide the colloids through convective forces. Convective forces can be thought as a positive feedback mechanism that helps to form and reinforce pattern, while Brownian motion act as a competing negative feedback mechanism to limit the growth of the pattern, as well as to increase the possibilities of bifurcation into different patterns, analogous to the competition observed in reaction-diffusion systems. By steering stochastic processes through these forces, we are able to gain control over the emergent pattern such as to form-deform-reform of a pattern, to change its shape and transport it spatially within seconds. This enables us to dynamically initiate and control large patterns comprised of hundreds of colloids. Further, by not relying on any specific chemical, optical or magnetic interaction, this new method is, in principle, completely independent of the material type being assembled.
Scale-dependent cyclone-anticyclone asymmetry in a forced rotating turbulence experiment
Gallet, Basile; Cortet, Pierre-Philippe; Moisy, Frédéric
2014-01-01
We characterize the statistical and geometrical properties of the cyclone-anticyclone asymmetry in a statistically-steady forced rotating turbulence experiment. Turbulence is generated by a set of vertical flaps which continuously inject velocity fluctuations towards the center of a tank mounted on a rotating platform. We first characterize the cyclone-anticyclone asymmetry from conventional single-point vorticity statistics. We propose a phenomenological model to explain the emergence of the asymmetry in the experiment, from which we predict scaling laws for the root-mean-square velocity in good agreement with the experimental data. We further quantify the cyclone-anticyclone asymmetry using a set of third-order two-point velocity correlations. We focus on the correlations which are nonzero only if the cyclone-anticyclone symmetry is broken. They offer two advantages over single-point vorticity statistics: first, they are defined from velocity measurements only, so an accurate resolution of the Kolmogorov sc...
Phenomenology of two-dimensional stably stratified turbulence under large-scale forcing
Kumar, Abhishek
2017-01-11
In this paper, we characterise the scaling of energy spectra, and the interscale transfer of energy and enstrophy, for strongly, moderately and weakly stably stratified two-dimensional (2D) turbulence, restricted in a vertical plane, under large-scale random forcing. In the strongly stratified case, a large-scale vertically sheared horizontal flow (VSHF) coexists with small scale turbulence. The VSHF consists of internal gravity waves and the turbulent flow has a kinetic energy (KE) spectrum that follows an approximate k−3 scaling with zero KE flux and a robust positive enstrophy flux. The spectrum of the turbulent potential energy (PE) also approximately follows a k−3 power-law and its flux is directed to small scales. For moderate stratification, there is no VSHF and the KE of the turbulent flow exhibits Bolgiano–Obukhov scaling that transitions from a shallow k−11/5 form at large scales, to a steeper approximate k−3 scaling at small scales. The entire range of scales shows a strong forward enstrophy flux, and interestingly, large (small) scales show an inverse (forward) KE flux. The PE flux in this regime is directed to small scales, and the PE spectrum is characterised by an approximate k−1.64 scaling. Finally, for weak stratification, KE is transferred upscale and its spectrum closely follows a k−2.5 scaling, while PE exhibits a forward transfer and its spectrum shows an approximate k−1.6 power-law. For all stratification strengths, the total energy always flows from large to small scales and almost all the spectral indicies are well explained by accounting for the scale-dependent nature of the corresponding flux.
cassiani, massimo; stohl, andreas; brioude, jerome
2014-05-01
The vertical gradient of air density has been included in a skewed probability density function formulation for turbulence in the convective boundary layer and the related drift term for Lagrangian stochastic particle modelling has been obtained based on the well-mixed condition. The formulation has been extended to include unsteady turbulence statistics. Tests were carried out to validate the model including consistency between forward and backward simulations and preservation of well-mixed state with unsteady conditions. The stationary state CBL drift term with density correction was incorporated in the FLEXPART/FLEXPART-WRF Lagrangian models. Currently only the steady state horizontally homogeneous drift term were included. To avoid numerical instability, using the steady homogenous drift in the presence of non-stationary and horizontally non-homogeneous conditions, a re-initialization procedure for particle velocity was used. The criteria for re-initialization and resulting errors were assessed.
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.
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Nickell, T.W.
1988-01-01
This study numerically analyzes combined radiative and natural or forced convective heat transfer between vertical parallel plates with two-dimensional discrete heat sources. The numerical method was verified by comparing its results with other published experimental data and the agreement was excellent. It is shown that radiative heat transfer is a significant and useful mode of heat transfer in combination with both natural and forced convection in this situation and cannot be neglected. Radiative heat transfer accounted for 50-60% or more of the total heat transfer in some cases, and usually approximately 30-35% on the top of a discrete heat source. This fact can be used to advantage in the thermal design of electronic circuit boards.
Directory of Open Access Journals (Sweden)
M. MOHANRAJ
2009-09-01
Full Text Available An indirect forced convection solar drier integrated with different sensible heat storage maternal has been developed and tested its performance for drying chili under the metrological conditions of Pollachi, India. The system consists of a flat plate solar air heater with heat storage unit, a drying chamber and a centrifugal blower. Drying experiments have been performed at an air flow rate of 0.25 kg/s. Drying of chili in a forced convection solar drier reduces the moisture content from around 72.8% (wet basis to the final moisture content about 9.1% in 24 h. Average drier efficiency was estimated to be about 21%. The specific moisture extraction rate was estimated to be about 0.87 kg/kWh.
Cao, Y.; Faghri, A.; Juhasz, A.
1991-01-01
Latent heat energy storage systems with both annular and countercurrent flows are modeled numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. It is found that the energy storage system with the countercurrent flow is an efficient way to absorb heat energy in a short period for pulsed power load space applications.
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Humphrey, J.A.C.
1982-01-01
A summary is provided of the first of three years of experimental and theoretical research on free-forced convection flows in cavity-type solar receivers. New experimental and theoretical results are presented and discussed. The implication of these findings, with respect to the future thrust of the research program, is clarified as well as is possible at the present time. Following various related conclusions a summary and tentative schedule of work projected for year two of research are presented.
Yang, Xuegeng; Mühlenhoff, Sascha; Nikrityuk, Petr A.; Eckert, Kerstin
2013-03-01
Magnetic fields are well-established in electrochemistry as an attractive tool to improve both the quality of the deposit as well as the deposition rate. The key mechanism is a mass transfer enhancement by Lorentz-force-driven convection. However, during electrolysis this convection interacts with buoyancy-driven convection, which arises from concentration differences, in a sometimes intriguing way. In the case of a Lorentz force opposing buoyancy, this is due to the growth of a bubble-like zone of less-concentrated cupric ion solution at the lower part of the vertical cathode when copper electrolysis is performed. If buoyancy is strong enough to compete with the Lorentz force, this zone rises along the cathode and causes surprisingly unsteady initial transient behaviour. We explore this initial transient under galvanostatic conditions by analyzing the development of the concentration and velocity boundary layers obtained by Mach-Zehnder interferometry and particle image velocimetry. Particular attention is also paid to higher current densities above the limiting current, obtained from potentiodynamic measurements, at which a chaotic advection takes place. The results are compared by scaling analysis.
Development of a Forced-Convection Liquid-Fluoride-Salt Test Loop
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Yoder Jr, Graydon L [ORNL; Wilson, Dane F [ORNL; Peretz, Fred J [ORNL; Wilgen, John B [ORNL; Romanoski, Glenn R [ORNL; Kisner, Roger A [ORNL; Holcomb, David Eugene [ORNL; Heatherly, Dennis Wayne [ORNL; Aaron, Adam M [ORNL
2010-01-01
A small forced-convection molten-fluoride-salt loop is being constructed at Oak Ridge National Laboratory to examine the heat transfer behavior of FLiNaK salt in a heated pebble bed. Objectives of the experiment include reestablishing infrastructure needed for fluoride-salt loop testing, developing a unique inductive heating technique for performing heat transfer (or other) experiments, measuring heat transfer characteristics in a liquid-fluoride-salt-cooled pebble bed, and demonstrating the use of silicon carbide (SiC) as a structural component for salt systems. The salt loop will consist of an Inconel 600 piping system, a sump-type pump, a SiC test section, and an air-cooled heat exchanger, as well as auxiliary systems needed to pre-heat the loop, transport salt into and out of the loop, and maintain an inert cover gas over the salt. A 30,000 Hz inductive heating system will be used to provide up to 250 kW of power to a 15 cm diameter SiC test section containing a packed bed of 3 cm graphite spheres. A SiC-to-Inconel 600 joint will use a conventional nickel/grafoil spiral wound gasket sandwiched between SiC and Inconel flanges. The loop system can provide up to 4.5 kg/s of salt flow at a head of 0.125 MPa and operate at a pressure just above atmospheric. Pebble Reynolds numbers of up to 2600 are possible with this configuration. A sump system is provided to drain and store the salt when not in use. Instrumentation on the loop will include pressure, temperature, and flow measurements, while the test section will be instrumented to provide pebble and FLiNaK temperatures.
Directory of Open Access Journals (Sweden)
Garg P.
2016-12-01
Full Text Available This paper studies the mathematical implications of the two dimensional viscous steady laminar combined free-forced convective flow of an incompressible fluid over a semi infinite fixed vertical porous plate embedded in a porous medium. It is assumed that the left surface of the plate is heated by convection from a hot fluid which is at a temperature higher than the temperature of the fluid on the right surface of the vertical plate. To achieve numerical consistency for the problem under consideration, the governing non linear partial differential equations are first transformed into a system of ordinary differential equations using a similarity variable and then solved numerically under conditions admitting similarity solutions. The effects of the physical parameters of both the incompressible fluid and the vertical plate on the dimensionless velocity and temperature profiles are studied and analysed and the results are depicted both graphically and in a tabular form. Finally, algebraic expressions and the numerical values are obtained for the local skin-friction coefficient and the local Nusselt number.
Chen, Bingyan; Fowler, Alex; Bhowmick, Sankha
2006-06-01
Trehalose is believed to offer desiccation protection to mammalian cells by forming stable glassy matrices. The goal of the current study was to explore the desiccation kinetics of thin films of trehalose-water solution under forced and natural convective conditions and to investigate the thermophysical state of mammalian cells at the bottom of the thin film. We developed a finite difference model based on the mass and energy conservation equations coupled to the water transport model from the cells. The boundary conditions were obtained from correlations or experimental measurements and the Gordon-Taylor equation was used to predict the glass transition temperature at every location. Results indicated that there are three distinct regimes for drying for both forced and natural convection, characterized by the slope of the moisture content plot as a function of time. Our results also indicate that the surface of the solution reached the glassy state in less than 10 min for the Reynolds (forced) numbers explored and approximately 30 min for some Rayleigh (natural convective) numbers; however, significant water was trapped at this instant. Larger drying force hastened quicker glass formation but trapped more water. The numerical model was capable of predicting the drying kinetics for the dilute region accurately, but deviated while predicting the other regimes. Based on these experimental validations of the model, the osmotic response of different cells located at the bottom of the solution with orders of magnitude difference in their membrane permeability (Lp) was predicted. The results suggested that extracellular glass formed around cells at the bottom of a trehalose-water solution by the propagation of glass into the solution; however it takes more than an order of magnitude time (approximately 7 min to >100 min for forced convective drying) to remove sufficient water to form glass around cells from the time when the first surface glass is formed. This is
Sakievich, Philip; Peet, Yulia; Adrian, Ronald
2016-11-01
At high Rayleigh numbers in moderate aspect-ratio cylindrical domains turbulent Rayleigh-Bénard convection (RBC) exhibits coherent large-scale motions with patterns like some of those found in laminar flow. In this work we show how the patterns of the largest scales in turbulent RBC affect the bias and convergence of the flow statistics at aspect-ratio 6.3 (diameter/ height). Large scale motions influence two of the finite-time statistical mean's inherent properties: 1) the orientation of the patterns changes so slowly that it may appear almost fixed during a finite averaging time interval, thereby imbedding a preferred azimuthal direction in the sampled data; 2) they also have at least two states associated with the occurrence of up and down motions near the center of the convection cell. We will present a novel technique for triggering additional states of RBC in DNS simulations that are targeted for improving the statistical convergence of the flow. This technique gently perturbs the flow so that the new variations of the large scale patterns can be sampled. Funding through U. S. National Science Foundation Grants CBET-1335731, CMMI-1250124 and XSEDE research allocation TG-CTS150039.
Visualization of Coherent Structures in Acoustically Forced,Gas-Particle Turbulent Round Jet
Institute of Scientific and Technical Information of China (English)
容易; 张会强; 王希麟; 陈昌麒
2004-01-01
To show the effects of the particles and forced disturbances on the instantaneous large-scale vortex structures in a gas-particle round jet, coherent structures in gas-particle turbulent round jets were investigated experimentally by flow visualization. The 45-μm and 350-μm diameter glass beads were used as the particles in the experiments. An acoustic speaker was used to introduce velocity perturbations at the jet exit. The Strouhal number based on the nozzle diameter, exit velocity, and forcing frequency was varied from 0.1 to 0.9. The Reynolds number was 9400. The coherent structures were visualized in unforced and forced single-phase jet flows and unforced and forced particle-laden jet flows with different diameter glass beads. The experimental results show that the particles have significant effects on the gas phase coherent structures. The coherent structures are controlled by the large 350-μm diameter particles, while the structures are mainly dominated by the forced disturbances in the flows with 45-μm diameter particles.
Effects of friction on forced two-dimensional Navier-Stokes turbulence
Blackbourn, Luke A. K.; Tran, Chuong V.
2011-10-01
Large-scale dissipation mechanisms have been routinely employed in numerical simulations of two-dimensional turbulence to absorb energy at large scales, presumably mimicking the quasisteady picture of Kraichnan in an unbounded fluid. Here, “side effects” of such a mechanism—mechanical friction—on the small-scale dynamics of forced two-dimensional Navier-Stokes turbulence are elaborated by both theoretical and numerical analysis. Given a positive friction coefficient α, viscous dissipation of enstrophy has been known to vanish in the inviscid limit ν→0. This effectively renders the scale-neutral friction the only mechanism responsible for enstrophy dissipation in that limit. The resulting dynamical picture is that the classical enstrophy inertial range becomes a dissipation range in which the dissipation of enstrophy by friction mainly occurs. For each α>0, there exists a critical viscosity νc, which depends on physical parameters, separating the regimes of predominant viscous and frictional dissipation of enstrophy. It is found that νc=[η'1/3/(Ckf2)]exp[-η'1/3/(Cα)], where η' is half the enstrophy injection rate, kf is the forcing wave number, and C is a nondimensional constant (the Kraichnan-Batchelor constant). The present results have important theoretical and practical implications. Apparently, mechanical friction is a poor choice in numerical attempts to address fundamental issues concerning the direct enstrophy transfer in two-dimensional Navier-Stokes turbulence. Furthermore, as relatively strong friction naturally occurs on the surfaces and at lateral boundaries of experimental fluids as well as at the interfaces of shallow layers in geophysical fluid models, the frictional effects discussed in this study are crucial in understanding the dynamics of these systems.
Reynolds-dependence of turbulent skin-friction drag reduction induced by spanwise forcing
Gatti, Davide
2015-01-01
This paper examines how increasing the value of the Reynolds number $Re$ affects the ability of spanwise-forcing techniques to yield turbulent skin-friction drag reduction. The control strategy is the streamwise-travelling waves of spanwise wall velocity (Quadrio {\\em et al. J. Fluid Mech.}, vol. 627, 2009, pp. 161--178). The study builds upon an extensive drag-reduction database created with Direct Numerical Simulation of a turbulent channel flow for two, 5-fold separated values of $Re$, namely $Re_\\tau=200$ and $Re_\\tau=1000$. The sheer size of the database, which for the first time systematically addresses the amplitude of the forcing, allows a comprehensive view of the drag-reducing characteristics of the traveling waves, and enables a detailed description of the changes occurring when $Re$ increases. The effect of using a viscous scaling based on the friction velocity of either the non-controlled flow or the drag-reduced flow is described. In analogy with other wall-based drag reduction techniques, like ...
Bogerd, Cornelis P; Brühwiler, Paul A; Heus, Ronald
2008-05-01
Both radiant and forced convective heat flow were measured for a prototype rowing headgear and white and black cotton caps. The measurements were performed on a thermal manikin headform at a wind speed of 4.0 m . s(-1) (s = 0.1) in a climate chamber at 22.0 degrees C (s = 0.05), with and without radiant heat flow from a heat lamp, coming from either directly above (90 degrees ) or from above at an angle of 55 degrees . The effects of hair were studied by repeating selected measurements with a wig. All headgear reduced the radiant heat gain compared with the nude headform: about 80% for the caps and 95% for the prototype rowing headgear (P convective heat loss was reduced more by the caps (36%) than by the prototype rowing headgear (9%) (P heat gain contributed maximally 13% to the net heat transfer, with or without headgear, showing that forced convective heat loss is the dominant heat transfer parameter under the chosen conditions. The results of the headgear - wig combinations were qualitatively similar, with lower absolute heat transfer.
Soucasse, Laurent
2013-01-01
Radiative transfer effects on natural convection flows are investigated in transitional and turbulent regimes. Air/H2O/CO2 mixtures contained in cubical differentially heated cavities are considered. Benchmark numerical simulations are carried out up to Ra=3x108 by coupling a spectral collocation method for the flow and a ray tracing method, associated with an ADF model, for radiation. In order to study the turbulent regime, a radiative transfer model based on spatial filtering is proposed: f...
Forced—Flow Convection for Liquid Methanol Flowing through Microchannels
Institute of Scientific and Technical Information of China (English)
X.F.Peng; B.X.Wang
1993-01-01
Experiments were conducted to investigate the single phase forced-flow convection of methanol flowing through microchannels with rectangular cross-section.The fully-developed turbulent convection regime was found to be initiated at about Re=1000-1500,The fully developed turbulent heat transfer can be predicted by the well-known Dittus-Boelter correlation with mere modification of the original empirical constant coefficient 0.023 to 0.00805.The transition and laminar heat transfer behaviors in microchannels are highly peculiar and complicated,and heavily affected by liquid temperature,velocity and microchannel size.
Parvin, Salma; Ahmed, Sajid; Chowdhury, Raju
2017-06-01
The present work investigates numerically the convective and radiative heat transfer performance and entropy generation of forced convection through a direct absorption solar collector (DASC). Four different fluids; Cu-water nanofluid, Al2O3-waternanofluid, TiO2-water nanofluid and pure water are used as the working fluid. Entropy production has been taken into account in addition to the collector efficiency and heat transfer enhancement. Penalty finite element method with Galerkin's weighted residual technique is used to solve the governing non-linear partial differential equations. Numerical simulations are performed for the variation of solar irradiation (I). The outcomes are presented in the form of isotherms, average output temperature, the average Nusselt number, collector efficiency, average entropy generation and Bejan number. The results present that the rate of heat transfer and collector efficiency enhance significantly for raising the values of I upto a certain range.
Anisotropic turbulence in weakly stratified rotating magnetoconvection
Giesecke, A
2010-01-01
Numerical simulations of the 3D MHD-equations that describe rotating magnetoconvection in a Cartesian box have been performed using the code NIRVANA. The characteristics of averaged quantities like the turbulence intensity and the turbulent heat flux that are caused by the combined action of the small-scale fluctuations are computed. The correlation length of the turbulence significantly depends on the strength and orientation of the magnetic field and the anisotropic behavior of the turbulence intensity induced by Coriolis and Lorentz force is considerably more pronounced for faster rotation. The development of isotropic behavior on the small scales -- as it is observed in pure rotating convection -- vanishes even for a weak magnetic field which results in a turbulent flow that is dominated by the vertical component. In the presence of a horizontal magnetic field the vertical turbulent heat flux slightly increases with increasing field strength, so that cooling of the rotating system is facilitated. Horizont...
Musielak, Z. E.; Rosner, R.; Ulmschneider, P.
1989-01-01
The source functions and the energy fluxes for wave generation in magnetic flux tubes embedded in an otherwise magnetic field-free, turbulent, and compressible fluid are derived. The calculations presented here assume that the tube interior is not itself turbulent, e.g., that motions within the flux tube are due simply to external excitation. Specific results for the generation of longitudinal tube waves are presented.
Chekmarev, Sergei F
2012-01-01
A simple analytical model for a turbulent flow is proposed, which considers the flow as a collection of localized spatial structures that are composed of elementary "cells" in which the state of the particles (atoms or molecules) is uncertain. The Reynolds number is associated with the ratio between the total phase volume for the system and that for the elementary cell. Calculating the statistical weights of the collections of the localized structures, it is shown that as the Reynolds number increases, the elementary cells group into the localized structures, which successfully explains the onset of turbulence and some other characteristic properties of turbulent flows. It is also shown that the basic assumptions underlying the model are involved in the derivation of the Navier-Stokes equation, which suggests that the driving force for the turbulent transition described with the hydrodynamic equations is essentially the same as in the present model, i.e. the tendency of the system to occupy a statistically do...
Turbulent transport in a strongly stratified forced shear layer with thermal diffusion
Garaud, Pascale
2015-01-01
This work presents numerical results on the transport of heat and chemical species by shear-induced turbulence in strongly stratified but thermally diffusive environments. The shear instabilities driven in this regime are sometimes called "secular" shear instabilities, and can take place even when the gradient Richardson number of the flow (the square of the ratio of the buoyancy frequency to the shearing rate) is large, provided the P\\'eclet number (the ratio of the thermal diffusion timescale to the turnover timescale of the turbulent eddies) is small. We have identified a set of simple criteria to determine whether these instabilities can take place or not. Generally speaking, we find that they may be relevant whenever the thermal diffusivity of the fluid is very large (typically larger than $10^{14}$cm$^2$/s), which is the case in the outer layers of high-mass stars ($M\\ge 10 M_\\odot$) for instance. Using a simple model setup in which the shear is forced by a spatially sinusoidal, constant-amplitude body-...
Turbulence-induced melting of a nonequilibrium vortex crystal in a forced thin fluid film
Perlekar, Prasad
2009-01-01
To develop an understanding of recent experiments on the turbulence-induced melting of a periodic array of vortices in a thin fluid film, we perform a direct numerical simulation of the two-dimensional Navier-Stokes equations forced such that, at low Reynolds numbers, the steady state of the film is a square lattice of vortices. We find that, as we increase the Reynolds number, this lattice undergoes a series of nonequilibrium phase transitions, first to a crystal with a different reciprocal lattice and then to a sequence of crystals that oscillate in time. Initially the temporal oscillations are periodic; this periodic behaviour becomes more and more complicated, with increasing Reynolds number, until the film enters a spatially disordered nonequilibrium statistical steady that is turbulent. We study this sequence of transitions by using fluid-dynamics measures, such as the Okubo-Weiss parameter that distinguishes between vortical and extensional regions in the flow, ideas from nonlinear dynamics, e.g., \\Poi...
Stably stratified turbulence in the presence of large-scale forcing
Rorai, C; Pouquet, A
2014-01-01
We perform two high resolution direct numerical simulations of stratified turbulence for Reynolds number equal to Re~25000 and Froude number respectively of Fr~0.1 and Fr~0.03. The flows are forced at large scale and discretized on an isotropic grid of 2048^3 points. Stratification makes the flow anisotropic and introduces two extra characteristic scales with respect to homogeneous isotropic turbulence: the buoyancy scale, L_B, and the Ozmidov scale, l_{oz}. The former is related to the number of layers that the flow develops in the direction of gravity, the latter is regarded as the scale at which isotropy is recovered. The values of L_B and l_{oz} depend on the Froude number and their absolute and relative size affect the repartition of energy among Fourier modes. By contrasting the behavior of the two simulated flows we identify some surprising similarities: after an initial transient the two flows evolve towards comparable values of the kinetic and potential enstrophy, and energy dissipation rate. Further...
Directory of Open Access Journals (Sweden)
Kathrin Wapler
2015-04-01
Full Text Available Two severe summer-time convective events in Germany are investigated which can be classified by the prevailing synoptic conditions into a strong and a weak forcing case. The strong forcing case exhibits a larger scale precipitation pattern caused by frontal ascent whereas scattered convection is dominating the convective activity in the weak forcing case. Other distinguished differences between the cases are faster movement of convective cells and larger regions with significant loss mainly due to severe gusts in the strong forcing case. A comprehensive set of various observations is used to characterise the two different events. The observations include measurements from a lightning detection network, precipitation radar, geostationary satellite and weather stations, as well as information from an automated cell detection algorithm based on radar reflectivity which is combined with severe weather reports, and damage data from insurances. Forecast performance at various time scales is analysed ranging from nowcasting and warning to short-range forecasting. Various methods and models are examined, including human warnings, observation-based nowcasting algorithms and high-resolution ensemble prediction systems. The analysis shows the advantages of a multi-sensor and multi-source approach in characterising convective events and their impacts. Using data from various sources allows to combine the different strengths of observational data sets, especially in terms of spatial coverage or data accuracy, e.g. damage data from insurances provide good spatial coverage with little meteorological information while measurements at weather stations provide accurate but pointwise observations. Furthermore, using data from multiple sources allow for a better understanding of the convective life cycle. Several parameters from different instruments are shown to have a predictive skill for convective development, these include satellite-based cloud-top cooling
DEFF Research Database (Denmark)
Taherian, Hessam; Yazdanshenas, Eshagh
2006-01-01
Due to scarcity of literature on forced-convection heat transfer in a solar collector with rhombic cross-section absorbing tubes, a series of experiments was arranged and conducted to determine heat transfer coefficient. In this study, a typical rhombic cross-section finned tube of flat......-plate collectors used as the test section. Two correlations were proposed for the Nusselt number as a function of the Reynolds number and the Prandtl number based on hydraulic diameter for various heat fluxes. The temperature distribution along the finned tube for the fluid and the wall were also illustrated....
Directory of Open Access Journals (Sweden)
M. R. Astaraki
2012-01-01
Full Text Available In the present study analytical solution for forced convection heat transfer in a circular duct with a special boundary condition has been presented, because the external wall temperature is a periodic function of axial direction. Local energy balance equation is written with reference to the fully developed regime. Also governing equations are two-dimensionally solved, and the effect of duct wall thickness has been considered. The temperature distribution of fluid and solid phases is assumed as a periodic function of axial direction and finally temperature distribution in the flow field, solid wall, and local Nusselt number, is obtained analytically.
The effect of Coriolis force on nonlinear convection in a porous medium
Directory of Open Access Journals (Sweden)
D. H. Riahi
1994-01-01
Full Text Available Nonlinear convection in a porous medium and rotating about vertical axis is studied in this paper. An upper bound to the heat flux is calculated by the method initiated first by Howard [6] for the case of infinite Prandtl number.
Forced Convective Heat Transfer in a Plate Channel Filled with Solid Particles
Institute of Scientific and Technical Information of China (English)
Pei－XueJiang; Ze－PeiRen; 等
1996-01-01
A numerical study of fluid flow and convective heat transfer in a plate channel filled with solid(metallic)perticles is presented in this paper,The study uses the thermal equilibrium model and a newly developed numerical model which does not assume idealized local thermal equilibrium between the solid particles and the fluid.The numerical simulation results are compared with the experimental data in reference[2].The paper investigates the effects of the assumption of local thermal equilibrium versus non-thermal equilibrium,the thermal conductivity of the solid particles and the particle diameter on convective heat transfer.For the conditions studied.the convective heat transfer and the temperature filed assuming local thermal equilibrium are much different from that for the non-thermal equilibrium assumption when the difference between the solid and fluid thermal conductivities is large,The relative values of the thermal conductivities of the solid particles and the fluid also have a profound effect on the temperature distribution in the channel.The pressure drop decreases as the particle diameter increases and the convective heat transfer coefficient may decrease of increase as the particle diameter increasws depending on the values of ε，λs，λf,λd,αu，ρu.
R-HPDC Process with Forced Convection Mixing Device for Automotive Part of A380 Aluminum Alloy
Directory of Open Access Journals (Sweden)
Bing Zhou
2014-04-01
Full Text Available The continuing quest for cost-effective and complex shaped aluminum castings with fewer defects for applications in the automotive industries has aroused the interest in rheological high pressure die casting (R-HPDC. A new machine, forced convection mixing (FCM device, based on the mechanical stirring and convection mixing theory for the preparation of semisolid slurry in convenience and functionality was proposed to produce the automotive shock absorber part by R-HPDC process. The effect of barrel temperature and rotational speed of the device on the grain size and morphology of semi-solid slurry were extensively studied. In addition, flow behavior and temperature field of the melt in the FCM process was investigated combining computational fluid dynamics simulation. The results indicate that the microstructure and pore defects at different locations of R-HPDC casting have been greatly improved. The vigorous fluid convection in FCM process has changed the temperature field and composition distribution of conventional solidification. Appropriately increasing the rotational speed can lead to a uniform temperature filed sooner. The lower barrel temperature leads to a larger uniform degree of supercooling of the melt that benefits the promotion of nucleation rate. Both of them contribute to the decrease of the grain size and the roundness of grain morphology.
Direct force wall shear measurements in pressure-driven three-dimensional turbulent boundary layers
Mcallister, J. E.; Tennant, M. H.; Pierce, F. J.
1982-01-01
Unique, simultaneous direct measurements of the magnitude and direction of the local wall shear stress in a pressure-driven three-dimensional turbulent boundary layer are presented. The flow is also described with an oil streak wall flow pattern, a map of the wall shear stress-wall pressure gradient orientations, a comparison of the wall shear stress directions relative to the directions of the nearest wall velocity as measured with a typical, small boundary layer directionally sensitive claw probe, as well as limiting wall streamline directions from the oil streak patterns, and a comparison of the freestream streamlines and the wall flow streamlines. A review of corrections for direct force sensing shear meters for two-dimensional flows is presented with a brief discussion of their applicability to three-dimensional devices.
Does the choice of the forcing term affect flow statistics in DNS of turbulent channel flow?
Quadrio, Maurizio; Hasegawa, Yosuke
2015-01-01
We seek possible statistical consequences of the way a forcing term is added to the Navier--Stokes equations in the Direct Numerical Simulation (DNS) of incompressible channel flow. Simulations driven by constant flow rate, constant pressure gradient and constant power input are used to build large databases, and in particular to store the complete temporal trace of the wall-shear stress for later analysis. As these approaches correspond to different dynamical systems, it can in principle be envisaged that these differences are reflect by certain statistics of the turbulent flow field. The instantaneous realizations of the flow in the various simulations are obviously different, but, as expected, the usual one-point, one-time statistics do not show any appreciable difference. However, the PDF for the fluctuations of the streamwise component of wall friction reveals that the simulation with constant flow rate presents lower probabilities for extreme events of large positive friction. The low probability value ...
Design of Test Loops for Forced Convection Heat Transfer Studies at Supercritical State
Balouch, Masih N.
Worldwide research is being conducted to improve the efficiency of nuclear power plants by using supercritical water (SCW) as the working fluid. One such SCW reactor considered for future development is the CANDU-Supercritical Water Reactor (CANDU-SCWR). For safe and accurate design of the CANDU-SCWR, a detailed knowledge of forced-convection heat transfer in SCW is required. For this purpose, two supercritical fluid loops, i.e. a SCW loop and an R-134a loop are developed at Carleton University. The SCW loop is designed to operate at pressures as high as 28 MPa, temperatures up to 600 °C and mass fluxes of up to 3000 kg/m2s. The R-134a loop is designed to operate at pressures as high as 6 MPa, temperatures up to 140 °C and mass fluxes in the range of 500-6000 kg/m2s. The test loops designs allow for up to 300 kW of heating power to be imparted to the fluid. Both test loops are of the closed-loop design, where flow circulation is achieved by a centrifugal pump in the SCW loop and three parallel-connected gear pumps in the R-134a loop, respectively. The test loops are pressurized using a high-pressure nitrogen cylinder and accumulator assembly, which allows independent control of the pressure, while simultaneously dampening pump induced pressure fluctuations. Heat exchangers located upstream of the pumps control the fluid temperature in the test loops. Strategically located measuring instrumentation provides information on the flow rate, pressure and temperature in the test loops. The test loops have been designed to accommodate a variety of test-section geometries, ranging from a straight circular tube to a seven-rod bundle, achieving heat fluxes up to 2.5 MW/m2 depending on the test-section geometry. The design of both test loops allows for easy reconfiguration of the test-section orientation relative to the gravitational direction. All the test sections are of the directly-heated design, where electric current passing through the pressure retaining walls of the
Properties of forced convection experimental with silicon carbide based nano-fluids
Soanker, Abhinay
-fluids. The nano-fluid properties were tested at three different volume concentrations; 0.55%, 1% and 1.6%. Thermal conductivity was measured for the three-volume concentration as function of temperature. Thermal conductivity enhancement increased with the temperature and may be attributed to increased Brownian motion of colloidal particles at higher temperatures. Measured thermal conductivity values are compared with results obtained by theoretical model derived in this work. Effect of temperature and volume concentration on viscosity was also measured and reported. Viscosity increase and related consequences are important issues for the use of nano-fluids. Extensive measurements of heat transfer and pressure drop for forced convection in circular pipes with nano-fluids was also conducted. Parameters such as heat transfer coefficient, Nusselt number, pressure drop and a thermal hydraulic performance factor that takes into account the gains made by increase in thermal conductivity as well as penalties related to increase in pressure drop are evaluated for laminar and transition flow regimes. No significant improvement in heat transfer (Nusselt number) compared to its based fluid was observed. It is also observed that the values evaluated for the thermal-hydraulic performance factor (change in heat transfer/change in pressure drop) was under unity for many flow conditions indicating poor overall applicability of SiC based nano-fluids.
Ahlers, Guenter; He, Xiaozhou
2014-01-01
We report on experimental determinations of the temperature field in the interior (bulk) of turbulent Rayleigh-Benard convection for a cylindrical sample with aspect ratio (diameter over height) of 0.50, both in the classical and in the ultimate state. The Prandtl number was close to 0.8. We find a "logarithmic layer" in which the temperature varies as A*ln(z/L) + B with the distance z from the bottom plate of the sample. The amplitude A varies with radial position r. In the classical state these results are in good agreement with direct numerical simulations (DNS); in the ultimate state there are as yet no DNS. A close analogy between the temperature field in the classical state and the "Law of the Wall" for the time-averaged down-stream velocity in shear flow is discussed.
Thornton, E. A.; Wieting, A. R.
1979-01-01
Conventional versus upwind convective finite elements, and lumped versus consistent formulations for practical conduction/forced convection analysis are evaluated on the basis of numerical studies, with finite element and finite difference lumped-parameter temperatures compared to closed-form analytical solutions for convection problems. Attention is given to two practical combined conduction and forced convection applications, stressing that the finite element method, showing superior accuracy, is competitive with the finite difference lumped-parameter method. Also considered are the computational time savings offered by the zero capacitance nodes procedure and comparative finite element and finite difference lumped-parameter computer times. The present study has reference to the design of actively cooled engine and airframe structures for hypersonic flight.
Cassiani, Massimo; Stohl, Andreas; Brioude, Jerome
2015-03-01
A correction for the vertical gradient of air density has been incorporated into a skewed probability density function formulation for turbulence in the convective boundary layer. The related drift term for Lagrangian stochastic dispersion modelling has been derived based on the well-mixed condition. Furthermore, the formulation has been extended to include unsteady turbulence statistics and the related additional component of the drift term obtained. These formulations are an extension of the drift formulation reported by Luhar et al. (Atmos Environ 30:1407-1418, 1996) following the well-mixed condition proposed by Thomson (J Fluid Mech 180:529-556, 1987). Comprehensive tests were carried out to validate the formulations including consistency between forward and backward simulations and preservation of a well-mixed state with unsteady conditions. The stationary state CBL drift term with density correction was incorporated into the FLEXPART and FLEXPART-WRF Lagrangian models, and included the use of an ad hoc transition function that modulates the third moment of the vertical velocity based on stability parameters. Due to the current implementation of the FLEXPART models, only a steady-state horizontally homogeneous drift term could be included. To avoid numerical instability, in the presence of non-stationary and horizontally inhomogeneous conditions, a re-initialization procedure for particle velocity was used. The criteria for re-initialization and resulting errors were assessed for the case of non-stationary conditions by comparing a reference numerical solution in simplified unsteady conditions, obtained using the non-stationary drift term, and a solution based on the steady drift with re-initialization. Two examples of "real-world" numerical simulations were performed under different convective conditions to demonstrate the effect of the vertical gradient in density on the particle dispersion in the CBL.
On Developments of k-τ and k-ω Models for Near-Wall Turbulence of Engineering Duct Flows
DEFF Research Database (Denmark)
Rokni, Masoud; Sundén, Bengt
2009-01-01
The performance of a modified k-tau model is assessed in predicting the turbulent flow and forced convective heat transfer in ducts with arbitrary cross-sections, under fully developed conditions. The presented model is based on more physical grounds using bounded time-scale, local turbulent...
Abdikamalov, E; Radice, D; Roberts, L F; Haas, R; Reisswig, C; Moesta, P; Klion, H; Schnetter, E
2014-01-01
We conduct a series of numerical experiments into the nature of three-dimensional (3D) hydrodynamics in the postbounce stalled-shock phase of core-collapse supernovae using 3D general-relativistic hydrodynamic simulations of a $27$-$M_\\odot$ progenitor star with a neutrino leakage/heating scheme. We vary the strength of neutrino heating and find three cases of 3D dynamics: (1) neutrino-driven convection, (2) initially neutrino-driven convection and subsequent development of the standing accretion shock instability (SASI), (3) SASI dominated evolution. This confirms previous 3D results of Hanke et al. 2013, ApJ 770, 66 and Couch & Connor 2014, ApJ 785, 123. We carry out simulations with resolutions differing by up to a factor of $\\sim$4 and demonstrate that low resolution is artificially favorable for explosion in the 3D convection-dominated case, since it decreases the efficiency of energy transport to small scales. Low resolution results in higher radial convective fluxes of energy and enthalpy, more ful...
Gibbs, J.A.; Fedorovich, E.; Eijk, A.M.J. van
2011-01-01
Weather Research and Forecasting (WRF) model predictions using different boundary layer schemes and horizontal grid spacings were compared with observational and numerical large-eddy simulation data for conditions corresponding to a dry atmospheric convective boundary layer (CBL) over the southern
Finite element methodology for transient conduction/forced-convection thermal analysis
Thornton, E. A.; Wieting, A. R.
1979-01-01
Finite element methodology for steady state thermal analysis of convectively cooled structures has been extended for transient analysis. The finite elements are based on representing the fluid passages by fluid bulk-temperature nodes and fluid-solid interface nodes. The formulation of the finite element equations for a typical flow passage is based on the weighted residual method with upwind weighting functions. Computer implementation of the convective finite element methodology using explicit and implicit time integration algorithms is described. Accuracy and efficiency of the methodology is evaluated by comparisons with analytical solutions and finite-difference lumped-parameter analyses. The comparative analyses demonstrate that finite element conduction/conduction methodology may be used to predict transient temperatures with an accuracy equal or superior to the lumped-parameter finite-difference method.
Ahmed, Mahmoud; Eslamian, Morteza
2015-01-01
Laminar natural convection in differentially heated (β = 0°, where β is the inclination angle), inclined (β = 30° and 60°), and bottom-heated (β = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and th...
Lee, Man
2012-02-22
A microchannel heat sink, integrated with pressure and temperature microsensors, is utilized to study single-phase liquid flow forced convection under a uniform heat flux boundary condition. Utilizing a waferbond-and-etch- back technology, the heat source, temperature and pressure sensors are encapsulated in a thin composite membrane capping the microchannels, thus allowing experimentally good control of the thermal boundary conditions. A three-dimensional physical model has been constructed to facilitate numerical simulations of the heat flux distribution. The results indicate that upstream the cold working fluid absorbs heat, while, within the current operating conditions, downstream the warmer working fluid releases heat. The Nusselt number is computed numerically and compared with experimental and analytical results. The wall Nusselt number in a microchannel can be estimated using classical analytical solutions only over a limited range of the Reynolds number, Re: both the top and bottom Nusselt numbers approach 4 for Re < 1, while the top and bottom Nusselt numbers approach 0 and 5.3, respectively, for Re > 100. The experimentally estimated Nusselt number for forced convection is highly sensitive to the location of the temperature measurements used in calculating the Nusselt number. © 2012 IOP Publishing Ltd.
Energy Technology Data Exchange (ETDEWEB)
Rosenfeld, Daniel [Hebrew Univ. of Jerusalem (Israel)
2015-12-23
Quantifying the aerosol/cloud-mediated radiative effect at a global scale requires simultaneous satellite retrievals of cloud condensation nuclei (CCN) concentrations and cloud base updraft velocities (Wb). Hitherto, the inability to do so has been a major cause of high uncertainty regarding anthropogenic aerosol/cloud-mediated radiative forcing. This can be addressed by the emerging capability of estimating CCN and Wb of boundary layer convective clouds from an operational polar orbiting weather satellite. Our methodology uses such clouds as an effective analog for CCN chambers. The cloud base supersaturation (S) is determined by Wb and the satellite-retrieved cloud base drop concentrations (Ndb), which is the same as CCN(S). Developing and validating this methodology was possible thanks to the ASR/ARM measurements of CCN and vertical updraft profiles. Validation against ground-based CCN instruments at the ARM sites in Oklahoma, Manaus, and onboard a ship in the northeast Pacific showed a retrieval accuracy of ±25% to ±30% for individual satellite overpasses. The methodology is presently limited to boundary layer not raining convective clouds of at least 1 km depth that are not obscured by upper layer clouds, including semitransparent cirrus. The limitation for small solar backscattering angles of <25º restricts the satellite coverage to ~25% of the world area in a single day. This methodology will likely allow overcoming the challenge of quantifying the aerosol indirect effect and facilitate a substantial reduction of the uncertainty in anthropogenic climate forcing.
Directory of Open Access Journals (Sweden)
G. V. Levina
2000-01-01
Full Text Available The work is concerned with the results of theoretical and laboratory modelling the processes of the large-scale structure generation under turbulent convection in the rotating-plane horizontal layer of an incompressible fluid with unstable stratification. The theoretical model describes three alternative ways of creating unstable stratification: a layer heating from below, a volumetric heating of a fluid with internal heat sources and combination of both factors. The analysis of the model equations show that under conditions of high intensity of the small-scale convection and low level of heat loss through the horizontal layer boundaries a long wave instability may arise. The condition for the existence of an instability and criterion identifying the threshold of its initiation have been determined. The principle of action of the discovered instability mechanism has been described. Theoretical predictions have been verified by a series of experiments on a laboratory model. The horizontal dimensions of the experimentally-obtained long-lived vortices are 4÷6 times larger than the thickness of the fluid layer. This work presents a description of the laboratory setup and experimental procedure. From the geophysical viewpoint the examined mechanism of the long wave instability is supposed to be adequate to allow a description of the initial step in the evolution of such large-scale vortices as tropical cyclones - a transition form the small-scale cumulus clouds to the state of the atmosphere involving cloud clusters (the stage of initial tropical perturbation.
Primary Issues of Mixed Convection Heat Transfer Phenomena
Energy Technology Data Exchange (ETDEWEB)
Chae, Myeong-Seon; Chung, Bum-Jin [Kyung Hee University, Yongin (Korea, Republic of)
2015-10-15
The computer code analyzing the system operating and transient behavior must distinguish flow conditions involved with convective heat transfer flow regimes. And the proper correlations must be supplied to those flow regimes. However the existing safety analysis codes are focused on the Light Water Reactor and they are skeptical to be applied to the GCRs (Gas Cooled Reactors). One of the technical issues raise by the development of the VHTR is the mixed convection, which occur when the driving forces of both forced and natural convection are of comparable magnitudes. It can be encountered as in channel of the stacked with fuel elements and a decay heat removal system and in VHTR. The mixed convection is not intermediate phenomena with natural convection and forced convection but independent complicated phenomena. Therefore, many researchers have been studied and some primary issues were propounded for phenomena mixed convection. This paper is to discuss some problems identified through reviewing the papers for mixed convection phenomena. And primary issues of mixed convection heat transfer were proposed respect to thermal hydraulic problems for VHTR. The VHTR thermal hydraulic study requires an indepth study of the mixed convection phenomena. In this study we reviewed the classical flow regime map of Metais and Eckert and derived further issues to be considered. The following issues were raised: (1) Buoyancy aided an opposed flows were not differentiated and plotted in a map. (2) Experimental results for UWT and UHF condition were also plotted in the same map without differentiation. (3) The buoyancy coefficient was not generalized for correlating with buoyancy coefficient. (4) The phenomenon analysis for laminarization and returbulization as buoyancy effects in turbulent mixed convection was not established. (5) The defining to transition in mixed convection regime was difficult.
Liljegren, James Christian
The development and validation of a new stochastic model for predicting the dispersion of a passive tracer in the convective boundary layer are presented. The methods and procedures employed and the results obtained from the Atterbury-87 field study of the dispersion of fog-oil smoke and hexachloroethane (HC) smoke are also described. The stochastic model represents a substantial improvement over previous modelling approaches in that it provides a more realistic treatment of the atmospheric turbulence, especially for the large-scale convective motions occurring in the daytime planetary boundary layer which are largely responsible for driving the dispersion process. Using the Langevin equation to model the Lagrangian velocities, the dispersion of a large number of passive tracer particles is computationally simulated. The shape of the resulting probability density function closely matches observations. The behavior of the autocorrelation of the modelled Lagrangian velocities also matches the non-exponential form computed from balloon-borne measurements by using the local integral time scale of the turbulence. The model was verified against data gathered from laboratory simulations of the boundary layer carried out in water tanks and wind tunnels as well as from actual field measurements. The predictions of the stochastic model were in agreement with the trends and magnitudes observed in the data, including the lift-off of the plume centerline from the ground due to the influence of the rising thermal updrafts. Comparisons of crosswind-integrated concentration and plume spread computed from the test data with the results of previous field studies of atmospheric dispersion indicated strong agreement. These comparisons revealed that the centerline of the smoke plume rose as the plume moved downwind due to the effects of thermal convection. Comparison of concentration data with the predictions of the stochastic model also showed good agreement and confirmed that the
The influence of convective current generator on the global current
Directory of Open Access Journals (Sweden)
V. N. Morozov
2006-01-01
Full Text Available The mathematical generalization of classical model of the global circuit with taking into account the convective current generator, working in the planetary boundary layer was considered. Convective current generator may be interpreted as generator, in which the electromotive force is generated by processes, of the turbulent transport of electrical charge. It is shown that the average potential of ionosphere is defined not only by the thunderstorm current generators, working at the present moment, but by the convective current generator also. The influence of the convective processes in the boundary layer on the electrical parameters of the atmosphere is not only local, but has global character as well. The numerical estimations, made for the case of the convective-unstable boundary layer demonstrate that the increase of the average potential of ionosphere may be of the order of 10% to 40%.
Institute of Scientific and Technical Information of China (English)
无
1996-01-01
An experimental investigation is described for the characteristics of convective boiling flow instabilities in horizontally helically coiled tubes using a steam-water two-phase closed circulation test loop at pressure from 0.5 MPa to 3.5MPa.Three kinds of oscillation are reported.density waves;pressure drop excorsions;thermal fluctuations.We describe their dependence on main system parameters such as system pressure,mass flowrate,inlet subcooling,compressible volume and heat flux.Utilising the experimental data together with conservation constraints,a dimensionless correlation is proposed for the occurrence of density waves.
Kahani, M.; Zeinali Heris, S.; Mousavi, S. M.
2014-05-01
Coiled tubes and nanofludics are two significant techniques to enhance the heat transfer ability of thermal equipments. The forced convective heat transfer and the pressure drop of nanofluid inside straight tube and helical coiled one with a constant wall heat flux were studied experimentally. Distilled water was used as a host fluid and Nanofluids of aqueous TiO2 nanoparticles (50 nm) suspensions were prepared in various volume concentrations of 0.25-2 %. The heat transfer coefficient of nanofluids is obtained for different nanoparticle concentrations as well as various Reynolds numbers. The experiments covered a range of Reynolds number of 500-4,500. The results show the considerable enhancement of heat transfer rate, which is due to the nanoparticles present in the fluid. Heat transfer coefficient increases by increasing the volume concentration of nanoparticles as well as Reynolds number. Moreover, due to the curvature of the tube when fluid flows inside helical coiled tube instead of straight one, both convective heat transfer coefficient and the pressure drop of fluid grow considerably. Also, the thermal performance factors for tested nanofluids are greater than unity and the maximum thermal performance factor of 3.72 is found with the use of 2.0 % volume concentration of nanofluid at Reynolds number of 1,750.
Qi, Mingfan; Kang, Yonglin; Zhu, Guoming; Li, Yangde; Li, Weirong
2017-01-01
A simple process so-called forced convection stirring casting (FCSC) was proposed to prepare large-diameter 7075 Al alloy ingots. The flow behavior, temperature, and composition fields of the melt in the FCSC process were simulated. The macromorphology, macrosegregation, microstructure, and mechanical properties of the ingots prepared by the FCSC were studied and compared with those prepared by normal casting (NC). The results showed that in the FCS device, the strong convection caused by the axial flow and circular flow rapidly promoted the uniformity of the temperature and composition fields of the melt. Microstructures of the FCSC ingots from the edge to the center were all nearly spherical grains, which were much finer and more uniform than that of the NC ingots. The rotation speed played an important role in the microstructure of the FCSC ingots, and the grains became finer and rounder as the speed increasing. The FCSC process effectively eliminated cracks, improved macrosegregation, and decreased the eutectic phase area fraction and the average grain boundary thickness of ingots. Mechanical properties of the ingots prepared by the FCSC are far better than that of the NC ingots.
Energy Technology Data Exchange (ETDEWEB)
Nanjundappa, C.E., E-mail: cenanju@hotmail.com [Department of Mathematics, Dr. Ambedkar Institute of Technology, Bangalore-560 056 (India); Shivakumara, I.S., E-mail: shivakumarais@gmail.com [Department of Mathematics, Bangalore University, Bangalore-560 001 (India); Prakash, H.N., E-mail: prakashahn83@gmail.com [Government Pre-University College, B H Road, Tumkur-572 102 (India)
2014-12-15
We investigate the influence of Coriolis force on the onset of thermomagnetic convection in ferrofluid saturating a porous layer in the presence of a uniform vertical magnetic field using both linear and weakly non-linear analyses. The modified Brinkman–Forchheimer-extended Darcy equation with Coriolis term has been used to describe the fluid flow. The linear theory based on normal mode method is considered to find the criteria for the onset of stationary thermomagnetic Convection and weakly non-linear analysis based on minimal representation of truncated Fourier series analysis containing only two terms has been used to find the Nusselt number Nu as functions of time. The range of thermal Rayleigh number R beyond which the bifurcation becomes subcritical increases with increasing Λ, Da{sup −1} and Ta. The global quantity of the heat transfer rate decreases by increasing the Taylor number Ta. The results obtained, during the above analyses, have been presented graphically and the effects of various parameters on heat and mass transfer have been discussed. Finally, we have drawn the steady streamlines for various parameters.
Energy Technology Data Exchange (ETDEWEB)
Magyari, E. [ETH-Zuerich, Institute of Building Technology, Zuerich (Switzerland); Barletta, A. [Universita di Bologna, Dipartimento di Ingegneria Energetica, Nucleare e del Controllo Ambientale (DIENCA), Bologna (Italy)
2007-12-15
Laminar forced convection flow of a liquid in the fully developed region of a circular duct with isothermal wall is analyzed. The effects of viscous dissipation as well as of temperature dependent viscosity are taken into account. The coupled momentum and energy equations are solved analytically by means of a power series method. Then, reference is made to the Poiseuille model for the temperature change of viscosity. For a fixed value of the axial pressure gradient along the duct, dual solutions are found for the velocity and temperature fields. Although dual solutions correspond to the same value of the axial pressure gradient, they lead in general to different values of the average fluid velocity, of the average fluid temperature and of the wall heat flux. It is shown that, for a given fluid and for a fixed duct radius, the absolute value of the axial pressure gradient has an upper bound above which no steady laminar solution can exist. (orig.)
Institute of Scientific and Technical Information of China (English)
YANG Xiao; LIU Xue-mei
2006-01-01
Based on the two-energy equation model, taking into account viscous dissipation due to the interaction between solid skeleton and pore fluid flow, temperature expressions of the solid skeleton and pore fluid flow are obtained analytically for the thermally developing forced convection in a saturated porous medium parallel plate channel,with walls being at constant temperature. It is proved that the temperatures of the two phases for the local thermal nonequilibrium approach to the temperature derived from the one-energy equation model for the local thermal equilibrium when the heat exchange coefficient goes to infinite. The temperature profiles are shown in figures for different dimensionless parameters and the effects of the parameters on the local thermal nonequilibrium are revealed by parameter study.
Guedda, M.; Benlahsen, M.; Sriti, M.; Achemlal, D.
2017-09-01
In this paper, we revisit a mathematical model representing a two-dimensional forced convection boundary-layer flow over a horizontal impermeable plate with a variable heat flux and viscosity. It is assumed that the fluid viscosity varies as an inverse linear function of temperature, the free stream velocity varies as an inverse linear of x and the wall heat flux varies with x as x^{λ}; where λ > -1 and x measures the distance along the surface. Analytical local similarity solutions are presented which reveal that there are two competing effects: λ and θe; where θe is the variable viscosity parameter. It has been shown that for θe > 0 dual solutions exist and boundary separation occurs, while a unique local similarity solution exists for any θe < 0.
Tatsumoto, H.; Shirai, Y.; Shiotsu, M.; Naruo, Y.; Kobayashi, H.; Inatani, Y.
2014-12-01
Forced convection heat transfer from a PtCo wire with a length of 120 mm and a diameter of 1.2 mm that was inserted into a vertically-mounted pipe with a diameter of 8.0 mm to liquid hydrogen flowing upward was measured with a quasi-steady increase of a heat generation rate for wide ranges of flow rate under saturated conditions. The pressures were varied from 0.4 MPa to 1.1 MPa. The non-boiling heat transfer characteristic agrees with that predicted by Dittus-Boelter correlation. The critical heat fluxes are higher for higher flow rates and lower pressures. Effect of Weber number on the CHF was clarified and a CHF correlation that can describe the experimental data is derived based on our correlation for a pipe.
Directory of Open Access Journals (Sweden)
M.M. Rahman
2012-12-01
Full Text Available This paper presents the numerical study on two-dimensional forced convection heat transfer across three in-line flat tubes confined in a channel under incompressible, steady-state conditions. This system is solved in body-fitted coordinates (BFC using the finite volume method (FVM. The constant heat flux is imposed on the surface of the tubes as the thermal boundary conditions. The range of the longitudinal pitch-to-diameter ratio (SL/Ds of 2.0–4.0 is considered, the Reynolds number varies within the range 25–300, and the Prandtl number is taken as 0.7. The temperature contours, local Nusselt number distributions at the tube surface and mean Nusselt number were analyzed. The strength of the heat transfer between the surface of the tubes and the air flow increases with an increase in Reynolds number and pitch-to-diameter ratio.
Energy Technology Data Exchange (ETDEWEB)
Luna, N. [Direccion de Operacion Petrolera, Direccion General de Exploracion y Explotacion de Hidrocarburos, Secretaria de Energia, 03100 Mexico DF (Mexico); Mendez, F. [Facultad de Ingenieria, UNAM, 04510 Mexico DF (Mexico)
2005-07-01
The steady-state analysis of conjugated heat transfer process for the hydrodynamically developed forced convection flow on a heated flat plate embedded in a porous medium is studied. The governing equations for the fluid-saturated porous medium are solved analytically using the integral boundary layer approximation. This integral solution is coupled to the energy equation for the flat plate, where the longitudinal heat conduction effects are taken into account. The resulting equations are then reduced to an integro-differential equation which is solved by regular perturbation techniques and numerical methods. The analytical and numerical predictions for the temperature profile of the plate and appropriate local and average Nusselt numbers are plotted for finite values of the conduction parameter, {alpha}, which represents the presence of the longitudinal heat conduction effects. (authors)
Study of the characteristics of forced homogeneous turbulence using band-pass Fourier filtering
Energy Technology Data Exchange (ETDEWEB)
Kareem, Waleed Abdel [Suez Canal University, Suez (Egypt)
2012-03-15
Simulations of forced homogeneous isotropic turbulence with resolutions of 128{sup 3} and 256{sup 3} using the Lattice Boltzmann method are carried out. The multi-scale vortical structures are identified using the band-pass Fourier cutoff filtering. Three fields at each simulation are extracted and their characteristics are studied. The vortical structures are visualized using the Q-identification method. A new lattice segmentation scheme to identify the central axes of the vortical structures is introduced. The central points of each vortex are identified and they are connected using the direction cosines technique. Results show that the Q-spectrum of the fine scale field survives at low and high wave-numbers. However, the large and intermediate Q-spectra survives till wave-numbers less than or equal to twice the used velocity cutoff wave-numbers. It is found that the extracted central axes clearly resemble the corresponding vortical structures at each scale. Using the central axes scheme, the radii and lengths of the vortical structures at each scale are determined and compared. It is also found that the radii of the identified vortical structures at each scale in both simulations are of the order of several times the Kolmogorov microscales.
Uhlmann, Markus
2016-01-01
We have performed interface-resolved direct numerical simulations of forced homogeneous-isotropic turbulence in a dilute suspension of spherical particles in the Reynolds number range Re-lambda=115-140. The solid-fluid density ratio was set to 1.5, gravity was set to zero, and two particle diameters were investigated corresponding to approximately 5 and 11 Kolmogorov lengths. Note that these particle sizes are clearly outside the range of validity of the point-particle approximation, as has been shown by Homann & Bec (2010). At the present parameter points the global effect of the particles upon the fluid flow is weak. We observe that the dispersed phase exhibits clustering with moderate intensity. The tendency to cluster, which was quantified in terms of the standard deviation of Voronoi cell volumes, decreases with the particle diameter. We have analyzed the relation between particle locations and the location of intense vortical flow structures. The results do not reveal any significant statistical cor...
Chrystie, Robin
2013-01-02
Temperature measurements have been taken on an acoustically forced lean premixed turbulent bluff-body stabilized flame. The burner used in this study is a test-bed to investigate thermoacoustic instability in gas-turbine engines at the University of Cambridge. Numerous experiments have been performed on the burner, one of which used two-line OH planar laser induced fluorescence to measure temperature. Here, we employ vibrational coherent anti-Stokes Raman scattering (CARS) of nitrogen as an alternative to measure temperature, circumventing the limitations of the former method. The use of nitrogen CARS avoids the problem of probing regions of the flame with low OH concentrations that resulted in erroneous temperature. Such an application of CARS showed that the results from previous efforts were systematically biased up to 47% close to the bluff-body. We also critically review the limitations of CARS used in our experiments, pertaining to spatial resolution and associated biasing further downstream from the bluff-body. Using the more accurate results from this work, more up-to-date computational fluid dynamical (CFD) models of the burner can be validated, with the aim of improved understanding and prediction of thermoacoustic instability in gas turbines. © 2013 Copyright Taylor and Francis Group, LLC.
TURBULENT TRANSPORT IN A STRONGLY STRATIFIED FORCED SHEAR LAYER WITH THERMAL DIFFUSION
Energy Technology Data Exchange (ETDEWEB)
Garaud, Pascale [Department of Applied Mathematics and Statistics, Baskin School of Engineering, University of California at Santa Cruz, 1156 High Street, Santa Cruz CA 95064 (United States)
2016-04-10
This work presents numerical results on the transport of heat and chemical species by shear-induced turbulence in strongly stratified, thermally diffusive environments. The shear instabilities driven in this regime are sometimes called “secular” shear instabilities, and can take place when the Richardson number of the flow is large, provided the Péclet number is small. We have identified a set of simple criteria to determine whether these instabilities can take place or not. Generally speaking, we find that they may be relevant whenever the thermal diffusivity of the fluid is very large (typically larger than 10{sup 14} cm{sup 2} s{sup −1}), which is the case in the outer layers of high-mass stars (M ≥ 10 M{sub ⊙}), for instance. Using a simple model setup in which the shear is forced by a spatially sinusoidal, constant-amplitude body-force, we have identified several regimes ranging from effectively unstratified to very strongly stratified, each with its own set of dynamical properties. Unless the system is in one of the two extreme regimes (effectively unstratified or completely stable), however, we find that (1) only about 10% of the input power is used toward heat transport, while the remaining 90% is viscously dissipated; (2) that the effective compositional mixing coefficient is well-approximated by the model of Zahn, with D ≃ 0.02κ{sub T}/J where κ{sub T} is the thermal diffusivity and J is the Richardson number. These results need to be confirmed, however, with simulations in different model setups and at higher effective Reynolds number.
Variability of radiatively forced diurnal cycle of intense convection in the tropical west pacific
Energy Technology Data Exchange (ETDEWEB)
Gray, W.M.; Sheaffer, J.D.; Thorson, W.B. [Colorado State Univ., Fort Collins, CO (United States)
1996-04-01
Strong differences occur in daytime versus nighttime (DVN) net radiative cooling in clear versus cloudy areas of the tropical atmosphere. Daytime average cooling is approximately -0.7{degrees}C/day, whereas nighttime net tropospheric cooling rates are about -1.5{degrees}C/day, an approximately two-to-one difference. The comparatively strong nocturnal cooling in clear areas gives rise to a diurnally varying vertical circulation and horizontal convergence cycle. Various manifestations of this cyclic process include the observed early morning heavy rainfall maxima over the tropical oceans. The radiatively driven DVN circulation appears to strongly modulate the resulting diurnal cycle of intense convection which creates the highest, coldest cloudiness over maritime tropical areas and is likely a fundamental mechanism governing both small and large scale dynamics over much of the tropical environment.
Forced convection to laminar flow of liquid egg yolk in circular and annular ducts
Directory of Open Access Journals (Sweden)
M. Bernardi
2009-06-01
Full Text Available The steady-state heat transfer in laminar flow of liquid egg yolk - an important pseudoplastic fluid food - in circular and concentric annular ducts was experimentally investigated. The average convection heat transfer coefficients, determined by measuring temperatures before and after heating sections with constant temperatures at the tube wall, were used to obtain simple new empirical expressions to estimate the Nusselt numbers for fully established flows at the thermal entrance of the considered geometries. The comparisons with existing correlations for Newtonian and non-Newtonian fluids resulted in excellent agreement. The main contribution of this work is to supply practical and easily applicable correlations, which are, especially for the case of annulus, rather scarce and extensively required in the design of heat transfer operations dealing with similar shear-thinning products. In addition, the experimental results may support existing theoretical analyses.
Jakkareddy, Pradeep S.; Balaji, C.
2017-02-01
This paper reports the results of an experimental study to estimate the heat flux and convective heat transfer coefficient using liquid crystal thermography and Bayesian inference in a heat generating sphere, enclosed in a cubical Teflon block. The geometry considered for the experiments comprises a heater inserted in a hollow hemispherical aluminium ball, resulting in a volumetric heat generation source that is placed at the center of the Teflon block. Calibrated thermochromic liquid crystal sheets are used to capture the temperature distribution at the front face of the Teflon block. The forward model is the three dimensional conduction equation which is solved within the Teflon block to obtain steady state temperatures, using COMSOL. Match up experiments are carried out for various velocities by minimizing the residual between TLC and simulated temperatures for every assumed loss coefficient, to obtain a correlation of average Nusselt number against Reynolds number. This is used for prescribing the boundary condition for the solution to the forward model. A surrogate model obtained by artificial neural network built upon the data from COMSOL simulations is used to drive a Markov Chain Monte Carlo based Metropolis Hastings algorithm to generate the samples. Bayesian inference is adopted to solve the inverse problem for determination of heat flux and heat transfer coefficient from the measured temperature field. Point estimates of the posterior like the mean, maximum a posteriori and standard deviation of the retrieved heat flux and convective heat transfer coefficient are reported. Additionally the effect of number of samples on the performance of the estimation process has been investigated.
Jakkareddy, Pradeep S.; Balaji, C.
2016-05-01
This paper reports the results of an experimental study to estimate the heat flux and convective heat transfer coefficient using liquid crystal thermography and Bayesian inference in a heat generating sphere, enclosed in a cubical Teflon block. The geometry considered for the experiments comprises a heater inserted in a hollow hemispherical aluminium ball, resulting in a volumetric heat generation source that is placed at the center of the Teflon block. Calibrated thermochromic liquid crystal sheets are used to capture the temperature distribution at the front face of the Teflon block. The forward model is the three dimensional conduction equation which is solved within the Teflon block to obtain steady state temperatures, using COMSOL. Match up experiments are carried out for various velocities by minimizing the residual between TLC and simulated temperatures for every assumed loss coefficient, to obtain a correlation of average Nusselt number against Reynolds number. This is used for prescribing the boundary condition for the solution to the forward model. A surrogate model obtained by artificial neural network built upon the data from COMSOL simulations is used to drive a Markov Chain Monte Carlo based Metropolis Hastings algorithm to generate the samples. Bayesian inference is adopted to solve the inverse problem for determination of heat flux and heat transfer coefficient from the measured temperature field. Point estimates of the posterior like the mean, maximum a posteriori and standard deviation of the retrieved heat flux and convective heat transfer coefficient are reported. Additionally the effect of number of samples on the performance of the estimation process has been investigated.
Baerenzung, J; Mininni, P D; Pouquet, A
2009-01-01
We present a study of spectral laws for helical turbulence in the presence of solid body rotation up to Reynolds numbers Re~1*10^5 and down to Rossby numbers Ro~3*10^-3. The forcing function is a fully helical flow that can also be viewed as mimicking the effect of atmospheric convective motions. We test in the helical case variants of a model developed previously (Baerenzung et al. 2008a) against direct numerical simulations (DNS), using data from a run on a grid of 15363 points; we also contrast its efficiency against a spectral Large Eddy Simulation (LES) (Chollet and Lesieur 1981) as well as an under-resolved DNS. The model including the contribution of helicity to the spectral eddy dissipation and eddy noise behaves best, allowing to recover statistical features of the flow. An exploration of parameter space is then performed beyond what is feasible today using DNS. At fixed Reynolds number, lowering the Rossby number leads to a regime of wave-mediated inertial helicity cascade to small scales. However, ...
Turbulence and turbulent mixing in natural fluids
Gibson, Carl H
2010-01-01
Turbulence and turbulent mixing in natural fluids begins with big bang turbulence powered by spinning combustible combinations of Planck particles and Planck antiparticles. Particle prograde accretion on a spinning pair releases 42% of the particle rest mass energy to produce more fuel for turbulent combustion. Negative viscosity and negative turbulence stresses work against gravity, creating mass-energy and space-time from the vacuum. Turbulence mixes cooling temperatures until a quark-gluon strong-force SF freeze-out. Gluon-viscosity anti-gravity ({\\Lambda}SF) exponentially inflates the fireball to preserve big bang turbulence information at scales larger than ct as the first fossil turbulence. Cosmic microwave background CMB temperature anisotropies show big bang turbulence fossils along with fossils of weak plasma turbulence triggered (10^12 s) as plasma viscous forces permit gravitational fragmentation on supercluster to galaxy mass scales (10^13 s). Turbulent morphologies and viscous-turbulent lengths a...
NATO Advanced Study Institute on Buoyant Convection in Geophysical Flows
Fedorovich, E; Viegas, D; Wyngaard, J
1998-01-01
Studies of convection in geophysical flows constitute an advanced and rapidly developing area of research that is relevant to problems of the natural environment. During the last decade, significant progress has been achieved in the field as a result of both experimental studies and numerical modelling. This led to the principal revision of the widely held view on buoyancy-driven turbulent flows comprising an organised mean component with superimposed chaotic turbulence. An intermediate type of motion, represented by coherent structures, has been found to play a key role in geophysical boundary layers and in larger scale atmospheric and hydrospheric circulations driven by buoyant forcing. New aspects of the interaction between convective motions and rotation have recently been discovered and investigated. Extensive experimental data have also been collected on the role of convection in cloud dynamics and microphysics. New theoretical concepts and approaches have been outlined regarding scaling and parameteriz...
The effects of outward forced convective flow on inward diffusion of potassium across human dentin.
Pashley, David H; Agee, Kelli; Zhang, Yi; Smith, Anita; Tavss, Edward A; Gambogi, Robert J
2002-08-01
To measure the inward flux of potassium across human dentin disks, in vitro, in the absence and then in the presence of a simulated pulpal pressure (PP), before and after brushing with two desensitizing dentifrices. Dentin discs were made from extracted unerupted third molars and etched with 6% citric acid to remove the smear layers. The hydraulic conductance (Lp or outward fluid movement) of the discs was measured before and after brushing for 2 minutes with two desensitizing dentifrices, Colgate Sensitive Maximum Strength or Sensodyne Fresh Mint, both dentifrices containing 5% potassium nitrate. The potassium flux was measured at a pressure of 0 cm H2O, and a simulated pulpal pressure of 20 cm H2O. The results showed the Colgate-brushed specimens had lower Lp (P Sensodyne or with saline. The Colgate-brushed specimens had a corresponding lower K+ flux, at a PP = 0 cm H2O, than specimens brushed with saline, but were not significantly different than specimens brushed with Sensodyne dentifrice. However, when the pressure was increased from 0 to 20 cm H2O, to simulate outward dentin fluid pressure, there were significant decreases in K+ flux across all three treatment groups, but the K+ flux of the Colgate-brushed specimens were affected the least, resulting in the Colgate-brushed specimens having significantly (P Sensodyne dentifrice-brushed groups. The magnitude of reduction in K+ flux in going from 0 cm H2O to 20 cm H2O was demonstrated to be linearly related to the magnitude of the Lp of the brushed specimens. Hence, a small Lp corresponded to a small reduction in K+ flux, and a correspondingly large K+ flux at 20 cm H2O. An explanation of this phenomenon is that a decrease in the Lp of dentin, produced by brushing, corresponds to a decreased outward dentin fluid flow. This lowers the outward K+ convective flow and also lowers the inward K+ diffusion. However, since the decrease of the outward K+ convective flow is greater than the decrease of the inward K
A zonally symmetric model for the monsoon-Hadley circulation with stochastic convective forcing
De La Chevrotière, Michèle; Khouider, Boualem
2016-09-01
Idealized models of reduced complexity are important tools to understand key processes underlying a complex system. In climate science in particular, they are important for helping the community improve our ability to predict the effect of climate change on the earth system. Climate models are large computer codes based on the discretization of the fluid dynamics equations on grids of horizontal resolution in the order of 100 km, whereas unresolved processes are handled by subgrid models. For instance, simple models are routinely used to help understand the interactions between small-scale processes due to atmospheric moist convection and large-scale circulation patterns. Here, a zonally symmetric model for the monsoon circulation is presented and solved numerically. The model is based on the Galerkin projection of the primitive equations of atmospheric synoptic dynamics onto the first modes of vertical structure to represent free tropospheric circulation and is coupled to a bulk atmospheric boundary layer (ABL) model. The model carries bulk equations for water vapor in both the free troposphere and the ABL, while the processes of convection and precipitation are represented through a stochastic model for clouds. The model equations are coupled through advective nonlinearities, and the resulting system is not conservative and not necessarily hyperbolic. This makes the design of a numerical method for the solution of this system particularly difficult. Here, we develop a numerical scheme based on the operator time-splitting strategy, which decomposes the system into three pieces: a conservative part and two purely advective parts, each of which is solved iteratively using an appropriate method. The conservative system is solved via a central scheme, which does not require hyperbolicity since it avoids the Riemann problem by design. One of the advective parts is a hyperbolic diagonal matrix, which is easily handled by classical methods for hyperbolic equations, while
A zonally symmetric model for the monsoon-Hadley circulation with stochastic convective forcing
De La Chevrotière, Michèle; Khouider, Boualem
2017-02-01
Idealized models of reduced complexity are important tools to understand key processes underlying a complex system. In climate science in particular, they are important for helping the community improve our ability to predict the effect of climate change on the earth system. Climate models are large computer codes based on the discretization of the fluid dynamics equations on grids of horizontal resolution in the order of 100 km, whereas unresolved processes are handled by subgrid models. For instance, simple models are routinely used to help understand the interactions between small-scale processes due to atmospheric moist convection and large-scale circulation patterns. Here, a zonally symmetric model for the monsoon circulation is presented and solved numerically. The model is based on the Galerkin projection of the primitive equations of atmospheric synoptic dynamics onto the first modes of vertical structure to represent free tropospheric circulation and is coupled to a bulk atmospheric boundary layer (ABL) model. The model carries bulk equations for water vapor in both the free troposphere and the ABL, while the processes of convection and precipitation are represented through a stochastic model for clouds. The model equations are coupled through advective nonlinearities, and the resulting system is not conservative and not necessarily hyperbolic. This makes the design of a numerical method for the solution of this system particularly difficult. Here, we develop a numerical scheme based on the operator time-splitting strategy, which decomposes the system into three pieces: a conservative part and two purely advective parts, each of which is solved iteratively using an appropriate method. The conservative system is solved via a central scheme, which does not require hyperbolicity since it avoids the Riemann problem by design. One of the advective parts is a hyperbolic diagonal matrix, which is easily handled by classical methods for hyperbolic equations, while
Raskutti, Sudhir; Ostriker, Eve C.; Skinner, M. Aaron
2016-10-01
Radiation feedback from stellar clusters is expected to play a key role in setting the rate and efficiency of star formation in giant molecular clouds. To investigate how radiation forces influence realistic turbulent systems, we have conducted a series of numerical simulations employing the Hyperion radiation hydrodynamics solver, considering the regime that is optically thick to ultraviolet and optically thin to infrared radiation. Our model clouds cover initial surface densities between Σ cl,0∼ 10--300 M⊙ pc-2, with varying initial turbulence. We follow them through turbulent, self-gravitating collapse, star cluster formation, and cloud dispersal by stellar radiation. All our models display a log-normal distribution of gas surface density Σ for an initial virial parameter αvir,0=2, the log-normal standard deviation is σln Σ =1-1.5 and the star formation rate coefficient ɛff,ρ=0.3-0.5, both of which are sensitive to turbulence but not radiation feedback. The net star formation efficiency (SFE) ɛfinal increases with Σcl,0 and decreases with α vir,0. We interpret these results via a simple conceptual framework, whereby steady star formation increases the radiation force, such that local gas patches at successively higher Σ become unbound. Based on this formalism (with fixed σln Σ), we provide an analytic upper bound on ɛfinal, which is in good agreement with our numerical results. The final SFE depends on the distribution of Eddington ratios in the cloud and is strongly increased by the turbulent compression of gas.
Numerical simulation of laminar forced convection of water-CuO nanofluid inside a triangular duct
Aghanajafi, Amir; Toghraie, Davood; Mehmandoust, Babak
2017-01-01
In this article, distilled water and CuO particles with volume fraction of 1%, 2% and 4% are numerically studied. The steady state flow regime is considered laminar with Reynolds number of 100, and nano-particles diameters are assumed 20 nm and 80 nm. The hydraulic diameter and the length of equilateral triangular channel are 8 mm and 1000 mm, respectively. The problem is solved for two different boundary conditions; firstly, constant heat flux for all sides as a validation approach; and secondly, constant heat flux for two sides and constant temperature for one side (hot plate). Convective heat transfer coefficient, Nusselt number, pressure loss through the channel, velocity distribution in cross section and temperature distribution on walls are investigated in detail. The fluid flow is supposed to be one-phase flow. It can be observed that nano-fluid leads to a remarkable enhancement on heat transfer coefficient. Furthermore, CuO particles increase pressure loss through the channel and velocity distribution in fully developed cross section of channel, as well. The computations reveal that the size of nano-particles has no significant influence on heat transfer properties. Besides, the study shows a good agreement between provided outcomes and experimental data available in the literature.
Energy Technology Data Exchange (ETDEWEB)
Aydin, Orhan; Avci, Mete [Karadeniz Technical University, Trabzon (Turkey). Department of Mechanical Engineering
2006-08-15
In this study, analytical solutions are obtained to predict laminar heat-convection in a Couette-Poiseuille flow between two plane parallel plates with a simultaneous pressure gradient and an axial movement of the upper plate. A Newtonian fluid with constant properties is considered with an emphasis on the viscous-dissipation effect. Both hydrodynamically and thermally fully-developed flow cases are investigated. The axial heat-conduction in the fluid is neglected. Two different orientations of the thermal boundary-conditions are considered: the constant heat-flux at the upper plate with an adiabatic lower plate (Case A) and the constant heat-flux at the lower plate with an adiabatic upper plate (Case B). For different values of the relative velocity of the upper plate, the effect of the modified Brinkman number on the temperature distribution and the Nusselt number are discussed. Comparison of the present analytical results for a special case with those available in the literature indicates an excellent agreement. (author)
Institute of Scientific and Technical Information of China (English)
WangShiping; ZhouXinqiu; 等
1995-01-01
In this paper,condensation performance in horizontal annulus was compared with a smooth tube,one Saw-Tooth Finned tube(STF tube),four Petal Shaped Fin tubes(PF tubes),using R113,R11 and their mixtures(vapor molar fractions of R11 at the test section inlet were 0.384,0.588and 0.809) as working fluid.The mass flux at the test section ranged from 15-220/m2s。Camera and video camera were used to shoot the flow pattern and condensation phenomena.The condensation transfer coefficient(hc) of mixtures were considerably lower than those of pure fluid,and did not change linearly with composition.The maximum degradation of measured hc from the ideal value were 23% for the smooth tube,65%for STF tube,67% for PF tubes,which occurred in the composition range of 0.4-0.6 vapor molar fraction of R11.For the condensation of mixture,R11 molar fraction from 38%to 81%,the PF tubes had the highest value of hc.which were 10-25% higher than those of STF tubes,and 480-580% higher than that of smooth tube,because the petal shaped fins of PF tubes could promote strong turbulence in the two phase flow,and reduce the mass transfer resistance.
Influence of forced convection on solidification and remelting in the developing mushy zone
Wu, M.; Vakhrushev, A.; Ludwig, A.; Kharicha, A.
2016-03-01
The mushy zone and solid shell formed during solidification of a continuous casting are mostly uneven, and this unevenness of shell growth might lead to surface defects or breakout. One known example is the unevenness of shell growth at the impingement point between the jet flow (coming from submerged entry nozzle) and the solidification front. This phenomenon is primarily understood as the local remelting caused by the superheat of the melt, which is continuously brought by the jet flow towards the solidification front. A recent study of the authors [Metall. Mater. Trans. B, 2014, in press] hinted that, in addition to the aforementioned superheat-induced local remelting (1), two other factors also affect the shell growth. They are (2) the advection of latent heat in the semi-solid mushy zone and (3) the enhanced dissipation rate of energy by turbulence in the bulk-mush transition region. This paper is going to perform a detailed numerical analysis to gain an insight into the flow-solidification interaction phenomena. Contributions of each of the above factors to the shell formation are compared.
Institute of Scientific and Technical Information of China (English)
WU Chunqiang; ZHOU Tianjun; SUN De-Zheng; BAO Qing
2011-01-01
Characteristics of the total clear-sky greenhouse effect (GA) and cloud radiative forcings (CRFs), along with the radiative-related water vapor and cloud properties simulated by the Spectral Atmospheric Model developed by LASG/IAP (SAMIL) are evaluated. Impacts of the convection scheme on the simulation of CRFs are discussed by using two AMIP (Atmospheric Model Inter-comparison Project) type simulations employing different convection schemes: the new Zhang-McFarlane (NZH) and Tiedtke (TDK) convection schemes. It shows that both the climatological GA and its response to El Nifio warming are simulated well, both in terms of spatial pattern and magnitude. The impact of the convection scheme on GA is not significant. The climatological longwav e CRF (LWCRF) and its response to El Nino warming are simulated well, but with a prominently weaker magnitude. The simulation of the climatology (response) of LWCRF in the NZH (TDK) run is slightly more realistic than in the TDK (NZH) simulation, indicating significant impacts of the convection scheme. The shortwave CRF (SWCRF) shows large biases in both spatial pattern and magnitude, and the results from the TDK run are better than those from the NZH run. A spuriously excessive negative climatological SWCRF over the southeastern Pacific and an insufficient response of SWCRF to El Nifio warming over the tropical Pacific are seen in the NZH run. These two biases are alleviated in the TDK run, since it produces vigorous convection, which is related to the low threshold for convection to take place. Also, impacts of the convection scheme on the cloud profile are discussed.
Urban, Pavel; Hanzelka, Pavel; Musilová, Věra; Králík, Tomáš; La Mantia, Marco; Srnka, Aleš; Skrbek, Ladislav
2014-05-01
We present experimental results on the heat transfer efficiency of cryogenic turbulent Rayleigh-Bénard convection (RBC) in a cylindrical cell 0.3 m in both diameter and height which has improvements with respect to various corrections connected with finite thermal conductivity of sidewalls and plates. The heat transfer efficiency described by the Nusselt number {\\rm{Nu}}={\\rm{Nu}}({\\rm{Ra}},Pr ) is investigated for the range of Rayleigh number {{10}^{6}}account. In contrast, if the mean temperature is determined as an arithmetic mean of the bottom and top plate temperatures, {\\rm{Nu}}({\\rm{Ra}})\\propto {\\rm{R}}{{{\\rm{a}}}^{\\gamma }} displays spurious crossover to higher γ that might be misinterpreted as a transition to the ultimate Kraichnan regime. The second step of our analysis, reported here for the first time, is to ignore the NOB effects affecting the top half of the RBC cell. We replace it by the inverted nearly OB bottom half in order to eliminate the boundary layer asymmetry. This leads to the effective temperature difference \\Delta {{T}_{{\\rm{eff}}}}=2({{T}_{{\\rm{b}}}}-{{T}_{{\\rm{c}}}}), where {{T}_{{\\rm{b}}}} denotes the bottom plate temperature, and to effective {\\rm{N}}{{{\\rm{u}}}_{{\\rm{eff}}}} and {\\rm{R}}{{{\\rm{a}}}_{{\\rm{eff}}}} values. The effective heat transfer efficiency obtained, showing no tendency of crossover to the ultimate regime up to 2\\times {{10}^{15}} in {\\rm{R}}{{{\\rm{a}}}_{{\\rm{eff}}}}, is reported and discussed.
Convective forcing of mercury and ozone in the Arctic boundary layer induced by leads in sea ice.
Moore, Christopher W; Obrist, Daniel; Steffen, Alexandra; Staebler, Ralf M; Douglas, Thomas A; Richter, Andreas; Nghiem, Son V
2014-02-06
The ongoing regime shift of Arctic sea ice from perennial to seasonal ice is associated with more dynamic patterns of opening and closing sea-ice leads (large transient channels of open water in the ice), which may affect atmospheric and biogeochemical cycles in the Arctic. Mercury and ozone are rapidly removed from the atmospheric boundary layer during depletion events in the Arctic, caused by destruction of ozone along with oxidation of gaseous elemental mercury (Hg(0)) to oxidized mercury (Hg(II)) in the atmosphere and its subsequent deposition to snow and ice. Ozone depletion events can change the oxidative capacity of the air by affecting atmospheric hydroxyl radical chemistry, whereas atmospheric mercury depletion events can increase the deposition of mercury to the Arctic, some of which can enter ecosystems during snowmelt. Here we present near-surface measurements of atmospheric mercury and ozone from two Arctic field campaigns near Barrow, Alaska. We find that coastal depletion events are directly linked to sea-ice dynamics. A consolidated ice cover facilitates the depletion of Hg(0) and ozone, but these immediately recover to near-background concentrations in the upwind presence of open sea-ice leads. We attribute the rapid recoveries of Hg(0) and ozone to lead-initiated shallow convection in the stable Arctic boundary layer, which mixes Hg(0) and ozone from undepleted air masses aloft. This convective forcing provides additional Hg(0) to the surface layer at a time of active depletion chemistry, where it is subject to renewed oxidation. Future work will need to establish the degree to which large-scale changes in sea-ice dynamics across the Arctic alter ozone chemistry and mercury deposition in fragile Arctic ecosystems.
Enhanced heat transfer of forced convective fin flow with transverse ribs
Energy Technology Data Exchange (ETDEWEB)
Chang, Shyy Woei; Chiou, Shyr Fuu [Thermal Fluids Laboratory, National Kaohsiung Institute of Marine Technology, No. 142, Hai-Chuan Road, Nan-Tzu District, Post code 811, Kaohsiung, Taiwan (China); Su, Lo May [Department of Electrical Engineering, Tung Fang Institute of Technology, Taiwan (China); Yang, Tsun Lirng [Department of Electrical Engineering, Fortune Institute of Technology, Taiwan (China)
2004-02-01
This experimental study investigates the heat transfers in three side-opened and bottom-sealed rectangular channels with two opposite walls roughened by 90 staggered ribs, which simulate the enhanced cooling passages in the fin-type heat sinks of electronic chip-sets. The various degrees of interactive effects due to the surface ribs, side-profile leakage flows and stream-wise weakened coolant flow are functionally related with Reynolds number (Re) and channel length-to-gap ratio (L/B), which unravel the considerable impacts on local and spatially averaged heat transfers over the rib-roughened fin surfaces. A selection of detailed heat transfer measurements over the rib-roughened fin surfaces illustrates the manner by which the isolated and interactive influences of Re and L/B-ratio affect the local and spatially averaged heat transfers. Relative to the heat transfer results acquired from the smooth-walled test channels, the augmentations of spatially averaged heat transfers generated by the present surface ribs are in the range of 140-200% of the flat fin reference levels. In conformity with the experimentally revealed heat transfer physics, a regression-type analysis is performed to develop the correlation of spatially-averaged Nusselt number over rib-roughened fin surface, which permits the individual and interactive effect of Re and L/B on heat transfer to be evaluated. A criterion for selecting the optimal length-to-gap ratio of a fin channel, which provides the maximum convective heat flux from the rib-roughened fin surface, is formulated as an engineering tool to assist the design activity for the cooling device of electronic chip-sets. (authors)
Directory of Open Access Journals (Sweden)
Sundus Hussein Abd
2012-01-01
Full Text Available In this research, an experimental study was conducted to high light the impact of the exterior shape of a cylindrical body on the forced and free convection heat transfer coefficients when the body is hold in the entrance of an air duct. The impact of changing the body location within the air duct and the air speed are also demonstrated. The cylinders were manufactured with circular, triangular and square sections of copper for its high thermal conductivity with appropriate dimensions, while maintaining the surface area of all shapes to be the same. Each cylinder was heated to a certain temperature and put inside the duct at certain locations. The temperature of the cylinder was then monitored. The heat transfer coefficient were then calculated for forced convection for several Reynolds number (4555-18222.The study covered free convection impact for values of Rayleigh number ranging between (1069-3321. Imperical relationships were obtained for all cases of forced and free convection and compared with equations of circular cylindrical shapes found in literature. These imperical equations were found to be in good comparison with that of other sources.
Direct and inverse energy cascades in a forced rotating turbulence experiment
Campagne, Antoine; Moisy, Frédéric; Cortet, Pierre-Philippe
2014-01-01
We present experimental evidence for a double cascade of kinetic energy in a statistically stationary rotating turbulence experiment. Turbulence is generated by a set of vertical flaps which continuously injects velocity fluctuations towards the center of a rotating water tank. The energy transfers are evaluated from two-point third-order three-component velocity structure functions, which we measure using stereoscopic particle image velocimetry in the rotating frame. Without global rotation, the energy is transferred from large to small scales, as in classical three-dimensional turbulence. For nonzero rotation rates, the horizontal kinetic energy presents a double cascade: a direct cascade at small horizontal scales and an inverse cascade at large horizontal scales. By contrast, the vertical kinetic energy is always transferred from large to small horizontal scales, a behavior reminiscent of the dynamics of a passive scalar in two-dimensional turbulence. At the largest rotation rate the flow is nearly two-di...
Ferreira, Pedro O.; Pinho, Fernando T.; da Silva, Carlos B.
2016-12-01
A new subgrid-scale (SGS) model developed for large-eddy simulations (LES) of dilute polymer solutions, described by the finitely extensible nonlinear elastic constitutive equation closed with the Peterlin approximation, is presented. In this distortion similarity model (DSIM) the filtered conformation tensor evolution equation is based on the self-similarity of the polymer stretching terms, and on a global equilibrium of the trace of the conformation tensor, which is proportional to the elastic energy stored in the polymer molecules, while the SGS stresses are modelled with the classical Smagorinsky model. The DSIM closure is assessed in direct numerical simulations (DNS) of forced isotropic turbulence using classical a priori tests, and in a posteriori (LES) showing very good agreement with all the exact (filtered DNS) results. The DSIM model is simple to implement and computationally inexpensive and represents a major step forward in the numerical simulation of turbulent flows of Newtonian fluids with polymer additives.
Forced convection and transport effects during hyperbaric laser chemical vapor deposition
Energy Technology Data Exchange (ETDEWEB)
Maxwell, James L [Los Alamos National Laboratory; Chavez, Craig A [Los Alamos National Laboratory; Espinoza, Miguel [Los Alamos National Laboratory; Black, Marcie [Los Alamos National Laboratory; Maskaly, Karlene [Los Alamos National Laboratory; Boman, Mats [UPPSALA UNIV
2009-01-01
This work explores mass transport processes during HP-LCYD, including the transverse forced-flow of precursor gases through a nozzle to enhance fiber growth rates. The use of laser trapping and suspension of nano-scale particles in the precursor flow is also described, providing insights into the nature of the gas flow, including jetting from the fiber tip and thermodiffusion processes near the reaction zone. The effects of differing molecular-weight buffer gases is also explored in conjunction with the Soret effect, and it is found that nucleation at the deposit surface (and homogeneous nucleation in the gas phase) can be enhanced/ retarded, depending on the buffer gas molecular weight. To demonstrate that extensive microstructures can be grown simultaneously, three-dimensional fiber arrays are also grown in-parallel using diffractive optics--without delatory effects from neighboring reaction sites.
Turbulence and diffusion fossil turbulence
Gibson, C H
2000-01-01
Fossil turbulence processes are central to turbulence, turbulent mixing, and turbulent diffusion in the ocean and atmosphere, in astrophysics and cosmology, and in most other natural flows. George Gamov suggested in 1954 that galaxies might be fossils of primordial turbulence produced by the Big Bang. John Woods showed that breaking internal waves on horizontal dye sheets in the interior of the stratified ocean form highly persistent remnants of these turbulent events, which he called fossil turbulence. The dark mixing paradox of the ocean refers to undetected mixing that must exist somewhere to explain why oceanic scalar fields like temperature and salinity are so well mixed, just as the dark matter paradox of galaxies refers to undetected matter that must exist to explain why rotating galaxies don't fly apart by centrifugal forces. Both paradoxes result from sampling techniques that fail to account for the extreme intermittency of random variables involved in self-similar, nonlinear, cascades over a wide ra...
Directory of Open Access Journals (Sweden)
Nemat Dalir
2014-12-01
Full Text Available Entropy generation for the steady two-dimensional laminar forced convection flow and heat transfer of an incompressible Jeffrey non-Newtonian fluid over a linearly stretching, impermeable and isothermal sheet is numerically investigated. The governing differential equations of continuity, momentum and energy are transformed using suitable similarity transformations to two nonlinear coupled ordinary differential equations (ODEs. Then the ODEs are solved by applying the numerical implicit Keller’s box method. The effects of various parameters of the flow and heat transfer including Deborah number, ratio of relaxation to retardation times, Prandtl number, Eckert number, Reynolds number and Brinkman number on dimensionless velocity, temperature and entropy generation number profiles are analyzed. The results reveal that the entropy generation number increases with the increase of Deborah number while the increase of ratio of relaxation to retardation times causes the entropy generation number to reduce. A comparative study of the numerical results with the results from an exact solution for the dimensionless velocity gradient at the sheet surface is also performed. The comparison shows excellent agreement within 0.05% error.
Institute of Scientific and Technical Information of China (English)
李明春; 赵中亮; 静宇; 刘家涛; 吴玉胜
2013-01-01
To study the influence of the Soret and Dufour effects on the reactive characteristics of a porous packed bed with endothermic reactions and forced convection, a two-dimensional mathematical model considering the cross-diffusion effects was developed in accordance with the thermodynamics of irreversible processes and the lo-cal thermal non-equilibrium model. The simulation results were validated by comparing with experimental data. The influence of the Soret and Dufour effects on the heat transfer, mass transfer and endothermic chemical reaction in the non-thermal equilibrium packed bed is discussed. It was found that when the Peclet number reaches 1865, the maximum relative error of the concentration of gas product induced by the Soret effect is 34.7% and that of the solid fractional conversion caused by the Dufour effect is 10.8%at reaction time 160 s and initial temperature 1473 K. The differences induced by the Soret and Dufour effects are demonstrated numerically to increase gradually with the initial temperature of feeding gas and the Peclet number.
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Amnart Boonloi
2014-09-01
Full Text Available Thermal performance analysis for laminar forced convection in an isothermal wall square channel with 30° V-baffle is presented numerically. The parameters of the V-baffle, blockage ratio (b/H, BR, pitch ratio (P/H, PR, flow direction (V-Downstream and V-Upstream, and arrangement (in-line and staggered, are studied and compared with the previous works, 20° and 45° V-baffle. The Reynolds number based on the hydraulic diameter of the channel (Dh, Re = 100–2000, is used in range study. The results show that the flow configurations of 30° V-baffle are found similar as 20° and 45° V-baffle. The fully developed periodic flow and heat transfer are created around 7th-8th module, while the periodic flow and heat transfer profiles are found at 2nd module in all cases. Except for the periodic concept, the 30° V-baffle can help to reduce the pressure loss around 2.3 times in comparison with the 45° V-baffle at the maximum f/f0 value (BR = 0.3, PR = 1, V-Downstream. The optimum thermal enhancement factor for the 30° V-baffle is found around 4.25 at BR = 0.15, PR = 1, and Re = 2000 for V-Downstream case with in-line arrangement.
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Moh'd A. Al-Nimr
2004-06-01
Full Text Available Magnetic field effect on local entropy generation due to steady two-dimensional laminar forced convection flow past a horizontal plate was numerically investigated. This study was focused on the entropy generation characteristics and its dependency on various dimensionless parameters. The effect of various dimensionless parameters, such as Hartmann number (Ha, Eckert number (Ec, Prandtl number (Pr, Joule heating parameter (R and the free stream temperature parameter (ÃŽÂ¸Ã¢ÂˆÂž on the entropy generation characteristics is analyzed. The dimensionless governing equations in Cartesian coordinate were solved by an implicit finite difference technique. The solutions were carried out for Ha2=0.5-3, Ec=0.01-0.05, Pr=1-5 and ÃŽÂ¸Ã¢ÂˆÂž=1.1-2.5. It was found that, the entropy generation increased with increasing Ha, Ec and R. While, increasing the free stream temperature parameter, and Prandtl number tend to decrease the local entropy generation.
Directory of Open Access Journals (Sweden)
Hsien-Hung Ting
2015-01-01
Full Text Available This numerical study is aimed at investigating the forced convection heat transfer and flow characteristics of water-based Al2O3 nanofluids inside a horizontal circular tube in the laminar flow regime under the constant wall temperature boundary condition. Five volume concentrations of nanoparticle, 0.1, 0.5, 1, 1.5, and 2 vol.%, are used and diameter of nanoparticle is 40 nm. Characteristics of heat transfer coefficient, Nusselt number, and pressure drop are reported. The results show that heat transfer coefficient of nanofluids increases with increasing Reynolds number or particle volume concentration. The heat transfer coefficient of the water-based nanofluid with 2 vol.% Al2O3 nanoparticles is enhanced by 32% compared with that of pure water. Increasing particle volume concentration causes an increase in pressure drop. At 2 vol.% of particle concentration, the pressure drop reaches a maximum that is nearly 5.7 times compared with that of pure water. It is important to note that the numerical results are in good agreement with published experimental data.
A Split Forcing Technique to Reduce Log-layer Mismatch in Wall-modeled Turbulent Channel Flows
Deleon, Rey; Senocak, Inanc
2016-11-01
The conventional approach to sustain a flow field in a periodic channel flow seems to be the culprit behind the log-law mismatch problem that has been reported in many studies hybridizing Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulation (LES) techniques, commonly referred to as hybrid RANS-LES. To address this issue, we propose a split-forcing approach that relies only on the conservation of mass principle. We adopt a basic hybrid RANS-LES technique on a coarse mesh with wall-stress boundary conditions to simulate turbulent channel flows at friction Reynolds numbers of 2000 and 5200 and demonstrate good agreement with benchmark data. We also report a duality in velocity scale that is a specific consequence of the split forcing framework applied to hybrid RANS-LES. The first scale is the friction velocity derived from the wall shear stress. The second scale arises in the core LES region, a value different than at the wall. Second-order turbulence statistics agree well with the benchmark data when normalized by the core friction velocity, whereas the friction velocity at the wall remains the appropriate scale for the mean velocity profile. Based on our findings, we suggest reevaluating more sophisticated hybrid RANS-LES approaches within the split-forcing framework. Work funded by National Science Foundation under Grant No. 1056110 and 1229709. First author acknowledges the University of Idaho President's Doctoral Scholars Award.
Zhan, Shuiqing; Yang, Jianhong; Wang, Zhentao; Zhao, Ruijie; Zheng, Jun; Wang, Junfeng
2017-09-01
Numerical simulations of gas-liquid two-phase flows in aluminum electrolysis cells using the Euler-Euler approach were presented. The attempt was made to assess the performance and applicability of different interphase forces (drag, lift, wall lubrication, and turbulent dispersion forces) and turbulence models (standard k- ɛ, renormalization group k- ɛ, standard k- ω, shear stress transport k- ω, and Reynolds stress models). Moreover, three different bubble-induced turbulence models have been also analyzed. The simulated electrolyte velocity profiles were discussed by comparing with each other and against published experimental data. Based on the results of the validation of different interphase forces and turbulence models, a set consisting of the dispersed standard k- ɛ model, Grace drag coefficient model, Simonin turbulent dispersion force model, and Sato et al.'s bubble-induced effective viscosity model was found to provide the best agreement with the experimental data. The prediction results showed that the contributions of the lift force and the wall lubrication force can be neglected for the present bubbly flows.
Zhan, Shuiqing; Yang, Jianhong; Wang, Zhentao; Zhao, Ruijie; Zheng, Jun; Wang, Junfeng
2017-03-01
Numerical simulations of gas-liquid two-phase flows in aluminum electrolysis cells using the Euler-Euler approach were presented. The attempt was made to assess the performance and applicability of different interphase forces (drag, lift, wall lubrication, and turbulent dispersion forces) and turbulence models (standard k-ɛ, renormalization group k-ɛ, standard k-ω, shear stress transport k-ω, and Reynolds stress models). Moreover, three different bubble-induced turbulence models have been also analyzed. The simulated electrolyte velocity profiles were discussed by comparing with each other and against published experimental data. Based on the results of the validation of different interphase forces and turbulence models, a set consisting of the dispersed standard k-ɛ model, Grace drag coefficient model, Simonin turbulent dispersion force model, and Sato et al.'s bubble-induced effective viscosity model was found to provide the best agreement with the experimental data. The prediction results showed that the contributions of the lift force and the wall lubrication force can be neglected for the present bubbly flows.
Marcum, Jeremy W.; Olson, Sandra L.; Ferkul, Paul V.
2016-01-01
The axisymmetric rod geometry in upward axial stagnation flow provides a simple way to measure normal gravity blowoff limits to compare with microgravity Burning and Suppression of Solids - II (BASS-II) results recently obtained aboard the International Space Station. This testing utilized the same BASS-II concurrent rod geometry, but with the addition of normal gravity buoyant flow. Cast polymethylmethacrylate (PMMA) rods of diameters ranging from 0.635 cm to 3.81 cm were burned at oxygen concentrations ranging from 14 to 18% by volume. The forced flow velocity where blowoff occurred was determined for each rod size and oxygen concentration. These blowoff limits compare favorably with the BASS-II results when the buoyant stretch is included and the flow is corrected by considering the blockage factor of the fuel. From these results, the normal gravity blowoff boundary for this axisymmetric rod geometry is determined to be linear, with oxygen concentration directly proportional to flow speed. We describe a new normal gravity 'upward flame spread test' method which extrapolates the linear blowoff boundary to the zero stretch limit in order to resolve microgravity flammability limits-something current methods cannot do. This new test method can improve spacecraft fire safety for future exploration missions by providing a tractable way to obtain good estimates of material flammability in low gravity.
Drying of prickly pear cactus cladodes (Opuntia ficus indica) in a forced convection tunnel
Energy Technology Data Exchange (ETDEWEB)
Lopez, R.; de Ita, A.; Vaca, M. [Universidad Autonoma Metropolitana-Azcapotzalco, DePt. de Energia, Area de Termofluidos, Av. San Pablo 180, Col. Reynosa Tamaulipas, Del. Azcapotzalco, C.P. 02200, Mexico D.F. (Mexico)
2009-09-15
In this work we evaluated the kinetics of drying of Opuntia's cladodes observing two conditions: complete cladode with the protective cuticle of the intact product and with reduced cuticle (partially removed), using a drying tunnel with forced flow. The temperature of the air was set at 35, 45, and 60 C with velocities of 1.5 and 3.0 m/s. The conditions of the environment were controlled and maintained at 22 C and 30% of relative humidity. The results show that the drying time was considerably reduced when approximately 30% of the cuticle that protects the product was removed. Additionally, the temperature had greater influence than the velocity of the air. The numerical model that best describes the behavior of the drying process is the double logarithmic one, with the imposed restrictions of r close to the unit, the lowest possible {chi}{sup 2} and the RSEM tending to zero. The characteristic drying function of the product resulted in a third-grade exponential curve, where r and SD were the corresponding selection criteria. (author)
Energy Technology Data Exchange (ETDEWEB)
Sharma, Subash L., E-mail: sharma55@purdue.edu [School of Nuclear Engineering, Purdue University, West Lafayette, IN 47907-1290 (United States); Hibiki, Takashi; Ishii, Mamoru [School of Nuclear Engineering, Purdue University, West Lafayette, IN 47907-1290 (United States); Brooks, Caleb S. [Department of Nuclear, Plasma, and Radiological Engineering, University of Illinois, Urbana, IL 61801 (United States); Schlegel, Joshua P. [Nuclear Engineering Program, Missouri University of Science and Technology, Rolla, MO 65409 (United States); Liu, Yang [Nuclear Engineering Program, Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (United States); Buchanan, John R. [Bechtel Marine Propulsion Corporation, Bettis Laboratory, West Mifflin, PA 15122 (United States)
2017-02-15
Highlights: • Void distribution in narrow rectangular channel with various non-uniform inlet conditions. • Modeling of void diffusion due to bubble collision force. • Validation of new modeling in adiabatic air–water two-phase flow in a narrow channel. - Abstract: The prediction capability of the two-fluid model for gas–liquid dispersed two-phase flow depends on the accuracy of the closure relations for the interfacial forces. In previous studies of two-phase flow Computational Fluid Dynamics (CFD), interfacial force models for a single isolated bubble has been extended to disperse two-phase flow assuming the effect in a swarm of bubbles is similar. Limited studies have been performed investigating the effect of the bubble concentration on the lateral phase distribution. Bubbles, while moving through the liquid phase, may undergo turbulence-driven random collision with neighboring bubbles without significant coalescence. The rate of these collisions depends upon the bubble approach velocity and bubble spacing. The bubble collision frequency is expected to be higher in locations with higher bubble concentrations, i.e., volume fraction. This turbulence-driven random collision causes the diffusion of the bubbles from high concentration to low concentration. Based on experimental observations, a phenomenological model has been developed for a “turbulence-induced bubble collision force” for use in the two-fluid model. For testing the validity of the model, two-phase flow data measured at Purdue University are utilized. The geometry is a 10 mm × 200 mm cross section channel. Experimentally, non-uniform inlet boundary conditions are applied with different sparger combinations to vary the volume fraction distribution across the wider dimension. Examining uniform and non-uniform inlet data allows for the influence of the volume fraction to be studied as a separate effect. The turbulence-induced bubble collision force has been implemented in ANSYS CFX. The
The Decay of Forced Turbulent Coflow of He II Past a Grid
Babuin, S.; Varga, E.; Skrbek, L.
2014-04-01
We present an experimental study of the decay of He II turbulence created mechanically, by a bellows-induced flow past a stationary grid in a 7×7 mm2 superfluid wind tunnel. The temporal decay L( t) originating from various steady-states of vortex line length per unit volume, L 0, has been observed based on measurements of the attenuation of second-sound, in the temperature range 1.17 Kbench-mark Oregon towed grid experiments and, despite our turbulence being non-homogeneous, find strong similarities.
Energy Technology Data Exchange (ETDEWEB)
Saez, M.
1998-10-20
The aim of our study is to establish a reliable database for improving thermal hydraulic codes, in the field of turbulent flows with buoyancy forces. The flow considered is mixed convection in the Reynolds and Richardson number range: Re = 10{sup 3} to 6.10{sup 4} and Ri = 10{sup -4} to 1. Experiments are carried out in an upward turbulent flow between vertical parallel plates at different wall temperatures. Part 1 gives a detailed database of turbulent mixed flow of free and forced convection. Part 2 presents the installation and the calibration system intended for probes calibration. Part 3 describes the measurement technique (constant temperature probe and cold-wire probe) and the method for measuring the position of the hot-wire anemometer from the wall surface. The measurement accuracy is within 0.001 mm in the present system. Part 4 relates the development of a method for near wall measurements. This correction procedure for hot-wire anemometer close to wall has been derived on the basis of a two-dimensional numerical study. The method permits to obtain a quantitative correction of the wall influence on hot-wires and takes into account the velocity profile and the effects the wall material has on the heat loss. Part 5 presents the experimental data obtained in the channel in forced and mixed convection. Results obtained in the forced convection regime serve as a verification of the measurement technique close to the wall and give the conditions at the entrance of the test section. The effects of the buoyancy force on the mean velocity and temperature profiles are confirmed. The buoyancy strongly affects the fluid structure and deforms the distribution of mean velocity. The velocity profiles are asymmetric. The second section of part 5 gives an approach of analytical wall functions with buoyancy forces, on the basis of the experimental data obtained in the test section. (author)
Raskutti, Sudhir; Skinner, M Aaron
2016-01-01
Radiation feedback from stellar clusters is expected to play a key role in setting the rate and efficiency of star formation in giant molecular clouds (GMCs). To investigate how radiation forces influence realistic turbulent systems, we have conducted a series of numerical simulations employing the {\\it Hyperion} radiation hydrodynamics solver, considering the regime that is optically thick to ultraviolet (UV) and optically thin to infrared (IR) radiation. Our model clouds cover initial surface densities between $\\Sigma_{\\rm cl,0} \\sim 10-300~M_{\\odot}~{\\rm pc^{-2}}$, with varying initial turbulence. We follow them through turbulent, self-gravitating collapse, formation of star clusters, and cloud dispersal by stellar radiation. All our models display a lognormal distribution of gas surface density $\\Sigma$; for an initial virial parameter $\\alpha_{\\rm vir,0} = 2$, the lognormal standard deviation is $\\sigma_{\\rm ln \\Sigma} = 1-1.5$ and the star formation rate coefficient $\\varepsilon_{\\rm ff,\\bar\\rho} = 0.3-...
Gokoglu, S. A.; Rosner, D. E.
1984-01-01
A simple engineering correlation scheme is developed to predict the variable property effects on dilute species laminar forced convection mass transfer applicable to all vapor molecules or Brownian diffusing small particle, covering the surface to mainstream temperature ratio of 0.25 T sub W/T sub e 4. The accuracy of the correlation is checked against rigorous numerical forced convection laminar boundary layer calculations of flat plate and stagnation point flows of air containing trace species of Na, NaCl, NaOH, Na2SO4, K, KCl, KOH, or K2SO4 vapor species or their clusters. For the cases reported here the correlation had an average absolute error of only 1 percent (maximum 13 percent) as compared to an average absolute error of 18 percent (maximum 54 percent) one would have made by using the constant-property results.