Analogy between thermal convective and magnetohydrodynamic instabilities
Energy Technology Data Exchange (ETDEWEB)
Valdmanis, Ya.Ya.; Kukainis, O.A.
1977-01-01
An examination is made of the analogy between thermo-convective instability and instability produced by various electromagnetic forces both in steady and alternating thermal and electromagnetic fields. An example is given for calculating an assumed bubble instability which could occur in an alternating magnetic field. 17 references.
Transient convective instabilities in directional solidification
Meca, Esteban
2010-01-01
We study the convective instability of the melt during the initial transient in a directional solidification experiment in a vertical configuration. We obtain analytically the dispersion relation, and perform an additional asymptotic expansion for large Rayleigh number that permits a simpler analytical analysis and a better numerical behavior. We find a transient instability, i.e. a regime in which the system destabilizes during the transient whereas the final unperturbed steady state is stable. This could be relevant to growth mode predictions in solidification.
Toward a unified theory of atmospheric convective instability
Shirer, H. N.
1982-01-01
A nonlinear three-dimensional truncated spectral model of shallow and moist Boussinesq convection indicates that parallel instability and thermal forcing are linked, in view of the fact that only one convective mode exists in which either or both mechanisms are operating to generate convection in the planetary boundary layer. It is also established that the wind field causes two-dimensional roll convection formation, an alignment of the convection with the wind in a preferred manner, and a propagation speed that is related to the wind component perpendicular to the roll axis. Latent heating is responsible for the decrease of the critical value of the environmental lapse rate in accordance with the slice method stability criterion. When only the upper part of the upward branch is moist and all of the downward branch is dry, latent heating also causes a finite-amplitude convective solution for Rayleigh number values lower than the critical value of linear analysis.
Electro-convective versus electroosmotic instability in concentration polarization.
Rubinstein, Isaak; Zaltzman, Boris
2007-10-31
Electro-convection is reviewed as a mechanism of mixing in the diffusion layer of a strong electrolyte adjacent to a charge-selective solid, such as an ion exchange (electrodialysis) membrane or an electrode. Two types of electro-convection in strong electrolytes may be distinguished: bulk electro-convection, due to the action of the electric field upon the residual space charge of a quasi-electro-neutral bulk solution, and convection induced by electroosmotic slip, due to electric forces acting in the thin electric double layer of either quasi-equilibrium or non-equilibrium type near the solid/liquid interface. According to recent studies, the latter appears to be the likely source of mixing in the diffusion layer, leading to 'over-limiting' conductance in electrodialysis. Electro-convection near a planar uniform charge selective solid/liquid interface sets on as a result of hydrodynamic instability of one-dimensional steady state electric conduction through such an interface. We compare the results of linear stability analysis obtained for instabilities of this kind appearing in the full electro-convective and limiting non-equilibrium electroosmotic formulations. The short- and long-wave aspects of these instabilities are discussed along with the wave number selection principles.
Nonlinear instability and convection in a vertically vibrated granular bed
Shukla, P.; Ansari, I.H.; van der Meer, Roger M.; Lohse, Detlef; Alam, M.
2014-01-01
The nonlinear instability of the density-inverted granular Leidenfrost state and the resulting convective motion in strongly shaken granular matter are analysed via a weakly nonlinear analysis of the hydrodynamic equations. The base state is assumed to be quasi-steady and the effect of harmonic
Nonlinear instability and convection in a vertically vibrated granular bed
Shukla, P.; Ansari, I.H.; Meer, van der R.M.; Lohse, D.; Alam, M.
2014-01-01
The nonlinear instability of the density-inverted granular Leidenfrost state and the resulting convective motion in strongly shaken granular matter are analysed via a weakly nonlinear analysis of the hydrodynamic equations. The base state is assumed to be quasi-steady and the effect of harmonic shak
Magneto-convective instabilities in horizontal cavities
Mistrangelo, Chiara; Bühler, Leo
2016-02-01
A linear stability analysis is performed to investigate the onset of convective motions in a flat cavity filled with liquid metal. A volumetric heat source is uniformly distributed in the fluid and a horizontal magnetic field is imposed. Walls perpendicular to the magnetic field are thermally insulating, and the top wall is isothermal and the bottom adiabatic. When a magnetic field is applied, electromagnetic forces tend to transform 3D convective flow structures into quasi-2D rolls aligned to the magnetic field. By integrating 3D equations along magnetic field lines, a quasi-2D mathematical model has been derived. A dissipation term in the 2D equations accounts for 3D viscous effects in boundary layers at Hartmann walls perpendicular to the magnetic field. The influence of various parameters on flow stability is investigated. The flow is stabilized by increasing the magnetic field intensity or the electric conductance of Hartmann walls and by reducing the aspect ratio of the cavity. Numerical simulations are performed to verify the analytical results and to describe the main convective flow patterns in the non-linear regime.
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.
Suppression of Instability in Rotatory Hydromagnetic Convection
Indian Academy of Sciences (India)
Joginder S Dhiman
2000-08-01
Recently discovered hydrodynamic instability [1], in a simple Bénard configuration in the parameter regime 02 > 1 under the action of a nonadverse temperature gradient, is shown to be suppressed by the simultaneous action of a uniform rotation and a uniform magnetic field both acting parallel to gravity for oscillatory perturbations whenever $(\\mathscr{Q}_1/^2+\\mathscr{J}/^4) > 1$ and the effective Rayleigh number $\\mathcal{R}$(1-02) is dominated by either 274(1 + 1/1)/4 or 274/2 according as 1 ≥ 1 or 1 ≤ 1 respectively. Here 0 is the temperature of the lower boundary while 2 is the coefficient of specific heat at constant volume due to temperature variation and 1, $\\mathcal{R}, \\mathscr{Q}$ and $\\mathscr{T}$ respectively denote the magnetic Prandtl number, the Rayleigh number, the Chandrasekhar number and the Taylor number.
Pattern Formation and Secondary Instabilities for Convection in Porous Media
Behringer, R. P.; Fiering, J.
1996-11-01
In recent work(Howle et al. Nature 362), 230 (1993); JFM to appear we showed that the pattern formation for convection in porous media could be studied by means of a simple shadowgraph if there is a line of sight through the medium which does not encounter curved fluid-medium interfaces. This work focused primarily on pattern formation near onset and was carried out in relatively small aspect ratios, Γ (the ratio of the horizontal dimension of the layer to the height, d). We present new studies with both higher Γ, and at high enough Rayleigh number R to encounter the secondary instabilities for the convection roll patterns. In a Γ = radius/d = 10.7 cylindrical experiment with a medium made from stacked bars, we find well aligned convection rolls with wavelength, λ in good agreement with theory. The alignment direction is determined by the periodic structure of the medium. In two other experiments, also with bar stackings for the medium, we have studied the secondary instabilities. The experiments have Γ = 4 and 8, respectively (with square planform), and consist of 8 and 4 layers of bars. In both cases, the instability is to cross rolls. In the Γ = 8 case, the strong symmetry of the system leads to heteroclinic orbits between different patterns.
Instability and Route to Chaos in Porous Media Convection
Directory of Open Access Journals (Sweden)
Peter Vadasz
2017-05-01
Full Text Available A review of the research on the instability of steady porous media convection leading to chaos, and the possibility of controlling the transition from steady convection to chaos is presented. The governing equations consisting of the continuity, the extended Darcy, and the energy equations subject to the assumption of local thermal equilibrium and the Boussinesq approximation are converted into a set of three nonlinear ordinary differential equations by assuming two-dimensional convection and expansion of the dependent variables into a truncated spectrum of modes. Analytical (weak nonlinear, computational (Adomian decomposition as well as numerical (Runge-Kutta-Verner solutions to the resulting set of equations are presented and compared to each other. The analytical solution for the transition point to chaos is identical to the computational and numerical solutions in the neighborhood of a convective fixed point and deviates from the accurate computational and numerical solutions as the initial conditions deviate from the neighborhood of a convective fixed point. The control of this transition is also discussed.
Maxwellianization of electron distribution functions by convective instabilities in presheaths
Baalrud, S. D.; Hegna, C. C.; Callen, J. D.
2008-11-01
Langmuir's paradox is a measurement of anomalous electron scattering where a Maxwellian electron velocity distribution function was measured much closer to a boundary than the electron collision length in a stable plasma; here one should expect truncation corresponding to the sheath energy. In this paper we theoretically analyze the presheath region that is present in Langmuir paradox-relevant plasmas (TeTi). It is shown that the ion-acoustic instability is present throughout the presheath causing convective amplification of thermal fluctuations. A collision operator for the plasma kinetic equation including instabilities in a finite space-time domain is derived [1] which shows that electron scattering can be dominated by wave-particle interactions in the presheath. The modified collision operator satisfies the Boltzmann H-theorem, so the only equilibrium is a Maxwellian which is achieved at a rate depending on collisionality. Wave-particle scattering shrinks the electron collision length to within a few cm for these discharges suggesting that one should expect a Maxwellian at the location of previously reported measurements. [1] S.D. Baalrud, J.D. Callen, C.C. Hegna, UW-CPTC 08-4, June 2008 (sub. to Phys. Plasmas).
Sharma, P; Quataert, E; Parrish, I J
2009-01-01
Using a linear stability analysis and two and three-dimensional nonlinear simulations, we study the physics of buoyancy instabilities in a combined thermal and relativistic (cosmic ray) plasma, motivated by the application to clusters of galaxies. We argue that cosmic ray diffusion is likely to be slow compared to the buoyancy time on large length scales, so that cosmic rays are effectively adiabatic. If the cosmic ray pressure $p_{cr}$ is $\\gtrsim 25 %$ of the thermal pressure, and the cosmic ray entropy ($p_{\\rm cr}/\\rho^{4/3}$; $\\rho$ is the thermal plasma density) decreases outwards, cosmic rays drive an adiabatic convective instability analogous to Schwarzschild convection in stars. Global simulations of galaxy cluster cores show that this instability saturates by reducing the cosmic ray entropy gradient and driving efficient convection and turbulent mixing. At larger radii in cluster cores, the thermal plasma is unstable to the heat flux-driven buoyancy instability (HBI), a convective instability genera...
An {sup 3}He-DRIVEN INSTABILITY NEAR THE FULLY CONVECTIVE BOUNDARY
Energy Technology Data Exchange (ETDEWEB)
Van Saders, Jennifer L.; Pinsonneault, Marc H., E-mail: vansaders@astronomy.ohio-state.edu, E-mail: pinsono@astronomy.ohio-state.edu [Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States)
2012-06-01
We report on the discovery of an instability in low-mass stars just above the threshold ({approx}0.35 M{sub Sun }) where they are expected to be fully convective on the main sequence (MS). Non-equilibrium {sup 3}He burning creates a convective core, which is separated from a deep convective envelope by a small radiative zone. The steady increase in central {sup 3}He causes the core to grow until it touches the surface convection zone, which triggers fully convective episodes in what we call the 'convective kissing instability'. These episodes lower the central abundance and cause the star to return to a state in which it has a separate convective core and envelope. These periodic events eventually cease when the {sup 3}He abundance throughout the star is sufficiently high that the star is fully convective, and remains so for the rest of its MS lifetime. The episodes correspond to few percent changes in radius and luminosity, over Myr to Gyr timescales. We discuss the physics of the instability, as well as prospects for detecting its signatures in open clusters and wide binaries. Secondary stars in cataclysmic variables (CVs) will pass through this mass range, and this instability could be related to the observed paucity of such systems for periods between two and three hours. We demonstrate that the instability can be generated for CV secondaries with mass-loss rates of interest for such systems and discuss potential implications.
Analysis of strange-mode instability with time-dependent convection in hot massive stars
Sonoi, Takafumi
2013-01-01
We carry out nonadiabatic analysis of strange-modes in hot massive stars with time-dependent convection (TDC) for the first time. Although convective luminosity in envelopes of hot massive stars is not as dominative as in stars near the red edge of the classical Cepheid instability strip in the Hertzsprung-Russell (H-R) diagram, we have found that the strange-mode instability can be affected by the treatment of convection. However, existence of the instability around and over the Humphreys-Davidson (H-D) limit is independent of the treatment. This implies that the strange-mode instability could be responsible for the lack of observed stars over the H-D limit regardless of uncertainties on convection theories.
Convective instability of sludge storage under evaporation and solar radiation
Tsiberkin, Kirill; Tatyana, Lyubimova
2014-05-01
The sludge storages are an important part of production cycle at salt manufacturing, water supply, etc. A quality of water in the storage depends on mixing of pure water and settled sediment. One of the leading factors is thermal convection. There are two main mechanisms of the layer instability exist. First, it is instability of water due to evaporation from the free surface [1]. It cools the water from upside, increases the particles concentration and leads to the instability in the near-surface layer. Second, the sediment absorbs a solar radiation and heats the liquid from below making it unstable in the near-bottom area. We assume the initial state is the mechanical equilibrium. The water and sediment particles are motionless, the sediment forms a uniform sludge layer of thickness z0, there are no evaporation and heating by solar energy, and the temperature has a linear profile is determined by fixed upper and bottom temperatures of the layer. Taking into account the evaporation and solar radiation absorption, we obtain a non-stationary solution for the temperature using Fourier series method. The local temperature gradients increases rapidly with time, and local Rayleigh number can be estimated by thermal conduction length Lt: Raloc(z,t) = gβ(δT(z,t)/δz)L4t-/νΞ , Lt ~ √Ξt, (1) where g is gravity acceleration, β, ν and Ξ are thermal volume expansion coefficient, kinematic viscosity and thermal conductivity of the liquid, respectively. Raloc* reaches the critical value at finite time t* and water motion begins. The maximal power of solar radiation in visible band equals 230 Wt/m2 at the latitude of "Uralkalii" salt manufacturer (Berezniki, Perm Region, Russian Federation). We neglect IR and UV radiation because of its huge absorption by water [2]. The evaporation speed is found using results for shallow water reservoir [3] and meteorological data for Berezniki [4]. We get the t*~ 6 · 102 s (10 min) for the layer of 1 m depth and t*~ 2 · 103 s (40
Noise-sustained convective instability in a magnetized Taylor-Couette flow
Energy Technology Data Exchange (ETDEWEB)
Liu, Wei [Los Alamos National Laboratory
2008-01-01
The helical magnetorotational instability of the magnetized Taylor-Couette flow is studied numerically in a finite cylinder. A distant upstream insulating boundary is shown to stabilize the convective instability entirely while reducing the growth rate of the absolute instability. The reduction is less severe with larger height. After modeling the boundary conditions properly, the wave patterns observed in the experiment turn out to be a noise-sustained convective instability. After the source of the noise resulted from unstable Ekman and Stewartson layers is switched off, a slowly-decaying inertial oscillation is observed in the simulation. We reach the conclusion that the experiments completed to date have not yet reached the regime of absolute instability.
Noise-Sustained Convective Instability in a Magnetized Taylor-Couette Flow
Liu, Wei
2008-01-01
The helical magnetorotational instability of the magnetized Taylor-Couette flow is studied numerically in a finite cylinder. A distant upstream insulating boundary is shown to stabilize the convective instability entirely while reducing the growth rate of the absolute instability. The reduction is less severe with larger height. After modeling the boundary conditions properly, the wave patterns observed in the experiment turn out to be a noise-sustained convective instability. After the source of the noise resulted from unstable Ekman and Stewartson layers is switched off, a slowly-decaying inertial oscillation is observed in the simulation. We reach the conclusion that the experiments completed to date have not yet reached the regime of absolute instability.
Noise-Sustained Convective Instability in a Magnetized Taylor-Couette Flow
Energy Technology Data Exchange (ETDEWEB)
W. Liu
2009-02-20
The helical magnetorotational instability of the magnetized Taylor-Couette flow is studied numerically in a finite cylinder. A distant upstream insulating boundary is shown to stabilize the convective instability entirely while reducing the growth rate of the absolute instability. The reduction is less severe with larger height. After modeling the boundary conditions properly, the wave patterns observed in the experiment turn out to be a noise-sustained convective instability. After the source of the noise resulted from unstable Ekman and Stewartson layers is switched off, a slowly-decaying inertial oscillation is observed in the simulation. We reach the conclusion that the experiments completed to date have not yet reached the regime of absolute instability.
Aitova, E. V.; Bratsun, D. A.; Kostarev, K. G.; Mizev, A. I.; Mosheva, E. A.
2016-12-01
The development of convective instability in a two-layer system of miscible fluids placed in a narrow vertical gap has been studied theoretically and experimentally. The upper and lower layers are formed with aqueous solutions of acid and base, respectively. When the layers are brought into contact, the frontal neutralization reaction begins. We have found experimentally a new type of convective instability, which is characterized by the spatial localization and the periodicity of the structure observed for the first time in the miscible systems. We have tested a number of different acid-base systems and have found a similar patterning there. In our opinion, it may indicate that the discovered effect is of a general nature and should be taken into account in reaction-diffusion-convection problems as another tool with which the reaction can govern the movement of the reacting fluids. We have shown that, at least in one case (aqueous solutions of nitric acid and sodium hydroxide), a new type of instability called as the concentration-dependent diffusion convection is responsible for the onset of the fluid flow. It arises when the diffusion coefficients of species are different and depend on their concentrations. This type of instability can be attributed to a variety of double-diffusion convection. A mathematical model of the new phenomenon has been developed using the system of reaction-diffusion-convection equations written in the Hele-Shaw approximation. It is shown that the instability can be reproduced in the numerical experiment if only one takes into account the concentration dependence of the diffusion coefficients of the reagents. The dynamics of the base state, its linear stability and nonlinear development of the instability are presented. It is also shown that by varying the concentration of acid in the upper layer one can achieve the occurrence of chemo-convective solitary cell in the bulk of an almost immobile fluid. Good agreement between the
Long-Wavelength Rupturing Instability in Surface-Tension-Driven Benard Convection
Swift, J. B.; Hook, Stephen J. Van; Becerril, Ricardo; McCormick, W. D.; Swinney, H. L.; Schatz, Michael F.
1999-01-01
A liquid layer with a free upper surface and heated from below is subject to thermocapillary-induced convective instabilities. We use very thin liquid layers (0.01 cm) to significantly reduce buoyancy effects and simulate Marangoni convection in microgravity. We observe thermocapillary-driven convection in two qualitatively different modes, short-wavelength Benard hexagonal convection cells and a long-wavelength interfacial rupturing mode. We focus on the long-wavelength mode and present experimental observations and theoretical analyses of the long-wavelength instability. Depending on the depths and thermal conductivities of the liquid and the gas above it, the interface can rupture downwards and form a dry spot or rupture upwards and form a high spot. Linear stability theory gives good agreement to the experimental measurements of onset as long as sidewall effects are taken into account. Nonlinear theory correctly predicts the subcritical nature of the bifurcation and the selection between the dry spot and high spots.
Rivera, Gustavo; Diamessis, Peter
2016-11-01
The shoaling of an internal solitary wave (ISW) of depression over gentle slopes is explored through fully nonlinear and non-hydrostatic simulations based on a high-accuracy deformed spectral multidomain penalty method. As recently observed in the South China Sea, in high-amplitude shoaling ISWs, the along-wave current can exceed the wave celerity resulting in convective instabilities. If the slope is less than 3%, the wave does not disintegrate as in the case of steeper slope shoaling but, instead, maintains its symmetric shape; the above convective instability may drive the formation of a turbulent recirculating core. The sensitivity of convective instabilities in an ISW is examined as a function of the bathymetric slope and wave steepness. ISWs are simulated propagating over both idealized and realistic bathymetry. Emphasis is placed on the structure of the above instabilities, the persistence of trapped cores and their potential for particle entrainment and transport. Additionally, the role of the baroclinic background current on the development of convective instabilities is explored. A preliminary understanding is obtained of the transition to turbulence within a high-amplitude ISW shoaling over progressively varying bathymetry.
Volponi, Francesco
2013-01-01
Discs with a rotation profile depending on radius and height are subject to an axisymmetric linear instability, the vertical shear instability. Here we show that non-axisymmetric perturbations, while eventually stabilized, can sustain huge exponential amplifications with growth rate close to the axisymmetric one. Transient growths are therefore to all effects genuine instabilities. The ensuing angular momentum transport is positive. These growths occur when the product of the radial times the vertical wavenumbers (both evolving with time) is positive for a positive local vertical shear, or negative for a negative local vertical shear. We studied, as well, the interaction of these vertical shear induced growths with a convective instability. The asymptotic behaviour depends on the relative strength of the axisymmetric vertical shear (s_v) and convective (s_c) growth rates. For s_v > s_c we observed the same type of behaviour described above - large growths occur with asymptotic stabilization. When s_c > s_v th...
Primary instabilities in convective cells due to nonuniform heating
Mancho, A. M.; Herrero, H.; Burguete, J.
1997-09-01
We study a convection problem in a container with a surface open to the air and heated by a long wire placed at the bottom. Coupled buoyancy and thermocapillarity effects are taken into account. A basic convective state appears as soon as a temperature gradient with horizontal component different from zero is applied. It consists of two big rolls that fill the convective cell and are parallel to the heater. A numerical solution allows us to determine this basic state. A linear stability analysis on this solution is carried out. For different values of the applied temperature gradient the basic rolls undergo a stationary bifurcation. The thresholds depend on the fluid properties, on the geometry of the heater, and on the heat exchange on the free surface. This confirms the results obtained in recent experiments.
Inviscid instability of the Batchelor vortex: Absolute-convective transition and spatial branches
Olendraru, Cornel; Sellier, Antoine; Rossi, Maurice; Huerre, Patrick
1999-07-01
The main objective of the study is to examine the spatio-temporal instability properties of the Batchelor q-vortex, as a function of swirl ratio q and external axial flow parameter a. The inviscid dispersion relation between complex axial wave number and frequency is determined by numerical integration of the Howard-Gupta ordinary differential equation. The absolute-convective nature of the instability is then ascertained by application of the Briggs-Bers zero-group-velocity criterion. A moderate amount of swirl is found to promote the onset of absolute instability. In the case of wakes, transition from convective to absolute instability always takes place via the helical mode of azimuthal wave number m=-1. For sufficiently large swirl, co-flowing wakes become absolutely unstable. In the case of jets, transition from absolute to convective instability occurs through various helical modes, the transitional azimuthal wave number m being negative but sensitive to increasing swirl. For sufficiently large swirl, weakly co-flowing jets become absolutely unstable. These results are in good qualitative and quantitative agreement with those obtained by Delbende et al. through a direct numerical simulation of the linear response. Finally, the spatial (complex axial wave number, real frequency) instability characteristics are illustrated for the case of zero-external flow swirling jets.
Vincze, Miklos; Borcia, Ion; Harlander, Uwe; Le Gal, Patrice
2016-12-01
A water-filled differentially heated rotating annulus with initially prepared stable vertical salinity profiles is studied in the laboratory. Based on two-dimensional horizontal particle image velocimetry data and infrared camera visualizations, we describe the appearance and the characteristics of the baroclinic instability in this original configuration. First, we show that when the salinity profile is linear and confined between two non-stratified layers at top and bottom, only two separate shallow fluid layers can be destabilized. These unstable layers appear nearby the top and the bottom of the tank with a stratified motionless zone between them. This laboratory arrangement is thus particularly interesting to model geophysical or astrophysical situations where stratified regions are often juxtaposed to convective ones. Then, for more general but stable initial density profiles, statistical measures are introduced to quantify the extent of the baroclinic instability at given depths and to analyze the connections between this depth-dependence and the vertical salinity profiles. We find that, although the presence of stable stratification generally hinders full-depth overturning, double-diffusive convection can lead to development of multicellular sideways convection in shallow layers and subsequently to a multilayered baroclinic instability. Therefore we conclude that by decreasing the characteristic vertical scale of the flow, stratification may even enhance the formation of cyclonic and anticyclonic eddies (and thus, mixing) in a local sense.
Large-Scale Flow and Spiral Core Instability in Rayleigh-Benard Convection
Aranson, I S; Steinberg, V; Tsimring, L S; Aranson, Igor; Assenheimer, Michel; Steinberg, Victor; Tsimring, Lev S.
1996-01-01
The spiral core instability, observed in large aspect ratio Rayleigh-Benard convection, is studied numerically in the framework of the Swift-Hohenberg equation coupled to a large-scale flow. It is shown that the instability leads to non-trivial core dynamics and is driven by the self-generated vorticity. Moreover, the recently reported transition from spirals to hexagons near the core is shown to occur only in the presence of a non-variational nonlinearity, and is triggered by the spiral core instability. Qualitative agreement between the simulations and the experiments is demonstrated.
Absolute and convective instabilities in a one-dimensional Brusselator flow model
DEFF Research Database (Denmark)
Kuznetsov, S.P.; Mosekilde, Erik; Dewel, G.
1997-01-01
The paper considers a one-dimensional Brusselator model with a uniform flow of the mixture of reaction components. An absolute as well as a convective instability can arise for both the Hopf and the Turing modes. The corresponding linear stability analysis is presented and supported by the results...
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...
Onset of convective instabilities in under-ice melt ponds.
Hirata, Sílvia C; Goyeau, Benoît; Gobin, Dominique
2012-06-01
The onset of double-diffusive natural convection in under-ice melt ponds is investigated through a linear stability analysis. The three-layer configuration is composed by a fluid layer (melt pond) overlying a saturated porous medium (ice matrix), which in turn overlies another fluid layer (under-ice melt pond). Water density inversion is taken into account by adopting a density profile with a quadratic temperature dependence and a linear concentration dependence. We show that the key parameter affecting stability is the depth of the ice matrix, while the depths of the upper and lower fluid layers play a marginal role. A Hopf bifurcation is observed in the whole range of parameters studied, and the size of the convection cells depends on ice permeability. The influence of the external temperature gradient is investigated by means of the definition of an extra thermal parameter accounting for the relative position of the density maximum. It is shown that convection is favored by larger temperature gradients, which occur during Arctic summer.
EVIDENCE OF ACTIVE MHD INSTABILITY IN EULAG-MHD SIMULATIONS OF SOLAR CONVECTION
Energy Technology Data Exchange (ETDEWEB)
Lawson, Nicolas; Strugarek, Antoine; Charbonneau, Paul, E-mail: nicolas.laws@gmail.ca, E-mail: strugarek@astro.umontreal.ca, E-mail: paulchar@astro.umontreal.ca [Département de Physique, Université de Montréal, C.P. 6128 Succ. Centre-ville, Montréal, Qc H3C 3J7 (Canada)
2015-11-10
We investigate the possible development of magnetohydrodynamical instabilities in the EULAG-MHD “millennium simulation” of Passos and Charbonneau. This simulation sustains a large-scale magnetic cycle characterized by solar-like polarity reversals taking place on a regular multidecadal cadence, and in which zonally oriented bands of strong magnetic fields accumulate below the convective layers, in response to turbulent pumping from above in successive magnetic half-cycles. Key aspects of this simulation include low numerical dissipation and a strongly sub-adiabatic fluid layer underlying the convectively unstable layers corresponding to the modeled solar convection zone. These properties are conducive to the growth and development of two-dimensional instabilities that are otherwise suppressed by stronger dissipation. We find evidence for the action of a non-axisymmetric magnetoshear instability operating in the upper portions of the stably stratified fluid layers. We also investigate the possibility that the Tayler instability may be contributing to the destabilization of the large-scale axisymmetric magnetic component at high latitudes. On the basis of our analyses, we propose a global dynamo scenario whereby the magnetic cycle is driven primarily by turbulent dynamo action in the convecting layers, but MHD instabilities accelerate the dissipation of the magnetic field pumped down into the overshoot and stable layers, thus perhaps significantly influencing the magnetic cycle period. Support for this scenario is found in the distinct global dynamo behaviors observed in an otherwise identical EULAG-MHD simulations, using a different degree of sub-adiabaticity in the stable fluid layers underlying the convection zone.
Convective instability in inhomogeneous media: impulse response in the subcritical cylinder wake
Marais, Catherine; Barkley, Dwight; Wesfreid, José Eduardo
2010-01-01
We study experimentally the impulse response of a cylinder wake below the critical Reynolds number of the B\\'enard-von K\\'arm\\'an instability. In this subcritical regime, a localized inhomogeneous region of convective instability exists which causes initial perturbations to be transiently amplified. The aim of this work is to quantify the evolution resulting from this convective instability using two-dimensional particle image velocimetry in a hydrodynamic tunnel experiment. The velocity fields allow us to describe the evolution of wave packets in terms of two control parameters: the Reynolds number and the magnitude of the imposed perturbation. The temporal evolution of energy exhibits a transient algebraic growth at short times followed by an exponential decay.
A viscous-convective instability in laminar Keplerian thin discs
Malanchev, Konstantin; Shakura, Nikolai
2016-01-01
Using the anelastic approximation of linearized hydrodynamic equations, we investigate the development of axially symmetric small perturbations in thin Keplerian discs. Dispersion relation is found as a solution of general Sturm-Liouville eigenvalue problem for different values of relevant physical parameters (viscosity, heat conductivity, disc semithickness). The analysis reveals the appearance of overstable mode for Prandtl parameter higher than some critical value. These modes have a viscous-convective nature and can serve as a seed for turbulence in astrophysical discs even in the absence of magnetic fields.
Absolute and Convective Ion Beam Instability Studied through Green's Function
DEFF Research Database (Denmark)
Jensen, Vagn Orla; Michelsen, Poul; Hsuan, H. C. S.
1974-01-01
A Vlasov plasma with a double‐humped, unstable ion velocity distribution function is considered. A δ function in space is assumed as the initial perturbation and the plasma response to this perturbation is calculated, i.e., the Green's function for the problem is found. The response can be divide...... into two parts: a self‐similar, damped part of the form t−1h(x/t), and an unstable, exponentially growing part. The conditions for absolute and convective growth of the latter are discussed....
Sato, Tomonori; Kimura, Fujio
2005-05-01
Convective rainfall often shows a clear diurnal cycle. The nighttime peak of convective activity prevails in various regions near the world's mountains. The influence of the water vapor and convective instability upon nocturnal precipitation is investigated using a numerical model and observed data. Recent developments in GPS meteorology allow the estimation of precipitable water vapor (PWV) with a high temporal resolution. A dense network has been established in Japan. The GPS analysis in August 2000 provides the following results: In the early evening, a high-GPS-PWV region forms over mountainous areas because of the convergence of low-level moisture, which gradually propagates toward the adjacent plain before midnight. A region of convection propagates simultaneously eastward into the plain. The precipitating frequency correlates fairly well with the GPS-PWV and attains a maximum value at night over the plain. The model also provides similar characteristics in the diurnal cycles of rainfall and high PWV. Abundant moisture accumulates over the mountainous areas in the afternoon and then advects continuously toward the plain by the ambient wind. The specific humidity greatly increases at about the 800-hPa level over the plain at night, and the PWV reaches its nocturnal maximum. The increase in the specific humidity causes an increase of equivalent potential temperature at about the 800-hPa level; as a result, the convective instability index becomes more unstable over the plain at night. These findings are consistent with the diurnal cycle of the observed precipitating frequency.
Absolute versus convective helical magnetorotational instability in a Taylor-Couette flow
Priede, JÄnis
2008-01-01
We analyze numerically the magnetorotational instability of a Taylor-Couette flow in a helical magnetic field (HMRI) using the inductionless approximation defined by a zero magnetic Prandtl number (Pm=0). The Chebyshev collocation method is used to calculate the eigenvalue spectrum for small amplitude perturbations. First, we carry out a detailed conventional linear stability analysis with respect to perturbations in the form of Fourier modes that corresponds to the convective instability which is not in general self-sustained. The helical magnetic field is found to extend the instability to a relatively narrow range beyond its purely hydrodynamic limit defined by the Rayleigh line. There is not only a lower critical threshold at which HMRI appears but also an upper one at which it disappears again. The latter distinguishes the HMRI from a magnetically-modified Taylor vortex flow. Second, we find an absolute instability threshold as well. In the hydrodynamically unstable regime before the Rayleigh line, the t...
Vincze, Miklos; Harlander, Uwe; Gal, Patrice Le
2016-01-01
A water-filled differentially heated rotating annulus with initially prepared stable vertical salinity profiles is studied in the laboratory. Based on two-dimensional horizontal particle image velocimetry (PIV) data, and infrared camera visualizations, we describe the appearance and the characteristics of the baroclinic instability in this original configuration. First, we show that when the salinity profile is linear and confined between two non stratified layers at top and bottom, only two separate shallow fluid layers can be destabilized. These unstable layers appear nearby the top and the bottom of the tank with a stratified motionless zone between them. This laboratory arrangement is thus particularly interesting to model geophysical or astrophysical situations where stratified regions are often juxtaposed to convective ones. Then, for more general but stable initial density profiles, statistical measures are introduced to quantify the extent of the baroclinic instability at given depths and to analyze t...
Convective instability and mass transport of diffusion layers in a Hele-Shaw geometry
Backhaus, Scott; Ecke, R E
2010-01-01
We consider experimentally the instability and mass transport of a porous-medium flow in a Hele-Shaw geometry. In an initially stable configuration, a lighter fluid (water) is located over a heavier fluid (propylene glycol). The fluids mix via diffusion with some regions of the resulting mixture being heavier than either pure fluid. Density-driven convection occurs with downward penetrating dense fingers that transport mass much more effectively than diffusion alone. We investigate the initial instability and the quasi steady state. The convective time and velocity scales, finger width, wave number selection, and normalized mass transport are determined for 6,000
Flow instability of buoyant-Marangoni convection in the LEC GaAs melt
Institute of Scientific and Technical Information of China (English)
2008-01-01
Flow transitions and instabilities have significant effects on the quality of the crystals. The flow and heat transfer in the LEC GaAs melt are numerically studied by a time-dependent and three-dimensional turbulent flow model. The effects of the change of the buoyancy and Marangoni force on the flow state are analyzed by changing the temperature difference between the crystal and the crucible walls. The results show that the flow will transform from axisymmetric steady flow to non-axisymmetric oscillatory flow when the temperature difference exceeds the critical value, and that the mechanism of the transition is attributed to the Marangoni instability. The critical temperature differences for the flow transitions corresponding to different melt depth H are numerically predicted. Several important characteristics of the non-axisymmetric buoyant-Marangoni convection are numerically observed and compared with that of the non-axisymmetric mixed convection coupled with crystal rotation.
Rayleigh-Taylor finger instability mixing in hydrodynamic shell convection models
Mocak, Miroslav
2010-01-01
Mixing processes in stars driven by composition gradients as a result of the Rayleigh-Taylor instability are not anticipated. They are supported only by hydrodynamic studies of stellar convection. We find that such mixing occurs below the bottom edge of convection zones in our multidimensional hydrodynamic shell convection models. It operates at interfaces created by off-center nuclear burning, where less dense gas with higher mean molecular weight is located above denser gas with a lower mean molecular weight. We discuss the mixing under various conditions with hydrodynamic convection models based on stellar evolutionary calculations of the core helium flash in a 1.25 Msun star, the core carbon flash in a 9.3 Msun star, and of oxygen burning shell in a star with a mass of 23 Msun. We simulate the hydrodynamic behavior of shell convection during various phases of stellar evolution with the Eulerian hydrodynamics code HERAKLES in two and three spatial dimensions. Initial models for this purpose are obtained by...
Experimental study of morphological and convective instabilities: The MEPHISTO space program
Favier, J. J.; Rouzaud, A.
It is now well established that the morphological instability, i.e. the transition during solidification from a planar L/S interface to a cellular one, is strongly influenced by convection. The most recent theories on this topic, which are very advanced, suffer from the lack of experimental tests because uncontrolled convective effects cannot be avoid on the ground. Moreover the check of all the pertinent solidification parameters are not controlled in the same time or measured in real time. After a review of the main 1g experiments and their own limitations, we describe a new apparatus (MEPHISTO) which allows : in-situ measurements of the main solidification parameters and in particular the undercooling at the solidification front in real time by a non perturbative method. real time supervision of convective motions influence on crystal growth. First results obtained under 1g condition are presented : hydrodynamic scaling laws testing, instabilities detection, transient phenomena. Main space results are then anticipated including the role of g-jitters.
Convective Instability and Mass Transport of the Diffusion Layer in CO2 Sequestration
Backhaus, S.
2011-12-01
The long-term fate of supercritical (sc) CO2 in saline aquifers is critical to the security of carbon sequestration, an important option for eliminating or reducing the emissions of this most prevalent greenhouse gas. scCO2 is less dense than brine and floats to the top of the aquifer where it is trapped in a metastable state by a geologic feature such as a low permeability cap rock. Dissolution into the underlying brine creates a CO2-brine mixture that is denser than brine, eliminating buoyancy and removing the threat of CO2 escaping back to the atmosphere. If molecular diffusion were the only dissolution mechanism, the CO2 waste stream from a typical large coal-fired electrical power plant may take upward of 10,000 years to no longer pose a threat, however, a convective instability of the dense diffusion boundary layer between the scCO2 and the brine can dramatically increase the dissolution rates, shortening the lifetime of the scCO2 waste pool. We present results of 2D and 3D similitude-correct, laboratory-scale experiments using an analog fluid system. The experiments and flow visualization reveal the onset of the convective instability, the dynamics of the fluid flows during the convective processes, and the long-term mass transfer rates.
Case study of convective instability observed in airglow images over the Northeast of Brazil
Carvalho, A. J. A.; Paulino, I.; Medeiros, A. F.; Lima, L. M.; Buriti, R. A.; Paulino, A. R.; Wrasse, C. M.; Takahashi, H.
2017-02-01
An intense activity of ripples during the nighttime was observed in airglow images over São João do Cariri (36.5° W, 7.4° S) on 10 October 2004 which lasted for two hours. Those ripples appeared simultaneously with the crossing of a mesospheric front and medium scale gravity waves. The ripples occurred ahead of the mesospheric front and their phase front were almost parallel to the phase of the mesospheric front and were almost perpendicular to the phase front of the gravity wave. Using wind measurements from a meteor radar located at São João do Cariri and simultaneous vertical temperature profiles from the TIMED/SABER satellite, on the night of the events and within the imager field of view, the atmospheric background environment in the mesosphere and lower thermosphere (MLT) was investigated in order to understand the instability process that caused the appearance of the ripples. Dynamic and convective instabilities have been pointed out as responsible for creation of ripples in the MLT. The observed ripples were advected by the neutral wind, they occurred into a region with negative lapse rate of the potential temperature and the Richardson number was negative as well. According to these characteristics, the ripple structures could be generated in the MLT region due to the predominance of convective instability.
Gopalakrishnan, S. S.; Carballido-Landeira, J.; De Wit, A.; Knaepen, B.
2017-01-01
The relative role of convection and diffusion is characterized both numerically and experimentally for porous media flows due to a Rayleigh-Taylor instability of a horizontal interface between two miscible solutions in the gravity field. We show that, though globally convection dominates over diffusion during the nonlinear regime, diffusion can locally be as important as convection and even dominates over lateral convection far away from the fingertips. Our experimental and numerical computations of the temporal evolution of the mixing length, the width of the fingers, and their wavelength are in good agreement and show that the lateral evolution of fingers is governed by diffusion.
Dynamics of fingering convection I: Small-scale fluxes and large-scale instabilities
Traxler, A; Garaud, P; Radko, T; Brummell, N
2010-01-01
Double-diffusive instabilities are often invoked to explain enhanced transport in stably-stratified fluids. The most-studied natural manifestation of this process, fingering convection, commonly occurs in the ocean's thermocline and typically increases diapycnal mixing by two orders of magnitude over molecular diffusion. Fingering convection is also often associated with structures on much larger scales, such as thermohaline intrusions, gravity waves and thermohaline staircases. In this paper, we present an exhaustive study of the phenomenon from small to large scales. We perform the first three-dimensional simulations of the process at realistic values of the heat and salt diffusivities and provide accurate estimates of the induced turbulent transport. Our results are consistent with oceanic field measurements of diapycnal mixing in fingering regions. We then develop a generalized mean-field theory to study the stability of fingering systems to large-scale perturbations, using our calculated turbulent fluxes...
Allali, Karam; Belhaq, Mohamed; El Karouni, Kamal
2012-04-01
The influence of a time-dependent gravity on the convective instability of reaction fronts in porous media is investigated in this paper. It is assumed that the time-dependent modulation is quasi-periodic with two frequencies σ1 and σ2 that are incommensurate with each other. The model consists of the heat equation, the equation for the depth of conversion and the equations of motion under the Darcy law. The convective threshold is approximated performing a linear stability analysis on a reduced singular perturbation problem using the matched asymptotic expansion method. The reduced interface problem is solved using numerical simulations. It is shown that if the reacting fluid is heated from below, a stabilizing effect of a reaction fronts in a porous medium can be gained for appropriate values of amplitudes and frequencies ratio σ={σ2}/{σ1} of the quasi-periodic vibration.
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.
Revisiting the theoretical DBV (V777 Her) instability strip: the MLT theory of convection
Córsico, A H; Bertolami, M M Miller; Garc\\'\\ia-Berro, E
2008-01-01
We reexamine the theoretical instability domain of pulsating DB white dwarfs (DBV or V777 Her variables). We performed an extensive $g$-mode nonadiabatic pulsation analysis of DB evolutionary models considering a wide range of stellar masses, for which the complete evolutionary stages of their progenitors from the ZAMS, through the thermally pulsing AGB and born-again phases, the domain of the PG1159 stars, the hot phase of DO white dwarfs, and then the DB white dwarf stage have been considered. We explicitly account for the evolution of the chemical abundance distribution due to time-dependent chemical diffusion processes. We examine the impact of the different prescriptions of the MLT theory of convection and the effects of small amounts of H in the almost He-pure atmospheres of DB stars on the precise location of the theoretical blue edge of the DBV instability strip.
BUOYANCY INSTABILITY IN THE NATURAL CONVECTION BOUNDARY LAYER AROUND A VERTICAL HEATED FLAT PLATE
Institute of Scientific and Technical Information of China (English)
颜大椿; 张汉勋
2002-01-01
A systematic research on the buoyancy instability in the natural convection boundary layer was conducted, including the basic characteristics such as its spectral components, wave length and velocity, the location of its critical layer,and amplitude distributions of the triple independent eigenmodes with the linear instability theory, the growth rates of its temperature and velocity fluctuations and the corresponding neutral curves for the buoyancy eigenmode were also obtained.Results indicated that the neutral curve of the velocity fluctuation had a nose shape consistent with that obtained in the numerical calculation, but for the temperature fluctuation, a ring-like region could be measured at a lower Grashof number before the nose-shaped main portion of the neutral curve.
Convective-absolute nature of ripple instabilities on ice and icicles
Camporeale, Carlo; Vesipa, Riccardo; Ridolfi, Luca
2017-05-01
Film hydrodynamics is crucial in water-driven morphological pattern formation. A prominent example is given by icicle ripples and ice ripples, which are regular patterns developing on freezing-melting inclined surfaces bounding open-channel flows. By a suitable mathematical model based on conservation principles and the use of the cusp map method, in this paper we address the convective-absolute nature of these two kinds of instabilities. The obtained results show that icicle ripples, which develop at inverted (overhang) conditions, have subcentimetric wavelengths which are unstable when the Reynolds number of the liquid flow (Re ) is small and the supercooling is intensive. With the increase in Re , the instability switches from absolute to convective. Ice ripples instead exhibit the opposite dependance on Re and are highly affected by the surface slope. In addition, the evaluation of the so-called absolute wave number, which is responsible for the asymptotic impulse response, suggests a different interpretation of some recent experiments about ice ripples.
Gelfgat, Alexander
2015-01-01
Transition from steady to oscillatory buoyancy convection of air in a laterally heated cubic box is studied numerically by straight-forward time integration of Boussinesq equations using a series of gradually refined finite volume grids. Horizontal and spanwise cube boundaries are assumed to be either perfectly thermally conducting or perfectly thermally insulated, which results in four different sets of thermal boundary conditions. Critical Grashof numbers are obtained by interpolation of numerically extracted growth/decay rates of oscillations amplitude to zero. Slightly supercritical flow regimes are described by time-averaged flows, snapshots, and spatial distribution of oscillations amplitude. Possible similarities and dissimilarities with two-dimensional instabilities in laterally heated square cavities are discussed. Arguments for grid and time step independence of the results are given.
de Valença, Joeri C.; Kurniawan, Aziz; Wagterveld, R. Martijn; Wood, Jeffery A.; Lammertink, Rob G. H.
2017-03-01
We investigate the influence of buoyancy on electroconvection at an ion-exchange membrane in an aqueous electrolyte solution. Electrokinetic instabilities (EKIs) and Rayleigh-Bénard (RB) convection are both known to mix the appearing concentration gradient layer and overcome the limiting current arising from diffusional limitations. The different physics, as well as the interplay between them, are investigated by electrical, flow, and concentration characterization. In the buoyancy stable orientation, an EKI mixing layer, having a low concentration, grows till saturated size. In the buoyancy unstable orientation, RB occurs and dominates the advective transport due to the large system size. When current density i 5 ilim EKI starts before RB and hastens the onset of RB. Upon onset of RB, EKI is suppressed while the overall resistance is still decreased. The onset times of EKI and RB could be predicted using a simple diffusion-migration model based on Fick's second law.
Instability onset and mixing by diffusive Rayleigh-Benard Convection in a Hele-Shaw Cell
Ehyaei, Dana; Kiger, Ken
2012-11-01
The injection and eventual dissolution of carbon dioxide in deep saline aquifers has suggested as an effective means of carbon sequestration. Typical injection conditions produce a buoyantly stable source of CO2 layered on top of the brine, whose dissolution is greatly accelerated by the onset of dissolution-driven, negatively buoyant, convective plumes that develop at the interface. The current work is a study conducted within a Hele-shaw cell, as an analogue for porous media, using working fluids that are mixtures of methanol and ethylene glycol diffusing in water, imitating the convective behavior of CO2 in the brine. The underlying physics of the flow are examined by measuring the velocity field directly via PIV, using appropriate methods to allow quantitative measurement in this thin-gap flow. This technique allows for detailed measurement of the entire evolution of the velocity and vorticity field during onset, growth and saturation of the instabilities. Features of the flow, the mechanisms that govern it and accurate time scales form onset time to later time mixings would be discussed for different Rayleigh numbers ranging from 2000 to 15000.
Zhou, S.-Q.; Qu, L.; Lu, Y.-Z.; Song, X.-L.
2014-02-01
In the present study, the classical description of diffusive convection is updated to interpret the instability of diffusive interfaces and the dynamical evolution of the bottom layer in the deep Arctic Ocean. In the new consideration of convective instability, both the background salinity stratification and rotation are involved. The critical Rayleigh number of diffusive convection is found to vary from 103 to 1011 in the deep Arctic Ocean as well as in other oceans and lakes. In such a wide range of conditions, the interface-induced thermal Rayleigh number is shown to be consistent with the critical Rayleigh number of diffusive convection. In most regions, background salinity stratification is found to be the main hindrance to the occurrence of convecting layers. With the new parameterization, it is predicted that the maximum thickness of the bottom layer is 1051 m in the deep Arctic Ocean, which is close to the observed value of 929 m. The evolution time of the bottom layer is predicted to be ~ 100 yr, which is on the same order as that based on 14C isolation age estimation.
Convection under rotation for Prandtl numbers near 1: K{umlt u}ppers-Lortz instability
Energy Technology Data Exchange (ETDEWEB)
Hu, Yuchou; Ecke, Robert E. [Condensed-Matter and Thermal Physics and Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Ahlers, Guenter [Department of Physics and Center for Nonlinear Sciences, University of California at Santa Barbara, Santa Barbara, California 93106 (United States)] Pesch, Werner [Physikalisches Institut der Universitaet Bayreuth, 95440 Bayreuth (Germany)
1998-11-01
The K{umlt u}ppers-Lortz (KL) instability in Rayleigh-B{acute e}nard convection rotated about a vertical axis was studied experimentally using optical-shadowgraph imaging in the rotating frame for dimensionless rotation rates 6{lt}{Omega}{lt}20. Two cylindrical convection cells with radius-to-height ratios {Gamma}=40 and 23 were used. The cells contained CO{sub 2} at 33.1 bar and 16.6 bar with Prandtl numbers {sigma}=0.93 and {sigma}=0.83, respectively. Numerical solutions of the Boussinesq equations with parameter values corresponding to the experiments were obtained for comparison. For {Gamma}=40 and 8{lt}{Omega}{lt}10.5, the initial pattern above onset was time dependent. Its dynamics revealed a mixture of sidewall-nucleated domain-wall motion characteristic of the KL instability and of dislocation-defect motion. For {Omega}{gt}10.5, spontaneous formation of KL domain walls away from the sidewall was observed. For 8{lt}{Omega}{lt}12, there were differences between the two cells very close to onset, but for {epsilon}{approx_gt}0.02 the systems were qualitatively similar. For {Omega}{approx_gt}12 there was no qualitative difference in the behavior of the two cells at any {epsilon}. The average size of a domain containing rolls of approximately the same orientation decreased with increasing {Omega}, and the time dependence speeded up and became dominated by domain-wall propagation. The numerical solutions were qualitatively similar, although there was a tendency for the domains to be larger at the same {epsilon} and {Omega}. The replacement of domains of one orientation by those with another led to a rotation in Fourier space which was characterized by a rotation frequency {omega}{sub a} in the frame rotating at angular velocity {Omega}. Quantitative experimental measurements of {omega}{sub a}, of a correlation length {xi}, and of a domain-switching angle {Theta}{sub s} as functions of {epsilon}{equivalent_to}{Delta}T/{Delta}T{sub c}{minus}1 and {Omega} are
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...
Nandukumar, Yada
2015-01-01
We investigate oscillatory instability and routes to chaos in Rayleigh-B\\'enard convection of electrically conducting fluids in presence of external horizontal magnetic field. Three dimensional direct numerical simulations (DNS) of the governing equations are performed for the investigation. DNS shows that oscillatory instability is inhibited by the magnetic field. The supercritical Rayleigh number for the onset of oscillation is found to scale with the Chandrasekhar number $\\mathrm{Q}$ as $\\mathrm{Q}^{\\alpha}$ in DNS with $\\alpha = 1.8$ for low Prandtl numbers ($\\mathrm{Pr}$). Most interestingly, DNS shows $\\mathrm{Q}$ dependent routes to chaos for low Prandtl number fluids like mercury ($\\mathrm{Pr} = 0.025$). For low $\\mathrm{Q}$, period doubling routes are observed, while, quasiperiodic routes are observed for high $\\mathrm{Q}$. The bifurcation structure associated with $\\mathrm{Q}$ dependent routes to chaos is then understood by constructing a low dimensional model from the DNS data. The model also shows...
Oscillatory instability of convection in ferromagnetic nanofluid and in transformer oil
Krauzina, Marina T.; Bozhko, Aleksandra A.; Krauzin, Pavel V.; Suslov, Sergey A.
2016-12-01
Stability of a mechanical equilibrium of ferromagnetic nanofluid and transformer oil in a spherical cavity carved inside a Plexiglas block heated from below is investigated experimentally. It is shown that in a fluid left at rest prior to the start of experiment the onset of convection is delayed and the convective motion arises abruptly acquiring a finite equilibrium amplitude at supercritical temperature differences. Convection has an oscillatory character associated with a precession of a vortex axis in the equatorial plane. A hysteresis is detected in the reverse transition to a quiescent state. Oscillations in the investigated ferro-nanofluid have been observed during the complete experimental runs (up to several weeks). Similar oscillations in transformer oil have been detected only in the beginning of experimental runs. They were observed to eventually decay resulting in a stationary convection.
Sherman, James P.; She, Chiao-Yao
2006-06-01
One thousand three hundred and eleven 15-min profiles of nocturnal mesopause region (80 105 km) temperature and horizontal wind, observed by Colorado State University sodium lidar over Fort Collins, CO (41°N, 105°W), between May 2002 and April 2003, were analyzed. From these profiles, taken over 390 h and each possessing vertical resolution of 2 km, a statistical analysis of seasonal variations in wind shears, convective and dynamical instabilities was performed. Large wind shears were most often observed near 100 km and during winter months. Thirty-five percent of the winter profiles contained wind shears exceeding 40 m/s per km at some altitude. In spite of large winds and shears, the mesopause region (at a resolution of 2 km and 15 min) is a very stable region. At a given altitude, the probability for convective instability is less than 1.4% for all seasons and the probability for dynamic instability (in the sense of Richardson number) ranges from 2.7% to 6.0%. Wind shear measurements are compared with four decades of chemical release measurements, compiled in a study by Larson [2002. Winds and shears in the mesosphere and lower thermosphere: results from four decades of chemical release wind measurements. Journal of Geophysical Research 107(A8), 1215]. Instability results are compared with those deduced from an annual lidar study conducted with higher spatial and temporal resolution at the Starfire Optical Range (SOR) in Albuquerque, NM, by Zhao et al. [2003. Measurements of atmospheric stability in the mesopause region at Starfire Optical Range, NM. Journal of Atmospheric and Solar-Terrestrial Physics 65, 219 232], and from a study by Li et al. [2005b. Characteristics of instabilities in the mesopause region over Maui, Hawaii. Journal of Geophysical Research 110, D09S12] with 19 days of data acquired from Maui Mesosphere and Lower Thermosphere (Maui MALT) Campaign . The Fort Collins lidar profiles were also analyzed using 1-h temporal resolution to compare
Allali, K.; Belhaq, M.
This work gives an overview on the effect of vertical periodic and QP gravitational modulations on the convective instability of reaction fronts in porous media. The model consists of the heat equation, the equation for the depth of conversion and the equations of motion under the Darcy law. Attention is focused on two cases. The case of a periodic gravitational vibration with a modulated amplitude, and the case of quasi-periodic vibration having two incommensurate frequencies. In both cases the heating is acted from below such that the sense of reaction is opposite to the gravity sense. The convective instability threshold is obtained by reducing the original reaction-diffusion problem to a singular perturbation one using the matched asymptotic expansion. The obtained reduced problem is then solved numerically after performing the linear stability analysis of the steady-state solution for the interface. It is shown that in the case of the modulation of the periodic vibration amplitude, a destabilizing effect of reaction fronts can be gained for a frequency modulation equal to half the frequency of the vibration, whereas a stabilizing effect is observed when the frequency of the modulation is twice that of the vibration. In the case of a quasi-periodic gravitational vibration it is indicated that for appropriate values of amplitudes and frequencies ratio of the quasi-periodic excitation, a stabilizing effect of reaction fronts can be successfully achieved.
Convective instability in a ferrofluid layer with temperature-modulated rigid boundaries
Energy Technology Data Exchange (ETDEWEB)
Singh, Jitender [Department of Mathematics, Guru Nanak Dev University, Amritsar 143005 (India); Bajaj, Renu, E-mail: sonumaths@gmail.com, E-mail: rbajaj@pu.ac.in [Department of Mathematics, Panjab University, Chandigarh 160014 (India)
2011-04-15
Under terrestrial as well as gravity-free conditions, a time-periodic modulation in temperatures of two horizontal rigid planes containing an initially quiescent ferrofluid layer induces time-periodic oscillations in the fluid layer at the onset of instability. This results in a series of patterns of time-periodically oscillating magnetoconvective rolls, along the vertical. The onset of instability in the ferrofluid layer is either a harmonic response or a subharmonic response depending upon the modulation. The instability is found to be significantly affected by the application of magnetic field across the ferrofluid layer. Under modulation, subcritical instabilities are found to occur in the form of subharmonic response. Also, the onset of instability in the ferrofluid layer when it is driven solely by the magnetic forces alone is found to heavily depend upon the frequency of modulation, the effect being greatest for the low-frequency modulation and negligible for the case of high-frequency modulation. The gravity-free limit is also evaluated as a function of the magnetic susceptibility, under modulation. To carry out this extensive study, the classical Floquet theory is utilized.
Lahiri, T.; Pal Majumder, T.; Ghosh, N. K.
2014-07-01
Commercialization of ferroelectric liquid crystal displays (FLCDs) suffers from mechanical and electro-convective instabilities. Impurity ions play a pivotal role in the latter case, and therefore we developed a mean-field type model to understand the complex role of space charges, particularly ions in a ferroelectric liquid crystal. Considering an effective ion-chirality relation, we obtained a modified Poisson-Boltzmann equation for ions dissolved into a chiral solvent like the ferroelectric smectic phase. A nonuniform director profile induced by the mean electrostatic potential of the ions is then calculated by solving an Euler-Lagrange equation for a helically twisted smectic state. A combination of effects resulting from molecular chirality and an electrostatically driven twist created by the ions seems to produce this nonuniform fluctuation in the director orientation. Finally, both theoretical and experimental points of view are presented on the prediction of this mean-field model.
Analytical and experimental study of instabilities in buoyancy-driven convection in porous media
Energy Technology Data Exchange (ETDEWEB)
Georgiadis, J.G.; Behringer, R.; Johnson, G.A.
1992-04-01
During the second year of support under the DOE grant, significant progress was made in two directions: (1) Visualization of structure and tow field in randomly packed beds via Magnetic Resonance Imaging, and (2) Shadowgraphic visualization of natural convection in porous systems. This report describes the activities in detail, cite publications which resulted from this project, and conclude with plans for the last phase of the experimental investigation.
The competition of convective and absolute instabilities in rotating-disk flow transition
Imayama, Shintaro; Alfredsson, P. Henrik; Lingwood, R. J.
2014-11-01
The main objective of this experimental study is to investigate laminar-turbulent transition mechanisms in the rotating-disk boundary-layer flow. Lingwood (1995) found that the flow becomes locally absolutely unstable above a critical Reynolds number and suggested that absolutely unstable travelling waves triggered nonlinearity leading to transition. However, the growth of convectively unstable stationary vortices is also a possible alternative route if the surface roughness of the disk is sufficiently large. The convectively unstable stationary vortices are attributed to an inviscid crossflow mechanism. Flow-visualization studies and hot-wire measurements of the rotating-disk boundary layer typically capture 28-32 stationary vortices in the transition regime (e.g. Imayama et al. 2014). The hot-wire measurements presented here were performed on a smooth glass disk with a diameter of 474 mm. To excite stationary vortices disk-shaped roughness elements with a diameter of 2 mm and a height of 5 micron were put on the disk at a radial position of 110 mm. In the presentation, the details of the convectively unstable stationary vortices in the rotating-disk boundary layer are shown and compared with travelling waves and similarities/differences in the turbulent transition discussed. This work is supported by the Swedish Research Council (VR) and the Linné FLOW Centre.
Rayleigh-Bénard convection instability in the presence of temperature variation at the lower wall
Directory of Open Access Journals (Sweden)
Jovanović Miloš M.
2012-01-01
Full Text Available This paper analyzes the two-dimensional viscous fluid flow between two parallel plates, where the lower plate is heated and the upper one is cooled. The temperature difference between the plates is gradually increased during a certain time period, and afterwards it is temporarily constant. The temperature distribution on the lower plate is not constant in x-direction, and there is longitudinal sinusoidal temperature variation imposed on the mean temperature. We investigate the wave number and amplitude influence of this variation on the stability of Rayleigh-Benard convective cells, by direct numerical simulation of 2-D Navier-Stokes and energy equation.
Convective and absolute instabilities in counter-rotating spiral Poiseuille flow
Energy Technology Data Exchange (ETDEWEB)
Langenberg, J.; Heise, M.; Pfister, G. [University of Kiel, Institute of Experimental and Applied Physics, Kiel (Germany); Abshagen, J. [University of Kiel, Leibniz-Institute of Marine Science, Kiel (Germany)
2004-11-01
We present results of an experimental study on the stability of Taylor-Couette flow in case of counter-rotating cylinders and an imposed axial through flow. We are able to confirm results form recent numerical investigations done by Pinter et al. [24] by measuring the absolute and convective stability boundaries of both propagating Taylor vortices (PTV) and spiral vortices (SPI). Thus our work shows that these theoretical concepts from hydrodynamic stability in open flows apply to experimental counter-rotating Taylor-Couette systems with an imposed axial through flow. (orig.)
A viscous-convective instability in laminar Keplerian thin discs. II. Anelastic approximation
Shakura, N
2015-01-01
Using the anelastic approximation of linearised hydrodynamic equations, we investigate the development of axially symmetric small perturbations in thin Keplerian discs. The sixth-order dispersion equation is derived and numerically solved for different values of relevant physical parameters (viscosity, heat conductivity, disc semi-thickness and vertical structure). The analysis reveals the appearance of two overstable modes which split out from the classical Rayleigh inertial modes in a wide range of the parameters in both ionized and neutral gases. These modes have a viscous-convective nature and can serve as a seed for turbulence in astrophysical discs even in the absence of magnetic fields.
Convection instability of non-Newtonian Walter's nanofluid along a vertical layer
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Galal M. Moatimid
2017-04-01
Full Text Available The linear stability of viscoelastic nanofluid layer is investigated. The rheological behavior of the viscoelastic fluid is described through the Walter's model. The normal modes analysis is utilized to treat the equations of motion for stationary and oscillatory convection. The stability analysis resulted in a third-degree dispersion equation with complex coefficients. The Routh–Hurwitz theory is employed to investigate the dispersion relation. The stability criteria divide the plane into several parts of stable/unstable regions. This shows some analogy with the nonlinear stability theory. The relation between the elasticity and the longitudinal wave number is graphically analyzed. The numerical calculations show that viscoelastic flows are more stable than those of the Newtonian ones.
The driving factors of electro-convective instability in concentration polarization
Rubinstein, Isaak
2016-01-01
Ionic current through a charge-selective interface in a binary electrolyte is a basic element of many electrochemical engineering and microfluidic processes. Such current passage is diffusion-limited: it induces a decrease of electrolyte concentration towards the interface (concentration polarization, CP), expressed in the saturation of current upon increasing voltage at some value (limiting current, LC). With further increase of voltage, this saturation breaks down (overlimiting conductance, OLC). In open systems OLC is mediated by a microscale vortical flow which develops as a result of electroconvective instability (ECI) of quiescent CP near LC. Electroconvection (EC) is a flow driven by the electric force acting either upon the space charge of the interfacial EDL (electroosmosis, EO) or the residual space charge of the quasielectroneutral bulk (bulk EC). There are two types of EO, the equilibrium and the nonequilibrium one. The former relates to the action of the tangential electric field upon the space c...
Prediction of traffic convective instability with spectral analysis of the Aw–Rascle–Zhang model
Energy Technology Data Exchange (ETDEWEB)
Belletti, Francois, E-mail: francois.belletti@berkeley.edu [Department of Electrical Engineering and Computer Sciences, University of California, Berkeley (United States); Huo, Mandy, E-mail: mhuo@berkeley.edu [Department of Physics, University of California, Berkeley (United States); Department of Mathematics, University of California, Berkeley (United States); Litrico, Xavier, E-mail: xavier.litrico@lyonnaise-des-eaux.fr [LyRE, R& D center of SUEZ environnement, Bordeaux (France); Bayen, Alexandre M., E-mail: bayen@berkeley.edu [Department of Electrical Engineering and Computer Sciences, University of California, Berkeley (United States); Department of Civil and Environmental Engineering, University of California, Berkeley (United States); Institute of Transportation Studies, University of California, Berkeley (United States)
2015-10-09
Highlights: • We linearize and diagonalize the ARZ model. We give a Froude number for traffic. • Spectral domain transfer functions are derived and decompose the model. • The linearized system is convectively unstable in the free-flow regime. • We conduct experiments with the linearized model on the NGSIM dataset. • We show that the linearization does not destroy the accuracy of the model. - Abstract: This article starts from the classical Aw–Rascle–Zhang (ARZ) model for freeway traffic and develops a spectral analysis of its linearized version. A counterpart to the Froude number in hydrodynamics is defined that enables a classification of the nature of vehicle traffic flow using the explicit solution resulting from the analysis. We prove that our linearization about an equilibrium is stable for congested regimes and unstable otherwise. NGSIM data for congested traffic trajectories is used so as to confront the linearized model's predictions to actual macroscopic behavior of traffic. The model is shown to achieve good accuracy for speed and flow. In particular, it accounts for the advection of oscillations on boundaries into the interior domain where the PDE under study is solved.
The instability of a horizontal magnetic field in an atmosphere stable against convection
Parker, E. N.
1979-01-01
The theoretical problem posed by the buoyant escape of a magnetic field from the interior of a stably stratified body bears directly on the question of the present existence of primordial magnetic fields in stars. This paper treats the onset of the Rayleigh-Taylor instability of the upper boundary of a uniform horizontal magnetic field in a stably stratified atmosphere. The calculations are carried out in the Boussinesq approximation and show the rapid growth of the initial infinitesimal perturbation of the boundary. This result is in contrast to the extremely slow buoyant rise of a separate flux tube in the same atmosphere. Thus for instance, at a depth of 1/3 of a solar radius beneath the surface of the sun, a field of 100 G develops ripples over a scale of 1000 km in a characteristic time of 50 years, whereas the characteristic rise time of the same field in separate flux tubes with the same dimensions is 10 billion years. Thus, the development of irregularities proceeds quickly, soon slowing, however, to a very slow pace when the amplitude of the irregularities becomes significant. Altogether, the calculations show the complexity of the question of the existence of remnant primordial magnetic fields in stellar interiors.
Yano, T.; Nishino, K.; Ueno, I.; Matsumoto, S.; Kamotani, Y.
2017-04-01
This paper reports the sensitivity of hydrothermal wave (HTW) instability of Marangoni convection to the interfacial heat transfer in liquid bridges (LBs) of high Prandtl number fluids (Pr = 67, 112, and 207) formed under the microgravity environment on the International Space Station. The data for instability are collected for a wide range of AR and for TC = 15 and 20 °C, where AR is the aspect ratio (=height/diameter) of the LB and TC is the cooled disk temperature. A significant decrease in critical oscillation frequency as well as an appreciable decrease in the critical Marangoni number is observed for AR > 1.25. This drastic change of instability mechanisms is associated with the reversal of axial traveling direction of HTWs and roll-structures as reported previously. It is found that this reversal is closely related to the interfacial heat transfer, which is evaluated numerically through accounting for both convective and radiative components. A heat transfer ratio, QI/QH, is introduced as a dimensionless parameter for interfacial heat transfer, where QI and QH are the heat transfer rates at the LB-gas and LB-heated disk interfaces, respectively. It is found that HTWs travel in the same direction as the surface flow for QI/QH > 0 (heat-loss condition) while in the opposite direction for QI/QH alters slightly but appreciably the basic temperature and flow field, the alteration that is not accounted for in the previous linear stability analyses for an infinite LB.
Energy Technology Data Exchange (ETDEWEB)
Georgiadis, J.G.; Behringer, R.; Johnson, G.A.
1992-04-01
During the second year of support under the DOE grant, significant progress was made in two directions: (1) Visualization of structure and tow field in randomly packed beds via Magnetic Resonance Imaging, and (2) Shadowgraphic visualization of natural convection in porous systems. This report describes the activities in detail, cite publications which resulted from this project, and conclude with plans for the last phase of the experimental investigation.
Institute of Scientific and Technical Information of China (English)
陈出新; 郭孝城
2003-01-01
Magnetoconvective instabilities in a rapidly rotating, electrically conducting fluid layer heated from below in the presence of a non-uniform, horizontal magnetic field are investigated. It was first shown by Chandrasekhar that an overall minimum of the Rayleigh number may be reached at the onset of magnetoconvection when a uniform basic magnetic field is imposed. In this paper, we show that the properties of instability can be quite different when a non-uniform basic magnetic field is applied. It is shown that there is an optimum value of the Elsasser number provided that the basic magnetic field is a monotonically decreasing or increasing function of the vertical coordinate. However,there exist no optimum values of the Elsasser number that can give rise to an overall minimum of the Rayleigh number at the onset of magnetoconvection if the imposed basic magnetic field has an infiexion point.
Parker, E. N.
1987-01-01
The dynamics of thermal shadows which develop in the convective zone of a star around an insulating obstacle such as a horizontal band in intense magnetic field are studied. The depth of the shadow on the cool side of the obstacle is found to depend largely on the width of the obstacle multiplied by the temperature gradient. Thermal shadows pressing fields up to 10,000 G downward against the bottom of the convective zone are produced by the broad bands of the azimuthal field in the sun's convective zone. In the third part, the time-dependent accumulation of heat beneath a thermal barrier simulating such a band in the lower convective zone of the sun is considered. The resulting Rayleigh-Taylor instability is shown to cause tongues of heated gas to penetrate upward through the field, providing the emerging magnetic fields that give rise to the activity of the sun.
Gelfgat, Alexander Yu.
2017-08-01
Transition from steady to oscillatory buoyancy convection of air in a laterally heated cubic box is studied numerically by straight-forward time integration of Boussinesq equations using a series of gradually refined finite volume grids. Horizontal and spanwise cube boundaries are assumed to be either perfectly thermally conducting or perfectly thermally insulated, which results in four different sets of thermal boundary conditions. Critical Grashof numbers are obtained by interpolation of numerically extracted growth/decay rates of oscillation amplitude to zero. Slightly supercritical flow regimes are described by time-averaged flows, snapshots, and spatial distribution of the oscillation amplitude. Possible similarities and dissimilarities with two-dimensional instabilities in laterally heated square cavities are discussed. Break of symmetries and sub- or supercritical character of bifurcations are examined. Three consequent transitions from steady to the oscillatory regime, from the oscillatory to the steady regime, and finally to the oscillatory flow, are found in the case of perfectly insulated horizontal and spanwise boundaries. Arguments for grid and time-step independence of the results are given.
Energy Technology Data Exchange (ETDEWEB)
Garcia Velarde, M.
1977-07-01
Thermo convective instabilities in horizontal fluid layers are discussed with emphasis on the Rayleigh-Bernard model problem. Steady solutions and time-dependent phenomena (relaxation oscillations and transition to turbulence) are studied within the nonlinear Boussinesq-Oberbeck approximation. Homogeneous steady solutions, limit cycles, and inhomogeneous (ordered) spatial structures are also studied in simple reaction-diffusion systems. Lastly, the non-periodic attractor that appears at large Rayleigh numbers in the truncated Boussinesq-Oberbeck model of Lorenz, is constructed, and a discussion of turbulent behavior is given. (Author) 105 refs.
Energy Technology Data Exchange (ETDEWEB)
Bau, H.H. [Univ. of Pennsylvania, Philadelphia, PA (United States)
1995-12-31
Using stability theory, numerical simulations, and in some instances experiments, it is demonstrated that the critical Rayleigh number for the bifurcation (1) from the no-motion (conduction) state to the motion state and (2) from time-independent convection to time-dependent, oscillatory convection in the thermal convection loop and Rayleigh-Benard problems can be significantly increased or decreased. This is accomplished through the use of a feedback controller effectuating small perturbations in the boundary data. The controller consists of sensors which detect deviations in the fluid`s temperature from the motionless, conductive values and then direct actuators to respond to these deviations in such a way as to suppress the naturally occurring flow instabilities. Actuators which modify the boundary`s temperature/heat flux are considered. The feedback controller can also be used to control flow patterns and generate complex dynamic behavior at relatively low Rayleigh numbers.
Convection in Type 2 supernovae
Energy Technology Data Exchange (ETDEWEB)
Miller, D.S.
1993-10-15
Results are presented here from several two dimensional numerical calculations of events in Type II supernovae. A new 2-D hydrodynamics and neutrino transport code has been used to compute the effect on the supernova explosion mechanism of convection between the neutrinosphere and the shock. This convection is referred to as exterior convection to distinguish it from convection beneath the neutrinosphere. The model equations and initial and boundary conditions are presented along with the simulation results. The 2-D code was used to compute an exterior convective velocity to compare with the convective model of the Mayle and Wilson 1-D code. Results are presented from several runs with varying sizes of initial perturbation, as well as a case with no initial perturbation but including the effects of rotation. The M&W code does not produce an explosion using the 2-D convective velocity. Exterior convection enhances the outward propagation of the shock, but not enough to ensure a successful explosion. Analytic estimates of the growth rate of the neutron finger instability axe presented. It is shown that this instability can occur beneath the neutrinosphere of the proto-neutron star in a supernova explosion with a growth time of {approximately} 3 microseconds. The behavior of the high entropy bubble that forms between the shock and the neutrinosphere in one dimensional calculations of supernova is investigated. It has been speculated that this bubble is a site for {gamma}-process generation of heavy elements. Two dimensional calculations are presented of the time evolution of the hot bubble and the surrounding stellar material. Unlike one dimensional calculations, the 2D code fails to achieve high entropies in the bubble. When run in a spherically symmetric mode the 2-D code reaches entropies of {approximately} 200. When convection is allowed, the bubble reaches {approximately} 60 then the bubble begins to move upward into the cooler, denser material above it.
Convection in Type 2 supernovae
Energy Technology Data Exchange (ETDEWEB)
Miller, Douglas Scott [Univ. of California, Davis, CA (United States)
1993-10-15
Results are presented here from several two dimensional numerical calculations of events in Type II supernovae. A new 2-D hydrodynamics and neutrino transport code has been used to compute the effect on the supernova explosion mechanism of convection between the neutrinosphere and the shock. This convection is referred to as exterior convection to distinguish it from convection beneath the neutrinosphere. The model equations and initial and boundary conditions are presented along with the simulation results. The 2-D code was used to compute an exterior convective velocity to compare with the convective model of the Mayle and Wilson 1-D code. Results are presented from several runs with varying sizes of initial perturbation, as well as a case with no initial perturbation but including the effects of rotation. The M&W code does not produce an explosion using the 2-D convective velocity. Exterior convection enhances the outward propagation of the shock, but not enough to ensure a successful explosion. Analytic estimates of the growth rate of the neutron finger instability axe presented. It is shown that this instability can occur beneath the neutrinosphere of the proto-neutron star in a supernova explosion with a growth time of ~ 3 microseconds. The behavior of the high entropy bubble that forms between the shock and the neutrinosphere in one dimensional calculations of supernova is investigated. It has been speculated that this bubble is a site for γ-process generation of heavy elements. Two dimensional calculations are presented of the time evolution of the hot bubble and the surrounding stellar material. Unlike one dimensional calculations, the 2D code fails to achieve high entropies in the bubble. When run in a spherically symmetric mode the 2-D code reaches entropies of ~ 200. When convection is allowed, the bubble reaches ~60 then the bubble begins to move upward into the cooler, denser material above it.
3D Convection-pulsation Simulations with the HERACLES Code
Felix, S.; Audit, E.; Dintrans, B.
2015-10-01
We present 3D simulations of the coupling between surface convection and pulsations due to the κ-mechanism in classical Cepheids of the red edge of Hertzsprung-Russell diagram's instability strip. We show that 3D convection is less powerful than 2D convection and does not quench the radiative pulsations, leading to an efficient 3D κ-mechanism. Thus, the 3D instability strip is closer to the observed one than the 1D or 2D were.
A Wildfire-relevant climatology of the convective environment of the United States
Brian E. Potter; Matthew A. Anaya
2015-01-01
Convective instability can influence the behaviour of large wildfires. Because wildfires modify the temperature and moisture of air in their plumes, instability calculations using ambient conditions may not accurately represent convective potential for some fire plumes. This study used the North American Regional Reanalysis to develop a climatology of the convective...
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.
Modulation instability: The beginning
Zakharov, V. E.; Ostrovsky, L. A.
2009-03-01
We discuss the early history of an important field of “sturm and drang” in modern theory of nonlinear waves. It is demonstrated how scientific demand resulted in independent and almost simultaneous publications by many different authors on modulation instability, a phenomenon resulting in a variety of nonlinear processes such as envelope solitons, envelope shocks, freak waves, etc. Examples from water wave hydrodynamics, electrodynamics, nonlinear optics, and convection theory are given.
Rotating convection in a viscoelastic magnetic fluid
Energy Technology Data Exchange (ETDEWEB)
Pérez, L.M. [Departamento de Fíisica y Matemática Aplicada, Universidad de Navarra, 31080 Pamplona (Spain); Laroze, D., E-mail: dlarozen@uta.cl [Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica (Chile); Díaz, P. [Departamento de Ciencias Físicas, Universidad de La Frontera, Casilla 54 D, Temuco (Chile); Martinez-Mardones, J. [Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso (Chile); Mancini, H.L. [Departamento de Fíisica y Matemática Aplicada, Universidad de Navarra, 31080 Pamplona (Spain)
2014-09-01
We report theoretical and numerical results on convection for a magnetic fluid in a viscoelastic carrier liquid under rotation. The viscoelastic properties are given by the Oldroyd model. We obtain explicit expressions for the convective thresholds in terms of the parameters of the system in the case of idealized boundary conditions. We also calculate numerically the convective thresholds for the case of realistic boundary conditions. The effects of the rheology and of the rotation rate on the instability thresholds for a diluted magnetic suspension are emphasized. - Highlights: • Ferrofluids. • Thermal convection. • Viscoelastic model. • Realistic boundary conditions.
Evaporative instabilities in climbing films
Hosoi, A. E.; Bush, John W. M.
2001-09-01
We consider flow in a thin film generated by partially submerging an inclined rigid plate in a reservoir of ethanol or methanol water solution and wetting its surface. Evaporation leads to concentration and surface tension gradients that drive flow up the plate. An experimental study indicates that the climbing film is subject to two distinct instabilities. The first is a convective instability characterized by flattened convection rolls aligned in the direction of flow and accompanied by free-surface deformations; in the meniscus region, this instability gives rise to pronounced ridge structures aligned with the mean flow. The second instability, evident when the plate is nearly vertical, takes the form of transverse surface waves propagating up the plate.
Theory of electrohydrodynamic instabilities in electrolytic cells
Bruinsma, R.; Alexander, S.
1990-01-01
The paper develops the theory of the hydrodynamic stability of an electrolytic cell as a function of the imposed electric current. A new electrohydrodynamic instability is encountered when the current is forced to exceed the Nernst limit. The convection is driven by the volume force exerted by the electric field on space charges in the electrolyte. This intrinsic instability is found to be easily masked by extrinsic convection sources such as gravity or stirring. A linear stability analysis is performed and a dimensionless number Le is derived whose value determines the convection pattern.
Layer Formation in Sedimentary Fingering Convection
Reali, J F; Alsinan, A; Meiburg, E
2016-01-01
When particles settle through a stable temperature or salinity gradient they can drive an instability known as sedimentary fingering convection. This phenomenon is thought to occur beneath sediment-rich river plumes in lakes and oceans, in the context of marine snow where decaying organic materials serve as the suspended particles, or in the atmosphere in the presence of aerosols or volcanic ash. Laboratory experiments of Houk and Green (1973) and Green (1987) have shown sedimentary fingering convection to be similar to the more commonly known thermohaline fingering convection in many ways. Here, we study the phenomenon using 3D direct numerical simulations. We find evidence for layer formation in sedimentary fingering convection in regions of parameter space where it does not occur for non-sedimentary systems. This is due to two complementary effects. Sedimentation affects the turbulent fluxes and broadens the region of parameter space unstable to the $\\gamma$-instability (Radko 2003) to include systems at l...
Destabilization of free convection by weak rotation
Gelfgat, Alexander
2011-01-01
This study offers an explanation of a recently observed effect of destabilization of free convective flows by weak rotation. After studying several models where flows are driven by a simultaneous action of convection and rotation, it is concluded that the destabilization is observed in the cases where centrifugal force acts against main convective circulation. At relatively low Prandtl numbers this counter action can split the main vortex into two counter rotating vortices, where the interaction leads to instability. At larger Prandtl numbers, the counter action of the centrifugal force steepens an unstable thermal stratification, which triggers Rayleigh-B\\'enard instability mechanism. Both cases can be enhanced by advection of azimuthal velocity disturbances towards the axis, where they grow and excite perturbations of the radial velocity. The effect was studied considering a combined convective/rotating flow in a cylinder with a rotating lid and a parabolic temperature profile at the sidewall. Next, explana...
Instability in Shocked Granular Gases
Sirmas, Nick; Radulescu, Matei
2013-01-01
Shocks in granular media, such as vertically oscillated beds, have been shown to develop instabilities. Similar jet formation has been observed in explosively dispersed granular media. Our previous work addressed this instability by performing discrete-particle simulations of inelastic media undergoing shock compression. By allowing finite dissipation within the shock wave, instability manifests itself as distinctive high density non-uniformities and convective rolls within the shock structure. In the present study we have extended this work to investigate this instability at the continuum level. We modeled the Euler equations for granular gases with a modified cooling rate to include an impact velocity threshold necessary for inelastic collisions. Our results showed a fair agreement between the continuum and discrete-particle models. Discrepancies, such as higher frequency instabilities in our continuum results may be attributed to the absence of higher order effects.
Instability in shocked granular gases
Sirmas, Nick; Falle, Sam; Radulescu, Matei
2014-05-01
Shocks in granular media, such as vertically oscillated beds, have been shown to develop instabilities. Similar jet formation has been observed in explosively dispersed granular media. Our previous work addressed this instability by performing discrete-particle simulations of inelastic media undergoing shock compression. By allowing finite dissipation within the shock wave, instability manifests itself as distinctive high density non-uniformities and convective rolls within the shock structure. In the present study we have extended this work to investigate this instability at the continuum level. We modeled the Euler equations for granular gases with a modified cooling rate to include an impact velocity threshold necessary for inelastic collisions. Our results showed a fair agreement between the continuum and discrete-particle models. Discrepancies, such as higher frequency instabilities in our continuum results may be attributed to the absence of higher order effects.
Convection in horizontally shaken granular material
Saluena, Clara; Poeschel, Thorsten
1998-01-01
In horizontally shaken granular material different types of pattern formation have been reported. We want to deal with the convection instability which has been observed in experiments and which recently has been investigated numerically. Using two dimensional molecular dynamics we show that the convection pattern depends crucial on the inelastic properties of the material. The concept of restitution coefficient provides arguments for the change of the behaviour with variing inelasticity.
Convection in Drying and Freezing Ground
Faizal, Mir
2012-01-01
In this paper we analyse the drying of a soil composed of particles, water and solute impurities, and study the occurrence of convective instabilities during evaporation. We find that the main driving force for instability is the formation of a concentration gradient at the soil surface due to the evaporation of water. A similar phenomenon may occur during the thawing of frozen ground in Arctic regions.
3-D nonlinear evolution of MHD instabilities
Energy Technology Data Exchange (ETDEWEB)
Bateman, G.; Hicks, H. R.; Wooten, J. W.
1977-03-01
The nonlinear evolution of ideal MHD internal instabilities is investigated in straight cylindrical geometry by means of a 3-D initial-value computer code. These instabilities are characterized by pairs of velocity vortex cells rolling off each other and helically twisted down the plasma column. The cells persist until the poloidal velocity saturates at a few tenths of the Alfven velocity. The nonlinear phase is characterized by convection around these essentially fixed vortex cells. For example, the initially centrally peaked temperature profile is convected out and around to form an annulus of high temperature surrounding a small region of lower temperature. Weak, centrally localized instabilities do not alter the edge of the plasma. Strong, large-scale instabilities, resulting from a stronger longitudinal equilibrium current, drive the plasma against the wall. After three examples of instability are analyzed in detail, the numerical methods and their verification are discussed.
Nonlinear simulations of the convection-pulsation coupling
Gastine, T
2011-01-01
In cold Cepheids close to the red edge of the classical instability strip, a strong coupling between the stellar pulsations and the surface convective motions occurs. This coupling is by now poorly described by 1-D models of convection, the so-called "time-dependent convection models" (TDC). The intrinsic weakness of such models comes from the large number of unconstrained free parameters entering in the description of turbulent convection. A way to overcome these limits is to compute two-dimensional direct simulations (DNS), in which all the nonlinearities are correctly solved. Two-dimensional DNS of the convection-pulsation coupling are presented here. In an appropriate parameter regime, convective motions can actually quench the radial pulsations of the star, as suspected in Cepheids close to the red edge of the instability strip. These nonlinear simulations can also be used to determine the limits and the relevance of the TDC models.
Amplitude equations for isothermal double diffusive convection
Energy Technology Data Exchange (ETDEWEB)
Becerril, R.; Swift, J.B. [Center for Nonlinear Dynamics and Department of Physics, University of Texas, Austin, Texas 78712 (United States)
1997-05-01
Amplitude equations are derived for isothermal double diffusive convection near threshold for both the stationary and oscillatory instabilities as well as in the vicinity of the codimension-2 point. The convecting fluid is contained in a thin Hele-Shaw cell that renders the system two dimensional, and convection is sustained by vertical concentration gradients of two species with different diffusion rates. The locations of the tricritical point for the stationary instability and the codimension-2 point are found. It is shown that these points can be made well separated (in the Rayleigh number R{sub s} of the slow diffusing species) as the Lewis number varies. Hence the behavior near these points should be experimentally accessible. {copyright} {ital 1997} {ital The American Physical Society}
On the convective overstability in protoplanetary discs
Latter, Henrik
2015-01-01
This paper explores the driving of low-level hydrodynamical activity in protoplanetary-disc dead zones. A small adverse radial entropy gradient, ordinarily stabilised by rotation, excites oscillatory convection (`convective overstability') when thermal diffusion, or cooling, is neither too strong nor too weak. I revisit the linear theory of the instability, discuss its prevalence in protoplanetary discs, and show that unstable modes are exact nonlinear solutions in the local Boussinesq limit. Overstable modes cannot grow indefinitely, however, as they are subject to a secondary parametric instability that limits their amplitudes to relatively low levels. If parasites set the saturation level of the ensuing turbulence then the convective overstability is probably too weak to drive significant angular momentum transport or to generate vortices. But I also discuss an alternative, and far more vigorous, saturation route that generates radial `layers' or `zonal flows' (witnessed also in semiconvection). Numerical ...
Multicloud convective parametrizations with crude vertical structure
Energy Technology Data Exchange (ETDEWEB)
Khouider, Boualem [University of Victoria, Mathematics and Statistics, PO BOX 3045 STN CSC, Victoria, BC (Canada); Majda, Andrew J. [New York University, Department of Mathematics and Center for Atmosphere/Ocean Sciences, Courant Institute, New York, NY (United States)
2006-11-15
Recent observational analysis reveals the central role of three multi-cloud types, congestus, stratiform, and deep convective cumulus clouds, in the dynamics of large scale convectively coupled Kelvin waves, westward propagating two-day waves, and the Madden-Julian oscillation. The authors have recently developed a systematic model convective parametrization highlighting the dynamic role of the three cloud types through two baroclinic modes of vertical structure: a deep convective heating mode and a second mode with low level heating and cooling corresponding respectively to congestus and stratiform clouds. The model includes a systematic moisture equation where the lower troposphere moisture increases through detrainment of shallow cumulus clouds, evaporation of stratiform rain, and moisture convergence and decreases through deep convective precipitation and a nonlinear switch which favors either deep or congestus convection depending on whether the troposphere is moist or dry. Here several new facets of these multi-cloud models are discussed including all the relevant time scales in the models and the links with simpler parametrizations involving only a single baroclinic mode in various limiting regimes. One of the new phenomena in the multi-cloud models is the existence of suitable unstable radiative convective equilibria (RCE) involving a larger fraction of congestus clouds and a smaller fraction of deep convective clouds. Novel aspects of the linear and nonlinear stability of such unstable RCE's are studied here. They include new modes of linear instability including mesoscale second baroclinic moist gravity waves, slow moving mesoscale modes resembling squall lines, and large scale standing modes. The nonlinear instability of unstable RCE's to homogeneous perturbations is studied with three different types of nonlinear dynamics occurring which involve adjustment to a steady deep convective RCE, periodic oscillation, and even heteroclinic chaos in
Nonlinear evolution of drift instabilities
Energy Technology Data Exchange (ETDEWEB)
Lee, W.W.; Krommes, J.A.; Oberman, C.R.; Smith, R.A.
1984-01-01
The nonlinear evolution of collisionless drift instabilities in a shear-free magnetic field has been studied by means of gyrokinetic particle simulation as well as numerical integration of model mode-coupling equations. The purpose of the investigation is to identify relevant nonlinear mechanisms responsible for the steady-state drift wave fluctuations. It is found that the saturation of the instability is mainly caused by the nonlinear E x B convection of the resonant electrons and their associated velocity space nonlinearity. The latter also induces energy exchange between the competing modes, which, in turn, gives rise to enhanced diffusion. The nonlinear E x B convection of the ions, which contributes to the nonlinear frequency shift, is also an important ingredient for the saturation.
The Role of the Magnetorotational Instability in the Sun
Kagan, Daniel
2014-01-01
We calculate growth rates for nonaxisymmetric instabilities including the magnetorotational instability (MRI) throughout the Sun. We first derive a dispersion relation for nonaxisymmetric instability including the effects of shear, convective buoyancy, and three diffusivities (thermal conductivity, resistivity, and viscosity). We then use a solar model evolved with the stellar evolution code MESA and angular velocity profiles determined by Global Oscillations Network Group (GONG) helioseismology to determine the unstable modes present at each location in the Sun and the associated growth rates. The overall instability has unstable modes throughout the convection zone and also slightly below it at middle and high latitudes. It contains three classes of modes: large-scale hydrodynamic convective modes, large-scale hydrodynamic shear modes, and small-scale magnetohydrodynamic (MHD) shear modes, which may be properly called MRI modes. While large-scale convective modes are the most rapidly growing modes in most o...
National Convective Weather Diagnostic
National Oceanic and Atmospheric Administration, Department of Commerce — Current convective hazards identified by the National Convective Weather Detection algorithm. The National Convective Weather Diagnostic (NCWD) is an automatically...
Model of oscillatory instability in vertically-homogeneous atmosphere
Directory of Open Access Journals (Sweden)
P. B. Rutkevich
2009-02-01
Full Text Available Existence and repeatability of tornadoes could be straightforwardly explained if there existed instability, responsible for their formation. However, it is well known that convection is the only instability in initially stable air, and the usual convective instability is not applicable for these phenomena. In the present paper we describe an instability in the atmosphere, which can be responsible for intense vortices. This instability appears in a fluid with Coriolis force and dissipation and has oscillatory behaviour, where the amplitude growth is accompanied by oscillations with frequency comparable to the growth rate of the instability. In the paper, both analytical analysis of the linear phase of the instability and nonlinear simulation of the developed stage of the air motion are addressed. This work was supported by the RFBR grant no. 09-05-00374-a.
Rayleigh-Bénard convection in a vertical annular container near the convection threshold.
Wang, Bo-Fu; Wan, Zhen-Hua; Ma, Dong-Jun; Sun, De-Jun
2014-04-01
The instabilities and transitions of flow in an annular container with a heated bottom, a cooled top, and insulated sidewalls are studied numerically. The instabilities of the static diffusive state and of axisymmetric flows are investigated by linear stability analysis. The onset of convection is independent of the Prandtl number but determined by the geometry of the annulus, i.e., the aspect ratio Γ (outer radius to height) and radius ratio δ (inner radius to outer radius). The stability curves for onset of convection are presented for 0.001≤δ≤0.8 at six fixed aspect ratios: Γ=1, 1.2, 1.6, 1.75, 2.5, and 3.2. The instability of convective flow (secondary instability), which depends on both the annular geometry and the Prandtl number, is studied for axisymmetric convection. Two pairs of geometric control parameters are chosen to perform the secondary instability analysis-Γ=1.2, δ=0.08 and Γ=1.6, δ=0.2-and the Prandtl number ranges from 0.02 to 6.7. The secondary instability exhibits some similarities to that for convection in a cylinder. A hysteresis stability loop is found for Γ=1.2, δ=0.08 and frequent changes of critical mode with Prandtl number are found for Γ=1.6, δ=0.2. The three-dimensional flows beyond the axisymmetry-breaking bifurcations are obtained by direct numerical simulation for Γ=1.2, δ=0.08.
Krysinski, Tomasz
2013-01-01
This book presents a study of the stability of mechanical systems, i.e. their free response when they are removed from their position of equilibrium after a temporary disturbance. After reviewing the main analytical methods of the dynamical stability of systems, it highlights the fundamental difference in nature between the phenomena of forced resonance vibration of mechanical systems subjected to an imposed excitation and instabilities that characterize their free response. It specifically develops instabilities arising from the rotor-structure coupling, instability of control systems, the se
Energy Technology Data Exchange (ETDEWEB)
K.Y. Ng
2003-08-25
The lecture covers mainly Sections 2.VIII and 3.VII of the book ''Accelerator Physics'' by S.Y. Lee, plus mode-coupling instabilities and chromaticity-driven head-tail instability. Besides giving more detailed derivation of many equations, simple interpretations of many collective instabilities are included with the intention that the phenomena can be understood more easily without going into too much mathematics. The notations of Lee's book as well as the e{sup jwt} convention are followed.
Evaporative Instability in Binary Mixtures
Narayanan, Ranga; Uguz, Erdem
2012-11-01
In this talk we depict the physics of evaporative convection for binary systems in the presence of surface tension gradient effects. Two results are of importance. The first is that a binary system, in the absence of gravity, can generate an instability only when heated from the vapor side. This is to be contrasted with the case of a single component where instability can occur only when heated from the liquid side. The second result is that a binary system, in the presence of gravity, will generate an instability when heated from either the vapor or the liquid side provided the heating is strong enough. In addition to these results we show the conditions at which interfacial patterns can occur. Support from NSF OISE 0968313, Partner Univ. Fund and a Chateaubriand Fellowship is acknowledged.
A New And Fundamental View Of Organized Tropical Convection
Webster, P. J.; Toma, V. E.
2012-12-01
During the last decade, a paradigm has emerged to explain the existence of tropical organized convection. Based on the projection of spatial and temporal patterns of observed convection onto dispersion relationships of equatorially trapped very shallow modes (h=10-30 m, where h is the equivalent depth of a shallow fluid) the convectively coupled equatorial mode (CCEM) theory has developed. However, there is an incompleteness and some inconsistencies in the theory that need to be addressed. Whereas the horizontal structure of these shallow modes appears similar to that observed, the vertical structure consistent with small h requires a high vertical wave number. This is not observed. Second, basic scaling of the tropics, as initially undertaken by Charney in the 1960s suggests an extremely stable vertical structure, far more stable than equivalent scales at higher latitudes. In fact, at the scales of observed organized convection in the tropics (about 106m) the atmosphere is essentially barotropic to high approximation resulting in almost complete lack of communication between the upper and lower troposphere. The CCEM theory suggests that the observed modes are consistent with existing convection but there is no explanation of how the convection forms and organizes in this very stable tropical environment. It is also noted that there are discrete genesis regions of organized convection formation within the tropics and that organized convection does not occur indiscriminately. Based on these factors we propose that organized convection occurs through regional instabilities of the basic state in which vortex tube stretching overcomes the inherent stability restriction. The instabilities determine the spatial and temporal scales of the convective phenomena. We provide examples of instabilities. Further, in certain regions, influences from higher latitudes may be important. In the end, CCEMs appears as a result and not an explanation or a cause of organized convection.
Stability Analysis of Convection in the Intracluster Medium
Gupta, Himanshu; Pessah, Martin E; Chakraborty, Sagar
2016-01-01
We use the machinery usually employed for studying the onset of Rayleigh--B\\'enard convection in hydro- and magnetohydro-dynamic settings to address the onset of convection induced by the magnetothermal instability and the heat-flux-buoyancy-driven-instability in the weakly-collisional magnetized plasma permeating the intracluster medium. Since most of the related numerical simulations consider the plasma being bounded between two `plates' on which boundary conditions are specified, our strategy provides a framework that could enable a more direct connection between analytical and numerical studies. We derive the conditions for the onset of these instabilities considering the effects of induced magnetic tension resulting from a finite plasma beta. We provide expressions for the Rayleigh number in terms of the wave vector associated with a given mode, which allow us to characterize the modes that are first to become unstable. For both the heat-flux-buoyancy-driven-instability and the magnetothermal instability...
A test of time-dependent theories of stellar convection
Gastine, T
2011-01-01
Context: In Cepheids close to the red edge of the classical instability strip, a coupling occurs between the acoustic oscillations and the convective motions close to the surface.The best topical models that account for this coupling rely on 1-D time-dependent convection (TDC) formulations. However, their intrinsic weakness comes from the large number of unconstrained free parameters entering in the description of turbulent convection. Aims: We compare two widely used TDC models with the first two-dimensional nonlinear direct numerical simulations (DNS) of the convection-pulsation coupling in which the acoustic oscillations are self-sustained by the kappa-mechanism. Methods: The free parameters appearing in the Stellingwerf and Kuhfuss TDC recipes are constrained using a chi2-test with the time-dependent convective flux that evolves in nonlinear simulations of highly-compressible convection with kappa-mechanism. Results: This work emphasises some inherent limits of TDC models, that is, the temporal variabilit...
Convective mixing in vertically-layered porous media: The linear regime and the onset of convection
Ghorbani, Zohreh; Riaz, Amir; Daniel, Don
2017-08-01
We study the effect of permeability heterogeneity on the stability of gravitationally unstable, transient, diffusive boundary layers in porous media. Permeability is taken to vary periodically in the horizontal plane normal to the direction of gravity. In contrast to the situation for vertical permeability variation, the horizontal perturbation structures are multimodal. We therefore use a two-dimensional quasi-steady eigenvalue analysis as well as a complementary initial value problem to investigate the stability behavior in the linear regime, until the onset of convection. We find that thick permeability layers enhance instability compared with thin layers when heterogeneity is increased. On the contrary, for thin layers the instability is weakened progressively with increasing heterogeneity to the extent that the corresponding homogeneous case is more unstable. For high levels of heterogeneity, we find that a small change in the permeability field results in large variations in the onset time of convection, similar to the instability event in the linear regime. However, this trend does not persist unconditionally because of the reorientation of vorticity pairs due to the interaction of evolving perturbation structures with heterogeneity. Consequently, an earlier onset of instability does not necessarily imply an earlier onset of convection. A resonant amplification of instability is observed within the linear regime when the dominant perturbation mode is equal to half the wavenumber of permeability variation. On the other hand, a substantial damping occurs when the perturbation mode is equal to the harmonic and sub-harmonic components of the permeability wavenumber. The phenomenon of such harmonic interactions influences both the onset of instability as well as the onset of convection.
Internal Wave Generation by Convection
Lecoanet, Daniel Michael
internal gravity wave spectrum, using the Lighthill theory of wave excitation by turbulence. We use a Green's function approach, in which we convolve a convective source term with the Green's function of different internal gravity waves. The remainder of the thesis is a circuitous attempt to verify these analytical predictions. I test the predictions of Chapter 2 via numerical simulation. The first step is to identify a code suitable for this study. I helped develop the Dedalus code framework to study internal wave generation by convection. Dedalus can solve many different partial differential equations using the pseudo-spectral numerical method. In Chapter 3, I demonstrate Dedalus' ability to solve different equations used to model convection in astrophysics. I consider both the propagation and damping of internal waves, and the properties of low Rayleigh number convective steady states, in six different equation sets used in the astrophysics literature. This shows that Dedalus can be used to solve the equations of interest. Next, in Chapter 4, I verify the high accuracy of Dedalus by comparing it to the popular astrophysics code Athena in a standard Kelvin-Helmholtz instability test problem. Dedalus performs admirably in comparison to Athena, and provides a high standard for other codes solving the fully compressible Navier-Stokes equations. Chapter 5 demonstrates that Dedalus can simulate convective adjacent to a stably stratified region, by studying convective mixing near carbon flames. The convective overshoot and mixing is well-resolved, and is able to generate internal waves. Confident in Dedalus' ability to study the problem at hand, Chapter 6 describes simulations inspired by water experiments of internal wave generation by convection. The experiments exploit water's unusual property that its density maximum is at 4°C, rather than at 0°C. We use a similar equation of state in Dedalus, and study internal gravity waves generation by convection in a water
Energy Technology Data Exchange (ETDEWEB)
Kaufman, A.N.; Brizard, A.J.; Morehead, J.J. [Lawrence Berkeley National Lab., CA (United States); Tracy, E.R. [College of William and Mary, Williamsburg, VA (United States)
1997-12-31
The resonant interaction of a negative-energy wave with a positive-energy wave gives rise to a linear instability. Whereas a single crossing of rays in a nonuniform medium leads to a convectively saturated instability, we show that a double crossing can yield an absolute instability.
DEFF Research Database (Denmark)
D'Angelo, N.
1967-01-01
A recombination instability is considered which may arise in a plasma if the temperature dependence of the volume recombination coefficient, alpha, is sufficiently strong. Two cases are analyzed: (a) a steady-state plasma produced in a neutral gas by X-rays or high energy electrons; and (b) an af...
Review of two-phase instabilities
Energy Technology Data Exchange (ETDEWEB)
Kang, Han Ok; Seo, Han Ok; Kang, Hyung Suk; Cho, Bong Hyun; Lee, Doo Jeong
1997-06-01
KAERI is carrying out a development of the design for a new type of integral reactors. The once-through helical steam generator is important design features. The study on designs and operating conditions which prevent flow instability should precede the introduction of one-through steam generator. Experiments are currently scheduled to understand two-phase instability, evaluate the effect of each design parameter on the critical point, and determine proper inlet throttling for the prevention of instability. This report covers general two-phase instability with review of existing studies on this topics. The general classification of two phase flow instability and the characteristics of each type of instability are first described. Special attention is paid to BWR core flow instability and once-through steam generator instability. The reactivity feedback and the effect of system parameters are treated mainly for BWR. With relation to once-through steam generators, the characteristics of convective heating and dryout point oscillation are first investigated and then the existing experimental studies are summarized. Finally chapter summarized the proposed correlations for instability boundary conditions. (author). 231 refs., 5 tabs., 47 figs
National Convective Weather Forecast
National Oceanic and Atmospheric Administration, Department of Commerce — The NCWF is an automatically generated depiction of: (1) current convection and (2) extrapolated signficant current convection. It is a supplement to, but does NOT...
Stabilization meshless method for convection-dominated problems
Institute of Scientific and Technical Information of China (English)
ZHANG Xiao-hua; OUYANG Jie; WANG Jian-yu
2008-01-01
It is well-known that the standard Galerkin is not ideally suited to deal with the spatial discretization of convection-dominated problems. In this paper, several tech- niques are proposed to overcome the instability issues in convection-dominated problems in the simulation with a meshless method. These stable techniques included nodal re- finement, enlargement of the nodal influence domain, full upwind meshless technique and adaptive upwind meshless technique. Numerical results for sample problems show that these techniques are effective in solving convection-dominated problems, and the adaptive upwind meshless technique is the most effective method of all.
Pattern formation in rotating Bénard convection
Fantz, M.; Friedrich, R.; Bestehorn, M.; Haken, H.
1992-12-01
Using an extension of the Swift-Hohenberg equation we study pattern formation in the Bénard experiment close to the onset of convection in the case of rotating cylindrical fluid containers. For small Taylor numbers we emphasize the existence of slowly rotating patterns and describe behaviour exhibiting defect motion. Finally, we study pattern formation close to the Küppers-Lortz instability. The instability is nucleated at defects and proceeds through front propagation into the bulk patterns.
Redeker, J; Vogt, P M
2011-01-01
Carpal instability can be understood as a disturbed anatomical alignment between bones articulating in the carpus. This disturbed balance occurs either only dynamically (with movement) under the effect of physiological force or even statically at rest. The most common cause of carpal instability is wrist trauma with rupture of the stabilizing ligaments and adaptive misalignment following fractures of the radius or carpus. Carpal collapse plays a special role in this mechanism due to non-healed fracture of the scaphoid bone. In addition degenerative inflammatory alterations, such as chondrocalcinosis or gout, more rarely aseptic bone necrosis of the lunate or scaphoid bones or misalignment due to deposition (Madelung deformity) can lead to wrist instability. Under increased pressure the misaligned joint surfaces lead to bone arrosion with secondary arthritis of the wrist. In order to arrest or slow down this irreversible process, diagnosis must occur as early as possible. Many surgical methods have been thought out to regain stability ranging from direct reconstruction of the damaged ligaments, through ligament replacement to partial stiffening of the wrist joint.
Nonadiabatic analysis of strange-modes in hot massive stars with time-dependent convection
Directory of Open Access Journals (Sweden)
Sonoi Takafumi
2015-01-01
Full Text Available We carry out nonadiabatic analysis of strange-modes in hot massive stars with time-dependent convection (TDC. We find that the instability of the modes excited at the Fe bump is weaker with TDC than with frozen-in convection (FC. But the instability still remains with TDC, and could be a possible candidate for the trigger of luminous blue variable (LBV phenomena.
Experimental and numerical investigation of wave ferrofluid convection
Energy Technology Data Exchange (ETDEWEB)
Bozhko, A.A. [Department of Physics, Perm State University, Bukirev Str. 15, 614990 Perm (Russian Federation)]. E-mail: bozhko@psu.ru; Putin, G.F. [Department of Physics, Perm State University, Bukirev Str. 15, 614990 Perm (Russian Federation); Tynjaelae, T. [Department of Energy and Environmental Engineering, Lappeenranta Univeristy of Technology, P.O. Box 20, Lappeenranta 53851 (Finland); Sarkomaa, P. [Department of Energy and Environmental Engineering, Lappeenranta Univeristy of Technology, P.O. Box 20, Lappeenranta 53851 (Finland)
2007-09-15
The stability of buoyancy-driven shear flow in an inclined layer of a ferrocolloid is investigated for different values of inclinations and homogeneous longitudinal magnetic fields. Near the onset of Rayleigh convection of ferrofluid layer inclined with respect to gravity, the wave oscillatory regimes were observed in experiments and numerical simulations. Visualization of convection patterns is provided by a temperature-sensitive liquid crystal film. As experiments testify, the origin of traveling wave regimes in ferrofluid is due to concentration gradients caused by gravity sedimentation of the magnetic particles. To study the effects of initial concentration gradient of particles, on convective instabilities, finite volume numerical simulations using a two-phase mixture model were carried out for the same setup. The most fascinating effect in ferrofluid convection is spontaneous formation of localized states, those where the convection chaotically focuses in confined regions and is absent in the remainder of cavity.
Numerical simulations of evaporative instabilities in sessile drops of ethanol on heated substrates
Semenov, Sergey; Carle, Florian; Medale, Marc; Brutin, David
2015-11-01
The work is focussed on numerical simulations of thermo-convective instabilities in evaporating pinned sessile droplets of ethanol on heated substrates. Computed evaporation rate of a droplet is validated against parabolic flight experiments and semi-empirical theory presented here. To the best authors' knowledge, this is the first study which combines theoretical, experimental and computational approaches in convective evaporation of sessile droplets. The influence of gravity level on evaporation rate and contributions of different mechanisms of vapor transport (diffusion, Stefan flow, natural convection) are shown. The qualitative difference (in terms of developing thermo-convective instabilities) between steady-state and unsteady numerical approaches is demonstrated.
FINANCIAL INSTABILITY AND POLITICAL INSTABILITY
Directory of Open Access Journals (Sweden)
Ionescu Cristian
2012-12-01
Full Text Available There is an important link between the following two variables: financial instability and political instability. Often, the link is bidirectional, so both may influence each other. This is way the lately crisis are becoming larger and increasingly complex. Therefore, the academic environment is simultaneously talking about economic crises, financial crises, political crises, social crises, highlighting the correlation and causality between variables belonging to the economic, financial, political and social areas, with repercussions and spillover effects that extend from one area to another. Given the importance, relevance and the actuality of the ones described above, I consider that at least a theoretical analysis between economic, financial and political factors is needed in order to understand the reality. Thus, this paper aims to find links and connections to complete the picture of the economic reality.
Marangoni Convection in Binary Mixtures
Zhang, J; Oron, A; Behringer, Robert P.; Oron, Alexander; Zhang, Jie
2006-01-01
Marangoni instabilities in binary mixtures are different from those in pure liquids. In contrast to a large amount of experimental work on Marangoni convection in pure liquids, such experiments in binary mixtures are not available in the literature, to our knowledge. Using binary mixtures of sodium chloride/water, we have systematically investigated the pattern formation for a set of substrate temperatures and solute concentrations in an open system. The flow patterns evolve with time, driven by surface-tension fluctuations due to evaporation and the Soret effect, while the air-liquid interface does not deform. A shadowgraph method is used to follow the pattern formation in time. The patterns are mainly composed of polygons and rolls. The mean pattern size first decreases slightly, and then gradually increases during the evolution. Evaporation affects the pattern formation mainly at the early stage and the local evaporation rate tends to become spatially uniform at the film surface. The Soret effect becomes i...
An instability in neutron stars at birth
Burrows, Adam; Fryxell, Bruce A.
1992-01-01
Calculations with a two-dimensional hydrodynamic simulation show that a generic Raleigh-Taylor-like instability occurs in the mantles of nascent neutron stars, that it is possibly violent, and that the standard spherically symmetric models of neutron star birth and supernova explosion may be inadequate. Whether this 'convective' instability is pivotal to the supernova mechanism, pulsar nagnetic fields, or a host of other important issues that attend stellar collapse remains to be seen, but its existence promises to modify all questions concerning this most energetic of astronomical phenomena.
A possible theory for the interaction between convective activities and vortical flows
Directory of Open Access Journals (Sweden)
N. Zhao
2011-10-01
Full Text Available Theoretical studies usually attribute convections to the developments of instabilities such as the static or symmetric instabilities of the basic flows. However, the following three facts make the validities of these basic theories unconvincing. First, it seems that in most cases the basic flow with balance property cannot exist as the exact solution, so one cannot formulate appropriate problems of stability. Second, neither linear nor nonlinear theories of dynamical instability are able to describe a two-way interaction between convection and its background, because the basic state which must be an exact solution of the nonlinear equations of motion is prescribed in these issues. And third, the dynamical instability needs some extra initial disturbance to trigger it, which is usually another point of uncertainty. The present study suggests that convective activities can be recognized in the perspective of the interaction of convection with vortical flow. It is demonstrated that convective activities can be regarded as the superposition of free modes of convection and the response to the forcing induced by the imbalance of the unstably stratified vortical flow. An imbalanced vortical flow provides not only an initial condition from which unstable free modes of convection can develop but also a forcing on the convection. So, convection is more appropriately to be regarded as a spontaneous phenomenon rather than a disturbance-triggered phenomenon which is indicated by any theory of dynamical instability. Meanwhile, convection, particularly the forced part, has also a reaction on the basic flow by preventing the imbalance of the vortical flow from further increase and maintaining an approximately balanced flow.
Convective wave front locking for a reaction-diffusion system in a conical flow reactor
DEFF Research Database (Denmark)
Kuptsov, P.V.; Kuznetsov, S.P.; Knudsen, Carsten
2002-01-01
We consider reaction-diffusion instabilities in a flow reactor whose cross-section slowly expands with increasing longitudinal coordinate (cone shaped reactor). Due to deceleration of the flow in this reactor, the instability is convective near the inlet to the reactor and absolute at the downstr...
The Role of the Magnetorotational Instability in the Sun
Kagan, Daniel; Wheeler, J. Craig
2014-05-01
We calculate growth rates for nonaxisymmetric instabilities including the magnetorotational instability (MRI) throughout the Sun. We first derive a dispersion relation for nonaxisymmetric instability including the effects of shear, convective buoyancy, and three diffusivities (thermal conductivity, resistivity, and viscosity). We then use a solar model evolved with the stellar evolution code MESA and angular velocity profiles determined by Global Oscillations Network Group helioseismology to determine the unstable modes present at each location in the Sun and the associated growth rates. The overall instability has unstable modes throughout the convection zone and also slightly below it at middle and high latitudes. It contains three classes of modes: large-scale hydrodynamic convective modes, large-scale hydrodynamic shear modes, and small-scale magnetohydrodynamic shear modes, which may be properly called MRI modes. While large-scale convective modes are the most rapidly growing modes in most of the convective zone, MRI modes are important in both stably stratified and convectively unstable locations near the tachocline at colatitudes θ grow faster than the corresponding axisymmetric modes; for some poloidal magnetic fields, the nonaxisymmetric MRI growth rates are similar to the angular rotation frequency Ω, while axisymmetric modes are stabilized. We briefly discuss the saturation of the field produced by MRI modes, finding that the implied field at the base of the convective zone in the Sun is comparable to that derived based on dynamos active in the tachocline and that the saturation of field resulting from the MRI may be of importance even in the upper convection zone.
The role of the magnetorotational instability in the sun
Energy Technology Data Exchange (ETDEWEB)
Kagan, Daniel; Wheeler, J. Craig, E-mail: kagan@astro.as.utexas.edu, E-mail: wheel@astro.as.utexas.edu [Department of Astronomy, University of Texas, Austin, TX 78712 (United States)
2014-05-20
We calculate growth rates for nonaxisymmetric instabilities including the magnetorotational instability (MRI) throughout the Sun. We first derive a dispersion relation for nonaxisymmetric instability including the effects of shear, convective buoyancy, and three diffusivities (thermal conductivity, resistivity, and viscosity). We then use a solar model evolved with the stellar evolution code MESA and angular velocity profiles determined by Global Oscillations Network Group helioseismology to determine the unstable modes present at each location in the Sun and the associated growth rates. The overall instability has unstable modes throughout the convection zone and also slightly below it at middle and high latitudes. It contains three classes of modes: large-scale hydrodynamic convective modes, large-scale hydrodynamic shear modes, and small-scale magnetohydrodynamic shear modes, which may be properly called MRI modes. While large-scale convective modes are the most rapidly growing modes in most of the convective zone, MRI modes are important in both stably stratified and convectively unstable locations near the tachocline at colatitudes θ < 53°. Nonaxisymmetric MRI modes grow faster than the corresponding axisymmetric modes; for some poloidal magnetic fields, the nonaxisymmetric MRI growth rates are similar to the angular rotation frequency Ω, while axisymmetric modes are stabilized. We briefly discuss the saturation of the field produced by MRI modes, finding that the implied field at the base of the convective zone in the Sun is comparable to that derived based on dynamos active in the tachocline and that the saturation of field resulting from the MRI may be of importance even in the upper convection zone.
Dielectrophoretic Rayleigh-Bénard convection under microgravity conditions.
Yoshikawa, H N; Tadie Fogaing, M; Crumeyrolle, O; Mutabazi, I
2013-04-01
Thermal convection in a dielectric fluid layer between two parallel plates subjected to an alternating electric field and a temperature gradient is investigated under microgravity conditions. A thermoelectric coupling resulting from the thermal variation of the electric permittivity of the fluid produces the dielectrophoretic (DEP) body force, which can be regarded as thermal buoyancy due to an effective gravity. This electric gravity can destabilize a stationary conductive state of the fluid to develop convection. The similarity of the DEP thermal convection with the Rayleigh-Bénard (RB) convection is examined by considering its behavior in detail by a linear stability theory and a two-dimensional direct numerical simulation. The results are analyzed from an energetic viewpoint and in the framework of the Ginzburg-Landau (GL) equation. The stabilizing effects of a thermoelectric feedback make the critical parameters different from those in the RB instability. The nonuniformity of the electric gravity arising from the finite variation of permittivity also affects the critical parameters. The characteristic constants of the GL equation are comparable with those for the RB convection. The heat transfer in the DEP convection is weaker than in the RB convection as a consequence of the feedback that impedes the convection.
Mechanisms initiating deep convection over complex terrain during COPS
Directory of Open Access Journals (Sweden)
Christoph Kottmeier
2008-12-01
Full Text Available Precipitating convection in a mountain region of moderate topography is investigated, with particular emphasis on its initiation in response to boundary-layer and mid- and upper-tropospheric forcing mechanisms. The data used in the study are from COPS (Convective and Orographically-induced Precipitation Study that took place in southwestern Germany and eastern France in the summer of 2007. It is found that the initiation of precipitating convection can be roughly classified as being due to either: (i surface heating and low-level flow convergence; (ii surface heating and moisture supply overcoming convective inhibition during latent and/or potential instability; or (iii mid-tropospheric dynamical processes due to mesoscale convergence lines and forced mean vertical motion. These phenomena have to be adequately represented in models in order to improve quantitative precipitation forecast. Selected COPS cases are analysed and classified into these initiation categories. Although only a subset of COPS data (mainly radiosondes, surface weather stations, radar and satellite data are used here, it is shown that convective systems are captured in considerable detail by sensor synergy. Convergence lines were observed by Doppler radar in the location where deep convection is triggered several hours later. The results suggest that in many situations, observations of the location and timing of convergence lines will facilitate the nowcasting of convection. Further on, forecasting of the initiation of convection is significantly complicated if advection of potentially convective air masses over changing terrain features plays a major role. The passage of a frontal structure over the Vosges - Rhine valley - Black Forest orography was accompanied by an intermediate suppression of convection over the wide Rhine valley. Further downstream, an intensification of convection was observed over the Black Forest due to differential surface heating, a convergence line
Plasma transport driven by the Rayleigh-Taylor instability
Ma, X.; Delamere, P. A.; Otto, A.
2016-06-01
Two important differences between the giant magnetospheres (i.e., Jupiter's and Saturn's magnetospheres) and the terrestrial magnetosphere are the internal plasma sources and the fast planetary rotation. Thus, there must be a radially outward flow to transport the plasma to avoid infinite accumulation of plasma. This radial outflow also carries the magnetic flux away from the inner magnetosphere due to the frozen-in condition. As such, there also must be a radial inward flow to refill the magnetic flux in the inner magnetosphere. Due to the similarity between Rayleigh-Taylor (RT) instability and the centrifugal instability, we use a three-dimensional RT instability to demonstrate that an interchange instability can form a convection flow pattern, locally twisting the magnetic flux, consequently forming a pair of high-latitude reconnection sites. This process exchanges a part of the flux tube, thereby transporting the plasma radially outward without requiring significant latitudinal convection of magnetic flux in the ionosphere.
Dark solitons, dispersive shock waves, and transverse instabilities
Hoefer, M A
2011-01-01
The nature of transverse instabilities to dark solitons and dispersive shock waves for the (2+1)-dimensional defocusing nonlinear Schrodinger equation / Gross-Pitaevskii (NLS / GP) equation is considered. Special attention is given to the small (shallow) amplitude regime, which limits to the Kadomtsev-Petviashvili (KP) equation. We study analytically and numerically the eigenvalues of the linearized NLS / GP equation. The dispersion relation for shallow solitons is obtained asymptotically beyond the KP limit. This yields 1) the maximum growth rate and associated wavenumber of unstable perturbations; and 2) the separatrix between convective and absolute instabilities. The latter result is used to study the transition between convective and absolute instabilities of oblique dispersive shock waves (DSWs). Stationary and nonstationary oblique DSWs are constructed analytically and investigated numerically by direct simulations of the NLS / GP equation. The instability properties of oblique DSWs are found to be dir...
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
Houdek, G
2010-01-01
In this short review on stellar convection dynamics I address the following, currently very topical, issues: (1) the surface effects of the Reynolds stresses and nonadiabaticity on solar-like pulsation frequencies, and (2) oscillation mode lifetimes of stochastically excited oscillations in red giants computed with different time-dependent convection formulations.
Secondary instability of wall-bounded shear flows
Orszag, S. A.; Patera, A. T.
1983-01-01
The present analysis of a secondary instability in a wide class of wall-bounded parallel shear flows indicates that two-dimensional, finite amplitude waves are exponentially unstable to infinitessimal three-dimensional disturbances. The instability appears to be the prototype of transitional instability in such flows as Poiseuille flow, Couette flow, and flat plate boundary layers, in that it has the convective time scales observed in the typical transitions. The energetics and vorticity dynamics of the instability are discussed, and it is shown that the two-dimensional perturbation without directly providing energy to the disturbance. The three-dimensional instability requires that a threshold two-dimensional amplitude be achieved. It is found possible to identify experimental features of transitional spot structure with aspects of the nonlinear two-dimensional/linear three-dimensional instability.
Convective cores in galactic cooling flows
Kritsuk, A G; Müller, E
2000-01-01
We use hydrodynamic simulations with adaptive grid refinement to study the dependence of hot gas flows in X-ray luminous giant elliptical galaxies on the efficiency of heat supply to the gas. We consider a number of potential heating mechanisms including Type Ia supernovae and sporadic nuclear activity of a central supermassive black hole. As a starting point for this research we use an equilibrium hydrostatic recycling model (Kritsuk 1996). We show that a compact cooling inflow develops, if the heating is slightly insufficient to counterbalance radiative cooling of the hot gas in the central few kiloparsecs. An excessive heating in the centre, instead, drives a convectively unstable outflow. We model the onset of the instability and a quasi-steady convective regime in the core of the galaxy in two-dimensions assuming axial symmetry. Provided the power of net energy supply in the core is not too high, the convection remains subsonic. The convective pattern is dominated by buoyancy driven large-scale mushroom-...
Stability analysis of Rayleigh-Bénard convection in a cylinder with internal heat generation
Wang, Bo-Fu; Zhou, Lin; Wan, Zhen-Hua; Ma, Dong-Jun; Sun, De-Jun
2016-07-01
The flow instabilities of Rayleigh-Bénard convection in a cylinder with effect of uniform internal heat source are investigated numerically. The instabilities of the static state and of axisymmetric flows are investigated by linear stability analysis. The convection threshold depends on the strength of internal heat source q and the aspect ratio of the cylinder Γ . The stability of axisymmetric flows is strongly affected by these two parameters, as well as the Prandtl number Pr. Depending on the value of q , three regimes are identified: weak internal heating, moderate internal heating, and strong internal heating regime. In a weak internal heating regime, the instability characteristics are similar to Rayleigh-Bénard convection. In a moderate internal heating regime, intense interaction of buoyancy instability and hydrodynamic instability result in complex instability curves. When q is large enough, the internal heating effect overwhelms the boundary heating effect. Specifically, the influence of Pr on instability is studied at a moderate internal heat strength q =6.4 . An extremely multivalued stability curve is observed. At most five critical Rayleigh numbers can be determined for the axisymmetry-breaking instability at a certain Prandtl number. An axisymmetric unsteady instability mode is observed as well. By nonlinear simulation, the oscillatory flow patterns are obtained, and the axisymmetry-breaking bifurcation of the unsteady toroidal flow is studied.
Stability analysis of Rayleigh-Bénard convection in a cylinder with internal heat generation.
Wang, Bo-Fu; Zhou, Lin; Wan, Zhen-Hua; Ma, Dong-Jun; Sun, De-Jun
2016-07-01
The flow instabilities of Rayleigh-Bénard convection in a cylinder with effect of uniform internal heat source are investigated numerically. The instabilities of the static state and of axisymmetric flows are investigated by linear stability analysis. The convection threshold depends on the strength of internal heat source q and the aspect ratio of the cylinder Γ. The stability of axisymmetric flows is strongly affected by these two parameters, as well as the Prandtl number Pr. Depending on the value of q, three regimes are identified: weak internal heating, moderate internal heating, and strong internal heating regime. In a weak internal heating regime, the instability characteristics are similar to Rayleigh-Bénard convection. In a moderate internal heating regime, intense interaction of buoyancy instability and hydrodynamic instability result in complex instability curves. When q is large enough, the internal heating effect overwhelms the boundary heating effect. Specifically, the influence of Pr on instability is studied at a moderate internal heat strength q=6.4. An extremely multivalued stability curve is observed. At most five critical Rayleigh numbers can be determined for the axisymmetry-breaking instability at a certain Prandtl number. An axisymmetric unsteady instability mode is observed as well. By nonlinear simulation, the oscillatory flow patterns are obtained, and the axisymmetry-breaking bifurcation of the unsteady toroidal flow is studied.
Natural convection in eccentric spherical annuli
Gallegos, A D
2015-01-01
A fluid between two spheres, concentric or not, at different temperatures will flow in the presence of a constant gravitational force. Although there is no possible hydrostatic state, energy transport is dominated by diffusion if temperature difference between the spheres is small enough. In this conductive regime the average Nusselt number remains approximately constant for all Rayleigh numbers below some critical value. Above the critical Rayleigh number, plumes appear and thermal convection takes place. We study this phenomenon, in particular the case where the inner sphere is displaced from the centre, using a two-component thermal lattice Boltzmann method to characterize the convective instability, the evolution of the flow patterns and the dependence of the Nusselt number on the Rayleigh number beyond the transition.
Hall Effect on Bénard Convection of Compressible Viscoelastic Fluid through Porous Medium
Directory of Open Access Journals (Sweden)
Mahinder Singh
2013-01-01
Full Text Available An investigation made on the effect of Hall currents on thermal instability of a compressible Walter’s B′ elasticoviscous fluid through porous medium is considered. The analysis is carried out within the framework of linear stability theory and normal mode technique. For the case of stationary convection, Hall currents and compressibility have postponed the onset of convection through porous medium. Moreover, medium permeability hasten postpone the onset of convection, and magnetic field has duel character on the onset of convection. The critical Rayleigh numbers and the wave numbers of the associated disturbances for the onset of instability as stationary convection have been obtained and the behavior of various parameters on critical thermal Rayleigh numbers has been depicted graphically. The magnetic field, Hall currents found to introduce oscillatory modes, in the absence of these effects the principle of exchange of stabilities is valid.
Observing Convective Aggregation
Holloway, Christopher E.; Wing, Allison A.; Bony, Sandrine; Muller, Caroline; Masunaga, Hirohiko; L'Ecuyer, Tristan S.; Turner, David D.; Zuidema, Paquita
2017-06-01
Convective self-aggregation, the spontaneous organization of initially scattered convection into isolated convective clusters despite spatially homogeneous boundary conditions and forcing, was first recognized and studied in idealized numerical simulations. While there is a rich history of observational work on convective clustering and organization, there have been only a few studies that have analyzed observations to look specifically for processes related to self-aggregation in models. Here we review observational work in both of these categories and motivate the need for more of this work. We acknowledge that self-aggregation may appear to be far-removed from observed convective organization in terms of time scales, initial conditions, initiation processes, and mean state extremes, but we argue that these differences vary greatly across the diverse range of model simulations in the literature and that these comparisons are already offering important insights into real tropical phenomena. Some preliminary new findings are presented, including results showing that a self-aggregation simulation with square geometry has too broad distribution of humidity and is too dry in the driest regions when compared with radiosonde records from Nauru, while an elongated channel simulation has realistic representations of atmospheric humidity and its variability. We discuss recent work increasing our understanding of how organized convection and climate change may interact, and how model discrepancies related to this question are prompting interest in observational comparisons. We also propose possible future directions for observational work related to convective aggregation, including novel satellite approaches and a ground-based observational network.
Pulsation driving and convection
Antoci, Victoria
2015-08-01
Convection in stellar envelopes affects not only the stellar structure, but has a strong impact on different astrophysical processes, such as dynamo-generated magnetic fields, stellar activity and transport of angular momentum. Solar and stellar observations from ground and space have shown that the turbulent convective motion can also drive global oscillations in many type of stars, allowing to study stellar interiors at different evolutionary stages. In this talk I will concentrate on the influence of convection on the driving of stochastic and coherent pulsations across the Hertzsprung-Russell diagram and give an overview of recent studies.
New layer thickness parameterization of diffusive convection in the ocean
Zhou, Sheng-Qi; Lu, Yuan-Zheng; Song, Xue-Long; Fer, Ilker
2016-03-01
In the present study, a new parameterization is proposed to describe the convecting layer thickness in diffusive convection. By using in situ observational data of diffusive convection in the lakes and oceans, a wide range of stratification and buoyancy flux is obtained, where the buoyancy frequency N varies between 10-4 and 0.1 s-1 and the heat-related buoyancy flux qT varies between 10-12 and 10-7 m2 s-3. We construct an intrinsic thickness scale, H0 =[qT3 / (κTN8) ] 1 / 4, here κT is the thermal diffusivity. H0 is suggested to be the scale of an energy-containing eddy and it can be alternatively represented as H0 = ηRebPr1/4, here η is the dissipation length scale, Reb is the buoyant Reynolds number, and Pr is the Prandtl number. It is found that the convective layer thickness H is directly linked to the stability ratio Rρ and H0 with the form of H ∼ (Rρ - 1)2H0. The layer thickness can be explained by the convective instability mechanism. To each convective layer, its thickness H reaches a stable value when its thermal boundary layer develops to be a new convecting layer.
Ionospheric Feedback Instability in the Coupling of Magnetosphere-Ionosphere
Institute of Scientific and Technical Information of China (English)
王旭宇; 曹晋滨
2003-01-01
The ionospheric feedback instability is discussed by using the conductivity argument. We give an exact quantitative description to show that the free energy for this instability comes from the reduction of the Joule dissipation produced by the pre-existing convection electric field through self-consistent changes in ionization and conducactive ionosphere is pumped into the magnetosphere, wlich is contrary to the usual case whereby energy carried electron E × B drift. The electron conductivity is controlled by the ion Perdersen conductivity rather than by the electrons Pedersen conductivity. We also provide a qualitative theoretical explanation to the intense aurora favoured by a lower ambient ionospheric conductivity in the ionospheric feedback instability.
Kakac, Sadik; Pramuanjaroenkij, Anchasa
2014-01-01
Intended for readers who have taken a basic heat transfer course and have a basic knowledge of thermodynamics, heat transfer, fluid mechanics, and differential equations, Convective Heat Transfer, Third Edition provides an overview of phenomenological convective heat transfer. This book combines applications of engineering with the basic concepts of convection. It offers a clear and balanced presentation of essential topics using both traditional and numerical methods. The text addresses emerging science and technology matters, and highlights biomedical applications and energy technologies. What’s New in the Third Edition: Includes updated chapters and two new chapters on heat transfer in microchannels and heat transfer with nanofluids Expands problem sets and introduces new correlations and solved examples Provides more coverage of numerical/computer methods The third edition details the new research areas of heat transfer in microchannels and the enhancement of convective heat transfer with nanofluids....
Transparent electric convection heater
Khalid, A.; Luck, J.L.
2001-01-01
An optically transparent electrically heated convection heater for use as a space heater in homes, offices, shops. Typically, said convection heater consists of a transparent layer 1 upon which is deposited a layer of a transparent electrically conductive material 2 such as indium-tin-oxide, electrodes 3 and 3a are formed on opposite edges of the transparent electrically conductive layer 2 and electrical wires 4 and 4a are connected to the electrodes. The transparent electrically conductive l...
Three-dimensional simulations of viscoelastic instability in polymeric filaments
DEFF Research Database (Denmark)
Rasmussen, Henrik Koblitz; Hassager, Ole
1999-01-01
The three-dimensional Langrangian integral method is used to simulate the elastic end-plate instability that occurs in the rapid extension of some polymeric filaments between parallel plates. It is demonstrated that the upper convected Maxwell model describes the essential features of the instabi...
Convection-driven pattern formation in lawn grasses
Thompson, Sally; Daniels, Karen
2009-11-01
Spatial patterns of 'dead' lawn grass have often been ascribed to Turing-type reaction-diffusion processes related to water scarcity. We present an alternative hypothesis: that the air within the grass canopy is unstable to a convective instability, such that chill damage caused by falling cold air is responsible for the creation of brown and green bands of grass. This hypothesis is consistent with several features of small-scale vegetation patterns, including their length scale, rapid onset and transient nature. We find that the predictions of a porous medium convection model based are consistent with measurements made for a particular instance of lawn-patterning in North Carolina.
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
Long Wavelength Limit of the Current Convective Instability.
1984-02-23
V " > v , velocity shear ( Satyanarayana and Ossakow...NR-5264 F/G 12/1 i ••>;y>vr>wv•>•’.’.•.’••,•.’.’•’•••’J." •’!ŕ *i-Wl’W.l 1 I’/1. *^^^^^^T^^^^^^^^ , I 1 ". m w\\ • I.I»J nn, i’ri L V i 12.8...025 MICROCOPY RESOLUTION TEST CHART NATIONAL »U«t AU Of STAMOANOS - ’MS • * &ä&ä^ .,.-. „•- .f .•- v ;•:•:•:•/•;•:•. ’ ^^^r^
The combined effect of precession and convection on the dynamo action
Wei, Xing
2016-01-01
To understand the generation of the Earth's and planetary magnetic fields, we investigate numerically the combined effect of precession and convection on the dynamo action in a spherical shell. The convection alone, the precession alone and the combined effect of convection and precession are studied at the low Ekman number at which the precessing flow is already unstable. The key result is that although the precession or convection alone is not strong to support the dynamo action the combined effect of precession and convection can support the dynamo action because of the resonance of precessional and convective instabilities. This result may interpret why the geodynamo maintains for such a long history compared to the Martian dynamo.
Buoyancy Instabilities in a Weakly Collisional Intracluster Medium
Kunz, Matthew W; Reynolds, Christopher S; Stone, James M
2012-01-01
The intracluster medium of galaxy clusters is a weakly collisional, high-beta plasma in which the transport of heat and momentum occurs primarily along magnetic-field lines. Anisotropic heat conduction allows convective instabilities to be driven by temperature gradients of either sign, the magnetothermal instability (MTI) in the outskirts of non-isothermal clusters and the heat-flux buoyancy-driven instability (HBI) in their cooling cores. We employ the Athena MHD code to investigate the nonlinear evolution of these instabilities, self-consistently including the effects of anisotropic viscosity (i.e. Braginskii pressure anisotropy), anisotropic conduction, and radiative cooling. We highlight the importance of the microscale instabilities that inevitably accompany and regulate the pressure anisotropies generated by the HBI and MTI. We find that, in all but the innermost regions of cool-core clusters, anisotropic viscosity significantly impairs the ability of the HBI to reorient magnetic-field lines orthogonal...
Saturation of the Magnetothermal Instability in Three Dimensions
Parrish, I J; Parrish, Ian J.; Stone, James M.
2006-01-01
In dilute astrophysical plasmas, thermal conduction is primarily along magnetic field lines, and therefore highly anisotropic. As a result, the usual convective stability criterion is modified from a condition on entropy to a condition on temperature. For small magnetic fields or small wavenumbers, instability occurs in any atmosphere where the temperature and pressure gradients point in the same direction. We refer to the resulting convective instability as the magnetothermal instability (MTI). We present fully three-dimensional simulations of the MTI and show that saturation results in an atmosphere with different vertical structure, dependent upon the boundary conditions. When the temperature at the boundary of the unstable layer is allowed to vary, the temperature gradient relaxes until the unstable region is almost isothermal. When the temperature at the boundary of the unstable region is fixed, the magnetic field is reoriented to an almost vertical geometry as a result of buoyant motions. This case exhi...
Stability analysis of convection in the intracluster medium
Energy Technology Data Exchange (ETDEWEB)
Gupta, H., E-mail: hiugupta@iitk.ac.in [Department of Physics, Indian Institute of Technology Kanpur, U.P. 208016 (India); Rathor, S.K., E-mail: skrathor@iitk.ac.in [Department of Physics, Indian Institute of Technology Kanpur, U.P. 208016 (India); Pessah, M.E., E-mail: mpessah@nbi.dk [Niels Bohr International Academy, Niels Bohr Institute, 2100, Copenhagen Ø (Denmark); Chakraborty, S., E-mail: sagarc@iitk.ac.in [Department of Physics, Indian Institute of Technology Kanpur, U.P. 208016 (India); Mechanics & Applied Mathematics Group, Indian Institute of Technology Kanpur, U.P. 208016 (India)
2016-07-15
We use the machinery usually employed for studying the onset of Rayleigh–Bénard convection in hydro- and magnetohydro-dynamic settings to address the onset of convection induced by the magnetothermal instability and the heat-flux-buoyancy-driven-instability in the weakly-collisional magnetized plasma permeating the intracluster medium. Since most of the related numerical simulations consider the plasma being bounded between two ‘plates’ on which boundary conditions are specified, our strategy provides a framework that could enable a more direct connection between analytical and numerical studies. We derive the conditions for the onset of these instabilities considering the effects of induced magnetic tension resulting from a finite plasma beta. We provide expressions for the Rayleigh number in terms of the wave vector associated with a given mode, which allow us to characterize the modes that are first to become unstable. For both the heat-flux-buoyancy-driven-instability and the magnetothermal instability, oscillatory marginal stable states are possible. - Highlights: • Stability analysis of the HBI and the MTI are presented taking into account the boundary conditions employed in the simulations. • It has been shown that the HBI doesn't set in as an oscillatory marginal state whereas the MTI can do so. • The HBI and the MTI criteria have been modified to include the affects of the magnetic tension.
Zero-Prandtl-number convection with slow rotation
Maity, Priyanka; Kumar, Krishna
2014-01-01
We present the results of our investigations of the primary instability and the flow patterns near onset in zero-Prandtl-number Rayleigh-B\\'enard convection with uniform rotation about a vertical axis. The investigations are carried out using direct numerical simulations of the hydrodynamic equations with stress-free horizontal boundaries in rectangular boxes of size $(2\\pi/k_x) \\times (2\\pi/k_y) \\times 1$ for different values of the ratio $\\eta = k_x/k_y$. The primary instability is found to...
On pattern formation in ferrocolloid convection
Energy Technology Data Exchange (ETDEWEB)
Bozhko, A [Department of Physics, Perm State University, Bukirev Str. 15, 614990 Perm (Russian Federation); Putin, G [Department of Physics, Perm State University, Bukirev Str. 15, 614990 Perm (Russian Federation); Tynjaelae, T [Department of Energy and Environmental Technology, Lappeenranta University of Technology, 53851 Lappeenranta (Finland); Meshin, M Dabagh [Department of Energy and Environmental Technology, Lappeenranta University of Technology, 53851 Lappeenranta (Finland); Jalali, P [Department of Energy and Environmental Technology, Lappeenranta University of Technology, 53851 Lappeenranta (Finland)
2007-04-15
Experimental studies and numerical simulations of stability of buoyancy-driven flows in a ferrocolloid for the cases of horizontal and inclined vertical orientation of a thin cylindrical cavity are performed. The influence of a homogeneous longitudinal magnetic field on convective instability and spatio-temporal patterns were also investigated. In the case of ferrocolloids the gradients of magnetic permeability may arise due to both temperature and particle concentration gradients. The particle mass flux in a classical form is summarized from the translation diffusion coefficient and the thermal diffusion ratio. However, the explanation for the observed self-oscillation regimes in magnetic fluid for the cavities of sufficiently large thickness is conditioned by the competition of density variations originating from the fluid thermal expansion and barometric sedimentation. The results prove that a uniform longitudinal magnetic field allows to control the stability and the shape of secondary convection motions at inclined orientation of layer. In a ferrocolloid the repeated transients involving localized roll convection and pure shear flow took place. Under action of uniform longitudinal magnetic field orientated perpendicular to flux velocity of shear motion on such long-wave transients can lead to complicated types of chaotic localized states or solitary vortices.
Interaction of Mesoscale Convection and Frontogenesis
Institute of Scientific and Technical Information of China (English)
彭加毅; 方娟; 伍荣生
2004-01-01
On the basis of the MM5 simulation data of the severe storm that occurred over the southeastern part of Hubei province on 21 July 1998, the interaction of mesoscale convection and frontogenesis is dealt with using the thermodynamical equation and frontogenetical function. The results show that the outbreak of the severe storm is closely related to the local frontogenesis. In fact, the interaction between the shearing instability of the low-level jet (LLJ) and the topographic forcing generates an gravity-inertia wave as well as local frontogenesis (the first front), which consequently induce the onset of the severe storm. From then on, owing to the horizontal and vertical advection of the potential temperature, the new frontogenesis (the second front) is formed to the northeast side of the severe storm, which initiates the second rain belt.Meanwhile, a two-front structure emerges over the southeastern part of Hubei province. Accompanied with the further intensification of the convection, the rain droplets evaporation cooling strengthens the first front and weakens the second front, resulting in single front structure over the southeastern part of Hubei province in the period of the strong convection.
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.
Electron heat flux instability
Saeed, Sundas; Sarfraz, M.; Yoon, P. H.; Lazar, M.; Qureshi, M. N. S.
2017-02-01
The heat flux instability is an electromagnetic mode excited by a relative drift between the protons and two-component core-halo electrons. The most prominent application may be in association with the solar wind where drifting electron velocity distributions are observed. The heat flux instability is somewhat analogous to the electrostatic Buneman or ion-acoustic instability driven by the net drift between the protons and bulk electrons, except that the heat flux instability operates in magnetized plasmas and possesses transverse electromagnetic polarization. The heat flux instability is also distinct from the electrostatic counterpart in that it requires two electron species with relative drifts with each other. In the literature, the heat flux instability is often called the 'whistler' heat flux instability, but it is actually polarized in the opposite sense to the whistler wave. This paper elucidates all of these fundamental plasma physical properties associated with the heat flux instability starting from a simple model, and gradually building up more complexity towards a solar wind-like distribution functions. It is found that the essential properties of the instability are already present in the cold counter-streaming electron model, and that the instability is absent if the protons are ignored. These instability characteristics are highly reminiscent of the electron firehose instability driven by excessive parallel temperature anisotropy, propagating in parallel direction with respect to the ambient magnetic field, except that the free energy source for the heat flux instability resides in the effective parallel pressure provided by the counter-streaming electrons.
Simulating deep convection with a shallow convection scheme
Directory of Open Access Journals (Sweden)
C. Hohenegger
2011-10-01
Full Text Available Convective processes profoundly affect the global water and energy balance of our planet but remain a challenge for global climate modeling. Here we develop and investigate the suitability of a unified convection scheme, capable of handling both shallow and deep convection, to simulate cases of tropical oceanic convection, mid-latitude continental convection, and maritime shallow convection. To that aim, we employ large-eddy simulations (LES as a benchmark to test and refine a unified convection scheme implemented in the Single-column Community Atmosphere Model (SCAM. Our approach is motivated by previous cloud-resolving modeling studies, which have documented the gradual transition between shallow and deep convection and its possible importance for the simulated precipitation diurnal cycle.
Analysis of the LES reveals that differences between shallow and deep convection, regarding cloud-base properties as well as entrainment/detrainment rates, can be related to the evaporation of precipitation. Parameterizing such effects and accordingly modifying the University of Washington shallow convection scheme, it is found that the new unified scheme can represent both shallow and deep convection as well as tropical and mid-latitude continental convection. Compared to the default SCAM version, the new scheme especially improves relative humidity, cloud cover and mass flux profiles. The new unified scheme also removes the well-known too early onset and peak of convective precipitation over mid-latitude continental areas.
Evaluating shoulder instability treatment
van der Linde, J.A.
2016-01-01
Shoulder instability common occurs. When treated nonoperatively, the resulting societal costs based on health care utilization and productivity losses are significant. Shoulder function can be evaluated using patient reported outcome measurements (PROMs). For shoulder instability, these include the
Jeans instability in superfluids
Energy Technology Data Exchange (ETDEWEB)
Hason, Itamar; Oz, Yaron [Tel-Aviv University, Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv (Israel)
2014-11-15
We analyze the effect of a gravitational field on the sound modes of superfluids. We derive an instability condition that generalizes the well-known Jeans instability of the sound mode in normal fluids. We discuss potential experimental implications. (orig.)
Mathematical models of convection
Andreev, Victor K; Goncharova, Olga N; Pukhnachev, Vladislav V
2012-01-01
Phenomena of convection are abundant in nature as well as in industry. This volume addresses the subject of convection from the point of view of both, theory and application. While the first three chapters provide a refresher on fluid dynamics and heat transfer theory, the rest of the book describes the modern developments in theory. Thus it brings the reader to the ""front"" of the modern research. This monograph provides the theoretical foundation on a topic relevant to metallurgy, ecology, meteorology, geo-and astrophysics, aerospace industry, chemistry, crystal physics, and many other fiel
Parameterizing convective organization
Directory of Open Access Journals (Sweden)
Brian Earle Mapes
2011-06-01
Full Text Available Lateral mixing parameters in buoyancy-driven deep convection schemes are among the most sensitive and important unknowns in atmosphere models. Unfortunately, there is not a true optimum value for plume mixing rate, but rather a dilemma or tradeoff: Excessive dilution of updrafts leads to unstable stratification bias in the mean state, while inadequate dilution allows deep convection to occur too easily, causing poor space and time distributions and variability. In this too-small parameter space, compromises are made based on competing metrics of model performance. We attempt to escape this “entrainment dilemma” by making bulk plume parameters (chiefly entrainment rate depend on a new prognostic variable (“organization,” org meant to reflect the rectified effects of subgrid-scale structure in meteorological fields. We test an org scheme in the Community Atmosphere Model (CAM5 with a new unified shallow-deep convection scheme (UW-ens, a 2-plume version of the University of Washington scheme. Since buoyant ascent involves natural selection, subgrid structure makes convection systematically deeper and stronger than the pure unorganized case: plumes of average (or randomly sampled air rising in the average environment. To reflect this, org is nonnegative, but we leave it dimensionless. A time scale characterizes its behavior (here ∼3 h for a 2o model. Currently its source is rain evaporation, but other sources can be added easily. We also let org be horizontally transported by advection, as a mass-weighted mean over the convecting layer. Linear coefficients link org to a plume ensemble, which it assists via: 1 plume base warmth above the mean temperature 2 plume radius enhancement (reduced mixing, and 3 increased probability of overlap in a multi-plume scheme, where interactions benefit later generations (this part has only been implemented in an offline toy column model. Since rain evaporation is a source for org, it functions as a time
Tewari, S. N.; Chopra, M. A.
1992-01-01
The influence of convection on the development of morphological instability at the liquid-solid interface during directional solidification in a positive thermal gradient has been examined in Pb-10 wt pct Sn and succinonitrile-1.9 wt pct acetone. The onset of interfacial breakdown occurs at higher growth speeds in the presence of convection. The linear stability analysis due to Favier and Rouzaud which uses the 'deformable' mass flow boundary layer concept shows a good agreement with the experimentally observed behavior.
Convection in axially symmetric accretion discs with microscopic transport coefficients
Malanchev, K L; Shakura, N I
2016-01-01
The vertical structure of stationary thin accretion discs is calculated from the energy balance equation with heat generation due to microscopic ion viscosity {\\eta} and electron heat conductivity {\\kappa}, both depending on temperature. In the optically thin discs it is found that for the heat conductivity increasing with temperature, the vertical temperature gradient exceeds the adiabatic value at some height, suggesting convective instability in the upper disc layer. There is a critical Prandtl number, Pr = 4/9, above which a Keplerian disc become fully convective. The vertical density distribution of optically thin laminar accretion discs as found from the hydrostatic equilibrium equation cannot be generally described by a polytrope but in the case of constant viscosity and heat conductivity. In the optically thick discs with radiation heat transfer, the vertical disc structure is found to be convectively stable for both absorption dominated and scattering dominated opacities, unless a very steep dependen...
Finite-sample-size effects on convection in mushy layers
Zhong, Jin-Qiang; Wells, Andrew J; Wettlaufer, John S
2012-01-01
We report theoretical and experimental investigations of the flow instability responsible for the mushy-layer mode of convection and the formation of chimneys, drainage channels devoid of solid, during steady-state solidification of aqueous ammonium chloride. Under certain growth conditions a state of steady mushy-layer growth with no flow is unstable to the onset of convection, resulting in the formation of chimneys. We present regime diagrams to quantify the state of the flow as a function of the initial liquid concentration, the porous-medium Rayleigh number, and the sample width. For a given liquid concentration, increasing both the porous-medium Rayleigh number and the sample width caused the system to change from a stable state of no flow to a different state with the formation of chimneys. Decreasing the concentration ratio destabilized the system and promoted the formation of chimneys. As the initial liquid concentration increased, onset of convection and formation of chimneys occurred at larger value...
CDM Convective Forecast Planning guidance
National Oceanic and Atmospheric Administration, Department of Commerce — The CDM Convective Forecast Planning (CCFP) guidance product provides a foreast of en-route aviation convective hazards. The forecasts are updated every 2 hours and...
Problems in Microgravity Fluid Mechanics: G-Jitter Convection
Homsy, G. M.
2005-01-01
This is the final report on our NASA grant, Problems in Microgravity Fluid Mechanics NAG3-2513: 12/14/2000 - 11/30/2003, extended through 11/30/2004. This grant was made to Stanford University and then transferred to the University of California at Santa Barbara when the PI relocated there in January 2001. Our main activity has been to conduct both experimental and theoretical studies of instabilities in fluids that are relevant to the microgravity environment, i.e. those that do not involve the action of buoyancy due to a steady gravitational field. Full details of the work accomplished under this grant are given below. Our work has focused on: (i) Theoretical and computational studies of the effect of g-jitter on instabilities of convective states where the convection is driven by forces other than buoyancy (ii) Experimental studies of instabilities during displacements of miscible fluid pairs in tubes, with a focus on the degree to which these mimic those found in immiscible fluids. (iii) Theoretical and experimental studies of the effect of time dependent electrohydrodynamic forces on chaotic advection in drops immersed in a second dielectric liquid. Our objectives are to acquire insight and understanding into microgravity fluid mechanics problems that bear on either fundamental issues or applications in fluid physics. We are interested in the response of fluids to either a fluctuating acceleration environment or to forces other than gravity that cause fluid mixing and convection. We have been active in several general areas.
A cryostat device for liquid nitrogen convection experiments
Dubois, Charles; Duchesne, Alexis; Caps, Herve
2015-11-01
When a horizontal layer of expansible fluid heated from below is submitted to a large vertical temperature gradient, one can observe convective cells. This phenomenon is the so-called Rayleigh-Bénard instability. In the literature, this instability is mainly studied when the entire bottom surface of a container heats the liquid. Under these conditions, the development of regularly spaced convective cells in the liquid bulk is observed. Cooling applications led us to consider this instability in a different geometry, namely a resistor immersed in a bath of cold liquid. We present here experiments conducted with liquid nitrogen. For this purpose, we developed a cryostat in order to be able to perform Particle Image Velocimetry. We obtained 2D maps of the flow and observed, as expected, two Rayleigh-Bénard convective cells around the heater. We particularly investigated the vertical velocity in the central column between the two cells. We compared these data to results we obtained with silicone oil and water in the same geometry. We derived theoretical law from classical models applied to the proposed geometry and found a good agreement with our experimental data. This project has been financially supported by ARC SuperCool contract of the University of Liege.
Internal Wave Generation by Convection
Lecoanet, Daniel
2016-01-01
In nature, it is not unusual to find stably stratified fluid adjacent to convectively unstable fluid. This can occur in the Earth's atmosphere, where the troposphere is convective and the stratosphere is stably stratified; in lakes, where surface solar heating can drive convection above stably stratified fresh water; in the oceans, where geothermal heating can drive convection near the ocean floor, but the water above is stably stratified due to salinity gradients; possible in the Earth's liq...
Rotating non-Boussinesq Rayleigh-Benard convection
Moroz, Vadim Vladimir
This thesis makes quantitative predictions about the formation and stability of hexagonal and roll patterns in convecting system unbounded in horizontal direction. Starting from the Navier-Stokes, heat and continuity equations, the convection problem is then reduced to normal form equations using equivariant bifurcation theory. The relative stabilities of patterns lying on a hexagonal lattice in Fourier space are then determined using appropriate amplitude equations, with coefficients obtained via asymptotic expansion of the governing partial differential equations, with the conducting state being the base state, and the control parameter and the non-Boussinesq effects being small. The software package Mathematica was used to calculate amplitude coefficients of the appropriate coupled Ginzburg-Landau equations for the rigid-rigid and free-free case. A Galerkin code (initial version of which was written by W. Pesch et al.) is used to determine pattern stability further from onset and for strongly non-Boussinesq fluids. Specific predictions about the stability of hexagon and roll patterns for realistic experimental conditions are made. The dependence of the stability of the convective patterns on the Rayleigh number, planform wavenumber and the rotation rate is studied. Long- and shortwave instabilities, both steady and oscillatory, are identified. For small Prandtl numbers oscillatory sideband instabilities are found already very close to onset. A resonant mode interaction in hexagonal patterns arising in non-Boussinesq Rayleigh-Benard convection is studied using symmetry group methods. The lowest-order coupling terms for interacting patterns are identified. A bifurcation analysis of the resulting system of equations shows that the bifurcation is transcritical. Stability properties of resulting patterns are discussed. It is found that for some fluid properties the traditional hexagon convection solution does not exist. Analytical results are supported by numerical
Mirouh, Giovanni M; Stellmach, Stephan; Traxler, Adrienne L; Wood, Toby S
2011-01-01
The process referred to as "semi-convection" in astrophysics and "double-diffusive convection in the diffusive regime" in Earth and planetary sciences, occurs in stellar and planetary interiors in regions which are stable according to the Ledoux criterion but unstable according to the Schwarzschild criterion. In this series of papers, we analyze the results of an extensive suite of 3D numerical simulations of the process, and ultimately propose a new 1D prescription for heat and compositional transport in this regime which can be used in stellar or planetary structure and evolution models. In a preliminary study of the phenomenon, Rosenblum et al. (2011) showed that, after saturation of the primary instability, a system can evolve in one of two possible ways: the induced turbulence either remains homogeneous, with very weak transport properties, or transitions into a thermo-compositional staircase where the transport rate is much larger (albeit still smaller than in standard convection). In this paper, we sho...
Relativistic Gravothermal Instabilities
Roupas, Zacharias
2014-01-01
The thermodynamic instabilities of the self-gravitating, classical ideal gas are studied in the case of static, spherically symmetric configurations in General Relativity taking into account the Tolman-Ehrenfest effect. One type of instabilities is found at low energies, where thermal energy becomes too weak to halt gravity and another at high energies, where gravitational attraction of thermal pressure overcomes its stabilizing effect. These turning points of stability are found to depend on the total rest mass $\\mathcal{M}$ over the radius $R$. The low energy instability is the relativistic generalization of Antonov instability, which is recovered in the limit $G\\mathcal{M} \\ll R c^2$ and low temperatures, while in the same limit and high temperatures, the high energy instability recovers the instability of the radiation equation of state. In the temperature versus energy diagram of series of equilibria, the two types of gravothermal instabilities make themselves evident as a double spiral! The two energy l...
Elastic instability in stratified core annular flow.
Bonhomme, Oriane; Morozov, Alexander; Leng, Jacques; Colin, Annie
2011-06-01
We study experimentally the interfacial instability between a layer of dilute polymer solution and water flowing in a thin capillary. The use of microfluidic devices allows us to observe and quantify in great detail the features of the flow. At low velocities, the flow takes the form of a straight jet, while at high velocities, steady or advected wavy jets are produced. We demonstrate that the transition between these flow regimes is purely elastic--it is caused by the viscoelasticity of the polymer solution only. The linear stability analysis of the flow in the short-wave approximation supplemented with a kinematic criterion captures quantitatively the flow diagram. Surprisingly, unstable flows are observed for strong velocities, whereas convected flows are observed for low velocities. We demonstrate that this instability can be used to measure the rheological properties of dilute polymer solutions that are difficult to assess otherwise.
Elastic instability in stratified core annular flow
Bonhomme, Oriane; Leng, Jacques; Colin, Annie
2010-01-01
We study experimentally the interfacial instability between a layer of dilute polymer solution and water flowing in a thin capillary. The use of microfluidic devices allows us to observe and quantify in great detail the features of the flow. At low velocities, the flow takes the form of a straight jet, while at high velocities, steady or advected wavy jets are produced. We demonstrate that the transition between these flow regimes is purely elastic -- it is caused by viscoelasticity of the polymer solution only. The linear stability analysis of the flow in the short-wave approximation captures quantitatively the flow diagram. Surprisingly, unstable flows are observed for strong velocities, whereas convected flows are observed for low velocities. We demonstrate that this instability can be used to measure rheological properties of dilute polymer solutions that are difficult to assess otherwise.
Shoulder instability; Schulterinstabilitaeten
Energy Technology Data Exchange (ETDEWEB)
Kreitner, Karl-Friedrich [Mainiz Univ. (Germany). Klinik und Poliklinik fuer Diagnostische und Interventionelle Radiologie
2014-06-15
In the shoulder, the advantages of range of motion are traded for the disadvantages of vulnerability to injury and the development of instability. Shoulder instability and the lesion it produces represent one of the main causes of shoulder discomfort and pain. Shoulder instability is defined as a symptomatic abnormal motion of the humeral head relative to the glenoid during active shoulder motion. Glenohumeral instabilities are classified according to their causative factors as the pathogenesis of instability plays an important role with respect to treatment options: instabilities are classified in traumatic and atraumatic instabilities as part of a multidirectional instability syndrome, and in microtraumatic instabilities. Plain radiographs ('trauma series') are performed to document shoulder dislocation and its successful reposition. Direct MR arthrography is the most important imaging modality for delineation the different injury patterns on the labral-ligamentous complex and bony structures. Monocontrast CT-arthrography with use of multidetector CT scanners may be an alternative imaging modality, however, regarding the younger patient age, MR imaging should be preferred in the diagnostic work-up of shoulder instabilities. (orig.)
Onset of Convection in a Nanofluid Saturated Porous Layer with Temperature Dependent Viscosity
Directory of Open Access Journals (Sweden)
I. S. Shivakumara
2014-04-01
Full Text Available The effect of nanofluid viscosity varying exponentially with temperature on the onset of convection in a layer of nanofluid saturated Darcy porous medium is investigated. The nanoparticle flux is zero condition on the boundaries is invoked to account for physically realistic situation. The resulting eigenvalue problem is solved numerically using the Galerkin method. It is observed that the instability sets in only as stationary convection and the occurrence of oscillatory convection is ruled out. The effect of viscosity parameter on the characteristics of stability is found to be significant and dual in nature. The onset of convection is hastened and the size of convection cells is enlarged with an increase in the value of modified diffusivity ratio, concentration Darcy-Rayleigh number, the modified particle density increment parameter and the Lewis number.
Numerical study of plume patterns in the chemotaxis-diffusion-convection coupling system
Deleuze, Yannick; Thiriet, Marc; Sheu, Tony W H
2015-01-01
A chemotaxis-diffusion-convection coupling system for describing a form of buoyant convection in which the fluid develops convection cells and plume patterns will be investigated numerically in this study. Based on the two-dimensional convective chemotaxis-fluid model proposed in the literature, we developed an upwind finite element method to investigate the pattern formation and the hydrodynamical stability of the system. The numerical simulations illustrate different predicted physical regimes in the system. In the convective regime, the predicted plumes resemble B\\'enard instabilities. Our numerical results show how structured layers of bacteria are formed before bacterium rich plumes fall in the fluid. The plumes have a well defined spectrum of wavelengths and have an exponential growth rate, yet their position can only be predicted in very simple examples. In the chemotactic and diffusive regimes, the effects of chemotaxis are investigated. Our results indicate that the chemotaxis can stabilize the overa...
Double-diffusive convection in a rotating cylindrical annulus with conical caps
Simitev, R D
2011-01-01
Double-diffusive convection driven by both thermal and compositional buoyancy in a rotating cylindrical annulus with conical caps is considered with the aim to establish whether a small fraction of compositional buoyancy added to the thermal buoyancy (or vice versa) can significantly reduce the critical Rayleigh number and amplify convection in planetary cores. It is shown that the neutral surface describing the onset of convection in the double-buoyancy case is essentially different from that of the well-studied purely thermal case, and does indeed allow the possibility of low-Rayleigh number convection. In particular, isolated islands of instability are formed by an additional "double-diffusive" eigenmode in certain regions of the parameter space. However, the amplitude of such low-Rayleigh number convection is relatively weak. At similar flow amplitudes purely compositional and double-diffusive cases are characterized by a stronger time dependence compared to purely thermal cases, and by a prograde mean zo...
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.
Elevator mode convection in flows with strong magnetic fields
Liu, Li; Zikanov, Oleg
2015-04-01
Instability modes in the form of axially uniform vertical jets, also called "elevator modes," are known to be the solutions of thermal convection problems for vertically unbounded systems. Typically, their relevance to the actual flow state is limited by three-dimensional breakdown caused by rapid growth of secondary instabilities. We consider a flow of a liquid metal in a vertical duct with a heated wall and strong transverse magnetic field and find elevator modes that are stable and, thus, not just relevant, but a dominant feature of the flow. We then explore the hypothesis suggested by recent experimental data that an analogous instability to modes of slow axial variation develops in finite-length ducts, where it causes large-amplitude fluctuations of temperature. The implications for liquid metal blankets for tokamak fusion reactors that potentially invalidate some of the currently pursued design concepts are discussed.
Elevator mode convection in flows with strong magnetic fields
Energy Technology Data Exchange (ETDEWEB)
Liu, Li; Zikanov, Oleg, E-mail: zikanov@umich.edu [Department of Mechanical Engineering, University of Michigan-Dearborn, 48128-1491 Michigan (United States)
2015-04-15
Instability modes in the form of axially uniform vertical jets, also called “elevator modes,” are known to be the solutions of thermal convection problems for vertically unbounded systems. Typically, their relevance to the actual flow state is limited by three-dimensional breakdown caused by rapid growth of secondary instabilities. We consider a flow of a liquid metal in a vertical duct with a heated wall and strong transverse magnetic field and find elevator modes that are stable and, thus, not just relevant, but a dominant feature of the flow. We then explore the hypothesis suggested by recent experimental data that an analogous instability to modes of slow axial variation develops in finite-length ducts, where it causes large-amplitude fluctuations of temperature. The implications for liquid metal blankets for tokamak fusion reactors that potentially invalidate some of the currently pursued design concepts are discussed.
Kelley, M. C.; Pfaff, R. F., Jr.; Dao, E. V.; Holzworth, R. H., II
2014-12-01
With the increase in solar activity, the Communications/Outage Forecast System satellite (C/NOFS) now goes below the F peak. As such, we now can study the development of Convective Ionospheric Storms (CIS) and, most importantly, large-scale seeding of the low growth-rate Rayleigh-Taylor (R-T) instability. Two mechanisms have been suggested for such seeding: the Collisional Kelvin-Helmholtz Instability (CKHI) and internal atmospheric gravity waves. A number of observations have shown that the spectrum of fully developed topside structures peaks at 600 km and extends to over 1000 km. These structures are exceedingly difficult to explain by CKHI. Here we show that sinusoidal plasma oscillations on the bottomside during daytime develop classical R-T structures on the nightside with the background 600 km structure still apparent. In two case studies, thunderstorm activity was observed east of the sinusoidal features in the two hours preceding the C/NOFS passes. Thus, we argue that convective tropospheric storms are a likely source of these sinusoidal features.
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...
Bejan, Adrian
2013-01-01
Written by an internationally recognized authority on heat transfer and thermodynamics, this second edition of Convection Heat Transfer contains new and updated problems and examples reflecting real-world research and applications, including heat exchanger design. Teaching not only structure but also technique, the book begins with the simplest problem solving method (scale analysis), and moves on to progressively more advanced and exact methods (integral method, self similarity, asymptotic behavior). A solutions manual is available for all problems and exercises.
Bachmann, Kurt T.
2000-01-01
I helped to complete a research project with NASA scientists Dr. David Hathaway (my mentor), Rick Bogart, and John Beck from the SOHO/SOI collaboration. Our published paper in 'Solar Physics' was titled 'The Solar Convection Spectrum' (April 2000). Two of my undergraduate students were named on the paper--Gavrav Khutri and Josh Petitto. Gavrav also wrote a short paper for the National Conference of Undergraduate Research Proceedings in 1998 using a preliminary result. Our main result was that we show no evidence of a scale of convection named 'mesogranulation'. Instead, we see only direct evidence for the well-known scales of convection known as graduation and supergranulation. We are also completing work on vertical versus horizontal flow fluxes at the solar surface. I continue to work on phase relationships of solar activity indicators, but I have not yet written a paper with my students on this topic. Along with my research results, I have developed and augmented undergraduate courses at Birmingham-Southern College by myself and with other faculty. We have included new labs and observations, speakers from NASA and elsewhere, new subject material related to NASA and space science. I have done a great deal of work in outreach, mostly as President and other offices in the Birmingham Astronomical Society. My work includes speaking, attracting speakers, giving workshops, and governing.
Thermal Vibrational Convection
Gershuni, G. Z.; Lyubimov, D. V.
1998-08-01
Recent increasing awareness of the ways in which vibrational effects can affect low-gravity experiments have renewed interest in the study of thermal vibrational convection across a wide range of fields. For example, in applications where vibrational effects are used to provide active control of heat and mass transfer, such as in heat exchangers, stirrers, mineral separators and crystal growth, a sound understanding of the fundamental theory is required. In Thermal Vibrational Convection, the authors present the theory of vibrational effects caused by a static gravity field, and of fluid flows which appear under vibration in fluid-filled cavities. The first part of the book discusses fluid-filled cavities where the fluid motion only appears in the presence of temperature non-uniformities, while the second considers those situations where the vibrational effects are caused by a non-uniform field. Throughout, the authors concentrate on consideration of high frequency vibrations, where averaging methods can be successfully applied in the study of the phenomena. Written by two of the pioneers in this field, Thermal Vibrational Convection will be of great interest to scientists and engineers working in the many areas that are concerned with vibration, and its effect on heat and mass transfer. These include hydrodynamics, hydro-mechanics, low gravity physics and mechanics, and geophysics. The rigorous approach adopted in presenting the theory of this fascinating and highly topical area will facilitate a greater understanding of the phenomena involved, and will lead to the development of more and better-designed experiments.
Inside the supernova a powerful convective engine
Herant, M; Hix, W R; Fryer, C F; Colgate, S A; Marc Herant; Willy Benz; Chris F Fryer; Stirling Colgate
1994-01-01
We present an extensive study of the inception of supernova explosions by following the evolution of the cores of two massive stars (15 Msun and 25 Msun) in two dimensions. Our calculations begin at the onset of core collapse and stop several 100 ms after the bounce, at which time successful explosions of the appropriate magnitude have been obtained. (...) Guided by our numerical results, we have developed a paradigm for the supernova explosion mechanism. We view a supernova as an open cycle thermodynamic engine in which a reservoir of low-entropy matter (the envelope) is thermally coupled and physically connected to a hot bath (the protoneutron star) by a neutrino flux, and by hydrodynamic instabilities. (...) In essence, a Carnot cycle is established in which convection allows out-of-equilibrium heat transfer mediated by neutrinos to drive low entropy matter to higher entropy and therefore extracts mechanical energy from the heat generated by gravitational collapse. We argue that supernova explosions are ne...
Interacting length scales in the reactive-infiltration instability
Szymczak, Piotr
2013-01-01
The reactive-infiltration instability, which develops when a porous matrix is dissolved by a flowing fluid, contains two important length scales. Here we outline a linear stability analysis that simultaneously incorporates both scales. We show that the commonly used "thin-front" model is a limiting case of a more general theory, which also includes convection-dominated dissolution as another special case. The wavelength of the instability is bounded from below, and lies in the range 1mm to 1km for physically reasonable flow rates and reaction rates. We obtain a closed form for the growth rate when the change in porosity is small.
Tidal instability in a rotating and differentially heated ellipsoidal shell
Cébron, David; Bars, Michael Le; 10.1111/j.1365-246X.2010.04712.x
2010-01-01
The stability of a rotating flow in a triaxial ellipsoidal shell with an imposed temperature difference between inner and outer boundaries is studied numerically. We demonstrate that (i) a stable temperature field encourages the tidal instability, (ii) the tidal instability can grow on a convective flow, which confirms its relevance to geo- and astrophysical contexts and (iii) its growth rate decreases when the intensity of convection increases. Simple scaling laws characterizing the evolution of the heat flux based on a competition between viscous and thermal boundary layers are derived analytically and verified numerically. Our results confirm that thermal and tidal effects have to be simultaneously taken into account when studying geophysical and astrophysical flows.
Cavitation Instabilities in Inducers
2006-11-01
gas handling turbomachines . The fluctuation of the cavity length is plotted in Fig.8 under the surge mode oscillation vi . The major differences...Cavitation Instabilities of Turbomachines .” AIAA Journal of Propulsion and Power, Vol.17, No.3, 636-643. [5] Tsujimoto, Y., (2006), “Flow Instabilities in
Instability in evolutionary games.
Directory of Open Access Journals (Sweden)
Zimo Yang
Full Text Available BACKGROUND: Phenomena of instability are widely observed in many dissimilar systems, with punctuated equilibrium in biological evolution and economic crises being noticeable examples. Recent studies suggested that such instabilities, quantified by the abrupt changes of the composition of individuals, could result within the framework of a collection of individuals interacting through the prisoner's dilemma and incorporating three mechanisms: (i imitation and mutation, (ii preferred selection on successful individuals, and (iii networking effects. METHODOLOGY/PRINCIPAL FINDINGS: We study the importance of each mechanism using simplified models. The models are studied numerically and analytically via rate equations and mean-field approximation. It is shown that imitation and mutation alone can lead to the instability on the number of cooperators, and preferred selection modifies the instability in an asymmetric way. The co-evolution of network topology and game dynamics is not necessary to the occurrence of instability and the network topology is found to have almost no impact on instability if new links are added in a global manner. The results are valid in both the contexts of the snowdrift game and prisoner's dilemma. CONCLUSIONS/SIGNIFICANCE: The imitation and mutation mechanism, which gives a heterogeneous rate of change in the system's composition, is the dominating reason of the instability on the number of cooperators. The effects of payoffs and network topology are relatively insignificant. Our work refines the understanding on the driving forces of system instability.
Robbins, G M; Masri, B A; Garbuz, D S; Greidanus, N; Duncan, C P
2001-10-01
Instability after total hip arthroplasty is a major source of patient morbidity, second only to aseptic loosening. Certain patient groups have been identified as having a greater risk of instability, including patients undergoing revision arthroplasty as early or late treatment for proximal femoral fractures.
Critical phenomena employed in hydrodynamic problems A case study of Rayleigh-Benard convection
Assenheimer, M; Assenheimer, Michel; Steinberg, Victor
1996-01-01
By virtue of Rayleigh-Benard convection, we illustrate the advantages of combining a hydrodynamic pattern forming instability with a thermodynamic critical point. This has already lead to many novel unexpected observations and is further shown to possess opportunities for the study of exciting fundamental problems in nonequilibrium systems.
Stability Analysis of Strange-Modes in Hot Massive Stars with Time-Dependent Convection
Sonoi, Takafumi
2014-01-01
We carry out a nonadiabatic analysis of strange-modes in hot massive stars with time-dependent convection (TDC). In envelopes of such stars, convective luminosity is not so dominant as that in envelopes of stars in the redder side of the classical instability strip. Around the Fe opacity bump, however, convection non-negligibly contributes to energy transfer. Indeed, instability of modes excited at the Fe bump is likely to be suppressed with TDC compared with the case of adopting the frozen-in convection approximation. But we make sure that unstable strange-modes certainly appear in hot massive stars even by taking into account TDC. We also examine properties of the strange-mode instability, which is related to destabilization of strange-modes without adiabatic counterparts. In this type of instability, the phase lag between density and pressure varies from 0 to $180^{\\circ}$ in an excitation zone unlike the case of the $\\kappa$-mechanism. In addition, we confirm by comparing models with $Z=0$ and $Z=0.02$ th...
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
Titan Balloon Convection Model Project
National Aeronautics and Space Administration — This innovative research effort is directed at determining, quantitatively, the convective heat transfer coefficients applicable to a Montgolfiere balloon operating...
Gravity-Driven Instability in a Liquid Film Overlying an Inhomogeneous Porous Layer
Institute of Scientific and Technical Information of China (English)
ZHAO Si-Cheng; LIU Qiu-Sheng; NGUYEN-THI Henri; BILLIA Bernard
2011-01-01
@@ A new model consisting of a liquid film overlying a saturated and inhomogeneous porous layer is investigated.We concentrate on effects of inhomogeneity on transition of instability modes.Influences of the averaged porosity and the gradient of porosity distribution on the instability behaviors of a liquid-porous layer system are emphasized.The average permeability of the porous layer is a key factor to determine the penetration of convection in the system.%A new model consisting of a liquid film overlying a saturated and inhomogeneous porous layer is investigated. We concentrate on effects of inhomogeneity on transition of instability modes. Influences of the averaged porosity and the gradient of porosity distribution on the instability behaviors of a liquid-porous layer system are emphasized.The average permeability of the porous layer is a key factor to determine the penetration of convection in the system.
Bidispersive-inclined convection
Mulone, Giuseppe; Straughan, Brian
2016-01-01
A model is presented for thermal convection in an inclined layer of porous material when the medium has a bidispersive structure. Thus, there are the usual macropores which are full of a fluid, but there are also a system of micropores full of the same fluid. The model we employ is a modification of the one proposed by Nield & Kuznetsov (2006 Int. J. Heat Mass Transf. 49, 3068–3074. (doi:10.1016/j.ijheatmasstransfer.2006.02.008)), although we consider a single temperature field only. PMID:27616934
Albarède, Francis; Van Der Hilst, Rob D
2002-11-15
We review the present state of our understanding of mantle convection with respect to geochemical and geophysical evidence and we suggest a model for mantle convection and its evolution over the Earth's history that can reconcile this evidence. Whole-mantle convection, even with material segregated within the D" region just above the core-mantle boundary, is incompatible with the budget of argon and helium and with the inventory of heat sources required by the thermal evolution of the Earth. We show that the deep-mantle composition in lithophilic incompatible elements is inconsistent with the storage of old plates of ordinary oceanic lithosphere, i.e. with the concept of a plate graveyard. Isotopic inventories indicate that the deep-mantle composition is not correctly accounted for by continental debris, primitive material or subducted slabs containing normal oceanic crust. Seismological observations have begun to hint at compositional heterogeneity in the bottom 1000 km or so of the mantle, but there is no compelling evidence in support of an interface between deep and shallow mantle at mid-depth. We suggest that in a system of thermochemical convection, lithospheric plates subduct to a depth that depends - in a complicated fashion - on their composition and thermal structure. The thermal structure of the sinking plates is primarily determined by the direction and rate of convergence, the age of the lithosphere at the trench, the sinking rate and the variation of these parameters over time (i.e. plate-tectonic history) and is not the same for all subduction systems. The sinking rate in the mantle is determined by a combination of thermal (negative) and compositional buoyancy and as regards the latter we consider in particular the effect of the loading of plates with basaltic plateaux produced by plume heads. Barren oceanic plates are relatively buoyant and may be recycled preferentially in the shallow mantle. Oceanic plateau-laden plates have a more pronounced
Institute of Scientific and Technical Information of China (English)
Rong Liu; Qiusheng Liu
2006-01-01
Classical theories have successfully provided an explanation for convection in a liquid layer heated from below without evaporation.However,these theories are inadequate to account for the convective instabilities in an evaporating liquid layer,especially in the case when it is cooled from below.In the present paper,we study the onset of Marangoni convection in a liquid layer being overlain by a vapor layer.A new two-sided model is put forward instead of the one-sided in evaporating liquid thin layers are investigated with a linear instability analysis.We define a new evaporation Biot number,Which is different from that in previous studies and discuss the influences of reference evaporating velocity and evaporation Biot number on the vapor-liquid system.At the end,we explain why the instability occurs even when an evaporating liquid layer is cooled from below.
Electrical imaging and fluid modeling of convective fingering in a shallow water-table aquifer
Dam, Remke L.; Eustice, Brian P.; Hyndman, David W.; Wood, Warren W.; Simmons, Craig T.
2014-02-01
Unstable density-driven flow can lead to enhanced solute transport in groundwater. Only recently has the complex fingering pattern associated with free convection been documented in field settings. Electrical resistivity (ER) tomography has been used to capture a snapshot of convective instabilities at a single point in time, but a thorough transient analysis is still lacking in the literature. We present the results of a 2 year experimental study at a shallow aquifer in the United Arab Emirates that was designed to specifically explore the transient nature of free convection. ER tomography data documented the presence of convective fingers following a significant rainfall event. We demonstrate that the complex fingering pattern had completely disappeared a year after the rainfall event. The observation is supported by an analysis of the aquifer halite budget and hydrodynamic modeling of the transient character of the fingering instabilities. Modeling results show that the transient dynamics of the gravitational instabilities (their initial development, infiltration into the underlying lower-density groundwater, and subsequent decay) are in agreement with the timing observed in the time-lapse ER measurements. All experimental observations and modeling results are consistent with the hypothesis that a dense brine that infiltrated into the aquifer from a surficial source was the cause of free convection at this site, and that the finite nature of the dense brine source and dispersive mixing led to the decay of instabilities with time. This study highlights the importance of the transience of free convection phenomena and suggests that these processes are more rapid than was previously understood.
Nonlinear helical MHD instability
Energy Technology Data Exchange (ETDEWEB)
Zueva, N.M.; Solov' ev, L.S.
1977-07-01
An examination is made of the boundary problem on the development of MHD instability in a toroidal plasma. Two types of local helical instability are noted - Alfven and thermal, and the corresponding criteria of instability are cited. An evaluation is made of the maximum attainable kinetic energy, limited by the degree to which the law of conservation is fulfilled. An examination is made of a precise solution to a kinematic problem on the helical evolution of a cylindrical magnetic configuration at a given velocity distribution in a plasma. A numerical computation of the development of MHD instability in a plasma cylinder by a computerized solution of MHD equations is made where the process's helical symmetry is conserved. The development of instability is of a resonance nature. The instability involves the entire cross section of the plasma and leads to an inside-out reversal of the magnetic surfaces when there is a maximum unstable equilibrium configuration in the nonlinear stage. The examined instability in the tore is apparently stabilized by a magnetic hole when certain limitations are placed on the distribution of flows in the plasma. 29 references, 8 figures.
Geoid Anomalies and Dynamic Topography from Time Dependent, Spherical Axisymmetric Mantle Convection
Kiefer, Walter S.; Kellogg, Louise H.
1998-01-01
Geoid anomalies and dynamic topography are two important diagnostics of mantle convection. We present geoid and topography results for several time-dependent convection models in spherical axisymmetric geometry for Rayleigh numbers between 10(exp 6) and 10(exp 7) with depth-dependent viscosity and mixtures of bottom and internal heating. The models are strongly chaotic, with boundary layer instabilities erupting out of both thermal boundary layers. In some instances, instabilities from one boundary layer influence the development of instabilities in the other boundary layer. Such coupling between events at the top and bottom of the mantle has been suggested to play a role in a mid-Cretaceous episode of enhanced volcanism in the Pacific. These boundary layer instabilities produce large temporal variations in the geoid anomalies and dynamic nd to the topography associated with the convection. The amplitudes of these fluctuations depend on the detailed model parameter,.% it of this but fluctuations of 30-50% relative to the time-averaged geoid and topography are common. The convective planform is strongly sensitive to the specific initial conditions. Convection cells with larger aspect ratio tend to have larger fractional fluctuations in their geoid and topography amplitudes, because boundary layer instabilities have more time to develop in long cells. In some instances, we observe low-amplitude topographic highs adjacent to the topographic lows produced by cold downwellings. We discuss applications of these results to several situations, including the temporal variability of m basis. hotspots such as Hawaii, the topography of subduction zone outer rises, and the topography of coronae on Venus.
Rapid PCR thermocycling using microscale thermal convection.
Muddu, Radha; Hassan, Yassin A; Ugaz, Victor M
2011-03-05
Many molecular biology assays depend in some way on the polymerase chain reaction (PCR) to amplify an initially dilute target DNA sample to a detectable concentration level. But the design of conventional PCR thermocycling hardware, predominantly based on massive metal heating blocks whose temperature is regulated by thermoelectric heaters, severely limits the achievable reaction speed(1). Considerable electrical power is also required to repeatedly heat and cool the reagent mixture, limiting the ability to deploy these instruments in a portable format. Thermal convection has emerged as a promising alternative thermocycling approach that has the potential to overcome these limitations(2-9). Convective flows are an everyday occurrence in a diverse array of settings ranging from the Earth's atmosphere, oceans, and interior, to decorative and colorful lava lamps. Fluid motion is initiated in the same way in each case: a buoyancy driven instability arises when a confined volume of fluid is subjected to a spatial temperature gradient. These same phenomena offer an attractive way to perform PCR thermocycling. By applying a static temperature gradient across an appropriately designed reactor geometry, a continuous circulatory flow can be established that will repeatedly transport PCR reagents through temperature zones associated with the denaturing, annealing, and extension stages of the reaction (Figure 1). Thermocycling can therefore be actuated in a pseudo-isothermal manner by simply holding two opposing surfaces at fixed temperatures, completely eliminating the need to repeatedly heat and cool the instrument. One of the main challenges facing design of convective thermocyclers is the need to precisely control the spatial velocity and temperature distributions within the reactor to ensure that the reagents sequentially occupy the correct temperature zones for a sufficient period of time(10,11). Here we describe results of our efforts to probe the full 3-D velocity and
Observing convection with satellite, radar, and lightning measurements
Hamann, Ulrich; Nisi, Luca; Clementi, Lorenzo; Ventura, Jordi Figueras i.; Gabella, Marco; Hering, Alessandro M.; Sideris, Ioannis; Trefalt, Simona; Germann, Urs
2015-04-01
Heavy precipitation, hail, and wind gusts are the fundamental meteorological hazards associated with strong convection and thunderstorms. The thread is particularly severe in mountainous areas, e.g. it is estimated that on average between 50% and 80% of all weather-related damage in Switzerland is caused by strong thunderstorms (Hilker et al., 2010). Intense atmospheric convection is governed by processes that range from the synoptic to the microphysical scale and are considered to be one of the most challenging and difficult weather phenomena to predict. Even though numerical weather prediction models have some skills to predict convection, in general the exact location of the convective initialization and its propagation cannot be forecasted by these models with sufficient precision. Hence, there is a strong interest to improve the short-term forecast by using statistical, object oriented and/or heuristic nowcasting methods. MeteoSwiss has developed several operational nowcasting systems for this purpose such as TRT (Hering, 2008) and COALITION (Nisi, 2014). In this contribution we analyze the typical development of convection using measurements of the Swiss C-band Dual Polarization Doppler weather radar network, the MSG SEVIRI satellite, and the Météorage lighting network. The observations are complemented with the analysis and forecasts of the COSMO model. Special attention is given to the typical evolutionary stages like the pre-convective environment, convective initiation, cloud top glaciation, start, maximum, and end of precipitation and lightning activity. The pre-convective environment is examined using instability indices derived from SEVIRI observations and the COSMO forecasts. During the early development satellite observations are used to observe the rise of the cloud top, the growth of the cloud droplet or crystals, and the glaciation of the cloud top. SEVIRI brightness temperatures, channel differences, and temporal trends as suggested by
Rotating thermal convection at very large Rayleigh numbers
Weiss, Stephan; van Gils, Dennis; Ahlers, Guenter; Bodenschatz, Eberhard
2016-11-01
The large scale thermal convection systems in geo- and astrophysics are usually influenced by Coriolis forces caused by the rotation of their celestial bodies. To better understand the influence of rotation on the convective flow field and the heat transport at these conditions, we study Rayleigh-Bénard convection, using pressurized sulfur hexaflouride (SF6) at up to 19 bars in a cylinder of diameter D=1.12 m and a height of L=2.24 m. The gas is heated from below and cooled from above and the convection cell sits on a rotating table inside a large pressure vessel (the "Uboot of Göttingen"). With this setup Rayleigh numbers of up to Ra =1015 can be reached, while Ekman numbers as low as Ek =10-8 are possible. The Prandtl number in these experiment is kept constant at Pr = 0 . 8 . We report on heat flux measurements (expressed by the Nusselt number Nu) as well as measurements from more than 150 temperature probes inside the flow. We thank the Deutsche Forschungsgemeinschaft (DFG) for financial support through SFB963: "Astrophysical Flow Instabilities and Turbulence". The work of GA was supported in part by the US National Science Foundation through Grant DMR11-58514.
Spondylolisthesis and Posterior Instability
Energy Technology Data Exchange (ETDEWEB)
Niggemann, P.; Beyer, H.K.; Frey, H.; Grosskurth, D. (Privatpraxis fuer Upright MRT, Koeln (Germany)); Simons, P.; Kuchta, J. (Media Park Klinik, Koeln (Germany))
2009-04-15
We present the case of a patient with a spondylolisthesis of L5 on S1 due to spondylolysis at the level L5/S1. The vertebral slip was fixed and no anterior instability was found. Using functional magnetic resonance imaging (MRI) in an upright MRI scanner, posterior instability at the level of the spondylolytic defect of L5 was demonstrated. A structure, probably the hypertrophic ligament flava, arising from the spondylolytic defect was displaced toward the L5 nerve root, and a bilateral contact of the displaced structure with the L5 nerve root was shown in extension of the spine. To our knowledge, this is the first case described of posterior instability in patients with spondylolisthesis. The clinical implications of posterior instability are unknown; however, it is thought that this disorder is common and that it can only be diagnosed using upright MRI.
Dynamics of fingering convection II: The formation of thermohaline staircases
Stellmach, S; Garaud, P; Brummell, N; Radko, T
2010-01-01
Regions of the ocean's thermocline unstable to salt fingering are often observed to host thermohaline staircases, stacks of deep well-mixed convective layers separated by thin stably-stratified interfaces. Decades after their discovery, however, their origin remains controversial. In this paper we use 3D direct numerical simulations to shed light on the problem. We study the evolution of an analogous double-diffusive system, starting from an initial statistically homogeneous fingering state and find that it spontaneously transforms into a layered state. By analysing our results in the light of the mean-field theory developed in Paper I, a clear picture of the sequence of events resulting in the staircase formation emerges. A collective instability of homogeneous fingering convection first excites a field of gravity waves, with a well-defined vertical wavelength. However, the waves saturate early through regular but localized breaking events, and are not directly responsible for the formation of the staircase....
Oscillatory convection and limitations of the Boussinesq approximation
Wood, Toby S
2016-01-01
We determine the asymptotic conditions under which the Boussinesq approximation is valid for oscillatory convection in a rapidly rotating fluid. In the astrophysically relevant parameter regime of small Prandtl number, we show that the Boussinesq prediction for the onset of convection is valid only under much more restrictive conditions than those that are usually assumed. In the case of an ideal gas, we recover the Boussinesq results only if the ratio of the domain height to a typical scale height is much smaller than the Prandtl number. This requires an extremely shallow domain in the astrophysical parameter regime. Other commonly-used "sound-proof" approximations generally perform no better than the Boussinesq approximation. The exception is a particular implementation of the pseudo-incompressible approximation, which predicts the correct instability threshold beyond the range of validity of the Boussinesq approximation.
Electrothermo Convection in a Porous Medium Saturated by Nanofluid
Directory of Open Access Journals (Sweden)
Ramesh Chand
2016-01-01
Full Text Available Thermal instability in a horizontal layer of nanofluid with vertical AC electric field in a porous medium is investigated. The flux of volume fraction of nanoparticles is taken to be zero on the isothermal boundaries and the eigenvalue problem is solved using the Galerkin method. Darcy model is used for the momentum equation. The model used for nanofluid incorporates the effect of Brownian diffusion and thermophoresis. Linear stability theory based upon normal mode technique is employed to find the expressions for Rayleigh number for stationary and oscillatory convection. Graphs have been plotted to study the effects of Lewis number, modified diffusivity ratio, concentration Rayleigh number, AC electric Rayleigh number and porosity on stationary convection.
A nonlinear model for rotationally constrained convection with Ekman pumping
Julien, Keith; Calkins, Michael A; Knobloch, Edgar; Marti, Philippe; Stellmach, Stephan; Vasil, Geoffrey M
2016-01-01
It is a well established result of linear theory that the influence of differing mechanical boundary conditions, i.e., stress-free or no-slip, on the primary instability in rotating convection becomes asymptotically small in the limit of rapid rotation. This is accounted for by the diminishing impact of the viscous stresses exerted within Ekman boundary layers and the associated vertical momentum transport by Ekman pumping. By contrast, in the nonlinear regime recent experiments and supporting simulations are now providing evidence that the efficiency of heat transport remains strongly influenced by Ekman pumping in the rapidly rotating limit. In this paper, a reduced model is developed for the case of low Rossby number convection in a plane layer geometry with no-slip upper and lower boundaries held at fixed temperatures. A complete description of the dynamics requires the existence of three distinct regions within the fluid layer: a geostrophically balanced interior where fluid motions are predominately ali...
Neutrino signature of supernova hydrodynamical instabilities in three dimensions.
Tamborra, Irene; Hanke, Florian; Müller, Bernhard; Janka, Hans-Thomas; Raffelt, Georg
2013-09-20
The first full-scale three-dimensional core-collapse supernova (SN) simulations with sophisticated neutrino transport show pronounced effects of the standing accretion shock instability (SASI) for two high-mass progenitors (20 and 27 M([Symbol: see text])). In a low-mass progenitor (11.2 M([Symbol: see text])), large-scale convection is the dominant nonradial hydrodynamic instability in the postshock accretion layer. The SASI-associated modulation of the neutrino signal (80 Hz in our two examples) will be clearly detectable in IceCube or the future Hyper-Kamiokande detector, depending on progenitor properties, distance, and observer location relative to the main SASI sloshing direction. The neutrino signal from the next galactic SN can, therefore, diagnose the nature of the hydrodynamic instability.
Experimental research on flow instability in vertical narrow annuli
Institute of Scientific and Technical Information of China (English)
WU Geping; QIU Suizheng; SU Guanghui; JIA Dounan
2007-01-01
A narrow annular test section of 1.5mm gap and 1800mm length was designed and manufactured, with good tightness and insulation. Experiments were carried out to investigate characteristics of flow instability of forced-convection in vertical narrow annuli. Using distilled water as work fluid, the experiments were conducted at pressures of 1.0～3.0 MPa, mass flow rates of 3.0～25 kg/h, heating power of 3.0～ 6.5kW and inlet fluid temperature of 20 ℃, 40 ℃ or 60℃. It was found that flow instability occured with fixed inlet condition and heating power when mass flow rate was below a special value. Effects of inlet subcooling, system pressure and mass flow rate on the system behavior were studied and the instability region was given.
Csernai, László P; Papp, G
1995-01-01
The evolution of dynamical perturbations is examined in nuclear multifragmentation in the frame of Vlasov equation. Both plane wave and bubble type of perturbations are investigated in the presence of surface (Yukawa) forces. An energy condition is given for the allowed type of instabilities and the time scale of the exponential growth of the instabilities is calculated. The results are compared to the mechanical spinodal region predictions. PACS: 25.70 Mn
Prediction of Algebraic Instabilities
Zaretzky, Paula; King, Kristina; Hill, Nicole; Keithley, Kimberlee; Barlow, Nathaniel; Weinstein, Steven; Cromer, Michael
2016-11-01
A widely unexplored type of hydrodynamic instability is examined - large-time algebraic growth. Such growth occurs on the threshold of (exponentially) neutral stability. A new methodology is provided for predicting the algebraic growth rate of an initial disturbance, when applied to the governing differential equation (or dispersion relation) describing wave propagation in dispersive media. Several types of algebraic instabilities are explored in the context of both linear and nonlinear waves.
Nield, Donald A
1992-01-01
This book provides a user-friendly introduction to the topic of convection in porous media The authors as- sume that the reader is familiar with the basic elements of fluid mechanics and heat transfer, but otherwise the book is self-contained The book will be useful both as a review (for reference) and as a tutorial work, suitable as a textbook in a graduate course or seminar The book brings into perspective the voluminous research that has been performed during the last two decades The field has recently exploded because of worldwide concern with issues such as energy self-sufficiency and pollution of the environment Areas of application include the insulation of buildings and equipment, energy storage and recovery, geothermal reservoirs, nuclear waste disposal, chemical reactor engineering, and the storage of heat-generating materials such as grain and coal Geophysical applications range from the flow of groundwater around hot intrusions to the stability of snow against avalanches
Nield, Donald A
2013-01-01
Convection in Porous Media, 4th Edition, provides a user-friendly introduction to the subject, covering a wide range of topics, such as fibrous insulation, geological strata, and catalytic reactors. The presentation is self-contained, requiring only routine mathematics and the basic elements of fluid mechanics and heat transfer. The book will be of use not only to researchers and practicing engineers as a review and reference, but also to graduate students and others entering the field. The new edition features approximately 1,750 new references and covers current research in nanofluids, cellular porous materials, strong heterogeneity, pulsating flow, and more. Recognized as the standard reference in the field Includes a comprehensive, 250-page reference list Cited over 2300 times to date in its various editions Serves as an introduction for those entering the field and as a comprehensive reference for experienced researchers Features new sections on nanofluids, carbon dioxide sequestration, and applications...
Dynamo action and magnetic buoyancy in convection simulations with vertical shear
Guerrero, G
2011-01-01
A hypothesis for sunspot formation is the buoyant emergence of magnetic flux tubes created by the strong radial shear at the tachocline. In this scenario, the magnetic field has to exceed a threshold value before it becomes buoyant and emerges through the whole convection zone. We follow the evolution of a random seed magnetic field with the aim of study under what conditions it is possible to excite the dynamo instability and whether the dynamo generated magnetic field becomes buoyantly unstable and emerges to the surface as expected in the flux-tube context. We perform numerical simulations of compressible turbulent convection that include a vertical shear layer. Like the solar tachocline, the shear is located at the interface between convective and stable layers. We find that shear and convection are able to amplify the initial magnetic field and form large-scale elongated magnetic structures. The magnetic field strength depends on several parameters such as the shear amplitude, the thickness and location ...
Influence of the Dufour effect on convection in binary gas mixtures
Hollinger, S; Hollinger, St.
1995-01-01
Linear and nonlinear properties of convection in binary fluid layers heated from below are investigated, in particular for gas parameters. A Galerkin approximation for realistic boundary conditions that describes stationary and oscillatory convection in the form of straight parallel rolls is used to determine the influence of the Dufour effect on the bifurcation behaviour of convective flow intensity, vertical heat current, and concentration mixing. The Dufour--induced changes in the bifurcation topology and the existence regimes of stationary and traveling wave convection are elucidated. To check the validity of the Galerkin results we compare with finite--difference numerical simulations of the full hydrodynamical field equations. Furthermore, we report on the scaling behaviour of linear properties of the stationary instability.
Sensitivity of the simulated precipitation to changes in convective relaxation time scale
Directory of Open Access Journals (Sweden)
S. K. Mishra
2010-10-01
Full Text Available The paper describes the sensitivity of the simulated precipitation to changes in convective relaxation time scale (TAU of Zhang and McFarlane (ZM cumulus parameterization, in NCAR-Community Atmosphere Model version 3 (CAM3. In the default configuration of the model, the prescribed value of TAU, a characteristic time scale with which convective available potential energy (CAPE is removed at an exponential rate by convection, is assumed to be 1 h. However, some recent observational findings suggest that, it is larger by around one order of magnitude. In order to explore the sensitivity of the model simulation to TAU, two model frameworks have been used, namely, aqua-planet and actual-planet configurations. Numerical integrations have been carried out by using different values of TAU, and its effect on simulated precipitation has been analyzed.
The aqua-planet simulations reveal that when TAU increases, rate of deep convective precipitation (DCP decreases and this leads to an accumulation of convective instability in the atmosphere. Consequently, the moisture content in the lower- and mid- troposphere increases. On the other hand, the shallow convective precipitation (SCP and large-scale precipitation (LSP intensify, predominantly the SCP, and thus capping the accumulation of convective instability in the atmosphere. The total precipitation (TP remains approximately constant, but the proportion of the three components changes significantly, which in turn alters the vertical distribution of total precipitation production. The vertical structure of moist heating changes from a vertically extended profile to a bottom heavy profile, with the increase of TAU. Altitude of the maximum vertical velocity shifts from upper troposphere to lower troposphere. Similar response was seen in the actual-planet simulations. With an increase in TAU from 1 h to 8 h, there was a significant improvement in the simulation of the seasonal mean precipitation. The
Propagating Instabilities in Solids
Kyriakides, Stelios
1998-03-01
Instability is one of the factors which limit the extent to which solids can be loaded or deformed and plays a pivotal role in the design of many structures. Such instabilities often result in localized deformation which precipitates catastrophic failure. Some materials have the capacity to recover their stiffness following a certain amount of localized deformation. This local recovery in stiffness arrests further local deformation and spreading of the instability to neighboring material becomes preferred. Under displacement controlled loading the propagation of the transition fronts can be achieved in a steady-state manner at a constant stress level known as the propagation stress. The stresses in the transition fronts joining the highly deformed zone to the intact material overcome the instability nucleation stresses and, as a result, the propagation stress is usually much lower than the stress required to nucleate the instability. The classical example of this class of material instabilities is L/"uders bands which tend to affect mild steels and other metals. Recent work has demonstrated that propagating instabilities occur in several other materials. Experimental and analytical results from four examples will be used to illustrate this point: First the evolution of L=FCders bands in mild steel strips will be revisited. The second example involves the evolution of stress induced phase transformations (austenite to martensite phases and the reverse) in a shape memory alloy under displacement controlled stretching. The third example is the crushing behavior of cellular materials such as honeycombs and foams made from metals and polymers. The fourth example involves the axial broadening/propagation of kink bands in aligned fiber/matrix composites under compression. The microstructure and, as a result, the micromechanisms governing the onset, localization, local arrest and propagation of instabilities in each of the four materials are vastly different. Despite this
THE UNSTABLE MODES OF NATURAL CONVECTION BOUNDARY LAYER
Institute of Scientific and Technical Information of China (English)
Tao Jianjun; Zhuang Fenggan; Yan Dachun
2000-01-01
The instability of natural convection boundary layer around a vertical heated flat plate is analyzed theoretically in this paper. The results illustrate that the “loop” in the neutral curve is not a real loop but a twist of the curve is the frequencywave number-Grashof number space, and there is only one unstable mode at small Prandtl numbers. Specially, when the Prandtl number is large enough two unstable modes will be found in the “loop” region. Along the amplifying surface intersection the two unstable modes have the same Grashof number, wave number and frequency but different amplifying rates. Their instability characteristics are analyzed and the criterion for determining the existence of the multi-unstable modes is also discussed.
Internally heated convection and Rayleigh-Bénard convection
Goluskin, David
2016-01-01
This Brief describes six basic models of buoyancy-driven convection in a fluid layer: three configurations of internally heated convection and three configurations of Rayleigh-Bénard convection. The author discusses the main quantities that characterize heat transport in each model, along with the constraints on these quantities. This presentation is the first to place the various models in a unified framework, and similarities and differences between the cases are highlighted. Necessary and sufficient conditions for convective motion are given. For the internally heated cases only, parameter-dependent lower bounds on the mean fluid temperature are proven, and results of past simulations and laboratory experiments are summarized and reanalyzed. The author poses several open questions for future study.
Calkins, Michael A.; Julien, Keith; Marti, Philippe
2015-01-01
The linear theory for rotating compressible convection in a plane layer geometry is presented for the astrophysically relevant case of low Prandtl number gases. When the rotation rate of the system is large, the flow remains geostrophically balanced for all stratification levels investigated and the classical (i.e. incompressible) asymptotic scaling laws for the critical parameters are recovered. For sufficiently small Prandtl numbers, increasing stratification tends to further destabilize the fluid layer, decrease the critical wavenumber and increase the oscillation frequency of the convective instability. In combination, these effects increase the relative magnitude of the time derivative of the density perturbation contained in the conservation of mass equation to non-negligible levels; the resulting convective instabilities occur in the form of compressional quasi-geostrophic oscillations. We find that the anelastic equations, which neglect this term, cannot capture these instabilities and possess spuriously growing eigenmodes in the rapidly rotating, low Prandtl number regime. It is shown that the Mach number for rapidly rotating compressible convection is intrinsically small for all background states, regardless of the departure from adiabaticity. PMID:25792951
Zero-Prandtl-number convection with slow rotation
Maity, Priyanka; Kumar, Krishna
2014-10-01
We present the results of our investigations of the primary instability and the flow patterns near onset in zero-Prandtl-number Rayleigh-Bénard convection with uniform rotation about a vertical axis. The investigations are carried out using direct numerical simulations of the hydrodynamic equations with stress-free horizontal boundaries in rectangular boxes of size (2π/kx) × (2π/ky) × 1 for different values of the ratio η = kx/ky. The primary instability is found to depend on η and Ta. Wavy rolls are observed at the primary instability for smaller values of η (1/√{3} ≤ η ≤ 2 except at η = 1) and for smaller values of Ta. We observed Küppers-Lortz (KL) type patterns at the primary instability for η = 1/√{3} and Ta ≥ 40. The fluid patterns are found to exhibit the phenomenon of bursting, as observed in experiments [K. M. S. Bajaj, G. Ahlers, and W. Pesch, "Rayleigh-Bénard convection with rotation at small Prandtl numbers," Phys. Rev. E 65, 056309 (2002)]. Periodic wavy rolls are observed at onset for smaller values of Ta, while KL-type patterns are observed for Ta ≥ 100 for η =√{3}. In case of η = 2, wavy rolls are observed for smaller values of Ta and KL-type patterns are observed for 25 ≤ Ta ≤ 575. Quasi-periodically varying patterns are observed in the oscillatory regime (Ta > 575). The behavior is quite different at η = 1. A time dependent competition between two sets of mutually perpendicular rolls is observed at onset for all values of Ta in this case. Fluid patterns are found to burst periodically as well as chaotically in time. It involved a homoclinic bifurcation. We have also made a couple of low-dimensional models to investigate bifurcations for η = 1, which is used to investigate the sequence of bifurcations.
Zero-Prandtl-number convection with slow rotation
Maity, Priyanka
2014-01-01
We present the results of our investigations of the primary instability and the flow patterns near onset in zero-Prandtl-number Rayleigh-B\\'{e}nard convection with uniform rotation about a vertical axis. The investigations are carried out using direct numerical simulations of the hydrodynamic equations with stress-free horizontal boundaries in rectangular boxes of size $(2\\pi/k_x) \\times (2\\pi/k_y) \\times 1$ for different values of the ratio $\\eta = k_x/k_y$. The primary instability is found to depend on $\\eta$ and $Ta$. Wavy rolls are observed at the primary instability for smaller values of $\\eta$ ($1/\\sqrt{3} \\le \\eta \\le 2$ except at $\\eta = 1$) and for smaller values of $Ta$. We observed K\\"{u}ppers-Lortz (KL) type patterns at the primary instability for $\\eta = 1/\\sqrt{3}$ and $ Ta \\ge 40$. The fluid patterns are found to exhibit the phenomenon of bursting, as observed in experiments [Bajaj et al. Phys. Rev. E {\\bf 65}, 056309 (2002)]. Periodic wavy rolls are observed at onset for smaller values of $Ta$...
Plumes in stellar convection zones
Zahn, J P
1999-01-01
All numerical simulations of compressible convection reveal the presence of strong downwards directed flows. Thanks to helioseismology, such plumes have now been detected also at the top of the solar convection zone, on super- granular scales. Their properties may be crudely described by adopting Taylor's turbulent entrainment hypothesis, whose validity is well established under various conditions. Using this model, one finds that the strong density stratification does not prevent the plumes from traversing the whole convection zone, and that they carry upwards a net energy flux (Rieutord & Zahn 1995). They penetrate to some extent in the adjacent stable region, where they establish a nearly adiabatic stratification. These plumes have a strong impact on the dynamics of stellar convection zones, and they play probably a key role in the dynamo mechanism.
Convective cooling of photovoltaics
Energy Technology Data Exchange (ETDEWEB)
Hodge, E.; Gibbons, C. [Energy Engineering Group, Mechanical Engineering Department, Cork Institute of Technology, Bishopstown, Cork (Ireland)
2004-07-01
Most solar cells presently on the market are based on silicon wafers, the so-called first generation technology. As this technology has matured costs have become increasingly dominated by material costs. In the last ten years, continuous work has brought the efficiency of standard cells to the 25% region. A switch to second generation or thin film technology cells now seems imminent. Thin film technology eliminates the silicon wafer and offer the prospect of reducing material and manufacturing costs, but they exhibit lower efficiencies of around 10% for a commercial device. Third generation or tandem cells are currently at a 'proof of concept' research level, with a theoretical conversion rate of 86.8% being asserted Whatever the material construction and manufacturing method of cells, the thermal effect of overheating will prevail in the semiconductor and it is accepted that a lowered temperature will bring about an increase in conversion efficiency. The aim of this project is to improve the efficiency of PV electrical output, by convectively cooling the cells through perforations in them. As the cells heat up they lose efficiency. As the panel heats up a loss in efficiency of 0.5% per C increase in temperature has been recorded. (orig.)
A climatology of potential severe convective environments across South Africa
Blamey, R. C.; Middleton, C.; Lennard, C.; Reason, C. J. C.
2017-09-01
Severe thunderstorms pose a considerable risk to society and the economy of South Africa during the austral summer months (October-March). Yet, the frequency and distribution of such severe storms is poorly understood, which partly stems out of an inadequate observation network. Given the lack of observations, alternative methods have focused on the relationship between severe storms and their associated environments. One such approach is to use a combination of covariant discriminants, derived from gridded datasets, as a probabilistic proxy for the development of severe storms. These covariates describe some key ingredient for severe convective storm development, such as the presence of instability. Using a combination of convective available potential energy and deep-layer vertical shear from Climate Forecast System Reanalysis, this study establishes a climatology of potential severe convective environments across South Africa for the period 1979-2010. Results indicate that early austral summer months are most likely associated with conditions that are conducive to the development of severe storms over the interior of South Africa. The east coast of the country is a hotspot for potential severe convective environments throughout the summer months. This is likely due to the close proximity of the Agulhas Current, which produces high latent heat fluxes and acts as a key moisture source. No obvious relationship is established between the frequency of potential severe convective environments and the main large-scale modes of variability in the Southern Hemisphere, such as ENSO. This implies that several factors, possibly more localised, may modulate the spatial and temporal frequency of severe thunderstorms across the region.
Neutrino beam plasma instability
Indian Academy of Sciences (India)
Vishnu M Bannur
2001-10-01
We derive relativistic ﬂuid set of equations for neutrinos and electrons from relativistic Vlasov equations with Fermi weak interaction force. Using these ﬂuid equations, we obtain a dispersion relation describing neutrino beam plasma instability, which is little different from normal dispersion relation of streaming instability. It contains new, nonelectromagnetic, neutrino-plasma (or electroweak) stable and unstable modes also. The growth of the instability is weak for the highly relativistic neutrino ﬂux, but becomes stronger for weakly relativistic neutrino ﬂux in the case of parameters appropriate to the early universe and supernova explosions. However, this mode is dominant only for the beam velocity greater than 0.25 and in the other limit electroweak unstable mode takes over.
DEFF Research Database (Denmark)
Langie, Sabine A S; Koppen, Gudrun; Desaulniers, Daniel
2015-01-01
, genome instability can be defined as an enhanced tendency for the genome to acquire mutations; ranging from changes to the nucleotide sequence to chromosomal gain, rearrangements or loss. This review raises the hypothesis that in addition to known human carcinogens, exposure to low dose of other...... scientists aware of the increasing need to unravel the underlying mechanisms via which chemicals at low doses can induce genome instability and thus promote carcinogenesis.......Genome instability is a prerequisite for the development of cancer. It occurs when genome maintenance systems fail to safeguard the genome's integrity, whether as a consequence of inherited defects or induced via exposure to environmental agents (chemicals, biological agents and radiation). Thus...
Mixing through shear instabilities
Brüggen, M
2000-01-01
In this paper we present the results of numerical simulations of the Kelvin-Helmholtz instability in a stratified shear layer. This shear instability is believed to be responsible for extra mixing in differentially rotating stellar interiors and is the prime candidate to explain the abundance anomalies observed in many rotating stars. All mixing prescriptions currently in use are based on phenomenological and heuristic estimates whose validity is often unclear. Using three-dimensional numerical simulations, we study the mixing efficiency as a function of the Richardson number and compare our results with some semi-analytical formalisms of mixing.
Observation of deep convection initiation from shallow convection environment
Lothon, Marie; Couvreux, Fleur; Guichard, Françoise; Campistron, Bernard; Chong, Michel; Rio, Catherine; Williams, Earle
2010-05-01
In the afternoon of 10 July 2006, deep convective cells initiated right in the field of view of the Massachusetts Institute Technology (MIT) C-band Doppler radar. This radar, with its 3D exploration at 10 min temporal resolution and 250 m radial resolution, allows us to track the deep convective cells and also provides clear air observations of the boundary layer structure prior to deep convection initiation. Several other observational platforms were operating then which allow us to thoroughly analyse this case: Vertically pointing aerosol lidar, W-band radar and ceilometer from the ARM Mobile Facility, along with radiosoundings and surface measurements enable us to describe the environment, from before their initiation to after the propagation of of one propagating cell that generated a circular gust front very nicely caught by the MIT radar. The systems considered here differ from the mesoscale convective systems which are often associated with African Easterly Waves, increasing CAPE and decreasing CIN. The former have smaller size, and initiate more locally, but there are numerous and still play a large role in the atmospheric circulation and scalar transport. Though, they remain a challenge to model. (See the presentation by Guichard et al. in the same session, for a model set up based on the same case, with joint single-column model and Large Eddy Simulation, which aims at better understanding and improving the parametrisation of deep convection initiation.) Based on the analysis of the observations mentioned above, we consider here the possible sources of deep convection initiation that day, which showed a typical boundary-layer growth in semi-arid environment, with isolated deep convective events.
Strange Attractors Characterizing the Osmotic Instability
Tzenov, Stephan I
2014-01-01
In the present paper a simple dynamical model for computing the osmotically driven fluid flow in a variety of complex, non equilibrium situations is derived from first principles. Using the Oberbeck-Boussinesq approximation, the basic equations describing the process of forward osmosis have been obtained. It has been shown that these equations are very similar to the ones used to model the free Rayleigh-Benard convection. The difference is that while in the case of thermal convection the volume expansion is driven by the coefficient of thermal expansion, the key role for the osmotic instability is played by the coefficient of isothermal compressibility. In addition, it has been shown that the osmotic process represents a propagation of standing waves with time-dependent amplitudes and phase velocity, which equals the current velocity of the solvent passing through the semi-permeable membrane. The evolution of the amplitudes of the osmotic waves is exactly following the dynamics of a strange attractor of Loren...
Heat flow control in thermo-magnetic convective systems using engineered magnetic fields
Lee, Jaewook; Nomura, Tsuyoshi; Dede, Ercan M.
2012-09-01
We present the design of a magnetically controlled convective heat transfer system. The underlying thermo-magnetic instability phenomenon is described, and enhanced convective fluid flow patterns are determined using non-linear programming techniques plus a design sensitivity analysis. Specifically, the magnetic fluid body force is computed by finding the optimal distribution and magnetization direction of a magnetic field source, where the objective is to minimize the maximum temperature of a closed loop heat transfer system. Sizeable fluid recirculation zones are induced by arranging magnetic field generation elements in configurations similar to Halbach arrays. Applications include improved heat flow control for electromechanical systems.
Wong, Teresa; Solomatov, Viatcheslav S.
2016-05-01
We perform numerical simulations of lithospheric failure in the stagnant lid regime of temperature-dependent viscosity convection, using the yield stress approach. We find that the time of failure can vary significantly for the same values of the controlling parameters due to the chaotic nature of the convective system. The general trend of the dependence of the time of lithospheric failure on the yield stress can be explained by treating lithospheric failure as a type of Rayleigh-Taylor instability. This study suggests that it is important to address not only the question of whether plate tectonics can occur on a planet but also when it would occur if conditions are favorable.
Convection in a nematic liquid crystal with homeotropic alignment and heated from below
Energy Technology Data Exchange (ETDEWEB)
Ahlers, G. [Univ. of California, Santa Barbara, CA (United States)
1995-12-31
Experimental results for convection in a thin horizontal layer of a homeotropically aligned nematic liquid crystal heated from below and in a vertical magnetic field are presented. A subcritical Hopf bifurcation leads to the convecting state. There is quantitative agreement between the measured and the predicted bifurcation line as a function of magnetic field. The nonlinear state near the bifurcation is one of spatio-temporal chaos which seems to be the result of a zig-zag instability of the straight-roll state.
Energy Technology Data Exchange (ETDEWEB)
Bhadauria, Beer S. [Babasaheb Bhimrao Ambedkar Univ., Lucknow (India). Dept. of Applied Mathematics and Statistics; Banaras Hindu Univ., Varanasi (India). Dept. of Mathematics; Srivastava, Atul K. [Banaras Hindu Univ., Varanasi (India). Dept. of Mathematics; Sacheti, Nirmal C.; Chandran, Pallath [Sultan Qaboos Univ., Muscat (Oman). Dept. of Mathematics
2012-01-15
The present paper deals with a thermal instability problem in a viscoelastic fluid saturating an anisotropic porous medium under gravity modulation. To find the gravity modulation effect, the gravity field is considered in two parts: a constant part and an externally imposed time-dependent periodic part. The time-dependent part of the gravity field, which can be realized by shaking the fluid, has been represented by a sinusoidal function. Using Hill's equation and the Floquet theory, the convective threshold has been obtained. It is found that gravity modulation can significantly affect the stability limits of the system. Further, we find that there is a competition between the synchronous and subharmonic modes of convection at the onset of instability. Effects of various parameters on the onset of instability have also been discussed. (orig.)
Directory of Open Access Journals (Sweden)
L. Hurd
2009-07-01
Full Text Available Resonant sodium lidar measurements from the transition region between the mesosphere and lower thermosphere have revealed frequently-occurring overturning events characterized by vertical scales of ~3–6 km and timescales of several hours. Larsen et al. (2004 proposed that a convective roll instability, similar to that found in the planetary boundary layer, is the likely mechanism responsible for the events. This type of instability requires an inflection point in the background winds near the center of the vortex roll with a low static stability region capped by an inversion. The earlier paper argued that the conditions required to support the instability are common in the altitude range where the features are found. In this paper, we use data from the University of Illinois sodium lidar that was located at the Starfire Optical Range near Albuquerque, New Mexico, and from the Maui/MALT Lidar Facility in Hawaii and present several cases that are used to examine the behavior of the inflection point in detail as a function of time during the evolution of the overturning event. In addition, we examine the background static stability conditions using the temperature data from the lidar.
Rotational effects on the negative magnetic pressure instability
Losada, Illa R; Kleeorin, N; Mitra, Dhrubaditya; Rogachevskii, I
2012-01-01
The surface layers of the Sun are strongly stratified. In the presence of turbulence with a weak mean magnetic field, a large-scale instability resulting in the formation of non-uniform magnetic structures, can be excited over the scale of many turbulent eddies or convection cells. This instability is caused by a negative contribution of turbulence to the effective (mean-field) magnetic pressure and has previously been discussed in connection with the formation of active regions and perhaps sunspots. We want to understand the effects of rotation on this instability in both two and three dimensions. We use mean-field magnetohydrodynamics in a parameter regime in which the properties of the negative effective magnetic pressure instability have previously been found to be in agreement with those of direct numerical simulations. We find that the instability is suppressed already for relatively slow rotation with Coriolis numbers (i.e. inverse Rossby numbers) around 0.2. The suppression is strongest at the equator...
Non-local convergence coupling in a simple stochastic convection model
Brenowitz, N. D.; Frenkel, Y.; Majda, A. J.
2016-06-01
Observational studies show a strong correlation between large-scale wind convergence and precipitation. However, using this as a convective closure assumption to determine the total precipitation in a numerical model typically leads to deleterious wave-CISK behavior such as grid-scale noise. The quasi-equilibrium (QE) schemes ameliorate this issue and smooth the precipitation field, but still inadequately represent the intermittent and organized nature of tropical convection. However, recent observational evidence highlights that the large-scale convergence field primarily affects precipitation by increasing the overall convective cloud fraction rather than the energetics of individual convective elements. In this article, the dynamical consequences of this diagnostic observation are studied using a simple one baroclinic mode stochastic model for convectively coupled waves. A version of this model is implemented which couples the stochastic formation of convective elements to the wind convergence. Linearized analysis shows that using the local convergence results in a classic wave-CISK standing instability where the growth rate increases with the wavenumber. However, using a large-scale averaged convergence restricts the instability to physically plausible scales. Convergence coupling is interpreted as a surrogate for the non-local effects of gregarious convection. In nonlinear stochastic simulations with a non-uniform imposed sea surface temperature (SST) field, the non-local convergence coupling introduces desirable intermittent variability on intraseasonal time scales. Convergence coupling leads to a circulation with a similar mean but higher variability than the equivalent parameterization without convergence coupling. Finally, the model is shown to retain these features on fine and coarse mesh sizes.
Effect of wave localization on plasma instabilities
Energy Technology Data Exchange (ETDEWEB)
Levedahl, W.K.
1987-01-01
The Anderson model of wave localization in random media is invoked to study the effect of solar-wind density turbulence on plasma processes associated with the solar type-III radio burst. ISEE-3 satellite data indicate that a possible model for the type-III process is the parametric decay of Langmuir waves excited by solar-flare electron streams into daughter electromagnetic and ion-acoustic waves. The threshold for this instability, however, is much higher than observed Langmuir-wave levels because of rapid wave convection of the transverse electromagnetic daughter wave in the case where the solar wind is assumed homogeneous. Langmuir and transverse waves near critical density satisfy the Ioffe-Riegel criteria for wave localization in the solar wind with observed density fluctuations {approximately}1%. Computer simulations using a linearized hybrid code show that an electron beam will excite localized Langmuir waves in a plasma with density turbulence. An action-principle approach is used to develop a theory of nonlinear wave processes when waves are localized. A theory of resonant particles diffusion by localized waves is developed to explain the saturation of the beam-plasma instability.
On the Onset of Thermocapillary Convection in a Liquid bridge
Shukla, Kedar
Thermo capillary convection refers to motion driven by the application of a temperature gradient along the interface. The temperature gradient may be large enough to cause oscillations in the basic state of the fluid. The vast majority of the liquid bridge investigations performed aboard on the sounding rockets or the space shuttles [1, 2] focused on the float zone processes because the process has been regarded as a candidate for the space based manufacturing of semiconductor materials. Although the buoyancy effect is avoided in the floating zone techniques during space operation, it experiences surface tension driven convection initiated by the temperature gradient along the free surface of the zone [3]. The appearance of the oscillatory thermo capillary convection couples with the solidification processes leads to the striations and results into the degradation of the crystals [4, 5]. The half zone consists of the liquid bridge held between two solid, planar end walls across which a temperature gradient is applied. Thus the basic state of thermo capillary convection consists of a single toroidal roll with the surface motion directed downwards from the hot upper disc to the cold lower one. Bennacer et al [6] studied how different axial profiles of the heat flux affect the flow patterns and transition from ax symmetric steady to ax symmetric oscillatory flow. The three dimensional instability of liquid bridges located between isothermal differentially heated disks were studied by several authors [7-14]. The interface deformation caused by the gravity jitters depends on the volume of the liquid bridge and cause changes in the physical properties of the liquid, which ultimately influence the basic state of the fluid [15-16]. The paper discusses Marangoni convection in a liquid bridge subject to g-jitters in a micro gravity environment. The parametric excitement of the liquid bridge with surface tension variation along with the free surface is considered. We will
Kinetic Theory of Instability-Enhanced Collisional Effects
Baalrud, Scott D.
2009-11-01
A generalization of the Lenard-Balescu collision operator is derived which accounts for the scattering of particles by instability amplified fluctuations that originate from the thermal motion of discrete particles (in contrast to evoking a fluctuation level externally, as is done in quasilinear kinetic theory) [1]. Emphasis is placed on plasmas with convective instabilities. It is shown that an instability-enhanced collective response results which can be the primary mechanism for scattering particles, being orders of magnitude more frequent than conventional Coulomb collisions, even though the fluctuations are in a linear growth phase. The resulting collision operator is shown to obey conservation laws (energy, momentum, and density), Galilean invariance, and the Boltzmann H-theorem. It has the property that Maxwellian is the unique equilibrium distribution function; again in contrast to weak turbulence or quasilinear theories. Instability-enhanced collisional effects can dominate particle scattering and cause strong frictional forces. For example, this theory has been applied to two outstanding problems: Langmuir's paradox [2] and determining Bohm's criterion for plasmas with multiple ion species [3]. Langmuir's paradox is a measurement of anomalous electron scattering rapidly establishing a Maxwellian distribution in gas discharges with low temperature and pressure. This may be explained by instability-enhanced scattering in the plasma-boundary transition region (presheath) where convective ion-acoustic instabilities are excited. Bohm's criterion for multiple ion species is a single condition that the ion fluid speeds must obey at the sheath edge; but it is insufficient to determine the speed of individual species. It is shown that an instability-enhanced collisional friction, due to streaming instabilities in the presheath, determines this criterion.[4pt] [1] S.D. Baalrud, J.D. Callen, and C.C. Hegna, Phys. Plasmas 15, 092111 (2008).[0pt] [2] S.D. Baalrud, J
A simplified nonlinear model of the Marangoni instability in gas absorption
Skurygin, E. F.; Poroyko, T. A.
2016-04-01
The process of gas absorption into initially motionless liquid layer is investigated. The convective instability caused by the temperature dependence of the surface tension. The critical time of transition of the process to unstable convective regime, as well as the intensity of mass transfer in a surface convection are estimated numerically. The mathematical model includes the equations of convective diffusion, thermal conduction and fluid motion. The problem was solved numerically in the two-dimensional formulation. In the coordinate along the interface the concentration of the absorbed substance is represented by three terms of the trigonometric Fourier series. A difference approximation of equations with an exponentially changing grid in the direction normal to the interface is used. The simulations results agree with the well-known experimental data on the absorption of carbon dioxide in water.
Shock instability in dissipative gases
Radulescu, Matei I.; Sirmas, Nick
2011-01-01
Previous experiments have revealed that shock waves in thermally relaxing gases, such as ionizing, dissociating and vibrationally excited gases, can become unstable. To date, the mechanism controlling this instability has not been resolved. Previous accounts of the D'yakov-Kontorovich instability, and Bethe-Zel'dovich-Thompson behaviour could not predict the experimentally observed instability. To address the mechanism controlling the instability, we study the propagation of shock waves in a ...
Nonlinear Farley-Buneman instability with Dust Impurities.
Atamaniuk, B.; Volokitin, A. S.
2009-04-01
The regimes of nonlinear stabilization of instability of low frequency waves in magnetized, weakly ionized and inhomogeneous ionospheric dusty plasma are considered. In the lower ionosphere in the E--region, a complex process transforms wind energy into currents creating the E--region electrojet. If these currents exceed a certain critical amplitude, a streaming instability called the Farley--Buneman or a collisional two-stream instability develops. When the number of cooperating waves remains small due to a competition of processes of their instability and attenuation, the turbulence appears in the result of their stochastic behavior. Then even system with finite number of interacting waves can realize a turbulent state in active media. At conditions when electrons are magnetized and characteristic time of density oscillations exceed the rate of electron ion collisions and electron dust collision the drift of electrons perpendicular to magnetic field is the main motion. Consequently, the main nonlinearity appears in result of convection of a density perturbation in one wave by another wave in the perpendicular to magnetic field and mathematically is expressed in a specific vector form The strong collisional damping of waves allow to assume that a typical perturbed state of plasma can be described as finite set of interacting waves. This allow to avoid difficulties of 3D simulations and to make full study of nonlinear stabilization and influence of the dust component in the conditions when the number of interacting waves keeps small by the strong competition of processes wave damping and instabilities Keywords: Dusty Plasmas, Farley-Buneman Instability, Nonlinear Stabilization. REFERENCES 1. M. Oppenheim and N. Otani, Geophysical Research Letters, 22, pp. 353-356, 1995. 2. A.V. Volosevich and C.V. Meister, Int. Journal of Geomagnetism and aeronomy, 3 pp.151-156, 2002 3. A. S. Volokitin and B. Atamaniuk, Reduced nonlinear description of Farley-Buneman instability
Convective stability of a vertical layer of magnetizable fluid in a uniform magnetic field
Energy Technology Data Exchange (ETDEWEB)
Bashtovoy, V.G.; Pavlinov, M.I.
1978-01-01
An infinitely large plane vertical layer of magnetizable fluid is considered, this layer being heated from below and bounded on both lateral surfaces by ferromagnetic half-spaces. The fluid and the ferromagnetic material on both sides have the same pyromagnetic coefficient. The possibility of overcoming a convective instability of such a fluid layer in a uniform magnetic field is demonstrated by a solution of the equilibrium equation. The result indicates that such a magnetic field raises the stability threshold to full stabilization of the fluid layer, with the instability range in terms of the Rayleigh number now having both a lower and an upper limit. 3 references.
Compressible Quasi-geostrophic Convection without the Anelastic Approximation
Calkins, M. A.; Marti, P.; Julien, K. A.
2014-12-01
Fluid compressibility is known to be an important, non-negligible component of the dynamics of many planetary atmospheres and stellar convection zones, yet imposes severe computational constraints on numerical simulations of the compressible Navier-Stokes equations (NSE). An often employed reduced form of the NSE are the anelastic equations, which maintain fluid compressibility in the form of a depth varying, adiabatic background state onto which the perturbations cannot feed back. We present the linear theory of compressible rotating convection in a local-area, plane layer geometry. An important dimensionless parameter in convection is the ratio of kinematic viscosity to thermal diffusivity, or the Prandtl number, Pr. It is shown that the anelastic approximation cannot capture the linear instability of gases with Prandtl numbers less than approximately 0.5 in the limit of rapid rotation; the time derivative of the density fluctuation appearing in the conservation of mass equation remains important for these cases and cannot be neglected. An alternative compressible, geostrophically balanced equation set has been derived and preliminary results utilizing this new equation set are presented. Notably, this new set of equations satisfies the Proudman-Taylor theorem on small axial scales even for strongly compressible flows, does not require the flow to be nearly adiabatic, and thus allows for feedback onto the background state.
Convection in axially symmetric accretion discs with microscopic transport coefficients
Malanchev, K. L.; Postnov, K. A.; Shakura, N. I.
2017-01-01
The vertical structure of stationary thin accretion discs is calculated from the energy balance equation with heat generation due to microscopic ion viscosity η and electron heat conductivity κ, both depending on temperature. In the optically thin discs it is found that for the heat conductivity increasing with temperature, the vertical temperature gradient exceeds the adiabatic value at some height, suggesting convective instability in the upper disc layer. There is a critical Prandtl number, Pr = 4/9, above which a Keplerian disc become fully convective. The vertical density distribution of optically thin laminar accretion discs as found from the hydrostatic equilibrium equation cannot be generally described by a polytrope but in the case of constant viscosity and heat conductivity. In the optically thick discs with radiation heat transfer, the vertical disc structure is found to be convectively stable for both absorption-dominated and scattering-dominated opacities, unless a very steep dependence of the viscosity coefficient on temperature is assumed. A polytropic-like structure in this case is found for Thomson scattering-dominated opacity.
Feedback control and heat transfer measurements in a Rayleigh-Bénard convection cell
Vial, M.; Hernández, R. H.
2017-07-01
We report experimental results on the heat transfer and instability onset of a Rayleigh-Bénard convection cell of aspect ratios 6:3:1 filled with a high Prandtl aqueous solution of glycerol under feedback control. We investigate the transient and stationary response of both local temperature readings and heat transfer fluxes on the Rayleigh Bénard cell in both conductive and convective states when we perform two independent feedback control actions on both hot and cold walls. We evaluate the performance of both controllers to maintain a temperature gradient independently if the system is below or above the convection threshold. As the convection cell can be rotated at 180° about the shorter axis of the cell, it was possible to perform transitions between thermal conduction and convection regimes and vice versa under a constant temperature difference maintained by both independent controllers. The experimental setup provided an accurate measurement of the critical Rayleigh number and the evolution of the Nusselt number as a function of the Rayleigh number in the moderately supercritical regime (R a convection pattern formed by 6 transverse rolls throughout the range of Rayleigh numbers.
Laboratory experiments on diffusive convection layer thickness and its oceanographic implications
Guo, Shuang-Xi; Zhou, Sheng-Qi; Qu, Ling; Lu, Yuan-Zheng
2016-10-01
We studied the thickness of diffusive convective layers that form when a linearly stratified fluid is subjected to heating from below in the laboratory. The thickness of the bottom convecting layer is much larger than subsequent layers. These thicknesses are systematically identified and used to examine the available convecting layer thickness parameterizations, which are consisted of the measured heat flux F (or thermal buoyancy flux qT), initial stratification N, density ratio Rρ, thermal diffusivity κT, etc. Parameterization with an intrinsic length scale >(qT3κ/TN8)1/4 is shown to be superior. Including the present laboratory convecting layer thicknesses and those observed in oceans and lakes, where layer thickness ranges from 0.01 to 1000 m, the parameterization is updated as H=C>(Rρ-1>)2>(qT3κ/TN8)1/4, where C = 38.3 for the bottom convective layer and 10.8 for the subsequent layers. Different prefactors are proposed to be attributed to different convective instabilities induced by different boundary conditions.
The onset of nonpenetrative convection in a suddenly cooled layer of fluid
Energy Technology Data Exchange (ETDEWEB)
Ihle, Christian F. [Program in Fluid Dynamics, Facultad de Ciencias Fisicas y Matematicas, Universidad de Chile, Blanco Encalada 2002 Of. 327, Santiago (Chile); Nino, Yarko [Departamento de Ingenieria Civil, Division de Recursos Hidricos y Medio Ambiente, Universidad de Chile, Av. Blanco Encalada 2002, Santiago (Chile)
2006-04-15
Conditions for the onset of nonpenetrative convection in a horizontal Boussinesq fluid layer subject to a step change in temperature are studied using propagation theory. A wide range of Prandtl numbers and two different kinematic boundary conditions are considered. It is shown that for high Rayleigh numbers, critical conditions for the onset of convective motion reproduce exactly those for the unsteady Rayleigh-Benard instability. Present results extend those of previous research and show a tendency of the rigid-rigid and free-rigid critical curves to converge for low Prandtl numbers. Comparison between present and previously reported results on critical conditions for the onset of instabilities and onset time using different methods yields good agreement on a middle to high Prandtl number range. A ratio of 10 between experimentally measured and theoretically predicted onset times is suggested for stress-free bounded systems. (author)
Genetic instability in Gynecological Cancer
Institute of Scientific and Technical Information of China (English)
ZHAO Qing-hua; ZHOU Hong-lin
2003-01-01
Defects of mismatch repair (MMR) genes also have beenidentified in many kinds of tumors. Loss of MMR functionhas been linked to genetic instability especially microsatelliteinstability that results in high mutation rate. In this review, wediscussed the microsatellite instability observed in thegynecological tumors. We also discussed defects in the DNAmismatch repair in these tumors and their correlation to themicrosatellite instability, as well as the gene mutations due tothe microsatellite instability in these tumors. From thesediscussion, we tried to understand the mechanism ofcarcinogenesis in gynecological tumors from the aspect ofgenetic instability due to mismatch repair defects.
Topology Optimization for Convection Problems
DEFF Research Database (Denmark)
Alexandersen, Joe
2011-01-01
This report deals with the topology optimization of convection problems.That is, the aim of the project is to develop, implement and examine topology optimization of purely thermal and coupled thermomechanical problems,when the design-dependent eects of convection are taken into consideration.......This is done by the use of a self-programmed FORTRAN-code, which builds on an existing 2D-plane thermomechanical nite element code implementing during the course `41525 FEM-Heavy'. The topology optimizationfeatures have been implemented from scratch, and allows the program to optimize elastostatic mechanical...
Instabilities in sensory processes
Balakrishnan, J.
2014-07-01
In any organism there are different kinds of sensory receptors for detecting the various, distinct stimuli through which its external environment may impinge upon it. These receptors convey these stimuli in different ways to an organism's information processing region enabling it to distinctly perceive the varied sensations and to respond to them. The behavior of cells and their response to stimuli may be captured through simple mathematical models employing regulatory feedback mechanisms. We argue that the sensory processes such as olfaction function optimally by operating in the close proximity of dynamical instabilities. In the case of coupled neurons, we point out that random disturbances and fluctuations can move their operating point close to certain dynamical instabilities triggering synchronous activity.
Modulation instability: The beginning
Noskov, Roman; Belov, Pavel; Kivshar, Yuri
2012-11-01
The study of metal nanoparticles plays a central role in the emerging novel technologies employing optics beyond the diffraction limit. Combining strong surface plasmon resonances, high intrinsic nonlinearities and deeply subwavelength scales, arrays of metal nanoparticles offer a unique playground to develop novel concepts for light manipulation at the nanoscale. Here we suggest a novel principle to control localized optical energy in chains of nonlinear subwavelength metal nanoparticles based on the fundamental nonlinear phenomenon of modulation instability. In particular, we demonstrate that modulation instability can lead to the formation of long-lived standing and moving nonlinear localized modes of several distinct types such as bright and dark solitons, oscillons, and domain walls. We analyze the properties of these nonlinear localized modes and reveal different scenarios of their dynamics including transformation of one type of mode to another. We believe this work paves a way towards the development of nonlinear nanophotonics circuitry.
Instability and internet design
Directory of Open Access Journals (Sweden)
Sandra Braman
2016-09-01
Full Text Available Instability - unpredictable but constant change in one’s environment and the means with which one deals with it - has replaced convergence as the focal problem for telecommunications policy in general and internet policy in particular. Those who designed what we now call the internet during the first decade of the effort (1969-1979, who in essence served simultaneously as its policy-makers, developed techniques for coping with instability of value for network designers today and for those involved with any kind of large-scale sociotechnical infrastructure. Analysis of the technical document series that was medium for and record of that design process reveals coping techniques that began with defining the problem and went on to include conceptual labour, social practices, and technical approaches.
Gravitational instabilities of superspinars
Pani, Paolo; Berti, Emanuele; Cardoso, Vitor
2010-01-01
Superspinars are ultracompact objects whose mass M and angular momentum J violate the Kerr bound (cJ/GM^2>1). Recent studies analyzed the observable consequences of gravitational lensing and accretion around superspinars in astrophysical scenarios. In this paper we investigate the dynamical stability of superspinars to gravitational perturbations, considering either purely reflecting or perfectly absorbing boundary conditions at the "surface" of the superspinar. We find that these objects are unstable independently of the boundary conditions, and that the instability is strongest for relatively small values of the spin. Also, we give a physical interpretation of the various instabilities that we find. Our results (together with the well-known fact that accretion tends to spin superspinars down) imply that superspinars are very unlikely astrophysical alternatives to black holes.
Huberman, B A; Huberman, Bernardo A; Youssefmir, Michael
1995-01-01
Recent developments in the global liberalization of equity and currency markets, coupled to advances in trading technologies, are making markets increasingly interdependent. This increased fluidity raises questions about the stability of the international financial system. In this paper, we show that as couplings between stable markets grow, the likelihood of instabilities is increased, leading to a loss of general equilibrium as the system becomes increasingly large and diverse.
Role of viscoelasticity in mantle convection models
Patocka, Vojtech; Cadek, Ondrej; Tackley, Paul
2015-04-01
A present limitation of global thermo-chemical convection models is that they assume a purely viscous or visco-plastic flow law for solid rock, i.e. elasticity is ignored. This may not be a good assumption in the cold, outer boundary layer known as the lithosphere, where elastic deformation may be important. Elasticity in the lithosphere plays at least two roles: It changes surface topography, which changes the relationship between topography and gravity, and it alters the stress distribution in the lithosphere, which may affect dynamical behaviour such as the formation of plate boundaries and other tectonics features. A method for adding elasticity to a viscous flow solver to make a visco-elastic flow solver, which involves adding advected elastic stress to the momentum equation and introducing an "effective" viscosity has been proposed (e.g. Moresi, 2002). The proposed method is designed primarily for a regional-scale numerical model which employs tracers for advection and co-rotation of the stress field. In this study we test a grid-based version of the method in context of thermal convection in the Boussinesq approximation. A simple finite difference/volume model with staggered grid is used, with the aim to later use the same method to implement viscoelasticity into StagYY (Tackley, 2008). The main obstacle is that Maxwell viscoelastic rheology produces instantaneous deformation if instantaneous change of the driving forces occurs. It is not possible to model such deformation in a velocity formulated convection model, as velocity undergoes a singularity for an instantaneous deformation. For a given Rayleigh number there exists a certain critical value of the Deborah number above which it is necessary to use a thermal time step different from the one used in viscoelastic constitutive equation to avoid this numerical instability from happening. Critical Deborah numbers for various Rayleigh numbers are computed. We then propose a method to decouple the thermal and
Convective modes in plasma with the strong shear of ExB drift velocity
Energy Technology Data Exchange (ETDEWEB)
Timofeev, A.V. [RRC ' Kurchatov Institute' , Moscow, Russia 123182 (Russian Federation)
2001-05-01
The convective modes of an inhomogeneously drifting plasma in a shear magnetic field (a generalization of Suydam's problem) is considered. It is shown that a sufficiently great shear of ExB velocity drift suppresses the instability in the case of an arbitrary 'magnetic hill'. This result can be considered again as a Rayleigh theorem analogue. (author). Letter-to-the-editor.
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.
Carpal instability nondissociative.
Wolfe, Scott W; Garcia-Elias, Marc; Kitay, Alison
2012-09-01
Carpal instability nondissociative (CIND) represents a spectrum of conditions characterized by kinematic dysfunction of the proximal carpal row, often associated with a clinical "clunk." CIND is manifested at the midcarpal and/or radiocarpal joints, and it is distinguished from carpal instability dissociative (CID) by the lack of disruption between bones within the same carpal row. There are four major subcategories of CIND: palmar, dorsal, combined, and adaptive. In palmar CIND, instability occurs across the entire proximal carpal row. When nonsurgical management fails, surgical options include arthroscopic thermal capsulorrhaphy, soft-tissue reconstruction, or limited radiocarpal or intercarpal fusions. In dorsal CIND, the capitate subluxates dorsally from its reduced resting position. Dorsal CIND usually responds to nonsurgical management; refractory cases respond to palmar ligament reefing and/or dorsal intercarpal capsulodesis. Combined CIND demonstrates signs of both palmar and dorsal CIND and can be treated with soft-tissue or bony procedures. In adaptive CIND, the volar carpal ligaments are slackened and are less capable of inducing the physiologic shift of the proximal carpal row from flexion into extension as the wrist ulnarly deviates. Treatment of choice is a corrective osteotomy to restore the normal volar tilt of the distal radius.
Chromosomal instability in meningiomas.
van Tilborg, Angela A G; Al Allak, Bushra; Velthuizen, Sandra C J M; de Vries, Annie; Kros, Johan M; Avezaat, Cees J J; de Klein, Annelies; Beverloo, H Berna; Zwarthoff, Ellen C
2005-04-01
Approximately 60% of sporadic meningiomas are caused by inactivation of the NF2 tumor suppressor gene on chromosome 22. No causative gene is known for the remaining 40%. Cytogenetic analysis shows that meningiomas caused by inactivation of the NF2 gene can be divided into tumors that show monosomy 22 as the sole abnormality and tumors with a more complex karyotype. Meningiomas not caused by the NF2 gene usually have a diploid karyotype. Here we report that, besides the clonal chromosomal aberrations, the chromosome numbers in many meningiomas varied from one metaphase spread to the other, a feature that is indicative of chromosomal instability. Unexpectedly and regardless of genotype, a subgroup of tumors was observed with an average number of 44.9 chromosomes and little variation in the number of chromosomes per metaphase spread. In addition, a second subgroup was recognized with a hyperdiploid number of chromosomes (average 48.5) and considerable variation in numbers per metaphase. However, this numerical instability resulted in a clonal karyotype with chromosomal gains and losses in addition to loss of chromosome 22 only in meningiomas caused by inactivation of the NF2 gene. In cultured cells of all tumor groups, bi- and multinucleated cells were seen, as well as anaphase bridges, residual chromatid strings, multiple spindle poles, and unseparated chromatids, suggesting defects in the mitotic apparatus or kinetochore. Thus, we conclude that even a benign and slow-growing tumor like a meningioma displays chromosomal instability.
Simulation of Rayleigh-Bénard convection using lattice Boltzmann method
Shan, X
1996-01-01
Rayleigh-Bénard convection is numerically simulated in two- and three-dimensions using a recently developed two-component lattice Boltzmann equation (LBE) method. The density field of the second component, which evolves according to the advection-diffusion equation of a passive-scalar, is used to simulate the temperature field. A body force proportional to the temperature is applied, and the system satisfies the Boussinesq equation except for a slight compressibility. A no-slip, isothermal boundary condition is imposed in the vertical direction, and periodic boundary conditions are used in horizontal directions. The critical Rayleigh number for the onset of the Rayleigh-Bénard convection agrees with the theoretical prediction. As the Rayleigh number is increased higher, the steady two-dimensional convection rolls become unstable. The wavy instability and aperiodic motion observed, as well as the Nusselt number as a function of the Rayleigh number, are in good agreement with experimental observations and the...
Plate-like convection induced by symmetries in fluids with temperature-dependent viscosity
Curbelo, Jezabel
2014-01-01
The study of instabilities in fluids in which viscosity experiences a transition at a certain temperature range is of great interest for the understanding of planetary interiors, since this phenomena models the melting and solidification of a magma ocean and thus is suitable for representing a lithosphere over a convecting mantle. To this end, we study a 2D convection problem in which viscosity depends on temperature by abruptly changing its value by a factor 400 within a narrow temperature gap at which magma melts. We perform a study which combines bifurcation analysis and time dependent simulations. Solutions such as limit cycles are found that are fundamentally related to the presence of symmetry. Sporadically during these cycles, through abrupt bursts, spontaneous plate-like behaviors that rapidly evolve towards a stagnant lid regime emerge. The plate-like evolution alternates motions towards either right or left, introducing temporary asymmetries on the convecting styles. Further time dependent regimes w...
Influence of through-flow on linear pattern formation properties in binary mixture convection
Jung, C; Büchel, P; Jung, Ch.
1996-01-01
We investigate how a horizontal plane Poiseuille shear flow changes linear convection properties in binary fluid layers heated from below. The full linear field equations are solved with a shooting method for realistic top and bottom boundary conditions. Through-flow induced changes of the bifurcation thresholds (stability boundaries) for different types of convective solutions are deter- mined in the control parameter space spanned by Rayleigh number, Soret coupling (positive as well as negative), and through-flow Reynolds number. We elucidate the through-flow induced lifting of the Hopf symmetry degeneracy of left and right traveling waves in mixtures with negative Soret coupling. Finally we determine with a saddle point analysis of the complex dispersion relation of the field equations over the complex wave number plane the borders between absolute and convective instabilities for different types of perturbations in comparison with the appropriate Ginzburg-Landau amplitude equation approximation. PACS:47.2...
Full computation of massive AGB evolution. I. The large impact of convection on nucleosynthesis
Ventura, P; Ventura, Paolo; Antona, Francesca D'
2004-01-01
It is well appreciated that the description of overadiabatic convection affects the structure of the envelopes of luminous asymptotic giant branch (AGB) stars in the phase of ``hot bottom burning '' (HBB). We stress that this important uncertainty in the modeling plays a role which is much more dramatic than the role which can be ascribed, e.g., to the uncertainty in the nuclear cross-sections. Due to the role tentatively attributed today to the HBB nucleosynthesis as the site of self-enrichment of Globular Clusters stars, it is necessary to explore the difference in nucleosynthesis obtained by different prescriptions for convection. We present results of detailed evolutionary calculations of the evolution of stars of intermediate mass during the AGB phase. We follow carefully the nucleosynthesis at the base of the external convective region, showing that very different results can be obtained according to the presciption adopted to find out the temperature gradient within the instability regions. We discuss ...
ENVIRONMENTAL ASPECTS OF THE INTENSIFICATION CONVECTIVE DRYING
Directory of Open Access Journals (Sweden)
A. M. Gavrilenkov
2012-01-01
Full Text Available Identified and analyzed the relationship of the intensity convective drying and air pollution emissions of heat. The ways to reduce the thermal pollution of the atmosphere at convective drying.
Convection in stellar envelopes a changing paradigm
Spruit, H C
1996-01-01
Progress in the theory of stellar convection over the past decade is reviewed. The similarities and differences between convection in stellar envelopes and laboratory convection at high Rayleigh numbers are discussed. Direct numerical simulation of the solar surface layers, with no other input than atomic physics, the equations of hydrodynamics and radiative transfer is now capable of reproducing the observed heat flux, convection velocities, granulation patterns and line profiles with remarkably accuracy. These results show that convection in stellar envelopes is an essentially non-local process, being driven by cooling at the surface. This differs distinctly from the traditional view of stellar convection in terms of local concepts such as cascades of eddies in a mean superadiabatic gradient. The consequences this has for our physical picture of processes in the convective envelope are illustrated with the problems of sunspot heat flux blocking, the eruption of magnetic flux from the base of the convection ...
Local Dynamical Instabilities in Magnetized, Radiation Pressure Supported Accretion Disks
Blaes, Omer M; Blaes, Omer; Socrates, Aristotle
2000-01-01
We present a general linear dispersion relation which describes the coupled behavior of magnetorotational, photon bubble, and convective instabilities in weakly magnetized, differentially rotating accretion disks. We presume the accretion disks to be geometrically thin and supported vertically by radiation pressure. We fully incorporate the effects of a nonzero radiative diffusion length on the linear modes. In an equilibrium with purely vertical magnetic field, the vertical magnetorotational modes are completely unaffected by compressibility, stratification, and radiative diffusion. However, in the presence of azimuthal fields, which are expected in differentially rotating flows, the growth rate of all magnetorotational modes can be reduced substantially below the orbital frequency. This occurs if diffusion destroys radiation sound waves on the length scale of the instability, and the magnetic energy density of the azimuthal component exceeds the non-radiative thermal energy density. While sluggish in this c...
Aerosol Effects on Instability, Circulations, Clouds, and Precipitation
Directory of Open Access Journals (Sweden)
Seoung-Soo Lee
2014-01-01
Full Text Available It is well known that increasing aerosol and associated changes in aerosol-cloud interactions and precipitation since industrialization have been playing an important role in climate change, but this role has not been well understood. This prevents us from predicting future climate with a good confidence. This review paper presents recent studies on the changes in the aerosol-cloud interactions and precipitation particularly in deep convective clouds. In addition, this review paper discusses how to improve our understanding of these changes by considering feedbacks among aerosol, cloud dynamics, cloud and its embedded circulations, and microphysics. Environmental instability basically determines the dynamic intensity of clouds and thus acts as one of the most important controls on these feedbacks. As a first step to the improvement of the understanding, this paper specifically elaborates on how to link the instability to the feedbacks.
Magnetohydrodynamic stability of natural convection in a vertical porous slab
Shankar, B. M.; Kumar, Jai; Shivakumara, I. S.
2017-01-01
The stability of the conduction regime of natural convection in an electrically conducting fluid saturated porous vertical slab is investigated in the presence of a uniform external transverse magnetic field. The flow in the porous medium is described by modified Brinkman-extended Darcy equation with fluid viscosity different from effective viscosity. The boundaries of the vertical porous slab are assumed to be rigid-isothermal and electrically non-conducting. The resulting stability equations are solved numerically using Galerkin method. The critical Grashof number Gc, the critical wave number αc and the critical wave speed cc are computed for a wide range of porous parameter σp, the ratio of effective viscosity to the fluid viscosity Λ, the Prandtl number Pr and the Hartmann number M. Based on these parameters, the stability characteristics of the system are discussed in detail. The presence of advective inertia is to instill instability on the flow in a porous medium and found that the magnetic field, porous parameter and ratio of viscosities have a stabilizing effect on both stationary and oscillatory wave instabilities. Besides, the value of Pr at which transition occurs from stationary to oscillatory mode of instability decreases with increasing M ,σp and Λ .
Shoulder instability; Schultergelenkinstabilitaet
Energy Technology Data Exchange (ETDEWEB)
Sailer, J.; Imhof, H. [Abteilung Osteoradiologie, Univ.-Klinik fuer Radiodiagnostik Wien (Austria)
2004-06-01
Shoulder instability is a common clinical feature leading to recurrent pain and limitated range of motion within the glenohumeral joint. Instability can be due a single traumatic event, general joint laxity or repeated episodes of microtrauma. Differentiation between traumatic and atraumatic forms of shoulder instability requires careful history and a systemic clinical examination. Shoulder laxity has to be differentiated from true instability followed by the clinical assessment of direction and degree of glenohumeral translation. Conventional radiography and CT are used for the diagnosis of bony lesions. MR imaging and MR arthrography help in the detection of soft tissue affection, especially of the glenoid labrum and the capsuloligamentous complex. The most common lesion involving the labrum is the anterior labral tear, associated with capsuloperiostal stripping (Bankart lesion). A number of variants of the Bankart lesion have been described, such as ALPSA, SLAP or HAGL lesions. The purpose of this review is to highlight different forms of shoulder instability and its associated radiological findings with a focus on MR imaging. (orig.) [German] Die Schultergelenkinstabilitaet ist haeufig fuer wiederholt auftretende Schmerzen sowie eine eingeschraenkte Beweglichkeit im Glenohumeralgelenk verantwortlich. Sie kann als Folge eines vorangegangenen Traumas, einer generellen Hyperlaxitaet oder infolge wiederholter Mikrotraumen entstehen. Die Differenzierung zwischen traumatischer und atraumatischer Form der Gelenkinstabilitaet erfordert eine sorgfaeltige Anamnese und eine genaue klinische Untersuchung. Die Gelelenklaxitaet als Differenzialdiagnose muss von der echten Instabilitaet unterschieden werden, die Instabilitaet wird dann im Rahmen des klinischen Status nach Grad und Richtung der glenohumeralen Translation unterteilt. Zur Diagnose knoecherner Laesionen werden das konventionelle Roentgen sowie die CT herangezogen. MRT sowie MR-Arthrographie dienen zur Detektion
Directional Solidification of a Binary Alloy into a Cellular Convective Flow: Localized Morphologies
Chen, Y.- J.; Davis, S. H.
1999-01-01
A steady, two dimensional cellular convection modifies the morphological instability of a binary alloy that undergoes directional solidification. When the convection wavelength is far longer than that of the morphological cells, the behavior of the moving front is described by a slow, spatial-temporal dynamics obtained through a multiple-scale analysis. The resulting system has a "parametric-excitation" structure in space, with complex parameters characterizing the interactions between flow, solute diffusion, and rejection. The convection stabilizes two dimensional disturbances oriented with the flow, but destabilizes three dimensional disturbances in general. When the flow is weak, the morphological instability behaves incommensurably to the flow wavelength, but becomes quantized and forced to fit into the flow-box as the flow gets stronger. At large flow magnitudes the instability is localized, confined in narrow envelopes with cells traveling with the flow. In this case the solutions are discrete eigenstates in an unbounded space. Their stability boundary and asymptotics are obtained by the WKB analysis.
Laboratory flow experiments for visualizing carbon dioxide-induced, density-driven brine convection
Energy Technology Data Exchange (ETDEWEB)
Kneafsey, T.; Pruess, K.
2009-09-01
Injection of carbon dioxide (CO{sub 2}) into saline aquifers confined by low-permeability cap rock will result in a layer of CO{sub 2} overlying the brine. Dissolution of CO{sub 2} into the brine increases the brine density, resulting in an unstable situation in which more-dense brine overlies less-dense brine. This gravitational instability could give rise to density-driven convection of the fluid, which is a favorable process of practical interest for CO{sub 2} storage security because it accelerates the transfer of buoyant CO{sub 2} into the aqueous phase, where it is no longer subject to an upward buoyant drive. Laboratory flow visualization tests in transparent Hele-Shaw cells have been performed to elucidate the processes and rates of this CO{sub 2} solute-driven convection (CSC). Upon introduction of CO{sub 2} into the system, a layer of CO{sub 2}-laden brine forms at the CO{sub 2}-water interface. Subsequently, small convective fingers form, which coalesce, broaden, and penetrate into the test cell. Images and time-series data of finger lengths and wavelengths are presented. Observed CO{sub 2} uptake of the convection system indicates that the CO{sub 2} dissolution rate is approximately constant for each test and is far greater than expected for a diffusion-only scenario. Numerical simulations of our system show good agreement with the experiments for onset time of convection and advancement of convective fingers. There are differences as well, the most prominent being the absence of cell-scale convection in the numerical simulations. This cell-scale convection observed in the experiments is probably initiated by a small temperature gradient induced by the cell illumination.
Breeze transients as triggers for the initiation of deep moist convection on mountains slopes
Giaiotti, D.; Stel, F.; Gladich, I.; Giacomini, A.
2010-09-01
Deep moist convection frequency is characterized by seasonal and diurnal cycles, related to the interplay between the different elements which force and sustain it: onset, instability, wind pattern. Besides the instability diurnal cycles, the low level triggers initiating the convective motions are considered responsible for the modulations of the cycle. In some areas, like in the southern side of the Alpine ridge, the onset and the reversal of breezes produce microscale areas of flow convergence, at the lower levels, due breezes transients and breezes interaction with the orography. This work presents a numerical analysis of the convection initiation generated by breezes transients. The Weather Research and Forecasting model (WRF) has been used to make simplified case studies and real cases simulations in which orography, solar radiation exposure and sea surface temperature variations are considered. General results support the thesis that the daily modulation of thunderstorm development is significantly accounted by the trigger effect of breezes transients, when synoptic winds are negligible of course. The results produced by the large set of simulations are compared with the climatological data of convective cells development in the Northeastern Italian region.
How stratified is mantle convection?
Puster, Peter; Jordan, Thomas H.
1997-04-01
We quantify the flow stratification in the Earth's mid-mantle (600-1500 km) in terms of a stratification index for the vertical mass flux, Sƒ (z) = 1 - ƒ(z) / ƒref (z), in which the reference value ƒref(z) approximates the local flux at depth z expected for unstratified convection (Sƒ=0). Although this flux stratification index cannot be directly constrained by observations, we show from a series of two-dimensional convection simulations that its value can be related to a thermal stratification index ST(Z) defined in terms of the radial correlation length of the temperature-perturbation field δT(z, Ω). ST is a good proxy for Sƒ at low stratifications (SƒUniformitarian Principle. The bound obtained here from global tomography is consistent with local seismological evidence for slab flux into the lower mantle; however, the total material flux has to be significantly greater (by a factor of 2-3) than that due to slabs alone. A stratification index, Sƒ≲0.2, is sufficient to exclude many stratified convection models still under active consideration, including most forms of chemical layering between the upper and lower mantle, as well as the more extreme versions of avalanching convection governed by a strong endothermic phase change.
Natural convection from circular cylinders
Boetcher, Sandra K S
2014-01-01
This book presents a concise, yet thorough, reference for all heat transfer coefficient correlations and data for all types of cylinders: vertical, horizontal, and inclined. This book covers all natural convection heat transfer laws for vertical and inclined cylinders and is an excellent resource for engineers working in the area of heat transfer engineering.
Subcritical convection in an internally heated layer
Xiang, Linyan; Zikanov, Oleg
2017-06-01
Thermal convection in a horizontal layer with uniform internal heating and stress-free constant-temperature boundaries is analyzed numerically. The work is motivated by the questions arising in the development of liquid metal batteries, in which convection is induced by the Joule heating of electrolyte. It is demonstrated that three-dimensional convection cells exist at subcritical Rayleigh numbers.
Measurements of entropy-layer instabilities over cone-ogive-cylinders at Mach 6
Greenwood, Roger T.
. Results show a smooth variation of the location of this instability descent with nosetip angle. As the angle increases, the instability approaches the model further upstream. Cross-correlations between the surface transducer and hot-wire anemometry measurements confirm that the same instability is being measured at both locations. Cross-correlations between axially-displaced surface sensors were used to calculate an instability convection velocity that is approximately equal to the numerically-calculated flow velocity. And cross-correlations between azimuthally-displaced sensors show that the instability is primarily axisymmetric. The model angle of attack for all measurements was nominally zero. However, the actual angle of attack may vary by up to 0.1 degrees. The experimental results were also compared with mean-flow computations for several of the model configurations.
Progress in theory of instabilities in a rotating plasma
Mikhailovskii, A. B.; Lominadze, J. G.; Churikov, A. P.; Pustovitov, V. D.
2009-04-01
A review is given of the basic results of modern theory of instabilities in a rotating plasma. Both axisymmetric and nonaxisymmetric perturbations are considered. Main attention is given to the magnetorotational instability (MRI), discovered earlier by Velikhov, and the rotational-convective instability (RCI) discussed in a number of papers of astrophysical trend. For qualitative explanation of the results, a local approach is used which, with equilibrium plasma pressure gradient and/or nonsymmetry of perturbations, requires operation with nonlocal azimuthal perturbed magnetic field. The gravity and effects of pressure anisotropy are taken into account. In addition to hydrodynamic, the electrodynamic approach is formulated. The drift effects are considered. Analyzed are the ideal instabilities and those depending on the dissipative effects: viscosity and heat conductivity. The MRI is considered at presence of the charged dust particles. Besides the local approach, the nonlocal approach is formulated for the plasma model with a steplike profile of angular rotation frequency. Alongside with perturbations which frequencies are small compared to the ion cyclotron frequency, the perturbations are analyzed with frequencies larger than the ion cyclotron frequency. The latter corresponds to the Hall regime and subregime of nonmagnetized plasma.
Resistive double-diffusive instability in the dead-zones of protostellar disks
Latter, Henrik N; Balbus, Steven A
2010-01-01
We outline a novel linear instability that may arise in the dead-zones of protostellar disks, and possibly the fluid interiors of planets and protoplanets. In essence it is an axisymmetric buoyancy instability, but one that would not be present in a purely hydrodynamical gas. The necessary ingredients for growth include a negative radial entropy gradient (of any magnitude), weak magnetic fields, and efficient resistive diffusion (in comparison with thermal diffusion). The character of the instability is local, axisymmetric, and double-diffusive, and it attacks lengths much shorter than the resistive scale. Like the axisymmetric convective instability, it draws its energy from the negative radial entropy gradient; but by utilising the diffusing magnetic field, it can negate the stabilising influence of rotation. Its nonlinear saturated state, while not transporting appreciable angular momentum, could drive radial and vertical mixing, which may influence the temperature structure of the disk, dust dynamics and,...
Radiation Induced Genomic Instability
Energy Technology Data Exchange (ETDEWEB)
Morgan, William F.
2011-03-01
Radiation induced genomic instability can be observed in the progeny of irradiated cells multiple generations after irradiation of parental cells. The phenotype is well established both in vivo (Morgan 2003) and in vitro (Morgan 2003), and may be critical in radiation carcinogenesis (Little 2000, Huang et al. 2003). Instability can be induced by both the deposition of energy in irradiated cells as well as by signals transmitted by irradiated (targeted) cells to non-irradiated (non-targeted) cells (Kadhim et al. 1992, Lorimore et al. 1998). Thus both targeted and non-targeted cells can pass on the legacy of radiation to their progeny. However the radiation induced events and cellular processes that respond to both targeted and non-targeted radiation effects that lead to the unstable phenotype remain elusive. The cell system we have used to study radiation induced genomic instability utilizes human hamster GM10115 cells. These cells have a single copy of human chromosome 4 in a background of hamster chromosomes. Instability is evaluated in the clonal progeny of irradiated cells and a clone is considered unstable if it contains three or more metaphase sub-populations involving unique rearrangements of the human chromosome (Marder and Morgan 1993). Many of these unstable clones have been maintained in culture for many years and have been extensively characterized. As initially described by Clutton et al., (Clutton et al. 1996) many of our unstable clones exhibit persistently elevated levels of reactive oxygen species (Limoli et al. 2003), which appear to be due dysfunctional mitochondria (Kim et al. 2006, Kim et al. 2006). Interestingly, but perhaps not surprisingly, our unstable clones do not demonstrate a “mutator phenotype” (Limoli et al. 1997), but they do continue to rearrange their genomes for many years. The limiting factor with this system is the target – the human chromosome. While some clones demonstrate amplification of this chromosome and thus lend
Nonlinear evolution of mirror instability in the Earth's magnetosheath in pic simulations
Ahmadi, Narges
Mirror modes are large amplitude non-propagating structures frequently observed in the magnetosheath and they are generated in space plasma environments with proton temperature anisotropy of larger than one. The proton temperature anisotropy also drives the proton cyclotron instability which has larger linear growth rate than that of the mirror instability. Linear dispersion theory predicts that electron temperature anisotropy can enhance the mirror instability growth rate while leaving the proton cyclotron instability largely unaffected. Contrary to the hypothesis, electron temperature anisotropy leads to excitement of the electron whistler instability. Our results show that the electron whistler instability grows much faster than the mirror instability and quickly consumes the electron free energy, so that there is not enough electron temperature anisotropy left to significantly impact the evolution of the mirror instability. Observational studies have shown that the shape of mirror structures is related to local plasma parameters and distance to the mirror instability threshold. Mirror structures in the form of magnetic holes are observed when plasma is mirror stable or marginally mirror unstable and magnetic peaks are observed when plasma is mirror unstable. Mirror structures are created downstream of the quasi-perpendicular bow shock and they are convected toward the magnetopause. In the middle magnetosheath, where plasma is mirror unstable, mirror structures are dominated by magnetic peaks. Close to the magnetopause, plasma expansion makes the region mirror stable and magnetic peaks evolve to magnetic holes. We investigate the nonlinear evolution of mirror instability using expanding box Particle-in-Cell simulations. We change the plasma conditions by artificially enlarging the simulation box over time to make the plasma mirror stable and investigate the final nonlinear state of the mirror structures. We show that the direct nonlinear evolution of the mirror
Observational evidence for new instabilities in the midlatitude E and F region
Hysell, David L.; Larsen, Miguel; Sulzer, Michael
2016-11-01
Radar observations of the E- and F-region ionosphere from the Arecibo Observatory made during moderately disturbed conditions are presented. The observations indicate the presence of patchy sporadic E (Es) layers, medium-scale traveling ionospheric disturbances (MSTIDs), and depletion plumes associated with spread F conditions. New analysis techniques are applied to the dataset to infer the vector plasma drifts in the F region as well as vector neutral wind and temperature profiles in the E region. Instability mechanisms in both regions are evaluated. The mesosphere-lower-thermosphere (MLT) region is found to meet the conditions for neutral dynamic instability in the vicinity of the patchy Es layers even though the wind shear was relatively modest. An inversion in the MLT temperature profile contributed significantly to instability in the vicinity of one patchy layer. Of particular interest is the evidence for the conditions required for neutral convective instability in the lower-thermosphere region (which is usually associated with highly stable conditions) due to the rapid increase in temperature with altitude. A localized F-region plasma density enhancement associated with a sudden ascent up the magnetic field is shown to create the conditions necessary for convective plasma instability leading to the depletion plume and spread F. The growth time for the instability is short compared to the one described by [Perkins(1973)]. This instability does not offer a simple analytic solution but is clearly present in numerical simulations. The instability mode has not been described previously but appears to be more viable than the various mechanisms that have been suggested previously as an explanation for the occurrence of midlatitude spread F.
A transilient matrix for moist convection
Energy Technology Data Exchange (ETDEWEB)
Romps, D.; Kuang, Z.
2011-08-15
A method is introduced for diagnosing a transilient matrix for moist convection. This transilient matrix quantifies the nonlocal transport of air by convective eddies: for every height z, it gives the distribution of starting heights z{prime} for the eddies that arrive at z. In a cloud-resolving simulation of deep convection, the transilient matrix shows that two-thirds of the subcloud air convecting into the free troposphere originates from within 100 m of the surface. This finding clarifies which initial height to use when calculating convective available potential energy from soundings of the tropical troposphere.
Bhuiyan, M. A. E.; Wanik, D. W.; Scerbo, D.; Anagnostou, E. N.
2015-12-01
We have developed a tool, the Convection Risk Index (CRI), to represent the severity, timing and location of convection for select geographic areas. The CRI is calculated from the Convection Risk Matrix (CRM), a tabulation of numerous meteorological parameters which are categorized into four broad factors that contribute to convection (surface and lower level moisture, atmospheric instability, vertical wind shear, and lift); each of these factors have historically been utilized by meteorologists to predict the likelihood for development of thunderstorms. The CRM ascribes a specific threshold value to each parameter in such a way that it creates a unique tool used to calculate the risk for seeing the development of thunderstorms. The parameters were combined using a weighted formula and which when calculated, yields the Convection Risk Index 1 to 4 scale, with 4 being the highest risk for seeing strong convection. In addition, we also evaluated the performance of the parameters in the CRM and CRI for predicting the maximum wind speed in areas where we calculated the CRI using nonparametric tree-based model, Bayesian additive trees (BART). The use of the CRI and the predicted wind speeds from BART can be used to better inform emergency preparedness efforts in government and industry.We have developed a tool, the Convection Risk Index (CRI), to represent the severity, timing and location of convection for select geographic areas. The CRI is calculated from the Convection Risk Matrix (CRM), a tabulation of numerous meteorological parameters which are categorized into four broad factors that contribute to convection (surface and lower level moisture, atmospheric instability, vertical wind shear, and lift); each of these factors have historically been utilized by meteorologists to predict the likelihood for development of thunderstorms. The CRM ascribes a specific threshold value to each parameter in such a way that it creates a unique tool used to calculate the risk for
Modulational instability of nematic phase
Indian Academy of Sciences (India)
T Mithun; K Porsezian
2014-02-01
We numerically observe the effect of homogeneous magnetic field on the modulationally stable case of polar phase in = 2 spinor Bose–Einstein condensates (BECs). Also we investigate the modulational instability of uniaxial and biaxial (BN) states of polar phase. Our observations show that the magnetic field triggers the modulational instability and demonstrate that irrespective of the magnetic field effect the uniaxial and biaxial nematic phases show modulational instability.
Political Instability and Economic Growth
Swagel, Phillip; Roubini, Nouriel; Ozler, Sule; Alesina, Alberto
1992-01-01
This paper investigates the relationship between political instability and per capita GDP growth in a sample of 113 countries for the period 1950-1982. We define ?political instability? as the propensity of a government collapse, and we estimate a model in which political instability and economic growth are jointly determined. The main result of this paper is that in countries and time periods with a high propensity of government collapse, growth is significantly lower than otherwise. This ef...
Weibel instability with nonextensive distribution
Energy Technology Data Exchange (ETDEWEB)
Qiu, Hui-Bin; Liu, Shi-Bing [Strong-field and Ultrafast Photonics Lab, Institute of Laser Engineering, Beijing University of Technology, Beijing 100124 (China)
2013-10-15
Weibel instability in plasma, where the ion distribution is isotropic and the electron component of the plasma possesses the anisotropic temperature distribution, is investigated based on the kinetic theory in context of nonextensive statistics mechanics. The instability growth rate is shown to be dependent on the nonextensive parameters of both electron and ion, and in the extensive limit, the result in Maxwellian distribution plasma is recovered. The instability growth rate is found to be enhanced as the nonextensive parameter of electron increases.
Directory of Open Access Journals (Sweden)
Abdoulaye Gueye
2017-07-01
Full Text Available We analyze the thermal convection thresholds and linear characteristics of the primary and secondary instabilities for viscoelastic fluids saturating a porous horizontal layer heated from below by a constant flux. The Galerkin method is used to solve the eigenvalue problem by taking into account the elasticity of the fluid, the ratio between the viscosity of the solvent and the total viscosity of the fluid and the lateral confinement of the medium. For the primary instability, we found out that depending on the rheological parameters, two types of convective structures may appear when the basic conductive solution loses its stability: stationary long wavelength instability as for Newtonian fluids and oscillatory convection. The effect of the lateral confinement of the porous medium by adiabatic walls is to stabilize the oblique and longitudinal rolls and therefore selects transverse rolls at the onset of convection. In the range of the rheological parameters where stationary long wave instability develops first, we use a parallel flow approximation to determine analytically the velocity and temperature fields associated with the monocellular convective flow. The linear stability analysis of the monocellular flow is performed, and the critical conditions above which the flow becomes unstable are determined. The combined influence of the viscoelastic parameters and the lateral confinement on the characteristics of the secondary instability is quantified. The major new findings concerning the secondary instabilities may be summarized as follows: (i For concentrated viscoelastic fluids, computations showed that the most amplified mode of convection corresponds to oscillatory transverse rolls, which appears via a Hopf bifurcation. This pattern selection is independent of both the fluid elasticity and the lateral confinement of the porous medium. (ii For diluted viscoelastic fluids, the preferred mode of convection is found to be oscillatory
Generalized Convective Quasi-Equilibrium Closure
Yano, Jun-Ichi; Plant, Robert
2016-04-01
Arakawa and Schubert proposed convective quasi-equilibrium as a basic principle for closing their spectrum mass-flux convection parameterization. In deriving this principle, they show that the cloud work function is a key variable that controls the growth of convection. Thus, this closure hypothesis imposes a steadiness of the cloud work function tendency. This presentation shows how this principle can be generalized so that it can also encompasses both the CAPE and the moisture-convergence closures. Note that the majority of the current mass-flux convection parameterization invokes a CAPE closure, whereas the moisture-convergence closure was extremely popular historically. This generalization, in turn, includes both closures as special cases of convective quasi-equilibrium. This generalization further suggests wide range of alternative possibilities for convective closure. In general, a vertical integral of any function depending on both large-scale and convective-scale variables can be adopted as an alternative closure variables, leading to an analogous formulation as Arakawa and Schubert's convective quasi-equilibrium formulation. Among those, probably the most fascinating possibility is to take a vertical integral of the convective-scale moisture for the closure. Use of a convective-scale variable for closure has a particular appeal by not suffering from a loss of predictability of any large-scale variables. That is a main problem with any of the current convective closures, not only for the moisture-convergence based closure as often asserted.
Instabilities in mimetic matter perturbations
Firouzjahi, Hassan; Gorji, Mohammad Ali; Mansoori, Seyed Ali Hosseini
2017-07-01
We study cosmological perturbations in mimetic matter scenario with a general higher derivative function. We calculate the quadratic action and show that both the kinetic term and the gradient term have the wrong sings. We perform the analysis in both comoving and Newtonian gauges and confirm that the Hamiltonians and the associated instabilities are consistent with each other in both gauges. The existence of instabilities is independent of the specific form of higher derivative function which generates gradients for mimetic field perturbations. It is verified that the ghost instability in mimetic perturbations is not associated with the higher derivative instabilities such as the Ostrogradsky ghost.
Nanofluid bio-thermal convection: simultaneous effects of gyrotactic and oxytactic micro-organisms
Energy Technology Data Exchange (ETDEWEB)
Kuznetsov, A V, E-mail: avkuznet@eos.ncsu.edu [Department of Mechanical and Aerospace Engineering, North Carolina State University, Campus Box 7910, Raleigh, NC 27695-7910 (United States)
2011-10-15
This paper investigates the onset of nanofluid bio-thermal convection in a horizontal layer of finite depth for the case when the suspension contains two species of motile micro-organisms exhibiting different taxes, gyrotactic and oxytactic micro-organisms. The obtained instability problem is controlled by four agencies, namely by distributions of nanoparticles, gyrotactic and oxytactic micro-organisms and by the vertical temperature variation. The utilization of the linear instability theory makes it possible to decouple the effects of these agencies and obtain an eigenvalue equation that involves four Rayleigh numbers: the nanoparticle Rayleigh number, the bioconvection gyrotactic and oxytactic Rayleigh numbers, and the traditional thermal Rayleigh number. Each Rayleigh number represents the effect of one of the four aforementioned agencies. Previously obtained results are recovered for limiting cases. The effects of different agencies on the boundary of marginal non-oscillatory instability are investigated.
[Aspirin suppresses microsatellite instability].
Wallinger, S; Dietmaier, W; Beyser, K; Bocker, T; Hofstädter, F; Fishel, R; Rüschoff, J
1999-01-01
Nonsteroidal anti-inflammatory drugs (NSAIDs) exhibit cancer preventive effects and have been shown to induce regression of adenomas in FAP patients. In order to elucidate the probable underlying mechanism, the effect of NSAIDs on mismatch repair related microsatellite instability was investigated. Six colorectal cancer cell lines all but one deficient for human mismatch repair (MMR) genes were examined for microsatellite instability (MSI) prior and after treatment with Aspirin or Sulindac. For rapid in vitro analysis of MSI a microcloning assay was developed by combining Laser microdissection and random (PEP-) PCR prior to specific MSI-PCR. Effects of NSAIDs on cell cycle and apoptosis were systematically investigated by using flow cytometry and cell-sorting. MSI frequency in cells deficient of MMR genes (hMSH2, hMLH1, hMSH6) was markedly reduced after long-term (> 10 weeks) NSAID treatment. This effect was reversible, time- and concentration dependent. However, in the hPMS2 deficient endometrial cancer cell line (HEC-1-A) the MSI phenotype kept unchanged. According to cell sorting, non-apoptotic cells were stable and apoptotic cells were unstable. These results suggest that aspirin/sulindac induces a genetic selection for microsatellite stability in a subset of MMR-deficient cells and may thus provide an effective prophylactic therapy for HNPCC related colorectal carcinomas.
Instability of enclosed horizons
Kay, Bernard S
2013-01-01
We study the classical massless scalar wave equation on the region of 1+1-dimensional Minkowski space between the two branches of the hyperbola $x^2-t^2=1$ with vanishing boundary conditions on it. We point out that there are initially finite-energy initially, say, right-going waves for which the stress-energy tensor becomes singular on the null-line $t+x=0$. We also construct the quantum theory of this system and show that, while there is a regular Hartle-Hawking-Israel-like state, there are coherent states built on this for which there is a similar singularity in the expectation value of the renormalized stress-energy tensor. We conjecture that in 1+3-dimensional situations with 'enclosed horizons' such as a (maximally extended) Schwarzschild black hole in equilibrium in a stationary box or the (maximally extended) Schwarzschild-AdS spacetime, there will be a similar singularity at the horizon and that would signal an instability when matter perturbations and/or gravity are switched on. Such an instability ...
Libration driven multipolar instabilities
Cébron, David; Herreman, Wietze
2014-01-01
We consider rotating flows in non-axisymmetric enclosures that are driven by libration, i.e. by a small periodic modulation of the rotation rate. Thanks to its simplicity, this model is relevant to various contexts, from industrial containers (with small oscillations of the rotation rate) to fluid layers of terrestial planets (with length-of-day variations). Assuming a multipolar $n$-fold boundary deformation, we first obtain the two-dimensional basic flow. We then perform a short-wavelength local stability analysis of the basic flow, showing that an instability may occur in three dimensions. We christen it the Libration Driven Multipolar Instability (LDMI). The growth rates of the LDMI are computed by a Floquet analysis in a systematic way, and compared to analytical expressions obtained by perturbation methods. We then focus on the simplest geometry allowing the LDMI, a librating deformed cylinder. To take into account viscous and confinement effects, we perform a global stability analysis, which shows that...
Hydrodynamic instability in the open system of the iodate-arsenous acid reaction.
Pópity-Tóth, Éva; Pimienta, Véronique; Horváth, Dezső; Tóth, Ágota
2013-10-28
Hydrodynamic instability arising in horizontally propagating vertical chemical fronts leading to the formation of a single stable convection roll is investigated experimentally in the iodate-arsenous acid reaction for various stoichiometry. In the presence of a free surface, the tilted reaction front becomes more elongated due to the evaporation of the surface active iodine and the decrease in the surface tension during the reaction. The experimental conditions are then identified where Marangoni instability represents the driving force for the distortion of the reaction front at the surface.
Convective aggregation in realistic convective-scale simulations
Holloway, Christopher E.
2017-06-01
To investigate the real-world relevance of idealized-model convective self-aggregation, five 15 day cases of real organized convection in the tropics are simulated. These include multiple simulations of each case to test sensitivities of the convective organization and mean states to interactive radiation, interactive surface fluxes, and evaporation of rain. These simulations are compared to self-aggregation seen in the same model configured to run in idealized radiative-convective equilibrium. Analysis of the budget of the spatial variance of column-integrated frozen moist static energy shows that control runs have significant positive contributions to organization from radiation and negative contributions from surface fluxes and transport, similar to idealized runs once they become aggregated. Despite identical lateral boundary conditions for all experiments in each case, systematic differences in mean column water vapor (CWV), CWV distribution shape, and CWV autocorrelation length scale are found between the different sensitivity runs, particularly for those without interactive radiation, showing that there are at least some similarities in sensitivities to these feedbacks in both idealized and realistic simulations (although the organization of precipitation shows less sensitivity to interactive radiation). The magnitudes and signs of these systematic differences are consistent with a rough equilibrium between (1) equalization due to advection from the lateral boundaries and (2) disaggregation due to the absence of interactive radiation, implying disaggregation rates comparable to those in idealized runs with aggregated initial conditions and noninteractive radiation. This points to a plausible similarity in the way that radiation feedbacks maintain aggregated convection in both idealized simulations and the real world.Plain Language SummaryUnderstanding the processes that lead to the organization of tropical rainstorms is an important challenge for weather
USING SATELLITE OBSERVATION FOR EARLY WARNING OF CONVECTIVE STORM IN TEHRAN
Directory of Open Access Journals (Sweden)
E. Owlad
2015-12-01
Full Text Available Severe convective storms are responsible for large amount of damage each year around the world. They form an important part of the climate system by redistributing heat, moisture, and trace gases, as well as producing large quantities of precipitation. As these extreme and rare events are in mesoscale there is many uncertainty in predicting them and we can’t rely on just models. On the other hand, remote sensing has a large application in Meteorology and near real time weather forecasting, especially in rare and extreme events like convective storms that might be difficult to predict with atmospheric models. On second of June 2014, near 12UTC a sudden and strong convective storm occurred in Tehran province that was not predicted, and caused economic and human losses. In This research we used satellite observations along with synoptic station measurements to predict and monitor this storm. Results from MODIS data show an increase in the amount of cloudiness and also aerosol optical depth and sudden decrease in cloud top temperature few hours before the storm occurs. EUMETSAT images show the governing of convection before the storm occurs. With combining the observation data that shows Lake of humidity and high temperature in low levels with satellite data that reveals instability in high levels that together caused this convective, we could track the storm and decrease the large amount of damage.
Directory of Open Access Journals (Sweden)
Missoum Abdelkrim
2016-01-01
Full Text Available This study focuses on the numerical simulation of heat transfer by natural convection in a rectangular enclosure, filled with a liquid metal (low Prandtl number partially heated from below with a sinusoidal temperature. The value of the study lies in its involvement in the crystal growth for the manufacture of semiconductors and electronics cooling. Indeed, the occurrence of convection during crystal growth can lead to in homogeneities that lead to striations and defects that affect the quality of the crystals obtained by the Bridgman techniques or Chochrawlski. Temperature of the oscillations, due to the instabilities of the convective flow in the liquid metal, also induces non-uniform cooling in the solidification front. Convection is then studied in order to reduce it. A modelling of the problem in two dimensions was conducted using Comsol computer code that is based on the finite element method, by varying the configuration of the control parameters, namely, the Rayleigh number, the nature of fluid (Prandtl number and amplitude of temperature on heat transfer rate (Nusselt number on convective structures that appear.
Widespread Occurrence of Trenching Patterns in the Granulation Field: Evidence for Roll Convection?
Getling, A. V.; Buchnev, A. A.
2008-04-01
Time-averaged series of granulation images are analysed using COLIBRI, a purpose-adapted version of a code originally developed to detect straight or curvilinear features in aerospace images. The image-processing algorithm utilises a nonparametric statistical criterion that identifies a straight-line segment as a linear feature (lineament) if the photospheric brightness at a certain distance from this line on both sides is stochastically lower or higher than at the line itself. Curvilinear features can be detected as chains of lineaments, using a modified criterion. Once the input parameters used by the algorithm are properly adjusted, the algorithm highlights “ridges” and “trenches” in the relief of the brightness field, drawing white and dark lanes. The most remarkable property of the trenching patterns is a nearly universally present parallelism of ridges and trenches. Since the material upflows are brighter than the downflows, the alternating, parallel light and dark lanes should reflect the presence of roll convection in the subphotospheric layers. If the numerous images processed by us are representative, the patterns revealed suggest a widespread occurrence of roll convection in the outer solar convection zone. In particular, the roll systems could form the fine structure of larger scale, supergranular and/or mesogranular convection flows. Granules appear to be overheated blobs of material that could develop into convection rolls owing to instabilities of roll motion.
Pattern formation and mass transfer under stationary solutal Marangoni instability.
Schwarzenberger, Karin; Köllner, Thomas; Linde, Hartmut; Boeck, Thomas; Odenbach, Stefan; Eckert, Kerstin
2014-04-01
According to the seminal theory by Sternling and Scriven, solutal Marangoni convection during mass transfer of surface-active solutes may occur as either oscillatory or stationary instability. With strong support of Manuel G. Velarde, a combined initiative of experimental works, in particular to mention those of Linde, Wierschem and coworkers, and theory has enabled a classification of dominant wave types of the oscillatory mode and their interactions. In this way a rather comprehensive understanding of the nonlinear evolution of the oscillatory instability could be achieved. A comparably advanced state-of-the-art with respect to the stationary counterpart seemed to be out of reach a short time ago. Recent developments on both the numerical and experimental side, in combination with assessing an extensive number of older experiments, now allow one to draw a more unified picture. By reviewing these works, we show that three main building blocks exist during the nonlinear evolution: roll cells, relaxation oscillations and relaxation oscillations waves. What is frequently called interfacial turbulence results from the interaction between these partly coexisting basic patterns which may additionally occur in different hierarchy levels. The second focus of this review lies on the practical importance of such convection patterns concerning their influence on mass transfer characteristics. Particular attention is paid here to the interaction between Marangoni and buoyancy effects which frequently complicates the pattern formation even more. To shed more light on these dependencies, new simulations regarding the limiting case of stabilizing density stratification and vanishing buoyancy are incorporated.
Experimental investigation of horizontal convection
Muñoz Córdoba, Lucía
2015-01-01
Fluid circulation driven by buoyancy forces due to a thermal gradient on a horizontal boundary, known as horizontal convection, is experimentally studied. For that purpose, a methacrylate box with inner dimensions 300x150x150 mm3 (LxWxH) whose bottom is composed by a heat exchanger and a printed circuit board is lled with water. The heat exchanger provides a uniform temperature boundary condition while the printed circuit board provides a boundary condition of uniform heat ...
Ice Nucleation in Deep Convection
Jensen, Eric; Ackerman, Andrew; Stevens, David; Gore, Warren J. (Technical Monitor)
2001-01-01
The processes controlling production of ice crystals in deep, rapidly ascending convective columns are poorly understood due to the difficulties involved with either modeling or in situ sampling of these violent clouds. A large number of ice crystals are no doubt generated when droplets freeze at about -40 C. However, at higher levels, these crystals are likely depleted due to precipitation and detrainment. As the ice surface area decreases, the relative humidity can increase well above ice saturation, resulting in bursts of ice nucleation. We will present simulations of these processes using a large-eddy simulation model with detailed microphysics. Size bins are included for aerosols, liquid droplets, ice crystals, and mixed-phase (ice/liquid) hydrometers. Microphysical processes simulated include droplet activation, freezing, melting, homogeneous freezing of sulfate aerosols, and heterogeneous ice nucleation. We are focusing on the importance of ice nucleation events in the upper part of the cloud at temperatures below -40 C. We will show that the ultimate evolution of the cloud in this region (and the anvil produced by the convection) is sensitive to these ice nucleation events, and hence to the composition of upper tropospheric aerosols that get entrained into the convective column.
Takahashi, Kazuya; Yamamoto, Yu; Yamada, Shoichi
2016-01-01
The explosion mechanism of core-collapse supernovae has not been fully understood yet but multi-dimensional fluid instabilities such as standing accretion shock instability (SASI) and convection are now believed to be crucial for shock revival. Another multi-dimensional effect that has been recently argued is the asymmetric structures in progenitors, which are induced by violent convections in silicon/oxygen layers that occur before the onset of collapse, as revealed by recent numerical simulations of the last stage of massive star evolutions. Furthermore, it has been also demonstrated numerically that accretions of such non-spherical envelopes could facilitate shock revival. These two multi-dimensional may hence hold a key to successful explosions. In this paper, we performed a linear stability analysis of the standing accretion shock in core-collapse supernovae, taking into account non-spherical, unsteady accretion flows onto the shock to clarify the possible links between the two effects. We found that suc...
Iterative methods for stationary convection-dominated transport problems
Energy Technology Data Exchange (ETDEWEB)
Bova, S.W.; Carey, G.F. [Univ. of Texas, Austin, TX (United States)
1994-12-31
It is well known that many iterative methods fail when applied to nonlinear systems of convection-dominated transport equations. Most successful methods for obtaining steady-state solutions to such systems rely on time-stepping through an artificial transient, combined with careful construction of artificial dissipation operators. These operators provide control over spurious oscillations which pollute the steady state solutions, and, in the nonlinear case, may become amplified and lead to instability. In the present study, we investigate Taylor Galerkin and SUPG-type methods and compare results for steady-state solutions to the Euler equations of gas dynamics. In particular, we consider the efficiency of different iterative strategies and present results for representative two-dimensional calculations.
EFFECTS OF CONVECTIVE FLUID MOTION UPON OXIDE CRYSTAL GROWTH IN HIGH TEMPERATURE SOLUTION
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
@@ For understanding of the influence of convective flow on crystal growth, space high temperature in situ observation instrument (SHITISOI) is dedicated to visualize and record the whole growth process of oxide crystals in high temperature up to 1000°C. Model experiments using transparent liquids such as KNbO3 and a mix ture of Li2B4O7+KNbO3 were chosen to investigate effects on ground and in space.On the earth, an investigation of growth kinetics of KNbO3 crystal related to two different states of convection: diffusive-advective flow and diffusive-convective flow,has been performed. The per unit length of a step e is calculated from the exper imental data for two different states of convection. Analyses of these data show the effect of buoyancy convection is to enhance the sharpness of the interface. The growth of KNbO3 crystals from solution of KNbO3+Li2B4O7 was investigated in space. The streamlines of the steady thermocapillary convection in Li2B4O7 solvent was observed. Due to thermocapillary convection, KNbO3 crystal grains grew and filled the whole solution homogeneously. Earth-based quenching experiments are de signed in order to study polyhedral instability of KNbO3 crystal, which is controlled by diffusion mechanism limitation. In all cases, when the crystal was nucleated near air/solution surface, it lost its polyhedral stability and varied from polyhedrons to dedrites. The thickness of diffusion mechanism limitation layer is about 60μm.
On the instabilities in miscible fluids under horizontal oscillating forcing
Shevtsova, Valentina; Gaponenko, Yuri; Mialdun, Aliaksandr
2012-07-01
In this research the attention is focused on the mass transfer under vibrations. The pure vibrational effect can be observed in weightlessness only. However, experimental studies addressing vibrational phenomena in weightlessness are very limited. This study was initiated by preparation of VIPIL proposal in the response of ESA AO-2009 call and will continue in the frame of the preparation the experiment VIPIL on the ISS. The current research is an attempt to provide experimental and numerical evidence for the instability in miscible fluids in the case of horizontal vibrations parallel to the interface. We present the results of the pioneer experiment SOVICON on the observation of the interface behavior between miscible liquids and vibrational convection in reduced gravity, conducted in the 49th Parabolic Flight Campaigns organized by the European Space Agency. Two miscible liquid mixtures, water and isopropanol of different concentrations, are placed in a closed cell, submitted to horizontal and sinusoidal oscillations at different frequencies and amplitudes. For the certain set of control parameters the localized mean convective flows are emerged in the vicinity of the triple points: the contact point between solid vertical wall and two liquids. These localized convective patterns spread along the solid walls and provide a local mixing along the walls. This type of instability was studied numerically in the averaged approach for high frequency vibrations [1], [2]. During the microgravity experiments we observed another type of instability in the form of standing waves. Above a threshold, a relief appears at the interface between the two fluids. This instability occurs when the vibration amplitude and frequency are above a critical value which is set by the level of viscous dissipation in the liquid. In general, this viscous dissipation will have a few sources: motion in the bulk of the liquid, motion near the boundaries of the liquid container, motion of any
Bony instability of the shoulder.
Bushnell, Brandon D; Creighton, R Alexander; Herring, Marion M
2008-09-01
Instability of the shoulder is a common problem treated by many orthopaedists. Instability can result from baseline intrinsic ligamentous laxity or a traumatic event-often a dislocation that injures the stabilizing structures of the glenohumeral joint. Many cases involve soft-tissue injury only and can be treated successfully with repair of the labrum and ligamentous tissues. Both open and arthroscopic approaches have been well described, with recent studies of arthroscopic soft-tissue techniques reporting results equal to those of the more traditional open techniques. Over the last decade, attention has focused on the concept of instability of the shoulder mediated by bony pathology such as a large bony Bankart lesion or an engaging Hill-Sachs lesion. Recent literature has identified unrecognized large bony lesions as a primary cause of failure of arthroscopic reconstruction for instability, a major cause of recurrent instability, and a difficult diagnosis to make. Thus, although such bony lesions may be relatively rare compared with soft-tissue pathology, they constitute a critically important entity in the management of shoulder instability. Smaller bony lesions may be amenable to arthroscopic treatment, but larger lesions often require open surgery to prevent recurrent instability. This article reviews recent developments in the diagnosis and treatment of bony instability.
Cinerama sickness and postural instability
Bos, J.E.; Ledegang, W.D.; Lubeck, A.J.A.; Stins, J.F.
2013-01-01
Motion sickness symptoms and increased postural instability induced by motion pictures have been reported in a laboratory, but not in a real cinema. We, therefore, carried out an observational study recording sickness severity and postural instability in 19 subjects before, immediately and 45 min af
Marital instability after midlife.
Wu, Z; Penning, M J
1997-09-01
"Divorce in later life has been shown to produce dramatic declines in the economic, psychological, and physical well-being of marital partners. This study examines the prevalence and determinants of marital disruption after midlife using Becker's theory of marital instability. Using recent Canadian national data, the marital outcomes of women and men who were married as of age 40 are tracked across the remaining years of the marriage. Cox proportional hazard regression models indicate stabilizing effects of the duration of the marriage, the age at first marriage, the presence of young children, as well as of remarriage for middle-aged and older persons. Other significant risk factors include education, heterogamous marital status, premarital cohabitation, number of siblings, and region."
Patzelt, Felix
2015-01-01
Many complex systems exhibit extreme events far more often than expected for a normal distribution. This work examines how self-similar bursts of activity across several orders of magnitude can emerge from first principles in systems that adapt to information. Surprising connections are found between two apparently unrelated research topics: hand-eye coordination in balancing tasks and speculative trading in financial markets. Seemingly paradoxically, locally minimising fluctuations can increase a dynamical system's sensitivity to unpredictable perturbations and thereby facilitate global catastrophes. This general principle is studied in several domain-specific models and in behavioural experiments. It explains many findings in both fields and resolves an apparent antinomy: the coexistence of stabilising control or market efficiency and perpetual instabilities resembling critical phenomena in physical systems.
Structural and Material Instability
DEFF Research Database (Denmark)
Cifuentes, Gustavo Cifuentes
This work is a small contribution to the general problem of structural and material instability. In this work, the main subject is the analysis of cracking and failure of structural elements made from quasi-brittle materials like concrete. The analysis is made using the finite element method. Three...... use of interface elements) is used successfully to model cases where the path of the discontinuity is known in advance, as is the case of the analysis of pull-out of fibers embedded in a concrete matrix. This method is applied to the case of non-straight fibers and fibers with forces that have....... Numerical problems associated with the use of elements with embedded cracks based on the extended finite element method are presented in the next part of this work. And an alternative procedure is used in order to successfully remove these numerical problems. In the final part of this work, a computer...
Chiodi, Filippo; Claudin, Philippe
2012-01-01
The river bar instability is revisited, using a hydrodynamical model based on Reynolds averaged Navier-Stokes equations. The results are contrasted with the standard analysis based on shallow water Saint-Venant equations. We first show that the stability of both transverse modes (ripples) and of small wavelength inclined modes (bars) predicted by the Saint-Venant approach are artefacts of this hydrodynamical approximation. When using a more reliable hydrodynamical model, the dispersion relation does not present any maximum of the growth rate when the sediment transport is assumed to be locally saturated. The analysis therefore reveals the fundamental importance of the relaxation of sediment transport towards equilibrium as it it is responsible for the stabilisation of small wavelength modes. This dynamical mechanism is characterised by the saturation number, defined as the ratio of the saturation length to the water depth Lsat/H. This dimensionless number controls the transition from ripples (transverse patte...
Onset of Vibrational Convection in a Binary Fluid Saturated Non-Darcy Porous Layer Heated from Above
Directory of Open Access Journals (Sweden)
Saravanan S.
2012-07-01
Full Text Available A linear stability analysis is used to investigate the influence of mechanical vibration on the onset of thermosolutal convection in a horizontal porous layer heated and salted from above. Vibrations are considered with arbitrary amplitude and frequency. The Brinkman extended Darcy model is used to describe the flow and the Oberbeck-Boussinesq approximation is employed. Continued fraction method and Floquet theory are used to determine the convective instability threshold. It is found that the solutal Rayleigh number has the stabilizing effect. The existence of a closed disconnected loop of synchronous mode is predicted in the marginal curve for moderate values of solutal Rayleigh number and vibration amplitude.
Effects of thermoelectric-magneto convection on the solidified microstructures of Al-4.5% Cu alloy
Institute of Scientific and Technical Information of China (English)
ZHANG Wei-qiang; SHI Hai-fang; LIU Qing
2004-01-01
A magnetic field was used to directional solidification of dendritic Al-Cu alloys.The primary arm spacing of the dendrites is increased with increasing the external magnetic field strength. This microstructural coarsening is related to the thermoelectric current on the I/s interface, which is induced by difference of temperature on the interface. Within a magnetic field Lorentz force may be generated in front of the interface,causing a thermoelectric-magneto convection that makes the microstructure coarsened.This induced convection also leads to structural instability so that the secondary arms are well developed when grown in the external field.
The Effect of Nuclear Reaction Rates & Convective Mixing on the Evolution of a 6M$_{\\odot}$ Star
Halabi, Ghina M
2014-01-01
We present the evolution of a 6M$_{\\odot}$ star, of solar-like initial metallicity, and investigate the effects of key nuclear reaction rates, as well as the treatment of the convective mixing on its evolution along the Cepheid instability strip. In particular, we study the effect of recent estimates of the $^{14}$N(p,{\\gamma})$^{15}$O reaction on the formation and extension of the blue loop during core helium burning. We also investigate the effects induced on this blue loop by the adoption of non-standard convective mixing prescriptions, as well as the implications of modifying the Mixing Length Theory.
Interfacial instabilities in a stratified flow of two superposed fluids
Schaflinger, Uwe
1994-06-01
Here we shall present a linear stability analysis of a laminar, stratified flow of two superposed fluids which are a clear liquid and a suspension of solid particles. The investigation is based upon the assumption that the concentration remains constant within the suspension layer. Even for moderate flow-rates the base-state results for a shear induced resuspension flow justify the latter assumption. The numerical solutions display the existence of two different branches that contribute to convective instability: long and short waves which coexist in a certain range of parameters. Also, a range exists where the flow is absolutely unstable. That means a convectively unstable resuspension flow can be only observed for Reynolds numbers larger than a lower, critical Reynolds number but still smaller than a second critical Reynolds number. For flow rates which give rise to a Reynolds number larger than the second critical Reynolds number, the flow is absolutely unstable. In some cases, however, there exists a third bound beyond that the flow is convectively unstable again. Experiments show the same phenomena: for small flow-rates short waves were usually observed but occasionally also the coexistence of short and long waves. These findings are qualitatively in good agreement with the linear stability analysis. Larger flow-rates in the range of the second critical Reynolds number yield strong interfacial waves with wave breaking and detached particles. In this range, the measured flow-parameters, like the resuspension height and the pressure drop are far beyond the theoretical results. Evidently, a further increase of the Reynolds number indicates the transition to a less wavy interface. Finally, the linear stability analysis also predicts interfacial waves in the case of relatively small suspension heights. These results are in accordance with measurements for ripple-type instabilities as they occur under laminar and viscous conditions for a mono-layer of particles.
Scheck, L; Foglizzo, T; Kifonidis, K
2007-01-01
By 2D hydrodynamic simulations including a detailed equation of state and neutrino transport, we investigate the interplay between different non-radial hydrodynamic instabilities that play a role during the postbounce accretion phase of collapsing stellar cores. The convective mode of instability, which is driven by negative entropy gradients caused by neutrino heating or by time variations of the shock strength, can be identified clearly by the development of typical Rayleigh-Taylor mushrooms. However, in cases where the gas in the postshock region is rapidly advected towards the gain radius, the growth of such a buoyancy instability can be suppressed. In such a situation the shocked flow nevertheless can develop non-radial asymmetry with an oscillatory growth of the amplitude. This phenomenon was previously termed ``standing accretion shock instability'' (SASI) by Blondin et al. (2003). It is shown here that the oscillation period of the SASI observed in our simulations agrees well with the one estimated fo...
Beam instability Workshop - plenary sessions
Energy Technology Data Exchange (ETDEWEB)
NONE
2001-07-01
The purpose of this workshop was to provide a review of the mechanisms of limiting beam instabilities, their cures, including feedback, and beam measurement for synchrotron radiation light sources. 12 plenary sessions took place whose titles are: 1) challenging brilliance and lifetime issues with increasing currents; 2) limiting instabilities in multibunch; 3) experience from high currents in B factories; 4) longitudinal dynamics in high intensity/bunch; 5) Transverse instabilities for high intensity/bunch; 6) working group introduction from ESRF experience; 7) impedance modelling: simulations, minimization; 8) report on the broadband impedance measurements and modelling workshop; 9) feedback systems for synchrotron light sources; 10) beam instabilities diagnostics; 11) harmonic cavities: the pros and cons; and 12) experimental study of fast beam-ion instabilities at PLS. This document gathers the 12 articles that were presented during these sessions.
Equilibrium Electro-osmotic Instability
Rubinstein, Isaak
2014-01-01
Since its prediction fifteen years ago, electro-osmotic instability has been attributed to non-equilibrium electro-osmosis related to the extended space charge which develops at the limiting current in the course of concentration polarization at a charge-selective interface. This attribution had a double basis. Firstly, it has been recognized that equilibrium electro-osmosis cannot yield instability for a perfectly charge-selective solid. Secondly, it has been shown that non-equilibrium electro-osmosis can. First theoretical studies in which electro-osmotic instability was predicted and analyzed employed the assumption of perfect charge-selectivity for the sake of simplicity and so did the subsequent numerical studies of various time-dependent and nonlinear features of electro-osmotic instability. In this letter, we show that relaxing the assumption of perfect charge-selectivity (tantamount to fixing the electrochemical potential in the solid) allows for equilibrium electro-osmotic instability. Moreover, we s...
Gravitational Instabilities in Circumstellar Disks
Kratter, Kaitlin M
2016-01-01
[Abridged] Star and planet formation are the complex outcomes of gravitational collapse and angular momentum transport mediated by protostellar and protoplanetary disks. In this review we focus on the role of gravitational instability in this process. We begin with a brief overview of the observational evidence for massive disks that might be subject to gravitational instability, and then highlight the diverse ways in which the instability manifests itself in protostellar and protoplanetary disks: the generation of spiral arms, small scale turbulence-like density fluctuations, and fragmentation of the disk itself. We present the analytic theory that describes the linear growth phase of the instability, supplemented with a survey of numerical simulations that aim to capture the non-linear evolution. We emphasize the role of thermodynamics and large scale infall in controlling the outcome of the instability. Despite apparent controversies in the literature, we show a remarkable level of agreement between analyt...
Gravitational Instabilities in Circumstellar Disks
Kratter, Kaitlin; Lodato, Giuseppe
2016-09-01
Star and planet formation are the complex outcomes of gravitational collapse and angular momentum transport mediated by protostellar and protoplanetary disks. In this review, we focus on the role of gravitational instability in this process. We begin with a brief overview of the observational evidence for massive disks that might be subject to gravitational instability and then highlight the diverse ways in which the instability manifests itself in protostellar and protoplanetary disks: the generation of spiral arms, small-scale turbulence-like density fluctuations, and fragmentation of the disk itself. We present the analytic theory that describes the linear growth phase of the instability supplemented with a survey of numerical simulations that aim to capture the nonlinear evolution. We emphasize the role of thermodynamics and large-scale infall in controlling the outcome of the instability. Despite apparent controversies in the literature, we show a remarkable level of agreement between analytic predictions and numerical results. In the next part of our review, we focus on the astrophysical consequences of the instability. We show that the disks most likely to be gravitationally unstable are young and relatively massive compared with their host star, Md/M*≥0.1. They will develop quasi-stable spiral arms that process infall from the background cloud. Although instability is less likely at later times, once infall becomes less important, the manifestations of the instability are more varied. In this regime, the disk thermodynamics, often regulated by stellar irradiation, dictates the development and evolution of the instability. In some cases the instability may lead to fragmentation into bound companions. These companions are more likely to be brown dwarfs or stars than planetary mass objects. Finally, we highlight open questions related to the development of a turbulent cascade in thin disks and the role of mode-mode coupling in setting the maximum angular
Introduction to the focus issue: chemo-hydrodynamic patterns and instabilities.
De Wit, A; Eckert, K; Kalliadasis, S
2012-09-01
Pattern forming instabilities are often encountered in a wide variety of natural phenomena and technological applications, from self-organization in biological and chemical systems to oceanic or atmospheric circulation and heat and mass transport processes in engineering systems. Spatio-temporal structures are ubiquitous in hydrodynamics where numerous different convective instabilities generate pattern formation and complex spatiotemporal dynamics, which have been much studied both theoretically and experimentally. In parallel, reaction-diffusion processes provide another large family of pattern forming instabilities and spatio-temporal structures which have been analyzed for several decades. At the intersection of these two fields, "chemo-hydrodynamic patterns and instabilities" resulting from the coupling of hydrodynamic and reaction-diffusion processes have been less studied. The exploration of the new instability and symmetry-breaking scenarios emerging from the interplay between chemical reactions, diffusion and convective motions is a burgeoning field in which numerous exciting problems have emerged during the last few years. These problems range from fingering instabilities of chemical fronts and reactive fluid-fluid interfaces to the dynamics of reaction-diffusion systems in the presence of chaotic mixing. The questions to be addressed are at the interface of hydrodynamics, chemistry, engineering or environmental sciences to name a few and, as a consequence, they have started to draw the attention of several communities including both the nonlinear chemical dynamics and hydrodynamics communities. The collection of papers gathered in this Focus Issue sheds new light on a wide range of phenomena in the general area of chemo-hydrodynamic patterns and instabilities. It also serves as an overview of the current research and state-of-the-art in the field.
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
Particle sedimentation and diffusive convection in volcanic ash-clouds
Carazzo, G.; Jellinek, A. M.
2013-04-01
Understanding the longevity of volcanic ash-clouds generated by powerful explosive eruptions is a long standing problem for assessing volcanic hazards and the nature and time scale of volcanic forcings on climate change. It is well known that the lateral spreading and longevity of these clouds is influenced by stratospheric winds, particle settling and turbulent diffusion. Observations of the recent 2010 Eyjafjallajökull and 2011 Grimsvötn umbrella clouds, as well as the structure of atmospheric aerosol clouds from the 1991 Mt Pinatubo event, suggest that an additional key process governing the cloud dynamics is the production of internal layering. Here, we use analog experiments on turbulent particle-laden umbrella clouds to show that this layering occurs where natural convection driven by particle sedimentation and the differential diffusion of primarily heat and fine particles give rise to a large scale instability. Where umbrella clouds are particularly enriched in fine ash, this "particle diffusive convection" strongly influences the cloud longevity. More generally, cloud residence time will depend on fluxes due to both individual settling and diffusive convection. We develop a new sedimentation model that includes both sedimentation processes, and which is found to capture real-time measurements of the rate of change of particle concentration in the 1982 El Chichon, 1991 Mt Pinatubo and 1992 Mt Spurr ash-clouds. A key result is that these combined sedimentation processes enhance the fallout of fine particles relative to expectations from individual settling suggesting that particle aggregation is not the only mechanism required to explain volcanic umbrella longevity.
Mechanisms for convection triggering by cold pools
Torri, Giuseppe; Tian, Yang
2015-01-01
Cold pools are fundamental ingredients of deep convection. They contribute to organizing the sub-cloud layer and are considered key elements in triggering convective cells. It was long known that this could happen mechanically, through lifting by the cold pools' fronts. More recently, it has been suggested that convection could also be triggered thermodynamically, by accumulation of moisture around the edges of cold pools. A method based on Lagrangian tracking is here proposed to disentangle the signatures of both forcings and quantify their importance in a given environment. Results from a simulation of radiative-convective equilibrium over the ocean show that parcels reach their level of free convection through a combination of both forcings, each being dominant at different stages of the ascent. Mechanical forcing is an important player in lifting parcels from the surface, whereas thermodynamic forcing reduces the inhibition encountered by parcels before they reach their level of free convection.
On laminar convection in solar type stars
Bruevich, E A
2010-01-01
We present a new model of large-scale multilayer convection in solar type stars. This model allows us to understand such self-similar structures observed at solar surface as granulation, supergranulation and giant cells. We study the slow-rotated hydrogen star without magnetic field with the spherically-symmetric convective zone. The photon's flux comes to the convective zone from the central thermonuclear zone of the star. The interaction of these photons with the fully ionized hydrogen plasma with $T>10^5K$ is carried out by the Tomson scattering of photon flux on protons and electrons. Under these conditions plasma is optically thick relative to the Tomson scattering. This fact is the fundamental one for the multilayer convection formation. We find the stationary solution of the convective zone structure. This solution describes the convective layers responsible to the formation of the structures on the star's surface.
Effect of thermosolutal convection on directional solidification
Indian Academy of Sciences (India)
Suresh V Garimella; James E Simpson
2001-02-01
The impact of thermosolutal convection during directional solidification is explored via results of numerical investigations. Results from fully transient numerical simulations of directional solidification in a differentially heated cavity under terrestrial conditions and Bridgman crystal growth in space are discussed. The pivotal role of both thermal and solutal convection in the solidification process is illustrated by examining these two cases. In particular, radial and longitudinal macrosegregation resulting from this thermosolutal convection is discussed.
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.
Helioseismology challenges models of solar convection
Gizon, Laurent; 10.1073/pnas.1208875109
2012-01-01
Convection is the mechanism by which energy is transported through the outermost 30% of the Sun. Solar turbulent convection is notoriously difficult to model across the entire convection zone where the density spans many orders of magnitude. In this issue of PNAS, Hanasoge et al. (2012) employ recent helioseismic observations to derive stringent empirical constraints on the amplitude of large-scale convective velocities in the solar interior. They report an upper limit that is far smaller than predicted by a popular hydrodynamic numerical simulation.
Modeling of heat explosion with convection.
Belk, Michael; Volpert, Vitaly
2004-06-01
The work is devoted to numerical simulations of the interaction of heat explosion with natural convection. The model consists of the heat equation with a nonlinear source term describing heat production due to an exothermic chemical reaction coupled with the Navier-Stokes equations under the Boussinesq approximation. We show how complex regimes appear through successive bifurcations leading from a stable stationary temperature distribution without convection to a stationary symmetric convective solution, stationary asymmetric convection, periodic in time oscillations, and finally aperiodic oscillations. A simplified model problem is suggested. It describes the main features of solutions of the complete problem.
A Study of Detrainment from Deep Convection
Glenn, I. B.; Krueger, S. K.
2014-12-01
Uncertainty in the results of Global Climate Model simulations has been attributed to errors and simplifications in how parameterizations of convection coarsely represent the processes of entrainment, detrainment, and mixing between convective clouds and their environment. Using simulations of convection we studied these processes at a resolution high enough to explicitly resolve them. Two of several recently developed analysis techniques that allow insight into these processes at their appropriate scale are an Eulerian method of directly measuring entrainment and detrainment, and a Lagrangian method that uses particle trajectories to map convective mass flux over height and a cloud variable of interest. The authors of the Eulerian technique used it to show that the dynamics of shells of cold, humid air that surround shallow convective updrafts have important effects on the properties of air entrained and detrained from the updrafts. There is some evidence for the existence of such shells around deep convective updrafts as well, and that detrainment is more important than entrainment in determining the ultimate effect of the deep convection on the large scale environment. We present results from analyzing a simulation of deep convection through the Eulerian method as well as using Lagrangian particle trajectories to illustrate the role of the shell in the process of detrainment and mixing between deep convection and its environment.
Abelianization of QCD plasma instabilities
Arnold, Peter; Lenaghan, Jonathan
2004-12-01
QCD plasma instabilities appear to play an important role in the equilibration of quark-gluon plasmas in heavy-ion collisions in the theoretical limit of weak coupling (i.e. asymptotically high energy). It is important to understand what nonlinear physics eventually stops the exponential growth of unstable modes. It is already known that the initial growth of plasma instabilities in QCD closely parallels that in QED. However, once the unstable modes of the gauge fields grow large enough for non-Abelian interactions between them to become important, one might guess that the dynamics of QCD plasma instabilities and QED plasma instabilities become very different. In this paper, we give suggestive arguments that non-Abelian self-interactions between the unstable modes are ineffective at stopping instability growth, and that the growing non-Abelian gauge fields become approximately Abelian after a certain stage in their growth. This in turn suggests that understanding the development of QCD plasma instabilities in the nonlinear regime may have close parallels to similar processes in traditional plasma physics. We conjecture that the physics of collisionless plasma instabilities in SU(2) and SU(3) gauge theory becomes equivalent, respectively, to (i) traditional plasma physics, which is U(1) gauge theory, and (ii) plasma physics of U(1)×U(1) gauge theory.
Irradiated stars with convective envelopes
Lucy, L B
2016-01-01
The structure of low-mass stars irradiated by a close companion is considered. Irradiation modifies the surface boundary conditions and thereby also the adiabatic constants of their outer convection zones. This then changes the models' radii and luminosities. For short-period M dwarf binaries with components of similar mass, the radius inflation due to their mutual irradiation is found to be < 0.4%. This is an order of magnitude too small to explain the anomalous radii found for such binaries. Although stronger irradiation of an M dwarf results in a monotonically increasing radius, a saturation effect limits the inflation to < 5%.
Modulation instability of space-periodic oscillatory patterns
Nepomnyashchy, Alexander; Shklyaev, Sergey; Oron, Alexander
2011-11-01
Pattern selection and stability of regular (periodic in space) regimes is a classical problem with a number of applications in fluid dynamics. For steady bifurcations both competition of perfect periodic patterns and their stability with respect to slow modulations in space (e.g. Eckhaus or zigzag instabilities) are well studied. In contrast, in the case of Hopf bifurcation, usually only selection of patterns that possess a certain symmetry was analyzed (Silber & Knobloch, Nonlinearity, 1991; Roberts et al, Contemp. Math, 1986), whereas the set of Ginzburg-Landau equations was studied only in the one-dimensional case (rolls). Dealing with a wide class of problems, where the longwave oscillatory instability takes place, we consider a stability of regular oscillatory patterns that belong to either square or hexagonal lattices with respect to spatial modulations. By means of the multiple scale expansion, we derive instability criteria valid near the stability threshold. Useful classification of possible perturbations of a regular structure is introduced. As an example, the theory is applied to Marangoni convection in a layer of a binary mixture with the Soret effect. Domains of stability of space-periodic patterns are obtained.
Experiments on the global instability of confined axisymmetric dense wakes.
Li, Larry; Juniper, Matthew
2007-11-01
Recent theoretical studies [M. Juniper, J. Fluid Mech. 565, 171-195 (2006); M. Juniper and S. Candel, J. Fluid Mech. 482, 257-269 (2003)] predict that confinement increases the hydrodynamic instability of wakes by causing the transition from convective to absolute instability to occur at lower values of shear. Experimental evidence supporting this prediction is presented here for a confined, axisymmetric wake at density ratios, S ≡ ρ1 /ρ2> 1 (i.e. dense wake). The wake was produced by a pair of convergent nozzles mounted concentrically, one within the other, in a low-turbulence wind tunnel facility. Variations in S were achieved by employing two high density gases (S = 1.53 and 5.11) in the inner flow with air in the outer flow. For a fixed S, there existed a critical value of shear above which dominant peaks appeared abruptly in the near-wake velocity spectra, as quantified by hot-wire anemometry. Corresponding high-speed video sequences revealed large-scale, sinuous wake motions. Results on the confined wake's response to externally-applied, acoustic forcing are also presented. The presence of discrete spectral peaks and coordinated instability oscillations suggests the emergence of a self-sustained, global mode.
A synoptic climatology of derecho producing mesoscale convective systems in the North-Central Plains
Bentley, Mace L.; Mote, Thomas L.; Byrd, Stephen F.
2000-09-01
Synoptic-scale environments favourable for producing derechos, or widespread convectively induced windstorms, in the North-Central Plains are examined with the goal of providing pattern-recognition/diagnosis techniques. Fifteen derechos were identified across the North-Central Plains region during 1986-1995. The synoptic environment at the initiation, mid-point and decay of each derecho was then evaluated using surface, upper-air and National Center for Atmospheric Research (NCAR)/National Center for Environmental Prediction (NCEP) reanalysis datasets.Results suggest that the synoptic environment is critical in maintaining derecho producing mesoscale convective systems (DMCSs). The synoptic environment in place downstream of the MCS initiation region determines the movement and potential strength of the system. Circulation around surface low pressure increased the instability gradient and maximized leading edge convergence in the initiation region of nearly all events regardless of DMCS location or movement. Other commonalities in the environments of these events include the presence of a weak thermal boundary, high convective instability and a layer of dry low-to-mid-tropospheric air. Of the two corridors sampled, northeastward moving derechos tend to initiate east of synoptic-scale troughs, while southeastward moving derechos form on the northeast periphery of a synoptic-scale ridge. Other differences between these two DMCS events are also discussed.
Eskew, Matthew W.; Harrison, Jason; Simoyi, Reuben H.
2016-11-01
Oxidation reactions of thiourea by chlorite in a Hele-Shaw cell are excitable, autocatalytic, exothermic, and generate a lateral instability upon being triggered by the autocatalyst. Reagent concentrations used to develop convective instabilities delivered a temperature jump at the wave front of 2.1 K. The reaction zone was 2 mm and due to normal cooling after the wave front, this generated a spike rather than the standard well-studied front propagation. The reaction front has solutal and thermal contributions to density changes that act in opposite directions due to the existence of a positive isothermal density change in the reaction. The competition between these effects generates thermal plumes. The fascinating feature of this system is the coexistence of plumes and fingering in the same solution which alternate in frequency as the front propagates, generating hot and cold spots within the Hele-Shaw cell, and subsequently spatiotemporal inhomogeneities. The small ΔT at the wave front generated thermocapillary convection which competed effectively with thermogravitational forces at low Eötvös Numbers. A simplified reaction-diffusion-convection model was derived for the system. Plume formation is heavily dependent on boundary effects from the cell dimensions. This work was supported by Grant No. CHE-1056366 from the NSF and a Research Professor Grant from the University of KwaZulu-Natal.
Instability of ties in compression
DEFF Research Database (Denmark)
Buch-Hansen, Thomas Cornelius
2013-01-01
Masonry cavity walls are loaded by wind pressure and vertical load from upper floors. These loads results in bending moments and compression forces in the ties connecting the outer and the inner wall in a cavity wall. Large cavity walls are furthermore loaded by differential movements from...... exact instability solutions are complex to derive, not to mention the extra complexity introducing dimensional instability from the temperature gradients. Using an inverse variable substitution and comparing an exact theory with an analytical instability solution a method to design tie...
Microsatellite instability in bladder cancer
DEFF Research Database (Denmark)
Gonzalez-Zulueta, M; Ruppert, J M; Tokino, K;
1993-01-01
Somatic instability at microsatellite repeats was detected in 6 of 200 transitional cell carcinomas of the bladder. Instabilities were apparent as changes in (GT)n repeat lengths on human chromosome 9 for four tumors and as alterations in a (CAG)n repeat in the androgen receptor gene on the X...... chromosome for three tumors. Single locus alterations were detected in three tumors, while three other tumors revealed changes in two or more loci. In one tumor we found microsatellite instability in all five loci analyzed on chromosome 9. The alterations detected were either minor 2-base pair changes...
Futterer, B.; Egbers, C.; Dahley, N.; Koch, S.; Jehring, L.
2010-01-01
Physical mechanisms of thermally driven rotating fluids are important for a large number of geophysical problems, e.g. to explain the convection of the Earth's liquid outer core. Objective of the 'GeoFlow' experiment is to study stability, pattern formation, and transition to chaos of thermal convection in fluid-filled concentric, co-axially rotating spheres. This experiment is integrated in the Fluid Science Laboratory of the European COLUMBUS module on International Space Station. Fluid dynamics of the experiment was predicted with numerical simulations by means of a spectral code. In the non-rotating case the onset of convection bifurcated into steady fluid flow. Here patterns of convection showed co-existing states with axisymmetric, cubic and pentagonal modes. Transition to chaos was in the form of sudden onset. For the thermal convection in rotating spheres the onset of first instability showed an increase of modes for higher parameter regime. Transition was from steady via periodic to chaotic behaviour. Convection patterns of the experiment are observed with the Wollaston shearing interferometry. Images are in terms of interferograms with fringe patterns corresponding to special convective flows. A first glance at the images showed the classification of sub- and supercritical flow regimes. Aligned with numerical data a shift between experiment and numerical simulation was identified. Identification of convection patterns in interferograms was demonstrated for the example of a supercritical flow.
Directory of Open Access Journals (Sweden)
Karen Bouwer
2000-06-01
Full Text Available For Plantier, language constitutes reality and is male dominated. Readers of texts, she says, are at a disadvantage because the author imposes a logic that we must accept in order to understand the text. The discourses shaping our social reality have the same effect. Plantier has struggled against individual voices, discourses, and the very fabric of language informed by these discourses. "Subject to Instability" examines the impact on her generic evolution of a changing sense of self, of who her interlocutors are, and of those for whom she is speaking. I argue that her increasing attempt to juggle many different voices destabilizes her "monologic," poetical voice, resulting in a blurring of generic boundaries and eventually the abandonment of poetry. Recognizing that our entry into language is a form of alienation also unsettles Plantier because it undermines the very identity that allows her to speak for others. She concludes that each woman needs to become a Subject in her own right, but she continues to struggle against dominant discourses, modeling "resisting reader" strategies. If she can no longer practice "monologic steadfastness," this does not deter her from attempting to dismantle patriarchal language and striving to make her voice prevail over others.
Meng, Xuhui; Yang, Xiaofan; Guo, Zhaoli
2016-11-01
Geological storage of the CO2 in subsurface saline aquifers is a promising way to reduce CO2 emissions. During this process, CO2 first dissolves into pure brine. Then the acidic and denser mixture falls down under the gravity and reacts with the rock. In the present work, a microfluidic experiment is conducted to investigate the density-driven convection with dissolution in porous media. Moreover, the linear stability analysis and numerical simulations are further performed to investigate the interfacial instability. The results demonstrate that front instability can be triggered by the density contrast between the two miscible fluids, leading to the Rayleigh-Taylor instability. While this type of instability can be suppressed by the surface reaction between the fluid and solid phases, which prevents the transport of the denser fluid to the deeper region at the beginning. Over the long term, it is found that the interfacial instability can be influenced by the evolution of the porosity due to the dissolution, which will drive the transport of denser fluid further down. Our investigation shows that the transport of the reactive fluid in porous media depends on the competition among the density contrast, the chemical reaction rate and the evolution of the porosity/permeability.
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.
Atlantoaxial instability in Down's syndrome
J Gordon Millichap
1987-01-01
The radiographs and clinical evaluations of 90 children with Down’s syndrome were reassessed after an interval of 5 years in a study of atlantoaxial instability (AAI) at the Derbyshire Children’s Hospital and Infirmary, Derby, UK.
Intrinsic Instability of Coronal Streamers
Chen, Y; Song, H Q; Shi, Q Q; Feng, S W; Xia, L D; 10.1088/0004-637X/691/2/1936
2009-01-01
Plasma blobs are observed to be weak density enhancements as radially stretched structures emerging from the cusps of quiescent coronal streamers. In this paper, it is suggested that the formation of blobs is a consequence of an intrinsic instability of coronal streamers occurring at a very localized region around the cusp. The evolutionary process of the instability, as revealed in our calculations, can be described as follows: (1) through the localized cusp region where the field is too weak to sustain the confinement, plasmas expand and stretch the closed field lines radially outward as a result of the freezing-in effect of plasma-magnetic field coupling; the expansion brings a strong velocity gradient into the slow wind regime providing the free energy necessary for the onset of a subsequent magnetohydrodynamic instability; (2) the instability manifests itself mainly as mixed streaming sausage-kink modes, the former results in pinches of elongated magnetic loops to provoke reconnections at one or many loc...
Keyhole depth instability in case of CW CO2 laser beam welding of mild steel
Indian Academy of Sciences (India)
N Kumar; S Dash; A K Tyagi; Baldev Raj
2010-10-01
The study of keyhole (KH) instability in deep penetration laser beam welding (LBW) is essential to understand welding process and appearance of weld seam defects. The main cause of keyhole collapse is the instability in KH dynamics during the LBW process. This is mainly due to the surface tension forces associated with the KH collapse and the stabilizing action of vapour pressure. A deep penetration high power CW CO2 laser was used to generate KH in mild steel (MS) in two different welding conditions i.e. ambient atmospheric welding (AAW) and under water welding (UWW). KH, formed in case of under water welding, was deeper and narrower than keyhole formed in ambient and atmospheric condition. The number and dimensions of irregular humps increased in case of ambient and under water condition due to larger and rapid keyhole collapse also studied. The thermocapillary convection is considered to explain KH instability, which in turn gives rise to irregular humps.
Material Instabilities in Particulate Systems
Goddard, J. D.
1999-01-01
Following is a brief summary of a theoretical investigation of material (or constitutive) instability associated with shear induced particle migration in dense particulate suspensions or granular media. It is shown that one can obtain a fairly general linear-stability analysis, including the effects of shear-induced anisotropy in the base flow as well as Reynolds dilatancy. A criterion is presented here for simple shearing instability in the absence of inertia and dilatancy.
Instability following total knee arthroplasty.
Rodriguez-Merchan, E Carlos
2011-10-01
Background Knee prosthesis instability (KPI) is a frequent cause of failure of total knee arthroplasty. Moreover, the degree of constraint required to achieve immediate and long-term stability in total knee arthroplasty (TKA) is frequently debated. Questions This review aims to define the problem, analyze risk factors, and review strategies for prevention and treatment of KPI. Methods A PubMed (MEDLINE) search of the years 2000 to 2010 was performed using two key words: TKA and instability. One hundred and sixty-five initial articles were identified. The most important (17) articles as judged by the author were selected for this review. The main criteria for selection were that the articles addressed and provided solutions to the diagnosis and treatment of KPI. Results Patient-related risk factors predisposing to post-operative instability include deformity requiring a large surgical correction and aggressive ligament release, general or regional neuromuscular pathology, and hip or foot deformities. KPI can be prevented in most cases with appropriate selection of implants and good surgical technique. When ligament instability is anticipated post-operatively, the need for implants with a greater degree of constraint should be anticipated. In patients without significant varus or valgus malalignment and without significant flexion contracture, the posterior cruciate ligament (PCL) can be retained. However, the PCL should be sacrificed when deformity exists particularly in patients with rheumatoid arthritis, previous patellectomy, previous high tibial osteotomy or distal femoral osteotomy, and posttraumatic osteoarthritis with disruption of the PCL. In most cases, KPI requires revision surgery. Successful outcomes can only be obtained if the cause of KPI is identified and addressed. Conclusions Instability following TKA is a common cause of the need for revision. Typically, knees with deformity, rheumatoid arthritis, previous patellectomy or high tibial osteotomy, and
Midcarpal instability: a radiological perspective
Energy Technology Data Exchange (ETDEWEB)
Toms, Andoni Paul [Norfolk and Norwich University Hospital NHS Trust, Department of Radiology, Norwich, Norfolk (United Kingdom); Radiology Academy, Cotman Centre, Norwich, Norfolk (United Kingdom); Chojnowski, Adrian [Norfolk and Norwich University Hospital NHS Trust, Department of Orthopaedic Surgery, Norwich, Norfolk (United Kingdom); Cahir, John G. [Norfolk and Norwich University Hospital NHS Trust, Department of Radiology, Norwich, Norfolk (United Kingdom)
2011-05-15
Midcarpal instability (MCI) is the result of complex abnormal carpal motion at the midcarpal joint of the wrist. It is a form of non-dissociative carpal instability (CIND) and can be caused by various combinations of extrinsic ligament injuries that then result in one of several subtypes of MCI. The complex patterns of injury and the kinematics are further complicated by competing theories, terminology and classifications of MCI. Palmar, dorsal, ulna midcarpal instability, and capitolunate or chronic capitolunate instability are all descriptions of types of MCI with often overlapping features. Palmar midcarpal instability (PMCI) is the most commonly reported type of MCI. It has been described as resulting from deficiencies in the ulna limb of the palmar arcuate ligament (triquetrohamate-capitate) or the dorsal radiotriquetral ligaments, or both. Unstable carpal articulations can be treated with limited carpal arthrodesis or the ligamentous defects can be treated with capsulorrhaphy or ligament reconstruction. Conventional radiographic abnormalities are usually limited to volar intercalated segment instability (VISI) patterns of carpal alignment and are not specific. For many years stress view radiographs and videofluoroscopy have been the methods of choice for demonstrating carpal instability and abnormal carpal kinematics respectively. Dynamic US can be also used to demonstrate midcarpal dyskinesia including the characteristic triquetral ''catch-up'' clunk. Tears of the extrinsic ligaments can be demonstrated with MR arthrography, and probably with CT arthrography, but intact yet redundant ligaments are more difficult to identify. The exact role of these investigations in the diagnosis, categorisation and management of midcarpal instability has yet to be determined. (orig.)
Thermal Rayleigh-Marangoni convection in a three-layer liquid-metal-battery model
Köllner, Thomas; Boeck, Thomas; Schumacher, Jörg
2017-05-01
The combined effects of buoyancy-driven Rayleigh-Bénard convection (RC) and surface tension-driven Marangoni convection (MC) are studied in a triple-layer configuration which serves as a simplified model for a liquid metal battery (LMB). The three-layer model consists of a liquid metal alloy cathode, a molten salt separation layer, and a liquid metal anode at the top. Convection is triggered by the temperature gradient between the hot electrolyte and the colder electrodes, which is a consequence of the release of resistive heat during operation. We present a linear stability analysis of the state of pure thermal conduction in combination with three-dimensional direct numerical simulations of the nonlinear turbulent evolution on the basis of a pseudospectral method. Five different modes of convection are identified in the configuration, which are partly coupled to each other: RC in the upper electrode, RC with internal heating in the molten salt layer, and MC at both interfaces between molten salt and electrode as well as anticonvection in the middle layer and lower electrode. The linear stability analysis confirms that the additional Marangoni effect in the present setup increases the growth rates of the linearly unstable modes, i.e., Marangoni and Rayleigh-Bénard instability act together in the molten salt layer. The critical Grashof and Marangoni numbers decrease with increasing middle layer thickness. The calculated thresholds for the onset of convection are found for realistic current densities of laboratory-sized LMBs. The global turbulent heat transfer follows scaling predictions for internally heated RC. The global turbulent momentum transfer is comparable with turbulent convection in the classical Rayleigh-Bénard case. In summary, our studies show that incorporating Marangoni effects generates smaller flow structures, alters the velocity magnitudes, and enhances the turbulent heat transfer across the triple-layer configuration.
Naso, L; Bonanno, A; Paternò, L
2007-01-01
During the first 40 s after their birth, proto-neutron stars are expected to be subject to at least two types of instability: the convective instability and the neutron-finger one. Both instabilities involve convective motions and hence can trigger dynamo actions which may be responsible for the large magnetic fields in neutron stars and magnetars. We have solved the mean-field induction equation in a simplified one-dimensional model of both the convective and the neutron-finger instability zones. Although very idealized, the model includes the nonlinearities introduced by the feedback processes which tend to saturate the growth of the magnetic field (alpha-quenching) and suppress its turbulent diffusion (eta-quenching). The possibility of a dynamo action is studied within a dynamical model of turbulent diffusivity where the boundary of the unstable zone is allowed to move. We show that the dynamo action can be operative and that the amplification of the magnetic field can still be very effective. Furthermore...
Introductory Analysis of Benard-Marangoni Convection
Maroto, J. A.; Perez-Munuzuri, V.; Romero-Cano, M. S.
2007-01-01
We describe experiments on Benard-Marangoni convection which permit a useful understanding of the main concepts involved in this phenomenon such as, for example, Benard cells, aspect ratio, Rayleigh and Marangoni numbers, Crispation number and critical conditions. In spite of the complexity of convection theory, we carry out a simple and…
Spurious multiple equilibria introduced by convective adjustment
den Toom, M.; Dijkstra, H.A.; Wubs, F.W.
2011-01-01
The application of bifurcation analysis to ocean climate models is substantially hampered by difficulties associated with the use of convective adjustment, i.e. a parameterisation of convection in which the vertical diffusion of heat and salt is greatly enhanced whenever the water column becomes sta
Instability of enclosed horizons
Kay, Bernard S.
2015-03-01
We point out that there are solutions to the scalar wave equation on dimensional Minkowski space with finite energy tails which, if they reflect off a uniformly accelerated mirror due to (say) Dirichlet boundary conditions on it, develop an infinite stress-energy tensor on the mirror's Rindler horizon. We also show that, in the presence of an image mirror in the opposite Rindler wedge, suitable compactly supported arbitrarily small initial data on a suitable initial surface will develop an arbitrarily large stress-energy scalar near where the two horizons cross. Also, while there is a regular Hartle-Hawking-Israel-like state for the quantum theory between these two mirrors, there are coherent states built on it for which there are similar singularities in the expectation value of the renormalized stress-energy tensor. We conjecture that in other situations with analogous enclosed horizons such as a (maximally extended) Schwarzschild black hole in equilibrium in a (stationary spherical) box or the (maximally extended) Schwarzschild-AdS spacetime, there will be similar stress-energy singularities and almost-singularities—leading to instability of the horizons when gravity is switched on and matter and gravity perturbations are allowed for. All this suggests it is incorrect to picture a black hole in equilibrium in a box or a Schwarzschild-AdS black hole as extending beyond the past and future horizons of a single Schwarzschild (/Schwarzschild-AdS) wedge. It would thus provide new evidence for 't Hooft's brick wall model while seeming to invalidate the picture in Maldacena's ` Eternal black holes in AdS'. It would thereby also support the validity of the author's matter-gravity entanglement hypothesis and of the paper ` Brick walls and AdS/CFT' by the author and Ortíz.
Vector-Resonance-Multimode Instability
Sergeyev, S. V.; Kbashi, H.; Tarasov, N.; Loiko, Yu.; Kolpakov, S. A.
2017-01-01
The modulation and multimode instabilities are the main mechanisms which drive spontaneous spatial and temporal pattern formation in a vast number of nonlinear systems ranging from biology to laser physics. Using an Er-doped fiber laser as a test bed, here for the first time we demonstrate both experimentally and theoretically a new type of a low-threshold vector-resonance-multimode instability which inherits features of multimode and modulation instabilities. The same as for the multimode instability, a large number of longitudinal modes can be excited without mode synchronization. To enable modulation instability, we modulate the state of polarization of the lasing signal with the period of the beat length by an adjustment of the in-cavity birefringence and the state of polarization of the pump wave. As a result, we show the regime's tunability from complex oscillatory to periodic with longitudinal mode synchronization in the case of resonance matching between the beat and cavity lengths. Apart from the interest in laser physics for unlocking the tunability and stability of dynamic regimes, the proposed mechanism of the vector-resonance-multimode instability can be of fundamental interest for the nonlinear dynamics of various distributed systems.
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
Directory of Open Access Journals (Sweden)
Shivakumara I S,
2011-05-01
Full Text Available The linear stability analysis of Marangoni convection in a fluid layer with a boundary slab of finite conductivity is considered. The effects of various non uniform temperature gradients are investigated. The lower boundary is a considered to be a thin slab of finite conductivity instead of a regular rigid surface. At the contact surface between the thin slab and the fluid layer the thermalboundary conditions are used and the upper surface is considered to be free and insulating to temperature perturbation and also surface tension effects are allowed. The resulting eigen value problem is solved exactly. The critical values of the Marangoni numbers for the onset of Marangoni convection are calculated for different temperature profile and the latter is found to be critically dependent on the depth ratio and conductivity ratio. The effects of the thermal conductivity and the thickness of the solid plate on the onset of convective instability with different temperature profile arestudied in detail.
Convection of Moist Saturated Air: Analytical Study
Directory of Open Access Journals (Sweden)
Robert Zakinyan
2016-01-01
Full Text Available In the present work, the steady-state stationary thermal convection of moist saturated air in a lower atmosphere has been studied theoretically. Thermal convection was considered without accounting for the Coriolis force, and with only the vertical temperature gradient. The analytical solution of geophysical fluid dynamics equations, which generalizes the formulation of the moist convection problem, is obtained in the two-dimensional case. The stream function is derived in the Boussinesq approximation with velocity divergence taken as zero. It has been shown that the stream function is asymmetrical in vertical direction contrary to the dry and moist unsaturated air convection. It has been demonstrated that the convection in moist atmosphere strongly depends on the vapor mass fraction gradient.
Topology Optimisation for Coupled Convection Problems
DEFF Research Database (Denmark)
Alexandersen, Joe; Andreasen, Casper Schousboe; Aage, Niels
conduction governs in the solid parts of the design domain and couples to convection-dominated heat transfer to a surrounding fluid. Both loosely coupled and tightly coupled problems are considered. The loosely coupled problems are convection-diffusion problems, based on an advective velocity field from......The work focuses on applying topology optimisation to forced and natural convection problems in fluid dynamics and conjugate (fluid-structure) heat transfer. To the authors' knowledge, topology optimisation has not yet been applied to natural convection flow problems in the published literature...... and the current work is thus seen as contributing new results to the field. In the literature, most works on the topology optimisation of weakly coupled convection-diffusion problems focus on the temperature distribution of the fluid, but a selection of notable exceptions also focusing on the temperature...
Topology Optimisation for Coupled Convection Problems
DEFF Research Database (Denmark)
Alexandersen, Joe
This thesis deals with topology optimisation for coupled convection problems. The aim is to extend and apply topology optimisation to steady-state conjugate heat transfer problems, where the heat conduction equation governs the heat transfer in a solid and is coupled to thermal transport...... in a surrounding uid, governed by a convection-diffusion equation, where the convective velocity field is found from solving the isothermal incompressible steady-state Navier-Stokes equations. Topology optimisation is also applied to steady-state natural convection problems. The modelling is done using stabilised...... finite elements, the formulation and implementation of which was done partly during a special course as prepatory work for this thesis. The formulation is extended with a Brinkman friction term in order to facilitate the topology optimisation of fluid flow and convective cooling problems. The derived...
Convection in Oblate Solar-Type Stars
Wang, Junfeng; Liang, Chunlei
2016-01-01
We present the first global 3D simulations of thermal convection in the oblate envelopes of rapidly-rotating solar-type stars. This has been achieved by exploiting the capabilities of the new Compressible High-ORder Unstructured Spectral difference (CHORUS) code. We consider rotation rates up to 85\\% of the critical (breakup) rotation rate, which yields an equatorial radius that is up to 17\\% larger than the polar radius. This substantial oblateness enhances the disparity between polar and equatorial modes of convection. We find that the convection redistributes the heat flux emitted from the outer surface, leading to an enhancement of the heat flux in the polar and equatorial regions. This finding implies that lower-mass stars with convective envelopes may not have darker equators as predicted by classical gravity darkening arguments. The vigorous high-latitude convection also establishes elongated axisymmetric circulation cells and zonal jets in the polar regions. Though the overall amplitude of the surface...
Convection in Condensible-rich Atmospheres
Ding, Feng
2016-01-01
Condensible substances are nearly ubiquitous in planetary atmospheres. For the most familiar case-water vapor in Earth's present climate-the condensible gas is dilute, in the sense that its concentration is everywhere small relative to the noncondensible background gases. A wide variety of important planetary climate problems involve nondilute condensible substances. These include planets near or undergoing a water vapor runaway and planets near the outer edge of the conventional habitable zone, for which CO2 is the condensible. Standard representations of convection in climate models rely on several approximations appropriate only to the dilute limit, while nondilute convection differs in fundamental ways from dilute convection. In this paper, a simple parameterization of convection valid in the nondilute as well as dilute limits is derived and used to discuss the basic character of nondilute convection. The energy conservation properties of the scheme are discussed in detail and are verified in radiative-co...
Enhanced summer convective rainfall at Alpine high elevations in response to climate warming
Giorgi, Filippo; Torma, Csaba; Coppola, Erika; Ban, Nikolina; Schär, Christoph; Somot, Samuel
2016-08-01
Global climate projections consistently indicate a future decrease in summer precipitation over the European Alps. However, topography can substantially modulate precipitation change signals. For example, the shadowing effect by topographic barriers can modify winter precipitation change patterns, and orographic convection might also play an important role. Here we analyse summer precipitation over the Alpine region in an ensemble of twenty-first-century projections with high-resolution (~12 km) regional climate models driven by recent global climate model simulations. A broad-scale summer precipitation reduction is projected by both model ensembles. However, the regional models simulate an increase in precipitation over the high Alpine elevations that is not present in the global simulations. This is associated with increased convective rainfall due to enhanced potential instability by high-elevation surface heating and moistening. The robustness of this signal, which is found also for precipitation extremes, is supported by the consistency across models and future time slices, the identification of an underlying mechanism (enhanced convection), results from a convection-resolving simulation, the statistical significance of the signal and the consistency with some observed trends. Our results challenge the picture of a ubiquitous decrease of summer precipitation over the Alps found in coarse-scale projections.
A self-consistent linear-mode model of stellar convection
Macauslan, J.
1985-01-01
A normal-mode expansion of the linearized fluid equations in terms of small subset of spherical harmonics can provide a foundation for a physically motivated, self-consistent description of a solar-type convection zone. In the absence of dissipation, a second-order differential equation governs the radial dependence of the modes, so that interpretation of the effects on convection quantities of the normal-form 'potential well' is straightforward. The philosophy is quite different from the more recent work of Narasimha and Antia (1982): all envelopes presented here differ substantially from MLT envelopes, and therefore, from theirs, which are constructed to be consistent with MLT. The amplitude of all modes is set by a Kelvin-Helmholtz-('shear'-) instability argument unrelated to solar observations, with the result that the convection description may be considered to arise from 'first-hueristic-principles'. The thermodynamics modelled vaguely resemble the sun's, and more vigorously convective envelopes show some phenomena qualitatively like solar observations (e.g., atmospheric velocity spectra).
Energy Technology Data Exchange (ETDEWEB)
Luo, Kang; Yi, Hong-Liang, E-mail: yihongliang@hit.edu.cn; Tan, He-Ping, E-mail: tanheping@hit.edu.cn [School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001 (China)
2014-05-15
Transitions and bifurcations of transient natural convection in a horizontal annulus with radiatively participating medium are numerically investigated using the coupled lattice Boltzmann and direct collocation meshless (LB-DCM) method. As a hybrid approach based on a common multi-scale Boltzmann-type model, the LB-DCM scheme is easy to implement and has an excellent flexibility in dealing with the irregular geometries. Separate particle distribution functions in the LBM are used to calculate the density field, the velocity field and the thermal field. In the radiatively participating medium, the contribution of thermal radiation to natural convection must be taken into account, and it is considered as a radiative term in the energy equation that is solved by the meshless method with moving least-squares (MLS) approximation. The occurrence of various instabilities and bifurcative phenomena is analyzed for different Rayleigh number Ra and Prandtl number Pr with and without radiation. Then, bifurcation diagrams and dual solutions are presented for relevant radiative parameters, such as convection-radiation parameter Rc and optical thickness τ. Numerical results show that the presence of volumetric radiation changes the static temperature gradient of the fluid, and generally results in an increase in the flow critical value. Besides, the existence and development of dual solutions of transient convection in the presence of radiation are greatly affected by radiative parameters. Finally, the advantage of LB-DCM combination is discussed, and the potential benefits of applying the LB-DCM method to multi-field coupling problems are demonstrated.
The limiting form of symmetric instability in geophysical flows
Griffiths, Stephen
2017-04-01
The stability of parallel flow with vertical shear, density stratification and background rotation is of fundamental importance in geophysical fluid dynamics. For a flow with vertical shear Uz and buoyancy frequency N, the dominant instability is typically a symmetric instability (sometimes known as slantwise convection) when 1/4 linear stability problem has been well studied for the case of constant Uz and N, and has some interesting mathematical properties (e.g., non-separable governing PDE, an absence of normal mode solutions in rectangular domains). Here, for the first time, a general theory of symmetric instability is given when Ri varies smoothly with height, thinking of the more realistic case where an unstable layer with Ri 1. The mathematical theory is developed for horizontally periodic disturbances to a basic state with arbitrary smooth N(z), but constant Uz. An asymptotic analysis is used to derive expressions for the most unstable mode, which occurs in the limit of large cross-isentropic wavenumber and takes the form of solutions trapped within the unstable layer; the same result is derived using an interesting generalised parcel dynamics argument, which explicitly shows how the trapping is linked to vertical variations of the potential vorticity. A separate asymptotic analysis is given for the small wavenumber limit, where only one such trapped mode may exist, as expected from the spectral theory of the Schrödinger equation. These two limiting results are shown to be consistent with an exact solution of the linear stability problem that can be obtained for a special choice of N(z). The asymptotic analysis can be extended to allow for weak diffusion at arbitrary Prandtl number, yielding an explicit diffusive scale selection at large wavenumber. Numerical simulations show that these weakly diffusive modes dominate the early stages of the nonlinear evolution of the symmetric instability.
The effect of rotation on oscillatory double-diffusive convection (semiconvection)
Moll, Ryan
2016-01-01
Oscillatory double-diffusive convection (ODDC, more traditionally called semiconvection) is a form of linear double-diffusive instability that occurs in fluids that are unstably stratified in temperature (Schwarzschild unstable), but stably stratified in chemical composition (Ledoux stable). This scenario is thought to be quite common in the interiors of stars and giant planets, and understanding the transport of heat and chemical species by ODDC is of great importance to stellar and planetary evolution models. Fluids unstable to ODDC have a tendency to form convective thermo-compositional layers which significantly enhance the fluxes of temperature and chemical composition compared with microscopic diffusion. Although a number of recent studies have focused on studying properties of both layered and non-layered ODDC, few have addressed how additional physical processes such as global rotation affect its dynamics. In this work we study first how rotation affects the linear stability properties of rotating ODD...
Millennial-scale stable oscillations between sea ice and convective deep water formation
Saha, Raj
2015-01-01
During the last ice age there were several quasi-periodic abrupt warming events. The climatic effects of the so-called Dansgaard-Oeschger (DO) events were felt globally, although the North Atlantic experienced the largest and most abrupt temperature anomalies. Similar but weaker oscillations also took place during the interglacial period. This paper proposes an auto-oscillatory mechanism between sea ice and convective deep water formation in the north Atlantic as the source of the persistent cycles. A simple dynamical model is constructed by coupling and slightly modifying two existing models of ocean circulation and sea ice. The model exhibits mixed mode oscillations, consisting of decadal scale small amplitude oscillations, and a large amplitude relaxation fluctuation. The decadal oscillations occur due to the insulating effect of sea ice and leads to periodic ventilation of heat from the polar ocean. Gradually an instability builds up in the polar column and results in an abrupt initiation of convection an...
Natural convection between concentric spheres
Garg, Vijay K.
1992-01-01
A finite-difference solution for steady natural convective flow in a concentric spherical annulus with isothermal walls has been obtained. The stream function-vorticity formulation of the equations of motion for the unsteady axisymmetric flow is used; interest lying in the final steady solution. Forward differences are used for the time derivatives and second-order central differences for the space derivatives. The alternating direction implicit method is used for solution of the discretization equations. Local one-dimensional grid adaptation is used to resolve the steep gradients in some regions of the flow at large Rayleigh numbers. The break-up into multi-cellular flow is found at high Rayleigh numbers for air and water, and at significantly low Rayleigh numbers for liquid metals. Excellent agreement with previous experimental and numerical data is obtained.
Leconte, Jérémy; Hersant, Franck; Guillot, Tristan
2016-01-01
In an atmosphere, a cloud condensation region is characterized by a strong vertical gradient in the abundance of the related condensing species. On Earth, the ensuing gradient of mean molecular weight has relatively few dynamical consequences because N$_2$ is heavier than water vapor, so that only the release of latent heat significantly impacts convection. On the contrary, in an hydrogen dominated atmosphere (e.g. giant planets), all condensing species are significantly heavier than the background gas. This can stabilize the atmosphere against convection near a cloud deck if the enrichment in the given species exceeds a critical threshold. This raises two questions. What is transporting energy in such a stabilized layer, and how affected can the thermal profile of giant planets be? To answer these questions, we first carry out a linear analysis of the convective and double-diffusive instabilities in a condensable medium showing that an efficient condensation can suppress double-diffusive convection. This sug...
Elliptic and magneto-elliptic instabilities
Directory of Open Access Journals (Sweden)
Lyra Wladimir
2013-04-01
Full Text Available Vortices are the fundamental units of turbulent flow. Understanding their stability properties therefore provides fundamental insights on the nature of turbulence itself. In this contribution I briely review the phenomenological aspects of the instability of elliptic streamlines, in the hydro (elliptic instability and hydromagnetic (magneto-elliptic instability regimes. Vortex survival in disks is a balance between vortex destruction by these mechanisms, and vortex production by others, namely, the Rossby wave instability and the baroclinic instability.
Actively convected liquid metal divertor
Shimada, Michiya; Hirooka, Yoshi
2014-12-01
The use of actively convected liquid metals with j × B force is proposed to facilitate heat handling by the divertor, a challenging issue associated with magnetic fusion experiments such as ITER. This issue will be aggravated even more for DEMO and power reactors because the divertor heat load will be significantly higher and yet the use of copper would not be allowed as the heat sink material. Instead, reduced activation ferritic/martensitic steel alloys with heat conductivities substantially lower than that of copper, will be used as the structural materials. The present proposal is to fill the lower part of the vacuum vessel with liquid metals with relatively low melting points and low chemical activities including Ga and Sn. The divertor modules, equipped with electrodes and cooling tubes, are immersed in the liquid metal. The electrode, placed in the middle of the liquid metal, can be biased positively or negatively with respect to the module. The j × B force due to the current between the electrode and the module provides a rotating motion for the liquid metal around the electrodes. The rise in liquid temperature at the separatrix hit point can be maintained at acceptable levels from the operation point of view. As the rotation speed increases, the current in the liquid metal is expected to decrease due to the v × B electromotive force. This rotating motion in the poloidal plane will reduce the divertor heat load significantly. Another important benefit of the convected liquid metal divertor is the fast recovery from unmitigated disruptions. Also, the liquid metal divertor concept eliminates the erosion problem.
Budroni, M. A.
2015-12-01
Cross diffusion, whereby a flux of a given species entrains the diffusive transport of another species, can trigger buoyancy-driven hydrodynamic instabilities at the interface of initially stable stratifications. Starting from a simple three-component case, we introduce a theoretical framework to classify cross-diffusion-induced hydrodynamic phenomena in two-layer stratifications under the action of the gravitational field. A cross-diffusion-convection (CDC) model is derived by coupling the fickian diffusion formalism to Stokes equations. In order to isolate the effect of cross-diffusion in the convective destabilization of a double-layer system, we impose a starting concentration jump of one species in the bottom layer while the other one is homogeneously distributed over the spatial domain. This initial configuration avoids the concurrence of classic Rayleigh-Taylor or differential-diffusion convective instabilities, and it also allows us to activate selectively the cross-diffusion feedback by which the heterogeneously distributed species influences the diffusive transport of the other species. We identify two types of hydrodynamic modes [the negative cross-diffusion-driven convection (NCC) and the positive cross-diffusion-driven convection (PCC)], corresponding to the sign of this operational cross-diffusion term. By studying the space-time density profiles along the gravitational axis we obtain analytical conditions for the onset of convection in terms of two important parameters only: the operational cross-diffusivity and the buoyancy ratio, giving the relative contribution of the two species to the global density. The general classification of the NCC and PCC scenarios in such parameter space is supported by numerical simulations of the fully nonlinear CDC problem. The resulting convective patterns compare favorably with recent experimental results found in microemulsion systems.
Experimental and numerical investigation on two-phase flow instabilities
Energy Technology Data Exchange (ETDEWEB)
Ruspini, Leonardo Carlos
2013-03-01
Two-phase flow instabilities are experimentally and numerically studied within this thesis. In particular, the phenomena called Ledinegg instability, density wave oscillations and pressure drop oscillations are investigated. The most important investigations regarding the occurrence of two-phase flow instabilities are reviewed. An extensive description of the main contributions in the experimental and analytical research is presented. In addition, a critical discussion and recommendations for future investigations are presented. A numerical framework using a hp-adaptive method is developed in order to solve the conservation equations modelling general thermo-hydraulic systems. A natural convection problem is analysed numerically in order to test the numerical solver. Moreover, the description of an adaptive strategy to solve thermo-hydraulic problems is presented. In the second part of this dissertation, a homogeneous model is used to study Ledinegg, density wave and pressure drop oscillations phenomena numerically. The dynamic characteristics of the Ledinegg (flow excursion) phenomenon are analysed through the simulation of several transient examples. In addition, density wave instabilities in boiling and condensing systems are investigated. The effects of several parameters, such as the fluid inertia and compressibility volumes, on the stability limits of Ledinegg and density wave instabilities are studied, showing a strong influence of these parameters. Moreover, the phenomenon called pressure drop oscillations is numerically investigated. A discussion of the physical representation of several models is presented with reference to the obtained numerical results. Finally, the influence of different parameters on these phenomena is analysed. In the last part, an experimental investigation of these phenomena is presented. The designing methodology used for the construction of the experimental facility is described. Several simulations and a non
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...
Preparing for an Explosion: Hydrodynamic Instabilities and Turbulence in Presupernovae
Smith, Nathan
2013-01-01
Both observations and direct numerical simulations are discordant with predictions of conventional stellar evolution codes for the latest stages of a massive star's life prior to core collapse. We suggest that the problem lies in the treatment of turbulent convection in these codes, which ignores finite amplitude fluctuations in velocity and temperature, and their nonlinear interaction with nuclear burning. The hydrodynamic instabilities that may arise prompt us to discuss a number of far-reaching implications for the fates of massive stars. In particular, we explore connections to enhanced presupernova mass loss, unsteady nuclear burning and consequent eruptions, swelling of the stellar radius that may trigger violent interactions with a companion star, and potential modifications to the core structure that could dramatically impact calculations of the core-collapse mechanism itself. These modifications may be of fundamental importance to the interpretation of measured isotopic anomalies in meteorites, chang...
Three-Dimensional Simulations of SASI- and Convection-Dominated Core-Collapse Supernovae
Fernández, Rodrigo
2015-01-01
We investigate the effect of dimensionality on the transition to explosion in neutrino-driven core-collapse supernovae. Using parameterized hydrodynamic simulations of the stalled supernova shock in one-, two- (2D), and three spatial dimensions (3D), we systematically probe the extent to which hydrodynamic instabilities alone can tip the balance in favor of explosion. In particular, we focus on systems that are well into the regimes where the Standing Accretion Shock Instability (SASI) or neutrino-driven convection dominate the dynamics, and characterize the difference between them. We find that SASI-dominated models can explode with up to ~20% lower neutrino luminosity in 3D than in 2D, with the magnitude of this difference decreasing with increasing resolution. This improvement in explosion conditions originates in the ability of spiral modes to generate more non-radial kinetic energy than a single sloshing mode, increasing the size of the average shock radius, and hence generating better conditions for the...
Soret-driven ferro thermohaline convection
Energy Technology Data Exchange (ETDEWEB)
Vaidyanathan, G. [Department of Physics, Pondicherry Engineering College, Pillaichavady, Pondicherry 605014 (India)]. E-mail: gvn_pec@yahoo.com; Sekar, R. [Department of Mathematics, Pondicherry Engineering College, Pondicherry 605014 (India); Hemalatha, R. [Department of Mathematics, Bharathidassan Govt. College for Women, Pondicherry 605003 (India); Vasanthakumari, R. [Department of Mathematics, K.M. Center for PG studies, Lawspet, Pondicherry 605008 (India); Sendhilnathan, S. [Department of Physics, Sri Manakula Vinayagar Engineering College, Pondicherry 605107 (India)
2005-03-01
Soret-driven thermoconvective instability in multicomponent fluids has wide applications in heat and mass transfer estimations. In ferrofluids, three components, namely, core, surfactant and carrier fluids, coexist. In this paper, an attempt is made to obtain the condition for the onset of thermoconvective instability due to the Soret effect. Both stationary and oscillatory instabilities have been investigated. The principle of exchange of stability is used to determine mode of instability. A linear stability analysis is used. The results are presented numerically and graphically.
Complex Convective Thermal Fluxes and Vorticity Structure
Redondo, Jose M.; Tellez, Jackson; Sotillos, Laura; Lopez Gonzalez-Nieto, Pilar; Sanchez, Jesus M.; Furmanek, Petr; Diez, Margarita
2015-04-01
Local Diffusion and the topological structure of vorticity and velocity fields is measured in the transition from a homogeneous linearly stratified fluid to a cellular or layered structure by means of convective cooling and/or heating[1,2]. Patterns arise by setting up a convective flow generated by an array of Thermoelectric devices (Peltier/Seebeck cells) these are controlled by thermal PID generating a buoyant heat flux [2]. The experiments described here investigate high Prandtl number mixing using brine and fresh water in order to form density interfaces and low Prandtl number mixing with temperature gradients. The set of dimensionless parameters define conditions of numeric and small scale laboratory modeling of environmental flows. Fields of velocity, density and their gradients were computed and visualized [3,4]. When convective heating and cooling takes place the combination of internal waves and buoyant turbulence is much more complicated if the Rayleigh and Reynolds numbers are high in order to study entrainment and mixing. Using ESS and selfsimilarity structures in the velocity and vorticity fieds and intermittency [3,5] that forms in the non-homogeneous flow is related to mixing and stiring. The evolution of the mixing fronts are compared and the topological characteristics of the merging of plumes and jets in different configurations presenting detailed comparison of the evolution of RM and RT, Jets and Plumes in overall mixing. The relation between structure functions, fractal analysis and spectral analysis can be very useful to determine the evolution of scales. Experimental and numerical results on the advance of a mixing or nonmixing front occurring at a density interface due to body forces [6]and gravitational acceleration are analyzed considering the fractal and spectral structure of the fronts like in removable plate experiments for Rayleigh-Taylor flows. The evolution of the turbulent mixing layer and its complex configuration is studied
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.
Transient Mixed Convection Validation for NGNP
Energy Technology Data Exchange (ETDEWEB)
Smith, Barton [Utah State Univ., Logan, UT (United States); Schultz, Richard [Idaho National Lab. (INL), Idaho Falls, ID (United States)
2015-10-19
The results of this project are best described by the papers and dissertations that resulted from the work. They are included in their entirety in this document. They are: (1) Jeff Harris PhD dissertation (focused mainly on forced convection); (2) Blake Lance PhD dissertation (focused mainly on mixed and transient convection). This dissertation is in multi-paper format and includes the article currently submitted and one to be submitted shortly; and, (3) JFE paper on CFD Validation Benchmark for Forced Convection.
One type of hydrodynamic instability in joule heating of a fluid near an ion-selective surface
Nikitin, N. V.; Khasmatulina, N. Yu.; Ganchenko, G. S.; Kalaidin, E. N.; Kiriy, V. A.; Demekhin, E. A.
2016-06-01
The stability of the equilibrium state of an electrolyte in a horizontal microgap between two ionselective surfaces in an electric field is studied with the Joule heating of the fluid taken into account. It is established that the Joule heating can lead to instability at the potential differences, which are several times smaller than those in the isothermal case. The effects of microscale thermal instability differ from the Rayleigh-Benard thermal convection: the destabilization occurs upon heating in the upper part of the gap.
Interfacial instabilities and Kapitsa pendula
Krieger, Madison
2015-11-01
Determining the critera for onset and amplitude growth of instabilities is one of the central problems of fluid mechanics. We develop a parallel between the Kapitsa effect, in which a pendulum subject to high-frequency low-amplitude vibrations becomes stable in the inverted position, and interfaces separating fluids of different density. It has long been known that such interfaces can be stabilized by vibrations, even when the denser fluid is on top. We demonstrate that the stability diagram for these fluid interfaces is identical to the stability diagram for an appopriate Kapitsa pendulum. We expand the robust, ``dictionary''-type relationship between Kapitsa pendula and interfacial instabilities by considering the classical Rayleigh-Taylor, Kelvin-Helmholtz and Plateau instabilities, as well as less-canonical examples ranging in scale from the micron to the width of a galaxy.
Interfacial Instability during Granular Erosion.
Lefebvre, Gautier; Merceron, Aymeric; Jop, Pierre
2016-02-12
The complex interplay between the topography and the erosion and deposition phenomena is a key feature to model granular flows such as landslides. Here, we investigated the instability that develops during the erosion of a wet granular pile by a dry dense granular flow. The morphology and the propagation of the generated steps are analyzed in relation to the specific erosion mechanism. The selected flowing angle of the confined flow on a dry heap appears to play an important role both in the final state of the experiment, and for the shape of the structures. We show that the development of the instability is governed by the inertia of the flow through the Froude number. We model this instability and predict growth rates that are in agreement with the experiment results.
Laboratory blast wave driven instabilities
Kuranz, Carolyn
2008-11-01
This presentation discusses experiments involving the evolution of hydrodynamic instabilities in the laboratory under high-energy-density (HED) conditions. These instabilities are driven by blast waves, which occur following a sudden, finite release of energy, and consist of a shock front followed by a rarefaction wave. When a blast wave crosses an interface with a decrease in density, hydrodynamic instabilities will develop. Instabilities evolving under HED conditions are relevant to astrophysics. These experiments include target materials scaled in density to the He/H layer in SN1987A. About 5 kJ of laser energy from the Omega Laser facility irradiates a 150 μm plastic layer that is followed by a low-density foam layer. A blast wave structure similar to those in supernovae is created in the plastic layer. The blast wave crosses an interface having a 2D or 3D sinusoidal structure that serves as a seed perturbation for hydrodynamic instabilities. This produces unstable growth dominated by the Rayleigh-Taylor (RT) instability in the nonlinear regime. We have detected the interface structure under these conditions using x-ray backlighting. Recent advances in our diagnostic techniques have greatly improved the resolution of our x-ray radiographic images. Under certain conditions, the improved images show some mass extending beyond the RT spike and penetrating further than previously observed or predicted by current simulations. The observed effect is potentially of great importance as a source of mass transport to places not anticipated by current theory and simulation. I will discuss the amount of mass in these spike extensions, the associated uncertainties, and hypotheses regarding their origin We also plan to show comparisons of experiments using single mode and multimode as well as 2D and 3D initial conditions. This work is sponsored by DOE/NNSA Research Grants DE-FG52-07NA28058 (Stewardship Sciences Academic Alliances) and DE-FG52-04NA00064 (National Laser User
Telomere dysfunction and chromosome instability
Energy Technology Data Exchange (ETDEWEB)
Murnane, John P., E-mail: jmurnane@radonc.ucsf.edu [Department of Radiation Oncology, University of California San Francisco, 2340 Sutter Street, San Francisco, CA 94143-1331 (United States)
2012-02-01
The ends of chromosomes are composed of a short repeat sequence and associated proteins that together form a cap, called a telomere, that keeps the ends from appearing as double-strand breaks (DSBs) and prevents chromosome fusion. The loss of telomeric repeat sequences or deficiencies in telomeric proteins can result in chromosome fusion and lead to chromosome instability. The similarity between chromosome rearrangements resulting from telomere loss and those found in cancer cells implicates telomere loss as an important mechanism for the chromosome instability contributing to human cancer. Telomere loss in cancer cells can occur through gradual shortening due to insufficient telomerase, the protein that maintains telomeres. However, cancer cells often have a high rate of spontaneous telomere loss despite the expression of telomerase, which has been proposed to result from a combination of oncogene-mediated replication stress and a deficiency in DSB repair in telomeric regions. Chromosome fusion in mammalian cells primarily involves nonhomologous end joining (NHEJ), which is the major form of DSB repair. Chromosome fusion initiates chromosome instability involving breakage-fusion-bridge (B/F/B) cycles, in which dicentric chromosomes form bridges and break as the cell attempts to divide, repeating the process in subsequent cell cycles. Fusion between sister chromatids results in large inverted repeats on the end of the chromosome, which amplify further following additional B/F/B cycles. B/F/B cycles continue until the chromosome acquires a new telomere, most often by translocation of the end of another chromosome. The instability is not confined to a chromosome that loses its telomere, because the instability is transferred to the chromosome donating a translocation. Moreover, the amplified regions are unstable and form extrachromosomal DNA that can reintegrate at new locations. Knowledge concerning the factors promoting telomere loss and its consequences is
Stringy bounces and gradient instabilities
Giovannini, Massimo
2017-01-01
Bouncing solutions are obtained from a generally covariant action characterized by a potential which is a nonlocal functional of the dilaton field at two separated space-time points. Gradient instabilities are shown to arise in this context but they are argued to be nongeneric. After performing a gauge-invariant and frame-invariant derivation of the evolution equations of the fluctuations, a heuristic criterium for the avoidance of pathological instabilities is proposed and corroborated by a number of explicit examples that turn out to be compatible with a quasi-flat spectrum of curvature inhomogeneities for typical wavelengths larger than the Hubble radius.
Research on aviation fuel instability
Baker, C. E.; Bittker, D. A.; Cohen, S. M.; Seng, G. T.
1984-01-01
The problems associated with aircraft fuel instability are discussed. What is currently known about the problem is reviewed and a research program to identify those areas where more research is needed is discussed. The term fuel instability generally refers to the gums, sediments, or deposits which can form as a result of a set of complex chemical reactions when a fuel is stored for a long period at ambient conditions or when the fuel is thermally stressed inside the fuel system of an aircraft.
Mechanical Instabilities of Biological Tubes
Hannezo, Edouard; Prost, Jacques; Joanny, Jean-François
2012-07-01
We study theoretically the morphologies of biological tubes affected by various pathologies. When epithelial cells grow, the negative tension produced by their division provokes a buckling instability. Several shapes are investigated: varicose, dilated, sinuous, or sausagelike. They are all found in pathologies of tracheal, renal tubes, or arteries. The final shape depends crucially on the mechanical parameters of the tissues: Young’s modulus, wall-to-lumen ratio, homeostatic pressure. We argue that since tissues must be in quasistatic mechanical equilibrium, abnormal shapes convey information as to what causes the pathology. We calculate a phase diagram of tubular instabilities which could be a helpful guide for investigating the underlying genetic regulation.
Hydromagnetic Instabilities in Neutron Stars
Lasky, Paul D; Kokkotas, Kostas D; Glampedakis, Kostas
2011-01-01
We model the non-linear ideal magnetohydrodynamics of poloidal magnetic fields in neutron stars in general relativity assuming a polytropic equation of state. We identify familiar hydromagnetic modes, in particular the 'sausage/varicose' mode and 'kink' instability inherent to poloidal magnetic fields. The evolution is dominated by the kink instability, which causes a cataclysmic reconfiguration of the magnetic field. The system subsequently evolves to new, non-axisymmetric, quasi-equilibrium end-states. The existence of this branch of stable quasi-equilibria may have consequences for magnetar physics, including flare generation mechanisms and interpretations of quasi-periodic oscillations.
Political instability and illegal immigration.
Campos, J E; Lien, D
1995-01-01
"Economic theory suggests that transnational migration results from the push-pull effect of wage differentials between host and source countries. In this paper, we argue that political instability exacerbates the migration flow, with greater instability leading to relatively larger flows. We conclude then that an optimal solution to the illegal immigration problem requires proper coordination of immigration and foreign policies by the host country. A narrow preoccupation with tougher immigration laws is wasteful and may be marginally effective." Emphasis is on the United States as a host country.
Undulation Instability of Epithelial Tissues
Basan, Markus; Prost, Jacques; Risler, Thomas; 10.1103/PhysRevLett.106.158101
2011-01-01
Treating the epithelium as an incompressible fluid adjacent to a viscoelastic stroma, we find a novel hydrodynamic instability that leads to the formation of protrusions of the epithelium into the stroma. This instability is a candidate for epithelial fingering observed in vivo. It occurs for sufficiently large viscosity, cell-division rate and thickness of the dividing region in the epithelium. Our work provides physical insight into a potential mechanism by which interfaces between epithelia and stromas undulate, and potentially by which tissue dysplasia leads to cancerous invasion.
Wen, Baole
Buoyancy-driven convection in fluid-saturated porous media is a key environmental and technological process, with applications ranging from carbon dioxide storage in terrestrial aquifers to the design of compact heat exchangers. Porous medium convection is also a paradigm for forced-dissipative infinite-dimensional dynamical systems, exhibiting spatiotemporally chaotic dynamics if not "true" turbulence. The objective of this dissertation research is to quantitatively characterize the dynamics and heat transport in two-dimensional horizontal and inclined porous medium convection between isothermal plane parallel boundaries at asymptotically large values of the Rayleigh number Ra by investigating the emergent, quasi-coherent flow. This investigation employs a complement of direct numerical simulations (DNS), secondary stability and dynamical systems theory, and variational analysis. The DNS confirm the remarkable tendency for the interior flow to self-organize into closely-spaced columnar plumes at sufficiently large Ra (up to Ra ≃ 105), with more complex spatiotemporal features being confined to boundary layers near the heated and cooled walls. The relatively simple form of the interior flow motivates investigation of unstable steady and time-periodic convective states at large Ra as a function of the domain aspect ratio L. To gain insight into the development of spatiotemporally chaotic convection, the (secondary) stability of these fully nonlinear states to small-amplitude disturbances is investigated using a spatial Floquet analysis. The results indicate that there exist two distinct modes of instability at large Ra: a bulk instability mode and a wall instability mode. The former usually is excited by long-wavelength disturbances and is generally much weaker than the latter. DNS, strategically initialized to investigate the fully nonlinear evolution of the most dangerous secondary instability modes, suggest that the (long time) mean inter-plume spacing in
Convection in complex shaped vessel; Convection dans des enceintes de forme complexe
Energy Technology Data Exchange (ETDEWEB)
NONE
2000-07-01
The 8 november 2000, the SFT (Societe Francaise de Thermique) organized a technical day on the convection in complex shaped vessels. Nine papers have been presented in the domains of the heat transfers, the natural convection, the fluid distribution, the thermosyphon effect, the steam flow in a sterilization cycle and the transformers cooling. Eight papers are analyzed in ETDE and one paper dealing with the natural convection in spent fuels depository is analyzed in INIS. (A.L.B.)
NUMERICAL STUDY ON MIXED CONVECTIVE FLOW IN A SOLAR COLLECTOR
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
In a solar energy heat collector forced convection and free convection will occur concurrently. In this paper, the mixed convective flow was investigated. The dimensionless equation was derived and the results was verified by experiments. The numerical solution shows that error is less than 5% if the effect of free convection is ignored.
Secondary instabilities of linearly heated falling films
Institute of Scientific and Technical Information of China (English)
HU Jun; SUN Dejun; HU Guohui; YIN Xieyuan
2005-01-01
Secondary instabilities of linearly heated failing films are studied through three steps. Firstly, the analysis of the primary linear instability on Miladinova's long wave equation of the linearly heated film is performed. Secondly, the similar Landau equation is derived through weak nonlinear theory, and a two-dimensional nonlinear saturation solution of primary instability is obtained within the weak nonlinear domain. Thirdly, the secondary (three-dimensional) instability of the two-dimensional wave is studied by the Floquet theorem.Our secondary instability analysis shows that the Marangoni number has destabilization effect on the secondary instability.
1984-07-27
Coroniti Code 4187 (E. Szuszczewicz) Code 4187 (P. Rodriguez) Columbia University New York. New York 10027 University of ALaska ATTN: R. Taussig ...Llndman L. Thode 17 N NŔ I % % 4 TRW Space and Technology Group Temerin, Michael Space Science Dept. Space Science Lab. BuiLding R-1, Room 1170...and Astronomy Dr. Carl Fitz University of Minnesota MinneapoLis, MN 55455 Schulz, MichaeL Aerospace Corp. A6/2451, P.O. Box 92957 Los AngeLes
Scaling Anisotropy and Convective Instability of the Atmospheric Surface-Layer
Fitton, G. F.; Tchiguirinskaia, I.; Schertzer, D. J.; Lovejoy, S.
2013-12-01
In this study we use the scaling exponents, often called Hurst exponents, of the horizontal velocity and the temperature to classify the stability of the atmospheric surface-layer, including in the wake of a turbine. For this study we use two datasets for comparison. In the Growian experiment two 150m masts were constructed on coastal terrain with propeller anemometers positioned at the heights 10, 50, 75, 100, 125 and 150m measuring wind speed and direction. The measurements were taken at 2.5Hz over twenty-minutes with 300 measuring runs done in total. In addition, temperature was measured also at 2.5Hz over twenty minutes but only at the heights 10, 50, 100 and 150m. The second dataset consisted of three sonic anemometers positioned at 22, 23 and 43m on a single mast situated in a wind turbine test site in a mountainous part of Corsica France. The sonic anemometers measured three dimensional velocities and temperature at 10Hz over a period of six-months. The samples are separated into daily sub-samples, 180 in total. We find that the stability of the atmospheric surface-layer strongly depends on whether or not the temperature scales passively as the velocity. When the two scaling exponents remain of the same order, the scaling of both the velocity and temperature is consistent with surface-layer literature. However, when the scaling exponent of the temperature becomes larger than the scaling exponent of the velocity, the corresponding time-scales exhibit a strong, scaling anisotropy. To avoid shadow effects from masts, we are compelled to deal with samples whose `mean' velocity is near-perpendicular to the masts. The anisotropy of these samples turns out to be beyond a trivial component-wise anisotropy corresponding to pre-factors depending on the direction, i.e., the scaling exponents themselves (in particular the Hurst exponent) depend on the direction. We use a rotated frame of reference to better analyse this behaviour and put forward analytical expression of the angular variation of the Hurst exponent. We show that this scaling anisotropy has important consequences for first-order multifractal phase transitions; it decreases the critical order qD (the analog of the inverse of a critical temperature) at which the transitions occur, including that in the wake of a turbine. This decrease is so significant that these transitions may occur on a unique sample whereas usually they are expected only a very large number of samples since they correspond to a divergence of moments for an infinite number of samples. When these transitions happen, the stability of the atmospheric surface-layer cannot be captured with the help of Richardson number and the Obukhov-length whose statistics become wild.
Influence of Convective Momentum Transport on Tropical Waves
Zhou, L.
2012-12-01
Convective momentum transport (CMT) has been found to play an important role during the Madden-Julian Oscillation (MJO). Influences of CMT on tropical waves are analytically studied in a two-layer model, which captures the first-order baroclinic structure in the vertical. Since CMT is the momentum exchange between the lower and the upper troposphere during convection, the easterly and westerly vertical shears of background zonal winds lead to different CMT influences. Generally, CMT plays more important roles than a damping term to tropical waves. CMT is a critical factor for determining the meridional scale of tropical waves and leads to kinetic energy transfer against the direction of background wind shear in the vertical. CMT can also be favorable for internal instability and induce upscale momentum transfer. Specifically, due to CMT, the meridional scale in the two-layer model is wider than the Rossby radius of deformation (RL, the meridional scale of tropical waves in the classical theory) over the Indo-Pacific warm pool, but narrower than RL from the central to the eastern Pacific Ocean and over the Atlantic Ocean. Such variation is consistent with observations. CMT results in minor modifications to the speeds of Rossby waves, inertial gravity waves, and Kelvin waves. Nevertheless, CMT has significant influences on the mixed Rossby-gravity (MRG) waves, especially over the Indo-Pacific warm pool where the vertical wind shear in easterly. Westward propagating MRG waves with small wavenumber become unstable under the influence of CMT. The phase relation between the convergence and geopotential is no longer in quadrature, which is different from classical MRG waves. As a result, there is a net source of mechanical energy within one period and there is an upscale momentum transfer from the perturbed field to large scale velocities. This theoretical study sheds lights on the relation between CMT and slow variations in the atmosphere, including MJO.
Topology optimisation for natural convection problems
Alexandersen, Joe; Andreasen, Casper Schousboe; Sigmund, Ole
2014-01-01
This paper demonstrates the application of the density-based topology optimisation approach for the design of heat sinks and micropumps based on natural convection effects. The problems are modelled under the assumptions of steady-state laminar flow using the incompressible Navier-Stokes equations coupled to the convection-diffusion equation through the Boussinesq approximation. In order to facilitate topology optimisation, the Brinkman approach is taken to penalise velocities inside the solid domain and the effective thermal conductivity is interpolated in order to accommodate differences in thermal conductivity of the solid and fluid phases. The governing equations are discretised using stabilised finite elements and topology optimisation is performed for two different problems using discrete adjoint sensitivity analysis. The study shows that topology optimisation is a viable approach for designing heat sink geometries cooled by natural convection and micropumps powered by natural convection.
Convective Radio Occultations Final Campaign Summary
Energy Technology Data Exchange (ETDEWEB)
Biondi, R. [Atmospheric Radiation Measurement, Washington, DC (United States)
2016-03-01
Deep convective systems are destructive weather phenomena that annually cause many deaths and injuries as well as much damage, thereby accounting for major economic losses in several countries. The number and intensity of such phenomena have increased over the last decades in some areas of the globe. Damage is mostly caused by strong winds and heavy rain parameters that are strongly connected to the structure of the particular storm. Convection over land is usually stronger and deeper than over the ocean and some convective systems, known as supercells, also develop tornadoes through processes that remain mostly unclear. The intensity forecast and monitoring of convective systems is one of the major challenges for meteorology because in situ measurements during extreme events are too sparse or unreliable and most ongoing satellite missions do not provide suitable time/space coverage.
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.
Fingering Convection in Red Giants Revisited
Wachlin, F C; Althaus, L G
2014-01-01
Fingering (thermohaline) convection has been invoked for several years as a possible extra-mixing which could occur in Red Giant stars due to the modification of the chemical composition induced by nuclear reactions in the hydrogen burning zone. Recent studies show however that this mixing is not sufficient to account for the needed surface abundances. A new prescription for fingering convection, based on 3D numerical simulations has recently been proposed (BGS). The resulting mixing coefficient is larger than the ones previously given in the literature. We compute models using this new coefficient and compare them to previous studies. We use the LPCODE stellar evolution code with the GNA generalized version of the mixing length theory to compute Red Giant models and we introduce fingering convection using the BGS prescription. The results show that, although the fingering zone now reaches the outer dynamical convective zone, the efficiency of the mixing is not enough to account for the observations. The fing...
An Observational Investigation of Penetrative Convection
DEFF Research Database (Denmark)
Jensen, Niels Otto; Lenschow, D. H.
1978-01-01
Data taken during the Air Mass Transformation Experiment (AMTEX) by the NCAR Electra aircraft have proven useful for investigating the structure of thermals penetrating into the turbulent inversion layer which caps the convective mixed layer. Variances, covariances, spectra and cospectra...
Eye Formation in Rotating Convection
Oruba, L; Dormy, E
2016-01-01
We consider rotating convection in a shallow, cylindrical domain. We examine the conditions under which the resulting vortex develops an eye at its core; that is, a region where the poloidal flow reverses and the angular momentum is low. For simplicity, we restrict ourselves to steady, axisymmetric flows in a Boussinesq fluid. Our numerical experiments show that, in such systems, an eye forms as a passive response to the development of a so-called eyewall, a conical annulus of intense, negative azimuthal vorticity that can form near the axis and separates the eye from the primary vortex. We also observe that the vorticity in the eyewall comes from the lower boundary layer, and relies on the fact the poloidal flow strips negative vorticity out of the boundary layer and carries it up into the fluid above as it turns upward near the axis. This process is effective only if the Reynolds number is sufficiently high for the advection of vorticity to dominate over diffusion. Finally we observe that, in the vicinity o...
Eye formation in rotating convection
Oruba, L.; Davidson, P. A.; Dormy, E.
2017-02-01
We consider rotating convection in a shallow, cylindrical domain. We examine the conditions under which the resulting vortex develops an eye at its core; that is, a region where the poloidal flow reverses and the angular momentum is low. For simplicity, we restrict ourselves to steady, axisymmetric flows in a Boussinesq fluid. Our numerical experiments show that, in such systems, an eye forms as a passive response to the development of a so-called eyewall, a conical annulus of intense, negative azimuthal vorticity that can form near the axis and separates the eye from the primary vortex. We also observe that the vorticity in the eyewall comes from the lower boundary layer, and relies on the fact the poloidal flow strips negative vorticity out of the boundary layer and carries it up into the fluid above as it turns upward near the axis. This process is effective only if the Reynolds number is sufficiently high for the advection of vorticity to dominate over diffusion. Finally we observe that, in the vicinity of the eye and the eyewall, the buoyancy and Coriolis forces are negligible, and so although these forces are crucial to driving and shaping the primary vortex, they play no direct role in eye formation in a Boussinesq fluid.
Convective drying of sludge cake
Chen, Jianbo; Peng, Xiaofeng; Xue, Yuan; Lee, Duujong; Chu, Chingping
2002-08-01
This paper presented an experimental study on convective drying of waste water sludge collected from Beijing GaoBeiDian Sewage Treatment Plant, particularly on the correlation between the observed shrinkage dynamics of sludge cake and the drying curve. During the initial stage of drying the process resembles to that of a particulate bed, in which moisture diffuses and evaporates at the upper surface. Conventional drying theory assuming a diffusion-evaporating front interprets this period of drying. Consequently, owing to the very large shrinkage ratio of the dried cake, cracks emerges and propagates on and within the cake body, whence inducing evaporating channel that facilitates the water removal. This occurrence compensates the reduction of surface area for evaporation, whence extending the constant-rate period during the test. Afterwards, the cracks meet with each other and form isolated cake piles, while the subsequent drying occur mainly within these piles and the conventional theory fails. The transition between the drying on a plain cake layer and that on the isolated piles demonstrates the need to adopt distinct descriptions on these two regimes of drying for the sludge cake.
Convection in a vertical channel
Energy Technology Data Exchange (ETDEWEB)
Tisserand, J-C; Creyssels, M; Gibert, M; Castaing, B; Chilla, F, E-mail: Francesca.Chilla@ens-lyon.f [Universite de Lyon, ENS Lyon, UMR 5672 CNRS, 46 Allee d' Italie, 69364 Lyon Cedex 7 (France)
2010-07-15
The flow generated by heat convection in a long, vertical channel is studied by means of particle imagery velocimetry techniques, with the help of the thermal measurements from a previous paper (Gibert et al 2009 Phys. Fluids 21 035109). We analyse the mean velocity profiles and the Reynolds stresses, and compare the present results with the previous ones obtained in a larger cell and at a larger Reynolds number. We calculate the horizontal temperature profile and the related horizontal heat flux. The pertinence of effective turbulent diffusivity and viscosity is confirmed by the low value of the associated mixing length. We study the one-point and two-point statistics of both velocity components. We show how the concept of turbulent viscosity explains the relations between the local probability density functions (pdf) of fluctuations for temperature, vertical and horizontal velocity components. Despite the low Reynolds number values explored, some conclusions can be drawn about the small scale velocity differences and the related energy cascade.
Uncertainties in stellar evolution models: convective overshoot
Bressan, Alessandro; Marigo, Paola; Rosenfield, Philip; Tang, Jing
2014-01-01
In spite of the great effort made in the last decades to improve our understanding of stellar evolution, significant uncertainties remain due to our poor knowledge of some complex physical processes that require an empirical calibration, such as the efficiency of the interior mixing related to convective overshoot. Here we review the impact of convective overshoot on the evolution of stars during the main Hydrogen and Helium burning phases.
Uncertainties in Stellar Evolution Models: Convective Overshoot
Bressan, Alessandro; Girardi, Léo; Marigo, Paola; Rosenfield, Philip; Tang, Jing
In spite of the great effort made in the last decades to improve our understanding of stellar evolution, significant uncertainties remain due to our poor knowledge of some complex physical processes that require an empirical calibration, such as the efficiency of the interior mixing related to convective overshoot. Here we review the impact of convective overshoot on the evolution of stars during the main Hydrogen and Helium burning phases.
Convective Heat Transfer for Ship Propulsion.
1982-04-01
RD-A124 Wi CONVECTIVE HEAT TRANSFER FOR SHIP PROPULSION (U) ARIZONA 112 UNIV TUCSON ENGINEERING EXPERIMENT STATION PARK ET AL. 01 APR 82 1248-9 N814...395 CONVECTIVE HEAT TRANSFER FOR SHIP PROPULSION Prepared for Office of Naval Research Code 431 Arlington, Virginia Prepared by J. S. Park, M. F...FOR SHIP PROPULSION By J. S. Park, M. F. Taylor and D. M. McEligot Aerospace and Mechanical Engineering Department University of Arizona Tucson
Global instabilities and transient growth in Blasius boundary-layer flow over a compliant panel
Indian Academy of Sciences (India)
K Tsigklifis; A D Lucey
2015-05-01
We develop a hybrid of computational and theoretical approaches suited to study the fluid–structure interaction (FSI) of a compliant panel, flush between rigid upstream and downstream wall sections, with a Blasius boundary-layer flow. The ensuing linear-stability analysis is focused upon global instability and transient growth of disturbances. The flow solution is developed using a combination of vortex and source boundary-element sheets on a computational grid while the dynamics of a plate-spring compliant wall are couched in finite-difference form. The fully coupled FSI system is then written as an eigenvalue problem and the eigenvalues of the various flow- and wall-based instabilities are analysed. It is shown that coalescence or resonance of a structural eigenmode with either a flow-based Tollmien–Schlichting Wave (TSW) or wall-based travelling-wave flutter (TWF) modes can occur. This can render the nature of these well-known convective instabilities to become global for a finite compliant wall giving temporal growth of system disturbances. Finally, a non-modal analysis based on the linear superposition of the extracted temporal modes is presented. This reveals a high level of transient growth when the flow interacts with a compliant panel that has structural properties which render the FSI system prone to global instability. Thus, to design stable finite compliant panels for applications such as boundary-layer transition postponement, both global instabilities and transient growth must be taken into account.
Sensitivity of Tropical Cyclones to Parameterized Convection in the NASA GEOS5 Model
Lim, Young-Kwon; Schubert, Siegfried D.; Reale, Oreste; Lee, Myong-In; Molod, Andrea M.; Suarez, Max J.
2014-01-01
The sensitivity of tropical cyclones (TCs) to changes in parameterized convection is investigated to improve the simulation of TCs in the North Atlantic. Specifically, the impact of reducing the influence of the Relaxed Arakawa-Schubert (RAS) scheme-based parameterized convection is explored using the Goddard Earth Observing System version5 (GEOS5) model at 0.25 horizontal resolution. The years 2005 and 2006 characterized by very active and inactive hurricane seasons, respectively, are selected for simulation. A reduction in parameterized deep convection results in an increase in TC activity (e.g., TC number and longer life cycle) to more realistic levels compared to the baseline control configuration. The vertical and horizontal structure of the strongest simulated hurricane shows the maximum lower-level (850-950hPa) wind speed greater than 60 ms and the minimum sea level pressure reaching 940mb, corresponding to a category 4 hurricane - a category never achieved by the control configuration. The radius of the maximum wind of 50km, the location of the warm core exceeding 10 C, and the horizontal compactness of the hurricane center are all quite realistic without any negatively affecting the atmospheric mean state. This study reveals that an increase in the threshold of minimum entrainment suppresses parameterized deep convection by entraining more dry air into the typical plume. This leads to cooling and drying at the mid- to upper-troposphere, along with the positive latent heat flux and moistening in the lower-troposphere. The resulting increase in conditional instability provides an environment that is more conducive to TC vortex development and upward moisture flux convergence by dynamically resolved moist convection, thereby increasing TC activity.
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.
Seismic Sounding of Convection in the Sun
Hanasoge, Shravan; Sreenivasan, Katepalli R
2015-01-01
Our Sun, primarily composed of ionized hydrogen and helium, has a surface temperature of 5777~K and a radius $R_\\odot \\approx 696,000$ km. In the outer $R_\\odot/3$, energy transport is accomplished primarily by convection. Using typical convective velocities $u\\sim100\\,\\rm{m\\,s^{-1}}$ and kinematic viscosities of order $10^{-4}$ m$^{2}$s$^{-1}$, we obtain a Reynolds number $Re \\sim 10^{14}$. Convection is thus turbulent, causing a vast range of scales to be excited. The Prandtl number, $Pr$, of the convecting fluid is very low, of order $10^{-7}$\\,--\\,$10^{-4}$, so that the Rayleigh number ($\\sim Re^2 Pr$) is on the order of $10^{21}\\,-\\,10^{24}$. Solar convection thus lies in extraordinary regime of dynamical parameters, highly untypical of fluid flows on Earth. Convective processes in the Sun drive global fluid circulations and magnetic fields, which in turn affect its visible outer layers ("solar activity") and, more broadly, the heliosphere ("space weather"). The precise determination of the depth of sola...
Convection in Condensible-rich Atmospheres
Ding, F.; Pierrehumbert, R. T.
2016-05-01
Condensible substances are nearly ubiquitous in planetary atmospheres. For the most familiar case—water vapor in Earth’s present climate—the condensible gas is dilute, in the sense that its concentration is everywhere small relative to the noncondensible background gases. A wide variety of important planetary climate problems involve nondilute condensible substances. These include planets near or undergoing a water vapor runaway and planets near the outer edge of the conventional habitable zone, for which CO2 is the condensible. Standard representations of convection in climate models rely on several approximations appropriate only to the dilute limit, while nondilute convection differs in fundamental ways from dilute convection. In this paper, a simple parameterization of convection valid in the nondilute as well as dilute limits is derived and used to discuss the basic character of nondilute convection. The energy conservation properties of the scheme are discussed in detail and are verified in radiative-convective simulations. As a further illustration of the behavior of the scheme, results for a runaway greenhouse atmosphere for both steady instellation and seasonally varying instellation corresponding to a highly eccentric orbit are presented. The latter case illustrates that the high thermal inertia associated with latent heat in nondilute atmospheres can damp out the effects of even extreme seasonal forcing.
Convective overshoot at stiffly stable interfaces
Brown, Benjamin; Oishi, Jeffrey; Lecoanet, Daniel; Burns, Keaton; Vasil, Geoffrey
2016-11-01
Convective overshoot is an important non-local mixing and transport process in stars, extending the influence of turbulent stellar convection beyond the unstable portions of the atmosphere. In the Sun, overshoot into the tachocline at the base of the convection zone has been ascribed a major role in the storage and organization of the global-scale magnetic fields within the solar dynamo. In massive stars, overshooting convection plays an important role in setting the lifespan of the star by mixing fuel into the nuclear burning core. Here we narrowly consider the properties of convective overshoot across very stiff interfaces within fully compressible dynamics across convection zones with significant stratification. We conduct these studies using the Dedalus pseudospectral framework. We extend prior studies of overshoot substantially and find that the depth of overshoot in DNS simulations of a typical plume is well-predicted by a simple buoyancy equilibration model. The implications of this model, extended into the stellar regime, are that very little overshoot should occur under solar conditions. This would seem to sharply limit the role of the tachocline within the global solar dynamo.
Edge instabilities of topological superconductors
Hofmann, Johannes S.; Assaad, Fakher F.; Schnyder, Andreas P.
2016-05-01
Nodal topological superconductors display zero-energy Majorana flat bands at generic edges. The flatness of these edge bands, which is protected by time-reversal and translation symmetry, gives rise to an extensive ground-state degeneracy. Therefore, even arbitrarily weak interactions lead to an instability of the flat-band edge states towards time-reversal and translation-symmetry-broken phases, which lift the ground-state degeneracy. We examine the instabilities of the flat-band edge states of dx y-wave superconductors by performing a mean-field analysis in the Majorana basis of the edge states. The leading instabilities are Majorana mass terms, which correspond to coherent superpositions of particle-particle and particle-hole channels in the fermionic language. We find that attractive interactions induce three different mass terms. One is a coherent superposition of imaginary s -wave pairing and current order, and another combines a charge-density-wave and finite-momentum singlet pairing. Repulsive interactions, on the other hand, lead to ferromagnetism together with spin-triplet pairing at the edge. Our quantum Monte Carlo simulations confirm these findings and demonstrate that these instabilities occur even in the presence of strong quantum fluctuations. We discuss the implications of our results for experiments on cuprate high-temperature superconductors.
Lending sociodynamics and economic instability
Hawkins, Raymond J.
2011-11-01
We show how the dynamics of economic instability and financial crises articulated by Keynes in the General Theory and developed by Minsky as the Financial Instability Hypothesis can be formalized using Weidlich’s sociodynamics of opinion formation. The model addresses both the lending sentiment of a lender in isolation as well as the impact on that lending sentiment of the behavior of other lenders. The risk associated with lending is incorporated through a stochastic treatment of loan dynamics that treats prepayment and default as competing risks. With this model we are able to generate endogenously the rapid changes in lending opinion that attend slow changes in lending profitability and find these dynamics to be consistent with the rise and collapse of the non-Agency mortgage-backed securities market in 2007/2008. As the parameters of this model correspond to well-known phenomena in cognitive and social psychology, we can both explain why economic instability has proved robust to advances in risk measurement and suggest how policy for reducing economic instability might be formulated in an experimentally sound manner.
The Chemistry of Beer Instability
Stewart, Graham G.
2004-01-01
Brewing of beer, one of the oldest biotechnology industries was one of the earliest processes to be undertaken on commercial basis. Biological instability involves contamination of bacteria, yeast, or mycelia fungi and there is always a risk in brewing that beer can become contaminated by micro-organisms.
Size effects on cavitation instabilities
DEFF Research Database (Denmark)
Niordson, Christian Frithiof; Tvergaard, Viggo
2006-01-01
In metal-ceramic systems the constraint on plastic flow leads to so high stress triaxialities that cavitation instabilities may occur. If the void radius is on the order of magnitude of a characteristic length for the metal, the rate of void growth is reduced, and the possibility of unstable cavi...... as the void grows to a size well above the characteristic material length....
Waves and instabilities in plasmas
Chen Liu
1987-01-01
The topics covered in these notes are selective and tend to emphasize more on kinetic-theory approaches to waves and instabilities in both uniform and non-uniform plasmas, students are assumed to have some basic knowledge of plasma dynamics in terms of single-particle and fluid descriptions.
GENETIC INSTABILITY IN CERVICAL CARCINOMA
Institute of Scientific and Technical Information of China (English)
赵旻; 伍欣星; 邱小萍; 李晖; 戴天力; 谭云
2002-01-01
Objective: The role of human papillomavirus (HPV) in the development of cervical carcinoma has been clearly established but other factors could be involved in cervical tumorigenesis such as loss of heterozygosity (LOH) and microsatellite instability (MI). The aim of the present study was to investigate the genetic instability in cervical carcinoma tissues and provide evidence for discoveringnew tumor suppressor genes and screening diagnostic molecular marker of cervical carcinoma. Methods: Fifty primary cervical carcinoma samples from high-incidence area were analyzed by PCR for HPV16 infection, LOH and microsatellite instability. Results: HPV16 was detected in 88% of the cases. Sixty-six percent of total cases showed LOH with no more than 3 different loci per case. The highest frequency of the allelic loss was found in D18S474 (18q21, 40.5%). MI was detected in 4 cases (8%) only. Conclusion: Different percentages of LOH on specific chromosomal regions were found and MI was very infrequent in cervical carcinoma. The putative suppressor gene(s) could be located on specific chromosome regions such as 18q, and genetic instability could be involved in cervical tumorigenesis.
Archimedean Proof of the Physical Impossibility of Earth Mantle Convection
Herndon, J Marvin
2010-01-01
Eight decades ago, Arthur Holmes introducted the idea of mantle convection as a mechanism for continental drift. Five decades ago, continental drift was modified to become plate tectonics theory, which included mantle convection as an absolutely critical component. Using the submarine design and operation concept of "neutral buoyancy", which follows from Archimedes' discoveries, the concept of mantle convection is proven to be incorrect, concomitantly refuting plate tectonics, refuting all mantle convection models, and refuting all models that depend upon mantle convection.
Energy Technology Data Exchange (ETDEWEB)
Horiuchi, Ritoku; Sato, Tetsuya [Theory and Computer Simulation Center, National Inst. for Fusion Science, Toki, Gifu (Japan)
1999-06-01
Generation of anomalous resistivity and dynamical development of collisionless reconnection in the vicinity of a magnetically neutral sheet are investigated by means of a three-dimensional particle simulation. For no external driving source, two different types of plasma instabilities are excited in the current layer. The lower hybrid drift instability (LHDI) is observed to grow in the periphery of current layer in an early period, while a drift kink instability (DKI) is triggered at the neutral sheet in a late period as a result of the nonlinear deformation of the current sheet by the LHDI. A reconnection electric field grows at the neutral sheet in accordance with the excitation of the DKI. When an external driving field exists, the convective electric field penetrates into the current layer through the particle kinetic effect and collisionless reconnection is triggered by the convective electric field earlier than the DKI is excited. It is also found that the anisotropic ion distribution is formed through the anomalous ion heating by the DKI. (author)
Bigelbach, B. C.; Mullendore, G. L.; Starzec, M.
2014-10-01
We utilize the Weather Research and Forecasting (WRF) model with chemistry to simulate mass transport during the 2007 convective season in the U.S. Southern Great Plains at convection-allowing scale. Resolved storms are classified using an object-based classification scheme. This scheme uses model-derived radar reflectivity to classify storm type as quasi-isolated strong convection (QISC) or mesoscale convective system (MCS). Differences between QISCs and MCSs are investigated by analysis of two transport parameters for each convective object: the level of maximum detrainment (LMD) and the detrainment mass flux. Analysis of the mean LMD showed differences between the two regimes is statistically significantly different in May, as the mean QISC LMD is 440 m higher than the mean MCS LMD in May, and statistically insignificant in July where the mean QISC LMD is only 350 m higher. The detrainment flux per deeply convective object showed statistically significant differences between the two regimes in both May (MCS 4.8 times greater than QISC) and July (MCS 6.8 times greater than QISC). Over the entire study period, MCS storms accounted for 72% of the total mass detrainment, even though QISCs were twice as common as MCSs. However, differences in the detrainment flux per unit area of deep convection showed that QISCs exhibited stronger flux (1.1 times greater) than MCSs in both months. Analysis of tropopause-relative LMDs showed that QISCs detrained the maximum amount of mass closer to the tropopause altitude than MCSs for both months. However, only in May is the difference statistically significant (430 m closer).
Directory of Open Access Journals (Sweden)
Chand Ramesh
2015-12-01
Full Text Available Thermal instability in a horizontal layer of Oldroydian visco-elastic fluid in a porous medium is investigated. For porous medium the Brinkman–Darcy model is considered. A linear stability analysis based upon perturbation method and normal mode technique is used to find solution of the fluid layer confined between two free-free boundaries. The onset criterion for stationary and oscillatory convection is derived analytically. The influence of the Brinkman–Darcy, Prandtl–Darcy number, stress relaxation parameter on the stationary and oscillatory convection is studied both analytically and graphically. The sufficient condition for the validity of PES has also been derived.
Singlet and triplet instability theorems
Yamada, Tomonori; Hirata, So
2015-09-01
A useful definition of orbital degeneracy—form-degeneracy—is introduced, which is distinct from the usual energy-degeneracy: Two canonical spatial orbitals are form-degenerate when the energy expectation value in the restricted Hartree-Fock (RHF) wave function is unaltered upon a two-electron excitation from one of these orbitals to the other. Form-degenerate orbitals tend to have isomorphic electron densities and occur in the highest-occupied and lowest-unoccupied molecular orbitals (HOMOs and LUMOs) of strongly correlated systems. Here, we present a mathematical proof of the existence of a triplet instability in a real or complex RHF wave function of a finite system in the space of real or complex unrestricted Hartree-Fock wave functions when HOMO and LUMO are energy- or form-degenerate. We also show that a singlet instability always exists in a real RHF wave function of a finite system in the space of complex RHF wave functions, when HOMO and LUMO are form-degenerate, but have nonidentical electron densities, or are energy-degenerate. These theorems provide Hartree-Fock-theory-based explanations of Hund's rule, a singlet instability in Jahn-Teller systems, biradicaloid electronic structures, and a triplet instability during some covalent bond breaking. They also suggest (but not guarantee) the spontaneous formation of a spin density wave (SDW) in a metallic solid. The stability theory underlying these theorems extended to a continuous orbital-energy spectrum proves the existence of an oscillating (nonspiral) SDW instability in one- and three-dimensional homogeneous electron gases, but only at low densities or for strong interactions.
Entropy Production in Convective Hydrothermal Systems
Boersing, Nele; Wellmann, Florian; Niederau, Jan
2016-04-01
Exploring hydrothermal reservoirs requires reliable estimates of subsurface temperatures to delineate favorable locations of boreholes. It is therefore of fundamental and practical importance to understand the thermodynamic behavior of the system in order to predict its performance with numerical studies. To this end, the thermodynamic measure of entropy production is considered as a useful abstraction tool to characterize the convective state of a system since it accounts for dissipative heat processes and gives insight into the system's average behavior in a statistical sense. Solving the underlying conservation principles of a convective hydrothermal system is sensitive to initial conditions and boundary conditions which in turn are prone to uncertain knowledge in subsurface parameters. There exist multiple numerical solutions to the mathematical description of a convective system and the prediction becomes even more challenging as the vigor of convection increases. Thus, the variety of possible modes contained in such highly non-linear problems needs to be quantified. A synthetic study is carried out to simulate fluid flow and heat transfer in a finite porous layer heated from below. Various two-dimensional models are created such that their corresponding Rayleigh numbers lie in a range from the sub-critical linear to the supercritical non-linear regime, that is purely conductive to convection-dominated systems. Entropy production is found to describe the transient evolution of convective processes fairly well and can be used to identify thermodynamic equilibrium. Additionally, varying the aspect ratio for each Rayleigh number shows that the variety of realized convection modes increases with both larger aspect ratio and higher Rayleigh number. This phenomenon is also reflected by an enlarged spread of entropy production for the realized modes. Consequently, the Rayleigh number can be correlated to the magnitude of entropy production. In cases of moderate
Theory of Collisional Two-Stream Plasma Instabilities in the Solar Chromosphere
Madsen, Chad Allen; Dimant, Yakov; Oppenheim, Meers; Fontenla, Juan
2014-06-01
The solar chromosphere experiences intense heating just above its temperature minimum. The heating increases the electron temperature in this region by over 2000 K. Furthermore, it exhibits little time variation and appears widespread across the solar disk. Although semi-empirical models, UV continuum observations, and line emission measurements confirm the existence of the heating, its source remains unexplained. Potential heating sources such as acoustic shocks, resistive dissipation, and magnetic reconnection via nanoflares fail to account for the intensity, persistence, and ubiquity of the heating. Fontenla (2005) suggested turbulence from a collisional two-stream plasma instability known as the Farley-Buneman instability (FBI) could contribute significantly to the heating. This instability is known to heat the plasma of the E-region ionosphere which bears many similarities to the chromospheric plasma. However, the ionospheric theory of the FBI does not account for the diverse ion species found in the solar chromosphere. This work develops a new collisional, two-stream instability theory appropriate for the chromospheric plasma environment using a linear fluid analysis to derive a new dispersion relationship and critical E x B drift velocity required to trigger the instability. Using a 1D, non-local thermodynamic equilibrium, radiative transfer model and careful estimates of collision rates and magnetic field strengths, we calculate the trigger velocities necessary to induce the instability throughout the chromosphere. Trigger velocities as low as 4 km s^-1 are found near the temperature minimum, well below the local neutral acoustic speed in that region. From this, we expect the instability to occur frequently, converting kinetic energy contained in neutral convective flows from the photosphere into thermal energy via turbulence. This could contribute significantly to chromospheric heating and explain its persistent and ubiquitous nature.
Deal, E.; Carazzo, G.; Jellinek, M.
2013-12-01
The longevity of volcanic ash clouds generated by explosive volcanic plumes is difficult to predict. Diffusive convective instabilities leading to the production of internal layering are known to affect the stability and longevity of these clouds, but the detailed mechanisms controlling particle dynamics and sedimentation are poorly understood. We present results from a series of analog experiments reproducing diffusive convection in a 2D (Hele-Shaw) geometry, which allow us to constrain conditions for layer formation, sedimentation regime and cloud residence time as a function of only the source conditions. We inject a turbulent particle-laden jet sideways into a tank containing a basal layer of salt water and an upper layer of fresh water, which ultimately spreads as a gravity current. After the injection is stopped, particles in suspension settle through the cloud to form particle boundary layers (PBL) at the cloud base. We vary the initial particle concentration of the plume and the injection velocity over a wide range of conditions to identify and characterize distinct regimes of sedimentation. Our experiments show that convective instabilities driven as a result of differing diffusivities of salt and particles lead to periodic layering over a wide range of conditions expected in nature. The flux of particles from layered clouds and the thicknesses of the layers are understood using classical theory for double diffusive convection adjusted for the hydrodynamic diffusion of particles. Although diffusive convection increases sedimentation rates for the smallest particles (<30 μm) its overall effect is to extend the cloud residence time to several hours by maintaining larger particles in suspension within the layers, which is several orders of magnitude longer than expected when considering individual settling rates.
Effect of wave localization on plasma instabilities. Ph. D. Thesis
Energy Technology Data Exchange (ETDEWEB)
Levedahl, W.K.
1987-10-01
The Anderson model of wave localization in random media is involved to study the effect of solar wind density turbulence on plasma processes associated with the solar type III radio burst. ISEE-3 satellite data indicate that a possible model for the type III process is the parametric decay of Langmuir waves excited by solar flare electron streams into daughter electromagnetic and ion acoustic waves. The threshold for this instability, however, is much higher than observed Langmuir wave levels because of rapid wave convection of the transverse electromagnetic daughter wave in the case where the solar wind is assumed homogeneous. Langmuir and transverse waves near critical density satisfy the Ioffe-Reigel criteria for wave localization in the solar wind with observed density fluctuations -1 percent. Numerical simulations of wave propagation in random media confirm the localization length predictions of Escande and Souillard for stationary density fluctations. For mobile density fluctuations localized wave packets spread at the propagation velocity of the density fluctuations rather than the group velocity of the waves. Computer simulations using a linearized hybrid code show that an electron beam will excite localized Langmuir waves in a plasma with density turbulence. An action principle approach is used to develop a theory of non-linear wave processes when waves are localized. A theory of resonant particles diffusion by localized waves is developed to explain the saturation of the beam-plasma instability. It is argued that localization of electromagnetic waves will allow the instability threshold to be exceeded for the parametric decay discussed above.
Preparing for an explosion: Hydrodynamic instabilities and turbulence in presupernovae
Energy Technology Data Exchange (ETDEWEB)
Smith, Nathan; Arnett, W. David, E-mail: nathans@as.arizona.edu, E-mail: darnett@as.arizona.edu [Steward Observatory, University of Arizona, Tucson, AZ 85721 (United States)
2014-04-20
Both observations and numerical simulations are discordant with predictions of conventional stellar evolution codes for the latest stages of a massive star's life before core collapse. The most dramatic example of this disconnect is in the eruptive mass loss occurring in the decade preceding Type IIn supernovae. We outline the key empirical evidence that indicates severe pre-supernova instability in massive stars, and we suggest that the chief reason that these outbursts are absent in stellar evolution models may lie in the treatment of turbulent convection in these codes. The mixing length theory that is used ignores (1) finite amplitude fluctuations in velocity and temperature and (2) their nonlinear interaction with nuclear burning. Including these fluctuations is likely to give rise to hydrodynamic instabilities in the latest burning sequences, which prompts us to discuss a number of far-reaching implications for the fates of massive stars. In particular, we explore connections to enhanced pre-supernova mass loss, unsteady nuclear burning and consequent eruptions, swelling of the stellar radius that may trigger violent interactions with a companion star, and potential modifications to the core structure that could dramatically alter calculations of the core-collapse explosion mechanism itself. These modifications may also impact detailed nucleosynthesis and measured isotopic anomalies in meteorites, as well as the interpretation of young core-collapse supernova remnants. Understanding these critical instabilities in the final stages of evolution may make possible the development of an early warning system for impending core collapse, if we can identify their asteroseismological or eruptive signatures.
Experimental study of absolute instability over a rotating disk
Othman Bekhit, Hesham Abdel Ghafar
2005-07-01
A series of experiments were performed to study the absolute instability of Type I traveling cross-flow modes in the boundary layer on a smooth disk rotating at constant speed. The basic flow agreed with analytic theory, and the growth of natural disturbances matched linear theory predictions. Controlled temporal disturbances were introduced by a short-duration air pulse from a hypodermic tube located above the disk and outside the boundary layer. The air pulse was positioned just outboard of the critical radius for Type I cross-flow modes. A hot-wire sensor primarily sensitive to the azirnuthal velocity component, was positioned at different spatial locations on the disk to document the growth of disturbances produced by the air pulses. Ensemble averages conditioned on the air pulses revealed wave packets that evolved in time and space. Two amplitudes of air pulses were used. The lower amplitude produced wave packets with linear amplitude characteristics that agreed with linear-theory wall-normal eigenfunction distributions and spatial growth rates. The higher amplitude pulse produced wave packets that had nonlinear amplitude characteristics. The space-time evolution of the leading and trailing edges of the wave packets were followed well past the critical radius for the absolute instability based on Lingwood (1995). With the linear amplitudes, the absolute instability was dominated by the convective modes, agreeing with the linear DNS simulations of Davies and Carpenter (2003). With the nonlinear amplitudes, larger temporal growth of the wave packets existed which supports the finite amplitude analysis of Pier (2003), and more closely resembles the wave packet evolution in the experimental study of Lingwood (1996). This suggests that the disturbance levels in the experiment that was intended to demonstrate the linear analysis, were likely fuite.
Observation of Parametric Instability in Advanced LIGO
Evans, Matthew; Fritschel, Peter; Miller, John; Barsotti, Lisa; Martynov, Denis; Brooks, Aidan; Coyne, Dennis; Abbott, Rich; Adhikari, Rana; Arai, Koji; Bork, Rolf; Kells, Bill; Rollins, Jameson; Smith-Lefebvre, Nicolas; Vajente, Gabriele; Yamamoto, Hiroaki; Derosa, Ryan; Effler, Anamaria; Kokeyama, Keiko; Betzweiser, Joseph; Frolov, Valera; Mullavey, Adam; O`Reilly, Brian; Dwyer, Sheila; Izumi, Kiwamu; Kawabe, Keita; Landry, Michael; Sigg, Daniel; Ballmer, Stefan; Massinger, Thomas J; Staley, Alexa; Mueller, Chris; Grote, Hartmut; Ward, Robert; King, Eleanor; Blair, David; Ju, Li; Zhao, Chunnong
2015-01-01
Parametric instabilities have long been studied as a potentially limiting effect in high-power interferometric gravitational wave detectors. Until now, however, these instabilities have never been observed in a kilometer-scale interferometer. In this work we describe the first observation of parametric instability in an Advanced LIGO detector, and the means by which it has been removed as a barrier to progress.
Amplitude Equation for Instabilities Driven at Deformable Surfaces - Rosensweig Instability
Pleiner, Harald; Bohlius, Stefan; Brand, Helmut R.
2008-11-01
The derivation of amplitude equations from basic hydro-, magneto-, or electrodynamic equations requires the knowledge of the set of adjoint linear eigenvectors. This poses a particular problem for the case of a free and deformable surface, where the adjoint boundary conditions are generally non-trivial. In addition, when the driving force acts on the system via the deformable surface, not only Fredholm's alternative in the bulk, but also the proper boundary conditions are required to get amplitude equations. This is explained and demonstrated for the normal field (or Rosensweig) instability in ferrofluids as well as in ferrogels. An important aspect of the problem is its intrinsic dynamic nature, although at the end the instability is stationary. The resulting amplitude equation contains cubic and quadratic nonlinearities as well as first and (in the gel case) second order time derivatives. Spatial variations of the amplitudes cannot be obtained by using simply Newell's method in the bulk.
Periodic marangoni instability in surfactant (CTAB) liquid/liquid mass transfer.
Lavabre, Dominique; Pradines, Vincent; Micheau, Jean-Claude; Pimienta, Véronique
2005-04-21
Periodic Marangoni convective instability has been observed in a biphasic system during the mass transfer of cetyltrimethylammonium bromide (CTAB) from an aqueous to a dichloromethane organic phase. Visualization of the convective fluxes was possible thanks to the CTAB crystals that are formed in the aqueous phase at a temperature below the Krafft point. Surface tension and electrical potential oscillations have been shown to be correlated with the fluid motion. Surface tension measurements, representative of the adsorption state, showed fast adsorption during the convective stage, followed by a slower desorption process in the quiet stage. To account for the electrical potential data, two components need to be taken into account. In the quiet stage, the signal was comparable to surface tension, and the main contribution would result from the electrical double layer formed at the interface by charged surfactants. In the convective stage, the electrical potential was furthermore related to the velocity of the fluid in the aqueous layer. Perturbations of the charge distribution in the Gouy-Chapman layer due to tangential flows could be at the origin of the phenomenon.
Directory of Open Access Journals (Sweden)
Masanori Yamasaki
2011-01-01
Full Text Available This paper describes results from numerical experiments which have been performed as the author's first step toward a better understanding of the Madden-Julian oscillation (MJO. This study uses the author's mesoscale-convection-resolving model that was developed in the 1980s to improve parametrization schemes of moist convection. Results from numerical experiments by changing the SST anomaly in the warm pool area indicate that the period of the MJO does not monotonously change with increasing SST anomaly. Between the two extreme cases (no anomaly and strong anomaly, there is a regime in which the period varies in a wide range from 20 to 60 days. In the case of no warm pool, eastward-propagating Kelvin waves are dominant, whereas in the case of a strong warm pool, it produces a quasi-stationary convective system (with pronounced time variation. In a certain regime between the two extreme cases, convective activities with two different properties are strongly interacted, and the period of oscillations becomes complicated. The properties and behaviors of large-scale convective system (LCS, synoptic-scale convective system (SCS, mesoscale convective system (MCS, and mesoscale convection (MC, which constitute the hierarchical structure of the MJO, are also examined. It is also shown that cloud clusters, which constitute the SCS (such as super cloud cluster SCC, consist of a few MCS, and a new MCS forms to the west of the existing MCS. The northwesterly and southwesterly low-level flows contribute to this feature. In view of recent emphasis of the importance of the relative humidity above the boundary layer, it is shown that the model can simulate convective processes that moisten the atmosphere, and the importance of latent instability (positive CAPE, which is a necessary condition for the wave-CISK, is emphasized.
Dawn-dusk asymmetry in the Kelvin-Helmholtz instability at Mercury.
Paral, Jan; Rankin, Robert
2013-01-01
The NASA MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft entered orbital phase around Mercury on 18 March 2011. A surprising consistent feature in the data returned is large-scale vortices that form exclusively on the dusk side of the magnetosphere. Here we present global kinetic hybrid simulations that explain these observations. It is shown that vortices are excited by a Kelvin-Helmholtz instability near the subsolar point, which grows convectively along the dusk-side magnetopause. Virtual time series along a track approximating a flyby of the MESSENGER show correspondence with the satellite data; the data contain sawtooth oscillations in plasma density, flow and magnetic field, and exhibit the observed dawn-dusk asymmetry. It is shown that asymmetry between dawn and dusk at Mercury is controlled by the finite gyroradius of ions and by convection electric fields. Mercury's magnetosphere offers a natural laboratory for studying plasma regimes not present in other planetary magnetospheres or the laboratory.
Deciphering Core Collapse Supernovae Is Convection the Key?; 1, prompt convection
Mezzacappa, A; Bruenn, S W; Blondin, J M; Guidry, M W; Strayer, M R; Umar, A S
1996-01-01
We couple two-dimensional hydrodynamics to detailed one-dimensional multigroup flux-limited diffusion neutrino transport to investigate prompt convection in core collapse supernovae. Our initial conditions, time-dependent boundary conditions, and neutrino distributions for computing neutrino heating, cooling, and deleptonization rates are obtained from one-dimensional simulations that implement multigroup flux-limited diffusion neutrino transport and one-dimensional hydrodynamics. The development and evolution of prompt convection and its ramifications for the shock dynamics are investigated for both 15 and 25 solar mass models, representative of the two classes of stars with compact and extended iron cores, respectively. In the absence of neutrino transport, prompt convection develops and dissipates on a time scale $\\sim$15 ms for both models. Prompt convection seeds convection behind the shock, which causes distortions in the shock's sphericity, but on the average, the shock radius is not boosted significan...
Stability on time-dependent domains: convective and dilution effects
Krechetnikov, R.; Knobloch, E.
2017-03-01
We explore near-critical behavior of spatially extended systems on time-dependent spatial domains with convective and dilution effects due to domain flow. As a paradigm, we use the Swift-Hohenberg equation, which is the simplest nonlinear model with a non-zero critical wavenumber, to study dynamic pattern formation on time-dependent domains. A universal amplitude equation governing weakly nonlinear evolution of patterns on time-dependent domains is derived and proves to be a generalization of the standard Ginzburg-Landau equation. Its key solutions identified here demonstrate a substantial variety-spatially periodic states with a time-dependent wavenumber, steady spatially non-periodic states, and pulse-train solutions-in contrast to extended systems on time-fixed domains. The effects of domain flow, such as bifurcation delay due to domain growth and destabilization due to oscillatory domain flow, on the Eckhaus instability responsible for phase slips in spatially periodic states are analyzed with the help of both local and global stability analyses. A nonlinear phase equation describing the approach to a phase-slip event is derived. Detailed analysis of a phase slip using multiple time scale methods demonstrates different mechanisms governing the wavelength changing process at different stages.
A Multiple-Scale Analysis of Evaporation Induced Marangoni Convection
Hennessy, Matthew G.
2013-04-23
This paper considers the stability of thin liquid layers of binary mixtures of a volatile (solvent) species and a nonvolatile (polymer) species. Evaporation leads to a depletion of the solvent near the liquid surface. If surface tension increases for lower solvent concentrations, sufficiently strong compositional gradients can lead to Bénard-Marangoni-type convection that is similar to the kind which is observed in films that are heated from below. The onset of the instability is investigated by a linear stability analysis. Due to evaporation, the base state is time dependent, thus leading to a nonautonomous linearized system which impedes the use of normal modes. However, the time scale for the solvent loss due to evaporation is typically long compared to the diffusive time scale, so a systematic multiple scales expansion can be sought for a finite-dimensional approximation of the linearized problem. This is determined to leading and to next order. The corrections indicate that the validity of the expansion does not depend on the magnitude of the individual eigenvalues of the linear operator, but it requires these eigenvalues to be well separated. The approximations are applied to analyze experiments by Bassou and Rharbi with polystyrene/toluene mixtures [Langmuir, 25 (2009), pp. 624-632]. © 2013 Society for Industrial and Applied Mathematics.
Convective Regimes in Crystallizing Basaltic Magma Chambers
Gilbert, A. J.; Neufeld, J. A.; Holness, M. B.
2015-12-01
Cooling through the chamber walls drives crystallisation in crustal magma chambers, resulting in a cumulate pile on the floor and mushy regions at the walls and roof. The liquid in many magma chambers, either the bulk magma or the interstitial liquid in the mushy regions, may convect, driven either thermally, due to cooling, or compositionally, due to fractional crystallization. We have constructed a regime diagram of the possible convective modes in a system containing a basal mushy layer. These modes depend on the large-scale buoyancy forcing characterised by a global Rayleigh number and the proportion of the chamber height constituting the basal mushy region. We have tested this regime diagram using an analogue experimental system composed of a fluid layer overlying a pile of almost neutrally buoyant inert particles. Convection in this system is driven thermally, simulating magma convection above and within a porous cumulate pile. We observe a range of possible convective regimes, enabling us to produce a regime diagram. In addition to modes characterised by convection of the bulk and interstitial fluid, we also observe a series of regimes where the crystal pile is mobilised by fluid motions. These regimes feature saltation and scouring of the crystal pile by convection in the bulk fluid at moderate Rayleigh numbers, and large crystal-rich fountains at high Rayleigh numbers. For even larger Rayleigh numbers the entire crystal pile is mobilised in what we call the snowglobe regime. The observed mobilisation regimes may be applicable to basaltic magma chambers. Plagioclase in basal cumulates crystallised from a dense magma may be a result of crystal mobilisation from a plagioclase-rich roof mush. Compositional convection within such a mush could result in disaggregation, enabling the buoyant plagioclase to be entrained in relatively dense descending liquid plumes and brought to the floor. The phenocryst load in porphyritic lavas is often interpreted as a
Competing structural instabilities in cubic perovskites
Vanderbilt, D
1994-01-01
We study the antiferrodistortive instability and its interaction with ferroelectricity in cubic perovskite compounds. Our first-principles calculations show that coexistence of both instabilities is very common. We develop a first-principles scheme to study the thermodynamics of these compounds when both instabilities are present, and apply it to SrTiO$_3$. We find that increased pressure enhances the antiferrodistortive instability while suppressing the ferroelectric one. Moreover, the presence of one instability tends to suppress the other. A very rich $P$--$T$ phase diagram results.
An Interface Stretching-Diffusion Model for Mixing-Limited Reactions During Convective Mixing
Hidalgo, J. J.; Dentz, M.; Cabeza, Y.; Carrera, J.
2014-12-01
We study the behavior of mixing-limited dissolution reactions under the unstable flow conditions caused by a Rayleigh-Bénard convective instability in a two fluids system. The reactions produce a dissolution pattern that follows the ascending fluids's interface where the largest concentration gradients and maximum mixing are found. Contrary to other chemical systems, the mixing history engraved by the dissolution does not map out the fingering geometry of the unstable flow. The temporal scaling of the mixing Χ and the reaction rate r are explained by a stretching-diffusion model of the interface between the fluids. The model accurately reproduces the three observed regimes: a diffusive regime at which Χ, r ~ t-1/2; a convective regime of at which the interface contracts to the Batchelor scale resulting in a constant Χf and r independent of the Rayleigh number; and an attenuated convection regime in which Χ and r decay faster than diffusion as t-3/2 and t-1, respectevely, because of the decompression of the interface and weakened reactions caused by the accumulation of dissolved fluid below the interface.
Köllner, Thomas; Rossi, Maurice; Broer, Frauke; Boeck, Thomas
2014-11-01
A case of convection driven by chemical reactions is studied by linear stability theory and direct numerical simulations. In a plane aqueous layer of glucose, the methylene-blue-enabled catalytic oxidation of glucose produces heavier gluconic acid. As the oxygen is supplied through the top surface, the production of gluconic acid leads to an overturning instability. Our results complement earlier experimental and numerical work by Pons et al. First, we extend the model by including the top air layer with diffusive transport and Henry's law for the oxygen concentration at the interface to provide a more realistic oxygen boundary condition. Second, a linear stability analysis of the diffusive basic state in the layers is performed using an optimal perturbation approach. This method is appropriate for the unsteady basic state and determines the onset time of convection and the associated wavelength. Third, the nonlinear evolution is studied by the use of three-dimensional numerical simulations. Three typical parameters sets are explored in detail showing significant differences in pattern formation. One parameter set for which the flow is dominated by viscous forces, displays persistently growing convection cells. The other set with increased reaction rate displays a different flow regime marked by local chaotic plume emission. The simulated patterns are then compared to experimental observations.
Multi-Dimensional Radiation/Hydrodynamic Simulations of Protoneutron Star Convection
Dessart, L; Livne, E; Ott, C D
2005-01-01
Based on multi-dimensional multi-group radiation hydrodynamic simulations of core-collapse supernovae with the VULCAN/2D code, we study the physical conditions within and in the vicinity of the nascent protoneutron star (PNS). Conclusions of this work are threefold: First, as before, we do not see any large-scale overturn of the inner PNS material. Second, we see no evidence of doubly-diffusive instabilities in the PNS, expected to operate on diffusion timescales of at least a second, but instead observe the presence of convection, within a radius range of 10-20 km, operating with a timescale of a few milliseconds. Third, we identify unambiguously the presence of gravity waves, predominantly at 200-300 ms past core bounce, in the region separating the convective zones inside the PNS and between the PNS surface and the shocked region. PNS convection is always confined to a region between 10 and 20 km, i.e., within the neutrinospheric radii for all neutrino energies above just a few MeV. We find that such motio...
Constraints on plate tectonics initiation from scaling laws for single-cell convection
Wong, Teresa; Solomatov, Viatcheslav S.
2016-08-01
The Earth is the only planet known to have plate tectonics, while other planets are covered with a stagnant lid. On the Earth, the initiation of subduction, which is thought to be the fundamental process for plate tectonics initiation, is caused not only by the negative buoyancy of the lithosphere but also by the forces from plate motions. However, for planets which do not have plate tectonics, the very first episode of lithospheric failure has to be caused by forces other than plate motions. Sublithospheric convection has been proposed as a possible mechanism that provides lithospheric instability through inducing stresses in the lithosphere, and lithospheric failure can occur when the yield stress is below a critical value. We test the applicability of scaling laws for the critical yield stress obtained in single-cell convection simulations to strongly time-dependent multi-cell systems. We show that with an appropriate choice of characteristic aspect ratio for the convective system, the scaling laws from single-cell simulations can be used to evaluate the conditions on the terrestrial planets in the inner Solar System for plate tectonics to exist. In agreement with previous studies, the estimated values for critical yield stress and coefficient of friction are much lower than the expected values for the Earth's lithosphere.
Wanstall, Taber; Hadji, Layachi
2016-11-01
The convective stability associated with carbon sequestration is modeled by adopting an unstably stratified basic profile having a step function density with top heavy carbon saturated layer overlying a lighter carbon free layer. The model takes into account the anisotropy in both permeability and carbon dioxide diffusion, and chemical reactions between the CO2 rich brine and host mineralogy. We carry out a linear stability analysis to derive the instability threshold parameters for a variety of CO2 boundary conditions. We solve for the minimum thickness of the carbon-rich layer at which convection sets in and quantify how its value is influenced by diffusion, anisotropy, permeability, reaction and type of boundary conditions. The discontinuity leads to convective concentration contours that have the shape of an asymmetric lens which we quantify by deriving and making use of the CO2 flux expressions at the interface. The linear problem is extended to the nonlinear regime, the analysis of which leads to the determination of a uniformly valid super critical steady solution.
Numerical simulation of helical-vortex effects in Rayleigh-Bénard convection
Directory of Open Access Journals (Sweden)
G. V. Levina
2006-01-01
Full Text Available A numerical approach is substantiated for searching for the large-scale alpha-like instability in thermoconvective turbulence. The main idea of the search strategy is the application of a forcing function which can have a physical interpretation. The forcing simulates the influence of small-scale helical turbulence generated in a rotating fluid with internal heat sources and is applied to naturally induced fully developed convective flows. The strategy is tested using the Rayleigh-Bénard convection in an extended horizontal layer of incompressible fluid heated from below. The most important finding is an enlargement of the typical horizontal scale of the forming helical convective structures accompanied by a cells merging, an essential increase in the kinetic energy of flows and intensification of heat transfer. The results of modeling allow explaining how the helical feedback can work providing the non-zero mean helicity generation and the mutual intensification of horizontal and vertical circulation, and demonstrate how the energy of the additional helical source can be effectively converted into the energy of intensive large-scale vortex flow.
Convective transport resistance in the vitreous humor
Penkova, Anita; Sadhal, Satwindar; Ratanakijsuntorn, Komsan; Moats, Rex; Tang, Yang; Hughes, Patrick; Robinson, Michael; Lee, Susan
2012-11-01
It has been established by MRI visualization experiments that the convection of nanoparticles and large molecules with high rate of water flow in the vitreous humor will experience resistance, depending on the respective permeabilities of the injected solute. A set of experiments conducted with Gd-DTPA (Magnevist, Bayer AG, Leverkusen, Germany) and 30 nm gadolinium-based particles (Gado CELLTrackTM, Biopal, Worcester, MA) as MRI contrast agents showed that the degree of convective transport in this Darcy-type porous medium varies between the two solutes. These experiments consisted of injecting a mixture of the two (a 30 μl solution of 2% Magnevist and 1% nanoparticles) at the middle of the vitreous of an ex vivo whole bovine eye and subjecting the vitreous to water flow rate of 100 μl/min. The water (0.9% saline solution) was injected at the top of the eye, and was allowed to drain through small slits cut at the bottom of the eyeball. After 50 minutes of pumping, MRI images showed that the water flow carried the Gd-DTPA farther than the nanoparticles, even though the two solutes, being mixed, were subjected to the same convective flow conditions. We find that the convected solute lags the water flow, depending on the solute permeability. The usual convection term needs to be adjusted to allow for the filtration effect on the larger particles in the form (1- σ) u . ∇ c with important implications for the modeling of such systems.
Properties of convective motions in facular regions
Kostik, R.; Khomenko, E. V.
2012-09-01
Aims: We study the properties of solar granulation in a facular region from the photosphere up to the lower chromosphere. Our aim is to investigate the dependence of granular structure on magnetic field strength. Methods: We used observations obtained at the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife) using two different instruments: the Triple Etalon SOlar Spectrometer (TESOS) to measure velocity and intensity variations along the photosphere in the Ba ii 4554 Å line; and, simultaneously, the Tenerife Infrared Polarimeter (TIP-II) to the measure Stokes parameters and the magnetic field strength at the lower photosphere in the Fe i 1.56 μm lines. Results: We find that the convective velocities of granules in the facular area decrease with magnetic field while the convective velocities of intergranular lanes increase with the field strength. Similar to the quiet areas, there is a contrast and velocity sign reversal taking place in the middle photosphere. The reversal heights depend on the magnetic field strength and are, on average, about 100 km higher than in the quiet regions. The correlation between convective velocity and intensity decreases with magnetic field at the bottom photosphere, but increases in the upper photosphere. The contrast of intergranular lanes observed close to the disk center is almost independent of the magnetic field strength. Conclusions: The strong magnetic field of the facular area seems to stabilize the convection and to promote more effective energy transfer in the upper layers of the solar atmosphere, since the convective elements reach greater heights.
Nowcasting of convective cells over Italian Peninsula
Directory of Open Access Journals (Sweden)
C. M. Medaglia
2005-01-01
Full Text Available The aim of the study is the individuation of convective cells over the Italian peninsula with the conjunction use of geostationary satellite data (METEOSAT, MSG satellite in the IR and WV channels and lightning data. We will use GCD (Global Convective Diagnostic algorithm developed at Aviation Weather Centre (AWC of NOAA (National Oceanic and Atmospheric Administration. This algorithm is based on the idea that a deep convective cloud will not have any significant moisture above it. This technique works quite well at identifying active deep convection and can be applied to all the world's geostationary satellites. However it does not always agree with lightning sensors. Low topped convection with lightning will be missed. We will extend the capabilities of GCD using lightning data. The new product will be validate over different cases in the central Italy using the C-band polarimetric radar of ISAC-CNR (Institute of Atmospheric Sciences and Climate-of the Italian National Research Council Rome.
Scaling regimes in spherical shell rotating convection
Gastine, T; Aubert, J
2016-01-01
Rayleigh-B\\'enard convection in rotating spherical shells can be considered as a simplified analogue of many astrophysical and geophysical fluid flows. Here, we use three-dimensional direct numerical simulations to study this physical process. We construct a dataset of more than 200 numerical models that cover a broad parameter range with Ekman numbers spanning $3\\times 10^{-7} \\leq E \\leq 10^{-1}$, Rayleigh numbers within the range $10^3 < Ra < 2\\times 10^{10}$ and a Prandtl number unity. We investigate the scaling behaviours of both local (length scales, boundary layers) and global (Nusselt and Reynolds numbers) properties across various physical regimes from onset of rotating convection to weakly-rotating convection. Close to critical, the convective flow is dominated by a triple force balance between viscosity, Coriolis force and buoyancy. For larger supercriticalities, a subset of our numerical data approaches the asymptotic diffusivity-free scaling of rotating convection $Nu\\sim Ra^{3/2}E^{2}$ in ...
Superparameterised convection in the EMAC model
Rybka, Harald; Tost, Holger
2017-04-01
Clouds in large-scale circulation models are often not well represented due to the large grid box size of these models. Especially convective clouds with a typical extension of a few kilometres only are subgrid-scale compared to the grid box size of the host models. To overcome this scale discrepancy in the chemistry climate model EMAC, a superparameterisation has been implemented, i.e. a cloud resolving model handling both large-scale as well as convective clouds. The gain for the substantial increase in computational costs is an increase in performance for the global precipitation distribution, especially in the tropics. Furthermore, the diurnal cycle of convective activity is much better represented by the superparameterisation compared to traditional convection schemes. We also provide results on the total water budget, e.g. integrated liquid and ice water as well as the partitioning between the two phases, which substantially differs between parameterised and superparameterised convection due to the explicit treatment of cloud microphysical processes in the latter scheme. Especially, this partitioning has implications for the atmospheric radiation budget and consequently also surface temperatures.
Quad-Bike Operational Instability
Directory of Open Access Journals (Sweden)
Ross H. Macmillan
2017-05-01
Full Text Available The stake-holders in the quad-bike (QB industry in Australia have failed to reach a satisfactory resolution of the present impasse that exists with respect to the causes and mitigation of the trauma suffered by riders due to QB instability. In an effort to provide purchasers with data enabling them to discriminate between safer and less safe machines, static longitudinal and lateral tests have been conducted by various interested parties; quasi-static lateral tests have also been conducted under some operational conditions. It is argued that while these static tests are valid, under many operating conditions QBs will not reach such unstable slopes due to poor traction. Further, these tests do not include the quasi-static and dynamic factors which also influence the processes associated with operational instability. For these reasons, the static tests do not provide an adequate basis for discrimination between safer and less safe machines.
Streaming Instabilities in Protoplanetary Disks
Youdin, A N; Youdin, Andrew N.; Goodman, Jeremy
2004-01-01
Interpenetrating streams of solids and gas in a Keplerian disk produce a local, linear instability. The two components mutually interact via aerodynamic drag, which generates radial drift and triggers unstable modes. The secular instability does not require self-gravity, yet it generates growing particle density perturbations that could seed planetesimal formation. Growth rates are slower than dynamical, but faster than radial drift, timescales. Growth rates, like streaming velocities, are maximized for marginal coupling (stopping times comparable dynamical times). Fastest growth occurs when the solid to gas density ratio is order unity and feedback is strongest. Curiously, growth is strongly suppressed when the densities are too nearly equal. The relation between background drift and wave properties is explained by analogy with Howard's semicircle theorem. The three-dimensional, two-fluid equations describe a sixth order (in the complex frequency) dispersion relation. A terminal velocity approximation allows...
Instability of supersymmetric microstate geometries
Eperon, Felicity C; Santos, Jorge E
2016-01-01
We investigate the classical stability of supersymmetric, asymptotically flat, microstate geometries with five non-compact dimensions. Such geometries admit an "evanescent ergosurface": a timelike hypersurface of infinite redshift. On such a surface, there are null geodesics with zero energy relative to infinity. These geodesics are stably trapped in the potential well near the ergosurface. We present a heuristic argument indicating that this feature is likely to lead to a nonlinear instability of these solutions. We argue that the precursor of such an instability can be seen in the behaviour of linear perturbations: nonlinear stability would require that all linear perturbations decay sufficiently rapidly but the stable trapping implies that some linear perturbation decay very slowly. We study this in detail for the most symmetric microstate geometries. By constructing quasinormal modes of these geometries we show that generic linear perturbations decay slower than any inverse power of time.
Buckling instability of squeezed droplets
Elfring, Gwynn J
2015-01-01
Motivated by recent experiments, we consider theoretically the compression of droplets pinned at the bottom on a surface of finite area. We show that if the droplet is sufficiently compressed at the top by a surface, it will always develop a shape instability at a critical compression. When the top surface is flat, the shape instability occurs precisely when the apparent contact angle of the droplet at the pinned surface is pi, regardless of the contact angle of the upper surface, reminiscent of past work on liquid bridges and sessile droplets as first observed by Plateau. After the critical compression, the droplet transitions from a symmetric to an asymmetric shape. The force required to deform the droplet peaks at the critical point then progressively decreases indicative of catastrophic buckling. We characterize the transition in droplet shape using illustrative examples in two dimensions followed by perturbative analysis as well as numerical simulation in three dimensions. When the upper surface is not f...
Stretching Folding Instability and Nanoemulsions
Chan, Chon U
2009-01-01
Here we show a folding-stretching instability in a microfluidic flow focusing device using silicon oil (100cSt) and water. The fluid dynamics video demonstrates an oscillating thread of oil focused by two co-flowing streams of water. We show several high-speed sequences of these oscillations with 30,000 frames/s. Once the thread is decelerated in a slower moving pool downstream an instability sets in and water-in-oil droplets are formed. We reveal the details of the pinch-off with 500,000 frames/s. The pinch-off is so repeatable that complex droplet patterns emerge. Some of droplets are below the resolution limit, thus smaller than 1 micrometer in diameter.
Instability of ties in compression
DEFF Research Database (Denmark)
Buch-Hansen, Thomas Cornelius
2013-01-01
Masonry cavity walls are loaded by wind pressure and vertical load from upper floors. These loads results in bending moments and compression forces in the ties connecting the outer and the inner wall in a cavity wall. Large cavity walls are furthermore loaded by differential movements from...... the temperature gradient between the outer and the inner wall, which results in critical increase of the bending moments in the ties. Since the ties are loaded by combined compression and moment forces, the loadbearing capacity is derived from instability equilibrium equations. Most of them are iterative, since......-connectors in cavity walls was developed. The method takes into account constraint conditions limiting the free length of the wall tie, and the instability in case of pure compression which gives an optimal load bearing capacity. The model is illustrated with examples from praxis....
Instability of supersymmetric microstate geometries
Energy Technology Data Exchange (ETDEWEB)
Eperon, Felicity C.; Reall, Harvey S.; Santos, Jorge E. [Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA (United Kingdom)
2016-10-07
We investigate the classical stability of supersymmetric, asymptotically flat, microstate geometries with five non-compact dimensions. Such geometries admit an “evanescent ergosurface”: a timelike hypersurface of infinite redshift. On such a surface, there are null geodesics with zero energy relative to infinity. These geodesics are stably trapped in the potential well near the ergosurface. We present a heuristic argument indicating that this feature is likely to lead to a nonlinear instability of these solutions. We argue that the precursor of such an instability can be seen in the behaviour of linear perturbations: nonlinear stability would require that all linear perturbations decay sufficiently rapidly but the stable trapping implies that some linear perturbation decay very slowly. We study this in detail for the most symmetric microstate geometries. By constructing quasinormal modes of these geometries we show that generic linear perturbations decay slower than any inverse power of time.
Modern management of patellar instability.
Rhee, Shin-Jae; Pavlou, George; Oakley, Jeremy; Barlow, David; Haddad, Farres
2012-12-01
Recurrent patellofemoral instability is a disabling condition, attributed to a variety of anatomical aetiologies. Trochlear dysplasia, patella alta, an increased tibial tubercle trochlear groove distance of greater than 20 mm and soft tissue abnormalities such as a torn medial patellofemoral ligament and inadequate vastus medialis obliquus are all factors to be considered. Management of this condition remains difficult and controversial and knowledge of the functional anatomy and biomechanics of the patellofemoral joint, a detailed history and clinical examination, and an accurate patient assessment are all imperative to formulate an appropriate management plan. Surgical treatment is based on the underlying anatomical pathology with an aim to restore normal patellofemoral kinematics. We summarise aspects of assessment, treatment and outcome of patellofemoral instability and propose an algorithm of treatment.
Mechanical Instabilities of Biological Tubes
Hannezo, Edouard; Prost, Jacques; 10.1103/PhysRevLett.109.018101
2012-01-01
We study theoretically the shapes of biological tubes affected by various pathologies. When epithelial cells grow at an uncontrolled rate, the negative tension produced by their division provokes a buckling instability. Several shapes are investigated : varicose, enlarged, sinusoidal or sausage-like, all of which are found in pathologies of tracheal, renal tubes or arteries. The final shape depends crucially on the mechanical parameters of the tissues : Young modulus, wall-to-lumen ratio, homeostatic pressure. We argue that since tissues must be in quasistatic mechanical equilibrium, abnormal shapes convey information as to what causes the pathology. We calculate a phase diagram of tubular instabilities which could be a helpful guide for investigating the underlying genetic regulation.
Fluctuations and Instability in Sedimentation
Guazzelli, Élisabeth
2011-01-21
This review concentrates on the fluctuations of the velocities of sedimenting spheres, and on the structural instability of a suspension of settling fibers. For many years, theoretical estimates and numerical simulations predicted the fluctuations of the velocities of spheres to increase with the size of the container, whereas experiments found no such variation. Two ideas have increased our understanding. First, the correlation length of the velocity fluctuations was found experimentally to be 20 interparticle separations. Second, in dilute suspensions, a vertical variation in the concentration due to the spreading of the front with the clear fluid can inhibit the velocity fluctuations. In a very dilute regime, a homogeneous suspension of fibers suffers a spontaneous instability in which fast descending fiber-rich columns are separated by rising fiber-sparse columns. In a semidilute regime, the settling is hindered, more so than for spheres. © 2011 by Annual Reviews. All rights reserved.
Placing Marangoni instabilities under arrest
Bhamla, M Saad
2016-01-01
Soap bubbles occupy the rare position of delighting and fascinating both young children and scientific minds alike. Sir Isaac Newton, Joseph Plateau, Carlo Marangoni, and Pierre-Gilles de Gennes, not to mention countless others, have discovered remarkable results in optics, molecular forces and fluid dynamics from investigating this seemingly simple system. We present here a compilation of curiosity-driven experiments that systematically investigate the surface flows on a rising soap bubble. From childhood experience, we are familiar with the vibrant colors and mesmerizing display of chaotic flows on the surface of a soap bubble. These flows arise due to surface tension gradients, also known as Marangoni flows or instabilities. In Figure 1, we show the surprising effect of layering multiple instabilities on top of each other, highlighting that unexpected new phenomena are still waiting to be discovered, even in the simple soap bubble.
Circulation in blast driven instabilities
Henry de Frahan, Marc; Johnsen, Eric
2016-11-01
Mixing in many natural phenomena (e.g. supernova collapse) and engineering applications (e.g. inertial confinement fusion) is often initiated through hydrodynamic instabilities. Explosions in these systems give rise to blast waves which can interact with perturbations at interfaces between different fluids. Blast waves are formed by a shock followed by a rarefaction. This wave profile leads to complex time histories of interface acceleration. In addition to the instabilities induced by the acceleration field, the rarefaction from the blast wave decompresses the material at the interface, further increasing the perturbation growth. After the passage of the wave, circulation circulation generated by the blast wave through baroclinic vorticity continues to act upon the interface. In this talk, we provide scaling laws for the circulation and amplitude growth induced by the blast wave. Numerical simulations of the multifluid Euler equations solved using a high-order accurate Discontinuous Galerkin method are used to validate the theoretical results.
Organised convection embedded in a large-scale flow
Naumann, Ann Kristin; Stevens, Bjorn; Hohenegger, Cathy
2017-04-01
In idealised simulations of radiative convective equilibrium, convection aggregates spontaneously from randomly distributed convective cells into organized mesoscale convection despite homogeneous boundary conditions. Although these simulations apply very idealised setups, the process of self-aggregation is thought to be relevant for the development of tropical convective systems. One feature that idealised simulations usually neglect is the occurrence of a large-scale background flow. In the tropics, organised convection is embedded in a large-scale circulation system, which advects convection in along-wind direction and alters near surface convergence in the convective areas. A large-scale flow also modifies the surface fluxes, which are expected to be enhanced upwind of the convective area if a large-scale flow is applied. Convective clusters that are embedded in a large-scale flow therefore experience an asymmetric component of the surface fluxes, which influences the development and the pathway of a convective cluster. In this study, we use numerical simulations with explicit convection and add a large-scale flow to the established setup of radiative convective equilibrium. We then analyse how aggregated convection evolves when being exposed to wind forcing. The simulations suggest that convective line structures are more prevalent if a large-scale flow is present and that convective clusters move considerably slower than advection by the large-scale flow would suggest. We also study the asymmetric component of convective aggregation due to enhanced surface fluxes, and discuss the pathway and speed of convective clusters as a function of the large-scale wind speed.
Tropical convection and climate sensitivity
Williams, Ian Nobuo
Surface temperature has become a popular measure of climate change, but it does not provide the most critical test of climate models. This thesis presents new methods to evaluate climate models based on processes determining the climate sensitivity to radiative forcing from atmospheric greenhouse gases. Cloud radiative feedbacks depend on temperature and relative humidity profiles in addition to surface temperature, through the dependence of cloud type on boundary layer buoyancy. Buoyancy provides a reference to which the onset of deep convection is invariant, and gives a compact description of sea surface temperature changes and cloud feedbacks suitable for diagnostics and as a basis for simplified climate models. This thesis also addresses uncertainties in climate sensitivity involving terrestrial ecosystem responses to global warming. Different diagnostics support different conclusions about atmospheric transport model errors that could imply either stronger or weaker northern terrestrial carbon sinks. Equilibrium boundary layer concepts were previously used in idealized tropical climate models, and are extended here to develop a diagnostic of boundary layer trace gas transport and mixing. Hypotheses linking surface temperature to climate and precipitation sensitivity were tested in this thesis using comprehensive and idealized climate model simulations, and observational datasets. The results do not support the thermostat hypothesis that predicts deep cloud cover will increase with radiative forcing and limit sea surface temperatures to the maximum present-day warm pool temperature. Warm pool temperatures increased along with or even faster than the tropical average over the past several decades, while diagnosed deep cloud cover has not significantly increased, in agreement with global warming simulations. Precipitation sensitivity also depends on more than surface temperature alone, including thermodynamic profiles and air-sea temperature differences. The
MD 751: Train Instability Threshold
Carver, Lee Robert; Metral, Elias; Salvant, Benoit; Levens, Tom; Nisbet, David; Zobov, M; CERN. Geneva. ATS Department
2016-01-01
The purpose of this MD is to measure the octupole current thresholds for stability for a single bunch, and then make an immediate comparison (with the same operational settings) for a train of 72 bunches separated by 25ns. From theory, the expected thresholds should be similar. Any discrepancy between the two cases will be of great interest as it could indicate the presence of additional mechanisms that contribute to the instability threshold, for example electron cloud.
Polygonal instabilities on interfacial vorticities
Labousse, Matthieu
2015-01-01
We report the results of a theoretical investigation of the stability of a toroidal vortex bound by an interface. Two distinct instability mechanisms are identified that rely on, respectively, surface tension and fluid inertia, either of which may prompt the transformation from a circular to a polygonal torus. Our results are discussed in the context of three experiments, a toroidal vortex ring, the hydraulic jump, and the hydraulic bump.
Instability of colliding metastable strings
Energy Technology Data Exchange (ETDEWEB)
Hiramatsu, Takashi [Kyoto Univ. (Japan). Yukawa Inst. for Theoretical Physics; Eto, Minoru [Yamagata Univ. (Japan). Dept. of Physics; Kamada, Kohei [Deutsches Elektronen-Synchrotron (DESY), Hamburg (Germany); Kobayashi, Tatsuo [Kyoto Univ. (Japan). Dept. of Physics; Ookouchi, Yutaka [Kyoto Univ. (Japan). Dept. of Physics; Kyoto Univ. (Japan). The Hakubi Center for Advanced Research
2013-04-15
We investigate the collision dynamics of two metastable strings which can be viewed as tube-like domain walls with winding numbers interpolating a false vacuum and a true vacuum. We find that depending on the relative angle and speed of two strings, instability of strings increases and the false vacuum is filled out by rapid expansion of the strings or of a remnant of the collision.
Gravitational instabilities in astrophysical fluids
Tohline, Joel E.
1990-01-01
Over the past decade, the significant advancements that have been made in the development of computational tools and numerical techniques have allowed astrophysicists to begin to model accurately the nonlinear growth of gravitational instabilities in a variety of physical systems. The fragmentation or rotationally driven fission of dynamically evolving, self-gravitating ``drops and bubbles'' is now routinely modeled in full three-dimensional generality as we attempt to understand the behavior of protostellar clouds, rotating stars, galaxies, and even the primordial soup that defined the birth of the universe. A brief review is presented here of the general insights that have been gained from studies of this type, followed by a somewhat more detailed description of work, currently underway, that is designed to explain the process of binary star formation. A short video animation sequence, developed in conjunction with some of the research being reviewed, illustrates the basic-nature of the fission instability in rotating stars and of an instability that can arise in a massive disk that forms in a protostellar cloud.
Microphysics of cosmic ray driven plasma instabilities
Bykov, A M; Malkov, M A; Osipov, S M
2013-01-01
Energetic nonthermal particles (cosmic rays, CRs) are accelerated in supernova remnants, relativistic jets and other astrophysical objects. The CR energy density is typically comparable with that of the thermal components and magnetic fields. In this review we discuss mechanisms of magnetic field amplification due to instabilities induced by CRs. We derive CR kinetic and magnetohydrodynamic equations that govern cosmic plasma systems comprising the thermal background plasma, comic rays and fluctuating magnetic fields to study CR-driven instabilities. Both resonant and non-resonant instabilities are reviewed, including the Bell short-wavelength instability, and the firehose instability. Special attention is paid to the longwavelength instabilities driven by the CR current and pressure gradient. The helicity production by the CR current-driven instabilities is discussed in connection with the dynamo mechanisms of cosmic magnetic field amplification.
Effect of hall currents on thermal instability of dusty couple stress fluid
Directory of Open Access Journals (Sweden)
Aggarwal Amrish Kumar
2016-09-01
Full Text Available In this paper, effect of Hall currents on the thermal instability of couple-stress fluid permeated with dust particles has been considered. Following the linearized stability theory and normal mode analysis, the dispersion relation is obtained. For the case of stationary convection, dust particles and Hall currents are found to have destabilizing effect while couple stresses have stabilizing effect on the system. Magnetic field induced by Hall currents has stabilizing/destabilizing effect under certain conditions. It is found that due to the presence of Hall currents (hence magnetic field, oscillatory modes are produced which were non-existent in their absence.
Marangoni-Bénard Instability with the Exchange of Evaporation at Liquid-Vapour Interface
Institute of Scientific and Technical Information of China (English)
LIU Rong; LIU Qiu-Sheng; HU Wen-Rui
2005-01-01
@@ A new two-sided model rather than the one-sided model in previous works is put forward. The linear instability analysis is performed on the Marangoni-Bénard convection in the two-layer system with an evaporation interface.We define a new evaporation Biot number which is different from that in the one-sided model, and obtain the curves of critical Marangoni number versus wavenumber. The influence of evaporation velocity and Biot number on the system is discussed and a new phenomenon uninterpreted before is now explained from our numerical results.
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.
A new conceptual model of convection
Energy Technology Data Exchange (ETDEWEB)
Walcek, C. [State Univ. of New York, Albany, NY (United States)
1995-09-01
Classical cumulus parameterizations assume that cumulus clouds are entraining plumes of hot air rising through the atmosphere. However, ample evidence shows that clouds cannot be simulated using this approach. Dr. Walcek suggests that cumulus clouds can be reasonably simulated by assuming that buoyant plumes detrain mass as they rise through the atmosphere. Walcek successfully simulates measurements of tropical convection using this detraining model of cumulus convection. Comparisons with measurements suggest that buoyant plumes encounter resistance to upward movement as they pass through dry layers in the atmosphere. This probably results from turbulent mixing and evaporation of cloud water, which generates negatively buoyant mixtures which detrain from the upward moving plume. This mass flux model of detraining plumes is considerably simpler than existing mass flux models, yet reproduces many of the measured effects associated with convective activity. 1 fig.
Magnetic field generation by intermittent convection
Chertovskih, R; Chimanski, E V
2016-01-01
Magnetic field generation by convective flows in transition to weak turbulence is studied numerically. By fixing the Prandtl number at P=0.3 and varying the Rayleigh number (Ra) as a control parameter in three-dimensional Rayleigh-Benard convection of an electrically conducting fluid, a recently reported route to hyperchaos involving quasiperiodic regimes, crises and chaotic intermittent attractors is followed, and the critical magnetic Prandtl number ($P_m^c$) for dynamo action is determined as a function of Ra. A mechanism for the onset of on-off intermittency in the magnetic energy is described, the most beneficial convective regimes for dynamo action are identified, and how intermittency affects the dependence of $P_m^c$ on Ra is discussed.
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.
Basics of lava-lamp convection
Gyüre, Balázs; Jánosi, Imre M.
2009-10-01
Laboratory experiments are reported in an immiscible two-fluid system, where thermal convection is initiated by heating at the bottom and cooling at the top. The lava-lamp regime is characterized by a robust periodic exchange process where warm blobs rise from the bottom, attach to the top surface for a while, then cold blobs sink down again. Immiscibility allows to reach real steady (dynamical equilibrium) states which can be sustained for several days. Two modes of lava-lamp convection could be identified by recording and evaluating temperature time series at the bottom and at the top of the container: a “slow” mode is determined by an effective heat transport speed at a given temperature gradient, while a second mode of constant periodicity is viscosity limited. Contrasting of laboratory and geophysical observations yields the conclusion that the frequently suggested lava-lamp analogy fails for the accepted models of mantle convection.
Solar convection and oscillations in magnetic regions
Jacoutot, L; Wray, A; Mansour, N N
2008-01-01
The goal of this research is to investigate how magnetic field affects the dynamics of granular convection and excitation of solar oscillations by means of realistic numerical simulations. We have used a 3D, compressible, non-linear radiative magnetohydrodynamics code developed at the NASA Ames Research Center. This code takes into account several physical phenomena: compressible fluid flow in a highly stratified medium, sub-grid scale turbulence models, radiative energy transfer between the fluid elements, and a real-gas equation of state. We have studied the influence of the magnetic field of various strength on the convective cells and on the excitation mechanisms of the acoustic oscillations by calculating spectral properties of the convective motions and oscillations. The results reveal substantial changes of the granulation structure with increased magnetic field, and a frequency-dependent reduction in the oscillation power in a good agreement with solar observations. These simulations suggest that the ...
Convection and Mixing in Giant Planet Evolution
Vazan, Allona; Kovetz, Attay; Podolak, Morris
2015-01-01
The primordial internal structures of gas giant planets are unknown. Often giant planets are modeled under the assumption that they are adiabatic, convective, and homogeneously mixed, but this is not necessarily correct. In this work, we present the first self-consistent calculation of convective transport of both heat and material as the planets evolve. We examine how planetary evolution depends on the initial composition and its distribution, whether the internal structure changes with time, and if so, how it affects the evolution. We consider various primordial distributions, different compositions, and different mixing efficiencies and follow the distribution of heavy elements in a Jupiter-mass planet as it evolves. We show that a heavy-element core cannot be eroded by convection if there is a sharp compositional change at the core-envelope boundary. If the heavy elements are initially distributed within the planet according to some compositional gradient, mixing occurs in the outer regions resulting in a...
Differential Rotation in Solar Convective Dynamo Simulations
Fan, Yuhong
2015-01-01
We carry out a magneto-hydrodynamic (MHD) simulation of convective dynamo in the rotating solar convective envelope driven by the solar radiative diffusive heat flux. The simulation is similar to that reported in Fan & Fang (2014) but with further reduced viscosity and magnetic diffusion. The resulting convective dynamo produces a large scale mean field that exhibits similar irregular cyclic behavior and polarity reversals, and self-consistently maintains a solar-like differential rotation. The main driver for the solar-like differential rotation (with faster rotating equator) is a net outward transport of angular momentum away from the rotation axis by the Reynolds stress, and we found that this transport is enhanced with reduced viscosity and magnetic diffusion.
Entropy in adiabatic regions of convection simulations
Tanner, Joel D; Demarque, Pierre
2016-01-01
One of the largest sources of uncertainty in stellar models is caused by the treatment of convection in stellar envelopes. One dimensional stellar models often make use of the mixing length or equivalent approximations to describe convection, all of which depend on various free parameters. There have been attempts to rectify this by using 3D radiative-hydrodynamic simulations of stellar convection, and in trying to extract an equivalent mixing length from the simulations. In this paper we show that the entropy of the deeper, adiabatic layers in these simulations can be expressed as a simple function of og g and log T_{eff} which holds potential for calibrating stellar models in a simple and more general manner.
Magnetic Fields in the Solar Convection Zone
Directory of Open Access Journals (Sweden)
Yuhong Fan
2009-12-01
Full Text Available Active regions on the solar surface are generally thought to originate from a strong toroidal magnetic field generated by a deep seated solar dynamo mechanism operating at the base of the solar convection zone. Thus the magnetic fields need to traverse the entire convection zone before they reach the photosphere to form the observed solar active regions. Understanding this process of active region flux emergence is therefore a crucial component for the study of the solar cycle dynamo. This article reviews studies with regard to the formation and rise of active region scale magnetic flux tubes in the solar convection zone and their emergence into the solar atmosphere as active regions.
Convective towers detection using GPS radio occultations
DEFF Research Database (Denmark)
Biondi, Riccardo; Neubert, Torsten; Syndergaard, S.
The tropical deep convection affects the radiation balance of the atmosphere changing the water vapour mixing ratio and the temperature of the upper troposphere and lower stratosphere. To gain a better understanding of deep convective processes, the study of tropical cyclones could play an import...... (ACES) payload on the International Space Station....... 1194 profiles in a time window of 3 hours and a space window of 300 km from the eye of the cyclone. We show that the bending angle anomaly of a GPS RO signal is typically larger than the climatology above the tropopause. Comparisons with co-located radiosondes, climatology of tropopause altitudes...... and GOES analyses will also be shown to support our hypothesis and to corroborate the idea that the bending angle anomaly can be used as an indicator of convective towers. The results are discussed in connection to the GPS radio occultation receiver which will be part of the Atomic Clock Ensemble in Space...
Differential rotation in solar convective dynamo simulations
Fan, Yuhong; Fang, Fang
2016-10-01
We carry out a magneto-hydrodynamic (MHD) simulation of convective dynamo in the rotating solar convective envelope driven by the solar radiative diffusive heat flux. The simulation is similar to that reported in Fan and Fang (2014) but with further reduced viscosity and magnetic diffusion. The resulting convective dynamo produces a large scale mean field that exhibits similar irregular cyclic behavior and polarity reversals, and self-consistently maintains a solar-like differential rotation. The main driver for the solar-like differential rotation (with faster rotating equator) is a net outward transport of angular momentum away from the rotation axis by the Reynolds stress, and we found that this transport is enhanced with reduced viscosity and magnetic diffusion.
Boiko, Andrey V; Grek, Genrih R; Kozlov, Victor V
2012-01-01
Starting from fundamentals of classical stability theory, an overview is given of the transition phenomena in subsonic, wall-bounded shear flows. At first, the consideration focuses on elementary small-amplitude velocity perturbations of laminar shear layers, i.e. instability waves, in the simplest canonical configurations of a plane channel flow and a flat-plate boundary layer. Then the linear stability problem is expanded to include the effects of pressure gradients, flow curvature, boundary-layer separation, wall compliance, etc. related to applications. Beyond the amplification of instability waves is the non-modal growth of local stationary and non-stationary shear flow perturbations which are discussed as well. The volume continues with the key aspect of the transition process, that is, receptivity of convectively unstable shear layers to external perturbations, summarizing main paths of the excitation of laminar flow disturbances. The remainder of the book addresses the instability phenomena found at l...
Non-Radial Instabilities and Progenitor Asphericities in Core-Collapse Supernovae
Mueller, B
2014-01-01
Since core-collapse supernova simulations still struggle to produce robust neutrino-driven explosions in 3D, it has been proposed that asphericities caused by convection in the progenitor might facilitate shock revival by boosting the activity of non-radial hydrodynamic instabilities in the post-shock region. We investigate this scenario in depth using 42 relativistic 2D simulations with multi-group neutrino transport to examine the effects of velocity and density perturbations in the progenitor for different perturbation geometries that obey fundamental physical constraints (like the anelastic condition). As a framework for analysing our results, we introduce semi-empirical scaling laws relating neutrino heating, average turbulent velocities in the gain region, and the shock deformation in the saturation limit of non-radial instabilities. The squared turbulent Mach number, , reflects the violence of aspherical motions in the gain layer, and explosive runaway occurs for ~0.3, corresponding to a reduction of t...
Curvature effects on axisymmetric instability of conduction regime in a tall air-filled annulus
Pécheux, J.; Le Quéré, P.; Abcha, F.
1994-10-01
This paper numerically studies curvature effects on the instability of the conduction regime of natural convection in a tall air-filled differentially heated annulus of vertical aspect ratio 16 by integrating the two-dimensional axisymmetric Navier-Stokes equations in the Boussinesq approximation. The numerical algorithm combines a pseudospectral Chebyshev space discretization with a second-order time-stepping scheme. It is shown that, in contrast with linear stability analysis of the conduction solution, the time-periodic cross-roll instability does not take place in finite aspect ratio cavities for small values of the radius ratio. For all values of the radius ratio transition to unsteadiness occurs through supercritical Hopf bifurcations. Extensive computations show very complex behaviors of the unsteady solutions depending on the radius ratio. The nature of the reverse transition to steady state that occurs for increasing value of the Rayleigh number is also found to depend strongly on the value of the radius ratio.
Buoyancy-driven instabilities of acid-base fronts: the case of a color indicator
Riolfo, L. A.; Kuster, S.; Trevelyan, P. M. J.; El Hasi, C.; Zalts, A.; Almarcha, C.; D'Onofrio, A.; de Wit, A.
2011-11-01
Buoyancy-driven hydrodynamic instabilities of acid-base fronts are studied both experimentally and theoretically in the case where an aqueous solution of a strong acid is put above a denser aqueous solution of a color indicator in the gravity field. The neutralization reaction between the acid and the color indicator as well as their differential diffusion modifies the initially stable density profile in the system and can trigger convective motion both above and below the initial contact line. The type of patterns observed as well as their wavelength and the speed of the reaction front are shown to depend on the value of the initial concentrations of the acid and of the color indicator and on their ratio. A reaction-diffusion model explains how the hydrodynamic instability scenarios change when the concentration of the reactants are varied.