Showing papers on "Rayleigh number published in 1990"

Effect of a centered conducting body on natural convection heat transfer in an enclosure

Abstract: The effect of a centered, square, heat-conducting body on natural convection in a vertical square enclosure was examined numerically. The analysis reveals that the fluid flow and heat transfer processes are governed by the Rayleigh and Prandtl numbers, the dimensionless body size, and the ratio of the thermal conductivity of the body to that of the fluid. For Pr = 0.71 and relatively wide ranges of the other parameters, results are reported in terms of streamlines, isotherms, and the overall heat transfer across the enclosure as described by the Nusselt number. Heat transfer across the enclosure, in comparison to that in the absence of a body, may be enhanced (reduced) by a body with a thermal conductivity ratio less (greater) than unity. Furthermore, the heat transfer may attain a minimum as the body size is increased. These and other findings are justified through a careful examination of the local heat and fluid flow phenomena.

Coherent structures in turbulent convection, an experimental study

Abstract: We present results from a visualization experiment in Rayleigh-Benard convection in water at high Rayleigh number. We distinguish three kinds of coherent structures in the flow: waves along the boundary layers, plumes, and spiraling swirls. The waves originate from the interaction of plumes with the boundary layers. The spiraling swirls appear to be the result of a shear instability of the viscous boundary layer. We describe the “life cycle” of these structures in the cell, and when we focus on the waves and characterize them quantitatively using local temperature measurements.

Direct numerical simulation of two-dimensional turbulent natural convection in an enclosed cavity

Abstract: A two-dimensional direct numerical simulation of the natural convection flow of air in a differentially heated square cavity was performed for a Rayleigh number of 10 10 . The simulation was commenced from isothermal and quiescent conditions and was allowed to proceed to a statistical steady state. Two-dimensional turbulence resulted without the introduction of random forcing. Good agreement of mean quantities of the statistically steady flow is obtained with available experimental results. In addition, the previously proposed (George & Capp 1979) − $\frac{1}{3}$ and $+\frac{1}{3}$ temperature and velocity variations in the buoyant sublayer are confirmed. Other statistics of the flow are consistent with available experimental data. Selected frames from a movie generated from the computational results show very clearly turbulence production via the sequence from initial instability, proceeding through transition, and eventually reaching statistical steady state. Prominent large-scale structures are seen to persist at steady state.

Temperature and velocity field regimes of convective motions in a rotating plane fluid layer

Abstract: The paper is a continuation of work published in Boubnov & Golitsyn (1986). We present new measurements of the temperature and velocity field patterns and their statistical characteristics. This allows us to classify regimes of convection in a plane rotating horizontal fluid layer in terms of Rayleigh and Taylor numbers. Within the irregular regimes geostrophic convection is found for which the Rossby number is much less than unity.In the regular regimes the mean temperature profiles are linear with height in the bulk of the fluid, the gradient being dependent mainly on rotation rate Ω and fluid depth h. These together with some dimensional arguments lead to the heat transfer relationship Nu ∝ Ra3 Ta−2 between Nusselt, Rayleigh and Taylor numbers. Experimental results by Rossby (1969) and theoretical work by Chan (1974) and Riahi (1977) suggested this dependence. The dependence on ωτ of the temperature power spectrum normalized by the variance was found to be universal at higher frequencies for all irregular convective motions, where τ is the timescale of the thermal boundary layer for cases with a small influence of rotation and with τ about three times larger (in numerical coefficient) for geostrophic convection. For irregular geostrophic regimes it is found that the temperature variance depends on rotation rate and heat flux, and is inversely proportional to the buoyancy parameter.Horizontal and vertical components of the velocity fields were measured for regular as well as irregular regimes, confirming, especially for geostrophic convection, the theoretical results by Golitsyn (1980). In conclusion some geophysical applications are briefly mentioned.

The relationship between plate velocity and trench viscosity in Newtonian and power‐law subduction calculations

Abstract: The relationship between oceanic trench viscosity and oceanic plate velocity is studied using a Newtonian rheology by varying the viscosity at the trench. The plate velocity is a function of the trench viscosity for fixed Rayleigh number and plate/slab viscosity. Slab velocities for non-Newtonian rheology calculations are significantly different from slab velocities from Newtonian rheology calculations at the same effective Rayleigh number. Both models give reasonable strain rates for the slab when compared with estimates of seismic strain rate. Non-Newtonian rheology eliminates the need for imposed weak zones and provides a self-consistent fluid dynamical mechanism for subduction in numerical convection models.

High-Rayleigh-number convection in a horizontal enclosure

Abstract: A review of the literature on natural convection in a horizontal layer heated from below shows the need for reliable data at high Rayleigh number (Ra) to determine the asymptotic Nusselt number (Nu) variation with Rayleigh number. The present study expands the data base by the use of an electrochemical mass transfer technique to determine the asymptotic dependence of the Sherwood number (Sh) on Ra at high Schmidt number (Sc). The results of the present study give Sh = 0.0659 for Sc ≈ 2750, 3 × 109 < Ra < 5 × 1012. Using the heat-mass transfer analogy, this indicates the high Prandtl number variation of Nu with Ra.

Numerical Computation of the Natural Convection Flow about a Horizontal Cylinder Using Splines

Abstract: The present work is devoted to the numerical study of laminar natural convection flow from a heated horizontal cylinder under diverse surface boundary conditions using the spline fractional step method. A general formulation to treat mixed boundary conditions using the spline approximation has been presented. Numerical solutions have been obtained by solving the Navier-Stokes and energy equations. The results for the isothermal boundary condition as well as for the uniform heat flux are in good agreement with published experimental data and with other solutions presently available in the literature. Some new computations at very high Rayleigh numbers indicate the existence of attached separation vortices in the downstream plume region, the appearance of these vortices being dependent on the values of the Biot number. All results were computed on a personal computer using unequally spaced grids that provided good results with a minimum number of computational points. The numerical scheme presented here app...

Hydrodynamic interactions in directional solidification

Abstract: A binary liquid undergoes unidirectional solidification. The one-dimensional steady state is susceptible to morphological instability that causes the solid/liquid interface to change from a planar state to a cellular pattern. This paper examines the effects on this transition of volume-change convection, buoyancy-driven convection or forced flows. It emphasizes how flows alter stability limits, create scale and pattern changes in morphology, and create, through coupling, new instabilities. Emphasis is placed on the physical mechanisms of the interactions.

An Analytical Study on Natural Convection in Isotropic and Anisotropic Porous Channels

Abstract: This paper is an analytical study on natural two-dimensional convection in horizontal rectangular channels filled by isotropic and anisotropic porous media. The channel walls, assumed to be impermeable and perfectly heat conducting, are nonuniformly heated to establish a linear temperature distribution in the vertical direction. The authors derive the critical Rayleigh numbers for the onset of convection and examine the steady flow patterns at moderately supercritical Rayleigh numbers. The stability properties of these flow patterns are examined against two-dimensional perturbations using a weakly nonlinear theory.

Transition to hard turbulence in thermal convection at infinite Prandtl number

Abstract: Direct numerical simulations of two‐dimensional high Rayleigh (Ra) number, base‐heated thermal convection in large aspect‐ratio boxes are presented for infinite Prandtl number fluids, as applied to the Earth’s mantle. A transition is characterized in the flow structures in the neighborhood of Ra between 107 and 108. These high Ra flows consist of large‐scale cells with strong intermittent, boundary‐layer instabilities. For Ra exceeding 107 it is found that the heat‐transfer mechanism changes from one characterized by mushroom‐like plumes to one consisting of disconnected ascending instabilities, which do not carry with them all the thermal anomaly from the bottom boundary layer. Plume–plume collisions become much more prominent in high Ra situations and have a tendency of generating a pulse‐like behavior in the fixed plume. This type of instability represents a distinct mode of heat transfer in the hard turbulent regime. Predictions of this model can be used to address certain issues concerning the mode of time‐dependent convection in the Earth’s mantle.

Numerical investigation of mixed convection heat transfer in a horizontal channel with a built-in square cylinder

Abstract: Structures of laminar wakes and heat transfer in the presence of thermal buoyancy art investigated from the numerical solution of complete Navier-Stokes and energy equations in a two-dimensional horizontal channel with a built-in square cylinder. Results show that mixed convection can initiate periodicity and asymmetry in the wake at lower Reynolds numbers than forced convection alone. For a given Reynolds number, the heating of the fluid in the channel is improved by mixed convection up to a certain Grashof number and deteriorates if the Grashof number is further increased.

Numerical simulations of soft and hard turbulence: Preliminary results for two-dimensional convection

Abstract: Results on the transition from soft to hard turbulence in simulations of two-dimensional Boussinesq convection are reported. The computed probability densities for temperature fluctuations are exponential in form in both soft and hard turbulence, unlike what is observed in experiments. In contrast, a change is obtained in the Nusselt number scaling on Rayleigh number in good agreement with the three-dimensional experiments.

CALCULATION OF THE TURBULENT BUOYANCY-DRIVEN FLOW IN A RECTANGULAR CAVITY USING AN EFFICIENT SOLVER AND TWO DIFFERENT LOW REYNOLDS NUMBER κ-ε TURBULENCE MODELS

Abstract: The buoyancy-driven flow in a tall rectangular cavity of 5:1 aspect ratio with a Rayleigh number of 4 ×1010 is calculated using finite volume methods. The CELS solver is extended to be able to handle large density variations. CELS is compared with SIMPLEC, and it is shown to be up to more than 4 times as fast as SIMPLEC. A modified form of a low Reynolds number κ-e turbulence model is developed. This model is consistent in its near-wall behavior, and it allows simulation of the decay of grid turbulence. The model developed by Lam and Bremhorst [1] is also tested. Both turbulence models are shown to predict the transitional and relaminarization regions according to experiments.

The phase diffusion and mean drift equations for convection at finite Rayleigh numbers in large containers

Abstract: We derive the phase diffusion and mean drift equations for the Oberbeck-Boussinesq equations in large aspect ratio containers. We are able to recover all the long wave instability boundaries (Eckhaus, zig-zag, skew-varicose) of straight parallel rolls found previously by Busse and his colleagues. We can calculate the wavenumber selected by curved patterns and find very close agreement with the dominant wavenumbers observed by Heutmaker and Gollub at Prandtl number 2.5 and by Steinberg, Ahlers and Cannell at Prandtl number 6.1. We find a new instability, the focus instability, which causes circular target patterns to destabilize and which, at sufficiently large Rayleigh numbers, plays a major role in the onset of time dependence. Further, we predict the values of the Rayleigh number at which the time dependent but spatially ordered patterns will become spatially disordered. The key difficulty in obtaining these equations is the fact that the phase diffusion equation appears as a solvability condition at order e (the inverse aspect ratio) whereas the mean drift equation is the solvability condition at order e2. Therefore, we had to use extremely robust inversion methods to solve the singular equations at order e and the techniques we use should prove to be invaluable in a wide range of similar situations. In addition to providing more details of comparisons between theory and experiment, we plan to make these techniques and the program to implement them available in paper II.

The Prandtl Number Effect on the Transition in Natural Convection Along a Vertical Surface

Abstract: As is often the case, the decision to embark on a new project was triggered by a set of coincidental and mutually reinforcing observations: (i) There is a definite lack of information (theoretical, numerical, experimental) on natural convection in low-Pr fluids (Lykoudis, 1989). This observation was stressed recently also by Wolff et al. (1988). (ii) Georgiadis (1989) drew attention to the line of work represented by Chao et al. (1982) and Bertin and Ozoe (1986), who showed numerically that the onset of Benard convection appears to be influence by the Prandtl number in the Pr < 1 range. (iii) The authors were struck by Bertin and Ozoe's (1986) discovery that they were unable to obtain numerically a steady flow below a certain Prandtl number (Pr = 0.001), even though the Rayleigh number based on height seemed sufficiently low (Ra = 2,800). The present study was motivated by these observations and the apparent suggestion that in low-Pr fluids the laminar flow expires at unexpectedly low Rayleigh numbers. This idea is particularly interesting if they think of the natural convection boundary layer near a vertical wall, for which the textbook teaches to associate the constant Ra {approx} 10{sup 9} with the heightmore » of transition to the turbulent flow, regardless of the Prandtl number (e.g., Incropera and DeWitt, 1985, p. 427). In order to test this idea, the authors reexamined the experimental record of observations on transition in vertical natural convection boundary layer flow. Indeed, the empirical data described next show that the Prandtl number has a strong influence on the transition Rayleigh number. In low-Pr fluids the transition occurs at Rayleigh numbers much lower than the often-mentioned Ra {approx} 10{sup 9}, while in high-Pr fluids the transition Rayleigh number is higher than 10{sup 9}. It appears that the constant Grashof number Gr {approx} 10{sup 9} (i.e., not Ra {approx} 10{sup 9}) marks the transition in the wide Pr range 0.001-1000.« less

Planforms in Thermal Convection With Internal Heat Sources at Large Rayleigh and Prandtl Numbers

Abstract: Laboratory experiments are used to examine the effects of large amounts of internal heat generation on Rayleigh-Benard convection planforms in a constant viscosity, high Prandtl number fluid. Internal heat generation is simulated by lowering the boundary temperatures at a constant rate. The Rayleigh number (RaT) based on the imposed temperature difference is l.5×105 and the Rayleigh number based on the amount of internal heat generation (RaH) is varied from 0 to 3.0×106. In the absence of internal heat generation the convection planforms are the symmetric spoke pattern in which connected spokes of ascending and descending flow have, on average, equal strength. With internal heat production, the planform is asymmetric with buoyancy concentrated in isolated descending arcuate trenches and circular plumes. The upwellings are nonbuoyant, although their planform is more connected than downwellings. The amount of time dependence is observed to increase with internal heat generation, and occurs by branching and propagation of the negatively buoyant trenches.

Effect of rigid boundaries on the onset of convective instability in a triply diffusive fluid layer

Abstract: The onset of convection in a triply diffusive, incompressible motionless Newtonian fluid layer of infinite horizontal extent bounded by rigid parallel walls is studied by means of a linear stability analysis. The qualitative features of most of the results carry over from the case of stress‐free boundaries studied earlier by Pearlstein, Harris, and Terrones [J. Fluid Mech. 202, 443 (1989)]. One striking feature that does not carry over from the stress‐free case, however, is the possibility of quasiperiodic bifurcation from the steady motionless state via two pairs of complex conjugate temporal eigenvalues with incommensurable imaginary parts crossing into the right half‐plane at the same combination of Rayleigh numbers. The impossibility of this type of quasiperiodic bifurcation from the rest state in the rigid case is discussed in terms of the topology of the disconnected neutral curves. The numerical method employed is suitable for the computation of disconnected oscillatory neutral curves in other stab...

The transition from two-dimensional to three-dimensional planforms in infinite-Prandtl-number thermal convection

Abstract: The stability of two-dimensional thermal convection in an infinite-Prandtl-number fluid layer with zero-stress boundaries is investigated using numerical calculations in three-dimensional rectangles. At low Rayleigh numbers (Ra 2.22 (aspect ratio 1.6) are time dependent for Ra > 4 × 104. For every case in which the initial condition was a time-dependent large-aspect-ratio roll, two-dimensional convection was found to be unstable to three-dimensional convection. Time-dependent rolls are replaced by either bimodal or knot convection in cases where the horizontal dimensions of the rectangular box are less than twice the depth. The bimodal planforms are steady states for Ra [les ] 105, but one case at Ra = 5 × 105 exhibits time dependence in the form of pulsating knots. Calculations at Ra = 105 in larger domains resulted in fully three-dimensional cellular planforms. A steady-state square planform was obtained in a 2.4 × 2.4 × 1 rectangular box. started from random initial conditions. Calculations in a 3 × 3 × 1 box produced steady hexagonal cells when started from random initial conditions, and a rectangular planform when started from a two-dimensional roll. An hexagonal planform started in a 3.5 × 3.5 × 1 box at Ra = 105 exhibited oscillatory time dependence, including boundary-layer instabilities and pulsating plumes. Thus, the stable planform in three-dimensional convection is sensitive to the size of the rectangular domain and the initial conditions. The sensitivity of heat transfer to planform variations is less than 10%.

Oscillatory motion in a side-heated cavity

Abstract: Direct numerical simulations of the transient flow in a side-heated cavity have been conducted for a Rayleigh Number of 2 × 10 9 , an aspect ratio of 1 and a Prandtl Number of 7.1. The results show the presence of both long-period and short-period oscillations. The long-period oscillation is a cavity-scale mode produced by the tilting of the isotherms. The short-period oscillations are shown to be the result of two distinct boundary-layer instabilities. Whereas the latter oscillations can produce large deviations in the observed temperature records, they are relatively shortlived and have only a minor influence on the evolution of the flow towards steady state. The suggestion of the existence of an internal hydraulic jump in such flows has been investigated and found to be incorrect.

Linear instability in viscoelastic fluid convection

Abstract: The onset of convection in a viscoelastic fluid that obeys the Jeffreys model is investigated. Two boundary conditions have been considered separately: free-free and rigid-rigid. The role played by the retardation time, characteristic of the Jeffreys model, is emphasised. The threshold values of the parameters (critical Rayleigh number, critical wavenumber, onset frequency, etc.) for stationary and oscillatory convection are obtained. The frontier between oscillatory and stationary convection is calculated and the possibility to obtain a codimension-two point is discussed.

Heat transfer and the onset of chaos in a spherical, axisymmetric, anelastic model of whole mantle convection

Abstract: An isochemical uni-phase model of whole mantle convection has been developed in terms of which factors influencing the onset of time dependent chaotic behavior may be assessed. The model is spherical but restricted in generality to the analysis of axisymmetric solutions. In this paper we have employed it to examine the impact of compressibility and sphericity on the nature and onset of time dependence. Particular attention has been given to an examination of the impact that the onset of time dependence has upon the power law relation that connects the heat transfer (represented by the Nusselt number) to the strength of the thermal forcing (represented by the Rayleigh number). In order to obtain these results very extensive numerical simulations were required and the results themselves should be rather useful in the context of models of the thermal history of the planet.

An explanation for the multivalued heat transport found experimentally for convection in a porous medium

Abstract: Convection experiments were conducted in a porous slab 3 m in diameter and 30 cm thick, using two quite different media: a filling of rubberized curled coconut fibre and of clear polymethylmethacrylate beads. The second experiment involved the successful use of a visualization scheme for the flows at the upper boundary. Convection began in a hexagonal pattern with a slight tendency to form into rolls, but became very complex, irregular and three-dimensional at higher Rayleigh numbers, without developing any obvious temporal instabilities. Above a Rayleigh number of 1000 a significant number of dendritic downwellings appeared, where smaller downwellings seemed to feed into larger areas such that the whole complex may have converged into a single downwelling plume. The visualization provides direct confirmation that the lateral scale of the convection decreases with increasing Rayleigh number, approximately as ([Ascr ] + C)−0.5.Nusselt number versus Rayleigh number curves were obtained for both experiments. The only feature they have in common is a central section where the slope on a log/log graph is slightly over 0.5. On the graph from the first experiment, this section is preceded by a slope close to 1 and followed by a slope close to 0.33. The temperature measured at a point in the fill 25 mm below the top boundary was unsteady at conditions representative of the upper two segments of the graph; sensitivity was insufficient to measure fluctuations at lower temperature differences. The Nusselt number for the bead fill jumps upward just above onset (where [Ascr ] = 4π2), rapidly settles to a slope of 0.52, and then gradually breaks upward again to a slope of greater than 1. Increases in conductivity and permeability close to the boundary are not a large enough fraction of the boundary-layer thickness to cause this. A new phenomenon, lateral thermal dispersion, appears to be responsible. It occurs because there is no constant separation distance between adjacent channels in a bead fill. Thermal exchange in the junction pores exceeds the average if the flow is fast enough, especially when the fluid is more conductive than the beads.A simple boundary-conduction theory can be matched to the uncontaminated results. It is based on relative scaling of the residence time of fluid in the boundary layer, and predicts Nusselt number growth as the 0.55 power of Rayleigh number toward the high values typical of major geothermal areas.

Numerical simulation of oscillatory convection in low-Pr fluids : a GAMM Workshop

Abstract: Benchmark Definition.- 1. Finite Difference Methods.- Fine Mesh Solutions Using Stream Function-Vorticity Formulation.- A Comparison of Velocity-Vorticity and Stream Function-Vorticity Formulations for Pr=0.- Buoyancy-Driven Oscillatory Flows in Shallow Cavities Filled With Low-Prandtl Number Fluids.- A Finite-Difference Method With Direct Solvers for Thermally-Driven Cavity Problems.- Contribution to the GAMM Workshop.- Low Prandtl Number Convection in a Shallow Cavity.- Numerical Simulation of Oscillatory Convection in Low Prandtl Number Fluids With the TURBIT Code.- Marangoni Flows in a Cylindrical Liquid Bridge of Silicon.- Numerical Simulation of Oscillatory Convection in a Low Prandtl Fluid.- Steady-State Natural Convection in a Rectangular Cavity Filled With Low Prandtl Number Fluids.- Numerical Simulation of Oscillatory Convection in Low Prandtl Number Fluids Using AQUA Code.- Pressure Correction Splitting Methods for the Computation of Oscillatory Free Convection in Low Pr Fluids.- Influence of Thermocapillarity on the Oscillatory Convection in Low-Pr Fluids.- 2. Finite Volume Methods.- Numerical Simulation of Oscillatory Convection in Low-Pr Fluids.- An Implicit Pressure Velocity Algorithm Applied to Oscillatory Convection in Low Prandtl Fluid.- Oscillatory Natural Convection in a Long Horizontal Cavity.- Contribution of the Heat-Transfer Group at DELFT University.- Numerical Simulation of Oscillatory Convection in Low Prandtl Fluids.- 3. Finite Element Methods.- Application of the N3S Finite Element Code to Simulation of Oscillatory Convection in Low Prandtl Fluids.- Two- and Three-Dimensional Finite Element Simulations of Buoyancy-Driven Convection in a Confined Pr=0.015 Liquid Layer.- Two and Three-Dimensional Study of Convection in Low Prandtl Number Fluids.- Numerical Simulation of Oscillatory Convection in Low Prandtl Fluids.- The Solution of the Boussinesq Equations by the Finite Element Method.- Numerical Simulation of Oscillatory Convection in Low Pr Fluids by Using the Galerkin Finite Element Method.- 4. Spectral Methods.- Oscillatory Convection in Low Prandtl Fluids: A Chebyshev Solution With Special Treatment of the Pressure field.- Contribution to the GAMM Workshop With a Pseudo-Spectral Chebyshev Algorithm on a Staggered Grid.- Spectral Calculations of Convection in Low-Pr Fluids.- Spectral Method for Two-Dimensional Time-Dependent Pr?0 Convection.- Steady-State Solution of a Convection Benchmark Problem by Multidomain Chebyshev Collocation.- 5. Synthesis.- Synthesis of Finite Difference Methods.- Synthesis of the Results With the Finite-Volume Method.- Analysis of Finite Element Results.- Analysis of Spectral Results.- General Synthesis of the Numerical Results.- 6. Stability Results.- Linear and Non-Linear Analysis of the Hadley Circulation.- A Bifurcation Analysis of Oscillatory Convection in Liquid Metals.- 7. Experimental Results.- A Laboratory Study of Oscillations in Differentially Heated Layers of Mercury.- Subharmonic Transitions in Convection in a Moderately Shallow Cavity.- Convection in a Shallow Cavity.- Conclusions.- List of Participants.- Support and Sponsoring Acknowledgements.

Transition to turbulence via spatiotemporal intermittency in one-dimensional Rayleigh-Bénard convection.

Abstract: Rayleigh-Benard convection is studied in quasi-one-dimensional geometries. Fixed and periodic boundary conditions are imposed using a rectangular and an annular cell, respectively. The destabilization process of the homogeneous convective pattern is studied for increasing Rayleigh number A. The first time-dependent behaviors are given by the appearance of coupled oscillators. At larger A values, the spatial breakdown appears through the propagation of spatial defects, which appear to be solitary waves. This spatiotemporal destabilization is followed at higher % by a spatiotem-poral intermittent regime, which corresponds to a dramatic decrease of the spatial coherence and to a mixing of turbulent patches within laminar domains. This last regime is studied within the frame of phase transitions. The statistical analysis evidences a second-order phase transition at least in the rectangular geometry (fixed boundary conditions), while this transition looks imperfect in the annu-lar geometry (periodic boundary conditions). Nevertheless, the essential qualitative features shown by theoretical and numerical models are observed in both geometries. Comparison with a simple model of directed percolation shows that the imperfect nature of the transition in the annulus could be the consequence of some mechanism of self-generation of the turbulent domains. This mechanism is, however, unknown but is probably related to the influence of the boundaries.

Steady-State Natural Convection in Empty and Partitioned Enclosures at High Rayleigh Numbers

Abstract: Steady-state natural convection, which occurs in building enclosures (Rayleigh numbers of 1010 ), was studied experimentally in a full-scale room and in a 1:5.5 small-scale physical model containing R114 gas. The model was geometrically similar, had the same Rayleigh number, and had the same dimensionless end wall temperatures as the full-scale room. Configurations were tested with the enclosure empty, with a vertical partition extending from the floor to midheight, and with the vertical partition raised slightly off the floor. For isothermal opposing end walls at different temperatures, excellent agreement was found between the full-scale room and the scale model in flow patterns, velocity levels, temperature distributions, and heat transfer, even though the radiation heat transfer was not scaled between the two models.

Chaotic, subduction-like downflows in a spherical model of convection in the Earth's mantle

Abstract: Model calculations are described for a compressible fluid in a three-dimensional spherical shell with 80 percent of the surface heat flow generated within the model mantle. The numerical solutions are strongly chaotic, with surface planforms dominated by long curvilinear downflows reminiscent of the descending slabs in the earth's mantle. The results suggest that descending slabs play an important part in driving mantle convection, and that their chaotic evolution may influence the spatial and temporal behavior of plates and thus the dispersal and aggregation of continents.