Coherent Structures in Compressible Convection
Abstract
An issue of some debate in numerical simulations of compressible convection is concerned with whether the flows break up into multiple cells in the vertical, or whether coherent flow structures exist which span much of the depth range. We have carried out a series of two dimensional simulations to determine whether vertically coherent flow patterns found in some of the earlier studies persist as the convective motions are required to carry an ever greater fraction of the total energy flux upward (in the limit of very "efficient" convection), or when the density contrast across the convective layer is high (factor of ~100), or when an eddy viscosity is employed instead of a constant dynamic viscosity. Such circumstances were thought by some to be responsible for the compressible convection breaking up into multiple cells, in a manner favored by mixinglength models. In all the cases that we have studied, the convection consists of flows coherent over the entire depth of the computational domain, with no tendency to form a series of cells in the vertical, contrary to the basic assumption of mixinglength approaches. The convective flows are often supersonic near the upper boundary. In some cases fluttering shocks form on the sides of the downflows, and significant modulation of the enthalpy and kinetic fluxes by vertical acoustic pulsations is observed. It appears that sufficient thermal diffusion and a high density contrast create a situation that favors the formation of supersonic regions near the upper boundary. Yet substantial thermal diffusion may also broaden the shocks into smooth transition regions, and large pressure fluctuations found near the top of downflows serve to decelerate the fast horizontal flows. Pressure fluctuations often can result in positive density fluctuations and thus cause buoyancy braking throughout the upflows, even though the fluid there is warmer than its surroundings. Since vigorous (and often supersonic) motions were encountered, we then turned to examine ways to account for effects of subgridscale (SGS) turbulence expected to coexist with such shearing flows. Preliminary experiments with a promising numerical technique known as largeeddy simulations (LES) were performed which incorporated SGS terms. These twodimensional LES simulations again yield flow patterns much like our other results. However, interesting properties of the SGS models that may affect gross structures of the flows (such as flow symmetry) have been revealed, though the overall SGS effects are modest within such two dimensional treatments. The SGS terms considered here can be incorporated readily into future threedimensional LES simulations of compressible convection.
 Publication:

Ph.D. Thesis
 Pub Date:
 1991
 Bibcode:
 1991PhDT.........5X
 Keywords:

 STELLAR THEORY;
 Physics: Astronomy and Astrophysics, Physics: Fluid and Plasma;
 Compressible Flow;
 Convection;
 Convection Cells;
 Convective Flow;
 Flow Distribution;
 Mixing Length Flow Theory;
 Shear Flow;
 Turbulence Models;
 Two Dimensional Models;
 Vertical Air Currents;
 Eddy Viscosity;
 Flux Density;
 Pressure Oscillations;
 Supersonic Flow;
 Thermal Diffusion;
 Fluid Mechanics and Heat Transfer