9220061 Parmentier The studies will explore the physics of high Rayleigh number, 3D thermal convection in an infinite Prandtl number fluid based on numerical experiments. Numerical experiments completed thus far for a volumetrically heated fluid cooled from above show that fluid below the thermal boundary layer at the top boundary is substantially non-adiabatic, contrary to the usual assumptions about the structure of thermal convection at high Ra. The structure of the flow consists of nearly axisymmetric plumes that are fed by the cold boundary layer. Linear structures, that would presumably represent subduction zones in the convecting mantle, appear only infrequently. The PI will carry out further numerical experiments that include heating from below as well as volumetric heating. The principal innovation is a multigrid iterative solver for buoyant viscous flow that is an order of magnitude or more faster than the FFT methods that have frequently been used in 3D convection problems. The PI will extend development of multigrid solvers to buoyant fiscous flow with variable viscosity. This will be carried out by solving equations formulated in primitive variable (pressures and velocity components) rather than in vector streamfunction and vorticity or some other potential function formulation. The first application of this new methodology will be to understand how temperature and depth dependent viscosity modify results of the constant viscosity, high Rayleigh number numerical experiments. ***
