Unraveling the Boundary Layers of High Rayleigh Number Convection Through Direct Numerical Simulations

We study the behavior of boundary layers in thermal convection at Rayleigh numbers ranging from 10^5 to 10^12 in a box of aspect ratio Γ = 4, and periodically extended sides. We use the GPU accelerated spectral element solver, NekRS, for our simulations. At the highest Rayleigh number, the simulations utilize nearly 3400 A100 GPUs of the JUWELS Booster facility, to cover as many as 40 free-fall time units, enabling us to obtain reliable statistics in steady-state. This demonstrates the excellent scalability of the solver. Moreover, since the focus of the study is on the boundary layers at the top and bottom walls, we ensure that the near-wall regions are well-resolved. We observe that the boundary layers have both shear dominated and plume dominated regions, with a nearly constant area-fraction between these regions. We also study the structure of wall shear-stress fields and their connection to plume structures emanating from the walls.