Towards spectral-element large-scale simulations of turbulent Rayleigh-Bénardconvection in round cells

The (potential) transition to the ultimate regime in turbulent Rayleigh-BénardConvection (RBC) hasstill not been shown conclusively through direct numerical simulation. Inthis talk, our developments towardssimulationsof RBCusingthe high-fidelity spectral-element code Neko are considered.The code has a specific focus onlarge-scale direct numerical simulations ofturbulenceand incorporates a modular multi-backend design,enabling performance portability across a wide range of GPUs and CPUs, as well as a GPU-optimized preconditioner with task overlapping for the pressure-Poisson equation and in-situ data compression.We carry out initialsimulations of RBC in a slender circular cell with aspect ratio 1:10 on both the LUMI and Leonardo supercomputers. Neko is able to strong scale to 16,384 GPUsfor a mesh with over 100M spectral elements, correspondingto more than 37B grid points.The Nusselt number and its dependence on Rayleigh numberRa for a few smaller simulations at lower Ra is discussed.We are currentlyunder way with carrying out and preparing simulations at even higher Rayleigh numbers, tonumericallyobservewhether and whenthe transition to the ultimate regime occurs.