Roughness as a Route to the Ultimate Regime of Thermal Convection
We use highly resolved numerical simulations to study turbulent Rayleigh-Benard convection in a cell with sinusoidally rough upper and lower surfaces in two dimensions for Pr = 1 and Ra = [4 x 10(6), 3 x 10(9)]. By varying the wavelength. at a fixed amplitude, we find an optimal wavelength lambda(opt) for which the Nusselt-Rayleigh scaling relation is (Nu - 1 proportional to Ra-0.483), maximizing the heat flux. This is consistent with the upper bound of Goluskin and Doering [J. Fluid Mech.