Optimal heat transport solutions for Rayleigh–Bénard convection

Abstract

Steady flows that optimize heat transport are obtained for two-dimensional Rayleigh–Bénard convection with no-slip horizontal walls for a variety of Prandtl numbers $\mathit{Pr}$ and Rayleigh number up to $\mathit{Ra}\sim 10^{9}$. Power-law scalings of $\mathit{Nu}\sim \mathit{Ra}^{{\it\gamma}}$ are observed with ${\it\gamma}\approx 0.31$, where the Nusselt number $\mathit{Nu}$ is a non-dimensional measure of the vertical heat transport. Any dependence of the scaling exponent on $\mathit{Pr}$ is found to be extremely weak. On the other hand, the presence of two local maxima of $\mathit{Nu}$ with different horizontal wavenumbers at the same $\mathit{Ra}$ leads to the emergence of two different flow structures as candidates for optimizing the heat transport. For $\mathit{Pr}\lesssim 7$, optimal transport is achieved at the smaller maximal wavenumber. In these fluids, the optimal structure is a plume of warm rising fluid, which spawns left/right horizontal arms near the top of the channel, leading to downdraughts adjacent to the central updraught. For $\mathit{Pr}>7$ at high enough $\mathit{Ra}$, the optimal structure is a single updraught lacking significant horizontal structure, and characterized by the larger maximal wavenumber.