Direct numerical simulations of optimal thermal convection in rotating plane layer dynamos

Abstract

The heat transfer behaviour of convection-driven dynamos in a rotating plane layer between two parallel plates, heated from the bottom and cooled from the top, is investigated. At a fixed rotation rate (Ekman number, $E=10^{-6}$ ) and fluid properties (thermal and magnetic Prandtl numbers, $Pr=Pr_m=1$ ), both dynamo convection (DC) and non-magnetic rotating convection (RC) simulations are performed to demarcate the effect of magnetic field on heat transport at different thermal forcings (Rayleigh number, $Ra=3.83\times 10^{9}\unicode{x2013}3.83\times 10^{10}$ ). In this range, our turbulence resolving simulations demonstrate the existence of an optimum thermal forcing, at which heat transfer between the plates in DC exhibits maximum enhancement, as compared with the heat transport in the RC simulations. Unlike any global force balance reported in the literature, the present simulations reveal an increase in the Lorentz force in the thermal boundary layer, due to stretching of magnetic field lines by the vortices near the walls with a no-slip boundary condition. This increase in Lorentz force mitigates turbulence suppression due to the Coriolis force, resulting in enhanced turbulence and heat transfer.