Wall modes in magnetoconvection at high Hartmann numbers

Three-dimensional turbulent magnetoconvection at a Rayleigh number of Ra=10⁷ in liquid gallium at a Prandtl number $Pr=0.025$ is studied in a closed square cell for very strong external vertical magnetic fields B₀ in direct numerical simulations which apply the quasistatic approximation. As B₀ , or equivalently the Hartmann number $Ha$ , are increased, the convection flow, which is highly turbulent in the absence of magnetic fields, crosses the Chandrasekhar linear stability limit for which thermal convection ceases in an infinitely extended layer and which can be assigned a critical Hartmann number Hac . Similar to rotating Rayleigh–Bénard convection, our simulations reveal subcritical sidewall modes that maintain a small but finite convective heat transfer for Ha>Hac . We report a detailed analysis of the complex two-layer structure of these wall modes, their extension into the cell interior, and a resulting sidewall boundary layer composition that is found to scale with the Shercliff layer thickness.