Effects of boundary conditions on the dynamics of convection-driven plane layer dynamos.

Rapidly rotating convection-driven dynamos are investigated under different kinematic and magnetic boundary conditions using direct numerical simulations. At a fixed rotation rate, represented by the Ekman number, E=5×10^-7, the thermal forcing is varied from 2 to 20 times its critical value (R=Ra/Ra_c=2-20, where Ra is the Rayleigh number), keeping the fluid properties constant (Pr=Pr_m=1, where Pr and Pr_m are the thermal and magnetic Prandtl numbers. For R=3, the horizontal and vertical velocities are higher with no-slip conditions compared to free-slip conditions at the wall. The structure and strength of the magnetic field produced by the dynamos, especially near the walls, depend on both velocity and magnetic boundary conditions. Though the leading-order force balance in bulk remains geostrophic, the Lorentz force becomes comparable to the Coriolis force inside the thermal boundary layer with no-slip, electrically conducting conditions. We also find enhanced heat transfer in the rotating dynamo convection, as compared with non-magnetic rotating convection, with the peak enhancement lying in the range R=3-4. The dynamo action may significantly enhance the heat transport for free-slip conditions by suppressing large-scale vortex formation. However, the peak enhancement is found at R=3 with no-slip, electrically conducting walls, which can be attributed to a local magnetorelaxation of the rotational constraint due to enhanced Lorentz force inside the thermal boundary layer.