Simulations of the kinematic dynamo onset of spherical Couette flows with smooth and rough boundaries

TL;DR

This study numerically investigates how boundary roughness affects the onset of magnetic field generation in spherical Couette flows, revealing that rough boundaries lower the dynamo threshold in turbulent regimes.

Contribution

It introduces a novel simulation approach comparing smooth and rough boundary conditions to understand their impact on dynamo onset in spherical flows.

Findings

Rough boundary conditions reduce the critical rotation rate for dynamo onset.

Boundary layer thickness influences the dynamo threshold.

Rough surfaces may enhance the feasibility of experimental dynamo setups.

Abstract

We study numerically the dynamo transition of an incompressible electrically conducting fluid filling the gap between two concentric spheres. In a first series of simulations, the fluid is driven by the rotation of a smooth inner sphere through no-slip boundary conditions, whereas the outer sphere is stationary. In a second series a volume force intended to simulate a rough surface drives the fluid next to the inner sphere within a layer of thickness one tenth of the gap width. We investigate the effect of the boundary layer thickness on the dynamo threshold in the turbulent regime. The simulations show that the boundary forcing simulating the rough surface lowers the necessary rotation rate, which may help to improve spherical dynamo experiments.