title: A comparison of no-slip, stress-free and inviscid models of rapidly rotating fluid in a spherical shell.
abstract: We investigate how the choice of either no-slip or stress-free boundary conditions affects numerical models of rapidly rotating flow in Earth's core by computing solutions of the weakly-viscous magnetostrophic equations within a spherical shell, driven by a prescribed body force. For non-axisymmetric solutions, we show that models with either choice of boundary condition have thin boundary layers of depth E(1/2), where E is the Ekman number, and a free-stream flow that converges to the formally inviscid solution. At Earth-like values of viscosity, the boundary layer thickness is approximately 1 m, for either choice of condition. In contrast, the axisymmetric flows depend crucially on the choice of boundary condition, in both their structure and magnitude (either E(-1/2) or E(-1)). These very large zonal flows arise from requiring viscosity to balance residual axisymmetric torques. We demonstrate that switching the mechanical boundary conditions can cause a distinct change of structure of the flow, including a sign-change close to the equator, even at asymptotically low viscosity. Thus implementation of stress-free boundary conditions, compared with no-slip conditions, may yield qualitatively different dynamics in weakly-viscous magnetostrophic models of Earth's core. We further show that convergence of the free-stream flow to its asymptotic structure requires E ≤ 10(-5).
authors.affiliation: School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Philip W
authors.initials: PW
authors.lastname: Livermore
authors.affiliation: School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Lewis M
authors.initials: LM
authors.lastname: Bailey
authors.affiliation: School of Mathematics, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Rainer
authors.initials: R
authors.lastname: Hollerbach