On the excitation of inertial modes in an experimental spherical Couette flow

TL;DR

This study investigates inertial modes in spherical Couette flow, identifying their properties and excitation mechanisms through numerical modeling and experimental observations, revealing critical conditions for mode excitation related to shear layer instabilities.

Contribution

The paper provides the first detailed identification of inertial modes in spherical Couette flow and links their excitation to critical layers and shear instabilities, enhancing understanding of flow dynamics.

Findings

Inertial modes are non-axisymmetric, retrograde, equatorially anti-symmetric.

Mode excitation occurs below a critical Rossby number.

Excitation is driven by instabilities associated with shear layers along the tangent cylinder.

Abstract

Spherical Couette flow (flow between concentric rotating spheres) is one of flows under consideration for the laboratory magnetic dynamos. Recent experiments have shown that such flows may excite Coriolis restored inertial modes. The present work aims to better understand the properties of the observed modes and the nature of their excitation. Using numerical solutions describing forced inertial modes of a uniformly rotating fluid inside a spherical shell, we first identify the observed oscillations of the Couette flow with non-axisymmetric, retrograde, equatorially anti-symmetric inertial modes, confirming first attempts using a full sphere model. Although the model has no differential rotation, identification is possible because a large fraction of the fluid in a spherical Couette flow rotates rigidly. From the observed sequence of the excited modes appearing when the inner sphere is slowed down by step, we identify a critical Rossby number associated with a given mode and below which it is excited. The matching between this critical number and the one derived from the phase velocity of the numerically computed modes shows that these modes are excited by an instability likely driven by the critical layer that develops in the shear layer staying along the tangent cylinder of the inner sphere.