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Excitation of inertial modes in an experimental spherical Couette flow

Michel Rieutord^*

Santiago Andrés Triana^† and Daniel S. Zimmerman^‡

Daniel P. Lathrop^§

Université de Toulouse, UPS-OMP, IRAP, Toulouse, France and CNRS, IRAP, 14 avenue E. Belin, 31400 Toulouse, France

Department of Physics and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742, USA

Department of Physics, Department of Geology, Institute for Physical Science and Technology, and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742, USA

^*michel.rieutord@irap.omp.eu
^†triana@umd.edu
^‡danzimmerman@gmail.com
^§lathrop@umd.edu

Phys. Rev. E 86, 026304 – Published 6 August, 2012

DOI: https://doi.org/10.1103/PhysRevE.86.026304

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 nonaxisymmetric, retrograde, equatorially antisymmetric 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, 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.

Article Text

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