Magnetic and Acoustic Investigations of Turbulent Spherical Couette Flow

Abstract

Title of dissertation: MAGNETIC AND ACOUSTIC INVESTIGATIONS OF TURBULENT

SPHERICAL COUETTE FLOW

Matthew M. Adams, Doctor of Philosophy, 2016

Dissertation directed by: Professor Daniel Lathrop

Department of Physics

This dissertation describes experiments in spherical Couette devices, using both gas and liquid

sodium. The experimental geometry is motivated by the Earth's outer core, the seat of the geodynamo, and

consists of an outer spherical shell and an inner sphere, both of which can be rotated independently to

drive a shear flow in the fluid lying between them. In the case of experiments with liquid sodium, we apply

DC axial magnetic fields, with a dominant dipole or quadrupole component, to the system.

We measure the magnetic field induced by the flow of liquid sodium using an external array of Hall effect

magnetic field probes, as well as two probes inserted into the fluid volume. This gives information about possible

velocity patterns present, and we extend previous work categorizing flow states, noting further information

that can be extracted from the induced field measurements. The limitations due to a lack of direct velocity

measurements prompted us to work on developing the technique of using acoustic modes to measure zonal flows. Using

gas as the working fluid in our 60~cm diameter spherical Couette experiment, we identified acoustic modes of the

container, and obtained excellent agreement with theoretical predictions. For the case of uniform rotation of the

system, we compared the acoustic mode frequency splittings with theoretical predictions for solid body flow, and

obtained excellent agreement. This gave us confidence in extending this work to the case of differential rotation,

with a turbulent flow state. Using the measured splittings for this case, our colleagues performed an inversion to

infer the pattern of zonal velocities within the flow, the first such inversion in a rotating laboratory experiment.

This technique holds promise for use in liquid sodium experiments, for which zonal flow measurements have historically

been challenging.