Magnetic and Acoustic Investigations of Turbulent Spherical Couette Flow
Magnetic and Acoustic Investigations of Turbulent Spherical Couette Flow
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Date
2016
Authors
Adams, Matthew Michael
Advisor
Lathrop, Daniel P
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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.