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Graduate Student Seminar - 3/29/2019

"The Three-Meter Model of the Earth's Core: Increasing Helicity Toward Achieving Dynamo Action"

by Rúben Rojas

Advisor:  Professor Dan Lathrop

Friday, March 29, 2019 -- 12:00 p.m.
Large Conference Room, 1207 Energy Research Facility

Abstract: The dynamo generation of magnetic fields in turbulent flows of conducting fluids and plasmas are important features of stars and planetary cores. Spherical Couette flows, which are shear-driven flows between two concentric and independently rotating shells, are one of the experimental approaches used to experimentally probe magnetohydrodynamics in similar geometries. However, dynamo states have yet not been achieved in these systems. Our goal is to achieve a dynamo state in our three-meter diameter spherical Couette model of the Earth’s core. Amplifications of the magnetic field between 10-30% have been observed, but without a self-sustaining dynamo. Numerical studies of Finke and Tilgner (Phys. Rev. E, 86:016310, 2012) suggest roughening the inner sphere, which can be achieved by adding baffles on the inner sphere. Those studies showed a reduction in the threshold for dynamo action by increasing the poloidal flows with respect to the zonal flows and hence increasing helicity. In this work, we use a 40-cm spherical Couette water apparatus to characterize the effect of different baffle designs on flow dynamics. We present velocity profiles and torque measurements, which give us insight into baffle design for roughening of the three-meter experiment. We propose a new baffle design that improve our chances of observing dynamo action by breaking the symmetry in the parameter space of the experiment and generating flows of different topology. We gratefully acknowledge support from NSF EAR-1417148, and helpful conversations with A. Tilgner and K. Moffatt.

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