20 Oct 2011

Laboratory Models of the Earth’s Core: The Liquid Sodium Sphere

Dr. Daniel Lathrop, University of Maryland

The University of Maryland Three Meter Geodynamo experiment is the largest and most powerful in a series of experiments designed to achieve a self-excited magnetic field from a turbulent liquid metal model of a planetary core. The Three Meter experiment consists of concentric, independently rotating spherical boundaries of radius ratio 0.35 with an outer boundary diameter of nearly 3 m. The gap between the spheres is filled with 13,000 L of liquid sodium metal. The device allows access to regimes of turbulent hydromagnetic flow dominated by rapid rotation (with Ekman number lower than 10^–8) and achieving high magnetic Reynolds number (R_m), exceeding that estimated in Earth’s core (300-500). We have undertaken a series of mechanical tests and hydrodynamic investigations in water, including a systematic study of flows forced by the precession of the container axis as Earth rotates. These precession-driven flows are dominated by the spinover inertial mode and exhibit internal shear layers. Additional experiments in the sheared flow of water between the spheres show a wide variety of phenomena, including turbulent bi-stability and unusual angular momentum transport.