Dynamo 3.5
Dynamo 3.5 is a modified version of Dynamo III. Its outer spherical vessel is 60 cm in diameter, can hold about 110 liters of sodium, and rotates, driven by an AC motor. Dynamo 3.5 differs from Dynamo III in that the fluid motions are initiated mechanically (instead of thermally) via an inner shaft that also rotates, independent of the outer vessel, driven by a separate AC motor. By forcing the fluid mechanically, we gain access to faster, more turbulent flows and new dynamics. Our data is comprised of magnetic field measurements, taken with a collection of Hall probes mounted near the surface of the sphere (red dots with arrows in the diagram). We've subjected the flow to various external magnetic fields; our current setup uses a pair of electromagnets oriented parallel to the axis of rotation that can produce about 400 G, as well as a second, smaller pair oriented transverse to the axis.
Spherical Coutte flow and inertial waves
For the first set of experiments in Dynamo 3.5, we constructed an inner sphere (20 cm in diameter) and mounted it on the inner shaft, concentric with the outer sphere, in the so-called spherical Couette geometry. The 1:3 radius ratio models the Earth's core. Applying a DC magnetic field, we see significant AC magnetic induction at some rotation rate combinations, and have been able to identify many of the induction patterns as inertial modes. Our results (Kelley et al 2007) are forthcoming in Geophysical and Astrophysical Fluid Dynamics.
S1T1 flow
More recently we have replaced the inner sphere with a propeller that pumps fluid upward along the axis of rotation. By adjusting the speed of the propeller and the speed of the outer sphere, we can tune the ratio of pumping to rotation, which allows us to seek the S1T1 dynamo mode predicted numerically by Dudley and James in 1989. Six baffles, each 2 cm thick and mounted on the inner surface of the sphere, reduce rotation of the fluid. Initial results from this configuration show wave-like behavior consistent with the dispersion relation that governs full-sphere Rossby waves. Currently under construction is an additional probe array (up to E7, as shown in diagram) allowing the unique identification of wave patterns with much finer structure (up to m=46) to verify the presence of Rossby waves.
Upcoming plans for Dynamo 3.5
- More carefully characterize the onset of inertial waves in spherical Couette flow, which seems to be associated with over-reflection at a shear layer.
- Reproduce the magnetorotational instability in spherical Couette flow, as observed in a similar experiment by Sisan et al.
- Use twin, back-to-back propellers to seek the S2T1 dynamo predicted by Dudley and James.
- Install a roughened inner spere.
- Implement direct velocity measurement of the flow.