Precessional states in a laboratory model of the Earth's core

A water‐filled three‐meter diameter spherical shell, geometrically similar to the Earth's core, shows precessionally forced flows. The precessional torque is supplied by the daily rotation of the laboratory by the Earth. We identify the precessionally forced flow to be primarily the spin‐over inertial mode, i.e., a uniform vorticity flow whose rotation axis is not aligned with the sphere's rotation axis. A systematic study of the spin‐over mode is carried out, showing that the amplitude depends on the ratio of precession to rotation rates (the Poincaré number), in marginal qualitative agreement with Busse's (1968) laminar theory. We find its phase differs significantly though, likely due to topographic effects. At high rotation rates, free shear layers are observed. Comparison with previous computational studies and implications for the Earth's core are discussed.