Angular Momentum Transport in Turbulent Spherical Couette Flow

Angular momentum transport in rapidly rotating fluids is a topic of great importance in geophysical and astrophysical contexts. We present experimental measurements of boundary torque, wall shear stress, pressure, and velocity in the turbulent shear flow of water between two spherical boundaries of radius ratio 0.35, geometrically similar to Earth's core. We achieve unprecedented Reynolds number (Re = (Ω _{i-Ω_o)L^2/ν} ) in this geometry, higher than 50 million, along with Ekman number ( ν /Ω L²) as low as 10^-7. As we vary the Rossby number (Ro=(Ω _{i-Ω_o)/Ω_o} ), we observe several turbulent flow transitions including bistable regimes of intermittent switching between adjacent states. These transitions have a dramatic effect on the angular momentum flux through the device as measured globally by the torques required to keep the boundaries at constant speed. The transitions are observed in all measured quantities, with a spatial redistribution of wall shear stress, large changes in local measurements of mean azimuthal velocity, and appearance and disappearance of system scale waves. The evidence seems consistent with the formation and destruction of zonal flow transport barriers to angular momentum. For all fixed Ro, as we increase Re, we find that the torque increases in a manner similar to other turbulent shear flows.