Turbulent drag in a rotating frame

What is the turbulent drag force experienced by an object moving in a rotating fluid? This open and fundamental question can be addressed by measuring the torque needed to drive an impeller at constant angular velocity $\omega$ in a water tank mounted on a platform rotating at a rate $\mathrm{\Omega }$. We report a dramatic reduction in drag as $\mathrm{\Omega }$ increases, down to values as low as $12$% of the non-rotating drag. At small Rossby number $Ro = \omega/\Omega$,thedecreaseindragcoefficient$K$ follows the approximate scaling law $K\sim Ro$, which is predicted in the framework of nonlinear inertial wave interactions and weak-turbulence theory. However, stereoscopic particle image velocimetry measurements indicate that this drag reduction rather originates from a weakening of the turbulence intensity in line with the two-dimensionalization of the large-scale flow.