Laboratory Studies of Pseudo - Periodic Forcing Due to Vortex Shedding from an Isolated Solid Obstacle in a Homogeneous Rotating Fluid

Abstract

Experimental data are presented on the behaviour of free vortices generated by differential rotation N of a vertical rod in a rotating homogeneous fluid. It is demonstrated that in the case of cyclonic differential rotation, the vortices have sharply-defined circular boundaries, with limiting radii determined by the values of N and the background rotation period T. The anticyclonic counterparts are shown to have no well-defined horizontal spatial structure. A theoretical model is proposed to account for these differences in terms of the different secondary flows induced in the interiors of the vortices by the Ekman pumping mechanism. Experimental data are presented in support of the model. Previous studies by the authors on wake asymmetry behind circular cylinders in rapidly rotating homogeneous flows are reviewed in the light of the free vortex data. It is shown that the effect of the asymmetry is to generate a net horizontal lift force on the cylinder, the magnitude of which is proportional to the inverse Rossby number Ro −1 . The components of the instantaneous lift forces which fluctuate with the Strouhal frequency and which show amplitude modulation are also discussed in relation to the structures of the cyclonic and anticyclonic eddies in the cylinder wake.