Experimental studies of lift and drag forces upon cylindrical obstacles in homogeneous, rapidly rotating fluids

Abstract

A series of laboratory measurements is described, in which simultaneous drag and lift forces have been measured directly on full and truncated cylinders moving uniformly through homogeneous rotating fluids. For the full cylinder experiments, it has been demonstrated that within the Reynolds ( Re ) and Rossby ( Ro ) number ranges 9.8 × 10 3 ≤ Re ≤5.6 × 10 4 and 0.1 ≤ Ro ≤0.9, the mean total lift coefficient and the mean lift coefficient because of wake asymmetry both increase with decreasing Rossby number Ro . For cases in which the fluid is non-rotating, the total lift coefficient is shown to be zero, and the values of the drag coefficient for the cylinder are shown to be in accord with existing drag for this geometry. When the value of the Reynolds number is sufficiently small, the presence of background rotation is shown to result in a two- to three-fold increase in the drag coefficient. Experiments with truncated cylinders show similar behaviour to the full cylinder cases with regard to the dependence of the drag coefficient upon Ro and Re . However, values of the lift coefficient are shown to be increased considerably over corresponding full cylinder cases throughout the Reynolds number range, particularly at low and intermediate values of the inverse Rossby number Ro −1 . The increase in lift is attributed to (1) the accentuation of wake asymmetry caused by vortex tube stretching as fluid passes below the truncated cylinder, and (2) the generation of an unbalanced Coriolis force on the volume of fluid displaced by the cylinder.