Flow past a circular cylinder in a rotating stratified fluid

Abstract

Experimental results are presented to show the development of the flow past a cylinder in a rotating stratified fluid. Both transient and fully developed flow states are investigated, for a range of values of Rossby number Ro and Burger number Bu and for full depth and truncated obstacles. The fully developed flows are shown to be of three main types, delineated respectively as attached eddy pairs, transitional, and vortex wakes, with the boundaries between each flow type being determined by the values of the Reynolds number, the Burger number, the aspect ratio and the ratio of the height of the obstacle to the depth of the fluid. For cases of homogeneous fluid flow, the critical Rossby and Reynolds numbers for vortex shedding are significantly lower than the values found by Boyer. The discrepancy is ascribed to aspect ratio effects and differences in the cylinder shear layer structures in the two studies. The weakening effects of density stratification upon characteristic features of rapidly rotating homogeneous flows are seen to be two-fold: firstly, the wake asymmetries associated with the unstratified flows are seen to be destroyed by the stratification, and, secondly, the retention times of starting eddies, the eddy growth times and the isolated eddy formation times are shown to no longer tune with the background rotation timescale but to scale with the advective timescale of the motion and the aspect ratio of the cylinder. Cross-stream velocity profiles taken at several downstream stations in the wake of the cylinder are used to show a linear increase in the momentum flux coefficient with increasing Re , in the range 55 < Re < 135. Velocity profile measurements taken above and downstream of the truncated mcylinder are presented. These show that the vertical distance over which the upper level fluid is influenced by the presence of the obstacle depends strongly upon the value of Bu , and is typically one-tenth of the obstacle height at the highest values of Bu . Finally, flow visualization studies of the motion in the vertical shear layers for Bu = 0 are compared and contrasted with other similar observations, and with theoretical understandings of the flow structure. The importance of the upper boundary condition is clarified.