Experimental Comparison of Unstratified and Stratified Drag Wakes of a Dimpled Sphere at Reynolds Number 105

The drag wake of a dimpled sphere is studied experimentally using stereo particle image velocimetry at a Reynolds number of 105 in both unstratified and stratified (Froude number ≃80) fluids at downstream distances of 2≤x/D≤15. More than eighty experiments were conducted, and both analyses of ensemble-mean wakes and statistics of individual experiment wake data are presented. Stratification is found to qualitatively change the ensemble-mean wake axial velocity defect immediately behind the sphere, taking a Gaussian shape without stratification and an oval shape with stratification. As x/D increases, the impact of stratification decreases up to the limit of the data at x/D=15. Analysis of individual experiment wakes indicates that most of the difference between unstratified and stratified ensemble-mean wakes at x/D>10 is because stratification reduces wake meandering in the vertical direction.