Flow Past a Rotating Cylinder at Low and High Rotation Rates

Flow past a rotating circular cylinder is studied experimentally. The experiments are carried out in a water tunnel at Reynolds numbers of 200, 300, and 400 and nondimensional rotation rates (ratio of surface speed of the cylinder to the free stream velocity), $α$⁠, varying from 0 to 5. The diagnostic is done by flow visualization using hydrogen bubble technique and quantitative measurements using a particle image velocimetry technique. We present the global view of the wake structure at the three Reynolds numbers and various rotation rates. Vortex shedding activity is observed to occur from $α=0$ to $α~1.95$⁠, after which it is suppressed. Reynolds number is found to have a strong effect on the wake morphology near the suppression rotation rate, $α=1.95$⁠. Interestingly, the vortex shedding activity again resumes in the range $4.34<α<4.70$ as first discovered numerically (Mittal and Kumar, 2003, “Flow past a rotating cylinder,” J. Fluid Mech., 476, 303) for Re = 200. The shed vortices are of one sign in this range of rotation rates. Experimental evidence of this new vortex shedding mode is presented, for the first time, at $α=4.45$ in the newly discovered window of rotation rates, using flow visualization and particle image velocimetry measurements. Strouhal number measurements and global wake patterns agree well with the computations of Mittal and Kumar at a Reynolds number of 200.