Vortex shedding in flows of dilute polymer solutions

Experiments on vortex shedding from a cylinder placed in uniform flows of low concentration polymer solutions are reported for Reynolds numbers from 50 to 150. The fluids used were aqueous solutions of polyethylene oxide (PEO) and Weissenberg numbers were O   (10−110−1). It is shown that even small amounts of PEO have a significant stabilizing effect on the von Kármán instability which is in turn destabilized by shear-thinning at higher concentrations. The presence of PEO is also observed to reduce the saturated vortex shedding frequency and cause a redistribution of velocity fluctuations in the attached shear layers. At higher concentrations further changes in the velocity fluctuations are noted in the wake. Downstream of the cylinder an overshoot in the time-average streamwise velocity is measured for the polymer solutions and an analogy is drawn to the negative wake behind a sphere falling in a viscoelastic fluid.