Hydrodynamic Damping and Quasi-Coherent Structures at Large Strokes Numbers

Abstract

Unsteady flow about solid and perforated cylinders at large Stokes numbers and very small Keulegan-Carpenter numbers has not been sufficiently understood to predict the hydroelastic response of compliant structures subjected to high frequency excitation. It is impossible to compute, difficult to quantify, and extremely difficult to visualize. Here, following a brief review of the previous research, new results for both forced and pluck-induced oscillations are presented. It is shown that: (a) the measured drag coefficients are larger (about double for smooth cylinders) than those predicted from the Stokes-Wang analysis (in the region of its applicability); (b) there is an unstable #ow regime in which the oscillatory boundary layer develops quasicoherent structures (QCS) over a range of K values before giving rise to Honji-type coherent structures (HTCS); (c) the drag coefficients for perforated cylinders are much larger than those for solid cylinders and do not follow the Stokes}Wang prediction even at small b. As expected, their inertia coefficients are considerably smaller than those for solid cylinders.