Modes of vortex shedding from a rotary oscillating plate

The wake of a two-dimensional rotary oscillating plate of chord to thickness ratio B/H = 3.0 was investigated by smoke wire visualization and hot-wire measurement in a wind tunnel at Re = 1500 and oscillation amplitudes and reduced frequencies in the ranges of alpha = 2.5 degrees-50 degrees and f(e)H/V-infinity = 0.02-0.25, respectively, where f(e) is oscillation frequency, V-infinity is on-coming wind speed. There are six modes of synchronized vortex shedding observed, including three new modes, i.e., 3P-three vortex pairs shed in one period, 2S(2)-two pairs of vortex twins shed in one period, and T-three vortices shed in one period of plate oscillation. The zone of every mode was found out and drawn on the amplitude-reduced frequency plane. Numerical simulation was carried out, and the simulated vorticity field, velocity profiles, and wake width were verified by experiment at typical oscillation conditions. At higher oscillation frequencies, the linear local unsteady term of inertial force dominates, and the simulated moment is an accurate linear function of rotary displacement and angular velocity. At lower frequencies, where 2T, 3P, and 2S(2) modes of vortex shedding occur, however, the nonlinear convective term of inertial force dominates, and the simulated fluctuating moment deviates from Scanlan's expression. Statistical results of numerical data show that the root-mean-square (r.m.s) value of fluctuating moment is an exponential function of single parameter eta = alpha f(e)H/V-infinity.