Characteristics of vortex shedding from a sinusoidally pitching hydrofoil at high Reynolds number

The paper describes experiments on a sinusoidally pitching NACA 0012 hydrofoil with reduced frequency 0.16 <= k <= 2.51 and angle of attack amplitude 6 degrees <= alpha(m) <= 34 degrees. It extends the chord Reynolds number range with respect to earlier studies to Re = 5.9 x 10(4), 1.2 x 10(5), 1.9 x 10(5), and 2.4 x 10(5). Lift, drag, and moment are directly measured and compared with predictions from linear inviscid theory, while wake flowfields are visualized by phase-locked particle image velocimetry. Three vortex shedding regimes, i.e., the undulating wake, the reverse von Karman vortex street, and the leading-edge vortex, are identified from the phase-averaged vorticity fields at different k and am. The nonlinear effect on statistical features of the force and moment responses gradually increases with am. The viscous effect leads to relatively large deviations of the measured responses from the linear inviscid models at low k. With increasing Re the measured values gradually approach those predicted by the models. Moreover, linear inviscid theory cannot capture the alpha(m)-dependent behavior of the drag and moment phase lags psi(d) and psi(m), respectively, resulting in a larger deviation from the measured values. A linear variation of psi(m) with alpha(m) is found at k and Re for which the reverse von Karman vortex street dominates. Within the trailing-edge shedding regimes of the undulating wake and the reverse von Karman vortex street, the force and moment responses are largely insensitive to the wake pattern at varied alpha(m). However, in the leading-edge vortex regime, at low k and large alpha(m), the flow remains no longer attached and the time-dependent pitching moment is found to be strongly affected by the leading-edge vortex over the wing.