Unsteady flows around a rigid high-aspect-ratio flat plate in pitching motion
Unsteady flows around a rigid high-aspect-ratio flat plate in pitching motionUnsteady flows around a rigid high-aspect-ratio pitching plate immersed in free stream inside a hydrodynamic tunnel test section and the resulting forces generated were investigated for relatively low Reynolds numbers in the order of 10^3. Detailed flow field measurements were performed using a high resolution particle image velocimetry (PIV) system to characterize the unsteady vortex structures in the wake and to evaluate the corresponding mean force on the pitching plate.Two main pitching amplitudes (10° and 15°) were investigated with a wide range of Strouhal numbers varying from about 0.05 to 0.7, which includes the narrow range in which several aquatic species evolve, and is therefore interesting for the study of artificial propulsion systems. Different wake topologies gradually appeared as the Strouhal number was increased, notably a classic von Karman wake, an inverted von Karman wake, generally considered as the thrust production signature and an asymmetric inverted von Karman wake. The forces exerted on the plate are extracted from the numerical simulation results and it is shown, that reliable predictions for possible thrust production can be inferred from a conventional experimental control volume analysis, only when besides the wake’s mean flow the contributions from the velocity fluctuation and the pressure term are taken into account.