Three-dimensional characteristics of the jet flows induced by a pitching plate in a quiescent fluid

Jet flows induced by pitching cantilever plates provide a power-efficient solution for fluid acceleration and cooling enhancement. In such applications, the time-averaged (mean) properties of the induced jet flows are of great importance. We report a combined experimental and numerical study on the three-dimensional (3-D) characteristics of the mean jet downstream of a harmonically pitching cantilever plate in a quiescent fluid. These characteristics are then correlated with the transient 3-D vortex structures emanated from the trailing and side edges. Our particle image velocimetry and 3-D numerical simulations reveal that the mean induced jet has two distinct regions - a shrinking region immediately downstream of the trailing edge followed by an abrupt expansion region - separated by a necking point. We investigate the transient 3-D wake vortex evolution downstream of the plate to help elucidate the physics underlying the geometry of the mean jet. Our observations suggest that the breakdown of the shed vortex structure and reorientation of the consequent substructures are the primary factors governing the shape of the jet. These factors in turn are controlled primarily by the plate width and the amplitude of the oscillations. The results presented in this study improve our understanding of the complicated 3-D geometry of the induced mean jet in oscillating plates and facilitate optimal design of devices that operate based on this principle, such as piezoelectric fans.