On the Unsteady Wake of a Rigid Plate under Constant Acceleration and Deceleration

Laboratory experiments are performed to explore distinct features of the unsteady wake induced by a rigid plate undergoing a series of accelerated and decelerated phases in a quiescent water tank. Each plate motion pattern is defined by a constant acceleration from the rest followed by the same magnitude deceleration until rest; i.e., both of them lasting the same time interval. Flow statistics are obtained for various plate acceleration/deceleration motions in a plane normal to the plate span using particle image velocimetry. Results revealed that the relative near-wake bulk flow underwent acceleration during 75% of the plate cycle; the associated maximum velocity was reached approximately at the middle of the deceleration phase in all the cases. Transversal velocity profiles in the near wake were significantly modulated by the magnitude and sign of the plate acceleration. The dynamics of the large-scale coherent motions shed by the plate was also dependent on plate acceleration pattern. However, the near-wake nondimensional circulation strength reached a plateau roughly at the end of the acceleration phase and was approximately conserved despite the modulation of the deceleration phase. In the early stage of the plate motion, the Lagrangian flow acceleration was dominant; however, the local acceleration dominated at the end of the cycle.