On performance of an oscillating plate underwater propulsion system with variable chordwise flexibility at different depths of submergence

In this work, an oscillating plate propulsor undergoing a combination of heave translation and pitch rotation is investigated experimentally. The oscillation kinematics are inspired by swimming mechanisms employed by fish and other marine animals. The primary focus was on the propulsive characteristics of such oscillating plates, which were studied by means of direct force measurements in the thrust-producing regime. Experiments were performed at constant Reynolds number and constant heave amplitude. By varying the Strouhal number, the depth of submergence and the chordwise flexibility of the plate, it was possible to investigate corresponding changes in the generated thrust and the hydromechanical efficiency. It was possible to establish a set of parameters, including the driving frequency of the system, the ratio of rigid to flexible segment length of the plate, and the range of Strouhal numbers that led to an overall increase in thrust and efficiency. The experiments, involving plates with various ratios of rigid to flexible segment lengths, showed that greater flexibility increased the propulsive efficiency and thrust compared to an identical motion of the purely rigid plate. By submerging the plate at different depths, it was observed that the proximity of the propulsor to the bottom of the channel led to an overall increase in the thrust coefficient across the oscillation frequencies considered. The flow visualization revealed the formation of large dynamic stall vortices that influenced the wake structure, and suggested that their constructive interaction with trailing edge vortices might lead to overall improvement of thrust and efficiency.