Hydrodynamic characteristics of two oscillating fins in series with heave-pitch coupled motions

Autonomous underwater gliders have rapidly become mature technologies in recent years. They have also been proved to be a successful tool for ocean sampling with an even wider range of future possibilities. An underwater glider is propelled by a buoyancy engine to adjust the difference between buoyancy and weight, combining with the lift induced by the wing. In general, an underwater glider can ascend and descend obliquely on a sawtooth trajectory but it lacks capability to move horizontally. Due to this reason, a concept design of biomimetic propulsor with two serial oscillating fins for enhancing the horizontal mobility of an underwater glider was proposed. The propulsor consists of a flapping fore fin acting as a leading edge vortex generator, and a flapping rear fin acting as a vortex manipulator. It was found in the previous paper that both thrust and efficiency can be improved significantly in comparing with the single fin model. However, the propulsion efficiency of the serial fins oscillating with only pitch motion is still not sufficient for practical use; even it has been enhanced by an added fore fin. Two oscillating fins in series with heave-pitch coupled motions are expected to have much higher propulsion efficiency. The investigations on its hydrodynamic characteristics via CFD simulations are conducted in the present paper. The rear-fin effects and the fore-fin contributions on propulsive performance are clarified. The optimal oscillating modes of the fore fin on enhancing the propulsion efficiency are discussed and their corresponding wake mechanisms are demonstrated. It is known that the optimal thrust coefficient and efficiency of the present two-fin model increase approximately 26% and 20% higher than the single fin model and their magnitudes reach 1.26 and 79.52%, respectively.