Numerical investigation on flapping hydrofoil for optimal propulsion performance using a very large eddy simulation method

In nature, the locomotion and maneuvering force of fish with thunniform mode crucially originate from the flapping foil as a principal component to produce thrust. Aiming to investigate the thrust performance and hydromechanical efficiency of flapping hydrofoil, a very large eddy simulation (VLES) method is introduced into dynamic mesh technique to solve the unsteady flow past flapping hydrofoil. The method can broaden the simulation of complex separated flow along with excellent compromise of accuracy and computing resources. In addition, the feasibility and validation of the method are verified to be well fit for dynamic mesh technique. The optimal propulsion performance is explored through varying the Strouhal (St) number and maximum angle of attack (α0) in an incoming flow condition of Re = 40,000. The temporal evolution of the angle of attack has a significant impact on lateral force coefficient, moment coefficient and the structure of shedding corotating vortices in the wake for high St number. The α0 exerts evident effect on the leading-edge separation. With the increase of St, the low-pressure values of suction surface become greater and the low-pressure areas become wider, hence producing more vortex-augmented thrusts and longer active time. The highest efficiency is equipped with the higher growth rate of thrust coefficient and moderate wake vortex strength. Furthermore, the temporal evolution of angle of attack has little effect, as does the leading-edge separation. In reality, an insight about high efficiency combined with high thrust should be considered in order to arrive to well-behaved propulsion system.