Numerical investigation on thrust production and unsteady mechanisms of three-dimensional oscillating wing

The thrust production of an oscillating wing in the low Reynolds number regime (75 <= Re <= 1000) was investigated via threedimensional numerical simulations, primarily focusing on exploring the effects of aspect ratio AR (3 <= AR <= 10) and heave-pitch phase difference phi (0 degrees <= phi <= 360 degrees) on thrust production and propulsion efficiency eta(p) by tracing the tip vortices. The propulsion characteristics were observed to be strongly dependent on phi and Re but weakly dependent on AR. The oscillating wing produces positive thrust + C-x for 67.5 degrees <= phi <= 135 degrees. The maximum + C-x is produced at phi = 112 degrees due to the flow features namely 'constructive wake deflection' and 'mild flow separation during stroke reversal'. The coherent vortex structures reveal that the horseshoe ring HSR formed in each stroke aids in producing positive thrust via momentum change. The shape and size of the HSR are affected by the AR of wing. The shape of HSR is nearly circular at low AR, whereas it is a rounded rectangle at high AR. The HSR is noticed to transport the flow of high magnitude in uvelocity when Re = 450 and 1000, that will increase the positive thrust.