Impacts of Unsteady Flow Environments on the Propulsive Performance of Oscillating Foils

A numerical study is conducted to understand the impact of an unsteady freestream on the aerodynamic performance of an oscillating airfoil. The unsteady flow environment is generated by placing a stationary inline circular cylinder array upstream of the oscillating airfoil. The dependence of thrust with variation of Reynolds numbers and Strouhal numbers is investigated, and it is revealed that the unsteady flow environment enhances thrust production of a pitching airfoil. This increased thrust production was related to an effective increase in the Reynolds number experienced by the airfoil. With airfoil-vortex interaction analysis, the increase in average thrust coefficient was shown to be caused by constructive interaction of freestream vortex structures and the oscillating airfoil. Drag-inducing interactions were also observed but were less common than thrust-increasing events, resulting in a higher average thrust. A simple scaling law is expanded to include the effects of unsteadiness, where thrust is found to be linearly dependent on turbulence intensity. It is demonstrated that the thrust generated by the pitching airfoil when operating in highly unsteady flow environments is more accurately represented as a function of Reynolds number, Strouhal number, and turbulence intensity.