A numerical study of hovering wings undergoing revolving or translating motions

Abstract

This paper presents simulations of revolving and translating wings undergoing reciprocating motions that are representative of hovering flight. A high-fidelity, implicit large-eddy simulation (ILES) approach is utilized to examine the vortex structure and unsteady loading generated on each wing to compare and contrast the mechanisms of lift production. For each stroke of the motion, the revolving wing produces a coherent and attached vortex system across the leading edge that remains in close proximity to the surface until the wing decelerates and flips at the end of the stroke. As the vortex collides with the wing, it breaks up rapidly before becoming entrained by the newly forming vortex loop on the reversed side. The back stroke produces effectively the same structure as the wing passes through its previously shed wake. The flow structure each stroke closely resembles that of a simple, unidirectionally revolving wing indicating that the wake capture effect is minimal. Alternatively, a translating hovering wing undergoing a rectilinear motion produces a leading edge vortex that develops into an arch-type vortex by lifting off at the wing mid-span and unpinning from the front edge of the wing to form legs that are anchored on the surface. The translating wing also generates about 32% less cycle-averaged lift as the revolving wing as the mechanism of lift is inherently different in the two cases. During rotation, the lift is attributed to the close proximity of the spanwise-oriented leading edge section of the vortex loop above the surface that creates persistent suction throughout the stroke. The majority of the lift on the translating wing, on the other hand, is due to the counter-rotating feet of the arch-type vortex that are anchored on the surface and produce diminishing suction through the stroke as the arch vortex weakens.