The Aerodynamics of Crescent Wings in Backward and Forward Sweep Orientations

Crescent wings are typically curved, backward swept wings of moderate aspect ratio and can be observed in fishes, birds and some aircraft. However, the crescent shape with a forward sweep also exists in some tropical fish. Here we provide results of numerical simulations using ANSYS-FLUENT on a NACA 0012 airfoil at a Re of 5.8e5 for backward swept, straight (i.e. unswept) and forward swept configurations. For unswept, mild, medium and strongly swept backward configurations our computational results agree well with existing experimental lift and drag measurements from 0 to 16 degrees AoA, which includes the sudden stall region. The simulations then provide new results for these four cases at up to 60 degrees AoA and all show stall recovery with a second lift peak at about 45 degrees AoA. Simulations of mild, medium and strongly swept forward cases generally show less lift pre-stall but more lift post-stall when compared to the backward swept cases. Notably, the stall behavior becomes much less sudden versus AoA for the three forward swept configurations, with minimal sudden stall for the most strongly swept case. In all seven cases studied, past 12 degrees AoA the flow becomes increasingly complex with leading edge separations, separation bubbles, and positive and negative vortices distributed along the span. These vortices are strongly dependent on the orientation of the leading edge of the wing - convex or concave relative to the oncoming flow - and hence on backward or forward sweep orientation of the wing.