The Aerodynamics of Crescent Wing Pairs in Backward and Forward Swept Orientations

Single crescent wings of backward-swept planform shape show classical "two-time stall" lift behavior vs. angle of attack, namely an initial linear lift increase, flowed by sudden stall (first stall), followed by increasing lift (recovery) and then a loss of lift (second stall). Single forward-swept crescent wings however show a very different "rolling stall" behavior marked by a reduction of first stall, an increase of lift in the recovery region and similar lift behavior at second stall. First stall can be essentially eliminated for strongly forward-swept wings. In this study we provide results of numerical simulations using ANSYS-FLUENT on a NACA 0012 airfoil at a Re of 5.8e5 for fixed pairs of crescent wings with variable separation distances of 0.25 to 2.0 chord lengths. We find that backward-swept wing pairs show a clear interference effect where the lift coefficient is dependent upon separation distance and differs from that of a single wing. This however is not the case for forward-swept wing pairs (as seen in some tropical fish) where the lift coefficient is fairly insensitive to separation distance, and is similar to that of a single wing. In all cases the flow is complex with leading edge separations, separation bubbles, and positive and negative vorticity distributed along the span. However, in spite of this, forward-swept cescent wing pairs show robust lift behavior with an insensitivity to separation distance, while backward-swept crescent wing pairs show a clear dependence on separation distance.