The three fundamental mechanisms for forward flight with flapping wings: dynamic stall, added mass reaction, wing-wake interactions

摘要

After introduction of the topic where we consider the role of viscosity on the differences between various locomotion strategies of living species, we present numerical simulations of an airfoil NACA 0012 experiencing a heaving motion using a 2D DNS code at Re=1000 with the objective to enlighten three fundamental unsteady mechanisms which govern aerodynamic efforts: dynamic stall, added mass reaction and wing-wake interactions. Dynamic stall occurs when aerodynamic incidence exceeds stall incidence. At low Reynolds numbers, we can observe the formation and shedding of a leading edge vortex creating a depression zone which greatly improves lift. Numerical simulations show that this phenomenon can be described at low frequencies thanks to a quasi steady analysis referencing to an airfoil which experiences the Von Karman regime. The second phenomenon, added mass reaction, has often been neglected. Added mass reaction is the counter reaction of the fluid which is displaced with the wing. At low Reynolds numbers, our numerical simulations show that this effect cannot be neglected and we show that it depends a lot on frequency. At high frequency, it clearly dominates circulatory forces. Finally, the third phenomenon, wing-wake interactions, is definitely unsteady. It deals with interactions between the wing and the depression zones of the previous strokes vortices. At last, we show that wing-wake interactions are very dependant on kinematics of the wing. As an example we show how it is possible to improve lift by capturing the leading edge vortex issuing from the previous stroke.