On the Role of Secondary Structures During Leading Edge Vortex Lift Off and Detachment on a Pitching and Plunging Flat Plate

Leading edge vortices (LEVs) occurring on unsteady airfoils induce strong forces during formation and detachment. In order to exploit the beneficial effect of an increased lift during growth and reduce the effect of the lift drop during detachment, a prolongation of the growth phase by active flow control is considered as a propulsion concept for future Micro Air Vehicles (MAVs). Profound understanding of the trigger mechanism for vortex detachment can serve as a basis to develop effective control approaches. This study investigates the impact of secondary structures on the vortex lift-off and detachment on a one-shot pitching and plunging flat plate at intermediate Reynolds numbers, on the grounds of various local (Eulerian) vortex identification methods and a complementary Lagrangian saddle-point tracking approach. It is found that for the investigated parameters, secondary structures upstream of the main vortex arise early within the cycle, causing the LEV to lift off from the airfoil, but to continue accumulating circulation from the feeding shear layer. Recirculation around the trailing edge of the airfoil was found to occur when the vortex terminated circulation accumulation, which confirmed the chord length to be characteristic for the ultimate detachment of the vortex from the feeding shear layer.