Novel Approach to Leading-Edge Vortex Suppression

A novel approach to reduce the peak lift and pitching moment on a plunging airfoil is investigated through force, moment, and velocity measurements. This approach, unlike previous investigations of delayed flow separation and leading-edge vortex suppression, uses forced separation through deployment of a minitab near the leading edge. The device can be activated for short time intervals during a gust encounter or unsteady maneuver at the expense of short-duration drag increase. Depending on the frequency and the amplitude of the wing motion and the mean angle of attack, roll-up of vorticity and the formation of a vortex can be delayed or even prevented. This change in the vortex dynamics provides effective lift and moment alleviation for post-stall angles of attack and for low reduced frequencies. In contrast, at low angles of attack, the separated shear layer may roll up for the manipulated flow, resulting in vortex shedding, and lift and nosedown pitching moment increase. These two distinct flow regimes cause decreased or increased lift force, with the most effective frequencies scaling with the reduced frequency. In contrast, the borderline between the two regions scales with the Strouhal number based on amplitude and, in particular, with the minimum effective angle of attack during the cycle. The transient response was studied by investigating impulsively started plunging oscillations. During the first cycle, lift reduction is achieved for all frequencies within the range tested.