The Mechanism of Lift and Pitching Moment Reversal Following an Actuator Burst over a Stalled Airfoil

Fluidic actuators such as pulsed jet actuators have been known for decades for their ability to reattach separated flow effectively and therefore, to increase the lift. However, in terms of the dynamic response to the actuation, the lift decreases initially right after the onsite of the actuation followed by the transition to lift enhancement. The initial lift decrease known as 'lift reversal' contributes to the slow response of the actuation system, which can be rather problematic for unsteady flow separation control where the control system's response time is vital.

In this work, we investigated the mechanism of the lift and pitching moment reversal following an impulse actuation on a stalled airfoil. By analyzing the relation between the flowfield evolution and its associated pressure and force variation, We found that a spatially localized high-pressure region caused by the vortex detachment is responsible for the lift and pitching moment reversal. Proper Orthogonal Decomposition (POD) analysis of the flowfield shows that POD mode 2 represents the flow structure corresponding to the vortex detachment, and the time coefficient of POD mode 2 closely tracks the time variation of the lift and pitching reversal.