Active control of flow over a pitching airfoil by leading-edge pulsation jets

This paper reports an experimental work on the active control of flow over a pitching NACA0012 airfoil model based on pulsation jets deployed at the leading edge. Reynolds number (Re) used is Re = 8 × 104, based on incoming freestream velocity (U∞) and chord length (c) of the airfoil model. Angle of attack (α) of the pitching airfoil model varies with time (t) in a sinusoidal function α(t) = αo + αmsin(2πft), where αo is the mean pitching angle, αm is the pitching amplitude, and f is the oscillation frequency. αo = 10°, αm = 15°, and f = 1.3 Hz (corresponding to a reduced ratio k = πfc/U∞ = 0.05) in this work. The pulsation jets at the leading edge are oriented at 45° with respect to the airfoil chord line and involve several control parameters, including momentum coefficient Cμ (= 0%–1.05%), non-dimensional pulsation frequency Fj* (= 0–0.91), and duty cycle DC (= 30%–50%). While aerodynamic forces on the airfoil model are measured by a load cell, flow fields over the airfoil model are captured via phase-locked particle image velocimetry technique. Results from force measurements indicate that lift coefficient of the airfoil model can be significantly enhanced by the pulsation jets after dynamic stall in the upstroke and in the entire downstroke, leading to a large reduction (up to 40%) of the hysteresis loop. Based on thorough examinations of instantaneous and phase-averaged flows and analyses of proper orthogonal decomposition, remarkable changes in the flow fields are observed in the presence of the pulsation jets, compared with those in the absence of perturbations. It is revealed that large-scale dynamic stall vortices from the pitching airfoil model are disturbed by small-scale coherent vortices induced by the pulsation jets. Moreover, an optimal pulsation frequency Fj* = 0.46 is identified, corresponding to the convection time of these vortices from the leading to trailing edges.