Unsteady actuation of counter-flowing wall jets for gust load attenuation

Counter-flowing wall jets actuated on the upper surface of an airfoil are investigated for the purpose of controlling gust encounters. For periodic and transient actuations, phase-averaged lift force and particle image velocimetry measurements are presented for a NACA 0012 airfoil, at a Reynolds number of 660,000, for a range of reduced frequencies and three jet locations, x _J /c=0.08, 0.60 and 0.95. For periodic actuation, amplitude of lift oscillations decrease and phase delay increase with increasing reduced frequency. The effect of reduced frequency on the amplitude and phase is more significant for blowing locations near the leading-edge and with increasing angle of attack. Transient actuation reveals the slow response of the separated flow, and therefore lift, with the delay becoming more pronounced for blowing near the leading-edge. Estimated time constants are similar to previous observations for forced separation and reattachment.