Separated flow response to rapid flap deflection

The aerodynamic response of a massively separated flow to rapid ramps in flap deflection angle is presented. A NACA0006 airfoil spanning the test section of a water tunnel is oriented at a fixed incidence of 20^◦ for Reynolds number Re = 40k. The airfoil is bisected about the mid-chord position resulting in a 50%-chord trailing-edge flap, and is equipped with a suite of motors to produce a range of flap deflection frequencies and amplitudes. The objective is to ascertain the susceptibility of a massively separated flow to mechanical actuation and the viability of rapid flap deflection as a control mechanism. Focus is given to a deflection amplitude of 2^◦ to minimize geometric deviation from the airfoil’s baseline planar configuration. The flap ramp-maneuver is completed in a fraction of a single convective time. The desired response to such motions is the evocation of transients conducive to enhanced lift generation. Two distinct transient responses are produced pending the direction of flap deflection. In a rapid ramp resulting in a net increase in airfoil camber the lift is increased instantaneously to modest values before relaxation to the steady lifting state of the final deflection angle. In effect, this mode expedites convergence to the steady state value of the final airfoil configuration. Reversing the ramp direction, resulting in a net increase in camber, provokes a response characterized by an initial reduction in lift prior to achieving desired lift gains, followed by relaxation to steady state. Both modes prove disruptive to the leading-edge shear layer and both are cause for roll-up of a leadingedge vortex. Through proper orthogonal decomposition, it is revealed that within the most energetic modes of the flow response exists a polarity dependence on ramp direction with potential implication on how to incite an instantaneous response in flow actuation.