PARAMETERS GOVERNING SEPARATION CONTROL WITH SWEEPING JET ACTUATORS

Abstract

Parameters governing separation control with sweeping jet actuators over a deflected flap are investigated experimentally on a generic "Multiple Flap Airfoil" (MFA). The model enables an extensive variation of geometric and aerodynamic parameters to aid the scaling of this novel flow control method to full-size applications.Sweeping jets exit from discrete, millimeter-scale nozzles distributed along the span and oscillate from side-to-side. The sweeping frequency is almost linearly dependent on the supplied flowrate per actuator. The measured thrust exerted by a row of actuators agrees well with vectored momentum calculations. Frequency and thrust measurements suggest that the jet velocity is limited to subsonic speeds and that any additional increase in flowrate causes internal choking of the flow.Neither the flowrate nor the momentum input is found to be a sole parameter governing the lift for varying distance between adjacent actuators. However, the product of the mass flow coefficient and the square root of the momentum coefficient collapses the lift onto a single curve regardless of the actuator spacing. Contrary to other actuation methods, separation control with sweeping jets does not exhibit any hysteresis with either momentum input or flap deflection. A comparison between sweeping and non-sweeping jets illustrates the superior control authority provided by sweeping jets. Surface flow visualization on the flap suggests the formation of counter-rotating pairs of streamwise vortices caused by the interaction of neighboring jets.The actuation intensity required to attach the flow increases with increasing downstream distance from the main element's trailing edge and increasing flap deflection. No obvious dependence of the ideal actuation location on actuator spacing, flap deflection, angle of attack, or actuation intensity is found within the tested range. Comparisons between experimental and numerical results reveal that the inviscid flow solution appears to be a suitable predictor for the effectively and efficiently obtainable lift of a given airfoil configuration. The flap size affects the achievable lift, the accompanying drag, and the required flap deflection and actuation intensity. By controlling separation, the range of achievable lift coefficients is doubled without significant penalty in drag even when considering a safety margin for the maximum applicable incidence.