Toward Understanding and Optimizing Separation Control Using Microjets

Flow separation in engine inlets and diffuser ducts and over external aerodynamic surfaces such as wings can significantly compromise performance. In a previous experimental study, we demonstrated the value of microjets for separation control. In this paper, we significantly broaden the range of conditions and parameters to explore the effects of microjet control on the efficiency of separation control over a backward-facing ramp. The parameters explored include the following: the freestream velocity, the ramp angle of attack, and the microjet pressure, location, and injection angle. Detailed velocity field investigations and unsteady pressure measurements have been conducted to study the effect of flow control over this parametric space. The results indicate that, by activating the actuator arrays in the immediate vicinity of the separation location, the control efficiency can be greatly enhanced, reducing the actuator mass flow needed dramatically. By correlating the unsteady surface pressure measurements with the measured velocity field, we demonstrate that, at least in the present geometry, the unsteady pressure alone can be used to 1) detect separation and hence identify which actuator(s) to activate, and 2) estimate the effect of control on a separated flowfield. Finally, based on the response of the flowfield to actuation, we propose some simple scaling laws for detecting and implementing separation control.