Combustion-Powered Actuation for Transitory Flow Control

Abstract

The susceptibility of wall-bounded and free shear flows to transitory, pulsed actuation having time scales that are significantly shorter than the flow’s characteristic convective time scale has been used for external and internal aerodynamic flow control. This approach, which exploits the disparity between the onset and relaxation time scales of the embedding flow response, requires brief, anharmonic high-impulse actuation that is difficult to achieve by conventional momentum-based electromechanical actuation. However, investigations over the past decade have demonstrated the utility of chemically based actuation in which controllable high-impulse jets are produced using small-scale combustion-powered actuation. The present paper comprises two parts. The beginning of the paper describes the development of this actuation technology. In the present implementation, high-speed pulsed actuation jets are engendered by the ignition of a mixture of gaseous fuel and air within a cubic-centimeter-scale combustion chamber. The dependence of the actuator’s performance and of the ensuing momentary actuation jet on the composition of the chemical species, the internal geometry, and the flow configuration are characterized using dynamic pressure measurements and flow measurements using particle image velocimetry. The latter part of the paper describes a few examples of aerodynamic flow control using combustion-based pulsed actuation for the mitigation of separated flows over external static and moving aerodynamic surfaces. A fundamental aspect of the receptivity to pulsed actuation is the disparity between the time scales of the attachment and relaxation processes such that a single actuation pulse having a characteristic time scale of 0.05 can lead to brief, partial collapse of the separated flow domain, coupled with a momentary increase in circulation on a time scale that is nearly 200 times longer; and that repetitive actuation results in sustained lift enhancement.