Measurement of Flow Perturbation Spectra in Mach 4.5 Corner Separation Zone

Abstract

Characterization of the flow over a flat-plate model with a 30 deg compression ramp was performed with low-enthalpy () and high-enthalpy () flow conditions for a wide range of unit Reynolds numbers (). Three measurement techniques were employed to measure the frequency spectrum of flow perturbations: a high-frequency Shack–Hartmann wavefront sensor (aerooptical method), high-frequency PCB™ pressure sensors, and a laser differential interferometer. The magnitude and frequency of flow oscillations measured by all three methods provide comprehensive and complementary results in determining spectra of gas disturbances within the flow. Of these measurement methods, the Shack–Hartmann wavefront sensor is shown to be the most suitable tool for analysis of the high-speed flow. Aerooptical measurements detect modification to the flow structure when plasma actuation is employed or when is varied. In this work, flow characterization by the Shack–Hartmann sensor at individual points over the ramp has shown three flow regimes depending on the unit Reynolds number: turbulent, transitional, and laminar. At higher , the freestream disturbances become strong enough to significantly affect the perturbations in the boundary-layer and the separation zone on the compression ramp. Frequency spectrum measurements during high-frequency plasma actuation () indicate amplification of the perturbations from the natural state that occur over the separation region.