Measurement of Flow Perturbations Occurring Both Naturally and by Plasma Induction over a Mach 4.5 Corner Separation Zone

This thesis documents the efforts to characterize the Mach 4.5 flow over a flat plate model with a 30° compression ramp over a range of unit Reynolds numbers (Rel = 4‧105 – 1‧107 m-1) in low enthalpy (T0 = 293 K) and high enthalpy (T0 = 800 – 1250 K) flow conditions. The compression ramp model was designed, fabricated, and implemented in a hypersonic test facility (ACT-1) to simulate small-scale scramjet inlet operation. Flow visualization was provided by a schlieren imaging system and planar laser Rayleigh scattering (PLRS) to characterize the flow structure with various Rel. Three measurement methods were used to determine the frequency spectrum of flow perturbations: a high-frequency Shack-Hartmann wavefront sensor, a laser differential interferometer, and high-frequency pressure sensors. All three methods provide complementary results to indicate the existence of dominant frequencies of perturbations in the flow. It is determined that of these three measurement tools, Shack-Hartmann wavefront sensing is the most beneficial for this flow analysis. These measurements detect modification to the flow perturbation spectra during plasma actuation and across varied Rel. Damping of the perturbations in certain frequency bands occurred in the spectra of measurements taken in the corner separation bubble. In this way, the spectra indicate dominant frequency bands of perturbations occurring in regions of the boundary layer and separation bubble. It is shown, that in the highest tested Rel cases, initial disturbances in the freestream develop into strong perturbations, which significantly affect the boundary layer and separation zone size over the compression ramp. Both amplification and damping effects from the natural state are seen within different bands of the spectra of perturbations measured within the flow occurring over the separation region of the compression ramp collected during high-frequency plasma actuation (100 kHz). The amplification observed leads to a fast laminar to turbulent transition immediately after flow reattachment and potential stabilization of the shock wave structure. This effect was not observed at F = 50 kHz plasma pulsing frequency.