An experimental investigation of unsteady separation shock wave motion in a plume-induced, separated flowfield

An experimental investigation of the unsteady separation shock wave motion in plume-induced, boundary layer separated (PIBLS) flowfields has been conducted. The PIBLS flowfields were created in a blowdown-type wind tunnel designed specifically to produce PIBLS in a planar, two-stream, supersonic flow. In this unique wind tunnel, separation of the freestream boundary layer upstream of the base plane was accomplished by utilizing an angle-induced separation geometry in the wind tunnel design in addition to operating the wind tunnel at jet-to-freestream static pressure ratios (JSPRs) greater than unity. In essence, the wind tunnel design consisted of a Mach 1.5 inner-jet flow angled at 40 degrees with respect to a Mach 2.5 freestream flow in the presence of a 0.5-inch thick base height. By throttling the stagnation pressure of the inner-jet flow, PIBLS flowfields, with nominal separation point locations ranging from two (JSPR $\approx$ 1.7) to six (JSPR $\approx$ 2.3) or more boundary layer thicknesses upstream of the base plane, were produced in the wind tunnel. The separation process associated with all of these PIBLS flowfields was observed by flow visualization techniques to be unsteady, and the separation shock wave that accompanied the separation process was found to exhibit large-scale (on the order of the incoming boundary layer thickness) motion in the streamwise direction.The primary objective of the current research program was to understand the unsteady characteristics of the separation shock wave motion present in the PIBLS flowfields by obtaining and analyzing detailed, non-intrusive experimental data including flow visualization photographs, surface flow visualization patterns, mean static pressure measurements, and instantaneous pressure fluctuation measurements throughout the region of shock wave motion (called the intermittent region). Since the vast majority of the statistical properties of the shock wave motion were computed from the fast-response pressure transducer measurements, the instantaneous pressure fluctuation measurements were of primary importance in the study. In recent years, similar measurements have been used to characterize the unsteady separation shock wave motion in shock wave boundary/layer interactions (SWBLIs) produced by solid boundary protuberances (i.e., compression ramps, circular cylinders, sharp- and blunt-edged fins, etc.). However, such data are virtually nonexistent in a plume-induced interaction and, therefore, the current data are quite unique.From standard time series and conditional analysis methods applied to the pressure fluctuation measurements, the statistical properties of the shock wave motion were determined over the intermittent region. The shock wave motion was found to be responsible for producing large pressure fluctuations over the intermittent region in these PIBLS flowfields. The standard deviation of the pressure fluctuations, when nondimensionalized by the local mean pressure, reached a maximum value of 0.22 near the middle of the intermittent region. The strength of the unsteady shock wave motion, determined as the ratio of the maximum standard deviation of the pressure fluctuations over the intermittent region to the mean pressure difference across the intermittent region, was calculated to be 0.43 for the current PIBLS flowfields. Both of these quantities demonstrate that the unsteady pressure loading caused by the shock wave motion has essentially the same magnitude in plume-induced separated flowfields as in SWBLI flowfields produced by solid boundary protuberances.U of I OnlyETDs are only available to UIUC Users without author permissio