Global Measurements of Axisymmetric Hypersonic Shock-Wave/Boundary-Layer Interactions

Measurement systems for identifying steady and unsteady phenomena in hypersonic shock-wave/boundary-layer interactions (SWBLI) have been developed and tested on a 7 degree half-angle circular cone/flare model in Mach-6 flow. The measurement techniques consist of infrared thermography and anodized-aluminum pressure-sensitive paint (AA-PSP). Global heat-flux data are calculated from the time-resolved temperature measurements carried out with the use of an infrared camera. Mean and fluctuating surface pressure measurements are obtained with the use of a temperature-corrected, high-frequency-response AA-PSP.

The steady aspect of the study was conducted in the Air Force Research Laboratory's Mach-6 Ludwieg Tube using infrared thermography. Measurements were performed at zero angle of attack on a total of 16 different nose bluntness/flare angle geometries at three different freestream unit Reynolds numbers. The boundary layer entering the interaction region was turbulent for the two sharper nosetips and laminar for the two blunter nosetips. Stanton-number contours and profiles were used to investigate the effect of nose bluntness, flare angle, and freestream unit Reynolds number on the length of boundary-layer separation, peak heating achieved upon reattachment, and thermal striations on the flare. The classical scaling analyses of Souverein et al. and Hung & Barnett for separation length and peak heating, respectively, have been extended for the use of axisymmetric cone/flare shock-wave/boundary-layer interactions. Scaling the peak heating with the density ratio across the separation shock wave, instead of the pressure ratio, significantly improves/simplifies the analysis.

The unsteady aspect of the work took place in Notre Dame's ACT-1 wind tunnel using AA-PSP. This AA-PSP was made in-house to provide the high frequency response required for this work. These proof-of-concept experiments were conducted to illustrate the AA-PSP's usefulness and effectiveness at measuring the unsteady structures inherent to many SWBLI. Measurements were performed at zero angle of attack on a single cone/flare geometry. One test was for a laminar boundary layer entering the interaction region, while the other was for a tripped turbulent boundary layer. The mean and fluctuating components of the surface pressure were used to provide multiple mean and instantaneous metrics of the locations of boundary-layer separation and reattachment shock feet. In general, all of these metrics were found to be in good agreement. Significant azimuthal structures were observed in the reattachment region due to the presence of streamwise counter-rotating vortices. Spectral analysis identified a smooth, low-frequency bandwidth characteristic of the shock-foot oscillations. Laminar coherence calculations indicated that the intermittent regions of separation, reattachment, and the recirculating separation bubble near the cone/flare junction are all relatively well correlated, and that the system exhibits a low-frequency breathing motion mostly driven by the reattachment fluctuations.