Background-Oriented Schlieren Measurements of a Sphere-Cone in the AFOSR--Notre Dame Large Mach-6 Quiet Tunnel

Studying supersonic flows requires experimental measurement techniques with high spatial and temporal resolution. Off-wall measurements are essential for assessing the topology of compressible flow fields, including features such as shock waves, boundary layers, and separated regions. Background-oriented schlieren (BOS) measurements were made on a $70^\circ$ half-angle sphere-cone in the AFOSR-–Notre Dame Large Mach-6 Quiet Tunnel to observe bow shock standoff distance and unsteadiness. The facility is a Ludwieg tube with a nozzle 0.63 meters in diameter, a driver tube 61 meters long, a fast-acting valve, and a nominal run time of 2 seconds. Background-oriented schlieren is a useful alternative to other off-wall measurement techniques since it provides quantitative data for comparatively easy setup and low cost. The facility, theory of BOS, data acquisition and reduction routines, and results are discussed. Bow shock standoff distance was measured and its unsteadiness was calculated via spectral methods for stagnation pressures from 172 to 827~kPa and model diameters from 60 to 220~mm.