Blockage and Noise Characterization of the AFOSR--Notre Dame Large Mach-6 Quiet Tunnel

Background-oriented schlieren (BOS) and stagnation pressure measurements were taken for a variable-diameter 70 deg. half-angle sphere-cone model in the AFOSR--Notre Dame Large Mach-6 Quiet Tunnel under conventional noise to determine tunnel start conditions. Model diameters ranging from 60--220mm were tested at driver tube pressures from 105--1035 kPa at three axial stations. At the farthest upstream model position, unstart occurred for model diameters 100mm and above. With the 80mm and smaller model diameters at this location, run times averaged 1340ms, a 68% increase over the typical 800ms, suspected to arise from a train of oblique shock waves that increase the diffuser efficiency and thus the pressure recovery and run duration. These long run times were investigated using BOS and by a 2-D inviscid computational fluid dynamics simulation, which suggest that a train of oblique shock waves exists for model diameters 80mm and smaller at least 30cm upstream of the nozzle exit and may cause the extended run times. With the model at the nozzle exit plane, unstart occurred for diameters 180mm and above. The sensitivity of the nozzle exit to the axial location of the 160mm model was also tested resulting in tunnel starts with the model nose located 3.8cm upstream of the nozzle exit. Moving the model 4.4cm upstream of the nozzle exit or more resulted in a failure to start. Tunnel noise was characterized with a pitot probe via a root-mean-square method and again using a power spectral density. These data were used to evaluate the effectiveness of the isolation ring designed to smooth the flow passage through the shutter valve. Extending the isolation ring resulted in average noise levels of 1.90% and 1.32% via the RMS and PSD methods, respectively. A scanning electron microscope was used to analyze two small impact craters on the blunt model surface.