Temperature variations in a free piston compression wind tunnel

This dissertation presents an investigation of the free stream stagnation temperature variations in the University of Southern Queensland (USQ) hypersonic wind tunnel (designated TUSQ), a short duration wind tunnel operated as a Ludwieg Tube with free piston compression heating. Because the facility is relatively new and because strong disturbances have previously been observed in similar facilities, a study to investigate the thermal characteristics of the hypersonic flow generated by TUSQ was needed. This study investigates the temporal and spatial thermal characteristics of the hypersonic flow produced in the TUSQ facility and relates these characteristics to the compression and flow discharge processes within the barrel. Quantification of the flow conditions produced in wind tunnels is important. Without such information, it is difficult to relate wind tunnel results to flight conditions or to perform meaningful computational simulations on the tested configuration. Three different versions of an aspirating thermocouple probe were developed for this work and a thin film heat flux gauge was also tested. Results with the Mach 6 nozzle show that the flow stagnation temperature decreases with time and thermodynamic simulations accurately reflect the majority of the observed temporal variations when flat plate boundary layer cooling is used to model the heat transfer in the barrel of the facility. Because the flat plate boundary layer cooling model provides a good match to the measured temperature on the nozzle centre line for the majority of the flow duration, it is concluded that significant mixing must have occurred across the diameter of barrel prior to flow discharge through the nozzle. Measurements in other facilities have indicated the existence of discrete, large scale thermal disturbance which propagated ahead of the piston and potentially compromised the test flow quality, but no such disturbance were detected at the centre line of the nozzle exit in the present work. The stagnation temperature measurements indicated a core flow region with a radius of almost 80mm near the start of the test flow. The maximum average spatial gradient of stagnation temperature was registered at about 150ms after the start of the test flow and had a value of approximately −0.45K/mm within the core flow region, indicating an average drop in stagnation temperature of about 20K over the core flow region at this time. Complementary pitot pressure measurements indicate core flow uniformity to within 2% and a core flow radius of at least 80 mm for the majority of the test flow duration of around 200ms. Mach number profiles deduced from the pitot pressure measurements are likewise uniform with a Mach number of 5.81 ± 0.05 for the majority of the test flow duration. A fully-developed turbulent pipe flow model was developed and stagnation temperature fluctuations in the TUSQ facility were estimated to be around 20 K. Although this value is large compared to results from previous experiments in a gun tunnel facility, the value obtained is consistent with the magnitude of the spatial variation in stagnation temperature within the core region of the nozzle exit flow at about 150 ms from the start of the flow. Relatively low frequency fluctuations in the stagnation point heat flux were observed and these appeared to correlate with the stagnation pressure fluctuations, but further effort in this area is required in order to resolve stagnation temperature fluctuations due to the turbulent mixing in the barrel.