Characterisation of the USQ hypersonic facility freestream

Characterisation of the USQ hypersonic facility
freestream

Hypersonic ground testing can make significant contributions to the design process for hypersonic flight vehicles. However, experimentation in conventional hypersonic ground testing facilities is complicated by the high levels of freestream fluctuations which are typically one-to-two orders of magnitude greater than in flight. This noisy test environment can have a significant impact on flow phenomena, such as boundary layer transition, and this leads directly to uncertainties in the prediction of essential hypersonic vehicle design parameters. To assess the noise level in ‘TUSQ’, the hypersonic wind tunnel at the University of Southern Queensland, the Mach 6 nozzle exit flow was characterised by measurements which provided: (1) the time-averaged and fluctuating components of Pitot pressure; (2) the time-averaged and fluctuating components of stagnation temperature; and (3) the fluctuating component of density. The Pitot pressure measurements were made using Kulite XTL-190M B-screen pressure transducers which were exposed directly to the flow. The stagnation temperature was determined from the experimental measurement of heat flux using microsecond response time coaxial surface junction thermocouples mounted in a stagnation point heat transfer gauge. A focused laser differential interferometer was designed for TUSQ, and this instrument was used to measure the freestream density fluctuations.

Using the Pitot pressure measurements and the measurements of the stagnation pressure in the nozzle reservoir (the barrel), the Mach number was found to decrease over the flow duration from 5.95 to 5.85. Through the measurement of stagnation temperature, the piston compression and the nozzle expansion of the test gas were found to be approximately isentropic for the first 65ms of hypersonic flow. Thereafter, the stagnation temperature reduces due to the heat lost to the cold barrel. Thermodynamic modelling based on the measured pressure history in the barrel combined with empirical heat transfer correlations can be used to simulate the stagnation temperature in TUSQ to within 2% of the actual value for t=0_150ms, increasing to within 5% at t=170ms. The heat transfer process in the barrel was found to significantly affect the fluctuations in the hypersonic freestream. For t less than 65ms, the freestream fluctuations of Pitot pressure, stagnation temperature and density were found to be broadband in nature, consistent with a disturbance environment dominated by the radiation of acoustic noise from the turbulent boundary layer on the nozzle walls. At t approximately equal to 65ms, a 3_4 kHz narrowband disturbance was detected in the barrel and in the freestream flow, and this disturbance remains superimposed on the broadband disturbance environment for the remainder of the test flow. Because the characteristics of the flow changed during the run, it is appropriate to specify two RMS Pitot pressure fluctuation magnitudes in the 300 Hz to 25 kHz bandwidth: 2.52% for t=5_65ms; and 2.86% for t=65_200ms for Reu = 6.94 x 106 m-1. The RMS Pitot pressure fluctuations in the TUSQ freestream are similar to comparable Ludwieg and blowdown facilities. RMS stagnation temperature fluctuations were resolved for f=4 Hz_5kHz and were found to increase throughout the flow period from approximately 1.5% at the start of the run to 2.4% at the termination of the nozzle flow. RMS freestream density fluctuations were determined for f=1_250kHz, increasing from 0.4% to 0.6% over he flow period. The bandwidth of the density fluctuation measurement was sufficient to resolve the classic Kolmogorov _5/3 rolloff in the inertial subrange.

Preliminary measurements of the boundary layer on a conical nose cylinder were made sing the focused laser differential interferometer. These measurements identified the second mode instabilities in the transitional boundary layer, and identified the amplification f the narrowband 3_4 kHz freestream fluctuations within the boundary layer. Further opportunities to explore boundary layer transition in the TUSQ facility are expected o arise in the near future, at which time the FLDI instrument can be deployed with improved focusing ability.