Hypersonic Wind Tunnel Flow Characterization, Condensation Detection, and Valve Modifications

Facility flow characterization and the optimization of freestream conditions, through the detection of condensation to set a minimum static temperature condition, leads to better data analysis. A static pressure probe and laser Rayleigh scattering were employed to measure the onset of condensation in three hypersonic wind tunnels: the Notre Dame Arcjet arc-heated tunnel (formerly ACT-1) at M=6 and Re=3 x 10^6 - 6 x 10^7 /m, the von Karman Institute (VKI) Longshot at M=14 and Re=1.5 - 3.5 x 10^6 /m, and the AFOSR-Notre Dame Large Mach-6 Quiet Tunnel (ANDLM6QT) at M=5.7 and Re=10 x 10^6 /m. Classical nucleation theory was employed to evaluate the critical cluster sizes and steady state nucleation rates of the flow conditions where condensation was measured. A computational analysis of the static pressure probe over the range of expected flow conditions was conducted using a Preconditioned/Pressure based Compressible Petrov-Galerkin Navier-Stokes solver in CFD++. Background Oriented Schlieren (BOS) of the static pressure probe was used to corroborate the CFD results.

In the ND Arcjet, a pitot-rake survey was conducted at multiple axial and radial stations for arc-off (stagnation temperature ~295 K) nitrogen flow in the Mach-6 nozzle. Stagnation pressures of 280 and 380 kPa were surveyed with the pitot rake. For a stagnation pressure of 380 kPa, the average measured Mach number at the nozzle exit plane was 4.75 and 6.35 for the Mach-4.5 and -6 nozzles, respectively. The flow of both nozzles exhibited slight angularity and asymmetry. Spectral content of noise in the flow for frequencies up to 127.5 kHz was measured by Kulite pressure transducers. The root-mean-squared of the pitot pressure was found to be 2.2 - 2.4% and 0.5 - 2.2% of the mean pitot pressure for the Mach-4.5 and -6 nozzles, respectively. Measurements with a static pressure probe in nitrogen indicated that condensation occurs in the Mach-6 nozzle and explains why the pitot survey results indicate a prematurely shortened axial length of the uniform flow region. Measurements with laser Rayleigh scattering in this facility employed an insufficiently intense laser.

In Longshot, condensation was detected with the static probe in two runs at T=42 K, p=64 Pa and T=45 K, p=65 Pa. Measurements with laser Rayleigh scattering in this facility were overwhelmed by sodium emission.

In ANDLM6QT, a pitot probe, pitot rake, static pressure probe, and laser Rayleigh scattering were deployed. The pitot probe was used to determine the freestream Mach number along the centerline and the pitot rake was used to measure the thickness of the boundary layer. Laser Rayleigh scattering was used to measure the onset of condensation characterized by increased particle size. Measurements indicate that with the aluminum surrogate nozzle, this facility currently operates at a conventional noise level of 2.6 %, a freestream Mach number of 5.7, and a boundary-layer thickness of 11 cm. With the static pressure probe, the flow was found to condense 6 - 9 K below the equilibrium saturation temperature for the four pressures tested. Laser Rayleigh scattering detected variation of the flow density. Using this technique, only 1 - 4 K supercooling was observed. Modifications to the fast-acting valve in ANDLM6QT have led to operability of the facility in noisy conditions, and steps are being taken to achieve quiet flow conditions.