Hypersonic and Shock Wind Tunnels
PSP application in hypersonic flows is more difficult because high enthalpy of flows may produce such a large temperature increase on a model that the temperature effect of PSP is overwhelming. Since hypersonic wind tunnels are usually short-duration tunnels, a very thin PSP coating is required not only to sustain high skin friction, but also to achieve a short response time. However, the luminescent emission from a very thin PSP layer is weak and thus a low SNR becomes a problem. The short run-time limits the exposure time of a CCD camera to collect photons and further reduces the possibility of improving the SNR. Kegelman et al. (1993) conducted PSP measurements on a 1/6-scale Pegasus launch vehicle model and a shock/boundary-layer interaction model in the NASA Langley Mach 6 High Reynolds Number Tunnel. The McDonnell Douglas PSP and TSP were used in their tests. The pressure distributions obtained using PSP on the Pegasus model were in qualitative agreement with the Navier-Stokes code results over most of the wing. However, considerable discrepancies between the PSP data and CFD results were observed near the leading edge and wing tip where high temperature generated by aerodynamic heating exceeded the workable temperature range of the paint. Quantitative PSP results, which were compared favorably with the pressure tap data, were obtained on the surface of a flat-plate model on which an oblique shock impinged; the accuracy of about 0.1 psia was reported.
Using fast-responding PSP formulations, Troyanovsky et al. (1993) carried out semi-quantitative PSP visualization in shock/body interaction in a Mach 8 shock tube with a duration of 0.1 s. Borovoy et al. (1995) measured the pressure distribution on a cylinder at Mach 6 in a shock wind tunnel with a duration of 40 ms, and achieved reasonable agreement with the theoretical solution and pressure transducer measurements. Jules et al. (1995) used a McDonnell Douglas PSP to study shock/boundary-layer interaction over a flat-plate/conical-fin configuration at Mach 6, showing a systematic shift compared to pressure tap measurements. Hubner et al. (1997, 1999, 2000, 2001) measured the pressure distributions on a wedge and an elliptic cone at Mach 7.5 in the Calspan hypersonic shock tunnel with a run-time of 7-8 ms. To reduce the temperature effect of PSP, they applied PSP directly on the metal model surface rather than a white basecoat. However, for a very thin layer of PSP without a white basecoat, the luminescent intensity of PSP was so low that only 5-12% of the CCD full-well capacity was utilized. Buck
(1994) discussed simultaneous temperature and pressure measurements on dyed ceramic models using luminescent materials in hypersonic wind tunnels.