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Single-Laser Krypton Tagging Velocimetry (KTV) Investigation of Air and N2 Boundary-Layer Flows Over a Hollow Cylinder in the Stevens Shock Tube

We investigate the boundary-layer profiles that form over a sharp, hollow cylinder in supersonic air and N2 flows with a Krypton Tagging Velocimetry (KTV) single-laser scheme. The supersonic flows are generated by the passage of the primary shock wave over the model in the Stevens Shock Tube. Eight experiments were performed in two gas mixtures: a) 99% N2/1% Kr at post-shock temperature T2=1300 K and pressure range P2 = 6.0 − 25 kPa; and, b) in 75% N2/20% O2/5% Kr at post-shock temperature T2 =1400 K and pressure range P2 = 2.7 − 19 kPa. This experimental design resulted in unit Reynolds numbers ranging from ≈1e5-1e6 m. This range of static conditions spans those of large-scale, high-enthalpy hypersonic impulse facilities, albeit at lower total enthalpy. The freestream pressure and temperature (but not the velocity) of large-scale facilities were reproduced to demonstrate KTV utility. The KTV data points over the hollow cylinder are mapped to wall-normal locations above a flat plate, enabling comparison with the similarity solution for compressible boundary-layer flow. Agreement between the similarity solution and experimental results is excellent. Compared to two-laser KTV schemes, our single-laser approach is simpler and more cost-effective but has a higher laser energy requirement. Single-laser KTV is implemented as follows. At the 212.6 nm wavelength, the write-laser pulse partially ionizes Kr via a (2+1) resonance-enhanced, multiphoton ionization (REMPI) process. The write step records the spontaneous emission resulting from the two-photon excitation. After a prescribed delay, the read step records the fluorescence resulting from the deionization process. The signal-to-noise-ratio (SNR) is sufficient to extract velocity profiles from single-shot, shock-tube experiments.