Single-Laser Krypton Tagging Velocimetry Investigation of Supersonic Air and N2 Boundary-Layer Flows over a Hollow Cylinder in a Shock Tube

Abstract

In this paper, we investigate the boundary-layer profiles that form over a sharp, hollow cylinder in supersonic air and N 2 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. The experiments are performed in two gas mixtures doped with Kr: 99% N 2 /1% Kr, to model N 2 , and 75% N 2 /20% O 2 /5% Kr, to model air. The experimental setup allows us to vary the pressure and Reynolds number from 3-25 kPa and 1.5 × 10 5 to 1.5 × 10 6 m −1 , respectively, while the Mach number is kept fixed at 1.7. The static temperature and pressure (but not the velocity) are representative of typical large-scale high-enthalpy hypersonic impulse facilities. The KTV data points over the hollow cylinder are mapped to corresponding wall-normal locations above a flat plate, which enables comparison with the similarity solution for compressible boundary-layer flow. Agreement between the similarity solution and experimental results is excellent. Relative to previous two-laser KTV schemes, the single-laser approach used in this work has the advantage of being simpler and more cost effective, but it has a higher laserenergy requirement, 10 mJ/pulse in these experiments. Single-laser KTV is implemented by increasing the energy of the write-laser pulse to a sufficient level such that the Kr becomes partially ionized via a (2 + 1) resonance-enhanced, multiphoton ionization (REMPI) process with an excitation wavelength of 212.6 nm. The write step records the fluorescence that results primarily from the spontaneous emission from the two-photon excitation. After a prescribed delay, the read step records the fluorescence that results from the transitions that follow the recombination process. The signal-to-noise ratio (SNR) is sufficient to extract velocity profiles from single-shot, shock-tube experiments. Two-photon absorption cross-section calculations and emission spectra are presented to justify the chosen excitation wavelength and support our understanding of the Kr excitation and emission scheme.

Freestream velocity-profile measurement in a large-scale, high-enthalpy reflected-shock tunnel

“…We found these levels of Kr doping to be sufficient in previous experiments (Mustafa et al. 2019b ). Referring to the energy level diagram in Fig.…”

“…The experimental conditions in Mustafa et al. ( 2019b ) showed that KTV could be performed at static conditions similar to the T5 freestream with 99% N /1% Kr, and 75% N /20% O /5% Kr gas mixtures. Relative to Mustafa et al.…”

“…( 2019b ), the experiments described in the present work are significantly more complex due to (a) the longer standoff distance from the KTV LIF to the camera; (b) lower write-laser power, 10 mJ/pulse in Mustafa et al. ( 2019b ) versus 3–4 mJ/pulse in this work; (c) timing complexity; (d) laser-power attenuation resulting from not triggering the laser setup at the designed repetition rate of 10 Hz; (e) freestream luminosity; and, (f) scheduling complications of single-shot-per-day experiments.…”

“…Recently, simplified KTV schemes were developed and demonstrated in an underexpanded-jet configuration (Mustafa and Parziale 2018 ) and in the flow following the incident shock in a shock tube (Mustafa et al. 2019b ). These simplified schemes used 1) a single dye laser for the write step with no read laser, or 2) a dye laser for the write step and a simple, continuous-wave (CW) laser diode to help boost signal-to-noise ratio (SNR) during the read step.…”

“…2017b , a , c , 2018 ; Mustafa and Parziale 2018 ; Mustafa et al. 2019a , b ; Shekhtman et al. 2020 ), VENOM (Hsu et al.…”