Laser Absorption Spectroscopy of Rb for enhanced hypersonic wind tunnel flow characterization
Creators: name:  E. DUDAS 
name:   E. ANFUSO  
name:   G. GROSSIR  
name:   O. CHAZOT 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT PHYSICS
WIND TUNNEL
MEASUREMENT TECHNIQUE

description: Ground testing experiments have always been a cornerstone for the development of new aerospace missions. This role has been further reinforced recently with the need to ensure a sustainable use of the space environment. This leads to space debris geometries being tested in hypersonic facilities in addition to the former set of rockets, cruise vehicles, and other re-entry spacecrafts. Meanwhile the need for accurate ground measurements has always been sought, now it is stressed one step further. In order to validate the Design for Demise (D4D) tools, which will serve as conception and design constraints regarding every bit of space hardware past their use-by time, a complete demise upon re-entering the Earth atmosphere will be required. The VKI Longshot hypersonic wind tunnel serves as a unique worldwide facility for ground testing measurements at high Mach numbers and high Reynolds numbers pertaining to atmospheric entries.1 It is currently established as a reference wind tunnel for investigating Earth re-entry aerothermodynamics in a low enthalpy, near-perfect gas environment free from chemistry effects. Extensive flow characterization test campaigns involving different sets of probes2 and advanced free-stream data rebuilding techniques3 have been reported over the past years. In line with these continuous efforts to provide accurate free-stream reference quantities and unprecedented high-quality data, we endeavour to further increase the confidence levels of the data issuing from ground facilities by involving novel, alternative flow diagnostic techniques. These efforts will also contribute to improve the characterization of high-speed flow expansions in presence of dense gas effects along with turbulence. We propose a non-intrusive measurement which will allow the determination of the free-stream flow velocity, and an insight into its translational temperature relying on a lineshape analysis method applied by Trinks et al.4 The experimental approach is based on tunable diode laser absorption spectroscopy (TDLAS) using rubidium (Rb) as a tracer gas. The spectroscopic challenges arise from the nature of the Longshot facility: less than 20 ms of test time per day, additional transients along the flow development, and the associated mechanical vibrations of the instrument. Investigations focus on the novel Longshot Mach 14 nozzle that has been recently designed, manufactured and commissioned5. This challenging experimental approach is presented and performances are illustrated with some preliminary results. This work is part of the Investigation of thermal non-equilibrium effects in rapid flow expansion project founded by AFOSR under grant FA8655-20-1-7024. (1) Grossir, G.; Puorto, D.; Ilich, Z.; Paris, S.; Chazot, O.; Rumeau, S.; Spel, M.; Annaloro, J. Aerodynamic Characterization of Space Debris in the VKI Longshot Hypersonic Tunnel Using a Free-Flight Measurement Technique. Exp. Fluids 2020, 61 (7), 163. https://doi.org/10.1007/s00348-020-02995-7. (2) Grossir, G.; Van Hove, B.; Paris, S.; Rambaud, P.; Chazot, O. Free-Stream Static Pressure Measurements in the Longshot Hypersonic Wind Tunnel and Sensitivity Analysis. Exp. Fluids 2016, 57 (5), 64. https://doi.org/10.1007/s00348-016-2137-5. (3) Grossir, G.; Dias, B.; Chazot, O.; Magin, T. E. High Temperature and Thermal Non-Equilibrium Effects on the Determination of Free-Stream Flow Properties in Hypersonic Wind Tunnels. Phys. Fluids 2018, 30 (12), 126102. https://doi.org/10.1063/1.5058098. (4) Trinks, O.; Beck, W. H. Application of a Diode-Laser Absorption Technique with the D2 Transition of Atomic Rb for Hypersonic Flow-Field Measurements. Appl. Opt. 1998, 37 (30), 70707075. https://doi.org/10.1364/AO.37.007070. (5) Kovacs, D. G.; Ilich, Z.; Grossir, G.; Chazot, O. Experimental Characterization of the VKI Longshot Mach 14 Contoured Nozzle. In AIAA SCITECH 2022 Forum; American Institute of Aeronautics and Astronautics: San Diego, CA Virtual, 2022. https://doi.org/10.2514/6.2022-1401.