The infrared tunable diode laser absorption spectroscopy (TDLAS) has now become a powerful tool to probe the free-stream-flows high-enthalpy wind tunnels like the ONERA-F4 or DLR-HEG.
Figure 1
TDLAS Principle
The laser beam crosses the flow with a small angle ( 60°) to the flow axis (Figure 1), in order to make a Doppler shift appear in the position of the absorption lines. The measurement of this shift makes it possible to deduce the velocity of the free flow, which is one of the most important parameters to calibrate these blowers.
Laser diodes with lead salt emitting around 5 µm are used to take advantage of strong absorptions of the fundamental lines of the species NO, CO, CO2 present around this wavelength (Figure 2). The laser emission linewidth is very narrow ( 10 -4 cm-1), so that it is possible to measure without correction ((of function of apparatus)) the thermal or collisional broadening and the Doppler shift of a molecular line and to deduce from it temperature, pressure and velocity of the gas.
Figure 2
Absorption lines of molecules of aerodynamic interest
matched to the tuning ranges of our diodes
This technique provides only integrated or average measurements
realised on the path of the laser beam. Thus, these measurements are valid only in the free stream flow where the transverse profiles (of density, speed, temperature...) are supposed to be constant - as they are in the probed wind tunnels. Another advantage of integrated measurements is the excellent signal/ noise ratio, which makes it possible to increase the frequency of the measurements. This is very interesting to follow the temporal evolutions of the parameters of strongly transient flows issued from hyperenthalpic wind tunnels - characteristic times are of the order of 1 ms in F4 and 0.1 ms in HEG. Figures 4 and 5 show typical evolutions of the parameters measured by the DLAS technique for runs in F4 and HEG (measurements on an NO absorption line around 5.4 µm).
Figure 2
TDLAS Measurements in F4
Figure 2
TDLAS Measurements in DLR's HEG shock tube
The comparison of the measurements with CFD results leads to similar
conclusions for F4 and HEG :
the velocity measured correlates well (within
10%) with theoretical values
the measured temperature is much
higher than its theoretical counterpart
References
Mohamed A. K., Rosier B., Henry D., Louvet Y., Varghese P. L., Tunable diode laser measurements on nitric oxide in a hypersonic wind-tunnel, AIAA Paper 95-0428
Mohamed A. K. , Rosier B., Sagnier P., Henry D., Louvet Y., Bize D., Application of Infrared Diode Laser Absorption Spectroscopy to the F4 High Enthalpy Wind Tunnel, Aerospace Science and Technology, n°4,241-250, 1998
Mohamed A. K., Beck W.H., Henry D., Bize D., Infrared diode laser absorption measurements in the HEG free stream flow, AIAA paper 98-2870
Beck W.H., Trinks O., Mohamed A., Characterization of high enthalpy flows using diode laser absorption spectroscopy of nitric oxide, rubidium and carbon monoxide, Paper 120, ISSW22 conference, London, July 1999
More...
Arroyo M. P., Langlois S., Hanson R. K., Diode laser absorption technique for simultaneous measurements of multiple gas dynamic parameters in high speed flows containing water vapor, Applied Optics, vol 33, n°15,20 May 1994
Upschulte B. L., Miller M. F., Allen M.G., Jackson K., Gruber M., Mathur T., Continuous water vapor mass flux and temperature measurements in a model scramjet combustor using a diode laser sensor, AIAA paper 0518, 1999
Beck W. H., Eitelberg G., Trinks O., Wollenhaupt M., Testing methodologies in the DLR High Enthalpy Shock Tunnel HEG, AIAA paper 2770, 1998
François G., Ledy J.P., Masson A., Onera high enthalpy wind-tunnel F4, 82nd Meeting of the Supersonic Tunnel Association, Wright Patterson AFB, Ohio, Oct. 16-18, 1994
Masson A. , The Onera F4 high enthalpy wind tunnel, to be published in Aerospace Science and Technology, 1999
Krek R.M., Eitelberg G., Classical Characterization of HEG(Nozzle,Free stream Flow Field), DLR report, DLR-IB 223-94 A50, Nov 1994
Vardavas I..M., Modelling reactive gas flows within shock tunnels, Australian J. Chem. 37, p157-177,1984
Ledy J.P., Prieur J., Improvement of the Onera hypersonic facility S4MA exhaust at the atmosphere at M = 6.4, 89th Supersonic Tunnel Association International Meeting (VKI), Brussels, 26-28 April 1998
Mohamed A. K., MSTP Phase 2 Progress Report. Infrared Diode Laser Absorption Spectroscopy in Wind Tunnels. I-Experimental Setup, II-Data Reduction Procedures (ESA -HT-TN-E34-701&702&703-ONER), Onera Technical Report n° 8 and 9 /7301 PY, 1996.
Sagnier Ph., Vérant J. L., Flow characterization in the Onera F4 high_enthalpy wind tunnel, AIAA J., Vol.36, N°4, pp.522-531, 1998
Sagnier Ph., Vérant J. L., On the validation of high enthalpy wind tunnel simulations, Aerospace Sciences and Technology, n°7, p. 425-437 , 1998
60°) to the flow 
