Physics, Instrumentation and Sensing

Optical diagnostics in hypersonic flows and in the upper atmosphere

Diagnostics by Electron Beam Fluorescence (EBF) in Hypersonics

Typical setup of EBF in a windtunnel
Visualisation of the electron beam and exploitation of its afterglow
to visualise a Mach 10 flow around the ESA EXPERT atmospheric reentry vehicle

Electron beam fluorescence is a well established technique to perform local and non intrusive measurements of density, vibrational and rotational temperatures and velocity in low density hypersonic flows (< 10 16 molecules/cm 3 ). We present here some applications in low density hypersonic wind tunnels using different types of electron guns developed at ONERA.

In a low density gas flow, the use of an energetic electron beam (typically 25 keV) induces a complicated set of excitations in the gas all along the beam. These excitations produce broadband fluorescence ranging from X ray to the infrared. Each molecular or atomic specie has its chraracteristic EBF signature in the form of characteristic vibrational bands or rotational emission lines from which measurements specific to that specie can be performed.

Figure 1 :
EBF vibrational bands of N2 and NO

For a given specie, the spectral analysis of its fluorescence provides the vibrational temperature (Figure 1) at medium resolution and the rotational temperature at higher spectral resolution. The density of that specie in the flow can be measured from the intensity of one or a few of its fluorescence lines. Velocity measurements can also be performed through Doppler shift of the radiative emission or by a time of flight method. For molecular Nitrogen, the main emissions in the UV and visible spectrum are mainly the first negative system N2+(1N) and the second positive system N2(2P) from which most of the measurements are performed. For NO, the most prominent bands are the gamma bands in the UV between 200 and 300 nm

The measurements can be useful in the following applications:

Validation of aerodynamic simulation codes from wind-tunnel testing
Gas-surface accommodation for shielding of reentry vehicules
Atmospheres of other planets

Parameter	Range
Flow Density	10¹³ -10¹⁶ molecules/cm³
Flow visualization	10¹³ -10¹⁷ molecules/cm³
Temperatures - of rotation T_r - of vibration T_v	as from a few Kelvins > 1000 K
Velocity (Doppler or Time of flight)	> 1000 m/s

Parameters which can be measured by EBF on nitrogen based flows

Results on Density Measurements using X Ray Radiation

For quantitative point measurement of density at higher densities and temperatures, X-ray emission, composed of Bremsstrahlung and characteristic radiation, is preferable because it is not subject to quenching and to spectral broadening (both depend on temperature and pressure and introduce non-linearity in the response). In principle, there are thus no physical limitations to employing this approach, provided the spatial resolution is much less than the electron mean free path. We present here a result of X-ray technique set to examine the air flow structure near a model (hollow cylinder with a ramp) specially designed for shock wave / boundary layer interaction studies. To avoid interference of strong X-ray radiation scattered from the model surface, the electron beam is passed through a tube inserted into the model and cut flush at the model surface. The density measurements can then be made down to 2 mm from the surface. Figure D.1 compares the experimental density profile obtained at position X/L = 0.76 to results obtained from DSMC and Navier-Stokes calculations.

Fig D.1 :
Density profile across the boundary layer of a holow cylinder

References

[1] MUNTZ E. P., The Electron Beam Fluorescence Technique, AGARDograph 132 (1968)

[2] ROTHE D. E, McCAA D., Emission Spectra of Molecular Gases Excited by 10 keV Electrons, Cornell Aeronautical Lab., Tech. Rep. N° 165 (1968)

[3] GOCHBERG L. A., The Electron Beam Fluorescence Technique in Hypersonic Aerothermodynamics , AIAA paper 94-2635 (1994)

[4] DELERY J., Shock interference in high Mach number flows, Aerospace Research, nm 3, (1994)

[5] CHANETZ B., COET M. C., Etude des Interférences de Chocs en Ecoulement Hypersonique, Onera Report RTS 32/4362AY (1993)

[6] EDNEY B., Anomalous heat transfer and pressure distributions on blunt bodies of hypersonic speeds in the presence of an impinging shock, Aeronautical Research Institute of Sweden, Report 115, Stockholm (1968)

[7] LEFEBVRE M., CHANETZ B., POT T., BOUCHARDY P., VARGHESE Ph., Measurements by Coherent Anti-Stokes Raman Scattering in the R5Ch Hypersonic Wind Tunnel, Aerospace Research, nm 4, (1994)

[8] BÜTEFISCH K. A., VENNEMANN D., The Electron-Beam Technique in Hypersonic Rarefied Gas Dynamics, Progress in Aerospace Science, Vol. 15, p217, Edited by D. KUCHEMANN, Pergamon Press Ltd., Oxford and New York (1974)

[9] MOHAMED A. K., Electron Beam Velocimetry , New Trends in Instrumentation for Hypersonic Research, NATO ASI Series E: Applied Science, 24, 275 (1993)

[10] PIPER L. G., COWLES L. M., RAWLINS W. T., State to State Excitation of NO (A²⁺,v'=0,1,2) by N₂(A³_u⁺,v'=0,1,2) , J.Chem.Phys., 85, p3369 (1986)

[11] WATEGAONKAR S. J., SETSER D. W., Excitation-Transfer Reactions from N₂(A³+_u) and CO(a³) to OH , J. Phys. Chem., 94, p7200 (1990)

[12] AJELLO J. M., PANG K. D., FRANKLIN B. O., A Study of Electron Impact Excitation of NO: The Middle Ultraviolet From 170 to 270 nm , J.Geophys.Res., 94, p9105 (1989)

[13] Masson A. , The Onera F4 high enthalpy wind tunnel, Aerospace Science and Technology, 1999

[14] Krek R.M., Eitelberg G., Classical Characterization of HEG(Nozzle,Free stream Flow Field), DLR report, DLR-IB 223-94 A50, Nov 1994

[15] Vardavas I..M., Modelling reactive gas flows within shock tunnels, Australian J. Physics. 37, p157-177,1984

Welcome to Onera, the French Aerospace Lab

Physics, Instrumentation and Sensing

Optical diagnostics in hypersonic flows and in the upper atmosphere

Diagnostics by Electron Beam Fluorescence (EBF) in Hypersonics

Results on Density Measurements using X Ray Radiation

References