Directed motion doppler shift effects on infrared band models of emission and absorption in a hypersonic conical flow

S. S. Penner, et al (Journal of Quantitative Spectroscopy and Radiative Transfer, Vol. 13, p. 1314, 1973) investigated the importance of directed-motion Doppler-shift effects in the measurement of hyper sonic gaseous flowfields using tunable laser-absorption spectroscopy. They concluded that the distortion of spectral-line shapes due to Doppler effects would prevent useful laser measurements at many conditions of interest. This work extends the foregoing analysis to lower spectral resolution, infrared band models, which are employed in conventional radiometer and spectrometer measurements. Although these instruments do not directly observe the profiles of individual spectral lines, they are sensitive to the Doppler-induced distortion of the line shapes.

This study begins with a comparative treatment of a Voigt profile spectral line with and without flowfield Doppler shifts. It first reproduces the earlier results of Penner, et al, and then generalizes this result to treat the curves of growth of a single line and the curves of growth for an infrared band model. The final step was to generate predicted CO₂ 4.3 micron-band spectra based on the obtained curves of growth to illustrate the significance of the directed-motion Doppler-shift effects.

For the particular conditions of this study, listed in Table 1 on page 48, it was shown that the inclusion of directed-motion Doppler shifts could increase the single-line curve of growth by up to 30 percent over that which was based on the neglect of this effect. The curve of growth for an infrared band model with this effect exhibited a maximum discrepancy of 26 percent over the curve of growth without this effect. Finally, comparison of the predicted band spectra showed the enhancement of the band-center absorption by 7 to 10 percent with the inclusion of directed-motion Doppler-shift effects.