Hypersonic‐helium‐flow‐field measurements with the resonant Doppler velocimeter

A low density of sodium atoms seeded into a hypersonic helium flow is used to quantitatively determine flow‐field properties. By observing fluorescence from a point as a single‐mode dye laser is tuned through the sodium D2 line, the absorption profile may be found. This profile is fitted to a properly weighted sum of Voigt functions to yield the velocity, temperature, and pressure at the observation point. The fluorescence is strong enough to use this technique for qualitative flow‐field visualization.