Luminescent Gas Flow Visualization for Low Density Wind Tunnels

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Abstract
At static pressures below a few millimeters of mercury the optical thickness of wind tunnel gas streams becomes insufficient for conventional flow visualization techniques in aerodynamic testing. In the University of California low density wind tunnel, where the free stream static pressure is of the order of one-tenth of one millimeter of mercury, use has been made of various gaseous afterglow phenomena in the visualization of low density flows. The present paper describes the successful application of afterglows in flows of nitrogen, air, argon, and helium. On the basis of recent tests the range of practicability of the enriched air glow technique was found to extend from Mach 2 to Mach 4. The argon and helium glows were found to be useful in the range Mach 1.3 to Mach 2. Discussion of the ranges of applicability and of other aspects of the observed glows is supported by representative glow photographs.