Measurement of Shock Wave Thickness by the Electron Beam Fluorescence Method

Shock wave thicknesses and density ratios have been measured in helium, argon, and nitrogen by means of the electron beam fluorescence method, over the range 1.5 < M < 17.4, in a low density wind tunnel. The shock thicknesses in argon and helium agreed well with Mott‐Smith theory at the higher Mach numbers and were between Navier‐Stokes and Mott‐Smith theories at the lowest Mach number. In nitrogen the measured shock thicknesses were considerably greater than the predictions of Navier‐Stokes theory. Measured density ratios across the shock wave were in good agreement with theory, in the lower density flow. Poorer agreement was found at higher flow densities, leading to estimates of upper bounds for the range of linear variation of fluorescence intensity with gas density. Density ratios for shocks produced in divergent free‐jet flows were found to be in better agreement with theory after a viscous curvature correction was applied. An anomalous dip in the fluorescence intensity was found ahead of helium sho...