The refraction of shock waves at a gaseous interface

Abstract

A programme of shock tube experiments has been conducted to study the refraction of plane shock waves at interfaces between two gases. Shocks of strength ζ = 0·85 (weak) and ζ = 0·30 (fairly strong) were allowed to impinge, at various angles of incidence, on interfaces between air/CO2 and air/CH4, and the resulting configurations were photographed through a Mach-Zehnder interferometer. From the interferograms, measurements were made of the strengths of the reflected waves, and of the angles of refraction, and the values were compared with the theoretical calculations of Polachek & Seeger (1951). Within the range of parameters for which the refraction model assumed by the theory is applicable–the so-called ‘regular refraction’ region–the observations were in excellent agreement with the theoretical predictions.When the study was extended to ranges of the parameters for which the theory is clearly inadequate, a succession of rather complex ‘irregular refraction’ patterns was observed. Although these configurations were highly interesting qualitatively, each of them involved curved shocks, non-uniform regions of flow, and other less simple processes which discouraged any formal theoretical analysis. On a less rigorous basis, however, it could be shown that these patterns were internally consistent, and that each represented a distortion of a regular refraction process which was reasonable under the prevailing aerodynamic conditions.Certain observations in these refraction experiments appear to be of some significance outside the specific problem. (i) The sensitivity of strong shock refractions to the values of the specific heat ratio λ for the two gases suggests a possible technique for the measurement of λ and its temperature dependence. (ii) Two of the irregular refraction patterns display a transition process which would be equally appropriate to the onset of the Mach configuration in the shock reflection problem. (iii) Some irregular refractions can be considered as special cases in the problem of the interaction of a shock and a boundary layer.