Experimental study of shock wave strengthening by a positive density gradient in a cryogenic shock tube

The strengthening of a shock wave propagating through an isobaric region of increasing density was determined. A new experimental configuration consisting of a pressure driven shock tube mounted vertically with its test section partially immersed in a cryogenic bath was used in this study. The shock wave created by the diaphragm rupture propagated past a region of uniform low density through a strong local density gradient and finally into a high-density zone. The Mach number of the shock emerging from the variable density region was measured and compared with results from approximate theoretical models. Mach numbers considerably higher than predicted were obtained and suggested the existence of shock strengthening processes not hypothesized in the previous theories but significant in this experimental configuration. Preliminary computations showed that multiple interactions between waves generated within the density gradient and flow nonuniformities due to the shock formation mechanism were of sufficient magnitude to explain the observed Mach numbers. This study contributes to the understanding of shock tube operation in the presence of a density gradient and provides a tool useful for experimentation in a cryogenic environment.