An experimental investigation of the reflected-shock pressure-time profiles for air, oxygen, nitrogen, argon, carbon dioxide and acetylene

Abstract

A review of the literature on the subject of normal reflection of shock waves at the end of a shock tube is given and indicates that, although many of the phenomena which influence the motion of the reflected shock wave have been identified, the analysis is by no means complete. Woods has given a theoretical description of the motion of the reflected shock for times not too far removed from the instant of reflection which includes most of the relevant influences, e.g., boundary-layer growth behind the primary shock wave and reflected shock-boundary-layer interaction. Experimental reflected-shock pressure-time profiles are described for air, oxygen, argon, nitrogen, carbon dioxide and acetylene for both short duration (~ 200 microseconds) and long duration (~ 4-10 milliseconds) recording times and the results compared with existing theories. Woods' theory is found to predict correctly the form of the pressure-time profile for the first few hundred microseconds after shock reflection, but for longer times the influence of shock interaction with a turbulent boundary layer in a shock tube and with a contact region rather than a contact surface require clarification. The pressure levels immediately after shock reflection are shown to agree closely with real-gas theory, but measurements of the pressure rise across the shock transmitted through the contact surface show a similar scatter and poor agreement with theory as was described by Holder and Schultz for reflected-shock studies in air. It is shown that reflected-shock waves in acetylene must be considered a separate study owing to the added complication of pyrolysis which occurs for incident shock Mach numbers greater than 4. Some streak schlieren photographs for acetylene showing the occurrence of carbon formation are shown. For all the gases except nitrogen the initial pressure range was 10-300 mm Hg, the driver gas was room temperature hydrogen and the experiments were carried out in a 2 in. internal diameter shock tube. The reflected-shock pressures were measured using a bar gauge, and a description of a modified bar gauge developed~especially for studying the long duration recordings shown in this report is given. The reflected-shock pressure-time profiles for nitrogen were obtained in the N.P.L. 2 in., 3 in. and 6 in. internal diameter shocktubes using hydrogen and helium gases at pressure up to 1800 lb/in² abs. In this latter case the shock pressures were measured using S.L.M. PZ.14 transducers.