Experimental investigation of weak irregular shock wave reflections

Abstract

An experimental investigation of weak shock wave reflections was conducted with the large scale shock tube in the Flow Research Unit, in the Mechanical Engineering Laboratory, at the University of the Witwatersrand, Johannesburg. The purpose of the study was to expand the current understanding of irregular shock wave reflections, especially the von Neumann reflection (vNR) and the Guderley reflection (GR). The experiments were conducted using a high sensitivity schlieren system for three Mach numbers in the region around M 1.10 with single-frame and multiple-frame cameras. The single-frame photographs were taken to visualise the sequences of expansion fans and shocklets behind the reflected shock wave, while the multiple-frame photographs were taken using a million-frame per second camera for the velocity calculation in the region near the triple point indicated in oblique shock wave theory. Most of the single-frame photographs show the first set of expansion fans and associated shocklet clearly, while a few of them show signs of the second set of expansion fans and shocklet. The third expansion fan is not very clear on some photographs, and there is no sign of the third shocklet at all. Most of the multiple-frame photographs were useful for the oblique shock calculations. The work done is the first time to obtain quantitative data on flow through a Guderley reflection. The lower burst pressure (1.8 bar) tests were proven to be most successful for the study while the higher burst pressure (3 bar) tests showed contradictions to the physical meaning of the flow behind the reflected shock wave. This is due to the assumption of plane shock waves in the oblique shock calculations, recognizing that the physical shock wave has a very large curvature and the substantial weakness of the reflected wave, being close to sonic conditions. Some primary computational fluid dynamics (CFD) simulations using an Euler code, were also conducted to inform the design of a new insert to vary the angle of the divergent section of the shock tube to improve the visibility of the expansion fans and shocklets behind the reflected shock wave. The new insert has not been tested due to time constraint.