An experimental and numerical investigation of fast transient gas dynamics

Abstract

An experimental and numerical investigation has been performed to study the evolution of shock waves undergoing a sudden expansion in one direction while restricted in the second. Experimental data are gathered and studied for shock waves undergoing the sudden 4:l area expansion in air for Mach numbers of 1.5 and 2.0. Detailed, time-accurate measurements of the shock wave and vortex core location as well as wall pressure data are presented. In addition, the evolving flow structure through the time-accurate flowfield imagery is also presented. The results of these experiments are compared to twodimensional numerical simulations specific to the Mach 1.5 and 2.0 initial conditions and geometry. The direct comparisons of the experimental work and numerical simulations provide insight into flowfield phenomena such as viscous dissipation and sh&&/vortex interaction. The data presented in this effort further elucidates key modeling questions by providing time-accurate flow visualization and pressure data of a two-dimensional shock wave undergoing a sudden expansion in a confined chamber. * Aerospace Engineer, Senior Member AIAA ’ Senior Principal Consultant, Associate Fellow AIAA ’ Research Engineer, Member AIAA ’ Professor & Department Chairman, Associate Fellow AIAA ‘I Associate Professor ’ Associate Professor, Senior Member AIAA ‘* Assistant Professor This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Background Gas dynamics of fast, transient characteristics includes issues such as shock wave reflections, vorticity production, shock-vortex interaction, energy exchange between shock and turbulence, and shock focusing (Chang and Kim, 1995). Figure 1 illustrates the sudden area expansion of a shock wave within a confined domain indicating these interesting fluid dynamic phenomena. These phenomena occur at time scales ranging from microseconds to milliseconds. This paper presents a focused look at sudden shock wave expansion within a confined chamber and the interaction of that shock wave with other fluid dynamic features. Both experimental and numerical tools have been adopted. Recent investigations of this type of gas dynamic flow, given by Jiang et al. (1997), present experimental and numerical simulations of a circular shock tube generated flow undergoing a sudden expansion. Figure 1 highlights the salient flow features of such a sudden expansion. Here it is seen that as the initial shock wave travels down the shock tube (Figure 1) it will undergo a sudden area expansion into a confined area (Figure lb-d), the flowfield gives rise to two important features. First, a “starting jet”, characterized by the formation of a vortex ring, Mach disk, and shear layer’ appear. Second, this precursor shock reflects off the confining walls of the expansion chamber and interacts with the starting jet flowfield. It is clearly seen in Figure 1 that this reflected shock splits once it interacts with the primary vortex ring, which gives rise to secondary shock waves. This shock/vortex/shear layer interaction is also shown in Figure 1. Here the reflected shock splits when it interacts with the primary vortex ring. It was also American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or published with permission of author(s) and/or author(s)’ sponsoring organization. observed by Jiang et al. (1997) that once the reflected shock wave passes through the developing shear layer that the shear layer splits, it is moved into the jet flow, and it is convected downstream. This allows another shear layer to form from the jet exit forming another weaker vortex ring. This is referred to as shear layer splitting and is largely an inviscid phenomenon (Jiang et al., 1997) Another phenomenon noted by Jiang et al. (1997) is the shock/shear layer interactions. It is noted that a vortex ring in the shear layer induced a shock wave and the shock wave separated the vortex ring from the shear layer. This is most often noted in strongly expanded free jets. Due to the strong influence of vortex dynamics and compressibility, the turbulence structure can be fruitfully assessed via an evaluation of viscous dissipation.