Techniques for creating precursors in shock tubes

The airblast precursor is a shock refraction effect formed by the interaction of a shock wave with a high-sound-speed layer. Various techniques are described for simulating precursors in small shock tubes. Three successful methods were developed to create high-sound-speed layers: helium layers contained by thin membranes, helium injected through a porous plate, and hot air layers formed by a heated foil technique. The flow was made visible by schlieren, shadow, and interferometer photography. Incident shock Mach numbers of M = 1.7 were used. The results indicate that strong precursors could be formed by these techniques. A prominent feature of the flow was an unstable, supersonic wall jet that developed large-scale, turbulent structures. Such experiments can be used to study the fluid mechanics of precursors and to evaluate the accuracy of computer simulations of this problem. (EMI)