<title>Holographic interferometric study of characteristics of a free-piston shock tunnel</title>

Abstract

The free piston shock tunnel (FPST) is one of the most useful ground test facilities for generating hot hypersonic flows. the piston motion depends upon its characteristics. However, the piston speed has never been measured continuously. In this study, a piston speed was measured continuously and precisely in 50 mm dia. and 2,000 mm length compression tube simulating FPST, using a piston motion velocity interferometer, which was developed specifically to measure a piston speed inside a compression tube. Then piston trajectory was obtained by integrating the measured speed. These experimental results were compared with the numerical predictions of a fourth order non-oscillatory scheme. Good agreement was obtained between the experimental and the numerical results. On the other hand, flows around a model suspended in hot hypersonic flows were visualized using a double-exposure holographic interferometry, because real gas effects of high enthalpy flows and driver gas contamination hamper quantitative comparison between measured and numerical results. Then driver gas contamination severely limits test time. For the initial installation of the FPST, the variation of shock stand-off distances also indicated if driver gas contamination was present. The duration time of steady-state flow in the test section was estimated from variation of the fringes in the shock layer, the shock stand-off distance and shock angle.