Abstract: Attenuation of incident shock waves in free piston shock tunnels is of great important, which not only reduces the enthalpy, pressure and steadiness, but also imposes constraints on valid testing time. In this study, an attempt of investigation of main reasons of attenuation of shock wave was carried out based on the structural features of free piston tunnels. After neglecting some random factors, two important factors of attenuation, viscosity of the boundary layer and the reflected expansion wave, were studied. The results show that the latter factor plays major role in free piston shock tunnels. When the main diaphragm ruptures, the front face of the piston is very close to the main diaphragm, which leads to reflected expansion wave to catch up with the interface or shock wave earlier. A new compression tube with an abrupt area change was proposed, which can delay the interaction between the reflected expansion wave and the interface (or shock wave). In the new compression tube, the steady pressure and temperature of driver gas could be formed in the end of the tube while the free piston decelerated to a safety speed.

Key words: free piston shock tunnel, incident shock wave, attenuation, compression tube, safe velocity