On the Piston Motion of a Free-Piston Shock Tube

Abstract

SummaryThe motion of a solid piston between the driver and test gases in a closed shock tube is studied. In particular, the piston stopping distance, i.e. the total length required to bring the piston to a stop, is calculated for various experimental conditions. Before the stop, the moving piston compresses the test gas, at first with its induced shock and then further with its own kinetic energy through the successive shock reflections. Thus, the moving piston can produce a reservoir of compressed gas with higher enthalpy than a shock of similar strength without the piston. Normal shock-wave theory for an ideal gas is used until the first reflected shock meets the piston. The subsequent shock reflections are approximated by a continuous compression in thermodynamic equilibrium. The calculated values for the piston stopping distance are found to compare favourably with the available experimental data, indicating that the approximations used are reasonable. Peak pressure and temperature of the compressed test gas are computed, and the effect of bleeding the gas reservoir through a nozzle is also included.