On the flow in an isentropic light piston tunnel

Abstract

A new form of short duration wind tunnel is described in which the test gas is heated employing isentropic compression by a free piston moving in a tube. The piston is propelled by gas from a high pressure reservoir and steady conditions are maintained during the running time by 'matching' the volumetric flow rate into the tube from the reservoir with that of the test gas issuing through the working section. The equations governing the compression and the steady running time are given. The effect of a finite piston mass producing pressure oscillations is considered and the equations for operation away from the 'matched' conditions are developed. Heat losses due to the vortex in front of the piston and the effect of the finite transit time of sound waves along the tube are discussed. Design charts and equations are given and experimental results from two such wind tunnels are presented which confirm the theoretical predictions.