Oscillation Phenomena of Pseudo-Shock Waves

In the previous paper, we presented the diffusion model of pseudo-shock waves, by which the time-mean static pressure distributions in pseudo-shocks could be explained. In the actual flow, however, the pseudo-shock wave is not steady, but oscillates about a mean position and causes the fluctuations of the local static pressures. In this paper the oscillation phenomena of pseudo-shocks in a straight pipe were experimentally investigated and the causes and the frequencies of oscillations were discussed. As the results, it was found that the oscillation of the pseudo-shock became stronger with the increasing Mach number and the maximum wall static pressure fluctuation induced by the oscillation amounted to about 60 per cent of the difference of static pressures before and behind the shock. Moreover, it was showed that this oscillating phenomena could be explained by the interaction of the shock with the small disturbances existing in the supersonic flow upstream the shock.