Radio Observations of Hypersonic Shock Waves

A SHOCK tube consists essentially of two sections, one containing a gas at high pressure and separated from the test gas in the other section by means of a diaphragm. A shock wave is produced in the test gas when the diaphragm is suddenly broken. As the initial pressure ratio is increased, the shock front attains a limiting velocity and the corresponding limiting shock Mach number is U/a1, where a1, a2 are the speeds of sound and γ1, γ2, the specific heat ratios (Cp/Cv) of the gases in the test and high-pressure sections respectively. Thus if both sections contained the same monatomic gas for which γ = Cp/Cv = 1.67 the limiting shock Mach number would be approximately 4. Greater shock Mach numbers could be achieved if the speed of sound in the high-pressure section were increased, for example, by using a light gas and by raising its temperature. Thus hydrogen is a suitable driver gas, and it could also be brought to a high temperature by burning oxygen in it. An alternative method of producing a high shock velocity is to introduce an intermediate section into the shock tube1. When the first diaphragm is broken, a shock wave is produced in the intermediate section, and the resulting column of hot gas flowing at high speed in this section then acts as the final driver to initiate a faster shock wave in the test section. In a previous communication2 we described the radio measurement of the velocity of shock waves by means of the Doppler effect. The value of this technique, which provides detailed measurement as the shock wave travels along the tube, is illustrated by its application to the study of methods for producing hypersonic flow.