Digital Measurements of the Speed Variations of Shock Waves by Means of Microwaves

The thermodynamic state of the flow within a diaphragm shock tube, or within that provided by a shock tube used to drive a tunnel, can be determined precisely only if the variations in shock front velocity within the tube are known accurately. A method of shock velocity measurement applicable to this problem has been developed which makes use of the Doppler effect measured in microwave signals reflected from the ionized shock front. The system employs electronic counting of a given number $(n)$ of periods of the Doppler beat between the emitted and reflected wave trains. Contiguous time measurements corresponding to consecutive shock traversals of the distance $n(λi/2)$ (wherein $λi$ is wavelength within the shock tube) are recorded by an appropriate number of sequentially operated counters. The microwave energy is introduced into the shock tube by a loop antenna which is located in such a way that it neither protrudes into the tube nor interferes with the flow of gas. A few typical experimental results demonstrate the applicability of the method at Mach numbers of 8 and above. The method provides velocity measurements over 85‐cm reference distances with limits of error of the order of one part in $103$⁠. Accuracy can be improved by employing higher‐frequency counters.