Instrument to Study Relaxation Rates behind Shock Waves

An instrument is described which combines the schlieren technique, a photomultiplier tube and an oscilloscope in a manner that enables one to measure the density distribution behind shock waves (in gases) produced in a shock tube. From these measured distributions one can determine the way in which the gas or gases relax to equilibrium and the time it takes to reach equilibrium after the gas's enthalpy is increased suddenly a calculable amount in passing through the shock wave. The theory of the instrument is discussed and its predicted performance verified experimentally by measuring vibrational heat capacity relaxation times behind shock waves in CO2 containing water vapor. The instrument in the reported tests demonstrated a sensitivity sufficient to record a change in atmospheric density of 12% over 1-mm distance and a space resolution of the density in the shock tube of 1/10 mm corresponding to times of the order of 1/10 μsec.