Anomalous relaxation and instability of shock waves in gases

The effect of shock wave instability on the emission characteristics is discussed for gases where the instability manifests itself in certain regimes in the form of perturbations of the front and the relaxation zone. The effect is due to anomalous relaxation which occurs in very nonequilibrium states, and unlike the normal relaxation it enhances the perturbations of the gasdynamic quantities. Results of some recent experiments are presented which clearly demonstrate the effect of instability in different gases and under different experimental conditions (ballistic and vacuum-jet equipment, shock tube). A theoretical interpretation of the effect in its general form is given. A quantitative analysis of the critical regimes is carried out for inert gases.