Spontaneous acoustic emission from strong shocks in diatomic gases

The stability properties of strong shock waves in diatomic ideal gases are investigated. Shock instabilities in diatomic gases may play a role in the dynamics of astrophysical molecular clouds, hypersonic flight applications, and inertial confinement schemes. It is shown that the dissociation processes alone do not give rise to shock instabilities. It is further demonstrated that the shock's front becomes unstable under spontaneous acoustic emission due to the ionization processes and only for those perturbations that are characterized by thermal nonequilibrium between the electrons and the heavy particles (atoms, ions, and molecules). To show that, the classical Dyakov-Kontorovich stability criterion is modified in order to take into account the effects of the perturbed electronic temperature. Numerical calculations for diatomic nitrogen indicate that the spontaneous acoustic instability occurs on the descending portion of the Hugoniot curve in the density-pressure plane. In addition, it is found that the threshold for spontaneous acoustic instability decreases with the upstream density.