Primary cause and mechanisms of structural instability of strong shock waves in gases

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This paper is aimed at finding the primary cause and mechanisms of flow structure rearrangement (stable/unstable) behind the front of strong shock waves (SW) in gases. Results of experimental and calculated investigations of strong shock waves in atomic gases obtained by the authors of this work and other researchers for the last several decades are analyzed. A unique analytical approach, referred to by the authors as the source of electrons method, is used, which makes it possible to determine the relationship between the kinetic processes of ionization of particles and the energy exchange in a non-equilibrium flow (avalanche ionization region) behind a shock wave in argon in a wide range of flow conditions: M = 10–30, pressure ahead of the shock wave front p1 = 1–25 torr. In addition to the basic processes, the following additional mechanisms are considered: (a) associative ionization in interatomic collisions with participation of highly excited atoms, (b) integral radiation losses, (c) convective energy transfer, and (d) electrons heating in the polarization electric field. It is found that the primary cause of flow structure instability in shock waves is the presence of flow boundaries responsible for (i) weak gas-dynamic perturbations and (ii) local (near-wall) inhomogeneity of the medium ahead of the shock wave front and behind it. The mechanism of structural instability of the shock wave in atomic gases at low Mach numbers is caused by a cardinal change in the relationship of the basic and additional kinetic and energy exchange processes between the ionized flow components behind the front, even in the case of a weak perturbation of the atomic–ionic component temperature. In the range of high Mach numbers, the instability mechanism is due to the thermal effect induced by wall and near-wall gas layers heating ahead of the SW front by precursor radiation and also due to possible energy release initiated by polarization of the electric field near the wall behind the shock wave front.