Criteria for acoustic instability in a gas with ambient vibrational and radiative nonequilibrium

When acoustic waves propagate in a gas, nonequilibrium phenomena can manifest themselves in either (or both) the ambient and perturbed states of the gas. To study the relatively unexamined effects of ambient nonequilibrium, the present paper uses a macroscopic, gas-dynamic approach to investigate the influence of such nonequilibrium, primarily as regards vibration and secondarily as regards associated radiation. In situations where the vibrational temperature exceeds the translational temperature, ambient vibrational nonequilibrium, from whatever source, is shown to cause instability (i.e., wave amplification) provided the vibrational relaxation time is a descreasing function of temperature (or increases at less than a specified rate) and the degree of ambient nonequilibrium is sufficiently high. Ambient radiative nonequilibrium will cause instability, in cases where the external-source temperature exceeds the gas temperature, provided the absorption coefficient is an increasing function of temperature and the rate of radiant energy input is sufficiently large