The isentropic thermal instability of media with a generalized heat-loss function and negative bulk viscosity condition are discussed. We obtain the nonlinear equation taking into account the nonlinear saturation of the isentropic instability. This equation describes the nonstationary evolution of acoustical waves in media with the isentropic instability. Its stationary solutions are investigated analytically. The most interesting solution is the self-sustained pulse. Using the numerical simulation of the nonlinear acoustical equation and the full system of one-dimensional nonstationary hydrodynamical equations, we showed the disintegration of the initial weak perturbation of compression into sequence of these self-sustained pulses in low-density PDRs.

No abstract

Acoustical instability of the inhomogeneous heat-emitted gas flow in dependence on the Mach number is investigated. Obtained acoustical equations for perturbations of velocity, density, pressure, and temperature have diff erent forms in inhomogeneous media. Growth increments for these perturbations diff er from each other as well. They are significantly dependent on the Mach number and the flow direction. The features of sub- and supersonic acoustics are considered.

Chemical active mixtures with irreversible reactions, vibrationally excited gases, and nonisothermal plasmas are examples of acoustically active nonequilibrium media. In such media it is possible the existence of stationary nonlinear structures that are different from the step-wise shock wave structures. In the first part of the present paper it is investigated the solutions of a general acoustical equation, describing in the second order perturbation theory a nonlinear evolution of wide spectrum acoustical disturbances in nonequilibrium media with one relaxation process. Its low- and high- frequency limits correspond to Kuramoto-Sivashinsky equation and the Burgers equation with a source, respectively. Stationary structures of general equation, the conditions of their establishment and all their parameters are found analytically and numerically. In acoustically active media it is predicted the existence of the stationary solitary pulse. Then, we consider 1-D relaxing gas dynamics system of equations with simple Landau-Teller model of relaxation. The possible stationary profiles are shown in nonequilibrium degree- stationary wave speed bifurcation diagram. The boundaries of this diagram are obtained in analytical forms. The field of weak shock wave instability is shown in this bifurcation diagram. Unstable shock wave disintegrates into the sequence of solitary pulses described by the general acoustical equation.