The Organized Vortex Structure in Relaxing Gas Flow

The flows of a weakly non-equilibrium compressible gas are satisfactorily described by the Navier – Stokes equations. The stress tensor in these equations contains two dissipative coefficients that are dynamic viscosity 1 η and bulk viscosity 2 η . The last one takes into account relaxation of the excited internal degrees of freedom of gas molecules. In paper [1] some experimental results on the bulk viscosity influence on the value of the critical Reynolds numbers of the laminar-turbulent transition (LTT) are presented. From the experimental data obtained it follows that, with increasing bulk viscosity, the value Rec of LTT also grows. In this case, registered change reached ten percent. However, the matter of the reliability of the obtained data have been debatable. The calculations, which were derived within the framework of linear stability theory in papers [2, 3], have shown that bulk viscosity influence on LTT is rather little. In [2] the calculations were carried out for air at Mach number 0 4;5 M = and relation of bulk viscosity to dynamic viscosity 2 1 / α η η = didn’t exceed unit. In paper [3] the estimations were obtained up to 30 α ≤ . At the same time the data of the experiments [1] correspond immediately to the nonlinear stage of transition in the turbulent regime at 7 α ≈ . It’s known from the modern scenarios of the transition and generation of turbulence are that these processes are essential nonlinear. They realize by means of origin, evolution and decay of certain vortex structures [4]. In boundary layers and pipes they are known λ -structures, in two-dimension free shear flows and jets they are two-dimensional vortices extended on the transversal coordinate. Under this interpretation the generation of turbulence can be considered as the process of LTT which is stochastically repeated in space and time. This fact allows one to assume that bulk viscosity influence on the development of perturbations at the nonlinear stage can be estimated by simulating the interaction of the solitary vortex structure with the basic (mean) flow. In this paper the simple model of the evolution of the spanwise vortex structure in a shear flow of non-equilibrium molecular gas is considered for such a purpose.