The aim of the paper is to study the unstable processes in free supersonic unstable jet flows. The direct method of solving a kinetic equation is used. For numerical solution the finitedifferent schemes are applied. To perform parallel computations a supercomputer MVS-100K is used. The maximum number of processors was 480. A mechanism of instability for a supercritical regime for 3D flows is investigated. Solutions of these problems are made for flows in a wide range of Knudsen number Kn with different aspect ratio of the orifice (square and rectangular forms). Mach number Ma = 1.4 and a ratio of pressure in the orifice and the background pressure np = 3.16. Comparison with results of known experiments is made. In calculations we have observed that for a subcritical regime with the large Knudsen numbers (small Reynolds numbers) a vorticity of the flow fields in the cross sections is equal zero. For supercritical regimes a system of streamwise pair vortices is obtained. That corresponds to theoretical and experimental data. The paper studies such a system of the Taylor-Görtler vortices in different cross-sections in an initial region of a jet. It presents results of numerous computations. When modeling a roughness in the nozzle orifice it has been found that the character of instability with disturbance of the symmetry in the cross-sections downstream is complicated. Based on direct method to solve the kinetic equation the paper, for the first time, studies in detail the character of 3D instability.

Previously, these problems were solved by continuum methods and also for some variants of flows was used the Direct Simulation Monte Carlo method. A potential field of application of the given results is theoretical and experimental investigations with more detailed, in comparison with previous studies, description of unstable supersonic flows, which show the similar features of the Taylor-Görtler instability. Based on conducted study the paper comes to conclusion that the direct methods of the kinetic approach allow us to describe appropriately a mechanism of the transition to the unstable flows in supersonic jets. We believe that a further study will make it possible to show up the characteristics of turbulence in free supersonic jets.