Instability and transition to turbulence in high-speed flows

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Rapid development of computer engineering resulted in great progress in numerical simulation of flow instabilities, the laminar-turbulent transition and turbulence. Direct Numerical Simulation (DNS) transitional and turbulent flows have become widely spread. It is used in one of the basic research areas where the facilities of the largest supercomputer centers are involved. However, numerical simulation of development of instabilities and the transition in high-speed and/or non-equilibrium flows is much less advanced than similar studies in low-speed equilibrium flows. Nevertheless, better understanding and possibility of controlling the transition to turbulence is one of the most important conditions of the development of promising hypersonic flying vehicles and also is of great importance for design of safer re-entry capsules.

It is planned that DNS computer codes based on high-resolution shock capturing techniques such as WENO (weighted essentially non-oscillatory) schemes will be developed the laboratory and applied to study this problem. The development of flow instability and the transition to turbulence in the boundary layer will be modeled at hypersonic velocities. Specific features of the transition with the so-called second mode of instability dominating in the flow at high Mach numbers will be examined. The planned investigations can only be successfully performed in close collaboration of numerical and experimental researchers within the laboratory, which involves the possibility of performing special experiments with the use of the most advanced methods of experimental diagnostics. Particular attention will be paid to studying the possibility of laminar-turbulent transition control. Stabilization of a hypersonic boundary layer with the use of ultrasound-absorbing porous coatings will be studied numerically and experimentally. It is known that such coatings can substantially delay the development of the second mode of instability, but later three-dimensional stages of the transition in the boundary layer on a sound-absorbing wall have not been simulated until now. The boundary conditions for numerical simulations will be developed on the basis of numerical analysis of non-equilibrium flow in the porous media by kinetic approach (DSMC and ES-BGK). It is also planned to study the possibility of controlling the laminar-turbulent transition by means of local heating, including the use of interference control where the development of the Tollmien-Schlichting waves is suppressed by destructive interference with perturbations generated by local heating of the surface.

The influence of real gas effects on generation and development of disturbances in the boundary layer will be studied. Relaxation effects may significantly influence transition to turbulence, causing the transition Reynolds number of flows to be increased by almost an order of magnitude. Transition delay caused by the flow non-equilibrium should be accounted in aerospace vehicle design. It is planned to study non-equilibrium aerophysics of high-temperature hypersonic gas flow containing both diatomic and polyatomic molecules. This problem is directly related to entry of space vehicles into atmospheres of Mars, Venus, Jupiter, Saturn, and their satellites. Owing to planned launches of space vehicles to these celestial bodies, the study of real gas effects on gas dynamics of the flow, thermal loads of reentry vehicles, and stability of the hypersonic boundary layer on their surface is one of the major priorities of the worldwide aerospace community for the next decade.

In the framework of the project the influence of the composition and the relative amount of polyatomic additives in the flow on the position of the laminar-turbulent transition and wave processes in the boundary layer will be determined. The project includes a comprehensive study of the emergence and evolution of disturbances in a hypersonic viscous shock layer at extremely high free-stream velocities (M = 12-21). The viscous shock layer is a flow region between the body surface and the bow shock wave. It is formed on the leading edges of hypersonic flying vehicles, where the local Reynolds number is still not too high and viscous forces dominate in the flow region behind the bow shock wave. Disturbances generated in the shock layer propagate further downstream and affect the evolution of instability and the laminar-turbulent transition in the hypersonic boundary layer of the flying vehicle as a whole. For this reason, the problem of controlling the intensity of disturbances in the hypersonic shock layer seems rather urgent. Generation of perturbations under the action of acoustic waves from the external flow (receptivity process) and blowing-suction on the plate surface will be considered. A method of flow control with sound-absorbing coatings will be tested for a flat plate at incidence. At non-zero angles of attack, disturbances of three instability modes (vortex, entropy, and acoustic modes) arise simultaneously in the shock layer, which substantially complicates controlling these disturbances.

To extend these results to the real flight conditions, non-equilibrium effects, due to the excitation of vibrational degrees of freedom of molecules and physical and chemical transformations, will be investigated. For this purpose, data on the characteristics of the disturbances generated by external perturbations in the high-temperature gas mixture flow around a flat plate will be resulted from experimental and computational investigations, the effectiveness of suppression of pulsations with sound-absorbing coating surface taking into account of real gas effects will be determined and the most effective sound-absorbing materials will be chosen.

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