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The role of physical-chemical processes in numerical simulation of spacecrafts entries into atmospheres

Team leader V.I. Sakharov (DSc)

Despite the significant number of high-speed flights of aircrafts in the Earth's atmosphere, the interest to the problems of gas dynamics that arise in their design, does not diminish. This is due to both the fundamental gasdynamic aspects of the problem and practical applications related to the need to calculate aerodynamic and heat transfer parameters in the design of future aircrafts in order to reduce the amount of costly experiments in wind tunnels. While solving the specific problems it is important to choose not only the gas-dynamic model, i.e. a system of equations, adequately describing the flow, but also a model of the gaseous environment, which includes a description of the physical and chemical processes occurring in the gas and on the surface and significantly affecting the macroscopic properties of the gas and the dynamics of the flowfield.

The molecular structure of the gas is not considered in classical gas dynamics, and the values associated with the molecular properties (specific heat transfer coefficients, etc.) are included in the gas dynamic equations as a variety of factors which can be calculated without the theory of molecular structure. High temperatures exclude an opportunity of the full description of aerodynamic problems in the framework of the adiabatic flow of perfect gas with constant specific heats.

If physical and chemical transformations: the excitation of vibrational and rotational degrees of freedom of the molecules, the dissociation (recombination) of the molecules, the ionization (neutralization), the excitation of electronic levels of atoms and molecules, the emission and absorption of radiant energy occur in the gas, it is necessary to use the idea of the gas particle properties , consider their internal structure and interactions. All of these processes have greater influence on the heat transfer and lesser on the drag.

Characteristic features of the high temperature polyatomic gas flows are the dissociation (recombination) and ionization (deactivation). The choice of the reaction mechanisms, which play a major role in the considered temperature range is important for correct calculation of flow fields and reduces the amount of required computations.

An important factor for the numerical simulation of re-entry problems for the Earth and other planets atmospheres is the choice of the kinetic models of the processes in the shock-heated gas near the body. Thus, the analysis of kinetic processes behind the bow shock wave in the air at speeds of 6-11 km/s, and consideration of the ionization processes along with the processes of dissociation allowed to estimate properly the length of the ionization relaxation zone behind the shock front (works of SA Losev and VA Polyansky at the Research Institute of Mechanics, Lomonosov Moscow State University).

A significant difference in the rates of chemical reactions and reactions of ionization in different kinetic models may influence the macroscopic parameters of gases in the calculation of high-temperature gas flows. Currently, there are numerous kinetic models of reactions in the air for mixtures containing CO2 and other gases.

Relaxation of the energy of the vibrational degrees of freedom of molecules is reduced to an exchange of energy between the vibrations and other degrees of freedom and to its redistribution among the vibrational degrees of freedom. This exchange becomes important when the vibrational degrees of freedom contain a considerable amount of energy and there is an intensive energy exchange with other degrees of freedom. In addition, the energy stored in the oscillations affects the translational temperature of the particles and the process of molecular dissociation.

The importance of the consideration of the nonequilibrium excitation of electronic states of atomic components in the problem of flow past bodies with velocities> 10 km / s was pointed out in the works of V.A.Gorelov et al (TsAGI), who proposed a simplified model of the stepwise ionization of atoms N and O, which takes into account the resonant exchange between electronic states of the levels of these components. The use of this model in the calculation leads to a better agreement of the electron temperature behind the shock front with experiment.

In the development of software systems designed for the calculation of hypersonic gas flows within the framework of Euler, Navier Stokes and Reynolds equations it is necessary to rely on specialized database of thermodynamic, kinetic and transport properties of individual gases and gas mixtures. Using the models of the gaseous environment of various complexity from perfect gas to multicomponent thermally and chemically nonequilibrium gas environment, optimal computer programs can be created for the solution of the specific problems . These programs combine a fairly complete description of the considered flow with the minimal computational burden.

These software systems can be oriented to the solution of a number of key problems of internal and external aerothermodynamics and are opened to researchers in terms of implementation of mathematical formulations and the software realization of processes, the account of which is required for specific tasks.