Numerical Simulation Of Hypersonic And High Temperature Gas Flowfields

The air particles in the high temperature shock layer of hypersonic re-entry vehicle undergo various physical and chemical processes such as vibrational excitation, dissociation and (possibly) ionization. The effects of these thermochemical phenomena ,will be termed"high temperature effects". High temperature effects have an important influence on the aerodynamic heating , the pitching moment characteristics and the communications of re-entry flight vehicles. The research of high temperature effects needs organic combination of the approaches of theoretical analysis, ground test and flight test. Though several high enthalpy facilies have been built in a number of countries, some difficulties are still existed due to the limitations of diagnostic methods and experimental flowfield analysis when studying high temperature effects in ground facilities. With the development of computer technology, CFD(Computational Fluid Dynamics) has gained popularity for studying high temperature effects. CFD can complete the theoretical analysis and the numerical calculation of the high temperature gas flow field around hypersonic vehicle and that associated with the flow fields in high enthalpy wind tunnel and around the model in the high enthalpy wind tunnel in relative short time, supplying a large amount of aerodynamic data and remedying the deficiency of the experiments and mesurments in high enthalpy facilities.For the purpose of studying high temperature effects in hypersonic and high temperature gas flowfields, the present thesis has developed a numerical software used for simulating the high temperature gas flow field around the hypersonic vehicle with complex configuration. The software is capable of solving two-dimensional and three-dimensional steady Euler and Navier-Stokes equations based on finite volume method, which has been validated by various of numerical tests and data. The approximate eigenvalues of viscous flux for thermochemical nonequilibrium flow are deduced independently. The catalytic wall boundary condition has been improved. For the high temperature gas flow field calculation on multi-block structure grids, the 1-1 blocking topology is applied. High temperature effects can be handled with frozen, chemical equilibrium, chemical nonequilibrium and thermochemical nonequilirium models in the software. The equilibrium constants method, free energy minimization method and approximate curve fits method have been used to determine the thermodynamic properties of the gas in chemical equilibrium flow. The thermochemical nonequilibrium model may be divided into three subclasses including the two-temperature model, the three-temperature model and the multi-temperature model(Each molecular species retains its own vibrational temperature). The main works have been carried out with the software including the study of the thermochemical nonequilibrium flows in hypersonic nozzle, the calculation and the correlation analysis of hypersonic aerodynamic characteristics for airfoils in high temperature gas flow and the numerical analysis of the high temperature gas flow over the original configuration of spacecraft, Hermes.The thesis consists of 6 chapters. Chapter 1 is the introduction including the background and simulation approach, state of the art and the development of research on high temperature effects, the challenge faced in numerical and ground based simulations. Finally, the main works of the thesis are outlined briefly.Chapter 2 consists of governing equations, physical models ,numerical method and the software validation. The thesis discusses the results of software validation study by making comparisons between experiments, previous and present CFD simulations. The test cases include: hemisphere, sharp cone, blunt cone, tranverse cylinder and blunt cone-flare. The results include pressure, skin-friction, heat flux, species mass fractions, vibrational temperature and electron number density, etc, which are in good agreement with the existing numerical and experimental results to some extent . It is shown that the numerical software has good reliability and precision. Chapter 3 focuses on the thermochemical nonequilibrium flows in hypersonic nozzle. Multi-temperature model testing calculations have been performed for the thermochemical nonequilibrium flow in three kinds of typical hypersonic nozzles. The results of flow variable such as translational temperature, vibrational temperatures of different molecular species, Mach number, different species mass fractions are presented,which are in good agreement with the existing 2D-axisymmetric Navier-Stokes equations solutions. It is shown that: (1)The velocity, pressure and temperature at exit of the nozzle would be increased because of chemical reaction, whereas density and Mach number would be decreased; (2)Though affecting the exit temperature, species mass fractions and Mach number in some degree, different chemical kinetic models have minor influence on the exit velocity and density; (3)Vibrational relaxation processes would result in an increment in Mach number and a decrement in temperature. As for the exit density, velocity and species mass fractions, the effects of thermal nonequilibrium are not obvious; (4)The exit temperature for the multi-temperature model simulation is slightly higher compared with the two-temperature model simulation. While the exit Mach number is slightly lower for the multi-temperature model simulation compared with the two-temperature model simulation; (5)The vibrational energy of all species described by a single vibrational temperature is not enough to simulate vibrational relaxation process for the hypersonic nozzle flow. The vibrational energy for each molecular species (at least for N2 and O2) should be modeled by its own vibrational temperature.Chapter 4 is the calculation and the correlation analysis of hypersonic aerodynamic characteristics for airfoils in high temperature gas flow. The effects of high temperature gas on the aerodynamic characteristics at hypersonic speeds are studied for airfoils in air. The calculations are performed on an airfoil similar to that used for the space shuttle orbiter, and ellipses of thickness ratios varying between 5 and 15%. For the airfoil, one flight condition is considered. For the ellipses, the calculations are carried out over a range of flight velocities and flight altitudes. To investigate the binary scaling parameter, aerodynamic characteristics for different chord lengths but with the same chord length-density product are calculated. It is shown that: (1) Lift and drag coefficients for the thermochemical nonequilibrium flow become systematically smaller than the perfect gas values as the flight velocity is increased. The center of pressure systematically moves forward as the flight velocity is increased; (2)The body thicknesses have little effect on the relative changes in lift and drag coefficients due to the high temperature effects, but they have a large effect on the center of pressure location. Generally speaking, the forward shift in the center of pressure is more obvious for blunt and thick airfoils than that for slender and thin airfoils; (3)For windward flow field which determine the aerodynamic characteristics of the ellipses, where chemical rate processes involving two collision partners dominate the chemical reaction processes, binary scaling duplication is validated.Chapter 5 is the numerical simulation of high temperature gas flows over the original configuration of the spacecraft, Hermes. The aerothermodynamics for the vehicle is obtained with various flow models. The high temperature effects on aerothermodynamic properties for the vehicle are obtained. Results of multi-block calculations for the flow over the Hermes space shuttle are also validated. Based on the numerical results, it is shown that: (1) The numerical software is capable of simulating high temperature gas flows over hypersonic vehicles with complex configuration; (2) The influence of thermal nonequilibrium phenomena on the shift in the center of pressure is obvious, which lead to the forward shift in the center of pressure further; (3) The location of center of pressure for the multi-temperature model simulation is close to the two temperature model simulation results; (4) The influence of the wall catalycity on the wall heating rate is notable.Chapter 6 is the concluding remarks. The dissertation works and the expectation for future research are briefly summarized.