Numerical Simulation Of Aerodynamic Heating In Hypersonic Flow In Near Space

Near space hypersonic vehicle has the characteristics of high efficiency of penetration and reconnaissance,which can greatly expand the battlefield space.The huge military and commercial economic value make it have a broad development prospect.When the aircraft flies in near space at hypersonic velocity,the intense compression of gas and the viscous block make the temperature rise sharply,which results in "real gas effect",such as dissociation,ionization and so on.The "real gas effect" brings a series of problems,"black barrier" and "thermal barrier",for example.Because aerodynamic heating is the boundary condition of thermal protection design for an aircraft,it is extremely important to find out the complex factors which affect the aerodynamic heating.In this thesis,two-dimensional axisymmetric numerical simulation is carried out for the aerodynamic heating problem of hypersonic vehicle.The research work is summarized as follows:1.A numerical simulation method for chemical equilibrium flow and chemical non-equilibrium flow is established under hypersonic conditions,which provides a robust numerical simulation tool for near space hypersonic aerothermal analysis.The chemical non-equilibrium flow model introduces a new form for the energy equation and the concept of the equivalent specific heat ratio.2.The real gas effect and the viscous effect of hypersonic chemical equilibrium flow passing a sphere are explored utilizing our code.Then the real gas effect and non-equilibrium effect of hypersonic chemical non-equilibrium flow passing a sphere are studied.The instability of numerical calculation of aerodynamic heating under high Mach number is also studied.The numerical results can be concluded as follows:(?)Under the chemical equilibrium condition,the viscosity has little influence on the stagnation temperature,pressure,density,shock wave standoff distance and the distribution of the density near the wall,but the temperature distribution near the wall surface is slightly different.(?)Under the chemical equilibrium condition,the pressure near wall surface calculated by AUSMPW(AUSM by pressure based weight functions)and AUSMPW+ scheme is commpared under high Mach numbers.When the Mach number is higher than 18,the AUSMPW scheme shows unstable solutions near wall surface,while the AUSMPW+ scheme can continue to give stable numerical solutions.(?)Whether it is in non-equilibrium flow or equilibrium flow,the real gas effect has significant influence on the structure of the property of flow field and the aerodynamic heating environment.Behind the shock wave,the temperature and the shock wave standoff distance decrease significantly,and the density increases greatly.But the pressure does not change much Behind the shock wave,the temperature decreases and the density increases gradually because of the non-equilibrium effect.The equilibrium flow is uniform across the shock wave,and there exists a great gradient for the flow field physical quantity in a very thin area.3.In view of the fact that there existing disagreement of selecting the first grid spacing in the calculation of the surface heat flux,the aerodynamic heating of a sphere is simulated under the condition of chemical non-equilibrium flow.The criteria for selecting the first grid spacing is studied.The numerical results show that the appropriate mesh generation can improve the capture quality of shock wave,but has little influence on the physical quantity of flow field.Compared with other physical quantities of flow field(temperature,pressure,density and velocity),aerodynamic heating simulation converges slower significantly.The grid distribution near the wall can affect the surface heat flux.When the first grid spacing decreases,the stagnation heat flux increases until it reaches convergence.The numerical results negate the"existence of the optimum value of the first grid spacing".Under different freestream conditions,different grids are required to predict the heat flux accurately.According to the calculation results,the selection of the mechanism of first grid spacing is proposed.4.In order to analyze the relationship between the variation of shock wave standoff distance behind the shock with Mach number,blunt body radius,freestream density and atmospheric altitude under hypersonic flow condition in near space,the variation of shock wave standoff distance with the increasing of Mach number is numerically simulated by chemical equilibrium and chemical non-equilibrium flow passing a sphere.The results show that:(?)In the chemical equilibrium flow,no matter how the blunt body radius,the freestream density or the atmospheric altitude change,the standoff distance decreases initially and then increases with the Mach number increasing.So there exists an inflection point for the variation of the standoff distance.The increase of the blunt body radius and the freestream density can make the standoff distance larger at the same Mach number,while the increase of the altitude is the opposite.At the same blunt body radius,the inflection point of Mach number for the minimum standoff distance decreases first and then increases with the increase in altitude.This is mainly related to the temperature changes in the atmosphere.According to numerical results,an equation is put forward to estimate the standoff distance by the atmospheric altitude in near space,the blunt body radius and the Mach number.(?)In the chemical non-equilibrium flow,when the atmospheric altitude,blunt body radius and freestream density are the same,the standoff distance decreases rapidly and then increases gradually with the increasing of Mach number.There also exists the minimum standoff distance in chemical non-equilibrium flow.But the Mach number which shows the minimum standoff distance is higher than that in chemical equilibrium flow at the same altitude.It illustrates that non-equilibrium state makes the compressibility of gas increase.The standoff distance computed by chemical non-equilibrium flow is larger than that by chemical equilibrium flow.And with the increasing of the altitude,the difference of the numerical results of standoff distance is getting larger between chemical non-equilibrium flow and chemical equilibrium flow.This is because the reduction of atmospheric density makes the non-equilibrium effect more remarkable.