Numerical Simulation For Heating Environment Of Gap In Hypersonic Flow

The hypersonic vehicle is subjected to extreme aerodynamic heating during a flight,and the surface of the aircraft is covered with the heat-resistant layer for thermal protection.Many gaps with different widths are inevitably left between these heat-resistant layers,and these gaps may cause local overheating,which leaves the risk of damage.Therefore,the numerical simulation of the local heating environment in the hypersonic flow is carried out on the theories of aerodynamic thermodynamics,heat transfer,thermoelasticity and fluid-structure coupling.This study will provide a guidance for the fine design of the thermal protection system.The shock layer caused by a hypersonic flow contains high-velocity compressible flow and low-velocity incompressible flow near the wall.For this flow characteristic,the two-dimensional compressible dimensionless N-S equations with the low velocity pretreatment and the Menter SST turbulence model are chosen as the flow control equation.The first-order difference scheme is used to discretize the time term.The second-order central difference scheme is used to discretize the viscous term of N-S equations,and the viscous and source terms of turbulent model.The AUSM+-up upwind scheme which is suitable for all-velocity flow is used to discretize the inviscid term of N-S equations,and the first-order upwind scheme is used for the inviscid term of the turbulence model.The flow control equation is discretized by using the LU-SGS implicit method without considering the rigidity of the source term of the turbulence model.The solution of the discretized equations are solved by FORTRAN language,and the test result shows that the program for a supersonic flow is effective of the capture of shock waves.On the assumption that the gap wall is a rigid isothermal wall,the physical model of the two-dimensional gap flow exposed to a hypersonic flow is established.Firstly,based on the calorically perfect gas model,the gap flow and thermal environment under the flow condition of Ma = 5,? = 0°?30°(interval 5°)are numerically simulated with the self-written program.Compared with the experimental results of ?=0°,the wall heat flux distribution agrees well with the experimental data,which verifies that the mathematical model and numerical method are reasonable and effective.Secondly,considering the real gas effect in a hypersonic flow,the Srinivasan equilibrium air fitting curve is used to describe the thermodynamic and transport properties of the gas,and extend the solving range of the program made for the calorically perfect gas.The gap flow and thermal environment under the flow condition of Ma?12?15.5,?=0°?30°(interval 10°)are numerically simulated.The numerical results show that the gap wall heat flux is basically "U" shaped,reaching the peak at the lip and decreasing rapidly along the depth of the gap.The peak heat flux at the windward lip increases with the increase of the attack angle of the air flow.There is no obvious difference between the equilibrium air model and the calorically perfect gas model in the distribution of the heat flux ratio on the gap wall.For the problem of elastic gap flow,the compressible N-S equations with low-velocity pretreatment are chosen as the flow control equations which are solved by the self-written calculation program for the chemical equilibrium air.The steady-state heat conduction equation and thermo-elastic mechanic equations are chosen as the control equations of the solid domain which is solved by the finite element method.The boundary conditions are exchanged at the fluid-solid coupling interface between the fluid domain and the solid domain.The solid domain provides the interface displacement and temperature for solving flow,and the fluid domain provides the interface pressure and heat flux for the solution of the solid domain.For a given initial interface position and temperature,the control equations of the fluid domain and solid domain are solved in turn,and the fluid mesh grids are updated by the technology of dynamic mesh.This process is repeated and until convergence.Based on the multi-field coupling algorithm,a thermal-fluid-structure coupling calculation program is written for elastic gap,and this program is verified by a test.For the problem of the heating environment of nosecap gap,the physical model of the elastic gap flow is established.The gap flow and its thermal environment are numerically simulated by the self-written program,and the influence of thermal expansion coefficient of C/C-SiC material and flow parameters(Mach number and angle of attack)on heat flux are analyzed.The numerical results show that increasing the thermal expansion coefficient in the thickness direction of the C/C-SiC material will cause the front wall of the gap to be exposed to the outflow more,which causes the heat flux on the front wall to increase,and the thermal expansion coefficient in plane of C/C-SiC material has an opposite effect.Increasing the Mach number and the attack angle will cause the temperature of the gas in the boundary layer to rise,which will cause the heat flux on the gap wall to increase.The heat transfer to the gap structure is mainly the heat conduction of the heat-resistant material,and the heat obtained from heat convection in the gap is not significant.