Study On Aerodynamic Heating And Flow Stability Of Hypersonic Boundary Layer

The design of hypersonic vehicle involves two basic physical quantities: aerodynamic force and aerothermal force.Whether the physical quantities can be obtained or not depends on whether the transition position can be predicted accurately.For high Mach number,gas temperature increases due to the increase of gas temperature after shock wave and the transformation of kinetic energy into thermodynamic energy in the boundary layer.Thermal properties of gas no longer satisfy the properties of calorically perfect gas,so the real gas effect at high temperature should be taken into account in calculation.The calculation of transition location,aerodynamic force and aerothermal force also need to consider the high temperature real gas effect.In addition,the prediction of transition position is also related to wall conditions.Isothermal and adiabatic wall conditions are no longer applicable for long time flight hypersonic vehicles.According to the actual flight conditions,the boundary conditions of convective heat transfer should be considered on the aircraft surface.The key problem is to put forward an appropriate formula for calculating convective heat transfer coefficient.Then,the temperature distribution of the wall at a certain time is obtained,and then the stability analysis and the calculation transition of the boundary layer around flat plate in hypersonic are carried out.Considering the real gas effect,the boundary conditions of convective heat transfer are taken to analyze the stability and transition prediction of the hypersonic plate boundary layer with Mach numbers of 4.5,6,8 and 10,respectively.The main research contents and conclusions are as follows:1?A suitable formula for calculating the convective heat transfer coefficient in the hypersonic plate boundary layer is studied and put forward.The influencing factors and characteristics of the convective heat transfer coefficient are analyzed.Considering the real gas effects such as air as calorically perfect gas,molecular vibration excitation(specific heat is a temperature function)and air molecule dissociation,the existence of similarity solutions for hypersonic laminar boundary layer is verified,and the accuracy of empirical formulas for convective heat transfer coefficient was checked basing on the similar solution.It is found that for hypersonic plate boundary layer,considering the real gas effects such as air dissociation,the relative error between the convective heat transfer coefficient calculated by the reference enthalpy method and the standard value is small.For example,the Mach number is less than 15,and the maximum relative error is less than 4%.The convective heat transfer coefficients with constant specific heat and variable specific heat are calculated by reference temperature method or reference enthalpy method,respectively.The relative error with the standard values can not be ignored when Mach number is higher than 4 and 7,respectively.The maximum relative error is more than 65% at the Mach number 15.In addition,with the increase of Mach number,the length of flow direction or the decrease of wall temperature,the value of convective heat transfer coefficient will increase.2?The calculation formula of convective heat transfer coefficient in hypersonic blunt cone boundary layer is studied.The real gas effect is considered according to the different surface temperature of the aircraft.The accuracy of the empirical formula of convection heat transfer coefficient is studied by using the direct numerical simulation results as the standard values.For blunt cone boundary layer,the relative error between the convective heat transfer coefficient and the standard value is small when the axis angle is less than 60 degrees,including the stagnation point.When the axis angle is higher than 60 degrees,the relative error between the convective heat transfer coefficient and the standard value increases sharply with the axis angle if the inflow pressure is still selected according to the plate model.A modified formula for selecting incoming pressure is proposed in this paper.The results show that the error between the convective heat transfer coefficient and the standard value can be reduced from 40% to less than 15% by using the proposed correction of inflow pressure.The modified inflow pressure formulas presented in this paper can improve the accuracy of empirical formulas for convective heat transfer coefficients in the case of calorically perfect gas,variable specific heat and high temperature dissociation.3?The wall temperature calculation method of hypersonic flat plate under convective heat transfer boundary conditions is studied.The wall temperature distribution at different time and its variation with time are given.The temperature relationship at different heights and different flow directions is also studied.The isothermal wall condition is only applicable to the initial flight time of the vehicle,while the adiabatic wall condition is hardly applicable.Because the wall temperature has a great influence on the prediction of transition position,it is necessary to adopt convective heat transfer boundary conditions in order to meet the actual flight conditions.For thin plates with Bi number less than 0.1,the results of lumped parameter method and heat balance method are compared.It is found that the wall temperature obtained by these two methods is basically the same.For a thick plate with Bi number higher than 0.1,the normal heat conduction in the plate should be considered,so the wall temperature obtained by the two methods is different.Therefore,when calculating the wall temperature distribution,the thermal balance method should be used to calculate the normal heat conduction in the plate.When calculating the wall temperature distribution at a certain time by lumped parameter method or heat balance method,it is found that there is a quantitative relationship between different heights and different flow direction positions.The wall temperature at any time,at any height and flow direction can be given with the quantitative relationship,and the calculation time can be reduced.4?The basic flow profile,stability analysis and transition location prediction of hypersonic plate boundary layer with real gas effect are calculated and studied.It is found that the basic flow profiles and neutral curves are quite different under different wall conditions,especially in the flow direction near the leading edge of the plate,and the growth rates of unstable waves are different at the same flow direction.By comparing the N values of disturbance waves under different boundary conditions,it is found that for the maximum disturbance amplitude of 2 m in the direction of flow,the difference between the disturbance amplitude under isothermal or adiabatic wall temperature conditions and the results under convective heat transfer boundary conditions is more than 40%,and the deviation under adiabatic wall temperature conditions tends to increase along the direction of flow.Therefore,it can be inferred that the transition position predicted by isothermal or adiabatic conditions on the wall for long time flight hypersonic vehicles should be quite different from the real situation,and the transition position predicted by convective heat transfer boundary conditions may be more accurate.