Study On Aerothermal Flutter Calculation Method Of Hypersonic Vehicle In Non-cruising State

Advanced hypersonic vehicles can autonomously control flight trajectories during high-speed flight.In the non-cruising state,the interaction of unsteady aerodynamics,aerothermal and structural vibration may cause thermal flutter.In this paper,the non-cruising thermal flutter calculation methods are studied from the aspects of flow field calculation,structural temperature field calculation and structural vibration transient response analysis.The “freezing” modal method and the multi-field transient coupling method are developed to calculate the non-cruising thermal flutter of hypersonic vehicle.Based on the finite volume method,a synchronous calculation method of hypersonic flow field-structure temperature field is established.The unified unsteady governing equation is solved by the dual-time marching method.A fully point-to-point grid on the interface is used,and virtual elements are constructed on both sides to solve the boundary derivative terms.The local time-stepping approach is introduced to accelerate convergence.The results of the synchronous calculation method are compared with those of the loose/tight coupling method and the experimental results.For the hypersonic aerodynamic heating problem with constant freestream,the predicted heat flow and temperature in the non-stationary region of the two numerical methods are in good agreements with the experimental results.For the hypersonic aerodynamic heating problem with variable freestream,the two numerical results are basically in agreements with the experimental values.The computed heat flux and temperature at the stagnation point of the tightly coupled method and the synchronous calculation method are closer to the experimental values than that of the loosely coupled method.On this basis,the influence of gas flow in cavity on the structure temperature field is studied.It is found that the heat transfer process of the gas in the cavity is very slow,which has a weak influence on the temperature distribution of the structure temperature field.Openings have a great influence on the structure temperature field,and the instantaneous temperature at the edge of the hole wall even exceeds the stagnation point temperature of the external flow field.Combining the established synchronous calculation method with the time-domain flutter calculation method of variable stiffness,a non-cruising thermal flutter calculation method is established.According to the fact that the characteristic time of structural vibration is much shorter than that of structural heat transfer,the idea of "freezing" modal is introduced.When solving the structural dynamic equation,the temperature field between track state points is neglected,and the temperature field and mode are kept unchanged.The method is applied to calculate the thermal flutter of the full-moving rudder in the course of trajectory motion,and the thermal flutter characteristics at the non-cruising point are obtained.It is found that both cold and hot modes exhibit coupled flutter of hinge mode and first-order bending mode.High temperature decreases the natural frequencies of each mode,increases the critical stiffness coefficient and reduces the flutter stiffness margin.Because the hinge mode is less affected by temperature,the frequency of the first bending modal of the thermal mode is closer to that of the hinge mode,and the critical stiffness coefficient of flutter increases greatly.The Nastran Restart function is used to realize the linear and non-linear transient solutions of time-varying loads.The RBF interpolation method is used to transfer the surface deformation of the structural elements to the surface mesh points of the flow field.A CFD/CSD coupled aeroelastic method is established and applied to the flutter calculation of a flat swept wing.The effects of geometric nonlinearity and material nonlinearity on the critical flutter velocity are studied.It is found that geometric nonlinearity changes the critical flutter velocity to some extent.The material nonlinearity is mainly manifested in the effect of temperature on the mechanical properties of materials.Combining the established synchronous calculation method with the solution method of transient response of non-linear structure,a non-linear transient multi-field coupling calculation method is established.By means of coordinate transformation,the deflection of rudder surface and structural vibration deformation in non-cruising state are superimposed to realize the dynamic mesh deformation of flow field.Based on this,a method for calculating thermal flutter of hypersonic vehicle in non-cruising state is established.Firstly,the structural temperature field at each state point in orbit is obtained by synchronous calculation method of flow-structure temperature field,and then the structural vibration response between adjacent state points in orbit is calculated by non-linear transient multi-field coupling method.The method is used to analyze the thermal flutter characteristics of the fully actuated rudder along the track 3-6 state points.It is found that the rudder flutter occurs between the state points of track 4-5.The flutter is manifested as the coupling between the hinge torsion mode and first-order bending mode,which is consistent with the calculation results by using the thermal flutter of “freezing” modal.By comparing the heat flux density and temperature of monitoring points with those from the synchronous calculation method without consideration of vibration,it is found that the temperature at monitoring points is reduced to some extent by vibration of the structure.Before the structural vibration has an obvious influence on the temperature field of the structure,the vibration has shown a divergent trend.From this point of view,the rationality of the established transient multi-field coupling thermal flutter calculation method is verified.