Aeroelastic Flutter Study Of Composite Structures In Supersonic Airflow

In recent years,a large number of supersonic and hypersonic vehicles have emerged,and the working environment is relatively harsh during flight.In order to work under complex conditions such as high temperature,corrosion and vibration,composite materials are widely used in the aerospace field.Compared with the previous situation,the boundary conditions of aircraft structures have changed,which brings challenge to the aeroelastic flutter problem of composite structures in complex flight environments.Flutter is a kind of self-excited vibration that occurs when the structure is coupled with aerodynamic force,inertial force and elastic force,which is an important aeroelastic problem.The structure generally exhibits a limit cycle motion with a limited magnitude when flutter occurs,which affects the fatigue strength,flight performance and safety.With the increase of the Mach number,the aerodynamic heating effect will reduce the structure stiffness and bring the thermal stress,strain and material ablation.So the thermal effect on the flutter stability must be considered in the aeroelastic study.In this paper,the vibration and aeroelastic behavior of composite plate and shells are studied.A unified method to determine the free vibration characteristics of plate and shell structures with elastic constraint and the connection based on artificial spring technology and Rayleigh-Ritz method is presented in this paper.The elastic constraint and the connection are simulated by arranging springs at the boundary and the joint.The displacement field is represented by different admissible functions,which are the free beam function,modified Fourier function and the characteristic orthogonal polynomial.Based on the Rayleigh-Ritz method,the unified solution form of natural frequency and mode function of structure is given.The correctness,convergence and computational efficiency of frequency for different admissible functions are compared.The free vibration of the structure under different spring stiffness is studied.In view of the previous problems focusing on classical boundary conditions,considering thermal strain caused by temperature rise,the flutter characteristics of composite plate in supersonic airflow are studied based on the modal function of elastically supported structure.The nonlinear displacement-strain relationship is described by von Kármán theory,and the first-order piston theory is used for the aerodynamic pressure.The nonlinear partial differential equation is derived,which are discretized into ordinary differential equations using the assumed modes method and Garlerkin method.The natural frequency,thermal buckling and flutter stability are studied using the frequency domain method.The effect of the airflow pressure on the nonlinear dynamic response is discussed.Considering the elastically supported cylindrical shell under the non-uniform temperature field,the temperature distribution along the thickness direction is obtained by solving the steady-state heat conduction equation,and the material property is a function of temperature and position coordinates.The aeroelastic equation is established based on the linear piston theory.The assumed modes method is used to discretize the system's partial differential equations.The modal functions are constructed by the Gram-Schmidt process in the Rayleigh-Ritz method.The effects of different volume fraction index,boundary constraints,temperature field on the buckling and flutter boundary are discussed.Establishing a fluid-solid-thermal coupling model of the plate in the hypersonic flow,the nonlinear dynamic behavior of the structure under non-uniform heating is studied by thermal-aeroelastic coupling iterative calculation.The Ecker's reference temperature method and heat flux equation are adopted to calculate the heat flux.Two-dimensional transient heat conduction along the chord and thickness direction is computed using a finite difference approach.In the coupling calculation,the influence of the real-time variation of structural material properties,thermal expansion coefficient and other parameters with temperature,as well as elastic deformation on aerodynamic heating and aerodynamic heating on the plate stiffness,are taken into account.Time-domain response of the structure under aero-thermal-aeroelastic coupling is calculated,which is compared with the nonlinear response of the plate under the stable temperature field.