Turbulence Modeling Based Flutter Derivative Identification Of Wing Structures And Power Spectral Method For Gust Response Analysis

Flutter and gust response are the main task of unsteady aerodynamics for aircrafts. Any progress and improvement in the area can be potentially a huge boost for designing larger and better performing aeronautical vehicles. The work presented in the thesis is aimed as an effort to combine the computational fluid dynamics calculation with the structural dynamics calculation to tackle the unsteady aerodynamics of aircraft wings.The classical Theodorsen function is the only analytic formulation available in unsteady aerodynamics theory for calculation of motion related aeroelastic forces. However the formulation is built on the two-dimensional potential flow theory and is applicable only for flat plate or thin airfoils oscillating in small amplitudes. For the design of modern backswept supper critical wings with winglets the theory is of no meaning. With the fast advance of computing techniques and computational algorithms, computational fluid dynamics (CFD) has become a powerful tool for solving unsteady aerodynamic problems. The first purpose of the thesis is to test the fluid-structural interaction algorithm and the mesh control method proposed by Sun et al. for solving unsteady aerodynamics of aircraft wings, particularly the identification of flutter derivatives of wings. The fluid-structure interaction is solved based on the block-iterative Gauss-Seidel method. The second objective of the work to assibilate the results from turbulence modeling based flutter derivative identification into the gust response calculation of aircraft wings using the pseudo-excitation method of Lin, which is a fast and accurate random vibration method called.As a first step the flutter derivatives of the NACA0012 airfoil are identified through solving the unsteady flow around moving airfoils using the RANS turbulence modeling; and compared with the Theodorsen function. The difference is found due to the inclusion of viscosity and finite thickness in the current calculation, indicating a much wilder application scope of the turbulence modeling based unsteady aerodynamics. To the knowledge of the author this is so far the first for the RANS having been used in this scenario. Once fully tested, the methods can be used to solve flutters of three-dimensional wings in different flow regimes, such as the transonic and supersonic vehicles.Based on the flutter derivatives identified from unsteady flow simulation, the gust encounter problems of aircrafts are attacked by solving power spectral densities of response of the dynamic system under the excitation of atmospheric turbulence, making use of the pseudo-excitation method. The aircraft wings are modeled using composite laminate elements. The Dryden turbulence spectrum is used to specify the gust excitation. The aeroelastic effects are considered by inclusion into the governing equations of motion of flutter derivatives identified from unsteady flow simulation. Then the pseudo-excitation method (PEM) is used to solve the response of system under the random vibration excitation. Using PEM the solution is converted to solving the responses of system under harmonic excitation and then integrating the response into standard deviations, which can be fast handled by most of the existing programs. A distinguished feature of using PEM is that various aeroelastic coupling and mechanical coupling between degrees of freedom have been automatically included. Up to this point, the unsteady flow calculation has been successfully combined with advanced structural dynamics calculation. A finite element program has been developed on DDJ, which is used for whole structure turbulence analysis of aircraft wings. It is shown that PEM combined with unsteady flow calculation is highly efficient for atmospheric turbulence response analysis of two-dimensional airfoils.