The Study On Nonliear Motion-induced Aerodynamic Force And Nonlinear Aerodynamic Stability Of Long-span Bridge Girder

This thesis reviewed the research of wind effects on the long span bridge, and summarized the current research on the linear and nonlinear Motion-Induced Aerodynamic Force (MIAF). Firstly, based on Taylor expansion of the equilibrium position, the mathematical model of nonlinear MIAF was developed under harmonic motions. Then, through the special wind tunnel tests, the nonlinear MIAF of thin airfoil and streamline box girder were obtained under the large amplitude oscillation, and the characteristics of nonlinear MIAF and aerodynamic hysteresis of the thin airfoil and streamline box girder were also discussed. In addition, the identification method of nonlinear aerodynamic parameters in the expression was proposed. Finally, the nonlinear aerodynamic analysis has been conducted using the nonlinear MIAF.The thesis research contains following contents:1. Reviewed the researches of wind effects on long span bridges, and summarized the current researches on the linear and nonlinear MIAF.2. Based on Taylor expansion of the equilibrium position, deduced the plural and real expression of nonlinear MIAF under1DOF and2DOF harmonic motions, and established the nonlinear MIAF mathematical model composed of multiple harmonic components.3. Through the special wind tunnel technology, conducted the tests of nonlinear MIAF of the thin airfoil and streamline box girder under the large amplitude oscillation, and found the remarkable higher harmonic components in the spectrum, which validated the theory that the nonlinear MIAF composed of multiple harmonic components. Discovered that the amplitude of nonlinear components in the MIAF increased with the increase of oscillation amplitude, and decreased with the increase of reduced velocity.4. Studied the characteristics of nonlinear MIAF and aerodynamic hysteresis, and found the exist of "figure8" loop in the aerodynamic moment hysteresis curve of the streamline box girder, that is, the aerodynamic moment has done not only positive work but also negative work in one period. Then discussed the possible vibration status of long span bridges under the large amplitude oscillation.5. Proposed the identification method of nonlinear aerodynamic parameters. This method could realized by nonlinear least-square iteration with the directly identified harmonic parameters as initial values. Studied the effect of different types of noise on the accuracy of identified parameters, and pointed out the serious effect of the noise which make signal offset on the identification accuracy of nonlinear harmonic components, and this type of noise could lead to the invalid results.6. Using the nonlinear MIAF and the4th order Runge-Kutta algorithm, performed the nonlinear aerodynamic stability analysis under special conditions, and found that the three types of motion mode including the convergence, the divergence and the large identical-amplitude oscillation could occur at the post-flutter status of long span bridges. Discussed the influence of different parameters in the nonlinear aerodynamic differential equation on motion type and the nonlinear aerodynamic stability, and also discussed the influence of turbulent flow on the nonlinear aerodynamic stability in brief. In addition, based on the features of nonlinear MIAF, interpreted the "hard flutter" and "soft flutter" phenomena in long span bridges.7. Discussed the outlook of research direction of nonlinear aerodynamics.