Study On Nonlinear Aerodynamic Stability Of Long-span Bridge In Turbulence Flow

In order to get accurate evaluation of the aerodynamic performance of long-span Bridge, the wind field characteristics at the bridge site become the precondition of the analysis of aerodynamic stability. The wind fields in the nature are stochastic turbulence process indeed, especially when the bridge site are located at mountainous valley terrains, the stochastic characteristics of wind field become more evident, as a result, the accurate description of the wind field characteristics at the bridge site possess great significance for the aerodynamic analysis of long-span bridge. The incoming turbulent flows not only induce buffeting of bridge structure, but also cause change of the aerodynamic stability of bridge. The effect of turbulence on the aerodynamic stability of bridge are complex process, it is an integration of the effect of turbulence on the self-excited forces and the spanwise correlation, and the effect of turbulence induced buffeting on the aerodynamic stability as well. In order to establish a quantificational analysis method for the study on the effect of the turbulence and the relevant wind tunnel test technology, the work summarized in follows are conducted:(1) Wind tunnel experiments were conducted to investigate the wind characteristics in the mountainous valley terrain with4simplified valley models and a model of an existing valley terrain. Furthermore, numerical simulations of wind field characteristics in mountainous valley terrain with large scale terrain models are carried out. By comparing the preliminary results obtained from the wind tunnel tests and the numerical simulation, some fundamental information are obtained which provide useful reference for the wind resistant design of bridge located in mountainous valley terrains.(2) Turbulent wind with large integral scale are generated in wind tunnel using the active grid turbulence generator. The effect of active grid turbulence on the derivatives of thin plate section model, section model of rectangular prism and streamline model with box girder section are investigated by the forced vibration method. The first benchmark model for evaluating the effect of turbulence on flutter stability is established. The results of test exhibit that the turbulent flow have slight effect on the streamline section model, while significant influence for the blunt body.(3) Through the analysis of pressures change in the surface of model, the mechanism of the change of the derivatives due to turbulence is disclosed. It indicate that the motion of separated and reattaching flow at the windward corner of the bluff structure cause the change of self-excited forces, then drive the derivatives to change. the effects of turbulence intensity and integral scales on the flutter derivatives as well as the spanwise correlation of aerodynamic forces are also studied.(4) A taut strip model that can exhibit the three dimensional characteristics of the wind induced vibration was fabricated. The model was tested in four grid turbulence wind fields for the investigation of the effect of turbulence on the spanwise correlation of aerodynamic forces. The results indicate the turbulence induce evident decrease of spanwise correlation of total aerodynamic forces, while the correlation of self-excited force exhibit near unity correlation along the spanwise direction.(5) The transient characteristics of the indicial function in the time-domain simulation of bridge deck self-excited aerodynamic forces are analyzed and compared, the reasonable predefining numerical range for exponential parameters in the fitting process are introduced. And a finite element program designed for the time domain analysis of bridge aerodynamic stability was developed, and the effect of turbulence on the nonlinear flutter stability of bridge was investigated using the time domain analysis program.(6) By comparing with the iterative static finite element procedure, the reliability of dynamic finite element methods for aerostatic stability analysis is validated. To investigate the effects of turbulence on aerostatic instability of bridge, the dynamic finite element method was adopted to perform bridge aerostatic stability analysis in time domain. Numerical results show that turbulence decreases the critical divergence velocity obviously, and higher turbulence. And, turbulence intensity and the spatial correlation of wind fluctuations also play an important role on that.