Nonlinear Self-Excited Forces Of Streamlined Box Deck And Nonlinear Flutter Response

The development of bridge wind resistance was reviewed, followed by the current research and acquisition approach of self-excited forces of bridge deck. Then based on computational fluid dynamic (CFD) simulations, the characteristics of the self-excited forces for both thin plate and streamlined box deck was studied. The relationship between the nonlinearity of self-excited forces and the typical characteristic of flow field was revealed by flow visualization. Under specified hypothesis, the dissertation developed a method for analysis of the nonlinear flutter response of 2D bridge deck through theoretical derivation. To take a step further, the influence of structural damping on the bridge nonlinear flutter response was discussed. This research focused on the following aspects:1. A multi-zone dynamic mesh deforming method was proposed. Combined with rigid grid technique and standard spring smoothing method, the quality of the computational grid could be well controlled even when the bridge deck was undergoing large amplitude oscillation.2. Self-excited forces for both thin plate and streamlined box deck under large amplitude oscillation was studied. The larger amplitude brought on stronger nonlinearity and the super-harmonic component of the aerodynamic self-excited forces would be increased. With the unvaried oscillation amplitude, a higher reduced wind speed induced stronger nonlinearity for self-excited forces of rotating motion, while for vertical bending motion the nonlinearity of self-excited forces would be stronger when the reduced wind speed is lower.3. Flow field around thin plate under lager amplitude oscillation was studied. It was found that flow separation was not the reason for the super-harmonic of the self-excited forces. Flow separation at the plate head had effect on the fundamental frequency amplitude of self-excited forces. There was a positive correlation between the percentages of the super harmonic component and the intensity of secondary vortex in flow.4. Flow field around streamlined box deck under lager amplitude oscillation was studied. The super harmonic component of self-excited forces appeared in synchronism with the reverse whirling vortex on the unilateral side of the deck.5. The bridge flutter was assumed as a simper harmonic motion coupled rotation and vertical bending. The flutter derivatives were expressed as functions of reduced frequency and motion amplitude which were called nonlinear flutter derivatives in this dissertation. Taking the phenomenon, where the total damping of the vibration system equals zero again, as a criterion for evaluating steady response of flutter, the dissertation build a basic method for nonlinear flutter analysis.6. The nonlinear amplitude response of Nanjing 4th bridge in flutter was investigated. The response could maintain on a limited level even though the flutter critical wind speed had been exceeded. The key factors that influencing the flutter response were the direct positive aerodynamic damping originated form A*2 and the coupled negative aerodynamic damping from A1* and H3* .7. The mathematical expression for structural damping ratio impacted the nonlinear flutter response. When the structural damping ratio was constant, it was incapable of suppressing the flutter divergence. When the structural damping ratio was linearly proportional to the amplitude of motion, the slop was an important factor for suppressing the flutter divergence. When the structural damping ratio is asymptotic, its flutter amplitude response cures were situated between that of the constant case and the linear case.