Aeroelastic Simulation Of Vortex Induced Vibration And Flutter Of Bridges By CFD Method

With the construction of many long-span cable-stayed bridges and suspension bridges, bridge structures are becoming slenderer and lighter. Long-span bridges are much more sensitive to wind loads. Bridge safety and durability are influenced by wind-induced vibration, which influences the safety and comfort of cars running on the bridge as well. So it’s very important to do some research on bridge aeroelastic phenomena. The study on this field helps a lot to improve bridge wind resistance and to control the wind-induced vibration.Computational fluid dynamics(CFD) is a new discipline based on classical fluid dynamics and numerical analysis. The superiority of using CFD simulation method to study bridge aeroelastic phenomena is obvious. It’s cheaper, time saving and easy to modify the parameters. So CFD simulation method is widely used in the area of bridge wind resistance study. This thesis is focused on the fluid-structure interaction of air and the bridge. The structure is regarded as a rigid moving body, whose dynamic motion equations are solved by a type of numerical solution called Newmark method. All the simulations and calculations are conducted in a commercial finite element calculation software, FLUENT. When the structure is moved to a new position, dynamic mesh is used to rebuild the meshes around it.Firstly, a2D square cylinder simulation model is established to study the turbulent near-wake flow around it. The results with different turbulence models are compared with the wind tunnel test result measured by Lyn. The k-co shear stress transport(SST) model is finally chosen because of its high precision in the aerodynamic parameter identification and flow field simulation. Then square cylinder with vertical vibration is simulated under different wind speeds. The strouhal number and coefficient of damping ratio are obtained and compared with the result of fixed square cylinder. Vortex-excited resonance is observed during the simulation.Secondly, a2D flat plate model with vertical and torsional degrees of freedom is established. The flutter derivatives of this flat plate are calculated. The flutter critical wind speed is then calculated by using the CFD simulation method mentioned above. The simulation result agrees very well with the wind speeds calculated by the Scanlan’s formula and Selberg’s formula. At the same time, by changing the structure damping ratio, its influence to the flutter critical wind speed is also studied. Then the aerodynamic damping ratio at wind speed Om/s is studied by changing the factors such as initial displacements, structure damping ratios and natural frequencies.Finally, based on a long-span suspension bridge, a2D main girder section model is established with the same scale ratio of its wind tunnel test. According to the model damping ratio measured by wind tunnel test and the aerodynamic damping calculated by CFD simulation, the structure damping ratio is computed. Then the model’s flutter critical wind speed is calculated by CFD simulation. The result is almost the same as the critical speed measured by wind tunnel test. The model’s resistance to vortex-induced vibration is also simulated. No vortex-induced vibration is observed during the whole process when the wind speed increases from2m/s all the way up to8.8m/s. The simulation agrees with wind tunnel test very well.The analysis results demonstrate that the free vibration method based on numerical simulation is a good way to study wind-induced vibrations. The turbulent flow past a square cylinder or a flat plate can be simulated and investigated by this CFD-CSD coupling method. The flutter critical wind speed of bridge section can be calculated and the result agrees well with the wind tunnel test. The bridge section’s resistance to vortex-induced vibration can also be well simulated. In a word, the CFD-CSD coupling method of this thesis has some reference meaning in the research on bridge wind resistance.