Characteristics Of Nonlinear Motion-induced Aerodynamic Force On A Rectangular Cylinder And A Streamlined Box Girder

Flutter is one of the controlling factors in wind-resistant design of long-span bridges.At present,the flutter calculation methodology is based on the approximation of linear motion-induced aerodynamic forces.Such an approach,can only predict the critical flutter speed,while post-critical flutter response amplitudes cannot be obtained.This practice is not compatible with modern design requirements for the wind-resistant design of future super-long span bridges.Therefore,it is necessary to explicitly account for the nonlinear characteristics of motion-induced aerodynamic forces.To this goal,the state-of-the-art in linear and nonlinear approximations for motion-induced aerodynamic forces is reviewed.Considering shortcomings in the relevant research,the nonlinear details of the motion-induced aerodynamic forces for a 5:1 rectangular cylinder(as a benchmark bluff section)and a streamlined box girder(deck section commonly used in modern long span bridges)are investigated.The main contents of this dissertation are as follows.1.Using the framework of Taylor series expansion,a single-degree-of-freedom nonlinear motion-induced aerodynamic force model is developed.This is combined with a parameter identification routine,as proposed by several researchers,and outputs are summarized.Further,the enhanced nonlinear motion-induced aerodynamic force model considering coupled heaving-pitching motions is put forward and potential improvements are suggested.2.A forced motion test rig,combining distributed pressure with displacement measurements,used throughout the experimental part of this work,is introduced.All details of the rig are discussed,including synchronization of pressure and displacement measurement and effects of pressure tube length.By evaluating the two-dimensionality of the section tests and comparing the observed Strouhal numbers and aerostatic coefficients to their equivalent theoretical values,the reliability of the test equipment is verified.And the method of extracting motion-induced aerodynamic force is explained.3.The nonlinear characteristics of motion-induced aerodynamic force for the 5:1rectangular cylinder and the streamlined box girder under single heaving,single pitching and coupling motion are investigated.It is found that the motion-induced aerodynamic force is almost fully correlated when tested sections are not stationary.When the pitching amplitude exceeds 8°and 10°,clear higher harmonics could be observed in the 5:1 rectangular cylinder and the streamlined box girder respectively.Variation of the heaving amplitude,within a broad testing range,has negligible effects on the nonlinearity of the motion-induced aerodynamic force.During coupled motion it is the pitching motion amplitude that dominates this nonlinearity.Interestingly,a superposition-type principle for the aerodynamic force(separating pitching to heaving effects)can apply in these two sections when they are in coupled motion.4.Time-averaged characteristics,spectral characteristics,spatio-temporal distributions and Proper Orthogonal Decomposition(POD)analysis have been performed for the fluctuating pressure fields of the 5:1 rectangular cylinder and the streamlined box girder.It was found that the nonlinearity presented in the motion-induced aerodynamic forces of the5:1 rectangular cylinder is closely related to the forward movement of the reattachment point.Variation of the motion amplitude has little influence on the flow pattern around the streamlined box girder.The nonlinearity shown in the motion-induced aerodynamic force of this section is closely related to the strong vortex activity near the wind nose and the inclined web.From the POD analysis,it was found that energy concentrates within the lower POD modes when motion amplitude increases.5.A revised nonlinear motion-induced aerodynamic force model is proposed based on the characteristics of motion-induced aerodynamic force and all corresponding model parameters are identified.A combination of the Newmark-? method and the Newton-Raphson iteration method is adopted in order to conduct the nonlinear coupled flutter analysis.Comparing the observed post-flutter response from wind tunnel tests to those predicted by the proposed revised model,the effectiveness and reliability of the new approach are validated.