Research On Parameter Identification Of The Vortex Induced Force Based On The Floating Frame Type Three Segments Model With Rectangular Section

The wind flow will seperate after flowing through some special section structures.And then periodic vortex shedding phenomenon occurs. The relation between the frequency and velocity of the vortex shedding can be expressed by Strouhal number. If the frequency of vortex shedding and the certain frequencies of the structure are equal or similar, vortex induced vibration will occur. Vortex induced vibration is a kind of self-excited and forced nature of vibration with the characteristics of amplitude limited and occuring at low wind speed. Although the vortex induced vibration is not devastating wind induced vibration, the periodic vibration will not only affect the comfort of drivers and pedestrians, it also will lead to the fatigue damage of the structure. The long span bridges are flexible structures with light weight and low damping, they are sensitive to the vortex induced vibration caused by wind. However, the mechanism of vortex induced vibration is not completely understood. There is no equation that has been set up to describe the vortex induced vibration perfectly. The vortex induced force is important data for the study of vortex induced vibration. But the existing research rarely directly tests the vortex induced force, mostly through the displacement response and parameter identification to get it indirectly. In view of this, based on the self-developed three segments model,the vortex induced force is tested in my research.And then the mechanism of vortex induced response is explored by analysising the vortex induced force.The main contents are as follow:(1)The floating frame type three segments model is designed based on ANSYS finite element analysis software. The sizes of the support frame, the suspension frame and the disturbance component are determined by the analysis based on ANSYS. At the same time, a modal analysis of the model is made to get the modal frequency to ensure that the local vibration will not appear in the later wind tunnel test.(2)Free hanging test of the section model in uniform flow condition is taken in the research. And the displacement response、the acceleration response and the dynamic force when the vortex induced vibration occurs on the suspended segment are measured. Then the vortex induced force is extracted through the force analysis.To evaluate the accuracy of the test of the vortex induced force, the displacement tested by experiment and calculated by the average acceleration method are compared. The result shows that,the calculated and measured displacement time history curves are in good agreement. At the same time, the VIV amplitudes under different wind speeds are statisticed on to get the locking range of rectangular section with aspect ratio of six.(3)Forced vibration test of the section model in uniform flow condition is also taken in the research.First, the total dynamic forces with and without wind conditons are tested. Then, the vortex induced force is obtained by subtraction of these two dynamic forces.After comparing the vortex induced forces got by free hanging test and by forced vibration test, it is found that the latter is bigger.(4) Parameters of Scanlan experience linear and nonlinear model are identificated according to the vibration response and the vortex induced force time history. After comparing,it is found that the parameters identificated based on the vortex induced force are more accuracte.(5) CFD numerical simulation analysis on the rectangular cross section with the aspect ratio of six is taken at the end of the research.The lift coefficient under the stationary condition and the response of the vortex induced force time history under vibration condition are obtained.At last, the result of simulation and measured result are compared. It shows that the locking range and the vortex induced force amplitude got by the CFD are similar to the measured.