Experimental Study On Model Scale Effect Of Vortex-induced Vibrations For 1:6 Rectangular Cross-sections

Long-span bridges represent important infrastructures in modern transportation system. The world is encountering an era of the construction of super long span bridge, and wind-resistant performance is one of the key issues during the design and construction of these bridges. The existing wind resistant design method has successfully avoided catastrophic failure of flutter instability, but vortex-induced vibration of steel box girder bridges at moderate wind is still an important problem that troubled wind resistant analysis. Performing the sprint-mounted dynamic wind tunnel test of scaled section model of bridge girders is the most common approach for vortex-induced vibrations. In such kinds of tests, the scale ratio of section models are usually 1/80~1/20 dependent on size of the test sections of wind tunnel. Some existing test results indicate different scale ratio leads to notable different VIV amplitudes of prototype, and the underlying mechanism is not fully understood. Under finical support from the National Natural Science Foundation, the model scale effect of vortex-induced vibrations for 1:6 rectangular cross-sections was studied in some detail. The investigation has been focused on the influence of length and width ratio of the model to the amplitude of vortex induced vibration, surface fluctuating pressure and its spanwise correlation. The main works are as follows:(1)The basic theory of flutter, galloping, ch attering and vortex-induced vibration were summarized, and several examples of the world famous bridge VIV were listed, then the research background and main content of this article were given.(2)Based on the harmonic force model of the vortex induced vibration, the main parameters that influence the amplitude of vortex induced vibration were described, including fluctuating lift coefficients, Strouhal number( St), the combined mass and damping parameter-Scrunton number and spanwise correlation of lift coefficients. At the same time, aerodynamic shape, Reynolds number, flow turbulence and other factors that could influence the above parameters were analyzed.(3)A large scale model(B=72cm, H=12cm, L=1.54m) and two small scale models(B=40 cm, H=6.7cm, L=1.54 m and L=2.30m) with the same side ratio B/H=6 are designed, which correspond to length-width ratios of L/B=2.14, 3.85 and 5.75, respectively. The vortex-induced vibration tests were carried out in uniform flow field that the Sc number are same. Two obvious vortex-induced vibration ranges for the three test models in the experiments was observed and the second vortex-induced vibration amplitude is larger than the first largest vortex-induced vibration amplitude. The maximum dimensionless amplitude of t he two small scale models are equal; the maximum dimensionless amplitude of the large scale model in the two vortex-induced vibration locking range are less than the dimensionless amplitude of two small scale models.(4)The variance of pressure coefficien t on the surface ? the variance of lift coefficient and the spanwise correlation of lift at the same dimensionless amplitude were studied for the three models, the results showed that the variance of pressure coefficient on the surface?the variance of lift coefficient and the spanwise correlation of lift of the large scale model(L/B=2.14) are less than the two small scale models(L/B=3.85,L/B=5.75). The influence of amplitude and turbulence to the spanwise correlation of lift was also studied.(5)Using CFD, through the two-dimensional numerical simulation to the rectangular of small scale and short model, and compared with the result of experimental test. It showed that the locking range got by the CFD is similar to the measured, but the maximum amplitude in t he second locking range has some difference.