A study on the effects of turbulence scale on flutter of long span bridges

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Engineering structures such as long span bridges and tall buildings are exposed to highly turbulent wind. Since long span bridges are susceptible to flutter, e. g. galloping, torsional flutter, vortex excitation, etc., the effects of turbulence on flutter have been a longstanding problem. The ratio, L_x/h, of turbulence integral scale L_x to the body size h is one of the measures which identify the turbulence effect. In the present research, galloping of rectangular prisms is considered. The principal purpose is to comprehensively understand the effects of turbulence scale on galloping over a wide range of L_x/h. Not only 2-D rectangular prisms but also 3-D prisms with one end fixed to the wall (3-D model) were used in order to attain larger values of L_x/h. Turbulent flows were generated by different grids with intensity u'/U about 10%, where u'is the r. m. s. of the u-component of velocity fluctuation and U is the free stream velocity.2-D model : Galloping is investigated on rectangu … More lar prisms with d/h of 0.6-3.0, where d is the depth and h is the height of the prism. (a) d/h=0.6-1.0 : Galloping occurs for both the uniform and turbulent flows with a slightly positive betaa, the aerodynamic growth rate of oscillation, where the scale effect is not significant. The oscillation due to buffeting is very weak. (b) d/h=2.0 : Galloping vanishes for the turbulent flows because of the promotion of reattachment of the separated-shear-layer to a side face, whereas galloping occurs for the uniform flow. With increasing L_x/h, betaa for the turbulent flows asymptotes to that for the smooth flow. This clearly shows that galloping is dependent on turbulence scale. (c) d/h=3.0 : Galloping vanishes for both the uniform and turbulent flows so that betaa negative. With increasing L_x/h, betaa decreases and tends to that for the uniform flow.3-D model : Galloping is investigated on rectangular prisms with d/h of 2.0 and 3.0. With increasing L_x/h, betaa for turbulent flows asymptotes to that for the smooth flow in a similar way to the 2-D model.Buffeting : In addition to galloping, models with d/h=2.0 and 3.0 at high reduced flow velocities are suffered from buffeting in turbulent flows with large Lx/h. The amplitude of buffeting increases rapidly at L_x/h of 2.0-3.0, but beyond around 10.0, the amplitude approaches an upper hmit. Less