Study on turbulence effects on flow fields around sharpedged bluff bodies

Flow patterns around bluff bodies have long been extensively studied, however the interaction mechanisms between flow fields and structures are still not fully understandable. Most structures in civil engineering are categorized as sharp-edge bluff-bodies because incoming flows into the bodies are obstructed due to their geometry, moreover flow separation will occur at certain locations at the leading edge of the bodies regardless of flow speed and Reynolds number. Flow structures also become more complicated when wind flow with turbulence. Past studies discovered that increase in turbulence intensity shortens reattachment length and significantly increases magnitude of negative pressures on side surfaces beneath separation region. It was also revealed that small-scale turbulence plays a significant role on flow structure around a sharp-edged bluff-body. Therefore, this study focuses on behavior and structure of flow fields around sharp-edge bluff bodies due to effect of small-scale turbulence using reduced turbulence intensity. Experiments conducted in the present study can be divided into three parts. First, surface pressures around a square cylinder and a rectangular cylinder with a slenderness ratio of 3.0 under flow with turbulence intensity ranging between 5 to 11% were measured. Both mean and fluctuating surface pressures were measured and consequently examined. Mean surface pressures can be integrated to be averaging forces acting on the body, while fluctuating surface pressures are directly responsible for buffeting which can cause local failure to the surface of a structure. The second experiment is flow visualization using Particle Image Velocimetry technique or PIV. Flow visualization illustrates invisible complicated flow structures of flow fields. Instantaneous flow images were continuously recorded by high-speed camera, thus flow velocity vectors surrounding the body were obtained. Finally, effects of flow with turbulence on an oscillating model will be considered. Experiment on a tower model of a long span bridge was conducted. The model will oscillate strongly at certain wind speeds; this phenomenon is called vortexinduced vibration because vortex shedding is responsible for the strong vibration. Mean surface pressures around a square cylinder and a rectangular cylinder with a slenderness ratio of 3 show that magnitude of negative pressures along side surfaces of the body increase with an increase in reduced turbulence intensity. Even difference between reduced turbulence intensity (Ir) and turbulence intensity (Iu) cannot be found in this study, however effect of small-scale turbulence can be explained by plotting surface pressure against length scale ratio when wind flow at constant turbulence intensity. It reveals that side surface pressures around forebody decrease with decrease of length scale ratio. This result corresponds well with decrease of shear layer due to small-scale turbulence in the experiment on PIV and an increase in fpeak in power spectral analysis. In conclusion, this study can show an effect of small-scale turbulence on flow around sharp-edge bluff-bodies, however the effect might be insignificant in reattachment region. It was assumed that effect of free-stream turbulence will be lessened or vanished in long rectangular cylinder