Aerodynamics of a flat girder: Effects of its aspect ratio and angle of attack

This study utilized wind tunnel experiments and large eddy simulations to profoundly investigate the aerodynamic characteristics and flow patterns surrounding flat girders with different aspect ratios (B/D = 2, 4, 7, and 10, where B and D are the length and height of the flat girder, respectively), which covers most of the engineering applications for typical long-span bridges. The test range of wind angle of attack (AOA) is [−12°, 12°]. The Reynolds number Re, based on the freestream velocity U∞ and D, was approximately 70,900. The results revealed that, the flow separation point of the flat girder gradually moved downstream, and an obvious separation bubble was generated at the leading wind fairing with an increase in AOA, especially for the cases of B/D = 4, 7, and 10. The aerodynamic performance for the case of B/D = 2 differed substantially from the other cases, experiencing the largest mean drag and the smallest fluctuation pressure in the range of −10° ≤ AOA ≤ 10°, which was attributed to different flow patterns. Specifically, leading-edge vortex shedding occurred in the case of B/D = 2, whereas impinging leading-edge vortex shedding (AOA = 0° and 4°) and a combination of impinging leading-edge vortex shedding with trailing-edge vortex shedding (AOA = 8°) were identified for the cases of B/D = 4 and 7, respectively, and dominated trailing-edge vortex shedding was determined for the case of B/D = 10. Additionally, the effects of AOA on aerodynamic characteristics showed relatively minor effects when |AOA| < 4° as compared to |AOA| > 4°. For the cases of B/D = 4 and 7, the thickness of the upper surface boundary layer and turbulence intensity were enhanced with an increase in AOA, resulting in a higher pressure fluctuation and multiple dominant peaks in the lift spectrum. However, this effect was significantly attenuated in the case of B/D = 10 due to the dissipation effect. This study may offer guidelines for relevant engineering applications.