LES prediction of aerodynamics and coherence analysis of fluctuating pressure on box girders of long-span bridges

• Flow separated from girder windward corners only reattaches on upper surface.
• Primary vortex is shed from the reattachment region on the upper surface.
• Vortex shedding is not synchronous along the girder span-wise direction.
• Vortex shedding energy is concentrated on a narrow band of frequency.
• Primary vortex shedding dominates the fluctuating pressure on girder surface.

The flow field around a box girder of a large span cable-stayed bridge, under Reynolds number of 1.27 × 105, is computed by using three-dimensional large eddy simulation (LES). The flow results under various angles of attack are obtained and compared with wind tunnel tests. The distribution of mean and RMS value of pressure around the girder surface are also provided. Investigation on fluctuating pressure on the girder surface is carried out through power spectrum density (PSD) and spatial coherent analysis. The research shows that the flow structure around the girder is characterized by the flow separated from the girder windward corners, reattachment on the girder upper surface, the primary vortices shedding within the reattachment region, vortex convection and alternatively shedding in the wake. The fluctuating pressure on the girder surface is clearly dominated by the presence of primary vortices. It is found that the primary vortices are not shed synchronously in girder span-wise direction, and the vortex shedding energy is concentrated on a narrow band of frequency. The fluctuating pressure on the girder lower surface, on the area upstream the reattachment region on girder upper surface is in phase, which is out of phase with that on the upper surface of leeward air fairing and area close to the upper leeward corner. The recognized flow mechanism may help to optimize the aerodynamic shape of girder cross section for long-span bridges against wind loading.