Numerical study of aerodynamic roles of bridge railings by immersed boundary method

Abstract

The roles of railings in determining the aerodynamic characteristics of bridge girders are investigated numerically. The Direct Forcing (DF) Immersed Boundary (IB) method is utilized to model the railings while the bridge girder is modeled by body-fitted mesh system. Two benchmark tests (flow around a circular cylinder at Re = 3900 and around an equipped bridge girder at 0 ° attack angle) are designed to validate the DF-IB method. It is shown that this method has sufficient accuracy to capture representative flow structures with fewer cells and little mesh topology change. The roles of railings are investigated at several wind attack angles up to ± 12°. Results show that railings mainly affect the upper surface of the girder section and their influence is sensitive to attack angle. For negative attack angles, only downstream railings influence the flow locally. For low positive attack angles, the separated vortex is suppressed by the railings and early reattachment is observed. When the attack angle is higher, separated flows generated from the highest horizontal bar and girder corner are mixed and transferred far from the upper surface, which results in a downstream boundary layer eruption. Thus, the upstream railing affects the flow over the entire upper surface, not just locally.