Investigation of vorticity and secondary flow strength near the protected bridge pier in a sharp bend

One notable feature of the flow dynamics along bend-shaped pathways is the presence of secondary flows, which introduce a degree of complexity to the flow behavior in this region. This study aimed to investigate the flow characteristics within a bend-shaped channel that features a central angle of 180º, incorporating an oblong pier with a collar. To collect velocity data within an equilibrium bed, a three-dimensional Vectrino velocimeter was employed. The findings indicated that the peak tangential velocity was observed in proximity to the convex bank and at the water surface level, suggesting an escalation in the pressure gradient in that area. During the examination of the collar with a thickness of 12 mm, the highest vorticity was observed in the counterclockwise direction at the 84º section of the bend. Furthermore, the peak strength of the secondary flow was observed during the examination of the collar with a thickness of 3 mm at the 70º section. It is noteworthy that increasing collar thickness from 0.06b to 0.24b reduced the peak secondary flow strength by approximately 30%, indicating a significant weakening of curvature and pier-induced secondary circulation. In both experiments, the secondary flow strength experienced a decline between 70 to 80º, followed by an increase thereafter. These findings provide practical guidance for optimizing collar design to mitigate flow-induced risks around bridge piers in sharp bends.