Influence of Froude Number on the Development and Evolution of Secondary Flows in a Sharply Curved Bend: An Experimental and Numerical Study

Abstract

Meandering rivers sculpt landscapes and foster diverse ecosystems, with secondary flows in bends exerting a pivotal influence on sediment transport and channel morphology. Although the Froude number typically remains below 0.3 in natural meanders, the interplay of secondary flows under these low-Froude conditions is still poorly understood. This study addresses this knowledge gap by systematically examining the influence of Froude numbers (Fr = 0.12-0.21) on secondary flow structures in sharply curved channels through high-resolution flume experiments and numerical simulations. Results reveal that even slight variations in Froude number can markedly alter vortex dynamics and secondary flow complexity, underscoring a delicate balance between inertial and turbulent forces. In particular, the stability of S2-type secondary flows depends on the precise alignment of advective, centrifugal, and turbulence-induced vorticity. Minor shifts in inertial forcing can rapidly destabilize S2, leading to significant changes in velocity distributions. Additionally, a time or spatial lag between the onset of secondary circulation and the point of maximum velocity inversion points to a dynamic, two-way feedback between the secondary flow and the main flow, evolving from robust vortex growth at lower Fr to flow decay at higher Fr. These findings advance our understanding of secondary flow mechanisms in natural rivers and offer practical insights for river engineering and flood management, informing more effective strategies for sediment control and bank stability.

Keywords: secondary flow, Froude number, Curved river bends, Vorticity dynamics, Streamwise velocity