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 shape landscapes and support diverse ecosystems, with secondary flows in bends playing a crucial role in sediment transport and channel morphology. The Froude number is a important parameter influencing flow behavior in bends, and for most meandering rivers, it typically ranges below 0.3. However, the transitional phases and interactions of secondary flows under low Froude number conditions remain poorly understood. This study addresses this gap by examining the influence of low Froude numbers (Fr = 0.12-0.21) on secondary flow structures in sharply curved river bends through high-resolution flume experiments and numerical simulations. Our findings demonstrated that even small changes in Froude number can notably alter vortex dynamics and secondary flow complexity, revealing a delicate balance between inertial and turbulent forces in curved channels. The stability of S2-type secondary flows hinges on the alignment of advection and centrifugal forces with turbulence-induced vorticity. Minor shifts in inertial forces can rapidly destabilize S2, reshaping velocity distributions. Additionally, A time or space lag between the onset of secondary circulation and maximum velocity inversion highlights a dynamic two-way feedback between secondary flows and the main flow, transitioning from strong vortex growth at low Fr to decay at higher Fr. These findings enhance the understanding of secondary flow dynamics in natural rivers, highlighting the balance between inertial and turbulent forces. The insights gained are valuable for river engineering and flood management, informing strategies for sediment control and bank stability.

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