Abstract: The meandering river, one of the most common types in nature, features various bends where the secondary flow is induced by curvature and interacts with turbulences, and usually develops into a complicated flow structure. Secondary flows are crucial to river evolution and nutrition material transport. Most previous numerical simulations and experiments focus on the small width-depth ratios and constant curvature of channel bends, but natural rivers tend to develop into a large width-depth ratio and variable curvature. This study conducts large eddy simulations (LES) of the flows in continuous sine-generated bends, and examines the hydrodynamic structure of large width-depth ratio bends under large Reynolds number conditions. The results show that at the cross-sections of zero curvature in the transition of two bends, the recirculation zone is the largest, and over the core region of these cross sections the secondary flow is transversely distributed most uniform. In the case of large width-depth ratio bends, this region can expand up to 15.5% of the cross-sectional area. The mainstream in this core region, relative to the small width-depth ratio case, is less effected by the curved boundaries, and the cross-sectional distributions of its time-averaged streamwise velocity component is closer to that in a straight, rectangular open channel, with the velocity peak located vertically very close to free surface and transversely within a range of 0.5-1.0 times the flow depth around the channel centerline. With an increasing width-depth ratio, Z-vorticity near the bend apex is increased, and the influence of bend curvature is extended further downstream. The secondary flow also changes the characteristics of flow turbulences in a meandering river. A larger width-depth ratio leads to less turbulent energy, and turbulent kinetic energy is the lowest in the core of the mainstream while the highest in shear layers.

Key words: sine-generated bend, large width-depth ratio, Reynolds number, large eddy simulation, turbulent structure