Secondary flow dynamics and turbulence in an ice-covered sharp bend channel at different subcritical froude numbers

Abstract This study examines ice cover effects on secondary flow and turbulence in a sharp bend with curvature ratio $$R/B = 1.5$$ , where R is the channel’s centerline radius and B the width. Three flow rates (1.44, 1.08, and 0.48 L/s) with Froude numbers 0.15, 0.12, and 0.05 were measured. Unlike previous ice-covered bend studies that often report ice- and bed-layer primary cells, here two distinct circulation cells developed in the upper layer: one near the inner bank and another near the channel center, while a circulation cell formed near the outer bank in the bed layer. Circulation was strongest at high flow and declined with decreasing discharge. Turbulent kinetic energy (TKE) increased with flow rate, but normalized TKE decreased, suggesting that turbulence was maintained at low flows by stronger relative shear beneath the ice. The turbulent structure parameter ( $$a_1$$ ) showed that turbulence at high flow was more uniformly distributed with reduced localized stress dominance, while low flow exhibited enhanced shear stress effects, forming localized regions of high turbulence. At lower flow, vertical shear stress ( $$\tau _{uw}$$ ) became greater relative to lateral-vertical shear stress ( $$\tau _{vw}$$ ), reflecting enhanced velocity gradient effects under reduced discharge. These results provide insights into how ice cover alters flow structures by intensifying mean turbulence beneath the ice and creating distinct secondary circulation and turbulence dynamics compared to open or partially covered channels.