P. Hu - State Key Laboratory of Water Resources and Hydropower Engineering, Wuhan University, Wuhan ۴۳۰۰۷۲, China
M. Yu - State Key Laboratory of Water Resources and Hydropower Engineering, Wuhan University, Wuhan ۴۳۰۰۷۲, China

ABSTRACT Secondary flow is a prominent feature of channel bends; it alters the streamwise velocity and bed shear stress distributions. Experiments were conducted to investigate the complex pattern of secondary flow in a narrow and sharp open-channel bend and the underlying mechanism of generation of multiple circulation cells. Compared with the moderate bends, the sharp bends are characteristic of multiple circulation cells from the ۹۰° section. In addition to the curvature-induced circulation cell (S۱) and turbulence-induced counter-rotation circulation cell (C۱) near the outer bank, another circulation cell (S۲) was observed near the inner bank and was attributed to flow separation. A term-by-term analysis of the vorticity equations indicates that the centrifugal term favours S۱ and C۱ while opposing S۲. The turbulence-related term accounts for the formation of C۱ and S۲. The advective transport term redistributes vorticity and maintains the existence of S۲. The dependence of secondary flow structure on Reynolds number and aspect ratio was also explored. With an increase in the Reynolds number from ۲۳۰۰۰ to ۳۷۰۰۰, both the strength and size of C۱ are reduced by ۵۰%, whereas the size of S۲ increases by ۲۰%, and its strength slightly decreases. With a decrease in the aspect ratio from ۳.۳ to ۲, the strengths of S۱, S۲, and C۱ are doubled, and the sizes of C۱ and S۲ increase by ۹۰% and ۲۰%, respectively.

Secondary flow, Vorticity transport equations, Open-channel bends, Aspect ratio, Reynolds number