An experimental study on main flow, secondary flow and turbulence in open-channel bends with emphasis on their interaction with the outer-bank geometry

This research focus on the influence of bank inclination and roughness on the near-bank flow patterns which is relevant for bank protection, bank erosion and design of stable river configuration. Foregoing studies have been carried out mostly in rectangular channels which are far from being representative of natural conditions. In straight-channel flows large scale vortical structures, such as secondary currents or circulation cells of Prandtl's second kind, play a fundamental role. In curved flows mainly two circulation cells are generated, the center-region cell and the outer-bank cell. The outer-bank cell has a fundamental role in protecting the outer-bank by constraining the center-region cell and thereby decreasing the downstream velocity and turbulence. It is still not clear how the secondary circulation cells interact with varying channel shape and wetted perimeter roughness distribution in straight as in curved flows. This work is a part of a joint research program. The project other three partners are: Delft University of Technology (TUD), WL | Delft Hydraulics (WL) and the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB). The main goal is to improve the understanding of flow underlying mechanisms and to improve existing hydrodynamic and morphodynamic tools. Despite the broader program where this research project is inserted, it is also self-contained. This research project is situated in the discipline of fluid dynamics applied to open-channel flows in the topic of dynamics of the mean-flow field and the turbulence in straight and curved channels. The two main goals are: i) To investigate, systematically in laboratory controlled conditions the influence of the outer bank inclination and roughness on the patterns of main flow, secondary flow, turbulence and outer-bank shear stress in straight and curved flows; ii) To give insight in the flow mechanisms responsible for the observed patterns. The channel width of B = 1.3 m, water depth of H = 0.16 m and a bulk velocity of U = 0.43 m/s were kept constant. The bank inclination was varied between 90°, 45° and 30° whereas the bank roughness was varied between smooth PVC, sand (d = 0.002 m) and d = 0.03 m materials (simulating riprap). The measurements were performed using the Acoustic Doppler Velocimetry Profiler (ADVP). The ADVP is fundamental for the accomplishment of these goals due to its accuracy and profiler capabilities. Straight channel results reveal that rectangular and trapezoidal channels have different flow patterns. The trapezoidal channels have less circulation cells than rectangular channels and a bed shear stress distribution with fewer oscillations. In trapezoidal channels the bed shear stress is higher than the cross-section averaged shear stress regardless the bank/bed roughness ratio. The bank shear stress value increases with outer-bank roughness. The experimental measurements were compared with methods of estimating mean and maximum bed and bank shear stresses, Chow (1959) and Knight et al. (1994). Chow (1959) wetted perimeter shear stress distribution is in agreement with the experiments for the trapezoidal channel experiments with homogeneous roughness distribution whereas for heterogeneous roughness distribution no agreement is verified. Knight (1994) estimations are in good agreement with measurements suggesting its applicability as engineering expedite process. In the curved flow experiments focus is given to the outer-bank roughness and inclination effect on the outer-bank flow region mainly on the outer-bank cell. Curved channel flow results reveal that the outer-bank roughness and inclination has a strong effect on the outer-bank cell, and as consequence, on the center-region cell and downstream velocity evolution along the bend. In all experiments the outer-bank cell constrains the outward limit of the center-region cell regardless its size or strength. In curved flows with rectangular channel, the outer-bank cell size increases with increasing outer-bank roughness and so further protect the bed close to the outer-bank. In curved flows with trapezoidal channel, the outer-bank cell is located over the outer-bank toe close to the free-surface even for low-bank inclination, however, the outer-bank basal zone is more exposed to higher shear stresses. In curved flows with trapezoidal channel, low outer-bank angle and varying bank roughness, the outer-bank cell does not increase with increasing outer-bank roughness. The mechanisms underlying the outer-bank cell are disclosed by analyzing the downstream vorticity equation main terms. The centrifugal term and the cross-stream shear term favor the outer-bank cell rotation sense but cross-stream turbulent anisotropy


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