Abstract
This study experimentally and numerically investigated the hydrodynamic characteristics of a 180° curved open channel over rough bed under the condition of constant downstream water depth. Three different sizes of bed particles (the small, middle and big cases based upon the grain size diameter D50) were selected for flume tests. Three-dimensional instantaneous velocities obtained by the acoustic Doppler velocimeter (ADV) were used to analyze hydrodynamic characteristics. Additionally, the Renormalization-Group (RNG) turbulence model was employed for numerical simulations. Experimental results show that rough bed strengthens turbulence and increases turbulent kinetic energy along curved channels. The power spectra of the longitudinal velocity fluctuation satisfy the classic Kolmogorov −5/3 law in the inertial subrange, and the existence of rough bed shortens the inertial subrange and causes the flow reach the viscous dissipation range in advance. The contributions of sweeps and ejections are more important than those of the outward and inward interactions over a rough bed for the middle case. Flow-3D was adopted to simulate flow patterns on two rough bed settings with same surface roughness (skin drag) but different bed shapes (form drag): one is bed covered with thick bottom sediment layers along the curved part of the flume (the big case) as the experimental condition, and the other one is uniform bed along the entire flume (called the big case_flat only for simulations). Numerical simulations reveal that the secondary flow is confined to the near-bed area and the intensity of secondary flow is improved for both rough bed cases, possibly causing more serious bed erosion along a curved channel. In addition, the thick bottom sediments (the big case), i.e., larger form drag, can enhance turbulence strength near bed regions, enlarge the transverse range of secondary flow, and delay the shifting of the core region of maximum longitudinal velocity towards the concave bank.
Highlights
At the conditions of the constant downstream water level at the tailgate, the thicker sediment layers lead to shallow water depth over the curved reach, i.e., hbig case > hmiddle case > hsmall case in general
For big case_flat, the water depth increases a little due to the bottom friction from rough bed, and the water depth of each section is approximately the same, i.e., uniform flow conditions, meaning that the friction exerted by the rough bed mainly balances gravity due to sloping bed
The longitudinal velocity for the big case_flat is smaller than that for the big case owing to larger water depths
Summary
Flow in curved open-channel follows a helicoidal path and induces a secondary flow, which can redistribute mass, momentum, boundary shear stress and thereby plays an important role in hydraulic engineering [1,2]. Considerable studies focused on the curved channel characteristics in different ways, including theoretical derivation, field observations and measurements, physical model experiments, and numerical simulations. Zeng et al [3] showed that the turbulent kinetic energy in the channel bend is significantly larger than that at the entrance and the exit according to laboratory experiments. Blanckaert [4] and Blanckaert and Vriend [5] found that water surface gradient and streamwise curvature variations are the major factors for velocity redistribution in sharp bend channels
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