Abstract
This work experimentally investigates the turbulent flow characteristics of developing and fully-developed flows over a rough bed channel that is subjected to downward seepage. Instantaneous 3D velocities were collected using an acoustic Doppler velocimeter (ADV) in the developing and fully-developed flow regions, along the channel centerline, to analyze different turbulent statistics. Observations revealed that the streamwise and vertical velocities were higher in developing flows, whereas the Reynolds shear stresses, and turbulence intensities, were smaller. The downward seepage would affect the velocity distributions and flow depth in both the developing and fully-developed regions. Therefore, new equations to represent the distribution of the turbulence intensities were proposed, and a comparison with the current literature is provided. The investigation of the Reynolds stress anisotropy tensors concludes that the degree of anisotropy in fully-developed flows is lower than for developing flows.
Highlights
The flow in open channels is mainly turbulent in nature
In the near-bed zone, the uncertainty errors for the velocities and the Reynolds shear stresses were lower than 5% and 12.5%, respectively, indicating acoustic Doppler velocimeter (ADV) data accuracy
Similar profiles were observed in the no-seepage and downward seepage conditions, but streamwise and vertical velocities increased under the influence of seepage
Summary
The flow in open channels is mainly turbulent in nature. In general, open-channel flows are spatially varied in the field due to the frequent variations of the boundary conditions arising from the alterations in flow discharge, as well as other channel parameters [1]. A high-velocity gradient is developed close to the channel boundary that is linked with the frictional stresses developed between the fluid particles and the rough surface [2]. The fluid layer close to a rough surface, where viscous properties are apparent, is termed the boundary layer. The viscous properties can be observed in a very thin layer, termed the viscous sublayer, in which the major portion of the velocity variation arises. In fully-developed turbulent flow, with respect to the velocity governing factors, the boundary layer is formed from the inner zone and the outer zone. It is important to investigate the fluid features in developing and developed open-channel flows to evaluate the fluid–sediment interface and sediment aggradation and degradation, which are closely related to the hydrodynamic features, such as the vertical profile of time-averaged velocities, turbulence intensities, Reynolds shear stresses, and turbulent kinetic energy [1]. The turbulent developing open-channel flow is a complex 3D flow governed by a seepage bed, the bed roughness, and other variables
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