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Comparison of Flow Properties in Developing and Developed Flow in Open Channels

Author(s): Subhadip Das; Ronald Barron; Ram Balachandar

Linked Author(s): Ronald Barron, Ram Balachandar

Keywords: Smooth open channel flow; Fully developed flow; Developing flow; PIV; UMZs

Abstract: To preserve consistency and commonality between different research groups investigating similar fluid-structure interaction problems, it is important to understand the influence of the fully developed approach flow. In smooth open channel flows, a fully developed state is generated by suitably tripping the near bed inflow. When the tripping is not adequate, the flow at the downstream location may deviate from the fully developed state. In the present study, the experiments are conducted with and without tripping, and the velocity measurements are carried out with a planar Particle Image Velocimetry (PIV) system. The Reynolds number based on the flow depth is constant at 39 000. The aspect ratio (channel width/flow depth) is kept sufficiently high to minimize the effects of the secondary currents. Under these conditions and for the given inflow length, the flow without a trip presents a developing open channel flow. The flow is tripped with a patch of coarse grain sand particles and the size of the trip is varied until a fully developed state is achieved. The mean velocity and Reynolds stresses appear to be similar between the developing and the fully developed flow. A good agreement between these two flow states is also achieved for the shear stress contribution from different quadrants. However, there is no free stream region theoretically present in the fully developed open channel flow and the interaction between the flow inside and outside of the boundary layer is not possible. Unlike the developing flow case, the state of flow in the fully developed condition is highly constrained due to the confinement of the free surface. Further analysis is carried out to observe the wall-normal distributions of fractional shear stress and the ratio of shear contribution from second and fourth quadrants (RQ2/Q4) and a clear deviation is identified close to the free surface. Finally, the variable RQ2/Q4 is conditionally averaged based on the number of momentum zones present in each instance and it shows the presence of vertical variability in the quadrant events for the fully developed flow, when compared to the profiles of the developing flow.


Year: 2022

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