Non-Uniform Interaction Effect of Flow and Rigid Submerged Vegetation on Turbulent Flow Characteristics

Document Type : Research Paper

Authors

1 Phd Candidate, Department of Civil Engineering, Islamic Azad University, Marvdasht Branch, Marvdasht, Iran

2 Assistant Professor, Department of Water Department, Islamic Azad University, Marvdasht Branch, Marvdasht, Iran

Abstract

Vegetation patches in riverbeds create shear instability, leading to the transfer of mass and momentum between different layers of vegetation. The presence of vegetation reduces flow velocity, narrows the river width, increases sedimentation in the riverbed, and consequently decreases discharge. The study conducted experiments in a straight rectangular channel measuring 14 meters in length, 90 cm in width, and 60 cm in depth, with a constant flow rate of 31.7 L/s. A 3D pool was constructed with a gravel bed and rigid submerged vegetated elements of 12 cm height and 10 cm diameter, with a fixed area density of 0.004 irregularly distributed on the bedform. Velocity fluctuations were measured at 13 cross-sections from the decelerating flow region to the accelerating flow region with a spatial interval of 20 cm using a downward-looking ADV device with a sampling frequency of 200 Hz over a 90-second period. The research revealed that the maximum flow speed occurred at y/h=0.52 for decelerating flow and at y/h=0.47 for accelerating flow. However, the depth of validity of the logarithmic law appeared to depend on the velocity profile measurement location and the non-uniform distribution of submerged rigid vegetated elements on the 3D bedform. Analysis of the velocity defect law showed wake strength coefficients of Π=-2.8 and Π=-5 in the decelerating and accelerating flow sections respectively. These sections, in the presence of submerged rigid vegetated elements, can impede flow, causing water to swirl and bend around them, leading to increased vorticity generation and the formation of complex flow patterns with rotating vortices. Turbulent intensities in the accelerating flow section, both in the flow direction and perpendicular to it, were consistently higher than those in the decelerating flow section. This suggests that the flow in these areas is significantly influenced by the presence of submerged rigid vegetation, in addition to the effects of increased flow velocity and favorable pressure gradient. Eddy patterns influenced by the 3D pool and submerged rigid vegetation can have ecological implications, creating micro-habitats with varying current velocities and turbidity levels, impacting the distribution of aquatic organisms and overall ecosystem health. Understanding these implications is crucial for applications such as river restoration, habitat enhancement, and water resource management.

Keywords

Main Subjects

References

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History

  • Receive Date: 25 June 2024
  • Revise Date: 28 July 2024
  • Accept Date: 17 August 2024

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