Numerical Investigation of Hydrodynamics in a U-Shaped Open Channel Confluence Flow with Partially Emergent Rigid Vegetation
2. Materials and Methods
3. Model Validations and Simulation Cases Set Up
3.1. Curved Open Channel Flow with Partially Rigid Vegetation
3.2. Curved Open Channel Confluence Flow
3.3. Numerical Simulation Cases Set Up
4. Results and Discussion
4.1. Steam-Wise Velocities and Separation Zone
4.2. Water Level
4.3. Cross-Section Secondary Flow
4.4. Bed Shear Stress
4.5. Flow Resistance
- The streamwise velocities of the convex bank were significantly greater than those of the concave bank and this velocity difference increased as the tributary discharge increased. The vegetation blocked the tributary flow to the mainstream causing an obvious decrease in velocities in the vegetation region. Moreover, the velocity difference between the convex bank and the concave bank was much larger. This change was more apparent with the higher vegetation solid volume fraction.
- The tributary flow impacted and deflected the main flow producing a separation zone with reversed smaller velocities on the concave bank after the confluence point. With the increase in tributary discharge, the separation zone became larger in both length and width. The vegetation near the concave bank played a role in blockage on the tributary flow and changed the mainstream deflecting direction. Compared to the non-vegetated cases, the separation zone was much smaller in length and width. Especially, with higher vegetation solid volume fraction (), the separation zone was divided into two parts, a smaller one right after the confluence point and a larger one on the second half of the curved reach after the confluence.
- The lowest water level point was located near the convex bank in the curved reach after the confluence point. Moreover, with the tributary discharge increased, the water level before the confluence increased while the water level after the confluence fell visibly. The lowest water level points also moved downstream as the tributary discharge increased. The vegetation brought resistance to the tributary flow causing backwater upstream of the confluence. The water level varied fast near the convex bank and tended to be gentle in the channel midline. With the same SVF (), a larger vegetation density brought more resistance to the tributary flow than a larger vegetation stem diameter.
- On the upstream cross-section right before the confluence point, there was only one main circulation cell located near the channel bed of the convex bank. While on the downstream cross-sections, besides the main circulation cell, there was another reversed smaller circulation on the concave bank. As the tributary discharge and vegetation SVF increased, the main circulation cell apparently shrunk and the concave-bank circulation moved towards the channel midline. Moreover, on the 180° cross-section, the circulation cells were much smaller than those in the curved reach. The maximum secondary flow strength was at the 90° cross-section where the confluence point was located.
- As the same trends of streamwise velocities, the region of maximum bed shear stress was always located near the convex bank along the whole curved reach. Compared to the non-vegetated case, the maximum bed shear stress was much larger in the vegetated case. Moreover, the bed shear stress in the non-vegetated region was also larger than that in the vegetated region. As the vegetation SVF increased, this difference became larger.
- The time-averaged drag coefficients of the vegetation decreased with the increase in stem Reynolds number and vegetation planting density. Along the arc length of the vegetation region, the largest local drag coefficient was located at the beginning of the region and decreased sharply in the middle reach, then it became a little bit larger downstream before the exit of the curved reach.
Data Availability Statement
Conflicts of Interest
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Shi, Z.; Jin, S. Numerical Investigation of Hydrodynamics in a U-Shaped Open Channel Confluence Flow with Partially Emergent Rigid Vegetation. Water 2022, 14, 4027. https://doi.org/10.3390/w14244027
Shi Z, Jin S. Numerical Investigation of Hydrodynamics in a U-Shaped Open Channel Confluence Flow with Partially Emergent Rigid Vegetation. Water. 2022; 14(24):4027. https://doi.org/10.3390/w14244027Chicago/Turabian Style
Shi, Zhengrui, and Sheng Jin. 2022. "Numerical Investigation of Hydrodynamics in a U-Shaped Open Channel Confluence Flow with Partially Emergent Rigid Vegetation" Water 14, no. 24: 4027. https://doi.org/10.3390/w14244027