Modeling of River Flow and Flow Dynamics Near Junctions

Date

2023-4-27

Author

Yönter, Tutku Ezgi

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In river networks, confluences are critical components where their hydrological, geomorphological, and ecological properties substantially impact the downstream characteristics of a river. Particular attention should be paid to the flow behavior in these regions since they are correlated with substantial alterations in flow dynamics, sediment transport/deposition, and riverbed morphology. The primary purpose of this study is to investigate river confluences by a) using the 1D hydraulic model, HEC-RAS, to understand large-scale model behavior, and b) utilizing the 3D CFD model, Flow-3D, to understand the flow behavior comprehensively near the confluence area. In the HEC-RAS model, a sensitivity analysis was carried out on the Ohio-Mississippi confluence, and hypothetical cases were generated and investigated by perturbing model parameters. A case study was also modeled at a nearly 9 km river reach with observed inputs to compare the model's capability in simulating a real case scenario with simple information. In the 3D CFD analysis, Flow-3D was utilized. After performing a mesh independence analysis, the performance of standard k-ε, k-ω, and RNG k-ε turbulence models was investigated with a 90° confluence channel geometry. After defining the most appropriate turbulence model and mesh size, the flow behavior at different confluence angles, 90°, 60°, and 30°, was investigated in the equilibrium flow state. Then, the effects introduced by flow ratio on flow dynamics were examined when the confluence angle is 90°. Specific attention was given to variables including turbulent kinetic energy, surface velocity, depth-averaged velocity, and Froude Number in plan view flow analyses; velocity magnitude, secondary velocity, and turbulent kinetic energy in cross-sectional flow analyses; velocity, Froude Number, flow depth, total hydraulic head, and turbulent kinetic energy in point location analyses. The effects of four discharge ratios, 0.429, 0.500, 0.600, and 0,667, were investigated. Consistent with the literature, it was found that the length and width of the recirculation zone decrease as the confluence angle decreases. Additionally, the magnitudes of the secondary velocities become more significant as the confluence angle increases from 30° to 90°. Furthermore, it was perceived that the flow ratio affects the features of the recirculation zone and influences the flow dynamics downstream of the confluence.

Subject Keywords

Numerical Modeling, HEC-RAS, CFD, River Confluence, Confluence angle, Sensitivity Analysis, Flow-3D, Discharge Ratio

URI

https://hdl.handle.net/11511/103971

Collections

Graduate School of Natural and Applied Sciences, Thesis