Abstract
In this study, numerical simulations using the Environmental Fluid Dynamics Code model were conducted to elucidate the effects of flow structures in the recirculation zone on solute storage based on the junction angle. Numerical simulations were performed at a junction angle of 30° to 90° with a momentum flux ratio of 1.62. The simulation results revealed that an increase in the junction angle caused the recirculation zone length and width to increase and strengthened the development of helical motion. The helical motion increased the vertical gradient of the mixing layer and the mixing metric of the dosage curves. The recirculation zone accumulated the solute as a storage zone, which formed a long tail in the concentration curves. The interaction between the helical motion and recirculation zone affected the transverse mixing, such that the transverse dispersion had a positive relationship with the helical motion intensity and a negative relationship with the recirculation zone size. Transverse mixing exhibited an inverse relationship with the mass exchange rate of the recirculation zone. These results indicate that the transverse dispersion is replaced by mixing due to strongly developed storage zones.
Highlights
The prediction of mixing characteristics of solute inputs in rivers affects decisionmaking management in the operation of hydraulic structures for the safety of local residents
The reproducibility of the flow structures associated with the junction angle of the designed confluence was evaluated based on numerical simulation results
The size of the recirculation zone increases as the junction angle increases, which has been reported in previous studies [12,23,38,53]
Summary
The prediction of mixing characteristics of solute inputs in rivers affects decisionmaking management in the operation of hydraulic structures for the safety of local residents. The complex topography of rivers and hydraulic structures such as dams and weirs generates storage zones that induce non-Fickian mixing [2]. Many previous studies have conducted parameter calibrations using concentration curves obtained from tracer experiments [7,8,9,10]. These calibrated parameters are associated with high uncertainties because the three-dimensional (3D) characteristics of solute mixing in actual tracer tests are translated into results for 1D storage model parameter values
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