Analysis of Storage Effects in the Recirculation Zone Based on the Junction Angle of Channel Confluence

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.

Influences of Momentum Ratio on Transverse Dispersion for Intermediate-Field Mixing Downstream of Channel Confluence

“…The recirculation zone size is known to be enlarged with the increase in the momentum ratio and confluence angle from experimental and simulation results [9,31,32,34]. The enlarged recirculation zone drives to increase the retention time of the trapped solute cloud, and thus, the pollutant is trapped and accumulates at the inner bank at a larger confluence angle (Figure 10b,c).…”

“…Thus, the channel width (W) is 10 m, and the width-to-depth ratio is about 3.0. The mesh size was referred to in the study by Shin et al [9], in which the grid sensitivity was checked for the 90 • confluence channel using EFDC by comparisons to the measurements by Weber et al [27]. The study suggested the grid size as ∆x < W/20 for grid independency and computational efficiency.…”

“…Inward the mixing interface, the flow structures have a helical motion that promotes transverse mixing [8,9]. The mixing interface is tilted by the helical flow downstream of the confluence, and the mixing interface is realigned toward the flow with the weaker momentum flux that induces transverse mixing change [1].…”

“…The tilted mixing interface leads to producing a steep concentration gradient and promotes transverse mixing [10]. The development of a strong helical motion increases the secondary flow intensity accompanying the transverse dispersion coefficient increase [9]. The second kind of secondary flow presenting the helical motion usually appears due to the channel curvature [11,12].…”

“…The dimensionless transverse dispersion coefficient increases to the range of 0.3-0.9 in conditions of the secondary flow for a meandering channel compared to the range for a straight channel, i.e., 0.15-0.3 [13]. For considering the significance of the secondary flow, previous studies included the sinuosity or relative strength of the secondary flow to de- Inward the mixing interface, the flow structures have a helical motion that promotes transverse mixing [8,9]. The mixing interface is tilted by the helical flow downstream of the confluence, and the mixing interface is realigned toward the flow with the weaker momentum flux that induces transverse mixing change [1].…”