Three-dimensional numerical study of flow structure in channel confluences

Abstract

Channel confluence is one of the important features of each river system and some hydraulic structures. The features that can dominantly control flow characteristics in a confluence are confluence angle, discharge, width ratios, and Froude number of flow. Several research studies have been conducted however a comprehensive three-dimensional (3-D) numerical study of flow characteristics in a confluence has not yet been reported. In the present study, SSIIM2.0, a 3-D numerical model, is validated and applied to investigate secondary currents, velocity distribution, flow separation, and water surface elevation in different conditions. The results of the present study illustrate that flow structure and water surface variations in a confluence are highly influenced by confluence angle, discharge, and width ratios as well as Froude number because of their effect on flow deflection, separation, and secondary currents. The graphs from the present study can be used to analyze water surface variation, velocity field, and flow separation dimensions in difference conditions for engineering designs.

Flow Patterns in an Open Channel Confluence with Increasingly Dominant Tributary Inflow

“…In the downstream channel, the separation zone is highly three-dimensional; hence, it is difficult to characterize its width or length in one number [6,[8][9][10]. Nevertheless, the separation zone gradually widens at the surface when the discharge ratio decreases, closely corresponding to the logarithmic regression provided by Best & Reid [35], even when the discharge ratio is outside their experimental range.…”

“…The secondary currents in the case where q = 0.25 show dissimilarities in comparison to those in the case of rectangular sections with similar width-to-depth ratio yet different Froude numbers, where one strong clockwise rotating cell is present [8][9][10]31]. The current simulations do show a clockwise cell, but it is located close to the bottom.…”

“…In the present simulations, the free surface is modeled by means of a so-called rigid lid. This approach has been applied to many types of open channel flows, see e.g., [16,17,23,24,26,27,29], although more complex surface tracking methods are also in use [9,13,25,31]. However, because of the small Froude number in the present experiments, the rigid lid is certainly a valid approach, and preferable because of reduced computational efforts.…”

“…Next to the separation zone, the merging flows passing through a narrowed cross-section are contracted, leading to increased velocities. From the very beginning of laboratory research on confluences, starting with Taylor [2], the ratio of incoming discharges has been recognized as a key parameter [1][2][3][4][5][6][7][8][9][10][11][12][13]. However, laboratory research was mainly performed with discharge ratios in the range of 0.1 ≲ q ≲ 0.9, at least for the case of asymmetrical confluences with a fixed concordant bed and subcritical flow throughout [12]; the type of confluence that will be further considered in this paper.…”

Effect of the Junction Angle on Turbulent Flow at a Hydraulic Confluence

“…For instance, the data of Biron et al [3] were taken into account for the simulations of Bradbrook et al [28] and Biron et al [10]. Shakibainia et al [29] used SSIIM 2.0, a RANS-based turbulent model, to reproduce the experiments of Shumate [18]. The same model and experimental test were used by Djordjevic [12] to study the effects of upstream planform curvature and bed elevation discordance between the tributary and main channels on the confluence hydrodynamics.…”