Density Effects at a Concordant Bed Natural River Confluence

Abstract

Confluences are locations of complex hydrodynamic conditions within river systems. The effects on hydrodynamics and mixing of temperature-induced density differences between incoming flows are investigated at a small-size, concordant bed confluence. To evaluate density effects, results of eddy-resolving simulations for a densimetric Froude number Fr = 4.9 (weak-density-effects cases) and Fr = 1.6 (strong-density-effects cases) are compared to results of simulations in which the densities of the incoming flows do not differ (no-density-effects cases). Flow patterns predicted for both weak-and strong-density-effects cases show that secondary flow develops with increasing distance from the confluence apex. The pattern of secondary flow is characterized by denser fluid on one side of the confluence moving near the bed toward the side of the downstream channel corresponding to the less dense fluid and the less dense fluid moving near the free surface in the opposite direction. This pattern of fluid motion is similar to a spatially evolving lock-exchange cross flow. In the strong-density-effects simulations, a cross-stream cell of secondary flow develops at the density interface between the flows, similar to interfacial billows generated in classical lock-exchange flows. Density effects increase global mixing with respect to corresponding no-density-effects cases regardless of whether the high-momentum stream contains the higher-density fluid or the lower-density fluid. When density effects are weak, the lock-exchange mechanism either reinforces the pattern of mixing associated with secondary flow induced by inertial forces, particularly helical motion, or opposes this pattern of mixing, depending on which tributary contains the denser fluid. When density effects are strong, flow from the upstream channel with the denser fluid moves under the flow from the upstream channel with the less dense fluid.

Advective Lateral Transport of Streamwise Momentum Governs Mixing at Small River Confluences

“…Recent work has also shown that density contrasts in which the KR is denser than the CS, as occurs for all events documented in this study, can enhance secondary flow. Modeling results indicate that this density effect becomes important when the densimetric Froude number of the flow is less than 5 (Horna ‐ (Horna‐Munoz) Munoz et al, 2020). Weak density effects related to buoyancy forces are apparent in simulations for KRCS3 (Horna Munoz et al, 2020).…”

“…of the flow is less than 5 (Horna -(Horna-Munoz) Munoz et al, 2020). Weak density effects related to buoyancy forces are apparent in simulations for KRCS3 (Horna Munoz et al, 2020). These effects contribute to mixing by intensifying the pattern of secondary flow associated with helical motion generated by flow curvature.…”

“…At asymmetrical confluences, the strength of secondary flow within flow from the lateral tributary generally increases with junction angle and momentum ratio (Constantinescu et al, 2016; Lewis & Rhoads, 2015; Sukhodolov & Sukhodolova, 2019). When the incoming flows have different bulk densities, buoyant forces act on the confluent flows, either reinforcing secondary currents associated with helical motion if flow from the lateral tributary has a lower density than flow in the straight main channel or acting against these secondary currents if the lateral‐tributary flow is denser than the main‐channel flow (Horna ‐ (Horna‐Munoz)Munoz et al, 2020; Ramón et al, 2013).…”

Near- and far-field structure of shallow mixing layers between parallel streams

“…see Cheng & Constantinescu 2018, 2020; Horna-Munoz et al . 2020) have shown that buoyancy effects due unequal density differences in the incoming streams are negligible for densimetric Froude numbers larger than 10, where the densimetric Froude number is defined with the mean velocity of the flow inside the main channel, downstream of the confluence apex, the mean flow depth inside the main channel and the relative density difference between the two streams. The present work is directly relevant for cases with negligible buoyancy effects where the mixing interface resembles a shallow mixing layer and a lateral lock-exchange flow does not develop downstream of the confluence apex.…”

Predicting Transverse Mixing Efficiency Downstream of a River Confluence

“…However, their findings were not systematically used to evaluate existing analytical transverse mixing solutions (as done by Rathbun & Rostad, 2004, and earlier works by Yotsukura, Beltaos, and others in the 1970s to 1980s), let alone to develop and improve them. On the other hand, high‐resolution numerical studies using computational fluid dynamics simulations (e.g., Cheng & Constantinescu, 2018; Constantinescu et al, 2016; Horna Munoz et al, 2020; Lyubimova et al, 2014; Shakibainia et al, 2010) have brought detailed insight on mixing processes downstream of confluences but their computational cost is still too high for engineering applications to large river domains.…”