Large‐scale turbulent mixing at a mesoscale confluence assessed through drone imagery and eddy‐resolved modelling

Abstract

Confluences are sites of intense turbulent mixing in fluvial systems. The large-scale turbulent structures largely responsible for this mixing have been proposed to fall into three main classes: vertically orientated (Kelvin-Helmholtz) vortices, secondary flow helical cells and smaller, strongly coherent streamwise-orientated vortices. Little is known concerning the prevalence and causal mechanisms of each class, their interactions with one another and their respective contributions to mixing. Historically, mixing processes have largely been interpreted through statistical moments derived from sparse pointwise flow field and passive scalar transport measurements, causing the contribution of the instantaneous flow field to be largely overlooked. To overcome the limited spatiotemporal resolution of traditional methods, herein we analyse aerial video of large-scale turbulent structures made visible by turbidity gradients present along the mixing interface of a mesoscale confluence and complement our findings with eddy-resolved numerical modelling. The fast, shallow main channel (Mitis) separates over the crest of the scour hole's avalanche face prior to colliding with the slow, deep tributary (Neigette), resulting in a streamwise-orientated separation cell in the lee of the avalanche face. Nascent large-scale Kelvin-Helmholtz instabilities form along the collision zone and expand as the high-momentum, separated near-surface flow of the Mitis pushes into them. Simultaneously, the strong downwelling of the Mitis is accompanied by strong upwelling of the Neigette. The upwelling Neigette results in $50% of the Neigette's discharge crossing the mixing interface over the short collision zone. Helical cells were not observed at the confluence. However, the downwelling Mitis, upwelling Neigette and separation cell interact to generate considerable streamwise vorticity on the Mitis side of the mixing interface. This streamwise vorticity is strongly coupled to the large-scale Kelvin-Helmholtz instabilities, which greatly enhances mixing. Comparably complex interactions between large-scale Kelvin-Helmholtz instabilities and coherent streamwise vortices are expected at other typical asymmetric confluences exhibiting a pronounced scour hole.

Aerial Observations and Numerical Simulations Confirm Density‐Driven Streamwise Vortices at a River Confluence

“…The variety of mixing patterns observed in the postconfluent reaches of natural confluences (Biron et al., 2019; Duguay et al., 2022; Horna‐Munoz et al., 2020; Lane et al., 2008; Sukhodolov & Sukhodolova, 2019) is largely attributed to the complex and little understood interactions of the four mentioned forms of coherent flow structures (KH instabilities, helical cells, episodic pulses, and SOVs). Therefore, a deeper understanding of SOVs, with a specific focus on how they form and interact with other coherent flow structures is necessary if confluence hydrodynamics are to be understood.…”

“…Inherently a feature of the subsurface flow, SOVs are difficult to view: The joining rivers often lack a turbidity contrast, rendering the SOVs “invisible” in aerial views, or when a sufficient turbidity gradient is present, waves and glare can occlude views of the subsurface turbulent billows (i.e., see Supporting Information Duguay et al. (2022)). Therefore, unlike KH instabilities, helical cells and episodic pulses, technological limitations and practical constraints have largely limited our understanding of SOVs to that which can be learned from eddy‐resolved numerical modeling.…”

“…The SOVs developed as back-toback counter-rotating vortices, one flanking each side of the mixing interface, each rotating in the opposite sense of the other. Recent eddy-resolved modeling has also predicted single-sided SOVs (Duguay et al, 2022) or has failed to detect SOVs entirely (Cheng & Constantinescu, 2020;Guillén Ludeña et al, 2017), suggesting they need not form in the dual back-to-back model discussed by Constantinescu et al (2011), nor are they universal to all confluences. SOVs are thought to arise from a downwelling of superelevated flow mechanism: as the opposing flows collide near the apex, a portion of their kinetic energy is converted to potential energy (i.e., superelevated surface), which subsequently reverts back to kinetic energy as the superelevated mass is accelerated toward the bed (Constantinescu et al, 2011;Horna-Munoz et al, 2020;Sukhodolov & Sukhodolova, 2019).…”

Comparative analysis of four types of mesoscale eddies in the Kuroshio-Oyashio extension region

“…In summary, mesoscale eddies can cause changes in the spatial distribution of ocean elements (such as heat, freshwater, nutrient and chlorophyll concentration) through their motions (horizontal movement, rotation and vertical pumping, Sasai et al, 2010;Kouketsu et al, 2015;Xu et al, 2019;Patel et al, 2020;Geng et al, 2021). In addition, mesoscale eddies also induce heat flux changes at the air-sea interface (Chelton, 2013;Frenger et al, 2013;Ma et al, 2015;Ma et al, 2016), subduction of modal water (Xu et al, 2016;Xu et al, 2014), and enhancement of mixing in the ocean interior (Ma and Wang, 2014b;Qi et al, 2020;Duguay et al, 2022).…”

Numerical Simulation of Confluence Flow in a Degraded Bed Under Different Turbulence Models with a Comparison of Rigid Lid and Volume of Fluid

“…However, WMLES with wall functions fading the role of this requirement by identifying the primary grid point within the log-law region (30 < z+ < 500). Although details of near-bed turbulence are reduced, the wall modelled approach has been widely applied to investigate large-scale turbulent phenomena within the outer region of the water column in flume and fluvial applications [30]. A velocity-based wall function was employed to ascertain the near-wall turbulent viscosity and bed shear stress caused by the rough solid boundary.…”