Large‐Scale Particle Image Velocimetry Reveals Pulsing of Incoming Flow at a Stream Confluence

Abstract

Despite widespread recognition that confluences are characterized by complex hydrodynamic conditions, few studies have mapped in detail spatial patterns of flow at confluences and variation in these patterns over time. Recent developments in large‐scale particle image velocimetry (LSPIV) have created novel opportunities to explore the spatial and temporal dynamics of flow patterns at confluences. This study uses LSPIV to map two‐dimensional flow structure at the water surface at a confluence and to examine variation in this structure over time. Results show that flow within the confluence is characterized by a large region of flow stagnation at the junction apex, a region of low velocities at the downstream junction corner, and a region of merging of the two flows along a mixing interface within the center of the confluence. Interaction between the incoming flows varies over time in the form of episodic pulsing in which one of the two tributary flows first decelerates and then subsequently accelerates into the confluence. The cause of this pulsing remains uncertain, but it may reflect unsteadiness in the water‐surface pressure‐gradient field as the two flows compete for space within the confluence. No large‐scale vortices are evident within the mixing interface for the particular flow conditions documented in this study, but such vortices do occur along the margins of the stagnation zone where shearing action between fast‐moving and slow‐moving fluid is strong. The results of the study provide insight into the time‐dependent dynamics of the spatial structure of flow at stream confluences.

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

“…While vertically orientated KH instabilities can be directly observed as swirls of debris or turbidity gradients on a confluence's surface (Biron et al., 2019; De Serres et al., 1999; Duguay et al., 2022; Lewis & Rhoads, 2015), helical cells are apparent only on cross‐sections of the mean flow measured using acoustic velocimetry (Rhoads & Johnson, 2018; Rhoads & Sukhodolov, 2001; Riley et al., 2015; Yuan et al., 2021). Recently, episodic pulses in the confluence's mixing interface have been identified with large‐scale particle image velocimetry (LSPIV) (Sabrina et al., 2021), a discovery highlighting the advantage of spatially resolved instantaneous measurement techniques. Though the causal mechanism of the pulses is not fully understood, they are likely due to unsteadiness in the water‐surface pressure‐gradient field as the flows compete for space within the confluence (Sabrina et al., 2021).…”

“…Recently, episodic pulses in the confluence's mixing interface have been identified with large‐scale particle image velocimetry (LSPIV) (Sabrina et al., 2021), a discovery highlighting the advantage of spatially resolved instantaneous measurement techniques. Though the causal mechanism of the pulses is not fully understood, they are likely due to unsteadiness in the water‐surface pressure‐gradient field as the flows compete for space within the confluence (Sabrina et al., 2021). Unlike KH instabilities, helical cells and episodic pulses, a fourth form of coherent flow structure—streamwise orientated vortices (SOVs)—have been suggested to exist yet have eluded detection in situ.…”

Impact of Density Gradients on the Secondary Flow Structure of a River Confluence

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A greater understanding of these structures is therefore of obvious interest. Four structures are often discussed: helical cells (secondary flow at the scale of the tributaries' widths), vertically orientated Kelvin-Helmholtz (KH) vortices (shear-induced instabilities along the mixing interface), episodic pulses (origins still not fully understood, see Sabrina et al (2021)) and streamwise orientated vortices (SOVs). SOVs were first discovered in numerical models as a pair of back-to-back, counter-rotating SOVs flanking each side of the mixing interface (Constantinescu et al, 2011) and their development is generally attributed to the downwelling of superelevated water, in a process strengthened by planform curvature (Sukhodolov & Sukhodolova, 2019).

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Hydrodynamic and dissolved oxygen–biochemical oxygen demand transport characteristics at the river confluence in China's largest alluvial plain—A modeling study

“…The stagnant flow phenomenon is common in large river confluence areas, where the flow is gentle and the water quality conditions are stable. Sabrina et al (2021) found a similar large region of flow stagnation at the junction apex by using large-scale particle image velocimetry. The distribution characteristics in the wet and dry periods were similar.…”

“…Sabrina et al. (2021) found a similar large region of flow stagnation at the junction apex by using large‐scale particle image velocimetry. The distribution characteristics in the wet and dry periods were similar.…”