Shallow mixing layers between non-parallel streams in a flat-bed wide channel

Cheng, Zhengyang; Constantinescu, George

doi:10.1017/jfm.2021.254

Cited by 19 publications

(14 citation statements)

References 40 publications

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“…2016; Pouchoulin et al. 2020; Proust & Nikora 2020; Cheng & Constantinescu 2021). The mixing layers are characterized by a spanwise profile of time-averaged streamwise velocity that typically exhibits an inflection point.…”

Section: Introductionmentioning

confidence: 99%

Shallow mixing layers over hydraulically smooth bottom in a tilted open channel

2022

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Shallow mixing layers (SMLs) behind a splitter plate were studied in a tilted rectangular open-channel flume for a range of flow depths and the initial shear parameter ${\lambda = (U_{2}-U_{1})/(U_{2}+U_{1})}$ , where $U_1$ and $U_2$ are streamwise velocities of the slow and fast streams, respectively. The main focus of the study is on (i) key parameters controlling the time-averaged SMLs; and (ii) the emergence and spatial development of Kelvin–Helmholtz coherent structures (KHCSs) and large- and very-large-scale motions (LSMs and VLSMs) and associated turbulence statistics. The time-averaged flow features of the SMLs are mostly controlled by bed-friction length scale $h/c_f$ and shear parameter $\lambda$ as well as by time-averaged spanwise velocities $V$ and momentum fluxes $UV$ , where $h$ and $c_f$ are flow depth and bed-friction coefficient, respectively. For all studied cases, the effect of shear layer turbulence on the SML growth is comparatively weak, as the fluxes $UV$ dominate over the spanwise turbulent fluxes. Initially, the emergence of KHCSs and their length scales largely depend on $\lambda$ . The KHCSs cannot form if ${\lambda \lessapprox 0.3}$ and the turbulence behind the splitter plate resembles that of free mixing layers. Further downstream, shear layer turbulence becomes dependent on the bed-friction number $S = c_f \delta _v /(4 h \lambda )$ , where $\delta _v$ is vorticity thickness. When $S \gtrapprox 0.01$ , the KHCSs are longitudinally stretched and the scaled transverse turbulent fluxes decrease with increasing $S$ . The presence and streamwise development of LSMs/VLSMs away from the splitter plate depends on the $\lambda$ -value, particularly when $\lambda > 0.3$ , resembling LSMs/VLSMs in conventional open-channel flows when $\lambda$ is small.

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Section: Introductionmentioning

confidence: 99%

Shallow mixing layers over hydraulically smooth bottom in a tilted open channel

2022

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“…The simulations were conducted with the same boundary conditions used in previous studies of confluent flows conducted using the same code (e.g. see Cheng & Constantinescu 2020 a , 2021; Horna-Munoz et al. 2020).…”

Section: Numerical Method Test Cases and Boundary Conditionsmentioning

confidence: 99%

“…Such events locally enhance mixing and modify the shape of the interface between the intrusions and the surrounding regions containing mostly unmixed, constant density fluid (figure 8). Although these billows resemble and have a similar effect on flow and mixing to the SOV cells forming at river confluences with a large confluence angle and small density contrast between the two tributaries (Constantinescu et al 2012;Constantinescu 2014;Cheng & Constantinescu 2021), their formation mechanism is totally different and entirely due to buoyancy effects. Another interesting observation is that the front of the near-bed intrusion in the Ri = 0.03 simulations is subject to relatively small oscillations in time (figure 8).…”

Section: Spatial Development Of the MI In The Mean Flowmentioning

confidence: 98%

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Shallow mixing interfaces between parallel streams of unequal densities

Cheng

Constantinescu²

2022

J. Fluid Mech.

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As opposed to the case of shallow mixing layers forming between parallel streams of unequal velocities and equal densities, the spatial development of the mixing interface (MI) between two parallel streams of unequal velocities and sufficiently large density contrast is controlled by the formation of a spatially developing, lock-exchange-like flow in transverse planes. Buoyancy effects are driven by the presence of a vertical density interface near the splitter plate. Eddy-resolving numerical simulations conducted in a wide and very long channel are used to investigate the mean flow structure and the effects of the lock-exchange-like flow and the associated coherent structures on mixing and the capacity of the flow to entrain sediment from the channel bed. When the two streams have unequal densities, quasi-two-dimensional Kelvin–Helmholtz (KH) vortices still form near the MI origin, but their coherence is lost over a much shorter distance compared with the case with no density contrast, and their cores are severely stretched in the transverse direction. A main cell of recirculating (cross-) flow forms in the mean flow, in between the fronts of the near-bed intrusion of heavier fluid and the free-surface intrusion of lighter fluid. The instantaneous flow fields contain streamwise-oriented vortical cells along the interface separating the regions containing heavier and lighter fluids. These vortical cells play an important role in enhancing mixing, similar to the KH billows forming in a classical lock-exchange flow. The regions of highest turbulence amplification are situated next to the boundaries of the main recirculating cell. Once the oscillating fronts of the intrusions get sufficiently close to the channel sidewalls, streamwise cells of secondary flow form near the channel boundaries. For cases with a strong density contrast, the free-surface mixing pattern is not a good indicator of mixing between the two streams. For the same flow velocities in the incoming channels, the two streams mix faster with increasing density difference between the two streams. This is because, as opposed to the KH vortices, coherent structures induced by the formation of the lock-exchange-like flow maintain their coherence and capacity to induce mixing at very large distances from the splitter plate. Meanwhile, the redistribution of the streamwise momentum leading to uniform, fully developed flow over the whole cross-section is delayed by buoyancy effects. Away from the splitter plate, the region with the highest sediment entrainment potential is situated next to the edge of the main recirculation cell on the high-speed side of the channel. For the same flow velocities in the incoming channels, the sediment entrainment capacity of the flow is much larger in the simulations conducted with density contrast between the incoming streams and peaks for the case when the faster stream contains the denser fluid.

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“…Other researchers have conducted numerical simulations using the Reynolds-Averaged Navier-Stokes (RANS) model to analyze 3D flow characteristics based on field measurements and laboratory experiments [21][22][23][24][25]. Additionally, detailed turbulence structures in the mixing interfaces of confluences have been analyzed through detached eddy simulations using the RANS model [26][27][28][29]. The junction angle influences near-field mixing by creating a recirculation zone and helical cells downstream of the confluence.…”

Section: Introductionmentioning

confidence: 99%

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

2021

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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.

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Shallow mixing layers between non-parallel streams in a flat-bed wide channel

Abstract: Abstract

Cited by 19 publications

References 40 publications

Shallow mixing layers over hydraulically smooth bottom in a tilted open channel

Shallow mixing layers over hydraulically smooth bottom in a tilted open channel

Shallow mixing interfaces between parallel streams of unequal densities

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

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