[1] River confluences are complex hydrodynamic environments where convergence of incoming flows produces complicated patterns of fluid motion, including the development of large-scale turbulence structures. Accurately simulating confluence hydrodynamics represents a considerable challenge for numerical modeling of river flows. This study uses an eddy-resolving numerical model to simulate the mean flow and large-scale turbulence structure at an asymmetrical river confluence with a concordant bed when the momentum ratio between the two incoming streams is close to 1. Results of the simulation are compared with field data on mean flow and turbulence structure. The simulation shows that the mixing interface is populated by quasi-two-dimensional eddies. Successive eddies have opposing senses of rotation. The mixing layer structure resembles that of a wake behind a bluff body (wake mode). Strong streamwise-oriented vortical (SOV) cells form on both sides of the mixing layer, a finding consistent with patterns inferred from the field data. The predicted mean flow fields show that flow curvature has an important influence on streamwise variation of circulation within the cores of the two primary SOV cells. These SOV cells, along with vortices generated by flow over a submerged block of sediment at one of the banks, strongly influence distributions of the streamwise velocity and turbulent kinetic energy downstream of the junction. Comparison of the eddy-resolving simulation results with predictions from the steady Spalart-Allmaras RANS model shows that the latter fails to predict important features of the measured distributions of streamwise velocity and turbulent kinetic energy because the RANS model underpredicts the strength of the SOV cells. Analysis of instantaneous and mean bed shear stress distributions indicates that the SOV cells enhance bed shear stresses to a greater degree than the quasi-two-dimensional eddies in the mixing interface.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010WR010018

Structure of turbulent flow at a river confluence with momentum and velocity ratios close to 1: Insight provided by an eddy‐resolving numerical simulation

Flow and scour around a vertical cylinder exposed to current are investigated by using a three-dimensional numerical model based on incompressible Reynolds-averaged Navier–Stokes equations. The mod...

https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.2014.0104

Numerical investigation of flow and scour around a vertical circular cylinder

[1] The flow and turbulence structure at stream confluences are characterized by the formation of a mixing interface (MI) and, in some cases, of streamwise-oriented vortical (SOV) cells flanking the MI. Depending on the junction angle and planform symmetry, as well as the velocity ratio across the MI, the MI can be in the Kelvin-Helmholtz (KH) mode or in the wake mode. In the former case, the MI contains predominantly co-rotating large-scale quasi two-dimensional (2-D) eddies whose growth is driven by the KH instability and vortex pairing. In the latter case, the MI is populated by quasi 2-D eddies with opposing senses of rotation. This study uses eddy resolving simulations to predict details of flow structure for KH- and wake-mode conditions at a confluence for which field measurements are available. Results indicate that SOV cells at this confluence, which occur in both modes, redistribute momentum and mass, enhancing the potential for entrainment of bed material beneath the cells and for extraction of fluid and suspended sediment from the MI. The simulations predict that the cores of some of the primary SOV cells are subject to large-scale bimodal oscillations toward and away from the MI that contribute to amplification of the turbulence close to the MI and enhance the capacity of the SOV cells to entrain sediment. At this confluence, which has a concordant bed and a large angle between the incoming streams - conditions that generate strong adverse lateral pressure gradients adjacent to the MI - the oscillating SOV cells interact with MI eddies to generate large bed friction velocities in the zone of scour immediately downstream of the confluence.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2012JF002452

Numerical analysis of the effect of momentum ratio on the dynamics and sediment-entrainment capacity of coherent flow structures at a stream confluence

Since 1971, when W. H. Grafs book, Hydraulics of Sediment Transport, was published, a number of other texts that also provide somewhat personal perspectives on water-sediment interactions in open-channel flows have appeared. The latest of these is Fluvial Processes in River Engineering, by Howard H. Chang. This text updates recent developments in the study of hydraulics and sediment transport of natural stream channels, but unlike the earlier books, it expands consideration to geomorphic processes and models of channel change. The author states that the book is intended as a text for senior and graduate-level engineering students. It appears that Chang has succeeded in tailoring the book for that audience, but it also appears that no fewer than two semesters of classroom time would be required to provide an adequate basis for understanding the considerable variety of subject matter covered.

Fluvial Processes in River Engineering

This paper uses results from detached eddy simulation to reveal the dynamics of large-scale coherent eddies in the flow around a circular pier with an equilibrium scour hole. This is important for the sediment transport because the local scour process is controlled to a large extent by the large-scale coherent structures present in the near-bed region. The present paper investigates the dynamics of these coherent structures, their interactions and their role in entraining sediment in the later stages of the scour process when the horseshoe vortex system is stabilized by the presence of a large scour hole. The pier Reynolds number was 2.06× 105 , outside the range of well-resolved large-eddy simulation (LES). Additionally, scale effects are investigated based on comparison with LES results obtained at a much lower Reynolds number of 16,000 in a previous investigation. The paper provides a detailed study of the dynamics of the main necklace vortices of the horseshoe vortex system, including an investigation...

https://ascelibrary.org/doi/pdf/10.1061/%28ASCE%29HY.1943-7900.0000101

Detached Eddy Simulation Investigation of Turbulence at a Circular Pier with Scour Hole

[1] The present study investigates the flow physics and the role played by the main coherent structures in the scouring processes around a vertical spur dike in a straight channel at conditions corresponding to the start (flat bed) of the scouring process. Large eddy simulation (LES) is performed at a relatively low channel Reynolds number (Re = 18,000), in the range where most flume studies with clear water scour conditions are conducted. Similar to these studies, the incoming flow is fully turbulent and contains realistic turbulence fluctuations. Visualization experiments are conducted to better understand the nature of the interactions between the dominant coherent structures playing a role in the erosion process. It is found that the structure of the horseshoe vortex (HV) system at the base of the spur dike changes considerably in time and in vertical sections perpendicular to the trajectory defined by the axis of the main necklace vortex. However, its intensity is the largest at vertical sections situated around the tip of the spur dike. It is in this region that the core of the main necklace vortex oscillates aperiodically between two preferred modes. In one of them (zero-flow mode), the necklace vortex is closer to the spur dike and more compact, and the near-bed jet flow beneath it is weak. In the other one (back-flow mode), a strong near-bed jet flow convects the primary necklace vortex away from the spur dike, and its core is more elongated and less compact. This explains the large amplification (by about 1 order of magnitude compared to the surrounding turbulent flow) of the turbulent kinetic energy and pressure fluctuations inside the HV system in the region situated around the tip of the spur dike and the double-peak distribution of the turbulent kinetic energy. Past the spur dike, in the legs of the necklace vortex, the intensity of the bimodal oscillations decreases such that they are not observed in spanwise sections situated at more than one channel depth behind the spur dike. It is found that the legs of the horseshoe vortices can interact, at times, with the vortex tubes shed in the detached shear layer (DSL) and with the tip of the spur dike. These events typically result in a significant change in the coherence of the HV system. The largest bed shear stress values in the mean flow are present in the strong acceleration region near the tip of the spur dike, but high bed shear stress values are also observed beneath the upstream part of the DSL. The bed shear stress fluctuations around the local mean values can be very high, especially in the region situated beneath the upstream part of the DSL. At random times, some of the vortices shed in the DSL merge or interact with eddies from the recirculation region. This leads to an increase in their strength and to a large increase of the bed shear stress along their path.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007WR006491

An investigation of the flow and scour mechanisms around isolated spur dikes in a shallow open channel: 1. Conditions corresponding to the initiation of the erosion and deposition process

[1] Results of a detached eddy simulation (DES) are used to better understand the effects of the mean flow three-dimensionality and secondary currents on turbulence and boundary shear stresses and the mechanisms through which the momentum and Reynolds stresses are redistributed in a strongly curved 193° bend with fixed deformed bed corresponding to the later stages of the erosion and sedimentation process. The ratio between the radius of curvature of the curved reach and the channel width is close to 1.3. The large channel curvature and the point bar induce flow separation near the inner bank and the formation of several strong separated shear layers (SSLs), where production by mean shear dominates. DES shows that in addition to the main cell of cross-stream circulation developing in the deeper part of the bend, several streamwise-oriented vortices (SOV) form at the inner bank. DES satisfactorily captures the distribution of the streamwise velocity and streamwise vorticity in relevant cross sections compared to experiment. Comparison with a Reynolds-averaged Navier-Stokes (RANS) simulation shows that DES predicts more accurately the velocity redistribution and cross-stream motions in the channel. This is because RANS significantly underpredicts the circulation and turbulence amplification inside the cores of the SOV vortices. DES is then used to clarify the influence of the SOV vortices and SSLs on the boundary shear stress. DES reveals the presence of several regions of large amplification of the pressure RMS fluctuations near the inner and outer banks, which can locally increase the bed erosion and affect the bank stability in the case of a bend with erodible banks. The mean flow bed shear stress distribution predicted by DES is significantly different than that predicted by RANS, while DES predictions of the mean flow are more accurate. This means that use of eddy-resolving techniques like DES in mobile bed simulations of flow in curved alluvial channels should result in more accurate predictions of bathymetry.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010WR010114

The structure of turbulent flow in an open channel bend of strong curvature with deformed bed: Insight provided by detached eddy simulation

The physics of the flow around a vertical-wall obstruction attached to one of the sidewalls of a straight channel is numerically investigated using detached eddy simulation (DES) at a high channel Reynolds number, Re=5×105 (case HR). In particular, the study investigates the role played by the large-scale coherent structures in the sediment entrainment processes at the bed for conditions close to the initiation of scour (flat bed) in a loose-bed channel. Scale effects are investigated by comparing the results of the present DES simulation with results from a large eddy simulation performed at a much lower Reynolds number, Re=18 000 (case LR). Similar to laboratory flume studies of flow and scour around in-stream obstructions, the incoming flow in the simulations was fully turbulent and contained unsteady velocity fluctuations. The main necklace vortex of the horseshoe vortex (HV) system forming near the upstream base of the flow obstruction was subject to bimodal large-scale oscillations. The intensity of...

An investigation of the dynamics of coherent structures in a turbulent channel flow with a vertical sidewall obstruction

[1] The flow and turbulence structure around a vertical-wall spur dike that extends over the whole depth of a straight channel are investigated using detached eddy simulation (DES). The channel Reynolds number in DES and the corresponding experiment is ReD = 2.4 × 105, which is typical of applications in small rivers and outside the range where well-resolved large eddy simulation (LES) can be conducted. The bathymetry at equilibrium scour conditions is obtained from a loose bed experiment. This paper discusses the main mechanisms which drive the growth of the scour hole upstream and downstream from the spur dike during the later stages of the scour process, and how these mechanisms change between the initial (flat bed conditions) and later stages of the scour process. Scale effects are investigated by comparing simulation results at ReD = 2.4 × 105 with those from simulations performed at a much lower Reynolds number, ReD = 18,000. Results show that while the structure of the horseshoe vortex (HV) system changes with respect to the case in which the bed is flat, the main necklace vortex of the HV system is still subject to large-scale aperiodic oscillations, similar to the ones observed in flows past in-stream bluff-body obstacles mounted on a flat surface. Present results show that the amplification of the horizontal vorticity within the lower part of the separated shear layer (SSL) and the associated formation of streaks of high-bed shear stress below the region where the SSL eddies are convected in the near-bed region is a general feature of high-Reynolds-number flow past a vertical-wall spur dike placed in a loose-bed channel at all stages of the scour process.

Flow and turbulence structure around a spur dike in a channel with a large scour hole

The turbulent flow structures around an isolated spur dike located at three different approach flow angles are investigated. Detached eddy simulation is used at a channel Reynolds number of 45,000. Experimental velocity measurements were also conducted using acoustic Doppler velocimetry for validating the simulations. It is found that the size and orientation of the horseshoe vortex system change considerably with the approach flow angle. The main necklace vortex is largest in size and most coherent for 90°. Within this orientation it has larger amplitude bimodal oscillations compared to the 60° and 120° cases. A new scour mechanism is defined for the 60° case which is expected to generate more scour upstream of the spur dike. The possible scour area around the spur dike is found to be the maximum in the 90° case which is roughly by 15% larger than that of the 120° case.

Mete Koken

Papers

An investigation of the flow and scour mechanisms around isolated spur dikes in a shallow open channel: 1. Conditions corresponding to the initiation of the erosion and deposition process

The structure of turbulent flow in an open channel bend of strong curvature with deformed bed: Insight provided by detached eddy simulation

An investigation of the dynamics of coherent structures in a turbulent channel flow with a vertical sidewall obstruction

Flow and turbulence structure around a spur dike in a channel with a large scour hole

Coherent structures around isolated spur dikes at various approach flow angles