Coherent Structures in Open-Channel Flows Over a Fixed Dune

2004 ◽  
Author(s):  
Wusi Yue ◽  
Ching-Long Lin ◽  
Virendra C. Patel
Keyword(s):  
Coherent Structures ◽  
Open Channel ◽  
Wall Layer ◽  
Mean Flow ◽  
Near Surface ◽  
Vortical Structures ◽  
Eddy Simulation ◽  
Numerical Predictions ◽  
Scale Models ◽  
Strong Shear

Turbulent open-channel flow over a two-dimensional laboratory-scale dune is studied using large eddy simulation. Free surface motion is simulated using a level set method. Two subgrid scale models, namely dynamic Smagorinsky and dynamic two-parameter models, are employed. The present numerical predictions of mean flow field and turbulence statistics are in good agreement with experimental data. Streaky structures are observed in the wall layer after flow reattachment. Quadrant two events dominate near-wall and near-surface motions. Coherent structures are produced behind the dune crest by strong shear layer riding over the recirculation zone. These tube-like vortical structures are transported downstream with the mean flow and most are destroyed before arriving at the next crest.

Coherent Structures In Open-Channel Flows Over a Fixed Dune

2005 ◽  
Vol 127 (5) ◽  
pp. 858-864 ◽  
Author(s):  
Wusi Yue ◽  
Ching-Long Lin ◽  
Virendra C. Patel
Keyword(s):  
Free Surface ◽  
Coherent Structures ◽  
Open Channel ◽  
Surface Deformation ◽  
Channel Flows ◽  
Mean Flow ◽  
Open Channel Flows ◽  
Near Surface ◽  
Numerical Predictions ◽  
The Mean

Turbulent open-channel flow over a two-dimensional laboratory-scale dune is studied using large eddy simulation. Free surface motion is simulated using level set method. Two subgrid scale models, namely, dynamic Smagorinsky model and dynamic two-parameter model, are employed for assessing model effects on the free surface flow. The present numerical predictions of mean flow field and turbulence statistics are in good agreement with experimental data. The mean flow can be divided into two zones, an inner zone where turbulence strongly depends on the dune bed geometry and an outer layer free from the direct influence of the bed geometry. Streaky structures are observed in the wall layer after flow reattachment. Quadrant two events are found to prevail in near-wall and near-surface motions, indicating the predominance of turbulence ejections in open-channel flows. Large-scale coherent structures are produced behind the dune crest by a strong shear layer riding over the recirculation zone. These quasistreamwise tubelike vortical structures are transported downstream with the mean flow and most are destructed before arriving at the next crest. Free surface deformation is visualized, demonstrating complex patterns of upwelling and downdraft.

scholarly journals Large-Eddy Simulation of Open-Channel Flow with Rigid Submerged Vegetation

10.29007/v24t ◽  
2018 ◽  
Author(s):  
Alessandro Monti ◽  
Mohammad Omidyeganeh ◽  
Alfredo Pinelli
Keyword(s):  
Large Eddy Simulation ◽  
Channel Flow ◽  
Coherent Structures ◽  
Open Channel ◽  
Open Channel Flow ◽  
Strong Impact ◽  
Streamwise Direction ◽  
Eddy Simulation ◽  
Large Structures ◽  
Large Eddy

A large-eddy simulation over a fully submerged, rigid canopy in an open-channel flow under transitional canopy flow regime has been carried out. The simulations revealed a flow structure characterized by the emergence of coherent structures, which are very elongated in the streamwise direction occupying a large portion of the flow inside and outside the canopy. These very large structures have a strong impact on the flow inside the canopy as well.

Study of turbulent mixing processes at a mesoscale confluence through aerial drone imagery and eddy-resolved modelling

2021 ◽  
Author(s):  
Jason Duguay ◽  
Pascale Biron ◽  
Thomas Buffin-Bélanger
Keyword(s):  
Flow Field ◽  
Suspended Sediment ◽  
Large Scale ◽  
Surface Flow ◽  
Mean Flow ◽  
Turbulent Structures ◽  
Mixing Processes ◽  
Near Surface ◽  
Vortical Structures ◽  
Instantaneous Flow Field

<p>The large-scale turbulent structures that develop at confluences fall into three main categories: vertically orientated (Kelvin-Helmholtz) vortices, large-scale secondary flow helical cells and smaller strongly coherent streamwise orientated vortices. The causal mechanisms of each class, how they interact with one another and their respective contributions to mixing is still unclear. Our investigation emphasises the role played by the instantaneous flow field in mixing at a mesoscale confluence (Mitis-Neigette, Quebec, Canada) by complementing aerial drone observations of turbulent suspended sediment mixing processes with results from a high-resolution eddy-resolved numerical simulation. The high velocity near-surface flow of the main channel (Mitis) separates at the crest of the scour hole before downwelling upon collision with the slower tributary (Neigette). Fed by incursions of lateral momentum of the Mitis, shear generated Kelvin-Helmholtz instabilities expand as they advect along the mixing-interface. As the instabilities shed, water from the deeper Neigette passes underneath the fast, over-topping Mitis, causing a large portion of the Neigette’s discharge to cross under the mixing-interface in a short distance. The remaining flow of the tributary crosses over inside large-scale lateral incursions farther downstream. The downwelling Mitis, upwelling Neigette and recirculatory cell interact to generate coherent streamwise vortical structures which assist in rapidly mixing the waters of the two rivers in the vicinity of the mixing-interface. Evidence of large-scale helical cells were not observed in the flow field. Results suggest that flow interaction with bathymetry, and both vertical and streamwise orientated coherent turbulent structures play important roles in mixing at confluences. Our findings strongly suggest that investigating mixing at confluences cannot be based solely on mean flow field variables as this approach can be misleading. Visualization of a confluence’s mixing processes as revealed by suspended sediment gradients captured in aerial drone imagery complemented with eddy-resolved numerical modelling of the underlying flow is a promising means to gain insights on the role of large-scale turbulent structures on mixing at confluences.</p>

Detached-eddy simulation of turbulent coherent structures around groynes in a trapezoidal open channel

2019 ◽  
Vol 32 (2) ◽  
pp. 326-336
Author(s):  
Jing-xin Zhang ◽  
Jian Wang ◽  
Xiang Fan ◽  
Dongfang Liang
Keyword(s):  
Coherent Structures ◽  
Open Channel ◽  
Detached Eddy Simulation ◽  
Eddy Simulation ◽  
Turbulent Coherent Structures

Coherent structures near the wall in a turbulent channel flow

1997 ◽  
Vol 332 ◽  
pp. 185-214 ◽  
Author(s):  
J. Jeong ◽  
F. Hussain ◽  
W. Schoppa ◽  
J. Kim
Keyword(s):  
Channel Flow ◽  
Coherent Structures ◽  
Turbulent Channel Flow ◽  
Normal Velocity ◽  
Reynolds Stresses ◽  
Hairpin Vortices ◽  
Vortical Structures ◽  
Instantaneous Flow Field ◽  
Strong Shear ◽  
Low Speed Streaks

Coherent structures (CS) near the wall (i.e.y+ ≤ 60) in a numerically simulated turbulent channel flow are educed using a conditional sampling scheme which extracts the entire extent of dominant vortical structures. Such structures are detected from the instantaneous flow field using our newly developed vortex definition (Jeong & Hussain 1995) - a region of negativeλ2, the second largest eigenvalue of the tensorSikSkj+ ΩikΩkj- which accurately captures the structure details (unlike velocity-, vorticity- or pressure-based eduction). Extensive testing has shown thatλ2correctly captures vortical structures, even in the presence of the strong shear occurring near the wall of a boundary layer. We have shown that the dominant near-wall educed (i.e. ensemble averaged after proper alignment) CS are highly elongated quasi-streamwise vortices; the CS are inclined 9° in the vertical (x, y)-plane and tilted ±4° in the horizontal (x, z)-plane. The vortices of alternating sign overlap inxas a staggered array; there is no indication near the wall of hairpin vortices, not only in the educed data but also in instantaneous fields. Our model of the CS array reproduces nearly all experimentally observed events reported in the literature, such as VITA, Reynolds stress distribution, wall pressure variation, elongated low-speed streaks, spanwise shear, etc. In particular, a phase difference (in space) between streamwise and normal velocity fluctuations created by CS advection causes Q4 ('sweep’) events to dominate Q2 ('ejection’) and also creates counter-gradient Reynolds stresses (such as Ql and Q3 events) above and below the CS. We also show that these effects are adequately modelled by half of a Batchelor's dipole embedded in (and decoupled from) a background shearU(y). The CS tilting (in the (x, z)-plane) is found to be responsible for sustaining CS through redistribution of streamwise turbulent kinetic energy to normal and spanwise components via coherent pressure-strain effects.

Size and effect on the mean flow of large-scale horizontal coherent structures in open-channel flows: an experimental studyThis paper is one of a selection of papers in this Special Issue in honour of Professor M. Selim Yalin (1925–2007).

2009 ◽  
Vol 36 (10) ◽  
pp. 1643-1655 ◽  
Author(s):  
Ana Maria Ferreira da Silva ◽  
Habib Ahmari
Keyword(s):  
Flow Velocity ◽  
Coherent Structures ◽  
Large Scale ◽  
Open Channel ◽  
Channel Flows ◽  
Flow Depth ◽  
Mean Flow ◽  
Velocity Measurements ◽  
Open Channel Flows ◽  
The Mean

The size of the largest horizontal coherent structures (HCSs) of turbulence in open-channel flows is investigated experimentally on the basis of three series of flow velocity measurements. These are further used to explore the dynamics and morphological consequences of HCSs. The flow velocity measurements were carried out in a 21 m long and 1 m wide channel, with a bed formed by sand with average grain size of 2 mm. The bed surface was flat. The turbulent and subcritical flow under investigation was uniform, with a flow depth of 4 cm. The bed slope of 0.0015 was such that, for the present flow depth, the bed shear stress acting on the bed was substantially below the threshold for initiation of motion, thus ensuring that the bed remained flat throughout the measurements. To the knowledge of the writers, this work is a first attempt to systematically investigate HCSs in open-channel flows. It should be viewed as an extension to the case of horizontal structures of work previously carried out by a number of authors on large-scale organized turbulence motion in open-channel flows, so far focusing exclusively on vertical coherent structures (VCSs). The horizontal burst length was found to be between five and seven times the flow width. A slight internal meandering of the flow caused by the superimposition of burst sequences on the mean flow was detectable. Both of these findings lend support to the longstanding belief expressed by many prominent researchers that the formation of large-scale river forms is directly related to the large-scale turbulence. In particular, the present measurements for the first time provide some direct evidence in support of hypotheses previously raised by Yalin and da Silva regarding the formation of alternate bars and meanders through the action of HCSs on the mean flow and the mobile bed and banks.

scholarly journals Turbulence Anisotropy Carried by Streaks in the Neutral Atmospheric Surface Layer

2008 ◽  
Vol 65 (8) ◽  
pp. 2631-2645 ◽  
Author(s):  
Thomas Dubos ◽  
Philippe Drobinski ◽  
Pierre Carlotti
Keyword(s):  
Coherent Structures ◽  
Eof Analysis ◽  
Spatial Correlations ◽  
Near Surface ◽  
Velocity Fluctuations ◽  
Eddy Simulation ◽  
Turbulence Anisotropy ◽  
Large Eddy ◽  
Horizontal Wave

Abstract The authors investigate the relationships between coherent structures and turbulence anisotropy in the neutral planetary boundary layer by means of empirical orthogonal function (EOF) analysis of large-eddy simulation (LES) data. The simulated flow contains near-surface transient streaks. The EOF analysis extracts the most energetic patterns from the velocity fluctuations based on their second-order spatial correlations. The scale and direction of streaks obtained from a level-by-level analysis of the LES flow field do correspond to that of the EOFs. It is found that two characteristics of the turbulence anisotropy depend on whether or not the velocity fluctuations with a given horizontal wave vector present distinct patterns: (i) the vertical extent up to which the turbulent kinetic energy (TKE) is concentrated and (ii) the ratio of the vertical TKE EV to the horizontal TKE EH. Although still present in the complete signal, this anisotropy is strongly emphasized when the signal is projected onto the EOF structures. Hence the coherent structures do indeed carry more anisotropy than the remaining turbulent fluctuations. Furthermore, at horizontal wave vectors where energetic patterns are dominant, the ratio EV/EH takes values close to 0.2, representative of the ratio EV/EH based on the total LES flow and on in situ measurements.

scholarly journals Comparisons of Subgrid-Scale Models for OpenFoam Large- Eddy Simulation

2021 ◽  
Vol 1802 (4) ◽  
pp. 042088
Author(s):  
Zhipeng Feng ◽  
Huanhuan Qi ◽  
Xuan Huang ◽  
Shuai Liu ◽  
Jian Liu
Keyword(s):  
Large Eddy Simulation ◽  
Subgrid Scale ◽  
Eddy Simulation ◽  
Scale Models ◽  
Large Eddy
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