scholarly journals Turbulence modifications induced by the bed mobility in intense sediment-laden flows

2016 ◽  
Vol 808 ◽  
pp. 469-484 ◽  
Author(s):  
T. Revil-Baudard ◽  
J. Chauchat ◽  
D. Hurther ◽  
O. Eiff
Keyword(s):  
Sediment Transport ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Shear Stresses ◽  
Wall Region ◽  
Law Of The Wall ◽  
Sediment Bed ◽  
Turbulent Schmidt Number ◽  
Turbulent Momentum ◽  
W Prime

An experimental dataset of high-resolution velocity and concentration measurements is obtained under intense sediment transport regimes to provide new insights into the modification of turbulence induced by the presence of a mobile sediment bed. The physical interpretation of the zero-plane level in the law of the wall is linked to the bed-level variability induced by large-scale turbulent flow structures. The comparison between intrinsic and superficial Reynolds shear stresses shows that the observed strong bed-level variability results in an increased covariance between wall-normal ($w^{\prime }$) and streamwise ($u^{\prime }$) velocity fluctuations. This appears as an additional Reynolds shear stress in the near-wall region. It is also observed that the mobile sediment bed induces an increase of turbulence kinetic energy (TKE) across the boundary layer. However, the increased contribution of interaction events ($u^{\prime }w^{\prime }>0$, i.e. quadrants I and III in the ($u^{\prime },w^{\prime }$) plane) induces a decrease of the turbulent momentum diffusion and an increase of the turbulent concentration diffusion in the suspension region. This result provides an explanation for the modification of the von Kármán parameter and the turbulent Schmidt number observed in the literature for intense sediment transport.

The structure of the near-wall region of two-dimensional turbulent separated flow

1991 ◽  
Vol 336 (1640) ◽  
pp. 5-17 ◽  
Keyword(s):  
Kinetic Energy ◽  
Shear Layer ◽  
Large Scale ◽  
Outer Region ◽  
Turbulence Kinetic Energy ◽  
Shear Stresses ◽  
Wall Region ◽  
Freestream Velocity ◽  
Outer Flow ◽  
Near Wall

The time-dependent structure of the wall region of separating, separated, and reattaching flows is considerably different than that of attached turbulent boundary layers. Large-scale structures, whose frequency of passage scales on the freestream velocity and shear layer thickness, produce large Reynolds shearing stresses and most of the turbulence kinetic energy in the outer region of the shear layer and transport it into the low velocity reversed flow next to the wall. This outer flow impresses a near wall streamwise streaky structure of spanwise spacing λ z simultaneously across the wall over a distance of the order of several λ z . The near wall structures produce negligible Reynolds shear stresses and turbulence kinetic energy.

scholarly journals PIV Measurements in the Atmospheric Boundary Layer within and above a Mature Corn Canopy. Part II: Quadrant-Hole Analysis

2007 ◽  
Vol 64 (8) ◽  
pp. 2825-2838 ◽  
Author(s):  
W. Zhu ◽  
R. van Hout ◽  
J. Katz
Keyword(s):  
Dissipation Rate ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Small Scale ◽  
Shear Stresses ◽  
Reynolds Stresses ◽  
Piv Measurements ◽  
Dissipation Rates ◽  
Scale Shear ◽  
Corn Canopy

Quadrant-hole (Q-H) analysis is applied to PIV data acquired just within and above a mature corn canopy. The Reynolds shear stresses, transverse components of vorticity, as well as turbulence production and cascading part of dissipation rates are conditionally sampled in each quadrant, based on stress and vorticity magnitudes. The stresses are representative of large-scale events, while the vorticity is dominated by small-scale shear. Dissipation rates (cascading energy fluxes) are evaluated by fitting −5/3 slope lines to the conditionally sampled and averaged spatial energy spectra, while the Reynolds stresses, vorticity, and production rates are calculated directly from the spatial distributions of two velocity components. The results demonstrate that sweep (quadrant 4) and ejection (quadrant 2) events are the dominant contributors to the Reynolds shear stress, consistent with previous observations. The analysis also shows a strong correlation between magnitudes of dissipation rate and vorticity. The dissipation rates and vorticity magnitudes are higher in quadrants 1 and 4, that is, when the horizontal component of the fluctuating velocity is positive, peaking in quadrant 1. Both are weakly correlated with the Reynolds stresses except for rare quadrant 1 events. However, the more frequently occurring quadrant 4 events are the largest contributors to the dissipation rate. The production rate inherently increases with increasing stress magnitude, but lacks correlation with vorticity. Quadrants 2 and 4 contribute the most to production. However, the contribution of quadrant 1 events to negative production should not be ignored above canopy. The results show a strong disconnection between small-scale- and large-scale-dominated phenomena.

scholarly journals The Impact of Wind on Flow and Sediment Transport over Intertidal Flats

2020 ◽  
Vol 8 (11) ◽  
pp. 910
Author(s):  
Irene Colosimo ◽  
Paul L. M. de Vet ◽  
Dirk S. van Maren ◽  
Ad J. H. M. Reniers ◽  
Johan C. Winterwerp ◽  
...  
Keyword(s):  
Sediment Transport ◽  
Tidal Flat ◽  
Large Scale ◽  
Tidal Flow ◽  
Interaction Model ◽  
Sediment Concentration ◽  
Momentum Balance ◽  
Shear Stresses ◽  
Intertidal Flats ◽  
The Impact

Sediment transport over intertidal flats is driven by a combination of waves, tides, and wind-driven flow. In this study we aimed at identifying and quantifying the interactions between these processes. A five week long dataset consisting of flow velocities, waves, water depths, suspended sediment concentrations, and bed level changes was collected at two locations across a tidal flat in the Wadden Sea (The Netherlands). A momentum balance was evaluated, based on field data, for windy and non-windy conditions. The results show that wind speed and direction have large impacts on the net flow, and that even moderate wind can reverse the tidal flow. A simple analytical tide–wind interaction model shows that the wind-induced reversal can be predicted as a function of tidal flow amplitude and wind forcing. Asymmetries in sediment transport are not only related to the tide–wind interaction, but also to the intratidal asymmetries in sediment concentration. These asymmetries are influenced by wind-induced circulation interacting with the large scale topography. An analysis of the shear stresses induced by waves and currents revealed the relative contributions of local processes (resuspension) and large-scale processes (advection) at different tidal flat elevations.

Reynolds stress and the physics of turbulent momentum transport

1990 ◽  
Vol 220 ◽  
pp. 99-124 ◽  
Author(s):  
Peter S. Bernard ◽  
Robert A. Handler
Keyword(s):  
Reynolds Stress ◽  
Reynolds Shear Stress ◽  
Turbulent Channel Flow ◽  
Local Approximation ◽  
Transport Processes ◽  
Momentum Transport ◽  
Wall Region ◽  
Mean Velocity ◽  
Fluid Particles ◽  
Turbulent Momentum

The nature of the momentum transport processes responsible for the Reynolds shear stress is investigated using several ensembles of fluid particle paths obtained from a direct numerical simulation of turbulent channel flow. It is found that the Reynolds stress can be viewed as arising from two fundamentally different mechanisms. The more significant entails transport in the manner described by Prandtl in which momentum is carried unchanged from one point to another by the random displacement of fluid particles. One-point models, such as the gradient law are found to be inherently unsuitable for representing this process. However, a potentially useful non-local approximation to displacement transport, depending on the global distribution of the mean velocity gradient, may be developed as a natural consequence of its definition. A second important transport mechanism involves fluid particles experiencing systematic accelerations and decelerations. Close to the wall this results in a reduction in Reynolds stress due to the slowing of sweep-type motions. Further away Reynolds stress is produced in spiralling motions, where particles accelerate or decelerate while changing direction. Both transport mechanisms appear to be closely associated with the dynamics of vortical structures in the wall region.

Radiofrequency plasma stabilization of a low-Reynolds-number channel flow

2014 ◽  
Vol 748 ◽  
pp. 663-691 ◽  
Author(s):  
Timothy J. Fuller ◽  
Andrea G. Hsu ◽  
Rodrigo Sanchez-Gonzalez ◽  
Jacob C. Dean ◽  
Simon W. North ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Channel Flow ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Low Reynolds Number ◽  
Plasma Heating ◽  
Shear Stresses ◽  
Exponential Factor ◽  
Turbulence Decay ◽  
Low Reynolds

AbstractThe effects of plasma heating and thermal non-equilibrium on the statistical properties of a low-Reynolds-number ($Re_{\tau } = 49$) turbulent channel flow were experimentally quantified using particle image velocimetry, two-line planar laser-induced fluorescence, coherent anti-Stokes Raman spectroscopy and emission spectroscopy. Tests were conducted at two radiofrequency plasma settings. The nitrogen, in air, was vibrationally excited to $T_{vib} \sim 1240\ \mathrm{K}$ and 1550 K for 150 W and 300 W plasma settings, respectively, while the vibrational temperature of the oxygen and the rotational/translational temperatures of all species remained near room temperature. The peak axial turbulence intensities in the shear layers were reduced by 15 and 30 % in moving across the plasma for the 150 and 300 W cases, respectively. The plasma did not alter the transverse intensities. The Reynolds shear stresses were reduced by 30 and 50 % for the 150 and 300 W cases. The corresponding Reynolds shear stress correlation coefficient was also reduced, which indicates that the large-scale structures were diminished. Finally, the plasma enhanced the turbulence decay in the zero-shear regions, where the power law decay $t^{-1/n}$ exponential factor $n$ decreased from 1.0 to 0.8.

Large-scale motions in a plane wall jet

2019 ◽  
Vol 877 ◽  
pp. 239-281 ◽  
Author(s):  
Ebenezer P. Gnanamanickam ◽  
Shibani Bhatt ◽  
Sravan Artham ◽  
Zheng Zhang
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Large Scale ◽  
Plane Wall ◽  
Shear Stresses ◽  
Wall Jet ◽  
Wall Region ◽  
Free Shear Layer ◽  
Large Scale Structures ◽  
Scale Structures

The plane wall jet (PWJ) is a wall-bounded flow in which a wall shear layer develops in the presence of extremely energetic flow structures of the outer free-shear layer. The structure of a PWJ, developing in still air, was studied with the focus on the large scales in the flow. Wall-normal hot-wire anemometry (HWA) measurements along with double-frame particle image velocimetry (PIV) measurements (wall-normal–streamwise plane) were carried out at streamwise distances up to $162b$, where $b$ is the slot width of the PWJ exit. The nominal PWJ Reynolds number based on exit parameters was $Re_{j}\approx 5940$. Comparisons with a zero-pressure-gradient boundary layer (ZPGBL) at nominally matched friction Reynolds number $Re_{\unicode[STIX]{x1D70F}}$ were also carried out as appropriate, to highlight key features of the PWJ structure. Consistent with previous work, the PWJ showed a dependence of the peak turbulent stresses on the jet exit Reynolds number. The turbulent production showed a peak corresponding to the near-wall cycle similar to the peak seen in the ZPGBL. However, another turbulent production peak was observed in the outer free-shear layer that was an order of magnitude larger than the inner one. Along with the change in sign of the viscous and Reynolds shear stresses, the PWJ was shown to have a region of very low turbulent production between these two peaks. The dissipation rate increased over the PWJ layer with a peak also in the outer region. Visualizations of the flow and two-point correlations reveal that the most energetic large-scale structures within a PWJ are vortical motions in the wall-normal–streamwise plane similar to those structures seen in free-shear layers. These structures are referred to as J (for jet) type structures. In addition two-point correlations reveal the existence of large-scale structures in the wall region which have a signature similar to those structures seen in canonical boundary layers. These structures are referred to as W (for wall) type structures. Instantaneous PIV realizations and flow visualizations reveal that these W type large-scale features are consistent with the paradigm of hairpin vortex packets in the wall region. The J type structures were seen to intrude well into the wall region while the W type structures were also seen to extend into the outer shear layer. Further, these large-scale structures were shown to modulate the amplitude of the finer scales of the flow.

scholarly journals MODELLING OF THE IMPACT OF A WAVE FARM ON NEARSHORE SEDIMENT TRANSPORT

2012 ◽  
Vol 1 (33) ◽  
pp. 66 ◽  
Author(s):  
Raul Gonzalez ◽  
Qingping Zou ◽  
Shunqi Pan
Keyword(s):  
Sediment Transport ◽  
Large Scale ◽  
Morphological Changes ◽  
Field Measurements ◽  
Integrated Modelling ◽  
Shear Stresses ◽  
Nearshore Sediment Transport ◽  
Wave Farm ◽  
The Impact ◽  
Modelling System

This paper presents the results from an integrated modelling system investigating the effects of a wave farm on nearshore sediment transport. Wave Hub project is a large scale demonstration site for the development of the operation of arrays of wave energy generation devices located at the southwest coast of the UK where multiple field measurements took place. The two-way coupled SWAN and ROMS models with nested modelling system were set up at the Wave Hub site and run with and without a wave farm. The model results show that the presence of the wave farm has significant impacts on the nearshore circulation, bed shear stresses and sediment transport. The morphological changes are also altered by the wave farm. The study is the key element for the wave resource characterization and environmental impact assessment of the wave farm.

Local isotropy and large structures in a heated turbulent jet

1979 ◽  
Vol 94 (4) ◽  
pp. 745-775 ◽  
Author(s):  
K. R. Sreenivasan ◽  
R. A. Antonia ◽  
D. Britz
Keyword(s):  
Large Scale ◽  
Reynolds Shear Stress ◽  
Turbulent Energy ◽  
Ensemble Averaging ◽  
Local Isotropy ◽  
Large Structures ◽  
Probability Densities ◽  
Temperature Signal ◽  
Turbulent Momentum ◽  
High Reynolds

Recent measurements of the skewness of the derivative of the temperature fluctuation θ, implying the breakdown of local isotropy even in high Reynolds number shear flows, are examined. Using the temperature signal in a slightly heated axisymmetric jet, a detailed quantitative analysis is made of the suggestion that the observed presence of a well-defined large-scale pattern of the temperature signal in these flows is responsible for this breakdown. A selective ensemble averaging technique is used for separating this pattern from fluctuations superposed on it. The technique is extended to extract the large-scale patterns in simultaneously measured axial (u), radial (v) velocity fluctuations, and the products uv, uθ and vθ, so that it is possible to separate contributions of these patterns from those of the superposed fluctuations to several important turbulent quantities. The mean shape of the patterns, their degree of anisotropy and correlation, and their contribution to turbulence intensities and Reynolds shear stress are obtained. Probability densities and spectra of these quasi-homogeneous superposed fluctuations are also obtained. Results show that the fluctuations are consistent with local isotropy and make the dominant contribution to the turbulence intensities, that the large-scale patterns are responsible for the observed skewness values of the derivative of v, and that the fluctuations may be responsible for a significant part of the turbulent momentum and heat transport, especially in the region of the jet where the turbulent energy production is substantial.

Turbulence collapse in a suction boundary layer

2016 ◽  
Vol 795 ◽  
pp. 356-379 ◽  
Author(s):  
T. Khapko ◽  
P. Schlatter ◽  
Y. Duguet ◽  
D. S. Henningson
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Large Scale ◽  
Shear Flows ◽  
Wall Region ◽  
Law Of The Wall ◽  
Logarithmic Scaling ◽  
Normal Transport ◽  
Observation Times ◽  
Near Wall

Turbulence in the asymptotic suction boundary layer is investigated numerically at the verge of laminarisation using direct numerical simulation. Following an adiabatic protocol, the Reynolds number $Re$ is decreased in small steps starting from a fully turbulent state until laminarisation is observed. Computations in a large numerical domain allow in principle for the possible coexistence of laminar and turbulent regions. However, contrary to other subcritical shear flows, no laminar–turbulent coexistence is observed, even near the onset of sustained turbulence. High-resolution computations suggest a critical Reynolds number $Re_{g}\approx 270$, below which turbulence collapses, based on observation times of $O(10^{5})$ inertial time units. During the laminarisation process, the turbulent flow fragments into a series of transient streamwise-elongated structures, whose interfaces do not display the characteristic obliqueness of classical laminar–turbulent patterns. The law of the wall, i.e. logarithmic scaling of the velocity profile, is retained down to $Re_{g}$, suggesting a large-scale wall-normal transport absent in internal shear flows close to the onset. In order to test the effect of these large-scale structures on the near-wall region, an artificial volume force is added to damp spanwise and wall-normal fluctuations above $y^{+}=100$, in viscous units. Once the largest eddies have been suppressed by the forcing, and thus turbulence is confined to the near-wall region, oblique laminar–turbulent interfaces do emerge as in other wall-bounded flows, however only transiently. These results suggest that oblique stripes at the onset are a prevalent feature of internal shear flows, but will not occur in canonical boundary layers, including the spatially growing ones.

2D and 3D CFD Investigations of Seabed Shear Stresses Around Subsea Pipelines

2013 ◽  
Author(s):  
Wenwen Shen ◽  
Terry Griffiths ◽  
Mengmeng Xu ◽  
Jeremy Leggoe
Keyword(s):  
Sediment Transport ◽  
Suspended Sediment ◽  
Interaction Model ◽  
Suspended Sediment Transport ◽  
Shear Stresses ◽  
Parametric Function ◽  
Ample Evidence ◽  
North West ◽  
Wide Range ◽  
Subsea Pipelines

For well over a decade it has been widely recognised that existing models and tools for subsea pipeline stability design fail to account for the fact that seabed soils tend to become mobile well before the onset of pipeline instability. Despite ample evidence obtained from both laboratory and field observations that sediment mobility has a key role to play in understanding pipeline/soil interaction, no models have been presented previously which account for the tripartite interaction between the fluid and the pipe, the fluid and the soil, and the pipe and the soil. There are numerous well developed and widely used theories available to model pipe-fluid and pipe-soil interactions. A challenge lies in the way to develop a satisfactory fluid-soil interaction algorithm that has the potential for broad implementation under both ambient and extreme sea conditions due to the complexity of flow in the vicinity of a seabed pipeline or cable. A widely used relationship by Shields [1] links the bedload and suspended sediment transport to the seabed shear stresses. This paper presents details of computational fluid dynamics (CFD) research which has been undertaken to investigate the variation of seabed shear stresses around subsea pipelines as a parametric function of pipeline spanning/embedment, trench configuration and wave/current properties using the commercial RANS-based software ANSYS Fluent. The modelling work has been undertaken for a wide range of seabed geometries, including cases in 3D to evaluate the effects of finite span length, span depth and flow attack angle on shear stresses. These seabed shear stresses have been analysed and used as the basis for predicting sediment transport within the Pipe-Soil-Fluid (PSF) Interaction Model [2] in determining the suspended sediment concentration and the advection velocity in the vicinity of pipelines. The model has significant potential to be of use to operators who struggle with conventional stabilisation techniques for the pipelines, such as those which cross Australia’s North West Shelf, where shallow water depths, highly variable calcareous soils and extreme metocean conditions driven by frequent tropical cyclones result in the requirement for expensive and logistically challenging secondary stabilisation measures.

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