Academic literature on the topic 'Sediment-turbulence interaction'

Abstract Even though the interaction between water movements and aquatic plant is crucial for the aquatic ecosystem management, the importance of water turbulence in this regard is not well documented. To add to our knowledge on the interaction between aquatic plant communities and water turbulence, this study examined turbulence, plant, sediment and water quality at the reed community (Phragmites australis) in the Lake Shinji, Japan. Observations were conducted along transects perpendicular to the shoreline. For each transect, reed communities were observed at land ward side, centre, water ward and the outside of the reed community. An elevated level of turbulence was observed outside compared to inside reed community, where the magnitude of turbulence decreased with distance into the community interior. A significant positive correlation was observed for turbulence and surface-dissolved oxygen where the latter was negatively correlated to reed density. Sediment composition was affected by water turbulence where the content of coarse particles positively correlated to turbulence. Accumulation of organic matter in anoxic sediments together with fine particles was observed under low turbulence. Our findings can offer insight into understanding the interactions between turbulence and aquatic plant communities.

Three-dimensional vortex structures involving obliquely descending eddies (ODE), produced by depth-induced breaking-waves, has been proved to be associated with local sediment suspension in the surf zone (Zhou et al., 2017); vertical velocity fluctuations around the ODEs induces sediment suspension near the bed. Otsuka et al. (2017) explained the mechanical contributions of the ODEs to enhance local sediment suspension under the breaking waves and modeled the vortex-induced suspension to predict the profile of the equilibrium sediment concentration in the surf zone. In order to predict local behaviors of sediment, however, sediment-turbulence interactions in the transitional turbulence under breaking waves need to be understood. The interaction may be described in terms of Schmidt number (Sc). Sc has been empirically determined for trivial steady flows such as open channel or pipe flows. In the surf zone where organized flows evolve into a turbulent bore, the interaction may vary with the transitional feature of turbulence during a wave-breaking process, and thus Sc may be variable in time and space. No appropriate Sc model has been proposed for the surf zone flow. A parametric study on the sediment motion with respect to the variation of Sc is required for better prediction of sediment transport in the surf zone. In this study, contributions of the sediment advection and diffusion in the vortex structure to the concentration are computationally investigated. Effects of Sc to the sediment suspension and diffusion process will be also discussed in this work.

Several researchers have studied turbulent structures, such as ejections, sweeps, and outwards and inwards interactions in flumes, where the streamwise velocity dominates over vertical and transversal velocities. However, this research presents an experimental study in which there are ejections associated with the interchange between surface and subsurface water, where the vertical velocity dominates over the streamwise component. The experiment is related to a surface alluvial stream that is polluted with fine sediment, which is percolated into the bed. The subsurface flow is modified by a lower permeability associated with the fine sediment and emerges to the surface current. Quasi-steady ejections are produced that drag fine sediment into the surface flow. Particle image velocimetry (PIV) measured the velocity field before and after the ejection. The velocity data were analyzed by scatter plots, power spectra, and wavelet analysis of turbulent fluctuations, finding changes in the distribution of turbulence interactions with and without the presence of fine deposits. The flow sediment ejection changes the patterns of turbulent structures and the distribution of the turbulence interactions that have been reported in open channels without subsurface flows.

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.

A basic framework characterising the interaction between aquatic flows and permeable sediment beds is presented here. Through the permeability Reynolds number ($Re_{K}=\sqrt{K}u_{\ast }/\unicode[STIX]{x1D708}$, where$K$is the sediment permeability,$u_{\ast }$is the shear velocity and$\unicode[STIX]{x1D708}$is the fluid viscosity), the framework unifies two classical flow typologies, namely impermeable boundary layer flows ($Re_{K}\ll 1$) and highly permeable canopy flows ($Re_{K}\gg 1$). Within this range, the sediment–water interface (SWI) is identified as a transitional region, with$Re_{K}$in aquatic systems typically$O(0.001{-}10)$. As the sediments obstruct conventional measurement techniques, experimental observations of interfacial hydrodynamics remain extremely rare. The use of refractive index matching here allows measurement of the mean and turbulent flow across the SWI and thus direct validation of the proposed framework. This study demonstrates a strong relationship between the structure of the mean and turbulent flow at the SWI and$Re_{K}$. Hydrodynamic characteristics, such as the interfacial turbulent shear stress, velocity, turbulence intensities and turbulence anisotropy tend towards those observed in flows over impermeable boundaries as$Re_{K}\rightarrow 0$and towards those seen in flows over highly permeable boundaries as$Re_{K}\rightarrow \infty$. A value of$Re_{K}\approx 1{-}2$is seen to be an important threshold, above which the turbulent stress starts to dominate the fluid shear stress at the SWI, the penetration depths of turbulence and the mean flow into the sediment bed are comparable and similarity relationships developed for highly permeable boundaries hold. These results are used to provide a new perspective on the development of interfacial transport models at the SWI.

The results of direct numerical simulations of the flow generated in a plane duct by a pressure gradient which is the sum of two terms are described. The first term of the pressure gradient is constant in space but it oscillates in time whereas the second term is constant both in space and in time. Therefore, a pulsating flow is generated, similar to that generated at the bottom of a monochromatic propagating surface wave when nonlinear effects are taken into account. The simulations are carried out for values of the parameters similar to those considered in previous investigations. It is shown that even a small constant pressure gradient influences the flow regime in the bottom boundary layer. In particular, turbulence strength is damped when the steady velocity component has the direction opposite to the oscillating velocity component whereas turbulence strength increases when the steady and oscillating components point in the same direction. Even though the flow is not exactly equal to that generated at the bottom of sea waves, where second order effects in the wave steepness induce a steady streaming in the direction of wave propagation, our results provide information on the interaction of the steady streaming with the oscillatory flow and are also relevant for investigating the dynamics of sediment close to the sea bottom. Indeed, since the turbulent eddies tend to pick-up the sediment from the bottom, it can be inferred that the triggering of turbulence enhances sediment transport towards the shore.

The critical conditions for incipient sediment motion induced by oscillating-grid generated turbulence interacting with a sloping sediment layer were investigated experimentally. Near-spherical monodisperse sediments were used throughout with relative densities of 1.2 and 2.5 and mean diameters(d)ranging between 80 and 1087 μm. Interaction characteristics were analysed in terms of the critical Shields parameter θc, defined using the peak root mean square (r.m.s) horizontal velocity component in the near-bed region. Bed slope effects on θc were investigated by tilting the bed (and the grid) at angles between 0 and the repose limit. In all cases, the grid was aligned to be parallel to the bed surface, so that the oscillation direction is always normal to bed surface. The measured values of θc on a horizontal bed were comparable to the values reported in the traditional Shields diagram with θc seen to increase monotonically for hydraulically smooth bedforms and to be approximately constant for hydraulically rough bedforms. To account for bed slope effects, the measured values of θc were compared with a force-balance model based on the conditions for incipient grain motion on a sloping bed. For hydraulically smooth bedforms, where the bed roughness is small compared to the boundary-layer depth, the model was derived to account for how viscous stresses act to damp the drag and lift forces acting on the near surface sediment. For hydraulically rough bedforms, where this viscous-damping effect is not present, the model assumes the standard approach with the drag and lift forces scaling with the square of the near-bed (inviscid)velocity scale. In both cases the model predicts the bedforms to become more mobile as the bed slope is increased. However, the damping effect of the viscous sublayer acts as a stabilizing influenced for hydraulically smooth bedforms, to reduce the rate at which the bed mobility increases with bed slope. The measured values of θc in the hydraulically rough bedforms were in agreement with the trends predicted by this model. However, measured θc in smooth bed cases were lower than predicted, and fall on the hydraulically rough trend when bed slope is < 20. When the bed slope reaches the repose limit, θc falls between the smooth-bed and rough-bed cases. Measurements of sediment trajectories due to the turbulence interacting with the bed were obtained, for a range of impact conditions. Observations of the sediment trajectories during the interaction show the individual sediment grains to be predominantly displaced in a circular 'splash'. Data showed that the ‘splash’ feature and particle entrainment within the turbulence structure was within one eddy turnover time.

2008/2009
In this work the turbulent field developing in case of local erosion around a 45° wing-wall bridge abutment was investigated numerically. Three different scour conditions were considered: beginning of the process, logarithmic phase and equilibrium stage. The flow field was computed using a wall-resolving large eddy simulation (a simulation where the near-wall viscous sub-layer is directly resolved) and the bathymetric data were taken from physical experiments with an equivalent geometry. The dynamics of the coherent structures forming around the obstacle and inside the scour-hole was investigated and its influence on the modeling of the problem and on the erosion process was discussed. The analysis suggested that the full dynamics of the vortex system should be directly solved since simple eddy-viscosity models, as the k-ε model in RANS approach, were found to be not suited for this kind of problem and since high-order statistics were found to be important for the evolution of the local scour. The results of the present study may be helpful to formulate new physical-based local scour models to be used for practical evaluation of the scour depth around bridge abutments.
XXII Ciclo
1981

Incipient motion criterion in sediment transport is very important, as it defines the flow condition that initiates sediment motion, and is also frequently employed in models to predict the sediment transport at higher flow conditions as well. In turbulent flows, even a reasonably accurate definition of incipient motion condition becomes very difficult due to the random nature of the turbulent process, which is responsible for sediment motion under incipient conditions. This work investigates two aspects, both of which apply to incipient sediment transport conditions. The first one deals with the role of turbulence in initiating sediment motion. The second part deals with the nature of sediment-fluid interaction for more general and complex flows where the number of sediment particles that form the rough surface is varied. The first part of this work that investigates the role of turbulence in initiating sediment motion, uses a video camera to simultaneously monitor and record the sediment (glass ball) motion and corresponding fluid velocity events measured by a three-component laser Doppler Velocimeter (LDV). The results of the single ball experiment revealed that the number of LDV flow measurements increase dramatically (more than four folds) just prior to the ball motion. The fluid mean velocity and its root-mean-square (rms) values also are significantly higher than the values that correspond to the flow conditions that yield no ball motion. The second part of the work, investigation of the fluid-sediment interaction, includes five tests with varying number of sediment particles. In order to understand the nature and extent of fluid-solid interaction, velocity profile measurements using the 3-D laser system were carried out at three locations for each of these five cases. Plots of mean velocities, rms quantities located the universal layer at about 1.5 ball diameters above the porous bed. However, at higher sediment particle concentrations, this distance reduced and the beginning of the universal layer approached the top of the porous bed.
Ph. D.

Le travail réalisé dans cette thèse a consisté en le développement et l'utilisation des modèles numériques pour étudier les interactions multi-échelles entre une éolienne offshore et la dynamique locale océanique et sédimentaire. Dans une première partie, les interactions entre le système couplé océan-sédiment et le sillage atmosphérique généré par une turbine éolienne offshore sont étudiées à l'aide d'un modèle numérique 2D développé au cours de la thèse et écrit en fortran. Ce modèle résout les équations de Barré-De-Saint-Venant pour l'océan et l'équation d'Exner pour le sédiment. Dans un seconde partie, le phénomène d'affouillement 3D autour d'un cylindre vertical est étudié à l'aide d'un modèle diphasique eulérien-eulérien, sedFoam, implémenté dans la boîte à outils numériques OpenFOAM. L'approche diphasique permet de tenir compte des processus de petite échelle en s'affranchissant des hypothèses classiquement faites pour la modélisation du transport sédimentaire, notamment la corrélation locale entre le flux de sédiments et la contrainte de cisaillement fluide sur le fond.Concernant l'impact du sillage atmosphérique généré par une turbine, nous avons montré que celui-ci peut générer des allées tourbillonnaires dans l'océan. La dynamique turbulente océanique est alors contrôlée par le paramètre de sillage S=Cd D/H, où D est le diamètre du sillage au point d'impact sur la surface de l'océan, Cd est le coefficient de la loi de friction quadratique entre l'océan et le fond et H la profondeur de l'océan. Une paramétrisation des flux turbulents basée sur S est proposée pour modéliser la dynamique océanique dans des modèles à plus grande échelle de type RANS (Reynolds Averaged Navier-Stokes). Les résultats montrent que la dynamique océanique a une rétro-action sur la puissance du vent disponible. Les résultats montrent également que la dynamique sédimentaire instantanée est couplée à la dynamique océanique. Cependant, les variations de l'élévation du fond marin sont faibles (mm/mois) et l'impact morphodynamique du sillage est négligeable.Concernant la simulation diphasique de l'affouillement, après une validation du modèle sur des configurations 1D et 2D, des simulations tridimensionnelles autour d'une pile cylindrique sont présentées. Dans un premier temps, une configuration sans sédiments est réalisée afin de valider la capacité du modèle de turbulence URANS (Unsteady Reynolds Averaged Navier-Stokes) développé dans ce travail de thèse à reproduire les structures tourbillonnaires responsables de l'affouillement comme le tourbillon en fer à cheval et le lâché tourbillonnaire à l'aval du cylindre. Ensuite, les premières simulations diphasiques 3D de l'affouillement autour du cylindre ont été réalisées en régime de transport de type lit-mobile. Ces simulations constitue un véritable challenge en terme calcul numérique à haute performance. La comparaison favorable des résultats de simulations avec les résultats expérimentaux de la littérature apporte la preuve de concept que l'approche diphasique est pertinente pour étudier des configurations d'écoulements complexes instationnaire et tridimensionnelle. Les résultats de simulation sont ensuite analysés pour étudier la relation entre le flux local de transport de sédiments, la valeur de la contrainte fluide sur le fond et la pente locale du lit sédimentaire. La déviation par rapport aux résultats obtenus en écoulement uniforme permet d'identifier les mécanismes prépondérant de transport associées au tourbillon en fer à cheval, à la pente de fond et aux tourbillons lâchés dans le sillage du cylindre. Les résultats obtenus montrent une sensibilité à la résolution numérique en particulier à l'aval du cylindre illustrant le besoin de réaliser des simulations des grandes échelles turbulentes diphasiques
The work undertaken in this PhD thesis was to develop and use numerical models to investigate the multi-scale interactions between an offshore wind turbine and the local ocean and sediment dynamics. First, the interactions between the coupled ocean-sediment system and the atmospheric wake generated by an offshore wind turbine are investigated using an idealized two-dimensional model developed during this Phd thesis and written in fortran. The model integrates the shallow water equations for the ocean together with the Exner equation for the sediment bed. In a second part, the 3D scour phenomenon around a vertical cylinder in a steady current is studied using a two-phase flow eulerian-eulerian solver, sedFoam, written within the framework of the numerical toolbox OpenFOAM. The two-phase flow approach accounts for small-scale processes by avoiding the traditional assumptions made for sediment transport modeling, such as a local corre- lation between the sediment flux and the fluid bed shear stress.Regarding the atmospheric wake generated by a turbine, the results shows that its impact on the ocean’s surface can generate vortices. The resulting turbulent ocean dynamics is controlled by the wake parameter S = CdD/H, where D is the wake diameter at the impact location on the ocean surface, Cd is the quadratic friction coefficient between the ocean and the sediment and H is the oceanic layer depth. A turbulence parameterization based on S is proposed, allowing for upscaling simulations in larger scales Reynolds Averaged Navier-Stokes (RANS) models. It is shown that the ocean dynamics has an effect on the available wind power. The results also show that the instantaneous sediment dynamics is strongly coupled with the ocean one but that the overall seabed elevation variations remain small (a few millimeters/month). The morphodynamic impact of the wake is thus negligible.Concerning the two-phase flow simulation of scour, sedFoam is first validated on 1D and 2D configurations. Then, 3D simulations around a vertical cylindrical pile are presented. At first, a validation of the Unsteady Reynolds Averaged Navier-Stokes (URANS) turbulence model developed in this work is performed on a configuration without sediment. The results show that the vortices structures responsible for scouring, the Horse Shoe Vortex (HSV) and the vortex-shedding in the lee of the cylinder are correctly reproduced. Then, 3D two-phase flow simulations of the scour around a cylindrical pile have been carried out in a live-bed configuration. This work is the first attempt to model 3D scour phenomenon using the two-phase flow approach. Such simulations represent a real challenge in terms of high performance computing. The good agreement between the numerical predictions and the literature experimental results provide the proof of concept that the two-phase flow approach can be used to study complex 3D and unsteady flow configurations. The relationship between the local bed shear stress, the sediment flux and the local sediment bed slope is further investigated. The deviation of the results from a uniform flow configuration is further analyzed to identify the relevant sediment transport mechanisms associated with the HSV, the slope in the scour mark and the vortex-shedding downstream of the cylinder. Finally, the numerical results show a grid sensitivity of the morphological predictions in the lee of the cylinder that are most probably related to small-scale resolved vortical structures. This highlights the need for two-phase flow Large Eddy Simulations on this configuration in the future

Thesis (Ph. D.)--Georgia Institute of Technology, 2006.
Title from PDF t.p. (viewed on Oct. 28, 2006). Weissburg, Marc, Committee Chair ; Dusenbery, David, Committee Member ; Hay, Mark, Committee Member ; Webster, Donald, Committee Member ; Blanton, Jackson, Committee Member. Includes bibliographical references (p. 108-119).

DNS coupled with a Point-Particle based model (PP) is used to study and predict particle-turbulence interactions in an open channel flow at Reynolds number of 811 (based on the friction velocity) corresponding to the experimental observations of [Righetti & Romano, JFM 2004]. Large particles of diameter 200 microns (8.1 in wall units) with average volume loading on the order of 0.001 are simulated using four-way coupling with closure models for drag, added mass, lift, pressure, and inter-particle/particle-wall collision forces. The point-particle model is able to accurately capture the effect of particles on the fluid flow in the outer layer where particles are under resolved. However, the dynamical interaction of particle-turbulence is under predicted in the near wall region where particles size are much larger than Kolmogorov scale and grid resolution in wall-normal direction, but smaller in both stream and span wise directions. It is conjectured that due to the large size particles compared to the Kolmogorov length scale near the bed, the effect of disturbances and deflections in the flow due to presence of such large particles is not captured using Lagrangian Point-Particle approach. For this configuration, the point-particle model is not appropriate in the near wall region and a hybrid resolved particle approach may be necessary.

The process of sediment flushing is simulated by a 3D numerical model in which sediment and flow interaction are reflected in the reservoirs. RANS equations are solved numerically by finite volume with a k-ε turbulence model. The convection-diffusion equation for the sediment concentration is solved. The Van-Rijn’s reference level concentration equation is adopted as a boundary condition. Bed changes are obtained by application of sediment continuity equation. The numerical results show that the flushing efficiency is related to the outflow discharge, initial conditions of water level and number and location of the bottom outlets.

Flow scour is the engineering term used to describe the erosion of a sediment bed due to fluid flow. Local scour occurs around objects placed in the path of flow, such as bridge piers and abutments. Severe damage or even failure of structures may occur if the amount of scour is too great. Due to the complexity of the fluid/structure interactions and cost of experiments, Computation Fluid Dynamics (CFD) methods are under development to predict the shape and depth of a scour hole. This study extends a previous 3-D iterative methodology, with several improvements to the scouring physics models, implemented in the commercial CFD software STAR-CCM+ to predict the scour hole formation around circular bridge piers. These improvements are inclusion of a variable critical shear stress (VCSS) for the initiation of motion of bed sediment, scouring normal to the sediment bed, and a sand slide model. Reynolds Averaged Navier-Stokes (RANS) equations and a k-ε turbulence model are used to resolve the flow field. The methodology uses a single phase implicit unsteady approach to obtain sediment bed shear stress values. Two moving boundary relations are employed to model the erosion and sand slide physics. One for the erosion rate is based upon an empirical correlation for critical shear stress combined with a sediment entrainment function of Van Rijn, and the other uses the slope of the sediment bed, to iteratively displace the sediment bed in a way that decreases slope as long as it exceeds the angle of repose of the sediment. This is accomplished by a user defined function to move the sediment bed at each time step and the mesh morphing procedure built into STAR-CCM+ to solve fluid-structure interaction problems to stretch the existing mesh to maintain cell quality throughout the flow domain as the bed is displaced. Simulation results have been compared to experimental data found in literature. It was found that simulations over predict the maximum scour depth by up to 35%, but show a large improvement in capturing the overall shape of the scour hole in comparison to models that do not include a sand slide model.

Most granular flows at environmental conditions are unsteady and exhibit a complex physical behavior. Dune formation and migration in the desert are controlled not only by the flow of saltating particles over the sand bed, but also by turbulent atmospheric airflow. In fact, sediments are transported by the atmospheric airflow within a thin layer only a few centimeters above the sandy surface. These jumping particles reach a maximum sediment mass flux level at a certain delay time (known as the “saturation time”) after the initial movement by sliding and rolling begins. Unlike sediment transport in water where the particles are lifted by the turbulent suspension, the saltating particles are kept alive in the layer mainly due to particle-particle and particle-bed collisions. In order to model this Aeolian transport of sand, Jenkins and Pasini [1] proposed a two-fluid model (one-dimensional and steady state) using Granular Kinetic Theory (GKT) to describe the solid-phase stress. The present work extends the original idea of Jenkins and Pasini [1] by using a more robust model of GKT for the kinetic/collisional contributions to the solid-phase stress tensor, together with a friction model activated for sustained contacts between particles. In addition, a standard k-ε turbulence model for the air and a drag model for the interaction between the phases are employed. A rectangular 2D geometry was chosen with a logarithmic profile for the inlet air velocity, along with an initial amount of sand at rest in the lower part of the simulation domain, resembling the particle saltating flow commonly seen in the vertical middle plane within saltation wind tunnels. This model is validated with experimental data from Liu and Dong [2] and the results given by Pasini and Jenkins [1]. A good estimation for the particle erosion and mass flux in the saltation layer is predicted, even though the profiles of mass flux and concentration within the transport layer are very thin and lower.

Marine Hydrokinetic Turbines (MHkT) are a new class of renewable energy devices that harvest the kinetic energy of the flowing water in rivers or tides. In these environments, the approach flow contains elevated levels of free-stream turbulence (FST) and large coherent structures, which affect the performance and structural loading of the turbine as well as the signature of the downstream wake. Very little is understood about these interactions and how they cross-couple to impact river morphology, flood conveyance, and sediment transport. The current work uses controlled laboratory experiments to investigate the effects of FST on both component and system level metrics of MHK turbines. Homogeneous, free-stream turbulence levels ranging from 1% to 10% were achieved by employing a Makita type active-grid turbulence generator that is placed at the entrance of the water channel test section and is equipped with motor controlled winglet shafts. For component level measurements, loads acting on an MHK turbine hydrofoil at angles of attack ranging from −40° to +40° were measured using a two-axis load cell. Turbulence was shown to influence the stall angle of the hydrofoil. Stall occurred at ∼17° in the laminar free-stream but was postponed to ∼25° in the turbulent free-stream. An increase in the lift was observed at all angles greater than 10° with the increase being more significant in the post-stall regime. The drag measured in the turbulent free-stream was higher at angles less than 20° and greater than 25 ° but was lower at points in between. Concluding from the obtained lift to drag ratios, FST was observed to decrease performance while operating at angles less than the stall angle, and increase performance in the post-stall regime. For the system level measurements, a comparison of performance characteristics that includes the mean and standard deviations of the power coefficient (CP), and thrust coefficient (CT) between the turbulent and the laminar free-stream cases was performed. The results show a ∼2 fold increase in standard deviation of CT; however, elevated levels of FST have a weak effect on mean performance characteristics.