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Freshwater mussel populations have been declining at an alarming rate around the world. Herein, we investigate whether changing flow conditions, as they affect juvenile freshwater mussel settling, could be a potential causative factor for this decline in the Dan River, North Carolina, USA. We deployed two uplooking velocity sensors on the riverbed between May and November 2019: one where mussels reside and another where they do not. From this data, we calculated shear velocity, which is a measure of the turbulence that acts to lift particles into suspension and acts against particle settling. We determined that a shear velocity less than 0.66 cm/s would be required to settle relatively large and dense juvenile mussels onto the riverbed; however, the lowest shear velocity we measured was 0.9 cm/s. Additionally, we estimated that juvenile freshwater mussels as large as 280-510 µm could always be suspended and not be able to settle onto the riverbed at these two locations. Therefore, the flow during May-November 2019 was high enough to prohibit recruitment of juvenile freshwater mussels at the sensor locations. Furthermore, we have identified that the magnitude of the lowest flows has increased since 2000, which may be exacerbating the decline in freshwater mussels.

Freshwater mussel populations have been declining at an alarming rate around the world. Herein, we investigate whether changing flow conditions, as they affect juvenile freshwater mussel settling, could be a potential causative factor for this decline in the Dan River, North Carolina, USA. We deployed two uplooking velocity sensors on the riverbed between May and November 2019: one where mussels reside and another where they do not. From this data, we calculated shear velocity, which is a measure of the turbulence that acts to lift particles into suspension and acts against particle settling. We determined that a shear velocity less than 0.66 cm/s would be required to settle relatively large and dense juvenile mussels onto the riverbed; however, the lowest shear velocity we measured was 0.9 cm/s. Additionally, we estimated that juvenile freshwater mussels as large as 280-510 µm could always be suspended and not be able to settle onto the riverbed at these two locations. Therefore, the flow during May-November 2019 was high enough to prohibit recruitment of juvenile freshwater mussels at the sensor locations. Furthermore, we have identified that the magnitude of the lowest flows has increased since 2000, which may be exacerbating the decline in freshwater mussels.

Deep-sea mining Industry and academia are presently investigating the potential of deep-seamining for valuable resources such as poly-metallic nodules. Nevertheless, the process of manoeuvring, excavating, and processing these resources poses a threat to the deep-sea environment through the generation of plumes of suspended fine-grained solids, which have the potential to damage benthic fauna by smothering or burying them. Assessing the impact of plumes requires an understanding of how far they disperse through bottomcurrents and how their suspended loads change over time. The affected area may be overestimated by current numerical simulations as they neglect the crucial process of flocculation, whereby fine particles aggregate into larger and faster-sinking clumps. The effect of flocculation on plume dispersion is still not well comprehended, engendering ambiguity in monitoring techniques dependent on optronic and acoustic sensors that are sensitive to particle size but do not take into account flocculation. To tackle these concerns, it is essential to undertake research that centres on studying flocculation characteristics in marine sediments. This includes carrying out controlled laboratory experiments to gain a better understanding of the critical conditions for flocculation. Additionally, it involves scaling up these experiments to replicate real-world plume dispersion and devising precise measuring methods for flocculated particle suspensions. This thesis focuses only on benthic sediment plumes..., Environmental Fluid Mechanics

Particle transport and deposition in turbidity currents is governed by the balance between turbulent suspension and gravitational settling, with settling velocity becoming dominant during the final rain-out phases of decelerated turbidity currents on lobes. Differential particle settling velocities play a role in the sorting of grains in turbidity currents; there is a preference of grains with higher settling velocities to be deposited first, yielding a settling-velocity gradient in vertical and longitudinal cross-sections through turbidite beds. If sediments contain little variation in particle shape and density (for example, siliciclastics), then settling velocity is dominantly controlled by grain size. Carbonate sediments, in contrast, are composed of non-skeletal and skeletal grains with various growth structures, producing a wide distribution of particle shapes (from spheroidal to platy, bladed and elongated forms). The present paper aims to constrain the extent to which shape-dependent differential settling velocities influence sorting mechanisms in carbonate turbidity currents. Experiments using natural skeletal sand were conducted to investigate the settling of carbonate grains in: (i) isolation; (ii) suspension clouds; and (iii) turbidity currents. Size, density and shape parameters, including Corey Shape Factor and Zingg diagrams, were analysed using high-resolution micro-computed tomography. The slower settling of non-spheroidal shapes was quantified. In the sinking suspensions, a sorting mechanism operated through differential velocities yielding an abundance of spheroidal grains at the base and enrichment in less-spheroidal grains towards the top of suspension deposits. This trend was also observed longitudinally in carbonate turbidity currents, for which enhanced advection lengths caused less spheroidal grains to be transported farther into the basin. The effect of particle shape becomes increasingly significant as grain size increases, in particula

Particle transport and deposition in turbidity currents is governed by the balance between turbulent suspension and gravitational settling, with settling velocity becoming dominant during the final rain-out phases of decelerated turbidity currents on lobes. Differential particle settling velocities play a role in the sorting of grains in turbidity currents; there is a preference of grains with higher settling velocities to be deposited first, yielding a settling-velocity gradient in vertical and longitudinal cross-sections through turbidite beds. If sediments contain little variation in particle shape and density (for example, siliciclastics), then settling velocity is dominantly controlled by grain size. Carbonate sediments, in contrast, are composed of non-skeletal and skeletal grains with various growth structures, producing a wide distribution of particle shapes (from spheroidal to platy, bladed and elongated forms). The present paper aims to constrain the extent to which shape-dependent differential settling velocities influence sorting mechanisms in carbonate turbidity currents. Experiments using natural skeletal sand were conducted to investigate the settling of carbonate grains in: (i) isolation; (ii) suspension clouds; and (iii) turbidity currents. Size, density and shape parameters, including Corey Shape Factor and Zingg diagrams, were analysed using high-resolution micro-computed tomography. The slower settling of non-spheroidal shapes was quantified. In the sinking suspensions, a sorting mechanism operated through differential velocities yielding an abundance of spheroidal grains at the base and enrichment in less-spheroidal grains towards the top of suspension deposits. This trend was also observed longitudinally in carbonate turbidity currents, for which enhanced advection lengths caused less spheroidal grains to be transported farther into the basin. The effect of particle shape becomes increasingly significant as grain size increases, in particula

The settling velocities of 66 microplastic particle groups, having both regular (58) and irregular (eight) shapes, are measured experimentally. Regular shapes considered include: spheres, cylinders, disks, square plates, cubes, other cuboids (square and rectangular prisms), tetrahedrons, and fibers. The experiments generally consider Reynolds numbers greater than 102, extending the predominant range covered by previous studies. The present data is combined with an extensive data set from the literature, and the settling velocities are systematically analyzed on a shape-by-shape basis. Novel parameterizations and predictive drag coefficient formulations are developed for both regular and irregular particle shapes, properly accounting for preferential settling orientation. These are shown to be more accurate than the best existing predictive formulation from the literature. The developed method for predicting the settling velocity of irregularly-shaped microplastic particles is demonstrated to be equally well suited for natural sediments in the Appendix.

Settling and sedimentation of fine-grained sediments is a physical phenomenon influenced by bio-geochemical processes that occurs in natural water bodies such as rivers, channels and estuaries. Of particular concern are estuaries maintained for navigation. The deepening of waterways to improve the navigability for ever larger container ships has the potential to intensify sedimentation and the accumulation of cohesive sediments, particularly within the estuarine turbidity maximum (ETM). For example, significant net sedimentation and accumulation are observed in the ETM of the Weser estuary even in the centre of the navigational channel, where high flow velocities favour sediment transport and erosion. Dredging is the main method of maintaining a channel, which requires large financial investment and also has potential negative environmental impacts. In engineering practice, numerical modelling of the ETM and accumulation zones is an important tool to improve channel maintenance. Knowledge of site-specific information on settling velocities is therefore essential, as underlying processes cannot be simulated universally valid yet and require sufficient local parametrisation. The research project FAUST (For An improved Understanding of estuarine Sediment Transport) addressed the challenge of net sedimentation and accumulation in the navigational channel by investigating the transport properties of cohesive sediments (mainly from the Weser estuary) in field and in laboratory studies. The conceptual design of the project FAUST has been presented in Patzke et al. (2019). Research on sediment erodibility, sediment fractions and density profiles has been published recently (Patzke et al. 2021; Patzke et al. 2022). The follow-up project ELMOD (Simulation and analysis of the hydrological and morphological development of the Tidal Elbe for the period from 2013 to 2018) focuses on sediment transport processes in the Elbe estuary. The findings and derived model parameterisations, Bundesanstalt für Wasserbau

The settling velocities of 66 microplastic particle groups, having both regular (58) and irregular (eight) shapes, are measured experimentally. Regular shapes considered include: spheres, cylinders, disks, square plates, cubes, other cuboids (square and rectangular prisms), tetrahedrons, and fibers. The experiments generally consider Reynolds numbers greater than 102, extending the predominant range covered by previous studies. The present data is combined with an extensive data set from the literature, and the settling velocities are systematically analyzed on a shape-by-shape basis. Novel parameterizations and predictive drag coefficient formulations are developed for both regular and irregular particle shapes, properly accounting for preferential settling orientation. These are shown to be more accurate than the best existing predictive formulation from the literature. The developed method for predicting the settling velocity of irregularly-shaped microplastic particles is demonstrated to be equally well suited for natural sediments in the Appendix.

This study presents morphological data and estimates of the settling velocity and drag coefficient of sea stars (Asterias rubens) caught in the Limfjord, Denmark. A geometric model describing the sea star is presented and the thickness and arm width are determined as linear functions of arm length. The volume and mass of the sea stars is well predicted by the geometric model and is in agreement with the experimental measurements. The mean sea star density is determined to be 1095 kg/m3, the mean drag coefficient is estimated to be 2.3 and the settling velocity is shown to vary with the square root of the its size.

The Krone–Partheniades (K-P) framework has been used for decades to quantify and analyze the sediment exchange at a water–bed interface. Measuring the erosion and deposition parameters that are part of this framework requires time-consuming field observations. Additionally, the erosion parameters are measured independently of deposition parameters, while in reality they are coupled. In numerical models applying the K-P framework these parameters are often assumed to be constant in time and mutually independent. In this study, we develop a relatively simple methodology to determine the erosion and deposition parameters, using conventional near-bed observations of bed level, sediment concentration and flow velocity. This methodology is subsequently applied to tripod observations collected in the Changjiang estuary, China, to compute continuous time-varying erosion and settling parameters. We propose a diagram to visualize the interdependency and accuracy of erosion and deposition parameters, which is the input for K-P framework models requiring this interdependency, Coastal Engineering, Environmental Fluid Mechanics

Particle settling velocity is a fundamental parameter in sedimentology and engineering, and has accordingly received much attention in the literature. Grain properties, such as shape and drag coefficient, which affect terminal settling velocity, also control the threshold of initiation of motion and sediment entrainment into suspension. Terminal settling velocity therefore provides insights into sediment dynamics in modern and past depositional environments and is important for marine engineering works. Despite the global importance of resedimented carbonates the study of particle hydrodynamics is strongly biased towards terrigenous sediments. This paper presents a review of the settling hydrodynamics of carbonate grains and associated particle properties, such as shape, grain size and density. For carbonate grains these parameters are more complex than for siliciclastic counterparts due to their common biogenic origin, introducing a wide range of morphologies, densities and abrasion products as a result of the skeletal nature of such grains. This review includes an extensive database of published composition-specific settling velocities, as well as densities of common carbonate constituents. The database includes corals, coralline red algae, bivalves, brachiopods, gastropods, Halimeda green algae, bryozoans, crinoids, echinoderms, Alcyonarian spicules, numerous benthic and planktic foraminifers, and fecal pellets. Grain density as a function of skeletal structure and mineralogy exerts another control on settling velocity, with unclarity in density definitions hampering effective communication in the literature. The variation in single-grain hydrodynamic behaviour implies careful application of previously proposed equations for the prediction of settling velocity of bulk sand. Despite a firm basis there is a need for additional systematic composition-specific investigations to enable the adequate prediction of carbonate particle hydrodynamics due to the broad spe

In this study, the ability of the aerobic biogranulation process to remove organic matter, total nitrogen, and total phosphorous from wastewater was investigated. The effect of organic matter load on the quality of the produced biogranules and settling velocity of the generated biomass was also investigated. Four different aerobic scenarios were studied in sequential batch bioreactors (SBR) to identify the best operating conditions. The experimental setup consisted of four bioreactors containing 300 mL samples (100 mL of sludge and 200 mL of sweet whey at four different concentrations) under constant stirring (150 rpm), room temperature, and controlled pH (e.g., 7.98 ± 0.25). The different organic matter loads were calculated as the chemical oxygen demand (COD) concentration at 1157 (A), 719 (B), 357 (C), and 202 mg/L (D), respectively. The process consisted of cycles of initial feast followed by 72 h of famine. During the experiments, COD, total nitrogen (TN), and total phosphorus (TP) were measured at the beginning (t0) and the end of the process. The results confirmed that the best conditions were found in the bioreactor with the highest COD load (1157 mg/L), which achieved 98.7 % reduction in organic matter, 45.6 % reduction in TP, and 99.4 % reduction in TN after 9 days with final average sludge settling velocity of 0.7 m/h. The biogranulation process achieved improved clarification of the supernatant, showing increased settling velocity in all of the bioreactors. Microscopic observation confirmed uniform morphology (e.g., spherical or cylindrical) with the biggest biogranules being observed in bioreactors A and B. Organic load was found to play a significant role in biogranules formation and the achievement of the highest organic matter removal, sludge volume reduction, and water clarification. The overall trend in final sludge settling velocity measured in the different bioreactors was C > B > D > A, and in all of them settling was faster than t

Transgenic crops appear as a response to the concern of supplying the high food demand produced bypopulation growth. In this regard, different authors have argued that transgenics contaminate traditionalcrops genetically, causing the loss of native agricultural biodiversity. Although the signicance of thiswork does not contemplate the affectation of traditional crops, the importance of this type of study andits possible use for subsequent research is explained. e aim of this work was to determine the averagedistance reached by transgenic maize pollen grains in 2016 in the municipality of Tierralta, Córdoba,using the Hysplit program. e results show that with a concentration of 10-9 mg/m3, the annual averagedistance reached by maize pollen grains was 581 m, recording the highest average in May with a value of799 m, and the minimum average in March with a value of 474 m. e analysis of results considered that,at the beginning of 2016, Colombia was undergoing the 'El Niño' phenomenon, causing the maize pollengrains to travel less compared to a neutral year or one subjected to the 'La Niña' phenomenon. is is dueto the weak winds that occurred in the Colombian Caribbean region. By contrast, in the event of a neutralyear or one with the 'La Niña' phenomenon, the particles could have traveled more as the wind strengthwould have been above normal., Los cultivos transgénicos aparecen como respuesta a la preocupación por abastecer la gran demandaalimenticia derivada del crecimiento poblacional. Sin embargo, diferentes autores han argumentadoque los transgénicos contaminan genéticamente los cultivos tradicionales y ocasionan la pérdida de labiodiversidad agrícola nativa. Aunque este trabajo no aborda la afectación de los cultivos tradicionales,explica la importancia de este tipo de estudios y su posible uso para investigaciones posteriores. Estainvestigación tuvo como objetivo determinar mediante el programa Hysplitla distancia promedio quealcanzaron los granos de polen de maíz transgénico durante el año 2016 en el municipio de Tierralta,Córdoba. En los resultados, la concentración fue de 10-9 mg/m3 y el promedio anual de la distanciaalcanzada por lo granos de polen de maíz fue de 500 m. El promedio máximo se registró en el mes de mayocon un valor de 799 m y el promedio mínimo se obtuvo en marzo con un valor de 474 m. Para el análisis delos resultados, se consideró que a comienzos del año 2016, el fenómeno deEl Niño en Colombia generóque los granos de polen de maíz viajaran menos que durante un añoneutro o un año con el fenómenode La Niña, debido a los vientos débiles que se presentaronen el Caribe colombiano. En caso de habersepresentado un año neutro o con La Niña, las partículashabrían viajado más porque la fuerza de los vientosestaría por encima de lo normal.

Transgenic crops appear as a response to the concern of supplying the high food demand produced bypopulation growth. In this regard, different authors have argued that transgenics contaminate traditionalcrops genetically, causing the loss of native agricultural biodiversity. Although the signicance of thiswork does not contemplate the affectation of traditional crops, the importance of this type of study andits possible use for subsequent research is explained. e aim of this work was to determine the averagedistance reached by transgenic maize pollen grains in 2016 in the municipality of Tierralta, Córdoba,using the Hysplit program. e results show that with a concentration of 10-9 mg/m3, the annual averagedistance reached by maize pollen grains was 581 m, recording the highest average in May with a value of799 m, and the minimum average in March with a value of 474 m. e analysis of results considered that,at the beginning of 2016, Colombia was undergoing the 'El Niño' phenomenon, causing the maize pollengrains to travel less compared to a neutral year or one subjected to the 'La Niña' phenomenon. is is dueto the weak winds that occurred in the Colombian Caribbean region. By contrast, in the event of a neutralyear or one with the 'La Niña' phenomenon, the particles could have traveled more as the wind strengthwould have been above normal., Los cultivos transgénicos aparecen como respuesta a la preocupación por abastecer la gran demandaalimenticia derivada del crecimiento poblacional. Sin embargo, diferentes autores han argumentadoque los transgénicos contaminan genéticamente los cultivos tradicionales y ocasionan la pérdida de labiodiversidad agrícola nativa. Aunque este trabajo no aborda la afectación de los cultivos tradicionales,explica la importancia de este tipo de estudios y su posible uso para investigaciones posteriores. Estainvestigación tuvo como objetivo determinar mediante el programa Hysplitla distancia promedio quealcanzaron los granos de polen de maíz transgénico durante el año 2016 en el municipio de Tierralta,Córdoba. En los resultados, la concentración fue de 10-9 mg/m3 y el promedio anual de la distanciaalcanzada por lo granos de polen de maíz fue de 500 m. El promedio máximo se registró en el mes de mayocon un valor de 799 m y el promedio mínimo se obtuvo en marzo con un valor de 474 m. Para el análisis delos resultados, se consideró que a comienzos del año 2016, el fenómeno deEl Niño en Colombia generóque los granos de polen de maíz viajaran menos que durante un añoneutro o un año con el fenómenode La Niña, debido a los vientos débiles que se presentaronen el Caribe colombiano. En caso de habersepresentado un año neutro o con La Niña, las partículashabrían viajado más porque la fuerza de los vientosestaría por encima de lo normal.

Particle settling velocity is a fundamental parameter in sedimentology and engineering, and has accordingly received much attention in the literature. Grain properties, such as shape and drag coefficient, which affect terminal settling velocity, also control the threshold of initiation of motion and sediment entrainment into suspension. Terminal settling velocity therefore provides insights into sediment dynamics in modern and past depositional environments and is important for marine engineering works. Despite the global importance of resedimented carbonates the study of particle hydrodynamics is strongly biased towards terrigenous sediments. This paper presents a review of the settling hydrodynamics of carbonate grains and associated particle properties, such as shape, grain size and density. For carbonate grains these parameters are more complex than for siliciclastic counterparts due to their common biogenic origin, introducing a wide range of morphologies, densities and abrasion products as a result of the skeletal nature of such grains. This review includes an extensive database of published composition-specific settling velocities, as well as densities of common carbonate constituents. The database includes corals, coralline red algae, bivalves, brachiopods, gastropods, Halimeda green algae, bryozoans, crinoids, echinoderms, Alcyonarian spicules, numerous benthic and planktic foraminifers, and fecal pellets. Grain density as a function of skeletal structure and mineralogy exerts another control on settling velocity, with unclarity in density definitions hampering effective communication in the literature. The variation in single-grain hydrodynamic behaviour implies careful application of previously proposed equations for the prediction of settling velocity of bulk sand. Despite a firm basis there is a need for additional systematic composition-specific investigations to enable the adequate prediction of carbonate particle hydrodynamics due to the broad spe

The Mekong river is one of the largest rivers in the world, which flows through six countries of Southeast Asia (China, Myanmar, Laos, Thailand, Cambodia and Vietnam). Its hydro-sedimentary regime is changing rapidly, as a consequence of a regional shift of land use (agriculture, road, etc.), damming, sand mining and climate changes, among others. This study assesses the behavior of particles transported in suspension in the Lower Mekong River (LMR), along approximately 1700 km from fluvial to estuarine environments. Suspended sediment properties were estimated, simultaneously with hydrodynamic conditions, during three field campaigns. In addition, further investigations were performed in the laboratory to assess the structures of particles (flocculated or not), their capacity to flocculate (and the impacts on siltation), under a wide range of sediment concentration (20–30,000 mg.L−1). This study confirms that suspended sediment transported in the LMR are predominantly (75% by volume) flocculi (or freshly eroded soils aggregates), with median aggregated particle size in the range 10–20 μm and median settling velocity of the order of 0.01–0.1 mm s−1. These flocculi are robust under the hydrodynamic conditions (turbulence and suspended sediment concentration – SSC) existing in the LMR. Laboratory investigations reveal the existence of a threshold sediment concentration (400 mg.L−1), beyond which flocculation and sedimentation increase of orders of magnitudes. Thus, concentration that exceeds this threshold might promote the formation of so-called fluid mud layers. Because of the nonlinear response of flocculation and sedimentation with SSC and considering the ongoing changes at a regional scale in the LMR, higher occurrence of fluid mud layers in the fluvial upstream waterbodies might be anticipated, and a lower occurrence in estuaries and alongshore where the concentration decrease. The geomorphology could be impacted, with an over, Accepted author manuscript, Mathematical Physics

We study the sedimentation of finite-size particles in quiescent wall-bounded Newtonian and shear-thinning fluids by interface resolved numerical simulations. The suspended phase consists of Non-Brownian rigid spherical particles with particle to fluid density ratio ρp/ρf=1.5 at three different solid volume fractions Φ=1%, 5% and 20%. Firstly, to focus on the effect of shear-thinning on the particle dynamics and interactions, the Archimedes number is increased for a single particle to have the same settling speed in the Newtonian fluid as in the shear-thinning fluid. Secondly, we consider fixed Archimedes and vary the shear-thinning properties of the fluid. Overall, we report a twofold effect of shear thinning. First and more important, the substantial increase of the particle sedimentation velocity in the shear-thinning case due to the increase of the shear rate around the particles, which reduces the local viscosity leading to a reduced particle drag. Secondly, the shear-thinning fluid reduces the level of particle interactions, causing a reduction of velocity fluctuations and resulting in particles sedimenting at approximately the same speed. Moreover, the mean settling velocities decrease with the particle concentration as a consequence of the hindering effect. Particles tend to sediment in the middle of the channel, preferentially positioning in the wake of neighbouring particles or aside them, resulting in lower levels of fluid velocity fluctuations in the gravity direction in the shear-thinning fluid., QC 20201013

Sediment transport is one key process in sedimentation problems. The dynamics of sediments in rivers will lead to an ecological change of the catchment. Among all complicated processes in sediment transport, the settling dynamics of solid particles in the water contribute to the water quality and geomorphology of a river channel directly. This paper aims at an investigation of the interaction between particle movement and water flow, with application to hydro-environment situations such as transport of fine sand sediment in natural streams. The Stokes number in this type of application is much smaller than unity at which enhancement of particle settling velocity by strong ambient turbulence has been observed in many numerical and laboratory studies. The present laboratory study is conducted to study the settling of sub-millimeter sized heavy particles in water with a relatively weak grid-generated turbulence. The two-phase flow is measured with the whole-field imaging techniques of particle image velocimetry (PIV) and particle tracking velocimetry (PTV). A two-camera PIV/PTV technique is used to obtain the instantaneous settling velocities of the solid particles and the turbulent water velocity field. Phase separation is based on an effective optical distinction of the two light scattering signals with fluorescent tagging on the solid particles. With this technique, it is found that in most cases of grid-generated turbulence the settling velocity of the heavy particles is slightly lower than the still water value. The falling heavy particles are also found to cause additional turbulence in the water with low ambient turbulence intensities. At one particular case of grid turbulence, the particles are found to fall with a depth-averaged settling velocity evidently higher than the still water value. The instantaneous turbulent fluid motions and particle trajectories tend to support the fast tracking effect to be related to the higher particle falling velocities. © 2020

Sediment transport is one key process in sedimentation problems. The dynamics of sediments in rivers will lead to an ecological change of the catchment. Among all complicated processes in sediment transport, the settling dynamics of solid particles in the water contribute to the water quality and geomorphology of a river channel directly. This paper aims at an investigation of the interaction between particle movement and water flow, with application to hydro-environment situations such as transport of fine sand sediment in natural streams. The Stokes number in this type of application is much smaller than unity at which enhancement of particle settling velocity by strong ambient turbulence has been observed in many numerical and laboratory studies. The present laboratory study is conducted to study the settling of sub-millimeter sized heavy particles in water with a relatively weak grid-generated turbulence. The two-phase flow is measured with the whole-field imaging techniques of particle image velocimetry (PIV) and particle tracking velocimetry (PTV). A two-camera PIV/PTV technique is used to obtain the instantaneous settling velocities of the solid particles and the turbulent water velocity field. Phase separation is based on an effective optical distinction of the two light scattering signals with fluorescent tagging on the solid particles. With this technique, it is found that in most cases of grid-generated turbulence the settling velocity of the heavy particles is slightly lower than the still water value. The falling heavy particles are also found to cause additional turbulence in the water with low ambient turbulence intensities. At one particular case of grid turbulence, the particles are found to fall with a depth-averaged settling velocity evidently higher than the still water value. The instantaneous turbulent fluid motions and particle trajectories tend to support the fast tracking effect to be related to the higher particle falling velocities. © 2020

Sediment transport is one key process in sedimentation problems. The dynamics of sediments in rivers will lead to an ecological change of the catchment. Among all complicated processes in sediment transport, the settling dynamics of solid particles in the water contribute to the water quality and geomorphology of a river channel directly. This paper aims at an investigation of the interaction between particle movement and water flow, with application to hydro-environment situations such as transport of fine sand sediment in natural streams. The Stokes number in this type of application is much smaller than unity at which enhancement of particle settling velocity by strong ambient turbulence has been observed in many numerical and laboratory studies. The present laboratory study is conducted to study the settling of sub-millimeter sized heavy particles in water with a relatively weak grid-generated turbulence. The two-phase flow is measured with the whole-field imaging techniques of particle image velocimetry (PIV) and particle tracking velocimetry (PTV). A two-camera PIV/PTV technique is used to obtain the instantaneous settling velocities of the solid particles and the turbulent water velocity field. Phase separation is based on an effective optical distinction of the two light scattering signals with fluorescent tagging on the solid particles. With this technique, it is found that in most cases of grid-generated turbulence the settling velocity of the heavy particles is slightly lower than the still water value. The falling heavy particles are also found to cause additional turbulence in the water with low ambient turbulence intensities. At one particular case of grid turbulence, the particles are found to fall with a depth-averaged settling velocity evidently higher than the still water value. The instantaneous turbulent fluid motions and particle trajectories tend to support the fast tracking effect to be related to the higher particle falling velocities. © 2020

This dissertation aims to better understand how floc aggregate characteristics and behaviors are modified under different local conditions and how such alterations impact the floc settling velocity, which is one of the most crucial parameters influencing sediment transport modeling. A series of laboratory experiments were conducted to examine the impact of suspended sediment concentration, mixes of clay and silt, and resuspension process to equilibrium floc size and floc settling velocity. In order to observe floc size evolution, a new floc imaging acquisition was first developed. This new method allows flocs in suspended sediment concentration up to C = 400 mg/L can be imaged non intrusively. This new method was applied in all three individual studies, which are composed of this dissertation. The first chapter investigates the behaviors of flocs under constant and decay suspended sediment concentrations within a steady turbulent suspension. In the constant-concentration set of experiments, floc size time series were measured for 12 h for each of the concentration C = 15, 25, 50, 100, 200, 300, and 400 mg/L. In the decay-concentration experiments, clear water was introduced to the mixing tank, simultaneously the suspension was drained out of the mixing tank at the same rate to make the suspended sediment concentration reduce while the turbulent shear was remained unchanged. The data shows that the equilibrium floc size is a weak, positive function of concentration. For example, in order to increase 20% of floc size (approximate 22 um) the concentration needs to be increased by 700% (going from 50 to 400 mg/L). The data also illustrates that during the decrease of concentration from C = 400 to 50 mg/L, the floc size responses to the changes of concentration in the order of 10 min or less. The second chapter examines how silt particles and clay aggregates interact in a turbulent suspension. Floc sizes and settling velocity of three different suspensions, i.e., pure cl

Background and Aims The settling velocity of diaspores is a key parameter for the measurement of dispersal ability in wind-dispersed plants and one of the most relevant parameters in explicit dispersal models, but remains largely undocumented in bryophytes. The settling velocities of moss spores were measured and it was determined whether settling velocities can be derived from spore diameter using Stokes' Law or if specific traits of spore ornamentation cause departures from theoretical expectations. Methods A fall tower design combined with a high-speed camera was used to document spore settling velocities in nine moss species selected to cover the range of spore diameters within the group. Linear mixed effect models were employed to determine whether settling velocity can be predicted from spore diameter, taking specific variation in shape and surface roughness into account. Key Results Average settling velocity of moss spores ranged from 0 center dot 49 to 8 center dot 52 cm s(-1). There was a significant positive relationship between spore settling velocity and size, but the inclusion of variables of shape and texture of spores in the best-fit models provides evidence for their role in shaping spore settling velocities. Conclusions Settling velocities in mosses can significantly depart from expectations derived from Stokes' Law. We suggest that variation in spore shape and ornamentation affects the balance between density and drag, and results in different dispersal capacities, which may be correlated with different life-history traits or ecological requirements. Further studies on spore ultrastructure would be necessary to determine the role of complex spore ornamentation patterns in the drag-to-mass ratio and ultimately identify what is the still poorly understood function of the striking and highly variable ornamentation patterns of the perine layer on moss spores.

Settling tests were performed on particles of NUKON fiberglass to relate the drag coefficient of the particles to the Reynolds number of the particles. A new method was developed to measure fiberglass particles. The projected area, projected perimeter, and average height of the particles are measured using this method. The measurements are used to calculate the measured drag coefficient and measured Reynolds number for the particles. Data collected was compared to previous studies that focused on the settling of sand grains. A predictive correlation that was developed for sand grains was applied towards the particles of fiberglass. Tests were run with dyed particles of fiberglass as a means of flow visualization. The results from these visualization tests were compared with results from the free settling of disks.

This study focuses on deposition processes from sediment laden buoyant river plumes in deltaic regions. The goal is to experimentally examine the effects of various physical phenomena influencing the rate at which sediment is removed from the plume. Previous laboratory and field measurements have suggested that, at times, sedimentation can take place at rates higher than that expected from individual particle settling (i.e., C{W}_{s}). Two potential drivers of enhanced sedimentation are flocculation and interfacial instabilities. We experimentally measured the sediment fluxes from each of these processes using two sets of laboratory experiments that investigate two different modes of instability, one driven by sediment settling and one driven by fluid shear. The settling-driven and shear-driven instability sets of experiments were carried out in a stagnant stratification tank and a stratification flume respectively. In both sets, continuous interface monitoring and concentration measurement were made to observe developments of instabilities and their effects on the removal of sediment. Floc size was measured during the experiments using a separate floc camera setup and image analysis routines. Results from the stratification tank experiments suggest that the settling-driven gravitational instabilities do occur in the presence of flocs, and that they can produce sedimentation rates higher than those predicted from floc settling. A simple cylinder based force balance approach adopting the concept of critical Grashof number was used to develop a model for the effective settling velocity under settling-driven instabilities that is a function of sediment concentration in the plume only. Results from the stratification flume experiments show that under shear instabilities, the effective settling velocity is greater than the floc settling velocity, and increases with plume velocity and interface mixing. The difference between effective and floc settling velocity was denote

Settling tests were performed on particles of NUKON fiberglass to relate the drag coefficient of the particles to the Reynolds number of the particles. A new method was developed to measure fiberglass particles. The projected area, projected perimeter, and average height of the particles are measured using this method. The measurements are used to calculate the measured drag coefficient and measured Reynolds number for the particles. Data collected was compared to previous studies that focused on the settling of sand grains. A predictive correlation that was developed for sand grains was applied towards the particles of fiberglass. Tests were run with dyed particles of fiberglass as a means of flow visualization. The results from these visualization tests were compared with results from the free settling of disks.

[Background and Aims] The settling velocity of diaspores is a key parameter for the measurement of dispersal ability in wind-dispersed plants and one of the most relevant parameters in explicit dispersal models, but remains largely undocumented in bryophytes. The settling velocities of moss spores were measured and it was determined whether settling velocities can be derived from spore diameter using Stokes' Law or if specific traits of spore ornamentation cause departures from theoretical expectations., [Methods] A fall tower design combined with a high-speed camera was used to document spore settling velocities in nine moss species selected to cover the range of spore diameters within the group. Linear mixed effect models were employed to determine whether settling velocity can be predicted from spore diameter, taking specific variation in shape and surface roughness into account., [Key Results] Average settling velocity of moss spores ranged from 0·49 to 8·52 cm s. There was a significant positive relationship between spore settling velocity and size, but the inclusion of variables of shape and texture of spores in the best-fit models provides evidence for their role in shaping spore settling velocities., [Conclusions] Settling velocities in mosses can significantly depart from expectations derived from Stokes' Law. We suggest that variation in spore shape and ornamentation affects the balance between density and drag, and results in different dispersal capacities, which may be correlated with different life-history traits or ecological requirements. Further studies on spore ultrastructure would be necessary to determine the role of complex spore ornamentation patterns in the drag-to-mass ratio and ultimately identify what is the still poorly understood function of the striking and highly variable ornamentation patterns of the perine layer on moss spores.

The aim of the present study is to investigate the sedimentation of large non-colloidal spherical particles in both quiescent and turbulent environments. To this aim, Direct Numerical Simulations are performed using an Immersed Boundary Method to account for the dispersed phase. The solid volume fractions considered are in the range φ = 0.005−0.010, while the solid to fluid density ratio ρp/ρf is set equal to 1.02. The particle diameter is chosen to be approximately 12 Komlogorov lengthscales in nominal conditions. The results show that the mean settling velocities Vt decrease in the turbulent cases. The overall drag is increased both by the non-linear finite Reynolds number behavior and by unsteady effects, which are negligible in quiescent cases., QC 20200616

This study presented in this book investigates micro- and macro- scale physical processes of a large-scale fine sediment estuarine system with a moderate tidal range as well as a highly seasonal-varying freshwater inflow. Based on a series measured, experimented and modeled results, the research highlights that (i) along-channel fresh-salt gradient near an estuarine turbidity maximum zone is a key parameter controlling local density stratification and sedimentation in the channel; (ii) the salinity-induced baroclinic pressure gradient forces are a major factor impacting internal velocity and suspended sediment concentration (SSC) structures; (iii) vertical profiles of current, salinity and SSC within a river plume are dependent on a correct prediction of the development of turbulence; (iv) both suspended particulate matter availability and local residual flow regime are of critical importance for trapping probability of sediment and the occurrence of fluid mud; (v) river discharge impacts the horizontal and vertical distribution of residual current; (vi) seasonally varying wind effect alters the residual currents near the riverine limit; (vii) seasonally varying mean sea level and wind climate jointly shape the saltwater intrusion length near the estuarine front., Hydraulic Engineering, Civil Engineering and Geosciences

This study presented in this book investigates micro- and macro- scale physical processes of a large-scale fine sediment estuarine system with a moderate tidal range as well as a highly seasonal-varying freshwater inflow. Based on a series measured, experimented and modeled results, the research highlights that (i) along-channel fresh-salt gradient near an estuarine turbidity maximum zone is a key parameter controlling local density stratification and sedimentation in the channel; (ii) the salinity-induced baroclinic pressure gradient forces are a major factor impacting internal velocity and suspended sediment concentration (SSC) structures; (iii) vertical profiles of current, salinity and SSC within a river plume are dependent on a correct prediction of the development of turbulence; (iv) both suspended particulate matter availability and local residual flow regime are of critical importance for trapping probability of sediment and the occurrence of fluid mud; (v) river discharge impacts the horizontal and vertical distribution of residual current; (vi) seasonally varying wind effect alters the residual currents near the riverine limit; (vii) seasonally varying mean sea level and wind climate jointly shape the saltwater intrusion length near the estuarine front., Hydraulic Engineering, Civil Engineering and Geosciences

The urban sprawl creates a gap between producers and consumers and the a sustainable circuitof nutrients and energy is difficult to maintain. Many times the waste that is created in urbanareas is not reused and the circuit is lost. In this project, wastewater treatment is looked atwith the view point that resource recovery is possible through energy production and reuse ofnutrients. In order to optimally run each process step at a wastewater treatment plant forimproved resource recovery, more knowledge is needed in order to not disregard the finaleffluent quality. The goal of this project was to develop a model in MATLAB/Simulink for achemically enhanced primary clarifier at a wastewater treatment plant. The potential ofproducing more biogas and reducing the aeration energy needed in the biological treatmentstep was looked at by focusing on describing the settling velocity of suspended solids.Experimental analysis on settling properties for solids was performed on sampled wastewaterentering the primary settler after changing the addition of chemicals prior in the process line.The wastewater samples were homogenized and then rapidly vacuum pumped up in a column.The solids in the column could thereafter settle and was retained in a cup at the bottom. Themass of total suspended solids (TSS) was classified in five different settling velocity classes,each class assigned a characteristic settling velocity. The experimental procedure followed theViCA's protocol (French acronym for Settling Velocity for Wastewater). A settler, much likethe secondary settler in the Benchmark Simulation Model No. 2 (BSM2), a 10 layer nonreactivetank was modeled. The mass balance in each layer of the settler was decided by thevertical solid flux in the tank and built on the characteristic settling velocity gained from theexperiments. Re-circulation of excess sludge from the subsequent steps at the plant showed toeffect the settling properties of the sludge in the primary settler. The component, När urbana områden växer uppstår svårigheter i att bibehålla ett hållbart kretslopp av energioch näringsämnen. Avståndet mellan producent och konsument ökar och många gångeråteranvänds inte det avfall som städerna producerar och det hållbara kretsloppet bryts. Dettaprojekt har fokuserat på resursåteranvändningen i avloppsvattenhanteringen genommöjligheterna som finns i energiproduktion i form av biogas samt återanvändning avnäringsämnen genom slamåterförsel. Mer kunskap behövs inom varje processteg för attoptimalt använda avloppsreningsverk för förbättrad resurs-återvinning så att inte utgåendevattenkvalitet blir lidande. Målet med projektet var att utveckla en modell iMATLAB/Simulink för primärsedimentering med kemisk fällning. Experimentelltanalyserades sedimentationsegenskaperna hos primärslam genom provtagning avavloppsvatten inkommande till försedimenteringen efter tillsatser av fällnings-kemikalier.Proverna homogeniserades och vakuumpumpades sedan snabbt upp i en kolonn. Detpartikulära materialet i kolonnen kunde därefter sedimentera och fångades upp i en kopp ibotten. Den sedimenterade massan av totalt suspenderat material (TSS) klassificerades i femolika sedimenteringshastighetsklasser och varje klass tilldelades en karakteristisksedimentationshastighet Det experimentella förfarandet följde ViCA’s protokoll (franskförkortning för sedimentationshastigheter för avloppsvatten). En modell av ensedimentationstank, ungefär som för sekundär-sedimenteringen i Benchmark SimulationModel No. 2 (BSM2), utvecklades som en 10 lager icke reaktiv tank. Massbalansen i varjelager bestämdes av det vertikala flödet av partiklar och beräknades med de experimentelltframtagna karakteristiska sedimentationshastigheterna. Återcirkulering av överskottsslam frånde efterföljande reningsstegen visade sig ha stor påverkan på slammetssedimentationsegenskaper i försedimenteringen. Typen av TSS-komponenter hade den störstainverkan på fördelningen av sedimentationshastigheter. Variationen i, I samarbete med forskningsgruppen ModelEAU, Quebec, Kanada

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Marine Science and Engineering 2 (2014): 413-436, doi:10.3390/jmse2020413., The Community Sediment Transport Modeling System (CSTMS) cohesive bed sub-model that accounts for erosion, deposition, consolidation, and swelling was implemented in a three-dimensional domain to represent the York River estuary, Virginia. The objectives of this paper are to (1) describe the application of the three-dimensional hydrodynamic York Cohesive Bed Model, (2) compare calculations to observations, and (3) investigate sensitivities of the cohesive bed sub-model to user-defined parameters. Model results for summer 2007 showed good agreement with tidal-phase averaged estimates of sediment concentration, bed stress, and current velocity derived from Acoustic Doppler Velocimeter (ADV) field measurements. An important step in implementing the cohesive bed model was specification of both the initial and equilibrium critical shear stress profiles, in addition to choosing other parameters like the consolidation and swelling timescales. This model promises to be a useful tool for investigating the fundamental controls on bed erodibility and settling velocity in the York River, a classical muddy estuary, provided that appropriate data exists to inform the choice of model parameters., Funding by the National Science Foundation (OCE-1061781 and OCE-0536572) supported Fall, Harris, Friedrichs, and Rinehimer.

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Marine Science and Engineering 2 (2014): 413-436, doi:10.3390/jmse2020413., The Community Sediment Transport Modeling System (CSTMS) cohesive bed sub-model that accounts for erosion, deposition, consolidation, and swelling was implemented in a three-dimensional domain to represent the York River estuary, Virginia. The objectives of this paper are to (1) describe the application of the three-dimensional hydrodynamic York Cohesive Bed Model, (2) compare calculations to observations, and (3) investigate sensitivities of the cohesive bed sub-model to user-defined parameters. Model results for summer 2007 showed good agreement with tidal-phase averaged estimates of sediment concentration, bed stress, and current velocity derived from Acoustic Doppler Velocimeter (ADV) field measurements. An important step in implementing the cohesive bed model was specification of both the initial and equilibrium critical shear stress profiles, in addition to choosing other parameters like the consolidation and swelling timescales. This model promises to be a useful tool for investigating the fundamental controls on bed erodibility and settling velocity in the York River, a classical muddy estuary, provided that appropriate data exists to inform the choice of model parameters., Funding by the National Science Foundation (OCE-1061781 and OCE-0536572) supported Fall, Harris, Friedrichs, and Rinehimer.

The urban sprawl creates a gap between producers and consumers and the a sustainable circuitof nutrients and energy is difficult to maintain. Many times the waste that is created in urbanareas is not reused and the circuit is lost. In this project, wastewater treatment is looked atwith the view point that resource recovery is possible through energy production and reuse ofnutrients. In order to optimally run each process step at a wastewater treatment plant forimproved resource recovery, more knowledge is needed in order to not disregard the finaleffluent quality. The goal of this project was to develop a model in MATLAB/Simulink for achemically enhanced primary clarifier at a wastewater treatment plant. The potential ofproducing more biogas and reducing the aeration energy needed in the biological treatmentstep was looked at by focusing on describing the settling velocity of suspended solids.Experimental analysis on settling properties for solids was performed on sampled wastewaterentering the primary settler after changing the addition of chemicals prior in the process line.The wastewater samples were homogenized and then rapidly vacuum pumped up in a column.The solids in the column could thereafter settle and was retained in a cup at the bottom. Themass of total suspended solids (TSS) was classified in five different settling velocity classes,each class assigned a characteristic settling velocity. The experimental procedure followed theViCA's protocol (French acronym for Settling Velocity for Wastewater). A settler, much likethe secondary settler in the Benchmark Simulation Model No. 2 (BSM2), a 10 layer nonreactivetank was modeled. The mass balance in each layer of the settler was decided by thevertical solid flux in the tank and built on the characteristic settling velocity gained from theexperiments. Re-circulation of excess sludge from the subsequent steps at the plant showed toeffect the settling properties of the sludge in the primary settler. The component, När urbana områden växer uppstår svårigheter i att bibehålla ett hållbart kretslopp av energioch näringsämnen. Avståndet mellan producent och konsument ökar och många gångeråteranvänds inte det avfall som städerna producerar och det hållbara kretsloppet bryts. Dettaprojekt har fokuserat på resursåteranvändningen i avloppsvattenhanteringen genommöjligheterna som finns i energiproduktion i form av biogas samt återanvändning avnäringsämnen genom slamåterförsel. Mer kunskap behövs inom varje processteg för attoptimalt använda avloppsreningsverk för förbättrad resurs-återvinning så att inte utgåendevattenkvalitet blir lidande. Målet med projektet var att utveckla en modell iMATLAB/Simulink för primärsedimentering med kemisk fällning. Experimentelltanalyserades sedimentationsegenskaperna hos primärslam genom provtagning avavloppsvatten inkommande till försedimenteringen efter tillsatser av fällnings-kemikalier.Proverna homogeniserades och vakuumpumpades sedan snabbt upp i en kolonn. Detpartikulära materialet i kolonnen kunde därefter sedimentera och fångades upp i en kopp ibotten. Den sedimenterade massan av totalt suspenderat material (TSS) klassificerades i femolika sedimenteringshastighetsklasser och varje klass tilldelades en karakteristisksedimentationshastighet Det experimentella förfarandet följde ViCA’s protokoll (franskförkortning för sedimentationshastigheter för avloppsvatten). En modell av ensedimentationstank, ungefär som för sekundär-sedimenteringen i Benchmark SimulationModel No. 2 (BSM2), utvecklades som en 10 lager icke reaktiv tank. Massbalansen i varjelager bestämdes av det vertikala flödet av partiklar och beräknades med de experimentelltframtagna karakteristiska sedimentationshastigheterna. Återcirkulering av överskottsslam frånde efterföljande reningsstegen visade sig ha stor påverkan på slammetssedimentationsegenskaper i försedimenteringen. Typen av TSS-komponenter hade den störstainverkan på fördelningen av sedimentationshastigheter. Variationen i, I samarbete med forskningsgruppen ModelEAU, Quebec, Kanada

Particulate wastes (uneaten feed and faeces) are assumed to cause the most intense impact on the benthic community beneath aquaculture cages. Settling velocity of uneaten feed pellets and faecal material, required as model input data, represents a key parameter for waste dispersion models. In this study, settling velocity rates of two commercial meagre (Argyrosomus regius) feed pellets (EFICO Sigma 578 — Biomar 9 and 12 mm) and faecal material from two size categories of cultured meagre (small: 0.821 ± 0.157 kg; large: 1.663 ± 0.371 kg) were determined. Settling velocity for Biomar 9 and 12 pellets followed a normal distribution, with mean values of 9.83 ± 0.17 cm s−1 (n = 78) and 9.67 ± 0.28 cm s−1 (n = 76) respectively and 9.75 ± 0.24 cm s−1 for all data (n = 154). On the contrary, faecal particle settling velocity data did not comply with normal distribution assumptions. Settling rates were not significantly different (P = 0.37) between the two meagre size categories, but significant variation (P b 0.001) in settling velocity between all interval categories was found. Settling velocities generally increase with particle size. Particles with slow settling velocities (b1 cm s−1 ) dominated samples (87.0%) and fast settling velocities (2–3 cm s−1 ) were uncommon, with values of 1.1%. Yet, particles with small settling velocities (b1 cm s−1 ) only account for 38% of the mass. Particles with medium settling velocities (1–2 cm s−1 ), in which frequency distribution was much lower (12%) had the highest mass distribution, 51%. Waste dispersion modelling scenarios based on specific meagre settling feed and faeces settling velocity data, resulted in a wider dispersion area and lower flux values, hence in a smaller severity of predicted deposition, when compared with model output scenarios based on non-specific sea bass (Dicentrarchus labrax) and sea bream (Sparus aurata) settling velocity data.

A macro-etching method has been used to analyze the distribution and amount of inclusions along billets and on cross sections. Main parameters that have been varied are holding time before casting and amount of liquid remaining after casting. The result show that short holding times, in the order of 10 minutes, give increased amount of inclusions in the beginning of the billets, but holding times in the range from 30 to 60 minutes do not show any significant differences. If the melt remaining in the furnace after casting is less than about 3000 kg, the inclusion density increases towards the end of the ingots. The distribution of inclusions over the cross section of billets show that most inclusions are found in the centre of the billets, however, at increased total amount of inclusions, they tend to appear evenly over the whole cross sections. The results are discussed based on convection in furnace and settling rates and convection at solidification front., Source: Scopus

A computational fluid dynamics simulation is presented of a basic open feedwell with shelf design using the flocculants, Rheomax 1050 and Magnafloc 336, with a fine-particle-size calcite (Omyacarb 5) to emphasise the impact of the flocculation process on flow fields at the feedwell exit. The simulations described use CFX-4.4 two-phase flow formulation incorporating equations for a population balance model of the flocculation process. The feedwell modelling results indicated that Rheomax 1050 produced the largest mean aggregate size at a solid fraction of 10 wt%, whereas Magnafloc 336 peak size was at, or below, a solid fraction of 5 wt%. The increased aggregate density for flocculation with Rheomax 1050 gave rise to higher settling velocities for this flocculant., A computational fluid dynamics simulation is presented of a basic open feedwell with shelf design using the flocculants, Rheomax 1050 and Magnafloc 336, with a fine-particle-size calcite (Omyacarb 5) to emphasise the impact of the flocculation process on flow fields at the feedwell exit. The simulations described use CFX-4.4 two-phase flow formulation incorporating equations for a population balance model of the flocculation process. The feedwell modelling results indicated that Rheomax 1050 produced the largest mean aggregate size at a solid fraction of 10 wt%, whereas Magnafloc 336 peak size was at, or below, a solid fraction of 5 wt%. The increased aggregate density for flocculation with Rheomax 1050 gave rise to higher settling velocities for this flocculant.

Mixed silicon and silicon carbide micropowders, obtained from the polycrystalline silicon ingot saw-cutting industry, were separated by a simple sequential sedimentation varying the pH with dilute HCl and NaOH solutions. The dispersion and aggregation of the microparticles is significantly influenced by pH, so their settlement velocity under the same hydraulic conditions is quite different. Silicon powders tend to remain in the upper section of the suspension because of their finer size and smaller density. From the originally received powder mixture of 30% Si with 70% SiC, after 4 sedimentation cycles at pH 8 a mixture of 62% Si with only 38% SiC was collected from the upper suspension, showing great potential for Si/SiC separation and recovery in a cost-effective way., Mixed silicon and silicon carbide micropowders, obtained from the polycrystalline silicon ingot saw-cutting industry, were separated by a simple sequential sedimentation varying the pH with dilute HCl and NaOH solutions. The dispersion and aggregation of the microparticles is significantly influenced by pH, so their settlement velocity under the same hydraulic conditions is quite different. Silicon powders tend to remain in the upper section of the suspension because of their finer size and smaller density. From the originally received powder mixture of 30% Si with 70% SiC, after 4 sedimentation cycles at pH 8 a mixture of 62% Si with only 38% SiC was collected from the upper suspension, showing great potential for Si/SiC separation and recovery in a cost-effective way.

The sedimentation behavior of silt particles in the hindered settling regime has been considered. Therefore, laboratory experiments are carried in especially designed settling columns. Silt-water mixtures are prepared and allowed to settle at various initial concentrations. During the settling process, a continuous vertical concentration profile is measured. Settling velocities are obtained from these profiles and are compared with the widely used Richardson and Zaki (1954) expression. This comparison showed that the Richardson and Zaki expression underestimates the settling velocities for the finest silts at high concentrations. For the coarsest silt the Richardson and Zaki expression performed will with the measured data. We anticipate that this is the effect of the particle size on the apparent viscosity of the settling silt-water mixture., Hydraulic Engineering, Civil Engineering and Geosciences

This research has tried to find a relation between particle settling velocity and shape. This was done by taking the settling velocity log-contrast of a non-spherical particle and an equivalent spherical particle. This log-contrast should be the logarithmic function of a certain shape factor. Settling data of several shapes were taken from previous authors and with those data several existing and new shape factors were tested to form a single relation. Recently a simple equation was developed which can explicitly predict the settling velocity of spherical particles for any Reynolds number. This equation was tested to be fairly accurate for spherical data. However, the spherical data showed a systematic error which needs to be resolved for the equation to be perfectly accurate. The equation also showed to be fairly accurate for non-spherical sieve diameter data. However sieve diameter data cannot be used to define the shape of a particle as there is no direct relation between sieve diameter and actual particle size and shape. In order to find a relation between particle settling velocity and shape, a correct shape factor must be found. For that matter several shape factors from previous authors were tested on data directly available from previous authors. The existing shape factors were proved to not be able to combine all investigated shapes into a single relation for settling velocity and a new shape factor was found with which a single relation might be formed. This new equation is a generalization of the Corey Shape Factor, the Boekhout Shape Factor: BSF=D_s/(?D_l?^n D_i^(1-n) ). However due to the small amount of data, important data gaps exist so that nothing can be concluded with certainty. Therefore more settling experiments should be done over a large range of Reynolds numbers to be able to conclude whether the Boekhout Shape Factor is the correct link between particle settling velocity and shape., Applied Geology, Geoscience and Engineering, Civil Engineering and Geosciences

The sedimentation behavior of silt particles in the hindered settling regime has been considered. Therefore, laboratory experiments are carried in especially designed settling columns. Silt-water mixtures are prepared and allowed to settle at various initial concentrations. During the settling process, a continuous vertical concentration profile is measured. Settling velocities are obtained from these profiles and are compared with the widely used Richardson and Zaki (1954) expression. This comparison showed that the Richardson and Zaki expression underestimates the settling velocities for the finest silts at high concentrations. For the coarsest silt the Richardson and Zaki expression performed will with the measured data. We anticipate that this is the effect of the particle size on the apparent viscosity of the settling silt-water mixture., Hydraulic Engineering, Civil Engineering and Geosciences

This research has tried to find a relation between particle settling velocity and shape. This was done by taking the settling velocity log-contrast of a non-spherical particle and an equivalent spherical particle. This log-contrast should be the logarithmic function of a certain shape factor. Settling data of several shapes were taken from previous authors and with those data several existing and new shape factors were tested to form a single relation. Recently a simple equation was developed which can explicitly predict the settling velocity of spherical particles for any Reynolds number. This equation was tested to be fairly accurate for spherical data. However, the spherical data showed a systematic error which needs to be resolved for the equation to be perfectly accurate. The equation also showed to be fairly accurate for non-spherical sieve diameter data. However sieve diameter data cannot be used to define the shape of a particle as there is no direct relation between sieve diameter and actual particle size and shape. In order to find a relation between particle settling velocity and shape, a correct shape factor must be found. For that matter several shape factors from previous authors were tested on data directly available from previous authors. The existing shape factors were proved to not be able to combine all investigated shapes into a single relation for settling velocity and a new shape factor was found with which a single relation might be formed. This new equation is a generalization of the Corey Shape Factor, the Boekhout Shape Factor: BSF=D_s/(?D_l?^n D_i^(1-n) ). However due to the small amount of data, important data gaps exist so that nothing can be concluded with certainty. Therefore more settling experiments should be done over a large range of Reynolds numbers to be able to conclude whether the Boekhout Shape Factor is the correct link between particle settling velocity and shape., Applied Geology, Geoscience and Engineering, Civil Engineering and Geosciences

Stokes’ law that is traditionally used for modeling the sedimentation of flocs, incorrectly assumes that the floc is solid and spherical. Consequently the settling rates of flocs cannot be estimated using the Stokes law. The application of fractal dimensions to study the internal structure and settling of flocs formed in lime softening process was investigated. An optical microscope with motorized stage was used to measure the fractal dimensions of lime softening flocs directly on their images in 2 and 3D space. The fractal dimensions of the lime softening flocs were 1.15-1.27 for floc boundary, 1.49-1.90 for cross-sectional area and 2.55-2.99 for floc volume. Free settling tests were used for indirect determination of 3D fractal dimension. The measured settling velocity of flocs ranged from 0.1 to 7.1 mm/s (average: 2.37 mm/s) for the flocs with equivalent diameters from 10µm to 260µm (average: 124 µm). Floc settling model incorporating variable floc fractal dimensions as well as variable primary particle size was found to describe the settling velocity of large (>60 µm) lime softening flocs better than Stokes’ law. Settling velocities of smaller flocs (<60 µm) could still be quite well predicted by the Stokes’ law. The variation of fractal dimensions with lime floc size in this study indicated that two mechanisms are involved in the formation of these flocs: cluster-cluster aggregation for small flocs (>60 µm) and diffusion-limited aggregation for large flocs (<60 µm). Therefore, the relationship between the floc fractal dimension and floc size appears to be determined by floc aggregation mechanisms. The settling velocity of lime softening flocs was also modeled by a general model that assumes multiple normally distributed fractal dimensions for each floc size. The settling velocities were in the range of 0-10mm/s and in good agreement with measured settling velocities (0.1-7.1mm/s). The Stokes’ law overestimates the settling velocity of lime flocs. It seems that

Stokes’ law that is traditionally used for modeling the sedimentation of flocs, incorrectly assumes that the floc is solid and spherical. Consequently the settling rates of flocs cannot be estimated using the Stokes law. The application of fractal dimensions to study the internal structure and settling of flocs formed in lime softening process was investigated. An optical microscope with motorized stage was used to measure the fractal dimensions of lime softening flocs directly on their images in 2 and 3D space. The fractal dimensions of the lime softening flocs were 1.15-1.27 for floc boundary, 1.49-1.90 for cross-sectional area and 2.55-2.99 for floc volume. Free settling tests were used for indirect determination of 3D fractal dimension. The measured settling velocity of flocs ranged from 0.1 to 7.1 mm/s (average: 2.37 mm/s) for the flocs with equivalent diameters from 10µm to 260µm (average: 124 µm). Floc settling model incorporating variable floc fractal dimensions as well as variable primary particle size was found to describe the settling velocity of large (>60 µm) lime softening flocs better than Stokes’ law. Settling velocities of smaller flocs (<60 µm) could still be quite well predicted by the Stokes’ law. The variation of fractal dimensions with lime floc size in this study indicated that two mechanisms are involved in the formation of these flocs: cluster-cluster aggregation for small flocs (>60 µm) and diffusion-limited aggregation for large flocs (<60 µm). Therefore, the relationship between the floc fractal dimension and floc size appears to be determined by floc aggregation mechanisms. The settling velocity of lime softening flocs was also modeled by a general model that assumes multiple normally distributed fractal dimensions for each floc size. The settling velocities were in the range of 0-10mm/s and in good agreement with measured settling velocities (0.1-7.1mm/s). The Stokes’ law overestimates the settling velocity of lime flocs. It seems that

The applications of thickeners in hydrometallurgy include pre-leach thickening to minimise water in leach feed slurry, countercurrent decantation thickening to separate and wash soluble metals from leach residue, separation of precipitated impurities or undesired metals for disposal, intermediate product recovery to concentrate mixed sulphides or hydroxides in underflow slurry and thickened tailings disposal. The selection and sizing of thickeners is discussed and the trend towards the use of high-density and paste thickeners is considered in relation to developments in the optimisation of flocculation technology., The applications of thickeners in hydrometallurgy include pre-leach thickening to minimise water in leach feed slurry, countercurrent decantation thickening to separate and wash soluble metals from leach residue, separation of precipitated impurities or undesired metals for disposal, intermediate product recovery to concentrate mixed sulphides or hydroxides in underflow slurry and thickened tailings disposal. The selection and sizing of thickeners is discussed and the trend towards the use of high-density and paste thickeners is considered in relation to developments in the optimisation of flocculation technology.

Single particle settling velocities through water fluidized beds of mono-sized glass spheres (dp = 0.645, 1.20, 1.94, 2.98 and 5 mm in diameter) were studied experimentally using a column, 40 mm in diameter. The settling spherical particles (Dp = 10 and 19.5 mm) had different densities (1237 to 8320 kg/m3), while the settling particles (Dp = 5 and 2.98 mm) were glass spheres. The pseudo-fluid model, which considers a liquid fluidized bed as a homogenous pseudo-fluid, predicts single particle settling velocities quite well if the ratio Dp/dp is larger than about 10. With decreasing ratio Dp/dp, the overall friction between the settling particle and the fluidized media increases. A method for predicting single particle settling velocities through a liquid fluidized bed is proposed and discussed. Following the approach of Van der Wielen et al. [L.A.M. Van der Wielen, M.H.H Van Dam, K.C.A.M. Van Luyben, On the relative motion of a particle in a swarm of different particles, Chem. Eng. Sci. 51 (2006) 995-1008], the overall friction is decomposed into a particle-fluid and a particle-particle component. The effective buoyancy force is calculated using the transition function proposed by Ruzicka [M.C. Ruzicka, On buoyancy in dispersion, Chem. Eng. Sci. 61 (2006) 2437-2446]. A simple model for predicting the collision force is proposed, as well as a correlation for the collision coefficient. The mean absolute deviation between the experimental and calculated slip velocities was 5.08%.

Single particle settling velocities through water fluidized beds of mono-sized glass spheres (dp = 0.645, 1.20, 1.94, 2.98 and 5 mm in diameter) were studied experimentally using a column, 40 mm in diameter. The settling spherical particles (Dp = 10 and 19.5 mm) had different densities (1237 to 8320 kg/m3), while the settling particles (Dp = 5 and 2.98 mm) were glass spheres. The pseudo-fluid model, which considers a liquid fluidized bed as a homogenous pseudo-fluid, predicts single particle settling velocities quite well if the ratio Dp/dp is larger than about 10. With decreasing ratio Dp/dp, the overall friction between the settling particle and the fluidized media increases. A method for predicting single particle settling velocities through a liquid fluidized bed is proposed and discussed. Following the approach of Van der Wielen et al. [L.A.M. Van der Wielen, M.H.H Van Dam, K.C.A.M. Van Luyben, On the relative motion of a particle in a swarm of different particles, Chem. Eng. Sci. 51 (2006) 995-1008], the overall friction is decomposed into a particle-fluid and a particle-particle component. The effective buoyancy force is calculated using the transition function proposed by Ruzicka [M.C. Ruzicka, On buoyancy in dispersion, Chem. Eng. Sci. 61 (2006) 2437-2446]. A simple model for predicting the collision force is proposed, as well as a correlation for the collision coefficient. The mean absolute deviation between the experimental and calculated slip velocities was 5.08%.

Single particle settling velocities through water fluidized beds of mono-sized glass spheres (dp = 0.645, 1.20, 1.94, 2.98 and 5 mm in diameter) were studied experimentally using a column, 40 mm in diameter. The settling spherical particles (Dp = 10 and 19.5 mm) had different densities (1237 to 8320 kg/m3), while the settling particles (Dp = 5 and 2.98 mm) were glass spheres. The pseudo-fluid model, which considers a liquid fluidized bed as a homogenous pseudo-fluid, predicts single particle settling velocities quite well if the ratio Dp/dp is larger than about 10. With decreasing ratio Dp/dp, the overall friction between the settling particle and the fluidized media increases. A method for predicting single particle settling velocities through a liquid fluidized bed is proposed and discussed. Following the approach of Van der Wielen et al. [L.A.M. Van der Wielen, M.H.H Van Dam, K.C.A.M. Van Luyben, On the relative motion of a particle in a swarm of different particles, Chem. Eng. Sci. 51 (2006) 995-1008], the overall friction is decomposed into a particle-fluid and a particle-particle component. The effective buoyancy force is calculated using the transition function proposed by Ruzicka [M.C. Ruzicka, On buoyancy in dispersion, Chem. Eng. Sci. 61 (2006) 2437-2446]. A simple model for predicting the collision force is proposed, as well as a correlation for the collision coefficient. The mean absolute deviation between the experimental and calculated slip velocities was 5.08%.