Settling Velocity Of Sediment Particles Research Articles

The Development of Explicit Equations for Estimating Settling Velocity Based on Artificial Neural Networks Procedure

This study proposes seven equations to predict the settling velocity of sediment particles with variations in grain size (d), particle shape factor (SF), and water temperature (T) based on the artificial neural network procedure. The data used to develop the equations were obtained from digitizing charts provided by the U.S. Interagency Committee on Water Resources (U.S-ICWR) and compiled from the measurement data of settling velocity from several sources. The equations are compared to three existing equations available in the literature and then analyzed using graphical and statistical analysis. The simulation results show the proposed equations produce satisfactory results. The proposed equations can predict the settling velocity of natural particle sediments, with diameters ranging between 0.05 mm and 10 mm in water with temperatures between 0 °C and 40 °C, and shape factor SF ranging between 0.5 and 0.95.

Numerical Assessment of Shear Boundary Layer Formation in Sewer Systems with Fluid-Sediment Phases

Numerical and empirical studies of soil slurry transport and deposition in urban stormwater sewers are few, presumably due to the difficulty of direct observation of soil slurry flow in stormwater pipes. Slurry in a sewer system includes both suspended load and bedload, but few studies have attempted to demarcate these two components. A boundary layer is a crucial determinant of sediment transport capacity. Stormwater runoff enters the sewer in turbulent flow, mostly mixed with soil slurry generated by rainfall. In this paper, we attempt analysis using ANSYS Fluent commercial CFD software. We describe the development of a numerical analytical methodology capable of predicting the flow of soil slurry in stormwater pipes, and propose a method for estimating the sediment–flow boundary layer. Using this model, we simulated stormwater runoff with a large content of soil slurry during a rainfall event. We investigated soil slurry transport and predict the formation of shear boundary layer by varying the inlet conditions (volume of soil slurry entering the stormwater sewer system) and by analyzing the flow velocity field and soil slurry volume fraction in the pipes under various experimental flow conditions. Based on the shear and settling velocity of sediment particles, we propose criteria for the formation of a shear boundary layer in stormwater pipes.

Experimental and numerical simulation research of sedimentation process in stationary column of aqueous suspension of solids

Our study proposes a new mathematical model for predicting the gravitational settling velocity of sediment particles, a measure to evaluate water clarity, to provide information to environmental engineers, first responders dealing with environmental emergencies or farmers, for the most rapid and efficient management responses. New and simplified simulation program are developed to the study sedimentation process. The mathematical model proposes a first-order differential equation who is solved by a numerical method algorithm. The proposed formula has the highest degree of predictive accuracy when compared with experimental data.

Estimation of settling velocity of sediment particles in estuarine and coastal waters

A model for estimating the settling velocity of sediment particles (spherical and non-spherical) in estuarine and coastal waters is developed and validated using experimental data. The model combines the physical, optical and hydrodynamic properties of the particles and medium to estimate the sediment settling velocity. The well-known Stokes law is broadened to account for the influencing factors of settling velocity such as particle size, shape and density. To derive the model parameters, laboratory experiments were conducted using natural flaky seashells, spherical beach sands and ball-milled seashell powders. Spectral light backscattering measurements of settling particles in a water tank were made showing a distinct optical feature with a peak shifting from 470-490 nm to 500–520 nm for particle populations from spherical to flaky grains. This significant optical feature was used as a proxy to make a shape determination in the present model. Other parameters experimentally determined included specific gravity (ΔSG), Corey shape factor (CSF), median grain diameter (D50), drag coefficient (Cd) and Reynolds number (Re). The CSF values considered ranged from 0.2 for flaky to 1.0 for perfectly spherical grains and Reynolds numbers from 2.0 to 105 for the laminar to turbulent flow regimes. The specific gravity of submerged particles was optically derived and used along with these parameters to estimate the sediment settling velocity. Comparison with the experiment data showed that the present model estimated settling velocities of spherical and non-spherical particles that were closely consistent with the measured values. Findings revealed that for a given D50, the flaky particles caused a greater decrease in settling velocity than the spherical particles which suggests that the particle shape factor has a profound role in influencing the sediment settling velocity and drag coefficients, especially in transitional and turbulent flow regimes. The present model can be easily adopted for various scientific and operational applications since the required parameters are readily measurable with the commercially available instrumentations.

Biofilm effects on size gradation, drag coefficient and settling velocity of sediment particles

Sediment particles are often colonized by biofilm in a natural aquatic ecological system, especially in eutrophic water body. A series of laboratory experiments on particle size gradation, drag coefficient and settling velocity were conducted after natural sediment was colonized by biofilm for 5, 10, 15 and 20 days. Particle image acquisition, particle tracking techniques of Particle Image Velocimetry and Particle Tracking Velocimetry were utilized to analyze the changes of these properties. The experimental results indicate that the size gradation, the drag force exerted on bio-particles, and the settling velocity of bio-particles underwent significant change due to the growth of biofilm onto the sediment surface. The study proposes a characteristic particle size formula and a bio-particle settling velocity formula based on the regression of experiment results, that the settling velocity is only 50% to 60% as the single particle which has the same diameter and density. However, biofilm growth causes large particle which the settling velocities are approximately 10 times larger than that of primary particles. These results may be specifically used in the low energy reservoir or lake environment.

Grain‐size distribution in open channel flow by mixing length approach

A mathematical model is developed by theoretical analysis for predicting the grain‐size distribution in suspension over sand and sand‐gravel beds. By including the viscous shear stress of sediment–fluid mixture and the Reynolds shear stress in terms of mixing length of sediment‐mixed fluid, a system of integrodifferential equations of velocity and suspension concentration is developed. The effect of hindered settling is considered in the settling velocity of sediment particle and the effect of particle–particle interaction is taken into account in the reference level. Apart from this, incipient motion probability, pick‐up probability and nonceasing probability of a sediment particle are included in the reference concentration. The proposed system of integrodifferential equations is converted into a system of first‐order differential equations, and the fourth‐order Runge–Kutta method has been adopted for solving the system of equations. The numerical solution of the proposed system reflects the behavior of fluid velocity and grain‐size distribution with changing vertical height over a sediment bed comprising nonuniform grain sizes. The computed values of velocity and grain‐size distribution are compared with a limited data set due to the unavailability of sufficient data in literature, and good agreement has been observed between them. The proposed model provides least errors in comparison to the other existing models in literature. Copyright © 2014 John Wiley & Sons, Ltd.

Grain-size distribution in suspension over a sand-gravel bed in open channel flow

Grain-size distributions of suspended load over a sand-gravel bed at two different flow velocities were studied in a laboratory flume. The experiments had been performed to study the influence of flow velocity and suspension height on grain-size distribution in suspension over a sand-gravel bed. The experimental findings show that with an increase of flow velocity, the grain-size distribution of suspended load changed from a skewed form to a bimodal one at higher suspension heights. This study focuses on the determination of the parameter βn which is the ratio of the sediment diffusion coefficient to the momentum diffusion coefficient of n th grain-size. A new relationship has been proposed involving βn, the normalizing settling velocity of sediment particles and suspension height, which is applicable for widest range of normalizing settling velocity available in literature so far. A similar parameter β for calculating total suspension concentration is also developed. The classical Rouse equation is modified with βn and β and used to compute grain-size distribution and total concentration in suspension, respectively. The computed values have shown good agreement with the measured values of experimental data.

Effect of bed roughness on grain-size distribution in an open channel flow

Grain-size distributions of suspended load were studied in a laboratory flume over five sediment beds having different values of bed roughness at three different flow velocities. The experiments had been performed to investigate the influence of bed roughness, flow velocity and suspension height on the grain-size distribution in suspension. This study focuses on the determination of the proportionality parameter βn in suspension which is the ratio of sediment diffusion coefficient to the momentum diffusion coefficient of the nth grain-size class. An empirical equation for βn has been proposed which is valid for a wide range of normalizing settling velocity of sediment particles and bed roughness. Also, the bed roughness effect is studied on the parameter β for total concentration in suspension and on the reference concentration, which is very important in suspension studies. The Rouse equation with modified βn and β was surveyed to know the grain-size distribution and total concentration in suspension that agreed well when compared with the experimental data.

Hindered settling with an apparent particle diameter concept

In this work a new expression has been developed to predict the settling velocity of a sediment particle which is dispersed in a sediment-fluid mixture during a turbulent flow. A concept of apparent particle diameter has been introduced and is defined by the diameter of the spherical volume in which the particle can move randomly after collision with other particles in suspension. The effect of suspension concentration is studied on the mass density of the sediment-fluid mixture. It has been shown that the settling velocity of sediment particle in a sediment-fluid mixture is a function of different characteristics of the sediment particle such as settling velocity in clear fluid, suspension concentration, relative mass density and Reynolds number. The model has shown good agreement when compared with previously published experimental data and it’s prediction accuracy is superior than the other existing models.

Numerical modeling of hyperpycnal flows in an idealized river mouth

Numerical experiments in an idealized river mouth are conducted using a three-dimensional hydrodynamics model (EFDC model) to examine the impacts of suspended sediment concentration (SSC), settling velocity of sediment and tidal mixing on the formation and maintenance of estuarine hyperpycnal flows. The standard experiment presents an illustrative view of hyperpycnal flows that carry high-concentrated sediment and low-salinity water in the bottom layer (>1.0 m in thickness) along the subaqueous slope. The structure and intra-tidal variation of the simulated hyperpycnal flows are quite similar to those previously observed off the Huanghe (Yellow River) mouth. Results from the three control experiments show that SSC of river effluents is the most important parameter to the formation of hyperpycnal flows. High SSC will increase the bulk density of river effluents and thus offset the density difference between freshwater and seawater. Low SSC of river effluents will produce a surface river plume, as commonly observed in most large estuaries. Both the settling velocity of sediment particles and the tidal mixing play an important role in maintaining the hyperpycnal flows. Increasing settling velocity enhances the deposition of sediment from the hyperpycnal layer and thus accelerates the attenuation of hyperpycnal flows, whereas increasing tidal mixing destroys the stratification of water column and therefore makes the hyperpycnal flows less evident. Our results from numerical experiments are of importance to understand the initiation and maintenance of hyperpycnal flows in estuaries and provide a reference to the rapidly decaying hyperpycnal flows off the Huanghe river mouth due to climatic and anthropogenic forcing over the past several decades.

A simple formula for predicting settling velocity of sediment particles

Based on the general relationship described by Cheng between the drag coefficient and the Reynolds number of a particle, a new relationship between the Reynolds number and a dimensionless particle parameter is proposed. Using a trial-and-error procedure to minimize errors, the coefficients were determined and a formula was developed for predicting the settling velocity of natural sediment particles. This formula has higher prediction accuracy than other published formulas and it is applicable to all Reynolds numbers less than 2×105.

Discussion of “Effect of Concentration on Settling Velocity of Sediment Particles” by Nian-Sheng Cheng

Discussion of “Effect of Concentration on Settling Velocity of Sediment Particles” by Nian-Sheng Cheng

Closure to “Effect of Concentration on Settling Velocity of Sediment Particles” by Nian-Sheng Cheng

Closure to “Effect of Concentration on Settling Velocity of Sediment Particles” by Nian-Sheng Cheng

Effect of Concentration on Settling Velocity of Sediment Particles

The drag coefficient for dispersed particles settling in a fluid is related to the particle Reynolds number with the same function as that used for a single particle falling in a clear fluid, after taking into account the effects of concentration both on the effective density of particles and on the viscosity of fluid-sediment mixture. A new method is developed for evaluating the settling velocity of natural sediment particles dispersed in a fluid; it agrees well with the available experimental data. The differences among various empirical expressions are illustrated using the present relationship.