Search

The experimental observations are presented on flow patterns and turbulence characteristics measured by an acoustic Doppler velocimeter within the developing (transient stage) scour hole around circular uniform and compound piers. Four series of experimental runs were conducted under the clear-water approach flow conditions. One experimental run was conducted around a uniform circular pier of diameter 114 mm, whereas the other three runs were conducted around a circular compound pier of diameter 114 mm and footing diameter 210 mm. In the series with a circular compound pier, the top surface of the footing was placed at three different elevations with respect to the general level of the channel bed, i.e., above the bed level, at the bed level, and below the level of the channel bed. Detailed measurements are presented on components of time-averaged velocity, turbulence intensity, and Reynolds shear stress around the pier in radial planes at 0°, 90°, and 180° from the flow axis. Flow structure around a circular compound pier in the presence of a scour hole was compared with flow structure similarly observed around a circular uniform pier by utilizing the observations made in radial planes at 0°, 30°, 60°, 90°, 120°, 150°, and 180° from flow axis. Diameter of the principal vortex upstream of the compound pier, when top surface of the footing is above the general level of the channel bed was 1.11 times as large as that for the circular uniform pier. However, size of the principal vortex is 0.85 times its size for the uniform pier, whereas the top surface of the footing was below the channel bed level. The components of turbulence intensities and Reynolds shear stress profiles in different radial planes are also compared around each of the pier models. [ABSTRACT FROM AUTHOR]

Bed-load measurements comprise an important component in the development of reliable formulas, in an effort to obtain the necessary constitutive relations between the amount of transported material and flow parameters. The uncertainty of such measurements is rather well known, being much more pronounced at lower transport rates. This uncertainty stems from the multitude of factors affecting bed-load transport and the lack of available trustworthy measuring technologies. Predictions of the limiting case of nearly zero bed-load transport, typically reported in literature as threshold of motion or critical condition, are even more challenging. The purpose of this contribution is twofold. First, to examine the sensitivity of bed-load transport measurements at conditions moderately higher than critical, to the presence of a rather unobtrusive trap, designed through several iterations. Even under relatively simple laboratory flume channel and flow conditions, it proved difficult to measure the bed-load transport rate in a completely unbiased way. Second, to develop a methodology, together with the appropriate instrumentation, for determining the condition of incipient motion. The nonintrusive approach described here proved to be reliable in detecting even the slightest movements of a particle. At the same time, it demonstrates the complexity of the problem due to the highly fluctuating nature of turbulent flow. [ABSTRACT FROM AUTHOR]

Hydraulic jumps on the steps of a stepped spillway were investigated analytically, physically, and numerically. Using classic hydraulic formulae, a conceptual prototype was designed. A large scale model was adapted and an experimental study was conducted to examine similarity of hydraulic jumps on each step, minimizing hydraulic jump length and maximizing discharge per unit width. A numerical model based on the 2D Reynolds averaged equations, where the free-surface is represented using a refined volume-of-fluid algorithm, the internal obstacles are described by means of the fractional area-volume obstacle representation method, and the turbulence is represented by a specially developed RNG (Renormalization Group) k-[variant_greek_epsilon] closure model was used for evaluating velocities and pressures, and for characterizing hydrodynamic forces on the baffles and sills. Preliminary design criteria are proposed for stepped spillways with hydraulic jump formation of simple shapes and adequate relations of critical depth/step height and the application of computational fluid dynamics to such problems is studied. [ABSTRACT FROM AUTHOR]

The direct numerical simulation of turbulent flows in a compound open channel is described. Mean flows and turbulence structures are provided, and are compared with numerical and measured data available in the literature. The simulated results show that twin vortices are generated near the interface of the main channel and the floodplain and that their maximum magnitude is about 5% of the bulk streamwise velocity. Near the interface, the simulated wall shear stress reaches a maximum, contrary to experimental data. A quadrant analysis shows that both sweeps and ejections become the main contributor to the production of Reynolds shear stresses near the interface. Through the conditional quadrant analysis, it is demonstrated how the directional tendency of dominant coherent structures determines the production of Reynolds shear stress and the pattern of twin vortices near the interface. In addition, the time-dependent characteristics of three-dimensional vortical structures in a compound open-channel flow were investigated using direct numerical simulation (DNS) data. [ABSTRACT FROM AUTHOR]

The use of computation fluid dynamics (CFD) for waste stabilization pond design is becoming increasingly common but there is a large gap in the literature with regard to validating CFD pond models against experimental flow data. This paper assesses a CFD model against tracer studies undertaken on a full-sized field pond and then on a 1:5 scale model of the same pond operated under controlled conditions in the laboratory. While the CFD tracer simulation had some discrepancies with the field data, comparison to the laboratory model data was excellent. The issue is, therefore, not in the way the model solves the problem, for example, the choice of turbulence model or differencing scheme, but rather with how accurately the physical conditions in the field are defined. Extensive survey of the sludge layer and transient input of changing flow rates, wind velocities, and temperature could allow closer alignment of CFD simulations to field data. However, in the practical application of CFD where a modification such as baffle installation results in a large change, then a simple pragmatic model, while not exact, can still provide valuable design insight. [ABSTRACT FROM AUTHOR]

The energy loss of the vertical drop with subcritical flow in the upstream channel has been studied experimentally with a physical model of 0.20 m drop height. Experimental data were collected in a range where few data exist. Based on a similarity between a turbulent surface jet and the flow over the drop, a model is developed using the theories of the shear layer and fully developed surface jet to estimate the energy loss. The results were compared with previous and current experimental data and were found to agree well with the proposed model. [ABSTRACT FROM AUTHOR]

Eulerian equations for the vertical flux and momentum of suspended particles in dilute sediment-laden open-channel flow in equilibrium have been derived using the two-fluid approach. Reynolds averaging has been applied in order to allow validation of individual terms with experimental data. Consideration of the various terms of the vertical momentum balance with experimental flume data indicates that the drag force has to be separated into a mean and a turbulent contribution with different timescales, respectively, the (gravitational) particle timescale and the integral turbulence timescale. The resulting formulation further provides a new theoretical closure for the turbulent Schmidt number. [ABSTRACT FROM AUTHOR]

A well-tested numerical model based on the Reynolds averaged Navier–Stokes equations with k-[variant_greek_epsilon] turbulence closure is further validated against three groups of experimental data on lock-exchange gravity flows. The model is then applied to study larger-scale turbidity currents with sudden-release and/or sustained inflow mechanisms. At the field-scale, sudden-release turbidity currents are found to be essentially unsteady with depth-averaged flow variables varying with time and distance from the upstream boundary. Turbidity currents with sustained inflows reach a quasi-equilibrium state in the body of the current. The simulation of a turbidity current initiated by sudden-release and then fed by sustained inflow reveals that, initially, two bore heads appear in the current with the second bore head resulting from sustained inflow and eventually catches up with the first one. Model results show that, depending on onset mechanism and slope, if the erosion rate of sediment exceeds the deposition rate, turbidity currents at the field scale can experience self-acceleration, a process predicted earlier by theory but not demonstrated in laboratory experiments. [ABSTRACT FROM AUTHOR]

The spreading of solutes or contaminants through water-distribution pipe networks is controlled largely by mixing at pipe junctions where varying flow rates and concentrations can enter the junction. Alternative models of solute mixing within these pipe junctions are presented in this paper. Simple complete-mixing models are discussed along with rigorous computational-fluid-dynamics models based on turbulent Navier–Stokes equations. In addition, a new model that describes the bulk-mixing behavior resulting from different flow rates entering and leaving the junction is developed in this paper. Comparisons with experimental data have confirmed that this bulk-mixing model provides a lower bound to the amount of mixing that can occur within a pipe junction, while the complete-mixing model yields an upper bound. In addition, a simple scaling parameter is used to estimate the actual (intermediate) mixing behavior based on the bounding predictions of the complete-mixing and bulk-mixing models. These simple analytical models can be readily implemented into network-scale models to develop predictions and bounding scenarios of solute transport and water quality in water-distribution systems. [ABSTRACT FROM AUTHOR]

This paper presents a method to predict depth-averaged velocity and bed shear stress for overbank flows in straight rectangular two-stage channels. An analytical solution to the depth-integrated Navier–Stokes equation is presented that includes the effects of bed friction, lateral turbulence, and secondary flows. The Shiono and Knight method accounts for bed shear, lateral shear, and secondary flow effects via three coefficients, f, λ, and Γ, respectively. A novel boundary condition at the internal wall between the main channel and the adjoined floodplain is proposed and discussed along with other conventional boundary conditions. The analytical solution using the novel boundary condition gives good prediction of both lateral velocity distribution and bed shear stress when compared with experimental data for different aspect ratios. [ABSTRACT FROM AUTHOR]

It is experimentally demonstrated that fitting the log law to the mean velocity profile in turbulent open-channel flow to determine the friction velocity, roughness length, and zero-plane displacement, can lead to relatively high standard errors in these parameters. It is also demonstrated that this approach, where the log law alone is used to estimate these parameters, may yield values of these parameters that are significantly different from those obtained when the friction velocity is determined independently of the log law. Although the statistical estimation of the three log law parameters from mean velocity data alone is frequently necessary, this procedure can lead to inaccurate and imprecise estimates of these quantities. [ABSTRACT FROM AUTHOR]

This note concerns variations of the friction factor in the two transitional regimes, one between laminar and turbulent flows and the other between fully smooth and fully rough turbulent flows. An interpolation approach is developed to derive a single explicit formula for computing the friction factor in all flow regimes. The results obtained for pipe flows give a better representation of Nikuradse’s experimental data, in comparison with other implicit formulas available in the literature. Certain modifications are also made for applying the obtained friction formula to open-channel flows. [ABSTRACT FROM AUTHOR]

To improve knowledge of the flow and scour processes associated with spur dikes, three-dimensional flow velocities were measured at 2,592 points using an acoustic Doppler velocimeter over a fixed flat bed with a trapezoidal shaped submerged spur dike in a laboratory flume. General velocity distribution and detailed near field flow structures were revealed by the measurements and numerical simulations performed using a free surface turbulent flow model with a k–[variant_greek_epsilon] closure scheme. The three-dimensional flow separation characterized in this study was found to yield forces on the bed that were significantly different from nonsubmerged vertical obstructions that have been measured in other studies. Values of bed shear stress derived from both measured and simulated values were similar but indicated that local scour would be initiated in one rather than in the two locations of initial local scour measured in previous experiments with a similar flow. [ABSTRACT FROM AUTHOR]

This paper describes the flow structures that occur in a 90° junction of four open channels with supercritical flow in two orthogonal inlet channels. An experimental facility was constructed to permit the measurement of flow rates, water depths, and the positions of hydraulic jumps in the channels. The various flow patterns which appear depend on the characteristics of the incoming flows and can be classified into three main types, depending on the location and shape of the hydraulic jumps that develop. These jumps can either be normal to the flow and located in the upstream channels, or can be oblique and confined within the junction. The explanation for the existence of various flow patterns is derived from previous studies of the rapid deflection of supercritical flow. A detailed description of each flow regime is provided, with information on the surface elevations, the behavior of the hydraulic jumps, the deflections of the incoming flows, and the formation and characteristics of recirculation zones. [ABSTRACT FROM AUTHOR]

Combining sewer junctions with a lateral inflow at 90° angle are commonly used in our sewer systems. A computational fluid dynamics (CFD) model based on Ansys CFX 10.0 was established to simulate fully surcharged flow at a 90° combining sewer junction. The model was carefully assessed by comparing its results with the measurements of detailed physical experiments. Good agreement was obtained between results of the computational model and of the laboratory experiments. The computational model was proved to be capable of simulating surcharged combining junction flow in the aspects of water depth, energy losses, velocity distributions, and turbulence. The verified CFD model was also used to investigate air entrainment and effects of the size of the junction chamber on the flow. Such CFD models can be used to optimize the design of sewer junctions and will also be useful in studying sediment transport at sewer junctions. [ABSTRACT FROM AUTHOR]

Results are presented from a series of laboratory experiments investigating the characteristic features of particle-laden, round, turbulent, buoyant jets discharged horizontally into stationary and coflowing receiving fluids. For the volumetric source concentrations of particles tested (∼ 0.1%), the presence of the particle load was found to have no significant influence on mean buoyant jet trajectories. Deposition patterns on the bed of the receiving water container indicated the existence of two separate sedimentation processes for discharges into stationary or coflowing ambients, namely (1) a relatively concentrated, narrow band of particle accumulation associated with near-source fallout from the buoyant jet margins; and (2) a broader and more disperse downstream depositional fan associated with particle fallout from the radially-expanding surface gravity current formed by the impingement of the buoyant jet with the free surface of the receiving fluid. Scaling arguments have been developed and applied successfully to deposition length scales associated with these sedimentation patterns, allowing the quantitative characteristics and parametric dependences of the deposition distributions to be established. [ABSTRACT FROM AUTHOR]

The commercial computational fluid dynamics model FLOW-3D (Flow Science, Inc., Santa Fe, N.M.) is used to simulate two-dimensional wave transformation and breaking across a naturally barred beach profile. Fine scale pressures and velocities are computed for a 35.5 min period over a two-dimensional beach profile measured during the 1990 Delilah field experiment. The model is driven by observed wave spectra obtained in 8 m water depth, and results compared with a cross-shore array of pressure sensors and current meters extending from near the shoreline to beyond the surf zone and the spatial distribution of wave breaking patterns obtained from video data. In the calculations, wave breaking is a natural consequence of the fluid dynamics and does not require the use of empirical formulations or breaking criteria. Good agreement between modeled and observed wave height transformation, mean cross-shore flow, and wave breaking variability suggests that the model can be used as a numerical laboratory to study the wave breaking and dissipation process in detail, and perhaps lead to improved parametrizations for more computationally efficient numerical models. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive analysis of suspended sediment transport in open channels under various flow conditions through a kinetic-model-based simulation. The kinetic model, accounting for both sediment-turbulence and sediment-sediment interactions, successfully represents experimentally observed diffusion and transport characteristics of suspended sediments with different densities and sizes. Without tuning any model coefficients, the nonmonotonic concentration distribution and the noticeable lag velocity with a negative value close to the wall are reasonably reproduced. Examination of flow conditions typical of suspension dominated rivers shows that the conventional method may overestimate or underestimate unit suspended-sediment discharge, depending on the Rouse number, sediment size, as well as shear velocity. The error may be less than 20% for dp<0.5 mm and might exceed 60% for dp>1.0 mm under typical flow conditions where shear velocity ranges from 1.0 to 12.5 cm/s and flow depth ranges from 1.0 to 5.0 m. [ABSTRACT FROM AUTHOR]

The flow hydrodynamics in a straight open channel containing a multiple-embayment groyne field on one of its sides is investigated numerically using large eddy simulation. The vertical groynes are fully emerged. The mean flow depth in the groyne region is about half that of the main channel and the length and width of the embayments are much larger than the mean depth in the embayment region. The model is validated using mean velocity and turbulent fluctuations data collected at the free surface in a previous experimental study. It is found that despite the fact that the flow inside the main recirculation eddy in the embayments can be characterized as being quasi-two dimensional, the flow inside the mixing layer region between the embayments and the channel is strongly nonuniform over the depth. As this region controls the mass exchange processes between the groyne field and the main channel, a three-dimensional description of the flow in this area is essential. The large-scale eddies that populate the mixing layer can penetrate the embayment region over lateral distances of the order of the channel depth. These eddies advect with them channel fluid inside the embayment. Eventually, the channel fluid is mixed with the embayment fluid by the small scales. The other main mixing mechanism is due to the injection of patches of high-vorticity mixing-layer fluid near the tip of the downstream groyne and their subsequent convection in the form of a wall-attached jet-like flow into the embayment, first parallel to the downstream groyne face and then to the sidewall. It is shown that on average, most of the fluid leaves the embayment region via the top layer of the embayment-channel interface (upstream half) and enters the embayment region at levels situated around the middepth (practically over the whole length of the embayment) of the interface surface. This explains why the mass exchange coefficients are overestimated when predicted using methods that employ floating particles as a tracer. The instantaneous bed shear stress inside the cavity is found to peak close to the downstream groyne face of each embayment and to show a high variability around the mean values due to the interaction of the mixing layer eddies with the tip of the groynes and the formation of the jet-like flow parallel to the droyne face. [ABSTRACT FROM AUTHOR]

The article discusses the paper "Turbulence Measurements with Acoustic Doppler Velocimeters," by Calos M. García, Mariano I. Cantero, Yarko Niñ, and Marcelo H. García. It states that the paper correctly pointed out that acoustic Doppler velocimetry (ADV) signal outputs include the combined effects of turbulent velocity fluctuations, Doppler noise, turbulent shear, and other disturbances. It provides further understanding of ADV turbulence measurement by arguing the effects of sampling duration and proximity of solid boundaries. It is noted that ADV signal outputs are affected by the proximity of solid boundaries, particularly when the sampling volumes is located less than 30 to 45 millimeter from the wall.

The development of a fully three-dimensional finite volume morphodynamic model, for simulating fluid and sediment transport in curved open channels with rigid walls, is described. For flow field simulation, the Reynolds-averaged Navier–Stokes equations are solved numerically, without reliance on the assumption of hydrostatic pressure distribution, in a curvilinear nonorthogonal coordinate system. Turbulence closure is provided by either a low-Reynolds number k-ω turbulence model or the standard k-[variant_greek_epsilon] turbulence model, both of which apply a Boussinesq eddy viscosity. The sediment concentration distribution is obtained using the convection-diffusion equation and the sediment continuity equation is applied to calculate channel bed evolution, based on consideration of both bed load and suspended sediment load. The governing equations are solved in a collocated grid system. Experimental data obtained from a laboratory study of flow in an S-shaped channel are utilized to check the accuracy of the model’s hydrodynamic computations. Also, data from a different laboratory study, of equilibrium bed morphology associated with flow through 90° and 135° channel bends, are used to validate the model’s simulated bed evolution. The numerically-modeled fluid and sediment transportation show generally good agreement with the measured data. The calculated results with both turbulence models show that the low-Reynolds k-ω model better predicts flow and sediment transport through channel bends than the standard k-[variant_greek_epsilon] model. [ABSTRACT FROM AUTHOR]

Lock-exchange gravity current flows produced by the instantaneous release of a heavy fluid are investigated by means of high-resolution two-dimensional (2D) large-eddy simulation (LES) using a dynamic subgrid scale model for Grashof numbers (Gr) up to 1012. The model is validated using 2D direct numerical simulation results for the classical lock-exchange gravity current flow in an infinite channel with no-slip walls. Then the model is applied to study lock-exchange flows for the case in which the heavier (lock) fluid is initially situated in between a vertical end wall and the lock barrier. Three cases with different aspect ratios of the initial lock fluid are considered for Grashof numbers of the order of 109–1010. It is found that 2D LES is able to capture most of the physics observed in experiments including the evolution of the head, as well as the formation of coherent billow structures at the apex of the head due to growth of the Kelvin-Helmholtz (KH) instabilities. However, due to the fact that three-dimensional instabilities cannot develop in 2D simulations, the coherence of these KH billows in the latter stages of the evolution of the current is found to be stronger compared to the experiment where mixing is sensibly higher at the interface between the current and the surrounding fluid. The LES simulations accurately capture the front and bore velocities in the slumping phase and predict that during the inviscid self-similar phase the front speed decay is proportional to t-1/3, which is consistent with theory, where t=time. A LES simulation at a Grashof number closer to the inviscid range (Gr=1012) shows that the evolution of the current and the structure of the head region are qualitatively similar to those observed at lower Grashof numbers (Gr>108). The energy balance shows that the higher front velocity in the Gr=1012 simulation is due to the fact that the amount of mixing (dissipation) decays with the Grashof number. This allows a larger fraction of the potential energy to be converted into kinetic energy. The flow fields are also used to obtain the distributions of the bed shear stress which determine the amount of sediment entrained by a compositional gravity current propagating over a loose bed. [ABSTRACT FROM AUTHOR]

A new theoretical approach is presented for the derivation of free surface profiles of two-dimensional steady and unsteady flows by solving the Reynolds-averaged Navier-Stokes equations applied to the turbulent flow regime. This approach enables us to compute the steady and unsteady curvilinear flows having small curvatures, such as free overfall and constant velocity surge. In addition, the applications of the theory to the second-order waves are illustrated through the problems of small height bore and second-order tide. [ABSTRACT FROM AUTHOR]

The power law velocity profile has been analyzed in terms of the envelope of the friction factor which gives the friction factor log law. The power law index α and prefactor C are shown as the function of the friction Reynolds number, as well as the function of the alternate variable the nondimensional friction velocity. The fully developed turbulent superpipe flow data of McKeon et al. and fully developed channel flow data of Zanoun et al. have been analyzed and the power law index α and prefactor C data have been estimated, first as a function of the friction Reynolds number and second as function of the nondimensional friction velocity. Based on analysis, several correlations have been proposed that have been supported by the data. [ABSTRACT FROM AUTHOR]

The goal of this paper is to discuss the spatial averaging concept in environmental hydraulics and develop it further by considering transport equations for fluid momentum, passive substances, and suspended sediments. The averaging theorems, the double-averaged (in time and in space) fluid momentum equation, and advection-diffusion equations for a passive substance and suspended sediments are introduced and their limitations and applications for modeling rough-bed flows, experimental design, and data interpretation are discussed. The suggested equations differ from those considered in terrestrial canopy aerodynamics and porous media hydrodynamics by accounting for roughness mobility, change in roughness density in space and time, and particle settling effects for the case of suspended sediments. We show that the form of the double-averaged equations may depend on the type of decomposition of flow variables and that this difference may have important implications for modeling. We also show that the suggested methodology offers better definitions for hydraulic characteristics, variables, and parameters such as flow uniformity, flow two dimensionality, and bed shear stress. [ABSTRACT FROM AUTHOR]

Understanding the hydraulics of flow in a compound channel with vegetated floodplains is very important for determining the stage-discharge curve and for supporting the management of fluvial processes. In this paper, the flow patterns over different types of vegetation, such as tree, shrub, and grass, are described, based on an experimental study. For vegetation on the floodplain, the authors choose plastic grass, duck feathers, and plastic straws as model grass, shrubs, and trees, respectively. A 3D acoustic Doppler velocimeter was used to measure the local flow velocities for different types of vegetation on the floodplain, and the total discharge and flume slope were measured independently. In the cases of nonvegetated floodplains, all measured streamwise velocity distributions followed the logarithmic distribution, but for vegetated floodplains, they followed an S-shaped profile, exhibiting three zones. For all cases, the fluctuating velocity followed a normal distribution. The influence of different types of vegetation on the distributions of the secondary currents, turbulence intensities, and Reynolds shear stresses were also analyzed. [ABSTRACT FROM AUTHOR]

We present a novel, mathematically equivalent representation of the Colebrook–White equation to compute friction factor for turbulent flow in rough pipes. This new form is simple, no iterative calculations are necessary, and is well suited for accurate friction factor estimation. A limiting case of this equation provided friction factor estimates with a maximum absolute error of 0.029 and a maximum percentage error of 1% over a 20×500 grid of [variant_greek_epsilon]/D and R values (10-6<=[variant_greek_epsilon]/D<=5×10-2; 4×103

Turbulent open-channel flow over a two-dimensional dune is studied using an established large-eddy simulation code. The free surface is approximated as a shear free boundary. Turbulence statistics and instantaneous flow structures are examined. Numerical results from two computational grids agree with each other, and are also in good agreement with recently obtained experimental data. The mean velocity profiles show significant changes along the dune and there is no region that conforms to the standard law-of-the-wall. Profiles of the Reynolds stresses show distinct peaks marking the shear layer that originates from flow separation at the dune crest. Secondary peaks found further from the dune are ascribed to the shear layer over the upstream dune. Details of the separated flow and development of the flow after reattachment are well predicted. Quadrant analysis of the Reynolds shear stress shows that turbulent ejections dominate the near-wall motions. Complex water surface flow structures are visualized. [ABSTRACT FROM AUTHOR]

The capability of acoustic Doppler velocimeters to resolve flow turbulence is analyzed. Acoustic Doppler velocimeter performance curves (APCs) are introduced to define optimal flow and sampling conditions for measuring turbulence. To generate the APCs, a conceptual model is developed which simulates different flow conditions as well as the instrument operation. Different scenarios are simulated using the conceptual model to generate synthetic time series of water velocity and the corresponding sampled signals. Main turbulence statistics of the synthetically generated, sampled, and nonsampled time series are plotted in dimensionless form (APCs). The relative importance of the Doppler noise on the total measured energy is also evaluated for different noise energy levels and flow conditions. The proposed methodology can be used for the design of experimental measurements, as well as for the interpretation of both field and laboratory observations using acoustic Doppler velocimeters. [ABSTRACT FROM AUTHOR]

This study evaluated fluctuations in the output signal of a densimeter, an in situ suspended-sediment measurement device based on a very sensitive differential pressure transducer. Although the densimeter produced accurate (less than 10% error) results in the laboratory for concentrations of 10–1,000 mg/L, there was a much larger fluctuation in the field test output signal. These fluctuations were hypothesized to have originated from fluctuations imparted by movement of bed material on the channel bottom. Measuring root mean square and mean velocities and corresponding pressure differences at locations in front of and behind a bluff body simulated movement of a body/bed form past a sampling point in a quasistatic manner. Position of the bluff body relative to the sample port resulted in differential pressure swings equivalent to 65,000 mg/L sediment concentration. Turbulent structure changed dramatically with position of the bluff body. Thus, pressure densitometry using a differential pressure transducer is likely not a reliable system for measuring sediment concentration under in-stream conditions, particularly near the channel bottom where bed forms and large objects may pass between the sample ports. [ABSTRACT FROM AUTHOR]

This paper reports the results of some further reflections by the writer on the nature of the flow physics in flooded channels where the floodplain flow overtops that in the main channel. A lot of the early conclusions drawn on this issue come from results of the Flood Channel Facility (FCF) program carried in the United Kingdom in the 1990s, but also from numerical work such as that of the writer. This paper takes this work further and reports that the FCF geometry may have led to conclusions that are dependent on a channel layout that is not fully representative of nature. It indicates that the flow structure evolves as a function of the channel width-to-depth ratio, and requires different turbulence model approaches to be computed accurately as this happens. In particular the standard k–[variant_greek_epsilon] is shown to become less adequate. It is also shown that the bank slope is influential in determining the flow structure, and that the flatter the slope the more likely it is to present increasing difficulties for modeling. [ABSTRACT FROM AUTHOR]

Most models of sediment transport are based on the hypothesis of a weak interaction between the fluid and the sediment phase, where the main flow is subjected to the mass and momentum conservation of the fluid phase, with small corrections due to the presence of the sediments. These models usually give a correct answer for conditions where the concentration of sediment is really low. In the case of high sediment transport, some models analyze the two subdomains separately, the high and the low concentration, and employ different constitutive equations. In the present model there is a two-phase description of the whole domain. The closure of the turbulence and the interaction between the sediments and the fluid introduce approximations, but results are consistent with experiments and previous models. [ABSTRACT FROM AUTHOR]

Mechanism of sediment transport is composed of complicated interactions between turbulent flow, particle motion, and bed configurations. Of particular significance is the interaction between turbulence and particle motion, although turbulence measurements of particle-laden two phase flow have been a problem for a long time, especially in the near-wall region. In this study, simultaneous measurements of both the particles and fluid (water) were conducted in particle-laden two phase open channel flows by means of a discriminator particle-tracking velocimetry. The mean velocity and turbulence characteristics for fluid and particles each were examined in comparison with those in clear-water (particle-free) flow, together with previous existing data measured by laser Doppler anemometer and phase Doppler anemometer. The relative velocity and the turbulence modulation, which are the most important topics in two phase-flow approach, were revealed by varying the particle diameter and specific density. The fluid-sweeps are more contributory to the motion of particles than the fluid ejections in the near-wall region. In turn, the particle-sweeps transport the high momentum to the carrier fluid and enhance the turbulence intensities of fluid. [ABSTRACT FROM AUTHOR]

This paper is written to mark the hundredth anniversary of the birth of Hans Albert Einstein (1904–1973). It casts his career as that of the archetypal researcher protagonist determined to master intellectually the way water flows and conveys alluvial sediment in rivers. In that effort, Einstein personified the mix of success and frustration experienced by many researchers who have attempted to formulate the complicated behavior of alluvial rivers in terms of mechanically based equations. His formulation of the relationship between rates of bed-sediment transport (especially bedload transport) and water flow comprised an innovative departure from the largely empirical approach that prevailed at the time. He introduced into that relationship the emerging fluid-mechanic concepts of turbulence and boundary layers, and concepts of probability theory. Inevitably the numerous complexities attending sediment transport mire formulation and prompt his use of several approximating compromises in order to make estimating bed-sediment transport practicable. His formulation nonetheless is a milestone in river engineering. [ABSTRACT FROM AUTHOR]

We analyze the general applicability of a recent explicit expression of the Colebrook–White equation for turbulent flow friction factor calculation. This explicit expression, which is based on the Lambert W function, is characterized by an exponential term which imposes restrictions on its use. These constraints have been expressed in terms of pipe roughness ([variant_greek_epsilon]/D) and the Reynolds number R that are required for friction factor calculation. These constraints were determined as 8.0666 ln(R)+([variant_greek_epsilon]/D)R<721.97 and 8.0666 ln(R)+([variant_greek_epsilon]/D)R<5731.83, respectively, for machines using single precision and double precision computations. Using the Lambert W function, an explicit equation relating R and [variant_greek_epsilon]/D was derived at the limiting case which allowed for a graphical representation of the applicability of the explicit form of the Colebrook–White equation in the R versus [variant_greek_epsilon]/D space. Before computing friction factors using the explicit Colebrook–White equation, a quick check must be performed to see if the desired combination of R and [variant_greek_epsilon]/D values satisfies the applicable constraint mentioned above. [ABSTRACT FROM AUTHOR]

The article adds to the information provided by the research conducted by Hans M. Tritico and Rollin H. Hotchkiss. The research by Tritico and Hotchkiss helps in the understanding of fluid mechanics in gravel bed rivers. Tritico and Hotchkiss evaluated the turbulence structure both in obstructed and unobstructed flows. This research conducted in the lowland river Venoge, in Switzerland, evaluates turbulence structure using a 3D acoustic Doppler velocity profiler.

The article discusses the highlights of the paper "Open Channel Flow Through Different Forms of Submerged Flexible Vegetation," by C. A. M. E. Wilson, T. Stoesser, P. D. Bates and A. Batemann Pinzen. This report focuses on ecological hydraulics issues at a vegetated stream. Authors investigated the effect of submerged vegetation on the turbulence intensities in a water flow at an open channel.

During the analysis of an open channel flow experiment, students were asked to determine whether the flow was laminar or turbulent. The array of answers highlighted the different information carried in fluid mechanics textbooks. It was noted that approximately 50% of the books did not mention the fact that the critical Reynolds number would be different in open channels as compared to circular pipes flowing full. [ABSTRACT FROM AUTHOR]

The article replies to comments on the paper "Open Channel Flow Through Different Forms of Submerged Flexible Vegetation," by C. A. M. E. Wilson, T. Stoesser, P. D. Bates and A. Batemann Pinzen. Researchers thank the discussers for their comments and appreciation of the work on the impact of plant forms on the velocity and turbulence structure in flexible submerged vegetation. They further explain on their use of one plant morphologic form per experimental study.

Discusses the mean flow and turbulence structure of open-channel flow through nonemergent vegetation. Use of an acoustic Doppler velocimeter; Measurement volume of laser Doppler velocimeter.