Three-Dimensional Numerical Simulation of Compound Channel Flows

A three-dimensional numerical study is presented for the calculation of turbulent flow in compound channels. The flow simulations are performed by solving the three-dimensional Reynolds-averaged continuity and Navier–Stokes equations with the k?ε turbulence model for steady-state flow. The flow equations are solved numerically with a general-purpose finite-volume code. The results are compared with the experimental data obtained from the UK Flood Channel Facility. The simulated distributions of primary velocity, bed shear stress, turbulent kinetic energy, and Reynolds stresses are used to investigate the accuracy of the model prediction. The results show that, using an estimated roughness height, the primary velocity distributions and the bed shear stress are predicted reasonably well for inbank flows in channels of high aspect ratio (width/depth ≥ 10) and for high overbank flows with values of the relative flow depth greater than 0.25.     

Three-Dimensional Numerical Simulation and Analysis of Flows around a Submerged Weir in a Channel Bendway

To improve navigation conditions for barges passing through river channels, many submerged weirs (SWs) have been installed along the bendways of many waterways by the U.S. Army Corps of Engineers. This paper presents results from three-dimensional numerical simulations that were conducted to study the helical secondary current (HSC) and the near-field flow distribution around one SW. The simulated flow fields around a SW in a scale physical model were validated using experimental data. The three-dimensional flow fields around a SW, the influence of the SW on general HSC, and the implication of effectiveness of submerged weirs to realign the flow field and improve navigability in bendways were analyzed. The numerical simulations indicated that the SW significantly altered the general HSC. Its presence induced a skewed pressure difference cross its top and a triangular-shaped recirculation to the downstream side. The over-top flow tends to realign toward the inner bank and therefore improves conditions for navigation.     

Three-Dimensional and Depth-Averaged Large-Eddy Simulations of Some Shallow Water Flows

Three-dimensional (3D) and two-dimensional (2D) depth-averaged (DA) large-eddy simulations (LES) are presented for three different shallow-water flows involving large-scale horizontal structures: a mixing layer, the flow around a circular cylinder, and the flow in a groyne field. The results are compared with each other and also with experiments. In the 3D-LES, most of the energy-containing turbulent motions, including the larger subdepth-scale motions, are resolved, while in the 2D-DA-LES the effect of the 3D subdepth-scale turbulence is represented by a quadratic bottom-friction model and a simple eddy-viscosity model. In the case of the mixing layer, an additional stochastic backscatter model is necessary to account for the energy transfer from the subdepth-scale turbulence to the 2D structures in order to generate the latter. The 3D-LES results are generally in good agreement with the experiments, including the evolution of the horizontal structures. The much more economic 2D-DA-LES are somewhat less realistic in detail but also produce results that are generally of sufficient accuracy for practical purposes.     

Three-Dimensional Numerical Study of Flows in Open-Channel Junctions

An open-channel junction flow is encountered in many hydraulic structures ranging from wastewater treatment facilities to fish passage conveyance structures. An extensive number of experimental studies have been conducted but a comprehensive three-dimensional numerical study of junction flow characteristics has not been performed and reported. In this paper, a three-dimensional numerical model is developed to investigate the open-channel junction flow. The main objective is to present the validation of a three-dimensional numerical model with high-quality experimental data and compare additional simulations with classical one-dimensional water surface calculations. The three-dimensional model is first validated using the experimental data of a 90° junction flow under two flow conditions. Good agreement is obtained between the model simulation and the experimental measurements. The model is then applied to investigate the effect of the junction angle on the flow characteristics and a discussion of the results is presented.     

Numerical and Experimental Study of Dividing Open-Channel Flows

Dividing flows in open channels are commonly encountered in hydraulic engineering systems. They are inherently three-dimensional (3D) in character. Past experimental studies were mostly limited to the collection of test data on the assumption that the flow was 1D or 2D. In the present experimental study, the flow is treated as 3D and test results are obtained for the flow characteristics of dividing flows in a 90°, sharp-edged, rectangular open-channel junction formed by channels of equal width. Depth measurements are made using point gauges, while velocity measurements are obtained using a Dantec laser Doppler anemometer over grids defined throughout the junction region. A 3D turbulence model is also developed to investigate the dividing open-channel flow characteristics. The predicted flow characteristics are validated using experimental data. Following proper model validation, the numerical model developed can yield design data pertaining to flow characteristics for different discharge and area ratios for other dividing flow configurations encountered in engineering practice. Energy and momentum coefficients based on the present 3D model yield more realistic energy losses and momentum transfers for dividing flow configurations. Data related to secondary flows provide information vital to bank stability, if the branch channel sides are erodible.     

Field Testing of a Simple Flume (SMBF) for Flow Measurement in Open Channels

In this paper the stage–discharge relationship of a new flume named SMBF (Samani, Magallanex, Baiamonte, Ferro), originally proposed by Samani and Magallanez and tested by Baiamonte and Ferro, for measuring flow discharge in open channels is reviewed. The flume is obtained inserting two semicylinders in a rectangular cross section. The results of some experimental runs carried out using horizontal flumes characterized by different values of the contraction ratio (ranging from 0.17 to 0.81) are used for determining the two coefficients of the power stage–discharge equation. The stage–discharge equation is tested using flow measurements carried out in the period between December 2004 and March 2006 in the Sicilian experimental SPA1 basin. Field testing of the SMBF flume is developed using discharge measurements carried out by a Khafagi–Venturi flume placed in the field measurement channel.     

Experimental and Numerical Investigation of Bed-Load Transport under Unsteady Flows

The dynamic behavior of bed-load sediment transport under unsteady flow conditions is experimentally and numerically investigated. A series of experiments are conducted in a rectangular flume (18?m in length, 0.80?m in width) with various triangular and trapezoidal shaped hydrographs. The flume bed of 8?cm in height consists of scraped uniform small gravel of D50 = 4.8??mm. Analysis of the experimental results showed that bed-load transport rates followed the temporal variation of the triangular and trapezoidal hydrographs with a time lag on the average of 11 and 30?s, respectively. The experimental data were also qualitatively investigated employing the unsteady-flow parameter and total flow work index. The analysis results revealed that total yield increased exponentially with the total flow work. An original expression which is based on the net acceleration concept was proposed for the unsteadiness parameter. Analysis of the results then revealed that the total yield increased exponentially with the increase in the value of the proposed unsteadiness parameter. Further analysis of the experimental results revealed that total flow work has an inverse exponential variation relation with the lag time. A one-dimensional numerical model that employs the governing equations for the conservation of mass for water and sediment and the momentum was also developed to simulate the experimental results. The momentum equation was approximated by the diffusion wave approach, and the kinematic wave theory approach was employed to relate the bed sediment flux to the sediment concentration. The model successfully simulated measured sedimentographs. It predicted sediment yield, on the average, with errors of 7% and 15% of peak loads for the triangular and trapezoidal hydrograph experiments, respectively.     

3D Numerical Simulation for Water Flows and Sediment Deposition in Dam Areas of the Three Gorges Project

This paper presents a three-dimensional (3D) mathematical model for suspended load transport in turbulent flows. Based on the stochastic theory of turbulent flow proposed by Dou, numerical schemes of Reynolds stresses for anisotropic turbulent flows are obtained. Instead of a logarithmic law, a specific wall function is used to describe the velocity profile close to wall boundaries. The equations for two-dimensional suspended load motion and sorting of bed material have been improved for a 3D case. Numerical results are in good agreement with the measured data of the Gezhouba Project. The present method has been employed to simulate sediment erosion and deposition in the vicinity of the Three Gorges Dam. The size distribution of the deposits and bed material, and flow and sediment concentration at different times and elevations, are predicted. The results agree well with the observations in physical experiments. Thus, a new method is established for 3D simulation of sediment motion in the vicinity of dams.     

Validation of a Three-Dimensional Numerical Code in the Simulation of Pseudo-Natural Meandering Flows

Validation of a three-dimensional finite volume code solving the Navier–Stokes equations with the standard k-ε turbulence model is conducted using a high quality and high spatial resolution data set. The data set was collected from a large-scale meandering channel with a self-formed fixed bed, and comprises detailed bed profiling and laser Doppler anemometer velocity measurements. Comparisons of the computed primary and secondary velocities are made with those observed and it is found that the lateral momentum transfer is generally under predicted. At the apices this results in the predicted position of the primary velocity maximum having a bias towards the channel center, compared to the position where it has been measured. Using a simplified two zone roughness distribution whereby a separate roughness height was prescribed for the channel center and channel sides relative to a single distributed roughness height, generally led to a slightly improved longitudinal velocity distribution; the higher velocities were located nearer to the outside of the bend. Improving both the free surface calculation and scheme for discretization of the convection terms led to no appreciable difference in the computed velocity distributions. A more detailed study involving turbulence measurements and bed form height distribution should discriminate whether using distributed roughness height is a precursor to using an anisotropic turbulence representation for the accurate prediction of three-dimensional river flows.     

3D Unsteady RANS Modeling of Complex Hydraulic Engineering Flows. I: Numerical Model

A general-purpose numerical method is developed for solving the full three-dimensional (3D), incompressible, unsteady Reynolds-averaged Navier-Stokes (URANS) equations in natural river reaches containing complex hydraulic structures at full-scale Reynolds numbers. The method adopts body-fitted, chimera overset grids in conjunction with a grid-embedding strategy to accurately and efficiently discretize arbitrarily complex, multiconnected flow domains. The URANS and turbulence closure equations are discretized using a second-order accurate finite-volume approach. The discrete equations are integrated in time via a dual-time-stepping, artificial compressibility method in conjunction with an efficient coupled, block-implicit, approximate factorization iterative solver. The computer code is parallelized to take full advantage of multiprocessor computer systems so that unsteady solutions on grids with 106 nodes can be obtained within reasonable computational time. The power of the method is demonstrated by applying it to simulate turbulent flow at R ? 107 in a stretch of the Chattahoochee River containing a portion of the actual bridge foundation located near Cornelia, Georgia. It is shown that the method can capture the onset of coherent vortex shedding in the vicinity of the foundation while accounting for the large-scale topographical features of the surrounding river reach.     

Direct Numerical Simulation of Turbulent Flow in a Square Duct: Analysis of Secondary Flows

A direct numerical simulation of turbulent flow in a square duct was performed for a Reynolds number based on bulk streamwise velocity and duct height equal to 4,440. The mechanism by which secondary flows are generated in a square duct was investigated. Two counterrotating secondary flows occur around the duct corner. These secondary flows were found to play a key role in momentum transfer between the corner and center of the duct. A conditional quadrant analysis was performed in the local maximum and minimum regions of the wall shear stress in order to characterize the pattern of the mean secondary flows.     

Modeling Study of the Flow past Irregularities in a Pressure Conduit

Results are presented to investigate the characteristics of turbulent flow in a pressure conduit, such as water supply pipes and flood discharging tunnels. The turbulent flow governing equations, the Reynolds-averaged Navier–Stokes equations, in conjunction with a k–ε turbulent model are numerically solved using SIMPLEC. The study focuses on the modeling and calculation of the flow velocity field, pressure distribution, and the incipient cavitation number of the surface irregularities in the conduit. Different types and sizes of irregularities are simulated for various incoming flow velocities. The computed results are in good agreement with laboratory experimental data.     

Case Study: Refinement of Hydraulic Operation of a Complex CSO Storage/Treatment Facility by Numerical and Physical Modeling

The performance of a combined sewer overflow (CSO) storage/treatment facility in North Toronto, Ont., Canada, was investigated by conjunctive numerical and physical (hydraulic) modeling. The main objectives of the study were to (1) assess the feasibility of increasing the hydraulic loading of the CSO facility without bypassing; and (2) establish a verified numerical model of the facility for future work. The numerical model [a commercial computational fluid dynamics (CFD), PHOENICS] was validated and verified using results from a hydraulic scale model (1:11.6). The results obtained show that the CFD model can simulate hydraulic conditions in the facility well, as demonstrated by accurate reproduction of the filling rate, water levels at various locations, flow velocities in feed pipes, and overflows from the inflow channel. Numerical simulations identified excessive local head losses and helped select structural changes to reduce such losses. The analysis of the facility showed that with respect to hydraulic operation, the facility is a complex, highly nonlinear hydraulic system. Within the existing constraints, a few structural changes examined by numerical simulation could increase the maximum treatment flow rate in the CSO storage/treatment facility by up to 31%.     

3D Unsteady RANS Modeling of Complex Hydraulic Engineering Flows. II: Model Validation and Flow Physics

A chimera overset grid flow solver is developed for solving the unsteady Reynolds-averaged Navier-Stokes (RANS) equations in arbitrarily complex, multiconnected domains. The details of the numerical method were presented in Part I of this paper. In this work, the method is validated and applied to investigate the physics of flow past a real-life bridge foundation mounted on a fixed flat bed. It is shown that the numerical model can reproduce large-scale unsteady vortices that contain a significant portion of the total turbulence kinetic energy. These coherent motions cannot be captured in previous steady three-dimensional (3D) models. To validate the importance of the unsteady motions, experiments are conducted in the Georgia Institute of Technology scour flume facility. The measured mean velocity and turbulence kinetic energy profiles are compared with the numerical simulation results and are shown to be in good agreement with the numerical simulations. A series of numerical tests is carried out to examine the sensitivity of the solutions to grid refinement and investigate the effect of inflow and far-field boundary conditions. As further validation of the numerical results, the sensitivity of the turbulence kinetic energy profiles on either side of the complex pier bent to a slight asymmetry of the approach flow observed in the experiments is reproduced by the numerical model. In addition, the computed flat-bed flow characteristics are analyzed in comparison with the scour patterns observed in the laboratory to identify key flow features responsible for the initiation of scour. Regions of maximum shear velocity are shown to correspond to maximum scour depths in the shear zone to either side of the upstream pier, but numerical values of vertical velocity are found to be very important in explaining scour and deposition patterns immediately upstream and downstream of the pier bent.     

Simulating Sediment Transport in a Flume with Forced Pool-Riffle Morphology: Examinations of Two One-Dimensional Numerical Models

One-dimensional numerical sediment transport models (DREAM-1 and DREAM-2) are used to simulate seven experimental runs designed to examine sediment pulse dynamics in a physical model of forced pool-riffle morphology. Comparisons with measured data indicate that DREAM-1 and -2 closely reproduce the sediment transport flux and channel bed adjustments following the introduction of fine and coarse sediment pulses, respectively. The cumulative sediment transport at the flume exit in a DREAM-1 simulation is within 10% of the measured values, and cumulative sediment transport at flume exit in a DREAM-2 simulation is within a factor of 2 of the measured values. Comparison of simulated and measured reach-averaged aggradation and degradation indicates that 84% of DREAM-1 simulation results have errors less than 3.3?mm, which is approximately 77% of the bed material geometric mean grain size or 3.7% of the average water depth. A similar reach-averaged comparison indicates that 84% of DREAM-2 simulation results have errors less than 7.0?mm, which is approximately 1.7 times the bed material geometric mean grain size or 11% of the average water depth. Simulations using measured thalweg profiles as the input for the initial model profile produced results with larger errors and unrealistic aggradation and degradation patterns, demonstrating that one-dimensional numerical sediment transport models need to be applied on a reach-averaged basis.     

Dynamic Routing of Flow Resistance and Alluvial Bed-Form Changes from the Lower to the Upper Regime

On the basis of a Strouhal number and the definition of the control factor, m, a new routing to calculate the energy slope in the lower and upper alluvial regimes is proposed. The control factor, m, representing the interactions in alluvial rivers, is reckoned as a bed-form index: while the flow evolves through transition, the control factor, m, decreases from m = 2, associated with two-dimensional fully developed dunes, to m = 1, associated primarily with in-phase waves. The way to predict the value of the control factor, m, is drawn from a previously published criterion for delineating the upper regime and is calibrated with experimental data. On several data from flumes and rivers, the routing is tested and compared with other methods from the literature. It appears that the new routing is the most robust because it allows researchers to obtain low averages of the discrepancy ratio for a wide range of ratios between the water depth and the median sediment diameter. On a selection of contrasted freshet events, the new routing allows for the capture of the primary dynamic of the flow resistance decrease.     

Numerical Modeling of River Flow for Ecohydraulic Applications: Some Experiences with Velocity Characterization in Field and Simulated Data

Information regarding the spatial and temporal organization of river flow is required for many applications in river management, and is a fundamental requirement in ecohydraulics. As an alternative to detailed field surveys and to mesohabitat reconnaissance schemes, potential exists to deploy numerical flow simulation as an assessment and design tool. A key question is the extent to which complex hydrodynamic models are really practical in river management applications. This paper presents experiences using sediment simulation in intakes with multiblock, a three-dimensional modeling code, in conjunction with a statistical approach for classifying the spatiotemporal dynamics of flow behavior. Even in a simple configuration, the model is able to replicate well flow structures which associate with the mesohabitat concepts used in field reconnaissance techniques. The model also captures spatiotemporal dynamics in flow and depth behavior at these scales. However, because the model shows differential performance between flow stages and between differing channel (bed form) units, the smaller-scale and discharge-dependent dynamics of some zones within the channel may be less-well represented, and the implications of this for future research are noted.     

Numerical Simulation of Flushing of Trapped Salt Water from a Bar-Blocked Estuary

Results of a numerical simulation investigating the complicated flushing process of an isolated trapped volume of salt water from a bar-blocked estuary are presented. A multiphase model, a part of the commercial code FLUENT 6.2, is applied. The governing equations together with initial and boundary conditions and the numerical scheme are described. The time-dependent salt-wedge position, vertical-density distribution, and proportion of total input kinetic energy converted into potential energy are examined for various incoming flow densimetric Froude number and estuary bed slope. The vertical position and thickness of the interfacial mixed layer between freshwater and salt water as well as the local gradient Richardson number are determined from simulated density profiles and velocity fields. The good agreement between the simulated and measured results indicates that the numerical model can be successfully applied to investigate the complex flushing process involving stratified flow.     

Case Study of an S-Shaped Spillway Using Physical and Numerical Models

The spillway studied in this paper was designed as an “S” shape in plan view, characterized in two curved conduits, steep slopes, and small curvature radiuses. An approach of the combination of physical and numerical models was adopted to study the hydraulic characteristics and examine the feasibility of the design. By setting proper sills whose specific layouts were determined by numbers of experiments at the bottom of the curved conduits, the flow pattern was significantly improved. The k–ε turbulence model was used to simulate the three-dimensional turbulent flow. The free water surface was determined by the volume of fluid method, and the governing equations were solved by the finite volume method. Simulated results of the free water surface and the velocity are in good agreement with measured data. It is shown that S shaped spillways are feasible in practical projects. Moreover, numerical simulations are useful for the design and analysis of S shaped spillways.