Hydrodynamic Behavior of Asymmetric Oscillatory Boundary Layers at Low Reynolds Numbers

An experimental and numerical study has been carried out to study the wave boundary layers under asymmetric waves. The experiments were conducted in an oscillating tunnel using a simple mechanical system to generate an asymmetric oscillatory motion similar to cnoidal waves. The velocities were measured by laser Doppler velocimetry and the bottom shear stress was calculated from the cross-stream velocity profile. A low Reynolds number k–ε model was used to predict the hydrodynamic properties of the cnoidal wave boundary layers. After validating the model with the experimental data, a series of numerical experiments were carried out to study the transitional behavior of these boundary layers by virtue of friction factor and phase difference between mean free-stream velocity and bottom shear stress. Finally a stability diagram was drawn to demarcate the laminar, transition, and fully turbulent regimes using the numerical results. The present study would be useful for the hydraulic and coastal engineers interested in calculating bottom shear stress in order to compute the sediment transport in coastal environments.     

Modeling Impacts of Thaw Lakes to Ground Thermal Regime in Northern Alaska

In northern Alaska, the ground is largely underlain by permafrost. Many engineering problems in this region can be attributed to the variations of ground thermal regime. Engineering projects such as construction of gas pipelines must be based on a good understanding of ground thermal regime and its interaction with seasonal climate changes. Numerical modeling is used to simulate a multimedia system with transient heat transfer in this research. The system includes a snow cover on the top, a shallow lake in the middle, and soils beneath the lake. The finite-element method is used for the spatial domain solution, and the finite-difference method is used for the temporal domain solution. The model is applied to three sites in northern Alaska for a nine-month period during the winter of 1995–1996. The result reveals the impacts of thaw lake on the ground thermal regime, the formation of the talik, as well as the formation of ice in the lake. The model is verified against field observations. The difference between the simulated and observed ice thickness is less than 3%.     

Using CFD Modeling to Improve the Inlet Hydraulics and Performance of a Storm-Water Clarifier

The feasibility of high-rate treatment of storm water achieving total suspended solids (TSS) removals in the range from 60 to 80% was studied using an available clarifier. The clarifier (3?m long, 1.4?m wide, and 2?m deep) was fitted with a removable lamella pack and had a limited flow capacity (surface load rate of 35?m/h). To achieve the desired removals of TSS, the clarifier required polymer feed (4?mg/L), which caused maintenance problems during intermittent storm-water treatment—laborious and costly cleaning of lamella plates after individual storm events. This problem posed the following challenge: was it feasible to avoid costly maintenance by removing the lamella pack and at the same time to retain the high TSS removals by improving the clarifier hydraulics by internal structural changes? The purpose of the paper is to evaluate such changes by focusing on different inlet configurations designed using computational fluid dynamics (CFD) simulations. This analysis resulted in adopting a U-tube duct inlet (inserted into the outer box of the original clarifier) with two special features: (1) three horizontal slot openings (width = 0.1?m) releasing flow into the clarifier and (2) a narrow slot opening in the bottom U bend allowing removal of grit. The flow release slots in the rising leg of the U tube were fitted, along the upper edge, with horizontal trailing plates protruding 0.15?m into the clarifier and forcing the flow to move horizontally. This clarifier design performed well, but storm-water grit accumulated at the bottom of the U tube, which had to be cleaned out after individual storms to avoid plugging. This issue was resolved by allowing grit to move into the sludge storage compartment of the clarifier through a narrow tilted slot opening in the U-tube bottom. The final clarifier design with polymer feed, without lamellas, produced TSS removals comparable to those in the original lamella clarifier (almost 80%), but at a higher surface loading rate (43?m/h, which was limited by the feed pump capacity). CFD modeling, in comparison to conventional methods of hydraulic design, served as a flexible and powerful tool providing distinct advantages with respect to the speed, efficiency and reduced cost of analysis, and a better understanding of the clarifier operation.     

Circulation in Stratified Lakes due to Flood-Induced Turbidity Currents

The river inflow in a natural lake with important suspended sediment load during floods, can impact water quality by mobilizing dissolved matters like phosphorous from deep to surface waters. Generally due to thermal stratification in prealpine lakes, the water column is stable. It does not mix vertically unless acted on by outside forces, for example, currents or winds. Since Lake Lugano has a strong thermal stratification, river inflow exhibits different modes of density currents, from surface flows and thermocline intrusion to bottom currents. Turbidity currents are the direct cause of the downward water flow, and at the same time at the origin of upward directed flow. In this study, the impact of river born turbidity currents in Lake Lugano under varying ambient conditions was investigated using field measurements at the inflow river and inside the lake, together with a full three-dimensional numerical model of the entire lake. The paper characterizes the induced circulation of the turbidity plume and gives some indications on the relevance of turbidity currents on the lake.     

Downscaling Model Resolution to Illuminate the Internal Wave Field in a Small Stratified Lake

This paper presents the application of hydrostatic and nonhydrostatic three-dimensional hydrodynamic models to a stratified lake. Focus was given to the multiscale response of the internal wave field to strong wind gusts exceeding 20?m?s?1. Simulations were performed using different horizontal grid resolutions with uniform grid sizes varying from 100×100 to 10×10?m. Results of the hydrostatic models were used to investigate the large-scale features of the internal wave motion. With the intent of investigating the high-frequency waves, observed results of these simulations were used as initial conditions for nonhydrostatic simulations using smaller grids. Wavelength of the high frequency waves decreased with grid resolution. However, none of the uniform grids were sufficiently fine to capture the waves of the highest frequency. Simulations performed using a nonuniform grid produced internal waves of similar frequency of the waves observed in the field. The simulations showed that these waves were shear unstable modes and that their vertical and horizontal length scales were in close agreement with results from linear stability analysis.     

Integrated Hydrodynamic and Water Quality Modeling System to Support Nutrient Total Maximum Daily Load Development for Wissahickon Creek, Pennsylvania

This paper presents a hydrodynamic and water quality modeling system for Wissahickon Creek, Pa. Past data show that high nutrient levels in Wissahickon Creek were linked to large diurnal fluctuations in oxygen concentration, which combining with the deoxygenation effect of carbonaceous biological oxygen demand (CBOD) causes violations of dissolved oxygen (DO) standards. To obtain quantitative knowledge about the cause of the DO impairment, an integrated modeling system was developed based on a linked environmental fluid dynamics code (EFDC) and water quality simulation program for eutrophication (WASP/EUTRO5) modeling framework. The EFDC was used to simulate hydrodynamic and temperature in the stream, and the resulting flow information were incorporated into the WASP/EUTRO5 to simulate the fate and transport of nutrients, CBOD, algae, and DO. The standard WASP/EUTRO5 model was enhanced to include a periphyton dynamics module and a diurnal DO simulation module to better represent the prototype. The integrated modeling framework was applied to simulate the creek for a low flow period when monitoring data are available, and the results indicate that the model is a reasonable numerical representation of the prototype.     

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.     

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.     

Modeling Salt Water Intrusion in Tanshui River Estuarine System—Case-Study Contrasting Now and Then

A vertical (laterally integrated) two-dimensional numerical model was applied to study the salt water intrusion in the Tanshui River estuarine system, Taiwan. The river system has experienced dramatic changes in the past half century because of human intervention. The construction of two reservoirs and water diversion in the upper reaches of the river system significantly reduces the freshwater inflow. The land subsidence within the Taipei basin and the enlargement of the river constriction at Kuan-Du have lowered the river bed. Both changes have contributed farther to the intrusion of tidal flow and salt water in the upstream direction. The model was reverified with the earliest available hydrographic data measured in 1977. The overall performance of the model is in reasonable agreement with the field data. The model was then used to investigate the change in salt water intrusion as a result of reservoir construction and bathymetric changes in the river system. The model simulation study reveals that significant salinity increases have resulted from the combined changes. It has been speculated by ecological researchers that the long-term increase in salinity might be the driving force altering the aquatic ecosystem structure in the lower reach of the estuary and the Kuan-Du mangrove swamp, particularly the enlargement of the mangrove area and the disappearance of freshwater marshes. However, concrete proof has not been available since no prototype salinity data were available prior to the reservoir construction. This case study offers the first quantitative estimate of the salinity changes due to human interference in this natural system.     

Modeling Depth-Averaged Velocity and Boundary Shear in Trapezoidal Channels with Secondary Flows

The Shiono and Knight method (SKM) offers a new approach to calculating the lateral distributions of depth-averaged velocity and boundary shear stress for flows in straight prismatic channels. It accounts for bed shear, lateral shear, and secondary flow effects via 3 coefficients—f,λ, and Γ—thus incorporating some key 3D flow feature into a lateral distribution model for streamwise motion. The SKM incorporates the effects of secondary flows by specifying an appropriate value for the Γ parameter depending on the sense of direction of the secondary flows, commensurate with the derivative of the term Hρ(UV)d. The values of the transverse velocities, V, have been shown to be consistent with observation. A wide range of boundary shear stress data for trapezoidal channels from different sources has been used to validate the model. The accuracy of the predictions is good, despite the simplicity of the model, although some calibration problems remain. The SKM thus offers an alternative methodology to the more traditional computational fluid dynamics (CFD) approach, giving velocities and boundary shear stress for practical problems, but at much less computational effort than CFD.     

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.     

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%.     

Steady and Unsteady Simulations of Turbulent Flow and Transport in Ultraviolet Disinfection Channels

The effects of unsteadiness in the turbulent flow through a staggered array of circular cylinders, modeling an ultraviolet disinfection system, are studied by means of solutions of the two-dimensional Reynolds-averaged Navier–Stokes equations incorporating the standard k–? turbulence model. Time averaging is applied to the unsteady solution, and the time-averaged characteristics are compared with a solution where a steady flow is a priori assumed, as well as with time-averaged measurements. Differences between the predictions of time-averaged and the steady-flow models are found to be largest in the entrance region of the array, and to decline in importance in the downstream direction. Comparison with measurements indicate that, while the time-averaged unsteady model predictions exhibited better agreement in some respects, the turbulent kinetic energy remained substantially underpredicted. Predictions of head losses through the array are also discussed.     

Numerical Study of the Transition Regime between the Skimming and Wake Interference Flows in a Water Flume by Using the Lattice-Model Approach

A numerical study to describe the transition regime between the skimming and wake interference flows due to the influence of an idealized bed roughness in a water flume was carried out here using the lattice model approach. The model reproduced the skimming, transition, and wake interference regimes for different aspect ratios that determine the bed roughness geometry. The simulated turbulent structures were visualized by drawing the trajectories of a large number of passive tracer particles released in the computational domain, and the results agreed with those reported by the research works. The dimensionless streamwise and vertical turbulent intensities were calculated at five test sections. The results obtained supported the visualized flow patterns permitting us to detect the presence of a shear layer developed at the top of the roughness element, whose strength varied according to the flow regime simulated.     

WAF Method and Splitting Procedure for Simulating Hydro- and Thermal-Peaking Waves in Open-Channel Flows

Hydro- and thermal-peaking waves, generated by hydroelectric power generation, have a strong impact on the ecological integrity of aquatic ecosystems. In order to reduce such effects, mitigation procedure must be studied and implemented. To this end a one-dimensional model which solves the coupling of hydrodynamics with heat transport is developed. The solution is obtained advancing simultaneously the hydrodynamic and thermal module with the same accuracy. For the numerical solution of the governing advection-reaction/diffusion problem a splitting procedure is adopted: the advection-reaction part is solved by means of the weight average flux (WAF) finite volume explicit method, while the diffusion part is solved using a nonlinear version of the implicit Crank-Nicolson method. The WAF method is extended to second-order in the presence of reaction terms. Numerical results are presented for different test examples, which demonstrate the accuracy and robustness of the scheme and its applicability in predicting temperature transport by shallow water flows. Application to the Adige River (Northern Italy) of this framework proves that the model is an effective tool for designing hydro- and thermal-peaking waves mitigation procedures.     

3D Numerical Modeling of Flow and Sediment Transport in Laboratory Channel Bends

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?ε 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?ε model.     

Analytical and Numerical Modeling Assessment of Capillary Barrier Performance Degradation due to Contaminant-Induced Surface Tension Reduction

Analytical and numerical models of capillary barrier performance commonly use hydraulic characteristics measured using pure water. However, the potential exists for an infiltrating solution to have a surface tension lower than that of pure water due to the presence of surface-active contaminants (surfactants). A lower surface tension solution may impact capillary barrier performance due to the dependence of capillarity on surface tension. An existing analytical solution for capillary diversion length (L) was modified to include the effect of surface tension reduction on steady-state capillary barrier performance during uniform and constant infiltration. The L for a surfactant-contaminated system was found to be less than for a pure water system and equal to L for a pure water system multiplied by the relative surface tension. Numerical modeling using HYDRUS-2D also showed that diversion was less in the surfactant-contaminated system and that the difference in the performance of the two systems was due to the fact that the fine layer in the capillary barrier retains less liquid when wetted with surfactant solution.     

Depth-Averaged Two-Dimensional Numerical Modeling of Unsteady Flow and Nonuniform Sediment Transport in Open Channels

A depth-averaged two-dimensional (2D) numerical model for unsteady flow and nonuniform sediment transport in open channels is established using the finite volume method on a nonstaggered, curvilinear grid. The 2D shallow water equations are solved by the SIMPLE(C) algorithms with the Rhie and Chow’s momentum interpolation technique. The proposed sediment transport model adopts a nonequilibrium approach for nonuniform total-load sediment transport. The bed load and suspended load are calculated separately or jointly according to sediment transport mode. The sediment transport capacity is determined by four formulas which are capable of accounting for the hiding and exposure effects among different size classes. An empirical formula is proposed to consider the effects of the gravity on the sediment transport capacity and the bed-load movement direction in channels with steep slopes. Flow and sediment transport are simulated in a decoupled manner, but the sediment module adopts a coupling procedure for the computations of sediment transport, bed change, and bed material sorting. The model has been tested against several experimental and field cases, showing good agreement between the simulated results and measured data.     

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.     

Improved Compartmental Modeling and Application to Three-Phase Contaminant Transport in Unsaturated Porous Media

The compartmental modeling approach has been widely used for simulating contaminant transport in porous media and surface waters. Yet a commonly used compartmental model that has only first-order accuracy may introduce considerable numerical errors under certain circumstances. Following a review of compartmental systems and compartmental modeling methodologies, performance and limitations of such a compartmental model are discussed. In particular, improvement approaches, including multipoint, high-order, linear, and nonlinear methods, are presented in detail. Finally, a number of testing problems are examined and various compartmental models that describe three-phase (dissolved, adsorbed, and vapor phases) contaminant transport in unsaturated porous media are compared with each other and also with standard numerical and analytical counterparts. The comparisons highlight the accuracy, applicability, and limitations of different compartmental models.