Large-Eddy Simulation of Turbulent Flow over a Fixed Two-Dimensional Dune

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.     

Large-Eddy Simulation of High Reynolds Number Turbulent Flow Past a Square Cylinder

Flow past a square cylinder at a Reynolds number of 21,400 has been studied numerically using the large-eddy simulation technique. A dynamic subgrid-scale stress model has been used for the small scales of turbulence. The time- and span-averaged axial and transverse velocities in the downstream of the cylinder are in good agreement with the experimental results. The distribution of turbulent normal and shear stresses is also well predicted. The coherent and incoherent components of turbulent fluctuations at some specified phases have been separated and their relative magnitudes downstream of the cylinder have been compared. The comparison shows more coherence in the near wake than the far wake, while the coherent and incoherent components are of comparable magnitude in the far wake. The far wake shows irregular phase-averaged structures.     

Case Study: Model to Simulate Regional Flow in South Florida

South Florida has a complex regional hydrologic system that consists of thousands of miles of networked canals, sloughs, highly pervious aquifers, open areas subjected to overland flow and sheet flow, agricultural areas and rapidly growing urban areas. This region faces equally complex problems related to water supply, flood control, and water quality management. Advanced computational methods and super fast computers alone have limited success in solving modern day problems such as these because the challenge is to model the complexity of the hydrologic system, while maintaining computational efficiency and acceptable levels of numerical errors. A new, physically based hydrologic model for South Florida called the regional simulation model (RSM) is presented here. The RSM is based on object oriented design methods, advanced computational techniques, extensible markup language, and geographic information system. The RSM uses a finite volume method to simulate two-dimensional (2D) surface and groundwater flow. It is capable of working with unstructured triangular and rectangular mesh discretizations. The discretized control volumes for 2D flow, canal flow and lake flow are treated as abstract “water bodies” that are connected by abstract “water movers.” The numerical procedure is designed to work with these and many other abstractions. An object oriented code design is used to provide robust and highly extensible software architecture. A weighted implicit numerical method is used to keep the model fully integrated and stable. A limited error analysis was carried out and the results were compared with analytical error estimates. The paper describes an application of the model to the L-8 basin in South Florida and the strength of this approach in developing models over complex areas.     

Influence of Mesh Geometry on Three-Dimensional Finite-Element Analysis of Tunnel Excavation

Finite element (FE) analysis has become an important tool for predicting building response to tunnel-induced ground movement. Because tunnel construction is a three-dimensional (3D) process, the trend is to apply 3D FE analysis to tunnel-soil-building interaction problems instead of applying the plane-strain models that are commonly used in engineering practice. Since 3D FE analyses require large amounts of computational resources, the geometric dimensions of the 3D models are often kept to a minimum to reduce calculation time. There is, however, a lack of published information concerning appropriate mesh dimensions. This paper investigates the influence of the geometry and the dimension of a 3D FE model on tunnel-induced surface settlement predictions. The paper shows how the vertical boundaries can influence the results. It demonstrates that reasonable results can be obtained by increasing the length of incremental tunnel excavation and by scaling back the settlement values to give a required tunnel volume loss. This study therefore not only highlights the limitations of 3D modeling but also shows its potential for engineering practice.     

Flood Simulation Using a Well-Balanced Shallow Flow Model

This work extends and improves a one-dimensional shallow flow model to two-dimensional (2D) for real-world flood simulations. The model solves a prebalanced formulation of the fully 2D shallow water equations, including friction source terms using a finite volume Godunov-type numerical scheme. A reconstruction method ensuring nonnegative depth is used along with a Harten, Lax, and van Leer approximate Riemann solver with the contact wave restored for calculation of interface fluxes. A local bed modification method is proposed to maintain the well-balanced property of the algorithm for simulations involving wetting and drying. Second-order accurate scheme is achieved by using the slope limited linear reconstruction together with a Runge-Kutta time integration method. The model is applicable to calculate different types of flood wave ranging from slow-varying inundations to extreme and violent floods, propagating over complex domains including natural terrains and dense urban areas. After validating against an analytical case of flow sloshing in a domain with a parabolic bed profile, the model is applied to simulate an inundation event in a 36?km2 floodplain in Thamesmead near London. The numerical predictions are compared with analytical solutions and alternative numerical results.     

Application of Several Depth-Averaged Turbulence Models to Simulate Flow in Vertical Slot Fishways

Vertical slot fishways are hydraulic structures which allow the upstream migration of fish through obstructions in rivers. The velocity, water depth, and turbulence fields are of great importance in order to allow the fish swimming through the fishway, and therefore must be considered for design purposes. The aim of this paper is to assess the possibility of using a two-dimensional shallow water model coupled with a suitable turbulence model to compute the flow pattern and turbulence field in vertical slot fishways. Three depth-averaged turbulence models of different complexity are used in the numerical simulations: a mixing length model, a k?ε model, and an algebraic stress model. The numerical results for the velocity, water depth, turbulent kinetic energy, and Reynolds stresses are compared with comprehensive experimental data for three different discharges covering the usual working conditions of vertical slot fishways. The agreement between experimental and numerical data is very satisfactory. The results show the importance of the turbulence model in the numerical simulations, and can be considered as a useful complementary tool for practical design purposes.     

Identification of Geologic Fault Network Geometry by Using a Grid-Based Ensemble Kalman Filter

Discrete geologic features such as faults and highly permeable embedded channels can significantly affect subsurface flow and transport characteristics. Therefore, they must be properly identified, parameterized, and represented in subsurface simulation models. In this work, we use an improved ensemble Kalman filter (EnKF) for history-matching fault network geometry from production data. EnKF is a sequential Monte Carlo data assimilation method that simultaneously propagates and updates an ensemble of model states, resulting in a set of calibrated model realizations that can be readily used for model prediction and uncertainty analysis. A pattern-based stochastic simulation algorithm was used to generate fault network realizations based on a priori fault trace data. The classic EnKF algorithm was enhanced with a grid-based covariance localization scheme to better handle non-Gaussian permeability distributions resulting from the presence of faults. Numerical experiments indicate that the modified EnKF can be a promising method for uncovering unmapped faults by using production data.     

Treatment of Natural Geometry in Finite Volume River Flow Computations

A method is proposed for the treatment of irregular bathymetry in one-dimensional finite volume computations of open-channel flow. The strategy adopted is based on a reformulation of the Saint-Venant equations. In contrast with the usual treatment of topography effects as source terms, the method accounts for slope and nonprismaticity by modifying the momentum flux. This makes it possible to precisely balance the hydrostatic pressure contributions associated with variations in valley geometry. The characteristic method is applied to the revised equations, yielding topographic corrections to the numerical fluxes of an upwind scheme. Further adaptations endow the scheme with an ability to capture transcritical sections and wetting fronts in channels of abrupt topography. To test the approach, the scheme is first applied to idealized benchmark problems. The method is then used to route a severe flood through a complex river system: the Tanshui in Northern Taiwan. Computational results compare favorably with gauge records. Discrepancies in water stage represent no more than a fraction of the magnitude of typical bathymetry variations.     

Conservative Scheme for Numerical Modeling of Flow in Natural Geometry

A numerical model is proposed to compute one-dimensional open channel flows in natural streams involving steep, nonrectangular, and nonprismatic channels and including subcritical, supercritical, and transcritical flows. The Saint-Venant equations, written in a conservative form, are solved by employing a predictor-corrector finite volume method. A recently proposed reformulation of the source terms related to the channel topography allows the mass and momentum fluxes to be precisely balanced. Conceptually and algorithmically simple, the present model requires neither the solution of the Riemann problem at each cell interface nor any special additional correction to capture discontinuities in the solution such as artificial viscosity or shock-capturing techniques. The resulting scheme has been extensively tested under steady and unsteady flow conditions by reproducing various open channel geometries, both ideal and real, with nonuniform grids and without any interpolation of topographic survey data. The proposed model provides a versatile, stable, and robust tool for simulating transcritical sections and conserving mass.     

Identification and Validation of a Discrete Element Model for Concrete

The use of a three-dimensional discrete element method (DEM) is proposed to study concrete structures submitted to dynamic loading. The aim of this paper is to validate the model first in the quasistatic domain, and second in dynamic compression, at the sample scale. A particular growing technique is used to set a densely packed assembly of arbitrarily sized spherical particles interacting together, representing concrete. An important difference from classical DEMs where only contact interactions are considered, is the use of an interaction range. First, the correct identification of parameters of the DEM model to simulate elastic and nonlinear deformation including damage and rupture is made through quasistatic uniaxial compression and tension tests. The influence of the packing is shown. The model produces a quantitative match of strength and deformation characteristics of concrete in terms of Young’s modulus, Poisson’s coefficient, and compressive and tensile strengths. Then, its validity is extended through dynamic tests. The simulations exhibit complex macroscopic behaviors of concrete, such as strain softening, fractures that arise from extensive microcracking throughout the assembly, and strain rate dependency.     

板坯结晶器流场的水模型实验和数值模拟

按1:2比例,通过多普勒激光测速仪(LDV)测试了水口插入深度130～170 mm、水口侧孔倾角 -30°~10°时640 mm×90mm 水模型结晶器的流场,并用FLUENT 软件进行了数值模拟。结果表明,浸入式 水口插入深度和水口侧孔倾角增大,有利于稳定钢液自由液面的波动,但增加了下流股的冲击深度,大量热钢 液下流,不利于夹杂物上浮。     

Two-Dimensional Simulation Model of Sediment Removal and Flow in Rectangular Sedimentation Basin

A steady, two-dimensional numerical model was created to study the hydrodynamics of a rectangular sedimentation basin under turbulent conditions. The strip integral method was used to formulate the flow equations, using a forward marching scheme for solving the governing partial differential equations of continuity, momentum, advection–diffusion, turbulent kinetic energy, and its dissipation. In this way the flow equations were converted to a set of ordinary differential equations (ODEs) in terms of the key physical parameters. These parameters, along with a set of shape functions, describe flow variables including the velocity, the concentration of suspended sediments, and both the kinetic energy and its dissipation rate. Four Gaussian distributions were investigated, one corresponding to each flow parameter. In order to calculate the turbulent shear stresses, a two-equation turbulence model (i.e., k-ε model) was used. A fourth order Runge–Kutta method numerically integrates the set of ODEs. Simulation results were compared with experimental data, and close agreement (generally within 5–10%) was observed.     

Validation and Optimization of an Equilibrium Model of a Hot-Lime-Softening Treatment System

This paper describes the development and validation of an equilibrium model of an industrial hot-lime-softening boiler-water-treatment unit for a large-scale nickel processing facility in which approximately 6.6?ML per hour of water is processed. In the industrial process, multiple water sources of varying quality are combined before the softening treatment, which makes control and optimization of the softening unit complicated and has brought about the necessity of a robust numerical model of water treatment. In this paper, the numerical thermodynamic and adsorption relations describing the softening treatment process are presented. Lime, magnesia, and soda ash additions are modeled. Emphasis has been placed on calcium, magnesium, and silica treatments as these are of most relevance to the industry. Jar tests described in this paper are used to determine adsorption relations, estimate statistical uncertainties, validate the model performance, and optimize the model parameters. Parameter estimations for equilibrium constants are undertaken and provide insights into the range of model validity and interactions between additions and softened water quality. Further jar testing is utilized to evaluate the effectiveness of using the model to numerically derive optimal chemical additions.     

Numerical Simulations of Nonlinear Short Waves Using a Multilayer Model

The multilayer model developed by Lynett and Liu is used for simulating the evolution of deep-water waves in a constant depth. The computational model is tested with experimental data for nonlinear monochromatic and biharmonic waves with kh values as high as 8.3. The experiments were conducted in a super wave tank with dimensions of 300?m×5?m×5.2?m located at Tainan Hydraulics Laboratory of National Cheng-Kung University. The nonlinearity of the waves tested, ka, range from 0.0627 to 0.1577. The overall comparisons between the multilayer model and the experiments are quite good, indicating that the multilayer model is adequate for both linear and nonlinear deep-water waves.     

Validation of a Source–Receiver Model for Road Traffic-Induced Vibrations in Buildings. II: Receiver Model

The objective of part II of this paper is to couple the validated source model for free-field traffic-induced vibrations, which has been presented in part I of the paper, to a receiver model that incorporates the structure and accounts for dynamic soil–structure interaction. The incident wave field is applied to the structure and the response is calculated using a subdomain formulation for dynamic soil–structure interaction. A finite element method is applied to the structure, while the unbounded soil domain is calculated with a boundary element method using the Green’s functions of a homogeneous or a layered half-space. The results of elaborate in situ experiments in and around a single family dwelling during the passage of a truck on joints in a concrete pavement and on a plywood unevenness, are used for the validation of the numerical prediction model. The predicted structural response during the passage of a truck at a speed v = 50?km/h is compared with the experimental results. The agreement between the numerical and the experimental results is very good for the passage on the plywood unevenness and satisfactory for the passage on the joints between the concrete plates.     

Population and Initial Validation of a Formal Model for Construction Safety Risk Management

The transient, unique, and complex nature of construction projects makes safety management exceptionally difficult. Most construction safety efforts are applied in an informal fashion under the premise that simply allocating more resources to safety management will improve site safety. Currently, there is no mechanism by which construction-site safety professionals may formally evaluate safety risk and select safety program elements for implementation. This paper introduces and validates a risk-based safety and health analytical model that can be used to evaluate expected risk given specific worker activities, strategically select highly effective safety program elements for implementation when resources are limited, and quantify resulting risk once the identified safety elements have been implemented. Specifically, the paper has three primary objectives: (1) introduce a risk-based construction safety and health analytical model; (2) validate relevant data used to populate the model; and (3) illustrate the applications of the model in practice. The findings of this research indicate that the values used to populate the model are reliable and that the model has the potential to significantly improve construction safety management.     

方坯连铸液体流动模拟实验

通过水模拟实验装置，对方坯连铸过程中的液体流动行为进行了实验和分析，结晶器内的液流状态分为冲击区和层流区，冲击区占据了结晶器的主要部分。     

Numerical Simulation of Shallow-Water Flow Using a Modified Cartesian Cut-Cell Approach

The Cartesian cut-cell method can be used to represent irregular and complex computational domains with less computational efforts by cutting the grid cells on the boundary surfaces in a background uniform Cartesian mesh. In this study, a modified Cartesian cut-cell grid technique is proposed to better represent complex physical geometries. A point shifting treatment was employed to determine the start and end points of a line segment in cut-cell grids. This led to an improved representation of sharply-shaped corners in surface polygons. Numerical simulation to solve a set of shallow-water equations was performed by incorporating a finite volume approach into the Cartesian cut-cell mesh. The advective fluxes at intercells were first estimated by a Harten, Lax and van Leer for contact wave approximate Riemann solver. In order to improve the model accuracy to the second order, a total variation diminishing-weighted average flux method was applied to work adaptively with the cut-cell mesh. The numerical model was then employed to simulate dam-break flow propagation in a small channel with a rectangular obstacle or a 45° bend. The numerical results show good agreement with available laboratory measurements.     

Model for Coupled Liquid Water Flow and Heat Transport with Phase Change in a Snowpack

The flow of liquid water in a snowpack is complex because of the coupled processes involved, including the phase change between liquid and solid, and the latent and sensible heat transfer processes. To properly describe the details of spatial and temporal changes in a snowpack it is necessary to include these coupled processes. This paper presents a numerical model of coupled liquid water flow and heat transport in a snowpack. The model is intended to quantify infiltration into a snowpack, and evaluate the potential for the formation of distinct heterogeneities in liquid water and heat transport properties in a snowpack. The numerical model solves the two-dimensional form of the governing coupled equations using a finite difference scheme. The governing equations assume thermodynamic equilibrium between the solid and liquid phases in the snowpack. Equations describing the metamorphosis of ice grains during liquid water flow are applied within the model, and the heat and liquid water transport properties of the snow are treated with relations identical to those used for mineral porous media. Sample solution results for an alternative formulation taken from the literature are used to test the present solution, and it is found that the present model yields similar results but with some distinct differences. The effect of direct coupling of the temperature with the liquid water pressure is presented in a simple horizontal freezing simulation, which is compared with the Stefan problem where liquid water is not redistributed. Overall the direct coupling and water redistribution is found to lead to greater front penetration in comparison to the Stefan formulation. For infiltration with gravity it is shown that grain size growth during infiltration leads to increased wetting front penetration.