Modeling of a Rough-Wall Oscillatory Boundary Layer Using Two-Equation Turbulence Models

The standard k?ω turbulence model and two versions of blended k?ω/k?ε models have been used to study the characteristics of a one-dimensional oscillatory boundary layer on a rough surface. The wall boundary condition for the specific dissipation rate of turbulent kinetic energy at the wall is specified in terms of a function based on wall roughness. A detailed comparison has been made for mean velocity, turbulent kinetic energy, Reynolds stress, and wall shear stress with the available experimental data. The three models predict the above properties reasonably well. In particular, the prediction of turbulent kinetic energy for the rough case by the blended models is much better than that for smooth oscillatory boundary layers as reported in previous studies. As a result of the present study, the use of one of the blended models in calculating the sediment transport in coastal environments may be recommended.     

Retracted: Fractal Dimensions of Fracture Surfaces

Compressive strength of porous materials, especially of cement gels, has been estimated by means of fractal dimension of fracture surfaces. The relationship between mechanical strength and fractal characteristics of porous gels has been derived and tested experimentally using samples of cement gels. The dimensions of fracture surfaces have been found to be general parameters independent on the fractal dimensions of inner material components.     

用扩张变换法测量金属断裂表面的分形维数

采用数学形态学中扩张变换的方法,对金属断口剖面迹线的分形维数进行了测量,并将结果与截岛法进行了比较。     

Turbulence Modeling of Flows over Circular Spillways

To predict the characteristics of flows over circular spillways, a turbulence model based on the Reynolds stress model (RSM) is presented. Circular spillways are used to regulate water levels in reservoirs. The flow over the spillway is rapidly varied with highly curvilinear streamlines. The isotropic eddy-viscosity models such as k-ε models are based on the Boussinesq eddy viscosity approximation that assumes the components of the turbulence Reynolds stress tensor linearly vary with the mean rate of strain tensor. Hence, they cannot very precisely predict the characteristics of flows over the spillway. On the other hand, the non-isotropic turbulence models such as the turbulence Reynolds stress models (RSM) that calculate all the components of the Reynolds stress tensor can accurately predict the characteristics of these flows. The k-ε models and RSM were applied in the present study to obtain the flow parameters such as the pressure and velocity distributions as well as water surface profiles. The previously published experimental results were used to validate the simulation predictions. For flow over a circular spillway, RSM appears to properly validate the characteristics of the flow under various conditions in the field, without recourse to expensive experimental procedures.     

Modeling Response of Flexible High-Aspect-Ratio Wings to Wind Turbulence

Analytical modeling of the effect of wind turbulence on flexible high-aspect-ratio aircraft wings typical of unmanned air vehicles (UAVs). The wind model is derived from the Kolmogorov power-law spectrum random field turbulence model invoking the Taylor “frozen-field” hypothesis. The aerodynamic model is based on the typical-section compressible attached-flow with Kutta–Joukowski boundary conditions. The gust loading—both lift and moment—is calculated explicitly for M = 0 and M = 1 as typical of subsonic and transonic flow. The gust loading intensity is shown to decrease as the speed increases, so that the turbulence effects are not significant at transonic speeds. To calculate the wing response we use the continuum two degree-of-freedom cantilever beam model of Goland and derive explicit expressions of the spectral density of both the plunge (bending) and pitch (torsion) response for M = 0. Numerical results are presented for two illustrative wings. Most of the turbulence energy is in the 0–10H range. Since flexibility and high aspect ratio push flutter modes and speeds down, turbulence can be a significant safety issue for UAVs in particular.     

薄板坯连铸中间包内抑湍器的模拟研究

控制中间包内钢液的合理流动对夹杂物的排除有重要影响,为此建立了模拟薄板坯连铸中间包流动情况的水模型。通过测定停留时间分布(RTD)曲线,研究了不同组合控流装置对中间包流体流动特性的影响。结果表明,结构及尺寸合理的抑湍器能延长水口响应时间及平均停留时间、提高活塞流区体积分数及降低死区体积分数;抑湍器与单坝组合的控流装置在控制流体流动方面效果极佳,而抑湍器与单墙单坝、单墙双坝组合的控流装置的控流效果不太理想。     

Definition of Raceway Boundary Using Fractal Theory

The particle velocity contours were obtained by tracking the tracer particles in the raceway region of the COREX melter gasifier model and the contours were irregular.According to the fractal theory,the fractal dimensions of different particle velocity contours were determined.Through the analysis of the fractal dimensions,a new method for precise determination of the raceway boundary was proposed.The results show that,when the velocity is less than 0.18m/s,the particles are located in the stagnant zone and the fractal dimensions of particle velocity contours are almost constant as 1.41;when the velocity increases from 0.18 to 0.83m/s,the particles are located in the rapid movement zone and the fractal dimensions decrease gradually from 1.41 to 1.05;when the velocity is greater than 0.83m/s,the particles are located in the cavity zone and the fractal dimensions are again almost constant as approaching to 1.00.Therefore,the velocity contour of 0.18m/s,which is critical to distinguish the rapid movement zone and stagnant zone,can be used to define the raceway boundary.Based on this method,the effect of blowing rate on raceway size was calculated and the results show that the penetration depth and height of the raceway increase with the increase of blowing rate.     

分形理论分析风口回旋区的边界

基于分形理论更能准确的界定风口回旋区边界。建立COREX熔化气化炉的半周三维冷态模型,利用高速摄影的方法跟踪冷态模型内示踪粒子的运动,得到冷模型观察面板处风口回旋区的颗粒运动信息。通过对大量颗粒运动信息的处理得到风口回旋区范围的颗粒速度标量场,最后运用分形理论对利用不同颗粒速度大小等值线界定的回旋区边界的“不规则”程度进行了研究。结果表明：在回旋区内部颗粒快速运动的空腔区,分维数基本不变,且接近于欧几里得维数1;在停滞区,分维数也基本不变,其数值大致为1.4;从空腔区到停滞区分维数逐渐增大;将停滞区分维数基本不变的速度值作为界定回旋区边界的标准,可以确定回旋区的形状和大小,并可通过余维相加定律计算出三维风口回旋区的内表面积;为风口回旋区的宏观动力学计算以及数值模拟提供准确的边界条件。     

Interaction of Particles and Near-Wall Lift in Slurry Pipelines

In pipeline transport of slurries, it is desirable that the solid particles should be suspended by the fluid phase. Within the body of the flow, turbulent diffusion promotes suspension, but this mechanism is not effective near the lower boundary. Under certain conditions, near-wall fluid lift can provide the necessary support for the particles. The Kutta-Zhukovski equation is used to relate the lift force to the shape of the fluid velocity profile. Comparison with experimental findings shows that this lift may be associated with profiles of solid concentration that show a decreased concentration as the bottom of the pipe is approached. Observed reductions of pressure gradients result from this “off-the-wall” lift force. In certain instances, the slurry pressure gradient is found to be less than that for the “equivalent fluid,” with favorable implications for pipeline economics.     

k–ε Turbulence Modeling of Density Currents Developing Two Dimensionally on a Slope

Dense underflows developing two dimensionally on a slope are simulated numerically. The k–ε model is used for the turbulence closure. The boundary-layer approximation renders the governing equations in the form of parabolic partial differential equations, which are easier to solve numerically than elliptic equations. Evolution of vertical structures of dense underflows is computed along the streamwise direction. Excellent similarity collapses of the computed vertical structures are obtained. The computed profiles of velocity and concentration are compared with measured data, resulting in good agreement. The impact of a parameter representing the stratification level in the k–ε model is investigated. Appropriate values of this parameter, yielding results that are nearly identical to the integral model, are proposed. Water entrainment coefficients are estimated from computed solutions, and are observed to fall within the range of previous measurements. Finally, by using the collapsed vertical structures, profile constants defined in the integral model are calculated, which assures that the top-hat assumption necessary for deriving the integral model is valid.     

Modeling Natural Convection in Copper Electrorefining: Describing Turbulence Behavior for Industrial-Sized Systems

A computational fluid dynamics (CFD) model of copper electrorefining is discussed, where natural convection flow is driven by buoyancy forces caused by gradients in copper concentration at the electrodes. We provide experimental validation of the CFD model for several cases varying in size from a small laboratory scale to large industrial scale, including one that has not been compared with a CFD model. Previously, the large-scale systems have been thought to be turbulent by some workers and modeled accordingly with k-ε type turbulence models, but others have not considered turbulence effects in their modeling. We find that the turbulence model does not predict turbulence exists; however, we analyze carefully the fluctuation statistics predicted for a transient model, finding that most cases considered do exhibit a type of turbulence, an instability related to the interaction between velocity and copper concentration fields. We provide a comparison of the extent of turbulence for various electrode heights, and gap widths, and we emphasize industrial-sized electrorefining cells.     

Modeling Weather-Sensitive Construction Activity Using Simulation

This paper proposes a framework for simulating weather-sensitive construction projects that are executed under extreme weather conditions. It applies the framework steps to enable simulating and planning pipeline construction activities under severe cold weather conditions. The uncertainties caused by weather, such as extreme cold, heat, wind, or precipitation, can significantly affect a project’s schedule and produce significant deviations from the baseline schedule. The proposed framework structures a project in the way an engineer would approach it, setting out a breakdown of work activities to quantify weather effects and account for their impact on the project baseline. The proposed weather-sensitive construction simulation framework is employed to determine the effects of weather on the construction process of high-density polyethylene (HDPE) pipe installation. The relevant simulation findings are reported to clarify the impact of extreme weather events on construction projects and to assist in project planning and decision support.     

Turbulence Measurement in Nonuniform Open-Channel Flow Using Acoustic Doppler Velocimeter (ADV)

Measurements of the mean and turbulence characteristics in nonuniform open-channel flows were carried out using a 3D acoustic Doppler velocimeter. Both accelerating and decelerating flows were investigated. Analyses based on the Reynolds equation and the continuity equation of 2D open-channel flow show that a flow will be in equilibrium if the pressure-gradient parameter β is a constant at different sections along the flow direction. The experimental data show that all the flows investigated are in equilibrium. The effect of the flow nonuniformity on the mean velocity and turbulence profiles was also examined. The study shows that (1) the log law is still valid for both accelerating and decelerating flows in the inner region. The Coles law can be used for the entire region, but the wake-strength parameter Π depends on the β-value; and (2) the turbulence intensities and the Reynolds stress decrease in accelerating flow and increase in decelerating flow, when compared with those in uniform flow. Finally, using the Reynolds equation and the continuity equation of 2D open-channel flow, the theoretical expressions for the distribution of vertical velocity and the Reynolds stress have been developed. The measured vertical velocity and the Reynolds stress profile compare well with the derived expressions.     

转炉水模分散相尺寸分布的分形表征

 运用图像处理方法获得了分散相粒径和分散相几何特征,建立了一个用几何特征计算分散相分布分形维数的数学模型。数据图像的提取来源于模拟转炉吹炼过程的二维水模型,主要利用高分辨率数码相机采集数据,通过图像处理软件对得到图像中分散相进行提取与粒度计算,应用粒度分布分形维数模型,研究了在硬吹和软吹条件下顶吹转炉钢滴分散相粒度分布与边界条件的关系,证明了粒度分布分形维数数学模型的合理性。     

顶吹转炉水模型炉渣和钢液分布的分形研究

通过对顶吹转炉二维水模型模拟实验,研究了顶吹过程中炉渣与钢液的混合情况。应用分形理论对炉渣在钢液中分布的分维数及无量纲面积进行了计算,为渣一金反应界面积的计算提供了一种新方法。实验结果表明：吹炼过程中,炉渣在钢液中的分布存在形变区和分裂区;枪位、气体流量及渣一金比对炉渣分布的影响较大;并得到了枪位、气体流量及渣金比三者对分散相炉渣分布分维数的影响及分布分维数对渣金反应界面无量纲面积影响的经验关系式。     

Effect of Turbulence Modeling on the Melt Flow and Inclusions Transport in a Steel Filtration Experiment

The current article deals with the effect of turbulence modeling on inclusions transport and melt flow in an induction crucible furnace (ICF), which was employed in order to investigate the efficiency and the performance of a ceramic filter. Furthermore, the influence of the discrete random-walk dispersion model on the behavior of inclusions in the ICF was investigated. Different turbulence models were employed in order to predict the turbulent melt flow. The numerical results show that the flow field is affected by the turbulence modeling method. Moreover, the distribution of the turbulent kinetic energy depends considerably on the choice of the turbulence model. In addition, the turbulence model and dispersion model also affect the inclusion transport in the melt. The filtration rate is also affected by the choice of the turbulence model.     

Evapotranspiration Modeling Using Linear Genetic Programming Technique

The study investigates the accuracy of linear genetic programming (LGP), which is an extension to genetic programming (GP) technique, in daily reference evapotranspiration (ET0) modeling. The daily climatic data, solar radiation, air temperature, relative humidity, and wind speed from three stations, Windsor, Oakville, and Santa Rosa, in central California, are used as inputs to the LGP to estimate ET0 obtained using the FAO-56 Penman-Monteith equation. The accuracy of the LGP is compared with those of the support vector regression (SVR), artificial neural network (ANN), and those of the following empirical models: the California irrigation management system Penman, Hargreaves, Ritchie, and Turc methods. The root-mean-square errors, mean-absolute errors, and determination coefficient (R2) statistics are used for evaluating the accuracy of the models. Based on the comparison results, the LGP is found to be superior alternative to the SVR and ANN techniques.     

Modeling Nonspherical Particles Using Multisphere Discrete Elements

In this paper axisymmetrical particles are modeled as multisphere discrete elements using a method in which particles are represented by overlapping spheres, fixed rigidly with respect to a local coordinate system. Contact detection is sphere-based and the resultant forces are transformed to the particle centroid to calculate the particle motion using standard discrete element method conventions. The multisphere method was used to model discharge of ellipse-shaped particles through an orifice in a flat-bottomed hopper and the simulations compared with physical experiments at the same scale. There was good agreement between the flow behavior of the simulated and physical particle assemblies for all orifice sizes. The rate of discharge and the vertical velocity profiles in the region of converging flow determined for the simulated and physical flows were in close agreement for flow from the larger orifices. A similar relationship between frequency of arch formation, particle mean diameter, and orifice diameter as for spheres was observed.