Feasibility of utilizing numerical viscosity from coarse grid CFD for fast turbulence modeling of indoor environments

Computational fluid dynamics (CFD) is a useful tool in building indoor environment study. However, the notorious computational effort of CFD is a significant drawback that restricts its applications in many areas and stages. Factors such as grid resolution and turbulence modeling are the main reasons that lead to large computing cost of this method. This study investigates the feasibility of utilizing inherent numerical viscosity induced by coarse CFD grid, coupled with simplest turbulence model, to greatly reduce the computational cost while maintaining reasonable modeling accuracy of CFD. Numerical viscosity introduced from space discretization in a carefully specified coarse grid resolution may have similar magnitude as turbulence viscosity for typical indoor airflows. This presents potentials of substituting sophisticated turbulence models with inherent numerical viscosity models from coarse grid CFD that are often used in fast CFD analysis. Case studies were conducted to validate the analytical findings, by comparing the coarse grid CFD predictions with the grid-independent CFD solutions as well as experimental data obtained from literature. The study shows that a uniform coarse grid can be applied, along with a constant turbulence viscosity model, to reasonably predict general airflow patterns in typical indoor environments. Although such predictions may not be as precise as fine-grid CFDs with well validated complex turbulence models, the accuracy is acceptable for indoor environment study, especially at an early stage of a project. The computing speed is about 100 times faster than a fine-grid CFD, which makes it possible to simulate a complicated 3-dimensional building in real-time (or near real-time) with personal computer.     

Performance-based analysis of cylindrical steel containment Vessels exposed to fire

This paper reports the results of a numerical investigation on the structural safety assessment of a nuclear cylindrical steel containment vessel exposed to an accidental external fire condition. A coupling procedure is proposed, linking the modeling of a nearly compressible flow with input energy given by a combustion model (CFD model) and a structural thermo-mechanical analysis (FEM model), allowing an accurate evaluation of a fluid-thermo-mechanical response for the entire duration of the simulated accident. The time-temperature evolution of the burned gases, resulting from the combustion of a hydraulic leakage pool, is performed by the CFD model. The FEM model is used to obtain (i) temperature variation and, (ii) thermo-mechanical behavior and ultimate strength of the vessel. The obtained results indicate that the suggested methodology can provide reliable fire-safety analyses, ensuring that the main safety (load-bearing and containment) functions of the installation are not impaired during accidental events.     

Computational fluid dynamics modeling of post-liquefaction soil flow using the volume of fluid method

Flow deformation of post-liquefaction soil during an earthquake can cause serious damage to engineering structures. To overcome the limitations of conventional deformation analysis methods based on solid mechanics for extremely large systems, a computational fluid dynamics (CFD) method is proposed to numerically simulate the flow behavior of post-liquefaction soil. The liquefied soil is assumed to be a viscous fluid, and the volume of fluid (VOF) model is used for interface tracking in the numerical scheme. The results of a modeling test conducted on liquefaction-induced ground flow to verify the validity of the method showed good agreement, indicating the proposed method is capable of being used to reproduce the flow behavior of post-liquefaction soil.     

Quality control of computational fluid dynamics in indoor environments

Computational fluid dynamics (CFD) is used routinely to predict air movement and distributions of temperature and concentrations in indoor environments. Modelling and numerical errors are inherent in such studies and must be considered when the results are presented. Here, we discuss modelling aspects of turbulence and boundary conditions, as well as aspects related to numerical errors, with emphasis on choice of differencing scheme and computational grid. Illustrative examples are given to stress the main points related to numerical errors. Finally, recommendations are given for improving the quality of CFD calculations, as well as guidelines for the minimum information that should accompany all CFD-related publications to enable a scientific judgment of the quality of the study.     

建筑排水立管数值模拟的初步探讨及应用

近十多年来,计算流体动力学(CFD)已发展到完全可以分析三维粘性湍流及漩涡运动等复杂问题.它不仅作为一个研究工具,而且还作为设计工具在水利工程、土木工程、环境工程、工业制造等领域发挥作用.文章通过具体算例,探讨了计算流体力学软件Fluent关键参数的设置,并将数值模拟的计算结果与经典立管试验结果进行了分析比较.结果表明...     

Determination of three dimensional hydraulic conductivities using a combined analytical/neural network model

Analysis of groundwater flow through fractured rock masses is an essential step in many engineering and environmental problems, such as in safety assessment of radioactive waste storages, hydrocarbon storage caverns and hydropower projects. The most important hydrological parameter in groundwater flow analysis is the hydraulic conductivity which is anisotropic and heterogeneous in the fractured rock masses. To analyze the groundwater flow correctly, some site investigations through boreholes must be carried out. One of the challenges in seepage analysis for an engineering project is how to determine the anisotropic and heterogeneous hydraulic conductivities of the fractured rock masses using the limited in situ investigation data. In this study, a new practical approach for the determination of three dimensional hydraulic conductivities of fractured rock masses is presented. Starting from rock fracture properties surveyed in six boreholes, the anisotropic hydraulic conductivities are estimated using the in situ injection test results and Oda’s theoretical model. A neural network method is then utilized to generate the three dimensional heterogeneous hydraulic conductivities based on the anisotropic hydraulic conductivities along the six boreholes. In order to evaluate the reliability of this approach, a 3D numerical seepage model using code FLAC3D is performed for a real project. The inflow values in a shaft obtained with the 3D numerical analysis are compared with the in situ measured flow. The result indicates that the derived hydraulic conductivity is acceptable.     

Validation of numerical predictions for liquefaction phenomenon – Lateral spreading in clean sands

During the last few decades, important efforts and developments in the computational modeling of geo-materials have contributed to an increase in the accuracy of the predictions of the dynamic responses of soil systems. Due to their catastrophic consequences, special emphasis has been placed on liquefaction-induced ground failures. However, the numerical tools for liquefaction modeling need to be continually assessed and validated in order to enhance their reliability and enable them to be included in design practices.Within that context, the main objective of this paper was to present a complete validation exercise that explores the capabilities of the numerical predictions to simulate the lateral spreading phenomenon in clean sands under a diverse range of densities and input motions. The validation exercise used the “Strain Space Multiple Mechanism Model” to simulate the lateral spreading phenomenon (although the methodology presented here might be applicable for the validation of other numerical tools as well), and was based on multiple, cross-checked, and high-quality physical models (centrifuge models) and element tests (hollow cylinder cyclic shear tests).Special focus was placed on the quantification of the median response and the associated variability of both physical and numerical models, including an analysis of the importance of the proper selection of validation metrics.The comparison showed that the numerical model is able to predict the displacements for the median trend and the 95% probability confidence bounds for PGA < 0.25 g.     

Subatmospheric pressure in a water draining pipeline with an air pocket

An air pocket’s behaviour inside of a pipeline during transient conditions is of great importance due to its effect on the safety of the hydraulic system and the complexity of modeling its behaviour. The emptying process from water pipelines needs more assessment because the generation of troughs of subatmospheric pressure may lead to serious damage. This research studies the air pocket parameters during an emptying process from a water pipeline. A well-equipped experimental facility was used to measure the pressure and the velocity change throughout the water emptying for different air pocket sizes and valve opening times. The phenomenon was simulated using a one-dimensional (1D) developed model based on the rigid formulation with a non-variable friction factor and a constant pipe diameter. The mathematical model shows good ability in predicting the trough of subatmospheric pressure value as the most important parameter which can affect the safety of hydraulic systems.     

水压致裂过程的三维数值模拟研究

基于RFPA数值分析方法和并行计算技术,建立了反映岩石细观损伤演化过程的三维渗流–应力–损伤耦合模型。对具有120万单元的方形岩石材料模型,进行了4组不同应力状态下水压致裂过程的三维大规模科学计算分析。计算结果分析表明：起裂压力与失稳压力并不重合,起始裂纹均为张性,裂纹扩展形式、表面平整度、走向、扩展失稳过程以及裂纹的空间分布形态受应力状态影响。当竖直方向为最大主应力方向时裂纹呈空间竖片分布,当水平应力差较大时裂纹表面形态平整,失稳到来较快;当竖直方向为最小主应力方向时裂纹的空间分布呈水平片状;不等的主应力情况下裂纹总是分布在最小主应力面内;当三向主应力相等时,裂纹起裂位置和扩展方向具有竞争趋势,空间分布不具规律,裂缝分支较多。数值模拟结果与物理实验结果有着较好的吻合,该研究对水压致裂工程设计有一定参考价值。     

岩体大应变黏塑性模型及其在矿山工程中的应用

 将传统黏塑性模型扩展到能够考虑有限应变效应,并试图用该模型结合非局部化方法描述应变局部化问题。局部化等效塑性变量的非局部化通过与材料特征尺度相关的数值加权平均获得,从而将材料的微细观效应考虑在内。该模型遵循以下假设：变形梯度可用极分解表示,材料存在弹性变形域并遵循最大塑性耗散原理的黏塑性调节形式。该模型具有考虑各向同性和运动硬化、软化的功能。通过不同的精化网格,分析有限应变条件下单元尺寸对非局部的敏感性。数值实验包括模拟无围压约束的岩石压缩实验的变形局部化并与FLAC3D的模拟进行对比分析,模拟深部巷道开挖后出现的大变形流变效应,数值结果表明,该黏塑性模型基本上可以克服目前应变局部化模型中存在的一些弊端。