Large-eddy simulation of human-induced contaminant transport in room compartments

A large-eddy simulation is used to investigate contaminant transport owing to complex human and door motions and vent-system activity in room compartments where a contaminated and clean room are connected by a vestibule. Human and door motions are simulated with an immersed boundary procedure. We demonstrate the details of contaminant transport owing to human- and door-motion-induced wake development during a short-duration event involving the movement of a person (or persons) from a contaminated room, through a vestibule, into a clean room. Parametric studies that capture the effects of human walking pattern, door operation, over-pressure level, and vestibule size are systematically conducted. A faster walking speed results in less mass transport from the contaminated room into the clean room. The net effect of increasing the volume of the vestibule is to reduce the contaminant transport. The results show that swinging-door motion is the dominant transport mechanism and that human-induced wake motion enhances compartment-to-compartment transport. PRACTICAL IMPLICATIONS: The effect of human activity on contaminant transport may be important in design and operation of clean or isolation rooms in chemical or pharmaceutical industries and intensive care units for airborne infectious disease control in a hospital. The present simulations demonstrate details of contaminant transport in such indoor environments during human motion events and show that simulation-based sensitivity analysis can be utilized for the diagnosis of contaminant infiltration and for better environmental protection.     

Large eddy simulation of compartment fire with solid combustibles

For properly describing practical building fire processes with solid combustibles, the pyrolysis kinetics model of solid combustibles and the large eddy simulation (LES) approach are applied to the simulation of the thermal decomposition of the polyurethane foam (PUF) slab and the space fire spread in a compartment. The instantaneous variations of the heat release rate of the PUF slab, the smoke temperature, and the smoke interface height with time are obtained under different ventilation conditions. They are in agreement with the measured data. The ventilation conditions have distinct effects on the interactions between the pyrolysis of the PUF slab and the space fire spread. Influenced by the space fire spread, the heat flux on the top plane of the PUF slab exhibits a non-uniform distribution. The PUF slab is consumed in an asymmetric manner.     

Using large eddy simulation to study particle motions in a room

As people spend most of their time in an indoor environment, it is important to predict indoor pollutant level in order to assess health risks. As particles are an important pollutant indoors, it is of great interest to study the airflow pattern and particle dispersion in buildings. This study uses large eddy simulation (LES) to predict three-dimensional and transient turbulent flows and a Lagrangian model to compute particle trajectories in a room. The motion of three different types of solid particles in a decaying homogeneous isotropic turbulent airflow is calculated. By comparing the computed results with the experimental data from the literature, the computational method used in this investigation is found to be successful in predicting the airflow and particle trajectories in terms of the second-order statistics, such as the mean-square displacement and turbulent intensity. This Lagrangian model is then applied to the study of particles' dispersion in a ventilated cavity with a simplified geometry for two ventilation scenarios. It is shown that light particles follow the airflow in the room and many particles are exhausted, while heavier particles deposit to the floor or/and are exhausted. PRACTICAL IMPLICATIONS: The results of this paper can be used to study dispersion of infectious diseases in enclosed spaces in which virus or bacteria are often attached to particles and transported to different rooms in a building through ventilation systems. In most of studies, the virus or bacteria have been considered to be gaseous phase so there is no slip between virus/bacteria and air. The results in this paper show that heavier particles are submitted to gravity and are sensitive to the ventilation strategy.     

Large eddy simulation of flow past rectangular-section cylinders: Side ratio effects

In this paper, the flow characteristics around rectangular-section cylinders with different side ratios (R=width/height) of 0.4-4 are studied for Re=10⁵. The effect of Reynolds number is also examined for some cases. Unsteady and three-dimensional computations are performed using large eddy simulation (LES) with two subgrid-scale models, the Smagorinsky (S-Model) and a one-equation model (OE-Model). An incompressible three-dimensional (3D) finite volume code with a collocated grid arrangement is used for solving the filtered Navier-Stokes equations. These equations are solved with an implicit fractional two-step method. A series of instantaneous and time- and spanwise-averaged resolved velocity, pressure, turbulent stresses, and streamlines are provided. Time-averaged global quantities such as the Strouhal number, the mean and the RMS values of drag force, the base suction pressure, the lift force, and the pressure coefficient are also calculated and compared with available experimental results.     

湍流反应流研究中的大涡模拟

针对湍流反应流分析高质量数值解所应具有的特征和准则，对比大涡模拟与N-S方程雷诺平均的能谱空间分解图，说明了大涡模拟的特点。使用FDS2程序对油池火进行模拟，以展示湍流反应流研究中的大涡模拟技术。     

Large eddy simulation of wind‐induced interunit dispersion around multistory buildings

Previous studies regarding interunit dispersion used Reynolds‐averaged Navier–Stokes (RANS) models and thus obtained only mean dispersion routes and re‐entry ratios. Given that the envelope flow around a building is highly fluctuating, mean values could be insufficient to describe interunit dispersion. This study investigates the wind‐induced interunit dispersion around multistory buildings using the large eddy simulation (LES) method. This is the first time interunit dispersion has been investigated transiently using a LES model. The quality of the selected LES model is seriously assured through both experimental validation and sensitivity analyses. Two aspects are paid special attention: (i) comparison of dispersion routes with those provided by previous RANS simulations and (ii) comparison of timescales with those of natural ventilation and the survival times of pathogens. The LES results reveal larger dispersion scopes than the RANS results. Such larger scopes could be caused by the fluctuating and stochastic nature of envelope flows, which, however, is canceled out by the inherent Reynolds‐averaged treatment of RANS models. The timescales of interunit dispersion are comparable with those of natural ventilation. They are much shorter than the survival time of most pathogens under ordinary physical environments, indicating that interunit dispersion is a valid route for disease transmission.     

密闭腔室火灾温度的大涡模拟

应用大涡模拟方法模拟全尺寸的密闭腔室火灾场景。提出应用平均燃烧效率的概念模拟密闭腔室火灾的方法,选取平均燃烧效率为0.75进行大涡模拟,得到密闭腔室室内温度场以及烟气运动的模拟数据。通过分析密闭腔室内竖直方向上的温度分布,发现在竖直方向上存在温度分层现象。将模拟结果与实验数据进行对比可以看出,大涡模拟可以较好地模拟密闭腔室内火灾的发生发展情况。     

Turbulence statistics in a fire room model by large eddy simulation

Fire and smoke movement in a room is influenced by the turbulence characteristics (such as Reynolds stress, turbulent heat flux, etc.) of the flow and temperature fields. In order to accurately predict fire and smoke movement by computational fluid dynamics (CFD), it is necessary to verify these turbulence quantities. The purpose of this study is to predict the turbulence structure of the flow and temperature fields due to a fire in the compartment by large eddy simulation (LES) using detailed experimental data to verify the simulation results. The results show reasonably good agreement with experimental data for both the mean flow properties and the turbulence quantities with the exception of the region near ceiling. This study provides useful information for verifying LES technique when applied to compartment fires.     

A new approach on zonal modeling of indoor environment with mechanical ventilation

This paper reports the development of a new zoning approach based on room air age, a parameter that indicates the mixing condition of the air. Zoning criteria are developed based on the deviation ratio of air age as well as location of the key source that is of concern (e.g., temperature, air pollutant). By integrating the zonal model with other models such as dynamic models for heat and moisture transfer, and source/sink models for air pollutant, the dynamic characteristics of indoor parameters such as air temperature, humidity, and pollutant concentrations can be simulated. A case study was presented for a displacement ventilated room, and simulation results using the new zonal model were compared with those using a computational fluid dynamics (CFD) model and a conventional zonal model. Results demonstrated that the new zonal model is more accurate in calculating the zonal temperature distributions than the conventional zonal model. The model is suitable for dynamic simulations (e.g., whole year) of indoor environmental parameters.     

中庭火灾烟气流动的大涡模拟

采用大涡模拟的方法,对中庭火灾烟气的流动过程进行了模拟,了解了中庭烟气的蔓延过程,得到了烟气的速度场和温度场、顶棚射流的速度和温度的详细结果。模拟结果表明,大涡模拟能比较准确地预测中庭内烟气的流动状态,可用于指导中庭建筑的防火设计。     

矢流洁净室二维流场的大涡模拟

采用大涡模拟技术对矢流洁净室二维流场进行了数值模拟,分析了模拟结果,确定了空态洁净室的送风口半径、回风口高度、送风速度等参数,指出湍流大涡模拟能准确得到洁净室流场中旋涡的位置和尺度,适用于对洁净空调多种工作环境的数值模拟。     

Experimental measurements and large eddy simulation of expiratory droplet dispersion in a mechanically ventilated enclosure with thermal effects

Understanding of droplet transport in indoor environments with thermal effects is very important to comprehend the airborne pathogen infection through expiratory droplets. In this work, a well-resolved Large Eddy Simulation (LES) was performed to compute the concentration profiles of monodisperse aerosols in non-isothermal low-Reynolds turbulent flow taking place in an enclosed environment. Good care was taken to ensure that the main dynamical features of the continuous phase were captured by the present LES. The particle phase was studied in both Lagrangian and Eulerian frameworks. Steady temperature and velocity were measured prior to droplet emission. Evolution of aerosol concentration was measured by a particle counter. Results of the present LES were to compare reasonably well with the experimental findings for both phases.     

深圳平安国际金融大厦风致响应大涡模拟分析

运用一种新的湍流脉动流场产生方法模拟了三种风场的湍流边界条件,采用一种新的大涡模拟的亚格子模型,基于Linux系统下软件平台Fluent 6.3的并行计算技术,对深圳平安国际金融大厦进行了全尺寸、高雷诺数（高达10×10⁸量级）的数值风洞模拟。计算了三种风场下建筑表面平均、脉动风压及风荷载时程数据。利用惯性风荷载（IWL）法得到三种风场下深圳平安金融大厦的基底等效静风荷载以及结构顶部峰值加速度响应。分析了不同的湍流来流对结构风压系数、风荷载及加速度响应的影响。分析结果表明：三种来流风场条件下,深圳平安金融大厦周围风场相差较大,来流的湍流强度越高,建筑物前方的脉动风速越高;顺风向等效风荷载主要受平均风速控制,横风向等效风荷载主要受脉动风控制;湍流强度越大,横风向等效风荷载越大;中国规范建议的湍流流场下,深圳平安金融大厦10年重现期顺风向、横风向峰值加速度响应满足居住者舒适度要求。     

大涡模拟及其在室内空气流动换热中的应用

介绍了国内外大涡模拟的亚格子湍流模式和在室内空气流动换热中的研究现状给出了大涡模拟室内空气流动的应用实例。认为大涡模拟有广阔的发展前景。     

Large eddy simulation of thermal stratification effect on convective and turbulent diffusion fluxes concerning gaseous pollutant dispersion around a high-rise model building

In this paper, the large eddy simulation (LES) approach is employed to investigate the role of different thermal stratification conditions (stable, neutral and unstable) in the air flow and gaseous pollutant dispersion processes around a high-rise non-isolated model building with 1:1:2 shape placed within a non-isothermal boundary layer. The simulation results are initially validated with available experimental measurements and then applied to study the characteristics of pollutant dispersion. Predictions show a low-velocity zone behind the model building. The major effect of a stable stratification on flow field is the formation of a weak recirculation flow in the wake region. Results represent an intense temperature gradient close to the sides of the model building for both stable and unstable conditions. Moreover, the convective and turbulent diffusion fluxes are compared under different thermal stratification conditions, and it is shown that the LES approach is capable of predicting the counter-gradient mechanism.     

Large eddy simulation of wind effects on a long-span complex roof structure

This paper presents a combined study of numerical simulations and wind tunnel tests for the determinations of wind effects on a long-span complex roof of the Shenzhen New Railway Station Building. The main objective of this study is to present an effective approach for the estimations of wind effects on a complex roof by computational fluid dynamics (CFD) techniques. A new inflow turbulence generator called the discretizing and synthesizing random flow generation (DSRFG) approach was applied to simulate inflow boundary conditions of a turbulent flow field. A new one-equation dynamic subgrid scale (SGS) model was adopted for the large eddy simulations (LES) of wind effects on the station building. The wind-induced pressures on the roof and turbulent flow fields around the station building were thus calculated based upon the DSRFG approach and the new SGS model integrated with the FLUENT software. In parallel with the numerical investigation, simultaneous pressure measurements on the entire station building were made in a boundary layer wind tunnel to determine the mean, fluctuating, and peak pressure coefficient distributions. The numerically predicted results were found to be consistent with the wind tunnel test data. The comparative study demonstrated that the recommended inflow turbulence generation technique and the new SGS model as well as the associated numerical treatments are useful tools for structural engineers to assess wind effects on long-span complex roofs and irregularly shaped buildings at the design stage.     

湍流边界层中低矮建筑绕流大涡模拟

通过对平板湍流边界层进行大涡模拟,采用拟周期边界条件维持湍流边界层厚度稳定,提取速度和压力时程作为低矮建筑绕流模拟之脉动入流边界条件,研究脉动入流下的低矮建筑绕流特性。研究结果表明：入流边界特性对网格变化适应性良好,其平均速度剖面、湍流强度、流速频谱特性基本符合空旷地貌风场特性;脉动入流下,建筑表面的平均风压系数、脉动风压系数的计算结果与风洞试验结果基本吻合。受雷诺数及湍流强度的影响,流动分离区负压与试验值存在一定差别;屋盖上分离区风压时程具有非高斯概率特性,尤以气流分离较剧烈的屋盖迎风边缘及屋盖两侧风压的非高斯特性明显,该特征与风洞试验基本一致;受非高斯特性的影响,建议峰值因子g取4.5~5.5。     

Inflow turbulence generation techniques for large eddy simulation of flow and dispersion around a model building in a turbulent atmospheric boundary layer

The present paper investigates the performance of various inflow turbulence generation techniques (ITGT) for large eddy simulation (LES) of flow and dispersion around a model building in a turbulent atmospheric boundary layer. Four different ITGT comprising 1 – no fluctuations, 2 – spectral method, 3 – vortex method and 4 – internal mapping, based on two basic methodologies (i.e. precursor and synthetic turbulence methods), are employed. These techniques are evaluated by considering their prediction accuracy, computational costs, complexity of implementation, inflow information required to operate and impacts on the flow downstream of the inlet, particularly in the wake region of the model building. Results indicate that the accuracy of LES predictions is greatly reliant on ITGT. It is shown that ITGT not only have significant effects on flow field vortical structures, but also influence frequency contents of velocity fluctuations, recirculation regions and plume shapes in the wake region.     

Numerical studies of indoor airflow and particle dispersion by large Eddy simulation

The indoor airflow and contaminant particle concentration in two geometrically different rooms have been investigated using the Large Eddy Simulation (LES) technique based on Renormalization Group (RNG) theory derived by Yakhot, Orszag, Yakhot and Israeli, Journal of Scientific Computing, 1989. The first room is without contaminant particles. Its simulated air phase velocity profiles are validated against the measurements of Posner, Buchanan and Dunn-Rankin, Energy and Buildings, 2003. A good agreement is achieved between the prediction and measured data. The LES model successfully captures the mean flow trends as well as instantaneous flow information, which is required for appropriate design and evaluation of a ventilation system. The second room has contaminant particles, which are simulated with a Lagrangian particle tracking model. In this case, the LES model provides acceptable prediction of the contaminant particle concentration, compared to the particle concentration decay measured by Lu, Howarth, Adam and Riffat, Building and Environment, 1996. The numerical results reveal that the particle-wall impact model has a considerable effect on the Lagrangian concentration prediction. It is proposed that further improvements to the particle-wall impact model are required to correctly predict the contaminant particle concentration through the Lagrangian model.