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.     

A passive scalar sub-grid scale model and its application to airflow simulation around a building

The airflow around a building is in high Reynolds number flow regime. It can be calculated by large eddy simulation method (LES). The air temperature around buildings is passive scalar. If the behavior of passive scalars is considered with sub-grid scale (SGS) model for LES, the calculation or the results will be improved greatly but it has not been implemented, till now. In this paper a passive scalar SGS model (PS-SGS) was derived based on the hypotheses about the sheet-like structure in Taylor scale. The kinetic-energy SGS model was improved with the derived PS-SGS. The validity of the model was checked out using the experimental data in reference. The field around a supposed two-dimensional building was estimated with the improved kinetic-energy SGS model. Karman vortex-street and some special behavior for passive scalars were distinguished with the derived model.     

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。     

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.     

Numerical simulation of dispersion around an isolated cubic building: Model evaluation of RANS and LES

Several studies have been carried out on CFD prediction based on a RANS (Reynolds Averaged Navier–Stokes equations) model for dispersion around buildings, but it was reported that a RANS computation often provides extremely high concentration, which are not observed in usual measurements. These results suggest that transient simulations such as the large-eddy simulation (LES) might be required to achieve more accurate results. Nevertheless, very few studies have evaluated the basic performance of LES in modeling the dispersion field for a simple configuration in comparison with the RANS model. Therefore, relative performance of these simulation methods for dispersion problem around buildings should be clarified in order to make it possible to choose a suitable numerical method for its purpose. The purpose of this study is to confirm the accuracy of LES in modeling plume dispersion near and around a simple building model and to clarify the mechanism for the discrepancy in relation to the RANS computation. Simple LES modeling gives better results than RNG modeling of the distribution of concentration, although the difference for mean velocity is not so large. The horizontal diffusion of concentration is well reproduced by LES. This tendency is closely related to the reproduction of unsteady periodic fluctuation around cubical forms in LES.     

Numerical simulation of pollutant dispersion around a building complex

The dispersion of exhausted pollutants from a building roof stack situated in the wake of a neighbouring tower has been studied using computational fluid dynamics (CFD) with the realizable k–? turbulence model for closure. Two scales are considered in this work, full-scale (1:1) and wind tunnel scale (1:200).Of primary interest are the distributions of the plume and of the pollutant concentrations on the building roof as well as on the leeward wall of the tower. Two stack heights and pollutant exhaust velocities have been considered for the distribution of pollutant concentrations in the neighbourhood of the building from which the pollutant is emitted. Results are compared with measurements from field and wind tunnel experiments to estimate the accuracy of simulations.     

Influence of cell geometry and mesh resolution on large eddy simulation predictions of flow around a single building

A number of research studies have employed a wide variety of test conditions to numerically assess flow field around buildings. In such studies, the employed mesh system and its resolution played a crucial role in determining the ultimate degree of solution accuracy. The objective of this study is to better establish a quantitative assessment of the influence of cell geometry and mesh resolution on the numerical predictions of large eddy simulation (LES) of the flow field around a single building. In order to achieve this objective, a number of mesh styles including conventional hexahedral-based mesh, tetrahedral-based mesh, and prism-based mesh have been considered. In addition, hybrid meshes of tetrahedron and hexahedron cells are also considered. In each mesh style, four grid resolutions were investigated: coarse, medium, fine, and very fine. Accuracy of the simulation has been assessed by applying comparisons to the experimental data available in literature. Moreover, quantitative grid convergence was calculated based on the grid convergence index, which accounts for the degree of grid refinement. Future studies are needed to analytically evaluate the influence of cell geometry on the solution accuracy and to more precisely evaluate other grid system options including various forms of hybrid configuration meshes.     

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.     

Three-dimensional numerical simulation of air flow around a cubic model by means of large eddy simulation

The unsteady, turbulent flowfield around a cubic model has been simulated by means of large eddy simulation (LES). The resolvable-scale flowfield has been obtained directly by solving the filtered, three-dimensional, time-dependent Navier-Stokes equations. The subgrid-scale motions were simulated by an SGS eddy viscosity model. The computed mean velocity dstributions and various turbulence statistics (i.e. turbulence intensity, velocity spectrum, integral scale, etc.) were compared to those obtained from wind-tunnel experiments to examine the accuracy of LES from the viewpoint of engineering applications. The correspondence between numerical simulation and wind-tunnel experiments is found to be good.     

大型超市能耗动态模拟研究

建立了大型超市能耗动态模拟的计算模型。以上海某大型超市为例,应用DeST软件模拟和实际调研分析相结合的方法,对该大型超市的全年逐时能耗进行了模拟,并利用模拟结果分析了该大型超市的能耗现状和结构。利用该建筑实测能耗数据对模拟结果进行了验证,表明该计算模型准确合理。     

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.     

风场大涡模拟中网格非匹配算法

计算风工程中对风场进行大涡模拟是从流体角度诠释风压产生、发展、分布的数值方法.风场需分割为多个子块,并根据风活跃程度分别指定不同的网格密度,在各块界面上网格非一一匹配.本文针对:三维子块,提出了一种搜集非匹配信息的算法,用以计算子块间的风流体通量.根据结构网格规律性,将公共界面上两个子块的网格线合并为一套网格,从而将公共界面划分为小面,形成联接信息进行全场联立求解.典型算例表明可在节省计算资源的同时保证精确性,同时实现网格密度平滑过渡,为大跨度结构风场模拟解决了一个基础问题.     

Scale-adaptive simulation of unsteady flow and dispersion around a model building: spectral and POD analyses

The present paper evaluates the relative performance of scale-adaptive simulation (SAS) in modelling unsteady concentration and flow fields around a model building relative to other transient simulations such as large eddy simulation (LES) and unsteady Reynolds-averaged Navier–Stokes (URANS) models. A novel application of proper orthogonal decomposition (POD) and time–frequency analysis is carried out in order to evaluate the transient behaviour and dominant structures of the flow fields predicted by SAS and LES. Results represent the outstanding performance of SAS in comparison with the URANS computation based on the SST k–ω model. This better performance is related to the accurate reproduction of unsteady fluctuations around the model building by SAS. In addition, the quantitative and qualitative agreements for the shapes and magnitudes of POD modes between SAS and LES confirm the LES-like behaviour of SAS in the wake region. However, in terms of computational performance, SAS imposes an extra CPU cost as compared with LES for the same grid resolution.     

CFD simulations of gas dispersion around high-rise building in non-isothermal boundary layer

Urban heat island phenomena and air pollution become serious problems in weak wind regions such as behind buildings and within street canyons, where buoyancy effect cannot be neglected. In order to apply CFD techniques for estimation of ventilation and thermal and pollutant dispersion in urban areas, it is important to assess the performance of turbulence models adopted to simulate these phenomena. As the first step of this study, we carried out wind tunnel experiments and CFD simulations of gas and thermal dispersion behind a high-rise building in an unstable non-isothermal turbulent flow. The standard k-ε model and a two-equation heat-transfer model as RANS models, and LES, were used for the CFD simulation. One of the important purposes of this study was to clarify the effect of inflow turbulence (both velocity and temperature) on flow field and gas/thermal dispersion for the LES calculation. Thus, LES calculations with/without inflow turbulence were conducted. The inflow turbulence was generated through a separate precursor simulation. The calculated results showed that both RANS models overestimated the size of the recirculation region behind the building and underestimated the lateral dispersion of the gas. Turbulent flow structures of LES with and without inflow turbulence were completely different. The LES result with inflow turbulence achieved better agreement with the experiment.     

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。     

Learning the fundamentals of building performance simulation through an experiential teaching approach

It is relatively easy to train architects and engineers to operate BPS tools and to produce results, but there is considerable evidence to show that it is quite difficult to produce accurate results, even for experienced users. I believe that BPS suffers from a credibility gap and that its full potential will only be realized once we adequately prepare users to effectively apply tools with full knowledge of their applicability, modelling limitations, and default methods and data, and provide them the skill set to scrutinize their results. An experiential approach for teaching the fundamentals of BPS through a learning spiral composed of four modes of learning has been devised. The approach and the learning outcomes that have been achieved through each of the learning modes are demonstrated in this article through examples extracted from the teaching of a semester-long post-graduate course.     

北京市某大型公共建筑三联供系统的优化

建立了以年度费用最小为目标函数的优化数学模型,对冷热电三联供系统的配置和运行方案进行了优化。结果表明,年度运行费用明显少于传统配置方案,但运行能耗却与传统配置方案相当。     

我国大型公共建筑能源管理的现状与前景

介绍了与既有大型公共建筑能源管理有关的基本概念和运作方式,总结了我国大型公共建筑能源管理的现状,对解决能源管理中存在的问题提出了建议。     

浅谈大空间火灾烟气运动规律的计算机模拟

分析并讨论了当前大尺度空间火灾烟气运动规律的计算机模拟研究现状。结果表明,采用大涡模拟方法结合多单元区域模拟是研究火灾烟气运动规律的发展趋势。     

Numerical simulation of CO2 dispersion in an auditorium

This paper presents the simulation of the air flow field in an auditorium. A CFD code is used for the numerical simulation of CO₂ dispersion caused by breathing of people present in the closed space. Two different ventilation systems, air induction and abduction flow rate, the number of the people, as well as the lighting equipment are taken into account. The geometry considered is three-dimensional and the turbulence phenomena are also accounted for.