New inflow boundary conditions for modelling the neutral equilibrium atmospheric boundary layer in computational wind engineering

Modelling neutral equilibrium atmospheric boundary layers (ABL) in CFD is an important aspect in computational wind engineering (CWE) applications. In this paper, new inflow boundary conditions are introduced from the viewpoint that these boundary conditions should satisfy the turbulence model employed. The new set of inflow turbulence boundary conditions is an approximate solution to the standard k-ε model transport equations. The capability of these boundary conditions to produce an equilibrium ABL is demonstrated by performing numerical simulations in an empty domain. The new inflow turbulence boundary conditions in this paper support future practical applications in CWE and future research in modelling equilibrium ABLs.     

重大水污染事件预警与应急技术

阐述了水污染及重大污染事件的含义,总结了突发性水污染事故的特点,分析了重大水污染的危害性,探讨了重大水污染事件的应急处理机制,指出应尽快建立并完善以国家确定的重要流域为单元的突发性水污染事件的应急处理机制,以防止突发性水污染的产生。     

Assessment of Numerical Simulation Capabilities of the Fire Dynamics Simulator (FDS 6) for Planar Air Curtain Flows

Computational Fluid Dynamics (CFD) results are discussed for momentum driven planar jet flows, resembling configurations in use for air curtain in the context of smoke control in building fire. The CFD package Fire Dynamics Simulator (FDS) is used. Special focus is given to the impact of grid resolution, synthetic turbulent inflow boundary condition and sub-grid scale eddy viscosity models. The computational results are compared with summarized literature data. Investigation of different set-ups of inlet boundary conditions, including the inlet duct length, velocity profile and method of generation of turbulence at the level of the inflow, reveals that the inlet boundary condition is the most influential factor governing the flow downstream. The FDS results successfully reproduce the planar jet flows, both in terms of mean variables and second-order statistics. ‘Reference’ results have been obtained with a fully developed turbulent flow emerging from a long inlet duct. By reducing the inlet duct length and applying the Synthetic Eddy Method (SEM) at the inflow boundary condition, the ‘reference’ results have been reproduced with a reduction in the computing times of approximately 20%. However, care must be taken when choosing the parameters of SEM, in particular the number of eddies and their length scale. The impact of turbulent viscosity model is noticeable, but not of primary importance for the flow at hand, provided that a sufficiently fine computational mesh is used.     

Effects of inflow conditions on mountainous/urban wind environment simulation

Inflow conditions play a key role in the Computational Fluid Dynamics (CFD) simulation of wind environment. Taking the micro wind climate of Hong Kong Kowloon Bay costal town as a research object, two kinds of widely used inflow condition determination methods are adopted to test their performances. One is to fit the velocity profile into the empirical (logarithmic/exponential) law, hereafter referred to as the Fitted Empirical Profile (FEP) method. The other is to interpolate the outflow velocities and turbulence properties from a pre-simulation of the upstream region, hereafter referred to as the Interpolated Multiscale Profile (IMP) method. The GIS data of this mountainous/ urban area are digitalized and simplified into the CFD geometry model. Computational treatments for numerical algorithms, domain size, grid systems and boundary conditions are carefully configured according to the published CFD Best Practice Guidelines (BPGs). By validating with one year of real scale wind measurement data from two meteorological stations, it is found that these two inflow conditions lead to considerably different results. Having less consideration for the blockage effects of terrain/buildings, the FEP method tends to predict higher wind speed. As more thermal effects are removed by increasing wind speed thresholds, the results of IMP method demonstrate an incremental agreement with the measurement data. Finally, the validated simulation results are applied to the spatial representativeness assessment of two meteorological stations. Both the point-to-surface consistency indicator and point-centered semivariance are employed. The results show that the meteorological stations show a good representation within a range of 400 m.     

突发性水污染事故的应急处理

近年来，我国城市水源地突发性污染事件日益增加，如何保护水源地生态环境安全和城市供水系统安全，以及如何快速、有效地处理突发性水污染事故已成为亟需解决的问题。介绍了突发性水污染事故应急处理的一般程序，重点讨论了应急措施、处理技术及区域污染物的处理方法。对于危及城市自来水厂区域的突发性污染事故，若在其处理能力范围内，可采用强化混凝、三级处理、备用水源地取水、开启与邻近城市间的备用供水管网等应急处理方法向城市供水；对于自来水厂取水口等遭受污染的水域，可采用吸附、化学沉淀、化学氧化、强化消毒等方法对不同污染物进行治理或隔离。     

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.     

Optimum model extent for numerical simulation of tunnel inflow in fractured rock

The objective of this study is the introduction of an optimum model extent in discontinuous rock masses for tunnel inflow assessment using numerical models. Tunnel groundwater inflow is an important problem in tunneling, and numerical simulation is widely used for estimating the amount of tunnel inflow. An adequate size of the model domain is of very high importance when using such models. On the one hand, if the tunnel boundary is too close to the outer model boundary, the simulated inflow rate into the tunnel is significantly overestimated (which will be shown in the present study). On the other hand, if the model domain is very large, models may become “unhandy”, and simulations become very expensive with respect to computer memory and CPU. In this technical note, an approach is presented that derives an optimum model extent for numerical simulation of tunnel inflow in fractured rock. The approach uses the two-dimensional universal distinct element code (UDEC). The impact of different model parameters, such as tunnel radius, groundwater level, joint spacing, joint dip/dip direction and joint aperture on the optimum model extent has been evaluated. Based on the results, an optimum model extent chart is presented that allows modelers a quick determination of the optimum model extent as a function of the most significant parameters, which are the tunnel depth under the groundwater level, tunnel radius and joint spacing.     

富水砂层盾构隧道洞门涌水涌砂处理措施研究

结合南昌轨道交通1号线一期工程施工实践,详细介绍了富水砂层盾构隧道洞门涌水涌砂处理措施,通过对涌水涌砂事故的分析总结了施工经验,为以后富水砂层中盾构隧道掘进防止洞门涌水涌砂提供了指导。     

Analytical solution for steady-state groundwater inflow into a drained circular tunnel in a semi-infinite aquifer: A revisit

This study deals with the comparison of existing analytical solutions for the steady-state groundwater inflow into a drained circular tunnel in a semi-infinite aquifer. Two different boundary conditions (one for zero water pressure and the other for a constant total head) along the tunnel circumference, used in the existing solutions, are mentioned. Simple closed-form analytical solutions are re-derived within a common theoretical framework for two different boundary conditions by using the conformal mapping technique. The water inflow predictions are compared to investigate the difference among the solutions. The correct use of the boundary condition along the tunnel circumference in a shallow drained circular tunnel is emphasized.     

不考虑开挖扰动影响的隧道涌水量预测模型试验研究

 采用大型施工及运营期矿山法隧道渗流模型试验系统,在不同的注浆圈和初衬渗透系数下,不考虑隧道开挖的扰动影响进行涌水量预测,并以Visual-modflow软件进行数值建模分析,结果表明：两者涌水量结果和规律基本吻合,随注浆圈或初衬的渗透系数减小,其试验平均涌水量值均呈非线性下降,各工况单次试验测试值的离散性也逐渐减小,而降低注浆圈渗透系数对涌水量的影响更为明显;改变注浆圈渗透系数,各工况连续采集涌水量结果呈先减小后增大至逐渐平衡的趋势,而改变初衬渗透系数则呈现涌水量逐渐减小直至平衡的趋势;非扰动开挖使地下水向隧道内汇集,隧道周围水力坡度明显增大,等水位线分布密集,掌子面附近等水位线沿隧道开挖方向凹陷;完善的防水体系(初衬及注浆圈)对保持地下水稳定起到显著作用,可有效减弱局部水力联系,减小渗流影响范围,地下水降幅也明显减小。研究方法及成果对类似工程设计和施工具有一定的指导作用。     

住宅小区风环境数值模拟

建立了某典型住宅小区风环境的物理和数学模型,并应用计算流体动力学方法求解了三种来流角度下的稳态三维湍流流场,给出了速度、压力、湍流度参数的分布;分析了街区涡流、巷道风及活塞风等效应,研究了风压作用下建筑群的迎风及背压区,讨论了湍流入流边界对合理湍流分布的作用;比较了不同来流角度下五个观测点位置的风环境参数。     

火灾与事故应急处置中的水体污染及防控措施

分析火灾及事故应急处置措施对于水环境的影响,明确消防部队应急处置措施与水环境污染的密切联系,介绍可能对水体造成污染的几种典型事故,并据此提出防控措施,以防止和减少应急处置措施对于水环境的污染.     

Consistent boundary conditions for flows within the atmospheric boundary layer

In computational wind engineering the neutrally stable atmospheric boundary layer (ABL) is often simulated using the standard k-ε model. The application of boundary conditions that are inconsistent with the profiles used at the inflow boundary causes streamwise gradients in the solution and prevents the simulation of a horizontally homogeneous boundary layer. In the present work these problems are overcome by applying a simple extension of the shear stress boundary condition at the top of the domain and by using one-dimensional models to generate inflow profiles in equilibrium with the ground boundary condition. This procedure allows the impact of the inconsistent boundary conditions to be quantitatively assessed. It is shown that inconsistent boundary conditions at the top of the domain result in erroneous streamwise gradients throughout the domain. These errors are reduced by enlarging the domain in the vertical direction but are not removed. The errors are also found in simulations with idealised and real topography included in the domain. A brief discussion of the impact of the errors on simulations of wind energy projects is given.     

CFD boundary conditions for contaminant dispersion,heat transfer and airflow simulations around human occupants in indoor environments

Indoor computational fluid dynamics (CFD) simulations can predict contaminant dispersion around human occupants and provide valuable information in resolving indoor air quality or homeland security problems. The accuracy of CFD simulations strongly depends on the appropriate setting of boundary conditions and numerical simulation parameters. The present study explores influence of the following three key boundary condition settings on the simulation accuracy: (1) contaminant source area size, (2) convective/radiative heat fluxes, and (3) shape/size of human simulators. For each of the boundary conditions, numerical simulations were validated with experimental data obtained in two different environmental chambers. In CFD simulations, a small release area of a contaminant point source causes locally high concentration gradients that require a very fine local grid system. This fine grid system can slow down the simulations substantially. The convergence speed of calculation is greatly increased by the source area enlargement. This method will not influence the simulation accuracy of passive point source within well-predicted airflow field. However, for active point source located within complicated airflow filed, such an enlargement should be carried out cautiously because simulation inaccuracy might be introduced. For setting thermal boundary conditions, convection to radiation heat flux ratio is critical for accurate CFD computations of temperature profiles around human simulators. The recommended convection to radiation (C:R) ratio is 30:70 for human simulators. Finally, simplified human simulators can provide accurate temperature profiles within the whole domain of interest. However, velocity and contaminant concentration simulations require further work in establishing the influence of simplifications on the simulation accuracy in the vicinity of the human simulator.     

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.     

城市燃气设施突发环境污染事故应急预案

针对企业目前对突发环境污染事故缺乏相应的应急预案和高效的应急系统的现状,阐述了突发环境污染事故应急系统的组成、企业应急机构的组成和职责。以中压燃气管网泄漏为例,论述了突发环境污染事故应急预案的制定。     

南海饮用水水源突发性污染应急处理系统的研发和建立

针对水源突发性污染频发的现状,开发和建立了南海饮用水水源突发性污染应急处理系统。在对北江水源地存在的污染物进行调查分析并确定目标污染物的基础上,研究了目标污染物的应急去除技术,建立水源水质预警系统和水厂应急强化处理系统。介绍了项目实施的技术思路以及取得的主要技术成果,可为同行业其他企业提供参考。     

Wirklichkeitsnahe Brandeinwirkungen für die Planung und Nachrechnung von Tunnelschalen

Realistic fire loads for design and re‐analysis of tunnel linings Meanwhile, the development of numerical systems for the simulation of flow phenomena has reached a level, which even allows the analysis of complex scenarios such as tunnel fires, within acceptable time‐periods. On the one hand, this provides the possibility to critically question fire‐curves from standards (derived and calibrated for the general case) against the background of tunnel‐specific temperature fields. On the other hand, both temperature and smoke development as well as expected structural impairment may be assessed on a relatively realistic basis without large‐scale conflagration tests being necessary. In addition to general options of non‐linear structural analyses (i.e. utilisation of load re‐distribution and activation of additional bedding reactions) [1] the simulation‐based evaluation of fire loads particularly provides a valuable potential for the re‐analysis of existing tunnel linings. This especially applies in such cases where the calculation with conventional fire‐curves requires extensive strengthening and retrofit, whereas more favourable loading conditions may be expected by taking into account the tunnel‐specific boundary conditions. In the present paper, firstly tunnel fire accidents and the derivation of fire‐curves are generally discussed. Then, the possibilities, the theoretical background and limits of CFD modelling of fire scenarios are presented and finally, major influence factors on the thermal loading (in particular the cross‐sectional size of the tunnel) are investigated by sensitivity studies.     

A semi-analytical method for analyzing the tunnel water inflow

This paper proposed a semi-analytical approach for analyzing the problems of the tunnel water inflow. The approach was developed by using the classical ground water theory. The groundwater problem in a finite domain is transformed to a problem in an infinite domain by the image method and the superposition principle. Using the convolution–deconvolution method, the time-variant inflow problem is transformed to a constant flow problem. The calculation formula was developed for the conceptualized hydro-geological models for the tunnel inflow problems. The computation is simple and fast in comparison with the other 2D and 3D numerical methods. The parameters required in the approach are few and easily estimated. Two cases of the tunnel water inflow were simulated by the approach. The calculated inflows agree well with the general trends of the measured inflow data.     

A new tunnel inflow classification (TIC) system through sedimentary rock masses

A new classification system with respect to the engineering geological characteristics of rock masses in different geological conditions were presented based on the authors’ experiences and observations. Rock mass composition (RMC), rock type (RT), clay-bearing content (CBC), unconfined compressive strength (UCS) and tunnel depth (TD) were found as the major factors affecting the tunnel inflow. In order to minimize judgmental bias and set up a basic database, data pertaining to these factors were compiled from 33 tunnels project with a total length of about 200 km thoroughly excavated in sedimentary rocks. The classification factors were rated using a combination of the analytic hierarchy process (AHP) and statistical methods. In order to cover all rock mass varieties and lessen the uncertainties, major factors were divided into categories of varying quality. Two statistical criteria were introduced to calculate the weighing of categories. The main advantage of this procedure is its capability of effectively predicting groundwater inflows in a vast variety of geological conditions especially from a single flow pathway such as a brittle fault zone to low permeable rock masses. The proposed classification was applied to the actual rock tunnels. It was revealed that the predicted values were in a good agreement with the actual field measurements and could provide quantitative measures of tunnel inflow. The proposed method could be more feasible for a reliable pre-assessment of groundwater inflows in the future tunnel construction projects under heterogeneous geological conditions. Furthermore, the most important factors as well as their combination are introduced for sedimentary rocks.