Three dimensional numerical simulations of long-span bridge aerodynamics, using block-iterative coupling and DES

The design of long-span bridges often depends on wind tunnel testing of sectional or full aeroelastic models. Some progress has been made to find a computational alternative to replace these physical tests. In this paper, an innovative computational fluid dynamics (CFD) method is presented, where the fluid-structure interaction (FSI) is solved through a self-developed code combined with an ANSYS-CFX solver. Then an improved CFD method based on block-iterative coupling is also proposed. This method can be readily used for two dimensional (2D) and three dimensional (3D) structure modelling. Detached-Eddy simulation for 3D viscous turbulent incompressible flow is applied to the 3D numerical analysis of bridge deck sections. Firstly, 2D numerical simulations of a thin airfoil demonstrate the accuracy of the present CFD method. Secondly, numerical simulations of a U-shape beam with both 2D and 3D modelling are conducted. The comparisons of aerodynamic force coefficients thus obtained with wind tunnel test results well meet the prediction that 3D CFD simulations are more accurate than 2D CFD simulations. Thirdly, 2D and 3D CFD simulations are performed for two generic bridge deck sections to produce their aerodynamic force coefficients and flutter derivatives. The computed values agree well with the available computational and wind tunnel test results. Once again, this demonstrates the accuracy of the proposed 3D CFD simulations. Finally, the 3D based wake flow vision is captured, which shows another advantage of 3D CFD simulations. All the simulation results demonstrate that the proposed 3D CFD method has good accuracy and significant benefits for aerodynamic analysis and computational FSI studies of long-span bridges and other slender structures.     

Effects of amplitude-dependent damping and time constant on wind-induced responses of super tall building

Full-scale measurements of wind-induced responses of a 79-storey tall building, Di Wang Tower, were conducted during the passages of several typhoons. The amplitude-dependent damping ratios of the super tall building were obtained from the measurements. A Monte Carlo simulation procedure was developed in this study to generate fluctuating along-wind and across-wind forces acting on this building. The wind-induced responses of Di Wang Tower were numerically evaluated in time domain on the basis of the generated fluctuating wind forces and the established finite element model of the building. The predicted dynamic responses of the building using the actual amplitude-dependent damping characteristics were compared to those computed with constant damping parameters assumed by the structural designers to evaluate the adequacy of current design practices and to investigate the effect of amplitude-dependent damping on the wind-induced responses. Finally, the effect of time constant on the wind-induced responses of Di Wang Tower was studied by comparing the time domain computational results with those from conventional spectral analysis method. Some of the research findings resulted from this combined experimental and numerical study are expected to be of interest and practical use to professionals and researchers involved in the design and analysis of super tall buildings.     

RBF neural networks for the prediction of building interference effects

Wind loads on tall buildings can be quite different from those on an isolated building due to neighboring building effects. With the increase of number of tall buildings in large cities, there is a growing attention to the interference effects among adjacent buildings under wind action. While wind tunnel tests are of importance in the understanding of the physical process, the general quantitative predictions of interference effects are difficult to reach owing to many variables involved. In the present paper, a radial basis function (RBF) neural network is proposed for its strong ability in nonlinear mapping and its higher training speed. Thus the RBF neural network is applied to evaluate the interference effects (expressed by interference factor, IF) by using experimental data obtained from many sources as training patterns. The results indicate that a very good agreement is found between the predicted IF values and the experimental counterparts.     

基于CFD/CSD方法的亚音速平板结构气动弹性分析

采用CFD/CSD双向流固耦合算法研究平板结构的气动弹性耦合特性.首先,采用CFD/CSD算法计算平板结构的颤振临界速度,并与已有文献中的实验结果进行比较验证.然后,分别对简支和固支边界条件的三维平板结构进行气动弹性特性分析,计算不同约束情况下流场分布的变化和平板结构的位移响应.同时还考虑加肋和结构材质对平板结构气动弹性特性的影响.     

Multidisciplinary Simulation of the Maneuvering of an Aircraft

A computational methodology for the simulation of the transient aeroelastic response of an unrestrained and flexible aircraft during high-G maneuvers is presented. The key components of this methodology are: (a) a three-field formulation for coupled fluid/structure interaction problems; (b) a second-order time-accurate and geometrically conservative flow solver for CFD computations on unstructured dynamic meshes; (c) a corotational finite element method for the solution of geometrically nonlinear and unrestrained structural dynamics problems; (d) a robust method for updating an unrestrained and unstructured moving fluid mesh; and (e) a second-order time-accurate staggered algorithm for time-integrating the coupled fluid/structure semi-discrete equations of motion. This computational methodology is illustrated with the simulation on a parallel processor of several three-dimensional high-G pullup maneuvers of the Langley Fighter in the transonic regime, using a detailed finite element aeroelastic model.     

Modelling of wind load on single and staggered dual buildings

A method of modelling numerically the wind loads on single and staggered dual buildings using Computational Fluid Dynamics is presented in this paper. Simulation of a turbulent boundary layer over test models was carried out at the Supercomputing and Visualization Unit, the National University of Singapore, using FLUENT 6.1.18. Turbulence was introduced at the inlet through a parallel auxiliary simulation and the computation of the flow advanced in time using Large Eddy Simulation with a ReNormalization Group subgrid-scale viscosity model. Wind velocities at different locations and wind pressures on the building faces were recorded. Subsequently the flow characteristics were examined and the force and moment spectra deduced. The results were compared with data from earlier wind tunnel experiments carried out at Virginia Polytechnic Institute and State University. It can be concluded from the study that numerical wind modelling on tall structures is a promising alternative to conventional tests in atmospheric boundary layer wind tunnels.     

Computational analysis of wind interactions for comparing different buildings sites in terms of natural ventilation

Nowadays building designers have to face up to new strategies to achieve the best sustainable building designs. Well planned natural ventilation strategies in building design may contribute to a significant reduction on building’s energy consumption. Natural ventilation strategies are conditioned to the particular location of each building. To improve natural ventilation performance of a building, the analysis of the influence of the location and the surrounding buildings on wind flow paths around the design building is a must. New computational tools such as Computational Fluid Dynamics (CFD) are particularly suited for modelling outdoor wind conditions and the influence on indoor air conditions prior to building construction. Hence, reliable methodologies are necessary to support building design decisions related to naturally ventilated buildings prior to construction.This paper presents a case study for the selection of the best future building location attending to natural ventilation behaviour inside the building, conditioned by different evolving environment. A validated CFD model is used to represent outdoor and indoor spaces. The methodology explains how to qualitatively and quantitatively analyze wind paths around and through a building to quantify the natural ventilation performance. The best location, from two real possible solutions, is then selected.     

高层建筑结构力学模型仿真系统

该文介络了高层建筑结构力学模型仿真的理论基础、关键技术和实现途径,并简述了高层建筑结构多媒体仿真系统MSTB的设计特点。     

远程CFD仿真及在建筑设计中的应用

数值仿真是评价建筑方案的有力手段，但一般工程人员不易使用。该文提出基于web service的CFD计算服务，将服务请求和实现分开，降低系统的耦合性。围绕远程计算的特性，介绍了基于客户端、web层、应用层和后台数据库的仿真平台系统架构，具体讨论了实现计算服务的统一接口及其描述。为减少用户对系统的干预，提出了计算伺服Agent以提高系统的智能性，并描述了其结构和工作原理。通过引入基于Java3D的交互式后处理界面以提高软件使用的友好性。最后开发了用以评价建筑群风环境的简单原型系统以检验方法的可行性。     

Multidisciplinary approach to aeroelastic studies of long-span bridges

This paper focuses on the design of long-span suspension bridges under aeroelastic constraints. Such challenging structures need to be protected against wind-induced instabilities as flutter. The authors envision that a set of scientific disciplines not currently used in bridge engineering may help along the design process and constitute a useful tool. First, the formulation of sensitivity analysis of flutter speed is described, indicating how this technique can be a guide for engineers making changes in the prototype; two examples of important bridges, as the Great Belt and Messina Strait, are used to demonstrate the capability of this approach. Then, the idea of producing computer animations to represent the aeroelastic deformation of bridges simulating virtual boundary layer wind tunnel testing is presented showing pictures of the Tacoma Narrows and Messina Bridges. Finally, the advantages of introducing distributed computing to make easier to implement the previously mentioned techniques are demonstrated. The authors have previously published papers related with sensitivity analysis in bridges or computer animations of the aeroelastic behaviour of suspension bridges. However, this is the first time that their comprehensive multidisciplinary approach is presented.     

Aeroelastic tailoring and scaling using Bacterial Foraging Optimisation

Bacterial Foraging Optimisation (BFO) is investigated in an attempt to evaluate its use in solving complex optimisation problems for aeronautical structures. A hybrid variant of BFOA, which incorporates meta-modelling techniques, is also proposed and employed. The efficiency and effectiveness of the methods are tested for tailoring a rectangular composite wing, aiming to maximise the flutter speed and for scaling a joined-wing aircraft, targeting to match aeroelastic responses between the physical prototype and wind tunnel model. The obtained results are compared with those found using a range of other biologically inspired optimisation methods (GA, PSO, ACO), proving that the social foraging behavior of motile bacteria is an effective tool for aeroelastic optimisation.     

Virtual wind tunnel: An alternative approach for the analysis of bridge behaviour under wind effects

The study of bridge responses under wind-induced loads is based upon full aeroelastic model testing or hybrid methods which use section model tests and subsequent computer analysis. Both methodologies present several strong points and some shortcomings, specially related with the visualization of the bridge dynamic behaviour. Nowadays, advances and improvements in computational power and computer aided design technologies make possible a new way towards the feasible design of long span bridges considering its aerodynamic and aeroelastic behaviour. The virtual wind tunnel (VWT) technique developed by the authors joins together accurate section model testing with computer aided design in order to obtain a detailed computer visualization of the complete bridge behaviour under wind flow. The results obtained for the Tacoma Narrows Bridge and the Messina Strait Bridge are presented.     

A methodology to assess the influence of local wind conditions and building orientation on the convective heat transfer at building surfaces

Information on the statistical mean convective heat transfer coefficient (CHTC_SM) for a building surface, which represents the temporally-averaged CHTC over a long time span (e.g. the lifetime of the building), could be useful for example for the optimisation of the performance of solar collectors and ventilated photovoltaic arrays or for preservation analysis of cultural heritage sites. A methodology is proposed to estimate the CHTC_SM for a building surface, by combining local wind climate information and information on the CHTC, namely CHTC-U₁₀ correlations, where U₁₀ is the mean wind speed at a height of 10 m above the ground. This methodology is applied to a cubic building for a specific wind climate, where the CHTC-U₁₀ correlations are obtained by means of CFD simulations (CFD code Fluent 6.3, realizable k-? turbulence model). It is shown that the CHTC_SM varied significantly with the orientation of the building surface due to the rather anisotropic wind conditions, where high values are found for surfaces oriented towards the prevailing wind directions, thus for windward conditions. Moreover, the evaluation of the CHTC_SM for other wind climates clearly shows that the local wind conditions also can have a significant impact on the overall magnitude of the CHTC_SM, where differences up to a factor 4 are found in this study. Different levels of complexity for determining the CHTC_SM value are also evaluated and it is found that the required number of CFD simulations can be reduced significantly by using more simplified methods to calculate the CHTC_SM, without compromising its accuracy. The applicability of the proposed methodology for other building-related applications is also discussed, for example to assess statistical mean pressure coefficients, wind-driven ventilation rates or convective mass transfer coefficients.     

Large-eddy simulation in a complex hill terrain enabled by a compact fractional step OpenFOAM® solver

We report computational fluid dynamics (CFD) code developments using the high-level programming syntax of the open source C++ library OpenFOAM®. CFD simulations utilizing the large-eddy simulation (LES) approach are carried out using the developed code in a real-world application. We investigate wind flowing over the Bolund hill, Denmark. In the present configuration a west–east wind meets the steep west side of the hill. Such conditions lead to flow separation at the location of a sharp cliff. A full scale simulation, with a simulation duration of over one month, is carried out on a supercomputer. Physically, about 45 min of real time is simulated in the LES enabling the statistical averaging of the results. The novelty of the paper consists of the following features: (1) we report validation results of the newly developed LES code for the Bolund hill case, (2) we show the high-level LES solver code in its entirety in a few tens of code lines which promotes transparency in CFD-code development in the OpenFOAM® environment, (3) the study is the first study to use LES in pointing out the complex 3d characteristics of the Bolund hill case with the computationally challenging west–east (270°) wind direction, and (4) based on the comparison with previous experimental data, and Reynolds averaged Navier–Stokes (RANS) simulations, the present LES gives so far the best match for the turbulent kinetic energy increase at the considered measurement positions.     

基于拓扑优化的筏形基础传力途径仿真

文章主要介绍了通过拓扑优化对筏形基础的传力途径进行仿真分析的基本原理和所涉及的关键问题.结合大型有限元软件ANSYS,建立了筏板-地基土共同作用体系的全三维有限元模型.并在此基础上,采用变密度法建立拓扑优化模型,选取序列线性规划法对某高层建筑的筏形基础结构进行了仿真分析.仿真结果较为清楚地反映了筏板内部应力分布的宏观规律.表明,应用有限元法进行筏形基础结构拓扑优化设计是一种有效的优化方法,可为高层建筑筏形基础设计提供重要的概念化设计参考.     

冷弯薄壁型钢蒙古包风雪作用下仿真研究

冷弯薄壁型钢蒙古包是一种新型房屋结构,结构质量轻、柔性大的特点决定了其控制荷载为风荷载与积雪荷载。通过计算流体动力学(CFD)方法,将蒙古包的风荷载体型系数与现行规范进行对比。模拟得到屋面的风荷载分布与不均匀雪荷载分布,定义2种风向角,将风荷载与风雪荷载分别作用于冷弯薄壁型钢蒙古包刚架,分析不均匀雪荷载与不同风向角对刚架的影响。研究表明,蒙古包由于其形状的特殊性使得风荷载体型系数出现变号情况;不均匀雪荷载分布使蒙古包在风场中的稳定性降低;30°风向角,即没有主平面刚架下的风向角为不利风向角,蒙古包刚架将在该风向角下,风雪荷载共同作用时率先发生失稳。     

高层建筑结构CAD系统的数据模式

用面向对象的分析方法对高层建筑结构CAD系统进行了分析,通过对基本对象及其之间关系,作用的抽象,运用面向对象的设计方法建立了高层建筑 基本对象模型,采用面向对象技术和关系数据库相结合的方法,构造和初步建立了高层建筑结构的工程数据库,能够较好地解决高层建筑结构集成化CAD系统的数据组织和管理问题。     

Wind modelling,validation and sensitivity study using Weather Research and Forecasting model in complex terrain

A physically-based wind model is applied to determine wind speed and direction and to conduct a model sensitivity analysis. The focus is the East African site of the Lake Turkana Wind Farm, characterized by complex terrain and high diurnal variability that creates a nocturnal jet of typically 15 m/s. Observations from three tall meteorological masts are compared with Weather Research and Forecast (WRF) model outputs. WRF is configured with four domains nested down to 900 m spatial resolution. The model is tested with initialization fields from two different sources, optimised using different grid configurations and parameterization schemes. Comparing model and data from 3 tall masts A, B and C yields that the primary source of error in WRF model simulation in a complex terrain is due to incorrect specification of boundary fields used to initialize the model. RMSEs achieved in this research are ≤2 m/s representing good model performance (Emery et al., 2001).     

Computational analysis of the performance of a venturi-shaped roof for natural ventilation: Venturi-effect versus wind-blocking effect

Computational Fluid Dynamics (CFD) is used to gain insight in the aerodynamic performance of a venturi-shaped roof (called VENTEC roof). The simulations are performed with the 3D steady Reynolds-averaged Navier–Stokes (RANS) equations and the Renormalisation Group k–ε model. A detailed analysis is conducted of the influence of the so-called venturi-effect and the wind-blocking effect on the aerodynamic performance of the VENTEC roof. The specific roof configuration is intended to create a negative pressure in the narrowest roof section (contraction) which can be used to partly or completely drive the natural ventilation of the building zones. The CFD simulations are based on a detailed grid-sensitivity analysis and on successful validation of the grid-independent results by comparison with experiments in an atmospheric boundary layer wind tunnel. The simulations show that the aerodynamic performance of the roof is governed by the balance between the so-called venturi-effect on the one hand and the wind-blocking effect on the other hand. The venturi-effect cannot act to its full extent because the flow is non-confined. The wind-blocking effect refers to the effect of the resistance exerted by the roof contraction on the air flow and the resulting tendency of the approaching wind to flow around and over the roof, rather than only being forced through the roof contraction. The results indicate that because of the wind-blocking effect, the highest contraction ratio does not provide the best aerodynamic performance and the largest negative pressure, which is a counter-intuitive result. The paper also provides a parametric analysis to optimise the roof contraction height and contraction ratio. The study in this paper illustrates the use of CFD to increase insight in building aerodynamics and to support sustainable building design.     

CFD mesh generation for biological flows: Geometry reconstruction using diagnostic images

A new thrust in the use of CFD techniques for simulation of biological flows has necessitated the demand for robust grid generation techniques to characterize the complex geometries. While the techniques of image manipulation required are simple, most researchers in this field use proprietary 3rd party software for image manipulation and grid generation. In the current study, we propose a simple MATLAB based grid generation techniques suitable for CFD studies of external and internal biological flows such as blood flow and respiration and flows around the human body. As an example, the flow inside two specific intracranial aneurysms is modeled by generating CFD grids from 3D rotational angiography images. Specific issues of modeling, such as boundary conditions and location of flow inlets and outlets, in relation to the reconstructed geometry are discussed. The reconstructed arterial geometry including the aneurysm matches the visual representation generated by the angiogram software (Leonardo software). The calculated CFD flow patterns also show a good correlation to the flow visualization presented by the Leonardo software. Areas of high pressure and wall shear stress are identified. The same technique is also used to generate the CFD grid of a human trachea to study the particle dispersion patterns during a human cough cycle. The fluid is modeled using an actual human cough signal with the particles simulating the influenza virus. The flow pattern out of the mouth along with the dispersion pattern of the particles is validated against similar human experimental studies to track the spread of the disease through cough. Work is also currently underway to use the present grid generation program to construct a superficial mesh of the human body from MRI/CAT scan images of cadavers. The goal is to build an accurate and scalable model of the human body surface with articulate joints which can be posed in any environment to model the air flow patterns around the body.