Improved parameterisation for the numerical modelling of air pollution within an urban street canyon

Numerical modelling for application to wind flow and dispersion in urban environments has noticeably progressed in recent years, to currently represent a widely used tool for simulating mechanical processes governing air pollution in complex geometries. In particular, Computational Fluid Dynamic (CFD) techniques based on RANS (Reynolds-Averaged Navier–Stokes equations) models, are extensively used to produce detailed simulations of the wind flow and turbulence in the urban canopy. However, several studies have indicated that RANS models, and in particular the widely used standard k–? turbulence model, are sensitive to the particular form of inlet profiles for turbulence and velocity. In the present study, simulations of the wind flow and dispersion within an idealised street canyon were carried out using the standard k–? turbulence model provided by the commercial software FLUENT. The aim of this study was to improve the standard k–? model performance by modifying the model parameters according to the chosen form of inlet profiles for velocity and turbulence. Capability of the model to reproduce real wind flow fields, turbulence and concentration patterns was evaluated by comparing the model results against recently published wind tunnel data. Results for turbulent kinetic energy and concentration showed that the redefinition of the default dispersive parameters can significantly enhance the model performance. The newly proposed parameterisations of the standard k–? turbulence model can be readily implemented within commercial CFD software packages, offering a reliable modelling tool for application to urban air pollution and other environmental studies.     

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.     

An investigation of ship airwakes using Detached-Eddy Simulation

Computational Fluid Dynamics simulations of ship airwakes have been performed using Detached-Eddy Simulation (DES) on unstructured grids. A generic simple frigate shape (SFS2) and a Royal Navy Type 23 Frigate (T23) have been studied at several wind-over-deck (WOD) conditions. A comprehensive validation exercise has been performed, comparing CFD results of the airwake calculated for the SFS2 with high quality wind tunnel data provided by the National Research Council of Canada. Comparisons of mean quantities and velocity spectra show good agreement, indicating that DES is able to resolve the large-scale turbulent structures which can adversely impact helicopter-ship operations. An analysis of the airwake flow topology at headwind and Green 45° conditions highlights the dominant flow features over the flight deck and it is shown that significant differences exist between the two WOD angles. T23 airwake data has been compared to full-scale experimental results obtained at sea. It is shown that the inclusion of an atmospheric boundary layer velocity profile in the CFD computations improves the agreement with full-scale data. Qualitative comparison between the simple frigate shape and T23 airwakes shows that large-scale flow patterns are similar; but subtle differences exist, particularly at more oblique WOD angles.     

Effects of superstructure flexibility on strength of reinforced concrete bridge decks

This research studies the impact of the relative rigidity between the concrete bridge deck and the remaining structural components of the bridge superstructure on the behavior of the concrete deck. The study uses non-linear 3-D FEM models, which are developed using ANSYS 5.7 software package. Experimental data from one-span non-composite bridge superstructure are used to validate and calibrate the proposed FEM models. A series of parametric studies is conducted with respect to three parameters: (a) composite action, (b) slenderness ratio, and (c) presence of diaphragms. The analysis results are discussed in detail and conclusions on the behavior of the bridge deck are presented.     

汽车换热器风室试验台的CFD分析

为给汽车前端和发动机舱内气流数值计算提供参考依据,基于FLUENT对某汽车换热器风室试验台进行建模和数值模拟;分析风室内部空气流动状况,针对流动特征,给出风室计算流体力学（Computational Fluid Dynamics,CFD）校核的评价．网格采用四面体结构,模型中采用三维不可压的雷诺平均N．S方程,速度压力耦舍采用SIMPLE方法．空间离散格式为2阶迎风格式,时间离散格式为2阶隐式．选用realizable k-ε占模型模拟风室内部空气的湍流流动．固体壁面采用无滑移边界条件和非平衡壁面函数边界条件．模型进口采用速度入口来给定风量,出口采用压力出口．比较计算结果与试验设计标准,喷嘴压差的相对偏差范围在5％以内,基本达到对设备的精度要求,对风室设计有一定指导意义．     

基于CFD的通信机房流场模型建立与分析

介绍了一种利用计算流体动力学技术分析通信机房流场分布的方法,找出合理的气流组织形式以达到节能的目的.对机房三维物理模型进行非结构化网格划分,选用标准的k-ε湍流模型,设定相应的边界条件,针对不同的空调风速工况,利用Fluent软件对机房内的温度场和速度场进行数值模拟与分析,研究得出合理的风速以降低能耗.该方法能够有效地建立通信机房的流场模型,对制定机房节能方案有重要的参考作用.     

Numerical simulation for aerodynamic derivatives of bridge deck

An improved domain decomposition method is proposed to compute aerodynamic derivatives of bridge deck based on commercial Computational Fluid Dynamic (CFD) code FLUENT. In this method, the computational region is discretized into rigid boundary layer mesh region, dynamic mesh region and static mesh region. A simplified formula used to control the body-fitted grid height is deduced from the standard wall function. Aerodynamic derivatives of a flat plate and two long span bridge decks are computed and compared with the theoretical values and the wind tunnel tests results. Numerical results show that several aerodynamic derivatives are influenced by the accessory structures of the bridge deck such as guardrails and inspecting vehicle rails, etc.     

A solution method for three-dimensional turbulent boundary layers on bodies of arbitrary shapes

This paper is concerned with the prediction of the three-dimensional turbulent flows around bodies of irregular but basically cylindrical shape. Examples of such bodies are: ship or submarine hulls or an aircraft fuselage.The solution method described uses a non-orthogonal coordinate system in which the surface of the body is arranged to coincide with a coordinate surface. The velocity components solved for are the axial, radial and circumferential components of the cylindrical system from which the coordinates are derived. The equations are solved by the finite difference method of [1] for three-dimensional, parabolic flows. Turbulence is accounted for via a two-equation model of turbulence developed in [2] and modelled in [3].Some preliminary solutions are presented for flow around the rear part of a ship's hull. The results seem to be qualitatively correct. However, no comparison with experimental data has yet been made.The present method can be easily extended to handle partially parabolic effects as described in [4].     

Low Reynolds number turbulent flows around a dynamically shaped airfoil

A computational investigation for flows surrounding a dynamically shaped airfoil, at a chord Reynolds number of 78,800, is conducted along with a parallel experimental effort. A piezo-actuated flap on the upper surface of a fixed airfoil is adopted for active control. The actuation frequency focused on is 500 Hz. The computational framework consists of a multi-block, moving grid technique, the eⁿ-based laminar-turbulent transition model, the two-equation turbulence closure, and a pressure-based flow solver. The moving grid technique, which handles the geometric variations in time, employs the transfinite interpolation scheme with a spring network approach. Comparing the experimental and computational results for pressure and velocity fields, implications of the detailed flap geometry, the flapping amplitude, turbulence modeling, and grid distributions on the flow structure are assessed. The effect of the flap movement on the separation location and vortex dynamics is also investigated.     

Holistic ship design optimization

Ship design is a complex endeavor requiring the successful coordination of many disciplines, of both technical and non-technical nature, and of individual experts to arrive at valuable design solutions. Inherently coupled with the design process is design optimization, namely the selection of the best solution out of many feasible ones on the basis of a criterion, or rather a set of criteria. A systemic approach to ship design may consider the ship as a complex system integrating a variety of subsystems and their components, for example, subsystems for cargo storage and handling, energy/power generation and ship propulsion, accommodation of crew/passengers and ship navigation. Independently, considering that ship design should actually address the whole ship’s life-cycle, it may be split into various stages that are traditionally composed of the concept/preliminary design, the contractual and detailed design, the ship construction/fabrication process, ship operation for an economic life and scrapping/recycling. It is evident that an optimal ship is the outcome of a holistic optimization of the entire, above-defined ship system over her whole life-cycle. But even the simplest component of the above-defined optimization problem, namely the first phase (conceptual/preliminary design), is complex enough to require to be simplified (reduced) in practice. Inherent to ship design optimization are also the conflicting requirements resulting from the design constraints and optimization criteria (merit or objective functions), reflecting the interests of the various ship design stake holders.The present paper provides a brief introduction to the holistic approach to ship design optimization, defines the generic ship design optimization problem and demonstrates its solution by use of advanced optimization techniques for the computer-aided generation, exploration and selection of optimal designs. It discusses proposed methods on the basis of some typical ship design optimization problems with multiple objectives, leading to improved and partly innovative designs with increased cargo carrying capacity, increased safety and survivability, reduced required powering and improved environmental protection. The application of the proposed methods to the integrated ship system for life-cycle optimization problem remains a challenging but straightforward task for the years to come.     

Smooth coverage path planning and control of mobile robots based on high-resolution grid map representation

This paper presents a new approach to a time and energy efficient online complete coverage solution for a mobile robot. While most conventional approaches strive to reduce path overlaps, this work focuses on smoothing the coverage path to reduce accelerations and yet to increase the average velocity for faster coverage. The proposed algorithm adopts a high-resolution grid map representation to reduce directional constraints on path generation. Here, the free space is covered by three independent behaviors: spiral path tracking, wall following control, and virtual wall path tracking. Regarding the covered region as a virtual wall, all the three behaviors adopt a common strategy of following the (physical or virtual) wall or obstacle boundaries for close coverage. Wall following is executed by a sensor-based reactive path planning control process, whereas the spiral (filling) path and virtual wall path are first modeled by their relevant parametric curves and then tracked via dynamic feedback linearization. For complete coverage, these independent behaviors are linked through a new path linking strategy, called a coarse-to-fine constrained inverse distance transform (CFCIDT). CFCIDT reduces the computational cost compared to the conventional constrained inverse distance transform (CIDT), which applies a region growing starting from the current robot position to find the nearest unexplored cell as well as the shortest path to it while constraining the search space. As for experimental validation, performance of the proposed algorithm is compared to those of conventional coverage techniques to demonstrate its completeness of coverage, energy and time efficiency, and robustness to the environment shape or the initial robot pose.     

Hydrodynamic design using a derivative-free method

A derivative-free shape optimization tool for computational fluid dynamics (CFD) is developed in order to facilitate the implementation of complex flow solvers in the design procedure. A modified Rosenbrocks method is used, which needs neither gradient evaluations nor approximations. This approach yields a robust and flexible tool and gives the capability of performing optimizations involving complex configurations and phenomena. The flow solver implemented solves the Reynolds-averaged Navier–Stokes equations (RANSE) on unstructured grids, using near-wall, low-Reynolds-number turbulence models. Free surface effects are taken into account by a pseudosteady surface tracking method. A mesh deformation strategy based on both lineal and torsional springs analogies is used to update the mesh while maintaining the quality of the grid near the wall for two-dimensional problems. A free-form-deformation technique is used to manage the mesh and the shape perturbations for three-dimensional cases. Two hydrodynamic applications are presented, concerning first the design of a two-dimensional hydrofoil in relation with the free-surface elevation and then the three-dimensional optimization of a hull shape, at full scale.     

A numerical model for wind loads simulation on long-span bridges

A numerical model of the air flow problem around the girder of a long-span bridge is presented. The model is based on a finite volume formulation and it is able to simulate steady and non-steady wind loading conditions on the structure under the simplifying assumption, which is plausible for bridges with long spans, of a two-dimensional-like approaching flow. For a given bridge deck cross-section the proposed model allows the numerical evaluation of the flutter derivatives, which is useful to characterize in an analytical way the stability conditions of the overall wind-induced bridge response. In order to obtain satisfactory accuracy and stability of the numerical solution, a two-equation k–ϵ RNG turbulence model and special boundary conditions are employed. The accuracy and applicability of the model to wind engineering problems are successfully assessed by computing the aerodynamic behaviour of some simple cross-section shapes. Numerical results are also obtained for typical long-span bridge cross-sections and the comparison with the available wind tunnel measurements shows a good agreement.     

Study of CFD variation on transport configurations from the second drag-prediction workshop

This paper describes and analyzes a series of nearly 90 CFD test cases performed as a contribution to the second Drag Prediction Workshop, held in Orlando, Florida in June 2003. Two configurations are included: DLR-F6 wing-body and wing-body-nacelle-pylon. The ability of CFD to predict the drag, lift, and pitching moment from experiment--including the “delta” arising from the addition of the nacelle and pylon--is assessed. In general, at a fixed angle of attack CFD overpredicts lift, but predicts the ΔC_L reasonably well. At low lift levels (C_L < 0.3), ΔC_D is 20-30 drag counts (30-45%) high. At the target lift coefficient of C_L  =  0.5, ΔC_D is overpredicted by between 11 and 16 counts. However, the primary contribution of this paper is not so much the assessment of CFD against experiment, but rather a detailed assessment and analysis of CFD variation. The series of test cases are designed to determine the sensitivity/variability of CFD to a variety of factors, including grid, turbulence model, transition, code, and viscous model. Using medium-level grids (6-11 million points) at the target lift coefficient, the maximum variation in drag due to different grids is 5-11 drag counts, due to code is 5-10 counts, due to turbulence model is 7-15 counts, due to transition is 10-11 counts, and due to viscous model is 4-5 counts. Other specific variations are described in the paper.     

On the effect of wind direction and urban surroundings on natural ventilation of a large semi-enclosed stadium

Natural ventilation of buildings refers to the replacement of indoor air with outdoor air due to pressure differences caused by wind and/or buoyancy. It is often expressed in terms of the air change rate per hour (ACH). The pressure differences created by the wind depend - among others - on the wind speed, the wind direction, the configuration of surrounding buildings and the surrounding topography. Computational Fluid Dynamics (CFD) has been used extensively in natural ventilation research. However, most CFD studies were performed for only a limited number of wind directions and/or without considering the urban surroundings. This paper presents isothermal CFD simulations of coupled urban wind flow and indoor natural ventilation to assess the influence of wind direction and urban surroundings on the ACH of a large semi-enclosed stadium. Simulations are performed for eight wind directions and for a computational model with and without the surrounding buildings. CFD solution verification is conducted by performing a grid-sensitivity analysis. CFD validation is performed with on-site wind velocity measurements. The simulated differences in ACH between wind directions can go up to 75% (without surrounding buildings) and 152% (with surrounding buildings). Furthermore, comparing the simulations with and without surrounding buildings showed that neglecting the surroundings can lead to overestimations of the ACH with up to 96%.     

3D ball skinning using PDEs for generation of smooth tubular surfaces

We present an approach to compute a smooth, interpolating skin of an ordered set of 3D balls. By construction, the skin is constrained to be C¹ continuous, and for each ball, it is tangent to the ball along a circle of contact. Using an energy formulation, we derive differential equations that are designed to minimize the skin’s surface area, mean curvature, or convex combination of both. Given an initial skin, we update the skin’s parametric representation using the differential equations until convergence occurs. We demonstrate the method’s usefulness in generating interpolating skins of balls of different sizes and in various configurations.     

基于CFD的集装箱船阻力性能优化

基于CFD方法实现船体型线的自动优化。应用船型参数化建模方法分析特征参数,提取设计变量,以兴波阻力最小为目标,分别采用Sobol算法和Tsearch算法实现船体型线的自动优化。将上述方法应用于5 100 TEU集装箱船的型线自动优化,运用Shipflow软件进行CFD数值计算。评估结果表明优化船型在弗劳德数Fr=0.26时总阻力减少3.62%,说明该方法可行。     

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.     

Practical and flexible path planning for car-like mobile robot using maximal-curvature cubic spiral

This paper presents a nonholonomic path planning method, aiming at taking into considerations of curvature constraint, length minimization, and computational demand, for car-like mobile robot based on cubic spirals. The generated path is made up of at most five segments: at most two maximal-curvature cubic spiral segments with zero curvature at both ends in connection with up to three straight line segments. A numerically efficient process is presented to generate a Cartesian shortest path among the family of paths considered for a given pair of start and destination configurations. Our approach is resorted to minimization via linear programming over the sum of length of each path segment of paths synthesized based on minimal locomotion cubic spirals linking start and destination orientations through a selected intermediate orientation. The potential intermediate configurations are not necessarily selected from the symmetric mean circle for non-parallel start and destination orientations. The novelty of the presented path generation method based on cubic spirals is: (i) Practical: the implementation is straightforward so that the generation of feasible paths in an environment free of obstacles is efficient in a few milliseconds; (ii) Flexible: it lends itself to various generalizations: readily applicable to mobile robots capable of forward and backward motion and Dubins’ car (i.e. car with only forward driving capability); well adapted to the incorporation of other constraints like wall-collision avoidance encountered in robot soccer games; straightforward extension to planning a path connecting an ordered sequence of target configurations in simple obstructed environment.     

Transition prediction for three-dimensional flows using parallel computation

A computational method to predict transition lines for general three-dimensional configurations is presented. The method consists of a coupled program system including a 3D Navier-Stokes solver, a transition module, a boundary layer code and a stability code. The newly developed transition module has been adapted to be used with parallel computation to account for the high computational demand for three-dimensional configurations. Detailed computations have been performed to show the ability of the Navier-Stokes code to provide three-dimensional boundary layer data of high accuracy needed for the stability analysis. A comprehensive investigation on general computational and parallel performance identifies the numerical effort for the transition prediction method. The procedure has been validated comparing the numerical results with experiments for the flow around an inclined prolate spheroid. Feasibility studies on generic transport aircraft have been performed to show the code’s capability to predict transition lines on general complex geometries.