Effects of freestream turbulence on streamwise pressure measured on a square-section cylinder

Wind tunnel experiments were performed to study the effects of freestream turbulence on the flow around a square cylinder. The results indicate that the pressure fluctuations on streamwise surfaces are produced mainly by periodic vortex shedding when turbulence intensity, σ_u/U, is less than 12%. As σ_u/U approaches 20%, vortex shedding weakens significantly and pressure fluctuations occur over a broad range of frequency. Flow visualization experiments show that extreme negative pressure peaks in highly turbulent flow are associated with intermittent reattachment of separated shear layers. The results also show that an increase in turbulence scale causes peak pressure coefficients to increase.     

近地台风风场特性及低矮房屋风荷载现场实测研究

通过研制的可移动平坡屋面实验房风压及台风风场现场实测系统,研究近地台风风场特性和低矮房屋表面风荷载分布规律。基于实验房获取到的10余次近地台风风速和风压实测数据,对近地台风风场湍流特征参数如湍流强度、阵风因子、湍流积分尺度及脉动风速功率谱等,按来流不同方位地貌状况进行分类研究;同时分析了斜向强风最不利工况下,屋面角部区域风压分布特征。分析结果表明: A、B、C类地貌条件下,台风顺风向湍流强度均值分别为0.13, 0.21, 0.32;阵风因子同湍流强度正相关,湍流积分尺度随湍流强度增加而减少;与季风相比,台风眼壁区域的顺风向脉动风速功率谱密度值略大于季风的实测值,而横风向脉动风速功率谱密度值显著大于季风的实测值;在低频和惯性子区范围,台风眼壁区域的顺风向脉动风速von Karman和Harris谱拟合值与实测值吻合较好;在斜向风作用下迎风屋檐角部边缘测点区域具有较高峰值负压和脉动风压,峰值负压系数达-13.5。     

Interference effects on local peak pressures between two buildings

Local peak pressure coefficients between two buildings were studied by using wind tunnel experiments for various locations, different height ratios of interfering building and wind directions. The measured local peak pressure coefficients were compared to those obtained previously from a study on an isolated building. This study also investigated interference effects for local peak pressures on a principal building with various configurations and different height ratios of an interfering building. The experimental results have been examined and presented from the viewpoint of cladding design. The results show that highest peak suctions on a principal building increased with increase in height ratios of the interfering building. The oblique configuration generated more severe peak suction than the tandem configuration. To examine the interference effects for local peak pressures in detail, interference factors for maximum positive and minimum negative peak pressures at each measurement point (i, j) of the principal building for all wind directions are presented and discussed.     

The influence of Helmholtz resonance on internal pressures in a low-rise building

This paper deals with an investigation of the phenomenon of Helmholtz resonance under oblique wind flow, and an examination of the applicability of the quasi-steady approach to internal pressures in buildings with a dominant opening. Studies on a 1:50 scale model of the Texas Tech University (TTU) test building in a boundary layer simulation show that ‘Helmholtz resonance under oblique wind flow’ produces an extremely strong response in internal pressure fluctuations, in comparison with that obtained under normal onset flow. It is verified that ‘eddy dynamics over the opening’ rather than ‘freestream turbulence’ is responsible for the intense excitation at oblique flow angles, implying that even if the Helmholtz resonance frequency were to be in the tail of the freestream turbulence spectrum, severe excitation would still be possible.Experimental measurements of internal pressures for a range of opening situations also reveal that the quasi-steady approach is inapplicable in the prediction of peak internal pressures. Furthermore, it is demonstrated that while the provisions of the Australian/New Zealand wind loading code—AS/NZS1170.2:2002, which is based upon the quasi-steady method, is adequate as far as mean internal pressures are concerned, it however underpredicts peak internal pressures in some situations. In particular, for the range of situations studied, measurements indicated that peak pressures were up to 25% higher than the AS/NZS1170.2:2002 provisions, in the case of openings in the positive pressure and sidewall regions. It is also shown that for openings located in the sidewall region, peak internal pressures could be just as extremely positive as it can be negative. It is suggested that in the calculation of internal pressures, the AS/NZS1170.2:2002 provide for the use of local pressure factors K_l, that are at present applied only to external pressure calculations. Secondly, the code should provide for internal pressure coefficients to be both negative and positive, when openings are located in sidewall regions. Finally, in order to account for the effects of additional fluctuations arising from Helmholtz resonance oscillations, the possibility of the use of an internal pressure factor K_i should be explored.     

Internal pressures - The dominant windward opening case - A review

A building with a dominant opening in a wall may experience internal pressures equal to, or greater than, the external wind pressures in a severe windstorm. This is a critical design case in severe wind events, during which accidental openings may be produced by windborne debris or by direct wind loading.This paper reviews previous work on the prediction of fluctuating and peak internal pressures arising from a dominant opening on a windward wall, and summarizes proposed, or current, code formulations in a unified non-dimensional format. Experimental data from a number of sources are plotted together, and are found to fall into two groups, depending on the turbulence intensity in the approach flow.It is shown that the proposed relationships for the ratio of internal to external pressure fluctuations differ considerably, depending on the assumptions made. An intermediate relationship, which agrees well with experimental data in approach flow with turbulence intensities representative of atmospheric wind flow in developed boundary layers, is presented, and a design example of its application is given.     

A laboratory experiment of shear-induced natural ventilation

When the wind direction is parallel to the opening façade, the wind shear near the building opening generates turbulence and entrains air across the opening. This kind of shear-induced ventilation cannot be predicted by the orifice equation because the time-averaged pressure difference across the opening is close to zero. This study uses wind tunnel experiments and the tracer gas decay method to investigate the ventilation rate of shear-induced ventilation. The influences of opening area A, external wind speed U and wind direction on the ventilation rates Q, of single-sided and two-sided openings are systemically examined. The experimental results indicate that the dimensionless ventilation rate, Q^* = Q/UA, of shear-induced ventilation is independent of the wind speed and opening area, and the value of Q^* of two-sided openings is larger than that of a single-sided opening. In addition, a cosine law was used to predict the ventilation rate of building with two-sided openings under various wind directions, and the results are compared with the prediction of the multizone ventilation model COMIS.     

紊流风特性参数对矩形结构表面平均风荷载的影响

为了得到风特性参数变化对结构表面风压分布的影响规律,在风洞中采用格栅紊流,分别形成了紊流强度相同但积分尺度不同与积分尺度相同但紊流强度不同的几种局部紊流风场,以此来研究紊流风特性参数对矩形结构表面风压分布规律的影响,并给出了考虑这些影响的修正公式。结果表明:紊流强度增大会使矩形结构表面正压区体型系数增大,负压区体型系数绝对值减小;正压区风压变化规律与结构尺度无关,负压区体型系数除了受来流紊流强度影响外,还存在明显的尺度效应;紊流积分尺度增大会使矩形结构表面体型系数绝对值也增大,但不同区域变化幅度没有规律,很难进行统一修正。     

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。     

Numerical simulation of the wind field around different building arrangements

A numerical model with the RNG κ−ε turbulence closure model and a pressure correction algorithm of SIMPLEC is used to examine three different building configuration effects on wind flow. Comparisons of computational results with experimental data have been carried out for the vertical velocity profiles at some measurement points. For the experimental study, the building arrangements were presented by 1:150 scale models and tested in a low-speed wind tunnel. It was found that the wind environment for two improved arrangements with lower interval-to-height ratio is better than that for the reference layout with higher aspect ratio in terms of the natural ventilation. The interference effect is more obvious for two improved arrangements than the reference one. The numerical results also show that changing wind direction from perpendicular to the building facades to a 45°-incidence angle has significant effect on the flow field for different configurations.     

A phenomenological model for the dynamic response of wind turbines to turbulent wind

To predict the average power output of a wind turbine, a response model is proposed which takes into account: (i) the delayed response to the longitudinal wind speed fluctuations; (ii) a response function of the turbine with arbitrary frequency dependence; (iii) wind fields of arbitrary turbulence intensity. In the limit of low turbulence intensity, the dynamical ansatz as proposed in 1992 by Rosen and Sheinman is reproduced. It is shown, how the response function of the turbine can be obtained from simulation experiments of a specific wind turbine. For two idealized situations the dynamic effect of fluctuating wind is estimated at turbulence intensities 0?I_u?0.5. At the special mean wind speed , the turbine response function is determined from simulation data published by Sheinman and Rosen in 1992 and 1994.     

Flow around a cube in a turbulent boundary layer: LES and experiment

We present a numerical simulation of flow around a surface mounted cube placed in a turbulent boundary layer which, although representing a typical wind environment, has been specifically tailored to match a series of wind tunnel observations. The simulations were carried out at a Reynolds number, based on the velocity U at the cube height h, of 20,000—large enough that many aspects of the flow are effectively Reynolds number independent. The turbulence intensity was about 18% at the cube height, and the integral length scale was about 0.8 times the cube height h. The Jenson number Je=h/z₀, based on the approach flow roughness length z₀, was 600, to match the wind tunnel situation. The computational mesh was uniform with a spacing of h/32, aiding rapid convergence of the multigrid solver, and the governing equations were discretised using second-order finite differences within a parallel multiblock environment. The results presented include detailed comparison between measurements and LES computations of both the inflow boundary layer and the flow field around the cube including mean and fluctuating surface pressures. It is concluded that provided properly formulated inflow and surface boundary conditions are used, LES is now a viable tool for use in wind engineering problems concerning flow over isolated bodies. In particular, both mean and fluctuating surface pressures can be obtained with a similar degree of uncertainty as usually associated with wind tunnel modelling.     

Wind tunnel tests of effects of atmospheric stability on turbulent flow over a three-dimensional hill

This paper presents a wind tunnel study on the turbulent structure of the airflow around a three-dimensional hill model placed in a boundary-layer flow. The effect of atmospheric stability: stable, neutral and unstable on the flow field of the boundary layer is examined. The wind velocity is measured with a three-dimensional laser doppler anemometer (LDA). Measurements analysis includes mean velocity, turbulent velocity, Reynolds stress and turbulence energy profiles around the hill. The main results are as follows: (1) The mean wind velocity does not vary with the stability at the hilltop; it reaches a maximum at the back of the hill, for the unstable case. (2) The turbulent velocity at the back of the hill reaches its peak value at the height of the hilltop. It takes maximum value for the stable boundary layer flow, and become smaller for the neutral flow and the unstable flow. Buoyancy production has little effect on the turbulence energy. (3) A clear peak of /U_H² is observed at a height near Z/H=1. The peak value becomes the largest for the stable case and the smallest for the unstable case.     

大跨度复杂屋盖结构风荷载的大涡模拟

应用一种新的湍流脉动流场产生方法DSRFG(Discretizing and Synthesizing Random Flow Generation)[1]模拟风场实际的湍流边界条件,采用一种新的大涡模拟(Large Eddy Simulation,LES)的亚格子模型[2],基于Linux系统下软件平台Fluent6.3的并行计算技术,对深圳新火车站进行了数值风洞模拟。并将屋盖的平均风压、脉动风压计算结果与风洞试验数据进行了比较,表明数值模拟很好地反映了大跨度屋盖表面风压的分布情况,由其得到的风压系数与风洞试验数据有较好的吻合。表明本文的DSRFG方法以及新的大涡模拟亚格子模型的数值模拟技术是一种很好的预测大型、复杂结构表面风荷载的有效方法。并为进一步发展在复杂湍流环境下大跨度屋盖结构的风荷载数值风洞技术提供参考。     

An experimental study on the wind pressure distribution of tapered super high‐rise buildings

Studies on the effect of different shape strategies on wind‐induced responses of super tall buildings have been extensive. However, little systematic research on the influence of aerodynamic shapes on wind pressure distributions of super high‐rise building having a height more than 500 m is reported in the literature. In this paper, a series of wind tunnel tests are conducted on models simulating tapered buildings taller than 500 m with an aspect ratio of 9:1 by applying synchronous pressure measurement technology to investigate the influence of different shape strategies on the wind force coefficients of the cross section (C_s) and on the peak negative pressure distributions on surfaces. The shape strategies considered include tapering of the cross section of a building along its height, chamfered modification, and opening ventilation slots. It is found that the wind force coefficient C_s increase with an increase of the tapering ratio. It is shown that chamfered modification can effectively reduce most of the wind force coefficients C_s to less than 0.9. As for peak wind pressures, a zone having a higher negative pressure is found to locate at the bottom of the side faces of the model. With an increase of the tapering ratio, the peak negative pressure of side faces of the model slightly decreases. Chamfered modification can significantly increase the peak negative pressure at the chamfered location. Furthermore, it is demonstrated that opening ventilation slots had less effect on C_s, but the peak negative pressure can significantly increase at the area of opening ventilation slots and adjacent areas.     

考虑特征湍流影响的体育场悬挑屋盖脉动风压谱模型

基于风洞试验对体育场悬挑屋盖的脉动风压谱进行系统研究,旨在得到适用于此类结构的脉动风压谱模型,为风振响应分析提供必要的信息。通过对屋盖表面脉动风压进行谱分析,可知在屋盖前缘处的风压谱与来流风速谱较接近,但屋盖后缘处则差异很大,表现出明显的漩涡脱落特征。因此脉动风压自谱采用来流谱与漩涡脱落谱相结合的形式来描述,并通过权数因子体现屋盖表面不同位置处流场作用的特点。对于脉动风压互谱则用指数衰减函数来表示,并确定了适用于悬挑屋盖的衰减系数。为验证所提出风压谱模型的有效性及特征湍流对风致效应的影响,对系列悬挑屋盖结构进行风振响应分析,风荷载时程分别采用风洞试验测得的风压时程、基于建议风压谱模型模拟生成的风压时程、按拟定常假设生成的风压时程。基于建议模型得到的响应结果与试验结果基本一致,基于拟定常假设的风振响应极值偏小10%～15%,均方根值偏小30%～40%,脉动风压谱建模中不可忽略特征湍流的影响。     

Pressures on a surface-mounted rectangular prism under varying incident turbulence

The pressure and load coefficients obtained from two groups of eight pressure taps on the upper surface of a surface-mounted prism are characterized in terms of their mean, rms, peak, probability distribution, peak correlations and durations. The prism is a 1:50 scale model of the WERFL experimental building at Texas Tech University. Results obtained with flows generated over seven different wind tunnel floor-roughness configurations in the boundary layer wind tunnel at Clemson University cover a wide range of turbulence intensities. The results presented include the spatial variation of the peak pressure and peak load coefficients, and their variations with incident turbulence. The stochastic characteristics of the peak coefficients are also addressed here. The results reveal that the distribution of the peak coefficients is in general well established by the Extreme Value Type I (Gumbel) distribution. Conditional sampling is employed to study the duration as well as the space and space-time correlations of the peaks. Analysis of the peaks reveals that those with the larger magnitudes are generally of longer duration.     

Applicability of LES to the turbulent wake of a rectangular cylinder—Comparison with PIV data

The applicability of the large eddy simulation (LES) technique to wake flows past a bluff body should be clarified in order to improve numerical accuracy for the estimation of aerodynamic forces and pressures acting on a bluff body. Here, we conduct both LES and particle image velocimetry (PIV) measurement for turbulent flows past rectangular cylinders with a depth/breadth ratio ranging from D/B=2.00 to 3.00 at Re=2000 and 20,000, which sensitively change turbulence structures due to flow separation and reattachment. For LES, we use the dynamic Smagorinsky-type SGS (sub-grid scale) model. First, at Re=2000 where pure turbulent viscosity without numerical viscosity is realized, the accuracy of the SGS modeling for the prediction of not only aerodynamic characteristics but also turbulence statistics in the wake of rectangular cylinders is examined in comparison with PIV data. Furthermore, at Re=20,000 where the numerical dissipation must be incorporated, we clarify the unfavorable effects of the numerical dissipation on the turbulence structures in the wake.     

Quasi-steady theory and point pressures on a cubic building

A quasi-steady method which uses observed mean pressure coefficients to predict the expected peak positive or negative pressures is developed. It is shown that in the case of wall pressures this involves calculating the joint probability of instantaneous wind direction and gust dynamic pressure. With roof pressures the situation is more complex since the pressures are also sensitive to elevation angles and so the joint probability also includes this angle. Comparison of these predictions with observed data from the Silsoe 6 m cube show reasonable agreement.     

应用CFD技术预测国家大剧院表面风压

基于定常Reynolds平均Navier-Stokes方程,采用RNG湍流模型,对中国国家大剧院穹顶在不同风向下的表面风压及周围风场进行了数值模拟。给出了0°风向角下建筑周围的流动图像和穹顶表面的压力系数分布,并分析了其基本特征。预测了不同风向下的顶部最大负压,并与风洞试验数据作了对比。计算了各个风向下的六个气动力分量,指出了它们随风向角的变化规律。     

低矮建筑风致局部瞬毁诱发次生灾害机理研究

基于缩尺比为1∶40低矮建筑在不同地貌条件下的风洞试验,研究因易损区局部风致破坏进而诱发低矮建筑屋面整体风毁的风载特性,分析屋面瞬间开孔所致瞬态峰值内压过冲效应,研究稳态阶段的内压分布特征及净风压极值分布规律。结果表明：建筑上游风场湍流度越大,屋面局部瞬间破坏所致过冲效应越明显;地貌对屋面迎风角部瞬间破坏所致过冲较分布区域的影响显著;屋面不同的区域开孔所致内压分布均匀,但内压值随风场湍流度增大呈增大趋势。风致建筑破坏所致内压系数试验值比我国现行荷载规范中建议取值大;屋面局部瞬毁进而诱发再次破坏主要分布在已损孔洞边缘及山墙部分。