Wind-tunnel modelling of the Silsoe Cube

1:40 scale wind-tunnel modelling of the Silsoe 6 m Cube at the University of Auckland is reported. In such situations, it is very difficult to model the full turbulence spectra, and so only the high-frequency end of each spectrum was matched. It is this small-scale turbulence that can directly interact with the local flow field and modify flow behaviour. This is illustrated by studying data from tests conducted in a range of European wind tunnels. It is recommended that spectral comparisons should be carried out by using turbulence-independent normalising parameter, such as plotting fS(f)/U² against reduced frequency f=nz/U. Using parameters such as the variance and integral length scale can easily mask major differences. It is noted that it is the size of the tunnel that limits the low-frequency end of the spectra, and so the longitudinal and transverse turbulence intensities were lower than in full scale. In spite of this similar pressure distributions are obtained. Some differences are observed and these are partially attributed to the reduced standard deviation of wind directions, which affects both the observed mean and peak pressures by reducing the band of wind directions occurring during a run centred on a particular mean direction. The reduced turbulence intensities also affect the peak-to-mean dynamic pressure ratio. However, since the missing turbulence is at low frequencies, the peak pressures appear to reduce in proportion. By expressing the peak pressure coefficient as the ratio of the extreme surface pressures to the peak dynamic pressure observed during the run, reasonable agreement is obtained. It is argued that this peak–peak ratio is also less sensitive to measurement system characteristics or analysis method, provided the measurement and analysis of the reference dynamic pressure is comparable with that used for the surface pressures.     

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。     

强台风“黑格比”登陆过程中近地风场特性

为准确研究低矮房屋的风载特性,基于建造在强台风"黑格比"登陆地点的低矮房屋原型实测房,对强台风"黑格比"登陆的全过程进行了监测,获取了在强台风登陆过程中近地风场及实测房屋表面风压的实测数据。通过对台风登陆过程的风速及风向、阵风因子、湍流度、湍流积分尺度和风速谱等参数的分析,研究了强台风"黑格比"登陆时的近地风场特性,同时介绍了实测房屋及测压系统。结果表明:强台风"黑格比"的最大瞬时风速达到56.7/s,10m in最大平均风速为40.2m/s;台风登陆前,风速及风向脉动变化很大;台风登陆后,湍流度及阵风因子明显减小;风速变大,阵风因子与湍流度有减小的趋势;实测风速谱与von Karman谱吻合较好。研究结果将增进对强台风近地风场的理解和认识,可为今后改进低矮房屋的抗风设计提供参考。     

超高层建筑风致响应及等效静力风荷载研究

以重庆宾馆为工程背景,制作了缩尺比为1∶300的试验模型,并进行了刚性模型同步测压风洞试验,采集了重庆宾馆建筑表面的脉动风压时程。风洞试验包括有周边建筑和无周边建筑两类工况。采用风洞试验的脉动风压时程数据,考虑该高层建筑2个主轴方向的前4阶弯曲模态,进行了其风致响应研究,得到了建筑顶部的位移响应和加速度响应,并进行了人体舒适度验算。采用惯性风荷载法,研究了建筑主轴方向的等效静力风荷载。结果表明:对于高度为300m的混凝土高层建筑,仅考虑1阶模态进行风致响应分析,位移响应能满足工程精度的要求,但加速度响应误差较大,至少应考虑前4阶模态;重庆宾馆10年重现期下建筑顶部的峰值加速度为0.144m/s2,满足舒适度限制要求;横风向平均风荷载较小,但惯性风荷载较大。     

A study of the mechanical behaviour of coal for pillar design

A series of triaxial compression tests was carried out on 61, 101, 146 and 300 mm diameter samples to investigate the effects of scale on the mechanical behaviour of coal. The “inherent variability” of laboratory coal strength data was found to be related to the degree of cleating or brightness of the coal samples. Using this observed relationship, a method was developed for estimating in situ coal seam strength based upon the “intact” properties of dull coal samples and seam brightness profiles. The peak strength criterion for in situ coal uses the parameters σ_c, m and s of the Hoek–Brown empirical strength criterion for rock masses. Unloading cycles were used to investigate the yield behaviour of the coal samples. The results show that, depending on the magnitude of the confining stress, the failure mechanism of coal alters significantly with a change from an axial splitting to a shearing mechanism. Measurements of the volumetric strain response also indicate an absence of scale dependence in the deformation behaviour of the three larger sample sizes. Unloading cycles were used as a means of investigating the development of recoverable (elastic) and irrecoverable (plastic) components of strain. The irrecoverable strain loci, for the larger sizes, were taken to be representative of the in situdeformation response of coal.     

Effect of silica fume and steel fibers on some properties of high-strength concrete

The main disadvantage of high-strength concrete is its highly brittle behavior and this can beovercome by adding fibers to the concrete. This would also improve some other mechanical properties of high-strength concrete such as tensile strength and compressive strength. These properties are not very well established for high-strength steel-fiber reinforced concrete (HSFRC) yet. In this study the influence of silica fume on the properties of HSFRC were investigated by using silica fume of two different percentages and three different hooked-end fibers namely, 30/0.50, 60/0.80 and 50/0.60 length/diameter (mm/mm). Fibers were added to concrete in three different volume percentages of 0.5, 1.0 and 2.0 by volume of concrete. The results indicated that there is a linear function between splitting tensile strength (F_splt) and volume percentage of fibers (V_f) [i.e. F_plt = A(V_f) + B, where A and B are correlation coefficients] as well as between splitting tensile strength (F_splt) and compressive strength (F_c) of plain series A concrete [i.e. F_splt = C (√F_c) + D, where C and D are correlation coefficients]. These relations can describe the development of splitting tensile strength of HSFRC containing no silica fume, 5% silica fume and 10% silica fume by weight of cement. On the other hand, although silica fume has an effect on compressive strength, volume percentage and aspect ratio of steel fibers has little effect.     

Field measurement and wind tunnel simulation of hurricane wind loads on a single family dwelling

During landfall of Hurricane Ivan on the Florida ’panhandle’ in 2004, pressure time-history data were recorded on multiple pressure sensors installed on the roofs of six single-family homes. An analysis approach was developed to determine the peak negative, mean, peak positive, and standard deviation of pressure coefficients for these datasets. This paper presents a comparison of the full scale pressure coefficients from one of these homes, which experienced sustained hurricane force winds, with the results of wind tunnel experiments on a 1:50 scale model of that home. It was determined that the wind tunnel and full-scale mean and rms pressure coefficients matched very closely at almost every monitored location on the roof, while the peak negative pressure coefficients in the wind tunnel study generally underestimated the full-scale values, consistent with observations from previous full-scale/wind tunnel comparative studies. Field-measured hurricane wind loads may prove useful for evaluating existing wind load provisions. However, recommendations in that regard are premature without the analyses of multiple homes in multiple storms, performed by more than one wind tunnel facility. Future work will focus on building such a joint study.     

Accounting for the uncertainties in the estimation of average shear wave velocity using V S– N correlations

Site-specific seismic hazard analysis is crucial for designing earthquake resistance structures, particularly in seismically active regions. Shear wave velocity ( V _S) is a key parameter in such analysis, although the economy and other factors restrict its direct field measurement in many cases. Various V _S–SPT– N correlations are routinely incorporated in seismic hazard analysis to estimate the value of V _S. However, many uncertainties question the reliability of these estimated V _S values. This paper comes up with a statistical approach to take care of such uncertainties involved in V _S calculations. The measured SPT– N values from all the critical boreholes were converted into statistical parameters and passed through various correlations to estimate V _S at different depths. The effect of different soil layers in the boreholes on the Vs estimation was also taken into account. Further, the average shear wave velocity of the top 30 m soil cover ( V _S30) is estimated after accounting for various epistemic and aleatoric uncertainties. The scattering nature of the V _S values estimated using different V _S– N correlations was reduced significantly with the application of the methodology. Study results further clearly demonstrated the potential of the approach to eliminate various uncertainties involved in the estimation of V _S30 using general and soil-specific correlations.     

Torsional response of base-isolated structures due to asymmetries in the superstructure

The torsional response of base-isolated structures when eccentricities are set in the superstructure is presented. Nonlinear dynamic analyses were used to study peak responses for different ratios of static eccentricities (e_s) between the center of mass and the center of rigidity for the superstructure. Unidirectional and bidirectional actions of selected ground motions typical of firm soils of the Mexican Pacific Coast were used in the study. An effective period range between 1.5 and 3.0 s (1.5 s≤T_I≤3.0 s) for the base-isolated structures was considered in the present study. Bilinear isolator systems with yield forces of 5% and 10% the weight of the complete structure (V_y/W=0.05 and V_y/W=0.10) and a postyield stiffness of 10% their elastic stiffness (k₂/k₁=0.10) were considered. Peak dynamic responses such as maximum isolator displacements and peak displacement ductility demands were studied and compared to the ones obtained for symmetric systems of reference for the different ground motions under consideration, assessing the importance of the relative value of e_s on those response quantities.     

Influence of site conditions on seismic design parameters for foundations as determined via nonlinear site response analysis

Site conditions, including geotechnical properties and the geological setting, influence the near-surface response of strata subjected to seismic excitation. The geotechnical parameters required for the design of foundations include mass density (ρ), damping ratio (β_s), shear wave velocity (V_s), and soil shear modulus (G_s). The values of the last three parameters are sensitive to the level of nonlinear strain induced in the strata due to seismic ground motion. In this study, the effect of variations in soil properties, such as plasticity index (PI), effective stress (σ′), over consolidation ratio (OCR), impedance contrast ratio (ICR) between the bedrock and the overlying strata, and depth of soil strata over bedrock (H), on seismic design parameters (β_s, V_s, and G_s) was investigated for National Earthquake Hazards Reduction Program (NEHRP) site classes C and D, through 1D nonlinear seismic site response analysis. The Morris one-at-a-time (OAT) sensitivity analysis indicated that β_s, V_s, and G_s were significantly influenced by variations in PI, while ICR affected β_s more than it affected V_s and G_s. However, the influence of H on these parameters was less significant. It was also found that variations in soil properties influenced seismic design parameters in soil type D more significantly than in soil type C. Predictive relationships for β_s, V_s, and G_s were derived based on the 1D seismic site response analysis and sensitivity analysis results. The β_s, V_s, and G_s values obtained from the analysis were compared with the corresponding values in NEHRP to determine the similarities and differences between the two sets of values. The need to incorporate PI and ICR in the metrics for determining β_s, V_s, and G_s for the seismic design of foundations was highlighted.     

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.     

倒角化迎风前缘对平屋盖表面风压特性的影响

通过刚性模型风洞测压试验，针对分离泡和锥形涡作用情况，研究了不同风向下倒角化迎风前缘对平屋盖表面风压幅值和脉动特性的影响。对比分析采用倒角化迎风前缘前后，平屋盖表面风压分布以及角部面积平均风压的变化。通过本征正交分解法，给出了平屋盖表面风压脉动的特征值和特征向量。从时域和频域角度，分析了倒角迎风前缘部位测点的风压特性。结果表明：采用倒角化迎风前缘后，分离泡和锥形涡作用区内风吸力单调递减，但迎风前缘附近风吸力可能增大；分离泡作用下，倒角化迎风前缘将增大屋盖角部面积平均风压均值；锥形涡作用下，其可减小屋盖角部面积平均风压的均值和极值，最大降幅分别为68%和82%；屋盖表面风压脉动区域减小至迎风前缘附近，且风压脉动能量降低，最大降幅出现在锥形涡作用下倒角半径较大的平屋盖表面；在倒角迎风前缘部位，极值风吸力和脉动风压谱峰值可超过其邻近区域；增大倒角半径，该部位的极值风吸力和低频风压脉动能量将有所降低。     

基于完全气动弹性模型的冷却塔风致响应风洞试验研究

风荷载是冷却塔设计的控制荷载,完全气动弹性模型风洞试验是研究其风致响应的有效途径。基于此,推导了冷却塔完全气动弹性模型相似关系并用有限元分析方法予以验证,据此相似关系设计并制作了某核电站200m高超大型冷却塔的1∶400完全气动弹性模型,并在风洞中模拟的B类风场对其风致响应进行测试,试验前对模型的动力特性进行检验。试验结果表明：完全气动弹性模型能较为精确模拟冷却塔结构的质量、刚度和阻尼相似;迎风面的风致变形较大,而其中又以喉部附近最大;背景响应占总响应的主导地位,动力放大效应不明显;风振系数值随高度的增加而减小,各控制点风振系数均值略小于DL/T 5339-2006《火力发电厂水工设计规范》规定值。     

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.     

不同地貌下几个典型屋盖的风压特性

在均匀流场、市郊地貌及城市地貌中对包括双坡、球壳及柱壳的几个典型屋盖进行了风洞试验,利用同步多点压力扫描技术得到屋盖的风压数据。将分布于屋盖的局部体型系数与建筑结构荷载规范进行了比较,讨论了不同地貌及风向角对屋盖风压的影响,分析了各风向角下的绝对值最大的负局部体型系数及出现的位置。发现位于双坡屋盖的屋角、屋檐及屋脊区域的风压远大于其它区域的风压,均匀流中这些屋盖的平均风压是市郊地貌屋盖的2至3倍,而市郊地貌屋盖的平均风压又是城市地貌屋盖的2至3倍。     

Buckling of super ellipsoidal shells under uniform pressure

This paper is concerned with the elastic buckling of super ellipsoidal shells under external uniform pressure. The middle surface of a super ellipsoidal shell is defined by the following equation: (x/a)²ⁿ+(y/b)²ⁿ+(z/c)²ⁿ=1, where n is an integer varying from unity to infinity. It is clear from the equation that the range of shell shapes covered sphere (n=1, a=b=c) to cube () and ellipsoid (n=1) to cuboid (n=∞). By adopting a recently proposed solid shell element for the buckling analysis, the critical buckling pressures of thin to thick super ellipsoidal shells are obtained and tabulated for engineers. The shell element allows for the effect of transverse shear deformation which becomes significant in thick shells. Their buckling shapes are also examined. In addition, a simple approximate formula for predicting the critical buckling pressure of thick spherical shells is proposed.     

Reduction of 3/4 open jet low-frequency fluctuations in the S2A wind tunnel

3/4 open jet wind tunnels are subjected to undesired low frequency pressure and velocity fluctuations at distinct flow velocities. The aim of this paper is to present recent research done in the full scale automotive aero-acoustic facility of the S2A wind tunnel complex in order to characterise the physical mechanism causing the fluctuations, to clarify their impact on the aerodynamic measurements and to validate solutions that allow high quality aerodynamic and acoustic measurements over the entire velocity range of the wind tunnel. Four areas of significant oscillations have been detected in the full scale S2A wind tunnel for wind velocities between 10 and 60 m/s. Those fluctuations are due to successive lock-ins of the wind speed dependent shear layer instability of the jet, with acoustic resonant frequencies of the wind tunnel. Various solutions have been tested in order to assess their impact on the fluctuations and on the quality of the aerodynamic measurements. A less protruding solution than classical vortex generator has been validated. It is made of two rectangular flaps along the upper edge of the nozzle exit, with a small plunging angle.     

Prise de pression C.S.T.B. omnidirectionnelle

A new pressure probe, for full scale measurements in the wind, is described in this paper. The C.S.T.B. pressure probe is based on the average of the mean pressures around a vertical cylinder associated with a disk in its wake. The probe gives a very stable mean pressure independent of the wind direction, inclination to the wind (in the range ±20°) and the wind speed levels. The probe pressure can very easily be adjusted to the static pressure given by a Pitot tube in a wind tunnel.     

Full-scale validation of vortex suppression techniques for mitigation of roof uplift

Widespread damage to roofs in hurricanes and other high wind events has highlighted the need for improving the wind performance of roofs, including both new construction and retrofit on existing buildings. Effective and economical methods to mitigate wind effects on roof components are sought to reduce accumulated losses from frequently recurring low- and moderate-intensity events. An experimental study was carried out in the 6-fan Wall of Wind (WoW) testing apparatus to assess the effectiveness of aerodynamic edge devices at full scale in reducing wind effects over the roof corner and edge regions. Two sets of tests, roof gravel scour testing and pressure testing, were carried out to visualize the development of edge vortices and compare roof uplift pressures with and without aerodynamic edge shapes, thus allowing the estimation of the effects of such shapes in alleviating wind damage of roofing material as well as high uplift pressures on roofs. Probabilistic estimates of peak pressures indicated that the use of such mitigation devices resulted in significant reductions in uplift within the instrumented area, which according to the US wind load specifications (ASCE 7-05) represents a corner section of the roof that will experience the highest suction. The largest reduction was achieved with the Flat Roof AeroEdge Guard (FRAG1). This research demonstrates the potential for substantial reduction of roof uplift pressures through the development and application of mitigation techniques to lessen or prevent hurricane-induced damage to new and existing roofs.     

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。