Atmospheric turbulence with respect to moving ground vehicles

A random-process model is developed for the turbulent wind with respect to a moving ground vehicle. The spatial and temporal variations of the wind turbulence component normal to the direction of motion of the vehicle are considered, and numerical results are given for the power spectral density, cross-correlation, square-root coherence and phase-lag functions.The power spectral density and aerodynamic-admittance functions of side force are evaluated, on the basis of a linear relationship between unsteady side force and turbulence velocity. A numerical example shows that significant excitation of vehicle suspension modes ($\text{f}{}_{\mathrm{n}}\text{\$}\text{?}\text{1}\text{Hz}$) may occur for vehicle speeds exceeding 40 m s^?1, the turbulence energy increasing approximately with the square of mean wind speed.The model gives valuable insight into the structure of turbulent wind with respect to moving vehicles, and should facilitate the interpretation of experimental data. The results also aid rational design of suspension systems for high-speed ground vehicles, to cope with loads due to wind turbulence.     

Simulation of mean flow and turbulence over a 2D building array using high-resolution CFD and a distributed drag force approach

A numerical simulation has been performed of the disturbed flow through and over a two-dimensional array of rectangular buildings immersed in a neutrally stratified deep rough-walled turbulent boundary-layer flow. The model used for the simulation was the steady-state Reynolds-averaged Navier-Stokes equations with linear and non-linear eddy viscosity formulations for the Reynolds stresses. The eddy viscosity was determined using a high-Reynolds number form of the k-ε turbulence-closure model with the boundary conditions at the wall obtained with a standard wall-function approach. The resulting system of partial differential equations was solved using the SIMPLE algorithm in conjunction with a non-orthogonal, colocated, cell-centered, finite volume procedure. The predictive capabilities of the high-resolution computational fluid dynamics (CFD) simulations of urban flow are validated against a very detailed and comprehensive wind tunnel data set. Vertical profiles of the mean streamwise velocity and the turbulence kinetic energy are presented and compared to those measured in the wind tunnel simulation.It is found that the performance of all the turbulence models investigated is generally good—most of the qualitative features in the disturbed turbulent flow field through and over the building array are correctly reproduced. The quantitative agreement is also fairly good (especially for the mean velocity field). Overall, the non-linear k-ε model gave the best performance among four different turbulence closure models examined. The turbulence energy levels within the street canyons and in the exit region downstream of the last building were underestimated by all four turbulence closure models. This appears to contradict the ‘stagnation point anomaly’ associated with the standard k-ε model which is a result of the excessive turbulence energy production due to normal straining. A possible explanation for this is the inability of the present models to account properly for the effects of secondary strains on the turbulence and/or for the effects of large-scale flapping of the strong shear layer at the canopy top.The results of the high-resolution CFD simulations have been used to diagnose values of the drag coefficient to be used in a distributed drag force representation of the obstacles in the array. Comparisons of the measured spatially-averaged time-mean mean velocity and turbulence kinetic energy in the array with predictions of the disturbed flow using the distributed drag force approach have been made.     

Effect of relative amplitude on bridge deck flutter

Self-excited wind forces on a bridge deck can be non-linear even when the vibration amplitude of the body is small. This phenomenon is evaluated in this paper. Experiments detecting the nonlinearity are performed first, with the concept of “relative amplitude”, i.e. the amplitude of the externally triggered free vibration relative to the envelope of the ambient response of an elastically supported rigid sectional model. Two types of sectional model, a twin-deck bluff model (model A) and a partially streamlined box girder model (model B) are tested with two extreme cases of relative amplitude. Based on the flutter derivatives of model B, a flutter boundary prediction is subsequently carried out on a cable-supported bridge to manifest the changes of critical flutter wind velocity due to different relative amplitudes. The effect of relative amplitude on flutter derivatives and on the flutter boundary reveals, from the structural point of view, a complex relationship between the self-excited forces and the “structural vibration noise” due to turbulence that is inherent in the interaction of the ambient wind with the structure. Although the aeroelastic forces are linear when the body motion due to an external trigger is not affected significantly by this turbulence, they are postulated to be nonlinear when this “vibration noise” cannot be neglected.     

来流湍流度对斜拉索风雨激振影响的理论研究

斜拉索风雨激振严重地影响斜拉桥的安全运营,而来流湍流度是影响风雨激振的一个重要因素。采用理论分析方法,研究来流湍流对斜拉索风雨激振响应的影响。引入2种类型的风场,首先,假设来流为简谐脉动风场,然后,采用谐波合成方法,将Kaimal风谱作为目标谱,模拟来流随机脉动风场。采用上述2种来流风场,在已建立的两质量三自由度理论模型的基础上,研究各种湍流度下拉索和水线的风雨激振振动规律。计算结果表明,当湍流度达到15%时,水线不能在拉索表面稳定地存在,风雨激振能得到有效的抑制;水线运动的脉动成分主要受来流风脉动成分的影响,拉索运动对水线运动有一定的影响,但影响较小;随着紊流度的增加,起振风速也增大。     

台风风场作用下体育场罩棚风压分布风洞试验研究

通过刚性模型测压风洞试验研究了台风风场高湍流、强变异性等特征对大跨结构风压分布特性的影响。以某体育场罩棚为原型制作1∶300刚性模型,进行了常规B类风场和台风风场作用下的测压对比试验。基于试验数据,从测点风压和总体升力角度对两类风场作用下体育场罩棚结构的风压分布总体特性进行了分析,重点比较了典型测点在典型风向角下的风压分布规律及相互关系。结果表明：两类风场作用下平均风压的分布规律基本类似,但各风向角下台风风场中的屋盖总体升力比B类风场增大8%~25%;台风风场的高湍流特性导致基于极值负风压求得的各风向角下屋盖总体升力比B类风场大27%~46%,各测点的极值风压均明显高于常规B类风场作用下的对应值,比值约为1.13~1.70,因此对于台风多发地区的大型体育场屋盖设计,必须考虑台风风场高湍流所致的脉动风压增大效应。     

Influence of low-frequency wind speed fluctuations on the aeroelastic stability of suspension bridges

Starting from the standard multimode approach to the aeroelastic instability of a long span suspension bridge, that gives the critical flutter speed in the ideal case of a wind constant both in time and in space (turbulence neglected), this paper describes a procedure to evaluate in a closed form the worst effects that can ever be expected as a consequence of a turbulence with a given spectrum. To this respect, only low-frequency wind speed fluctuations strongly correlated along the span are taken into account, neglecting the beneficial effects due to a loss of coherence or to higher frequency components. This allows to obtain a flutter speed certainly safe in comparison with any realistic turbulence field. The proposed formulation is then applied to the current design of the bridge on the Messina Strait (Italy).     

Measurement of extreme aerodynamic interference forces acting on circular cylinders in turbulent boundary layers

Extreme interference wind loads on circular cylinders are investigated in wind tunnel simulations of atmospheric boundary layers with high turbulence levels. Since for circular cylinders interference effects caused by vortex shedding are pronounced, the present work tries to clarify to which extent turbulence effects are reducing the interference in this case. Aerodynamic forces and moments were measured with a piezoelectric six-component wind tunnel balance. The balance is distinguished by a large dynamic range, low interferences and a high natural frequency. The numerous results include comparisons of mean values, r.m.s. values and extreme values of force coefficients with and without interference with reference to the velocity profiles and the turbulence levels in two different boundary layer simulations. Conclusions are drawn with regard to the application of the results in estimating wind loads on cylindrical structures.     

Savonius风力机叶片气动性能模拟与分析

针对Savonius阻力型风力发电机扰流流动特点,建立了二维不可压缩湍流模型,并对基于流体连续性方程和N-S方程及k-ε湍流模型的二维流场进行数值模拟计算.用ICEM构建了风力机叶片二维模型,完成了网格的划分,在FLUENT中模拟了流场内叶片截面的受力情况和速度分布情况,得到了Savonius风机的气动性能.     

Wind-induced failure of wind turbines

Models of loading and structural behaviour for use in lifetime prediction of propeller type wind turbines are discussed. The emphasis is on wind-induced loads, which are separated into periodic dynamic loads due to rotation in the spatially nonuniform mean windfield, and stationary stochastic loads due to wind turbulence. Turbulence is represented by auto- and cross-spectra of the turbulent wind speed fluctuations. The spectra are derived, however, for the turbulence as seen from points of a rotating wind turbine blade, which significantly changes the shape of the spectra. Structural response is calculated in the frequency domain and the stress responses appear as a combined periodic and stochastic signal. A fatigue model and a simple approach to extreme response calculations have been derived, which take into account both the periodic and the stochastic parts of the stress response. Assuming that failure occurs either by a slow accumulation of damage or by a sudden exceedence of a critical value for the stresses, the models can be used in the design phase when predicting the lifetime and assessing the safety of a wind-turbine structure. Finally, results from applying the model are presented and compared with available measured data.     

Model for power spectra of the blade of a wind turbine measured from the moving frame of reference

While the static aerodynamic loads on wind turbine blades seem to a large extent predictable, little work has been done to clarify the dynamic loads due to atmospheric turbulence. A simple kinematical model is here derived in order to predict the power spectrum of the turbulence component of the wind along the mean wind direction, observed from a frame of reference that is moving with constant angular velocity along the circumference of a circle perpendicular to the direction of the mean wind velocity. The results deviate significantly from those obtained when disregarding the rotation. The model is based on the assumption that the turbulence is stationary, homogeneous, and isotropic. Predictions of power spectra seem to be in good agreement with experiments. In particular, comparison with a spectrum of structural response for the Gedser wind turbine is interesting, because it demonstrate the applicability of the overall concept of the model. A technique is outlined to obtain cross-spectra between turbulence wind components measured at different points in the moving frame of reference.     

Thermal conditions and ventilation in an ideal city model of Hong Kong

Urban heat island can significantly increase the demand for cooling of buildings in cities. This paper investigates one of the main causes of the urban heat island phenomenon, i.e. reduced city ventilation. Two simple Hong Kong city models with relatively complex terrain were considered here under different atmospheric conditions. A 3D RNG k-? turbulence model was used for modeling turbulence effects. The simulation results showed that the influence of thermal stratification can be significant on city ventilation driven partially by thermal buoyancy. When the wind speed is relatively large, the impact of thermal stratification on air flow in city street canyons is minor. When the wind speed is small relative to the buoyancy force, the airflow in the street canyons is dependent on thermal stratification. When there is an adverse vertical temperature gradient, the greater the instability, the stronger the vertical mixing and the greater the flow rate caused by turbulence. The heat and pollutants can easily accumulate under stable atmospheric conditions when there is only a weak background wind or none at all.     

任意排列两方形断面高层建筑风致干扰机理研究

风致干扰效应是高层建筑群抗风设计中的常见难点问题之一。采用刚性模型测压试验,研究了均匀层流和两种大气边界层风场条件下任意排列两方形断面高层建筑的风致干扰效应,通过平均和脉动基底弯矩系数的干扰因子、风力系数、风压系数分布以及风荷载功率谱的研究,解释了其风致干扰效应的机理。结果表明,任意排列的两方形断面高层建筑风致干扰中,至少存在横风向静力干扰、顺风向静力干扰和横风向动力干扰三个值得注意的干扰区域。 窄道形成的加速效应使受扰结构上形成指向施扰建筑横风向平均吸力和阻塞形成的受扰建筑的横风向平均推力;遮挡效应使得受扰建筑承受指向位于上游的施扰建筑的顺风向风力;漩涡叠加增强位于尾流区受扰建筑上的横风向脉动荷载。不同风场的试验结果表明,提高来流的紊流度有助于减弱上述干扰效应。     

Monte Carlo simulation of wind velocity fields on complex structures

Monte Carlo simulation is becoming a fundamental tool for the design of complex and important wind-excited structures. A common application regards the time-domain dynamic analysis of multi-dof nonlinear structures whose excitation is calculated on the base of simulated wind velocity time-histories. The present paper describes a methodology for the simulation of wind velocity fields over large domains, possibly in zones characterised by complex topography. The modelling of turbulence in non-homogeneous flow condition and some computational aspects related to its simulation are discussed, proposing some strategies for reducing the calculation time. The simulation procedure is applied to the case of the Messina Strait bridge for which the three components of turbulence are simulated over a domain composed by 351 nodes.     

三边形桅杆风荷载谱模型试验研究

对三边形格构式桅杆进行了均匀流和两种紊流下的高频测力天平风洞试验,得到了顺风向、横风向和扭转向的气动力系数以及脉动风荷载谱。采用基于风速谱的数学模型对顺风向脉动风荷载谱进行拟合,验证了该经验公式在不同流场下的适用性。根据试验所得横风向和扭转向脉动风荷载功率谱曲线的特点,建立由紊流激励和旋涡脱落激励两部分组成的谱函数数学模型,最小二乘法拟合结果与风洞试验结果吻合良好。横风向脉动风荷载谱以紊流激励为主,紊流强度15%时旋涡脱落激励贡献仅占10%,扭转向脉动风荷载谱中旋涡脱落激励贡献明显增大,达到40%。     

On modelling of typhoon‐induced non‐stationary wind speed for tall buildings

Typhoon‐induced wind around tall buildings may not be stationary because it is a large body of rotating air. A new approach is thus proposed in this paper for characterizing typhoon‐induced wind speed. Typhoon‐induced non‐stationary wind speed is modelled as a deterministic time‐varying mean wind speed component plus a zero mean stationary fluctuating wind speed component. The time‐varying mean wind speed is naturally extracted from the measured wind speed time history using empirical mode decomposition (EMD). Wind characteristics described in the traditional approach based on a stationary wind model are redefined and extended in the non‐stationary wind model. The new approach is then applied to wind data measured at the Di Wang building during Typhoon York. The results show that most of recorded wind samples are non‐stationary but they can be decomposed into a time‐varying mean wind speed component plus a well‐behaved zero mean fluctuating wind speed component admitted as a stationary random process with Gaussian distribution. Other wind characteristics such as probability distribution of fluctuating wind speed, turbulence intensity, gust factor, and wind spectrum obtained by the new approach seem to be more realistic than those gained by the traditional approach. Copyright © 2004 John Wiley & Sons, Ltd.     

The use of cross-jets to simulate wind characteristics

The wind-tunnel simulation of natural wind characteristics is usually done with solid obstacles placed at the entrance of the working section and/or distributed on the floor. The approach presented in this paper uses an aerodynamic technique to achieve that purpose: a row of air jets directed perpendicularly to the longitudinal wind-tunnel stream.Some results agree reasonably well with natural wind measurements of mean wind velocity profile, intensity, scale, and power spectrum of longitudinal turbulence.The main advantages of the approach presented are: (i) the wind characteristics are controlled, in a rather wide range, by just opening or closing a valve; (ii) the artificially thickened boundary layer is obtained without solid obstacles and their blockage effects; there is no loss in flow velocity.     

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.     

不同断面宽度群体高层建筑的动力干扰效应(第1部分 顺风向响应)

基于高频底座力天平技术,研究了不同宽度比的两个和三个建筑物间的顺风向动力干扰效应。文中采用了神经网络、统计和谱分析等方法对干扰特性和机理进行了分析。结果显示,当受扰建筑位于上游施扰建筑物的尾流边界时,会产生较大的动力响应;并且两个施扰建筑物的联合干扰作用会比单个施扰建筑物的干扰作用强,在B类地貌下两个施扰建筑物测出的干扰因子(IF)会比单个施扰建筑的增加79%。位于上游的施扰建筑所脱落的旋涡会使受扰建筑产生涡激共振响应并且产生数倍于非共振情况的IF值,尤其对于小宽度的施扰建筑,在较小的折算风速时就会产生涡激共振问题。粗糙化地貌的高湍流度会对上游施扰建筑尾流的旋涡形成产生一定的抑制作用,在D类地貌下的IF值要远小于B类地貌情况,但在D类地貌下观察到IF值仍有2.2。     

紊流积分尺度对典型桥梁断面静力#br# 系数影响规律的风洞试验研究

为研究紊流积分尺度对典型桥梁断面静力系数影响的规律,文章通过在风洞中模拟紊流对典型桥梁断面进行测力试验。借助尖塔,格栅等方式模拟紊流的传统试验方法,难以模拟实际风场中的大紊流积分尺度。文章采用自主研制的主动控制翼栅,有效增大试验中风场的紊流积分尺度,通过控制主动控制翼栅的振动频率产生不同紊流积分尺度的风场。对单箱梁、双箱梁、桁架梁、边主梁、边箱梁五种典型桥梁断面在紊流下和均匀流下的静力系数进行了测定,其中紊流由主动控制翼栅产生。将五种典型桥梁断面在紊流下的静力系数和均匀流下的静力系数进行对比分析,结果表明在大紊流积分尺度下,阻力系数随紊流积分尺度增大而增大,且小于均匀流下的阻力系数值。结果也表明,在均匀流来测定桥梁断面静力系数对于实际工程应用是偏于安全的考虑,因此建议进行节段模型测力试验时,在均匀流中进行。     

Along-wind gust effect on elevated structures

The interest in the evaluation of the dynamic response of elevated structures to wind force has increased considerably in recent years. This paper introduces a random process approach and its application to these special structures. As an example, a skylight structure is analysed by random approach in regard to the gust effect. For comparison, the same effects are also evaluated by using some of the current available building codes.Results show that the effect due to the gust is greater than that due to the steady wind. Wind pressure obtained through the random approach are quite comparable with those obtained by using the building codes. However, the random approach presented in this paper provides more detailed procedure than the building code approach for evaluating gust effect on elevuated structures.