An experimental study of a high-rise building model in tornado-like winds

An experimental study was conducted to quantify the characteristics of wake vortex and flow structures around a high-rise building model as well as the resultant wind loads (both forces and moments) acting on the test model in tornado-like winds. In addition to measuring wind loads acting on the tested high-rise building model using a high-sensitivity load cell, a digital Particle Image Velocimetry (PIV) system was used to conduct detailed flow field measurements to quantify the evolution of the unsteady vortex and turbulent flow structures around the test model in tornado-like winds. The measurement results revealed clearly that the evolution of the wake vortex and turbulent flow structures around the test model as well as the resultant wind loads induced by tornado-like winds were significantly different from those in conventional straight-line winds. The detailed flow field measurements were correlated with the wind load measurement data to elucidate the underlying physics to gain further insight into the flow-structure interactions between the tested high-rise building model and tornado-like vortex. The new findings derived from the present study could be used to provide more accurate prediction of wind damage potential to built environment with the ultimate goal of reducing life loss, injury casualty, and economic loss that results from tornados.     

复杂工况下的大型风力机气动性能和尾迹研究Investigation of aerodynamic performance and wake of the large scale wind turbine in complicated conditions

在复杂工况下,大型风力机非定常特性会更严重,导致风力机气动性能变化和尾迹预测更加复杂。本文主要针对稳态偏航、动态偏航、风剪切和随机风速场等复杂工况,基于自由涡尾迹方法,嵌入复杂工况的模块,加入了动态失速模型和三维旋转效应模型修正,实现了复杂工况数值模拟计算,比较了不同复杂工况的气动载荷和尾迹形状。最后,得出了风力机在复杂工况下的气动性能、载荷和尾迹叶尖涡线特性,并计算出风力机在复杂工况下的气动载荷超调量和迟滞时间。对推进自由涡尾迹方法应用于风力机工程的大批工况载荷计算,提高大型风力机的载荷计算精度和设计水平等具有重要意义。     

锥形涡诱导下建筑物顶面风荷载

通过风洞模拟实验,研究了在锥形涡诱导下建筑物顶面风荷载的特性. 针对一个立方体形模 型,给出了在不同风向角下压力分布的系统结果,进而分析了产生这样结果的原因和流动机 理,以及建筑物顶面的分离流动结构随风向角改变而改变及其演化过程,并指出锥形涡的出 现是建筑物顶面局部出现峰值负压的主要原因. 实验结果还显示,在一定风向角下,由于分 离流动产生的锥形涡结构在屋顶局部可以诱导出时均负压峰值达$-1.0$以上;在风向角 $\beta =30^\circ$时,锥形涡强度达到最大,其诱导的顶面附近局部时均负压峰值可 达$-2.4$, 瞬时风压达$-5.7$以上,是导致建筑物屋顶在强风下损坏的根本原因.     

一种风力机气动计算的全自由涡尾迹模型

采用全自由方式建立风力机尾流场的涡尾迹模型,引入“虚拟周期”的概念,并发展一种自适应松弛因子方法,从而改善了自由尾迹迭代的稳定性,提高了迭代收敛速度。利用建立的自由涡尾迹模型,计算了风力机叶片的尾流场结构、气动性能及叶片载荷,并与实验结果进行了对比分析。结果表明,尖速比越大,自适应松弛因子方法对缩小模型计算时间越有效;全自由涡尾迹模型能准确给出风力机尾流场的结构,包括尾迹的扩张以及叶尖涡和叶根涡的产生、发展和耗散的过程,风轮扭矩与实验数据吻合;叶片载荷分布的计算结果在低风速下与实验值基本一致,但是在大风速下差别较大,说明需要一个准确的失速模型。     

Wind pressures on tapered and set-back tall buildings

Recent tall buildings tend to have irregular and unconventional shapes as a prevailing but unavoidable trend, which is very effective for suppressing across-wind responses. Suppression of across-wind responses is a major factor in safety and habitability design of tall buildings, and the so-called aerodynamic modification method is comprehensively used. While the effectiveness of aerodynamic modification in reducing wind loads has been widely reported, there have been few detailed investigations of pressure fluctuations. The purpose of the present work is to investigate the spatio-temporal characteristics of pressure fluctuations applied to height-modified tall buildings comprehensively, including differences of vortex formation and shedding mechanism based on the previously reported mechanism of a conventional square tall building. The results show that taper and set-back affect on the bandwidth of power spectra and position of peak frequencies. And through taper and set-back, the height at which the vortex begins to form moves up, and due to the small building dimension, the vortex component formed at that height sheds from the building more frequently before an inverted conical vortex is formed over the whole height.     

Vortex-induced dynamic loads on a non-spinning volleyball

An experiment on vortex-induced dynamic loads on a non-spinning Volleyball was conducted in a wind tunnel. The flow past the Volleyball was visualized, and the aerodynamic load was measured by use of a strain gauge balance. The separation on the Volleyball was measured with hot-film. The experimental results suggest that under the action of an unstable tail vortex system the separation region is changeable, and that the fluctuation of drag and lateral forces is the same order of magnitude as the mean drag, no matter whether the seam of the Volleyball is symmetric or asymmetric, with regard to the flow. Based on the experimental data a numerical simulation of Volleyball swerve motion was made.     

An experimental study on wind loads acting on a high-rise building model induced by microburst-like winds

In the present study, an experimental investigation is conducted to quantify the characteristics of the microburst-induced wind loads (i.e., both static and dynamic wind loads) acting on a high-rise building model, compared to those with the test model placed in conventional atmospheric boundary layer (ABL) winds. The experimental study is performed by using an impinging-jet-based microburst simulator available at Iowa State University. In additional to conducting flow field measurements to quantify the flow characteristics of the microburst-like wind, both mean and dynamic wind loads acting on the test model induced by the microburst-like wind are assessed in detail based on the quantitative measurements of the surface pressure distributions around the test model and the resultant aerodynamic forces. It is found that the microburst-induced wind loads acting on high-rise buildings would be significantly different from their counterparts in conventional ABL winds. Both the static and dynamic wind loads acting on the high-rise building model were found to change significantly depending on the radial locations and the orientation angles of the test model in respect to the oncoming microburst-like wind. The dynamic wind loads acting on the test model were found to be mainly influenced by the periodical shedding of the primary vortices and the high turbulence levels in the microburst-like wind. The findings derived from the present study are believed to be useful to gain further insight into the underlying physics of the flow–structure interactions of high-rise buildings in violent microburst winds for a better understanding of the damage potential of microburst winds to high-rise buildings.     

Dynamic wind loads and wake characteristics of a wind turbine model in an atmospheric boundary layer wind

An experimental study was conducted to characterize the dynamic wind loads and evolution of the unsteady vortex and turbulent flow structures in the near wake of a horizontal axis wind turbine model placed in an atmospheric boundary layer wind tunnel. In addition to measuring dynamic wind loads (i.e., aerodynamic forces and bending moments) acting on the wind turbine model by using a high-sensitive force-moment sensor unit, a high-resolution digital particle image velocimetry (PIV) system was used to achieve flow field measurements to quantify the characteristics of the turbulent vortex flow in the near wake of the wind turbine model. Besides conducting “free-run” PIV measurements to determine the ensemble-averaged statistics of the flow quantities such as mean velocity, Reynolds stress, and turbulence kinetic energy (TKE) distributions in the wake flow, “phase-locked” PIV measurements were also performed to elucidate further details about evolution of the unsteady vortex structures in the wake flow in relation to the position of the rotating turbine blades. The effects of the tip-speed-ratio of the wind turbine model on the dynamic wind loads and wake flow characteristics were quantified in the terms of the variations of the aerodynamic thrust and bending moment coefficients of the wind turbine model, the evolution of the helical tip vortices and the unsteady vortices shedding from the blade roots and turbine nacelle, the deceleration of the incoming airflows after passing the rotation disk of the turbine blades, the TKE and Reynolds stress distributions in the near wake of the wind turbine model. The detailed flow field measurements were correlated with the dynamic wind load measurements to elucidate underlying physics in order to gain further insight into the characteristics of the dynamic wind loads and turbulent vortex flows in the wakes of wind turbines for the optimal design of the wind turbines operating in atmospheric boundary layer winds.     

Modeling vortex-shedding effects for the stochastic response of tall buildings in non-synoptic winds

This study derives a model for the vortex-induced vibration and the stochastic response of a tall building in strong non-synoptic wind regimes. The vortex-induced stochastic dynamics is obtained by combining turbulent-induced buffeting force, aeroelastic force and vortex-induced force. The governing equations of motion in non-synoptic winds account for the coupled motion with nonlinear aerodynamic damping and non-stationary wind loading. An engineering model, replicating the features of thunderstorm downbursts, is employed to simulate strong non-synoptic winds and non-stationary wind loading. This study also aims to examine the effectiveness of the wavelet-Galerkin (WG) approximation method to numerically solve the vortex-induced stochastic dynamics of a tall building with complex wind loading and coupled equations of motions. In the WG approximation method, the compactly supported Daubechies wavelets are used as orthonormal basis functions for the Galerkin projection, which transforms the time-dependent coupled, nonlinear, non-stationary stochastic dynamic equations into random algebraic equations in the wavelet space. An equivalent single-degree-of-freedom building model and a multi-degree-of-freedom model of the benchmark Commonwealth Advisory Aeronautical Research Council (CAARC) tall building are employed for the formulation and numerical analyses. Preliminary parametric investigations on the vortex-shedding effects and the stochastic dynamics of the two building models in non-synoptic downburst winds are discussed. The proposed WG approximation method proves to be very powerful and promising to approximately solve various cases of stochastic dynamics and the associated equations of motion accounting for vortex shedding effects, complex wind loads, coupling, nonlinearity and non-stationarity.     

Simulation of Three-Dimensional Bluff-Body Flows Using the Vortex Particle and Boundary Element Methods

Recent contributions to the 3-D vortex method for bluff-body flows are presented. The numerical method--a vortex method combined with a boundary element method--is briefy reviewed. It is applied to direct numerical simulation (DNS) of the flow past a sphere (Re= 300, 500 and 1000). The on-going work to extend the method towards vortex-based large-eddy simulation (LES) for high Reynolds number flows is also presented. Preliminary results for the flow past a hemisphere are discussed.     

A random vortex simulation of wind-flow over a building

The random vortex method of Chorin¹ provides a numerical simulation of high-Reynolds number flow in two dimensions. The method can be used to model the viscous interaction of wind with a surface-mounted obstacle of arbitrary cross-section. In this paper the method has been used to investigate the flow of wind over common building shapes; an inlet profile is chosen to represent the stationary aspects of the atmospheric boundary layer. The evolution of flow over a short time-interval after flow initialization is depicted, and a mean value of pressure coefficient, C_p, is calculated over the building perimeter. Some comparison is made with published wind-tunnel measurements for the case of a surface-mounted square-section block and for a building model with 10° roof pitch.     

Using the perturbation method to solve the problem of separated incompressible flow past thin airfoils

A model for separated incompressible flow past thin airfoils in the neighborhood of the “shockless entrance” condition is constructed based on the averaging of the vortex shedding flow past the airfoil edges. By approximation of the vortex shedding by two vortex curves, determination of the average hydrodynamic parameters is reduced to a twofold solution of an integral singular equation equivalent to the equation describing steady-state nonseparated airfoil flow. In this case, the calculation time is two orders of magnitude smaller than the time required for the solution of the corresponding evolution problem. The results of a test calculation using the proposed method are in fair agreement with available results of calculations and experiments. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 3, pp. 49–63, May–June, 2006.     

Experimental analysis of a stochastic model for estimating wind-borne compact debris trajectory in turbulent winds

The impact of flying debris against building envelopes during high winds is a major source of structural damage. For example, damage produced by Hurricanes Katrina and Ike in the United States on the facades of tall buildings, located in urban areas, has been documented. It is therefore of relevance to analyze the vulnerability of tall buildings to debris-induced non-structural damage in the general context of performance-based wind engineering. In order to analyze the random trajectory of debris in highly turbulent winds, a numerical model combined with a probability-based algorithm was recently proposed by the authors (Moghim and Caracoglia, 2013). This model investigates the trajectory of “compact debris”, defined as point-mass objects of negligible mass moments of inertia and for which the aerodynamics is predominantly controlled by the drag force. The model replicates both the inherent randomness in debris properties and the effect of wind shear and atmospheric turbulence to estimate debris trajectory and the likelihood of impact against vertical building facades in a probabilistic setting.This paper describes the comparison between numerical model results and wind tunnel experiments. Tests were carried out in the Northeastern University׳s small scale wind tunnel in both smooth flow and grid-generated turbulent flow. The motion of spheres and cubes, simulating compact debris objects, was investigated in two dimensions (2D) on a vertical plane.The 2D motion of compact objects of various sizes was captured by a high-speed digital camera at different flow speeds. Experimental results showed to be consistent with numerical simulations. They also confirmed that not only mean flow speed but also turbulence features can have a non-negligible effect on the trajectory of compact objects.     

高层双塔楼绕流风场效应的数值预测

在一高层双塔楼静力风载风洞试验的基础上,采用时均数值模拟的方法,对其绕流风场进行了模拟,获得了风场的绕流特点和塔楼表面风压的分布规律．通过风压的模拟计算值与试验值的比较,表明以数值方法预测气动干扰强烈的绕流风场是准确和可行的．     

风嘴对旋转振荡板旋涡脱落的控制

5∶1矩形柱体被认为是通用桥面几何形状的代表,其简化模型可以用来进行风振控制的研究。风嘴作为一种常用的流动控制装置,能起到减阻和增加矩形板气动稳定性的效果,但控制装置对强迫振荡柱体的控制机理仍缺乏研究。研究者通过对节段模型施加强迫振动,在实验模型前部施加边缘型对称型风嘴,研究控制装置对矩形板尾流旋涡脱落的影响。通过流动显示结果,总结了施加风嘴后的四种旋涡脱落模式。并通过快速傅里叶变化频域分析法,得到实验模型后缘X/D = 10处的速度功率谱。施加风嘴控制能增大旋涡脱落频率,并抑制尾流旋涡脱落的能量。而数值模拟得到的不同实验工况下的升力均方根和力矩均方根显著减小,最大降幅分别为52%和23%,表明矩形板气动稳定性的提升和尾流不稳定性的减弱。

     

Aerodynamic unstable critical wind velocity for three-dimensional open cable-membrane structures

The aerodynamic unstable critical wind velocity for three-dimensional open cable-membrane structures is investigated. The geometric nonlinearity is introduced into the dynamic equilibrium equations of structures. The disturbances on the structural surface caused by the air flow are simulated by a vortex layer with infinite thickness in the structures. The unsteady Bernoulli equation and the circulation theorem are applied in order to express the aerodynamic pressure as the function of the vortex density. The vortex density is then obtained with the vortex lattice method considering the coupling boundary condition. From the analytical expressions of the unstable critical wind velocities, numerical results and some useful conclusions are obtained. It is found that the initial curvature of open cable-membrane structures has clear influence on the critical wind velocities of the structures.     

A HYBRID VORTEX METHOD FOR FLOWS OVER A BLUFF BODY

A hybrid vortex method was developed to simulate the two-dimensional viscous incompressible flows over a bluff body numerically. It is based on a combination of the diffusion–vortex method and the vortex-in-cell method by dividing the flow field into two regions. In the region near the body surface the diffusion–vortex method is used to solve the Navier–Stokes equations, while the vortex-in-cell method is used in the exterior domain. Comparison with results obtained by the finite difference method, other vortex methods and experiments shows that the present method is well adapted to calculate two-dimensional external flows at high Reynolds number. It is capable of calculating not only the global characteristics of the separated flow but also the evolution of the fine structure of the flow field with time precisely. The influence of the grid system and region decomposition on the results will also be discussed. © by 1997 John Wiley & Sons, Ltd.     

Y型柱体绕流特性及风荷载

通过风洞实验研究了二种典型尺寸的二维Y型柱体在不同风向角下的绕流特性及风荷载。实验结果表明:当风向顺Y型柱体某一肢时主要是来流脉动引起柱体较弱横向振动;当风向顺Y型柱体两肢分角线时则由涡脱落而引起柱体强烈的横向振动。来流湍流度的增加使脉动升力和阻力都大幅度增加,然而却使涡脱落引起的振动相对减弱。     

串列双圆柱绕流问题的数值模拟

本文运用有限体积方法,对绕串列放置的双圆柱的二维不可压缩流动进行了数值计算。为研究两圆柱不同间距对圆柱相互作用和尾流特征的影响,选取间距比Ｌ／Ｄ（Ｌ为两圆柱中心间的距离,Ｄ为圆柱直径）在１．５～５．０之间每隔０．５共八个有代表性的间距进行了计算模拟。计算均在Ｒｅ＝２００条件下进行。计算结果表明：对该绕流问题,流动特征在很大程度上取决于间距的大小。且间距存在一临界值,间距比从小于临界值变化到大于临界