Experimental and semi-analytical studies on the aerodynamic forces acting on a vehicle passing through the wake of a bridge tower in cross wind

In a strong cross wind, sudden change of wind velocity in the wake of a bridge tower causes rapid change of the aerodynamic forces acting on a passing vehicle, which may result in an accident due to driver’s miss-steering. This paper discusses and clarifies the transient characteristics of the aerodynamic forces acting on a ground vehicle in high cross winds when the vehicle is passing through the wake of a bridge tower using a scale model in a wind tunnel experiment. Various parameters, such as wind speed and direction, type of vehicle and tower, were considered in the study.The side force and yawing moment acting on the vehicle model were measured using strain gauges on the model-supporting system that was designed as two cantilever beams with high stiffness. Characteristics of the aerodynamic forces acting on the vehicle model were described and the prediction methods that considered the wind non-uniformity acting on the vehicle were proposed and their applicability was studied.For the side force, the quasi-steady forces based on the relative wind velocity acting at the c.g. of the vehicle model generally agreed with the measured results. Changes in side force were found to be more or less proportional to the wind speed distribution. For the yawing moment, however, the quasi-steady prediction did not agree well with the measured results due to the wind non-uniformity. Therefore, a modified prediction method was proposed that included the effects of wind non-uniformity in a simple manner. Depending on the case, the agreement of prediction and measured results was still not good quantitatively, but from the qualitative point of view, the proposed prediction method could reproduce the yawing moment peak that was similar to the experimental results.     

Experimental study of the wind forces on rectangular latticed communication towers with antennas

With today's expanding communication systems, a large number of lattice towers to support cellular antennas are being constructed in Brazil. Due to the lightweight of these structures, wind forces are the primary concern in the design. An experimental investigation on the subject was carried out at the Boundary Layer Wind Tunnel Laboratory, University of Western Ontario (UWO), Canada. Three section models were designed and constructed based on existing latticed towers built in Brazil. The wind incidence angle; the tower solidity; the shielding effect; the influence of the wind turbulence on the drag coefficient were analyzed. Measurements were made of the mean and RMS drag and crosswind forces. The results were compared with some existing codes and standards including the Canadian (NBCC, 1995), American (ASCE 7-95, 1995), Australian/New Zealand (AS/NZS 1170.2-2002), Australian (AS 3995-1994), British (BS8100, 1986), Eurocode 1 (European Committee for Standardization, 1995) and Brazilian (NBR 6123, 1988). It is a common approach to consider the wind forces on antennas independent of the lattice tower, without considering the effects of their presence on the computation of the wind forces. The question arises whether this is a good approach or not. These effects can be described by introducing an interference factor. This factor depends, among other things, on the tower solidity. Two models with different solidity were tested for wind incidence angle of 0 degrees and antenna dishes simulated with disks made of Styrofoam attached to the windward face. The results were compared with ESDU.     

Influence of floor motions in wind tunnels on the aerodynamics of road vehicles

The effect of a moving floor on the flow around a simplified car with a typical fastback geometry is investigated. Two large-eddy simulations of the flows with stationary and moving floors are made and both instantaneous and time-averaged results are compared. It is found that the floor motion reduces drag by 8% and lift by 16%. Changes in the flow are found to be global but are largest close to the floor and on the rear slanted surface of the vehicle. The wake flow is found to be relatively insensitive to the floor movement, in agreement with previous experimental observations. The periodicity of the flow events is found to be dependent on whether the floor is moving. Power spectral density of both the lift and the drag contain only one dominant frequency peak when the moving floor is adopted as compared to scattered spectra in the stationary floor case. Changes in the qualitative picture of the flow are limited to the flow near the floor and on the slanted surface of the body. However, changes in the surface pressure on the body and the history of the flow show the need of a moving floor in experimental and numerical simulations.     

Experimental investigations of the indoor natural ventilation for different building configurations and incidences

This paper presents an experimental investigation of the indoor natural ventilation in terms of wind pressures on the surfaces of cubic buildings of a street located within a high density urban area. Wind tunnel tests over 1:100 scale models for four typical building patterns of a highly populated urban area have been carried out. The variables of the experiments were the building configurations and the incident wind direction. The experimental data are presented in terms of wind pressure coefficient measured on the surfaces of the buildings. The study results gave the evidence that buildings configuration and wind direction are very important factors in determining the induced natural ventilation within urban domains since they characteristically influence the flow yielding differences in wind pressures.     

Wind tunnel tests on heavy road vehicles: Cross wind induced loads—Part 2

The sensitivity of heavy road vehicle aerodynamic coefficients to different testing parameters (scenario, vehicle type and turbulence intensity) is experimentally evaluated in this paper through wind tunnel experiments. The first part of the paper has investigated the aerodynamic loads (both stationary and non-stationary) acting on a high-sided lorry (VAN) in a flat ground scenario. The mean aerodynamic coefficients, the flow pattern around the vehicle and the aerodynamic admittance function have been assessed and compared for different wind turbulence conditions (boundary layer simulations). The present paper instead investigates the influence of the infrastructure scenario (flat ground, embankment, single and double viaduct), of the exposition (upwind or downwind) and of a trailed unit on the aerodynamic loads acting on the vehicle. Moreover, the mean aerodynamic coefficients and the aerodynamic admittance function of the high-sided lorry considered in the first part are compared with the ones of other heavy vehicles (tank truck, tractor-semitrailer combination and tractor-trailer combination) to assess the influence of the vehicle geometry.     

Experimental investigation of wake structure around an external rear view mirror of a passenger car

In order to investigate the wake structure around the external rear view mirror of a passenger car, velocity vector fields and spectra of velocity fluctuations in the mirror wake were measured by hot wire anemometry and laser Doppler velocimetry in a blow down wind tunnel at Re=200,000. Time-averaged velocity fields were measured on one streamwise vertical plane and three cross-sectional planes. In early wake, an alternating vortex was found along the upper and the lower edge line of the mirror housing, and in the downstream, a vortex wrap like a tip vortex was also found. A reverse flow region was observed at up to 1.2 times the mirror vertical width. In the moment measurements on the cross-sectional planes, peak values of the root-mean-square fluctuation velocity appeared near the tip edge of the mirror due to significant vortex shedding. Corresponding skewness and kurtosis factors clearly deviated from the Gaussian distribution, which shows mirror wake boundaries. Spectral characteristics of the mirror wake obtained on the cross-sectional planes showed that the frequencies of most of the vortex shedding in the mirror wake was less than 50 Hz, and that the peaks of the vortex shedding energy were near the outside edge of the mirror housing.     

Experimental investigation and CFD analysis of cross-ventilated flow through single room detached house model

Cross-ventilation is a complicated flow problem and difficult to control because wind exhibits a large degree of variation. The paper focuses on three items: a) to clarify and understand some of the basic characteristics of airflow as the influence of the opening size on the windward vortex and the leeward wake; b) to explore what information about the flow above the ground can be retrieved from pressure measurements on the ground; and c) to explore the accuracy of CFD. To meet these objectives, wind tunnel tests and CFD analyses were carried out. The studied object was a detached- house model provided with two openings. The size of these openings was changed in a wide range from narrow cracks to large openings. In the experiments, pressure measurements on the ground and PIV measurements were made. The internal flow was visualized with the sand erosion method. Pressure measurement on a floor surface is a relatively easy and an inexpensive method. In this experiment, the windward and leeward areas in particular were investigated to understand flow pattern and to confirm correspondence between flow pattern and recorded pressure on the ground. Those measurements show the difference in flow at different size openings in terms of the vortex on the windward side and the wake. When the size of the opening exceeded a certain value the near wake on the leeward side disappeared and on the windward side the vortex disappeared. The pressure distribution, flow pattern, and velocity profile are shown and compared between measurement and CFD.     

Wind tunnel tests on heavy road vehicles: Cross wind induced loads—Part 1

In the recent years, rollover has become an important safety issue for a large class of vehicles. Even though rollovers constitute a small percentage of all accidents, they have unproportionally large contribution to severe and fatal injuries. Under this point of view, rollover of heavy vehicles is particularly critical being associated with large traffic disruption, economic loss and risks connected to the transported goods. One of the main causes for heavy vehicles rollover is recognised to be cross wind. In order to determine which parameters (geometry and vehicle type, infrastructure scenario, turbulence conditions, etc.) most affect the aerodynamic loads acting on heavy vehicles, a comprehensive experimental campaign has been carried out in the Politecnico di Milano wind tunnel. The overall activity is presented in 2 papers. In this first paper attention is focused on a high-sided lorry in flat ground scenario. Mean aerodynamic forces and moments have been measured by means of a six-components dynamometric balance for different yaw angles and turbulence conditions. Moreover, in order to gain an insight of the flow pattern around the vehicle, pressure distribution on the vehicle surface has been measured. Finally, the vehicle aerodynamic admittance function has been assessed, for high turbulence conditions, to investigate the unsteady force/moment component. The second paper deals with the effect of infrastructure scenario (flat ground, embankment, double and single viaduct), of position (vehicle placed upwind or downwind) and of vehicle geometry/type (high-sided lorry with and without a trailed unit, tractor-semitrailer combination and tank truck) on the aerodynamic forces and moments, including both steady and unsteady components.     

Experimental and numerical study of wind pressures on irregular-plan shapes

This paper presents the results of a program of wind tunnel model tests on pressure distributions for irregular-plan shapes (L- and U-shaped models). The experiments were carried out in a closed-circuit wind tunnel and a multi-channel pressure measurement system was used to measure mean values of loads on 1:100 scale models. The same tests were carried out on a cube-shaped model as an experimental validation. The effectiveness of the model shape in changing the surface pressure distributions is assessed over an extended range of wind directions. The experimental data for the L- and U-shaped models showed different wall pressure distributions from those expected for single rectangular blocks. Furthermore, a Computational Fluid Dynamics (CFD) code was used to illustrate some particular cases and to provide a better understanding of the flow patterns around these irregular-plan models and of the pressure distributions induced on their faces. Computed pressure coefficients have also been compared with wind tunnel results for normal and oblique wind incidence. A general good agreement has been found for normal wind incidence whereas some differences have occurred for other directions.     

Experimental and numerical modelling of the three-dimensional incompressible flow behaviour in the near wake of circular cylinders

An experimental investigation was carried out to study the structure of the flow field around three-dimensional circular cylinders. The study of the flow field around an obstacle was performed in a wind tunnel using a particle image velocimetry (PIV) system. The flow of a fluid around an obstacle with a different velocity to the oncoming flow was examined. The results showed the dependence of the flow structure around the obstacle on its Reynolds number, and the spacing between a pair of obstacles. Detailed quantitative information of turbulence parameters in the vicinity of the obstacle was attained. Extensive wind tunnel experimental results are presented and compared with numerical simulation. A three-dimensional numerical model with Reynolds stress model (RSM) turbulence and a non-uniform grid system were used to examine the effects of a single cylinder and two cylinders in tandem on the flow. The principal objective was to analyse three-dimensional flow past a single cylinder and two circular cylinders placed in tandem by combining the application of a PIV experimental technique and an RSM turbulence model.For the case of two cylinders in tandem, the flow patterns are characterized in the gap region as a function of the distance between the cylinders. A good level of agreement was found between the experimental results of flow and numerical simulation.     

A new technique for identification of eighteen flutter derivatives using a three-degree-of-freedom section model

The prediction of flutter instability is of major concern for design of flexible structures. This necessitates the identification of aeroelastic parameters, known as flutter derivatives from wind tunnel experiments. The extraction of flutter derivatives becomes more challenging when the number of degrees of freedom (DOF) increases from two to three. Since the work in the field of identifying all 18 flutter derivatives has been limited, it has motivated the development of a new system identification method (iterative least squares method or ILS method) to efficiently extract the flutter derivatives using a section model suspended by a three-DOF elastic suspension system. The accuracy of a particular flutter derivative was determined by comparing the results obtained from all possible DOF combinations.     

某自卸车落物保护结构的仿真分析

用ANSYS软件对某自卸车落物保护结构建立有限元模型,进行了冲击强度计算,得到了在落物冲击载荷作用下结构的应力和变形,并对比相应标准进行了安全性分析,仿真结果证明该自卸车驾驶室强度能够保护驾驶员的人身安全。     

An experimental investigation of the wind environment and air quality within a densely populated urban street canyon

This paper presents an experimental investigation of wind flow characteristics and air quality along a street canyon located within a dense urban area. Four typical models of a highly populated urban area are studied and wind tunnel experiments are carried out over an extended range of the applied wind directions. The building patterns are represented by 1:100 scale models, where wind velocity and tracer gas concentrations are measured along the two sides of the street. The study results provide evidence that building configurations and wind directions are very important factors in determining both wind flow and pollutant dispersion characteristics within urban domains. Also, the results demonstrate that gaps between buildings are a very important factor to be considered by urban planners and designers, because, for a given building height, larger gaps induce more wind in urban canyons, thus improving the ventilation process.     

A computational study of the aerodynamic forces acting on a tractor-trailer vehicle on a bridge in cross-wind

This paper examines the effect of truck speed on the aerodynamic forces acting on a standard sized, North American transport truck travelling across a bridge under conditions of cross-wind. The objective is to establish a relationship between wind speed, truck speed and propensity for truck rollover that may be used to devise strategies for accident avoidance. Conditions of a moving truck travelling in both windward and leeward lanes were considered with a cross-wind speed of 120 km/h and truck speeds of 0-120 km/h. Using the calculated pressure distributions on the surface of the truck; the aerodynamic lift, drag and moment coefficients were determined for relative wind directions. The results show that the aerodynamic moment tending to overturn a truck in the windward lane of the bridge rises from approximately 120 kN m at low speeds (0-40 km/h) to 217 kN m at a truck speed of 120 km/h. For a truck in the leeward lane, the corresponding moments are substantially less, at 82 and 154 kN m, respectively. The 1.1 m barrier wall along the side of the bridge is a contributing factor to the aerodynamic difference between windward and leeward lanes.     

Wind tunnel simulation of the wind conditions inside bidimensional forest clear-cuts. Application to wind turbine siting

The flow field within bidimensional forest clear-cuts, oriented perpendicular to the wind direction, has been modelled in a wind tunnel, studying the effect of porosity and length of the leading forest and clear-cut. A simple foam model provides an effective simulation of the forest canopy flow, following similar turbulence characteristics found in other wind tunnel and field tests. Particle image velocimetry (PIV) measurements provide a clear picture of the displacement of the boundary layer above the porous canopy and the development of a cavity shear layer at the height of the forest. This shear layer originates high velocity gradients, turbulence intensities and wind gusts. The study of extreme winds is carried out using extreme value analysis on the PIV fields. The results are applied to study the technical feasibility of wind turbine siting in forests following the IEC61400-1 norm. With respect to the forest-free conditions, the turbulence intensity is increased by 4-9%, the wind energy is reduced by 5-20% and the extreme velocities are 15-30% higher. As a rule of thumb, the clearance between the rotor and the ground should be at least two times the forest height to avoid the cavity shear layer.     

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.     

考虑雷诺数效应的斜拉索气动力试验研究

由于大跨度斜拉桥斜拉索的风荷载在全桥风荷载中占很大比重,因此准确掌握斜拉索的风荷载对大桥的设计工作是十分重要的。通过风洞试验,测试斜拉索的阻力和升力,得到力系数随雷诺数的变化规律,证实在临界雷诺数区域有平均升力的产生,其最大升力系数远大于此雷诺数时对应的阻力系数,忽略升力的存在可能导致斜拉索风荷载的计算结果偏小;将阻力系数和升力系数合成为合力系数,并与规范值比较,发现只有当雷诺数超过一定数值之后,规范的算法才是安全的,雷诺数小于此值尤其是在临界雷诺数区域,合力系数比规范值超出较大。针对苏通长江公路大桥和南京长江第二大桥,利用试验结果和规范值,对斜拉索的风荷载分别进行计算分析,发现利用本文方法计算的合力并不随风速的增大单调增长,而是在一定的风速时取得极大值;利用静阵风风速计算风荷载,按桥规计算的结果在某些情况下偏小,建议规范适当考虑气动升力对斜拉索风荷载的贡献。     

Development of a shrouded wind turbine with a flanged diffuser

We have developed a wind turbine system that consists of a diffuser shroud with a broad-ring flange at the exit periphery and a wind turbine inside it. The flanged-diffuser shroud plays a role of a device for collecting and accelerating the approaching wind. Emphasis is placed on positioning the flange at the exit of a diffuser shroud. Namely, the flange generates a low-pressure region in the exit neighborhood of the diffuser by vortex formation and draws more mass flow to the wind turbine inside the diffuser shroud. To obtain a higher power output of the shrouded wind turbine, we have examined the optimal form of the flanged diffuser, such as the diffuser open angle, flange height, hub ratio, centerbody length, inlet shroud shape and so on. As a result, a shrouded wind turbine equipped with a flanged diffuser has been developed, and demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about 4-5 compared to a standard (bare) wind turbine. In a field experiment using a prototype wind turbine with a flanged diffuser shroud, the output performance was as expected and equalled that of the wind tunnel experiment.     

高空救援消防车电控系统的设计与开发

开发高空救援消防车电气系统,采用三菱FX2N 可编程控制器作为主控制器,FameView系列触摸屏开发整车电控系统HMI界面.利用慧鱼模型组件搭建了消防车模型,并制作控制平台.介绍系统的控制流程、硬件选型、PID调平控制策略和HMI界面开发.可实现车身声光警示、转台转动、臂架变幅、作业平台调平、实时监控和限位保护等功能.所提出的设计方案可应用于25 m及以下高空救援消防车电控系统开发,具有较高的性价比和应用价值.     

Aerodynamic design of a kinetic sculpture

This paper presents a novel application of galloping to the design of a kinetic sculpture intended to represent the shape and motion of dune grass. This 35 m sculpture will consist of a flexible composite mast with an elliptical profile head, deliberately designed to generate slow but large-amplitude cross-wind galloping oscillations. The galloping characteristics of triangular, triform and cruciform head cross-sections have been investigated using static and dynamic wind tunnel tests, and modelled successfully using a simple energy balance method. The effect of yaw angle on galloping of three-dimensional shapes is much more significant than for 2D prisms, with the formation of leading-edge vortices from the swept edges tending to suppress galloping. This results in a desirable ‘self-furling’ feature, with the sculpture ceasing to move in high winds as the mast bands backwards. A full scale prototype has been successfully tested, in advance of the installation of a group of six Dune Grass sculptures on the sea front at Blackpool.