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CONTENTS
Volume 19, Number 2, August 2014
 

Abstract
Separation bubble and conical vortices on a large-span flat roof were observed in this study through the use of flow visualization. The results indicated that separation bubble occurred when the flow was normal to the leading edge of the flat roof. Conical vortices that occur under the cornering flow were observed near the leading edge, and their appearance was influenced by the wind angle. When the wind changed from along the diagonal to deviating from the diagonal of the roof, the conical vortex close to the approaching flow changed from circular to be more oblong shaped. Based on the measured velocities in the conical vortices by flow visualization, a proposed two-dimensional vortex model was improved and validated by simplifying the velocity profile between the vortex and the potential flow region. Through measured velocities and parameters of vortices, the intensities of conical vortices and separation bubble on a large-span flat roof under different wind directions were provided. The quasi-steady theory was corrected by including the effect of vortices. With this improved two-dimensional vortex model and the corrected quasi-steady theory, the mean and peak suction beneath the cores of the conical vortices and separation bubble can be predicted, and these were verified by measured pressures on a larger-scale model of the flat roof.

Key Words
conical vortices; separation bubble; flat roof; flow model of vortex; prediction of suction beneath vortex cores; PIV experiment

Address
Jihong Ye: Key laboratory for RC&PC Structures of China Ministry of Education, Southeast University, Nanjing 210096, China
Xin Dong:Key laboratory for RC&PC Structures of China Ministry of Education, Southeast University, Nanjing 210096, China;
Tongji Architectural Design (Group) Co. Ltd, Shanghai 200092, China

Abstract
Wind-vehicle-bridge (WVB) interaction can be regarded as a coupled vibration system. Aerodynamic forces and moment on vehicles and bridge decks play an important role in the vibration analysis of the coupled WVB system. High-speed vehicle motion has certain effects on the aerodynamic characteristics of a vehicle-bridge system under crosswinds, but it is not taken into account in most previous studies. In this study, a new testing system with a moving vehicle model was developed to directly measure the aerodynamic forces and moment on the vehicle and bridge deck when the vehicle model moved on the bridge deck under crosswinds in a large wind tunnel. The testing system, with a total length of 18.0 m, consisted of three main parts: vehicle-bridge model system, motion system and signal measuring system. The wind speed, vehicle speed, test objects and relative position of the vehicle to the bridge deck could be easily altered for different test cases. The aerodynamic forces and moment on the moving vehicle and bridge deck were measured utilizing the new testing system. The effects of the vehicle speed, wind yaw angle, rail track position and vehicle type on the aerodynamic characteristics of the vehicle and bridge deck were investigated. In addition, a data processing method was proposed according to the characteristics of the dynamic testing signals to determine the variations of aerodynamic forces and moment on the moving vehicle and bridge deck. Three-car and single-car models were employed as the moving rail vehicle model and road vehicle model, respectively. The results indicate that the drag and lift coefficients of the vehicle tend to increase with the increase of the vehicle speed and the decrease of the resultant wind yaw angle and that the vehicle speed has more significant effect on the aerodynamic coefficients of the single-car model than on those of the three-car model. This study also reveals that the aerodynamic coefficients of the vehicle and bridge deck are strongly influenced by the rail track positions, while the aerodynamic coefficients of the bridge deck are insensitive to the vehicle speed or resultant wind yaw angle.

Key Words
crosswinds; moving-vehicle model; wind loads; wind-tunnel test; wind-vehicle-bridge system

Address
Yongle Li, Peng Hu, Mingjin Zhang and Haili Liao: Department of Bridge Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China
You-Lin Xu: Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong

Abstract
This paper presents a method of estimation of extreme wind. Assuming the extreme wind follows the Gumbel distribution, it is modeled through fitting an exponential function to the numbers of storms over different thresholds. The comparison between the estimated results with the Improved Method of Independent Storms (IMIS) shows that the proposed method gives reliable estimation of extreme wind. The proposed method also shows its advantage on the insensitiveness of estimated results to the precision of the data. The volume of extreme storms used in the estimation leads to more than 5% differences in the estimated wind speed with 50-year return period. The annual rate of independent storms is not a significant factor to the estimation.

Key Words
extreme wind speed; extreme value estimation; annual maximum; independent storms; weighted least-squares method

Address
Yi Hui and Zhengnong Li: College of civil engineering, Hunan University, Yuelushan, Changsha, Hunan, 410082, China
Qingshan Yang: Beijing\'s s Key Laboratory of Structural Wind Engineering and Urban Wind Environment, School of Civil Engineering, Beijing Jiaotong University, Beijing, 100044, China

Abstract
Ventilation and fire safety design in road tunnels are one of the most complex issues that need to be carefully considered and analysed in the designing stage of any tunnel project, as well as in case of any potential upgrade of ventilation and other fire safety systems in tunnels. Placement of road tunnels in space has an important influence on fire safety, especially when considering the effect of adverse wind conditions that significantly influence ventilation characteristics. The appropriate analysis of fire and smoke control is almost impossible without the use of modern simulation tools (e.g., CFD) due to a large number of influential parameters and consequently extensive data. The impact of the strong wind is briefly presented in this paper in the case of a longitudinally ventilated road tunnel Kastelec, which is exposed to various severe wind conditions that significantly influence its fire safety. The possibility of using CFD simulations in the analysis of the tunnel placement in space in terms of negative effect of wind influence on the tunnel ventilation is clearly indicated.

Key Words
CFD (computational fluid dynamics); wind loads; simulations; tunnel safety; longitudinal ventilation

Address
Simon Muhič:School of Technologies and Systems, Na Loko2, SI-8000 Novo mesto, Slovenia;
SimTec, Dr. Simon Muhič s. p., Stična 113, SI-1295 Ivančna Gorica, Slovenia
Mitja Mazej:School of Technologies and Systems, Na Loko2, SI-8000 Novo mesto, Slovenia

Abstract
Reported experimental and computational fluid dynamic (CFD) studies have demonstrated significant power augmentation of diffuser shrouded horizontal axis micro wind turbine compared to bare turbine. These studies also found the degree of the augmentation is strongly dependent on the shape and geometry of the diffuser such as the length and the expansion angle. However the study of flow field over the rotor blades in shrouded turbine has not received much attention. In this paper, CFD simulations of an experimental diffuser shrouded micro wind turbine have been carried out with the aim to understand the mechanisms underpinning the power augmentation phenomenon. The simulations provide insight of the flow field over the blades of bare wind turbine and of shrouded one elucidating the augmentation mechanisms. From the analysis, sub-atmospheric back pressure leading to velocity augmentation at the inlet of diffuser and lowering of the static pressure on the blade suction sides have been identified as the dominant mechanisms driving the power augmentation. And effective augmentation was achieved for y above certain value. For the case turbine it is y greater than = 2.

Key Words
coefficient of performance; computational fluid dynamics; diffuser shroud; micro horizontal axis wind turbine

Address
Seyed A Jafari and Buyung Kosasih:School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2500, Australia


Abstract
The characteristics of turbulent boundary layers over hilly terrain depend strongly on the hill slope and upstream condition, especially inflow turbulence. Numerical simulations are carried out to investigate the neutrally stratified turbulent boundary layer over two-dimensional hills. Two kinds of hill shape, a steep one with stable separation and a low one without stable separation, and two kinds of inflow condition, laminar and turbulent, are considered. An auxiliary simulation, based on the local differential quadrature method and the recycling technique, is performed to simulate the inflow turbulence to be imposed at the inlet boundary of the simulation with turbulent inflow, which preserves very well in the computational domain. A large separation bubble is established on the leeside of the steep hill with laminar inflow, while the reattachment point moves upstream under turbulent inflow condition. There is stable separation on the lee side of the low hill with laminar inflow, while not with turbulent inflow. Besides increase of turbulence intensity, inflow turbulence can efficiently enhance the speedup around hills.So in practice, it is unreasonable to study wind flow over hilly terrain without considering inflow turbulence.

Key Words
inflow turbulence; slope; turbulent boundary layer; two-dimensional hill; separation;speedup; numerical simulation

Address
Tong Wang: College of Civil Engineering, Shanghai Normal University, Shanghai 201418, China;
Department of Bridge Engineering, Tongji University, Shanghai 200092, China
Shuyang Cao and Yaojun Ge: Department of Bridge Engineering, Tongji University, Shanghai 200092, China


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