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CONTENTS
Volume 21, Number 1, July 2015
 

Abstract
Wind tunnel testing of a low-rise building with openings (holes) of different sizes and shapes on a roof corner is conducted to measure the internal and external pressures from the building model. Detailed analysis of the testing data is carried out to investigate the characteristics of the internal and external pressures of the building with different openings\' configurations. Superimposition of the internal and external pressures makes the emergence of positive net pressures on the roof. The internal pressures demonstrate an overall uniform distribution. The probability density function (PDF) of the internal pressures is close to the Gaussian distribution. Compared with the PDF of the external pressures, the non-Gaussian characteristics of the net pressures weakened. The internal pressures exhibit strong correlation in frequency domain. There appear two humps in the spectra of the internal pressures, which correspond to the Helmholtz frequency and vortex shedding frequency, respectively. But, the peak for the vortex shedding frequency is offset for the net pressures. Furthermore, the internal pressure characteristics indirectly reflect that the length of the front edge enhances the development of the conical vortices.The objective of this study aims to further understanding of the characteristics of internal, external and net pressures for low-rise buildings in an effort to reduce wind damages to residential buildings.

Key Words
low-rise building; wind tunnel testing; wind loading; internal pressure; external pressure; net pressure; separation bubble; conical vortex

Address
Yunjie Wang: College of Civil Engineering, Hunan University, Changsha 410082, P.R. China
Q.S. Li: Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong


Abstract
Oscillating motions of flexible cylinders are associated to some extent with the aerodynamic response of plants. Tandem motions of reeds with flexible stems in a colony are experimentally investigated using an array of flexible cylinders made of polydimethylsiloxane (PDMS). Consecutive images of flexible cylinders subjected to oncoming wind are recorded with a high-speed camera. To quantify oscillating motions, the average bending angle and displacement of flexible cylinders are evaluated using point-tracking method and spectral analysis. The tandem motions of flexible cylinders are closely related to the flow characteristics around the cylinders. Thus, the dynamic motions of a tandem arrangement of flexible cylinders are investigated with varying numbers of cylinders arranged in-line, numbers of cylinders in a group (behaving like a single body), and Reynolds numbers (Re). When the number of cylinders in a group increases, the damping effect caused by the support of downstream cylinders is pronounced. These results would be provide useful information on the tandem-arranged design of complex structures and energy harvesting devices.

Key Words
flexible cylinder; tandem arrangement; in-line cylinders; reconfiguration

Address
Sang Joon Lee, Jeong Jae Kim and Eunseop Yeom: Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyojadong, Pohang 790-784, South Korea

Abstract
The flutter stability of long-span suspension bridges during erection can be more problematic and more susceptible to be influenced by many factors than in the final state. As described in this paper, numerical flutter stability analyses were performed for the construction process of Zhongdu Bridge over Yangtze River using the commercial FE package ANSYS. The effect of the initial wind attack angle, the sequence of deck erection, the stiffness reduction of stiffening girders, the structural damping, and the cross cables are discussed in detail. It was found that the non-symmetrical sequence of deck erection was confirmed to be aerodynamically favourable for the deck erection of long-span suspension bridges and the best erection sequence should be investigated in the design phase. While the initial wind attack angle of -3o is advantageous for the aerodynamic stability, +3o is disadvantageous compared with the initial wind attack angle of 0o during the deck erection. The stiffness reduction of the stiffening girders has a slight effect on the flutter wind speed of the suspension bridge during erection, but structural damping has a great impact on it, especially for the early erection stages.

Key Words
long-span suspension bridges; flutter stability; deck erection; finite element (FE) model; ANSYS

Address
Yan Han, Shuqian Liu and Chunguang Li: School of Civil Engineering and Architecture, Changsha University of Science & Technology, Changsha, China, 410004
C.S. Cai:School of Civil Engineering and Architecture, Changsha University of Science & Technology, Changsha, China, 410004;
Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana, USA, LA 70803


Abstract
The shape of a tall building has significant impact on wind force generation and wind-induced dynamic response. To study the effect of recessed cavities, wind excitations on a wind-tunnel model of an H-section tall building were compared with those on a square-section building model. Characteristics of the fluctuating wind pressures on the side faces of the two tall buildings and their role in the generation of crosswind forces on the buildings were investigated with the space-time statistical tool of proper orthogonal decomposition (POD). This paper also compares the use of different pressure data sets for POD analysis in situations where pressures on two different surfaces are responsible for the generation of a wind force. The first POD mode is found to dominate the generation of crosswind excitation on the buildings.

Key Words
tall buildings; wind pressure; crosswind excitation; POD

Address
L. Cheng, K.M. Lam and S.Y. Wong: Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong

Abstract
Aerodynamic forces of vortex-induced vibration and galloping are going to be coupled when their onset velocities are close to each other, which will induce the cross-wind amplitudes of the structures increased continuously with ever-increasing wind velocities. The main purpose of the present work is going to propose an empirical formula to predict the response amplitude of VIV-galloping interaction. Firstly, two typical mathematical models for the coupled oscillations, i.e., Tamura & Shimada model and Parkinson & Corless model are comparatively summarized. Then, the key parameter affecting response amplitude is determined through comparative numerical simulations with Tamura & Shimada model. For rectangular cylinders with the side ratio from 0.5 to 2.5, which are actually prone to develop the VIV and galloping induced interaction responses, an empirical amplitude prediction formula is proposed after regression analysis on comprehensively collected experimental data with the predetermined key parameter.

Key Words
rectangular cylinders; VIV-Galloping interaction; amplitude prediction; empirical model

Address
Huawei Niu, Zhengqing Chen and Xugang Hua: Wind Engineering Research Center of Hunan University, Changsha, Hunan, China
Shuai Zhou:Wind Engineering Research Center of Hunan University, Changsha, Hunan, China;
China Railway Eryuan Engineering Group Co.Ltd, Chengdu, Sichuan, China

Abstract
This paper presents a numerical characterization of real railway overhead cables based on computational fluid dynamics (CFD). Complete analysis of the aerodynamic coefficients of this type of cross section yields a more accurate modelling of pressure loads acting on moving cables than provided by current approaches used in design. Thus, the characterization of certain selected commercial cables is carried out in this work for different wind speeds and angles of attack. The aerodynamic lift and drag coefficients are herein determined for two different types of grooved cables, which establish a relevant data set for the railway industry. Finally, the influence of this characterization on the fluid-structure interaction (FSI) is proved, the static behavior of a catenary system is studied by means of the finite element method (FEM) in order to analyze the effect of different wind angles of attack on the stiffness distribution.

Key Words
aerodynamic characteristics; overhead catenary line; CFD simulation; cable; lift force

Address
Cristina Sanchez-Rebollo: Institute for Research in Technology, Universidad Pontificia Comillas, c/ Santa Cruz de Marcenado 26, 28015 Madrid, Spain
Enrique Velez and Jesus R. Jimenez-Octavio: Department of Mechanical Engineering, Universidad Pontificia Comillas, c/ Alberto Aguilera 23, 28015 Madrid, Spain



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