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CONTENTS
Volume 9, Number 1, January 2006
 

Abstract
The response characteristics and suppression of flow-induced vibrations of rectangular prisms with various width-to-depth ratios were experimentally investigated. The prisms were rigid and elastically mounted at both ends to enable constrained torsional vibrations only. The present study focused on torsional vibrations, one of the three types of flow-induced vibrations generated in a rectangular prism. First, the response characteristics of torsional vibrations generated in rectangular prisms were investigated by free-vibration tests. It was found that the response characteristics of torsional vibrations generated in rectangular prisms could be classified into six patterns depending on the width-to-depth ratio. Next, the response characteristics of torsional vibrations observed in the free-vibration tests were reproduced by forced-vibration tests, and the mechanisms by which the three types of flow-induced vibrations, low-speed torsional flutter, vortex excitation and high-speed torsional flutter, are generated in the rectangular prisms were elucidated on the basis of characteristics of fluid forces and visualized flow patterns. Experiments were also carried out to establish an effective method for suppressing flow-induced vibrations generated in the rectangular prisms, and it was found that low-speed torsional flutter and high-speed torsional flutter could be suppressed by placing a small normal plate upstream of the prism, which results in suppression of the alternating rolling-up of the shear layers separating from the leading edges of the prism. It was also found that vortex excitation could be suppressed by placing a splitter plate downstream of the prism, which results in suppression of the generation of wake vortices.

Key Words
rectangular prism; width-to-depth ratio; torsional vibration; suppression; control normal plate.

Address
Department of Mechanical Engineering, Kitami Institute of Technology,165 Koen-cho, Kitami, Hokkaido, 090-8507, Japan

Abstract
The effect of buildings on flow in urban canopy is one of the most important problems in local/micro-scale meteorology. A large eddy simulation model is used to simulate the flow structure in an urban neighborhood and the bulk effect of the buildings on surrounding flows is analyzed. The results demonstrate that: (a) The inflow conditions affect the detailed flow characteristics much in the building group, including: the distortion or disappearance of the wake vortexes, the change of funneling effect area and the change of location, size of the static-wind area. (b) The bulk effect of the buildings leads to a loss of wind speed in the low layer where height is less than four times of the average building height, and this loss effect changes little when the inflow direction changes. (c) In the bulk effect to environmental fields, the change of inflow direction affects the vertical distribution of turbulence greatly. The peak value of the turbulence energy appears at the height of the average building height. The attribution of fluctuations of different components to turbulence changes greatly at different height levels, in the low levels the horizontal speed fluctuation attribute mostly, while the vertical speed fluctuation does in high levels.

Key Words
large eddy simulation; urban canopy; buildings; air flow.

Address
Ning Zhang; Department of Atmospheric Science, Nanjing University, Nanjing, 210093, ChinarnWeimei Jiang; Department of Atmospheric Science, Nanjing University, Nanjing, 210093, ChinarnKey Laboratory of Atmospheric Physics and Chemistry, Institue of Atmospheric Phyisics, ChinesernAcademy of Sciences, Beijing, 100029, ChinarnShiguang Miao; Beijing Meteorology Bureau, Beijing, 100089 China

Abstract
The paper describes a study about effects of upstream hills on design wind loads using two mathematical approaches: Computational Fluid Dynamics (CFD) and Artificial Neural Network (NN for short). For this purpose CFD and NN tools have been developed using an object-oriented approach and C++ programming language. The CFD tool consists of solving the Reynolds time-averaged Navier-Stokes equations and k - e turbulence model using body-fitted nearly-orthogonal coordinate system. Subsequently, design wind load parameters such as speed-up ratio values have been generated for a wide spectrum of two-dimensional hill geometries that includes isolated and multiple steep and shallow hills. Ground roughness effect has also been considered. Such CFD solutions, however, normally require among other things ample computational time, background knowledge and high-capacity hardware. To assist the end-user, an easier, faster and more inexpensive NN model trained with the CFD-generated data is proposed in this paper. Prior to using the CFD data for training purposes, extensive validation work has been carried out by comparing with boundary layer wind tunnel (BLWT) data. The CFD trained NN (CFD-NN) has produced speed-up ratio values for cases such as multiple hills that are not covered by wind design standards such as the Commentaries of the National Building Code of Canada (1995). The CFD-NN results compare well with BLWT data available in literature and the proposed approach requires fewer resources compared to running BLWT experiments.

Key Words
computational fluid dynamics; hills; neural network; speed-up ratio; turbulence; wind load; wind velocity.

Address
G. Bitsuamlak; RWDI Inc., Guelph, Ontario, CanadarnT. Stathopoulos; Centre for Building Studies, Concordia University, Montreal, Quebec, CanadarnC. Bedard; ETS-Ecole de Technologie Superieure, Montreal, Quebec, Canada

Abstract
Vertical stabilizer plates have been found to be an effective aerodynamic measure to improve the aerodynamic stability of bridges either with an open cross section or with a streamlined box cross section in wind tunnel testings and have been adopted in some long span bridges. By taking an open deck II-shaped section and a closed box section as examples, the mechanism of vertical stabilizer plates for improving aerodynamic stability are investigated by using numerical simulation based on Random Vortex Method. It is found that vertical stabilizer plates can increase the amplitude of the heaving motion, and decrease that of the rotational motion of the bridge decks.

Key Words
bridge deck; aerodynamic stability; aerodynamic measure; vertical stabilizer plate; random vortex method (RVM).

Address
State Key Laboratory for Disaster Reduction in Civil Engineering, Department of Bridge Engineering, Tongji University, Shanghai, China

Abstract
This paper presents a quasi-steady model of vibrations of two cylinders in a side-by-side arrangement. The cylinders have flexible support and equal diameters. The model assumes that both cylinders participate in the process of vibration, each of them having two degrees of freedom. The movement of cylinders is described by a set of four non-linear differential equations. These equations are evaluated on the basis of a numerical simulation and experimental data. Moreover many features of cylinder vibrations are found from numerical results and are described in this paper.

Key Words
aerodynamic interference; two circular cylinders; side-by-side arrangement; quasi-steady theory; numerical experiment.

Address
Department of Structural Mechanics, Faculty Civil and Sanitary Engineering, Lublin University of Technology, Nadbystrzycka 40, 20-618 Lublin, Poland


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