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CONTENTS
Volume 9, Number 2, March 2006
 

Abstract
Wind turbines are highly complex structures for numerical flow simulation. They normally comprise of a turbine mounted on a tower thus the movement of the turbine blades and the blade/tower interaction must be captured. In addition the ground effect should also be included. There are many more important features of wind turbines and it is difficult to include all of them. A simplified set of features is chosen here for both the turbine and the tower to show how the method can begin to identify the main points connected with wind turbine wake generation and tip vortex tower interaction. An approach to modelling the rotating blades of a turbine is proposed here. The model uses point forces based on blade element theory to model the blades and takes into account their time dependent motion. This means that local instantaneous velocities can be used as a basis for the blade element theory. The model is incorporated into a large eddy simulation code and, although many important features are left out of the model, the velocity/power performance relation is generally of the correct order of magnitude. Suggested improvements to the method are discussed.

Key Words
wind turbines; wind turbine/tower interaction; wakes; large eddy simulation.

Address
R. J. A. Howard; Departmento de Engenharia Mecanica, Universidade de Aveiro, Campo Universitaio de Santiago, 3810-193 Aveiro, PortugalrnJ. C. F. Pereira; Mechanical Engineering Department, LASEF, Instituto Superior Tecnico, Avenida Rovisco Pais, Lisbon 1049-001, Portugal

Abstract
This paper describes the characteristics of the fluctuating lift forces when a circular cylinder vibrates in the cross-flow direction. The response characteristics on elastically supported the circular cylinder was first examined by a free-vibration test. Next, flow-induced vibrations obtained by the free-vibration test were reproduced by a forced-vibration test, and then the characteristics of the fluctuating lift forces, the work done by the fluctuating lift, the behavior of the rolling-up of the separated shear layers were investigated on the basis of the visualized flow patterns. The main findings were that (i) the fluctuating lift forces become considerably large than those of a stationary circular cylinder, (ii) negative pressure generates on the surface of the circular cylinder when the rolling-up of separated shear layer begins, (iii) the phase between the fluctuating lift force and the cylinder displacement changes abruptly as the reduced velocity Ur increases, and (iv) whether the generating cross-flow vibration becomes divergent or convergent can be described based on the work done by the fluctuating lift force. Furthermore, it was found that the generation of cross-flow vibration can be perfectly suppressed when the small tripping rods are installed on the surface of the circular cylinder.

Key Words
circular cylinder; cross-flow vibration; vortex excitation; fluctuating fluid forces; displacement; separated shear layers; phase.

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

Abstract
Stonecutters Bridge of Hong Kong is a cable-stayed bridge with two single-column pylons each 298 m high and an aerodynamic twin deck. The total length of the bridge is 1596 m with a main span of 1018 m. The top 118 m of the tower will comprise structural steel and concrete composite while the bottom part will be of reinforced concrete. The bridge deck at the central span will be of steel whilst the side spans will be of concrete. Stonecutters Bridge has adopted a twin-girder deck design with a wide clear separation of 14.3 m between the two longitudinal girders. Although a number of studies have been conducted to investigate the aerodynamic performance of twin-girder deck, the actual real life application of this type of deck is extremely limited. This therefore triggered the need for conducting the present studies, the main objective of which is to investigate the performance of Stonecutters Bridge against flutter at its in-service stage as well as during construction. Based on the flutter derivatives obtained from the 1:80 scale rigid section model experiment, flutter analysis was carried out using 3-D finite element based single parameter searching method developed by the second author of this paper. A total of 6 finite element models of the bridge covering the in-service stage as well as 5 construction stages were established. The dynamic characteristics of the bridge associated with these stages were computed and applied in the analyses. Apart from the critical wind speeds for the onset of flutter, the dominant modes of vibration participating in the flutter vibration were also identified. The results indicate that the bridge will be stable against flutter at its in-service stage as well as during construction at wind speeds much higher than the verification wind speed of 95 m/s (1-minute mean).

Key Words
Stonecutters Bridge; flutter; multi-mode and single parameter searching.

Address
Michael C. H. Hui1),2) and Q. S. Ding 2); 1)Major Works Project Management Office, Highways Department, 6/F, Ho Man Tin Government Offices, 88 Chung Hau Street, Kowloon, Hong Kong, Chinarn2)State Key Laboratory for Disaster Reduction in Civil Engineering and Department of Bridge Engineering, Tongji University, Shanghai 200092, ChinarnY. L. Xu; Department of Civil and Structural Engineering, The Hong Kong Polytechnic University,rnHung Hom, Kowloon, Hong Kong, China

Abstract
The purpose of this paper is to find a more accurate method to evaluate pedestrian wind by computational fluid dynamics approach. Previous computational fluid dynamics studies of wind environmental problems were mostly performed by simplified models, which only use simple geometric shapes, such as cubes and cylinders, to represent buildings and structures. However, to have more accurate and complete evaluation results, various shapes of blocking objects, such as trees, should also be taken into consideration. The aerodynamic effects of these various shapes of objects can decrease wind velocity and increase turbulence intensity. Previous studies simply omitted the errors generated from these various shapes of blocking objects. Adding real geometrical trees to the numerical models makes the calculating domain of CFD very complicated due to geometry generation and grid meshing problems. In this case the function of Porous Media Condition can solve the problem by adding trees into numerical models without increasing the mesh grids. The comparison results between numerical and wind tunnel model are close if the parameters of porous media condition are well adjusted.

Key Words
CFD; porous media condition; pedestrian wind; wind tunnel.

Address
Department of Civil Engineering and Wind Engineering Research Center, Tamkang University, Tamsui, Taipei County, Taiwan

Abstract
For the slender and flexible cable supported bridges, identification of all the flutter derivatives for the vertical, lateral and torsional motions is essential for its stability investigation. In all, eighteen flutter derivatives may have to be considered, the identification of which using a three degree-of-freedom elastic suspension system has been a challenging task. In this paper, a system identification technique, known as covariance-driven stochastic subspace identification (COV-SSI) technique, has been utilized to extract the flutter derivatives for a typical bridge deck. This method identifies the stochastic state-space model from the covariances of the output-only (stochastic) data. All the eighteen flutter derivatives have been simultaneously extracted from the output response data obtained from wind tunnel test on a 3-DOF elastically suspended bridge deck section-model. Simplicity in model suspension and measurements of only output responses are additional motivating factors for adopting COV-SSI technique. The identified discrete values of flutter derivatives have been approximated by rational functions. rnKeywords: flutter derivative; wind tunnel test; stochastic subspace identification; rational function approximation.

Key Words
flutter derivative; wind tunnel test; stochastic subspace identification; rational function approximation.

Address
Shambhu Sharan Mishra; Department of Civil Engineering, NERIST, Nirjuli, Arunachal Pradesh 791109, IndiarnKrishen Kumar and Prem Krishna; Department of Civil Engineering, Indian Institute of Technology, Roorkee 247667, India


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