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CONTENTS
Volume 12, Number 1, January, 2009
 

Abstract
In this paper, the airflow around an ideal thin plate (hereafter referred to as ITP) with various ratios of central slot is simulated by using the finite-difference-method (FDM)-based Arbitrary-Lagrangian-Eulerian descriptions for the rigid oscillating body. The numerical procedure employs the second-order projection scheme to decouple the governing equations, and the multigrid algorithm with three levels to improve the computational efficiency in evaluating of the pressure equation. The present CFD method is validated through comparing the computed flutter derivatives of the ITP without slot to Theodorsen analytical solutions. Then, the unsteady aerodynamics of the ITP with and without central slot is investigated. It is found that even a smaller ratio of central slot of the ITP has notable effects on pressure distributions of the downstream section, and the pressure distributions on the downstream section will further be significantly affected by the slot ratio and the reduced wind speeds. Continuous increase of with the increase of central slot may be the key feature of the slotted ITP. Finally, flutter analyses based on the flutter derivatives of the slotted ITP are performed, and moreover, flutter instabilities of a scaled sectional model of a twin-deck bridge with various ratios of deck slot are investigated. The results confirm that the central slot is effective to improve bridge flutter stabilities, and that the flutter critical wind speeds increase with the increase of slot ratio.

Key Words
ITP; central slot; CFD; projection-2 scheme; multigrid method; unsteady pressure; flutter.

Address
Zhi-wen Zhu
Center of Wind Engineering, Hunan University, Changsha 410082, China
State Key Laboratory for Disaster Reduction in Civil Engineering,Tongji University, Shanghai 200092, China
Zheng-qing Chen
Center of Wind Engineering, Hunan University, Changsha 410082, China
Ming Gu
State Key Laboratory for Disaster Reduction in Civil Engineering,Tongji University, Shanghai 200092, China

Abstract
The logarithmic form for turbulent flow analysis guarantees the positivity of the turbulence variables as k and ? of the k-? model by using the natural logarithm of these variables. In the present study, the logarithmic form is incorporated into the finite element solution procedure for the unsteady turbulent flow analysis. A backward facing step flow using the standard k-? model and a flow around a 2D square cylinder using the modified k-? model (the Kato-Launder model) are simulated. These results show that the logarithmic form effectively keeps adequate balance of turbulence variables and makes the analysis stable during transient or unsteady processes.

Key Words
logarithmic form; k-? model; finite element method; backward facing step; square-cylinder.

Address
Hiroshi Hasebe
Department of Civil Engineering, CST, Nihon University,Kanda-Surugadai 1-8-14, Chiyoda-ku, Tokyo 101-8308, Japan
Takashi Nomura
Department of Civil Engineering, CST, Nihon University,Kanda-Surugadai 1-8-14, Chiyoda-ku, Tokyo 101-8308, Japan

Abstract
A method is presented to estimate the form drag and the base pressure on a triangular cylinder in the presence of blockage effect. The Strouhal number, which is found to increase with the flow constriction experimentally by Ramamurthy & Ng (1973), may be decoupled from the blockage effect when re-defined by using the velocity at flow separation and a theoretical wake width. By incorporating this wake width into the momentum equation by Maskell (1963) for the confined flow, a relationship between the form drag and the base pressure is derived. Independently, the experimental data of surface pressure from Ramamurthy & Lee (1973) are found to be independent of the blockage effect when expressed in terms of a modified pressure coefficient involving the pressure at separation. Using the potential flow model by Parkinson & Jandali (1970) and its subsequent development in Yeung & Parkinson (2000) for the unconfined flow, a linear relation between the pressure at separation and the form drag is formulated. By solving the two equations simultaneously with a specified blockage ratio and an apex angle of the triangular cylinder, the predictions of the drag and the base pressure are in reasonable agreement with experimental data. A new theoretical relationship for the Strouhal number, pressure drag coefficient and base pressure proposed in this study allows the confinement effect to be appropriately taken into consideration. The present approach may be extended to three-dimensional bluff bodies.

Key Words
blockage effect; bluff bodies; drag; base pressure.

Address
W.W.H. Yeung
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore

Abstract
The vortex-induced vibration of an -shaped bridge deck sectional model is studied in this paper via the wind tunnel experiment. The vibratory behavior of the model shows that there is a transition of the predominant vibration mode from the vertical to the rotational degree of freedom as the wind speed increases gradually or vice versa as the wind speed decreases gradually. The vertical vibration is, however, much weaker in the latter case than in the former. This is a phenomenon which is difficult to model by existing parametric models for vortex-induced vibrations. In order to characterize the aeroelastic property of the -shaped sectional model, a time domain force identification scheme is proposed to identify the time history of the aeroelastic forces. After the application of the proposed method, the resultant fluid forces are re-sampled in dimensionless time domain so that reduced frequency response function (RFRF) can be obtained to explore the properties of the vortex-induced wind forces in reduced frequency domain. The RFRF model is proven effective to characterize the correlation between the wind forces and bridge deck motions, thus can explain the aeroelastic behavior of the -shaped sectional model.

Key Words
force identification; bridge deck sectional model and vortex-shedding.

Address
Xin Zhang
School of Civil Engineering, Zhengzhou University, Zhengzhou, China
Zhanying Qian
Henan Provincial Agency of Quality Supervision for Construction Industry, Zhengzhou, China
Zhen Chen
Henan Provincial Agency of Quality Supervision for Construction Industry, Zhengzhou, China
Fanna Zeng
Henan Provincial Agency of Quality Supervision for Construction Industry, Zhengzhou, China

Abstract
A pulsed impinging jet is used to simulate the gust front of a thunderstorm downburst. This work concentrates on investigating the peak transient loading conditions on a 30 mm cubic model submerged in the simulated downburst flow. The outflow induced pressures are recorded and compared to those from boundary layer and steady wall jet flow. Given that peak winds associated with downburst events are often located in the transient frontal region, the importance of using a non-stationary modelling technique for assessing peak downburst wind loads is highlighted with comparisons.

Key Words
thunderstorm; downburst; gust front; ring vortex loading; non-stationary loads.

Address
M.S. Mason
School of Civil Engineering, University of Sydney, Sydney NSW, Australia
D.L. James
Department of Mechanical Engineering, Texas Tech University, Lubbock TX, USA
C.W. Letchford
School of Engineering, University of Tasmania, Hobart, Tasmania, Australia


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