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CONTENTS
Volume 1, Number 2, June 1998
 

Abstract
In this paper, the wake structures behind a square cylinder at the Reynolds number of 22,000 are simulated using the large eddy simulation and the main features of the wake structure associated with unsteady vortex-shedding are investigated. The Smagorinsky model is used for parametrization of the subgrid scales. The finite element method with isoparametric linear elements is employed in the computations. Unsteady computations are performed using the explicit method with streamline upwind scheme for the advection term. The time integration incorporates a subcycling strategy. No-slip condition is enforced on the wall surface. A comparative study between two-and three-dimensional computations puts a stress on the three-dimensional effects in turbulent flow simulations. Simulated three-dimensional wake structures are compared with numerical and experimental results reported by other researchers. The results include time-averaged, phase-averaged flow fields and numerically visualized vortex-shedding pattern using streaklines. The results show that dynamics of the vortex-shedding phenomenon are numerically well reproduced using the present method of finite element implementation of large eddy simulation.

Key Words
large eddy simulation; vortex-shedding; finite element method.

Address
Cooperative Institute for Research in Environmental Sciences, University of Colorado/NOAA, CO 80303, U.S.A.

Abstract
In recent years, the effects of a possible climate change have been discussed in regard to wind loading on buildings and structures. Simple scenarios based on the assumption of global warming suggest an in crease of storm intensities and storm frequencies and a possible re-distribution of storm tracks. Among recent publications, some papers seem to verify these scenarios while others deny the influence of climatic change. In an introductory step, the paper tries to re-examine these statements. Based on meteorological observations of a weather station in Germany, the existence of long-term trends and their statistical significance is investigated. The analysis itself is based on a refined model for the wind climate introducing a number of new basic variables. Thus, the numerical values of the design wind loads used in modern codes become more justified from the probabilistic point of view.

Key Words
climate change; extreme value analysis; overloading rist; directional variation; storm duration; design wind load concept.

Address
Ruhr-Universitat Bochum, Aeordynamik im Buawesen, 44780 Bochum, Germany

Abstract
Wind forces on structures are usually schematized by the sum of their mean static part and a nil mean fluctuation generally treated as a stationary process randomly varying in space and time. The multi-variate and multi-dimensional nature of such a process requires a considerable quantity of numerical procedures to carry out the dynamic analysis of the structural response. With the aim of drastically reducing the above computational burden, this paper introduces a method by means of which the external fluctuating wind forces on slender structures and structural elements are schematized by an equivalent process identically coherent in space. This process is identified by a power spectral density function, called the Generalized Equivalent Spectrum, whose expression is given in closed form.

Key Words
atmospheric turbulence; coherence; dynamic response; equivalent spectrum; multi-dimensional process; multi-variate process; wind loading.

Address
DISEG, Department of Structural and Geotechnical Engineering, University of Genova, Via Montallegro 1, 16145 Genova, Italy

Abstract
Torsional flutter occurs at 2D rectangular cylinders with side ratios B/D smaller than about 8 or 10. On the other hand, slender cylinders indicate the occurrence of coupled flutter, which means the coupled derivatives of slender cylinders have more significant role for flutter instability than that of bluffer ones. In this paper, based upon so called

Key Words
flutter instabilities; coupled derivatives.

Address
Department of Global Environment Engineering, Kyoto University, Japan

Abstract
This paper discusses some of the findings arising from long-term monitoring of the wind effects on a transmission tower located on an exposed site in South-West England. Site wind speeds have been measured, together with the foundation loads, at the base of each of the four legs. The results show good correlation between the wind speeds and leg strains (loads) for a given wind direction, as expected, for wind speeds in excess of 10m/s. Comparisons between the measured strains and those determined from the UK Code of Practice for lattice towers (BS8100), for the same wind speed and direction, show that the Code over-estimates most of the measured foundation loads by a moderate amount of about 14% at the higher wind speeds. This tends to confirm the validity of the Code for assessing design foundation loads. A finite element analysis model has been used to examine the dynamic behaviour of the tower and conductor system. This shows that, in the absence of the conductor, the tower alone has similar natural frequencies of approximately 2.2Hz in the both the first(transversal) and second (longitudinal) modes, whilst for the complete system and conductor oscillations dominate, giving similar frequencies of approximately 0.1 Hz for both the first and second modes.

Key Words
transmission tower; wind loading; meteorology; structural analysis.

Address
Department of Civil Engineering, University of Surrey, Guildford GU2 5XH, U.K.


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