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CONTENTS
Volume 11, Number 4, July 2008
 

Abstract
A number of passive aerodynamic drag reduction methods were applied separately and then in different combinations on an intercity bus model, through wind tunnel studies on a 1:20 scale model of a Mercedes Benz Tourismo 15 RHD intercity bus. Computational fluid dynamics (CFD) modelling was also conducted in parallel to assist with flow visualisation. The commercial CFD package CFXTM was used. It has been found that dramatic reductions in coefficient of drag (CD) of up to 70% can be achieved on the model using tapered and rounded top and side leading edges, and a truncated rear boat-tail. The curved front section allows the airflow to adhere to the bus surfaces for the full length of the vehicle, while the boat-tails reduce the size of the low pressure region at the base of the bus and more importantly, additional pressure recovery occurs and the base pressures rise, reducing drag. It is found that the CFD results show remarkable agreement with experimental results, both in the magnitude of the force coefficients as well as in their trends. An analysis shows that such a reduction in aerodynamic drag could lead to a significant 28% reduction in fuel consumption for a typical bus on intercity or interstate operation. This could translate to a massive dollar savings as well as significant emissions reductions across a fleet. On road tests are recommended.

Key Words
bus; aerodynamics; drag; fuel consumption; vehicle aerodynamics.

Address
Department of Mechanical Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand

Abstract
Large eddy simulations have been performed within and over different types of urban building arrays. This paper adopted three dimensionless parameters, building frontal area density (lf), the variation degree of building height (sh), and the staggered degree of building range (rs), to study the systematic influence of building spacing, height and layout on wind and turbulent characteristics. The following results have been achieved: (1) As lf decrease from 0.25 to 0.18, the mean flow patterns transfer from

Key Words
large eddy simulation; turbulent flow characteristics; urban building arrays; urban meteorology.

Address
Department of Atmospheric Sciences, Nanjing University, Nanjing, 210093, China

Abstract
Three-dimensional engineering simulations of momentum-driven tornado-like vortices are conducted to investigate the flow dynamics dependency on swirl ratio and the possible relation with real tornado Fujita scales. Numerical results are benchmarked against the laboratory experimental results of Baker (1981) for a fixed swirl ratio: S = 0.28. The simulations are then extended for higher swirl ratios up to S = 2 and the variation of the velocity and pressure flow fields are observed. The flow evolves from the formation of a laminar vortex at low swirl ratio to turbulent vortex breakdown, followed by the vortex touch down at higher swirls. The high swirl ratios results are further matched with full scale data from the Spencer, South Dakota F4 tornado of May 30, 1998 (Sarkar, et al. 2005) and approximate velocity and length scales are determined.

Key Words
tornado-like vortices; swirl ratio; Fujita scale.

Address
Alan G. Davenport Wind Engineering Group, The Boundary Layer Wind Tunnel Laboratory, The University of Western Ontario, Faculty of Engineering, London, Ontario, Canada

Abstract
When first commissioned, the 1.6 km span 275kV Severn Crossing Conductor experienced large amplitude vibrations in certain wind conditions, but without ice or rain, leading to flashover between the conductor phases. Wind tunnel tests undertaken at the time identified a major factor was the lift generated in the critical Reynolds number range in skew winds. Despite this insight, and although a practical solution was found by wrapping the cable to change the aerodynamic profile, there remained some uncertainty as to the detailed excitation mechanism. Recent work to address the problem of dry inclined cable galloping on cable-stayed bridges has led to a generalised quasi-steady galloping formulation, including effects of the 3D geometry and changes in the static force coefficients in the critical Reynolds number range. This generalised formulation has been applied to the case of the Severn Crossing Conductor, using data of the static drag and lift coefficients on a section of the stranded cable, from the original wind tunnel tests. Time history analysis has then been used to calculate the amplitudes of steady state vibrations for comparison with the full scale observations. Good agreement has been obtained between the analysis and the site observations, giving increased confidence in the applicability of the generalised galloping formulation and providing insight into the mechanism of galloping of yawed and stranded cables. Application to other cable geometries is also discussed.

Key Words
galloping; critical Reynolds number; yawed cable; stranded cable; transmission line; time history analysis.

Address
Department of Civil Engineering, University of Bristol, Queen\'s Building, University Walk, Bristol, UK

Abstract
In this paper, the along-wind, across-wind as well as torsional dynamic wind loads on three kinds of lattice tower models are investigated using the base balance technique in a boundary layer wind tunnel. The models were specially designed, and their fundamental frequencies in the directions of the three principal axes are still in the frequency range of the spectra of wind loads on lattice towers. In order to clear contaminations to the spectra of wind loads induced by model resonance, the generalized force spectra of the first mode of the models in along-wind, across-wind and torsional directions were derived based on measured base moments of the models. The RMS generalized force coefficients are also obtained by removing the contributions of model resonance. Finally, the characteristics of the 3-D dynamic wind loads, especially those of the across-wind dynamic loads, on the three kinds of lattice towers are presented and discussed.

Key Words
lattice tower; wind load; wind tunnel test; generalized force spectrum.

Address
Lianghao Zou and Shuguo Liang; School of Civil and Building Engineering, Wuhan University, Wuhan, 430072, P.R. China
Q. S. Li; Department of Building and Construction, City University of Hong Kong, Hong Kong
Lin Zhao and Yaojun Ge; Department of Bridge Engineering, Tongji University, Shanghai, 200092, P.R. China

Abstract
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Key Words
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Address
College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310032, P.R. China


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