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CONTENTS
Volume 16, Number 6, June 2013
 

Abstract
The aim of the paper is to use optimization and advanced numerical computation of a sail fiber-reinforced composite model to increase the performance of a yacht under wind action. Designing a composite-shell system against the wind is a very complex problem, which only in the last two decades has been approached by advanced modeling, optimization and computer fluid dynamics (CFDs) based methods. A sail is a tensile structure hoisted on the rig of a yacht, inflated by wind pressure. Our objective is the multiple criteria optimization of a sail, the engine of a yacht, in order to obtain the maximum thrust force for a given load distribution. We will compute the best possible yarn thickness orientation and distribution in order to minimize the total fiber volume with some displacement constraints and in order to leave the most uniform stress distribution over the whole structure. In this paper our attention will be focused on computer simulation, modeling and optimization of a sail-shape mathematical model in different regatta and wind conditions, with the purpose of improving maneuverability and speed made good.

Key Words
wind action; finite element analysis; sail; composites; fiber material

Address
R. Nascimbene : EUCENTRE, Via Ferrata 1, Pavia, Italy

Abstract
Rational Functions are used to express the self-excited aerodynamic forces acting on a flexible structure for use in time-domain flutter analysis. The Rational Function Approximation (RFA) approach involves obtaining of these Rational Functions from the frequency-dependent flutter derivatives by using an approximation. In the past, an algorithm was developed to directly extract these Rational Functions from wind tunnel section model tests in free vibration. In this paper, an algorithm is presented for direct extraction of these Rational Functions from section model tests in forced vibration. The motivation for using forced-vibration method came from the potential use of these Rational Functions to predict aerodynamic loads and response of flexible structures at high wind speeds and in turbulent wind environment. Numerical tests were performed to verify the robustness and performance of the algorithm under different noise levels that are expected in wind tunnel data. Wind tunnel tests in one degree-of-freedom (vertical/torsional) forced vibration were performed on a streamlined bridge deck section model whose Rational Functions were compared with those obtained by free vibration for the same model.

Key Words
flutter analysis; time-domain method; rational function approximation; forced vibration; long-span bridges

Address
Bochao Cao : 2271 Howe Hall, Department of Aerospace Engineering, Iowa State University, Ames, IA, 50011-2271, USA ; Department of Mechanics and Engineering Science, Fudan University, PR China
Partha P. Sarkar: 2271 Howe Hall, Department of Aerospace Engineering, Iowa State University, Ames, IA, 50011-2271, USA

Abstract
Interference effects are of considerable concern for group hyperboloidal cooling towers, but evaluation methods and results are different from each other because of the insufficient understanding on the structure behavior. Therefore, the mechanical performance of hyperboloidal cooling tower shell under wind loads was illustrated according to some basic properties drawn from horizontal rings and cantilever beams. The hyperboloidal cooling tower shell can be regarded as the coupling of horizontal rings and meridian cantilever beams, and this perception is beneficial for understanding the mechanical performance under wind loads. Afterwards, the mean external latitude wind pressure distribution, CP(O), was artificially adjusted to pursue the relationship between different CP(O) and wind-induced responses. It was found that the maximum responses in hyperboloidal cooling tower shell are primarily dominated by the non-uniformity of CP(O) but not the local pressure amplitude CP or overall resistance/drag coefficient CD. In all the internal forces, the maximum amplitude of meridian axial tension shows remarkable sensitivity to the variation of CP(O) and it\'s also the controlling force in structure design, so it was selected as an indicator to evaluate the influence of CP(O) on responses. Based on its sensitivity to different adjustment parameters of CP(O), an comprehensive response influence factor, RIF, was deduced to assess the meridian axial tension for arbitrary CP(O).

Key Words
hyperboloidal cooling towers; latitude wind pressure distribution; mechanical performance; evaluation indicator; response influence factor

Address
Jun-Feng Zhang : School of Civil Engineering, Zhengzhou University, Zhengzhou 450001, China;
State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China
Yao-Jun Ge and Lin Zhao:State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China

Abstract
This study is aimed at formulating an empirical equation for the across-wind fluctuating moment and spectral density coefficient, which are needed to estimate the across-wind dynamic responses of tall buildings, as a function of the side ratios of buildings. In order to estimate an empirical formula, wind tunnel tests were conducted on aero-elastic models of the rectangular prisms with various aspect and side ratios in turbulent boundary layer flows. In this paper, criteria for the across-wind fluctuating moment and spectral density are briefly discussed and the results are analyzed mainly as a function of the side ratios of the buildings. Finally, empirical formulas for the across-wind fluctuating moment coefficient and spectral density coefficient according to variation of the aspect ratio are proposed.

Key Words
tall building; across-wind load; aspect ratio; side ratio; fluctuating moment; power spectral density

Address
Young-Cheol Ha : Department of Architecture, Kumoh National Institute of Technology, Gumi, South Korea

Abstract
In wind load calculations based on pressure measurements, the concept of \'tributary area\' is usually used. The literature has less guidance for a systematic computational methodology for calculating tributary areas, in general, and for scattered pressure taps, in particular. To the best of the authos\'s knowledge, there is no generic mathematical equation that helps calculate the tributary areas for irregular pressure taps. Traditionally, the drawing of tributary boundaries for scattered and intensively distributed taps may not be feasible (a time and resource consuming task). To alleviate this problem, this paper presents a proposed numerical approach for tributary area calculations on rectangular surfaces. The approach makes use of the available coordinates of the pressure taps and the dimensions of the surface. The proposed technique is illustrated by two application examples: first, quasi-regularly distributed pressure taps, and second, taps that have scattered distribution on a rectangular surface. The accuracy and the efficacy of the approach are assessed, and a comparison with a traditional method is presented.

Key Words
tributary area; pressure taps; data interpolation; three-dimensional mesh; delaunay triangulation; MATLAB

Address
Aly Mousaad ALY : Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana, USA

Abstract
This paper reviews the current state-of-the-art in the numerical evaluation of wind loads on buildings. Important aspects of numerical modeling including (i) turbulence modeling, (ii) inflow boundary conditions, (iii) ground surface roughness, (iv) near wall treatments, and (vi) quantification of wind loads using the techniques of computational fluid dynamics (CFD) are summarized. Relative advantages of Large Eddy Simulation (LES) over Reynolds Averaged Navier-Stokes (RANS) and hybrid RANS-LES over LES are discussed based on physical realism and ease of application for wind load evaluation. Overall LES based simulations seem suitable for wind load evaluation. A need for computational wind load validations in comparison with experimental or field data is emphasized. A comparative study among numerical and experimental wind load evaluation on buildings demonstrated generally good agreements on the mean values, but more work is imperative for accurate peak design wind load evaluations. Particularly more research is needed on transient inlet boundaries and near wall modeling related issues.

Key Words
wind loads; building; computational fluid dynamics; turbulence; ABL; RANS; LES; hybrid LES-RANS; and validation

Address
Agerneh K. Dagnew : Laboratory for Wind Engineering Research, International Hurricane Research Center/Department of Civil and Environmental Engineering, Florida International University, Miami, Florida 33174, USA
Girma T. Bitsuamlak: 2Associate Director WindEEE Research Institute, Department of Civil and Environmental Engineering, University of Western Ontario in London, Boundary Layer Wind Tunnel Laboratory Rm. 105, ON, Canada, N6A 5B9


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