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CONTENTS
Volume 22, Number 4, April 2016
 

Abstract
Weibull distribution is a suitable distribution to use in modeling the life time data. It has been found to be a exact fit for the empirical distribution of the wind speed measurement samples. In brief this paper consist of important properties and characters of Weibull distribution. Also we discuss the application of Weibull distribution to wind speed measurements and derive an expression for the probability distribution of the power produced by a wind turbine at a fixed location, so that the modeling problem reduces to collecting data to estimate the three parameters of the Weibull distribution using Maximum likelihood Method.

Key Words
three-parameter Weibull distribution; mean; variance; maximum likelihood method; wind power

Address
C.V. Seshaiah and K. Sukkiramathi: Department of Mathematics, Sri Ramakrishna Engineering College, Coimbatore, 641022, India

Abstract
The mechanical and aerodynamic effect of building shape plays a dominate role in the pedestrian level wind environment. These effects have been presented in numerous studies and are available in many wind codes. However, most studies have focused on wind flow around conventional buildings and are limited to few wind directions. The present study investigated wind circulation in the re-entrant corners of cross-shaped high-rise buildings from various wind directions. The investigation focused on the pedestrian level wind environment in the re-entrant corners with different aspect ratios of building arrangements. Ninety cases of case study arrangements were evaluated using wind tunnel experimentation. The results show that for adequate wind circulation in the re-entrant corners, building orientations and separations play a critical role. Furthermore, in normal wind incident directions and at a high aspect ratio, poor wind flow was observed in the re-entrant corners. Moreover, it was noted that an optimized building orientation and aspect ratio significantly improved the wind flow in re-entrant corners and through passages. In addition, it was observed that oblique wind incident direction increased wind circulation in the re-entrant corners and through passages.

Key Words
wind tunnel experiment; re-entrant corner; wind circulation; wind incident direction

Address
Qureshi M. Zahid Iqbal: Department of Architecture and Civil Engineering, College of Science and Engineering,
City University of Hong Kong, Hong Kong
A.L.S. Chan: Division of Building Science and Technology, College of Science and Engineering,
City University of Hong Kong, Hong Kong



Abstract
In this paper, a shear deformation plate theory based on neutral surface position is developed for free vibration analysis of functionally graded material (FGM) plates. The material properties of the FGM plates are assumed to vary through the thickness of the plate by a simple power-law distribution in terms of the volume fractions of the constituents. During manufacture, defects such as porosities can appear. It is therefore necessary to consider the vibration behavior of FG plates having porosities in this investigation. The proposed theory is based on assumption that the in-plane and transverse displacements consist of bending and shear components, in which the bending components do not contribute toward shear forces and, likewise, the shear components do not contribute toward bending moments. The neutral surface position for a functionally graded plate which its material properties vary in the thickness direction is determined. The equation of motion for FG rectangular plates is obtained through Hamilton\'s principle. The closed form solutions are obtained by using Navier technique, and then fundamental frequencies are found by solving the results of eigenvalue problems. Numerical results are presented and the influences of the volume fraction index and porosity volume fraction on frequencies of FGM plates are clearly discussed.

Key Words
functionally graded material; porosities; free vibration; plate theory; neutral surface position

Address
Fethi Mouaici: Department of Civil Engineering, Material and Hydrology Laboratory, University of Sidi Bel Abbes,
Faculty of Technology, Algeria
Samir Benyoucef and Abdelouahed Tounsi Department of Civil Engineering, Material and Hydrology Laboratory, University of Sidi Bel Abbes,
Faculty of Technology, Algeria;
Laboratoire des Structures et Matériaux Avancés dans le Génie Civil et Travaux Publics, BP 89 Cité Ben M\'hidi, 22000 Sidi Bel Abbes, Algeria

Abstract
In both structural and environmental wind engineering, the vertical variation of wind direction is important as it impacts both the torsional response of the high-rise building and the pedestrian level wind environment. In order to systematically investigate the vertical variation of wind directions (i.e., the so-called \'twist effect\') induced by hills with idealized geometries, a series of wind-tunnel tests was conducted. The length-to-width aspect ratios of the hill models were 1⁄3, 1/2, 1, 2 and 3, and the measurements of both wind speeds and directions were taken on a three-dimensional grid system. From the wind-tunnel tests, it has been found that the direction changes and most prominent at the half height of the hill. On the other hand, the characteristic length of the direction change, has been found to increase when moving from the windward zone into the wake. Based on the wind-tunnel measurements, a descriptive model is proposed to calculate both the horizontal and vertical variations of wind directions. Preliminarily validated against the wind-tunnel measurements, the proposed model has been found to be acceptable to describe the direction changes induced by an idealized hill with an aspect ratio close to 1. For the hills with aspect ratios less than 1, while the description of the vertical variation is still valid, the horizontal description proposed by the model has been found unfit.

Key Words
descriptive model; hill terrain; wind characteristics; wind-tunnel test

Address
A.U. Weerasuriya and K.T. Tse: Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
Z.Z. Hu: Department of Civil Engineering, Tsinghua University and Graduate School at Shenzhen, Tsinghua University, P.R. China
S.W. Li: Division of Ocean Science and Technology, Graduate School at Shenzhen, Tsinghua University, Shenzhen, P.R. China



Abstract
Though hilly topography influences both wind speeds and directions aloft, only the influence on wind speeds, i.e. the speed-up effect, has been thoroughly investigated. Due to the importance of a model showing the spatial variations of wind directions above hilly terrains, it is worthwhile to systematically assess the applicability and limitations of the model describing the influence of hilly topographies on wind directions. Based on wind-tunnel test results, a model, which describes the horizontal and vertical variations of the wind directions separately, has been proposed in a companion paper. CFD (Computational Fluid Dynamics) techniques were employed in the present paper to evaluate the applicability of the proposed model. From the investigation, it has been found that the model is acceptable for describing the vertical variation of wind directions by a shallow hill whose primary-to-secondary axis ratio (aspect ratio) is larger than 1. When the overall hill slope exceeds 20, the proposed model should be used with caution. When the aspect ratio is less than 1, the proposed model is less accurate in predicting the spatial variation of wind directions in the wake zone in a separated flow. In addition, it has been found that local slope of a hill has significant impact on the applicability of the proposed model. Specifically, the proposed model is only applicable when local slope of a hill varies gradually from 0 (at the hill foot) to the maximum value (at the mid-slope point) and then to 0 (at the hill top).

Key Words
computation; topography; wind characteristics; direction changes

Address
S.W. Li: 1Division of Ocean Science and Technology, Graduate School at Shenzhen, Tsinghua University, Shenzhen, P.R. China
Z.Z. Hu: Division of Ocean Science and Technology, Graduate School at Shenzhen, Tsinghua University, Shenzhen, P.R. China;
Department of Civil Engineering, Tsinghua University, Beijing, P.R. China
K.T. Tse and A.U. Weerasuriya: Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong




Abstract
The suitability of Computational Fluid Dynamics (CFD) simulations on the built environment for the purpose of estimating average roughness characteristics and for studying wind flow patterns within the environment is assessed. Urban models of various levels of complexity are considered including an empty domain, array of obstacles arranged in regular and staggered manners, in-homogeneous roughness with multiple patches, a semi-idealized built environment, and finally a real built environment. For each of the test cases, we conducted CFD simulations using RANS turbulence closure and validated the results against appropriate methods: existing empirical formulas for the homogeneous roughness case, empirical wind speed models for the in-homogeneous roughness case, and wind tunnel tests for the semi-idealized built environment case. In general, results obtained from the CFD simulations show good agreement with the corresponding validation methods, thereby, giving further evidence to the suitability of CFD simulations for built environment studies consisting of wide-ranging roughness. This work also provides a comprehensive overview of roughness modeling in CFD-from the simplest approach of modeling roughness implicitly through wall functions to the most elaborate approach of modeling roughness explicitly for the sake of accurate wind flow simulations within the built environment.

Key Words
CFD; built environment; wind flow; roughness; urban models

Address
Daniel S. Abdi and Girma T. Bitsuamlak: Department of Civil and Environment Engineering, University of Western Ontario, London, ON, Canada


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