Analysis of airflow over building arrays for assessment of urban wind environment Azli Abd Razak a, * , Aya Hagishima b , Naoki Ikegaya b , Jun Tanimoto b a Faculty of Mechanical Engineering, Universiti Teknologi MARA, 40450 Shah Alam Selangor, Malaysia b Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen, Kasuga-shi, Fukuoka 816-8580, Japan article info Article history: Received 21 May 2012 Received in revised form 21 July 2012 Accepted 8 August 2012 Keywords: Large eddy simulation Frontal area ratio Plan area ratio Aspect ratio Pedestrian wind speed abstract Large eddy simulation (LES) of the airflows around various types of block arrays was performed to estimate the pedestrian wind environment. Five types of uniform staggered block arrays with different aspect ratios and an array with a nonuniform height were selected for the simulations. The simulation accuracy was validated by comparing the drag coefficient and wind profiles with those of previous work. The characteristics of the spatially averaged mean wind profiles of the arrays were analyzed on the basis of the calculation results. This study reveals that the frontal area ratio, which is the product of the plan area ratio and building aspect ratio, is the most important parameter in estimating the pedestrian wind environment. In addition, a simple exponential equation was derived for predicting the pedestrian wind speed as a function of the frontal area ratio, which is applicable to various building aspect ratios and amounts of height variability. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Human comfort in a high-density city located at low or mid latitude is affected by recent climate change, such as urban heat island and global warming as well as local or regional air pollution; therefore, it has become a major concern. Since the airflow within an urban canopy layer is a driving force for not only the indoor ventilation of urban buildings but also transfer of heat, water vapor, and other types of scalar variables in urban streets, assessing the effects of urban geometry on the airflow is an important issue for urban planning and building design in order to plan a more comfortable and healthy city. To this end, numerous researches have focused on the relationships between the geometry of an urban array and aerodynamic parameters such as the drag coeffi- cient (C d ), displacement height (d), and aerodynamic roughness length (z o ) [1]. Here, C d is a dimensionless parameter of the total surface shear stress scaled by the kinetic energy of fluid (e.g. [2],), z o and d are included in the logarithmic law for flow speed in an inertial sublayer over a rough surface under neutral condition, and depend on roughness geometry [3]. Experimental results have revealed that C d and z o of urban-like block arrays show peaks against the roughness packing density such as the frontal area ratio (ratio of building frontal area to ground surface area, hereafter l f ) or plan area ratio (ratio of building roof to ground surface area, here- after l p ) [4,5], according to the transition of the flow regime [6]. Another experimental work has reported that arrays with nonuniform heights have different tendencies for C d , z o , and d with respect to the roughness density [7]. Computational simulations of the airflow around block arrays have also contributed to the determination of urban aerodynamic parameters under various geometric conditions [8]. The sectional drag coefficients in urban canopy modeling have also been estimated on the basis of simu- lation results [9]. In addition, it is interesting how turbulent flow characteristics are affected by the urban canopy geometry, which assists in understanding the turbulent exchange process of momentum and scalars near the surface region [10,11]. Furthermore, numerous researches have focused on the flow inside the canopy. The mean flow profiles and mean flow patterns over block arrays have been extensively studied using both wind tunnel experiments and numerical simulations [12e14]. Some studies have shown that the vertical profile of the mean wind velocity is sensitive to l p , and the mean wind speed inside the canopy decreases with l p [8,12,13]. On the other hand, numerical simulations have shown that the mean flow patterns within various block arrays depend on the layout [8,14] and height variability of the blocks [15]. The ventilation in a pedestrian space has also been investigated by numerous researchers from the viewpoint of scalar dispersion. For example, Ikegaya et al. [16] studied the scalar transfer coeffi- cients of the street surfaces of block arrays. Michioka [17] analyzed * Corresponding author. Tel.: þ60 3 5543 6203; fax: þ60 3 55435160. E-mail address: azlirazak@salam.uitm.edu.my (A. Abd Razak). Contents lists available at SciVerse ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv 0360-1323/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.buildenv.2012.08.007 Building and Environment 59 (2013) 56e65