Aerodynamic Parameters of Urban Building Arrays with Random Geometries

Abstract

It is difficult to describe the flow characteristics within and above urban canopies using only geometrical parameters such as plan area index (λ p ) and frontal area index (λ f ) because urban surfaces comprise buildings with random layouts, shapes, and heights. Furthermore, two types of 'randomness' are associated with the geometry of building arrays: the randomness of element heights (vertical) and that of the rotation angles of each block (horizontal). In this study, wind-tunnel experiments were conducted on seven types of urban building arrays with various roughness packing densities to measure the bulk drag coefficient (C d ) and mean wind profile; aerodynamic parameters such as roughness length (z o ) and displacement height (d) were also estimated. The results are compared with previous results from regular arrays having neither 'vertical' nor 'horizontal' randomness. In vertical random arrays, the plot of C d and z o versus λ f exhibited a monotonic increase, and z o increased by a factor of almost two for λ f = 48-70%. C d was strongly influenced by the standard deviation of the height of blocks (σ ) when λ p ≥ 17%, whereas C d was independent of σ when λ p = 7%. In the case of horizontal random arrays, the plot of the estimated C d against λ f showed a peak. The effect of both vertical and horizontal randomness of the layout on aerodynamic parameters can be explained by the structure of the vortices around the blocks; the aspect ratio of the block is an appropriate index for the estimation of such features.

Evaluation of Urban Local-Scale Aerodynamic Parameters: Implications for the Vertical Profile of Wind Speed and for Source Areas

“…This suggests z d can be greater than the average roughness-element height (e.g. Jiang et al 2008;Xie et al 2008;Hagishima et al 2009;Zaki et al 2011;Millward-Hopkins et al 2011;Tanaka et al 2011;Kanda et al 2013), with a peak z 0 up to five times greater and displaced to higher λ f (Hagishima et al 2009;Zaki et al 2011). Roughness-element staggering, orientation and most importantly height heterogeneity therefore need to be considered in morphometric calculations; especially in complex city centres, such as the current study site (Sect.…”

“…Staggered and non-uniformly oriented groups of roughness elements generate a larger drag force than regular arrays, causing a more pronounced peak in z 0 , as well as larger values of z d (Macdonald 2000;Cheng et al 2007;Hagishima et al 2009;Zaki et al 2011;Claus et al 2012). Roughness-element height variability also influences flow and turbulent characteristics, as the taller roughness elements generate a disproportionate amount of drag (Xie et al 2008;Mohammad et al 2015b).…”

“…The Kan method uses large-eddy simulations for real urban areas in Japan (107 grid squares of size 1000 m (x) by 1000 m (y) by 600 m (z) with a 2-m resolution) and 23 simple arrays from the literature (Cheng et al 2007;Hagishima et al 2009;Leonardi and Castro 2010;Zaki et al 2011). Horizontally-averaged turbulent statistics, surface drag and wind-speed profiles were derived for each model grid and aerodynamic parameters determined through a least squares regression.…”

Zero-Plane Displacement Height in a Highly Built-Up Area of Tokyo

“…d is the point of effect for wind drag (Jackson 1981), and a higher wind speed at a higher altitude has a greater effect on buildings. Zaki et al (2011) also showed that d is larger than mean roughness height in canopies of heterogeneous height.…”

“…Most morphologic methods are based on the results of experiments with roughness obstacles of homogeneous height. Zaki et al (2011) conducted wind tunnel experiments and showed that height variation would make d larger than in the case of homogeneous roughness. Buildings in Tokyo are much more heterogeneous than those in Europe.…”