Wind-Direction Effects on Urban-Type Flows

Abstract

Practically all extant work on flows over obstacle arrays, whether laboratory experiments or numerical modelling, is for cases where the oncoming wind is normal to salient faces of the obstacles. In the field, however, this is rarely the case. Here, simulations of flows at various directions over arrays of cubes representing typical urban canopy regions are presented and discussed. The computations are of both direct numerical simulation and large-eddy simulation type. Attention is concentrated on the differences in the mean flow within the canopy region arising from the different wind directions and the consequent effects on global properties such as the total surface drag, which can change very significantly-by up to a factor of three in some circumstances. It is shown that for a given Reynolds number the typical viscous forces are generally a rather larger fraction of the pressure forces (principally the drag) for non-normal than for normal wind directions and that, dependent on the surface morphology, the average flow direction deep within the canopy can be largely independent of the oncoming wind direction. Even for regular arrays of regular obstacles, a wind direction not normal to the obstacle faces can in general generate a lateral lift force (in the direction normal to the oncoming flow). The results demonstrate this and it is shown how computations in a finite domain with the oncoming flow generated by an appropriate forcing term (e.g. a pressure gradient) then lead inevitably to an oncoming wind direction aloft that is not aligned with the forcing term vector.

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

“…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).…”

Are Urban-Canopy Velocity Profiles Exponential?

“…) along alternate rows in the figure. At right plan view of DIPLOS array of h × 2h × h cuboids (Castro et al 2016) directions and in a half-channel with H = 4h, have also been reported (Claus et al 2012); grid sizes were typically Δ = h/25 over the canopy height. A 'random height' extension of the case of the staggered cube array with λ p = 0.25 was that initially studied experimentally by Cheng and Castro (2002) and subsequently computationally by Xie et al (2008) .…”

Measurements and Computations of Flow in an Urban Street System

“…Note that the lateral force normal to the forcing direction must inevitably be zero in 561 a numerical computation, as explained by Claus et al (2012). We therefore conclude 562 that small numerical inaccuracies are sufficient to produce the asymmetry in W and,…”