Modelling Street-Scale Flow and Dispersion in Realistic Winds—Towards Coupling with Mesoscale Meteorological Models

Abstract

Further to our previous work -simulations of flow and dispersion in an oblique wind over the DAPPLE site (Xie ZT & Castro IP, Atmospheric Environment, 2009, Vol.43, 2174-2185)-large-eddy simulations of flows and dispersion over the same site in a wind perpendicular to Marylebone Road and the windward surfaces of most of the buildings were performed. The DAPPLE site is located at the intersection of Marylebone Road and Gloucester Place in central London. In order to investigate the effects of wind direction on flows and dispersion, the velocity and scalar fields in the perpendicular wind were compared with those in the oblique wind. Furthermore, realistic wind conditions measured on the BT Tower at 190 m above street level were processed and used to drive the numerical simulations of flows and dispersion at the DAPPLE site. This leads to significant predictive improvements of the dispersion compared with field measurements, which provides validation and confidence for coupling mesoscale meteorological models, e.g. the UK Met Office's Unified Model and the NCAR's Weather Research & Forecasting Model, with the street-scale large-eddy simulation of urban environments.

Wind-Direction Effects on Urban-Type Flows

“…Overall, the canopy flow tends to be directed along the streets, although when these are of very limited extent there are often very significant changes in local wind direction with height. The finding that near the bottom of the canopy average wind directions can be largely independent of the wind direction aloft is very similar to the recent result of Xie (2011) in the context of the DAPPLE field site in central London (Dobre et al 2005). He found that wind directions of both 90 • (along the Marylebone Road) and 51 • (quite close to the prevailing south-west wind direction) led to very similar near-surface average wind directions.…”

CFD simulation of near-field pollutant dispersion in the urban environment: A review of current modeling techniques

“…According to previous studies comparing RANS and LES for dispersion modeling around buildings (Xie and Castro, 2009;Dejoan et al, 2010;Santiago et al, 2010;Tominaga and Stathopoulos, 2010, 2011Salim et al, 2011;Gousseau et al, 2011a,b), LES gives better results than RANS for calculating the distribution of concentration, although the difference between LES and RANS results for mean velocity is not as large. This is because the horizontal and vertical diffusions of concentration are reproduced well by LES, due mainly to the reproduction of unsteady concentration fluctuations around the building in comparison to RANS computations.…”

“…In several studies, CFD has been applied to pollutant dispersion around actual building groups in urban areas based on RANS (Moon et al, 1997;Hanna et al, 2006;Tseng et al, 2006;Patnaik et al, 2007;Neofytou et al, 2008;Baik et al, 2009;Zhang et al, 2009;Pontiggia et al, 2010Pontiggia et al, , 2011 and LES (Xie and Castro, 2009;Xie, 2011;Liu et al, 2011). Although reasonable qualitative outcomes have been obtained from these studies, the results must be evaluated quantitatively.…”

“…They found that the five CFD models they compared produce similar wind flow patterns, as well as good agreement with winds observed during a field experiment. Recently, a relevant tracer dispersion experiment conducted in London has been referred to in several CFD simulations (Xie and Castro, 2009;Xie, 2011;Xie et al, 2013). The London experiment formed part of the Engineering and Physical Sciences Research Council (EPSRC), United Kingdom-funded Dispersion of Air Pollution and its Penetration into the Local Environment (DAPPLE) project, which involves field measurements (Arnold et al, 2004;Wood et al, 2009), wind-tunnel experiments (Carpentieri et al, 2009), computational modeling, meteorological measurements, and model development and evaluation.…”

Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

“…Many of the most detailed numerical studies have been limited to flow perpendicular to a group of regular obstacles [66]. Some recent numerical studies have attempted to address this lack of realistic incoming wind and turbulence (e.g., [67,68]). With the development of computational resources, more sophisticated CFD models (either RANS or LES), e.g., a 3D version of the model from Ca et al [45] with complete treatment of solar radiation, heat transfer and shadowing and radiative trapping effects, should be developed to provide more comprehensive and realistic information of the urban flow and dispersion.…”