Modelling dispersion of traffic pollution in a deep street canyon: Application of CFD and operational models

doi:10.1016/j.atmosenv.2009.01.038

Atmospheric Environment

Volume 43, Issue 14, May 2009, Pages 2303-2311

https://doi.org/10.1016/j.atmosenv.2009.01.038 Get rights and content

Abstract

In this study, numerical modelling of the flow and concentration fields has been undertaken for a deep street canyon in Naples (Italy), having aspect ratio (i.e. ratio of the building height H to the street width W) H/W = 5.7. Two different modelling techniques have been employed: computational fluid dynamics (CFD) and operational dispersion modelling. The CFD simulations have been carried out by using the RNG k–ɛ turbulence model included in the commercial suite FLUENT, while operational modelling has been conducted by means of the WinOSPM model. Concentration fields obtained from model simulations have been compared with experimental data of CO concentrations measured at two vertical locations within the canyon. The CFD results are in good agreement with the experimental data, while poor agreement is observed for the WinOSPM results. This is because WinOSPM was originally developed and tested for street canyons with aspect ratio H/W ≌ 1. Large discrepancies in wind profiles simulated within the canyon are observed between CFD and OSPM models. Therefore, a modification of the wind profile within the canyon is introduced in WinOSPM for extending its applicability to deeper canyons, leading to an improved agreement between modelled and experimental data. Further development of the operational dispersion model is required in order to reproduce the distinct air circulation patterns within deep street canyons.

Introduction

Despite the significant reduction of vehicle emission factors, air pollution in urban areas is still a significant environmental problem mainly associated with road traffic. As a consequence, many recent papers have focused on the modelling and monitoring of air quality in urban areas (Vardoulakis et al., 2003). Air quality models can be divided into two main categories: parametric and numerical models. A description of the main phenomena (fluid dynamics, thermal effects, mass transfer, chemical reaction and turbulence) determining the pollutant dispersion and transformation in urban areas or in a single street requires the use of numerical models at different scales (PHOENICS, FLUENT, MERCURE, MEMO, CALGRID, etc.). However, numerical models are computationally expensive and can only be reliably used by experts. Parametric models (STREET, CPBM, OSPM, etc.) have been developed as relatively simple tools that require less expertise and computational resources. The main problem of parametric models is their validation, as they include several empirical parameters often derived from experimental data. Moreover, the validity of results is limited to street geometries and dispersion conditions similar to those for which the validation was carried out.

One of the barriers for the wide application of operational traffic pollution models is the different structure and characteristics of the urban canopy. If a single street canyon is considered, its geometry can be easily parameterised and only few dimensions have to be considered (street length, width and buildings height, distance between crossroads, roof shape, etc.). The simplification obtained scaling down from the urban to the single street scale helps identify the dominant dispersion processes in built-up areas. However, using a parametric model for a street canyon with geometry different from that for which the model has been originally validated may cause large errors in the assessment of pollutant concentrations.

One of the main parameters characterizing the geometry of a street canyon is the aspect ratio (AR) defined as the ratio of average building height and the average street width AR = H/W. On the basis of the AR ratio, a street canyon can be classified into three main categories: (i) regular street canyon with AR in the range 0.7–1.5, (ii) deep (or narrow) street canyon with AR > 1.5, and (iii) low-rise street canyon with AR < 0.7. In the first case (and for roof-top wind >2.0 m s⁻¹) the wind velocity component vertical to the street axis generates a single large vortex inside the canyon. In the second case, two or more counter-rotating vortices could be formed (Sini et al., 1996). Finally, in the third case, a single large vortex may be formed inside the canyon, with its shape and location depending on the height of the canyon walls and the roof-top wind speed (Vardoulakis et al., 2007). For oblique roof-level wind directions, a spiral wind flow is induced inside the canyon. The flow along the street axis becomes the dominant pollutant transport mechanism for wind direction deviating by only 15° from the perpendicular direction (Savory et al., 2004). Also in this case a difference between regular and deep street canyon still exists because in deep street canyons a large reduction of mean horizontal velocity is observed at street level (Sini et al., 1996). Therefore, assuming the same external conditions (wind speed and direction) the flow field inside a regular street canyon (AR ≅ 1.0) may be quite different from that of a deep street canyon (AR > 1.7).

The increasingly popular way to describe the flow field in a street canyon is by using computational fluid dynamic (CFD) models, while the development of wind tunnel reduced-scale models is less common. Another approach is to solve the continuity and momentum conservation equations through some simplifying assumptions in the case of vortex-like flows (Hotchkiss and Harlow, 1973). The simulation of the flow field in the canyon is the first step in any dispersion model application.

One of the most popular and validated parametric models is the Operational Street Pollution Model (OSPM) (Berkowicz, 2000). OSPM is a practical tool for estimating traffic-related pollution dispersion in urban street canyon-type geometries. The model is based on similar principles as the CPBM-model by Yamartino and Wiengand (1986). The parameterisations of flow and dispersion conditions were empirically derived from extensive analysis of experimental data and model tests. It is important to note that OSPM has been mainly tested for regular street canyons (AR ≅ 1), for which skimming of the flow and vortex formation are observed under perpendicular roof-level wind conditions (Oke, 1988, Sini et al., 1996). Air quality monitoring data are usually available for full-scale regular street canyons with AR ≅ 1 or less (Chan and Kwok, 2000, Baik and Kim, 2002, Vardoulakis et al., 2002, Tsai and Chen, 2004). This geometry is representative of many built-up areas and city centres in continental Europe. WinOSPM (the Windows version of OSPM) has also been successfully tested for two low-rise street canyons (AR ≅ 0.6) in the U.K. (Vardoulakis et al., 2007).

It must be noted that in many old city centres in the Mediterranean area, aspect ratios of narrow busy streets can easily exceed the value of 2, creating very poor dispersion conditions for traffic-related air pollution. In this paper, a deep street canyon (AR = 5.7), representative of many Mediterranean city centre geometries, is studied in detail. The results of a one-week continuous monitoring campaign of carbon monoxide (CO) have been modelled using the CFD code FLUENT and the operational dispersion model WinOSPM with the aim to compare and evaluate the performance of both modeling techniques.

Section snippets

Experimental data

Experimental data have been obtained during a one-week continuous monitoring campaign of CO in Via Nardones, Naples (Italy) (Murena and Favale, 2007). The geometric features of the selected street canyon (Fig. 1) are: W = 5.8 m, H = 33.0 m, so that AR = 5.7. The orientation of the street axis is in the direction 70°–250° (i.e. from E–NE to W–SW). CO concentrations were measured at h₁ = 2.5 m and h₂ = 25.0 m on the south side of the street during the period 14–20 June 2006. During the same period, hourly

Concentration fields

In Fig. 4, a comparison between CO concentrations measured at 2.5 m (south side) and modelled with CFD (south and north side) is reported. Simulation results cover one day of the campaign, the 15th June from 8:00 to 22:00, because CO emission rates in the street canyon were unknown from 0:00 to 8:00 and from 22:00 to 24:00 (traffic flow was not measured during those hours). Fig. 4 shows good correlation between experimental and CFD modelled CO concentrations apart from the value at 16:00. If

Conclusions

CO concentrations monitored within a deep street canyon have been modelled with sufficient accuracy by a CFD model. Extensive use of CFD for modelling pollutant dispersion in deep street canyons is not, however, feasible because of the intrinsic complexity of CFD models and the high computational cost. For this reason, a parametric model (WinOSPM) was tested for the same canyon, but the initial agreement with observed CO concentrations was poor. WinOSPM model has been successfully used to

References (29)

J.J. Baik et al.
On the escape of pollutants from urban street canyons
Atmospheric Environment
(2002)
L.Y. Chan et al.
Vertical dispersion of suspended particulates in urban area of Hong Kong
Atmospheric Environment
(2000)
D. Hamlyn et al.
A numerical study of the flow field and exchange processes within a canopy of urban-type roughness
Atmospheric Environment
(2005)
S.J. Jeong et al.
Application of the k–ɛ turbulence model to the high Reynolds number skimming flow field of an urban street canyon
Atmospheric Environment
(2002)
J. Kim et al.
Effects of inflow turbulence intensity on flow and pollutant dispersion in an urban street canyon
Journal of Wind Engineering and Industrial Aerodynamics
(2003)
J.-J. Kim et al.
A numerical study of the effects of ambient wind direction on flow and dispersion in urban street canyons using the RNG k–ɛ turbulence model
Atmospheric Environment
(2004)
J. Kukkonen et al.
Evaluation of the OSPM model combined with an urban background model against the data measured in 1997 in Runeberg Street, Helsinki
Atmospheric Environment
(2003)
M.O. Letzel et al.
High resolution urban large-eddy simulation studies from street canyon to neighbourhood scale
Atmospheric Environment
(2008)
R.N. Meroney et al.
Study of line source characteristic for 2-D physical modelling of pollutant dispersion in street canyons
Journal of Wind Engineering and Industrial Aerodynamics
(1996)
F. Murena et al.
Continuous monitoring of carbon monoxide in a deep street canyon
Atmospheric Environment
(2007)

F. Murena et al.

Monitoring CO concentration at leeward and windward sides in a deep street canyon

Atmospheric Environment

(2008)

T.R. Oke

Street design and urban canopy layer climate

Energy and Buildings

(1988)

S.V. Patankar et al.

A calculation procedure for heat, mass and momentum transfer in three-dimensional parabolic flows

International Journal of Heat and Mass Transfer

(1972)

J.F. Sini et al.

Pollutant dispersion and thermal effects in urban street canyons

Atmospheric Environment

(1996)

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Modelling dispersion of traffic pollution in a deep street canyon: Application of CFD and operational models

Abstract

Introduction

Section snippets

Experimental data

Concentration fields

Conclusions

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Journal of Wind Engineering and Industrial Aerodynamics

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Journal of Wind Engineering and Industrial Aerodynamics

Atmospheric Environment

Atmospheric Environment

Energy and Buildings

International Journal of Heat and Mass Transfer

Atmospheric Environment