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Article: On the correlation of air and pollutant exchange for street canyons in combined wind-buoyancy-driven flow

TitleOn the correlation of air and pollutant exchange for street canyons in combined wind-buoyancy-driven flow
Authors
KeywordsAir quality
Computational fluid dynamics (CFD)
k - ε{lunate} turbulence model
Reynolds-averaged Navier-Stokes (RANS) equations
Street canyons
Wind-buoyancy-driven flow
Issue Date2009
PublisherPergamon. The Journal's web site is located at http://www.elsevier.com/locate/atmosenv
Citation
Atmospheric Environment, 2009, v. 43 n. 24, p. 3682-3690 How to Cite?
AbstractThe ventilation and pollutant transport in a two-dimensional (2D) street canyon of building-height-to-street-width (aspect) ratio h/b = 1 under different unstable stratifications were examined. To characterize the combined wind-buoyancy-driven flow and pollutant transport at different Richardson number Ri, a computational fluid dynamics (CFD) model based on the Reynolds-averaged Navier-Stokes (RANS) equations with the Renormalization Group (RNG) k - ε{lunate} turbulence model was adopted. Unlike the isothermal condition, a secondary recirculation is initiated at the ground-level windward corner of the street canyon once the unstable stratification is switched on (Ri < 0). It traps the ground-level pollutant leading to elevated pollutant concentration there. As Ri further decreases, the enlarging secondary recirculation enables direct pollutant removal from its core to the shear layer that offsets the ground-level pollutant accumulation. The ventilation and pollutant removal performance under different unstable stratifications are compared by the air (ACH) and pollutant (PCH) exchange rates, and pollutant retention time (τ). Both the mean and turbulent components of ACH are found to increase with decreasing Ri, suggesting that unstable stratification promotes ventilation in street canyons. Moreover, the CFD results agree well with our theoretical model that ACH2 varies linearly with Ri. Turbulent transport originally dominates the pollutant removal under isothermal condition. However, progressive domination of pollutant removal by mean wind can be observed with decreasing stability (decreasing Ri from 0 to -10.6). The critical value is estimated to be Ri = -8, below which mean wind is the major pollutant removal carrier. Reduction in τ is also observed with decreasing Ri. Hence, in unstable stratification, pollutant resides shorter time in the street canyon compared with its isothermal counterpart, and the ventilation and pollutant removal are more favorable. © 2009 Elsevier Ltd. All rights reserved.
Persistent Identifierhttp://hdl.handle.net/10722/59129
ISSN
2015 Impact Factor: 3.459
2015 SCImago Journal Rankings: 1.999
ISI Accession Number ID
Funding AgencyGrant Number
University of Hong Kong200802159008
Funding Information:

This project is partly supported by the University Research Committee Seed Funding Programme of Basic Research 200802159008 of the University of Hong Kong.

References

 

DC FieldValueLanguage
dc.contributor.authorCheng, WCen_HK
dc.contributor.authorLiu, CHen_HK
dc.contributor.authorLeung, DYCen_HK
dc.date.accessioned2010-05-31T03:43:22Z-
dc.date.available2010-05-31T03:43:22Z-
dc.date.issued2009en_HK
dc.identifier.citationAtmospheric Environment, 2009, v. 43 n. 24, p. 3682-3690en_HK
dc.identifier.issn1352-2310en_HK
dc.identifier.urihttp://hdl.handle.net/10722/59129-
dc.description.abstractThe ventilation and pollutant transport in a two-dimensional (2D) street canyon of building-height-to-street-width (aspect) ratio h/b = 1 under different unstable stratifications were examined. To characterize the combined wind-buoyancy-driven flow and pollutant transport at different Richardson number Ri, a computational fluid dynamics (CFD) model based on the Reynolds-averaged Navier-Stokes (RANS) equations with the Renormalization Group (RNG) k - ε{lunate} turbulence model was adopted. Unlike the isothermal condition, a secondary recirculation is initiated at the ground-level windward corner of the street canyon once the unstable stratification is switched on (Ri < 0). It traps the ground-level pollutant leading to elevated pollutant concentration there. As Ri further decreases, the enlarging secondary recirculation enables direct pollutant removal from its core to the shear layer that offsets the ground-level pollutant accumulation. The ventilation and pollutant removal performance under different unstable stratifications are compared by the air (ACH) and pollutant (PCH) exchange rates, and pollutant retention time (τ). Both the mean and turbulent components of ACH are found to increase with decreasing Ri, suggesting that unstable stratification promotes ventilation in street canyons. Moreover, the CFD results agree well with our theoretical model that ACH2 varies linearly with Ri. Turbulent transport originally dominates the pollutant removal under isothermal condition. However, progressive domination of pollutant removal by mean wind can be observed with decreasing stability (decreasing Ri from 0 to -10.6). The critical value is estimated to be Ri = -8, below which mean wind is the major pollutant removal carrier. Reduction in τ is also observed with decreasing Ri. Hence, in unstable stratification, pollutant resides shorter time in the street canyon compared with its isothermal counterpart, and the ventilation and pollutant removal are more favorable. © 2009 Elsevier Ltd. All rights reserved.en_HK
dc.languageengen_HK
dc.publisherPergamon. The Journal's web site is located at http://www.elsevier.com/locate/atmosenven_HK
dc.relation.ispartofAtmospheric Environmenten_HK
dc.subjectAir qualityen_HK
dc.subjectComputational fluid dynamics (CFD)en_HK
dc.subjectk - ε{lunate} turbulence modelen_HK
dc.subjectReynolds-averaged Navier-Stokes (RANS) equationsen_HK
dc.subjectStreet canyonsen_HK
dc.subjectWind-buoyancy-driven flowen_HK
dc.titleOn the correlation of air and pollutant exchange for street canyons in combined wind-buoyancy-driven flowen_HK
dc.typeArticleen_HK
dc.identifier.openurlhttp://library.hku.hk:4550/resserv?sid=HKU:IR&issn=1352-2310&volume=&spage=&epage=&date=2009&atitle=On+the+correlation+of+air+and+pollutant+exchange+for+street+canyons+in+combined+wind-buoyancy-driven+flowen_HK
dc.identifier.emailLiu, CH:chliu@hkucc.hku.hken_HK
dc.identifier.emailLeung, DYC:ycleung@hku.hken_HK
dc.identifier.authorityLiu, CH=rp00152en_HK
dc.identifier.authorityLeung, DYC=rp00149en_HK
dc.description.naturelink_to_subscribed_fulltext-
dc.identifier.doi10.1016/j.atmosenv.2009.04.054en_HK
dc.identifier.scopuseid_2-s2.0-67649207622en_HK
dc.identifier.hkuros157267en_HK
dc.relation.referenceshttp://www.scopus.com/mlt/select.url?eid=2-s2.0-67649207622&selection=ref&src=s&origin=recordpageen_HK
dc.identifier.volume43en_HK
dc.identifier.issue24en_HK
dc.identifier.spage3682en_HK
dc.identifier.epage3690en_HK
dc.identifier.eissn1873-2844-
dc.identifier.isiWOS:000268609000004-
dc.publisher.placeUnited Kingdomen_HK
dc.identifier.scopusauthoridCheng, WC=24283028300en_HK
dc.identifier.scopusauthoridLiu, CH=36065161300en_HK
dc.identifier.scopusauthoridLeung, DYC=7203002484en_HK

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