influence of thermal effects on the wind field wit

Boundary-LayerMeteorol(2009)131:223–243DOI10.1007/s10546-009-9368-4ARTICLEInfluenceofThermalEffectsontheWindFieldWithintheUrbanEnvironmentR.Dimitrova·Jean-FrançoisSini·K.Richards·M.Schatzmann·M.Weeks·E.PerezGarcía·C.BorregoReceived:23January2008/Accepted:24February2009/Publishedonline:11March2009©SpringerScience+BusinessMediaB.V.2009AbstractMicrometeorologicalconditionsinthevicinityofurbanbuildingsstronglyinfluencetherequirementsthatareimposedonbuildingheatingandcooling.Thegoalofthepresentstudy,carriedoutwithintheAdvanceToolsforRationalEnergyUsetowardsSus-tainability(ATREUS)Europeanresearchnetwork,istheevaluationofthewindfieldaroundbuildingswithwallsheatedbysolarradiation.Twocomputationalfluiddynamics(CFD)codeswerevalidatedagainstextensivewind-tunnelobservationstoassesstheinfluenceofthermaleffectsonmodelperformance.ThecodeselectedfromthisvalidationwasusedtosimulatethewindandtemperaturefieldsforasummerdayinaspecificregionofthecityofLisbon.Forthisstudy,themeteorologicaldataproducedbyanon-hydrostaticmesoscaleatmosphericmodel(MM5)wereusedasboundaryconditionsforaCFDcode,whichwasfurtherappliedtoanalyzetheeffectsoflocalroughnesselementsandthermodynamiccon-ditionsontheairflowaroundbuildings.TheCFDmodellingcanalsoprovidetheinflowparametersforaHeating,VentilationandAirConditioning(HVAC)system,usedtoevaluatethebuildingenergybudgetsandtopredictperformanceoftheair-conditioningsystem.Themainfindingofthepresentthree-dimensionalanalysesisthatthermalforcingassociatedwiththeheatingofbuildingscansignificantlymodifylocalpropertiesoftheairflow.KeywordsCodevalidation·Computationalfluiddynamics·Microscalemeteorologicalmodels·ThermaleffectsR.Dimitrova·J.-F.Sini(B)LaboratoiredeMécaniquedesFluides,EcoleCentraledeNantes,1,ruedelaNoë,BP92101,44321NantesCedex3,Francee-mail:[e-mail]·M.SchatzmannMeteorologicalInstitute,UniversityofHamburg,Hamburg,GermanyM.Weeks·E.PerezGarcía·C.BorregoDepartmentofEnvironmentandPlanning,UniversityofAveiro,Aveiro,PortugalR.DimitrovaCenterforEnvironmentalFluidDynamics,ArizonaStateUniversity,Tempe,AZ,USA123224R.Dimitrovaetal.1IntroductionTheinfluenceofairflowaroundbuildingsandtheeffectsofurbanheatingonlocalmicro-climate(i.e.theurbanheatislandeffect)havebeenaddressedinnumerousstud-ies,withalimitednumberfocussedonthethermaleffectsrelatedtothewallheatingontheflowregimeincloseproximitytobuildings(NakamuraandOke1988;Oke1988;Ruck1993;Mestayeretal.1995;Sinietal.1996;KimandBaik1999,2001;Santamourisetal.1999;Ueharaetal.2000;Loukaetal.2001;Kovar-Panskusetal.2002a,b;Huizhietal.2003;Bohnenstengeletal.2004;Xieetal.2007).Thenumericalresultsobtaineduntilnowremainpredominantlyinconsistentwithfieldandlaboratoryexperiments,whichinvestigatedtheinfluenceofthermalwalleffectsontheflowpatternaroundbuildings.Numericalstudiesofsolarradiationeffectsonthemodificationofclassicalisothermalflowwithinstreetcanyonsshowthattheflowpatternprimarilydependsonthewindspeedattheroof-toplevel,theexposureoftheheatingwall(s),andthecanyonaspectratioH/W,whereWisthestreetcanyonmeanwidthandHisthecharacteristicheightofbuildings.Previousstudiesweremainlyfocusedontwo-dimensionalflowsunderawidevarietyofstreetcanyonaspectratios,coveringdifferenttypesofflowregimes:skimmingflow,wakeinterferenceflowandisolatedroughnessflow(Oke1988).ForcanyonswithanaspectratioH/W≈0.5,thedisturbedairflowhasinsufficientdistancetoreadjustbeforeencounteringthedownwindbuilding,resultinginwakeinterferenceflow.Inthecaseofregularcanyons(H/W≈1),thebulkofthelarge-scaleflowglidesoverthestreetproducingaskimmingflow,whichischaracterisedbytheformationofasinglevortexwithinthecanyon(Hunteretal.1992).Intheskimmingflowregime,whichappearsinrelativelydeepstreetcanyons(H/W1),thetransitionfromaone-vortextoatwo-vortexregime,andfromatwo-vortextoathree-vortexregime,occursastheaspectratioincreases.Differentstreetcanyonaspectratiosthatinfluencetheflow,togetherwiththeheatedwalls,dependonthewinddirection,i.e.leewardorwindward.Theimpactofthevorticesinsidethecanyondependsonthewindspeedattheroof-toplevel,buttheirshapeandstrengthcanbeaffectedbythermaleffectsinducedbydifferentialheatingofthewallsand/orthebottomofthecanyon(Mestayeretal.1995;Sinietal.1996;KimandBaik1999,2001;Loukaetal.2001;Xieetal.2007).Ithasbeenshownthatthethermally-inducedairflowincloseproximitytoaheatedverticalsurfaceeitherstrengthensorweakensamechanicallyforcedflow,producingasinglevortex(H/W=1)ortwocounter-rotatingvortices(H/W=2).Notethatindeepercanyons(H/W3)theflowmodificationismorecomplex.Inthiscase,thehorizontalflowspeedatthegroundlevelistoolowandthereforethermally-drivenupwardmotionsplitsthelowestvortexintheflowformingtwoadjacentcounter-rotatingvortices(Kovar-Panskusetal.2002a;Xieetal.2007).Otherstudiesshowthatincreasesofthewall-airtemperaturedifferenceproduces,insomecases,additionalvortices.Theseresultsarerelatedtothegroundlevelorwindwardwallheat-ing(Mestayeretal.1995;Sinietal.1996;KimandBaik1999,2001),andclearlysuggestthatthecross-canyonvortexstructureisinfluencedbycanyonheating,whichdependsontheheatingwallexposure,thecanyonratio,andthedifferenc