Numerical study on the effects

BuildingandEnvironment41(2006)94–108NumericalstudyontheeffectsofaspectratioandorientationofanurbanstreetcanyononoutdoorthermalcomfortinhotanddryclimateFaziaAli-Toudert
,HelmutMayerMeteorologicalInstitute,UniversityofFreiburg,Werderring10,D-79085Freiburg,GermanyReceived25October2004;receivedinrevisedform5November2004;accepted4January2005AbstractThispaperdiscussesthecontributionofstreetdesign,i.e.aspectratio(orheight-to-widthratio,H/W)andsolarorientation,towardsthedevelopmentofacomfortablemicroclimateatstreetlevelforpedestrians.Theinvestigationiscarriedoutbyusingthethree-dimensionalnumericalmodelENVI-met,whichsimulatesthemicroclimaticchangeswithinurbanenvironmentsinahighspatialandtemporalresolution.ModelcalculationsarerunforatypicalsummerdayinGhardaia,Algeria(32.401N,3.801E,469ma.s.l.),aregioncharacterizedbyahotanddryclimate.Symmetricalurbancanyons,withvariousheight-to-widthratios(i.e.H/W¼0.5,1,2and4)anddifferentsolarorientations(i.e.E–W,N–S,NE–SWandNW–SE),havebeenstudied.Specialemphasisisplacedonahumanbio-meteorologicalassessmentofthesemicroclimatesbyusingthephysiologicallyequivalenttemperature(PET).Theresultsshowcontrastingpatternsofthermalcomfortbetweenshallowanddeepurbanstreetsaswellasbetweenthevariousorientationsstudied.Acomparisonofallcasestudiesrevealsthatthetimeandperiodofdayduringwhichextremeheatstressoccurs,aswellasthespatialdistributionofPETsatstreetlevel,dependstronglyonaspectratioandstreetorientation.Thisiscrucialsinceitwilldirectlyinfluencethedesignchoicesinrelationtostreetusage,e.g.streetsplannedexclusivelyforpedestrianuseorincludingmotortraffic,andalsothetimeoffrequentationofurbanspaces.Bothinvestigatedurbanfactorscanmitigateextremeheatstressifappropriatelycombined.Thesolaraccessindoorshasbeenbrieflydiscussedasanadditionalcriterionindesigningthestreetbyincludingwinterneedsforsolarenergy.r2005ElsevierLtd.Allrightsreserved.Keywords:Thermalsensation;Pedestriancomfort;Urbandesign;Streetmicroclimate;Aridclimate;ENVI-met;Numericalsimulation;PET1.IntroductionThedesignofastreetisakeyissueinbioclimaticurbandesignmethodology[1,2].Thestreetcanbeseenastheinterfaceofarchitecturalandurbanscalesasitconsistsofthesharedsurfacesbetweenthebuildingandtheopenurbancanopy.Hence,thestreetaffectsbothoutdoorandindoormicroclimates,andasaresultinfluencesthethermalsensationofpeopleaswellastheenergyconsumptioninurbanbuildings.Whileindoorthermalcomfortiswelldocumented,availableknowledgeoncomfortoutdoorsismuchmorelimited.Thislackisprobablyduetothedifferentwaysurbanclimatologistsandurbandesignershavedealt,sofar,withtheissueofunderstandingtherelationshipbetweenbuildingsandurbanclimate.Fortunately,outdoorcomforthasrecentlybecomeasharedissuebetweenbothdisciplineswithclearevidenceofmorecollaboration.Thismaybeobservedinrecentliteratureandrelatedscientificforums,e.g.PLEAandICUCconferences.ARTICLEINPRESS:10.1016/j.buildenv.2005.01.013
Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](F.Ali-Toudert).Themaindifficultyfacedbytheplannerinthedesignofastreetistheconflictbetweentheseasonalneeds.Indeed,thenecessaryprotectionfromthesuninthesummerandtheneedforsolaraccessinthewinterimply,theoretically,compactnessandopennesstothesky,respectively.Oke[1]claimedthata‘‘zoneofcompatibility’’,whichensuresacompromisebetweenapparentlyconflictingobjectivesindesigningastreet,canbefound.However,furtherinvestigationisrequiredforprovidingquantitativeinformationaboutoptimalstreetformsinordertoregulatetheclimatecomfort.2.BackgroundonoutdoorthermalcomfortThemainmicroclimaticfeaturesofanurbanstreetcanyonhavealreadybeenclarified,mainlybystudiesconductedinmid-latitudecities.Thesestudiesfocusedprincipallyontheenergybalance,e.g.[3–6],oronthermalandwindflowmodificationswithinastreetcanyon,e.g.[7–10].Theheight-to-widthratioandstreetorientationwithrespecttothesunwerefoundtohaveagreatinfluenceonthetimingandmagnitudeoftheenergyregimeofindividualurbansurfaces[3].Theamountofheattransferredfromthecanyonfacetstoairassensiblefluxdependsuponthesurfacetemperature,whichisinturndirectlyaffectedbytheexposuretothesun,andthelargertheirradiatedtotalareaofthecanyon,thegreatertheradiantenergydissipated[3,4].Thesurfacetemperatureismarkedlylowerforshadedareas,typicallyfavouredbyhighaspectratios[4,7,8,10].Theairtemperaturewithinastreetcanyonisaffectedbytheenergyregimeoftheurbancanyonsurfaces.However,temperatureofcanyonairshowssmallchangesincomparisontoabove-roofairforcanyonsclosetounity(H/WE1)[3,4,7],exceptadjacenttoirradiatedcanyonsurfaceswhereairisnoticeablywarmer[7].Fordeeperstreets,airstratificationcantakeplaceinthecanyonvolumewiththeairbeingcoolerbyafewdegreesatstreetlevel,e.g.[8,10].Yet,theverticalstratificationofthesurfacetemperaturesismuchhigherthantheairverticalgradient[8].Windflowwithinacanyonisasecondarycirculationdrivenbytheambientairaboveroof[7,8]anditsalterationin-canyondependsstronglyonthevelocityandangleofincidenceofthefreeair.Windspe