Utilising Calibration to Quality Assure CFD Models for ... · Accordingly, calibrating the CFD modelling by using temperature readings from within this space relates directly with

Proceedingsof9thWindsorConference:MakingComfortRelevantCumberlandLodge,Windsor,UK,7-10April2016.NetworkforComfortandEnergyUseinBuildings,http://nceub.org.uk

UtilisingCalibrationtoQualityAssureCFDModelsforPredictingThermalComfortinNaturallyVentilatedBuildingsDesignedforHighOccupancies

JuliaThompson1,Dr.MichaelDonn2,andEProfGeorgeBaird3.

1VictoriaUniversityofWellington,SchoolofArchitectureandDesign,[email protected]([email protected])3([email protected])

AbstractThispaperdiscussesapplicationofComputationalFluidDynamics(CFD)softwaretopredictoccupantcomfortin large auditoria. CFD simulates three-dimensional airflow within a space. The modelling process isdocumented for a case study ofWellington’s Opera House, New Zealand. Constructed in 1913, the OperaHouse was designed to ventilate without mechanical assistance while seating 2,000 people. Original plansshow a sliding roof and sliding ceiling system above themain seating. This system is no longer in use and,despitecomfortcomplaints,nomechanicalsystemhasbeenadded.Anupgradetothenon-existentventilationsystemintheOperaHouseisimminent.TocalibratetheCFDsimulationoftheexistingsituationtemperaturesensors were located throughout the auditorium. Recordings from awarm eveningwhen the Buildingwasempty and real time external data from theWellingtonWeather Station provide the calibration data. TheabilityofaCFDmodel to reproduce the stratifiedairflowwithin the spacehasbeenassessedbycomparingpredictedtomeasuredtemperatures.ThegoalofthisexerciseistoqualityassuretheCFDsimulationtobuildareliablemodel,andthereforeenableadegreeoftrustforupcomingiterationswithmeasureddatafromtheforthcomingArtsFestivalwhen theauditorium is in regularuse.Thegoal is toprovidea toolwithwhich toexploreoccupancycomfortifnaturalventilationisreintroducedtotheOperaHouse.

Keywords:CFD,NaturalVentilation,Simulation,Calibration,WellingtonOperaHouse

1 IntroductionLarge, institutional, unreinforced masonry buildings designed in the early 1900s areprevalentthroughoutNewZealand’stownsandcities.Withriskofseismicactivity,manyofthese high occupancy buildings are scheduled for strengthening renovations. The seismicstrengtheningprocess createsanopportunity to improveotheraspectsof thesebuildingssimultaneously, such as internal environment quality and therefore occupancy comfort.Manyofthesebuildingsarefromanerawhennaturalventilationwasapartoftheirdesign,butthathassincebeencompromisedorreplacedwithmechanicalsystems.Understandingthe intended airflow of these original systems may enable their restoration. A secondincentivefordevelopingthisunderstanding isthepotentialthatabetterunderstandingofthesetraditionalsolutionsmayenablemoreeffectivedesignofnaturalventilationsystemsfornewbuildingswithasimilarpurposeandscale.

ComputationalFluidDynamics(CFD)softwarehasthepotential topredictairflowwithinaspace,allowingasimulatedanalysisofanexistingbuilding’sairflow.InordertoutiliseCFDsimulation in theearly stagesofdesign, adegreeof trust isneeded regarding the results

Windsor Conference 2016 - Making Comfort Relevant - Proceedings 361 of 1332

produced by the software. Before design iterations can be tested, in order to alter andultimately improve the airflowwithin a building, the existing situationmust be simulatedandcalibratedagainstreality.

Thispaperdocumentstheprocessofcalibrationfortheexistingsituationofonecasestudy,Wellington’sOperaHouse.WhilethisstudyisbasedonCFDanalysis,ultimatelythemeasureofsuccessforsubsequentdesignalterationsisthecomfortofpeoplewithintheauditorium.Accordingly,calibratingtheCFDmodellingbyusingtemperaturereadings fromwithinthisspacerelatesdirectlywiththepurposeoftheCFDmodellingprocess,andallowscalibrationutilisingameasurethatismorefeasibletoassessthanairmovement.

1.1 TheCaseStudy–Wellington’sOperaHouseLocated inNewZealand’scapitalcity,Wellington’sOperaHousewasconstructed in1913.Originallydesignedwithacapacityof2,141people,theOperaHouseboastedaninnovativeslidingroofandceilingsystemdesignedbyAustralianarchitectWilliamPitt(Sawyer,2009).

TheOperaHouseisachallengingcasestudy,duetoitshighoccupancy,complexdesign,andheritagevalue.ThefreshairdesignwassimilartootherbuildingsinAustralia,Europe,andNorthAmericaaroundthistime.OriginalplansoftheOperaHouseshowtheslidingroofandceiling systemabove themain seatingarea, as canbe seen in figure1. This voidwas theBuilding’smainopeningtofreshair.

Within15yearsofthebuilding’sconstructionthissystemwasnolongerinuse;theslidingroof was rumoured to have only ever opened once (Adnett, 1927). The reason for thisinnovativesystem’slackofuseisnotdocumented,andthereforeitisunknownwhetheritwasasuccessfulventilationsystem.

Since construction, several renovations have been undertaken, including three stages ofmajor seismic strengthening between 1977 and 1982. Existing penetrations in theauditoriumwere covered in or reduced to improve the structural integrity (Christianson,1983).Thehatchinginfigure2belowshowsthepenetrationsintheoriginalconstructionoftheOperaHousethatwereblockedduringthe1977-85upgrades.TheserenovationshavemaskedthenaturalventilationsystemsexpectedtobeincludedinaPittdesignofthisera.

The existing Opera House has a capacity of 1,381 people, and is on theWellington CityCouncil’slistofearthquakedamagepronebuildings(PWV,2015).Therearethreelevelsofseating; the stalls, dress circle, and grand circle. The Building is currently in use forperformances. The Opera House has an impending seismic renovation, easy accessibility,and a history of an innovative natural ventilation system. Therefore, a trustworthycalibratedCFDsimulationoftheexistingOperaHouseispotentiallyuseful.


Figure1.LongitudinalSectionoftheoriginalplansfortheOperaHouse,showingtheslidingroofandceilingsystem(Pitt,1913)

Figure2.Strengtheninglayoutsofthe1977-85seismicrenovationsoftheWellingtonOperaHouse(Christianson,1983)

2 MethodologyThe tolerance allowance for calibrating simulatedmodelswith reality has been explored,andtheprocessofpresimulationcalibrationselected.Temperatureswerethenrecordedatvarious locations in the Opera House building, and a subsequent simulation model wascreatedusingboundaryconditionsfromtherecordingprocess.

Throughoutthis investigation,CFDanalysiswasundertakenusingtheselectedsoftwareofAutodesk’s Simulation CFD. Initial simulations of the original design of the Opera House,including the external opening in the ceiling and roof, showed airmovement downwardsintothespacethroughtheroofopening(figure3).Thedirectionofairflowintheseinitialexercises raisedquestions regarding thepotential performanceof this ventilation system,andaccuracyofthesimulationmodelling.Clearly,acalibrationprocesswasneeded.


Figure3.InitialCFDsimulationoftheoriginaldesignoftheOperaHouse,showingairmovemententeringthroughtheroofandceilingvoid

2.1 CalibrationTolerancesIn order to compare modelled data with measured data for long term hour-by-hoursimulations, statistical comparison techniques are often used to determine whether themodelledoutputsarewithinacceptablecalibrationtolerances(ASHRAE,2002).

TheMeanBiasError(MBE)andtheCoefficientofVariationoftheRootMeanSquaredError(CV(RMSE) are the two statistical indices used to determine compliance with calibrationtolerances.AccordingtoASHRAEGuideline14(2002),amodel isdeclaredtobecalibratedwhenitwillproduce:

- MBEwithin±10%,andCV(RMSE)within±30%,whenusinghourlydata,

- MBEwithin±5%,andCV(RMSE)within±15%,whenusingmonthlydata.

TheMBEmeasureshowcloselymodelledenergyconsumptionalignswithreality.However,ifsomehoursofmodelleddataexceedthemeasureddataandotherhoursarelessthanthemeasured data, the errors can become offset. Consequently, the CV(RMSE) is needed todetermine the proportion of variation that occurs between the modelled and measureddata(ASHRAE,2002).

The ‘Measurement and Verification Guidelines’ of the U.S. Department of Energy statecalibration tolerances can be set on an individual project basis, dependent on theappropriatelevelofeffortrequiredtoalignanenergymodelwithreality(Nexant,Inc.,2008).Minimumacceptablecalibrationtoleranceshowever,giveninASHRAEGuideline14,arealsoreferredtobytheU.S.DepartmentofEnergy(ASHRAE,2002).

The issuefor thisproject is thatCFDmodelsrepresentadynamicairflowasasinglemassbalancewithinaspace.Calibrationrequiressomeknowledgeofthe‘boundaryconditions’.Inthiscase,knowledgeofhowmuchairiscominginfromoutside,andgoingoutelsewherethroughthefabric;howmuchheatgainthere is intheauditorium(ifthere isanaudienceand the lighting ison forexample);andwhatare the temperaturesof thesurfaces in thespace.Choosingfirst,measurementsinanemptybuildingfromacalmdaysothepressuresdrivingtheairflowinandoutofthebuildingaremerelytemperaturedifferences,enablesa


simplemodel calibration. Choosing next awindy day,when externalwind speeds can bemeasured (fromwind tunnel tests and fromother CFD analyses) enables a second,morecomplexmodel to be calibrated. Choosing then to add complexity of internal heat gainsfrompeopleandlights,andtoexaminethetimebasedmatchofchangesastheauditoriummovesfromemptytofull,isthefinalstageofcalibration.

As thewholeprocess isaseriesofsinglecasestudies, theASHRAEGuideline14doesnotoutlineaparticularly suitableprocess as it focuseson simulationswherehundreds, if notthousands of hours of data for a year are available.What is proposed, in the absenceofother independently verified guidelines is that initially the calibration goal shouldbe thatthe average deviation of all temperature sensors together is less than 10%, and that noindividualsensorshouldbemorethan30%‘out’.

2.2 TemperatureRecordingMethodologyThemodelof theOperaHousehadnotpreviouslybeensimulated; thereforeaprocessofpre-simulationcalibrationwasundertaken.Pre-simulationcalibration involvesupdatinganexisting model or creating a new model informed by measured data (Raftery, Keane, &Costa,2009).Themodellingtakesintoconsiderationwinddrivenventilationduetoexternalwindspeedsandtemperatures,aswellasbuoyancydrivenventilationfromtheheatgainofoccupants.Therefore, thefollowingfactorsneedtobeconsideredduringtherecordingofmeasurements,sotheycanbereplicatedforsubsequentsimulations:

§ Externalweatherconditions:- Windspeed,- Winddirection,- Temperature.

§ Occupancynumbers,§ Locationofmeasurementpoints.

CFDmodellingisatoolusedtoassessairflowwithinaspace,howeverthemovementofairandquantity incoming from theexternal environmentalsodictate temperaturewithinanunconditionedinternalenvironment.WhilemeasuringairflowdirectlyispossiblewithlaserDoppler anemometry, the cost of and lack of accessibility to this equipment is a majorlimitation.Asanalternative,temperature-measuringdeviceswereavailablewiththeabilitytorecordinformationindifferentlocationsthroughoutthespacesimultaneouslyandoveralongperiod,allowingforamorecomprehensiveandthree-dimensionalpictureofthespacetobeanalysed.

The temperature sensors used were Testo Loggers 175-H2. Fifteen of these units werelocatedthroughouttheauditoriumspaceoftheOperaHouse,asseeninfigure4below.Thelocations of the sensors were selected due to the anticipated airflows in the space andrelevancetooccupancycomfort: intheroofspace, inthestagearea,andatthefrontandback of each of the three layers of seating, and beside a representative selection of themajoropeningstothefoyer,whichinturnopenstotheoutdoors.


Figure4.ThelocationsofthetemperaturerecordingdeviceswithintheOperaHouse


Forthepurposeofthisinitialcalibrationfeasibilitytest,thedevicesremainedinthebuildingfor4days,andthedatausedforanalysiswereselectedduringtheunoccupiedstateofthebuilding.

2.3 ModellingMethodology–RevitGeometryThe basemodel for CFD analysis was created using Autodesk Revit software. As seen infigure 5, the geometry of the space has been simplified and internal elements were notincluded.

Figure5.AxonometricandlongitudinalsectionviewsoftheRevitModeloftheOperaHouseusedasthebasefortheCFDSimulationanalysis

Themodel isasimplifiedabstractionofreality,reflectingelements includingthethicknessofthewalls,locationofopenings,angleofseatinglevels,andconstructiontype.Thisallowstheprocessofcalibrationtobeundertakenmoreefficiently.Increasingcomplexitywithinamodel not only extends simulation time but, also makes it difficult to attributeinconsistenciesbetweenrealityandsimulationresultstospecificelements.TheaimoftheRevitgeometryistocreatearepresentativemodel,whichwillproducebelievableresultsinaCFDanalysis,minimisingmodellingtimeaswellassimulationtime.Thismodellingprocessalso allows new elements such as seating to be added as they are required, should itbecome necessary to add elements to evaluate whether these elements help align theresultsmorecloselywithreality.

2.4 ModellingMethodology–simulatedinAutodeskSimulationCFDAutodesk’sSimulationCFDhasbeenselected,as it isaccessible to industryand isable tomodel wind-driven, and buoyancy-driven internal natural ventilation. This software canimport from external geometry modelling programmes such as Revit, is comparativelyreasonablypriced,andhasminimallearningtimewithlearningsupportavailable(Autodesk,2015).

2.4.1 BoundaryConditionsInordertomodeltheboundaryconditionscloselywithrealweatherconditionsduringthetimethemeasurementswereundertaken,informationhasbeenusedfromtheWellingtonWeather Station (Windfinder, 2016). The external wind speed and air temperature areneeded to inform the modelling. The temperature from the weather station has beendirectlyappliedastheambienttemperatureoftheCFDsimulation.


2.4.2 WindInordertopredicttheaveragewindspeedonsiteatthetimeoftherecordings,theratiobetween theaveragewind speedat the locationof theOperaHouseand theairporthasbeen interpreted from a 2004 wind tunnel study completed by Opus InternationalConsultants. This study is themost recent tohavebeenundertaken in the vicinity of theOperaHouse,anareaofWellingtonwhereverylittledevelopmenthasoccurredinthelast12yearsthatwouldaffecttheairflow.

Forthehourofthe31stJanuaryat6pm,thewindspeedrecordedatWellingtonAirportwas7.2m/sat190degrees(south).ThepointonthesouthernsideoftheOperaHousebuildingfromthe2004windtunnel test reportshows3.0m/saveragewindspeedrecordedat thelocation for a 21m/s speedwind blowing from185 degrees at the Airport; and a 5.2m/saveragewindspeedrecordedfora22m/sspeedwindfrom200degreesattheAirport. Inorder to estimate the wind from 190 degrees, the assumption of 3.73m/s at site for a21.33m/swindattheAirporthasbeenmade.Thus,thewindatthesiteispredictedas17.5%ofthewindattheairport.

Accordingly, 7.2m/s at 190 degrees at theWellington Airport equates to 1.26m/s at 190degreesonsite.ThisvaluehasbeenusedintheSimulationCFDmodellingprocess.

3 Results–TemperatureRecordingDevicesThetemperatureloggingdevicesremainedintheOperaHouseforfourdaysforthisinitialtrial.Asseen in figure6belowdue to the thermalmassof thebuilding the temperatureswithinthemainauditoriumspace,excludingtheroofspace,donotchangedramaticallyaredelayedcomparedwiththeexternalconditions.

Figure7showsthetrendofthedifferencebetweentheexternaltemperatureandinternaltemperatures loggedateach location. The two sensors located in the roof spacehad thegreatestdifferencewiththeexternaltemperatureduringthelateafternoon.

Figure6.OverviewofthefourandahalfdaysoftemperaturerecordingsinthespaceoftheOperaHousein

comparisonwithsimultaneousexternaltemperatures


Figure7.Thetrendofthetemperaturedifferencebetweentheexternalconditions,andtheinternaltemperaturerecordings

Due to the steady state of CFD simulation, in order to utilise the data one specific timeneededtobeselected.Tocalibratethesimplemodelandreviewitsalignmentwithrealityan unoccupied timewas selected for this analysis. Therewere no people in the space tocreateheatgain,andthestateoftheauditoriumspaceremainedconstantforthreehourseithersideoftheselecteddatatimepoint.

Table1.ResultsfromtheTestoLoggersfor6pmSundaythe31stJanuary2016,withexternalweatherconditionsof17oCand7.2m/swindat190degrees(south)atWellingtonAirportWeatherStation

Location(referfigure4) DeviceNumber Temperature[ºC]Level1 ONE 20.20Level1 TWO 19.40Level1 THREE 19.80Level1 FIVE 19.80Stage SIX 19.90Level2 SEVEN 20.00Level2 EIGHT 20.30Level2 NINE 20.80Level2 TEN 19.70Level3 ELEVEN 20.40Level3 TWELVE 22.30Level3 THIRTEEN 21.40Level3 FOURTEEN 20.60Roof FIFTEEN 24.40Roof SIXTEEN 27.70


Figure8.ThetemperaturesatvariouspointsrecordedbytheTestoLoggersduringanexternaltemperatureof17oC

The stratificationof air temperatures recordedwithin the spacewhileunoccupied canbeseeninfigure8.Aswasexpectedfromtheanalysis, lowertemperatureswereobservedinlevel1ofthespace,andatthesideoflevel2nearesttheopening.Thebackofthespaceinlevel 3 experienced the highest recorded temperature inside the seated area of theauditoriumduringthisscenario.

4 Results–SimulationCFDAnalysisTheSimulationCFDanalysisoftheOperaHousehasresultedinanevaluationofthespacethatalignswiththetrendshownbytheTestoLoggermeasurements.Asseen in figure10below, stratification of air in the space can be seen, at similar rates and locations to theTestoLoggerresultsseeninfigure8.

Figure9.LongitudinalSectionfromtheCFDSimulationanalysisofthetemperatureswithinthewholebuildingspaceshowingstratificationbetweenthelayersofseating.


Figure10.PerspectivefromtheCFDSimulationanalysisofthetemperatureswithintheauditoriumseatingspace,showingstratificationbetweenthelayersofseatingwithcomparativeresultstoTestoLogger

measurementsshowninFigure8

5 ConclusionThe ability of a CFD model to reproduce stratified airflow within the space has beenassessedbycomparingsimulationresultstomeasuredtemperatures.

TheprocessforcalibratingCFDsimulationoftheexistingsituationoftheOperaHouseforonescenariohasbeenundertaken.Thenextstepinthecalibrationprocesstoensurequalityassurance will include specific analysis of individual measurement locations, as well asfurtherrealtimemeasuringandsimulationmodellingfordifferentweatherconditionsandoccupancy scenarios. Further analysis of the space at difference occupancies, wind andtemperaturesituationsisrequiredinordertofullytrustthemodellingandunderstandthesensitivityofthedifferentelements.However,thefeasibilityoftheprocesshasbeenproveninordertoproduceasimplemodelwhichdisplaysbelievableresults.

ThevalueofthecalibratedCFDmodels is theirpotential for futureapplication,examiningthe feasibility of reintroducing natural ventilation to modern buildings. By following thiscalibrationprocess,theresultsoftheCFDsimulationscanbequalityassuredandthereforeenableadegreeoftrustforupcomingiterations.Thesemodelscanalsobeusedtopredictoccupancycomfort foranyproposedreintroductionofanaturalventilationsystemto theOperaHouse.A furtheroutcomeof thisprocess is toenable investigationofwhether theoriginalslidingroof/ceilingnaturalventilationsystemmighthavebeensuccessful.

AcknowledgmentsPositively Wellington Venues, for permission to place devices in the Wellington OperaHouse.

Opus International Consultants Ltd,Wellington, for providing a wind tunnel study of thecasestudyarea.


Multi-MediaSystemsLtdforaccesstotheWellingtonOperaHouse.

BRANZ,forassistingwithfundingthisresearch.

VictoriaUniversity,Wellington,forprovidingtemperature-recordingdevicesforthestudy.

ReferencesAdnett,S.,1927.File:00001:237:6/1754Pt1.WellingtonCityCouncilArchives,BarkerStreet,Wellington.ASHRAE, 2002.ASHRAEGuideline 14-2002,Measurement of Energy andDemand Savings.Atlanta:AmericanSocietyofHeating,RefrigeratingandAir-ConditioningEngineers,Inc.Autodesk, 2015. Self-Paced Fundamentals Training. 2015 Autodesk Inc, viewed 2015https://knowledge.autodesk.com/support/cfd/learn-explore/caas/CloudHelp/cloudhelp/2014/ENU/SimCFD/files/GUID-C1E9EBEA-7DF-478A-A795-24F337C93DF3-htm.htmlChristianson,J.,1983.StateOperaHouse-Upgrading.BulletinoftheNewZealandNationalSocietyforEarthquakeEngineering,175-178.Nexant, Inc., 2008. M&V Guidelines: Measurement and Verification for Federal EnergyProjects.Boulder:U.S.DepartmentofEnergy.Opus International Consultants Ltd, 2004.Wind Tunnel Study of aModified DevelopmentProposalfor37-39DixonStreet,Wellington.Wellington:OpusInternationalConsultantsLtd.Pitt,W.,1913.ProposedTheatreforWellingtonOperaHouseCo.Ltd.Melbourne.PWV - Positively Wellington Venues, 2015, Opera House. Positively Wellington Venues,viewed2015,http://www.pwv.co.nz/our-venues/opera-house/Raftery, P., Keane, M., & Costa, A., 2009. Calibrating of a Detailed Simulation Model toEnergy Monitoring System Data: A Methodology and Case Study. Eleventh InternationalIBPSAConference(pp.1199-1206).Glasgow,Scotland,2009,Glasgow:InternationalBuildingPerformanceSimulationAssociation.Sawyer, T., 2009. Setting the Scene; The Theatre Drawings ofWilliam Pitt. The La TrobeJournal,No.84,77-91.Windfinder, 2016. Real time wind & weather report; Wellington Airport/Lyall Bay.Windfinder,viewed2016,http://www.windfinder.com/report/wellington


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Utilising Calibration to Quality Assure CFD Models for ... · Accordingly, calibrating the CFD modelling by using temperature readings from within this space relates directly with