10 years of joint research at DLR and TU Braunschweig ... · 10 er i reer TU rueig wr ie irr eig i...

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Submit Manuscript | http://medcraveonline.com Nomenclature Quantities BPR bypass ratio Δh relative distance of noise source to observer location, [m] ΔL level difference, [dB] EPNL effective perceived noise level, [dB] I sound level intensity, [dB] L Aeq time-weighted, equivalent continuous sound pressure level, [dB] L A;max maximum, A-weighted sound pressure level, [dB] SEL sound exposure level, [dB] V flow velocity, [m/s] Abbreviations: CEAS, Council of European Aerospace Societies; DLR, German Aerospace Center; Empa, Swiss Federal Laboratories for Materials Science and Technology; ICAO, International Civil Aviation Organization; PANAM, Parametric Aircraft Noise Analysis Module, DLR noise prediction tool; PrADO, Preliminary Aircraft Design and Optimization, TU BS aircraft design synthesis code; SHADOW, DLR tool for noise shielding calculation; TU BS, Technical University of Braunschweig, Germany Introduction Several non-technical and technical measures to fight aircraft noise have been formulated by the International Civil Aviation Organization (ICAO). 1 The so-called ICAO “Balanced Approach” is derived from the definition of the time weighted, equivalent continuous sound pressure level (L Aeq ). 10 1 10 log 10 10 1 i Aeq i N SEL L g T i = = (1) This equation yields the L Aeq at one location within a characterization time T, e.g., T could be selected as all calendar days within one year. Simulated L Aeq at multiple locations around an airport can directly be translated into isocontour areas and so-called noise protection zones. 2 The predicted level at an arbitrary ground location is directly affected by the so-called non-technical ICAO measures (number of flight events N and time weighting g i of each flight “i”) and the so-called technical ICAO measures (sound exposure level SEL i for each flight “i”). These measures have to be applied and assessed at the same time according to ICAO. In 2007, prior to the joint research activity of the German Aerospace Center (DLR) and the Technical University of Braunschweig, no simulation process had been known to the authors that would allow to investigate the technical measures of the ICAO “Balanced Approach” in sufficient detail, i.e., also allowing to assess new technologies or novel vehicle concepts. Aircraft design synthesis processes were available since decades but did not include noise prediction capabilities. Available ideas toward low-noise aircraft concepts had not been subject to a complete noise immission assessment. It can be summarized, that in 2007 many ideas toward low- noise aircraft concepts had been published, yet the methods to assess the corresponding noise immission along a flight procedure were not available. Consequently, available concepts were not the result of any large design study or parameter variation hence would not represent optimal solutions. Consequently, the focus of the presented research activities of DLR and TU BS was a simulation process for the detail assessment of aircraft design and flight procedure. A new process had Aeron Aero Open Access J. 2019;3(2):89104. 89 ©2019 Bertsch et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and build upon your work non-commercially. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Volume 3 Issue 2 - 2019 Lothar Bertsch, 1 Wolfgang Heinze, 2 Sebastien Guerin, 3 Markus Lummer, 4 Jan Delfs 5 1 German Aerospace Center (DLR), Institute of Aerodynamics and Flow Technology, Gottingen 2 TU Braunschweig (TU BS), Institute of Aircraft Design and Lightweight Structures, Braunschweig 3 German Aerospace Center (DLR), Institute of Propulsion Technology, Berlin 4 German Aerospace Center (DLR), Institute of Aerodynamics and Flow Technology, Braunschweig 5 German Aerospace Center (DLR), Institute of Aerodynamics and Flow Technology, Braunschweig Correspondence: Lothar Bertsch, German Aerospace Center (DLR), Institute of Aerodynamics and Flow Technology, Gottingen, Germany, Email Received: May 08, 2019 | Published: May 31, 2019 Abstract This review article is based on an invited keynote presentation at the 22nd Workshop of the Aeroacoustics Specialists Committee of the CEAS. The event was held in September 2018 in Amsterdam with the main focus on the relation between aircraft design and noise impact. This article reviews the last years of joint research activities between the German Aerospace Center (DLR) and the Technical University of Braunschweig (TU BS) in the field of low-noise aircraft design. The joint research was initiated around 2008 between the DLR Institute of Aerodynamics and Flow Technology and the Institute of Aircraft Design and Lightweight Structures at TU BS. Around that time, DLR was developing a first version of an aircraft noise prediction tool. This tool has then consequently been implemented as a module into the aircraft design synthesis code of the TU BS. In 2012, for the first time, a fully automated and fully parametric aircraft design process with integrated noise prediction capabilities was established, i.e., including a full approach and departure flight simulation. The main focus lies on conventional tube-and-wing aircraft with turbofan or turboprop engine concepts. Ever since 2012, the tools have been under constant development and the simulation chain for low-noise aircraft design has been applied to various aircraft concepts. This article is comprised of a description of the tool development from 2008 until early 2018 and a selected application example. Some lessons learned and a brief outlook on future developments and applications conclude this review. Aeronautics and Aerospace Open Access Journal Review Article Open Access

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Page 1: 10 years of joint research at DLR and TU Braunschweig ... · 10 er i reer TU rueig wr ie irr eig i e aciee 90 Coprig 2019 er e Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years

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NomenclatureQuantities

BPR bypass ratio

Δh relativedistanceofnoisesourcetoobserverlocation,[m]

ΔL leveldifference,[dB]

EPNLeffectiveperceivednoiselevel,[dB]

Isoundlevelintensity,[dB]

LAeq time-weighted,equivalentcontinuoussoundpressure level,[dB]

LA;maxmaximum,A-weightedsoundpressurelevel,[dB]

SELsoundexposurelevel,[dB]

V flowvelocity,[m/s]

Abbreviations: CEAS, Council of European AerospaceSocieties; DLR, German Aerospace Center; Empa, Swiss FederalLaboratories for Materials Science and Technology; ICAO,International Civil Aviation Organization; PANAM, ParametricAircraftNoiseAnalysisModule,DLRnoisepredictiontool;PrADO,PreliminaryAircraftDesignandOptimization,TUBSaircraftdesignsynthesiscode;SHADOW,DLRtoolfornoiseshieldingcalculation;TUBS,TechnicalUniversityofBraunschweig,Germany

IntroductionSeveralnon-technicalandtechnicalmeasurestofightaircraftnoise

havebeenformulatedbytheInternationalCivilAviationOrganization(ICAO).1Theso-calledICAO“BalancedApproach”isderivedfrom

the definition of the time weighted, equivalent continuous soundpressurelevel(LAeq).

10

110 log 10 101

iAeq i

N SELL g

T i

= ⋅ ⋅∑ =

(1)

ThisequationyieldstheLAeqatonelocationwithinacharacterizationtimeT,e.g.,Tcouldbeselectedasallcalendardayswithinoneyear.SimulatedLAeqatmultiplelocationsaroundanairportcandirectlybetranslatedintoisocontourareasandso-callednoiseprotectionzones.2 Thepredictedlevelatanarbitrarygroundlocationisdirectlyaffectedby the so-called non-technical ICAO measures (number of flighteventsNand timeweightinggi ofeachflight“i”)and theso-calledtechnicalICAOmeasures(soundexposurelevelSELiforeachflight“i”).Thesemeasureshavetobeappliedandassessedatthesametimeaccording to ICAO. In 2007, prior to the joint research activity oftheGermanAerospaceCenter (DLR) and theTechnicalUniversityof Braunschweig, no simulation process had been known to theauthorsthatwouldallowtoinvestigatethetechnicalmeasuresoftheICAO“BalancedApproach”insufficientdetail,i.e.,alsoallowingtoassess new technologies or novel vehicle concepts.Aircraft designsynthesisprocesseswereavailablesincedecadesbutdidnotincludenoise prediction capabilities. Available ideas toward low-noiseaircraftconceptshadnotbeensubjecttoacompletenoiseimmissionassessment.Itcanbesummarized,thatin2007manyideastowardlow-noiseaircraftconceptshadbeenpublished,yetthemethodstoassessthecorrespondingnoiseimmissionalongaflightprocedurewerenotavailable.Consequently,availableconceptswerenottheresultofanylargedesignstudyorparametervariationhencewouldnotrepresentoptimalsolutions.Consequently,thefocusofthepresentedresearchactivitiesofDLRandTUBSwasasimulationprocessforthedetailassessmentofaircraftdesignandflightprocedure.Anewprocesshad

Aeron Aero Open Access J. 2019;3(2):89‒104. 89©2019 Bertsch et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and build upon your work non-commercially.

10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve?

Volume 3 Issue 2 - 2019

Lothar Bertsch,1 Wolfgang Heinze,2 Sebastien Guerin,3 Markus Lummer,4 Jan Delfs5

1German Aerospace Center (DLR), Institute of Aerodynamics and Flow Technology, Gottingen2TU Braunschweig (TU BS), Institute of Aircraft Design and Lightweight Structures, Braunschweig3German Aerospace Center (DLR), Institute of Propulsion Technology, Berlin4German Aerospace Center (DLR), Institute of Aerodynamics and Flow Technology, Braunschweig5German Aerospace Center (DLR), Institute of Aerodynamics and Flow Technology, Braunschweig

Correspondence: Lothar Bertsch, German Aerospace Center (DLR), Institute of Aerodynamics and Flow Technology, Gottingen, Germany, Email

Received: May 08, 2019 | Published: May 31, 2019

Abstract

Thisreviewarticleisbasedonaninvitedkeynotepresentationatthe22ndWorkshopoftheAeroacousticsSpecialistsCommitteeoftheCEAS.TheeventwasheldinSeptember2018inAmsterdamwiththemainfocusontherelationbetweenaircraftdesignandnoiseimpact.ThisarticlereviewsthelastyearsofjointresearchactivitiesbetweentheGermanAerospaceCenter (DLR)and theTechnicalUniversityofBraunschweig(TUBS) in thefieldoflow-noiseaircraftdesign.Thejointresearchwasinitiatedaround2008betweentheDLRInstituteofAerodynamicsandFlowTechnologyandtheInstituteofAircraftDesignandLightweightStructuresatTUBS.Aroundthattime,DLRwasdevelopingafirstversionofanaircraftnoisepredictiontool.ThistoolhasthenconsequentlybeenimplementedasamoduleintotheaircraftdesignsynthesiscodeoftheTUBS.In2012,forthefirsttime,afullyautomatedandfullyparametricaircraftdesignprocesswithintegratednoisepredictioncapabilitieswasestablished,i.e.,includingafullapproachanddepartureflightsimulation.Themain focus lies on conventional tube-and-wing aircraftwith turbofan or turbopropengineconcepts.Eversince2012,thetoolshavebeenunderconstantdevelopmentandthesimulationchainforlow-noiseaircraftdesignhasbeenappliedtovariousaircraftconcepts.Thisarticle iscomprisedofadescriptionof the tooldevelopmentfrom2008untilearly2018 and a selected application example. Some lessons learned and a brief outlook onfuturedevelopmentsandapplicationsconcludethisreview.

Aeronautics and Aerospace Open Access Journal

Review Article Open Access

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Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

tobedeveloped inorder to applyandassess these technical ICAOmeasures simultaneously. Although the process was developed toassessICAO’stechnicalmeasures,itisofcourseapplicabletoassesstheremainingnon-technicalmeasuresaswell.

Several tools foroverall aircraftnoisepredictionwereavailableand documented in the literature in 2007, e.g., INM3 or NASA’sANoPP.4,5Dependingontheiroriginandtheircoreareaofapplication,available tools have been separated into two categories, i.e., the“best-practicesimulationtools”andthe“scientificsimulationtools”(e.g., definition according toRef.6).The “best-practice” toolsworkwith extensive correlations of measurement data and are thereforenotdirectlyapplicabletoassessnoveltechnology.Moreover,“best-practice”toolssimulatetheoverallaircraftasonenoisesourcewhichisindirectcontrasttoanynoveltechnologythatcanusuallyonlybeassociatedwithacertainindividualnoisesourcesormechanism.Bysimplysubtractingaconstantdeltalevelfromthe(measured)overallaircraft noise emission in a database, individual modifications tocertainnoisesourcecanobviouslynotbecaptured.Only“scientificsimulationtools”withadetailedparametricmodelingofeachsourcemechanismcanbeappliedtoinvestigatehowaircraftnoiseimmissionis affected by novel technology, flight path modification,1 and/orvehicledesign.

In2007,availableandknownresearchactivitiesinthecontextof“scientificsimulationtools”wereassociatedwithoneormoreofthefollowing shortcomings. First, the acousticswas not available as adesignconstraintwithintheaircraftdesignphase.Onlyasubsequentnoiseassessmentofpredefinedandalreadyfinalconceptshadbeenavailable.Consequently,nodesignvariationsoroptimizationswerepossibleandthesolutionspacewasverylimitedfromthebeginning.The second shortcoming had been the incomplete consideration ofrelevant disciplines or interactions. For example, modifications tocertain aircraft or engine noise components to reduce noise wereassumed without consideration of the impact on other aircraftdisciplinesandontheoverallflightperformance.Thethirdsignificantshortcoming had been the insufficient assessment of the problem.Often,theknownandpublishedresultswereassociatedwithoneoracombinationofthefollowinglimitations.Thenoiseassessmentwaslimitedtotheemissionsituation,toacertainaircraftcomponent,toaspecificnoisemetriconly,tocertificationnoiselevels,and/ortoafixedoperatingcondition.Theselimitationspreventedtheidentificationofoptimalsolutions,couldfalsifyconclusionsduetoinadequatemetricorproblemdefinition,andmostcertainlycouldresultinsolutionsthatarenotvalidduringvaryingoperatingconditionsalonganyrealisticflightwithitscorrespondingnoisesourceranking.

MotivationIn2008itwasdecidedtojoinforcesofDLRandTUBSinthe

field of aircraft design and noise prediction.TheTUBS is knownfor their aircraft designprocessPrADO,7‒9whereasDLRbrings itsmethods,data,andexpertiseinflightsimulation,enginedesign,andcomprehensiveaircraftnoisepredictiontothetable.Theoverallgoalwasdefinedasajointsimulationprocessforlow-noiseaircraftdesign.Basedonthestatusquoasdescribedintheintroduction,certainmainrequirementstowardsuchanovelsimulationprocessweredefined.

Itwasdecidedthatareasonablelow-noisedesignprocesswouldrequirethefollowingcharacteristics:1Thisisespeciallyimportantifatypicalapproachwithitsvaryingnoisesourcedominanceisevaluated.

a) Allrelevantnoisesourcesandmajorinteractioneffectsaretakenintoconsideration.

b) Individual noise sources and their contribution to the overallvehicle noise - a.k.a. noise source breakdown or noise sourceranking - are specifically assessed under varying operatingconditions.

c) Emissionsituationandimmissionalongfullydetailedapproachanddepartureproceduresareevaluated,i.e.,againundervaryingoperatingconditions.

d) Multiple noise metrics at different and widespread groundlocationshavetobeconsidered.

e) Noisepredictionhastobeincorporatedasearlyaspossibleintheaircraftdesignprocess,i.e.,withintheconceptualaircraftdesignphase.

The last characteristic is of utmost importance.Onlywithin theconceptual design, relevant aircraft design parameters, e.g., engineselectionandengineinstallationonboard,canbeinfluencedaccordingto a chosen low-noise constraint. By definition, these basic designparametersaredetermined in theconceptualdesignphaseand thenhavetobekeptfixedthroughallsubsequentdesignphases.Yet,theseparametershaveadirectandmajorinfluenceonthenoisegenerationofthefinalvehicle,i.e.,certaindesignconstraintsdirectlydefinetheacoustic performance of the vehicle. Exemplary design parametersand the overall influence on the noise generation of correspondingcomponents or the overall aircraft are shown in Figure 1. Severalmajor design parameters can be directly assigned to the soundintensityIofcertaincomponentsortheoverallvehicle.Consequently,adirectintegrationofnoisepredictionwithinthedesignprocesshasbeenthemaingoalofajointresearchactivity.

The development of the overall simulation process is describedinthenextsection.Afteraselectedapplicationexample, thearticleisconcludedbyasummarywithlessonslearnedandabriefoutlooksection.

Simulation processThis section about the simulation process is divided into three

chronologicalworkingperiods,i.e.,“bringingthedisciplinestogether”(2007-2008), “toward a fully automated low-noise design process”(2009-2012), and “focus on noise immission” (2013-early 2018).Eachdescription is initiatedwitha timelineofmajordevelopmentsand applications and is concluded with a status overview of thesimulation process at the end of eachworking period (Figure 2, 3and 5).An overview chart with all developments and applications(including corresponding literature references) is provided in theAppendix(Figure5).

ThedescriptionofthefirstworkingperiodstartswithabriefstatusquoatDLRandTUBSintheyear2007.Inspiredbyknownresearchactivities andavailablepublications, e.g.,Ref.,10 the cooperationofDLRandTUBSwas initiated in2008. It isdemonstratedhow therelateddisciplines,i.e.,mainlyaircraftdesignandnoiseassessment,werebroughttogetherasdescribedinRefs.11,12Thesecondworkingperiod dealtwith developments between2009 and2012 leading toan automatedaircraft designprocess asdescribed inRefs.6,13‒22Thethirdworkingperiodstarted in2013. It includedadditionalupdatesandupgradestotheprocesstowarditscurrentstatuswiththefocusonnoiseimmissioninearly2018,seeRefs.23‒39

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10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve?

91Copyright:

©2019 Bertsch et al.

Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

Figure 1 Influence of aircraft design on the final vehicle’s acoustic performance.

Figure 2 Status of simulation capability in 2008 (first year of joint research).

Figure 3 Status of simulation capability in 2012.

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10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve?

92Copyright:

©2019 Bertsch et al.

Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

Figure 4 Fan noise shielding on board of the DLR Low-Noise Aircraft (LNA).

Figure 5 Status quo of simulation capability (early 2018).

“Bringing the disciplines together” (2007-2008)

In 2007 DLR has released a first version of a comprehensiveaircraftnoisepredictiontoolreferredtoasParametricAircraftNoiseAnalysis Module (PANAM) and introduced in Ref.11 The mainfeatureofthispredictiontoolisthecapabilitytopredicttheoverallaircraft immission as a combination of individual noise sourcesunderconsiderationofmovingsourceandsoundpropagationeffects.Furthermore, the predictions account for both operational inputparametersandforaircraftandenginedesignrelatedinput.Therefore,appropriateparametricmodels for relevant individualnoisesourceswere selected and finally incorporated into the tool. The models

capture themost relevantphysics-relatedeffectsbasedonavailableinputdataofadequatecomplexity.

TheselectedairframenoisesourcemodelscomefromDLRandaredescribedinRefs.40‒45 inmoredetail.Theairframeof theinitialPANAM version was comprised of clean wing, flap, slat, spoiler(fully empirical), and landing gear contribution. The engine noiseof conventional turbofan engines can adequately be described byits two dominating noise sources, i.e., fan and jet noise. For bothcontributions, the most commonly accepted models were selected.FannoiseisdescribedwiththeHeidmannfannoisemodelaccordingtoRef.46withsomemodifiedcoefficients tocapturethe2007statusquooffandesignandrelatedtechnologies.Thejetnoiseismodeled

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10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve?

93Copyright:

©2019 Bertsch et al.

Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

with amethodbyStone according toRef.47 Simplemodels for thenoise generation in the engine core, e.g., combustion noise, wereavailable but not accounted for due to the strong dominance of jetandfanonoverallenginenoise.Futurereductionofnowdominatingnoisesourceswillultimatelyincreasetheinfluenceoftheseneglectednoisesources.Therefore,PANAMhasamodularsetupallowingforastraight-forwardimplementationofneworrevisedsourcemodels.TheresultingcomputationalcostsoftheinitialPANAMversionarewellsuitableinthecontextofmultipledesigniterationswithinalargesimulationprocess(Figure2).

All selected source models have in common that the predictednoiseemissionisdefinedbyoperationalandgeometricalparameters,whichcanbesubjecttoalownoiseparametertradestudy.Obviously,theinputparameterrequirementdependsontheselectednoisesourcemodels and this input has to be known in order to initiate a noiseprediction.ForPANAMtheinputdatacanbeassignedtothefollowinggroups: aircraft design parameters, engine design and performanceparameters, operating conditions, i.e., flight data, and parametersdescribingthescenario(flighttrackroutingandselectionofobserverlocations).Allof therequired inputcanbeuserspecified,basedonmeasurements,and/orbegeneratedbyexternalsimulationtools,i.e.,oftenhigh-fidelitysimulationtools.Thescenariosettingsareselectedby the user and defined according to the application scenario, e.g.,reproductionofameasurementsetupforthecomparisonofpredictionversusmeasurement.

The immission results of the prediction process are comprisedof the standardmetrics, e.g., SPL,EPNL, or SEL, at arbitrary andscenario-specific observer arrangements. In addition to these noiseimmission results, the results of all preceding simulation steps canoptionallybestoredandfurtherprocessed,e.g.,foraninvestigationofnoiseemissionfromspecificsourcesalongasimulatedflight.Theresultsofprecedingsimulationstepsincludethetimehistoryofeachpredicted level, i.e., the emission or immission of arbitrary noisesourcesoracombinationofsourcescanbeanimatedovertime.11

As stated in the motivation section, the overall goal of thejoint research is the direct integration of the aforementioned noisepredictionasasimulationmodulewithinanadequateaircraftdesignprocess.SuchamodularprocesswasavailableattheTUBS(withoutconsideration of noise), i.e., the Preliminary Aircraft Design andOptimization code (PrADO).7‒9 PrADO is a design synthesis codethatenablesaniterativeexecutionoftask-relateddesignmodulesuntilpredefineddesignvariableshavereachedconvergence.Theindividualmodulesoperatewithacommondatabaseandcanstraightforwardlybe replacedbymodulesofhigherfidelity that address the identicaltask or discipline. The 2007 version of PrADO had to be updatedsothattherequiredbasicinputforanoisepredictionwithPANAMcouldhavebeengenerated.Yet,notallinputparametersasrequiredbythenoisesourcemodelswereavailablewithintheaircraftdesignprocessatthatpoint.Missingparametersordataofinadequatequalityforthenoisepredictionhadtobemanuallyprocessedfromexternaldatasources.Pre-computedenginedatawasprovidedfromexternalhighly-specializedtools,e.g.DLRtoolVarcycle.48Theflightdatawasnotgeneratedwithinthesimulationprocessatthattimebutalsohadtocomefromexternalsources.Datawasextractedfromaground-basedflightsimulator49inordertorealizeafirstPANAMcomparisonwithactualfly-overmeasurements.11The2008comparisondemonstratedasatisfyingagreementbetweenmeasurementsandpredictions.

Bringing together the disciplines aircraft design and noiseprediction allows to directly identify the parameters that have the

most significant impact on the noise immission. These parameterscan thenbemodifiedunderconsiderationof the implicationsonallother disciplineswithin the design process.A feasible and reliableconclusioncannowbemadebecauseallrelevantimplications,i.e.,so-calledsnowballeffects,areaccountedfor.Lowcomputationalcostsofanyofthesenoisesourcemodelsallowsforaquickoverallassessmentandlargeparametervariationswhichcanultimatelybecombinedintoalow-noisedesignoptimization.Thestatusoftheinitialsimulationprocess in thefirst year of the cooperation is depicted inFigure2,i.e.,modificationstothe2007PANAMVersionarehighlightedinredcolor.Atthispoint,externalinputdatawasanessentialrequirementto execute the simulation, i.e., the flight path (extracted from aground-based flight simulator run or from the flight data recorder)andtheenginedata(externalsimulationtoolVarcycle48).TheinitialapplicationwasareproductionofaDLRfly-overnoisecampaignsothatpredictionscouldbecomparedwithactualmeasurementsalongtheapproachandflightprocedures.

“Toward a fully automated low-noise design process” (2009-2012)

The initial process was constantly upgraded between 2009 and2012.Thestatusofthe2012simulationprocessisdepictedinFigure3andmodifications to the2008statusarehighlighted in redcolor.ThemajordevelopmentsandapplicationsarepresentedinAppendix(Figure 5).Most importantly, PrADO had to be upgraded in orderto generate the required input data for a PANAM computationand to avoid the need for additional and external input data, i.e.,especially the recordedflight trajectory.Consequently, a newflightsimulationmodulewasdevelopedtoaccountforthespecificaircraftflightperformancealongapproachanddeparture, i.e.,resultinginasimplifiedbutphysics-basedflightpathforarbitraryPrADOconcepts.Aircraftspecificenginesettingsandangleofattackareadaptedalonga fixed and predefined altitude and velocity profile. Consequently,the flight trajectory was no fixed input anymore but would reflecttheunderlyingflightperformanceoftheaircraftunderinvestigation.Theupdatemadetheprocessdirectlyapplicabletomodifiedornewaircraftdesignsbysimplyreplacingthepreviouslyrequiredflightdataby simulateddata.According to the specificflight performance, anindividualapproachanddepartureflightwas simulatedandanoisepredictionalongbothflightswasinitiated.

The capability to assess noise immission beyond the definedcertification situations was an important requirement in order toenable a more realistic picture of the community noise exposure.Multiple acousticmetrics at arbitraryobserver locations and arrayswere considered. Yet, due to the simplicity of the module, noinvestigationoflow-noiseflighttrajectorieswaspossibleatthattimebutapredefinedprocedurewassimulated.

Thesecondmajorupgradewasrelated to theenginesimulation.The PrADO thermodynamic cycle of the engine was improved toyieldtherequiredinputparametersforPANAM,e.g.,thejetexhaustvelocities and temperatures. Furthermore, a simple approximationof the fan rotational speed under defined operating conditionswasincludedbasedonbasicfandesignparameters.13Thisupdateallowedthesimulationofdifferentengineconcepts.Yet,ifhigh-qualityengineperformancedatawasavailableforanexistingengine,itwaspreferredovertheconceptualdesigndataforthenoiseprediction.

Automatically generating the required input parameters foran arbitrary aircraft within PrADO significantly enhanced theapplicability compared to the initial process as depicted in Figure

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10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve?

94Copyright:

©2019 Bertsch et al.

Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

2. In addition to the aircraft geometry, the corresponding flighttrajectoryandtherequiredenginedatacouldnowbegeneratedwithineach simulations run.Whereas the initial process had been limitedtoreproducinganexistingandmeasuredcase, i.e.,noisepredictionof preselected aircraft along a recorded flight path, the upgrade in2009enabledasimulationandvariationofanyoftherequiredinputparameters.Thisisreflectedbyapplicationoftheprocesstowardsthedesignandtheoverallevaluationofaquietshorttake-offandlandingvehicleconcept.14,15

Around 2010, a new tool was implemented into the process,DLR’s shielding tool SHADOW.50,51 The tool can be applied toinvestigate the acoustic shieldingdue to engine installation effects,i.e.,thenoisesourceisnotaffectedbutthesoundradiationtorelevantemissionanglescanbereduced.ThefirstapplicationoftheshieldingassessmentwasthenoisepredictionofDLR’sformerflyingtestbedATTAS. The aircraft is subject to engine noise shielding with itsengines mounted over the wing. TheATTASwas simulated alonga newly developed noise abatement flight procedure, the so-calledHelical NoiseAbatement Procedure (HeNAP). For this study, thenoise prediction tools were embedded in a distributed simulationenvironmentinordertocombinethenoisepredictionwithanexternalhigh-fidelityflightsimulationtool.17Thiscomputationalinvestigationfinally led to a flyover noise campaign duringwhich the predictednoise level reductions and dislocationwas demonstrated.18Overall,integration of SHADOW into the process significantly increasedtheapplicabilityoftheprocesstowardmoreunconventionalvehicledesigns with prevailing noise shielding effects. Exemplary, theshieldingeffectonthefannoiseemissiondirectivityonboardoftheDLRLow-NoiseAircraft (LNA) isdepicted inFigure4.Currently,interfacestoshieldingtoolsofevenhigherfidelityarediscusseddueto recent success in developing and applying these advanced toolstowardoverallaircraftnoiseassessment,e.g.,seeRef.52

To readily investigate novel concepts with more complexairframe architectures, e.g., multiple tailplanes, another upgrade tothesimulationprocesshadbecomenecessaryin2010.Thepreviousairframedefinitionbyrepresentativegeometries,i.e.,averagedwing,controlsurface,andhigh-liftelements,wasnolongerapplicable.Anautomated segmentation was implemented in PrADO to separatethe lift andcontrol surfaces intoacoustical relevant segments.Thisacoustical segmentation directly generates the required parametersfor a PANAM noise prediction of arbitrary and novel airframearchitectures.

Ultimately, the three upgrades (flight simulation, engine deck,airframe segmentation) and theSHADOWimplementation resultedin an automated and fully parametric aircraft design process withintegrated noise prediction capabilities.19 Such an embeddednoise prediction within the conceptual design enabled the directcomparabilityofvariousvehicleconceptsincludingtheacousticsatanadequatelevelofcomplexityanddetail.Theprocesswasappliedtoinvestigatenewideasrangingfromairtrafficroutingtolow-noiseaircraftdesignasdescribedintheliterature.20‒22

“Focus on noise immission” (2013-early 2018)

Duringthisthirdreportperiod,thesimulationprocessasdepictedin Figure 3 has been further developed.Modifications to the 2012statusarehighlightedinredcolor.Interfacestoexternalaerodynamicinput data were defined and continuously implemented into bothPrADO(aerodynamicsandweights)andPANAM(acoustics).

Based on dedicated model-scale experiments and high-fidelitysimulation, the effect of novel low-noise high-lift elements onthe overall aircraft flight performance and acoustics were thenevaluated.23,24 Later during the third period, the interfaces wereupdated to also account for external engine data fromhigh-fidelitysimulation.35 The external engine geometry and performance datacouldnowdirectlybeprocessedwithinPrADOinordertoevaluatetheimpactontheaircraftdesignandflightperformance.Atthispoint,allrequiredinputparametersforanoisepredictioncouldbegeneratedwithinthesimulationprocess.

Duringearlyapplicationsoftheprocessitbecameobvious,thattheproposeddesign-to-noisemethodologymightnotleadtoanoptimalsolution. Detailed modifications to individual components onlyshowedabenefitifthesecomponentsweredominantwithrespecttoothernoisesources.Consequently,itwasdecidedthatthemainfocusofa feasible low-noiseactivityneeds tobeshifted from improvingnoise emission-with subsequent immission assessment only-to adirect immission assessment. Furthermore, it was decided that theassessment should not be limited to the aircraft noise certificationlocations,e.g.,cf.assessmentbyNASAin2015.53Theoverallaircraftnoise as received on the ground26 can only be effectively reducedbased on a direct immission assessment. Interfaces to aTUDelft/NLRactivityofaircraftnoiseAuralizationweredefinedinorder toinvestigate and better understand the noise immission situation.27 Similar conceptshavebeenunder investigationaround that timeatRWTHAachen54 and at NASA.55 Up to 2015, all interfaces wereupdatedinorderforPANAMtoremainanembeddedsimulationtoolinDLR’scontinuously improvingdistributedsimulationframeworkRemote Component Environment2(RCE), later described in Ref.25 Furthermore,additionalnoisesourcemodelswereimplementedintoPANAMforapplicationtowarddifferentvehicleconcepts,i.e.,aflapsideedgemodel28,56,57andapropellernoisesourcemodel.58

The initial PrADO module for approach and departure flightsimulation as described previously was replaced by an advancedversion, see Ref.33 With this replacement, it became possible toidentify tailored flight procedures for each individual aircraft, i.e.,including the noise relevant high-lift setting and engine operationalongtheflights.Controllingthetimingofhigh-liftandlandinggeardeployment,theangleofattack,andtheenginethrustsettingallowstocomeupwithmorerealisticflightprocedures.Thenewmodulealsoallowstoquicklyassesstheimpactofcertainparametersonthenoiseimmissionandultimatelyallowstodefinelow-noiseflightproceduresforeachindividualaircraftasearlyaswithintheconceptualdesignphase.Forexample,modifyingahigh-liftsystemforreducednoiseemissionmightdecreasethemaximumliftcoefficientofthevehiclewhichwoulddirectly result in increasedflightvelocitiesduring theapproach.Obviously, any advantageous effect of a low-noisehigh-liftsystemwoulddirectlybecounteractedbythisgeneralincreaseinairframenoiseduetoahigherflightvelocity(Figure3).

From 2016 on, three specific research topics have been underdetailedinvestigation:

(1) Predictionuncertaintyassessment,

(2) Largeairportsimulationscenarios,and

(3)Low-annoyanceaircraftdesign.

In the context of these new research topics, the Swiss FederalLaboratories for Materials Science and Technology (Empa)2https://www.dlr.de/sc/en/desktopdefault.aspx/tabid-5625/

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10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve?

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Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

significantlycontributestotheongoingDLRandTUBraunschweigcooperation:

1.Essential requirement for any reliable and comprehensive noiseassessment is the understanding of inherent result uncertainties.Thisisespeciallyimportantifdifferentsimulationtoolsofvaryingfidelityand/orexperimental resultsareassembledand translatedforanoverallassessment.Itisessentialtodeterminetheinfluenceand reliability of individual part results, e.g., noise emission ofonehigh-liftelementthatarethenfurtherprocessedandcombinedwith other part results to describe the overall noise immission.Adedicated studyhasbeen launched in cooperationwithEmpato assess and quantify the uncertainties that are associatedwithPANAMresults.BasedonpreviousworkbySchäfferetal.59andThomann60anuncertaintymodulehasbeendeveloped.Theoveralluncertainties of the predicted noise immission are defined as acombinationofinputdata,modeling,andpropagationuncertaintyasdescribedinRefs.36,37

2.The goal of this research is an overall assessment of selectedlow-noisetechnologiesandvehicleconceptswithinlargeairportscenarios.Aninitialsimulationprocessofsuchanoverallscenarioassessmentisdefinedbyathree-stagedsimulationapproach,i.e.,the component level, the single aircraft level, and the systemscenario level as described inRef.38 The single aircraft level isthereby comprised of the entire simulation process of theDLRand TU BS cooperation as depicted in Ref.3 The individualtechnologies and components are provided from the componentlevel(basedontheirhigh-fidelitysimulationresults),transferredtothesingleaircraftlevel(herethetechnologiesandcomponentsare installed on an aircraft and the overall noise immission ispredicted),tofinallyintegratethenewvehiclesintoascenario(theoverallnoiseimmissionaroundtheairportispredictedaccordingtothescenario,seeRef.61).Obviously,itwasnecessarytocombinevarioussimulationtoolsofstronglydifferentfidelitylevelsinorderto enable this overall assessment. High-fidelity computationalanalysis of specific low-noise technologywas transferred downto the overall aircraft design to enable a noise assessment ofthis individual vehicle on a mid-fidelity level. Thereafter, theindividualvehicleonitsflighttrajectorywasimplementedintoalargeairportscenariowhichfurthermorereducedthelevelofdetailandfidelity.Themaingoalofthisactivityistokeepasmuchofthehigh-fidelityinformationandphysicswhenreducingoveralldetailandmovingfromcomponenttosingleflightdowntothescenario.Asafinalstep,theinitialresultsofthisnewprocessstillhavetobe subject toanuncertaintyassessmentasdescribedpreviously.At the time of the initial application as described in Ref,38 theuncertaintyassessmenthadstillbeenunderdevelopment.

3.The most recent upgrade and corresponding applicationof the process as depicted in Figure 5 is low-annoyance orperception influenced aircraft design. Based on initial work onAuralization27andinspiredbythegroundbreakingworkofRizzi,62 apilot project ImpactDrivEnAssessmentofLow-noise aircraft(IDeAL)wasinitiatedwithEmpa.UnderEmpalead,Auralizationswere performed for several novel DLR aircraft designs alongdifferent approach flight procedures as described in Ref.39 Theresulting sound samples have then been used for listening testsinorder toassess theannoyanceof the test subjects.Aproofofconcept was demonstrated (see Ref.39) and initial results of thelistening tests are very promising and scheduled for publicationinearly2019.

Selected application exampleArecentstudy50of2018hasbeenselectedinordertodemonstrate

themain simulationcapabilities that areavailableafter10yearsofjointresearch.Themaincapabilitiesarethefollowing:

a. aircraftdesignprocesswithintegratednoiseassessment

-Assessmentalongaircraftspecificapproachanddepartureflights,

-Applicationtolow-noise(shielded)architectures.

b. processingofexternalinputdatafromhigh-fidelitysimulationorexperiment(incorporatedinnoisepredictionandaircraftdesign)

-High-liftaerodynamics,weights,andacoustics,

-Engineperformancedeckandgeometrydetails.

c. uncertaintyassessmentofpredictionresults

Thefocusof thatstudyliesontheenginereplacementonboardof conventional and low-noise tube-and-wing aircraft architectures.Theresultingnoiseimmissionisassessedforvehicleswithreferenceversus geared turbofan (GTF) engine concepts. Furthermore, theefficiency of additional low-noise airframemeasures is studied foreachvehicleconcept.ResultsofthisinvestigationwerepublishedinRefs.23,24,35andarenowpresentedhere todemonstrate theavailablesimulation capabilities. In addition towhatwas presented in 2018,thenewlydevelopeduncertaintymoduleisappliedtothepredictionresults according to Refs.36,37 A direct comparison of the enginereplacements,35 andamoredetailedassessmentof leveldifferencesdue to the modifications including prediction uncertainties arepresented. For all predictions, approach and departure flights areconsideredandallvehiclesaresimulatedalongindividualanddetailedflightprocedures.

In a first section, the vehicles and technologies are introduced.A second section discusses the impact of engine replacement onperformance and noise immission. And finally, a third section isdedicatedtoadetailedstudyoftheeffectofindividualtechnologiesandmodificationsunderconsiderationofpredictionuncertainties.

Vehicles and technologies

Thereferenceaircraftisreferredtoas“zero”35andisaconventionaltube-and-wingaircraftwithturbofanenginesofbypassratio(BPR)6.ThedesignisbasedonthefollowingTopLevelAircraftRequirements(TLAR):4000kmrange,180Pax,1890kgpayloadandacruiseMachnumberof0.8.Thelow-noiseaircraftarchitecturesaredesignedfora reducedcruiseMachnumberof0.7 to counteract adverse effectsoncruiseaerodynamicsdue to theselectedengine integration.ThisreferencevehicleisthensubjecttocertainmodificationsasdepictedinFigure6,i.e.,modificationstotheengine(replacementofBPR6withBPR12engine),theaircraftarchitecture(aircraftadaptationtoBPR12engine&fannoiseshielding),and/ortheairframe(low-noisehigh-lift & low-noise landing gear). For simplicity, the followingabbreviations are introduced: “+gtf” for the engine replacement,“+adapt” for the aircraft adaption to the new engine, “+shield” forthe fan noise shielding architecture, and “+airf” for the additionallow-noise airframemeasures. The resulting vehicle variants of the2018studyaresummarizedinFigure7.Startingfromthereferencevehicle“zero”,theselectedtechnologiesanddesignmodificationsareapplied.Onthelefthandside,vehicleswithconventionaltube-and-wing architecture are listed. In afirst designmodification (“+gtf”),the engine is simply replaced on board of the “zero”-resulting invehicle “neo”. In a second design iteration (“+adapt”), the vehicle

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10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve?

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Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

layoutisfurthermoreadaptedtotheGTFconcept-leadingtovehicleconcept “neodapt”.All three vehicles can furthermore be equippedwith low-noise airframe technology (modification “+airf”). On theright hand side, the low-noise vehicle architectures are listed. The“V2 (af)” is based on the “zero”-also with conventional engines-but significantly modified in order to shield the engine fan noise(“+shield”).Theshieldingconceptandvehiclearchitectureisbasedonpreviousfindingsfromalargedesignstudy,seeRef.31.Thesecond

low-noisevehicleisanewdesignreferredtoas“fanex”.Thisvehiclewith fan noise shielding (“+shield”) is furthermore equipped withGTF engines (“+gtf”) and is adapted to this new engine concept(“+adapt”).Bothlow-noisevehiclesaredirectlyequippedwithlow-noise airframe technology (“+airf”). The layouts of aircraft design“zero”, “neopdapt”, “V-2 (af)”, and “fanex” are depicted in theAppendix(Figures1-3).

Figure 6 Selected technologies for engine (“+gtf”), architecture (“+adapt”, “+shield”), and airframe (“+airf”).

Figure 7 Vehicle concepts and applied technologies.

Impact of engine replacement on performance and noise

To directly investigate the impact of an engine replacement,comparative assessments of conventional vehicles, i.e., “zero” vs.“neodapt”, and low-noise vehicles, i.e., “V2 (af)” vs. “fanex”, arepresented.Replacing the reference engine ofBPR 6with a gearedturbofan engineofBPR12has significant implicationson the fuelrequirementsalong thedesignmission.Asignificantly reducedfuelrequirementalongthedesignmissionisfurtherexploitedbyadaptingtheaircraftdesignaccordingly.The transported fuelmassonboardis reduced according to the fuel requirement and consequently lessfueltanksareinstalled.Maintainingaconstantwingloadingdirectlyenabledareductionofwingareaandconsequentlyadown-sizingof

thetail.IterationofthePrADOdesignmodulesaccountedforseveralsideeffects,e.g.,downsizingofthelandinggear,whichfurthermorereducedtheoverallaircraftweight.Insummary,theadaptionoftheaircraft design results in the following changes to key design andperformance parameters of the reference aircraft “zero”:wing area-8.1%,maximum take-offweight -7.5%, operational emptyweight-2.7%, design mission fuel requirement -27.1%, and the directoperating costs per flight -7.7%. These modifications (“+adapt”)arerelevantforvehicles“neodapt”and“fanex”.Overall, theDirectOperatingConsts(DOC)perflightcanbereducedbyalmost8%ifthereferenceengineisreplacedbytheBPR12engineandthevehicleis adapted. This cost reduction is almost constant for the enginereplacementonboardoftheconventionalandlow-noisevehicles.

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10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve?

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Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

AdetailedassessmentoftheenginenoiseemissionforthereferenceandtheBPR12engineshowasignificantnoisereductionpotentialfor the engine replacement.The effect on vehicle noise immissionandflightperformanceisadirectpredictionresultofthesimulationprocesscomprisedofPrADO,SHADOW,andPANAM.TheairframemeasuresareaccountedforbymodifyingtheavailableairframenoisesourceswithinPANAM.Thenewhigh-liftconceptandtheselectedlandinggearmeshfairingareaccountedforbysubtractingspecifiedandconstantleveldifferencesΔdBfromthecorrespondingPANAMemission spectra of each specific component.The low-noise droopnose is accounted for by -6 dBand the trailing edgemodificationsby -5 dB with respect to the PANAM predictions, respectively.Furthermore,theimpactonthecomponentweightandaerodynamicperformanceisaccountedforinPrADOhencehasadirectimpactontheaircraftflightperformancealong theentireflight, i.e., includingapproachanddeparture, seeRefs.23,24 formoredetails.The landinggear mesh fairing is approximated by -3 dB delta with respect tothePANAMpredictionsfor the landinggear.For thismodification,only the level difference has been accounted for and any potentialimplicationonlandinggeardragorincreasedweightisassumedtobeofneglectablerelevance.

According to the introduction, the SEL isocontour areas wereselectedhere.Figure8(a)showsthesignificantlydifferentisocontourareasfor“zero”and“neodapt”,thetwovehicleswiththeconventionalarchitectureasdepictedinAppendix(Figure1&2).Isocontourareasaremainlyreduceddue to theenginereplacement (“+gtf”).Furtherreductioncanbeachievedbytheaircraftdesignmodification(“+adapt”)andalteredflightperformanceofthe“neodapt”.Thereducedweightand thrust excess due to theBPR 12 engine enables a steeper andfasterclimboutofthe“neodapt”comparedtothe“zero”.Asimilarobservation can bemade for the low-noise aircraft architectures asdepictedinFigure3andFigure4.Figure8(b)alsoshowsasignificantareareductiondespitethefactthatbothvehiclesarealreadydesignedfor low-noise, i.e., thefannoise isalreadyeffectivelyshielded.Forboth aircraft architectures the engine replacement (“+gtf”) is mostpromisingtowardreductionofnoiseisocontoursalongdepartureandapproachprocedures.

Detail assessment of level differences

Incontrast tothebaselinestudy,35adetailedassessmentofleveldifferences-includingpredictionuncertainties-duetothemodificationsis presented here. Two specific observers have been selected forfurtherinvestigation.Theseobserversareintentionallynotplacedatthecertificationpointsbutat6and15kmafterbrake-release(markedwithwhitesymbolsinFigures8(a)and8(b))whichisamuchmorerepresentativesituationwithrespecttocommunitynoiseannoyancearoundanymajorairport.

For the “neodapt” significant level reductions compared tothe reference vehicle “zero” are predicted, i.e., due to “+gtf” and“+adapt”.Significantlevelreductionscanbefoundinallmajornoisemetrics:SELandLA,max(mainlyduetojetnoisereduction),andEPNL(mainlyduetofannoisereduction),seeTable1.Asignificantbenefitcanalsobeidentifiedforanenginereplacementonboardofthelow-noise vehicles, i.e., comparing “V-2 (af)” and “fanex”. The levelsaresignificantlyreducedforthe“fanex”,seeTable1,buttheEPNLdifferencesare lesspronouncedcompared to thedifferencesamongthe vehicles with a conventional architecture. This can directly beattributedtoanalreadyprevailingfannoisereductionduetoshielding

effectsandthereforelesserdominanceofthetonalfannoise.Itcanfurthermorebedemonstratedthattheenginereplacementsignificantlyincreasestheimportanceandeffectivenessofanylow-noisemeasuretotheairframe(“+airf”).Thisisobviouslyaveryimportantfindingfor thenoise immission associatedwith typical approach situations(no isocontour areas are displayed here but the interested reader isreferredtoRef.35formoredetail).

Table 1 Influence of engine replacement on noise: conventional (“neodapt” vs. “zero”) and low-noise aircraft architectures (“fanex” vs. “V-2 (af)”)

case/metricΔ(neodapt-zero) Δ(fanex-V-2(af))

x=6km x=15km x=6km x=15km

LA,max [dB] -3.5 -4.9 -4.1 -3.0

EPNL [dB] -6.2 -5.8 -4.1 -2.8

SEL [dB] -3.1 -3.1 -3.5 -2.3

Tospecificallyidentifytheeffectoftheappliedlow-noisemeasures“+gtf”, “+adapt”, “+airf”, and “+shield”, the level differences inLA,maxarefurtherassessed.TheuncertaintyassessmentasintroducedinRefs.36,37isthenappliedtothesepredictedleveldifferences.Nowthe effect of eachmeasure can be assessed under consideration oftheknownpredictionuncertainties.Thedepartureflightscenarioasdescribed above is assessed here but a detailed investigation of anapproach is furthermore included to directly compare the differentlow-noisemeasures.

The three contributors to the overall uncertainty of thepredicted immission are input uncertainty, modeling uncertainty,and propagation uncertainty. The input parameter uncertainty isnegligible compared to the modeling and propagation uncertaintyfor this application. The input data comes with only very smalluncertainties because it is generated by high-fidelity simulation orcomesfromexperimentaldata, i.e.,airframeandenginenoisedata.Forthemodelinguncertaintiesoftheappliednoisesourcemodels,thefollowingvaluesareassumed.

a. airframenoisesources

-Landinggear&high-lift:umodof1.4dB

-Otherairframesources:umodof1dB

b. enginenoisesources

-Referencefan:umodof3.6dB(includinganadditionalumodof3dBduetofannoiseshielding)

-Gearedturbofan:umodof4.2dB(includinganadditionalumodof3dBduetofannoiseshielding)

-Jet:umodof1.5dB

Yet, toassess leveldifferences,covarianceshave tobe includedtoaccountforcorrelatedterms.Acorrelationfactorof0.8isassumedhere.Asimpleestimatefor theuncertaintiesdueto thepropagationisincludedhereasproposedinSchäfferetal.59andThomann.60Theeffectofpropagationontheoverall immissionuncertaintybecomesquietlargewithincreasingpropagationdistance.

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10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve?

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Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

(a) Conventional aircraft architectures

(b) Low-noise aircraft architecturesFigure 8 SEL contours along simulated departure: Influence of engine replacement for different aircraft architectures.

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10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve?

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Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

Figure 9 shows the level differences at each observer locationrelative to the referenceaircraft“zero”due toselectedmeasuresasdepicted in Figure 6, i.e., “+gtf”, “+airf”, and “+shield”. Differentcolors for thebarplotshavebeenselected intentionally inorder toemphasize the fact that eachbarplot represents either an approach(green) or a departure (red) situation hence to avoid a directcomparisonofthetwosituations.

Figure 9(a) shows 3.4±1.9 dB level reduction compared to thereferenceiftheengineisreplacedandthevehicleisadapted(“+gtf”and“+adapt”),i.e.,acomparisonofvehicles“zero”and“neodapt”.At15kmdistancealevelreductionof4±1.7dBispredicted,seeFigure9(b).Evenifpredicteduncertaintiesfor the“+gtf”modificationaretakenintoaccount,theleveldifferencesaresignificantcomparedtothereference.Asexpected,theadditionallow-noiseairframe(“+airf”)doesnothaveanysignificanteffectonthenoiseimmissionalongthedeparture.Iftheaircraftarchitectureisfurthermoremodifiedtoexploitfan noise shielding (“+shield”) a tremendous reduction between11.4±1.7 and 14.9±2.5 dB is predicted at 6 and 15 km distance,respectively.Evenconsiderationofpredictionuncertaintieswillyieldsignificantnoisereductionpotentialatbothobserverlocationsalongthedeparture.

Fortheapproachcase,depictedinFigures9(c)and9(d),low-noisemeasurestotheairframe(“+airf”)aremoreeffectiveduetoairframenoiserelevance.Theenginereplacementstillyields3.0±1.8dBlevelreductionatthefurtherobserverlocationcomparedtothereferenceseeFigure9(c).Thiscanbeattributedtothesignificantreductionofprevailing fan noise contribution associated with the “zero”.Withthis reduction, it becomes very efficient to further reduce the nowdominating airframe noise. The “+airf” measures reduce the noiseimmissionby7.5±1.9dBatthefurtherobserverlocation;seeFigure9(c). The effect of “+gtf” at the closer observer is still significantbutreducedtoalevelreductionof1.1±2.2dB,seeFigure9(d).Thisis due to the now dominating landing gear contributionwhich canfurthermorebereducedby3dBforthe“+airf”measures,i.e.,resulting

inanimmissionreductionof4.0±2.2dBat6kmbeforetouch-down.Fortheapproachcase,asmallereffectofenginenoiseshieldingcanbeobservedduetothemoredominatingairframenoisecontribution.At the most distant observer location, the noise reduction due to“+shield”is lessthan0.5dBcomparedtothereductionthatcanbeachievedbysimplyreplacingtheengine(“+gtf”).Thecloserobserverissubjecttomoreenginenoisethanthedistantobserver,hencemoreeffect due to “+shield” is experienced. Taking uncertainties intoconsideration,norelevantleveldifferencecanbeidentifiedbetween“+shield”and“+gtf”forbothobserversituationsalongtheapproach.

The level differences and the associated uncertainties varysignificantly along a simulated flight. Furthermore, the situationalonganapproachisobviouslysignificantlydifferenttoadeparturesituation. Multiple observer locations have to be considered inorder to draw any final conclusions of novel technologies. For thepresentedapplicationcaseitisdemonstratedthatthecombinationofenginereplacement(“+gtf”),vehicleadaptation(“+adapt”),andnoiseshielding (“+shield”) is most promising toward future low-noiseaircraftconcepts.Significantlyreducingtheenginenoisecontributionmakesmeasurestotheairframe(“+airf”)especiallyeffectiveduringapproach.

Theapplicationexampleclearlyindicatesthatalow-noiseaircraftdesigncanonlybeidentifiedbyadetailedimmissionanalysiswithinthedesignprocessratherthanbysubsequentnoiseassessmentoffinalvehicledesigns.Forexample,optimizingthenoiseemissionofahigh-liftconceptwithonlyasubsequentnoiseimmissionassessmentwillnot result in an optimal low-noise aircraft design.23,24 Furthermore,noconclusionofresultsshouldbedrawnifrelevantsnowballeffectsarenotconsidered.Forexample,modificationofenginebypassratioandassessmentof thenoise immissionwithoutpropermodelingofthe new engine performance is meaningless. If the impact of newtechnology on the flight performance is not accounted for in earlydesignstages, itcanbeassumedthat thefinal resultsmightchangedramaticallyaccordingtoadifferentthrustsettingorflightvelocity.

(a) dep at x=6km (b) dep at x=15km (c) app at x=-15km (d) app at x = -6 km

Figure 9 Influence of low-noise measures on ΔLA,max and associated uncertainty at selected observer locations along departure (dep) or approach (app) flight path.

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10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve?

100Copyright:

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Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

Summary & lessons learnedDLR and TU BS have developed an aircraft design synthesis

processwithintegratednoisepredictioncapabilities.Beforethejointactivity, acoustic assessment was not available within conceptualaircraftdesignbuthadtobeappliedasasubsequentnoiseassessmentofpredefinedconcepts.Asaconsequence,thepotentialsolutionspacewas henceforth significantly restricted. The noise of existing andfutureaircraftcannowbepredictedasearlyaswithintheconceptualdesignphase-atthemomenttheapplicationislimitedtotube-and-wing architectures. For a comprehensive immission prediction,vehiclesaresimulatedalongindividualflighttrajectorieswhichcanbe subject to low-noiseoptimizationaswell.Already in thedesignphaseofnovelandnon-existingvehiclesandtechnologies,itisnowpossible to identifypromisingflight trajectories forminimumnoiseimmission.To avoid a partial and incomplete assessment,multiplemetrics are evaluated for arbitrary arrays of observers.The engineinstallation effects are accounted for by a specificDLR simulationtool, SHADOW, but simulation results of higher fidelity can beimplemented and processed as well. Dedicated interfaces enable adirectconsiderationofexternalinputdatawithinthePrADOsimulationprocess,e.g.,byconsiderationofmodifiedaircraft/enginedesign,andwithinthenoisepredictionwithPANAM,e.g.,considerationofdeltalevelsforselectednoisesources.Theoverallprocessisunderconstantdevelopment. New findings with respect to engine noise shieldingsimulation are implemented as well as advanced noise sourcemodelssuchasthenewDLRfannoisemodelPropNoise,seeRef.63 AdvancedconceptssuchasdistributedpropulsionorBoundaryLayerIngestion(BLI)requireadditionalresearchactivitiessincenomaturemodelsareavailableyet.Theoverallprocesshasbeensubjecttoanuncertainty assessment.So far, focus of the uncertainty assessmentliesonconventionaltube-and-wingarchitecturesbutwillbeupdatedwithrecentfindingsofnoveltechnologiesandconcepts.

Several lessons learnedcanbedocumentedfor the last10yearsofthejointresearch.Themostimportantlessonlearnedistoensurethecompletenessofconsidereddisciplinesandrelevantinteractions.An adequate noise immission assessment cannot be limited to theemissionsituation,tocertainaircraftorenginecomponents,tospecificnoisemetrics,nortoafixedoperatingcondition.Itisrecommendedtoultimatelyassesstheimmissionalongtypicalflightproceduresatmultipleobserverlocationsandevaluatingmultiplenoisemetricsinordertodrawanyconclusionwithrespecttonoveltechnologies.

Anabsolutemust-haveforanyfeasiblenoisepredictionisahigh-qualitysimulationofthethermodynamicenginecyclethatincludesasufficiently accuratemodeling of aerodynamics.No adequate inputdata for any noise prediction should be solely based on simplifiedmethods and tools. It is demonstrated that these engine parametershaveahugeinfluenceonthepredictednoise,seeFigure1forsomeexemplaryparameters.Anotherabsolutemust-haveisaccesstoFDR

whencomparingpredictionswithfly-overmeasurements.Withoutthepreciseknowledgeofatleastthemostdominatingparameters,i.e.,thehigh-liftandlandinggearpositions,theflightvelocity,andthethrustsetting, no such comparison should be initiated.An estimation oftheachievablelevelofagreementbetweensimulationandmeasurednoiselevelscanbeaddedtothelessonslearned.Aperfectagreementisconsideredasapurecoincidencebecauseofseveralshortcomingsinherent for anyoverallnoiseprediction, i.e., not capturing scalinglaws, insufficient modeling capabilities, and oversimplification ofgeometries.Thebestagreementcanbeexpectedformaximumlevelsorcomparativeanalysesingeneral(trendassessment).64‒66

Futureactivitieswillfocusonelectrificationamongothertopics.Afirststeptowardstheassessmentofsuchelectricaircraftistherevisitofpropellernoisesimulationcapabilities.Theimplementednoisesourcemodelshavetobeassessedfortheirapplicabilitytowardnewaircraftconceptswithmodernturbopropenginesandinstallationeffectsthatwill distort the inflow. It canbe expected that electrificationof thepropulsion system might result in significantly different approachprocedures due to a now possible engine-out option compared tothe flight-idle of conventional turbofan engines or maybe even byenablingwind-millingtorechargetheenergystorageonboard.

Furthermore, the new perception influenced design activitieswith Empa will be continued with a focus on novel vehicles andunconventional flight procedures. The main focus of the futurework liesona feedback loop from listening testsback to thenoisemodelingandaircraftdefinition.The resultsof these listening testscanultimatelybeusedtoimprovetheoverallpredictioncapabilities.

AcknowledgmentsThe authors highly appreciate the support of several colleagues

andstudentsatDLRandEmpaovertheyears.Specialthanksgotoformer students Philip Krammer (2008), Christopher Kott (2009-2010), and Raihan Ramdjanbeg (2014-2015). Furthermore, theauthors thank JoshuaAmmermann for his contribution as studentemployee(2016-today).PhDstudentsJasonBlinstrub(2014-today)andMarcKoch(2016-today)contributesignificantlytotheongoingdevelopment and their work and effort toward low-noise aircraftdesignresearchactivitiesishighlyappreciated.Theauthorsareverygrateful for thescientificsupportof the followingDLRcolleagues:Werner Dobrzynski (retired), TomOtten, FlorianWolters,MichaelPott-Pollenske, and Karl-Stephane Rossignol. The expertise andsupport ofEmpa colleaguesBeat Schäffer andReto Pieren towarduncertaintyassessmentandauralizationofthenoisepredictionresultsisgreatlyappreciated.

Conflicts of interestAuthorsdeclarethatthereisnoconflictofinterest.

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Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

Appendix

Figure 1Vehicledesign“zero”.35 Figure 2Vehicledesign“neodapt”.35

Figure 3Vehicledesign“V-2(af)”35 Figure 4Vehicledesign“fanex”.35

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102Copyright:

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Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

Figure 5Timelineofdevelopments(redboxes)andapplications(greenboxes)withliteratureallocation.

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Citation: Bertsch L, Heinze W, Guerin S, et al. 10 years of joint research at DLR and TU Braunschweig toward low-noise aircraft design-what did we achieve? Aeron Aero Open Access J. 2019;3(2):89‒104. DOI: 10.15406/aaoaj.2019.03.00085

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