Table Of ContentNotes on Numerical Fluid Mechanics
and Multidisciplinary Design 134
Charles Mockett
Werner Haase
Dieter Schwamborn Editors
Go4Hybrid: Grey Area
Mitigation for Hybrid
RANS-LES Methods
Results of the 7th Framework Research
Project Go4Hybrid, Funded by the
European Union, 2013–2015
European Commission
Research Directorate General
Notes on Numerical Fluid Mechanics
and Multidisciplinary Design
Volume 134
Series editors
Wolfgang Schröder, Lehrstuhl für Strömungslehre und Aerodynamisches Institut,
Aachen, Germany
e-mail: offi[email protected]
BendiksJanBoersma,DelftUniversityofTechnology,CADelft,TheNetherlands
e-mail: [email protected]
Kozo Fujii, The Institute of Space and Astronautical Science, Kanagawa, Japan
e-mail: fujii@flab.eng.isas.jaxa.jp
Werner Haase, Neubiberg, Germany
e-mail: [email protected]
Ernst Heinrich Hirschel, Zorneding, Germany
e-mail: [email protected]
Michael A. Leschziner, Imperial College of Science Technology and Medicine,
London, UK
e-mail: [email protected]
Jacques Periaux, Paris, France
e-mail: [email protected]
Sergio Pirozzoli, Università di Roma “La Sapienza”, Roma, Italy
e-mail: [email protected]
Arthur Rizzi, KTH Royal Institute of Technology, Stockholm, Sweden
e-mail: [email protected]
Bernard Roux, Technopole de Chateau-Gombert, Marseille Cedex, France
e-mail: [email protected]
Yurii I. Shokin, Siberian Branch of the Russian Academy of Sciences,
Novosibirsk, Russia
e-mail: [email protected]
About this Series
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More information about this series at http://www.springer.com/series/4629
Charles Mockett Werner Haase
(cid:129)
Dieter Schwamborn
Editors
Go4Hybrid: Grey Area
Mitigation for Hybrid
RANS-LES Methods
Results of the 7th Framework Research
Project Go4Hybrid, Funded by the European
–
Union, 2013 2015
123
Editors
CharlesMockett DieterSchwamborn
CFDSoftware E+F GmbH DeutschesZentrumfürLuft-undRaumfahrt
Berlin e.V.(DLR)
Germany Institut für Aerodynamik und
Strömungstechnik
WernerHaase Göttingen, Niedersachsen
Aeronautics Consultants Germany
Neubiberg
Germany
ISSN 1612-2909 ISSN 1860-0824 (electronic)
NotesonNumerical Fluid MechanicsandMultidisciplinary Design
ISBN978-3-319-52994-3 ISBN978-3-319-52995-0 (eBook)
DOI 10.1007/978-3-319-52995-0
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Foreword
It is observed in many technical applications that RANS models show inherent
technology limitations for certain types of flows (for example, flows with
large/massive separation, flows with strong mixing zones, flows with strong
interactionsofdifferent flowsfeatures).Forthisreason,there isacontinuousdrive
within the turbulence community to augment RANS models with Scale-Resolving
Simulation (SRS) capabilities. Motivated by the high cost of conventional Large
EddySimulation(LES)andWallModeledLES(WMLES)forwallboundarylayer
simulations, hybrid RANS-LES models have been developed over the past two
decades. From the numerous different hybrid approaches, the Go4Hybrid project
was mainly centered around models of the DES family (Spalart 2009), which are
the most prominently used hybrid RANS-LES models in industrial simulations
today.
The original goal in the development of hybrid models was the formulation of
methodswhereall(!)attachedandmildlyseparatedboundarylayersarecoveredin
RANS-modeandwhereonlydetachedregionsaretreatedbyLES.Agoodexample
isanaircraftwiththelandinggearextracted,whereonlythehighlyunstablemixing
zonebehindthelandinggearwouldbecoveredbyLESwhereasallboundarylayers
around the aircraft are covered by RANS. Even this seemingly simple goal poses
significant challenges. The first one isthe ‘shielding’ ofthe RANS boundary layer
against any unwanted influences from the LES formulation. In the original DES
formulation, this aspect was only partly achieved. This resulted in highly prob-
lematic effects on the solution, especially Grid Induced Separation (GIS) (Menter
and Kuntz 2003). GIS appeared as a result of the reduction of the eddy-viscosity
inside the RANS boundary layers due to the impact of the grid-dependent LES
modelformulation.Especiallyingeneralindustrialflowsimulations,safeshielding
is a major requirement for a reliable model. Improved shielding has subsequently
beenachievedthroughaddingshielingfunctionsasproposedbyMenterandKuntz
(2003), finally resulting in Delayed Detached Eddy Simulation (DDES) (Spalart
et al. 2006; Gritskevich et al. 2012). But even with DDES, the shielding is not
perfectandcanbebrokenunderpracticalconditionsandcarehastobeexercisedby
theuser.WhilemanystudiesfocusontheLESsideofhybridRANS-LESmodels,
v
vi Foreword
the importance of maintaining the integrity of attached RANS boundary layers
cannot be over-stressed. Experience over the last decades has shown that most
industrial users cannot reliably adhere to complex grid generation requirements
related to this issue, partly due to a lack of understanding of the details of the
methods,partlyduetoalackofsupportfromthemeshingtools,butmostlydueto
the shear complexity of the geometry of their application. When dealing with
hybrid RANS-LES methods, one is therefore well advised to consider their ability
of shielding the boundary layers from unwanted LES influences. Several model
formulations within this book address this issue as a central point of their design.
The second critical aspect with respect to global hybrid models (meaning the
user selects one model formulation for the entire domain and the model itself
determines where to act in RANS and where in LES mode) is the fidelity with
whichtheyswitchfromRANStoLESmode.‘Switching’doesnotonlymean that
the model formulation changes, but that there is a sufficiently rapid appearance of
unsteady resolved turbulence structures in the ‘transitional’ zone between RANS
andLES.Inglobalhybridmodels,this‘transition’istriggeredbyaninstability.The
speedwithwhichresolvedturbulencestructuresappeardependsbothonthetypeof
flowandonthemodelformulation.Forhighlyunstableflows,e.g.flowspastbluff
bodies,allmodelstendtoswitchswiftlyfromRANStoLESmode.However,many
more technical flows are dominated by Separating Shear Layers (SSL). The most
prominent example is a backward facing step flow, where the attached boundary
layerupstreamofthesteptransitionsintoafreemixinglayerpastthestep.TheSSL
does not push the flow very hard into LES mode and the fidelity with which this
transfer happens depends strongly on the details of the model formulation (as well
asonmeshingandnumericsdetails).TheoriginalDES/DDESmodelfamilyshows
a fairly slow RANS-LES transition, due to the relatively high LES eddy-viscosity
levels under such conditions. It is interesting to note that this deficiency is linked
closely with the desire of achieving reliable shielding of the attached RANS
boundary layers. So, to a certain degree, there is a balance between shielding and
rapid RANS-LES transition.
When using global hybrid models, one should also never forget that the
‘RANS-LES’ transition is an inherently unphysical process. In the real world, the
upstream turbulent boundary layer already injects ‘resolved’ turbulence into SSL
thereby suppressing the 2D-transitional process observed in global hybrid models.
The optimization of global hybrid models reduces the extent of this unphysical
‘transition’ zone. However, there are scenarios where the flow instability is not
strongenoughtopushthesimulationfromRANStoLESevenwithoptimalmodel
formulations. In such cases, zonal methods need to be developed, where the user
selects the RANS and LES zones during pre-processing. More importantly, algo-
rithms are employed at the RANS-LES interfaces to convert modeled turbulence
intoresolvedturbulence.Thisistypicallyachievedthroughsyntheticallygenerated
turbulence based on the upstream RANS solution. This is a much more complex
technology, both from the formulation of the methods themselves as well as with
regards to the user interaction required.
Foreword vii
The Go4Hybrid project has addressed all of the above issues by different
degrees. The main focus was devoted to global hybrid RANS-LES methods and
this is also the area, where the most substantial progress has been achieved.
However, numerous interesting aspects of zonal methods have been investigated
and those findings will contribute in the longer term to improved model formula-
tions for industrial CFD simulations.
As an observer and reviewer to the project, I enjoyed being part of this very
activeandcooperativegroup.Manyusefuldiscussionstookplaceduringtheproject
andmuchusefulworkhasbeenconductedintherelativelyshortprojecttimeframe.
The project was managed in a very goal-oriented manner and a good balance
between model advancement and solidification of findings through
proof-of-concept experiments was achieved. Finally, complex test cases demon-
strated that some of the more mature methods are ready for industrial use.
To me, as an active turbulence modeler, the findings of the Go4Hybrid project
go beyond just a number of ready-to-use new model formulations, but offer a
substantial wealth of inspiring ideas and new modeling elements on the subject.
Iamconvincedthatevery readerinterestedinthistopicwillsimilarly benefitfrom
the book at hand.
Florian Menter
Senior Fellow, ANSYS Germany GmbH
Preface
Computational fluid dynamics (CFD) is nowadays applied extensively in all
aerodynamics-basedtopicsofaircraftdesign,developmentandoptimization.Since
standardCFDapproachesstilllackaccuracyinareasofhighlynonlinear,unsteady
flows close to the borders of the flight envelope, the aeronautical industry is
increasingly willing to apply more costly scale-resolving methods, if such are able
to provide a real predictive alternative for critical situations. While Large Eddy
Simulation(LES)maybeaviableoptionincertainareas,itisstillfartoocostly—if
notimpossible—toapplyittohighReynoldsnumberflowsaboutevenmoderately
complexconfigurations.Thus,thefamilyofHybridRANS-LESMethods(HRLM)
currently appears to be the best candidate for the next generation of CFD methods
to increase solution fidelity at an industrially feasible expense.
HRLM have been proven to perform considerably better than conventional
Reynolds-Averaged Navier-Stokes (RANS or URANS) approaches in situations
with strong flow separation, but they are less effective once they have to deal with
weakly unstable1 flows, e.g. thin separation regions or shear layers in general. In
such cases, resolved structures develop only very slowly, resulting in areas where
thetotalamountofturbulence(bothinmodeledandresolvedterms)isunphysically
low.Theseso-called“GreyAreas”oftenleadtoresultsthatareworsethanthoseof
RANS simulations.
Unfortunately, such grey area situations appear in many of the flows important
close to the borders of the flight envelope, e.g. near maximum lift. Accordingly,
thereisastrongnecessitytomitigatethegreyareainordertoprovidetheindustry
with Hybrid RANS-LES approaches that are trustworthy for relevant flow situa-
tions. Precisely this was the primary objective of the Go4Hybrid project, which
focusedontwomainaspects:toprovideviableextensionstoHRLMmitigatingthe
grey area problem in non-zonal approaches and to improve embedded methods,
such that they are applicable to arbitrarily complex geometries.
1Note,thatunstablerefersheretoaneasyswitchingfromanunresolvedtoaresolvedmodelingof
turbulence.
ix
x Preface
The improvements derived in the course of the project have been assessed
againstbothfundamentalandmorecomplextestcasesandthustheeditorssincerely
hopethatthisbookprovidesahelpfulcompendiumforthereaderinterestedinthis
kind of method and grey area mitigation in particular. In this respect also the
CommonAssessmentPlatform—toourknowledgeanewideaintheframeworkof
European projects—may provide interesting insight into the behavior of the dif-
ferent approaches.
The whole outcome of the Go4Hybrid project described in this book and the
goals achieved showcase the excellent cooperation between the partners involved.
Theircollaboration—fosteredbythefinancialsupportoftheEuropeanUnion–has
not only benefitted the organizations involved, but will also lead to a wide dis-
semination of the knowledge acquired both through education of students and the
closelinkstotheindustry.Thelatterareparticularlyreflectedinthelonglistoften
Industrial Observers and two Associate Partners of Go4Hybrid.
This volume compiles all technical work and achievements obtained in the
Go4Hybrid project, starting with an overview of the project in Part I “The
Go4HybridProject”,followedbethepresentationoftheapproachesusedinPartII
“PresentationofApproaches”.InPartIII“ResultsforFundamentalTestCases”and
Part IV “Results for Complex Test Cases” all relevant results are presented and
discussed. An activity was set up within Go4Hybrid referred to as the “Common
Assessment Platform” (CAP) with the goal to implement and assess the most
promising partner methods developed in the project using a common CFD code.
The major findings of this endeavor can be found in Part V “The Common
Assessment Platform”. Last but not least, Part VI contains the “Conclusions”
drawn. All references (used in the different parts and chapters) can be found at the
end of the book.
ThanksandcongratulationsareduetotheGo4Hybridpartners,whoseexpertise
and open collaboration made the project both clearly successful as well as highly
enjoyable. Special thanks are also due to the Associate Partners and Industrial
Observers,someofwhomcontributedactivelytolivelydiscussionsinthemeetings
as well as to the final workshop.
AdditionalthanksareduetoD.Knörzer,theEuropeanCommission’sScientific
Officertotheproject, whowas always verysupportiveoftheprojectandprovided
every help necessary.
Lastbutnotleast,theeditorsofthisbookwouldliketoexpresstheirgratitudeto
W. Schröder, the General Editor of the Springer series ‘Notes on Numerical Fluid
MechanicsandMultidisciplinaryDesign’,aswellastoA.Feldhusen-Hoffmannfor
their help and editorial advice, and of course to the Springer team around L. di
Cecco.
Berlin, Germany Charles Mockett
München, Germany Werner Haase
Göttingen, Germany Dieter Schwamborn
November 2016
