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Grey Area Mitigation for Hybrid RANS-LES Methods

European Union
Complete with results
Geo-spatial type
Total project cost
€1 310 254
EU Contribution
€993 410
Project Acronym
STRIA Roadmaps
Vehicle design and manufacturing (VDM)
Transport mode
Airborne icon
Transport sectors
Passenger transport,
Freight transport


Call for proposal
Link to CORDIS
Background & Policy context

The aeronautical industry lacks confidence in the accuracy of computational fluid dynamics (CFD) in areas of highly non-linear, unsteady flows close to the flight envelope borders, which demands advanced approaches and methods. The family of Hybrid RANS-LES Methods (HRLM) is the best candidate for the next generation of CFD methods for increased fidelity at industrially-feasible expense. While HRLM have been proven to perform considerably better than conventional (U)RANS approaches in situations with strong or massive flow separation, they are hampered by the Grey Area issue once they have to deal with thin separation regions and where shear layer instabilities are weaker.
As exactly these areas are of high importance for aircraft performance (lift, loads) the acceptance of HRLM strongly depends on the ability to mitigate the extent of the Grey Area (GA).


With the new/advanced Grey Area mitigation approaches, the Go4Hybrid project offers hybrid RANS-LES methods that improve predictive capability with increased flexibility and reduced user decision load. Hence, the incentive for future use of these highly sophisticated methods is in line with a considerably high impact:

  • Progress beyond the state-of-the-art for non-zonal methods is achieved by the development and demonstration of generally-applicable extensions to mitigate the Grey Area problem, thereby extending their applicability to important industrial flows at the performance frontiers.
  • For embedded methods, a focus will be placed on improving methods so that they are applicable to arbitrary complex geometries, as opposed to many existing techniques that require e.g. canonical boundary layer assumptions or homogeneous flow directions and are hence fundamentally less flexible.

In general, development work will pursue a number of key goals contributing to extended applicability, improved accuracy, increased flexibility, reduced user decision load and increased Technology Readiness Level of hybrid approaches.


Parent Programmes
Institution Type
Public institution
Institution Name
The European Commission
Type of funding
Public (EU)
Specific funding programme


High-fidelity air flow simulations

EU-funded researchers have refined an innovative mathematical method to support simulations that can replace costly test flights in the safety assessment of new aircraft designs.

The aeronautical industry lacks confidence in the accuracy of computational fluid dynamics (CFD) close to the edges of the flight envelope. Existing algorithms are still inadequate to comply with stringent requirements related to showing maximum lift and stability.

Although relatively large vortices can be accurately reproduced, calculating low-level turbulence involves modelling that is far from ideal, supplemented by costly physical testing. Between the two extremes lies a grey area where both test methodologies need to be combined.

The GO4HYBRID (Grey area mitigation for hybrid RANS-LES methods) project aimed to refine such hybrid large eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) methods. Researchers explored different approaches to improving their predictive accuracy and user friendliness.

Focus was set on improving hybrid RANS-LES methods so that they are applicable to arbitrary complex geometries, as opposed to existing techniques assuming canonical boundary layer or uniform air flow. During the three-year project lifetime the researchers achieved a breakthrough in the field of complex CFD simulations.

Significant improvements to methods such as detached eddy simulations have opened the door to increased reliability of turbulent structures calculations at feasible computational cost. These have proven suitable for weakly separated flows typical in maximum lift conditions.

Among the challenges addressed was lift optimisation using hybrid turbulence-resolving simulations based on a very large number of parameters. In addition, researchers developed a new strategy to optimise noise emission attenuation for airliners.

By extending the capabilities of RANS-LES methods, GO4HYBRID has increased their technology-readiness level and the aeronautical industry's confidence in their predictions. Importantly, knowledge gained through the mitigation of the grey area problem will help reduce design cycles and time-to-market of innovative aircraft designs.


Lead Organisation
Cfd Software - Entwicklungs- Und Forschungsgesellschaft Mbh
Wolzogenstrasse 4, 14163 Berlin, Germany
EU Contribution
€191 200
Partner Organisations
Totalforsvarets Forskningsinstitut
Gullfossgatan, 164 90 Stockholm, Sweden
Organisation website
EU Contribution
€15 141 036
Totalforsvarets Forskningsinstitut
Gullfossgatan, 164 90 Stockholm, Sweden
Organisation website
EU Contribution
€165 750
New Technologies And Services
Dobrolyubov Ave., 14, SAINT-PETERSBURG, 197198, Russia
EU Contribution
€97 987
Manchester Metropolitan University
All Saints Building, Oxford Road, MANCHESTER, M15 6BH, United Kingdom
Organisation website
EU Contribution
€112 690
Stichting Centrum Voor De Ontwikkeling Van Transport En Logistiek In Europa
Van Nelleweg 1, 3044 BC Rotterdam, Netherlands
Organisation website
EU Contribution
€142 500
Office National D' Etudes Et De Recherches Aérospatiales
29, avenue de la Division Leclerc, BP72 CHÂTILLON CEDEX, France
Organisation website
EU Contribution
€142 599
Deutsches Zentrum Fr Luft Und Raumfahrt E.v
Linder Hoehe, 51147 KOELN, Germany
Organisation website
EU Contribution
€140 685


Technology Theme
Computer-aided design and engineering
Next generation computational fluid dynamics (CFD)
Development phase

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