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Advanced Turbulence Simulation for Aerodynamic Application Challenges

Tags: Air

State of the Art - Background

Substantial resources have been invested over the years into Computational Fluid Dynamics (CFD). These investments (many of them made in the framework of European programmes) have resulted in a remarkable progress in the use of Computational Fluid Dynamics (CFD) for the design of new aircraft by the European airframe industry, significantly reducing the reliance on wind-tunnel and flight tests. This trend has resulted in the progressive shift of CFD priorities from numerics to flow physics and, ultimately, to turbulence modelling, which has become the weakest link in the CFD-based design chain. This is highlighted by the fact that, on the one hand, the theoretical capabilities for simulating full aircraft configurations with deployed flaps and landing gear are available, while, on the other hand, maximum-lift prediction of much simpler configurations or flow and noise predictions for an isolated landing gear fail due to turbulence modelling defects. It is clear that strengthening this link is crucial to satisfying the urgent needs of the European aerospace industries. CFD-aided design procedures for the analysis of turbulent aerodynamic flows must be improved to a level that is sufficient to resolve numerous problems directly related to the 'Green Aircraft Challenge', e.g. reliable evaluation of innovative drag and noise reducing concepts, and high lift systems allowing steeper take-off and landing.


ATAAC aims at improving the current turbulence modelling/simulation approaches available in CFD methods for aerodynamic flows. As Large Eddy Simulation (LES) will not be affordable for the high Reynolds numbers typical of real-life flows in the next decades, ATAAC focuses on approaches below the LES level, namely Differential Reynolds Stress Models (DRSM), advanced Unsteady Reynolds Average Navier-Stokes (RANS) models, including Scale-Adaptive Simulation (SAS), Wall-Modelled LES, and different hybrid RANS-LES coupling schemes. The project resources will concentrate exclusively on flows for which current models fail to provide sufficient accuracy, e.g. stalled flows, high lift applications, swirling flows (delta wings, trailing vortices) or buffet. The assessment and improvement process will follow thoroughly conceived roadmaps linking practical goals with corresponding industrial application challenges and with modelling issues through 'stepping stones' represented by appropriate generic test cases.

The final goals are:

- to recommend one or at most two 'best' DRSM for conventional RANS and Unsteady RANS;

- to provide a small set of hybrid RANS-LES and SAS methods that can be used as 'reference' turbulence-resolving approaches in future CFD design tools;

- to formulate best practice guidelines for the recommended models with clear indications of areas of applicability and uncertainty for aerodynamic applications in industrial CFD.

Description of Work

The project consists of four work packages (WP).

WP1 is dedicated to managing the project and dissemination and exploitation activities. This WP also includes the management of the website, which hosts the database and is also used for the management, communication and dissemination processes. The remaining three work packages deal with scientific and technical work.

WP2 is dedicated to the work on the different types of models under consideration and their improvement in terms of both physics and numerical efficiency.

WP3 serves the assessment of the chosen models and their improvement based on the roadmaps by employing small sets of basic and challenging applications from the fields of both aerodynamics and aero-acoustics.

WP4 is dedicated to the gathering and preservation of the knowledge gained in the project and the appraisal of the models based on the results of WP2 and WP3. This work package will run over the complete time of the project ensuring also the critical supervision of the assessment process in WP3, the adherence to quality guidelines, as well as the identification and minimisation of uncertainties. Thus it will guarantee a sound final assessment of all project results leading to the recommendation of 'standard' or 'reference' approaches from the different model strands and to a concise set of best practice guidelines.

Expected Results

Besides a set of critically assessed and improved turbulence models and approaches with the main results of ATAAC are one (or at most two) 'best' Reynolds-Stress models for conventional RANS and Unsteady RANS, as well as a small set of hybrid RANS-LES and SAS methods that can be used as 'reference' turbulence-resolving approaches in future CFD design tools. These will be supplemented by recommendations for their usage and their range of applicability and uncertainty, which will be documented in the best practice guidelines.

Contributing to reliable industrial CFD tools, ATAAC will thus have a direct impact on the predictive capabilities in design and optimisation. This is of utmost importance due to the clear tendency of the airframe industry to base their design cycles much more upon numerical simulation and to perform experiments with a significantly reduced frequency at a later point in the cycle. Increasing the trust in reliable prediction, ATAAC will thus directly contribute to the development of greener aircraft (Greening of Air Transport) as well as to improving cost efficiency.

Detached Eddy Simulation of Three-Element Airfoil showing turbulent structures
Detached Eddy Simulation of Three-Element Airfoil showing turbulent structures