Europe seeks to reduce aircraft development and operating costs in the short and long term. This must be accomplished both through improved aircraft performance and through reduction in maintenance and direct operating costs. To reach these objectives, the aeronautical industry needs improved models based on a deeper understanding of the physics, which in turn must be acquired using the most advanced experimental and modeling methods. While this is true for all the aspects of the design and operation of an aircraft, it is particularly true for aerodynamics.
Although aerodynamics has made tremendous progress in the last century, it still lacks reliable turbulence models (which are also crucial for many other industrial design problems) and the understanding to develop them. The search for these models is a very active domain for research and improvement. In fact, turbulence remains one of the great unsolved riddles of engineering and natural sciences, nowhere more so than for flow near surfaces.
The WALLTURB project was a challenging research programme within the objectives of FP6 in Aeronautics and of strong industrial interest for the intermediate and long term.The global aim of WALLTURB is to progress significantly in the understanding and modelling of near wall turbulence in Boundary Layers.
In this respect, the WALLTURB objectives are the following:
- to advance the knowledge about and the prediction of wall-bounded turbulent flows;
- to put a common database, shared by the WALLTURB partners, the existing relevant data they have about near wall turbulence (from both experiments and DNS);
- to generate by experiment, and by complementary DNS, equivalent data for the Adverse Pressure Gradient Turbulent Boundary Layer physics (including separated flow cases), and to put them in the common database;
- to use database to improve near wall turbulence models such RANS, LES and LODS, and especially to understand their relative strengths and weaknesses.
The work programme was divided into six Work Packages.
- Work Package 2 focused on the experiments and DNS performed during the project;
- Work Package 3 was the kernel of the project responsible for the management and processing of the eight different databases that were provided by the partners for common use;
- Work Package 4 was concerned mostly with the classical and industrial RANS approach and aimed at improving the physical content of the models, especially for Adverse Pressure Gradient flows;
- Work Package 5 was devoted to the improvement of LES modelling near the wall, and especially the investigation of new models for this region;
- Work Package 6 will investigate the possibilities of the fairly recent Low Order Dynamical Systems approach, and of its coupling with LES in the near-wall region.
The project methodology included:
- generating and analysing new data on near wall turbulence;
- extracting physical understanding from these data;
- putting more physics in the near wall RANS models;
- developing better LES models near the wall;
- investigating alternative models based on Low Order Dynamical Systems (LODS).
The main outputs of the programme were:
- a detailed database of results on the flow structure of turbulent boundary layers in both zero and adverse pressure gradient, suitable for both physical analysis and turbulence model validation;
- improved RANS model capable of coping properly with the near-wall region of the turbulent boundary layer;
- improved LES models with a better modelling of the near-wall region of the turbulent boundary layer; and;
- LODS models, representative of the very near-wall region of the turbulent boundary layer, which can be coupled, as boundary conditions, to the LES models.
The WALLTURB project was an opportunity to establish an up-to-date and very comprehensive set of databases on wall turbulence. These databases include both experimental and DNS data which are complementary. They have given access to all scales of near wall turbulence, both in the ZPG and APG cases. Even separation has been addressed by USur. This set of databases has already proven to be helpful in support of the modelling work in WP4 to WP6, and as well to have contributed to the theoretical understanding of turbulence. It will certainly continue to be used to support both theoretical and numerical investigations in the near future.
Dynamical systems representative of the near wall turbulence were built and compared to the real flow. All the problems encountered could not be solved, but the results available give good hope that low order dynamical systems can represent fairly correctly the main features of near wall turbulence. The question of coupling such a system to a LES solver is still open but is worth investigating in a future project.
Significant work has been performed on turbulence modelling, leading to the improvement of existing models and to the development of new ones. This is particularly true in RANS modelling, where models developed in the frame of the project have been tested by Airbus in its in-house code with some good success. The work done on LES has shown the potential of the method and its applicability to adverse pressure gradient flows of industrial interest. This is surely a modelling approach of the future which should see industrial partners entering into the game soon. The Low Order Dynamical System approach has more long term perspectives but has shown its ability to represent relatively faithfully the main features of near wall turbulence. The coupling with a LES solver is still an open question.