Reducing the extent of turbulent flow by delaying laminar-turbulent transition on an aircraft wing is of considerable practical interest because it reduces the friction drag. In supersonic flow, it also contributes towards satisfying the strict requirements on emission and noise. In this project, fundamental, numerical and experimental investigations were carried out for evaluating the capabilities of several control techniques on supersonic civil aircraft wings.
The general objectives of the SUPERTRAC project were to explore the possibilities of skin friction drag reduction on supersonic aircraft wings by delaying laminar-turbulent transition.
The following laminar flow techniques were tested:
- micron-sized roughness elements;
- suction at the wall (Laminar Flow Control); and
- pressure gradient optimisation (Natural Laminar Flow).
In addition, the problem of preventing leading edge contamination was addressed.
To support these investigations, three models will be used: two 'physical' models with a simple geometry, which will be tested in supersonic wind tunnels, and one 'numerical' (and more realistic) model, which will be used for computations only.
At the end of the project, much original information will be available:
- experimental data based on the effects of suction, micron-sized roughness elements and anti-contamination devices;
- advanced numerical tools for the design of these control systems;
- statements concerning the efficiency of the various control techniques investigated; and
- definition of the best 3D wing shape and estimation of the benefits.
The project was divided into six Work Packages.
In Work Package 1 (Specifications), the industrial partners provided a quantitative definition of the objectives, as well as the preliminary definition of a fully 3D wing, which was used as a reference shape ('numerical' model).
The objective of Work Package 2 was to define a simple model (swept wing of constant chord) equipped with micron-sized roughness elements and anti-contamination devices. This model was manufactured and tested in the S2 wind tunnel of the Modane-Avrieux ONERA centre.
Work Package 3 ran in parallel with Work Package 2. Another swept wing of constant chord, equipped with a suction panel in the leading edge region, was designed, manufactured and tested in the RWG wind tunnel of DLR Göttingen.
Work Package 4 used the 'numerical' model defined in Work Package 1. The objectives were:
- to numerically investigate the concept of Natural Laminar Flow Control by shape optimisation; and
- to analyse the compatibility of the different control techniques, in particular those of Work Packages 2 and 3. This will result in the definition of the best compromise for skin friction drag reduction.
The results of Work Packages 2 to 4 will be summarised in Work Package 5 by the industrial partners, who will provide a quantification of the benefits and recommendations for practical applicability to future supersonic aircraft wings.
Work Package 6 is devoted to the management and the exploitation of the project.
SUPERTRAC provided information of practical/industrial interest concerning the possibilities of laminar flow control at supersonic speeds. Some of them are extrapolations of results already established in the transonic regime. As a final achievement of the project, the 'best' supersonic 3D wing has been defined and the expected benefits (in term of drag and fuel consumption reduction) have been estimated. It is clear that for the large sweep angle wing considered here, NLF alone is not sufficient for obtaining significant skin friction gains. However the application of a small amount of suction makes it possible to increase the laminar flow extent in a significant manner. Of course, many technological problems, such as the compatibility with leading edge high lift devices or the effect of surface imperfections need to be studied. These issues were out of the scope of the present project but could be addressed in future projects dealing with laminarity at high speed.
When SUPERTRAC started, there were practically no published results concerning the possibilities to laminarize a supersonic wing. Therefore a large part of the numerical and experimental studies performed in the framework of the project can be considered as innovative.
The transition control by MSR is a new approach, which had never been validated in Europe, at least for supersonic conditions. The computations allowed a critical assessment of the capabilities of this concept. A strategy for the use of nonlinear Parabolized Stability Equations (PSE) was developed by the partners, so that systematic applications of this control technique are now possible, at least numerically.