Flow of a 3-element-airfoil (High Lift Airfoil from the LEISA project)
This three-element airfoil, called DLR-F15, has been investigated in the DLR project LEISA (Low noise exposing integrated design for start and approach, 2005-2008), which combined and focused activities in the research areas of high lift system design, flow control and aero-acoustic design methods. The underlying contour is a three-element airfoil by Airbus, while the modified positions of slat and flap were developed by DLR. With these optimised positions the flow remains attached on the flap for high Reynolds numbers close to flight conditions.
However, for the low Reynolds number considered in this test case, flow separation is observed on the flap. This separation and the airframe acoustic noise generated by the flow at the slat trailing edge are the main physical phenomena and also the main modelling challenges.
Being able to reliably predict the effects of innovative concepts for high lift systems would be a major progress directly related to the “Green Aircraft Challenge”.
While the RANS/URANS modelling approach may be sufficient if the focus is on prediction of flow separation on the flap, turbulence resolving approaches like DES (or other hybrid RANS-LES approaches) have to be used if the focus is also on acoustics.
Measurements used in this project were performed in the low-speed wind-tunnel Braunschweig (NWB), the acoustic wind tunnel Braunschweig (AWB) and the cryogenic wind tunnel Cologne (KKK).
Even for small incidence angles the wind tunnel side walls influence the flow. For incidence angles larger or equal 12°, flow separation on the wind tunnel side walls occurs even on the main wing element. In simulations that do not include the wind tunnel, these sidewall effects need to be considered, by using an adapted incidence angle. The larger the sidewall effects, the larger and less reliable become the corrections. However, for the angle of attack of 7° considered here the corrections are less than 1°.
- Wild, J., Pott-Pollenske, M., “An Integrated Design Approach for low Noise exposing high-lift devices”, AIAA paper 2006-2843, 2006