Near- to Midfield-Wake: Experimental Characterisation

The wake of wide body aircraft is a hazard to following aircraft in approach, climb, and cruise. Thus, a large amount of experimental research effort has been devoted to instability and breakdown of vortices. However, investigations of the instabilities of vortices in the wake far field very often are lacking the link to the near field and are inconclusive as to the aerodynamic, whereas investigations of the wake in a wind tunnel very often miss the link to the mid- and far field at a sufficiently high Reynolds-number. For this reason, the wake of an Airbus A-300 with different flap settings was investigated in a joint ONERA/DLR-project in the catapult of ONERA Lille. The Particle Image Velocimetry was chosen as a non-intrusive testing technique. In a particular setting of the landing flaps, a premature breakdown of the vortex cores was observed. The origin of the breakdown was understood as a Raylleigh-Ludwieg instability, which occurs if destabilising centrifugal forces dominate stabilising pressure forces.