During take-off and landing, powerful vortices are created behind an aircraft. Such vortices result from pressure differences between the upper and lower surfaces of the wing whenever a wing generates lift. The lift-generating process leads to the formation of a vortex sheet along the trailing edge which due to its unstable character finally rolls up into a single vortex on each side of the wing, thus forming a pair of counter-rotating vortices.
The Airbus 340 model mounted on a streamlined traverse rig.
At take-off and landing, the vortices are particularly strong: the aircraft lift coefficient is rather high because flight speed is low.
While the vortices are found to dissipate very slowly, high tangential velocities are measured in them, even at a distance of several kilometers behind the aircraft.
Such vortices constitute a hazard to following aircraft which may encounter them. This is particularly important during landing when aircraft are flying in trail to approach the same runway. To assure a maximum of safety during flight, pilots must strictly adhere to proven safe separation distances between landing aircraft.
The investigation of vortices from realistic aircraft models and real aircraft in flight is the main goal of the European C-Wake project (C for "Characterisation" and "Control"). Another goal is to search for a means of destroying vortices more quickly by controlling them.
In the C-Wake research programme a number of experiments were conducted in the small towing tank of the Hamburgische Schiffbau-Versuchsanstalt (HSVA) using an underwater model of the Airbus A340 - an aircraft which has been in service for some years.
The model was submerged to a depth of 1 meter and towed at a speed of 3 m/s.
The model had a wing span of 1.2 meters (scale 1:48) and was suspended at a depth of 1 meter below the surface. Mounted to the carriage with a streamlined strut, the model was dragged at a speed of 3 meters per second through the 80 meter long and 5 meter wide tank, a facility normally used for naval investigations. At a particular station along the tank where test conditions had stabilised, the flowfield development over time was measured using a custom designed Particle Image Velocimetry measurement system from Dantec Dynamics. The size of the measuring plane was 0.44 x 0.61 meters.
As the aircraft model passed the measurement rig, both the camera and light sheet started to move slowly downward to track the vortices in their downward motion. The results are depicted in the figure above.
The figure shows the projection of the path of the vortices and the vortex structure as given by the vorticity parameter.Vorticity, a measure of vortex strength, was calculated from the flowfield for each of the 150 frames obtained during a run. Integrating vorticity yields "circulation", a more common parameter for characterising a vortex.
Dr.-Ing. Klaus Hünecke
Airbus Deutschland GmbH