Aircraft wake vortex evolution in ground proximity is investigated numerically with large eddy simulations (LES). The simulations are performed with different modifications of the ground surface in order to trigger rapid vortex decay or to simulate the landing of an aircraft. The impact of environmental turbulence in terms of turbulent winds is taken into account, where wall-resolved and wall-modeled LES are performed for low and high Reynolds number cases, respectively. In order to understand wake vortex decay mechanisms in ground proximity the interaction of primary and secondary vortices is thoroughly investigated. The results show that vortex decay is initiated and accelerated with obstacles at the ground. In order to optimize obstacle shape and size we show that we can achieve a similar effect with relatively small plates as with large block-shaped barriers. Concerning large Reynolds numbers we show that turbulence effects triggered by the ground can not be modeled by a simple wall model. As a first approximation of landing we use a ramp at the ground and show that the flow disturbances are similar to the result of flat ground with obstacles. In particular two kinds of so-called end effects are superposed: pressure waves in the vortex core and the propagation of the secondary vortex structures.

Large-eddy simulations of a coherent counter-rotating vortex pair in different environments are performed. The environmental background is characterized by varying turbulence intensities and stable temperature stratifications. Turbulent exchange processes between the vortices, the vortex oval, and the environment, as well as the material redistribution processes along the vortex tubes are investigated employing passive tracers that are superimposed to the initial vortex flow field. It is revealed that the vortex bursting phenomenon, known from photos of aircraft contrails or smoke visualization, is caused by collisions of secondary vortical structures traveling along the vortex tube which expel material from the vortex but do not result in a sudden decay of circulation or an abrupt change of vortex core structure. In neutrally stratified and weakly turbulent conditions, vortex reconnection triggers traveling helical vorticity structures which is followed by their collision. A long-lived vortex ring links once again establishing stable double rings. Key phenomena observed in the simulations are supported by photographs of contrails. The vertical and lateral extents of the detrained passive tracer strongly depend on environmental conditions where the sensitivity of detrainment rates on initial tracer distributions appears to be low.

Wake-vortex evolution during approach and landing of a long range aircraft is investigated. The simulations cover final approach, touchdown on the tarmac, and the evolution of the wake after touchdown. The wake is initialized using a high fidelity Reynolds-averaged Navier-Stokes solution of the flow field around an aircraft model. The aircraft in high-lift configuration with deployed flaps and slats is swept through a ground fixed domain. The further development of the vortical wake is investigated by large-eddy simulation until final decay. The results show the formation of a pronounced shear layer at the ground and an increase in circulation in ground proximity, caused by the wing in ground effect. Disturbances at disconnected vortex ends, so-called end effects, appear after touchdown and propagate along the wake vortices against the flight direction. They lead to a circulation decay of the rolled-up wake vortices, combined with a growth of the core radius to 300% of its initial value. After touchdown wake vortices are subjected to strong three-dimensional deformations and linkings with the ground. The complete vortex evolution, including roll-up and decay, is accelerated in ground proximity. Additionally the effect of a plate line installed in front of the runway is studied with this method. The plates cause disturbances of the vortices propagating to either side and interacting with the end effects. The plate line further accelerates the vortex decay, reducing the circulation rapidly by another 25% of its initial value.

No abstract

No abstract