ENGINEERING AND CFD

As an unavoidable consequence of lift aircraft generate a pair of counter-rotating and long-lived wake vortices that may pose a potential risk to following aircraft. The highest risk to encounter wake vortices prevails in ground proximity, where the vortices cannot descend below the glide path but tend to rebound due to the interaction with the ground surface. Prediction of wake vortex drift, descent, and decay, and the resulting minimum separations between consecutive aircraft is essential for an effective, resource-efficient, and safe guidance and planning of air traffic.

Highly resolving large-eddy simulations (LES) conducted on GCS system SuperMUC of LRZ Garching provide valuable in-sights in the physics of wake vortex behaviour during different flight phases and under various environmental conditions. For example, wall-resolved large eddy simulations were employed to investigate the behaviour of wake vortices and single vortices in ground proximity for a variety of crosswind and headwind conditions [5]. A thorough analysis of the simulations demonstrates that vortex descent, rebound, ascent and decay characteristics are controlled by the interaction of the vortices with secondary vorticity detaching from the ground, the redistribution of vorticity within the boundary layer, and the interaction of the vortices with the environmental turbulence.

A hybrid RANS/LES method was developed that introduces the flowfield around an aircraft model obtained from high-fidelity Reynolds-averaged Navier–Stokes (RANS) simulations into an LES environment that simulates the vortical wake until its decay [3]. This so-called one-way coupling method enables valuable insights into the roll-up process behind specific aircraft geometries and configurations and the consequences on the resulting vortex properties. Hybrid RANS/LES have been applied to investigate effects of the detailed geometry of an A320 aircraft in high-lift configuration on wake vortex dynamics during approach and landing [10]. The hybrid simulations conducted together with the DLR Institute of Aerodynamics and Flow Technology reveal a strong coherent vortex emerging from the engine-landing-gear system interacting with the main vortices and the tailplane vortices (see Fig. 1). Similar vortex structures are also frequently observed in lidar measurement data collected at Vienna airport.

Further, the hybrid method has been applied to investigate the jet-vortex interaction in cruise flight [9]. It is found that the engine jets perturb the coherency of the wake flow such that the relatively strong engine vortices are quickly becoming turbulent. As a consequence, the development of the usually occurring sinusoidal instability is impeded delaying the decay of the entire vortex system.

Even when adhering to separation standards, aircraft regularly experience wake vortex encounters during final approach. To mitigate the risk of wake encounters and thereby to improve runway capacity, so-called plate lines have been developed. Wake vortices generated by landing aircraft induce secondary vortices at the plates’ surfaces that approach the primary vortices and trigger premature wake vortex decay. Hybrid LES were used to better understand the underlying vortex dynamics and to investigate the impact of crosswind and headwind [1, 2, 4]. Fig. 2, left, depicts the complex vortex system generated by an A340 aircraft on final approach and Fig. 2, right, illustrates the disturbances caused by the plate line situated on the left of the picture. A LES based optimization study resulted in plate dimensions of 4.5 m height and 9 m length, consisting of 8 plates and a plate separation of 20 m [7]. Encounter simulations employing LES flowfields reveal a significant reduction of the maximal vortex impact on the encountering aircraft after the installation of plate lines [6, 11].

DLR and Austro Control partnered to deploy two plate lines in front of runway 16 of Vienna International Airport (see Fig. 3) and to conduct a six-month measurement campaign employing three lidars for wake vortex characterization and a comprehensive suite of meteorological instrumentation [13]. A plate line design was established that proved to be compatible with airport requirements such as obstacle clearance, frangibility, stability against wake vortices and storms, interference with the localizer, and visibility of the approach lighting. The average of the lifetime reductions of the long-lived wake vortices for different aircraft types amounts to about 30%. This endeavour has been bestowed the ATM Awards: Second Place and named a finalist in the inaugural Maverick Innovation Awards presented by World ATM Congress 2020.