Is the assumption of straight vortices valid for encounter hazard assessment?

Abstract

Wake vortices can be strongly deformed during the decay process depending on the atmospheric conditions in terms of turbulence and thermal stratification. Nevertheless, vortices are mostly assumed to be straight for encounter hazard assessment. The validity of this assumption was never verified. For this reason, a simulation study was conducted in order to investigate the influence of vortex deformation on the wake encounter characteristics. Encounters of a single aisle, medium range transport aircraft of the “Medium” category behind a generator aircraft of the “Heavy” category were simulated with straight vortices and with realistically deformed vortices derived from large-eddy-simulation. All relevant parameters, which influence the encounter characteristics, like encounter angles and positions were varied within a wide range. In order to cover all kinds of vortex deformation encounters with different vortex ages from 16 to 136 seconds were simulated. Hence, all relevant phases during the vortex decay from nearly straight and wavy vortices to vortex rings were considered. The parameter variation study revealed that on average the impact on the encountering aircraft is less with deformed vortices than with straight vortices of comparable strength. Especially with vortex rings the encountering aircraft is exposed to a much smaller impact on average. Also, strong encounters are less probable to occur with deformed vortices. However, the results also show a stronger aircraft response during encounters with wavy vortices shortly prior to vortex linking than with straight vortices of comparable strength. Furthermore, vortex deformation shifts the encounter angles, at which the strongest encounters occur, towards greater angles. The study reveals that the assumption of vortices being straight is indeed conservative for young and very old vortices. However, in the stage of wavy vortices such assumption might underestimate the encounter hazard.