Investigation of Encounters with Deformed Wake Vortices using a Monte-Carlo Simulation Methodology

Abstract

Flight simulation is an important tool for investigating the impact of a wake-vortex flow field on aircraft response and on the severity of wake vortex encounters as it allows to evaluate a huge variety of encounter parameters under controlled conditions without risk, at low costs and time-efficient. The majority of simulation studies has used a pair of counter-rotating straight vortices to represent the vortex wakes. However, as part of the decay process, the vortices develop a periodic long-wave deformation due to Crow instability that may lead to a breakup into vortex rings. The paper describes a methodology to perform Monte-Carlo simulations of aircraft encounters with such perturbed vortices. A high-fidelity six degree-of-freedom aircraft simulation is coupled with a wake encounter simulation that includes models of wavy vortices and vortex rings, that are fitted to wake shapes from pre-computed large-eddy simulations. Application of the methodology is demonstrated by assessing aircraft bank angle upsets from encounters with straight and deformed vortices for different test setups. First results indicate that the impact of wake deformation is strongly affected by the simulated encounter scenario. For a full-scale Monte-Carlo simulation with uniform parameter distributions and assuming the same and constant vortex strength, vortex rings exhibit a lower probability of experiencing bank angle upsets above a specific threshold, whereas only small differences between straight and wavy vortices are observed.

Sensitivity Analysis of Aircraft Encounters with Deformed Wake Vortices (Invited)

“…It was indicated in Ref. 17, that the impact of vortex deformation on encounter severity is less significant if only encounters with a targeted intersection of the vortex core are considered. If such a scenario is the basis of investigation, only the changes in encounter geometry will impact the aircraft upsets.…”

“…Identification of the vortex shapes based on Jeong and Hussain's λ 2 criterion 20 and fitting of the shape parameters is described in Ref. 17. The LES cover wake decay at low turbulence and different levels of atmospheric thermal stratification (given by the dimensionless Brunt-Väisälä frequency N * = N t 0 with time scale t 0 = 2πb 2 0 /Γ 0 ), but only wake shapes from the reference case with neutral stratification (N * = 0) are used here, because those conditions allow for vortex rings to develop before the vortex structure begins to dissolve due to the interaction with the atmosphere.…”