No abstract

Today, wake turbulence related aircraft separations are a limiting factor for the capacity of airports worldwide. Detailed knowledge about the severity of potential wake encounters is necessary for a safe revision of the current ICAO separation requirements. Vortex encounter simulation is an important part in building an understanding how the encountering aircraft is adversely affected by the vortex-induced disturbances. Whereas most research has focused on aircraft encounters with a pair of straight vortices, this paper describes the simulation of encounters with perturbed vortices as they are observed under various atmospheric conditions. A method to account for unsteady aerodynamic effects during computation of vortex-induced forces and moments is presented. These effects are becoming especially important for vortex-induced disturbances of higher frequency as they appear during encounters with perturbed vortices. In combination with existing models for vortex deformation due to long-wave Crow instability, the realism of encounter simulations is increased. The simulation models are suitable for real-time and fast-time applications such as piloted simulator experiments and fast-time Monte Carlo simulations. First results of a simulator study with airline pilots are discussed. Nomenclature c = chord length a 1 …c 2 = vortex ring shape parameters A V = deformation amplitude for wavy vortices b = generator aircraft wing span b V = span of straight vortex pair C L = lift curve slope dv = induced velocity due to an infinitesimal straight vortex segment k = reduced frequency m = generator aircraft weight n = vortex ring counter N* = nondimensional Brunt-Väisälä frequency r = distance to vortex center r C = vortex core radius r V = position vector from origin of vortex coordinate system to point on vortex axis s = local variable for vortex rings t 0 = time of incremental gust step change T V = deformation parameter for vortex rings V = airspeed V t = tangential vortex velocity v V , w V = vortex-induced velocities w g = gust downwash  V ,  V = lateral end vertical encounter angles  = local vortex-induced angle of attack  = vortex circulation  = wavy vortex wave number * = nondimensional deformation growth rate * = nondimensional eddy dissipation rate  V = spatial phase shift of vortex deformation  1 ,  2 = lag states of unsteady downwash  V = wavelength of Crow instability  V = inclination angle of wavy vortex deformation t* = nondimensional times  = frequency of disturbance Subscripts g = gust i = strip number qs = quasi-steady step = step change us = unsteady V = vortex

Simulation of aircraft wake-vortex encounters is regularly applied in the research toward revised aircraft separation minima. Most encounter flight simulation studies have used a pair of straight counter-rotating vortices. However, after being shed by the generating aircraft, the wake vortices begin to develop a growing deformation due to the long-wave Crow instability, especially under low-turbulent atmospheric conditions when vortices decay slowly. In this study, two methods for simulation of aircraft encounters with such perturbed wake vortices are described and compared. The first method uses a vortex simulation model, which is based on analytical models of the deformation that consider the large-scale vortex shapes. The second method applies vortex-velocity fields from high-fidelity largeeddy simulations. It offers the highest possible level of realism and is used as the reference. The resulting aircraft responses induced by wavy vortices and vortex rings are reproduced with good quality by the vortex simulation model. The comparison of both methods shows that the vortex simulation model captures the overall impact of vortex deformation on the aircraft upsets. It can be used for future safety assessments that require a large number of encounter simulations.

No 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.