Simulation of Aircraft Encounters with Perturbed Vortices Considering Unsteady Aerodynamic Effects

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

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

“…5, to simulate piloted encounters with perturbed wake vortices during final approach. 10 The results confirmed the reduced severity of vortex rings found by the prior studies, both in terms of objective aircraft bank angle upsets as well as subjective pilot severity assessment. Meanwhile, encounters with wavy and straight vortices were rated equally severe by the pilots.…”

“…Additional details are provided in Ref. 10. The impact of those effects depends on the disturbance frequency.…”

“…1 The flight simulator was recently used in a piloted simulator campaign of aircraft encounters with perturbed vortices. 10 Although the aircraft is not in active service, pilots with type ratings for modern sidestick-controlled transport aircraft (e. g., A320, A330) accustomed quickly with the aircraft and found manual control to be mostly identical.…”

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

“…However, it was also described in Ref. 15 that bank angle upsets induced by wavy vortices exhibit a wider range and may in some cases even exceed those induced by straight vortices.…”

“…In addition, the uncertainties in vortex modeling, i. e. whether to include wake deformation in safety assessments or to use a simple straight vortex pair, are assumed to be larger than the inaccuracies of highly sophisticated aircraft flight simulation models. Aircraft encounters with wavy vortices and vortex rings have been analyzed by means of offline simulations 12,13 and piloted tests in certified training and research flight simulators 14,15 or by in-flight simulation, 16 using either simplified models of the deformed vortices or three-dimensional fields of vortex-induced velocities from prior Large-Eddy Simulation (LES). Those studies indicate a reduced encounter severity for the vortex ring phase after linking.…”