Engineering Notes ENGINEERING NOTES are short manuscripts describing new developments or important results of a preliminary nature. These Notes should not exceed 2500 words (where a figure or table counts as 200 words). Following informal review by the Editors, they may be published within a few months of the date of receipt. Style requirements are the same as for regular contributions (see inside back cover).
The decay of wing-tip vortices under the in uence of turbulence in a stably strati ed atmosphere is discussed by means of large-eddy simulations. The vortices originate from a B-747 aircraft in cruise. Atmospheric turbulence and turbulence originating from the boundary layer around the aircraft are distinguished. The former is weak and anisotropic with eddy sizes in the order of the wing span, whereas the latter is wrapped around the vortices with the maximum intensity at the core diameter. During their descent, the parallel vortex tubes approach each other because strati cation and turbulence detrain mass into the ambient air. The atmospheric eddies deform the trailing vortices such that their spacing varies. This, in turn, yields different mutually induced velocities that amplify the deformation quickly according to Crow's instability theorem. The bent vortex tubes link after about 1.5 min and form rings. The continuous trail of turbine exhaust is reorganized in a row of single puffs. Without any atmospheric turbulence the vortices approach each other but remain parallel. They start to dissolve after 2 min when they touch. This dissolution is triggered by the small-scale boundary-layer turbulence. The exhaust trail remains aligned along the ight track.
To mitigate the risk of wake vortex encounters during final approach, 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. Each plate line consists of several upright plates that are installed underneath the approach glide path. While the plate line extends perpendicular to the flight direction, its individual plates are oriented in parallel to the runway centerline. In order to obtain the approval of the authorities for the installation of the plate lines at runway 16 of Vienna International Airport, the plate design had to comply with airport requirements like obstacle clearance, stability, and frangibility. During a six-month campaign wake vortex behavior with and without plates was measured simultaneously by three lidars completed by a comprehensive suite of meteorological instrumentation. A preliminary analysis of 430 measured wake vortex evolutions indicates that the plate lines reduce the lifetime of longlived vortices in a safety corridor along the final approach on average by more than 30%. More comprehensive analyses are underway and will be presented at the AVIATION Forum 2020 in Reno.
Large-scale distortion of aircraft wake vortices appears to play a crucial role for aircraft safety during approach and landing. Vortex distortion is investigated based on large eddy simulations of wake vortex evolution in a turbulent atmosphere. A vortex identification method is developed that can be adapted to the vortex scales of interest. Based on the identified vortex center tracks, a statistics of vortex curvature radii is established. This statistics constitutes the basis for understanding, modeling, and exploiting mitigation effects of vortex distortion on wake vortex encounters.
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