The looming crisis of air traffic capacity - can vortex dynamics help?
Fazle Hussain
Department of Mechanical Engineering
University of Houston
Abstract:
By 2025, the air traffic capacity will become about threefold, demanding a tripling of runways at world's major airports. Forced by safe aircraft separation, this challenge is not only at takeoffs and landings, but also during cruise in the crowded skies. We propose a method of breaking up the trailing vortices and hence inducing their rapid decay so that safe aircraft separation can be significantly reduced, thus minimizing the need for additional runways and flight delays.
This crisis has motivated us to study via direct numerical simulation the evolution of a vortex column embedded in fine-scale turbulence. We explore three potential mechanisms for core perturbation growth: (a) centrifugal instability due to circulation overshoot inherent to a turbulent vortex, (b) Kelvin wave growth due to resonance with the external turbulence, and (c) transient growth in the normal-mode-stable vortex. We show that both (a) and (b) can indeed occur resulting into growth of perturbations, but are inherently self-limiting, thus not likely useful for inducing vortex breakup. We explore the mechanisms involved in possible transient growth and show that transient growth can produce many orders of magnitude amplification of the core perturbations and hence possible breakup of trailing vortices - particularly at Reynolds numbers of aircraft trailing vortices.