Large-eddy simulations of the vortex-pair breakup in aircraft wakes

We have utilized large-eddy simulations to simulate the transition from an aircraft wake dominated by the interaction of vortex-pair dynamics and engine plume buoyancy to one dominated by atmospheric dispersion. Our investigation concentrates on the period from a few seconds to several minutes after the wake is generated, during which the essentially two-dimensional vortex-pair is broken up into a variety of three-dimensional eddies. Our initial conditions are taken from near-wake simulations for three aircraft: the B-737, the B-747, and the ER-2. Comparisons of our simulations, for conditions representative of the atmosphere in the upper troposphere and the lower stratosphere, with wake photographs taken from the ground show very similar features. In the absence of wind shear, the vortex pair go through a linking instability that is affected both by engine buoyancy and ambient turbulence. The resultant series of vortex rings continue to descend, leaving the wake with the appearance of a series of suspended puffs. When the mean atmospheric shear is sufficiently large, the ambient vorticity can erode one member of the vortex pair, leading to a breakup mode with a much shorter wavelength.