LARGE-EDDY SIMULATIONS OF THE VORTEX-PAIR BREAKUP IN AIRCRAFT WAKES

We have utilized large-eddy simulations to simulate the transition fro m an aircraft wake dominated by the interaction of vortex-pair dynamic s and engine plume buoyancy to one dominated by atmospheric dispersion . Our investigation concentrates on the period from a few seconds to s everal minutes after the wake is generated, during which the essential ly two-dimensional vortex-pair is broken up into a variety of three-di mensional eddies. Our initial conditions are taken from near-wake simu lations for three aircraft: the B-737, the B-747, and the ER-2. Compar isons of our simulations, for conditions representative of the atmosph ere in the upper troposphere and the lower stratosphere, with wake pho tographs taken from the ground show very similar features. In the abse nce of wind shear, the vortex pair go through a linking instability th at is affected both by engine buoyancy and ambient turbulence. The res ultant series of vortex rings continue to descend, leaving the wake wi th the appearance of a series of suspended puffs. When the mean atmosp heric shear is sufficiently large, the ambient vorticity can erode one member of the vortex pair, leading to a breakup mode with a much shor ter wavelength.