DNS of Aircraft Wake Vortices: The Effect of Stable Stratification on the Development of the Crow Instability
A numerical experiment is performed to determine the likelihood that the Crow instability will mitigate the potentially hazardous effects of the buoyancy-induced rebound of initially parallel line vortices. Parameters are chosen to correspond to wake vortices downstream of an elliptically loaded wing with large span (typical of the A380) landing in very stable conditions. The DNS is initiated by perturbing the vortex pair into the shape of the linearly most unstable Crow eigenmode, with a maximum displacement of 1% of the initial distance between the pair. Under these conditions, the Crow instability progresses fast enough to break the two dimensionality of the vortex system before it returns to its original elevation. This suggests that in many cases the Crow instability will prevent the rebounding vorticity from being a serious danger to following aircraft. Whether or not this will always happen in practice is an open question, requiring further investigation.
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This work was done as part of the UK Turbulence Consortium, sponsored by the Engineering and Physical Sciences Research Council (Grant EP/D044073/1). The computations were done on the EPSRC HPCx and HECToR clusters. We are grateful to Dr Philippe Spalart for his contribution to this study.