Influence of Vortex Core on Wake Vortex Sound Emission

Abstract

A possible sound emission mechanism of aircraft wake vortices has been identified by using both measurement data and theoretical results. This mechanism relates consistently to a dominant frequency of sound pressure matching the rotation frequency of a Kirchhoff vortex. The rotation of the Kirchhoff vortex is due to the self-induction inside the vortex core. Numerical simulations are necessary for a more realistic wake consisting of a counterrotating vortex pair with inviscid ground effects and shear flow. The simulations are carried out using a vortex particle method. A far-field vortex sound formulation, based on asymptotic expansions, is developed to calculate acoustic pressure from the wake vortices. The simulation results confirm that the frequency of the wake sound emission is essentially the same as the classical Kirchhoff vortex, even under the influences of an inviscid ground effect or a weak-shear cross wind. The aforementioned results suggest that the identified mechanism should be fairly robust, which demonstrates its persistency under several different environmental conditions.