Resonance in axisymmetric jet under controlled helical fundamental and axisymmetric subharmonic forcing

Abstract

An axisymmetric jet is forced with two helical fundamental waves of identical frequency spinning in the opposite directions and an additional axisymmetric subharmonic wave. The subharmonic component rapidly grows downstream from resonant interaction with the fundamental, significantly depending on the initial phase difference between the subharmonic and the fundamental. The variation of the subharmonic amplitude with the initial phase difference shows a cusplike shape. The amplification of the subharmonic results in a vortex pairing of helical modes. Furthermore, the azimuthal variation of the amplification induces an asymmetric jet cross section. When the initial subharmonic is imposed with an initial phase difference close to a critical value, the jet cross section evolves into a three-lobed shape. The lobes are generated by vortex pairing. Therefore, the conclusion is made that the initial phase difference between the fundamental and the subharmonic plays an important role in controlling the jet cross section