Vortex structures in the near field of a transversely forced jet

We investigate the effect of transverse acoustic forcing on the formation of vortex structures in the near field of an axisymmetric jet using stereoscopic particle image velocimetry. The jet is placed at different locations between the pressure anti-node and node within a standing wave, and velocity and vorticity fields were measured in the $x-r$ plane. At the pressure anti-node, the jet response exhibited an axisymmetric mode, $m=0$, as harmonic fluctuations in pressure and the streamwise velocity components result in the periodic formation of vortex rings at the forcing frequency. As the jet was moved away from the anti-node, the shear layer roll-up and resulting vortex structures become increasingly asymmetric and three-dimensional due to time-varying spatial pressure gradients across the jet exit. The location where the transverse and streamwise velocity fluctuations were of equal magnitude coincided with sudden change in the jet response, characterised by shear layer roll-up and resulting vortex structures either side of the jet being in anti-phase. At the pressure node, harmonic transverse oscillations of the jet were observed forming vortices of equal circulation on either side of the jet in anti-phase. Meandering of the potential core was also observed.