Measurement of vortex ring characteristics during pairing in a forced subsonic air jet

The pairing of vortex rings in an acoustically excited jet is experimentally investigated. The acoustical forcing of the jet allows a series of particle image velocimetry (PIV) measurements to be performed at a frequency slightly lower than the frequency at which the vortex rings are shed in the shear layer. This technique, here called stroboscopic PIV, provides a time-resolved evolution of the velocity and vorticity fields during the threading and merging of the vortex rings, with a view to the prediction of the sound produced by this interaction. The evolution of the characteristics of the cores of the vortex rings during pairing is determined using an automated vortex detection algorithm. Three stages are distinguished in the evolution of the rings, based on the relative acceleration and deformation of their cores. The conservation of the circulation during the merging is addressed. The accuracy of the measurements is estimated, being a crucial issue for the future prediction of the sound generation based on the experimental aescription of the flow field. (Author)