Influence of a counter rotating vortex pair on the stability of a jet in across flow: an experimental study by flow visualizations

The loss of stability of a flow resulting from the interaction of a transve rse jet and a crossflow has been studied experimentally by means of several nonintrusive optical methods (Laser-Induced Fluorescence, Particle Streak Velocimetry). In particular, flow visualizations have allowed measurement o f the characteristic velocity of rotation of a counter-rotating vortex pair , which is the prevailing system of this complex flow. It is shown that the elliptical geometry of the cross sections of these vortex structures can c ause the loss of stability of the jet according to the theory of Landman an d Saffman (1987). The unstable jet state is characterized by the generation of transverse unsteady rotating structures on the jet boundaries. These tr ansverse eddies are clearly identifiable and their regularity of occurrence is satisfactorily explained by the theory of Landman and Saffman. Therefor e, this study shows for the first time that the longitudinal structures mus t be taken into account in the mechanism of stability arising from the meet ing of a jet and a cross stream.