Control of local vorticity generation and shear layer instabilities in transverse jets*

The present experimental study examines the effects of strategic positioning of small tabs in the periphery of the exit plane of a gaseous jet in crossflow (JICF) as a means of controlling vorticity generation and mixing.  Tab location can significantly influence the dynamics of the shear layer instability as quantified via acetone planar laser induced fluorescence (PLIF) imaging and stereo particle image velocimetry (PIV). When the JICF is created with a low jet-to-crossflow momentum flux ratio J, typically below 6-10, which in the absence of tabs is associated with a globally unstable upstream shear layer (USL), the tab can significantly weaken the instability.  When the tab is placed in the jet's upstream region, for example, the USL can become convectively unstable.  The effects of tabs on the JICF at a much higher values of J are not quite as significant, but further weakening of the convectively unstable USL does have an influence on cross-sectional symmetry and hence vorticity evolution and mixing.  Proper orthogonal decomposition (POD) enables quantification of changes in the shear layer dynamics, providing evidence for the potential to tailor local vorticity and mixing via simple geometrical alterations.