Dye visualisation of the formation of a synthetic jet in water (top left to bottom right), whose non-dimensional stroke length exceeds the circulation limit of the primary vortex. Consequently, excess circulation is shed resulting in an unstable shear column, followed by the roll-up of secondary (and tertiary) vortices. Composite image; not enhanced.
Dye flow visualisation of vortex shedding in a synthetic jet —

Dye visualisation of the formation of a synthetic jet in water (top left to bottom right), whose non-dimensional stroke length exceeds the circulation limit of the primary vortex. Consequently, excess circulation is shed resulting in an unstable shear column, followed by the roll-up of secondary (and tertiary) vortices. Composite image; not enhanced.

https://fluids.ac.uk/files/10_Mark_Jabbal_Instability_in_a_synthetic_jet.1543871226.jpg