Principle of the instability

    Here again this kind of instability can be interpreted as a double Kelvin-Helmholtz instability like for Stroual instability.

    The flow arrives from a hole,. So a positive vortex sheet develops at the top of the shearing layer instead a negative one at the bottom of the layer. In, fact there is a double line of vortices which grows up in space to give turbulence (part c of the following figure).
 
 

    This kind of configuration is more unstable than slip-stream like for Stroual instability then it is more easy to visualize it.

    Like this configuration is not very stable, instability in a jet is not difficult to obtain and it is well known nowadays.
 
 

    The most principal example of situation where this instability appears is the example of boosters in general : the exhaust gazes are ejected by a jet to produce push at the engine taking-off.

    We can find also such instability in Nature when a river emerges in a lake.