A new mechanism of small-scale transition in a plane mixing layer: Core dynamics of spanwise vortices

A new mechanism of small-scale transition via core dynamics instability (CDI) in an incompressible plane mixing layer, a transition which is not reliant on the presence of longitudinal vortices ('rib') and which can originate much earlier the rib-induced transition is presented. Both linear stability analysis and direct numerical simulations are used to describe CDI growth and subsequent transition in terms of vortex dynamics and vortex line topology. It is found that energetic CDI is excited by subharmonic oblique modes of shear layer instability after pairing, when adjacent rolls with out-of-phase undulations merge. It is also found that accelerated energy transfer to high wave numbers precedes the development of roll internal intermittency; this transfer occurs prior to the development of strong opposite-signed spanwise vorticity and granularity of the roll vorticity distribution.