COHERENT STRUCTURES AND INCOHERENT TURBULENCE.

What are coherent structures? What roles do these structures play in transport phenomena and production of incoherent turbulence? This paper briefly addresses these questions as well as the concepts of 'preferred modes' and 'negative production'. Studies of coherent structures in mixing layers suggest that the saddle is the most crucial to the dynamics of the structures. It is concluded that the saddle region or the braid is not a continuous sheet but consists of spatially-separated 'ribs' which have significant vorticity aligned with the diverging separatrix. The stretching of the ribs induces entrainment and produces incoherent turbulence which is then transported away from the saddle to feed the large-scale coherent structures. Similar mechanisms appear to be central to entrainment and production in all turbulent shear flows.