Coherent structures and incoherent turbulence

The role of coherent structures in the heat, mass, and momentum transport phenomenon and the concepts of preferred modes and negative production are studied analytically. It is suggested that the saddle region, or the braid, is not a continuous sheet but consists of spatially separated ribs that 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 are noted to be central to entrainment and production in all turbulent shear flows.