SMALL-SCALE INTERMITTENCY IN TURBULENCE

The present theoretical and experimental knowledge of the intense inte rmittent events in high Reynolds number turbulence is reviewed. An att empt is made to relate the two main streams of research in this area: the multifractal description and the coherent filaments identified in numerical simulations. It is concluded that, although both approaches can be expressed in a common language, they are inconsistent in detail and both are incomplete. While the multifractal approach is a kinemat ic description without dynamics, the present understanding of the scal ing and dynamics of the coherent vortices requires the existence of st able laminar structures of arbitrarily large Reynolds numbers, and pro bably represents only one of a hierarchy of intermittent objects. It i s suggested that the vortices may not survive at very high values of R ex, and that the problem extends to all the intermittent structures of turbulence. It is shown that the filaments should dominate the struct ure functions in a range of scales that goes from beyond the Kolmogoro v scale for p < 4 to the whole inertial range if the scaling exponent zeta(p) --> infinity (usually expected as p --> infinity.) (C) Elsevie r, Paris.