INTERMITTENCY IN TURBULENCE

The phenomenon of turbulent intermittency and its characteristic features are briefly reviewed. Intermittent spottiness of turbulent activity has been modelled in various ways including dynamical models. Here we report on results from approximate solutions of the Navier-Stokes equation itself, based on a Fourier superposition with a geometrically scaling subset of wave vectors (''Fourier-Weierstrass ansatz''). Turbulent flow with Reynolds numbers up to 10(7) can be treated. The resolution comprises more than 3 orders of magnitude. The probability density function for the velocity fluctuations becomes stretched exponential with decreasing scales; intermittency in time and structures in space show up in the solution. Remarks on the restricted usefulness of Kolmogorov's refined similarity hypothesis are added. We furthermore offer a new type of scaling law for the stretching exponent for the partial derivative 1u1 probability density, in terms of an algebraic decrease in powers of the logarithm of Re(lambda).