THE STRUCTURE OF INTENSE VORTICITY IN ISOTROPIC TURBULENCE

The structure of the intense-vorticity regions is studied in numerically simulated homogeneous, isotropic, equilibrium turbulent flow fields at four different Reynolds numbers, in the range Re(lambda) = 35-170. In accordance with previous investigators this vorticity is found to be organized in coherent, cylindrical or ribbon-like. vortices ('worms'). A statistical study suggests that they are simply especially intense features of the background, O(omega'), vorticity. Their radii scale with the Kolmogorov microscale and their lengths with the integral scale of the flow. An interesting observation is that the Reynolds number gamma/nu, based on the circulation of the intense vortices, increases monotonically with Re(lambda), raising the question of the stability of the structures in the limit of Re(lambda) --> infinity. Conversely, the average rate of stretching of these vortices increases only slowly with their peak vorticity, suggesting that self-stretching is not important in their evolution. One- and two-dimensional statistics of vorticity and strain are presented; they are non-Gaussian and the behaviour of their tails depends strongly on the Reynolds number. There is no evidence of convergence to a limiting distribution in this range of Re(lambda), even though the energy spectra and the energy dissipation rate show good asymptotic properties in the higher-Reynolds-number cases. Evidence is presented to show that worms are natural features of the flow and that they do not depend on the particular forcing scheme.