Three Dimensional Feature of Coherent Fine Scale Eddies in Homogeneous Isotropic Turbulence.

Abstract

Three dimensional structure of coherent fine scale eddies in turbulence is investigated by using a DNS data base of homogeneous isotropic turbulence. The axes of coherent fine scale eddies are determined and characteristics are clarified. The axis has several nodes which are identified as the minima of the second invariant on the axis. Along the axis, the maximum azimuthal velocity and axial velocity show relatively large fluctuations, while diameters are nearly constant. At each node, the axis bends with a large angle. Magnitude of the advection velocity and direction of movement of the coherent fine scale eddy also change at these nodes. The probability density function of the length of segment between neighboring nodes shows a peak at the length of the Taylor micro scale. The segment length shows a strong correlation with the maximum second invariant and increases with the increase of the second invariant with a 1/2∼2/3 power. Probability density function of advection velocity of each segment shows a peak at 1.25 times of the r.m.s. velocity fluctuation, which means that coherent fine scale eddies are very active in turbulence.