THREE-DIMENSIONAL CHARACTERISTICS OF THE COHERENT FINE SCALE EDDY IN TURBULENCE

Abstract

This paper presents the three-dimensional characteristics of coherent fine scale eddies related to their axis in homogeneous isotropic turbulence in order to understand the detailed features of the fine scale eddy in turbulence. The coherent fine scale eddy and their axis are identified by an auto tracking algorithm from the Direct Numerical Simulation (DNS) database, where DNS is performed based on a spectral method. It is shown that the axis of each fine scale eddy contains several nodes, in which these nodes are identified by the minima of second invariant (Q) of the velocity gradient tensor on the axis. It is also shown that the number of nodes increases for strong and relatively long eddy but it decreases for weak and short eddy. At each node the axis bends with a large angle. The maximum azimuthal velocity and axial velocity of coherent fine scale eddy show relatively large fluctuations along their axis, while the diameter is nearly constant along the entire axis of the fine scale eddy. The coherent fine scale eddies have large advection velocity but the dissipation rate along the axis of the eddy is not so large.