Identification of Coherent Fine Scale Structure in Turbulence

Theoretical description of intermittent character in small scale motion have been one of the most important subjects in turbulence research. Theorists have made efforts to establish a theory of fine scale structure by assuming various type of structure as a fine scale structure (Townsend, 1951; Tennekes, 1968; Lundgren, 1982; Pullin and Saffman, 1993; Saffman and Pullin, 1994). Most of them are based on an assumption that many tubelike or sheet-like vortices are embedded in turbulence randomly. Each vortex is considered to be an analytical solution of Navier-Stokes equations. Recent studies by direct numerical simulations of turbulence have found the evidence supporting theoretical expectations for coherent fine scale structures (Kerr, 1985; She et al., 1990; Vincent and Meneguzzi, 1991; Ruetsch and Maxey, 1991; Ruetsch and Maxey, 1992; Jimenez et al., 1993; Vincent and Meneguzzi, 1994). In homogeneous turbulence, high vorticity regions are supposed to be a candidate of fine scale structure (She et al., 1990; Vincent and Meneguzzi, 1991; Jimenez et al., 1993). However, definitions of fine scale structure based on vorticity magnitude are not clear and educed fine scale structures depend on the threshold. The objectives of this study are to specify fine scale structures in homogeneous isotropic turbulence and to investigate a scaling law and Reynolds number dependence of fine scale structures. DNS database of decaying homogeneous isotropic turbulence are analyzed by using a new identification method of fine scale structures.