INTERACTION OF HELICAL MODES IN FORMATION OF VORTICAL STRUCTURES IN DECAYING ISOTROPIC TURBULENCE

A transition process from vortex layers into vortex tubes through the Kelvin-Helmholtz instability in decaying homogeneous turbulence is stu died numerically in terms of the helical decomposition. For smooth ini tial conditions high-vorticity regions with positive and negative heli city generally take the form of blobs. In the early stage with almost inviscid dynamics these blobs become flatter and layer-like, approachi ng simultaneously each other. In the next stage a pair of high-vortici ty strips with opposite-signed helicity attach each other. It forms a precursor of a vortex tube with an internal dipole structure, which co rresponds to the surrounding dipoles of high energy-dissipation. At la ter times two high-vorticity tubes with opposite-signed helicity join in series into a single vortex tube. They are separated again by the v iscous diffusion. The recovery to a reflectional symmetry state is als o studied using a strongly helical initial condition.