Turbulence is filled with vortical structures, namely, filamentary object (vortex tube) and ribbon-like object (vortex sheet). A conventional scenario for formation of the vortex tube along vortex sheet is a rolling-up of single vortex sheet due to the Kelvin-Helmholz instability. Our analysis, however, showed that the tube is often formed through an interaction of multiple vortex sheets. In this talk, we will present the results for this formation process obtained using DNS data for homogeneous isotropic and shear turbulence. The formation process consisted of the following steps. 1) formation of a recirculation region with low pressure at its center by multiple sheets, 2) stretching of the sheets by the recirculating flow, 3) reorientation of the direction of vorticity vectors along the sheets, 4) concentration of low pressure region and formation of the tube core region, 5) entrainment of the sheets by the tube.
As a result, a vortical structure similar to the stretched spiral vortex (Lundgren 1982, Pullin and Saffman 1993, Pullin and Lundgren 2001) was formed. Depending on the alignment of the vorticity vectors on the sheets and tube, multiple modes of configuration were found. Likelihood for survival of each mode, the effects of shear and compressibility on the formation of these modes, and an implication of these modes for the energy spectrum will be discussed.
