A Formation Process of Turbulent Structure and Its Reynolds-number Dependence in Homogeneous Isotropic Turbulence

Abstract

A process for formation of the vortex tube along the vortex sheet was studied using the direct numerical simulation (DNS) data for incompressible homogeneous isotropic turbulence. The vortex sheet, in which the vorticity and strain are comparably large, was educed using a new identification method. In an early stage of time development, the large scale vortex sheets, the lateral extend of which was several times of integral scale, were formed. Then, compression of the strain rate took place at certain locations along the vortex sheet, and the transverse vorticity was generated in turn and the transverse vorticity accumulated to form the core region of the tube. A consistency of this process with an estimate obtained using the governing equations of strain and vorticity was presented. It was shown that during this process, an intense energy cascade took place along the vortex sheet. We assessed the dependence of the occurrence of this transformation process on Reynolds number. It was shown that the frequence of its occurrence increased at a high Reynolds number, and more abundant vortex tubes were generated than at a low Reynolds number. These vortex tubes formed a cluster structure along the vortex sheet, and its implication for generation of the inertial subrange was discussed.