This paper analyzes mechanism of generation of an axial flow of a vortex in homogeneous isotropic turbulence. The axial flow can be supposed to be generated by the pressure gradient through vortical region or normal to swirl plane, while a bundle of vorticity line has been revealed to be associated with the generation. The present study analyzes eigenvalues of azimuthal component of the vorticity vector, Hessians of the axial flow, and the pressure gradient in swirl plane defined by vortex space based on local flow geometry, and investigates their relations. Joint probability density functions between these quantities suggest that swirling of the vorticity lines in swirl plane gives significant effect in the generation of axial flow. Since the rotation of the vorticity line can be expressed by the Laplacian of the axial flow mathematically, the viscous term of the axial flow in the Navier–Stokes equation may have an important role.
SAKURAI et al. (Wed,) studied this question.