A Geometrical Characteristic of a Vortex Birth in Terms of Local Flow Geometry in Isotropic Homogeneous Turbulence

The present study investigates geometrical characteristics of a vortex birth, i.e., bran-new vortex in terms of the local flow geometry in an isotropic homogeneous decaying turbulence in the Direct Numerical Simulation. We capture the transition of the geometry in the vortex birth excluding vortex generation derived from transfer of a vortical region. We analyse with several physical quantities associated with the invariant and detail local geometry (Nakayama, FDR 2014). The swirlity that specifies the unidirectionality and intensity of the azimuthal flow monitors the flow transition where the swirlity changes from a negative to positive value. The quantity of the vortical flow symmetry shows that a vortex is born with much skewed swirling motion. On the other hand, the sourcity associated with the symmetry and intensity of the radial flow indicates that a born vortex has a tendency to have symmetric radial flow such as complete inflow or outflow in all directions in the swirl plane, because the intensity of radial flow is still dominant in the vortex birth. An inflow vortex has a feature to develop stronger that an outflow vortex.