A vortex identification method based on strain and enstrophy production invariants

Abstract

A new vortex identification method is proposed for extracting vortical structures from homogeneous isotropic turbulence. The method is compared with other identification schemes such as the high rotational method ([Formula: see text]), the vorticity magnitude method ([Formula: see text]), the negative eigenvalue method ([Formula: see text]) and the normalized vorticity method ([Formula: see text]). A new normalization method based on the probability distribution function (PDF) of the identification invariants is also introduced. In addition, a modification for the discriminant criterion known as the [Formula: see text] method is carried out and it is denoted as the modified delta method ([Formula: see text]). The velocity of the isotropic turbulent field is simulated using the lattice Boltzmann method with resolution [Formula: see text]. The new identification method depends on the higher-orders of the invariants of the velocity gradient tensor as well as the strain rate and the enstrophy production terms. The elongated tube-like vortices are extracted successfully using the new method and several features of the vortices are demonstrated and compared with the vortical structures that are extracted using the [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] identification methods. The recommended normalization method enabled the justification of the visualization threshold value to be within the order of unity and the threshold value [Formula: see text] is used in all identification methods. A remarkably similar geometrical worm-like vortices are extracted and a high similarity between the identification methods is observed and statistically studied.