A vortex identification method based on strain and enstrophy production invariants

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 (Q), the vorticity magnitude method (ω), the negative eigenvalue method (λ2) and the normalized vorticity method (Ω). 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 Δ method is carried out and it is denoted as the modified delta method (Δm). The velocity of the isotropic turbulent field is simulated using the lattice Boltzmann method with resolution 2563. 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 Q, ω, λ2, Ω, Δ and Δm 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 0.55 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.