Evaluation of vortex identification methods based on two- and three-dimensional swirling strengths

Modified normalized Rortex/vortex identification method

“…6,[11][12] Due to the clear physical meaning of the Rortex vector, it has gained a lot of research attentions. [13][14][15][16][17][18][19][20][21] Although the iso-surfaces of magnitude of the Rortex vector is very effective to capture the coherent vortex structures, the determination of the threshold is still case-dependent. [16][17][18] Recently, using the idea of the widely used Ω method, 6,10,[17][18][19][20][21][22] a normalized Rortex/vortex identification method is developed by Dong et al 23 The new method denoted by Ω 𝑅𝑅 has the several important advantages, including: (1) Ω 𝑅𝑅 is a dimensionless relative quantity from 0 to 1, which can be used to do statistics and correlation analysis directly; (2) Ω 𝑅𝑅 can distinguish the vortex from high vorticity concentration with high shear and exclude boundary layers; (3) Ω 𝑅𝑅 is robust to threshold change and can empirically be set as 0.52 to visualize the vortex structures; (4) Ω 𝑅𝑅 has the capability of capturing both strong and weak vortices simultaneously.…”