Liutex and Proper Orthogonal Decomposition for Coherence Structure in the Wake of Micro Vortex Generator

In 2016, a new criterion called Omega vortex identification was proposed by Liu et al., which has many advantages according to several studies. However, all the existing vortex identification methods are just a scalar and they are contaminated by shear. For the first time in history, a new vortex vector with both local rotation axis and rotation strength - called Liutex (with no shear) was introduced by Liu et al. in 2018. In this study, we load data directly from Liutex vector (Lx, Ly, Lz) instead of velocity (u, v, w). Moreover, proper orthogonal decomposition (POD) method is applied to analyze the coherent structure of the first eighth POD modes in the wake of micro vortex generator at Ma = 2.5 and Reθ = 5760 by using Liutex. Pairing of POD modes is clearly found, which is the sign of Kevin-Helmholtz instability. The observation of the mode paring strongly supports the new mechanism of MVG for reduction of flow separation, which is the interaction between the shock and the spanwise vortex rings generated by K-H instability.