Rortex and comparison with eigenvalue-based vortex identification criteria

Abstract

Most of the currently popular Eulerian vortex identification criteria, including the Q criterion, the Delta criterion and the Lambda_ci criterion, are based on the analysis of the velocity gradient tensor. More specifically, these criteria are exclusively determined by the eigenvalues of the velocity gradient tensor or the related invariants and thereby can be regarded as eigenvalue-based criteria. However, these criteria have been found to be plagued with two shortcomings: (1) these criteria fail to identify the swirl axis or orientation; (2) these criteria are prone to contamination by shearing. In this paper, an alternative eigenvector-based definition of Rortex is introduced. The real eigenvector of the velocity gradient tensor is used to define the direction of Rortex as the possible axis of the local fluid rotation, and the rotational strength obtained in the plane perpendicular to the possible local axis is defined as the magnitude of Rortex. This alternative definition is mathematically equivalent to our previous one but allows a much more efficient implementation. Furthermore, a complete and systematic interpretation of scalar, vector and tensor versions of Rortex is presented to provide a unified and clear characterization of the instantaneous local rigidly rotation. By relying on the tensor interpretation of Rortex, a new decomposition of the velocity gradient tensor is proposed to shed light on the analytical relations between Rortex and eigenvalue-based criteria. It can be observed that shearing always manifests its effect on the imaginary part of the complex eigenvalues and consequently contaminates eigenvalue-based criteria, while Rortex can exclude the shearing contamination and accurately quantify the local rotational strength.