Vortex axis tracking by iterative propagation (VATIP): a method for analyzing three-dimensional turbulent structures

Study of turbulent vortices in DNS data relies heavily on direct visual inspection, anecdotal observations, and intuitive arguments. Quantitative analysis is limited by the lack of computational tools for the objective detection and extraction of vortex structures. Much progress has been made in the identification criteria of vortices, which, however, only label spatial regions belonging to vortices, without any information on the identity, topology, and shape of individual vortices. This latter information requires vortex tracking techniques and existing methods so far only focused on quasi-linear streamwise vortices. In this study, a new tracking algorithm is proposed which propagates along the vortex axis-lines and iteratively search for new directions for growth. It is the first tracking method designed for general three-dimensional vortices. The method is tested in transient flow fields with specific vortex types as well as DNS. A new procedure is also proposed that classifies vortices into commonly-observed shapes, including quasi-streamwise vortices, hairpins, hooks, and branches, based on their axis-line topology. The study offers a new tool-set for the fundamental study of the dynamics of turbulent coherent structures.