Vortex axis tracking by iterative propagation (VATIP): analyzing three-dimensional vortex structures in viscous and viscoelastic turbulent flows

Study of turbulent vortices in DNS relies heavily on 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. Despite much progress in the development of vortex identification criteria (which shows the vortices without distinguishing their individualities), vortex tracking requires a separate step and existing techniques only targeted quasi-linear 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. This new method is then applied to both viscous and viscoelastic turbulent channel flows for analyzing the distribution of vortex size, shape, and location. Introducing polymer additives suppresses vortex lift-up process and fundamentally change the vortex regeneration dynamics.