Filtering of FTLE for visualizing spatial separation in unsteady 3D flow

Abstract

In many cases, feature detection for flow visualization is structured in two phases: first candidate identification, and then filtering. With this paper, we propose to use the directional information contained in the finite-time Lyapunov exponents (FTLE) computation, in order to filter the FTLE field. In this way we focus on those separation structures that delineate flow compartments which develop into different spatial locations, as compared to those that separate parallel flows of different speed. We provide a discussion of the underlying theory and our related considerations. We derive a new filtering scheme and demonstrate its effect in the context of several selected fluid flow cases, especially in comparison with unfiltered FTLE visualization. Since previous work has provided insight with respect to the studied flow patterns, we are able to provide a discussion of the resulting visible separation structures.