- Journal Article
We derive measures of local material stretching and rotation that are computable from individual trajectories without reliance on other trajectories or on an underlying velocity field. Both measures are quasi-objective: they approximate objective (i.e., observer-independent) coherence diagnostics in frames satisfying a certain condition. This condition requires the trajectory accelerations to dominate the angular acceleration induced by the spatial mean vorticity. We illustrate on examples how quasi-objective coherence diagnostics highlight elliptic and hyperbolic Lagrangian coherent structures even from very sparse trajectory data. A vast amount of discrete tracer trajectory data is available for fluid flows in the laboratory and in nature. All features of tracer trajectories, however, including their shape, velocities, and looping, depend on the reference frame of the observer even though the coherent flow structures one often tries to infer from tracers are observer indifferent. Here, we develop quasi-objective coherent structure diagnostics that can be computed from individual single tracer trajectories without reliance on other trajectories. A quasi-objective diagnostic approximates an objective diagnostics in all frames of reference that satisfy certain conditions we derive. We show how the single-trajectory quasi-objective scalar fields highlight material coherent structures even from sparse trajectory data sets on which multi-trajectory objective diagnostics tend to perform poorly. Show more
Journal / seriesChaos
Pages / Article No.
Organisational unit03973 - Haller, George / Haller, George
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