24/09/2012: Lagrangian coherent structures in oceanic and laboratory flows

Lagrangian coherent structures in oceanic and laboratory flows

Montag, 24.9.2012, HG E 23

Florian Huhn, Dipl. Phys.
Non-Linear Physics Group, Faculty of Physics
University of Santiago de Compostela, Spanien

Most flows in engineering and geoscience are unsteady and transport in these time- dependent aperiodic velocity fields can be highly chaotic. We present several examples of transport problems where the concept of Lagrangian coherent structures (LCS) turns out to be extremely useful to highlight the geometry of mixing in quasi twodimensional unsteady flows. We show how LCS control the transport of reactive tracers, e.g., the chlorophyll concentration in a plankton bloom in the ocean and a chemical reaction in the laboratory. As a further application, we demonstrate that LCS also serve to understand the pathways of surface drifters in an estuary and give a footprint of the water exchange processes of the estuary with the shelf. Discrete time-dependent velocity fields for these studies are obtained from satellite data, a hydrodynamic coastal model or direct measurements via Particle Image Velocimetry (PIV). In all cases the extracted LCS act as barriers to transport and determine the local mixing dynamics.
The LCS approach seeks to extract those flow structures that determine the transport. In the twodimensional case, LCS are lines that separate different regions of the flow containing dynamically different trajectories. LCS are related to the stable and unstable manifolds of hyperbolic regions in the flow and define the building blocks of the mixing geometry.