Lagrangian Coherent Structures are known to drive biological dynamics,
from plankton to top predators, thus it is very important to be able
to characterize them in realistic three dimensional flows. We have
used the Finite Size Lyapunov Exponent (FSLE) to identify coherent
structures in a three dimensional turbulent velocity field. The FSLE
is a measure of particle dispersion in fluid flows and the ridges of
this scalar field locate regions of the velocity field where strong
exponential separation between particles occur. These regions are
referred to as Lagrangian Coherent Structures (LCS). We have used as a
test case for our algorithm a canonical turbulent velocity field that
is the turbulent flow between two parallel stationary plates driven by
a pressure gradient in the mean flow direction. The results show
a complicated pattern of thin LCS, that show like filaments in 2D
plots and that move with the flow. These results serve to validate our
algorithm and we plan to use it in more realistic application such as
oceanic flows.