  To facilitate the understanding and to quantitatively assess the material
transport in fluids, a modern characterisation method has emerged in fluid
dynamics in the last decades footed in dynamical systems theory. It allows to
examine the most influential material lines which are called Lagrangian
Coherent Structures (LCS) and order the material transport into dynamically
distinct regions at large scales which resist diffusion or mixing. LCS reveal
the robust skeleton of material surfaces and are essential to assess material
transport in time-dependent flows quantitatively. Candidates of LCS can be
estimated and visualised from finite-time stretching and folding fields by
calculating the Finite-Time Lyapunov Exponents (FTLE).
  In this contribution, we provide an OpenFOAM function object to compute FTLE
during CFD simulation. This enables the OpenFOAM community to assess the
geometry of the material transport in any flow quantitatively on-the-fly using
principally any OpenFOAM flow solver.
