The Many Roles of Lagrangian Coherent Structures in Fluid Mixing

Abstract:

Understanding, characterizing, and modeling hydrodynamic mixing and transport in unsteady and turbulent flows remains a tremendous challenge despite decades of work. A wide array of techniques have been applied to mixing, ranging from statistical mechanics to stochastic modeling to decompositions of the flow field into discrete coherent structures. Recent years have seen significant progress in applying the tools and methods of dynamical systems theory to turbulent mixing. Such methods are usually applied in the Lagrangian framework, and are based on studying advection directly. Using data from quasi-two-dimensional laboratory experiments and numerical simulations, I will discuss the roles played by one particular mixing diagnostic: so-called Lagrangian Coherent Structures (LCSs). LCSs are defined to be distinguished material lines that are the dominant barriers to fluid transport and that organize advection. Here, I will demonstrate that they also play other roles. In particular, I will show that they separate regions of the flow field with distinct spectral dynamics, that they act as aligning structures for transported anisotropic particles, and that they can be attractors for self-motile particles.