Elastic chains, inertial particles and turbulent flow structures

Small-scale turbulence is riddled with intense, interacting, vortical and straining flow structures, which are closely related to its characteristic intermittent and non-gaussian statistics. These structures distinguish turbulence from simple random flow fields, and must impact various aspects of turbulent transport. In this talk, I will show how flow structures govern (i) the transport of elastic filaments and (ii) the nature of collisions between inertial particles. Using an elastic bead-spring chain as a simple model for an extensible filament, I will show how the interaction between fluid drag and elasticity leads to a preferentially sampling of vortical regions. This is in stark contrast to the situation for inertial particles, whose density causes them to be centrifuged out of vortices. In this latter case, we will see how collisions between particles are also strongly influenced by flow structures: Collisions are predominantly head-on in straining regions, but grazing in vortical regions. Moreover, vorticity and strain can conspire, in the form of vortex-strain worm-rolls, to produce rapid and violent collisions. These results provide clues to the importance of turbulent flow structures in the rapid growth of aggregates in natural processes, such as the initiation of rain in warm clouds.