Dynamics of a long chain in turbulent flows: Impact of vortices

TL;DR

This study investigates how long bead-spring chains behave in turbulent flows, revealing their tendency to sample vortex structures differently in 2D and 3D turbulence, influenced by chain flexibility and length.

Contribution

It demonstrates that flexible chains preferentially sample vortices in both 2D and 3D turbulence, with the chain's length and stiffness affecting sampling behavior.

Findings

In 2D turbulence, chains collapse into vortices.

In 3D turbulence, chains align with vortex axes and remain extended.

Chain flexibility and length influence vortex sampling behavior.

Abstract

We show and explain how a long bead-spring chain, immersed in a homogeneous, isotropic turbulent flow, preferentially samples vortical flow structures. We begin with an elastic, extensible chain which is stretched out by the flow, up to inertial-range scales. This filamentary object, which is known to preferentially sample the circular coherent vortices of two-dimensional (2D) turbulence, is shown here to also preferentially sample the intense, tubular, vortex filaments of 3D turbulence. In the 2D case, the chain collapses into a tracer inside vortices. In 3D, on the contrary, the chain is extended even in vortical regions, which suggests that it follows axially-stretched tubular vortices by aligning with their axes. This physical picture is confirmed by examining the relative sampling behaviour of the individual beads, and by additional studies on an inextensible chain with adjustable bending-stiffness. A highly-flexible, inextensible chain also shows preferential sampling in 3D, provided it is longer than the dissipation scale, but not much longer than the vortex tubes. This is true also for 2D turbulence, where a long inextensible chain can occupy vortices by coiling into them. When the chain is made inflexible, however, coiling is prevented and the extent of preferential sampling in 2D is considerably reduced. In 3D, on the contrary, bending stiffness has no effect, because the chain does not need to coil in order to thread a vortex tube and align with its axis.