  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.
