A string of tracers, interacting elastically, in a turbulent flow is shown to have a dramatically different behaviour when compared to the non-interacting case. In particular, such an elastic chain shows strong preferential sampling of the turbulent flow unlike the usual tracer limit: an elastic chain is trapped in the vortical regions. The degree of preferential sampling and its dependence on the elasticity of the chain is quantified via the Okubo-Weiss parameter. The effect of modifying the deformability of the chain, via the number of links that form it, is also examined. PACS numbers: 47.27.Gs, 05.20.Jj The development of Lagrangian techniques, in experiments and theory, has lead to major advances in our understanding of the complexity of turbulent flows, especially at small scales [1][2][3]. What makes this possible is the use of tracer particles which uniformly sample the flow and hence access the complete phase space in which the dynamics resides. This feature of tracers depends, crucially, on the assumption that the particles remain inertia-less and point-like. When some of these assumptions are relaxed, it may lead to dissipative particle dynamics and preferential sampling of the structures in a flow [4][5][6][7][8][9][10]. This is, for instance, the case for heavy, inertial particles, which show small-scale clustering and concentrate away from vortical regions. Various phenomena can influence the properties of inertial clustering in turbulence, such as gravity [11,12], turbophoresis [13,14], or the non-Newtonian nature of the fluid [15]. Preferential sampling in turbulent flows may also emerge as a result of the motility of particles, as in the case of gyrotactic [16], interacting [17], or jumping [18] micro-swimmers.We now propose a novel mechanism for preferential sampling in turbulent flows which is induced by extensibility. A simple model of an extensible object which retains enough internal structure is a chain of tracers with an elastic coupling between the nearest neighbours. We show, remarkably, that turning on such elastic interactions amongst tracers leads to very different dynamics: unlike the case of non-interacting tracers, an elastic chain preferentially samples vortical regions of the flow. We perform a systematic study of this phenomenon and quantify, via the Okubo-Weiss parameter, the level of preferential sampling and its dependence on the elasticity and deformability of the chain.Harmonic chains have been at the heart of several important problems in the areas of equilibrium and nonequilibrium statistical physics. These have ranged from problems in crystalline to amorphous transitions [19], electrical and thermal transport both in and out-ofequilibrium [20], as well as understanding structural properties of disordered and random systems [21]. Given the ubiquity and usefulness of the elastic chain, it is sur-prising that the effect of a turbulent medium on long chains has not been studied as extensively as in other areas of non-equilibrium statistical physics.There is another reason...
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 three-dimensional (3D) turbulence. In the 2D case, the chain collapses into a tracer inside vortices. In the 3D case on the contrary, the chain is extended even in vortical regions, which suggests that the chain 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 three dimensions, 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 two dimensions is considerably reduced. In three dimensions, 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. This article is part of the theme issue ‘Fluid dynamics, soft matter and complex systems: recent results and new methods’.
The role of the spatial structure of a turbulent flow in enhancing particle collision rates in suspensions is an open question. We show and quantify, as a function of particle inertia, the correlation between the multiscale structures of turbulence and particle collisions: Straining zones contribute predominantly to rapid head-on collisions compared to vortical regions. We also discover the importance of vortex-strain worm-rolls, which goes beyond ideas of preferential concentration and may explain the rapid growth of aggregates in natural processes, such as the initiation of rain in warm clouds. * jrpicardo@icts.res.in;
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