Complex Particles in Turbulent Flows

While still moving with the fluid, such particles also possess additional degrees of freedom. Several studies have thus investigated the orientation and rotation of anisotropic solid particles (such as small rigid fibres or ice crystals), the stretching of elastic polymers, the bending of small flexible fibres, as well as their fragmentation. Complex-shape particles such as crosses, jacks, and helicoids have also been used to gain information on the local properties of the turbulent velocity field.

The study of objects with sizes in the inertial range, or even exceeding the velocity integral scale, is very challenging, as a finite-size object indeed samples multiple scales of the flow. If in addition the object is deformable, its conformation is strongly coupled with its displacement, further complicating its dynamics. The importance of such aspects is well known for filaments in quiescent and creeping flows. In turbulence, they have only recently begun to be studied in the case of long fibres.

The dynamics of active particles, such as swimming bacteria, spermatozoa, plankton, or anguilliform juvenile fishes, has also attracted much attention in recent years. The focus has been on how the orientation and deformation dynamics of such self-propelled particles interferes with the carrier turbulent flow. Another interesting question is how particle motility can possibly lead to the formation of 'active' aggregates and collective motions.