Flexible fibers in turbulent channel flow

In this paper we investigate the dynamics of small flexible fibers in turbulent channel flow. We aim at examining the translational and rotational behavior of fibers with different elongation (parameterized by the aspect ratio) and inertia (parameterized by the Stokes number) with and without fiber-fluid coupling. We use a Eulerian-Lagrangian approach based on direct numerical simulation of turbulence to describe fiber dynamics. Fibers, which are longer than the Kolmogorov length scale, are modelled as chains of sub-Kolmogorov rods connected through ball-and-socket joints that enable bending and twisting. Different mass and volume fractions are considered to investigate two-way coupling effects. Velocity, orientation and concentration statistics, extracted from simulations at shear Reynolds number Re_τ=300 are presented to give insights into the complex fibers-turbulence interactions that arise when non-sphericity and deformability add to fiber inertia. Flexible fibers in the dilute regime appear to undergo the same inertia-driven mechanisms that govern preferential concentration of spherical particles in bounded flows. We show that momentum coupling provides an additional bias, which may produce significant quantitative modifications in the statistics of both phases.