Orientational Dynamics of Long Flexible Fibers in Wall-Bounded Turbulence

In this paper, we study numerically the role of fiber length and flexibility on the orientational dynamics of slender fibers in turbulent channel flow. We consider fibers of different flexibility at varying aspect ratio, up to lengths being comparable to the channel height. These fibers are constructed by constraining a large number of sub-Kolmogorov rods in a single chain, alongside a bending stiffness torque that allows to prescribe a finite value of the fiber rigidity. To perform our analysis, we carried out a series of one-way coupled direct numerical simulations of a fiber-laden channel flow at fixed shear Reynolds number: $Reτ=300$⁠, based on the half height of the channel. By calculating the orientational statistics of the suspended fibers, we find that shorter fibers, with length $O(10−1)$ when normalized by the channel half height, tend to exhibit a nearly-isotropic orientation distribution near the channel center, as would fibers suspended in homogeneous isotropic turbulence. As the fiber length is increased (up to values comparable to the channel half height), however, deviations from the isotropic orientation distribution become more and more significant. When the fibers are more rigid, these deviations are dampened and it is also observed that the tumbling rate of the fiber is lowered on average.