Slender flexible fibers in wall-bounded turbulence

We study the dynamics of flexible fibers in turbulent channel flow by performing direct numerical simulations at shear Reynolds numbers up to Re=1200 and tracking fibers with large aspect ratio (AR, up to 200), length L extending up to the inertial range of the flow, and Stokes numbers spanning two orders of magnitude (from St= 0.1 to St=11). For each combination of (AR, L, St) values, different fiber rigidities are considered to compare fibers with moderate stiffness (Young modulus E_Y=10⁵ in dimensionless units) with stiff-less ones (E_Y=0). We also account for the effects of fluid inertia on the forces and torques experienced by the fibers, thus going beyond the commonly used Jeffery torques. We focus on the conditions under which the inertial contribution becomes relevant given the intermittent nature of the flow, comparing the drift time (order of a few periods of rotation) with the typical time of the fluid velocity fluctuations. We show that the fibers in the bulk of the flow orient with the local strain, align with the vorticity - as in homogeneous isotropic turbulence - and experience a tumbling rate comparable to that of rigid fibers. Near the walls, vorticity orients with the spanwise direction while flexible fibers align with the mean flow. This orthogonality determines a stronger contribution of the flow rotation to the tumbling rate. The most probable deformed shapes define a bi-variate probability space, suggesting that two main deformation patterns exist: eyelash bending and compressive buckling.