Slender flexible fibers in turbulent channel flow

The orientation and deformation of slender flexible fibers in turbulent channel flow is investigated numerically at varying fiber inertia, aspect ratio and bending stiffness. We use a Euler-Lagrangian approach based on DNS of the flow, combined to a rod-chain pointwise representation of the fibers. The simulations are performed using a pseudo-spectral solver 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 turbulent 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=10⁴ in dimensionless units) with stiffless fibers (E_Y=0). 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. The time spent by fibers in either bent state before being stretched again by the flow is found to scale with the fiber rotation timescale.