Kinematics of rigid fibers in a turbulent channel flow

Measurements of concentrations, translational velocities, orientations and rotation rates of seven, straight and rigid fiber types in a fully developed turbulent water channel flow (one-way coupled, Re_τ=435) are reported. The aim was to study inertial and length effects on fiber kinematics. Fibers were characterized by Stokes numbers and mean, normalized lengths ranging between 0.02 ¡ St ¡ 0.34 and 16 ≤L¯⁺≤ 66.8. Except for the longest (L¯⁺ = 66.8) fibers, a depletion layer was observed near the wall, its width increasing with decreasing St (for a given length). Away from the wall, fibers lagged the fluid, increasingly so with increasing St and L¯⁺ due to increased drag. In the buffer layer, fiber excess velocities decreased with L¯⁺ while inertial effects were small. Irrespective of fiber length, all fibers strongly aligned with the streamwise direction in the near-wall region. Fiber rotation rates peak in the buffer layer, and remain nearly constant in the log-layer and wake region. However, peak values decreased with increasing L¯⁺. Our results suggest intricate coupling between fiber inertia and length in conjunction with the governing turbulence scales.