On inertial effects of long fibers in wall turbulence: fiber orientation and fiber stresses

Inertia effects of large-aspect-ratio fibers have been investigated in wall turbulence. The turbulent flow field in a plane channel was obtained from a direct numerical simulation. The translational and rotational motion of the rigid fibers were obtained by a Lagrangian approach, first for inertial fibers with Stokes number St = 10, 1.0 and 0.1 and thereafter for massless fibers, which correspond to St = 0. All simulations were one-way coupled. The fiber orientation statistics and the normal components of the fiber stress tensor turned out to be almost independent of the fiber inertia all the way from the channel wall to the center for St ≤  1.0. This observation suggested that fiber inertia plays a negligible role for Stokes number below unity and the gap between inertial fibers and massless fibers has been bridged. The massless particle approach appears as a viable alternative to mimic the orientation and stress tensor of fibers with only modest inertia.