Shear-induced migration and orientation of rigid fibers in an oscillatory pipe flow

The spatial and orientation distributions of fibers in suspension were measured during oscillatory flow within a circular pipe. The fibers were rigid and non-colloidal, and two aspect ratios (length L to diameter d ratios) of L/d = 11 and 23 were tested; the suspending fluid was viscous, Newtonian, and density matched to the particles. As with shear-induced migration of spheres in parabolic flows, fibers in the concentrated suspensions migrated toward the center of the pipe. The migration was similar for the fibers, irrespective of the aspect ratio, at the same dimensionless number density n(0)L(2)d ( n(0) is the number of fibers per unit volume of the bulk suspension), rather than at the same volume fraction. The extent of migration was maximum at n(0)L(2)d = 0.84 for both aspect ratios. The orientation distribution of the fibers was spatially dependent. Fibers near the center of the channel aligned closely with the flow direction, while fibers near the pipe wall had an enhanced probability of aligning in the vorticity direction.