Near-wall convection in a sedimenting suspension of fibers

The sedimentation of a fiber suspension near a vertical wall is investigated numerically. Initially, the near‐wall convection is an upward backflow, which originates from the combined effects of the steric‐depleted layer and a hydrodynamically depleted region near the wall. The formation of the hydrodynamically depleted region is elucidated by a convection‐diffusion investigation, in which fibers are classified according to the different directions in which they drift. For fibers with sufficiently large aspect ratio, the initial near‐wall backflow keeps growing. However, the backflow reverses to downward flow at later times if the aspect ratio is small. This is due to the fiber‐wall interactions which rotate fibers to such angles that make fibers drift away from the wall, inducing a dense region and a correspondingly downward flow outside the initial backflow. Moreover, the steric‐depleted boundary condition is of secondary importance in the generation and evolution of the near‐wall convection. © 2014 American Institute of Chemical Engineers AIChE J 60: 4253–4265, 2014