  Long, flexible fibers transported by a turbulent channel flow sample
non-linear variations of the fluid velocity along their length. As the fibers
tumble and collide with the boundaries, they bounce off with an impulse that
propels them toward the center of the flow, similar to pole vaulting. As a
result, the fibers migrate away from the walls, leading to depleted regions
near the boundaries and more concentrated regions in the bulk. These higher
concentrations in the center of the channel result in a greater net flux of
fibers than what was initially imposed by the fluid. This effect becomes more
pronounced as fiber length increases, especially when it approaches the channel
height.
