Impact of initial fiber states on different fiber dynamic patterns in the laminar channel flow

Abstract

The dynamics of an elastic fiber with various initial states in a laminar channel flow is investigated using the immersed boundary-lattice Boltzmann method. Fiber-wall collisions are solved by adding a repulsive force in the model. Our simulation results demonstrate that the initial fiber state closely relates to the stability of the considered conveyance system. Different initial states may lead to different dynamic patterns in the downstream. The fiber is found to go straight forward along a horizontal path when it is horizontally and symmetrically placed at the channel centerline. Breaking this symmetry by varying the fiber's initial vertical position or orientation will induce the instability of the system, which then causes deviations and fluctuations in fiber's conveyance path. No matter how large the deviation occurs in the upstream, the fiber is always found to migrate to the channel central region in the downstream and would eventually settle in a vertical position slightly away from the channel centerline. Moreover, the off-centerline distance that a fiber settles depends on its dynamic pattern rather than a specific initial fiber state. For our system, there are two kinds of dynamic patterns observed in the downstream channel. In the first pattern, the fiber eventually reaches its equilibrium state and is observed to do a translational motion. In the second pattern, no equilibrium state is observable, and the fiber is found to do a periodically tumbling motion. The fiber's eventual conveyance speed depends on the vertical position it eventually settles and can be roughly approximated by the local flow velocity.