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Article

Simulation of finite-size fibres in turbulent channel flow.

Authors: Do-Quang, M., Amberg, G.A., Brethouwer, G.B., Johansson, A.V.J.
Document Type: Article
Pubstate: Published
Journal: Physical Review E
Volume: 89   013006
Year: 2014

Abstract

The dynamical behavior of almost neutrally buoyant finite-size rigid fibres or rods in turbulent channel flow is studied by direct numerical simulations. The time evolution of the fibre orientation and translational and rotational motions in a statistically steady channel flow is obtained for three different fibre lengths. The turbulent flow is modeled by an entropy lattice Boltzmann method and the interaction between fibres and carrier fluid is modeled through an external boundary force method. Direct contact and lubrication force models for fibre-fibre interactions and fibre-wall interaction are taken into account to allow for a full four-way interaction. The density ratio is chosen to mimic cellulose fibres in water. It is shown that the finite size leads to fibre-turbulence interactions that are significantly different from earlier reported results for point-like particles (e.g. elongated ellipsoids smaller than the Kolmogorov scale). An effect that becomes increasingly accentuated with fibre length is an accumulation in high-speed regions near the wall, resulting in a mean fibre velocity that is higher than the mean fluid velocity. The simulation results indicate that the finite-size fibres tend to stay in the high-speed streaks due to collisions with the wall. In the central region of the channel, long fibres tend to align in the spanwise direction. Closer to the wall the long fibres instead tend to toward to a rotation in the shear plane, while very close to the wall they become predominantly aligned in the streamwise direction.