The sedimentation of fibre suspensions at low Reynolds number is
studied using two
different, but complementary, numerical simulation methods: (1) Monte Carlo
simulations, which consider interparticle hydrodynamic interactions at
all orders
within the slender-body theory approximation (Mackaplow & Shaqfeh 1996),
and (ii)
dynamic simulations, which consider point–particle interactions and
are accurate for
suspension concentrations of nl3=1, where n
and l are the number density and
characteristic half-length of the fibres, respectively. For homogeneous,
isotropic
suspensions, the Monte Carlo simulations show that the hindrance of the
mean
sedimentation speed is linear in particle concentration up to at least
nl3=7. The speed
is well predicted by a new dilute theory that includes the effect of two-body
interactions. Our dynamic simulations of dilute suspensions, however, show
that
interfibre hydrodynamic interactions cause the spatial and orientational
distributions
to become inhomogeneous and anisotropic. Most of the fibres migrate into
narrow
streamers aligned in the direction of gravity. This drives a downward convective
flow
within the streamers which serves to increase the mean fibre sedimentation
speed. A
steady-state orientation distribution develops which strongly favours fibre
alignment
with gravity. Although the distribution reaches a steady state, individual
fibres
continue to rotate in a manner that can be qualitatively
described as a flipping between
the two orientations aligned with gravity. The simulation results are in
good agreement
with published experimental data.