  We numerically investigate the effects of fiber aspect ratio, roughness,
flexibility, and flow inertia on the rheology of concentrated suspensions. We
perform direct numerical simulations modeling the fibers, suspended in an
incompressible Newtonian fluid, as continuous flexible slender bodies obeying
the Euler-Bernoulli beam equation. An immersed Boundary Method (IBM) is
employed to solve for the motion of fibers. In concentrated suspensions, fibers
come into contact due to the presence of asperities on their surface. We assume
a normal load-dependent friction coefficient to model contact dynamics and
friction, which successfully recovers the shear-thinning behaviour observed in
experiments. First, we report the shear rate dependent behavior and the
increase in the suspension viscosity with the increasing volume fraction of
fibers, fiber roughness, and rigidity. The increase in the viscosity is
stronger at a lower shear rate and finite inertia. Simulation results indicate
that for the concentrated suspensions, contact stresses between fibers form the
dominant contribution to the viscosity. Moreover, we find the first normal
stress difference to be positive and to increase with the Reynolds number for
flexible fibers. Lastly, we explore the divergence of viscosity for different
aspect ratios and roughness of the fibers and predict the jamming volume
fraction by fitting the data to the Maron-Pierce law. The jamming volume
fraction decreases with increasing the aspect ratio and roughness. We conclude
that the contact forces and interparticle friction become one of the crucial
factors governing the rheology of flexible fiber suspensions at high
concentrations.
