  The immersed boundary method is a numerical and mathematical formulation for
solving fluid-structure interaction problems. It relies on solving fluid
equations on an Eulerian fluid grid and interpolating the resulting velocity
back onto immersed structures. To resolve slender fibers, the grid spacing must
be on the order of the fiber radius, and thus the number of required grid
points along the filament must be of the same order as the aspect ratio.
Simulations of slender bodies using the IB method can therefore be intractable.
A technique is presented to address this problem in the context of Stokes flow.
The velocity of the structure is split into a component coming from the
underlying fluid grid, which is coarser than normally required, and a component
proportional to the force (a drag term). The drag coefficient is set so that a
single sphere is represented exactly on a grid of arbitrary meshwidth. Implicit
treatment of the drag term removes some of the stability restrictions normally
associated with the IB method. This comes at a loss of accuracy, although tests
are conducted that show 1-2 digits of relative accuracy can be obtained on
coarser grids. After its accuracy and stability are tested, the method is
applied to two real world examples: fibers in shear flow and a suspension of
fibers. These examples show that the method can reproduce existing results and
make reasonable predictions about the viscosity of an aligned fiber suspension.
