  We present a general simulation approach for fluid-solid interactions based
on the fully-Eulerian Reference Map Technique (RMT). The approach permits the
modeling of one or more finitely-deformable continuum solid bodies interacting
with a fluid and with each other. A key advantage of this approach is its ease
of use, as the solid and fluid are discretized on the same fixed grid, which
greatly simplifies the coupling between the phases. We use the method to study
a number of illustrative examples involving an incompressible Navier-Stokes
fluid interacting with multiple neo-Hookean solids. Our method has several
useful features including the ability to model solids with sharp corners and
the ability to model actuated solids. The latter permits the simulation of
active media such as swimmers, which we demonstrate. The method is validated
favorably in the flag-flapping geometry, for which a number of experimental,
numerical, and analytical studies have been performed. We extend the flapping
analysis beyond the thin-flag limit, revealing an additional destabilization
mechanism to induce flapping.
