  We present an improved method for computing incompressible viscous flow
around suspended rigid particles using a fixed and uniform computational grid.
The main idea is to incorporate Peskin's regularized delta function approach
[Acta Numerica 11 (2002) 1] into a direct formulation of the fluid-solid
interaction force in order to allow for a smooth transfer between Eulerian and
Lagrangian representations while at the same time avoiding strong restrictions
of the time step. This technique was implemented in a finite-difference and
fractional-step context. A variety of two- and three-dimensional simulations
are presented, ranging from the flow around a single cylinder to the
sedimentation of 1000 spherical particles. The accuracy and efficiency of the
current method are clearly demonstrated.
