  We present and analyze a penalization method wich extends the the method of
[1] to the case of a rigid body moving freely in an incompressible fluid. The
fluid-solid system is viewed as a single variable density flow with an
interface captured by a level set method. The solid velocity is computed by
averaging at avery time the flow velocity in the solid phase. This velocity is
used to penalize the flow velocity at the fluid-solid interface and to move the
interface. Numerical illustrations are provided to illustrate our convergence
result. A discussion of our result in the light of existing existence results
is also given. [1] Ph. Angot, C.-H. Bruneau and P. Fabrie, A penalization
method to take into account obstacles in incompressible viscous flows, Numer.
Math. 81: 497--520 (1999)
