  Unfitted finite element methods have emerged as a popular alternative to
classical finite element methods for the solution of partial differential
equations and allow modeling arbitrary geometries without the need for a
boundary-conforming mesh. On the other hand, the efficient solution of the
resultant system is a challenging task because of the numerical
ill-conditioning that typically entails from the formulation of such methods.
We use an adaptive geometric multigrid solver for the solution of the mixed
finite cell formulation of saddle-point problems and investigate its
convergence in the context of the Stokes and Navier-Stokes equations. We
present two smoothers for the treatment of cutcells in the finite cell method
and analyze their effectiveness for the model problems using a numerical
benchmark. Results indicate that the presented multigrid method is capable of
solving the model problems independently of the problem size and is robust with
respect to the depth of the grid hierarchy.
