As you are not interested in the deformation of you object, I would really recommend to use a mesh Which only contains the fluid domain. You can use Gmsh to create such a mesh, see for instance the tutorial I have created at my homepage . To load this mesh into dolfin, see for instance Transitioning from mesh.xml to mesh.xdmf, from dolfin-convert to meshio or Converter from GMSH to XDMF (with physical groups).
Also, please supply minimal working code examples that reproce your error message, as described in Read before posting: How do I get my question answered?