Strange Behaviour of Porous Domain (Pictures Included)
 

I'm modelling how the placement of stents in arteries affects blood flow at different angles of side wall branches when the stent crosses the opening (using idealised geometries).

http://www.mayfieldclinic.com/Images/flow_diverter.jpg

I can't model the stent itself because the scale of the mesh would require a supercomputer to solve. I am trying to use a porous domain with a directional loss model to calculate the resistive effects of the stent and hence the reductions of blood flow into adjacent side branches.

A couple of my simulations worked (second picture) but then the porous domain started acting very strange (first picture). Any ideas what could be going wrong? I've tried remeshing, redefining the domains but no matter what I do the flow at the interface of the blood vessel/stent domains is going crazy. I can even get converged solutions (it takes a very long time) but they include these 'interesting' flow features.

Thanks for any help