Meshing around domain interface

Tom S

Hi guys, I'm new to CFX and trying to model atmospheric flows around a forest canopy. The aim is to have a rectangular porous domain representing the forest on the floor of a much larger domain representing the surrounding atmosphere.

As an initial test I have defined both domains as fluid domains and am checking to see that adding the forest domain does not disturb the flow. Unfortunately I get noticeable flow disturbances around the forest domain. I am assuming this is numerical diffusion due to meshing problems, but I cannot figure out how to resolve it. If I refine the mesh massively around the interface the disturbance is reduced, but at the expense of long solution times. It is worrying me that even when the mesh is contiguous through the domain interface (using a multi body part for the two domains) the mesh distortions caused by the interface affect the flow.

Any help / suggestions appreciated.

Tom