Does anyone have any thoughts about modelling very thin walls in CFX5. I have a situation where a thin-walled duct protrudes into a box. I want to be able to model the flow as it develops in the duct and then jets into the box. I used to be able to separate blocks in CFX5 using WALLS but now I seem to have to resort to a gap of finite width between the inner and outer sides of the duct.

CFX-5 does not yet support thin surfaces, so you are correct that your wall needs a finite thickness. The thickness should be larger (say by an order of magnitude) than the global model tolerance. But you don't need to have a finite thickness for the entire wall, though... you can (and probably should to get a nicer mesh) taper to a point at its end (so each wall actually looks like a wedge.)

Hope this helps, phil