heat conducting solid within a fluid medium?
 

How would I model a solid object inside an air duct?

My problem is that I don't know how to model a solid block that sits in an air duct. I can create the duct in STAR and define it as my fluid region, but how do I put a solid object inside? The only way I can think of is doing an extrude cut up into the duct to create an 'obstruction' that models the block. However, when I split the surfaces and assign parts to regions, all of the faces of the block become part of the fluid region. I need the obstruction to be defined as a seperate solid region.

So the next thing I thought of was to select all of the boundaries that define the block in the fluid region, and creat a new region based on these boundaries. I defined this as the solid region. The problem with this is that all of these boundaries got deleted out of the fluid region. I need to define interfaces between the fluid/solid for each face right? However I cannot do this since an interface requires that a boundary be shared between regions, but when I defined the block faces as a new solid region, they are removed from the fluid region like I said, so now they cannot be shared anymore.

There are no tutorials on such a simple case, and I cannot learn about interfaces between solid/fluid regions because all of the tutorials have you import volume meshes, not actually create geometries. Can someone help?

First of all there are several ways how you can build your regions. This is one possibility.
Create your bodies at the 3D-CAD-modeler. Be sure that you get two seperate bodies (body interaction should be None, not imprint, not merge). Assign them to the parts level. Now you can use the substract boolean operation to cut out the little box from the big box. (The big box should be selected as target part). Now you should have a new part. This part plus the little box part could now assigned as regions.