Importing 3D geometry into COMSOL

Hello,

Happy new year everyone!

I need help using this software. I want to know how to select my flow area/domain , which needs fluid to flow through my imported complex geometry(say from AUTOCAD).

The geometry is something like a complex pipe with a specific thickness. So when i import the geometry from AUTOCAD to COMSOL, the fluid is flowing through the thickness of my solid geomtery instead of flowing through it. Can anyone help me with this? how do i select the inner area fir my fluid flow/properties.

I need the flow to flow through the inner part, instead of the thickness of the solid geometry.

Can someone please help me with it?

Thanks in Advance