Here is a relevant discussion that used faces to represent walls.

In your case, you would have a solid block to represent the wind tunnel of air, say 200 m long x 200 m wide x 100 m tall.  Then you create a solid cylinder with a 30 m diameter on the center of that 200 x 100 m floor to the height of the open tank.  Now you subtract that cylinder from the block, but keep the tool.  Now you have two solid bodies that don't interfere and you can turn on shared topology.  In Meshing, use Named Selections to name the cylindrical face Wall-tank, name the top circular face Interior, and name the bottom faces Floor.  Name one end of the wind tunnel Inlet and the other end Outlet. The other three sides of the wind tunnel can be named Symmetry.  Make sure both solid bodies are set to Fluid in the Properties.

After you mesh, in Fluent, you will end up with an open topped zero thickness tank that you can compute the pressure on the cylindrical face and transfer that to a Structural shell model.