Download the geometry from here and open it.
Once successfully imported, enable the Compressible flow (ideal gas) option and press Finish. In most situations, air behaves closely enough to an ideal gas that it can be modeled as one.
Press Geometry in the Workflow panel and select Edit Geometry in the Geometry template. In the Prepare tab of the Model Editor, select the Volume Extract tool. Use the Select Faces tool to select the faces at the opposite ends of the duct. Lastly, use Select Seed Face to select a face inside the duct. This can be any face on the inside of the duct. Press the green checkmark and your interior flow volume will be extracted.
Below is what the flow volume will look like when the imported solid is hidden.
Now that the geometry of the flow volume has been created, we can suppress the imported solid geometry from the physics calculation. Right click on the Solid in the geometry Structure tree and select Suppress for Physics. Exit the modeler to continue with the simulation.