In this step, we assign Air to the internal flow volume.
1. To start, hide the casing by holding Ctrl and middle clicking on it:
2. Select the flow volume by left clicking on it:
3. Click Edit from the Materials context panel:
4. On the Materials quick edit dialog, verify the Type is Fluid, and the Name is Air.
5. Click Apply.
What it should look like...
6. To verify that the air material is properly assigned, check the Design Study Bar and the part coloring: