@iosili, since the simulation involves multiple domains with different material properties, you should create a multi-region mesh. However, the geometry is still missing the region occupied by air. I would suggest you to modify the geometry in the following way:
- Open your geometry in a CAD modeler of your choice (e.g. Onshape)
- Create a box that occupies the region where airflow occurs.
- From this box, subtract the other geometry entities - at the end of this step, you will have three disjoint regions: air, the smaller disc and the larger disc. These three regions share some boundaries with each other.
Upload this geometry to SimScale, and follow this link for more information on how to generate a multi-region mesh.
Please let me know if you have more questions or issues.