[ansys_matt]

I just installed fluent 16.2 and I noticed I am not able to remove a material even after I added a new material to the materials list. I was modeling with methane-air and wanted to switch to methane-air-2step, so I added that new material to the materials menu. When I tried to delete methane-air I get the error:

Error: delete-material: still in use by interior_fluids.
Error Object: methane-air

So I went to Interior_Fluids(fluid) menu and tried to set the Material Name to methane-air-2step in the dropdown menu like I have always done in past versions of Fluent, but I found out there is no other choice in the dropdown menu besides the methane-air.

What is going on?? How can I delete the one and add the other?