Hello, This can be done in Fluent meshing while selecting boundaries. It is easier while using the water tight workflow. In the boundaries tab, just choose the fluid domain to internal.

Thanks Rahkumar.nUsing the watertight workflow usually ends unsuccessfully when generating my surface mesh due to the complex geometry, do you know how to solve this issue?. Adding to it, I used the same local sizing which worked (after exporting the sizefield and re-importing the geometry with a surface mesh). nIs there a way to achieve the same without using the workflow?nAlso, I would appreciate on which of the geometry methods I attempted would work better as I have tried different combinations using designmodeler and spaceclaimn