Hi josh_shao,
Let me preface my comments, I’m no expert in your field, just a novice. I know it seems a little crazy to delete the body you are interested in, but that’s how it works for me when I’m using CFD for my aerodynamics.
I did find these recent comments that may be of use to you…from a Simscale support engineer ggiraldof simscale.com/users/ggiraldof
I hope one of these links works for you as it seemed to be similar to your issue of heat/fluid…The topic/question was [Deleting bodies and assigning materials]
(Deleting bodies and assigning materials)
Deleting bodies and assigning materials - Using SimScale - SimScale CAE Forum
Deleting bodies and assigning materials - #2 by ggiraldof
All the best,
Kind regards Ted