Dear developers，
I will put my model in the follows.As you can see,the number of block cells account for 98.03%.In general, cells outside the fluid domain won’t effect CFD calculation.But the speed of calculation has increased obviously when I decrease the number of block cells.
I want to know how can I remove the block cells？Or how can I solve the problem that the speed of iteration is too slow？In fact, the number of fluid cells has only 64240.What surprised me is that the simulation time was only 0.1s after one night.
Thanks!
5.mfx (10.1 KB)
5_2023-03-11T194430.506087.zip (29.5 KB)
I would be grateful for the trouble if you give an example 
That is because of the structured mesh. Ideally you should be able to turn on the re-indexing to only loop over active fluid cells, but it somehow crashes. We are looking into it.
Yes, as you said, it does break down