can you please share your project with the forum so that we can have a look at it.
However, what you can try to do is to use the local element refinement in order to treat these two surfaces individually and decrease the cell size so that the gap stays open.
Here’s my project
I didnt mention that I was using the Hex-dominant Parametric mode
What’s the reasoning for using hex dominant parametric?
What I would in general suggest you is to use the standard meshing algorithm. since this can produce a much cleaner mesh, then the hex dominant parametric.
You can use this by creating the flow volume as described here:
How to Create Flow Volume Extraction? | Knowledge Base | SimScale,
You can then like you have done so far used the refinements to control the element size.
for the gap between the tire and the wheel arch, I think it’s important to understand these types of situations. In order to accurately capture this gabe we would need the boundary layer cells and at least 3 cells between the layers. That would mean that we roughly need a region refinement of 1mm fro this to be accurately captured.