Grid Independence of Boundary layer
 

Hello,

I posted this question in the StarCCM+ forum since that is my application software, but the question is a general one about the boundary layer.

My question:
When I increase the mesh resolution in the BL, it has the result of reducing the mesh size of the last BL mesh, in comparison to the first mesh volume size of the core flow. Is this a problem, when there is a large step change between the last prism layer mesh, and first core mesh volume?

Model:
I'm modeling channel flow looking at the boundary layer on one wall that starts from a BL trip (modeled with surface roughness near the beginning of the channel). I am using a Reynolds stress turbulence model. I'm confused in how/the need to do a grid independence study for the boundary layer.

Grid independence methodology:
First I refined my core (outside the BL) as needed until I was happy with grid independence for the bulk flow. For these different runs of different core mesh size, my boundary layer mesh was always the same-- y+=~0.85, with 20 prism layers across the BL thickness (as calculated from normal flat plate turbulent BL growth).

Now, any insight into the proper methodology to reach grid independence in the BL? Do I, as stated above, just increase the number of cells in the BL, while keeping the first mesh size the same to preserve y+=~1.0 as needed for the turbulence model?

Alternatively, I could also reduce the size of the first mesh, so now it would be even less than y+=~0.85 and it is more refined all the way across the BL.

Also since there seems to be two variables to change (first mesh size, and number of cells across the boundary layer mesh), any advice to know when the BL mesh is sufficient? Since it isn't like bulk flow where you just refine it until your variables of interest aren't changing.

Any insight would be appreciated. Thanks!