Add boundary layers to a wall

Build the prism stack that resolves the near-wall flow, sized from the y+ you want rather than from a first-cell height you would have to work out yourself.

Runs on the free tier.

Steps

1. Select Boundary layers.

2. Set Target y+. The hint tells you which range applies to the Wall treatment chosen under Models — wall functions and a resolved low-Re wall want different things, and the two are not interchangeable.

3. Read the block underneath. It works forwards from the reference velocity, the reference length and the fluid to give the Reynolds number, the first layer height, the total thickness of the stack and the surface cell size. Nothing here is typed in; it follows from the target and the case.

4. Under Stack, set:

   • Layers — how many prisms.
   • Expansion — the growth ratio between them.

   A warning appears if the stack becomes as thick as the surface cell, or if the first-cell aspect ratio gets high. Both are real problems: the outermost layer then sits against a much larger cell.

5. Press Generate mesh — the button is on this pane as well as the toolbar.

6. Come back and read Coverage. It reports the percentage achieved after the last mesh. Below the 70% target, Apply coverage recommendation appears and adjusts the settings for you.

Scope

The stack is one setting for the case, not one per patch, and coverage is reported against the wall it was applied to. If different walls need different treatment, use a second simulation rather than expecting a per-patch control here.

Check it worked

• Coverage reports 70% or better.
• Quality has not degraded — layer addition is the step most likely to push skewness up.
• Add a y+ entry under Result control before the run, and confirm afterwards that the achieved y+ is in the range the wall treatment needs.

Why y+ decides whether the turbulence model is being used correctly: Concepts.