[GrN]

Hey everybody,

I'm wondering how to determine the first cell height for laminar flow.

For turbulence the case is quite clear to me, calculating around the y+ value and so on but when it comes down to laminar flow I never found a solid guidance on how to determine the first cell height next to the wall.

I'm feeling kinda dumb as it seems to be rather simple and I'm just missing some obvious stuff. Maybe someone in here might wants to shed some light into this darkness?

Thanks in advance!

[tesorieri]

In this post by andy_ you'll find the information you need.
Hope it helps!

The modelling assumptions used with turbulence models usually require the first cell to be placed around a particular y+ location. For some it is deep within the laminar sublayer where the turbulent stresses are insignificant (y+ < 1) for others it is in the equilibrium layer (y+ ~ 30) where the assumption that turbulent energy production balances turbulent energy destruction and the transport terms are negligible.

Laminar flows have no turbulence models and hence have none of these constraints on y+ for the first cell. What is required for the first cell and every other cell in the grid is that the gradients in the flow are adequately resolved. This is a function of the discretisation scheme and how well the gradients need to be resolved at the cell location. For example, a fully developed flow in a pipe may require only 1 cell to be fully resolved with a higher order numerical scheme or tens of cells for a low order scheme.

A resolution criteria based on y+ is going to be rather limited in applicability compared to a normal one based on gradients of the solution variables.