# Mesh Independence Study - Layering disappears across study meshes

#41

@DaleKramer

I hope you get the results that you want. But bear in mind that there is another side of CFD - discretization schemes and linear solver settings. If you still run into problems, share your project here so I can have a look.

#42

Look out, I plan to take you up on that offer at some point

Dale

#43

Before you leave, I am a little unsure of what you mean here. Are you talking the plane surface refinement levels ānormalā to flow dimension or āalongā the flow dimension?

Dale

#44

@DaleKramer

Sorry my bad. In the refinement settings, you can define refinement levels on the fuselage, the wings, and landing gears, etc. You can also define refinement levels on a box that covers the plane, so that the cells within the box will have at least that refinement level. For example, with a background mesh of 0.5m generated from blockMesh, you can define level 8 refinement on the plane, and level 6 refinement on a box that covers the plane. Now, if you define a level 9 refinement on the box, then the entire plane will also have level 9 refinement, finer than what you need creating a lot of cells.

When doing mesh independency study, you can increase the level of refinement of that box to 7, but you donāt need to increase the level on the plane. If there are areas on the plane that are apparently not described by the surface mesh properly, then you refine it.

#45

Ok, so would you consider these āspaceā refinement levels sufficient for the 15 layer boundary?

#46

Yes, I think the space refinement is enough. However, you want to increase a parameter called ānCellsBetweenLevelsā to 4 or 5 so that the transition between levels is smoother. At the moment it seems to be 2 or 3. Also, you donāt want 15 layers in this case, because:

1. using an averaged y+ = 50 first layer, with a growth rate of 1.2, 4 of these layers will yield an overal y+ = 50+50 * 1.2+50 * 1.2 * 1.2+50 * 1.2 * 1.2 * 1.2 = 268.4. A turbulent boundary layer ranges from y+ = 100 to 300. I think 4~5 layers is sufficient.
2. even with 15 layers, the transition between the final layer and the adjacent cell is still abrupt ā very large volume size change between the two. You want to avoid such behaviour.

#47

Sorry, I used that previous image only to see what kind of space refinement levels were required with respect to the final BL layer. That is not how I have layered my yPlus 30-100 mesh. The 30-100 mesh only has 3 layers like this:

Is this better?

Dale

#48

@DaleKramer

That looks much better.

#49

I think Iām off to the races now ā¦

#50

Hi Dylan, @dylan

Could you explain how your formula, that calculates a yPlus value for the 4th boundary layer, was derived and what purpose does it serve?

Is it somehow a guide for how many layers to choose based on a particular 1st layer y+ value and layer expansion ratio?

If that is the case, is this layering setup somehow a most optimized setup for a desired 1st layer y+ and layer expansion ratio?

Your formula seems to determine the yPlus value of each layer ( YP(x) ), and that value is based on the 1st layer yPlus value ( YP(1) ) and an exponential progression of the expansion ratio (ER). The formula can be summarized as:

YP(x) = YP(x-1) + YP(1)*ER^YP(x)

(derived from your equation for the 4th layer y+ = 50+50 * 1.2+50 * 1.2 * 1.2+50 * 1.2 * 1.2 * 1.2 = 268.4)

With 1.2 ER and a 1st layer y+ of 50, here is the progression of your formula:

I thought a desired yPlus was used in the online yPlus calculator in order to determine ONLY the 1st layer thickness.

I can not find any information of yPlus values being assigned to any layer but the first Layer.

When I view my yPlus values of a converged simulation, they are only on the geometry surface, the 2nd and further away layers are mapped at yPlus=0, like this 6 layer BL:

I am confused

Thanks,
Dale