Boundary layer question


#1

Can someone explain in detail howto control and check the boundary layer.
For one of my wings i can calculate or use a calculator to get my distance for desired y+ 1.
https://www.cfd-online.com/Tools/yplus.php

But how to use the inflate boundary layer option to get this value? and how to check after meshing?
Should the final thickness be the value for my first cell? Or should i just make sure that the final layer thickness cover my distance calculated?

And a last thing , if we do use inflate boundary layer value calculated for ,say y+ 1, how will wall treatment for no-slip at surface do for my simulation?

Bests JT


#2

Hi @jan2re!

Maybe my colleagues @Get_Barried or @vgon_alves can give you a detailed description of the problem and how to efficiently solve that. Good question by the way! I will probably post an instruction for that later on that week into the SimWiki!

Cheers!

Jousef


#3

Hi @jan2re,

Boundary layer inflation under your mesh refinements “Inflate Layers” is how you control it.

Under the result control in the simulation tab there is a y+ result control that you can enable that allows you to quickly check the y+ throughout the geometry when post-processing.

You first need to calculate out your first layer cell size using the y+ calculator you posted. From there, determine a suitable expansion ratio, typically 1.2 to 1.5, and the number of layers you want to inflate to. You then need to calculate out the final layer size and the overall layer thickness and state those into the mesh refinement. Do note to put your minimum overall layer size to much less than the overall layer size if not the mesher just deletes off the layers.

Stated above. Another way is to enable “surface with edges” in ParaView and measure the first cell layer height then compare that to the calculated value through the link you posted to see if it matches. You can also see it through the meshing log, towards the final part of the log where it states the layers and if they are generated. If all are 0 that means the layers are not generated.

No it is the final layer thickness, not the first cell. You don’t want to inflate only 1 cell as the cell ratio between that really small cell and the outside cell will cause inaccuracies.

By calculated distance I assume you mean the output from the y+ calculator. That just gives your first layer cell height and not your final layer cell height. So your first cell should be at or below this amount.

Wall functions for no-slip means that your y+ required to capture the viscous effects near the surface is now 30 to 300 and not less than 30. Even if you do set a y+ to be less than 30 it would likely not yield very different results. However, if you were to use full resolution, then you would want a y+ of less than 1. Do note that full resolution for some reason has instability when used with the k-eps model and as far as I know is only stable for the k-w SST turbulence model.

Last thing, snappyHexMesh tends to not like very small layers for some reason due to the quality control. So in order to achieve the exact layers you want, you need to disable relative layer size and follow this post to ensure your layers don’t get deleted.

Cheers.

Regards,
Barry

EDIT: The statement I made “Even if you do set a y+ to be less than 30 it would likely not yield very different results” is incorrect as if a standard wall function is used as is in the case for SimScale, the y+ must strictly be between 30 and 300 if not behavior near the wall may act differently or incorrectly. Refer to the documentation for a statement on this.


#4

Nice one Barry :+1:

We should really transform this into a step-by-step instruction - I believe that a lot of people will benefit from this! I hope that Barry could help you @jan2re! For any further questions make sure to contact us whenever you like.

All the best!

Jousef


#5

Excellent , i am impressed over the quality of answer given in this forum. Thank you , @Get_Barried for your service.