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Y plus in buffer layer in StarCCM+

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Old   May 10, 2022, 23:04
Default Y plus in buffer layer in StarCCM+
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Hamed
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Hi all,

I have simulated a ship in a virtual wind tunnel, and I want to show Y+ in the buffer layer. As can be seen in the attached I have drawn Y plus for the surface of my model, even though I was wondering how do I have to indicate it for the buffer layer specifically? any help would be appreciated.

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Old   May 11, 2022, 04:26
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That's not how yplus contours work in any code, nor y+ in general. The contour you see is just a nondimensional version of your tau_wall (actually, its absolute value).

This nondimensional value is based on the y+ definition using the wall distance in the cell adjacent to the point of the surface where you see the contour.

So, for a given grid and solution, there is a single y+ value to show, the one related to your near wall cell and its tau_wall. That value might or not end up in the buffer layer, but this mostly depends from your grid.

Now, if you want to have a nice plot of the u+ vs y+ for a given profile extracted along a normal to the wall in a given point, you need to extract the tau_wall at that point and do the change of variable yourself.

You can also divide the yplus you have by the local cell wall distance and you obtain a quantity that can just be multiplied by any distance and return your new y+ value in all the locations
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Old   May 11, 2022, 05:30
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Dear Sbaffini,

Thanks for your prompt reply. However, I was wondering if there is any built-in feature in Starccm to do that?

Actually, I don't know how to implement this part of your message
"Now, if you want to have a nice plot of the u+ vs y+ for a given profile extracted along a normal to the wall in a given point, you need to extract the tau_wall at that point and do the change of variable yourself."

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Old   May 11, 2022, 06:09
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Well, the profile part is typically simple and common to several software, yet I have zero knowledge of starccm.

I don't think there is any code automating this. In order to do this manually you need to take care of two issues:

1) When you pick up a general point on a surface, the relevant velocity to extract along the normal n is the one parallel to the wall: u - (u*n)n

2) The tau_wall has to be extracted at the intersection between the line you extract and the wall
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Old   May 11, 2022, 08:04
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Star gives you tau at the wall and y+ and u* at the wall adjacent cells. Star does give you the distance to the nearest wall for a cell in the interior volume but it does not give you y+,u+, nor u* there. There isn't a general framework for how to compute y+ and u+ for an arbitrarily complex 3D flow, this part you have to do yourself.
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Old   May 12, 2022, 13:58
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If you have access the to Siemens Support Center site, you might want to take a look at this article:

https://support.sw.siemens.com/en-US...00037693_EN_US

If the link doesn't work, you can search for an article titled:

How to plot y+ and u+ boundary layer profiles normal to a wall
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