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Old   March 2, 2012, 21:55
Default yPlusRAS Question
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Daniel
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Hello all,

I am running a 2-D airfoil simulation at M=0.3, Re = 6e6, and I want to set up my simulation with y+ ~ 30 to use the SST turbulence model effectively. When I run yPlusRAS, I get the following values for yPlus: min: 2.89107 max: 48.1331 average: 18.4332. I plotted yPlus vs x/c to find the locations where y+ deviates most from the desired value, and now I want to plot y vs x/c to find out how I can alter the mesh to achieve a more consistent y+. However, when I run yPlusRAS the data I get in the 'y' file is not of much use:

Code:
wing
    {
        type            calculated;
        value           uniform 1e-15;
    }
When I run sample on the y field, I get 1e-15 all around the airfoil. This is not accurate though, as seen when I try to calculate y+ from wallShearStress. I used these equations:

y+ = u* y / nu
u* = sqrt(tau_w / rho)
tau_w = mu * (du/dy)@y=0

I calculate a different tau_w than OF when I use OF's wallGradU for (du/dy)@y=0;
I calculate a different y+ than OF when I use OF's tau_w & y.

I think it is clear that y is not being calculated properly, at least not output to file properly, and it does not make sense to me that tau_w is not reproducible. I believe that y+ is being reported properly, but I am frustrated that I cannot reproduce it. I think y is wrong because this website:

http://geolab.larc.nasa.gov/APPS/YPlus/

Suggests I need a y of 0.0001356 m to get y+ = 30, and that is the number I used to generate the mesh. Has anyone tried to calculate y+ manually like this and succeeded? I would appreciate any advice that could be offered.

Thanks,
Dan
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Old   March 3, 2012, 12:18
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Eysteinn Helgason
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Hi Dan,

Have you seen this thread?
yPlusRANS gives you ystar ( and yes this is confusing),
you should be able to calculate yPlus using the tool from Niklas in post 8.

Cheers.
Eysteinn
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Old   March 3, 2012, 19:22
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Daniel
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Thanks Eysteinn,

The file "y" that it produces is identical. The file "yPlus" that it produces gives me lower values of y+, that are zero in some cases. Unfortunately, without an accurate list of y values it is not possible to check the y+ calculation manually. Any ideas?

yPlusRAS: min: 2.70987 max: 40.6757 average: 17.2329
yPlus: min: 0 max: 36.9822 average: 9.29129

A second issue is the calculation of tau_w: I cannot reproduce this from dU/dx, which concerns me for two reasons: 1) I use tau_w in determining y+ manually, and 2) I use tau_w in determining Cd. Any advice?

Thanks,
Dan
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Old   March 6, 2012, 05:07
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Eysteinn Helgason
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Hi Dan,

Sorry for my late answer,

I think the y values are given in the cell centers which should explain
your value on the wing. I get a list in my y file with values for the cells.

So you can use the spreadsheet view in paraview and select a cell to get the exact cell value and then compare to the yPlus value
This was useful for me: http://www.paraview.org/Wiki/Data_Selection

/Eysteinn
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Old   April 15, 2012, 17:18
Default Overview yPlus methods
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Arnout
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Hi,

I spend some hours to find out what the yPlusRAS utility is doing. I calculated yplus with four different methods:

1) yPlusRAS utility
2) ystar from the k at the wall: y*= Cmu^0.25*k^0.5*rho*y_wall /mu.
3) yplus using the wallShearStress utility: y+=tau^0.5*rho*y_wall /mu
4) yplus using the wallgradU utility: y+=(mu*du/dy)^0.5*rho*y_wall /mu

Be carefull:
- The tau openfoam is calculating is nu*du/dy. Multiplying with rho will give the wallShear stress in Pa as in most formulas in books and on the web.
- Use the cell center (height of the cell devided by two) for y_wall.

I get exactly the same values for method 1) and 2). Method 3) and 4) are giving different results, see plot.

Looking in the code, the wallGradU utility is using:
wallGradU.boundaryField()[patchi] = -U.boundaryField()[patchi].snGrad();

which is the same as in the yplus utility going around on this forum. The wallShearStress utility is using:

wallShearStress.boundaryField()[patchi] = (-mesh.Sf().boundaryField()[patchi]/mesh.magSf().boundaryField()[patchi]) & Reff.boundaryField()[patchi];

But what is correct? I don't know yet. However, I think I will use the yPlusRAS utility since fluent is doing the same.
Attached Images
File Type: gif y_plus.gif (17.7 KB, 229 views)
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Old   September 22, 2013, 11:08
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Daniel
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Hello Arnout,

For 3) and 4) to be the same, you must consider mut (compressible) or nut (incompressible). See this thread:

wallShearStress, wallGradU are inconsistent

As for 1 & 2 and 3 & 4 being different from each other, that is the subject of many threads - yPlusRAS calculates y* and not y+.

Regards,
Dan
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Old   January 8, 2015, 14:23
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Hasan K.J.
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Quote:
Originally Posted by dancfd View Post
Hello Arnout,

For 3) and 4) to be the same, you must consider mut (compressible) or nut (incompressible). See this thread:

wallShearStress, wallGradU are inconsistent

As for 1 & 2 and 3 & 4 being different from each other, that is the subject of many threads - yPlusRAS calculates y* and not y+.

Regards,
Dan
Dear Dan,

So using the corrected yPlus Utility from here: Problems with YPlusRAS and wallShearStress
calculates the right y+ values for a completed simulation am I right ?

My question is how do we now determine the Y distance which will be varying through out the airfoil and implement it within the mesh ? for capturing the flow better closer to the airfoil.

Kind Regards,
Hasan K.J
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Old   January 15, 2015, 22:37
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Daniel
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Hello Hasan,

It has been a while since I looked at it but I believe you are correct, that utility will calculate y+ in place of y*, though the Fluent guide states that the two should be similar under some conditions - I do not recall what conditions, though - suggest you check the guide.

As for determining the wall distance of the first layer of cells, you could calculate the minimum required distance to achieve the maximum desired y+ and apply that single value all over the airfoil, or calculate it by cell on the airfoil (probably unnecessary and difficult to mesh).

Hope this helps. Good luck!

Dan
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