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y+ calculation (for no wall function walls) (and its BC)

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Old   March 4, 2011, 05:59
Default y+ calculation (for no wall function walls) (and its BC)
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Dear experts,

When i use wall function i can see y+ by yPlusRAS and yPlusLES command. it calculate min, max and mean of y+ and write it on terminal.
But when y+<5 i don't need wall function and i set wall as zerograDient.
but by above commands no y+ would be written on terminal:
Code:
Time = 0.47
Calculating wall distance

Writing wall distance to field y

Reading field U

Reading/calculating face flux field phi

Selecting incompressible transport model Newtonian
Selecting RAS turbulence model LRR
LRRCoeffs
{
    Cmu             0.09;
    Clrr1           1.8;
    Clrr2           0.6;
    C1              1.44;
    C2              1.92;
    Cs              0.25;
    Ceps            0.15;
    sigmaEps        1.3;
    couplingFactor  0;
}

    no nutWallFunction patches
Writing yPlus to field yPlus

End
in yPlus file it is:
Code:
    lowerWall
    {
        type            calculated;
        value           uniform 0;
How can i see y+ now?

it may because it is zeroGradient nut, K, epsilon, R so what BC should be used when no need to wall function. U is set value 0 not zeroGradien.
Regards
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Old   March 4, 2011, 07:27
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Use wallGradU and formula 7.24 from pope:

tau_wall = rho*nu*(dU/dy)_wall

However, this will only work if first y+ is really small, say around 1. You can use this is an wall-resolved LES or DNS.
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Old   March 4, 2011, 07:31
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sorry, double post

Last edited by stevenvanharen; March 4, 2011 at 08:35.
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Old   March 4, 2011, 08:09
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sorry, double post

Last edited by stevenvanharen; March 4, 2011 at 08:35.
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Old   March 4, 2011, 09:02
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Quote:
Originally Posted by stevenvanharen View Post
Use wallGradU and formula 7.24 from pope:

tau_wall = rho*nu*(dU/dy)_wall

However, this will only work if first y+ is really small, say around 1. You can use this is an wall-resolved LES or DNS.
Thanks.

But this formula won't give me y+.

i want see y+ value by yPlusRAS or yPlusLES
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Old   March 4, 2011, 09:12
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yPlusLES works fine for me.

Why does it not work for you?
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Old   March 4, 2011, 10:58
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yPlusRAS works when the patch setting be nutWallFunction.

When it be set as zeroGradient above output is seen.
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Old   March 7, 2011, 05:39
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Quote:
Originally Posted by maysmech View Post
Thanks.

But this formula won't give me y+.

i want see y+ value by yPlusRAS or yPlusLES
Not true. The formula is indeed suitable for giving you the y+ value at the wall, as:

y+=(y/nu)*sqrt(tau_wall/rho)

Youll'have simply to know the height y of your first near wall node and then you can calculate y+ wherever you want, by sampling wallGradU or directly wallShearStress (both of these utilities work also without wall functions as bcs)

Regards

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Old   March 7, 2011, 07:17
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My problem is yPlusRAS dosen't work for me.
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Old   March 7, 2011, 08:18
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Quote:
Originally Posted by maysmech View Post
How can i see y+ now?
Is this you or not? However, yPlusRAS doesn't work if you don't use a wallfunction for nut...Sorry

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Old   March 7, 2011, 10:01
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Thanks Vesselin
It is strange because yPlusLES works for a LES case without wall function but yPlusRAS doesn't work for a LRR case without wall function.
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