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-   -   y+ calculation (for no wall function walls) (and its BC) (https://www.cfd-online.com/Forums/openfoam/85678-y-calculation-no-wall-function-walls-its-bc.html)

maysmech March 4, 2011 04:59
y+ calculation (for no wall function walls) (and its BC)
 
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

stevenvanharen March 4, 2011 06:27
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.

stevenvanharen March 4, 2011 06:31
sorry, double post

stevenvanharen March 4, 2011 07:09
sorry, double post

maysmech March 4, 2011 08:02
Quote:
Originally Posted by stevenvanharen (Post 297906)
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

stevenvanharen March 4, 2011 08:12
yPlusLES works fine for me.

Why does it not work for you?

maysmech March 4, 2011 09:58
yPlusRAS works when the patch setting be nutWallFunction.

When it be set as zeroGradient above output is seen.

vkrastev March 7, 2011 04:39
Quote:
Originally Posted by maysmech (Post 297924)
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

V.

maysmech March 7, 2011 06:17
My problem is yPlusRAS dosen't work for me.

vkrastev March 7, 2011 07:18
Quote:
Originally Posted by maysmech (Post 297900)
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

V.

maysmech March 7, 2011 09:01
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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