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Near wall treatment in k-omega SST

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Old   November 12, 2014, 08:38
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Yes, it changes. I put all information in the excel file, enclosed here


Quote:
Originally Posted by RodriguezFatz View Post
So your y+ changes along the plate?
Attached Files
File Type: xlsx info.xlsx (57.9 KB, 13 views)
File Type: txt yplus.txt (5.9 KB, 25 views)
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Old   November 12, 2014, 09:00
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This looks strange. Is "wall_dist" the first cell distance in "m"? Is this exported from Fluent or from your meshing tool?
I am asking because, did you write down the OMG_vis formula without wall units? It's
OMG_vis = 6 * nu / (0.072 y^2)
So for constant y (in m) you should have a constant omega wall value...
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Old   November 12, 2014, 09:20
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Yes, "wall_dist" is the adjcent cell to the wall in "m" and it is exported from Fluent.
The OMG_vis formula is based on y+ not y and y+ is a nondimensional parameter. Moreover, Omega depents on wall shear, too; so that it has not constant value along the plate.
The main problem is that what formula Fluent uses for the Omega value of the wall.

Quote:
Originally Posted by RodriguezFatz View Post
This looks strange. Is "wall_dist" the first cell distance in "m"? Is this exported from Fluent or from your meshing tool?
I am asking because, did you write down the OMG_vis formula without wall units? It's
OMG_vis = 6 * nu / (0.072 y^2)
So for constant y (in m) you should have a constant omega wall value...
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Old   November 12, 2014, 09:26
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Quote:
Originally Posted by behest View Post
The OMG_vis formula is based on y+ not y and y+ is a nondimensional parameter. Moreover, Omega depents on wall shear, too; so that it has not constant value along the plate.
Write down the OMG_vis formula in dimensional units and not in non-dimensional units like you use it. You will see, that OMG_vis does not depend on any flow values at all. It just depends on the wall distance of your first cell (in "m") and on dynamic viscosity.
You will get
OMG_vis = 6 * nu / (0.072 y^2)
No y+, no wall shear.
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Old   November 12, 2014, 09:37
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Yes, you are right. Then, how is Fluent calculate the omega of the wall?
Actually, the same formula is written in Fluent theory guide,

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Originally Posted by RodriguezFatz View Post
Write down the OMG_vis formula in dimensional units and not in non-dimensional units like you use it. You will see, that OMG_vis does not depend on any flow values at all. It just depends on the wall distance of your first cell (in "m") and on dynamic viscosity.
You will get
OMG_vis = 6 * nu / (0.072 y^2)
No y+, no wall shear.
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Old   November 12, 2014, 09:43
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Are you 100% sure about the grid? Is the wall distance really constant?
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Old   November 12, 2014, 09:58
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Yes, I am sure, because my case study is a flat plate and I use hexahedral elements (structural mesh) and cells have the same wall distance.
I enclosed two figures of the mesh

Quote:
Originally Posted by RodriguezFatz View Post
Are you 100% sure about the grid? Is the wall distance really constant?
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File Type: jpg mesh,leading edge of plate.jpg (70.3 KB, 109 views)
File Type: jpg mesh.jpg (21.4 KB, 108 views)
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Old   November 13, 2014, 07:55
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I remember when I checked these values for one of my cases everything was alright. There must be some mistake in the data. Can you upload the whole fluent case?
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Old   November 13, 2014, 08:14
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Yes, sure, you can find the cas and data file in the Dropbox, linked here:

https://www.dropbox.com/sh/rg87mkr95...MyjuKLP-a?dl=0

Quote:
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I remember when I checked these values for one of my cases everything was alright. There must be some mistake in the data. Can you upload the whole fluent case?
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Old   November 13, 2014, 08:59
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Ali, you should try to understand my post #14 in this thread (very first post of me). I think you can not plot omega on the surface, because Fluent doesn't let you. You are currently plotting interpolated values. Try this: plot "k" along the plate. You will see, that it isn't zero everywhere. This doesn't make sense, because k-boundary condition is zero. This can only be explained by interpolation errors during post-processing.

I also found in the userguide:

In ANSYS FLUENT you can choose to display the computed cell-center values or values that have been interpolated to the nodes. By default, the Node Values option is turned on, and the interpolated values are displayed. For line contours or profiles, node values are always used. If you are displaying filled contours or profiles and you prefer to display the cell values, turn the Node Values option off. Filled contours/profiles of node values will show a smooth gradation of color, while filled contours/profiles of cell values may show sharp changes in color from one cell to the next.

For face-only functions (e.g., Wall Shear Stress), the cell values that are displayed for boundary zone surfaces will actually be the face values. This is only true in the case of boundary zone surfaces created for postprocessing, where the actual cell values are used for the part of the surface which lies in the interior. These face values are more
accurate, as face-only functions are computed on the faces and not on the cells.
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Old   April 21, 2015, 03:38
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Hi!

I don't understand this:

Quote:
Wall functions should only be used for 30 < y+ < 100-300, which means that the first cell is in the log layer and not in the sublayer. If you want to use kOmega_SST without a wall function, y+ should be around 1.
I mean, if you run the motorbike tutorial (simpleFoam), and then you run the yPlusRAS utility (you can do it because it uses wallfunctions for every variable) you get:

Code:
/*---------------------------------------------------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
Build  : 2.3.0-f5222ca19ce6
Exec   : yPlusRAS -latestTime
Date   : Apr 21 2015
Time   : 09:20:56
Host   : "usuario-SATELLITE-P50-A-14G"
PID    : 6279
Case   : /home/usuario/OpenFOAM/usuario-2.3.0/run/tutorials/incompressible/simpleFoam/motorBike
nProcs : 1
sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE).
fileModificationChecking : Monitoring run-time modified files using timeStampMaster
allowSystemOperations : Disallowing user-supplied system call operations

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time

Create mesh for time = 500

Time = 500
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 kOmegaSST
kOmegaSSTCoeffs
{
    alphaK1         0.85034;
    alphaK2         1;
    alphaOmega1     0.5;
    alphaOmega2     0.85616;
    gamma1          0.5532;
    gamma2          0.4403;
    beta1           0.075;
    beta2           0.0828;
    betaStar        0.09;
    a1              0.31;
    b1              1;
    c1              10;
    F3              false;
}

Patch 3 named lowerWall y+ : min: 4.88086 max: 3607.62 average: 206.955

Patch 5 named motorBike_frt-fairing:001%1 y+ : min: 1.19946 max: 832.435 average: 124.919

Patch 6 named motorBike_windshield:002%2 y+ : min: 10.6911 max: 166.337 average: 75.398

Patch 7 named motorBike_rr-wh-rim:005%5 y+ : min: 4.1793 max: 512.061 average: 163.725

Patch 8 named motorBike_rr-wh-rim:010%10 y+ : min: 2.58583 max: 509.935 average: 196.126

Patch 9 named motorBike_fr-wh-rim:011%11 y+ : min: 5.40472 max: 597.576 average: 187.444

Patch 10 named motorBike_fr-wh-brake-disk:012%12 y+ : min: 107.562 max: 338.746 average: 216.073

Patch 11 named motorBike_frame:016-shadow%13 y+ : min: 35.4755 max: 730.857 average: 258.606

Patch 12 named motorBike_rear-susp:014%14 y+ : min: 0.253153 max: 414.543 average: 77.3381

Patch 13 named motorBike_rear-susp:014-shadow%15 y+ : min: 0.300651 max: 344.091 average: 72.7773

Patch 14 named motorBike_frame:016%16 y+ : min: 35.5895 max: 782.373 average: 252.4

Patch 15 named motorBike_rr-wh-rim:005-shadow%17 y+ : min: 6.08735 max: 483.529 average: 174.497

Patch 16 named motorBike_rr-wh-chain-hub:022%22 y+ : min: 12.6166 max: 518.445 average: 200.842

Patch 17 named motorBike_rearseat%24 y+ : min: 4.59619 max: 681.697 average: 145.558

Patch 18 named motorBike_frt-fairing%25 y+ : min: 4.95551 max: 305.27 average: 75.9044

Patch 19 named motorBike_windshield%26 y+ : min: 7.26132 max: 450.868 average: 94.9765

Patch 20 named motorBike_headlights%27 y+ : min: 11.7862 max: 197.488 average: 65.7203

Patch 21 named motorBike_driversseat%28 y+ : min: 1.01499 max: 753.983 average: 127.026

Patch 22 named motorBike_rear-body%29 y+ : min: 0.596934 max: 993.403 average: 116.239

Patch 23 named motorBike_fuel-tank%30 y+ : min: 1.739 max: 566.51 average: 84.194

Patch 24 named motorBike_exhaust%31 y+ : min: 3.85593 max: 935.894 average: 203.403

Patch 25 named motorBike_rr-wh-rim%32 y+ : min: 2.09283 max: 497.315 average: 73.341

Patch 26 named motorBike_fr-mud-guard%33 y+ : min: 6.73648 max: 640.736 average: 131.08

Patch 27 named motorBike_fr-wh-rim%34 y+ : min: 4.9542 max: 746.132 average: 166.426

Patch 28 named motorBike_fr-wh-brake-disk%35 y+ : min: 4.49047 max: 568.912 average: 184.304

Patch 29 named motorBike_fr-brake-caliper%36 y+ : min: 40.765 max: 675.114 average: 327.912

Patch 30 named motorBike_fr-wh-tyre%37 y+ : min: 3.78615 max: 717.634 average: 116.777

Patch 31 named motorBike_hbars%38 y+ : min: 29.7385 max: 803.726 average: 350.639

Patch 32 named motorBike_fr-forks%39 y+ : min: 4.94379 max: 767.731 average: 163.808

Patch 33 named motorBike_chain%40 y+ : min: 1.35618 max: 801.723 average: 291.154

Patch 34 named motorBike_rr-wh-tyre%41 y+ : min: 2.02377 max: 621.427 average: 94.479

Patch 35 named motorBike_square-dial%42 y+ : min: 12.783 max: 20.3879 average: 18.4621

Patch 36 named motorBike_round-dial%43 y+ : min: 133.509 max: 461.298 average: 269.324

Patch 37 named motorBike_dial-holder%44 y+ : min: 4.75498 max: 447.694 average: 145.443

Patch 38 named motorBike_rear-susp%45 y+ : min: 0.533745 max: 690.342 average: 158.851

Patch 39 named motorBike_rear-brake-lights%46 y+ : min: 66.039 max: 679.236 average: 293.376

Patch 40 named motorBike_rear-light-bracket%47 y+ : min: 4.5641 max: 589.924 average: 154.913

Patch 41 named motorBike_frame%48 y+ : min: 0.825434 max: 937.666 average: 135.254

Patch 42 named motorBike_rear-mud-guard%49 y+ : min: 3.30992 max: 553.535 average: 110.729

Patch 43 named motorBike_rear-susp-spring-damp%50 y+ : min: 13.8833 max: 329.802 average: 90.9432

Patch 44 named motorBike_fairing-inner-plate%51 y+ : min: 1.26159 max: 331.501 average: 77.0047

Patch 45 named motorBike_clutch-housing%52 y+ : min: 0.284496 max: 528.746 average: 89.331

Patch 46 named motorBike_radiator%53 y+ : min: 105.444 max: 1107.61 average: 652.528

Patch 47 named motorBike_water-pipe%54 y+ : min: 109.441 max: 719.566 average: 342.032

Patch 48 named motorBike_water-pump%55 y+ : min: 58.1246 max: 400.726 average: 216.617

Patch 49 named motorBike_engine%56 y+ : min: 0.095713 max: 864.928 average: 90.6738

Patch 50 named motorBike_rear-shock-link%57 y+ : min: 16.4089 max: 252.338 average: 134.633

Patch 51 named motorBike_rear-brake-fluid-pot-bracket%58 y+ : min: 2.38783 max: 795.973 average: 126.167

Patch 52 named motorBike_rear-brake-fluid-pot%59 y+ : min: 70.8986 max: 461.902 average: 232.822

Patch 53 named motorBike_footpeg%60 y+ : min: 80.5822 max: 573.279 average: 304.165

Patch 54 named motorBike_rr-wh-chain-hub%61 y+ : min: 4.50327 max: 354.193 average: 162.007

Patch 55 named motorBike_rear-brake-caliper%62 y+ : min: 13.9359 max: 808.401 average: 285.541

Patch 56 named motorBike_rider-helmet%65 y+ : min: 5.23685 max: 622.124 average: 109.845

Patch 57 named motorBike_rider-visor%66 y+ : min: 39.7826 max: 160.656 average: 121.427

Patch 58 named motorBike_rider-boots%67 y+ : min: 1.5236 max: 732.281 average: 128.206

Patch 59 named motorBike_rider-gloves%68 y+ : min: 3.46863 max: 891.645 average: 276.421

Patch 60 named motorBike_rider-body%69 y+ : min: 1.42095 max: 782.554 average: 99.6377

Patch 61 named motorBike_frame:0%70 y+ : min: 7.67993 max: 623.53 average: 266.79

Patch 62 named motorBike_frt-fairing:001-shadow%74 y+ : min: 1.71476 max: 885.956 average: 126.73

Patch 63 named motorBike_windshield-shadow%75 y+ : min: 9.79227 max: 503.034 average: 104.015

Patch 64 named motorBike_fr-mud-guard-shadow%81 y+ : min: 6.33128 max: 511.078 average: 129.128

Patch 65 named motorBike_fr-wh-brake-disk-shadow%83 y+ : min: 6.77294 max: 525.776 average: 173.026

Patch 66 named motorBike_rear-mud-guard-shadow%84 y+ : min: 2.3761 max: 535.362 average: 102.709

Patch 67 named motorBike_rear-susp-spring-damp-shadow%85 y+ : min: 12.0661 max: 370.146 average: 116.931

Patch 68 named motorBike_radiator-shadow%86 y+ : min: 123.77 max: 941.179 average: 598.408

Patch 69 named motorBike_rear-shock-link-shadow%87 y+ : min: 19.2968 max: 294.77 average: 145.051

Patch 70 named motorBike_rear-brake-fluid-pot-bracket-shadow%88 y+ : min: 2.29904 max: 427.28 average: 131.788

Patch 71 named motorBike_rr-wh-chain-hub-shadow%89 y+ : min: 6.38424 max: 360.01 average: 170.899

Writing yPlus to field yPlus

End
As you can see, there are many different values of y+... I am a little bit confused
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Old   April 21, 2015, 04:21
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yPlus value is calculated for each cell next to the wall, each cell has it's y+ value, that's why you have min/max and avg values.
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Old   April 21, 2015, 04:26
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I know..

But they are not in the supposed interval, that's because I posted that
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Old   April 21, 2015, 05:00
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Quote:
Originally Posted by Yuby View Post
I know..

But they are not in the supposed interval, that's because I posted that
You have to consider that the tutorials are not for academic analysis. They are there to show the capabilities of OpenFOAM. However, when you design your cases, you will need to take care of numerics yourself. You will also find LES cases running with low res schemes in the tutorials, which does not mean this should be done like that in real life. Keep in mind the tutorial cases should run fast on many difference computers.
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Old   April 21, 2015, 05:08
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Infinite thanks, Roman!

If I use the same wall functions of the motorbike tutorial (nutKwallfunction, kqRwallfunction and omegawallfunction) and standard k omega model what values of y+ I should have? And for standard k epsilon with epsilonwallfunction?
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Old   April 21, 2015, 09:00
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I know..

But they are not in the supposed interval, that's because I posted that
Oh, sorry, I misunderstood your question.
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Old   April 22, 2015, 03:19
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Quote:
Originally Posted by Yuby View Post
Infinite thanks, Roman!

If I use the same wall functions of the motorbike tutorial (nutKwallfunction, kqRwallfunction and omegawallfunction) and standard k omega model what values of y+ I should have? And for standard k epsilon with epsilonwallfunction?
For all these wall functions, one should consider to have the y+ between 35 and 150, at about 35 the log layer starts. If you have more than 150, let's say up to 300, this could sometimes also work, but I wouldn't necessarily rely on it. The best way of finding out is to run a simple case (channel flow, pipe flow in 3D) against experimental values and see what gives good results and what doesn't

More information on wall functions and near wall behavior can be found in

1. Wilcox, D. C. (2006). Turbulence modeling for CFD (3rd ed.). La Canada Calif.: DCW Industries.

2. Pope, S. B. (2000). Turbulent flows. Cambridge [u.a.]: Cambridge Univ. Press.

3. Schlichting, H., & Gersten, K. (2000). Boundary Layer Theory (8th Revise). Darmstadt: Springer Berlin Heidelberg.

4. Bredberg, J. (2000). On the Wall Boundary Condition for Turbulence Models. Göteborg, Sweden. (gives explanation of wall functions in OpenFOAM)

5. Schiestel, R. (2008). Modeling and Simulation of Turbulent Flows. London, UK: ISTE. http://doi.org/10.1002/9780470610848
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Old   June 9, 2015, 09:35
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Quote:
Originally Posted by Arnoldinho View Post
Yes, you are right. Although regarding different sources, you also get different limiting values for the viscous sub-layer (0 < y+ < 1, 5) and the log-layer (30, 70 < y+ < 100, 200, 300). I also remember a thread where OF has been tested as low re model without wall functions: an y+ value < 0.2 lead to 'wrong' results as well. For the buffer layer (e.g. 5 < y+ < 30), a blending is needed.

Arne
Hello Arne,

It's the first time I hear about y+ values below 0.2 leading to wrong results. Do you have more information about this?

Best regards,

Kate
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Old   March 8, 2017, 13:48
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Helllo,
So by having all in hand,
now in modelling high-speed flow with K-w sst, if your first grid is in Y+=150 (log layer). and you are interested in studying the vorticity of separated flow downstream of a backstep (Backward Facing Step), does this putting first grid point on log layer make any problem?
what is the difference between Fluent and OpenFoam in this?
Can anyone introduce me some papers in this regard?

Thanks
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