__ wall function for low yPlus __
Dear Foamers,
Happy new year.:) Please tell me which wall function i should use to define wall B.C when my y+ is: Patch 4 named wall y+ : min: 0.0516154 max: 2.20514 average: 0.344511 now, i use nutWallFunction for nut kqRWallFunction for k epsilonWallFunction for epsilon but i think they dont give true answers. Thanks in advance, 
mmmm....I would use no wall functions, your yplus values are too low to be in the log layer:
http://www.cfdonline.com/Wiki/Wall_functions 
Hello, maysam,
I suppose you are using one of the kepsilon based turbulence models? I can only guess but if you're not already using the LowReynolds corrected versions of the kepsilon model you'll obtain poor results when integrating these turbulence models down to the viscous sublayer (which is clearly the case looking at your y+values). See D.C. Wilcox's book "Turbulence Modeling for CFD" as a reference. If your actually using the LowReynolds models then you shouldn't use wall functions, like Steven said in the post above. These models are supposed to work with more stringent boundary conditions (see http://www.cfdonline.com/Wiki/LowRe_kepsilon_models as a reference). Greetings, Felix 
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the case (hydrocyclone) is not a low Re problem and i want run it with ke, LRR and LES. i think its low y+ is result of is small mesh at near wall. now, what is your suggestion about Wall boundary field settings for k, epsilon and nut? Best, Maysam 
you mean i should set zseroGradient B.C for K, epsilon and nut for wall ?
wall { type zeroGradient; } Is it a suitable B.C? 
Hello, Maysam,
I guess the term "LowReynolds model" is a bit misleading. These models are actually suitable for any Reynolds number (as long as the flow is turbulent, of course). These LowReynolds kepsilonmodels are standard kepsilon models with additional viscous damping functions to improve the models' results for nearwall regions. They're called "LowReynolds", because these models require an near wall y+ of the order of 1 (as in your case) and this is usually too costly for engineering applications with high reynolds numbers. But if you insist in using the standard (HighRe) kepsilon model for your case without using logwallfunctions (i.e. y+>30), these would be suitable BCs, according to my knowledge: nut: fixedValue 0; OR nutWallFunction; (this WF is continuous!) k and epsilon: fixedValue 1e10; (has to be nonzero to avoid division by zero errors!) Let me repeat: The results will probably be inaccurate, especially the wall shear stress and heat transfer values. And this is not a BC problem, this is a problem of the kepsilon model itself. Please refer to Wilcox' book regarding that issue. Greetings, Felix. 
Thanks,
What is your suggestion for high y+. I changed my mesh to a coarse one and run it with ke. y+ is near 70. 
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Regards V. 
turbulent flat plate
Hi,
I am trying to simulate the turbulent flat plate flow. I am using a yPlusRAS utility which gives me the following Patch 3 named plate y+ : min: 63.4497 max: 90.5452 average: 73.1344 I am using kqRWallFunction for k, omegaWallFunction for omega, and nutWallFuction for nut. However, I am not getting the expected results, eg. the k value is not zero at the plate. I am not sure whether I am correctly putting up the BC. Can you post me your BC file, so that i can cross check if i am setting up the case correctly? Quote:

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Hope this helps V. 
low y+ values
Hi,
Thanks for your reply. I compared my results to the different posts in the forum, and they are coming out quite close. The problem that now I am having is that when i decrease the y+ by increasing the cell grading, my solution does not converge. In my case, as y+ approaches 30, the residuals blow up. Is there something I'm missing in my solution. Quote:

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Regards V. 
low y+ values
3 Attachment(s)
Hello V,
Thanks yet again. I am attaching the three files for your review (blockMesh, system files, and BC). These are the files for the case where I get the nonconverged solutions. yPlus utility gives me the avg. value of y+ as 43.62 and the min y+ as 30.643. Quote:

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2) Assign an initial value for k and omega at the plate (as you have done with nut, but this time using the same values of the internal field initialization) 3) Underrelax omega and put the underrelaxation factors for k and omega to 0.5 (instead of 0.7) I'm quite sure that the main problems are in point 1 and 3. Hope this helps V. 
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