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Old   December 2, 2003, 07:16
Default grid independence
  #1
chanchala
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Hi,

If you found the grid independent solution for a certain geometry using say, standard wall functions and k-epsilon turbulence model, would this mesh still be grid independent if you change to Re-stress model or non-equilibrium wall functions? Or would you have to carry out a grid indepence test again?

thanks, Chanchi
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Old   December 2, 2003, 07:21
Default Re: grid independence
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chanchala
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Another question,

With non-equilibrium wall functions is there a recommended y+ value for the cells at the wall?
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Old   December 2, 2003, 16:32
Default Re: grid independence
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ap
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The y+ range is the same of standard wall-functions (y+ between 30 and 60).

For the enhanced wall treatment, you should have y+ close to 1 and, anyway, lower than 4.

Hi

ap
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Old   December 3, 2003, 13:20
Default Re: grid independence
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AMV
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Why are you so much bothered about grid independence. In whatever model you are using, you will definitely find that if your entire range of y+ is less than say 500 for a grid and less than 100 for another grid, the results like velocity profile at outlet etc would match considerably. Thus the only thing you should make sure is that your y+ is < 500. Also, unless its your research topic, dont go too much details of near wall flows, as nobody knows which approach is better. If you are asking engineering solutions, then k-epsilon with std wall functions is best approach.
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Old   December 8, 2003, 12:11
Default Re: grid independence
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chanchala
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Hi AMV (and ap),

Thanks for your comments. I'm doing a PhD and my pipe geometry is very curved so I thought it might be interesting to check what the effect of the different near wall models may be since the flow is governed by the curvature of the walls. It's only as a point of interest rather than a problem actually.

thanks, chanchi
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Old   December 9, 2003, 12:20
Default Re: grid independence
  #6
AM Vaidya
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If this is the case, then you should be really careful and should do proper exercise. Have you read following ? 1) Chen H.C., Patel V.C., Near wall turbulence models for complex flows including separation, AIAA, 26, No.6, June 1988. 2)Amano R.S., Development of a turbulence near-wall model and its application to separated and reattached flows, Numerical Heat Transfer, 7, 59 - 75, 1984. 3)Chieng C.C. and Launder B.E., On the calculation of turbulent heat transport downstream from an abrupt pipe expansion, Numerical Heat Transfer, 3, 189 -207, 1980. 6)Launder B.E. and Spalding D.B., The numerical computation of turbulent flows, Computational Methods in Applied Mechanics and Engineering, 3, 269-289, 1974.
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Old   December 9, 2003, 12:22
Default Re: grid independence
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AM Vaidya
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I forgot to give your answer in previous answer.

You will have to carry out grid independence test for each model separately.
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Old   December 9, 2003, 12:58
Default Re: grid independence
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chanchala
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Hi Thanks so much for those references. I have reserved a few books from the library on the subject but they seem to always be out! I'll get these immediately.

chanchi
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Old   December 9, 2003, 13:04
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AM Vaidya
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great . you r welcome for any future discussion .
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Old   January 11, 2004, 00:07
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s
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i think it is not neccesary, then we must carry out mesh study for every boudnary condition.
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Old   January 11, 2004, 06:45
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AMV
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strictly speaking yes
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Old   January 11, 2004, 07:50
Default Re: grid independence
  #12
simple
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how can u say that . do u have any reference
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