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wall function in backward step

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Old   March 10, 2005, 08:39
Default wall function in backward step
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Did somebody solved the turbulent backward step problem in fluent tutorial. In the tutorial they use k-omega model but never checked the y plus values and they used the wall function. I see that yplus values are less than 10 this gives that wall function becomes wrong. Can anybody comment on it. pp
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Old   March 10, 2005, 10:14
Default Re: wall function in backward step
Andrew Garrard
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This is a tutorial problem we set students all the time. The reason that the Y-plus drops below the desired value is that at the re-connection point the velocity vector of the fluid is changed to be more perpendicular with lower surface. It will be at this point you notice the y+ drop. I like this tutorial because it gets the following point across

In CFD you make a number of assumptions. Even if the Y+ values are in the guide range there are still assumptions made, that don't hold up in a number of real life applications. Because it is only a few cells that drop below the desired Y+ value you can tolerate this as it will still give you a very good solution to the problem. If you want a solution to match experiments exactly, you should probably not use numerical modeling.

You have to decide on an engineering tolerence that you consider accetiable. You will not get your model to match the experiment exactly. The more effort you put in, the closer the more accurate the solution will be, but what increse in accuracy warrents that effort?
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Old   March 10, 2005, 10:35
Default Re: wall function in backward step
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Is it acceptable to use wall function in LES calculations.
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