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Y+ Values for a sperating flow over a hill

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Old   July 7, 2005, 09:49
Default Y+ Values for a sperating flow over a hill
  #1
Dave
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Hey, I'm somewhat new to the whole CFD thing and though im learning quickly i've got to a part that has stumped me! see i have this 2d model where there is a small hill that wil produce a seperation in flow. I've opted to use a k-e viscous model as thats what dassault aviation used for a similar model. And according to the userguide an RNG would give better results! anyway.. i've been told that i want a y+ value of between 30 and 300 so that it follows the law of the wall. The problem i find is that when you get to the parts where it seperates the y+ values just dip to values around 0 -5 depending on the grid. The only grid that i've managed to get higher that 30 values for y+ is one that is rather large relative to the model. hence im loosing accuracy due to the large boundary layer mesh! people have said i want a fine B.L mesh though all that i've tried give me really small y+. Its realy stumped me! any suggestions? would the low y+ values at the speration points really effect the result so much?
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Old   July 8, 2005, 00:16
Default Re: Y+ Values for a sperating flow over a hill
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zxaar
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well if you have yplus near to 3 (manual say 1 would be better), what you can do is use enhanced wall treatment for it. where it needs the mesh in the yplus you are talking about.
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Old   July 9, 2005, 23:18
Default Re: Y+ Values for a sperating flow over a hill
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anindya
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1) Use enhanced wall treatment as Zxaar mentioned. For this will need a finer mesh.

2) Adapt the grid that you are already using for Y+ so that you Y+ lies between say 30-60. Then use non-equilibrium wall funtions.

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