# Fluent:Y+

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 May 4, 2006, 10:14 Fluent:Y+ #1 justin Guest   Posts: n/a Does Fluent have the function to control Y+>30 or other alternative way? Thanks Justin

 May 4, 2006, 19:51 Re: Fluent:Y+ #2 Benzamia Guest   Posts: n/a Heloo you make Plot---- XY plot (a) Under Y Axis Function, select Turbulence... and Wall Yplus. (b) In the Surfaces list, select wall-bottom and wall-top. (c) Deselect Node Values and click Plot. Wall Yplus is available only for cell values. The values of y+ are dependent on the resolution of the grid and the Reynolds number of the ow, and are meaningful only in boundary layers. The value of y+ in the wall-adjacent cells dictates how wall shear stress is calculated. When you use the Spalart-Allmaras model, you should check that y+ of the wall-adjacent cells is either very small (on the order of y+ = 1), or approximately 30 or greater. Otherwise, you will need to modify your grid. where y is the distance from the wall to the cell center,  is the molecular viscosity,  is the density of the air, and w is the wall shear stress. Figure 3.12 indicates that, except for a few small regions (notably at the shock and the trailing edge), y+ > 30 and for much of these regions it does not drop signi - cantly below 30. Therefore, you can conclude that the grid resolution is acceptable. sincerly ;

 May 5, 2006, 04:46 Re: Fluent:Y+ #3 Kamarul Arifin Guest   Posts: n/a I don't really understand the question. But if you want to control Y+>30 in Fluent, you can adapt the grid by setting the minimum Y+ to be greater 30.

 May 5, 2006, 07:01 Re: Fluent:Y+ #4 justin Guest   Posts: n/a Thanks for your reply. Sorry, if my question is not clear. Firstly the convergence is got, the rang of y+ is from 40-1000 and then I do y+ adaption, but after the convergence, the previous Y+ which is larger than 30 become smaller than 30. I am confused that the mininum allowed y+ didn't work. Regards Justin

 May 5, 2006, 08:41 Re: Fluent:Y+ #5 Jason Guest   Posts: n/a That's because adaption doesn't work by adjusting cell size... you define what you want to do to pick the cells for adaption, and Fluent then splits those cells in half. There's no magic behind what Fluent's doing. For most of the turbulence models 30-300 is a decent range to stay within, so instead of adapting cells with y+>30, adapt y+>200 or so... to get the cells within the desired range. If you're trying to get exact values you'll have to remesh in Gambit, and you'll spend several iterations going between the solution and the mesh to get exact ranges. Hope this helps, and good luck, Jason

 May 5, 2006, 09:07 Re: Fluent:Y+ #6 justin Guest   Posts: n/a oh,I see. That is the reason the default min allowed is 50, not 30.

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