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May 26, 2008, 09:16 
★ how to generate gird accoring to Y+?

#1 
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Hi, in Fluent, before calculating the flow field, how to generate gird according to Y+?
for example, if Re=20,000, may I use "enhanced wall treatment" ? if so, how can I set the grid points near the wall? and how many points? thanks 

May 26, 2008, 09:22 
Re: ★ how to generate gird accoring to Y+?

#2 
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for complicated geometry, it needs much time to generate grid, but I have no idea what is the Y+ when I place grid points. for example, if 50501000 is used, the Y+ is 5 after the solution is converged. Now, what is the guidelines for generate gird again? or using "enhanced wall treatment" ?


May 26, 2008, 20:48 
Re: ★ how to generate gird accoring to Y+?

#3 
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You can make a coarse grid, solve it initially, then adapt the grid according to y+ values. For example adapt all cells where y+ (or y*) is above 5.
EWT is fine when wall cells are very fine, i.e. y+=1. May be worth starting off the solution with standard wall functions, then adapt the grid and switch to EWT once y+ values are alot smaller. Hope this helps, Phil 

May 28, 2008, 02:35 
Re: ★ how to generate gird accoring to Y+?

#4 
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thank you Phil.
i first computed with use standard wall functions, and plot the Y+ along the wall, most of Y+ are less than 4, but some of them above 4,up to 12. In this case, what can I do ? 

May 28, 2008, 12:23 
Re: ★ how to generate gird accoring to Y+?

#5 
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conan,
go into the adapt menu and select yplus/ystar. This facility will adapt(refine) the grid at only the areas where y+ is above a certain range. You can then refine all grid points where y+ >2 say, but 12 isn't actually very high so you may not actually want to do this. For standard wall functions you want y+>30, but as close to 30 as possible. For EWT you want on the order of y+=1. I think you only need to adapt your grid if the dense regions are of particular interest  it won't make a huge difference to the results. Phil 

May 29, 2008, 03:38 
Re: ★ how to generate gird accoring to Y+?

#6 
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thank you, Phil.
in some papers, there mentioned "to resolve the near wall viscous region, 10grid points were placed in boundary near wall", in FLUENT, as I known, to generate boundary grid, first distance to wall and the scaling ratio should be given. how did they do so that Y+ being around unity ? 

May 29, 2008, 03:44 
Re: ★ how to generate gird accoring to Y+?

#7 
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also, by the way.
if i adapt grid with standard wall function, in the adapt menu, should I choose ystar not yplus? and should the min be set 30, and max be 60 ? if i adapt grid with enhanced wall treatment, in the adapt menu, should I choose yplus not ystar? and should the min be set 0.5, and max be 1 ? thank you 

May 29, 2008, 08:31 
Re: ★ how to generate gird accoring to Y+?

#8 
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I don't think you can really make the initial grid according to y+ as its a flow dependent/solution dependant value. There's probably an equation for it but to be honest I don't think you need to worry so much about the initial grid with standard wall functions. They aren't all that accurate anyway and basically just model a boundary layer within the wall adjacent cells. They are valid for y+ of around 30 unpwards. I'd just make the original grid as good quality as possible, and hexahedral.
I always just use y+, with y+ of around 1 for EWT, where possible. I don't advise going to millions of cells just to resolve y+ of 1. 

May 29, 2008, 11:17 
Re: ★ how to generate gird accoring to Y+?

#9 
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thank you, Phil. what is your email? i want to discuss further with you.
Maybe my original grid is not so good. Even if I adapt the grid in adapr menu, the results is almost not improved. 

June 6, 2008, 08:44 
Re: ★ how to generate gird accoring to Y+?

#10 
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Results might not change because the Standard wall functions are not so bad for simple cases. But in one of your messages I saw y+ obtained is 4 while using standard wall function which is not a good thing to have, because condition to use SWF is that you should not have any node in viscous sublayer. In fact, first node from wall should be placed where you may have wall y+ >30 (Phil has mesntioned it somewhere). y+<5 is good enough to explore any of the turbulence model. You can only try to get y+~1.
In your case, refinement based upon adaption suggested by Phil should work. But with current y+ <4, you may use near wall treatment option. 

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