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-   -   y+ (http://www.cfd-online.com/Forums/fluent/112708-y.html)

kato February 3, 2013 12:49

y+
 
hi every one
i have two question
what bet is need for wall function model become true?
what bet is need for (y+)in wall function become true?

cocobi February 3, 2013 19:48

Hi, Kato,

You need y+ whether you use wall function or not.
In order to use the function, y+ should be put between 30 and 300.
To get a reasonable result without the function, the value of your mesh should be near 1.

Wall function can be used when the flow become a streamline near the wall, so if something like separation or shock happen, it can not be used and your mesh should be fine enough to make y+ near 1.

I am a beginner of Fluent.... I just asked similar question a few days ago. So, I just shared that. If you are a beginner too, you would be better to try to find my question I uploaded recently and might get more helpful information for the first step....

Good luck!

Engr.RZA February 4, 2013 04:16

Here is the thumb rule...

For the Low Re Turbulence models..must have y+ value between 1 to 5

For the high Re Turbulence models..y+ value should be greater the 30 as they use wall functions..

ghost82 February 4, 2013 12:42

Also, if you have y+ in a range between 1 and 300 you can better switch to scalable wall function (available in newer fluent versions).

kato February 4, 2013 13:34

y+
 
Quote:

Originally Posted by ghost82 (Post 405949)
Also, if you have y+ in a range between 1 and 300 you can better switch to scale wall function (available in newer fluent versions).


hi
thank you
if y+ for major wall is 1 but y+ for a little zone of wall is 25
is the msh true?

ghost82 February 5, 2013 05:30

Quote:

Originally Posted by kato (Post 405969)
hi
thank you
if y+ for major wall is 1 but y+ for a little zone of wall is 25
is the msh true?

Use scalable wall function or refine the grid in the "little zone".

cocobi February 14, 2013 07:39

Quote:

Originally Posted by Engr.RZA (Post 405841)
Here is the thumb rule...

For the Low Re Turbulence models..must have y+ value between 1 to 5

For the high Re Turbulence models..y+ value should be greater the 30 as they use wall functions..

Hi, Engr. RZA

Your explanation makes me confused a little.
As far as I know, wall function cannot be used for the flow having something like separation or shock and usually, this kind of flow can be developed to a transonic or supersonic flow with high Re. So, I believe that the simulation for the high Re flow would be better fine enough to have y+ in around 1. In opposite, for the streamline flow which doesn't have a discontinuity and is a low Re in general, wall function can be used and y+ should be in a range between 30 and 300 to use the function.

That's what I know,

Could you please explain what's wrong?

Thanks,

RodriguezFatz February 14, 2013 08:28

Dear cocobi,

Nothing is wrong, you are right, but you misinterpret the word's origins:
"High" and "low" refer to the local turbulent Reynolds number (defined by the local turbulent length scale), not the global one (of your Pipe, domain, ...). Near the wall this Re gets small because turbulence is damped, far away it gets high. Thus, if you want to resolve the near wall region you need a turbulence model that is valid for low Re regions. If you don't do that, you can use a high Re number model.

Edit: You can read this: http://www.tfd.chalmers.se/~lada/pos...ndium_turb.pdf

diamondx February 14, 2013 09:06

dear all,
Sorry for jumping in ,i was always wondering what that length in the equation means ? i know it is for a flat plat or for an airfoil, but what if i have a complex geometry like a cylinder inside a wind tunnel, and that cylinder contains hole for example ?? i mean that length is not always applicable ?

RodriguezFatz February 14, 2013 09:09

You mean the "L" in "Re"?

diamondx February 14, 2013 09:12

the boundary layer length ??

RodriguezFatz February 14, 2013 09:15

Wait. You wrote "that length in the equation means". I was asking which length in which equation you ment. Did you mean the "L" in the equation "Re = L * u * rho / mu" ?

diamondx February 14, 2013 09:38

In the tool section of CFD online there is yplus calculator and it asks for boundary layer length ... What should be this length if the geometry is comple thanks for your replly

RodriguezFatz February 14, 2013 11:30

Diamondx, as I understand it, every part in such a setup has its own boundary layer length, with Re, y+ and all the other things.

diamondx February 14, 2013 11:58

good that's how tend to explain. so i choose the part wich required the smallest height make it for all the mesh

RodriguezFatz February 14, 2013 12:01

I am not sure that I understood you right. I think since you have to mesh every part with its own inflation layer anyway, you can use an appropriate y+ for each of these parts.

cocobi February 14, 2013 21:20

Quote:

Originally Posted by RodriguezFatz (Post 407806)
Dear cocobi,

Nothing is wrong, you are right, but you misinterpret the word's origins:
"High" and "low" refer to the local turbulent Reynolds number (defined by the local turbulent length scale), not the global one (of your Pipe, domain, ...). Near the wall this Re gets small because turbulence is damped, far away it gets high. Thus, if you want to resolve the near wall region you need a turbulence model that is valid for low Re regions. If you don't do that, you can use a high Re number model.

Edit: You can read this: http://www.tfd.chalmers.se/~lada/pos...ndium_turb.pdf

Dear RodrigueFatz,

Thank you for your explanation.
Now, I understand and thanks for the material, as well.
It will be helpful.

Have a good day!


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