[kato]

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]

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]

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]

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]

Originally Posted by ghost82

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]

Originally Posted by kato

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]

Originally Posted by Engr.RZA

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]

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]

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]

You mean the "L" in "Re"?

[diamondx]

the boundary layer length ??

[RodriguezFatz]

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]

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]

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]

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

[RodriguezFatz]

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]

Originally Posted by RodriguezFatz

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!