[mohibanwar]

Hello Members,
Hope you guys are Enjoying the day.I need your serious help and suggestions.Kindly help me out.

I got stuck in the problem of near wall turbulence treatment of my case.As i have resolved my near boundary mesh very fine as required for my Multi phase case and i got the Y+ values between 1 and 5,that mean in viscous sub layers.
Now my problem is that,Theory suggest me to use enhance wall treatment and i want to use standard wall treatment as it give me values near to my experiment?So can any body suggests me whether standard wall treatment can be used for viscous sub layer having a very fine boundary mesh with Y+ between 1 and 5?
I am Using standard k-e model for turbulence.
Kindly help me out as literature has totally confused me.
Thanks

[LuckyTran]

If you want to use standard wall treatment, then it is best that the mesh is not in the buffer layer (either y+<5 everywhere or y+>30 everywhere). The standard wall treatment does not have the extra blending that is available in enhanced wall treatment option. Otherwise, standard wall treatment is okay for a fine mesh that is globally y+<5.

edit* actually you shouldn't use the standard wall treatment with y+ <30

[mohibanwar]

Helllo Lucky Tran,
Thanks a lot for your kind response and i am very glad to listen to your good suggestions.
Can you provide me with any reference which state that we can use very fine mesh with standard wall function with y+<5 because up till now i have read many references but all suggest me to go for enhanced wall treatment which give me very bad results in my case and fluent manual is dealing with Y-star not the y+.
So after your suggestion i am more confident to use but i need some references to justify it as i do not have it now.

[LuckyTran]

I mistakenly thought that in Fluent, the standard wall function was also a two-layer approach. I am surprised to learn that it is not (despite reference the work of Launder & Spalding). Since that is not the case, it is definitely not recommended to use standard wall functions on a low y+ mesh. You may want to try scalable or non-equilibrium wall functions instead. Maybe someone that knows better can answer whether Fluent does or does not use a two-layer approach (which is basically required if you want to use low y+ grids).

The difference is in the epsilon equation, in the two-layer approach a different equation for epsilon is used near walls.

One would expect of course that EWT is best, but in your case it is odd for standard wall functions to give better results.

[mohibanwar]

Hello Lucky Tran,
Thanks a lot once again for having a detail study on my case and brief response given to me.I appreciate your kindness.
Actually that what i have confirmed from the theory already and i am pretty sure on that i can not use the standard wall function with fine mesh.
Can you give me another favor about the Non-Equilibrium wall function that can be used in my case or not as according to your statement Non-equilibrium wall function is a two layer function so kindly give me your suggestions and if you can give me some reference.I am very thankful.
I am once again thankful to you.
Regards,
Khan

[hotboy]

Originally Posted by LuckyTran

If you want to use standard wall treatment, then it is best that the mesh is not in the buffer layer (either y+<5 everywhere or y+>30 everywhere). The standard wall treatment does not have the extra blending that is available in enhanced wall treatment option. Otherwise, standard wall treatment is okay for a fine mesh that is globally y+<5.

Many people suggest that when use standad wall treatment ,it is better to have the 30<y+<300 .Is that right?Thank you very much!

[Blanco]

Yes, usually a small range is suggested, like 30<y+<100, but sometimes people tend to relax the higher limit even if it would be useful to keep it at its original value.

[mohibanwar]

Originally Posted by Blanco

Yes, usually a small range is suggested, like 30<y+<100, but sometimes people tend to relax the higher limit even if it would be useful to keep it at its original value.

Hello Blanco,
Do you wanna mean that we can use standard wall function for fine mesh of Y+ between 1 and 5?

[Blanco]

Hello mohibanwar,
No I was answering to the last question from hotboy about standard wall treatment. As far as fine meshes are concerned, I agree with previous posts from others where it is cited that we can't use standard wall function with such a low y+~1÷5

[mohibanwar]

Originally Posted by LuckyTran

I mistakenly thought that in Fluent, the standard wall function was also a two-layer approach. I am surprised to learn that it is not (despite reference the work of Launder & Spalding). Since that is not the case, it is definitely not recommended to use standard wall functions on a low y+ mesh. You may want to try scalable or non-equilibrium wall functions instead. Maybe someone that knows better can answer whether Fluent does or does not use a two-layer approach (which is basically required if you want to use low y+ grids).

The difference is in the epsilon equation, in the two-layer approach a different equation for epsilon is used near walls.

One would expect of course that EWT is best, but in your case it is odd for standard wall functions to give better results.

Hello Lucky Tran,
Thanks a lot once again for having a detail study on my case and brief response given to me.I appreciate your kindness.
Actually that what i have confirmed from the theory already and i am pretty sure on that i can not use the standard wall function with fine mesh.
Can you give me another favor about the Non-Equilibrium wall function that can be used in my case or not as according to your statement Non-equilibrium wall function is a two layer function so kindly give me your suggestions and if you can give me some reference.I am very thankful.
I am once again thankful to you.
Regards,
Khan