## Fluids

#### Y+ question

TAGGED: ,

• javat33489
Subscriber
Hi all.
I'm doing aerodynamic calculations. Speed 0.2 M, angle of attack 4 degrees.
I made test calculations on three types of turbulence: K omega SST, SA and K epsilon standart wall.
I got the following Y+ scores:
SA
K-omega SST
K-eps

According to the rules for model SA, Y+ must either be less than or equal to 1 or greater than 30. Is SA best suited?

There is no information about the boundary layer in the fluent tutorial. I did without it. Is my decision correct?
All three solutions are roughly similar, but they have different Y +, only SA is suitable, am I right?
Ansys Employee
• aitor.amatriain
Subscriber
┬¿According to the rules for model SA, Y+ must either be less than or equal to 1 or greater than 30┬¿
• javat33489
Subscriber
Ansys tutorial

• aitor.amatriain
Subscriber
Okay, I get that. SA is very old and after checking it now I understand the reason of that message.
SA will work well depending on how many cells have y+<30 and where are they located. The min and max values are a good starting point, but you should check the "y+ distribution".
There is no information about the boundary layer in the fluent tutorial. I did without it. Is my decision correct?
I do not know what is the purpose of the tutorial. In SA, f you have y+<1 everywhere you are resolving the flow well, while if you have y+>30 then you are relying on the wall functions.
All three solutions are roughly similar, but they have different Y +, only SA is suitable, am I right?
k-omega SST includes the optimum wall treatment, therefore (for similar y+) it should be the most accurate model.
• javat33489
Subscriber
Thank you very much for the answer.
But I do not understand. What should I do next?
I have very high Y+ scores. For SST model from 13 to 34871. I will check the Y+ distribution.
For SA, if the majority of cells >30, then do I need to build a boundary layer? If <30, then I leave the solution how did it turn out?
For SST, according to the rules, it should be 1 I'm a little confused.
• aitor.amatriain
Subscriber
If you want maximum accuracy you should follow the guidelines. If not, for k-omega SST if you have 13 In any case, what are you simulating? How many cells does your mesh have? What is the Reynolds number?

• javat33489
Subscriber
I solve the problem of aircraft aerodynamics.
I didn't use the boundary layer.
I got the following indicators y +
cells 0-300

cells 1000-3000

all others up to 33k ...

Please tell me the expression formula to display% for cells, for example, how many percent of Y + cells will have up to 300?
I have very high Y+ scores and it scares me