[mihylee]

Hi, all

I want to know what is the suitable value of yPlus for Transition SST mode.

I have tested 2D NACA0012 with Transition SST model.
I seperated my fluid zone into laminar and turbulence zone so that my simulation can be solved more accurately.

My lift and drag values were really close to experimental data (5% of error) which made me happy.

Furthermore, I plotted yPlus graph but it seems my graph looks tricky?!
please check: https://ibb.co/k6ChyXs
The value is between 2.5 and 22.5.

Does this value mean my result is not validated?

Kindly looking for answers,

Thank you

[LuckyTran]

What's tricky about it? Looks reasonable. If you don't like 20, then use smaller cells.

[mihylee]

Hi, LuckyTran

Thank you for your reply.

I read some posts which said the reasonable range for yplus of SST turbulence model is 20<y+<300 for SST turbulence model.

However, I could not find any clue or posts regarding transition SST model.
Can you please explain why my yplus values are reasonable??

[LuckyTran]

You have some wall y+ which is 0 in the laminar region, spikes to some high value and then decreases towards the trailing edge as the boundary layer becomes thicker. That's why I say it looks reasonable.

If you're trying to get y+ > 20, then use a coarser mesh. That should be easy.

Originally Posted by mihylee

I read some posts which said the reasonable range for yplus of SST turbulence model is 20<y+<300 for SST turbulence model.

I disagree completely because the point of the SST model is to take advantage of the k-w model which works really nicely for y+ <1. If you run SST targeting wall y+>20, then it's not really an SST but just a bad k-epsilon model. But that doesn't mean that it's wrong to do it this way. Although I disagree SST should be 20<y+<300, I also don't think this problem really needs requires a low y+ to get a decent result. So I do think using a high y+ is a good idea.

You're using a transitional SST model, the transitional model just adds an intermittency factor to help model the transitional region. You have fixed a laminar zone which constrains this somewhat. I like the idea to fix a laminar zone, but now you're not really using the transitional model the way it likes to be run either. And you probably could've achieved the same result using a regular SST model since it looks like your transition happens over only a few cells.

[mihylee]

Thank you so much,

Your explanation made me understand the concept of y+ value.