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February 5, 2019, 05:34 |
Yplus value for Transition SST
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#1 |
New Member
MH Lee
Join Date: May 2018
Posts: 12
Rep Power: 7 |
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 |
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February 5, 2019, 10:17 |
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#2 |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,674
Rep Power: 66 |
What's tricky about it? Looks reasonable. If you don't like 20, then use smaller cells.
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February 5, 2019, 10:24 |
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#3 |
New Member
MH Lee
Join Date: May 2018
Posts: 12
Rep Power: 7 |
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?? |
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February 5, 2019, 10:45 |
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#4 | |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,674
Rep Power: 66 |
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. Quote:
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. |
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February 6, 2019, 04:30 |
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#5 |
New Member
MH Lee
Join Date: May 2018
Posts: 12
Rep Power: 7 |
Thank you so much,
Your explanation made me understand the concept of y+ value. |
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