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Re Number and Y+ when using K-Omega SST |
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April 20, 2018, 03:21 |
Re Number and Y+ when using K-Omega SST
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#1 |
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Eric Lee
Join Date: Jul 2017
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Hello,
I'm analyzing the turbulence cased by the pantograph of high-speed trains, and one of the review of my paper told me that I should use K-omega SST model NOT the standard k-epsilon model. I understand that SST is better than the standard K-epsilon model, for example, when the flow separation is the interest. But, in my case, the flow speed is above 300km/h, which means a high Re number and as far as I know SST model is suitable for low Re number cases. Moreover, the Y+ value should be maintained within <1 when using SST model to my knowledge, but I frankly don't understand why my y+ values still range from 20~300. I would really appreciate if you could share your knowledge on this. |
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April 20, 2018, 03:37 |
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#2 |
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Gert-Jan
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You are mixing up the different models.
SST is some kind of a hybrid model that can handle full range of Y+. From less than 1 and up to 300. On one side of the spectrum it behaves like k-omega. On the otherside it behaves likes k-epsilon. Maybe you could consider it as a universal model. So, with SST, don't let Y+ be larger than 300 and you're more or less safe. |
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April 20, 2018, 07:38 |
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#3 |
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Eric Lee
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Thank you for such a clear explanation!
I tried to look up at various sources about this but no one explained easily like you. I guess now I have to work on reducing Y+ values for few regions that overly exceed 300. Again, thank you! |
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April 20, 2018, 09:08 |
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#4 |
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Gert-Jan
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Maybe it is a bit too crude. If someone has a more detailed view, do not hesitate to post a comment..........
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April 22, 2018, 07:35 |
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#5 |
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Glenn Horrocks
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Do a sensitivity study If the simulation is not sensitive to the boundary layer then it won't matter (yes, many types simulations don't care what near wall model you use).
So run a model with y+ being large and another with y+<300 and see if it makes a difference.
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October 21, 2018, 12:12 |
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#6 |
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Federico Agustín Caccia
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Can somebody provide some reference on the y+<300 limit? Thanks!
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October 21, 2018, 16:45 |
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#7 |
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Glenn Horrocks
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There is no limit at y+=300. The upper limit of allowable y+ values is problem dependent and needs to be determined case by case with a sensitivity analysis.
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October 21, 2018, 19:13 |
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#8 |
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Federico Agustín Caccia
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October 22, 2018, 03:47 |
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#9 |
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Gert-Jan
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In pinciple you have to perform multiple calculations with different grids and see how results (velocity profiles, pressures, forces, etc) change with grid density. Meanwhile, see what Y-plus you obtained in each calculation and how results are affected.
The results depends a lot on the turbulence model you apply and the geometry you have. If it is streamlined or blunt, etc. |
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October 22, 2018, 08:45 |
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#10 | |
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Federico Agustín Caccia
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Quote:
Great! I think this is what I usually do, trying to get convergence in results. For example, using k-epsilon, I usually get result convergence once the grid has y+ under 50/70. But I haven't found this limit changing with problems, for example, I haven't seen a problem in which the results don't converge until y+<10. So I thought that upper limits were more general. |
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October 23, 2018, 20:18 |
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#11 | |
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Quote:
No, that's not correct. Decide are the wall functions of the k-e model. k-e model with scalable or enhanced wall treatment and fine mesh is even better than k-o sst. k-o sst only good with high y+ (<300) and large meshgrid size without energy therms. When the flow separation is the interest, the k-o sst is very bad, because it is not working with a fine mesh (y+ < 11). Standard k-o or k-e with enhanced wall treatment and y+ < 4 recommend. Flow seperation results are prooly resolution with a y+ > 11. |
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July 18, 2022, 04:16 |
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#12 | |
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Arijit Saha
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Quote:
For SST k-omega it switches from k-omega near wall to k-epsillon in free-stream with some blend. In my opinion, even when the y+ near wall is in log law region the SST k-omega uses k-epsilon term to model the flow which is kinda similar to standard k-epsilon. Please let me know, if I stated anything wrong. |
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