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January 8, 2010, 23:34 
which yplus (SST)

#1 
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Hi,
I've read every post on yplus values in this forum, but still I'm not clear what to shoot for. Are these assumptions correct? 1. yplus needs to be less than 200 (CFXHelp) 2. only for yplus < 1 can SST use it's full potential (however it works also for any higher value) 3. yplus can't be too small But now what do I go for? If I shoot for yplus = 100 in CFXMesh, the mesh I get looks like this: If I go for yplus = 10, my mesh looks like this: From a very inexperienced view it seems like the last mesh is not very good as it transitions from very small elements to huge ones. If I would go for yplus = 1, it would be even worse? So what should I go for? I can't make the mesh around it much smaller, since I have already quite a lot of elements (3 mill). For what it's worth, my object is a car. Can anyone tell me how I should proceed? Thanks! 

January 9, 2010, 15:28 
yplus

#2 
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Fatema Zandi Goharrizi
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hi
it based on the turbulent model that you want to use for problem due to the charactristics that it has. for example for the most popular flow problems kepsilon is used and as in cfx help the yplus should be in the range of 20 and 100 so if the problem force you to obtain the parameters in sub layer and ... and you choose komega or sst model your yplus limits is different that is in the cfx solver help i hope it could be useful good luck 

January 9, 2010, 18:31 

#3 
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thanks, you're right. the cfx help only states that yplus has to be smaller than 200. that's it. but that still leaves me with a huge range of options and I just can't imagine that it doesn't matter which one I take.


January 10, 2010, 03:39 

#4 
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Fatema Zandi Goharrizi
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ANSYS CFXSolver Modeling Guide  Turbulence and NearWall Modeling  Modeling Flow Near the Wall 
Solver Yplus and Yplus Guidelines for Mesh Generation One of the most essential issues for the optimal performance of turbulence models is the proper resolution of the boundary layer. In this section, two criteria are suggested for judging the quality of a mesh:
Minimum Node Spacing The goal is to determine the required near wall mesh spacing, , in terms of Reynolds number, running length, and a target value. A < 200 is acceptable if you are using the automatic wall treatment, if not, continue to read the advice below. After running a solution, the value of (in particular, the value given by the solver variable Yplus, representing the value for the first node from the wall) should agree with: Note Here, y+ refers to the solver variable Yplus (not the solver variable Solver Yplus), which is stored at each node on a wall boundary. For details, see Solver Yplus and Yplus. Note Here, lowRe model means using a fine mesh and one of the models (which include the SST model). The models do accept coarser meshes, due to the automatic nearwall treatment for these models. this is from help you have to choos turbulent model first then you have automatic wall function or standard wall function or wall function then can control the limitations good luck 

January 10, 2010, 03:49 

#5 
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Hi,
thanks for your answer and posting the help here. I am using SST with automatic wall functions. I based my assumptions partly on this text. Are they correct? "The models do accept coarser meshes, due to the automatic nearwall treatment for these models" > SST has automatic wall function, so I can have a coarser mesh. "yplus < 200 is acceptable if you are using the automatic wall treatment" > SST has automatic wall function, so yplus needs to be small than 200. So far so good  but I still don't know, what I'm supposed to go for? I do have yplus < 200 since that is the only obvious regulation, but how small is good for me? I just don't know and I really don't find any advice in the help file on that. 

January 10, 2010, 04:56 

#6 
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Fatema Zandi Goharrizi
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Hi
your welcome I'm not so experinced in cfd but I know if you choos sst so, y+<2 or 1 not sure I can't open the link for text you sent. 

January 10, 2010, 05:42 

#7 
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Attesz
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Hi,
I recommend you not to use this way to get yplus in CFXmesher. It's a better way to set the height of the boundary layer, and then set the number of elements in it and its aspect ratio. After a simulation run, you should check the yplus values (which needs to be between 20 or 200, or above 1), and then modify the mesh. The second mesh is not so good quality, because there is a big size different between the last boundary layer cell and the first tetrahedra. The first mesh is better. Anyway, if you have ICEM CFD, i recommend you to use that, because it's more controlable. This type of mesh you can generate more easier, and can get a better quality. Regards, Attesz 

January 10, 2010, 06:04 

#8 
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Thanks for your answer.
So you say that  I should prefer similar element sizes over certain yplus values (which I take from you statement, that the second mesh is better)  I should have yplus between 20 and 200 The last point is the most critical to me. With simulation models like k,emodel, I know that yplus has to be in a certain range, e.g. 20 to 200. But with automatic wall functions (which are part of k,wmodels like SST), some people claim that yplus should be below 1. So which is it for SST? yplus < 1 or between 20 and 200?? 

January 10, 2010, 06:22 

#9 
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Attesz
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Here, lowRe model means using a fine mesh and one of the komega models (which include the SST model). The models do accept coarser meshes, due to the automatic nearwall treatment for these models. Minimum Number of Nodes Goal A good mesh should have a minimum number of mesh points inside the boundary layer in order for the turbulence model to work properly. As a general guideline, a boundary layer should be resolved with at least: Nnormal= 10 for wall function 15 for lowRe model where Nnormal is the number of nodes in the boundary layer in the direction normal to the wall. a strict lowReynolds number implementation of the model would also require a near wall grid resolution of at least y+ < 2. This condition cannot be guaranteed in most applications at all walls. For this reason, a new near wall treatment was developed by ANSYS CFX for the komega based models that allows for a smooth shift from a lowReynolds number form to a wall function formulation. This near wall boundary condition, named automatic near wall treatment in ANSYS CFX, is used as the default in all models based on the omegaequation (standard k.omega, Baseline komga, SST, omegaReynolds Stress). To take advantage of the reduction in errors offered by the automatic switch to a lowRe near wall formulation, you should attempt to resolve the boundary layer using at least 10 nodes when using these models. From CFX help, You can see, that both value range is ok, but from the last sentence, it's recommend to use y+<1 or at least yplus<2. But If you have a complex geometry, it's not to easy, to reach it everywhere... Attesz 

January 10, 2010, 11:07 

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Thank you, that was very clear and answers my question. There is one more problem though  if I go for yplus < 2 I get even a huger size difference between inflation elements and regular elements.
So would you recommend going for yplus < 2 (while meshing, don't know if I'll achieve them in the end  like in picture 2) or just accept yplus of about 100, but having similar sized cells (like in picture 1)? Which mesh will be better? Thanks again for all your help! 

January 10, 2010, 11:20 

#11 
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Attesz
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Hi sanchezz,
The huge size difference between inflation elements and regular elements means a big aspect ratio, so the changes in flow will be not resolved there. The recommened aspect ratio is about 1.31.5, but maybe you can get bigger. To avoid it, you can set the aspect ratio bigger (1.51.6) when generating boundary layer mesh, and set a lower mesh size value near the wall using "Face sizing" or "Edge Sizing". Unfortunately, you have to generate always the mesh, and then modify the values and again and again...thats why I'm not using cfx mesher. Reading the meshing forum's topics here can be useful http://www.cfdonline.com/Forums/ansysmeshing/ Good luck, Attesz 

January 10, 2010, 15:43 
height of boundary layer

#12  
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Fatema Zandi Goharrizi
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Quote:
dear attesz could you tell me please how I can calculate height of boundary layer due to the formula from Irving H. Shames book: Integral(1u/(u max))*dy,o,∞) 

January 10, 2010, 16:05 

#13 
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Attesz
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Hi zandi,
unfortunately I have no exact idea. I'm not so experienced in the theoretical background of CFD. But check these: boundary thickness 1.jpg boundary thickness.jpg However, you can calculate the boundary layer thickness with this simple form only in simple flows, but it's a good estimate. Usually I make the b.l.mesh by my own estimate. After simulation I check the yplus values, and then modify the thickness values, or the aspect ratios...it's not easy. But in some work, due the complexity of the problem, you can not reach everywhere yplus<1... 

January 10, 2010, 16:29 

#14  
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Fatema Zandi Goharrizi
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Quote:
thank you very much but the formula in the first piture is not so clear could you send me it in the more quality or exact formula please 

January 11, 2010, 00:29 

#16 
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Hi,
just wanted to let you know that I talked to a ANSYS support guy and he basically confirmed attesz statement: 1. yplus as small as possible (just like any mesh and with automatic wall functions, it can't be too fine) 2. yplus < 1 is good 3. yplus should by no means exceed 200 Thanks for the advice on using different aspect ratios  my mesh looks much better now. Thanks for your help! 

January 11, 2010, 03:32 

#17  
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Fatema Zandi Goharrizi
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Quote:
thank you very very much 

January 11, 2010, 05:45 

#18 
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Attesz
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you're welcome, good work!
Attesz 

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