November 15, 2018 at 10:23 amabd fsSubscriber
I am really stuck and I do not know what I can do to solve the problem of y +.
I have a y + <1 but there is a reader of 2 with the document or I compare with my results I use the function adaptation region and thanks to it I got y + <1 but it is always shifted
the model is K-omga SST
goal is : separation flow
figure 2 articl
how can I decrease the value of y + to find the same value of the article?
November 15, 2018 at 2:04 pmKeyur KanadeAnsys Employee
Can you please give more details about your mesh, models etc?
Please insert images.
November 15, 2018 at 2:59 pm
November 15, 2018 at 3:42 pmDrAmineAnsys Employee
I cannot understand the issue? What is reader and document??
Why are you not generating a pure hex mesh? Do not exaggerate with yplus keeping the boundary layer unresolved.
And ensure deep convergence: check reports, imbalances and residuals.
You have better results as in the articles if I judge it from the qualitative perspective.
November 15, 2018 at 4:18 pmabd fsSubscriber
thank you for reply
your sign is very important, my goal is to decrease the value of y +, to find the same article value
even in the report they worked with a triangular mesh far from wall and at paroin level they used inflation
this is the source of the article:
all the results are right except y +
November 15, 2018 at 4:40 pmDrAmineAnsys Employee
Yplus doe not only depend on the first cell centroid distance but also on the wall stress velocity. Are you using same boundary condition at inlet and elsewhere?
November 15, 2018 at 4:44 pmDrAmineAnsys Employee
If you do not want to go throughly through the paper then you need to make a boundary adaption here which I do not like. Go to adapt boundary adaption select the number of cells nesr the wall and adapt once.
By the way the plot is just showing yplus nothing more..
November 15, 2018 at 8:50 pmabd fsSubscriber
Thank you for sharing these information here.
November 16, 2018 at 1:53 pmJosé MantovaniSubscriber
Hello abd fs.
Try to make a structured mesh with a y+< 1 for the first cell near wall.
November 16, 2018 at 2:17 pm
November 18, 2018 at 10:10 amabd fsSubscriber
November 18, 2018 at 2:57 pmKarthik RAdministrator
Are you plotting this at the wall?
As Amine point out, the wall y+ value depends not only on the first cell centroid distance, but also on other parameters used to generate the grid. Please attempt to generate the grid exactly as it was created by the article you point to. If the authors of the article are using Fluent, I'd suggest you replicate the steps exactly as they have done. If you are in doubt about a particular step (such as setting used for the grid), please contact the authors. Another thought, which version of Fluent are you using for your model and how does this compare with the article you refer to?
November 18, 2018 at 4:59 pmabd fsSubscriber
Thanks for your fast reply which is really appreciated.
I plotted y + at the wall level,
I contacted author but no answer, for that I shared my problem here and I gave the same value y + that the author gives
I fixed all the problems and I found the same results it remains that the value of y +, and all thanks to this group,Keep helping me liké this.
November 18, 2018 at 8:28 pmDrAmineAnsys Employee
Have you started looking in boundary adaption? You could try reducing the spacing of the first cell in the pre-processing?
But: Why do you want to have the same plot as the author? What is the motivation here?
November 18, 2018 at 8:56 pmabd fsSubscriber
Dear AMINE . Thanks for reply.
I think if y + is just that I can capture the physical phenomena (boundary layer,Boundary Layer thickness,Displacement thickness....)
regarding the adaptation of the limit, I do not know how to do it because my goal and adapted an area very close to wall.
could you advice me please ?
if you need info. please let me know
November 18, 2018 at 9:01 pmDrAmineAnsys Employee
You have already yplus smaller than one and good number of inflation layers. Yplus alone is not enough: Yplus and the number of cells parallel to the wall (prism layers)are important to resolve the laminar regime and the boundary layer.
Regarding adaption: Go to adapt boundary adaption select the number of cells near the wall and adapt once. I recommend however doing that step in Meshing or the tool you use for creating the mesh.
What you really need is a consistent mesh refinement strategy to obtain grid independent results.
November 18, 2018 at 9:29 pmabd fsSubscriber
You as an expert how did you find that I have many layers of inflation.
could you please inform me?
November 18, 2018 at 9:48 pmDrAmineAnsys Employee
Counting the prism layers which are parallel to the wall. How did you create your mesh?
November 18, 2018 at 10:00 pm
November 19, 2018 at 7:13 amKeyur KanadeAnsys Employee
As Amine has pointed out, please do mesh refinement in meshing rather than using adaption in Fluent.
Also you may need to do mesh independent study by doing several iterations. You will need to create mesh witn 15 layers, 20 layers etc with different face/edge sizing.
November 19, 2018 at 10:29 am
November 19, 2018 at 10:37 pmKarthik RAdministrator
Might I ask you one question - what is your premise behind matching the wall y+ values from your reference. I am trying to understand your thoughts because you are able to replicate all other results. Are you trying to better your modeling approach through this study?
Also, can you comment on the difference between your model vs. the model that you are trying to compare your results with?
November 19, 2018 at 11:01 pmabd fsSubscriber
Dear Sir Kremella,
All the results I've already found is thanks to this group.
if I have a good y + I can study the viscous sublayer, buffer layer and turbulent layer.
what is your point of view on the velocity vector that I launched under
(Are you trying to better your modeling approach through this study?) yes
November 20, 2018 at 6:09 amDrAmineAnsys EmployeeYou are already having all ingredient to capture the viscous layer. Just think about mesh sensitivity study.
November 20, 2018 at 10:28 amabd fsSubscriber
I did lots of the Sensitivity Analysis of some items, e.g. mesh size parameters, and the inflation parameters
I start the simulation with a very coarse mesh and a poor residuals convergence Then when everything looks fine I try to refine the inflation, but despite all this work the resolution does not change
1-Is there another criteria should be selected to pick the most beneficial mesh size?
I think the problem does not depend on mallaige
2-How the wall stress velocity influenced on the value of y +?
November 20, 2018 at 3:43 pmDrAmineAnsys Employee
1/You go to pre-processing and you reduce the mesh size if your inflatin (first layer thickness or total layer thickness)
2/yplus=delta_y*u_tau/viscosity uplus=u/u_tau with u_tau = SQRT (Wall shear stress/density)
November 20, 2018 at 6:43 pmabd fsSubscriber
I searched in web but no information on this procedure
what is preprocessing and where I can find in ANSYS
if possible, pictures that can help me.
November 20, 2018 at 6:50 pmDrAmineAnsys Employee
Which pre-processor are you using?
November 20, 2018 at 6:56 pmabd fsSubscriber
November 20, 2018 at 7:08 pmDrAmineAnsys Employee
That is a quite old Release. Please update to the most actual ones. I assume you are working with ANSYS Meshing. Now how did you create the mesh? If you will and can answer the question then you might have already found the way to change the number of inflation layers and the thickness of the first layer /or total.
November 20, 2018 at 7:29 pmabd fsSubscriber
Dear AMINE ,
Yes i work with ansys meshing.
1-I created a triangular mesh throughout the area but I have refined the ramp area;
2- I created inflation at the wall level with a well defined first layer thickness and with 15 layers.
I hope I answered your questions
it's the first time in my life that I use fluent ANSYS
November 20, 2018 at 7:53 pmJosé MantovaniSubscriber
Try to make a quadrilateral structured mesh.
November 20, 2018 at 8:18 pmabd fsSubscriber
I know you worked with the same subject but with a 90 ° angle, and you have used a quadrilateral mesh and thanks to your questions in this group I have successfully solved many problems.
Our goal is to find the same results with a triangular mesh, and I know that this type of mesh is difficult to apply, but there are people who have made the same triangular mesh and they are finding good results.
November 20, 2018 at 8:29 pmDrAmineAnsys Employee
So now reduce the thickness of the first layer.
November 20, 2018 at 8:32 pmabd fsSubscriber
here is the value I give 1e-3, do I have to decrease this value,
November 20, 2018 at 8:46 pmDrAmineAnsys Employee
If you want to decrease yplus value then yes!
November 20, 2018 at 11:14 pm
November 21, 2018 at 5:58 amDrAmineAnsys EmployeeSwitch Off node values for plot and check again.
November 21, 2018 at 9:56 am
November 21, 2018 at 10:09 amDrAmineAnsys Employee
I will first of all try to use an actual release before suggesting any further steps. I also have large doubts about the way you created the mesh as the new plot is not in line with your first graph. You can plot quantities related to yplus along the wall to see which one is depicting the same behavior (cell distance, velocity, wall shear stress, viscosity..)
November 21, 2018 at 11:49 am
November 21, 2018 at 2:19 pmDrAmineAnsys Employee
November 21, 2018 at 2:27 pmabd fsSubscriber
Thank you so mush
November 21, 2018 at 3:55 pm
November 21, 2018 at 5:47 pmabd fsSubscriber
November 23, 2018 at 6:36 amKeyur KanadeAnsys Employee
The original questions regarding y+ are answered and topic is marked as solved.
If you have questions on meshing, can you please create a separate thread for it. This will help others on forum.
November 23, 2018 at 8:53 amabd fsSubscriber
I solved the problem of y + and mesh
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