Learning Forum

[abd fs]

Dear all,

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

                                                         figure1:my work

                                                                              figure 2 articl

how can I decrease the value of y + to find the same value of the article?

Best regrat

SER

 

[Keyur Kanade]

Can you please give more details about your mesh, models etc?

Please insert images. 

[abd fs]

Dear kkande,

At the wall I used inflation with 15 maximum layer. After using region adaptation I got to the curve above.

model k-omega sst 

Best regrads,

ser

[DrAmine]

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.

[abd fs]

Dear amine,

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:

https://www.sciencedirect.com/science/article/pii/S1631072114000977

all the results are right except y +

Best regrads,

SER

[DrAmine]

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?

[DrAmine]

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..

[abd fs]

Dear AMINE,

Thank you for sharing these information here.

I will try this and post here the results

Best regards,

SER

[José Mantovani]

Hello abd fs.

Try to make a structured mesh with a y+< 1 for the first cell near wall. 

Hugs,

Mantovani. 

[abd fs]

Dear  Montovani,

Thanks for your reply, now i am facing problem in reducing the values of Y plus , i am using SST turbulence model for simulating flow.

here are some pictures :

could you advice me please ? 

if you need info. please let me know

Best regards,

SER

[abd fs]

Someone can help me? With some tips, comments?

 

Thanks.

SERRAR

[Karthik R]

Hello,

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?

Thank you.

Best Regards,

Karthik

[abd fs]

Dear Kremella,

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.

Best regrds,

Ser

[DrAmine]

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?

[abd fs]

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

Best regrds,

Ser

[DrAmine]

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. 

[abd fs]

Dear AMINE,

You as an expert how did you find that I have many layers of inflation.

could you please inform me?

Best regrds,

Ser

[DrAmine]

Counting the prism layers which are parallel to the wall. How did you create your mesh?

[abd fs]

Dear AMINE,

before the region adaptation I have 15 left

after the region adaptation I have 

could you advice me please ? 

if you need info. please let me know

Best regrds,

Ser

[Keyur Kanade]

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. 

[abd fs]

Dear kkanade,

I did the refinement of mesh and I made the mesh sensitivity test but I always get the same results.

could you take a look at  velocity profile vector before the ramp. after adaptation region

here is the velocity profile that I inject into the domain inlet:

if you need info. please let me know

Best regrds,

Ser

 

[Karthik R]

Hello,

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?

Thank you.

Best Regards,

Karthik

[abd fs]

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

 

Best regrds,

Ser

 

[DrAmine]

You are already having all ingredient to capture the viscous layer. Just think about mesh sensitivity study.

[abd fs]

Dear AMINE,

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 +?

Best regrds,

Ser

 

[DrAmine]

Hey,

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)

 

[abd fs]

Dear AMINE,

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.

Best regrds,

Ser

[DrAmine]

Which pre-processor are you using? 

[abd fs]

 

Dear AMINE,

ANSYS 15.0

Best regrds,

Ser

 

[DrAmine]

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. 

[abd fs]

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

if you need info. please let me know

Best regrds,

Ser

[José Mantovani]

Hello.

Try to make a quadrilateral structured mesh. 

Hugs,

Mantovani. 

[abd fs]

Dear Mantovani,

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.

Best regrds,

Ser

 

[DrAmine]

So now reduce the thickness of the first layer.

[abd fs]

Dear AMINE,

here is the value I give 1e-3, do I have to decrease this value,

Best regards,

ser

[DrAmine]

If you want to decrease yplus value then yes!

[abd fs]

Dear AMINE,

after several tries I arrived to find this curve without the use of adaptation of region and that good, it remains only to eliminate the zig-zag.

could you advise me please?

Best regrds,

Ser

[DrAmine]

Switch Off node values for plot and check again.

[abd fs]

Dear AMINE,

do you mean, I have to turn off this part which is red,

 if yes,nothing changes

Best regards,

Ser

[DrAmine]

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..)

[abd fs]

Dera AMINE,

I changed the mesh a bit and I increased the number of layers and finally I got this curve

Best regards,

Ser

 

[DrAmine]

Super

[abd fs]

Dear AMINE,

Thank you  so mush

Best regards,

Ser

 

[DrAmine]

Please mark this topic as solved and highlight the answer which helped to resolve the issue

[abd fs]

ok

[Keyur Kanade]

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. 

[abd fs]

Dear kkanade,

I solved the problem of y + and mesh 

Bes regrads,

SER