[ghorrocks]

My recommendation is to use SST unless you know a specific reason not to.

In your case the rotational effects might cause a problem, but I would try with the curvature correction model with SST before I considered any other turbulence model.

[ngoc_tran_bao]

Thank you so much for your recommend, Ghorrocks. I have tried to refine the mesh at all walls by creating 10 layers of inflation to get the value Y+ of 1 and apply SST turbulence model. It is harder to get convergence than K-e model but I believe it will be alright. By the way, I have another question about head loss, you know beside friction there are many types of head loss in the pump such as volumetric loss, hydraulic loss and so on. Is there any way to apply those loss in our simulation i order to make the result to be more accurate, Sir?

[ghorrocks]

The whole idea of CFD is that you do not need to define these factors as the CFD model works it out based on the flow conditions.

If you wish to apply all those loss factors then you are doing a system level model, not CFD.

[ozturk]

Hello,
I am analizing 3d naca airfoil. I use k-w SST and my Re 4.1e+7. I want to learn drag and lift over airfoil. Airflow velocity is 210 m/s. I desired y+ value 1 and calculate my first wall (infilation) height is 5.2e-6 . My infilation number is 45. After I runned my case, I plotted my y+ value over airfol is between 0,05 and 0,68. Is this value suitable for calculating right drag and lift?
Thank you.
Regards

[ghorrocks]

You cannot say from a simple y+ number whether your mesh is adequate. You should do a mesh sensitivity study to determine whether your mesh is accurate enough for your purposes or not.

[ozturk]

Thanks for your quick reply,
You said I should do a mesh sensitivity study; I should run different mesh elements number.
I calculated my case between 2000000 and 4500000 elements and then my max skewness nearly 0.95 . Before I said you, I desired my y+ value 1; but my results between 0.05 and 0.65, it is under 1 . It is not exactly 1. I want to ask these:
1. My solution model k-w STT is true?
2. My desired y+ value 1 is true?
3. After calculation I have to catch 1 y+ value, if it is lower 1, is it problem?
Thank you.
Regards.

[ghorrocks]

y+<1 is a guide for what is required for SST to do integration to the wall with reasonable accuracy. But note that if you can use wall functions (and many flows can) then you are aiming for y+>11. So do not simply assume you need a small y+ and wall functions are much easier to use and work for many flows.

In my experience as the mesh is refined and the y+ gets smaller the results get more accurate. But there will be a point where numerical round off errors start becoming important and that means that further mesh refinement will lead to divergence. So there is a lower limit on mesh refinement. And the lower limit also varies between simulations.

[ozturk]

I understood that: I think my way is true. Because I do not use k-epsilon ( wall function ). I use k-w STT so for my y+ value 1 is right desired value.
Thank you again.
Regards

[ghorrocks]

Because you are using SST does NOT mean you then have to have y+=1. If you are using automatic wall functions it will automatically transition from wall functions to integrating to the wall. You should try a y+ which will give wall functions and see if it works in your situation.

[ozturk]

Hi,
Again I am here
Maybe you can remember my model is 3d airfoil. I analize cd nad cl coefficient. I want to ask that I write the area which looking airfoil through y axis. I know this is true for cl coefficient but I am not sure for cd.
For cd it maybe looking through x axis.
Thank you.

[Blauwal]

Hi everyone!

I'm barely new in CFD and currently working with cfx on my first study. Because i have to write a report for my university I would like to explain why I took a y + value of 1.

I tried to understand the theory of delta y the viscose sublayer etc. but in short words or better related on any paper is there anything you could recommend? i do not want to explain it in detail but relate me on something.


Thank you in advance!!!

[karachun]

https://www.youtube.com/watch?v=fJDYtEGMgzs