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y+ plotting into the Volume with OpenFoam 3.0.1 / plotting a viscosity with OF 3.0.1

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Old   July 25, 2016, 20:27
Question y+ plotting into the Volume with OpenFoam 3.0.1 / plotting a viscosity with OF 3.0.1
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Björn
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Dear Foamers,

I have to do an y+ / u+ plot on the NASA 2D Zero Pressure Gradient Flat Plate Case.
As far as i can see, I have everything I need for my calculations, except for a viscosity.

I have searched the Forum, but couldn't find a solution, suitable to my remaining time on the task.

I'm using the rhoSimple solver on a subsonic, turbulent air flow with the kOmegaSST RAS model.

It would really make my day if anybody got an usefull hint for me.
... One could consider me as a beginner

This is my first post, so don't hesitate to tell me about everything i did wrong in detail, in a kind way.
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Old   July 26, 2016, 03:50
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The command "yPlus" will write the cell-y+ to a specific field called "yPlus".
After starting paraview using the paraFoam-command, you will then be able to activate said field in the "fields" tab on the left. After that, you can use either the "Plot on Intersection Curves" filter in paraview in order to create the plot directly in paraview. Or you can use the "Integrate Variables" filter followed by the "Save data" option to export the points an their corresponding y+ values.
I hope there is an option that suits your needs.

Cheers
Alex
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Old   July 26, 2016, 06:39
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Hello Kina!

Thanks for your quick reply!
As far as i understand it, the "yPlus" command will write the current y+ values for the first cell layer of the volume, for all defined walls, into a file.

What i need to know is "what would the y+ be like, if the first cell was bigger in the wall normal direction (only at one point of the wall, not even all along it)".

I think also a Reynolds number within the Volume should do just fine for my calculations, using the formulas "1." and "5." from this page:
http://www.cfd-online.com/Wiki/Y_plu...nce_estimation

Or am I on a completely wrong track here..?


Greetings

Björn
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Old   July 27, 2016, 11:43
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Well, yes and no. The viscosity is just the viscosity of your fluid that is defined in the TransportProperties file. This is the nominal viscosity that you are estimating the initial y+ with. However, since the first cell layer is very very influencing in terms of near wall viscosity, velocity distribution and such, the REAL y+ calculation can only be read out of the solver when you have a convergent solution. That's why we can't simply take the velocity field of a y+ = 30 solution and extrapolate down to y+ = 1 since this can change everything and crash your theoretical approach.

Cheers
Alex
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Old   August 20, 2016, 14:25
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Hello Kina,

it has been a while and I have written the theory part of my work now, thanks for your help, I understand a lot more now of what you tried to tell me, it was really helpful!
It looks like that in the end it really was just, to simply take the viscosity as a given constant.
This thread can be considered as closed.
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