# Step-by-Step Tutorial: Homework of Session 2

#173

How can I calculate k and omega value by myself?

Regards!

Solver Error
#174

Hi @jbin

Here you will find some good and detailed instructions about the calculation of K and Omega.
https://www.cfd-online.com/Wiki/Turbulence_free-stream_boundary_conditions

We also prepared a speadsheet based tool, which you can access here:

Cheers,

#175

Greetings,

I’m working on homework 2 and again; facing errors. Since I’ve done the Step by step tutorial multiple times and the error indicates to me that there’s a problem in CAD geometry; I’m not sure how to continue and solve this. I saw someone having the same problem on the forum but he was moved to a private discussion. If anyone knows a solution to this please answer, it would mean a world to me since I’ve put down this project on a hold a few weeks ago because of a similar error and lack of time. I will post the name of errors in my Event log alongside with my project link.
Thanks in forward,

Mislav Bošnjak

Event log:
2016-12-28 14:15
Illegal triangles were found after surface tesselation. There could be a problem with the CAD geometry. Trying to proceed anyway.
2016-12-28 14:15
The tesselated surface is not closed. There could be a problem with the CAD geometry (such as self-intersections). Please inspect your geometry. Trying to proceed anyway.
2016-12-28 14:41
Run finished.
2016-12-28 14:46
Mesh check finished.
2016-12-28 14:46
Number of nodes: 10383175
2016-12-28 14:46
Number of faces: 26733859
2016-12-28 14:46
Number of volumes: 8246127
2016-12-28 14:46
Number of tetrahedra: 112
2016-12-28 14:46
Number of hexahedra: 6490000
2016-12-28 14:46
Number of prisms: 188650
2016-12-28 14:46
Number of pyramids: 0
2016-12-28 14:46
Number of polyhedra: 1558877
2016-12-28 14:46
Mesh quality check failed. The mesh is not OK.
2016-12-28 14:46
Mesh import started.
2016-12-28 14:47
Mesh bounding box diagonal length: 29.6296

#176

also tried to mesh it also with less cores but I also could not run the project.

I will provide you with a mesh that is working: Session 2 - Mesh

If you have questions, feel free to ask.

Best,

Jousef

#177

@jousefm
Hi!

Thank You a lot!
I will start immidiately on trying with this one.

Sincerely,
Mislav Bošnjak

#178

on my project … i made a mesh but thrice … it says my simulation was canceled
the error always was "The job execution was aborted, possibly due to a numerical instability. Review the log to identify reasons: unphysically large field values, extremely small time step size, etc. Modifying numerical settings and time step size could resolve the issue. "
i changed the run time to 6000 and 4000 in the 2nd adn thurd run but it still showed an error

#179

I would like to know why the value for k is 0.06 in the tutorial because the spreadsheet outputs 1.5 m2/s2.

Acording to this link the turbulence intensity in a flow around a car should be below 1%. If you type 0.01 as the turbulence intensity, the result for k is 0.06, but then the result for omega is way below 44.7, which is the value in the tutorial and in the spreadsheet when you use 0.05 for the turbulence intensity. Basically the k and omega values in the spreadsheet never match with the values from the tutoral.

#180

Hi @eduardocclc,

as you may know these are the default values of the simulation setup and you are absolutely right that you have to adapt the values according to the formulae you posted. The purposes of the workshops are not only that users learn how to set up a simulation but also learn to be sceptical and question the results (“multicoloured pictures say nothing about the accuracy”). So your scepticism is absolutely legit!

You could do a study on how the default values behave compared to the values you get when applying the formulae (in the sense of being strict with the use of them). You can give it a try and see if this has a tremendous impact on the results.

Best,

Jousef

#181

Yeah, I will do that and see what is the impact of theses values.

By the way, I have another question. How can I calculate the mass flow rate through the radiator?
I tried to apply an area integral on the surface of the radiator but the simulation ends with an error.
It doesn´t seems that I can use area integrals on porous media zones. I only manage to use it on the inlet.

#182

Good question @eduardocclc! Never tried that myself before.

#183

Hi! my meshing operation is failing continuously. i cant seem to solve the problem. it says illegal maxfacedff = 0 . i have initially meshed this cad on a diffrent project in the same method but i can’t now . could you please help me out ?

#184

Hi @pdarda,

make sure you share your project here. If you do not know how that works, please have a look at this post: How to share a project .

I will have a look at that. Maybe just a small mistake in the settings.

Best,

Jousef

#185

Hello, I went through the tutorial to improve my skills. And I cant quite understand why we have to use a rotating wall condition and a mrf on top of that. Why is it not enough to have a rotating wall boundary condition? Or why cant i simply wrap a control volume around the whole tire (not just rim) and create a mrf for that neglecting the rotating wall condition. I remember performing a centrifugal pump simulation. No rotating wall condition was used there, only mrf.

#186

Hi @kingdeking and thanks for your question!