CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Timestep and laplacian heat transfer

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 30, 2021, 14:32
Default Timestep and laplacian heat transfer
  #1
New Member
 
George
Join Date: Oct 2020
Posts: 24
Rep Power: 4
Dicanic is on a distinguished road
Hi all,



I am finding some strange results when testing OpenFOAM against analytical solutions. The scenario I am testing is 1D ss with generation. I am using gauss linear for my gradSchemes and gauss linear corrected for my laplacian schemes.


This is a snipet from my fvOptions where power is generated:


// ***** Power *****
const scalar powerlvl = 1e6; // Watts/m3


// Apply power *************************************************
forAll(V, i)
{
eSource[i] -= powerlvl*V[i]; // (W per cell)
}


My question is how does a small timestep produce the wrong solution?
Dicanic is offline   Reply With Quote

Old   November 30, 2021, 14:38
Default
  #2
New Member
 
George
Join Date: Oct 2020
Posts: 24
Rep Power: 4
Dicanic is on a distinguished road
Figure for reference: https://ibb.co/C13590S
Dicanic is offline   Reply With Quote

Old   November 30, 2021, 23:52
Default
  #3
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,064
Rep Power: 60
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Are you sure it doesn't have anything to do with not using the same mesh?
LuckyTran is online now   Reply With Quote

Old   December 1, 2021, 00:57
Default
  #4
New Member
 
George
Join Date: Oct 2020
Posts: 24
Rep Power: 4
Dicanic is on a distinguished road
I used three different meshes, but I can see how this figure would mislead only showing two with different timesteps. Suffice to say, even the coarsest mesh of 400 cells correctly matched the analytical solution when the diffusion number was greater than 5 or so. Still a little confused and worried over this but I'll update once I figure it out. It might have something to do with my residual control or tolerance.
Dicanic is offline   Reply With Quote

Old   December 2, 2021, 11:26
Default Solution!
  #5
New Member
 
George
Join Date: Oct 2020
Posts: 24
Rep Power: 4
Dicanic is on a distinguished road
So I figured out my solver tolerances were too low and a steady state wasn't actually achieved, the solver just stopped. Meaning, changing the tolerances from the tutorial cases is probably a good idea if you want to reach analytical level accuracy.



You can read about it here under 4.5.1.1 Solution tolerances:
https://cfd.direct/openfoam/user-guide/v6-fvsolution/
Dicanic is offline   Reply With Quote

Reply

Tags
heat transfer

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On



All times are GMT -4. The time now is 11:38.