CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM > OpenFOAM Running, Solving & CFD

undetermined cells in mesh cause passive scalar transport equation to diverge

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 3, 2019, 20:07
Default undetermined cells in mesh cause passive scalar transport equation to diverge
  #1
Member
 
Anonymous
Join Date: Aug 2016
Posts: 75
Rep Power: 10
backscatter is on a distinguished road
Hi,


Recently, I noticed that having high aspect ratio cells away from the wall leads to a very low determinant for those cells. It has absolutely no effect on flow field (UEqn) but when I add a passive scalar transport equation to the simulation, the presence of these underdetermined cells leads to the blow-up of the scalar equation (only) with the momentum equation still converging.

To circumvent this issue, I alter my mesh such that I get rid of these underdetermined cells away from the wall. That helps and doesn't cause scalar field to blow up.

Has anybody noticed this kind of behavior before?
If yes, is there anything we could do than to alter the mesh?
I have tried all kinds of bounded schemes. They don't help.

PS:To get the idea of what I am talking about look at my old thread:
TEqn (passive scalar) diverges all of a sudden
backscatter is offline   Reply With Quote

Old   February 6, 2019, 15:53
Default
  #2
Senior Member
 
Join Date: Jul 2013
Posts: 124
Rep Power: 13
wildfire230 is on a distinguished road
Just to double check, once you introduce a scalar transport equation, there are at least two new conditions on the solver in order to have stability and convergence. A new limit on the time step is imposed, and the grid must be sufficiently refined in order to converge. If the time step is too large the velocity and pressure may still converge, while the scalar value could blow up, and if the grid is too coarse the scalar may blow up no matter how small the time step is.
wildfire230 is offline   Reply With Quote

Old   February 6, 2019, 16:13
Default
  #3
Member
 
Anonymous
Join Date: Aug 2016
Posts: 75
Rep Power: 10
backscatter is on a distinguished road
Quote:
Originally Posted by wildfire230 View Post
Just to double check, once you introduce a scalar transport equation, there are at least two new conditions on the solver in order to have stability and convergence. A new limit on the time step is imposed, and the grid must be sufficiently refined in order to converge. If the time step is too large the velocity and pressure may still converge, while the scalar value could blow up, and if the grid is too coarse the scalar may blow up no matter how small the time step is.



Thanks for the comment. I knew that would be the case but in the back of my mind I was thinking if the undetermined cells might be the cause. It turns out that reducing the time step by an order of magnitude does the job. In the regions where Reynolds Analogy breaks down, the turbulent Prandtl number decreases to a large extent and that increases the turbulent thermal diffusivity, thereby making the time scale of thermal transport small- which, of course, the timestep suited for solving momentum equation could not resolve.
backscatter is offline   Reply With Quote

Old   February 20, 2022, 09:42
Default
  #4
Member
 
George
Join Date: Dec 2020
Posts: 31
Rep Power: 5
damon707 is on a distinguished road
Hello all,

How exactly did you manage to eliminate the cells with low determinant?

Kind regards,
George
damon707 is offline   Reply With Quote

Reply


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


Similar Threads
Thread Thread Starter Forum Replies Last Post
[snappyHexMesh] SnappyHexMesh running killed! Mark JIN OpenFOAM Meshing & Mesh Conversion 7 June 14, 2022 02:37
[mesh manipulation] Importing Multiple Meshes thomasnwalshiii OpenFOAM Meshing & Mesh Conversion 18 December 19, 2015 19:57
[snappyHexMesh] No layers in a small gap bobburnquist OpenFOAM Meshing & Mesh Conversion 6 August 26, 2015 10:38
Source Term on Scalar Transport Equation - crashing simulation alessio.nz OpenFOAM 19 February 8, 2011 08:38
Is it possible to use divergence in the source term of an scalar transport equation? jannnesss CFX 0 January 8, 2010 20:53


All times are GMT -4. The time now is 01:26.