# Energy residual changing between two values

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

 November 2, 2017, 10:34 Energy residual changing between two values #1 New Member   Join Date: Oct 2017 Posts: 26 Rep Power: 2 Hello all, I am simulating combustion in a flare. For this I am using the transient solver, the solution has a hard time converging in the steady state solver. But here is the problem: My energy residual keeps switching between two values. If I change the timestep the values change, but it still switches between two values(so for a timestep of 1e-5s it switches between 1,311e-4 to 3,843e-4 and with a timestep of 1e-6 it switches between 1,423e-5 to 4,562e-5). If I reduce the residual far enough (0,5 or lower) it gets stuck on one value. How do I solve this? Thanks!

 November 2, 2017, 12:45 #2 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,620 Rep Power: 26 Are you sure that you are not just seeing the normal convergence behavior of a transient simulation? High residuals at the beginning of a time step that are then reduced to lower values during the inner iterations... __________________ Please do not send me CFD-related questions via PM

November 3, 2017, 02:13
#3
New Member

Join Date: Oct 2017
Posts: 26
Rep Power: 2
Quote:
 Originally Posted by flotus1 Are you sure that you are not just seeing the normal convergence behavior of a transient simulation? High residuals at the beginning of a time step that are then reduced to lower values during the inner iterations...
Maybe... I'm just wierded out because it starts jumping between two values after some iterations. For each timestep the same values!

 November 3, 2017, 04:52 #4 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,620 Rep Power: 26 You can upload a screenshot if you want. Until then I suspect the residuals look like this. __________________ Please do not send me CFD-related questions via PM

November 3, 2017, 05:01
#5
New Member

Join Date: Oct 2017
Posts: 26
Rep Power: 2
Quote:
 Originally Posted by flotus1 You can upload a screenshot if you want. Until then I suspect the residuals look like this.
If only...
It converges a little bit, and then it starts changing between the same two values in one timestep. So basically it starts to oscillate between two set values untill the maximum iterations/timestep is reached. And then it does it again for the next timestep.

 November 3, 2017, 05:33 #6 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,620 Rep Power: 26 A picture is worth a thousand words... __________________ Please do not send me CFD-related questions via PM

 November 3, 2017, 06:09 #7 New Member   Join Date: Oct 2017 Posts: 26 Rep Power: 2 It looks worse than it is. The first few timesteps look like the image you uploaded, it became so irregular after the spark ignition kicked in. I could not replicate the "jumping between two set values", but this is definitly how it looked. What do you think?

 November 3, 2017, 06:26 #8 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,620 Rep Power: 26 Looks fine to me. The time step size might be too high which causes a high level for the continuity residual and poor convergence in general. __________________ Please do not send me CFD-related questions via PM

November 3, 2017, 06:28
#9
New Member

Join Date: Oct 2017
Posts: 26
Rep Power: 2
Quote:
 Originally Posted by flotus1 Looks fine to me. The time step size might be too high which causes a high level for the continuity residual and poor convergence in general.
I tried decreasing the time step to 1e-8... Even then it does not converge good. I also changed the relaxation factors, no joy there either. Maybe it's so unstable that normal convergence cannot be achieved?

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post AlmostSurelyRob OpenFOAM Running, Solving & CFD 28 November 10, 2017 16:45 chrizzl OpenFOAM Running, Solving & CFD 13 March 28, 2017 05:49 xsa OpenFOAM Running, Solving & CFD 3 November 7, 2016 06:07 Lord Kelvin OpenFOAM Running, Solving & CFD 8 March 28, 2016 11:08 jonmec OpenFOAM Running, Solving & CFD 3 July 28, 2011 05:24

All times are GMT -4. The time now is 15:24.