Residuals converge initially, then stop converging

andrew-c · April 4, 2023, 12:43

I am running a transient, 3D multiphase simulation of water vaporization using the species model (with species mass transfer mechanism).

I start my simulations by letting the model come to steady state (~10s of flowtime) with the energy equation off. Then, I turn on the energy equation to continue.

However, I am having trouble converging my residuals at each timestep. They converge for quite some time and generally decrease, but then start rising significantly afterwards (see attached image). They don't usually diverge, but I feel I can't trust the results with such high residuals (continuity in particular is over 1e-1 in most cases).

My simulation is eulerian multiphase, realizable k-e. I have tried implicit and explicit, and have varied the timesteps from 1e-1 down to 1e-8, but this behavior occurs each time.

Any suggestions would be great. Thanks!

2023-04-04 11_29_07-J_Fluent Parallel Fluent@DESKTOP-TTQLM5V [3d, dp, pbns, eulerian, spe, rke,.jpg

NickFL · April 4, 2023, 16:31

The residuals show us very little here. You need to look at the flow field just before and just as it starts to diverge. What is different about the flow? What phenomenon is occurring that wasn't occurring before? Is something reaching an outlet? Do you have backflow? Is a large amount of fluid changing phase all at once? So many possibilities.

andrew-c · April 6, 2023, 13:12

Thank you Nick, I appreciate your response!

I will take another look and see what else occurs when the residuals start to increase. If I find more info and have questions, I will reply again to this thread.

April 4, 2023, 12:43	Residuals converge initially, then stop converging	#1
andrew-c New Member Andrew Join Date: Dec 2022 Posts: 29 Rep Power: 3	I am running a transient, 3D multiphase simulation of water vaporization using the species model (with species mass transfer mechanism). I start my simulations by letting the model come to steady state (~10s of flowtime) with the energy equation off. Then, I turn on the energy equation to continue. However, I am having trouble converging my residuals at each timestep. They converge for quite some time and generally decrease, but then start rising significantly afterwards (see attached image). They don't usually diverge, but I feel I can't trust the results with such high residuals (continuity in particular is over 1e-1 in most cases). My simulation is eulerian multiphase, realizable k-e. I have tried implicit and explicit, and have varied the timesteps from 1e-1 down to 1e-8, but this behavior occurs each time. Any suggestions would be great. Thanks! 2023-04-04 11_29_07-J_Fluent Parallel Fluent@DESKTOP-TTQLM5V [3d, dp, pbns, eulerian, spe, rke,.jpg

April 4, 2023, 16:31		#2
NickFL Senior Member Join Date: Jun 2009 Location: Technische Universität Chemnitz Posts: 107 Rep Power: 17	The residuals show us very little here. You need to look at the flow field just before and just as it starts to diverge. What is different about the flow? What phenomenon is occurring that wasn't occurring before? Is something reaching an outlet? Do you have backflow? Is a large amount of fluid changing phase all at once? So many possibilities. CFDKareem and andrew-c like this.

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April 6, 2023, 13:12		#3
andrew-c New Member Andrew Join Date: Dec 2022 Posts: 29 Rep Power: 3	Thank you Nick, I appreciate your response! I will take another look and see what else occurs when the residuals start to increase. If I find more info and have questions, I will reply again to this thread.