Transient Adaptive Time-stepping - Divergence?

Time4Tea · June 10, 2016, 15:11

Hi, I'm trying to do a transient simulation in FLUENT, using adaptive time-stepping, but I seem to be having a problem with solution stability. I'm trying to model incompressible internal flow through a valve and the set-up that I have seems to work fine with fixed time steps. However, when I try to change to using adaptive time steps, it calculates the first time step ok, but when it tries to start the second time step, I immediately get an error saying 'divergence detected in AMG solver ...' and the solver falls over.

I thought the adaptive time-stepping should just be varying the time-step size, so I'm not sure why the solution should suddenly become unstable like this. Has anyone else experienced similar behavior? Any ideas on how I can resolve it? Thanks in advance!

Time4Tea · June 14, 2016, 09:45

I didn't get an answer to this question, so perhaps I can try to re-phrase it?:

Does anyone have any experience of using adaptive time-stepping for transient simulations in FLUENT? If so, are there any general guidelines I should be aware of for achieving a stable solution?

June 10, 2016, 15:11	Transient Adaptive Time-stepping - Divergence?	#1
Time4Tea Senior Member Lee Strobel Join Date: Jun 2016 Posts: 133 Rep Power: 9	Hi, I'm trying to do a transient simulation in FLUENT, using adaptive time-stepping, but I seem to be having a problem with solution stability. I'm trying to model incompressible internal flow through a valve and the set-up that I have seems to work fine with fixed time steps. However, when I try to change to using adaptive time steps, it calculates the first time step ok, but when it tries to start the second time step, I immediately get an error saying 'divergence detected in AMG solver ...' and the solver falls over. I thought the adaptive time-stepping should just be varying the time-step size, so I'm not sure why the solution should suddenly become unstable like this. Has anyone else experienced similar behavior? Any ideas on how I can resolve it? Thanks in advance!

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June 14, 2016, 09:45		#2
Time4Tea Senior Member Lee Strobel Join Date: Jun 2016 Posts: 133 Rep Power: 9	I didn't get an answer to this question, so perhaps I can try to re-phrase it?: Does anyone have any experience of using adaptive time-stepping for transient simulations in FLUENT? If so, are there any general guidelines I should be aware of for achieving a stable solution?