Question on OpenFOAM and implicit time discretization

me3840 · March 10, 2016, 18:32

Hello folks,

OpenFOAM has quite a few implicit time schemes which are used pretty heavily. But what I don't understand is that often in practice and a lot of advice on this forum says you should still try to get a CFL of 1, even though the implicit scheme shouldn't require that.

I have found, though, that in many instances without this, the code blows up. For my higher-speed cases, this can happen with dt=1e-4s. With much smaller timesteps, 1e-5s or 1e-6, the code doesn't blow up. Does this represent some kind of instability in the scheme, or am I missing something?

Thanks.

akidess · March 11, 2016, 04:05

The time marching is stable, the system coupling is not.

March 10, 2016, 18:32	Question on OpenFOAM and implicit time discretization	#1
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 25	Hello folks, OpenFOAM has quite a few implicit time schemes which are used pretty heavily. But what I don't understand is that often in practice and a lot of advice on this forum says you should still try to get a CFL of 1, even though the implicit scheme shouldn't require that. I have found, though, that in many instances without this, the code blows up. For my higher-speed cases, this can happen with dt=1e-4s. With much smaller timesteps, 1e-5s or 1e-6, the code doesn't blow up. Does this represent some kind of instability in the scheme, or am I missing something? Thanks.

March 11, 2016, 04:05		#2
akidess Senior Member Anton Kidess Join Date: May 2009 Location: Germany Posts: 1,377 Rep Power: 30	The time marching is stable, the system coupling is not. __________________ On twitter @akidTwit Spend as much time formulating your questions as you expect people to spend on their answer.

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