MaxCo with interFoam

sega · January 5, 2009, 05:39

Hello again.

Thinking about the maxCo entry in controlDict I'm asking myself what this entry is good for if I'm using Euler implicit discretisation for the temporal derivative, which should be unconditionally stable for ALL timesteps.

What is maxCo limiting in this case?
The velocity field?

Greetings.

caw · January 5, 2009, 08:36

Hi Sebastian,

yes, maxCo is based on the velocity field. The stability is limited by the PISO algorithm, which is stable for Co up to 1 i think. But Co should be lower for interFoam, it should be around 0.25. To overcome this, you can use subtimestepping for MULES. A good choice is maxCo = 0.5 and two subtimesteps for gamma.

Concerning Euler: even if Euler is unconditionally stable, this does NOT mean that it is unconditionally accurate ;-)

Best regards
Christian

sega · January 6, 2009, 05:05

Thank you, Christian. That makes sense.
Who can tell the correct Co-limit for PISO?

January 5, 2009, 05:39	Hello again. Thinking about	#1
sega Senior Member Sebastian Gatzka Join Date: Mar 2009 Location: Frankfurt, Germany Posts: 729 Rep Power: 20	Hello again. Thinking about the maxCo entry in controlDict I'm asking myself what this entry is good for if I'm using Euler implicit discretisation for the temporal derivative, which should be unconditionally stable for ALL timesteps. What is maxCo limiting in this case? The velocity field? Greetings. __________________ Schrödingers wife: "What did you do to the cat? It's half dead!"

January 5, 2009, 08:36	Hi Sebastian, yes, maxCo is	#2
caw Member Christian Winkler Join Date: Mar 2009 Location: Mannheim, Germany Posts: 63 Rep Power: 17	Hi Sebastian, yes, maxCo is based on the velocity field. The stability is limited by the PISO algorithm, which is stable for Co up to 1 i think. But Co should be lower for interFoam, it should be around 0.25. To overcome this, you can use subtimestepping for MULES. A good choice is maxCo = 0.5 and two subtimesteps for gamma. Concerning Euler: even if Euler is unconditionally stable, this does NOT mean that it is unconditionally accurate ;-) Best regards Christian

January 6, 2009, 05:05	Thank you, Christian. That mak	#3
sega Senior Member Sebastian Gatzka Join Date: Mar 2009 Location: Frankfurt, Germany Posts: 729 Rep Power: 20	Thank you, Christian. That makes sense. Who can tell the correct Co-limit for PISO? __________________ Schrödingers wife: "What did you do to the cat? It's half dead!"

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