CFL at CFX

phPatras · August 5, 2018, 07:40

Hi
My question is that: as the implicit scheme is unconditionally stable for any size of time step and the transient scheme of CFX are implicit what the meaning CFL

ghorrocks · August 5, 2018, 08:12

Stability: see https://flow3d.com/resources/cfd-101...rical-methods/

CFL: https://en.wikipedia.org/wiki/Couran...Lewy_condition

Google is your friend.

phPatras · August 5, 2018, 08:30

Thanks for your reply.
But as i wrote from "W. Malalasekera and Versteeg" the implicit scheme (as of CFX) UNCONDITIONALLY STABLE....or better "not require the Courant number to be small but but neither very big... eg 100"

ghorrocks · August 5, 2018, 19:05

That is correct. In terms of numerical stability of the time step scheme implicit schemes are unconditionally stable. Instead the maximum time step size is determined by how accurately the numerical scheme can follow the modelled transients. Note that if the scheme cannot follow the modelled transients accurately that this is can lead to numerical instability - but this is a different form of instability to that related directly to the time step size. This is discussed in more detail in the link I posted (and Malalasekera and Versteeg probably discuss it as well).

August 5, 2018, 07:40	CFL at CFX	#1
phPatras New Member Mentzos Join Date: Dec 2010 Location: Athens Posts: 22 Rep Power: 15	Hi My question is that: as the implicit scheme is unconditionally stable for any size of time step and the transient scheme of CFX are implicit what the meaning CFL

August 5, 2018, 08:12		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Stability: see https://flow3d.com/resources/cfd-101...rical-methods/ CFL: https://en.wikipedia.org/wiki/Couran...Lewy_condition Google is your friend. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

August 5, 2018, 19:05		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	That is correct. In terms of numerical stability of the time step scheme implicit schemes are unconditionally stable. Instead the maximum time step size is determined by how accurately the numerical scheme can follow the modelled transients. Note that if the scheme cannot follow the modelled transients accurately that this is can lead to numerical instability - but this is a different form of instability to that related directly to the time step size. This is discussed in more detail in the link I posted (and Malalasekera and Versteeg probably discuss it as well). phPatras likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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August 5, 2018, 08:30		#3
phPatras New Member Mentzos Join Date: Dec 2010 Location: Athens Posts: 22 Rep Power: 15	Thanks for your reply. But as i wrote from "W. Malalasekera and Versteeg" the implicit scheme (as of CFX) UNCONDITIONALLY STABLE....or better "not require the Courant number to be small but but neither very big... eg 100"