CFL-Number

Zaktatir · October 5, 2010, 04:52

I know CFX is an implicit solver nevertheless i wanted to know which best practice range for CFL number does exist for multiphase flows.
thanks

Julienne

stumpy · October 5, 2010, 09:27

There's no recommended range. For example, as you refine your mesh your CFL number would change, but in general you wouldn't need to adjust your timestep.

joey2007 · October 8, 2010, 13:35

From theory we know, that an implicit solution is stable independent from the time step. I guess does not include source terms from reaction or multiphase. For that reason in that applications you may require smaller time steps.

ghorrocks · October 9, 2010, 05:54

To expand on the previous comments: Explicit solvers have a stability limit at a Courant number of 1. You simply can't go faster than that. But CFX is an implicit solver so this restriction does not apply (but a very common mistake is to forget this and run CFX with a Courant number <1).

So the timestep required for CFX should be determined with a sensitivity study on a case by case basis. The allowable timestep is often much bigger than 1.0, but exactly how big is OK depends on the simulation.

October 5, 2010, 04:52	CFL-Number	#1
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	I know CFX is an implicit solver nevertheless i wanted to know which best practice range for CFL number does exist for multiphase flows. thanks Julienne

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October 5, 2010, 09:27		#2
stumpy Senior Member Join Date: Apr 2009 Posts: 531 Rep Power: 21	There's no recommended range. For example, as you refine your mesh your CFL number would change, but in general you wouldn't need to adjust your timestep.

October 8, 2010, 13:35		#3
joey2007 Senior Member Join Date: Mar 2009 Location: Europe Posts: 169 Rep Power: 17	From theory we know, that an implicit solution is stable independent from the time step. I guess does not include source terms from reaction or multiphase. For that reason in that applications you may require smaller time steps.

October 9, 2010, 05:54		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	To expand on the previous comments: Explicit solvers have a stability limit at a Courant number of 1. You simply can't go faster than that. But CFX is an implicit solver so this restriction does not apply (but a very common mistake is to forget this and run CFX with a Courant number <1). So the timestep required for CFX should be determined with a sensitivity study on a case by case basis. The allowable timestep is often much bigger than 1.0, but exactly how big is OK depends on the simulation.