Transient simulation in CFX

BalanceChen · August 20, 2011, 04:41

Hi all, I am doing transient simulation of a compressor using CFX 12.1. I set the time step to be 20 steps in a passage. But the tips show up as "The Maximum mach num exceed 3" after a few steps,then the mach number go larger and larger until it flows up.

I try to set the time step smaller, i.e. 30 steps in a passage, or to use the adaptive timesteps, but problem is still there.

I have no idea now, what should i do next?

Thanks a lot,

BlanceChen

mauricio · August 21, 2011, 19:11

a high Mach number means an acceleration of fluid in your domain. Try simplyfying a bit the model and also trying a different turbulence model and options.

Clasen · August 26, 2011, 11:58

Probably you have a sharp corner or some geometry ending in tangent.
As Mauricio told, try to simplify your geometry in these points building right angles.

BalanceChen · August 27, 2011, 23:33

In a casual test, I do a simulation both in Win CFX 12.1 and Linux CFX 12.1, the problem still appears in the Linux CFX 12.1, but fine with Win CFX12.1. Is it the problem of the Linux CFX12.1? Both sets of the simulation are exactly the same, except the CPU used to compute.

ghorrocks · August 28, 2011, 07:40

It is very unusual but not impossible for a simulation to have different results in Windows and Linux. If you can isolate what is causing it you can report it as a bug - but this will take some detective work.

Also be aware that you are not running the current version of the software (V13 SP2). There is a good chance your bug has been fixed in the newer releases so I suggest you upgrade.

BalanceChen · August 28, 2011, 22:45

I do another test by an example transient simulation on the Linux CFX12.1, found no problem there. Then it should be some mistake i made in my simulation settings, or maybe the grid is not good enough? Which will be the most possible to cause such problem...It is really unbelievable that the same simulation can be continued on the Win CFX, but stopped on Linux CFX.

ghorrocks · August 29, 2011, 19:36

As I said, as you appear to have found a bug in CFX I would stop going round in circles exploring this bug and upgrade to the most recent version where it is probably fixed. I don't know about you but I do not have time to waste chasing bugs which have probably already been fixed.

August 20, 2011, 04:41	Transient simulation in CFX	#1
BalanceChen Member Join Date: Sep 2010 Posts: 45 Rep Power: 16	Hi all, I am doing transient simulation of a compressor using CFX 12.1. I set the time step to be 20 steps in a passage. But the tips show up as "The Maximum mach num exceed 3" after a few steps,then the mach number go larger and larger until it flows up. I try to set the time step smaller, i.e. 30 steps in a passage, or to use the adaptive timesteps, but problem is still there. I have no idea now, what should i do next? Thanks a lot, BlanceChen

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August 21, 2011, 19:11		#2
mauricio Member Mauricio Labarca Join Date: Aug 2009 Posts: 35 Rep Power: 17	a high Mach number means an acceleration of fluid in your domain. Try simplyfying a bit the model and also trying a different turbulence model and options.

August 26, 2011, 11:58		#3
Clasen New Member Join Date: Jul 2011 Posts: 2 Rep Power: 0	Probably you have a sharp corner or some geometry ending in tangent. As Mauricio told, try to simplify your geometry in these points building right angles.

August 27, 2011, 23:33		#4
BalanceChen Member Join Date: Sep 2010 Posts: 45 Rep Power: 16	In a casual test, I do a simulation both in Win CFX 12.1 and Linux CFX 12.1, the problem still appears in the Linux CFX 12.1, but fine with Win CFX12.1. Is it the problem of the Linux CFX12.1? Both sets of the simulation are exactly the same, except the CPU used to compute.

August 28, 2011, 07:40		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	It is very unusual but not impossible for a simulation to have different results in Windows and Linux. If you can isolate what is causing it you can report it as a bug - but this will take some detective work. Also be aware that you are not running the current version of the software (V13 SP2). There is a good chance your bug has been fixed in the newer releases so I suggest you upgrade.

August 28, 2011, 22:45		#6
BalanceChen Member Join Date: Sep 2010 Posts: 45 Rep Power: 16	I do another test by an example transient simulation on the Linux CFX12.1, found no problem there. Then it should be some mistake i made in my simulation settings, or maybe the grid is not good enough? Which will be the most possible to cause such problem...It is really unbelievable that the same simulation can be continued on the Win CFX, but stopped on Linux CFX.

August 29, 2011, 19:36		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	As I said, as you appear to have found a bug in CFX I would stop going round in circles exploring this bug and upgrade to the most recent version where it is probably fixed. I don't know about you but I do not have time to waste chasing bugs which have probably already been fixed.