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BalanceChen August 20, 2011 03:41

Transient simulation in CFX
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,


mauricio August 21, 2011 18:11

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 10:58

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 22:33

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 06:40

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 21:45

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 18:36

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.

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