Automatic Case Modification Flow-Time doesnt add up

alphaNOVA · September 23, 2023, 15:45

Hello,

im using the automatic case modification in Fluent to start my calculations with a low cournat number. This is also set in the original settings. The original setting and my two other modification steps each use the "Flow-Time" specification on how long they should run. According to the Ansys Documentation the flow time i enter in this dialog is the flow time for how long the simulation will be carried out.
I set my Original setup to run for 7.5e-5s, the first modification for 17.5e-5s and the third on 1.825e-3 seconds. So i would expect the simulation running in total 2.5e-3 seconds. When i check flow time at the end of a simulation it varies between 3.5e-3 and 4.5e-3 seconds.
Did i understand the flow-time setting wrong or is there anything else i need to look out for in order to make the times add up? (simulating up to 4.5e-3 seconds nearly doubles my total required computation time - which i dont really can afford).

LuckyTran · September 23, 2023, 16:39

You don't add the flow time settings.

If you specify a flow time of 7.5e-5s then Fluent stops at or after 7.5e-5 s. If you then modify it to 1.825e-3 s then it stops at or as as soon as it can after 1.825e-5. It doesn't try to stop at 2.5e-3 and you shouldn't expect it to stop at 2.5e-3 based on this setting.

Now the question is why it vastly overshoots. Probably another one of your million automatic settings like allowing the Courant number maximum to be too high.

4.5e-3 seconds is clearly something you can afford because you did it without even trying!

alphaNOVA · September 28, 2023, 05:22

Thank you for your input.
I am still not sure what caused it but i swapped to fixed time steps and now it seems to work the way i expected.

Because i can afford it on a coars test mesh with a single run does not mean its suitable for a larger parameter study.

September 23, 2023, 15:45	Automatic Case Modification Flow-Time doesnt add up	#1
alphaNOVA New Member Marc Join Date: Sep 2023 Posts: 11 Rep Power: 2	Hello, im using the automatic case modification in Fluent to start my calculations with a low cournat number. This is also set in the original settings. The original setting and my two other modification steps each use the "Flow-Time" specification on how long they should run. According to the Ansys Documentation the flow time i enter in this dialog is the flow time for how long the simulation will be carried out. I set my Original setup to run for 7.5e-5s, the first modification for 17.5e-5s and the third on 1.825e-3 seconds. So i would expect the simulation running in total 2.5e-3 seconds. When i check flow time at the end of a simulation it varies between 3.5e-3 and 4.5e-3 seconds. Did i understand the flow-time setting wrong or is there anything else i need to look out for in order to make the times add up? (simulating up to 4.5e-3 seconds nearly doubles my total required computation time - which i dont really can afford).

September 23, 2023, 16:39		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,673 Rep Power: 65	You don't add the flow time settings. If you specify a flow time of 7.5e-5s then Fluent stops at or after 7.5e-5 s. If you then modify it to 1.825e-3 s then it stops at or as as soon as it can after 1.825e-5. It doesn't try to stop at 2.5e-3 and you shouldn't expect it to stop at 2.5e-3 based on this setting. Now the question is why it vastly overshoots. Probably another one of your million automatic settings like allowing the Courant number maximum to be too high. 4.5e-3 seconds is clearly something you can afford because you did it without even trying! alphaNOVA likes this.

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September 28, 2023, 05:22		#3
alphaNOVA New Member Marc Join Date: Sep 2023 Posts: 11 Rep Power: 2	Thank you for your input. I am still not sure what caused it but i swapped to fixed time steps and now it seems to work the way i expected. Because i can afford it on a coars test mesh with a single run does not mean its suitable for a larger parameter study.