Time Step Size

Car · December 19, 2018, 05:26

Hello everyone,

I need to simulate a lime fouling process. I want information about the fouling layer thickness, the changing of the heat conductivity and the temperature that appear during a year.

I already got the model. Now I face the problem of the calculation time. I wonder if I can set the time step up to an hour, if I start the simulation with a small time step size (~0.0001) and slowly increase the time step size as the simulation converges.

Carolin

LuckyTran · December 19, 2018, 10:57

There isn't a time-step ramp option. You have to script the changes or change it manually.

AlexanderZ · December 19, 2018, 23:05

As LuckyTran mentioned you may do it using UDF, take a look into DEFINE_DELTAT macro
More information in ANsys Fluent Customization manual

best regards

Car · December 20, 2018, 03:13

Thank you for your answer!

I think I didn't pose the question properly. What I want to know is if it will affect the results' accuracy if I set the time step size to 1800 sec.

Has anyone a suggestion?

AlexanderZ · December 20, 2018, 03:20

It depends on your model
How fast physical processes you model comparing to timestep you want?

best regards

Car · December 21, 2018, 06:13

Dear AlexanderZ,

I'm not sure if I understand your question, so I try to explain my model a bit.

I use DEFINE_PROPERTY to define the thick walls thermal conductivity. The conductivity slowly decreases (as a consequence of the growing fouling layer which has a lower conductivity than the wall).
Due to this UDF I thought the temperature of the fluid should slowly decrease. But this isn't the case. I found the temperature increasing just a little (about 0.001 K in a month). I wonder why this happens and I think it might be a problem with the time step size.

As you can see, the change in the system is very small. Regarding your last answer am I right to think that the time step size should therefore be very small as well?

Thank you for your help!

Carolin

AlexanderZ · December 23, 2018, 20:55

Does your wall have thickness? (it should have)
Did you put any heatsource on the wall? (probably, you should put)
What about your mesh, did you resolve boundary layer?
If all abouve is correct and you have such a slow change in temperature, then I would increase timestep, if I were you.

best regards

December 19, 2018, 05:26	Time Step Size	#1
Car New Member Caro Join Date: Jul 2018 Location: Germany Posts: 14 Rep Power: 7	Hello everyone, I need to simulate a lime fouling process. I want information about the fouling layer thickness, the changing of the heat conductivity and the temperature that appear during a year. I already got the model. Now I face the problem of the calculation time. I wonder if I can set the time step up to an hour, if I start the simulation with a small time step size (~0.0001) and slowly increase the time step size as the simulation converges. Carolin

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December 19, 2018, 10:57		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,680 Rep Power: 66	There isn't a time-step ramp option. You have to script the changes or change it manually.

December 19, 2018, 23:05		#3
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	As LuckyTran mentioned you may do it using UDF, take a look into DEFINE_DELTAT macro More information in ANsys Fluent Customization manual best regards

December 20, 2018, 03:13		#4
Car New Member Caro Join Date: Jul 2018 Location: Germany Posts: 14 Rep Power: 7	Thank you for your answer! I think I didn't pose the question properly. What I want to know is if it will affect the results' accuracy if I set the time step size to 1800 sec. Has anyone a suggestion?

December 20, 2018, 03:20		#5
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	It depends on your model How fast physical processes you model comparing to timestep you want? best regards

December 21, 2018, 06:13		#6
Car New Member Caro Join Date: Jul 2018 Location: Germany Posts: 14 Rep Power: 7	Dear AlexanderZ, I'm not sure if I understand your question, so I try to explain my model a bit. I use DEFINE_PROPERTY to define the thick walls thermal conductivity. The conductivity slowly decreases (as a consequence of the growing fouling layer which has a lower conductivity than the wall). Due to this UDF I thought the temperature of the fluid should slowly decrease. But this isn't the case. I found the temperature increasing just a little (about 0.001 K in a month). I wonder why this happens and I think it might be a problem with the time step size. As you can see, the change in the system is very small. Regarding your last answer am I right to think that the time step size should therefore be very small as well? Thank you for your help! Carolin

December 23, 2018, 20:55		#7
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	Does your wall have thickness? (it should have) Did you put any heatsource on the wall? (probably, you should put) What about your mesh, did you resolve boundary layer? If all abouve is correct and you have such a slow change in temperature, then I would increase timestep, if I were you. best regards