transient

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  • Last Post 01 June 2020
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reza1374 posted this 31 May 2020

1-in transient method. in every time step should converge? or example in 100 time step,5 step dont converge is problem and make error in calculate?

2- in transient solve: residuals just descending or in addition to descending they must move in straight line?

3- my transient run time 0-20 sec. but article consider time average for 5-20 sec. because after 5 sec achieve to steady state. can i see steady state from residuals in transient run.

4- for my run , at first 5 sec calculate and after finish 5 sec. i active time statistic for time average from 5-20 sec

Kalyan Goparaju posted this 01 June 2020

Hello, 

1. A good way to judge the choice of dt is to observe the number of iterations ANSYS Fluent needs to converge at each time step. The ideal number of iterations per time step is 5–10. If ANSYS Fluent needs substantially more, the time step is too large. If ANSYS Fluent needs only a few iterations per time step, dt should be increased. Frequently a time-dependent problem has a very fast “startup” transient that decays rapidly. Therefore, it is often wise to choose a conservatively small dt for the first 5–10 time steps. dt may then be gradually increased as the calculation proceeds. 

2. The residuals only make sense within a time-step. The overall behavior of residuals does not really matter. 

3. In order to see steady-state, you should have a few surface/point monitors to observe their behavior with time. After the initial transience, these monitors most likely will exhibit a statistically stationary behavior, where their mean value no longer varies with time. 

4. It is recommended to start collecting statistics only after the statistically stationary state is reached. 

Thanks,

Kalyan

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