# Questions about increasing the number of time steps and time step sizes

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June 8, 2020, 18:46
Questions about increasing the number of time steps and time step sizes
#1
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Chris Hunter
Join Date: May 2020
Posts: 17
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Hello everyone,

I have some questions about my simulation. I use Fluent 6.3.26 to simulate plasma interaction with liquid (water). When I tried time step size 1 s with different number of time steps (e.g., 100, 300, 600, etc.), I could get stable results. But the result (electric potential) seems too high than I expected. Because the average electric potential shown in the figure is around -5.e+10 V. But I only applied -2.5e+3 V. See the attachment in the first figure.

So I reduced my time step size to 1e-4 s and used a small number of time steps (less than 100), the result seems to make sense. However, ff I increased the number of time steps to 150 or even larger, then I have errors for my simulation: under convergence history of a UDS: 1.#QNBe+00.

My questions are:

1. why at high time step, the result is higher than expected?

2. why I choose a smaller time step size, the result is reasonable but the increase of number of time step leads to simulation errors: under convergence history of a UDS: 1.#QNBe+00.

I am wondering if you could help me with this.

Thanks a lot,
mcc007
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June 9, 2020, 01:25
#2
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Lolita
Join Date: Aug 2016
Posts: 115
Rep Power: 9
Quote:
 Originally Posted by mcc007 Hello everyone, I have some questions about my simulation. I use Fluent 6.3.26 to simulate plasma interaction with liquid (water). When I tried time step size 1 s with different number of time steps (e.g., 100, 300, 600, etc.), I could get stable results. But the result (electric potential) seems too high than I expected. Because the average electric potential shown in the figure is around -5.e+10 V. But I only applied -2.5e+3 V. See the attachment in the first figure. So I reduced my time step size to 1e-4 s and used a small number of time steps (less than 100), the result seems to make sense. However, ff I increased the number of time steps to 150 or even larger, then I have errors for my simulation: under convergence history of a UDS: 1.#QNBe+00. I guess it depends on the Courant number. try this https://www.simscale.com/blog/2017/08/cfl-condition/ My questions are: 1. why at high time step, the result is higher than expected? 2. why I choose a smaller time step size, the result is reasonable but the increase of number of time step leads to simulation errors: under convergence history of a UDS: 1.#QNBe+00. I am wondering if you could help me with this. Thanks a lot, mcc007
I guess it depends on the courant number. try this: https://www.simscale.com/blog/2017/08/cfl-condition/

June 9, 2020, 02:35
#3
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Chris Hunter
Join Date: May 2020
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Quote:
 Originally Posted by rupak504 I guess it depends on the courant number. try this: https://www.simscale.com/blog/2017/08/cfl-condition/
Thanks for your reply. I checked that page. It seems doesn't work. Any suggestions? Thank you.

 June 15, 2020, 06:34 Temporal Discretization #4 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 36 Just like space discretization, temporal discretization has its own rules. You cannot use long, straight lines to draw a curve or a circle. Smaller are better. But that doesn't mean way too small. You need to look at the time-scales of the phenomena taking place in the case you have. If those are of the order of milli-second, then 1e-4 is good but if those have large time-scales, then you can use a larger time-step. E.g., if you want to study melting of ice kept at room temperature, you don't expect any significant changes even over 5 seconds. So, you use a time-step of 15-20 second. As far as iterations are concerned, it should never be more than 40. If the case requires more than 40 iterations to converge in each time-step, then the time-step is too larger and should be reduced, but the number of iterations should not be increased. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

June 15, 2020, 16:04
#5
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Chris Hunter
Join Date: May 2020
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Quote:
 Originally Posted by vinerm Just like space discretization, temporal discretization has its own rules. You cannot use long, straight lines to draw a curve or a circle. Smaller are better. But that doesn't mean way too small. You need to look at the time-scales of the phenomena taking place in the case you have. If those are of the order of milli-second, then 1e-4 is good but if those have large time-scales, then you can use a larger time-step. E.g., if you want to study melting of ice kept at room temperature, you don't expect any significant changes even over 5 seconds. So, you use a time-step of 15-20 second. As far as iterations are concerned, it should never be more than 40. If the case requires more than 40 iterations to converge in each time-step, then the time-step is too larger and should be reduced, but the number of iterations should not be increased.
Make sense. It's very helpful. Thank you.

 Tags number of time steps, time step size