# Question about reasonable steps of "model mech substep" command

**URL:** https://forum.itascainternational.com/t/question-about-reasonable-steps-of-model-mech-substep-command/1389
**Category:** 3DEC
**Created:** [March 31, 2024, 2:50am UTC](https://forum.itascainternational.com/t/question-about-reasonable-steps-of-model-mech-substep-command/1389 "2024-03-31T02:50:59Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![ShanLee](https://avatars.discourse-cdn.com/v4/letter/s/b4bc9f/32.png) [@ShanLee](https://forum.itascainternational.com/u/ShanLee)
#### Post date: [March 31, 2024, 2:50am UTC](https://forum.itascainternational.com/t/question-about-reasonable-steps-of-model-mech-substep-command/1389/1 "2024-03-31T02:50:59Z")

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Hi everyone,

In my project, I would like to do THM simulation with three sub-cycles. There are HM coupling, TH coupling and HM coupling with certain time ratio of three cycles. Firstly, I use 100 mechancial substep to do simulation.

I noticed that after mechaical and thermal balance. The time ratio of fluid time (by fluid.time command) and mechanical time (by mech.time command) become much less than 100. To speed up simulation, is it reasonable to substitute mechanical substep with this value? If the answer is not, how to determine reasonable substeps with program information I can get?

Thank you all.

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### Author: ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)
#### Post date: [April 1, 2024, 6:55pm UTC](https://forum.itascainternational.com/t/question-about-reasonable-steps-of-model-mech-substep-command/1389/2 "2024-04-01T18:55:38Z")

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If you specify a mechanical substep of 100, it will run 100 mechanical steps _ **or** _ it will run until it has solved mechanically, whichever occurs first. So if not much is changing with your fluid or thermal, it may only run a few mechanical steps. So you probably don’t need to change the substeps.

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### Author: ![ShanLee](https://avatars.discourse-cdn.com/v4/letter/s/b4bc9f/32.png) [@ShanLee](https://forum.itascainternational.com/u/ShanLee)
#### Post date: [April 3, 2024, 6:13am UTC](https://forum.itascainternational.com/t/question-about-reasonable-steps-of-model-mech-substep-command/1389/3 "2024-04-03T06:13:16Z")

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Thanks for your reply, I’ll take this into consideration when simulating.