# Controlling time step

**URL:** <https://community.geodynamics.org/t/controlling-time-step/3751>\
**Category:** ASPECT\
**Tags:** time-step\
**Created:** [November 24, 2024, 12:51pm UTC](https://community.geodynamics.org/t/controlling-time-step/3751 "2024-11-24T12:51:38Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Neta](https://avatars.discourse-cdn.com/v4/letter/n/91b2a8/32.png) [@Neta](https://community.geodynamics.org/u/Neta)\
**Post date:** [November 24, 2024, 12:51pm UTC](https://community.geodynamics.org/t/controlling-time-step/3751/1 "2024-11-24T12:51:38Z")

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Hello,  
I’m rather new to ASPECT. I’m trying to run a simple model that simulates the development of A type Olivine undergoing simple shear, starting from zero strain and gradually increasing to a strain of 5. I attached the relevant prm file. The velocity vector (in subsection Velocity function, which is located within the subsection Prescribed Stokes solution) is defined is x10e-5;0;0; so the shear strain rate is 0.5\*(10e-5) = 5\*10e-6. The end time is 2e6 (seconds).  
Since strain increment = strain rate \* time step, I need to be able to control the time step size. If possible, I prefer to keep CFL as is. I read the documentation that there is a Time Stepping section although I haven’t been able to find it (and it also uses CFL) and I feel a bit lost in the pile of subsections and parameters. Any help would be appreciated.  
Thanks!  
Neta  
[drex.prm](https://community.geodynamics.org/uploads/short-url/6yAmok8AGJNchxli8jWOdMCAO9r.prm) (5.1 KB)

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**Author:** ![daniel.douglas](https://yyz2.discourse-cdn.com/flex036/user_avatar/community.geodynamics.org/daniel.douglas/32/2126_2.png) [@daniel.douglas](https://community.geodynamics.org/u/daniel.douglas)\
**Post date:** [November 24, 2024, 3:43pm UTC](https://community.geodynamics.org/t/controlling-time-step/3751/2 "2024-11-24T15:43:57Z")

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Hey Neta,

Thanks for posting on the forum! Navigating the manual can certainly be overwhelming for a code as big as ASPECT 😅. It sounds like the parameters that you’re interested in are actually [Global parameters](https://aspect-documentation.readthedocs.io/en/latest/parameters/global.html), specifically the parameters `Maximum time step size` and perhaps also `Maximum first time step size`. The `CFL number` parameter can also be changed here, though you shouldn’t need to adjust it if you tweak these two parameters.

Cheers,  
Daniel

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**Author:** ![Neta](https://avatars.discourse-cdn.com/v4/letter/n/91b2a8/32.png) [@Neta](https://community.geodynamics.org/u/Neta)\
**Post date:** [November 25, 2024, 9:36am UTC](https://community.geodynamics.org/t/controlling-time-step/3751/3 "2024-11-25T09:36:48Z")

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Hi Daniel,  
Thank you very much for your quick and kind response. I feel privileged to receive a reply from someone like you! This is exactly what I’ve been looking for.  
Thanks!!!  
Neta

‫בתאריך יום א׳, 24 בנוב׳ 2024 ב-17:54 מאת ‪Daniel Douglas via Computational Infrastructure for Geodynamics‬‏ \<‪[notifications@geodynamics.discoursemail.com](mailto:notifications@geodynamics.discoursemail.com)‬‏\>:‬