# Question about Time-Dependent Prescribed Boundary Velocities in Regional Models

**URL:** <https://community.geodynamics.org/t/question-about-time-dependent-prescribed-boundary-velocities-in-regional-models/4285>\
**Category:** ASPECT\
**Created:** [September 9, 2025, 10:12am UTC](https://community.geodynamics.org/t/question-about-time-dependent-prescribed-boundary-velocities-in-regional-models/4285 "2025-09-09T10:12:24Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![XHL-CUG](https://avatars.discourse-cdn.com/v4/letter/x/a9adbd/32.png) [@XHL-CUG](https://community.geodynamics.org/u/XHL-CUG)\
**Post date:** [September 9, 2025, 10:12am UTC](https://community.geodynamics.org/t/question-about-time-dependent-prescribed-boundary-velocities-in-regional-models/4285/1 "2025-09-09T10:12:25Z")

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Hello,

I am currently studying the tutorial on [Prescribing Velocities from Global Convection Models as Boundary Conditions in Regional Models — ASPECT 3.1.0-pre](https://aspect-documentation.readthedocs.io/en/latest/user/cookbooks/cookbooks/global_regional_coupling/doc/global_regional_coupling.html). and I have a question regarding time-dependent boundary velocity input.

Actually, I have already successfully run the example model. The `regional_velocity_files` directory contains four files (e.g., `chunk_3d_east.0.txt`), which appear to provide velocity data only for the initial time step (`t = 0`).

```auto
subsection Boundary velocity model
  set Tangential velocity boundary indicators = outer
  set Prescribed velocity boundary indicators = south:ascii data, west:ascii data, east:ascii data, north:ascii data
  subsection Ascii data model
    set Data directory = $ASPECT_SOURCE_DIR/contrib/python/scripts/regional_velocity_files/
    set Data file name = chunk_3d_%s.%d.txt
  end
end

```

My question is: **How can I extend this setup to provide time-varying boundary velocities over the full duration of a long simulation (e.g., end time = 200e3 years)?**

Specifically:

- Should I generate multiple data files with different time step indices (e.g., `chunk_3d_east.0.txt`, `chunk_3d_east.1.txt`, etc.)?

- Or, could I instead use a single file (e.g., rename `chunk_3d_east.0.txt` to `chunk_3d_east.txt`)

Any help is appreciated.

Thanks,

Haolin

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<div class="post-metadata">

**Author:** ![jbnaliboff](https://avatars.discourse-cdn.com/v4/letter/j/58956e/32.png) [@jbnaliboff](https://community.geodynamics.org/u/jbnaliboff)\
**Post date:** [September 9, 2025, 4:06pm UTC](https://community.geodynamics.org/t/question-about-time-dependent-prescribed-boundary-velocities-in-regional-models/4285/2 "2025-09-09T16:06:10Z")

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Hi @XHL-CUG,

Thank you for posting your question to the forum.

Here are a few links to documentation and examples on how to implement time-dependent velocity boundary conditions with the ascii data approach:

- Manual entry for the [Boundary velocity model / Ascii data model](https://aspect-documentation.readthedocs.io/en/latest/parameters/Boundary_20velocity_20model.html#subsection-boundary-velocity-model-ascii-data-model) - the key information is under the [Data file name](https://aspect-documentation.readthedocs.io/en/latest/parameters/Boundary_20velocity_20model.html#parameter-name-data-file-name), [Data file time step](https://aspect-documentation.readthedocs.io/en/latest/parameters/Boundary_20velocity_20model.html#parameter-name-data-file-time-step), and associated parameters within this subsection.
- The test cases [ascii\_data\_boundary\_velocity\_2d\_box\_time.prm](https://github.com/geodynamics/aspect/blob/main/tests/ascii_data_boundary_velocity_2d_box_time.prm) and [ascii\_data\_boundary\_velocity\_2d\_box\_time\_non\_equidistant.prm](https://github.com/geodynamics/aspect/blob/main/tests/ascii_data_boundary_velocity_2d_box_time_non_equidistant.prm)

Please let us know if you have additional questions after reviewing this information.

Cheers,

John

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<div class="post-metadata">

**Author:** ![XHL-CUG](https://avatars.discourse-cdn.com/v4/letter/x/a9adbd/32.png) [@XHL-CUG](https://community.geodynamics.org/u/XHL-CUG)\
**Post date:** [September 10, 2025, 7:16am UTC](https://community.geodynamics.org/t/question-about-time-dependent-prescribed-boundary-velocities-in-regional-models/4285/3 "2025-09-10T07:16:47Z")

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Hi John,

Thank you very much for your fast reply!

Sorry for my delayed response. Thank you for providing the links. I think I have solved the problem, but a new one has arisen.  
I’d like to use the boundary velocity (calculated by `extract_local_velocity.py`) in a box model. To do this, do I only need to convert the coordinates, or do I also need to transform the three velocity components? I’m a bit unsure how to handle the vector components correctly during this transformation.

Thanks,

Haolin

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<div class="post-metadata">

**Author:** ![jbnaliboff](https://avatars.discourse-cdn.com/v4/letter/j/58956e/32.png) [@jbnaliboff](https://community.geodynamics.org/u/jbnaliboff)\
**Post date:** [September 11, 2025, 4:28am UTC](https://community.geodynamics.org/t/question-about-time-dependent-prescribed-boundary-velocities-in-regional-models/4285/4 "2025-09-11T04:28:53Z")

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Hi @XHL-CUG,

Great to see someone is already using `extract_local_velocity.py`!

To your question, I will need to think a bit about what the best approach is to extract and project data from a spherical model onto the boundaries of a cartesian model. You could in theory do this all within ParaView or as an additional post processing (projection) step.

@daniel.douglas - Thoughts on this?

Cheers,

John