[aspect-devel] Partial velocity boundary conditions

[Rene Gassmoeller]

Hello Lev,
some of your questions have been discussed in a few github issues and
pull requests, but not directly in this form, so do not worry. The
github page is in general a good place to keep up to date with the
latest additions, because new features are often discussed there rather
than on the mailing list (e.g.
https://github.com/geodynamics/aspect/pulls or
https://github.com/geodynamics/aspect/issues).

Anyway, to your questions:

> I'm writing a velocity boundary conditions plugin, and  I was wondering if
> it's possible to only prescribe a certain component of velocity (say, along
> the x axis), leaving the other ones (y and z) unrestricted. Would it be a
> valid plugin?

It would be. In fact you do not need to change your plugin, because the
main code can decide to use a certain plugin only for some components of
the velocity. You can change the selection of velocity boundary
conditions into something like the following:

set Prescribed velocity boundary indicators = left x: your_plugin, top
xy: another_plugin

This is described in more detail in the documentation of the 'Prescribed
velocity boundary indicators' in the manual.

> Also, is it possible to prescribe some velocity to a part of a side wall,
> and have free slip for the rest of it (mix 2 types of b.c.)?

This is a bit more tricky, because it requires the geometry model to
provide two different boundary ids for a boundary (boundary conditions
are precsribed to boundaries according to their id or name). It will
probably require writing a new geometry model. In case you are looking
for a box with two different boundary conditions at the sides of the
box, take a look at the 'box with lithosphere boundary indicators'
geometry model of the development version (contributed by Anne Glerum).
This one essentially stacks two box geometries, and calls the upper part
of the boundary 'left/right lithosphere' and the lower part 'left/right'
as usual. These are just names though, you can adjust the depth of the
transition of the boundaries.

Hope that helped, let us know, if you have further questions.
Best regards,
Rene

> 
> Thanks in advance!
> 
> Best regards,
> Lev Karatun.
> 
> 
> 
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