[aspect-devel] internal velocity boundary conditions

[Timo Heister]

Magali,

this might not quite be what you want, but my student Ryan just
implemented a feature where you can turn off solving for the Stokes
system and supply a given velocity and pressure in the whole domain.
So, if you are able to describe the velocity as a function of the
location, you can run your model like this for a certain amount of
time before you snapshot and switch to a normal computation where
velocity is driven by temperature differences again.

Internal boundary conditions are a bit more complicated, especially
because you need to be careful to pose them in a compatible way.

On Tue, May 12, 2015 at 8:49 PM, Jonathan Perry-Houts
<jperryh2 at uoregon.edu> wrote:
> I actually had this on my list of hackathon items as well. It would be
> very useful.
>
> On 05/12/2015 05:46 PM, Thorsten Becker wrote:
>> Likewise not at the hackathon myself, but this exact application (for
>> the Tohoku setting) would be of great interest to my group as well...
>>
>> Thanks!
>>
>> Thorsten W Becker
>> geodynamics.usc.edu <http://geodynamics.usc.edu/~becker>
>>
>> On Tue, May 12, 2015 at 5:43 PM, Magali Billen <mibillen at ucdavis.edu
>> <mailto:mibillen at ucdavis.edu>> wrote:
>>
>>     Hello Everyone,
>>     Unfortunately I’m not going to make it to Hackathon to ask this
>>     question in person, but this way everyone
>>     can read (ignore?) and hear or contribute to the response.
>>
>>     The short form of my question is: Is it possible to define
>>     “internal” velocity boundary conditions in Aspect: that is
>>     can I fix the velocity at nodes inside the model domain? If the
>>     answer is yes, can someone comment on the basic steps
>>     that would be needed (e.g., need to flag these nodes as “boundary
>>     conditions” so they get handled properly during assembly of the
>>     solution matrices, then assign velocities,…).
>>
>>     Here is the background for why I ask this question:
>>
>>     One of the projects we are starting to develop in Aspect in my group
>>     is instantaneous models for a specific
>>     subduction zones. The key issue with these models is that we need to
>>     define a starting thermal structure
>>     that is based on the observed geometry of the subducted plate (e.g.,
>>     from seismicity). There are different
>>     was to do this, and I’ve done several of them for previous models
>>     completed using Citcom.
>>
>>     Based on that experience, and given the AMR capabilities of Aspect,
>>     I think the best (most accurate and easiest) way to
>>     define the starting thermal structure is to run model in which you
>>     have defined the surface of the plate
>>     INSIDE the model (going down into the mantle) and then define fixed
>>     velocities associated with this surface.
>>
>>     Note it is not necessary for the elements to conform to this surface
>>     (no distortion of the grid), we can use refinement of the grid to
>>     get accurate enough for our purposes.
>>
>>     Once the velocity conditions inside the mesh are defined together
>>     with the normal external boundary conditions and an initial
>>     temperature structure for the plates at the actual top of the mesh,
>>     then we would run this model forward in time to kinematically
>>     “subduct” the plate. This will allow us to create a smooth 3D
>>     starting temperature models for our instantaneous dynamically-driven
>>     models that follows the observed shape of the slab.
>>
>>     In addition to using this capability for the purpose describe above,
>>     this would allow Aspect to also run what is commonly referred to as
>>     “mantle wedge thermal models”, in which the subducted plate and
>>     overriding plates are really used as boundary conditions on the
>>     flow/temperature in the mantle between them. These models are
>>     commonly used to look at the detailed thermal structure and melting
>>     in the mantle wedge.
>>
>>     Cheers,
>>     Magali
>>
>>     --------------------------------------------------
>>     Professor of Geophysics & UCD Chancellor Fellow
>>     Chair, Geology Graduate Program
>>     Earth & Planetary Sciences Dept., UC Davis
>>     Davis, CA 95616
>>     2129 Earth & Physical Sciences Bldg.
>>     Office Phone: (530) 752-4169 <tel:%28530%29%20752-4169>
>>     http://mygeologypage.ucdavis.edu/billen/
>>     --------------------------------------------------
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
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>>
>>
>>
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-- 
Timo Heister
http://www.math.clemson.edu/~heister/