[aspect-devel] Flow through free surface
Rene Gassmoeller
rene.gassmoeller at mailbox.org
Thu Feb 1 00:14:02 PST 2018
Hi Lev,
>
> I was wondering if you could give me an idea of how hard it would be
> to allow material flow through the free surface? Right now I'm just
> able to dampen the elevation change by applying the diffusion
> equation, but is it possible to actually take material out of the
> system? Would the hardest part be figuring out if a point is inside a
> curved domain? Or is there an inherent limitation in Aspect that makes
> it not feasible in principle?
> To rephrase what I mean: is it possible to let the material move at
> the surface with Stokes velocity, and then draw the new top boundary
> on top of it, getting rid of material that was eroded away (falls
> outside of the new model boundaries), and filling in the now less deep
> trenches with crustal material?
As Wolfgang mentioned, currently the 'free' boundary means material
flows without resistance at the surface, and the surface moves with the
same speed (so there is no differential velocity). However, I do not see
a conceptual problem with keeping the boundary open (letting velocity
flow through), and moving the surface with a different 'mesh velocity'
(whatever way that is computed). This way all material moving out of the
domain should be lost, and all material entering the domain should enter
(with temperature and composition according to the boundary condition).
I guess that this should happen already with your diffusion.
> Essentially, right now I know how to adjust the velocity of the
> surface vertices according to the solution of the diffusion equation.
> But I don't know how to superimpose the new boundary on top of the
> advected material. Is it a realistic thing to do? How would I go about
> it?
You will want to modify the constraints in free_surface.cc:378, and this
would be the 'velocity' of the boundary (i.e. if you compute the new
position of the mesh vertices, you need to divide the vertex
displacement over this timestep by the timestep length).
Actually we are working on allowing something similar (though for a
completely different application) here:
https://github.com/geodynamics/aspect/issues/2067, maybe the discussion
helps for your problem.
Cheers,
Rene
On 01/30/2018 01:49 PM, Lev Karatun wrote:
> If someone could give me an idea of whether what I'm thinking of is
> realistic or not, I'd appreciate it!
>
> Best regards,
> Lev Karatun.
>
> 2018-01-17 20:50 GMT-05:00 Lev Karatun <lev.karatun at gmail.com
> <mailto:lev.karatun at gmail.com>>:
>
> Hi Wolfgang,
>
> * You want the boundary to be free and move according to
> whatever flow you get below
>
> My understanding is that this is the current implementation of
> free surface?
>
> For any more complicated procedure, it would be useful to
> first define exactly what equations you want to solve, and
> then (and *only* then) think about how you want to solve them.
>
> What I was thinking of doing is either something as simple as
> eroding all material that is above a specified elevation (in which
> case I don't even need to solve a system of equations -- correct
> me if I'm wrong please), or solving a diffusion equation (which (I
> think) I know how to solve).
>
> Essentially, right now I know how to adjust the velocity of the
> surface vertices according to the solution of the diffusion
> equation. But I don't know how to superimpose the new boundary on
> top of the advected material. Is it a realistic thing to do? How
> would I go about it?
>
> Best regards,
> Lev Karatun.
>
> 2018-01-17 16:03 GMT-05:00 Wolfgang Bangerth
> <bangerth at colostate.edu <mailto:bangerth at colostate.edu>>:
>
> On 01/16/2018 01:53 PM, Lev Karatun wrote:
>
>
> I was wondering if you could give me an idea of how hard
> it would be to allow material flow through the free
> surface? Right now I'm just able to dampen the elevation
> change by applying the diffusion equation, but is it
> possible to actually take material out of the system?
> Would the hardest part be figuring out if a point is
> inside a curved domain? Or is there an inherent limitation
> in Aspect that makes it not feasible in principle?
> To rephrase what I mean: is it possible to let the
> material move at the surface with Stokes velocity, and
> then draw the new top boundary on top of it, getting rid
> of material that was eroded away (falls outside of the
> new model boundaries), and filling in the now less deep
> trenches with crustal material?
>
>
> Mathematically, the free boundary is of course determined just
> so that no flux goes across it.
>
> I think what you want is essentially a two-step process:
> * You want the boundary to be free and move according to
> whatever flow you get below
> * Then you want to erode away or deposit material on top of it.
>
> The easiest way to implement may be exactly to do this: do
> one; then do the other. For any more complicated procedure, it
> would be useful to first define exactly what equations you
> want to solve, and then (and *only* then) think about how you
> want to solve them.
>
> Best
> W.
>
> --
> ------------------------------------------------------------------------
> Wolfgang Bangerth email:
> bangerth at colostate.edu <mailto:bangerth at colostate.edu>
> www:
> http://www.math.colostate.edu/~bangerth/
> <http://www.math.colostate.edu/%7Ebangerth/>
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--
Rene Gassmoeller
http://www.math.colostate.edu/~gassmoel/
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