[aspect-devel] Van Keken et al. 2008 subduction benchmark

[Max Rudolph]

Jonathan,
Thanks for your help. One solution to the large pressure oscillations near
the velocity discontinuity would be to also impose lithostatic pressure in
this region. Because there is rigid body translation in both the overriding
and downgoing plate in this benchmark, this is the exact solution.

The wedge shaped mesh might be a good solution for some problems. However,
we also want to calculate a steady-state wedge temperature field. Without
solving the energy equation in the downgoing plate and overriding plate, it
won't be possible to do this.

Max



On Thu, Oct 27, 2016 at 11:03 AM, Jonathan Perry-Houts <jperryh2 at uoregon.edu
> wrote:

> Hi Max,
>
> I don't know what's going wrong, but one thing that comes to mind is
> that constraining internal velocities is a bit weird in a finite element
> sense because we're prescribing solutions on particular nodes (i.e.
> setting rows in the mass matrix to the identity, and setting the RHS in
> those rows to some arbitrary value that doesn't represent rho*g
> anymore), but the rest of the assembly is still integrating over entire
> cells and expecting the usual RHS values.
>
> The reason the internal velocities thing is only described in the
> cookbook and not included in the code is because the number of ways that
> could go wrong (understandably) made Wolfgang nervous. It could be that
> the discontinuous pressure elements in the "use locally conservative
> discretization" option just make it easier to run into cases where one
> of the assumptions here is violated.
>
> Another way you might try the Van Keken benchmark would be making a
> geometry model (mesh) with one 45 degree angle, and just prescribing
> boundary conditions rather than internal velocities. I vaguely remember
> hearing Wolfgang say he had tried something like this and found that
> quadrilateral elements didn't work so well, though.
>
> -JPH
>
> On 10/27/2016 09:13 AM, Max Rudolph wrote:
> > I was wondering if anyone has been successful in setting up the Van
> > Keken et al. 2008 subduction benchmark using ASPECT? I have been using
> > this as a starting point to set up a more complicated corner flow model
> > and have not yet been successful.
> >
> > This benchmark solution requires a rigid overriding plate and kinematic
> > subducting plate. There is a discontinuity along the subduction
> > interface. I tried imposing kinematic boundary conditions on the left
> > and bottom of the box and using Jonathan Perry-Houts' prescribed
> > velocity plugin (cookbooks/prescribed_velocity), but the Stokes solver
> > fails to converge. Looking at the pressure field, it's not surprising to
> > see very large oscillations in this region where the velocity field is
> > discontinuous. It occurred to me that using the locally conservative
> > discretization might help. However, when the locally conservative
> > discretization is enabled, the internal prescribed velocities are no
> > longer enforced anywhere. Is it obvious why this might be the case?
> >
> > Regards,
> > Max
> >
> >
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