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

Thieulot, C.A.P. (Cedric) c.thieulot at uu.nl
Fri Dec 9 13:32:48 PST 2016

I believe that Scott’s advice still holds: if you look at subsection 2.3 of the paper, you’ll see that along the interface
between downgoing plate and overriding place there is no velocity discontinuity.

[cid:83FFAEC2-5940-4970-B26E-88C7176B394B at lan]

On 09 Dec 2016, at 20:12, Max Rudolph <maxwellr at gmail.com<mailto:maxwellr at gmail.com>> wrote:

Scott and Wolfgang,
I did not try the modification to allow a linear ramp. The problem that I encountered was not associated with the pressure field in the wedge. It was actually along the subduction interface where the downgoing plate meets the over-riding plate. There is a discontinuity in the velocity field there. The velocity field is locally divergence-free but it may not be divergence-free in the integral sense when interpolated using the finite element basis functions. One possible work-around would be to prescribe zero dynamic pressure throughout the rigid downgoing and over-riding plate (the correct solution). Doing so would mean prescribing all of the degrees of freedom for every node in the prescribed velocity region. I think that this would be a very straightforward modification of the prescribed velocity plugin. Maybe we can chat for a few minutes about this at the ASPECT dinner?


On Fri, Dec 9, 2016 at 1:22 AM, Scott King <sdk at vt.edu<mailto:sdk at vt.edu>> wrote:

Do you use a linear ramp something from the point where the slab meets the over-riding plate down into the wedge?   This *smooths* the discontinuity (at least as far as the wedge sees it).  With this the pressure fields in the wedge seems to become more smooth for many codes.   I always had to you a much larger "ramp" (25,-50 km) because my elements were not refined as much as Herr. Dr. Prof. van Keken.   This puts the discontinuity into the part of the domain where the velocities are entirely prescribed.  It seems to be ok for some codes because they don't "solve" any equation when all the velocities are prescribed.  Years of arguing and twiddling.


On Dec 8, 2016, at 10:25 PM, Wolfgang Bangerth <bangerth at tamu.edu<mailto:bangerth at tamu.edu>> wrote:

> On 10/27/2016 10:13 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?
> Max -- I haven't tried implementing the benchmark in ASPECT, but recall trying to do so in an early version of step-32 when I visited Scott in Blacksburg for the very first time many many years ago. I remember it to be very difficult because the benchmark definition really doesn't fit the finite element method.
> One of the issues with the use of the prescribed velocity is that the prescribed velocity needs to be divergence free on each cell. Is it, in your case? Even on cells where you only prescribe values at some nodes?
> Best
> W.
> --
> ------------------------------------------------------------------------
> Wolfgang Bangerth          email:                 bangerth at colostate.edu<mailto:bangerth at colostate.edu>
>                           www: http://www.math.colostate.edu/~bangerth/
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