[aspect-devel] Velocity boundary conditions - lithosphere ext/compression

Tue Jul 4 13:03:37 PDT 2017

Hi Philip,

I ran the model and the velocities certainly are showing interesting 
behavior. Over time the velocity magnitudes increase, but the flow 
pattern is basically reversing at each visualized output (1 Myr intervals).

While the velocity boundary conditions are constant, this change in flow 
pattern appears to reflect that different portions of the lithosphere 
(sides vs interior) are alternating between moving and up and down. 
These vertical motions are swamping out the signal from the applied 
velocity boundary conditions along the side walls.

In some ways this kind of behavior looks similar to "drunken seaman" 
behavior (instability associated with free surface), but I don't think 
that is the case. Rather, I think the model is having trouble to 
localize large-scale deformation in the lithosphere and this is part of 
the response. Significantly, I noticed that in the parameter file the 
velocities may be a bit lower than intended. The variable "cm" is set to 
"0.001". With the other specified values this gives a convergence 
velocity of 0.1 cm/year applied on each side in the upper 120 km. This 
value is quite low for continental convergence and I think if you 
increase this value to 1 cm/year it may eliminate some of the anomalous 
behavior. I would also try decreasing the CFL number to 0.2 or 0.1.

Aside from trying those fixes, may I ask if there is a specific end goal 
of these models? In other words, is this experiment designed to produce 
a model of long-term continental shortening? If so, different initial 
conditions may help.

If the goal is indeed to model some form of long-term convergence 
(continental collision, subduction, ...), is there a particular prior 
study (or studies) you have in mind? Always happy to help reproduce 
prior work, which can then be added as benchmarks cases to the ASPECT 
repository. A number of us are currently working on subduction problems, 
so lots of information, ideas, tips and parameter files to share.

A note to all new or potential ASPECT users - the offer to help with 
reproducing prior studies or discussing ideas for new setups is open to 
everyone.

Cheers,
John

*************************************************
John Naliboff
Assistant Project Scientist, CIG
Earth & Planetary Sciences Dept., UC Davis

On 07/04/2017 07:36 AM, HERON, PHILIP J. wrote:
>
> Hi John,
>
>
> thanks for the reply - I've attached the log.txt for the prm file I 
> sent, along with the error output.
>
>
> I have tried and played around with a great number of parameters to 
> try and get this to work, but I think it is good to get some outside 
> perspective.
>
>
> Point 1 - thanks for that, I've applied that change to the models.
>
>
> Point 2 - the vertical sides seems to be immune to the fluctuating 
> velocity, the interior of the model has the blow up in velocity.
>
>
> Point 3 - I'm just running a fixed mesh model now to see where it gets 
> to. Will keep you posted.
>
>
>
> Furthermore, I'm going to run a series of tests on reference viscosity 
> after noticing this post:
>
>
> https://github.com/geodynamics/aspect/issues/1814
>
> Issues with linear solver convergence due to choice of Reference 
> viscosity · Issue #1814 · geodynamics/aspect 
> <https://github.com/geodynamics/aspect/issues/1814>
> github.com
> I've had some issues with choosing the "right" reference viscosity (in 
> models with varying viscosity) in order to get convergence of the 
> linear system and was wondering if there might be some way o...
>
>
>
>
>
>
> Philip J. Heron
> Junior Research Fellow
> Dept. of Earth Sciences
> Durham University
> web: http://philheron.com <http://philheron.com/>
>
> ------------------------------------------------------------------------
> *From:* Aspect-devel <aspect-devel-bounces at geodynamics.org> on behalf 
> of John Naliboff <jbnaliboff at ucdavis.edu>
> *Sent:* 04 July 2017 01:22:47
> *To:* aspect-devel at geodynamics.org
> *Subject:* Re: [aspect-devel] Velocity boundary conditions - 
> lithosphere ext/compression
> Hi Philip,
>
> Yes, certainly happy to help on this front! A few things to discuss.
>
> 1. Unrelated to your issue, I completely forgot that there is a small 
> bug with the heat production term in this cookbook that Anne Glerum 
> pointed out to me a week or two ago. This will cause the crust to cool 
> more rapidly than it should over time, but it is not readily apparent 
> over 10's of Myr with advection on. This is unrelated to the anomalous 
> velocities in your model, but I will push a fix for this this evening 
> or tomorrow morning. Apologies to all who are currently using the 
> cookbook example. The quick fix is to change the "Initial 
> concentrations crust" parameter from 1 to 1e6. Alternatively, the 
> "fix" I'll push tomorrow will allow you to set different constant 
> heating rates for each compositional layer (see "compositional 
> heating" method).
>
> 2.Onto your issue. I've run a similar extension model (1000 x 600 km) 
> with similar boundary conditions along the sides (balanced inflow 
> outflow) and the velocity field was stable over time. Are your 
> horizontal velocities fluctuating throughout the model or near the 
> vertical sides?
>
> 3. In similar models I used a fixed mesh (no AMR). Have you tried the 
> model with a fixed mesh? Very likely to be unrelated, but might be 
> worth using a fixed mesh to see if the error occurs at a similar time 
> and place in the model.
>
> 3. I'm running your parameter file now, but in the meantime can you 
> send the log.txt file from the output folder?
>
> Regardless of the root issue, we can get this worked out quickly. If 
> the issue is not readily apparent, we can start from a similar working 
> input file and compare the differences.
>
> Thanks for sending in this question to the "devel" email list. Really 
> helpful for others to see where issues are cropping up!
>
> Cheers,
> John
> *************************************************
> John Naliboff
> Assistant Project Scientist, CIG
> Earth & Planetary Sciences Dept., UC Davis
> On 07/03/2017 08:14 AM, HERON, PHILIP J. wrote:
>>
>> Hello all,
>>
>>
>> I was just extending the 'continental_extension.prm' cookbook to 
>> include sub-lithosphere mantle, and have ran into issues of 
>> convergence. I wonder if anyone on the group could help!
>>
>>
>> I've attached the extended cont extension model, I was wondering if 
>> anyone could point me in the right directions of how to get it to 
>> converge. The issue is that after 10-15 timesteps velocities within 
>> the model start to blow up.
>>
>>
>> I feel that the temperature profile initially is fine. Although 
>> applying a variable horizontal velocity to the boundary, the initial 
>> horizontal velocity field is smooth. However, almost immediately the 
>> horizontal velocity starts to fluctuate.
>>
>>
>> Are there better techniques to be able to get this  model to 
>> converge? Am I missing something simple? I've played around with a 
>> few different things but with little luck.
>>
>>
>> Thanks in advance!
>>
>>
>> Phil
>>
>>
>>
>>
>>
>>
>>
>> Philip J. Heron
>> Junior Research Fellow
>> Dept. of Earth Sciences
>> Durham University
>> web: http://philheron.com <http://philheron.com/>
>>
>>
>>
>> _______________________________________________
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>> Aspect-devel at geodynamics.org
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>
>
>
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