[CIG-SHORT] Convergence

[Charles Williams]

Hi Tabrez,

Actually, aspect ratios of 50+ sound pretty high to me, although that  
may not be your problem.  The other question is what sort of Maxwell  
times you have compared to your time step size.  Are you using  
automatic or fixed time stepping?

Charles


On Dec 11, 2008, at 5:49 AM, Tabrez Ali wrote:

> Brad/Charles
>
> Thanks for your reply. Yes the residuals do increase after each time
> step. I have a low dipping fault which curves from very shallow dip to
> somewhat steeper dip (as in shallow subduction).
>
> If I specify slip on the steeper part then things work fine. Things  
> also
> work fine when I dont specify slip on this particular fault but have
> other faults/BC's in place (on the same mesh).
>
> Its slip on the shallow part which seems to mess it up.
>
> Also my mesh quality isnt that bad. I do have a few elements with  
> aspect
> ratios of 50+ (in cubit) but nothing greater than 100. And my material
> property for the fault is the same as used in the examples/benchmarks.
>
> Regards
> Tabrez
>
> Brad Aagaard wrote:
>> Tabrez-
>>
>> Does the solution blow up (residual increases) or just fail to  
>> converge
>> (residual approaches some value)? If the solution blows up,  
>> something is
>> wrong. Either the problem is not setup correctly or there is a bug.
>>
>> There are a number of things that can cause the solution to  
>> converge very
>> slowly. In addition to the mesh quality issue that Charles pointed  
>> out, some
>> other things you might look into include:
>>
>> (1) Do you get the same behavior with a coarser mesh?
>>
>> (2) Does the solution converge when you omit the fault in the  
>> parameters
>> (i.e., use the same mesh but don't have any fault interface  
>> conditions)? If
>> you have zero displacement BC, try simple compression without a  
>> fault.
>>
>> (3) What is Poisson's ratio? Do any cells have a Poisson ratio  
>> greater than
>> 0.45 (only values greater than about 0.48 should cause problems)?
>>
>> (4) What physical properties are you using for the fault (this  
>> affects the
>> conditioning of the system and can affect convergence if they are not
>> reasonable)? In a test problem, the solution converged slightly  
>> faster for
>> uniform physical properties for the fault (only used to condition  
>> the system)
>> compared with the actual 3-D variation.
>>
>> Brad
>>
>>
>> On Wednesday 10 December 2008 12:05:31 am Charles Williams wrote:
>>
>>> I would look at the element quality in the mesh.  You can do this in
>>> ParaView.  The low angles may be giving you poorly-formed elements,
>>> and this could cause problems.  I doubt that incorrect fault
>>> parameters is causing the problem, unless you're doing something
>>> that's giving you very large strains.
>>>
>>> Charles
>>>
>>> On Dec 10, 2008, at 6:43 PM, Tabrez Ali wrote:
>>>
>>>> Brad/PyLith Users
>>>>
>>>> It seems that whenever I use a low angle thrust fault (very low  
>>>> dip)
>>>> the
>>>> solution (to the quasi-static problem) after a certain number of  
>>>> time
>>>> steps fails to converge and blows up. Can wrong fault parameters
>>>> such as
>>>> up_dir or normal_dir also cause this?
>>>>
>>>> Regards
>>>> Tabrez
>>>> _______________________________________________
>>>> CIG-SHORT mailing list
>>>> CIG-SHORT at geodynamics.org
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>>>>
>>> Charles A. Williams
>>> Scientist
>>> GNS Science
>>> 1 Fairway Drive, Avalon
>>> PO Box 30368
>>> Lower Hutt  5040
>>> New Zealand
>>> ph (office): 0064-4570-4566
>>> fax (office): 0064-4570-4600
>>> C.Williams at gns.cri.nz
>>> NOTE NEW E-MAIL ADDRESS
>>>
>>
>>
>>
>
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Charles A. Williams
Scientist
GNS Science
1 Fairway Drive, Avalon
PO Box 30368
Lower Hutt  5040
New Zealand
ph (office): 0064-4570-4566
fax (office): 0064-4570-4600
C.Williams at gns.cri.nz
NOTE NEW E-MAIL ADDRESS


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