[CIG-SHORT] Convergence

[Tabrez Ali]

Charles

I am using automatic time stepping but the problem occurs even with a  
fixed time step (much smaller than the maxwell time).

I will experiment with mesh and recheck the fault parameters as Brad  
suggested.

Tabrez

On Dec 10, 2008, at 3:28 PM, Charles Williams wrote:

> 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
>>>>> http://geodynamics.org/cgi-bin/mailman/listinfo/cig-short
>>>>>
>>>> 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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