[CIG-SHORT] Convergence

[Tabrez Ali]

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
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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
>>     
>
>
>