[petsc-users] Non-uniform Cartisian Grid

[Zhenglun (Alan) Wei]

Dear folks,
     Here are several questions following up my previous ones:
1, I modified my code based on ex49.c. However, I found that if I don't 
call 'DMDASetUniformCoordinates' before calling 'DMDAVecGetArray', then 
an error comes up:
[0]PETSC ERROR: Null argument, when expecting valid pointer!
[0]PETSC ERROR: Null Object: Parameter # 2!
     The code do works if I add a call for 'DMDASetUniformCoordinates' 
in it. However, I feel it is a waste of calling 
'DMDASetUniformCoordinates' since what follows this calling is to change 
the coordinate to a non-uniform gird. Does it have any way to bypass 
this call or you think it is fine to keep it here.

2, Also, if I do by this way, the outputted mesh looks weird, as shown 
in the figure.
     I was trying to double the dy value manually in the 
SetNonUniformGrid(). However, the only mesh changed is just on the z=0 
and x=0 plane. Here I attached my code. Do I need to do anything else 
except DMDAVecRestoreArray() after I modified the coordinate information?

thanks,
Alan


On 2/26/2013 8:46 PM, Barry Smith wrote:
> On Feb 26, 2013, at 4:49 PM, "Zhenglun (Alan) Wei"<zhenglun.wei at gmail.com>  wrote:
>
>> Dear folks,
>>       I have one question for the ex49.
>>       It do show a way to 'get' the 'coord' by the 'DMDAVecGetArray'. Should I just modified the coordinate on the 'coord' in order to obtain a non-uniform grid? and then, use DMDAVecRestoreArray to link the modified 'coord' back to 'cda' so that the 'coord' will be updated to the mesh I want?
>     Yes, you can do this.
>
>
>> thanks,
>> Alan
>>
>> On 2/22/2013 3:20 PM, Matthew Knepley wrote:
>>> On Fri, Feb 22, 2013 at 3:57 PM, Zhenglun (Alan) Wei<zhenglun.wei at gmail.com>  wrote:
>>> Dear all,
>>> I hope you're having a nice day.
>>> I wonder if anyone could point out a example for me about the
>>> DMDASetCoordinates(). I found this
>>> (http://www.mcs.anl.gov/petsc/petsc-current/src/dm/impls/da/dacorn.c.html#DMDASetCoordinates)
>>> online; however it does not really demonstrate how to create the
>>> 'coordinate vector - c'.
>>> Also, I found that in the petsc sub-folder:
>>> /petsc-dev/src/dm/impls/patch has a patch.c, which I suppose that it
>>> does a local refinement for mesh. Does this a mature model that we can
>>> use or just a testing code?
>>>
>>> Look at KSP ex49. In the routine DMDACoordViewGnuplot2d() it reads out the coordinates.
>>> You could just as easily set them in this fashion.
>>>
>>>     Matt
>>>   
>>> thanks,
>>> Alan
>>>
>>> On 11/7/2012 6:55 PM, Barry Smith wrote:
>>>> On Nov 7, 2012, at 6:50 PM, Alan<zhenglun.wei at gmail.com>  wrote:
>>>>
>>>>> Dear folks,
>>>>> I hope you're having a nice day.
>>>>> I'm testing the Poisson solver in /src/ksp/ksp/examples/tutorials/ex45.
>>>>> It uses the 'DMDACreate3d' to generate a Cartisian structured grid and
>>>>> solve it with uniform grid size. However, I attempt to modify it to
>>>>> obtain solution with a Cartsian structured grid and non-uniform grid (It
>>>>> is not unstructured local refined mesh). Should I still use the
>>>>> 'DMDACreate3d' and implement different grid size in 'ComputeMatrix' and
>>>>> 'ComputeRHS'? In other words, does 'DMDACreate3d' itself include any
>>>>> mesh size information?
>>>>     You can use DMDASetCoordinates() and DMDAGetCoordinates() (called DMSetCoordinates() and DMGetCoordinates() in petsc-dev) to keep track of the coordinates but you are responsible for generating the correctly scaled entries in the matrix etc.
>>>>
>>>>     Barry
>>>>
>>>>> thanks,
>>>>> Alan
>>>
>>>
>>> -- 
>>> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
>>> -- Norbert Wiener

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