[Paraview] extract surface of multiblock mesh

Mon Apr 6 19:41:45 EDT 2009

Berk, forgive for delaying my response (I was out of my lab...)

Just a silly question:

What do you exactly mean by "Ghost cell"? This silly question is justified
by the fact that I thought that ghost cells only make sense when we have
subdomains _overlapped_. In these cases, the number of overlapped cell
layers should (or could) be considered as "ghost level".

In my case, the subdomains are not overlapped at all. They are just
partitioned by Metis and the solver can deal with the computations without
the need to use overlapped elements. Thus, if "ghost cells" can be simply
defined by "cells somehow touching the parallel interface (even those not
overlapped)" I think I could try tagging my parallel interface cells as
"ghost level 0" to see what happens...

Renato.

On Sun, Apr 5, 2009 at 11:53 PM, Chris Kees <
christopher.e.kees at usace.army.mil> wrote:

> Berk,
>
> Thanks for correcting me. I'll try adding the vtkGhostLevels to the XDMF
> and see how that goes.
>
> Chris
>
> On Apr 4, 2009, at 2:50 PM, Berk Geveci wrote:
>
>  The right way to deal with this situation is to mark the ghost cells
>> as ghost. If you create a cell array called vtkGhostLevels and assign
>> 0 to inside cells and 1 to ghost cells, you should not need
>> MergeBlocks or CleanToGrid. Note that this array has to be of type
>> unsigned char. There are actual benefits to keeping ghost levels since
>> some algorithms will produce better results.
>>
>> -berk
>>
>> On Thu, Apr 2, 2009 at 6:37 PM, Chris Kees
>> <christopher.e.kees at usace.army.mil> wrote:
>>
>>> I turned off the overlapping domain decomposition (ghost cells) for a
>>> simple
>>> problem and the sequence
>>>
>>> MergeBlocks->CleantoGrid->ExtractSurface->Clip
>>>
>>> shows just the physical boundary of the problem (clipped open so you can
>>> see
>>> inside). Also volume visualization and streamline calculation works with
>>> no
>>> processor boundary artifacts.
>>>
>>> From what I understand, there are no filters in paraview or abstractions
>>> in
>>> the XDMF data model  at this time that will allow paraview to read in
>>> overlapping blocks and really make use of the ghost cells correctly. For
>>> now
>>> truncating our output to only "owned" elements will solve our problems.
>>> Thanks again for the help.
>>>
>>> Chris
>>>
>>> On Mar 30, 2009, at 2:06 PM, Chris Kees wrote:
>>>
>>>  Thanks for  the help. I also tried suggestions from Paul, Ken, and Berk,
>>>> but it does seem that I'm stuck right now unless I provide ParaView with
>>>> more information. Since streamlines are computed correctly on the
>>>> current
>>>> multiblock mesh I just generated the mesh on a single processor and used
>>>> ExtractSurface->Clip on that mesh to visualize the geometry around the
>>>> streamlines from the multiblock grid.
>>>>
>>>> On the first method: Each of my UnstructuredGrids in the Multiblock Grid
>>>> is a subdomain in an overlapping decomposition of the domain. Each of
>>>> the
>>>> subdomains has several elements of overlap (the layer of ghost cells is
>>>> more
>>>> than one element thick).  Presumably the streamline generation works now
>>>> on
>>>> the multiblock grid because the overlap is loaded into ParaView. Is
>>>> there a
>>>> way I can just set a cell-centered attributed to identify the ghost
>>>> cells so
>>>> that surface extraction and volume visualization will work too?
>>>>  Currently
>>>> volume visualization of the multiblock grid shows only a single
>>>> subdomain
>>>> and volume visualization after MergeBlocks shows the whole domain but
>>>> with
>>>> overlap regions being more opaque.
>>>>
>>>> On your other method, we have both the external boundary mesh and a
>>>> pre-mesh polygonal representation of the boundaries available in the
>>>> simulator. You are suggesting that I just dump one of those to a valid
>>>> ParaView format as well, is that correct?
>>>>
>>>> Chris
>>>>
>>>> On Mar 30, 2009, at 9:14 AM, Jean Favre wrote:
>>>>
>>>>  Chris Kees wrote:
>>>>>
>>>>>>
>>>>>> So far I've tried MergeBlocks->ExtractSurface->FeatureEdges->Clip and
>>>>>> various permutations that I've seen in previous posts and the wiki,
>>>>>> but I always end up with the  surfaces on the interior of the tank as
>>>>>> if it still sees each subdomain as a closed surface.
>>>>>>
>>>>>
>>>>> In fact, it seems to me that ParaView does the best it can. Your
>>>>> unstructured mesh is partitioned in 512 pieces and [presumably], you
>>>>> did
>>>>> not specify ghost-cells at the partition boundaries. Without
>>>>> ghost-cells, ParaView has no information to help decide whether an
>>>>> outside face looks towards the outside world, or to another partition.
>>>>> I
>>>>> don't think any combination of filters would help you. Removing
>>>>> duplicate points may only remove duplicate fake boundaries, but these
>>>>> fake boundaries must be removed all together.
>>>>>
>>>>> I use two methods to achieve what you want. Ghost-cells, or another
>>>>> multi-piece object containing the different boundary types (solid,
>>>>> symmetries, inflow, outflow, etc) stored as vtkPolyData. These are read
>>>>> in from the models on disk.
>>>>>
>>>>> Jean --
>>>>> Swiss National Supercomputing Center
>>>>>
>>>>>
>>>>>
>>>>>
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