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July 23, 2009 12:24am UTC

exporting data from the boundary

Hi,

I am wondering if anyone exported somehow in COMSOL or postprocessed in (COMSOL with) MATLAB the results from an interior boundary (or from a particular boundary). If you could give me an advice on how to access data from a specific boundary, I would very much appreciate it.

Thank you,
Noemi

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July 23, 2009 4:23pm UTC in response to R. Faycal Hamou

Re: exporting data from the boundary

Yeah, I know that. Unfortunately I need to export this data OUT from COMSOL so that I can import it in COMSOL with MATLAB and use it for another model as a boundary condition information.

Thanks!
Noemi

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July 23, 2009 6:10pm UTC in response to Sumedh Joshi

Re: exporting data from the boundary

You are right, but my problem is that my boundary is a little bit complicated hence I cannot simply specify the points p from the boundary to use postinterp(fem, 'u', p), ... unless there is a way to extract all the points from the boundary?

Best,
Noemi

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September 2, 2009 7:29pm UTC in response to N Z

Re: exporting data from the boundary

Hi, how is it progressing ?

in the mean time I have also been stuck on how to easily access a "named" boundary from within matlab/script.

This gives me the opportunity to correct one comment I made earlier, you do not need to change any boundary property "just a little" to make a named group, it's in fact much easier to go to the group window, select new and define in a new name, and then return to the element selection window, select your items and add them to the new group.

To access the group indices from matlab was slightly trickier, but once I have found the way around, its rather "logical"

in the example below I have 2 application modes, I have defined a boundary (surface) of a 3D suctural model with a given name and then I use this for my plots:

I'm running V3.5a, have heard it will change and be much easier in V4, hope this is so,

my 'TopZernikeSurf' surface definition, from the history file is:

bnd.name = {'','Bottom_Surf','TopZernikeSurf'};
bnd.ind = [1,1,2,2,3,3,1,2,3,2,3,1];
appl.bnd = bnd;

and later I'm plotting with something like:

postcrossplot(fem, 2, [5,6,9,11], 'surfdata','z11*1e6', ...

Ideally I would like to write:

postcrossplot(fem, 2, 'TopZernikeSurf', 'surfdata','z11*1e6', ...

but this is not possible in v3.5a

so with some fiddling with matlab cells I get this to work:

postcrossplot(fem, 2, [find(fem.appl{1}.bnd.ind==strmatch('TopZernikeSurf',fem.appl{1}.bnd.name))], 'surfdata','z11*1e6', ...

in fact you can check that:

[5,6,9,11] = [find(fem.appl{1}.bnd.ind==strmatch('TopZernikeSurf',fem.appl{1}.bnd.name))]

I'm matching the string 'TopZernikeSurf' to the content of "fem.appl{1}.bnd.name" as I'm working on the 2D surfaces of a 3D body (you must adapt the names if you are running on other boundaries types) this gives me the index value to compare to the index list "fem.appl{1}.bnd.ind", and the "find" extracts the indexed list values I need. Note that the {1} is there because I have 2 different application modes and I'm addressing the first one, I expect it would also work if you have only one application, or you might need to smply drop the {1}.

I haven’t had much time to cross check how stable this is, certainly you must return to the GUI to redefine the elements of the group if you modify the geometry and rerun the calculation, but normally I do not expect to have to do this if the geometry is not changed.

Hope this might help you too

Ivar

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September 29, 2009 9:06am UTC in response to Ivar K

Re: exporting data from the boundary

Hi Ivar,

I've too have worked with similar questions in mind. I've found when modeling from the command prompt the optional output from geomcsg is very usful.

Imagine I have a complete geometry defined as a solid object;

>>fem.draw.s.objs = mygeom;

Then I have certain area of interest, which might be made up of several boundaries in the analyzed geometry. If I add an extra draw object for the area of interest I can later know the indices of boundaries making up this area. The draw object is not necessary to define the geometry, it simply acts as a hook to fetch indices of interest, and should overlap with parts of the existing 'mygeom'.

>>fem.draw.c.objs = c1;
>>fem.draw.c.name = mybound;
...
>>[fem.geom,st,ct] = geomcsg(fem);

This line creates a solid table 'st', and curve table 'ct' which contains the indices of draw objects in the order which they are assigned above. So the indices to my draw object is found by;

>>mybound_ind = find(ct(:,find(strcmp(fem.draw.c.name,'mybound'))));

This way I can assign boundary conditions and postprocess with 'mybound_ind'.

Cheers
Lars Ottar

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October 30, 2009 2:42pm UTC in response to Ivar K

Re: exporting data from the boundary

Problem with the postcrossplot is that it actually doesn't return a proper post data struct. If you use it on a line the 'd'-field won't appear.

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November 1, 2009 7:57am UTC in response to Grunde Waag

Re: exporting data from the boundary

Hi

could it be that your "dl" should be written "Dl" with un uppercase "D" ?

normally Comsol is rather case sensitve

My way around is to index the COMSOL documents, and then search i.e. posteval to have a look at different examples scattered in the different books, this helps to identify how to use the functions


Good luck
Ivar

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