[Gmsh] ... electric field lines ...

Matt Koch mkoch at gvtc.com
Tue Oct 10 03:43:59 CEST 2006 

... let me know how I can help ...

... I am no good at C++ ...

... I am fairly good at ANSI C, but tend to have a very non-standard 
coding style (stemming from Fortran and Scilab) ...

Regards,

Matt Koch

Christophe Geuzaine wrote:

> mkoch at gvtc.com wrote:
>
>> OK,
>>
>> so, what I suggested below seems to work OK.
>>
>> The problem is, Plugin(StreamLines) seems to allow to start the 
>> field  lines on a rectangular grid only. That makes the plot look 
>> relatively  ugly, unless the grid is very fine, but then it takes a 
>> long time to  calculate.
>>
>> Instead, I think what would be needed is an arbitrary definition of  
>> starting points, such as on irregular electrode or ground surfaces. 
>> I  realize that might be hard to implement, especially when trying to 
>> go  through the simplified Plugin user interface.
>>
>> On the other hand, for GetDP, surfaces must usually be defined, and  
>> thus it may be possible in GMsh to somehow "recall" these surfaces 
>> and  thus their geometry points?
>
>
>
> That sounds like a good idea: we could maybe specify a geometrical 
> entity--and the plugin would extract either the geometrical points or 
> the points from the mesh to start the plugin. Or we could use another 
> view as the input...
>
>
>
>>
>> Regards,
>>
>> Matt Koch
>>
>> ----- Message from mkoch at gvtc.com ---------
>>      Date: Fri,  6 Oct 2006 10:10:37 -0500
>>      From: mkoch at gvtc.com
>> Reply-To: mkoch at gvtc.com
>>   Subject: Re: [Gmsh] ... electric field lines ...
>>        To: mkoch at gvtc.com
>>        Cc: Christophe Geuzaine <cag32 at case.edu>, mattkoch at scitex.us,  
>> gmsh at geuz.org
>>
>>
>>> Hi All,
>>>
>>> I just looked into StreamLines.cpp, and a I am not even going to
>>> pretend I understand what goes on in there. However, in GetDP
>>> PostProcessing, could one not define a new vector field as F/|F|,
>>> rather than F itself? And then indeed the StreamLines Plugin should
>>> apply, as long as one does not interpret time in there as literal time,
>>> but rather as some sort of independent space parameter?
>>>
>>> My concern is, that upon perusing StreamLines.cpp, I did not see any
>>> obvious reference to an ODE solver, such as the one from GSL. Instead,
>>> I see on lines 289 through 294 and below a reference to something that
>>> looks like some sort of homemade Runge-Kutta algorithm? What am I
>>> missing?
>>>
>>> Thanks,
>>>
>>> Matt Koch
>>>
>>> ----- Message from mkoch at gvtc.com ---------
>>>     Date: Fri,  6 Oct 2006 09:20:41 -0500
>>>     From: mkoch at gvtc.com
>>> Reply-To: mkoch at gvtc.com
>>>  Subject: Re: [Gmsh] ... electric field lines ...
>>>       To: Christophe Geuzaine <cag32 at case.edu>
>>>       Cc: mattkoch at scitex.us, gmsh at geuz.org
>>>
>>>
>>>> Hi All,
>>>>
>>>> I think the Plugin(StreamLines) is different from what we would need
>>>> for a Plugin(FieldLines), because of the field normalization:
>>>>
>>>> StreamLines:
>>>> dx/dt = vx(x,y,z)
>>>> dy/dt = vy(x,y,z)
>>>> dz/dt = vz(x,y,z)
>>>>
>>>> FieldLines:
>>>> dx/ds = Fx(x,y,z)/|F(x,y,z)|
>>>> dy/ds = Fy(x,y,z)/|F(x,y,z)|
>>>> dz/ds = Fz(x,y,z)/|F(x,y,z)|
>>>>
>>>> Nevertheless, it should be possible to copy the StreamLines plugin 
>>>> to a
>>>> FieldLines plugin and add this normalization to it. I'll take a 
>>>> look at
>>>> the .CPP file later on today. However, I hope that most of the 
>>>> stuff in
>>>> there is in pure C, as I am virtually illiterate in C++.
>>>>
>>>> Regards,
>>>>
>>>> Matt Koch
>>>>
>>>>
>>>> ----- Message from cag32 at case.edu ---------
>>>>    Date: Thu, 05 Oct 2006 18:57:50 -0400
>>>>    From: Christophe Geuzaine <cag32 at case.edu>
>>>> Reply-To: Christophe Geuzaine <cag32 at case.edu>
>>>> Subject: Re: [Gmsh] ... electric field lines ...
>>>>      To: mattkoch at scitex.us
>>>>      Cc: gmsh at geuz.org
>>>>
>>>>
>>>>> Matt Koch wrote:
>>>>>
>>>>>> Hi All,
>>>>>>
>>>>>> this seems to be a simple matter of solving the following    
>>>>>> equations for F = (Fx,Fy,Fz) as the vector (yeah right, 
>>>>>> NOTHING    is EVER  simple):
>>>>>>
>>>>>> dx/ds = Fx/|F|
>>>>>> dy/ds = Fy/|F|
>>>>>> dz/ds = Fz/|F|
>>>>>>
>>>>>> with |F| = sqrt(Fx^2 + Fy^2 + Fz^2)
>>>>>>
>>>>>> Even a dumb Runge-Kutta approach could probably do. I might could 
>>>>>> give
>>>>>
>>>>> Isn't this pretty much what we do in Plugin(StreamLines)? There we
>>>>> solve dX(t)/dt = V, with X(0) chosen as a bunch of points in space 
>>>>> and
>>>>> V(x,y,z) interpolated from the vector view.
>>>>>
>>>>>> it a try, but I'd need to know how to write plugins (how to     
>>>>>> programmatically access fields calculated by GetDP, how to     
>>>>>> programmatically draw in GMsh, and such), and perhaps get 
>>>>>> some     feedback from y'all on how long it might take to write 
>>>>>> a   "typical"   plugin? Would some sort of Runge-Kutta driver 
>>>>>> or   other ODE solver   be accessible to a plugin, or would that 
>>>>>> have   to be developed  from  scratch, or pulled in from GSL, for 
>>>>>> example?
>>>>>
>>>>> Have a look at Plugin/StreamLines.cpp in the Gmsh source code: you'll
>>>>> see that it's actually pretty simple to write a plugin.
>>>>>
>>>>>
>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> Matt Koch
>>>>>>
>>>>>>
>>>>>> Christophe Geuzaine wrote:
>>>>>>
>>>>>>> Matt Koch wrote:
>>>>>>>
>>>>>>>> Hello there!
>>>>>>>>
>>>>>>>> what a wonderful program GMsh is! It takes a little while to   
>>>>>>>> get   used to its GUI, but once you get the hang of it, it 
>>>>>>>> is    extremely useful and intuitive! However, one thing I have 
>>>>>>>> not    been able to figure out yet is how to make electric 
>>>>>>>> field lines    from an  electric field vector solution. I 
>>>>>>>> looked into the    StreamLines  PlugIn, but that seems to only 
>>>>>>>> work off of a    velocity field. Any  thoughts?
>>>>>>>
>>>>>>>
>>>>>>> Maybe this could be a good idea for a new plugin?
>>>>>>>
>>>>>>>
>>>>>>>> Thanks,
>>>>>>>>
>>>>>>>> Matt Koch
>>>>>>>> mattkoch at scitex.us
>>>>>>>> www.scitex.us
>>>>>>>> _______________________________________________
>>>>>>>> gmsh mailing list
>>>>>>>> gmsh at geuz.org
>>>>>>>> http://www.geuz.org/mailman/listinfo/gmsh
>>>>>>>>
>>>>>>>
>>>>>
>>>>> -- 
>>>>> Christophe Geuzaine
>>>>> Assistant Professor, Case Western Reserve University, Mathematics
>>>>> http://www.case.edu/artsci/math/geuzaine
>>>>
>>>>
>>>> ----- End message from cag32 at case.edu -----
>>>
>>>
>>> ----- End message from mkoch at gvtc.com -----
>>
>>
>>
>> ----- End message from mkoch at gvtc.com -----
>>
>>
>>
>>
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