[Getdp] Floating potentials in 3D: long execution times

[Christophe Geuzaine]

Hi John - Thanks for the feedback. The two issues you mention (slow 
generation of GroupsOfNodesOf and slow assembly) are indeed known, but 
seem to be especially bad in your case. How many nodes do you have on 
the floating potential surfaces?


On 25/05/11 17:54, John_V wrote:
> I am using GetDP to calculate the potential in a 3D geometry of mixed
> conductors and insulators. The calculation is repeated frequently with
> different charge distributions, so CPU time is important. Even though my
> mesh has ~400k nodes, with some experimenting I have been able to get
> good solutions in about 90 seconds, provided all the conducting surfaces
> have Dirichlet boundary conditions.
>
> I need also to be able to deal with floating conductors, however. My
> first attempt at this was to treat the floating conductor as if it were
> a dielectric with very large dielectric constant. The problem with this
> approach is that it degrades the condition of the matrix. I have to use
> the ILUP preconditioner (~12 minutes to compute) instead of the faster
> ILU0 one (0.3 seconds).
>
> The discussion of floating potentials in the manual gave me hope of a
> better alternative. Bernhard Kubicek's "Capacitor2D" example on the Wiki
> showed the way, The method is very elegant, based I think on basis
> functions for groups of nodes as described in the paper by Dular,
> Legros, and Nicolet in IEEE Trans. Magnet. 34 (1998). I have implemented
> this for my 3D geometry.
>
> As I hoped, doing it this way let me return to the fast ILU0
> precondition. However, time gained here is lost elsewhere. Execution
> times are VERY large. It requires a total of 57 minutes CPU time for
> this method on my mesh.
>
> The two largest contributors to the 57 minutes are preprocessing
> (especially generating GroupsOfNodesOf, 27.5 minutes) and generating the
> system of equations (29 minutes).
>
> What is GetDP doing during all that time? Are these operations
> inherently this time-consuming?
>
> John
>
>
>
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-- 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://www.montefiore.ulg.ac.be/~geuzaine