fenics team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Jun 28, 2010 at 02:54:47PM +0200, Lorenzo Isella wrote:
> Dear All,
> I am trying to migrate from Comsol to Fenics.
> Now, one problem I need to be able to tackle with Fenics (a problem
> which is conceptually simple) is the following.
> Consider (in 3D) a large box (computational domain). To fix the ideas,
> let us say that the box has size L=30 and it stretches from -15 to 15
> along x, y and z. Inside the box I have what in aerosol science is
> called an aggregate i.e. static configuration of hard spheres, where
> every sphere is in touch with at least another sphere.
> To fix the ideas, consider, as a minimal example two spheres having both
> R=1 (in some units) and whose centres are in positions (-1,0,0) and
> (1,0,0).
> Now, the problem I would like to solve is a diffusion problem (we can
> set D=1 if we want)
>
> D\nabla^2\rho=0 with boundary conditions
> \rho=1 on the sides of the box
> and \rho=0 on the aggregate surface.
> No computations need to be carried out for the inside of the spheres.
>
> I need to be able to calculate then fairly standard quantities like the
> concentration \rho(x,y,z) inside the box and the diffusive flux
> orthogonal to the aggregate surface.
> I know that this specific example, being in the end of the day a linear
> chain, is axisymmetric and hence 2D, but the more general cases I have
> in mind (to be tackled when and if I get started with Fenics) are not.
> Can anyone point me in the right direction, maybe with a minimal script
> I can then study and fine-tune according to my needs?
> It should not be tremendously complicated for someone who is experienced
> in running much more complicated simulations with Fenics.
> Any help is appreciated.
> Cheers
>
> Lorenzo

This sounds like you just want to solve Poisson's equation. Is that
right? Then the steps are:

1. Copy the Poisson demo

2. Create the geometry for your problem (box minus spheres). You should
be able to do that easily either in Netgen (where you can say things
like box - sphere - sphere) or directly in DOLFIN (see the demo in
demo/mesh/smoothing).

--
Anders

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