Re: [ff3d-users] Setting boundary conditions!!

[Stephane Del Pino]

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Le mardi 25 Mai 2004 22:30, Arun Viswanathan a écrit :
> Hi all,
>
> sorry to be bothering you all again.  But please do
> take some time of to go through the issue here. I'd
> appreciate as many suggestions as possible, coz I need
> all the help possible here.
>
> The problem that I am trying to solve involves a
> container carrying several drums, each drum consisting
> of gases. Incase any of the drums leaks, I would like
> to determine the concentration of the gas (as it
> diffuses into the rest of the container) in different
> pats of the container.
>
> 1. The trivial case to consider here is if there are
> no drums, and I just want to determine the
> concentration in different parts of the container. In
> this case, the boundary condition will be that the
> rate of change of concentration in the direction
> normal to the container's walls is zero. So my
> question is, how to set the condition as: dC/dx = 0
> for a wall perpendicular to the x-axis and dC/dz -= 0
> for a wall perpendicular to the z-axis??
You have to use the syntax
        dnu(C) = 0 on M foo;
where foo is one of xmin,xmax,ymin,ymax,zmin,zmax.
But since this condition is 0, you can omit it, it will be naturally taken 
(see FEM theory for details).

> 2. The not-so-trivial case would involve the drums
> within the container. Here, any gas that leaks (from a
> drum) would find its way around the container (through
> diffusion). But the additional boundary conditions I'd
> have to provide here would be, if the gas hits a wall
> of any of the drums, then the change in concentration
> normal to that direction would be zero. How do I set
> these conditions for the drums' walls??
This is exactly the same thing. Imposing nothing will provide homogeneous 
Neumann conditions.

Regards,
Stephane.
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