Re: [Libmesh-users] Time-dependent Natural BCs in Newmark System

[John P. <jwp...@gm...>]

Hi Andrew,

Sorry for the lack of response.  I don't think any of the active developers
are very familiar with the Newmark system, hence their silence.


On Wed, Sep 18, 2013 at 1:43 PM, Andrew Davis <and...@gm...> wrote:

> Hi all,
>
> Apologies if this is somewhere in the documentation and I missed it.  I am
> trying to update the "Newmark system and the Wave Equation" example.  I
> have successfully updated the example to use time dependent Dirichlet
> (essential) boundary conditions and I am working on doing something similar
> in the Neumann (natural) case.
>
> The problem with just updating the assemble function to make the Neumann
> BCs time dependent in the example is the matrix is only assembled once so
> instead of having time dependent boundary conditions the initial evaluation
> is treated as constant Neumann BCs.  Doing a reinit() and assemble() each
> timestep doesn't work for the Newmark case (and seems inefficient).
>

It sounds like the Newmark system (as it's implemented in libmesh) may be
incompatible with what you are attempting to do?  I think you'd be better
off with either implementing something more general (either with FEMSystem
or by following one of the other time-dependent examples) or generalizing
the existing Newmark system class so that it actually does what you want.


>  Finally, I tried to change the "fill_dirichlet_bc" function to
> "fill_neumann_bc".  The new version ingrates the Neumann BC over the
> boundary sides of each boundary element and adds the result to the RHS
> vector.  This seems to make the BC time dependent but the wrong magnitude.
>

Not sure what you mean by "wrong magnitude" exactly.  Note that the current
fill_dirichlet_bc() function uses the penalty method to enforce a Dirichlet
boundary condition, which is not appropriate for Neumann...

-- 
John