Re: [Libmesh-users] Starting out with libmesh

[Roy S. <roy...@ic...>]

On Wed, 28 Mar 2018, Jorge Lopes wrote:

> \nabla V = Source(\phi)
> The source depends on the function \phi.

The more complicated math probably makes the software questions
easier: even in the implicit integration case you'll probably want to
solve as a fully coupled system and then consider decoupling in the
solver, rather than setting up two independent systems and iterating
between them.  The fem_system (if you want to use it) and
systems_of_equations examples may be good places to start.

> I'd say we start with explicit to see if it runs and simulates
> smoothly and then, if there are numerical problems we can switch to
> implicit.
> We have some freedom here, at this point either is fine.

I'm afraid you may have performance problems to worry about too, in
the explicit case: libMesh users originally all used implicit time
stepping, which meant there wasn't much attention placed on optimizing
any per-time-step costs which are negligable compared to a linear
solve, which means that if you're trying to avoid linear solves by
doing lots more time steps you may see those costs show up... which
tends to push libMesh users toward implicit rather than explicit time
stepping whenever both are reasonable options, which perpetuates the
cycle.

So if you *do* want to start with explicit time stepping, keep us in
the loop.  There are may be opportunities for library optimization
that we just haven't bothered with because the developers who could
make those improvements don't have great benchmark codes with which to
work on the problem.
---
Roy