Re: [Libmesh-devel] [Libmesh-users] Adding one more equation...

[David K. <dknez@MIT.EDU>]

OK, so I've got a simple version of this working.

I've attached a patch, as well as some new files for the instantiation 
of FEScalar (I sent this earlier, but didn't zip it up, so it didn't 
make it under the 40kb limit)

Some notes:
- As a first cut, I used lots of hacks in dof_map.h that will only work 
in serial
- I used Roy's idea of letting the Order of SCALAR indicate how many 
scalar variables we want -- I guess then it would be good to make sure 
that we only add one SCALAR to a system, but I haven't done that yet...
- I didn't (yet) make any changes to the sparsity pattern, so adding to 
the last row(s) and column(s) of the matrix will be slow
- Plotting the SCALAR variables gives garbage at the moment, although I 
guess it should be easy to change GMVIO etc to plot the constant value 
of the SCALAR

I've attached a test example (lagrange_mult.C) for this functionality, 
using a Lagrange multiplier to solve a pure (homogeneous) Neumann 
problem, and enforcing that \int_\Omega u \dx = 5.

- Dave



Roy Stogner wrote:
> 
> On Sat, 18 Apr 2009, David Knezevic wrote:
> 
>> So, I'd really like to add this system.add_variable(foo,SCALAR)
>> functionality to libMesh. I've been poking around the library, but I'm
>> not really sure how to get started, so I could definitely use some
>> pointers...
>>
>> It seems to me that the important details for this are in DofMap, for
>> storing and retrieving the dof index of the scalar variable, as well as
>> for setting the sparsity pattern/number of non-zeros...?
> 
> That's right.
> 
>> Some thoughts I've had are:
>>
>> - Add a new enum SCALAR to FEFamily (or alternatively, a new enum SCALAR
>> to Order)?
> 
> FEFamily.  The Order would then specify how many scalars to add.
> 
> Maybe next write an FE<SCALAR> specialization?  Something that acted
> sort of like a discontinuous monomial element?  That might help
> avoiding the need to add special cases to a lot of loops in the
> library later.
> 
>> - Short circuit all the loops over elements for DOF counting in DofMap
>> for SCALAR variables,
> 
> If the FE<SCALAR> claimed to have zero dofs per element, DofMap would
> basically skip right over it... but I'm not sure that's an intuitive
> behavior; this might be a loop you really do want a special case on.
> 
>> and instead store the dof index of each SCALAR variable in a vector
>> in DofMap?
> 
> Right.
> 
>> - However, I can't see where in the code one should compute the dof
>> index of a SCALAR variable in the first place?
> 
> That's all in dof_map.C.
> 
>> - Ben, regarding setting the nonzero count and the number of rows that
>> you mentioned; where is this controlled? In SparsityPattern?
> 
> In SparseMatrix, I believe, although the code to build the sparsity
> pattern is in DofMap.
> ---
> Roy
>