Note that we don't have a matrix to condense in our matrix free scheme ;-)
I ultimately think that we're going to use a BoundaryMesh to do
this... we'll just loop over the nodes in the boundary mesh and set
their residual as a post-processing step.... and avoid all of these
problems (including the triangle/tet problem).
Derek
On Thu, Jun 12, 2008 at 2:24 PM, Benjamin Kirk <[EMAIL PROTECTED]> wrote:
> Many, many ages ago we did Lagrange BCs ('cus there weren't any other types
> of elements yet!) strongly. The trick is to do it at the element
> matrix/vector level before inserting into the global matrix.
>
> There is actually a member function in DenseMatrix to do this:
>
> Ke.condense(i,j,val,Fe);
>
> is what you want. This will impose val strongly by doing the right thing at
> the element level. Of course you can add these and instead of getting
>
> val=foo
>
> at the linear system level you get
>
> N*val = N*foo
>
>
> There is another, important issue though that John learned via suboptimal
> convergence on triangular meshes... Usually we only concern ourselves with
> BCs on elements whose sides intersect the boundary, e.g. NULL neighbors...
> If you are going to impose the value strongly through the element matrix
> approach you have to do it for all elements which touch the DOF.
>
>
> Finally, you can effectively accomplish the same thing with penalty using
> add() instead of set(), just add 1e30 to the global matrix diagonal and
> 1e30*value to the RHS, all the other entries will "go away" in floating
> point.
>
> -Ben
>
>
>
> On 6/12/08 3:12 PM, "Derek Gaston" <[EMAIL PROTECTED]> wrote:
>
>> It's kind of hard to see how to go from dofs_on_side back to the
>> mapping to Re()... you have to know what "node number" those dofs map
>> to for the current element. Or am I missing something?
>>
>> Derek
>>
>> On Thu, Jun 12, 2008 at 2:08 PM, Roy Stogner <[EMAIL PROTECTED]> wrote:
>>>
>>> On Thu, 12 Jun 2008, Derek Gaston wrote:
>>>
>>>> Of course putting all my thoughts together in an email made me realize
>>>> what I needed to do... all I need to do is use the usual side
>>>> assembly... and call elem->is_node_on_side(i,side)... and if it is
>>>> then that dof is on the side... then I can just set
>>>> Re(i)=soln(dof_indices[i])-bc_value like I want to... overriding
>>>> whatever values were assembled into Re before I ever add it into the
>>>> residual...
>>>
>>> This looks good, but you might find it easier in the long run to use
>>> FE::dofs_on_side() rather than Elem::is_node_on_side; the latter might
>>> make it too easy to write code that only works on isoparametric
>>> Lagrange elements.
>>> ---
>>> Roy
>>>
>>
>> -------------------------------------------------------------------------
>> Check out the new SourceForge.net Marketplace.
>> It's the best place to buy or sell services for
>> just about anything Open Source.
>> http://sourceforge.net/services/buy/index.php
>> _______________________________________________
>> Libmesh-users mailing list
>> [email protected]
>> https://lists.sourceforge.net/lists/listinfo/libmesh-users
>
>
-------------------------------------------------------------------------
Check out the new SourceForge.net Marketplace.
It's the best place to buy or sell services for
just about anything Open Source.
http://sourceforge.net/services/buy/index.php
_______________________________________________
Libmesh-users mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/libmesh-users
