On Thu, Jun 12, 2008 at 4:18 PM, Derek Gaston <[EMAIL PROTECTED]> wrote:
> Ah - that is good info... I thought the BoundaryMesh just had pointers to
> the other mesh's data structures... kind of blows that idea out of the
> water...
>
> So how are other people doing boundary conditions with tri's and tets? With
> Dirichlet you can just use a penalty to swamp everything out. But with
> Nuemann?
Neumann bcs are already in terms of an edge (or face) integral. So
these elements with zero support (only a node) on the boundary don't
figure into the Neumann case anyway.
-J
> Derek
>
> On Jun 12, 2008, at 3:02 PM, John Peterson wrote:
>
>> On Thu, Jun 12, 2008 at 3:46 PM, Derek Gaston <[EMAIL PROTECTED]> wrote:
>>>
>>> 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).
>>
>> I know this is still in the idea stage, but I thought I would mention
>> -- currently when you use the BoundaryInfo object to sync() with a
>> BoundaryMesh, the BoundaryMesh makes its own *copy* of the nodes from
>> the original mesh. (The reason for this was so that the BoundaryMesh
>> could survive on its own, even if the original Mesh was destroyed.
>> Also you can alter the BoundaryMesh independently of the original if
>> you desire.)
>>
>> A better approach might be to define a boundary node_iterator in the
>> style of the other node iterators... but I'm sure you will cross this
>> bridge when you come to it.
>>
>> -J
>>
>>
>>
>>>
>>> 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
>>>>>>
>>>>>
>>>>>
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