ffc team mailing list archive

[jhoffman@xxxxxxxxxxx]

> On Wed, Aug 31, 2005 at 05:57:50PM +0200, jhoffman@xxxxxxxxxxx wrote:
>> > I'm planning to add a list of predefined operators and operands to FFC
>> > to simplify and beautify the notation anyway, so this would fit well
>> > into that category.
>> >
>> > Operators that will be added include grad(), rot() and div().
>> >
>> > Operands that will be added include h (mesh size) and d (space
>> dimension).
>> >
>> > Any other suggestions?
>>
>> I find it useful to be able use some more geometry information (in
>> particular when we start integrating on surfaces):
>>
>> vol - volume of cell
>> area(i) - area for local face i
>> hface(i) - h for local face i
>> ...
>> same for local edges?
>
> I can understand if you need h (mesh size) and v (volume), but do you
> really need area of a specific local face?
>
>> It may also be useful to be able to identify a local face from a
>> collection of vertices; that is, while visiting a cell in the assembly
>> one
>> might want to get information about the local faces.
>
> I think all this messy stuff should be hidden. We need to do this when
> we implement integrals over boundaries, but everything that needs to
> be visible in FFC is the new operator
>
>     *ds
>
> Karin Kraft and Johan Jansson have already implemented a prototype
> version of boundary integrals in DOLFIN as far as I understand, so I
> only need to implement the remaining portions in FFC. I will probably
> also need to request some new features from FIAT so I can tabulate
> basis functions on faces/edges of the reference cells.
>
>> On the other hand, we may avoid a lot of this since we intend to
>> assemble
>> separately over faces (and edges), and then vol and h may be reduced to
>> the corresponding functions for the face when looping over the faces.
>
> Do you need both vol and h at the same time in the stabilization?

No, this is not for the stabilization. It is for applying conditions on
surfaces, such as weak boundary conditions. But if we do the assembly over
faces, we can take "h" to mean the "face h", so maybe this can remain
hidden. But typically we may want to formulate a form including both
volume and surface integrals, and then one would like to include an h_face
in the surface integral. But as I said; this may be avoided by hiding
"h_face" behind the regular "h". On the other hand, we have both dx and
ds.

/Johan