dolfin team mailing list archive

[Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx>]

On 22 November 2011 21:15, Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx> wrote:
> On 22 November 2011 21:07, Anders Logg <logg@xxxxxxxxx> wrote:
>> On Tue, Nov 22, 2011 at 07:34:38PM +0000, Garth N. Wells wrote:
>>> On 22 November 2011 19:12, Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx> wrote:
>>> > Hi,
>>> >
>>> > The IntervalCell::facet_area in DOLFIN currently returns 0.0, while
>>> > the facet determinant (a scale factor) 'det = 1.0;'
>>> > in the generated code for the tabulate_tensor function in 1D (facet integrals).
>>> >
>>> > Is 0.0 the correct return value, or should it return 1.0?
>>> >
>>>
>>> I would think 1.0.
>
> That's what I'm leaning towards too.
>
>> Facet is the thing of dimension one lower than the cell dimension so
>> in this case 1 - 1 so it would mean the area of an endpoint. So I
>> believe 0 is correct.
>
> I see your 'point' :), but is it consistent with how we evaluate
> integrals on endpoints?

Running this script:

from dolfin import *

cells = [interval, triangle, tetrahedron]
meshes = [UnitInterval(1), UnitSquare(1,1), UnitCube(1,1,1)]

for cell, mesh in zip(cells, meshes):
    c = Constant(1, cell)
    print
    print cell
    print "volume:  ", assemble(c*dx, mesh=mesh)
    print "surface: ", assemble(c*ds, mesh=mesh)

produces:

<interval cell in R1>
volume:   1.0
surface:  2.0

<triangle cell in R2>
volume:   1.0
surface:  4.0

<tetrahedron cell in R3>
volume:   1.0
surface:  6.0

If we want surface = 0.0 for the interval, 'det' must be set to zero
in the generated code.
However, doing so will mean that f*ds etc. becomes zero too, which is
not very useful.

Kristian

> Kristian
>
>> --
>> Anders
>>
>>
>>> Garth
>>>
>>> > Kristian
>>> >
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>>> >
>>>
>>>
>>>
>>
>