[firedrake] Projecting CG1 function into DG0 function

[David Ham]

If you're worried about performance, then you can notice that the LHS
matrix is diagonal (because D is in DG0) so you can avoid matrix assembly
and solve with:

D.assign(assemble(div(q) * e * dx)/assemble(e * dx))

(ie just divide by the diagonal entries of the matrix).

On Thu, 12 Nov 2015 at 11:08 David Ham <David.Ham at imperial.ac.uk> wrote:

> Basically:
>
> d = TrialFunction(D.function_space())
> e = TestFunction(D.function_space())
>
> solve(d * e * dx == div(q) * e * dx, D)
>
> One might have to think a little about what value this gives at the
> boundary, but I think it's OK.
>
> On Thu, 12 Nov 2015 at 11:00 Justin Chang <jychang48 at gmail.com> wrote:
>
>> Hi all,
>>
>> Perhaps this may be a simple question, but say I have this bilinear and
>> linear form:
>>
>> a = grad(u)*D*grad(v)*dx
>> L = F*v*dx
>>
>> where u,v is trial/test function on CG1 space, and D and F are
>> coefficients that live in DG0 space.
>>
>> Say I have a velocity vector function q (CG1) on the same mesh and want D
>> to be the element-wise divergence of said velocity.
>>
>> How would I formulate D in Firedrake?
>>
>> Thanks,
>> Justin
>>
>
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