Question #137463 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/137463
Status: Open => Answered
Anders Logg proposed the following answer:
I might be missing something here, but it looks like you compare
different things: the full jump and the jump in only the normal
component.
--
Anders
On Sat, Dec 11, 2010 at 08:51:26PM -0000, JG wrote:
> New question #137463 on DOLFIN:
> https://answers.launchpad.net/dolfin/+question/137463
>
> Hello -- I seem to get different answers when I project a discontinuous
> vector function component by component or using the vector DG function space.
> I expected the difference to be machine precision. What could I be doing
> wrongly? Thanks, --Jay
>
> from dolfin import *
>
> degree=1
> n=10
>
> mesh = UnitSquare(n,n)
> Q = Expression(("(x[1]>=0.5? 10.0 : 0.0)","(x[0]>=0.5? 100.0 : 0.0)"))
>
> VDG = VectorFunctionSpace(mesh, "DG", degree)
> DG = FunctionSpace(mesh, "DG", degree)
>
> q = project(Q, VDG, degree)
>
> qx = project(Q[0], DG, degree)
> qy = project(Q[1], DG, degree)
>
> print 'q and (qx,qy) differs by: %e, %e' % \
> ( sqrt(abs(assemble( dot(q[0]-qx,q[0]-qx)*dx, mesh=mesh))), \
> sqrt(abs(assemble( dot(q[1]-qy,q[1]-qy)*dx, mesh=mesh))) )
>
> n = FacetNormal(mesh)
> print 'jump(q)=%e, jump(qx)=%e, jump(qy)=%e' %\
> ( sqrt(abs(assemble( jump(q,n)*jump(q,n)*dS, mesh=mesh))), \
> sqrt(abs(assemble( jump(qx)*jump(qx)*dS, mesh=mesh))), \
> sqrt(abs(assemble( jump(qy)*jump(qy)*dS, mesh=mesh))) )
>
> Output:
>
> q and (qx,qy) differs by: 5.215406e-08, 3.885747e-07
> jump(q)=3.516078e-14, jump(qx)=1.931411e-07, jump(qy)=2.697398e-06
>
>
>
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