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[Anders Logg <question137463@xxxxxxxxxxxxxxxxxxxxx>]

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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