Zero matrix

Hello,

Upon running the following code,  using dsf1 or dsf2, I get a matrix of zeroes. However, I am anticipating a non-zero matrix. What can be causing the error?

from ngsolve import *
from netgen.geom2d import SplineGeometry
ngsglobals.msg_level=1

def topBottom():
    geometry = SplineGeometry()
    pnts     = [ (0, 0),  (0.5, 0), (1, 0), (1, 1), (0.5, 1), (0, 1)  ]
    pnums    = [geometry.AppendPoint(*p) for p in pnts]
    lines = [ (pnums[0], pnums[1], "gamma1",  1, 0),
              (pnums[1], pnums[2], "gamma2",  2, 0),
              (pnums[2], pnums[3], "gamma2",  2, 0),
              (pnums[3], pnums[4], "gamma2",  2, 0),
              (pnums[4], pnums[5], "gamma1",  1, 0),
              (pnums[5], pnums[0], "gamma1",  1, 0),
              (pnums[1], pnums[4], "gammaf", 1, 2)]
    for p1,p2,bc,left, right in lines:
        geometry.Append( ["line", p1, p2], bc=bc, leftdomain=left, rightdomain=right)
    geometry.SetMaterial(1, "Domain1")
    geometry.SetMaterial(2, "Domain2")
    return geometry
mesh = Mesh(topBottom().GenerateMesh (maxh=0.1))
n = specialcf.normal(mesh.dim)

V = VectorL2(mesh, order=1, dgjumps=True)
Q = H1(mesh, order=1, definedon=mesh.Boundaries("gammaf"), dirichlet="gamma1|gamma2")
X=FESpace([V,Q])
(u,p),(v,q)=X.TnT()

dsf1=dx(element_boundary=True,definedon=mesh.Boundaries("gammaf"))
dsf2=dx(skeleton=True,definedon=mesh.Boundaries("gammaf"))

a=BilinearForm(X,check_unused=False)
a+=(p*(v-v.Other())*n+q*(u-u.Other())*n)*dsf2

_,_,vals = a.Assemble().mat.COO()
npvals = vals.NumPy()
print('assemble values = ', (npvals*npvals).sum())

Hi Dori,

you are integrating over an internal interface, and you want to access functions from the left, the right, and the interface, right ? 
This is not fully supported in NGSolve. Internal interfaces are treated as faces inside the domain, and 'u' and 'u.Other()' refer to the left and right volume elements.

Way out:
Q1 = H1(mesh, order=1)
Q = Compress(Q1, active_dofs=Q1.GetDofs(mesh.Boundaries("gammaf")))

Then you can access functions from Q in the volume, but you pay only for dofs at the interface.

- Joachim