# How do I get the area of internal facets for DG methods?

Asked by
Martin Sandve Alnæs

I'm trying to implement a DG scheme that has a penalty term

h_F = min_{T \in (T+,T-)} |T| / |dT|

I've tried to implement this in PyDOLFIN as:

h_T = CellSize(mesh)

h_dT = FacetArea(mesh)

h_p = h_T('+') / h_dT('+')

h_m = h_T('-') / h_dT('-')

# h_F = min(h_p, h_m) = h_p < h_m ? h_p: h_m

h_F = conditional(lt(h_p, h_m), h_p, h_m)

but FacetArea is only defined for exterior facets.

Is there something corresponding for interior facets?

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