Re: [Geopdes-users] question on a simple patch test

[Rafael V. <va...@im...>]

Dear Hoang-Giang Bui,
to be honest, I'm not an expert of patch tests (not a fan, either). I 
have tried your test, and asked a colleague to run the same with another 
code (igatools), and the results are identical. It seems that you only 
get sigma_xy=0 exactly in the case of an affine transformation. Even in 
a simple bilinear case (2x2 elements), equivalent to FEM, if you move 
the central control point the computed sigma_xy is not zero anymore (but 
it converges when you refine).

So, I guess that this is not a problem of GeoPDEs, but about the 
understanding of (isoparametric) quadrilateral elements.

Best,
Rafa