Hi Dave, This term can't be integrated exactly because it is not polynomial, so my first guess would be that different quadrature rules are used for those two different expressions (if you don't suggest a quadrature rule then FFC tries to guess one from the expression). Try fixing the quadrature rule in that term and see if the results are still different.
all the best --Colin ________________________________ From: [email protected] [[email protected]] on behalf of David Bernstein [[email protected]] Sent: 01 September 2013 20:35 To: [email protected] Subject: [FEniCS] Question about nonlinear problem Hi Everyone, Recently I wrote a fenics python code for solving a nonlinear advection diffusion equation which worked very well. However, I came across one issue which I couldn't resolve and which is illustrated by the attached code (simpler than original project but still seems to contain the problem). The example problem is identical to the one described here http://fenicsproject.org/documentation/tutorial/nonlinear.html except that the nonlinear term q(u) is (1-u)^(-2), i.e., the PDE is grad(grad(u)/(1-u)^2) = -f The attached code gives different results if q is written as "(1-u)**(-2)" and "(1/(1-u))**2". Is there a reason for this? I would have thought they should be equivalent. Thanks, Dave _______________________________________________ fenics mailing list [email protected] http://fenicsproject.org/mailman/listinfo/fenics
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