On Tue, Sep 20, 2016 at 7:49 PM, JAEKWANG KIM <jaekwan...@gmail.com> wrote:

> Can I ask more about "the limits of our implementation of our quadrature
> formulas?".
> I wonder when it usually happens.
> Once I calculated drag coefficient, with Q1, my error is 10% compare to
> exact solution.
> However, I can 1.7% error when I use Q2 which is significant decrease!
> At Q3, I get 0.4% and At Q4 it starts to increase again 0.5%.
> If I go higher, then my error is more than 100%..... I really want to
> figure out why this happens....
> To summarize
> From Q1~Q3... it shows significant decrease in error
> but it is not anymore at Q4 and Q5


If you are using FE_Q space with uniformly spaced support points, then
there could be problem at higher degrees. Just to check, you should use
Gauss nodes, e.g.



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