You would indeed expect significantly better results with higher polynomial 
degree on the same mesh, if the solution is sufficiently regular.
What kind of mesh are you using? Are you resolving all the boundary layers 
suitably? Are you using a Mapping of the same order as your velocity ansatz 
space is?


Am Sonntag, 18. September 2016 17:57:16 UTC+2 schrieb JAEKWANG KIM:
> Hello, I am a starter of dealii and am learning a lot these days with the 
> help of video lectures and tutorial examples. 
> I modified step-22 code (stokes flow code) into my own problem, the flow 
> around sphere.
> and I intend to evaluate the drag force (which is analytically given by 
> stokes equation) 
> My code reached quite close to the value since the absolute error  : 
> abs(drag_calculated-drag_exact)/drag_exact is around 10^(-3)
> However, I expected that if I input higher 'degree' I will receive more 
> accurate result, but it didn't
> Obviously Q2 is better than Q1. and Q3 is better than Q2. But Q4 or Q4 is 
> not better than Q2 or Q3? 
> Is there any reason on this? 
> (To be specific, if i say degree 2 , that mean I use (2+1) for velocity, 
> (2) for pressure, and (2+2) for Gauss integral....
> Thank you 
> Jaekwang Kim  

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