Dear Jaekwank,

In addition to what Daniel and Wolfgang said: One does definitely benefit 
from going to higher degrees and deal.II is able to handle this (the 
accurate boundary representation by a mapping is one thing that is easily 
forgotten). In a recent preprint, we considered the flow around a cylinder, 
a standard benchmark test for the incompressible Navier-Stokes equations:
Go to page 23, Table 1, to see how the solution quality increases as the 
polynomial degree is increased from 4 to 7.


On Sunday, September 18, 2016 at 5:57:16 PM UTC+2, JAEKWANG KIM wrote:
> 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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