Jaekwang,
Have a look at step-10. There you see how the order of the mapping
influences the error.
Basically, you have to set the mapping for your FEValues objects, when you
compute error via integrate_difference
and when you project or interpolate some Function to an FE Vector.

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Best,
Daniel
Am Dienstag, 20. September 2016 18:10:11 UTC+2 schrieb JAEKWANG KIM:
>
>
>
> 2016년 9월 18일 일요일 오전 10시 57분 16초 UTC-5, JAEKWANG KIM 님의 말:
>>
>>
>> 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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