[Nek5000-users] Smoothness of the derivatives

[nek5000-users at lists.mcs.anl.gov]

Hi Philipp!

Thank you for your answer!

In fact I have almost the DNS resolution ( the grid spacing is comparable with the Kolmogorov scale from the literature data) and rather small error in the dissipation field. But I want to see a smooth dissipation, so I really want to try to filter it.

As I understood filtering procedure is in  dssum subroutine. But looking at it didn't help to me to realize the process of the filtering.
So could you explain, please, how can I choose the filter to be used? Cause now I think that my filter doesn't go beyond one element and it is the reason why it  makes my dissipation field worse .

Best regards,
Vlad




>Понедельник, 25 июня 2018, 23:05 +07:00 от nek5000-users at lists.mcs.anl.gov:
>
>Hi,
>I guess your only chance is to increase the resolution, which will make 
>the discontinuities of the derivatives decrease. This is one of the 
>characteristics of spectral methods; you will see a lack of resolution 
>directly in the fields. Similar resolution with other methods will not 
>give you more accurate results, but they appear nicer (smoother).
>
>Indeed, filtering does not help much. But if you want to try, make sure 
>you use a filter that actually goes beyond one element (e.g. using a 
>Laplace operator).
>Philipp
>
>On 2018-06-25 17:54,  nek5000-users at lists.mcs.anl.gov wrote:
>> Because I need to analyze the dissipation field
>> 
>> понедельник, 25 июня 2018г., 20:58 +07:00 от 
>>  nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>:
>> 
>>     This is correct, derivatives in SEM are not C0. Your code forces the
>>     field to be C0 but it does not make it smooth across elements.
>>     Typically filtering doesn’t help. Why do you want a smooth
>>     derivative field?
>> 
>> 
>>      > On 25 Jun 2018, at 12:55, " nek5000-users at lists.mcs.anl.gov
>>     <mailto:nek5000-users at lists.mcs.anl.gov>"
>>     < nek5000-users at lists.mcs.anl.gov
>>     <mailto:nek5000-users at lists.mcs.anl.gov>> wrote:
>>      >
>>      > Hi, Neks!
>>      >
>>      > 1) I am doing a DNS study of turbulent pipe jets and looking at
>>     turbulent kinetic energy budgets.
>>      >
>>      > I take derivatives via gradm1 subroutine, for example:
>>      >
>>      > call gradm1(uxdx,uxdy,uxdz,ux)
>>      >
>>      > After all, if I look at my dissipation field (via outpost), I see
>>     some wrong values on the surface between spectral elements.
>>      > I want to filter them using:
>>      >
>>      > call col2(uxdx,bm1,ntot)
>>      > call dssum(uxdx,lx1,ly1,lz1)
>>      > call col2(uxdx,binvm1,ntot)
>>      >
>>      > call dsavg(dxux)
>>      >
>>      > for every array. But conversely it leads to an increase in gaps.
>>     What am I doing wrong?
>>      >
>>      > 2) How does a filter-function in dssum look and how much points
>>     does it use for filtering? 
>>      >
>>      > Best regards,
>>      > Vlad.
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-- 
С уважением,

Иващенко Владислав, ИТ СО РАН
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