[Nek5000-users] Smoothness of the derivatives

Tue Jun 26 09:03:12 CDT 2018

Vlad,

dssum will just sum up the shared points between elements - so no filtering. Note, currently there is no easy accessable mechanism in place to make your field smooth. As pointed out already modal filtering won't help.

Cheers,
Stefan

-----Original message-----
> From:nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
> Sent: Tuesday 26th June 2018 15:34
> To: nek5000-users at lists.mcs.anl.gov
> Subject: Re: [Nek5000-users] Smoothness of the derivatives
> 
> 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 <mailto: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> <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>
> >     <mailto:nek5000-users at lists.mcs.anl.gov>"
> >     <nek5000-users at lists.mcs.anl.gov <mailto: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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