[Nek5000-users] Smoothness of the derivatives

[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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