[Nek5000-users] Derivatives problem

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

Dear Paul,

Sorry for the very late reply but I was carrying out various checks to see
where the problem was coming from.

It seems that there are some strange values at the boundaries of the
spectral elements in the instantaneous fields (i.e. in the original nek
dumps prior to any post-processing). We are using Visit 2.7.0. The problem
is evident when the data limits are changed. Some examples:

x-velocity, zoomed in, default data limits:
https://www.dropbox.com/s/ewrlwyjqmg8kfx4/L1_xvel_slice_zoom_default_range.png?dl=0

x-velocity, zoomed in, reduced data limits:
https://www.dropbox.com/s/1wy8j6mu4z20lyf/L2_xvel_slice_zoom_reduced_range.png?dl=0

same as above with superimposed mesh:
https://www.dropbox.com/s/mcungp7vr2un3da/L3_xvel_slice_zoom_reduced_range_w_mesh.png?dl=0

Similar results are obtained with the y-vel, z-vel and pressure.

We have also carried out statistics on the above (this is where we started
noticing about the problem at the spectral element boundaries). We imagine
the problems we see are directly linked to the above. Some results, when
averaged, seem to make the problem disappear whilst others still evidently
have the problem: (note that the following are Paraview visualizations)

averaged x-velocity (over time and in the z/spanwise/homogeneous direction)
, zoomed in, default data limits:
https://www.dropbox.com/s/rbvhpfcth4hst96/L4_STAT_avg_x_zoom_default_range.png?dl=0
(even when changing the data limits we could not spot any strange values at
the boundaries)

Turbulent kinetic energy - default data limits:
https://www.dropbox.com/s/529cdukvl8t86va/L5_TKE.png?dl=0
(even when changing the data limits we could not spot any strange values at
the boundaries)

Problem - Production - default data limits:
https://www.dropbox.com/s/qyygzzmg3r3fbve/L6_PROD.png?dl=0

Problem - Dissipation - default data limits:
https://www.dropbox.com/s/q27097z5wsi57a5/L7_DISSIPATION.png?dl=0

For a less resolved case, the problem also becomes evident in, for example,
the averaged velocities and averaged squared fluctuations. The problem is
not seen for these statistics in the well resolved case  BUT is still seen
(as shown above) for the Production case... At least we imagine that the
problem is not due to mesh resolution... We thought it was a problem in our
post-processing script but since we see this problem in the instantaneous
dumps too we are a bit confused!

Some details about the simulation:
Channel flow with a lower curved wall, periodic boundary conditions in the
x-direction (streamwise), periodic boundary conditions in the z-direction
(spanwise) and top and bottom walls. Initial condition is zero and a
forcing is imposed in the x-direction to sustain flow. Re=10000 in .rea.
Find .box and SIZE files in the link below. We used a routine in usrdat2()
to modify the geometry and create a bump.

You can find the .box and SIZE files attached here:
http://we.tl/slw0QxA6wr

Let me know if you need any more information..

Thank you very much for your help and time,
JP
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