[Nek5000-users] Unrealistic values at element edges

Wed Dec 10 11:34:21 CST 2014

Tony,

The wiggles in the quantities your plotting are generally indicative of marginal
resolution for the given Reynolds number.  They will go away with increased resolution.
Lambda2 is not continuous a priori, but converges to a continuous function with increased
resolution (exponentially fast).   Are you using PnPn or PnPn-2 ?   Pressure will generally
look better with PnPn (and be better, in the marginally resolved, i.e., LES, case).

For lambda2 it might be possible to do some filtering to get a smoother plot...(same for
other quantities).   I would suggest going that route --- there are several filtering strategies
but I'm not certain which would be best for your particular case.     You might start with the
g_filter() routine in navier5.f with a diagonal scaling of the wavenumbers like:

  ( 1  1  1   1   1 ... 1  .99 .95  .90 )

say... which reduces the energy in the top 3 modes.  There is also filter_d2()
that you can experiment with.

Paul

________________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
Sent: Wednesday, December 10, 2014 10:28 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Unrealistic values at element edges

Dear Neks,

Sorry that the image wasn't shown. I've attached it here. Hope someone can help.

Thanks and regards,

Tony

________________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov <nek5000-users-bounces at lists.mcs.anl.gov> on behalf of nek5000-users-request at lists.mcs.anl.gov <nek5000-users-request at lists.mcs.anl.gov>
Sent: 10 December 2014 16:23
To: nek5000-users at lists.mcs.anl.gov
Subject: Nek5000-users Digest, Vol 70, Issue 4

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Today's Topics:

   1. Unrealistic values at element edges
      (nek5000-users at lists.mcs.anl.gov)

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Message: 1
Date: Wed, 10 Dec 2014 16:23:02 +0000
From: nek5000-users at lists.mcs.anl.gov
To: "nek5000-users at lists.mcs.anl.gov"
        <nek5000-users at lists.mcs.anl.gov>
Subject: [Nek5000-users] Unrealistic values at element edges
Message-ID:
        <mailman.11813.1418228589.3424.nek5000-users at lists.mcs.anl.gov>
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Dear Neks,

I've been running turbulent channel and pipe flows using nek5000 for a while. However, recently when I created movies of lambda2 structures and pressure iso-surfaces in channel and pipe flows I observed some "strange" values at the edge between elements.

[cid:cec5092f-41be-4ae9-a468-a5fa9ab9f7ff]

This is lambda2 structures in turbulent pipe flow.

It is clearly visible from the picture that at some slices in streamwise direction, the values don't seem to be realistic and it feels like the flow is passing through some filters. Interestingly, when I plotted the mesh together with the variables, the vertical noise lines happened to be at the edge between adjacent elements, which is particularly clear in lambda2 plot. Same behaviour was also found in 2D pressure plots and in turbulent channel flow cases. Has anyone observed similar things before in visualisation?

I used both Paraview and VisIt for visualisation and the same thing was observed, so it shouldn't be a problem of software. In this case, I'm just wondering why the result is like this or is it because I set something wrong in my simulations?

Any comments will be appreciated and thank you very much in advance.

Best regards,

Tony
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