Element imprint on Q-Criterion Isosurfaces

[Alex Tsolovikos]

Hi,

I am working on a zero pressure gradient turbulent boundary layer and I'm interested in plotting Q criterion isosurfaces. However, when I compute the Q criterion with paraview, there is a clear discontinuity of the isosurfaces between elements (see attached image).

The simulation appears to be well-resolved (Δx+, Δz+ < 5, Δy+ of first node < 0.1 and it matches the statistics of other similar DNS simulations). 

I know that there is a gradient discontinuity between elements that causes this issue. How can I reduce its effect, besides increasing the grid size? Would increasing the polynomial order (rather than num of elements) help? Is there any smoothing filter in paraview that I can use?

Thanks in advance,

Alex

[Fischer, Paul]

Hi Alex,

I don't have an answer yet but, out of curiosity, what order are you

currently using?

Paul

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From: nek...@googlegroups.com <nek...@googlegroups.com> on behalf of Alex Tsolovikos <atsol...@gmail.com>
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To: Nek5000 <nek...@googlegroups.com>
Subject: [nek5000] Element imprint on Q-Criterion Isosurfaces

 

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[Alex Tsolovikos]

Hi Paul,

I am currently using N = 7.

Alex

[Philipp Schlatter]

Hi,

indeed, this is a problem that I have seen also in the past, in various codes/methods. What I typically did (in spectral codes) was to improve the physical-space representation by (high-order) interpolation. For instance, for the video here (https://www.youtube.com/watch?v=dWe3fnfo9WQ&t=0s) I doubled the resolution in physical space in each direction by spectral interpolation prior to going to VTK.

Another thing that helps a bit is to use the loop subdivision filter in paraview (not sure whether Visit has something similar).

Finally, for SEM-type meshes I would image that interpolating to a uniform mesh prior to VTK may also help to avoid the (linear) interpolation on non-equidistant meshes for the various filters.

What I doubt will help is simple element-based filters/smoothing.

Philipp

[Alex Tsolovikos]

Hi Philipp,

Thanks for the tips. I interpolated to a uniform grid and then computed Q-criterion and that seems to do the trick! (See attached.)

Thanks,

Alex

[Philipp Schlatter]

Great!
Just to be sure: Did you compute Q before or after interpolation on a
uniform grid? If you did it afterwards; how did you compute Q on the
uniform mesh?

Philipp

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[Alex Tsolovikos]

Hi Philipp,

I first interpolate the velocity field to a uniform grid and then compute the Q criterion with the interpolated velocity field.

I do both in Paraview as follows:

1) Create a FastUniformGrid (with the desired num of grid points)

2) Transform the FastUniformGrid to the desired size

3) ResampleWithDataset from the original Nek5000 grid to the transformed FastUniformGrid

4) Use the Gradient filter to compute the Q criterion

I also tried exporting the Nek5000 directly to a uniform grid by setting ifreguo = .true., but that did not help get rid of the element imprint.

Is there any other way to interpolate to a uniform grid in Nek5000 besides setting ifreguo = .true. or using history points?

Best,

Alex