I have used mostly gmsh. Without seeing your exact details, make sure you are defining your boundary conditions and the domain in gmsh first in the geometry file. Then once you have specified the boundary conditions and the domain you can export the mesh to use in Nektar++. I'm sending you a text file where I have basically defined my boundary conditions and domain in a gmsh file. 

The 'Physical Line(1)' is my inlet which consists of a few curves, 'Physical Line(2)' is the outlet and so on.

Physical Surface(0) is the domain, you have to manually specify every surface that makes up the domain.

If you still have issues I can try to assist you. Or we could perhaps talk on zoom if I think I can help with your issue.

Matt

On Tue, May 9, 2023 at 11:26 AM Ehsan Asgari <eh.asgari@gmail.com> wrote:

No worries. I hope you will get satisfactory results.

I had a question though, how did you generate your 2-D mesh? Did you use Gmsh or other commercial packages? Because I 

have difficulty converting my 2-D mesh to Nektar format.

syavash 

On Tue, May 9, 2023, 18:47 Matt Duran <matt.duran60@gmail.com> wrote:

Hello Syavash,


That is very useful to know, 60% overprediction of drag certainly could fit with some of the strange results I'm experiencing. I will attempt to recreate some of the 3D results you have suggested to me from Jiang et al then.

I appreciate the help,
Matt

On Tue, May 9, 2023 at 2:50 AM Ehsan Asgari <eh.asgari@gmail.com> wrote:

Hi Matt

I noticed you are simulating the benchmark cylindrical flow at Re=3900. As you may see in literature, the flow and the wake at this Re are highly 3-D and 2-D representative will lead to overprediction of drag, as high as 60%. So, I would not expect to have a physical vortex shedding with this 2-D setup.

Kind regards

syavash 

On Tue, May 9, 2023, 04:09 Lahooti, Mohsen <m.lahooti@imperial.ac.uk> wrote:

Hi Matt,

The vortices certainly look odd. But I wouldn't expect that much of symmetry at this Re either.

Nevertheless, I noticed in your condition file no expansions are set. This means you are using the default expansion in the mesh file? I would explicitly set the expansion in the condition file.

Also, I would suggest using high order out flow for the outlet boundary

From your plots, the inlet and sides are also too close. Particularly the side walls can affect your vortex structure 

After your 2D is sorted out, you also might want to try 3DH1D too

We have a tutorial for flow over cylinder (https://www.nektar.info/notebooks//tutorials/basics-incns-solver/) that might be helpful. It is quite through, and you might find it helpful 

Hope this helped

Cheers

Mohsen

Mohsen Lahooti

Lecturer in Fluid-Structure Interaction 

Mechanical Engineering 

Newcastle University

School of Engineering 

Stephenson Building 

Newcastle upon Tyne

NE1 7RU

UK

---

From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Matt Duran <matt.duran60@gmail.com>
Sent: Tuesday, May 9, 2023 1:24:44 AM
To: nektar-users <nektar-users@imperial.ac.uk>
Subject: [Nektar-users] Turbulent cylinder flow simulation errors

 

This email from matt.duran60@gmail.com originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list to disable email stamping for this address.

 

Hello Nektar++ users,

I have been experiencing unusual results while simulating cylinder flow with the incompressible Navier-Stokes Solver. When running the simulation at higher Reynolds numbers such as Re = 3900, the flow does not appear as would be expected.  The early wake looks too unsymmetric, with one of the first vortex growing to an unusually large size before shedding.  After running the simulation for a long period of time the vortices still look quite unusual, lacking the symmetry that would be expected in the vortex street.

In trying to solve this issue I have checked the turbulent length scale and ensured I have enough mesh resolution. I also have used a small time scale of 0.0001.

 

I was wondering whether anyone had advice on what I might be doing wrong or how I may attempt to fix the issue. I have included some of the results of the vorticity contours and the .xml conditions in hopes someone recognizes why the simulation is not behaving as expected. 

Thank you,

Matt Duran

PhD student

University of Central Florida

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