# Periodic boundary conditions (C21 C22 C23)

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 November 30, 2021, 14:45 Periodic boundary conditions (C21 C22 C23) #1 New Member   Vahid Join Date: Jan 2011 Posts: 23 Rep Power: 14 Dear All, Hello, Can someone explains C21 to C23 Periodic boundary conditions in DIVEMesh. I wanted to assign periodic boundary conditions to the side walls of the numerical wave tank (i.e. Surfside2 and Surfside3), so the inclined waves that exit the domain, enter once again from the other side of the NWT. Is this possible in Reeef3D? Thanks in advance Vahid

 November 30, 2021, 16:23 #2 Super Moderator   Hans Bihs Join Date: Jun 2009 Location: Trondheim, Norway Posts: 225 Rep Power: 16 Hi Vahid, the periodic boundary conditions are suitable for LES simulations. The NWT is not adapted to deal with periodicity. But that should not be a problem for wave simulations, I don't think it is that useful. Let me know what type of simulation you want to perform. Then I could suggest a good alternative approach. __________________ Hans Bihs Team REEF3D www.reef3d.com

November 30, 2021, 17:08
#3
New Member

Vahid
Join Date: Jan 2011
Posts: 23
Rep Power: 14
Hi Hans,

Thank you for the response.

I am trying to model the wave propagation on a sloped bed.
I have used the beach type of boundary condition for the side walls of NWT (upper and lower boundaries in the image I attached).
However, as can be seen on the image (the red rectangle), the inclined waves close to the side walls (on the constant elevation part of the bed) are curved toward the walls, which is not correct.
I have changed the side boundaries into no-slip wall and should see the results.

Since in real prototype the waves are infinite along the beach, I was wondering if there is this capability to assign the periodic boundary condition to the side walls, so that all the waves that exit the domain from the upper side once again enters from the lower side of the domain (in other words the wave field will not be influenced by the side walls)?

Best Regards,
Vahid
Attached Images
 Output2.jpg (79.3 KB, 27 views)

 November 30, 2021, 17:28 #4 Super Moderator   Hans Bihs Join Date: Jun 2009 Location: Trondheim, Norway Posts: 225 Rep Power: 16 Hi Vahid, with this type of scenario, I would recommend not trying to have perfect waves in the whole domain. Instead, assign a working zone where you need the correct waves. Around the working zone, use wave generation and numerical beach relaxation zones as needed. Another option would be to turn the wave generation relaxation zone so that it is parallel to the incoming wave front. While it maybe sound like a natural solution for this type of case, periodic boundary conditions would only work well if the free surface would be 100% symmetric, which is actually not the case. __________________ Hans Bihs Team REEF3D www.reef3d.com

 November 30, 2021, 17:51 #5 New Member   Vahid Join Date: Jan 2011 Posts: 23 Rep Power: 14 Hi Hans, Can you explain more about the working zone? I can imagine to model a more wider area so I can skip the wave deformations close to side walls. Is this what you mean? Best Regards, Vahid

 December 1, 2021, 06:20 #6 Super Moderator   Hans Bihs Join Date: Jun 2009 Location: Trondheim, Norway Posts: 225 Rep Power: 16 Hi Vahid, with working zone I mean that you accept that the waves show some wall effects outside a given area. Have you tried to turn the relaxation zone? I think this will improve the curved waves on the right side wall. __________________ Hans Bihs Team REEF3D www.reef3d.com

 August 29, 2022, 09:05 #7 New Member   Fabio Pierella Join Date: Aug 2013 Location: Trondheim Posts: 6 Blog Entries: 1 Rep Power: 11 Hi REEF3D team, I am trying to simulate waves on a long shoaling beach (about 10-15 km long, a few km wide, with a 1:100 slope), with 3D incoming waves, and with the REEF3D::FNPF solver with periodic boundary conditions on the sides. Periodic BC would allow me to run a long, "converged" simulation of a 3D sea state, limiting the sideways extension of the domain. Two questions: 1- Is this feasible with the current FNPF solver? 2- Do you see any troubles with this setup, or any alternative paths I could take? Best and thanks for your help, Fabio

August 29, 2022, 17:52
#8
New Member

Fabio Pierella
Join Date: Aug 2013
Location: Trondheim
Posts: 6
Blog Entries: 1
Rep Power: 11
Dear all,

I here post some exciting updates

I have managed to set up a simulation, modifying slightly Tutorial 8.6.
I have simulated a 2000m x 2000m domain with irregular 3D waves and periodic BCs. To check periodicity, I have duplicated the view of the domain in Paraview.

The solver works as expected, and seems very robust to me
I attach two screenshots of the free surface elevation, one at t=0s (initial) and t=100s.

In the first, you can see the initial wave field which is not periodic.
In the second, you see that some spurious waves are generated at the domain boundary due to the enforcement of the periodic BC. We can still see a discontinuity in the wave field in the generation zone (bottom) but in the rest of the domain the free surface elevation is periodic.

My conclusion is that this setup would work perfectly out of the box, provided that one was able to provide the software with a 3D spectrum periodic in the y-direction.

Any thoughts or comments on this conclusion?

Fabio
Attached Images
 eta.0000.jpg (31.3 KB, 17 views) eta.0100.png (189.1 KB, 14 views)

 August 31, 2022, 10:47 #9 Super Moderator   Hans Bihs Join Date: Jun 2009 Location: Trondheim, Norway Posts: 225 Rep Power: 16 Hi Fabio, great to see your results! Though at the moment, FNPF does not support periodic boundary conditions. What you describe as spurious waves are actually reflections. So far, we have only implemented true periodic BCs for CFD. What I would recommend to do is to employ relaxation zone beaches at the sides of the basin. For the future: do you think periodic BCs would be useful to have in FNPF? Would this approach actually work? __________________ Hans Bihs Team REEF3D www.reef3d.com

 Tags boundary, conditions, periodic

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