Problems with wave BC without a relaxation zone

Hisham · October 7, 2013, 07:28

Dear Foamers,

I have been trying to model waves in a 3-D tank using the boundary conditions from waves2Foam (i.e. waveAlpha and waveVelocity). Because I do not need to absorb waves at the inlet, I have not configured a relaxation zone at the inlet (left). The use of the BCs only is OK for 2D flumes. However, when I run a 3D geometry (with a symmetry plane) the case crashes from an artificial high velocity near the water surface (air phase) and very close to the BC. The error is gone if a small relaxation zone is used at the inlet near the BC (1 m. long).

The error also happens for:
- Wall BC instead of the symmetry BC and
- Use of codedBC for wave BC instead of waves2Foam BCs (not a waves2Foam problem)

I have tried to neglect the air phase convection as well as relax the air velocities at each time step (set to zero). This seems to take the case for more steps further but still fails in the end.
The mesh is structured (check mesh OK). I also tried different schemes/solvers (but not all).

I am using OF-2.1.1 and thinking of trying a different version but have doubts if the version is the problem!

I would appreciate help with this problem!

Best regards,
Hisham

ngj · October 17, 2013, 14:39

Hi Hisham,

I have first seen your post now.

I have had the same problems many time, and I am sorry to tell you that I do not have a solution for it. Therefore, I have not developed/tested for this type of use in waves2Foam for many years. The only suggestion I have is to look in the literature for absorbing boundary conditions with reflection compensation. As this works without relaxation zones, a solution might be available.

If you find a solution, please tell me, such that I can improve waves2Foam.

Kind regards,

Niels

Hisham · October 17, 2013, 15:06

Quote:

Originally Posted by ngj

Hi Hisham,

I have first seen your post now.

I have had the same problems many time, and I am sorry to tell you that I do not have a solution for it. Therefore, I have not developed/tested for this type of use in waves2Foam for many years. The only suggestion I have is to look in the literature for absorbing boundary conditions with reflection compensation. As this works without relaxation zones, a solution might be available.

If you find a solution, please tell me, such that I can improve waves2Foam.

Kind regards,

Niels

Hi Niels,

I really appreciate your response. I have doubts that an absorbing boundary can solve that because I tried to increase the domain length to avoid reflection, but the case crashes too soon for any reflection to take place (at least that is what I experience or understand/think you mean by reflection). I can totally understand your idea about reflection because of solvers that use active absorption and are reported to work! Nevertheless!!

I wonder if it will work if I were to use two relaxation zones on both sides like the 3DTank tutorial (but for a small length). I will try the tutorial without the inlet zone.

As for my problem I am developing a "hack" for it (while admiring beauty of waves2Foam

) as a new waveTheory class that I will send to you after it's published. However, it is still a relaxation approach.

Best regards,
Hisham

ngj · October 17, 2013, 18:57

Cool, looking forward to your contribution.

I hope that your project are still on track

Kind regards,

Niels

haze_1986 · January 21, 2015, 08:23

Hi Hisham, I am having a similar problem. Using groovyBC or IHFoam to generate waves (just Stokes I into a 3D harbour), the simulations did not crash but sampling the inlet point I am never able to obtain the specified Stokes I waves. I have not tried waves2foam but do you think inlet relaxation zones could be the answer to my question?

October 7, 2013, 07:28	Problems with wave BC without a relaxation zone	#1
Hisham Senior Member Hisham Elsafti Join Date: Apr 2011 Location: Braunschweig, Germany Posts: 257 Blog Entries: 10 Rep Power: 17	Dear Foamers, I have been trying to model waves in a 3-D tank using the boundary conditions from waves2Foam (i.e. waveAlpha and waveVelocity). Because I do not need to absorb waves at the inlet, I have not configured a relaxation zone at the inlet (left). The use of the BCs only is OK for 2D flumes. However, when I run a 3D geometry (with a symmetry plane) the case crashes from an artificial high velocity near the water surface (air phase) and very close to the BC. The error is gone if a small relaxation zone is used at the inlet near the BC (1 m. long). The error also happens for: - Wall BC instead of the symmetry BC and - Use of codedBC for wave BC instead of waves2Foam BCs (not a waves2Foam problem) I have tried to neglect the air phase convection as well as relax the air velocities at each time step (set to zero). This seems to take the case for more steps further but still fails in the end. The mesh is structured (check mesh OK). I also tried different schemes/solvers (but not all). I am using OF-2.1.1 and thinking of trying a different version but have doubts if the version is the problem! I would appreciate help with this problem! Best regards, Hisham alia likes this.

October 17, 2013, 14:39		#2
ngj Senior Member Niels Gjoel Jacobsen Join Date: Mar 2009 Location: Copenhagen, Denmark Posts: 1,900 Rep Power: 37	Hi Hisham, I have first seen your post now. I have had the same problems many time, and I am sorry to tell you that I do not have a solution for it. Therefore, I have not developed/tested for this type of use in waves2Foam for many years. The only suggestion I have is to look in the literature for absorbing boundary conditions with reflection compensation. As this works without relaxation zones, a solution might be available. If you find a solution, please tell me, such that I can improve waves2Foam. Kind regards, Niels __________________ Please note that I do not use the Friend-feature, so do not be offended, if I do not accept a request.

October 17, 2013, 18:57		#4
ngj Senior Member Niels Gjoel Jacobsen Join Date: Mar 2009 Location: Copenhagen, Denmark Posts: 1,900 Rep Power: 37	Cool, looking forward to your contribution. I hope that your project are still on track Kind regards, Niels __________________ Please note that I do not use the Friend-feature, so do not be offended, if I do not accept a request.

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January 21, 2015, 08:23		#5
haze_1986 Senior Member Join Date: Jul 2011 Posts: 120 Rep Power: 14	Hi Hisham, I am having a similar problem. Using groovyBC or IHFoam to generate waves (just Stokes I into a 3D harbour), the simulations did not crash but sampling the inlet point I am never able to obtain the specified Stokes I waves. I have not tried waves2foam but do you think inlet relaxation zones could be the answer to my question?