Negitive Mesh Element Volume Due to a Heave Plate

Cluain · April 2, 2012, 06:06

Hi all,
I am simulating ocean waves in a 2 dimensional wave tank. I have a model in the tank and I am struggling with negative mesh elements around it due to the model moving up and down. The model has a heave plate and the simulation works when the heave plate is not there. But when I add the heave plate I get negative mesh elements.
A heave plate is a thin horizontal plate that reduces the vertical motion of the vessel
Like the square heave plates on the bottom of the circular columns in the image http://cdni.wired.co.uk/659x425/w_z/wave_inline.jpg
Does anyone have any ideas how to make the mesh accommodate that?
Also I am looking for a good meshing tutorial as I need to improve my mesh in general. I need to mesh the 2 dimensional model well. (to get the go ahead to do a 3D model simulation)

ghorrocks · April 4, 2012, 07:49

I recommend you save a series of time steps leading up the time step where you get the mesh folding. Make sure you include the mesh in the results file. Then you can look at your mesh and identify exactly where it is folding. When you know where the problem is then you have a chance to fix it.

Cluain · April 4, 2012, 11:12

It is folding in the first timestep, so I am going to reduce the timestep by a factor of 100 and see how that goes. How would you go about fixing it? Is there any merit in a finer mesh or should I be increasing the mesh flexibility?

ghorrocks · April 4, 2012, 17:49

Yes, use a finer time step and write each time step to a output file so you can see how the mesh is moving. Do not remesh until you have identified the problem.

If you cannot fix it by remeshing or a different motion strategy (eg using GGIs) then consider mesh flexibility. Remeshing and using GGIs is a much better approach so do them first.

April 2, 2012, 06:06	Negitive Mesh Element Volume Due to a Heave Plate	#1
Cluain Member Join Date: Oct 2011 Posts: 32 Rep Power: 14	Hi all, I am simulating ocean waves in a 2 dimensional wave tank. I have a model in the tank and I am struggling with negative mesh elements around it due to the model moving up and down. The model has a heave plate and the simulation works when the heave plate is not there. But when I add the heave plate I get negative mesh elements. A heave plate is a thin horizontal plate that reduces the vertical motion of the vessel Like the square heave plates on the bottom of the circular columns in the image http://cdni.wired.co.uk/659x425/w_z/wave_inline.jpg Does anyone have any ideas how to make the mesh accommodate that? Also I am looking for a good meshing tutorial as I need to improve my mesh in general. I need to mesh the 2 dimensional model well. (to get the go ahead to do a 3D model simulation)

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April 4, 2012, 07:49		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	I recommend you save a series of time steps leading up the time step where you get the mesh folding. Make sure you include the mesh in the results file. Then you can look at your mesh and identify exactly where it is folding. When you know where the problem is then you have a chance to fix it.

April 4, 2012, 11:12		#3
Cluain Member Join Date: Oct 2011 Posts: 32 Rep Power: 14	It is folding in the first timestep, so I am going to reduce the timestep by a factor of 100 and see how that goes. How would you go about fixing it? Is there any merit in a finer mesh or should I be increasing the mesh flexibility?

April 4, 2012, 17:49		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	Yes, use a finer time step and write each time step to a output file so you can see how the mesh is moving. Do not remesh until you have identified the problem. If you cannot fix it by remeshing or a different motion strategy (eg using GGIs) then consider mesh flexibility. Remeshing and using GGIs is a much better approach so do them first.