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Overset: refine background mesh without resetting the mesh

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Old   May 13, 2014, 18:31
Default Overset: refine background mesh without resetting the mesh
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Hello,

I have a little issue when I try to refine my mesh in an overset mesh simulation:

My simulation is halfway through and my overset mesh is fine and I do not want to change it. However, my moving body is now entering a section of the background mesh that I did not make fine enough in the first place so I need to refine the mesh there in order to continue the simulation without it crashing.

My problem is that every time I add a volume control and refine my background mesh, the overset mesh (moving body) is going back to its original position after remeshing...

Is there a way to refine my background mesh without affecting anything (so my moving body does not go back to its initial position) ?

thanks!
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Old   May 14, 2014, 03:53
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if you are using the older region based meshing method this is very easy by just setting the mesh continuum of the moving region to none then doing a remesh with the new mesh size etc settings

with the newer parts based meshing method is it a little more work:
transform the geometry part of the moving part or parts from the initial coordinate system to the body system
remesh
transform the geometry part of the moving part from the body system back to the initial coordinate system - this gets the part back to where is was in case another mesh refine is required

this same method is very handy to refine the mesh of simpler dfbi cases without overset mesh when wanting to go from a coarser mesh to finer or to say re-align the water surface mesh with the free surface in a vof marine case
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Old   May 15, 2014, 00:21
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Just turn the mesh continuum to 'none' in the overset region and it shouldn't move.
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Old   May 15, 2014, 12:35
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It works great, thanks guys!!

now, how far can this method go, i.e. I have a moving boat in a shallow water and closed channel, can I modify the geometry itself of the bottom of the channel far away from the boat (for example introduce a bump or a deep section) and continue running my simulation ? Or because I have a given quantity of water (since my channel is closed), doing this would not work and computations will diverge ?

I would like to gradually make the channel shallower, simulating a boat approaching a beach.

thanks again!
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Old   May 15, 2014, 21:42
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You'll get jumps in the results when you switch geometries, but I don't think it'll diverge.
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Old   May 16, 2014, 05:54
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and if you simply want the channel to get shallower you could probably add motion to that region like some rotation and translation as a function of time then there is no need to remesh that region
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Old   May 19, 2014, 14:48
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Thanks for you help!

Ping, but if I do that, depending on how I move the background, it would disturb the water surface (more or less) I suppose ? I actually have a flat bottom and would like to replace it with a geometry that might be hard to simulate by moving the bottom, but thanks for the hint, this will help me somewhere else!

Another little issue related the the mesh:
I ran a simulation with a thin mesh overset and a coarse background (but still thin enough so it does not crash, and gives acceptable results), got the loads on that boat, all good.
I then ran the exact same simulation, with a much thinner mesh in the overset and in the background regions, but now I got loads that are oscillating (too much to be acceptable), and are less accurate than with the first mesh. Have you ever encountered that before ?

thanks for your help!
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