Overset mesh motion issues

astrotoma · March 24, 2020, 10:17

Dear all,

I am facing issues when running a model with an overset mesh and DFBI enabled. As you can see from the attached image of a simple test simulation of a floating cube, which shows the volume fraction on a section plane and the cube surface, the overset mesh seems to "drag" the fluid along while there seems to be no effect on the free surface in the background mesh.

Has anyone experienced this? I encountered the same problem in the "Lifeboat with overset mesh" tutorial.

ping · March 28, 2020, 21:19

it looks like your two regions have not been interfaced so do this and then use the overset mesh representation and overset status field functions to check the quality of the interfacing. you don’t need elongated cells since these are just used to save cell numbers in the free surface zone to create more cells vertically that horizontally

ping · March 30, 2020, 04:13

for a newby to overset mesh large cubic cells will work well and give quick debugging of basic problems that are happening in this case. and there is no reason to have grids initially aligned eg one region could use polyhedral cells although this is not recommendend for the HRIC physics used in star when wanting a good free surface.

once those issues are resolved then the user can introduce some mesh refinement in the vertical direction and and possible grid refinement, small time steps, 2nd order physics etc.

flat elongated cells are used to save cell numbers when trying to capture waves in vof free surface cases and then only in the free surface zone. for example when trying to capture large droplets that break away from the free surface eg in sloshing and separators then cubic cells are recommended in those locations.

so back to the problem in this case - from the image it looks like the two fluid regions have not been attached to each other using an overset mesh interface.

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March 28, 2020, 21:19		#2
ping Senior Member Ping Join Date: Mar 2009 Posts: 556 Rep Power: 20	it looks like your two regions have not been interfaced so do this and then use the overset mesh representation and overset status field functions to check the quality of the interfacing. you don’t need elongated cells since these are just used to save cell numbers in the free surface zone to create more cells vertically that horizontally

March 30, 2020, 04:13		#3
ping Senior Member Ping Join Date: Mar 2009 Posts: 556 Rep Power: 20	for a newby to overset mesh large cubic cells will work well and give quick debugging of basic problems that are happening in this case. and there is no reason to have grids initially aligned eg one region could use polyhedral cells although this is not recommendend for the HRIC physics used in star when wanting a good free surface. once those issues are resolved then the user can introduce some mesh refinement in the vertical direction and and possible grid refinement, small time steps, 2nd order physics etc. flat elongated cells are used to save cell numbers when trying to capture waves in vof free surface cases and then only in the free surface zone. for example when trying to capture large droplets that break away from the free surface eg in sloshing and separators then cubic cells are recommended in those locations. so back to the problem in this case - from the image it looks like the two fluid regions have not been attached to each other using an overset mesh interface.