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Problem with global ODEs and DAEs physic

Posted May 24, 2012, 4:04 a.m. EDT Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers Version 4.2a 3 Replies

Emma Sundling
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Hi, we are trying to simulate a floating ponton in water with waves and to make the ponton move we would like to try to solve the ODE to calculate the displacement with the help of the force (calculated by integrating the boundary over the stress) but we can not make the global ODE mode work.

We have add the physic global ODE and just try to solve the simplest case when ydtt = 0 with initial values 0 and 0 but it do not converege, even though it is not connected to the other ODEs in the program. So it seems like something is wrong with our ODE

I have attached the file with the ponton.

Please help us =)
3 Replies Last Post May 24, 2012, 8:08 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago May 24, 2012, 7:26 a.m. EDT
Hi

I'm astonished you do not have any prescribed displacement nodes for your ALE, everything seems moving. A velocity of "0" does NOT mean its necessarily fixed.

Wha not start with the sloshing tankl example of the model library, replace the gravity generator by your moving wave-boom and then at the end, once you have waves, add your floating object

--
Good luck
Ivar
Hi I'm astonished you do not have any prescribed displacement nodes for your ALE, everything seems moving. A velocity of "0" does NOT mean its necessarily fixed. Wha not start with the sloshing tankl example of the model library, replace the gravity generator by your moving wave-boom and then at the end, once you have waves, add your floating object -- Good luck Ivar
Emma Sundling
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Posted: 1 decade ago May 24, 2012, 7:46 a.m. EDT
Hi, thanks for your quick respond

But if I use the slooshing tank how should I make the floating object move? Is it determined in the ALE, like prescirbed mesh displacement or is it something else?

And what is the gravity generator in the slooshing tank, we have worked a lot with it but we did not make any progress, that is why we tried another approach.

Another question: we manage to solve the ODE ydtt=0 if we use tw studies and solve the problems separetly, solving laminar flow and moving mesh in study 1 and the global ODE in study 2, but then the answers do not seem to be connected and we can not make the ponton move anyway. Is there some way we can connect them, so that in the result it takes the both solutions from the two studies?

Thank you for considering our questions
Hi, thanks for your quick respond But if I use the slooshing tank how should I make the floating object move? Is it determined in the ALE, like prescirbed mesh displacement or is it something else? And what is the gravity generator in the slooshing tank, we have worked a lot with it but we did not make any progress, that is why we tried another approach. Another question: we manage to solve the ODE ydtt=0 if we use tw studies and solve the problems separetly, solving laminar flow and moving mesh in study 1 and the global ODE in study 2, but then the answers do not seem to be connected and we can not make the ponton move anyway. Is there some way we can connect them, so that in the result it takes the both solutions from the two studies? Thank you for considering our questions
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago May 24, 2012, 8:08 a.m. EDT
Hi

my suggestion was to first get the waves generated by the moving submerged boom (that is how I interprete it, then add the floating ponton. But anyhow you need gravity to counter react with the boyance force, if your object is floating then you have a force equilibrium.

What you cannot simulate easily, is the appearance of "air" under the ponton, as this reppresents a change in topology and that COMSOL (nor any other FEM programme I know about) cannot handle automatically. But perhaps with a levvel set 2 phase approach its also possible, but that is a step further.

My suggestions, take one step at the time get first your waves running as wanted, then add next tstep ;)
--
Good luck
Ivar
Hi my suggestion was to first get the waves generated by the moving submerged boom (that is how I interprete it, then add the floating ponton. But anyhow you need gravity to counter react with the boyance force, if your object is floating then you have a force equilibrium. What you cannot simulate easily, is the appearance of "air" under the ponton, as this reppresents a change in topology and that COMSOL (nor any other FEM programme I know about) cannot handle automatically. But perhaps with a levvel set 2 phase approach its also possible, but that is a step further. My suggestions, take one step at the time get first your waves running as wanted, then add next tstep ;) -- Good luck Ivar

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