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Buoyancy and the 6DOF solver (FLUENT)

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Old   July 26, 2012, 04:20
Default Buoyancy and the 6DOF solver (FLUENT)
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i2a
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Hello Everybody,

Background: Object fully immersed (single phase) in water and falling under gravity. I am using the 6DOF solver (properties for which, are defined using a UDF as usual) in FLUENT.

Question: Do I have to explicity apply the buoyant force in the 6DOF properties UDF e.g prop[SDOF_LOAD_F_Y] = 100 N. I can easily get a number - because the body is fully immersed.

My Understanding: As far as I can understand the 6DOF solver should take care of Buoyancy automatically. As, in certain cases (object falling under gravity from air to water), the situation can get quite complicated and may require an interesting UDF for Buoyant Force calculation, which will be far from elegant for the multiphase users. Moreover if am not mistaken buoyancy is just the force due to the pressure difference between the upper and lower faces of the body (thinking of a square). As we are solving for pressure in FLUENT it (Buoyancy) should be automatically accounted for by the 6DOF solver.

Reason for the Thread: I think (do not have any experimental number) my object is falling with somewhat higher velocity. Hence the question.

Thanks very much,
Awaiting.







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Old   August 24, 2012, 09:19
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Hy,

I'm also simulating argon bubbles rising in liquid tin at constant temperature (500K) and constant atmospheric pressure. I do it in Fluent with VOF scheme. I also include surface tension and wetting angle on the wall between argon and tin. This model is very simple so it should not be any problems: after some time bubbles should rise to the surface because of densities difference ( buoyancy force against gravity force ). But it does not happen (the bubbles sink to the bottom of the tube and stay there till the end of the simulation) so I understood that here buoyancy force is somehow neglected... I would really appreciate if someone help with that issue!
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Old   August 24, 2012, 09:32
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Hy,

I'm also simulating argon bubbles rising in liquid tin at constant temperature (500K) and constant atmospheric pressure. I do it in Fluent with VOF scheme. I also include surface tension and wetting angle on the wall between argon and tin. This model is very simple so it should not be any problems: after some time bubbles should rise to the surface because of densities difference ( buoyancy force against gravity force ). But it does not happen (the bubbles sink to the bottom of the tube and stay there till the end of the simulation) so I understood that here buoyancy force is somehow neglected... I would really appreciate if someone help with that issue!
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Old   May 2, 2014, 03:25
Default Buoyancy not included in single-phase 6dof simulations
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Jakob
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Hi,
We have been struggling with the free fall motion of a flat plate in water.
We did some simulations by letting the plate fall freely parallel to the direction of gravity and monitored the position as function of time (using the motion history option provided with the 6dof solver).

The results show that a plate parallel to the gravity direction fall at almost 9.8 m/s^2 , suggesting that buoyancy is not included.

We then applied a external force using prop[SDOF_LOAD_F_Y] corresponding to the buoyancy and found the results to be reasonable.

If you have any question, please let me know.

Best regards,
Jakob
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