Rigid body

stefano rigosi · July 7, 2011, 14:21

Dear all,
I am dealing with floating body in the real sea state condition, which means irregular wave, high unsteadiness and wave-body interaction.
Anybody knows how CFX 13 computes the dynamic behaviour of a floating body??
Does it takes into account non-linear damping coefficient, the added mass of inertia and so on.
I would like to have some more information about this new topic, since in the Help it is not well explained.

I have got another easy question. Why when dealing with water free surface and two phases it is better to implement SST as turbulent model instead of k-e??

Thanks

ghorrocks · July 7, 2011, 20:02

CFX has a 6DOF solver. For details see the documentation.

What is the non-linear damping coefficient you refer to? The body is coupled to the fluid around it. There is no damping coefficients, the damping is modelled directly.

Mass - obviously, yes this is modelled.

Free surface and SST/k-e - I do not know where you got that one from. SST is generally a more widely applicable turbulence model than k-e, but that applies to single phase models as well.

stefano rigosi · July 8, 2011, 16:32

Dear Glenn,

Thank you for your quick answer.
Since I did the tutorial of a floating buoy in CFX13 I found that they suggest to use SST turbulence model, but they do not justify this choice.
I think, it is more robust for the free surface modelling, but I am not sure.

Moreover, I have some problem in my simulations. I used a timestep of 0,01 sec. and when I monitor the force and the torque acting on the rigid body I found very changeable values, such as +100 and -20 between two timesteps.
What can I do apart from reducing the timesteps, that leads to much longer simulations??

Thanks,

ghorrocks · July 10, 2011, 07:38

No, SST is recommended as the general purpose turbulence model. This recommendation is general, not specific to free surface models.

The rapidly oscillating forces suggest the 6DOF solver is not happy. Sometimes this can be helped with a smaller timestep, but there may be options in the solver to help damp oscillations. I don't know, I am not very familiar with the 6DOF solver.

July 7, 2011, 14:21	Rigid body	#1
stefano rigosi New Member stefano rigosi Join Date: Dec 2010 Posts: 4 Rep Power: 16	Dear all, I am dealing with floating body in the real sea state condition, which means irregular wave, high unsteadiness and wave-body interaction. Anybody knows how CFX 13 computes the dynamic behaviour of a floating body?? Does it takes into account non-linear damping coefficient, the added mass of inertia and so on. I would like to have some more information about this new topic, since in the Help it is not well explained. I have got another easy question. Why when dealing with water free surface and two phases it is better to implement SST as turbulent model instead of k-e?? Thanks

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Rigid body	Birkov	CFX	4	May 25, 2012 07:26
Turbulent solver rigid body mechanics	philippose	OpenFOAM Running, Solving & CFD	53	July 16, 2011 14:35
Rigid Body State Variables in Solid Immersed Simulation	Hamidreza	CFX	1	October 19, 2009 07:14
rigid body motion	antonello	FLUENT	3	July 28, 2004 04:25
What type of boundary for a moving rigid body?	Ryan	FLUENT	5	April 13, 2004 12:55

July 7, 2011, 20:02		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	CFX has a 6DOF solver. For details see the documentation. What is the non-linear damping coefficient you refer to? The body is coupled to the fluid around it. There is no damping coefficients, the damping is modelled directly. Mass - obviously, yes this is modelled. Free surface and SST/k-e - I do not know where you got that one from. SST is generally a more widely applicable turbulence model than k-e, but that applies to single phase models as well.

July 8, 2011, 16:32		#3
stefano rigosi New Member stefano rigosi Join Date: Dec 2010 Posts: 4 Rep Power: 16	Dear Glenn, Thank you for your quick answer. Since I did the tutorial of a floating buoy in CFX13 I found that they suggest to use SST turbulence model, but they do not justify this choice. I think, it is more robust for the free surface modelling, but I am not sure. Moreover, I have some problem in my simulations. I used a timestep of 0,01 sec. and when I monitor the force and the torque acting on the rigid body I found very changeable values, such as +100 and -20 between two timesteps. What can I do apart from reducing the timesteps, that leads to much longer simulations?? Thanks,

July 10, 2011, 07:38		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	No, SST is recommended as the general purpose turbulence model. This recommendation is general, not specific to free surface models. The rapidly oscillating forces suggest the 6DOF solver is not happy. Sometimes this can be helped with a smaller timestep, but there may be options in the solver to help damp oscillations. I don't know, I am not very familiar with the 6DOF solver.