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-   -   6dof (https://www.cfd-online.com/Forums/star-ccm/79562-6dof.html)

Birkov August 27, 2010 03:04

6dof
 
Hi to all,

I have little problem with 6DOF moving. I have cube in cube model and I followed star-ccm+ tutorial to set up my problem but there is one problem. As 6DOF object I choosed inner cube and set all needed properties.
When I run solver,region starts falling, not just cube which is selected as 6DOF.
Could you give me any suggestions?

Thank you very much.

B.

Edit: SOLVED

Henry Arrigo September 1, 2010 03:12

Hi Brikov,
I suggest you to check out your rigid body 's property for solving this problem. may be the weight and buoyancy don t match.

Birkov September 1, 2010 03:17

Hi,
I already solved this problem. I used filed function instead Flatvofwave. But thanks for reply.

B.

Henry Arrigo September 1, 2010 03:39

Good luck. :)

Henry Arrigo September 1, 2010 03:52

I v a problem with solving a DFBI case too, do you have any idea plz? I want too run a boat with thrust force in a domain. is that possible? I mean instead of setting a velocity inlet i want to move the boat using a force acting on it. this case is very helpful to study maneuvering. I v already solved a simple case, but the results are not reliable for example free surface deformation is unreasonable and drag force plot is strange too, but as i expected the whole domain moves in the direction of applied force, and also boat motions seems to be rational. do you have any idea how to come up with this case?
thank you :)

tobe September 8, 2010 02:09

You can add forces and moments to a hull in 6DOF but stil lock the motion in the required axis. This will give better solutions instead of moving the model as time steps are very critical.

Henry Arrigo September 8, 2010 14:39

Hi tobe,
Thanks for your reply. I still confused about your suggestion:
Why should I lock the motion in required direction? I want to see the boat raising up and moving down but if I lock the motions I won t see how does the boat respond to the fluid incidence.
Also I don t know what kind of B.Cs I have to use for the domain? for the case which I mentioned above I used mass flow inlet with flow rate equal to zero for all six faces of the cubic domain, but seems it has not worked out correctly.

tobe September 8, 2010 19:54

You can choose which motions to lock and which ones to have free, so you can lock logitudanal motion but have it free to trim and sink.
As to boundary conditions the roof we usually have as a non-slip wall, we usually only model 1/2 the boat so the there is a sym plane and the rest are velocity inlets.

What motions are you trying to model ?

Henry Arrigo September 9, 2010 06:51

Although I had in mind to study heave and pitch motions but preferably I would like to let the boat be free in six directions. I think for some cases like turning tests it is mandatory to do so. Does n 't it need to move forward when a force acts behind it? I cant make a sense about that!
For the velocity inlets, what magnitude of velocity should I assign to them? for mass flow inlet I used zero. Can I use zero for this case too?

tobe September 9, 2010 20:10

You can certianly have zero for your velocity inlets, if you are using VOF waves then they need to be set to VOF waves. What sort of time steps and cell sizes are you trying to use?

The boat can have a force applied with it fixed in that identical axis.

Henry Arrigo September 10, 2010 05:33

Implicit unsteady with 0.01s timestep size.
About the gird, here is the report of volume range: Volume range: [3.9930201368e-009, 9.4388723373e-002] m^3.


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