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-   -   2 translating bodies in waves (http://www.cfd-online.com/Forums/star-ccm/121811-2-translating-bodies-waves.html)

frownless August 5, 2013 21:11

2 translating bodies in waves
 
Hi everybody,
I am trying to simulate a floating object in waves with a free weight attached to a spring on top of it. My ideal situation would be the boat tutorial results, with a weight on a spring that translates just perpendicular to the boats top surface. The weight doesnt interact with the water directly, it only has gravity and the linear spring acting on it, and the boat has the spring, gravity and the waves acting on it. My initial idea was to create the 6dof dfbi boat, then attach a second one dof translating body in the boats frame of reference with a linear spring body coupling. But it turns out you cant use one dof bodies, or any translating bodies, in moving reference frames. Also it seems you are not allowed to run two 6dof bodies. Does anyone know of a clever way to do this? Can momentum be carried over a region? Should I be doing something with morphing or do I have to resort to co simulations?
To start with I would be happy to restrict the angle to purely vertical so that the spring is always acting parallel to gravity, but I eventually would like to extent the model to include at least one degree of rotation on the boat, as done in the tutorial.

Any help at all is appreciated. Thank you in advance

frownless August 5, 2013 21:22

1 Attachment(s)
Pictorial representation attached in case my description didnt make any sense! Thanks!

ping August 6, 2013 19:23

i imagine this will be quite possible but it needs more thought and somes tests.
you can have any number of 6DOF bodies - you are maybe refering to the basic rigid body motion method when only one is allowed. if you change to morphing or overset moving mesh then it eliminates this restriction - take a look at some of the videos of boats crossing, 3 airplanes manouvering etc.
a first step would be to use the body spring force feature - the ends of the springs can be defined in the moving body coordinate system.

frownless August 7, 2013 01:19

Thanks ping! Ah I see, I thought I needed rotating/ translating bodies and didnt know morphing motions could be used to do other non prescribed motions such as translation and rotations. Also, with the springs, I understand that they can act in the moving coordinate system, but because of gravity always acting in the lab, wont I get into trouble with that motion? Can I restrict the motion of the weight with physical barriers (ie the walls or a rail) so that the off axis gravity force gets transferred into a moment force on the boat?

ping August 12, 2013 02:49

i think you need to explore the wide range of dfbi features and try a few things in a very simple test model. i think it is possible to define a child coordinate system in the body system which diatates the direction of motion of the weight - use this as the initial coordinate system for the weight. since it is a child of the body system it will move with the body so that you can then control which degrees of freedom are freed for the weight - eg the x direction in the new coodinate system - this solves you problem.

Yage September 25, 2014 13:48

Hi frownless,

I am also interested in this topic, could I know you outcome?

Regards,
Yage

dai549592484 October 8, 2014 05:07

Dear Ping

Do you know whether it is possible to control the damping of such kind of a device?
For example, in this case, apply an active damping to the mass (which is connected to a spring).

Thanks in advance
Best Regards
Dai


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