# Rotating a high speed craft about its CG

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October 17, 2013, 15:26
Rotating a high speed craft about its CG
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mojtaba maali amiri
Join Date: Oct 2013
Location: Tehran
Posts: 7
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Dear all
I have been trying to rotate a high speed craft about its center of gravity in order to find its hydrodynamic coefficients. In attached you can find the image of this vessel in CFX environment. In fact, for deriving the hydrodynamic coefficients of a vessel two sets of simulation are conducted, pure heave( motion in z direction) and pure pitch( rotation about y axis). Pure heave simulation was carried out successfully by defining an appropriate expression. However, I have encountered some difficulties in pure pitch simulation.
I need some useful expressions which can be used to do the pure rotation about an perpendicular axis on a rigid body( in this case a vessel).
I would be so grateful to take some helpful advice from you,
Thank You
Mojtaba

Attached Images
 Vessel.jpg (35.9 KB, 24 views)

Last edited by Mojtabam; October 18, 2013 at 10:06.

 October 20, 2013, 20:04 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,327 Rep Power: 110 If I was doing this I would generate a mesh at all the different rotations. I think there is a new morphing thing which might be able to help here but I am not familiar with it.

 October 21, 2013, 04:53 #3 New Member   mojtaba maali amiri Join Date: Oct 2013 Location: Tehran Posts: 7 Rep Power: 6 Dear Mr. Glenn Horrocks Thank you for your reply. Actually I conducted this simulation by defining some appropriate Expressions which considered the new position of mesh instead of displacement. New X= if(t>Timeoffset ,((x-X0)-Total Mesh Displacement X)*cos(0.1 [rad]* cos(1 [s^-1] * t + Offphase))+((y-Y0)-Total Mesh Displacement Y)*sin(0.1 [rad]* cos(1 [s^-1] * t + Offphase))+X0 , 0 [m]) New Y= if(t>Timeoffset ,((y-Y0)-Total Mesh Displacement Y) + Y0, 0 [m]) New Z= if(t>Timeoffset ,-((x-X0)-Total Mesh Displacement X)*sin(0.1 [rad]* cos(1 [s^-1] * t + Offphase))+((z-Z0)-Total Mesh Displacement Z)*cos(0.1 [rad]* cos(1 [s^-1] * t + Offphase))+Z0, 0 [m])

 October 21, 2013, 05:19 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,327 Rep Power: 110 To do this you need a transient simulation with moving mesh. This is a very inefficient way of doing it and will take a very long time. A series of steady state runs as I described will be far more efficient.

 October 21, 2013, 06:50 #5 New Member   mojtaba maali amiri Join Date: Oct 2013 Location: Tehran Posts: 7 Rep Power: 6 However I need to do my simulations in transient way. because I need time history of the forces acting on the vessel. I dont think that steady state simulation would be helpful for me.

 October 21, 2013, 06:55 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,327 Rep Power: 110 I thought you were deriving the motion coefficients. Can't they be done steady state?

 October 21, 2013, 07:14 #7 New Member   mojtaba maali amiri Join Date: Oct 2013 Location: Tehran Posts: 7 Rep Power: 6 Motion coefficients are derived from the forces equations acting on the vessel. these forces must be a function of time. so it is essential to do the simulation in a transient way.

 October 21, 2013, 08:09 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,327 Rep Power: 110 I will take your word for it. How are you modelling the vessel? As for your original question - wouldn't a transformation matrix do what you want (http://en.wikipedia.org/wiki/Transformation_matrix)?

 October 21, 2013, 08:35 #9 New Member   mojtaba maali amiri Join Date: Oct 2013 Location: Tehran Posts: 7 Rep Power: 6 the vessel is considered a rigid body with no degree of freedom and no slip wall for its surface. For modeling the free surface the same expressions as for Buoy which you can find it in CFX tutorial.Transformation matrix is used for changing the coordinate system but here we have time.

 October 21, 2013, 17:30 #10 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,327 Rep Power: 110 ? Then you make the transform matrix a function of time. Isn't that obvious?

 October 21, 2013, 17:43 #11 New Member   mojtaba maali amiri Join Date: Oct 2013 Location: Tehran Posts: 7 Rep Power: 6 Dear Mr G. Horroks I must admit that I did not totally understand your suggestion. I would be so grateful to you if you could give me a vivid explanation of your suggestion.

 October 21, 2013, 18:02 #12 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,327 Rep Power: 110 I will be a little more vivid: For a simple 2D rotation you get.... make theta=a*sin(t) and it will rock side to side as a function of time. If you want a more complex transformation look at the wikipedia reference I quoted. In your post #3 you have a function which looks similar, but has several extra parameters. Is there something wrong with these functions?

 October 21, 2013, 19:05 #13 New Member   mojtaba maali amiri Join Date: Oct 2013 Location: Tehran Posts: 7 Rep Power: 6 I think it would be a good idea to use transformation matrices. I am going to make use of this idea, and I think it would save more time for me. The Expression in post #3 is actually like the following: New X= if(t>Timeoffset ,((x-X0)-Total Mesh Displacement X)*cos(alpha)+((y-Y0)-Total Mesh Displacement Y)*sin(alpha)+X0 , 0 [m]) Timeoffset is set by the user, for example I set this variable, 0. alpha= 0.1 [rad]* cos(1 [s^-1] * t + Offphase) the vessel will be rotating with an amplitude of motion equal to 0.1 rad and frequency equal to 1 s^-1.

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