# Calculate natural frequency

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 May 10, 2011, 06:21 Calculate natural frequency #1 Senior Member   omid Join Date: Apr 2011 Posts: 102 Rep Power: 8 Hi all I have a rigid sphere that jointed with elastic rod. Diameter of sphere is 20cm and the rod have 50cm length an 1cm diameter. The sphere is submerged. How can i calculate the natural frequency of this system? Best regards.

 May 10, 2011, 09:00 #2 Senior Member   Join Date: Apr 2009 Posts: 532 Rep Power: 14 Use a Modal analysis in Workbench. The submerged regions will need a layer of acoustic elements added to account for the fluid. This approach neglects viscous effects. I would contact ANSYS support for the details.

 May 11, 2011, 04:59 #3 Senior Member   omid Join Date: Apr 2011 Posts: 102 Rep Power: 8 Thanks stumpy I use it, but in Modal analysis i can't define any properties of fluid, i think submergence will affect the natural frequency of system, How can i do this? Best regards

 May 11, 2011, 06:39 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,808 Rep Power: 107 If viscosity will affect the modal frequencies then I think you will have to do it as a normal 3D simulation. It will be a long simulation. Only do it this way when you are completely sure modal analysis is inadequate as that will be so much easier.

 May 13, 2011, 05:08 #5 Senior Member   omid Join Date: Apr 2011 Posts: 102 Rep Power: 8 Thanks ghorrocks How can i do this 3D simulation?

 May 13, 2011, 07:06 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,808 Rep Power: 107 Do the CFX tutorials and all will become clear.

 May 13, 2011, 09:47 #7 Senior Member   Join Date: Apr 2009 Posts: 532 Rep Power: 14 There some details on the modal approach here: http://www.idac.co.uk/enews/articles..._Acoustics.pdf You would have to use a Command Snippet to define the FLUID30 elements, so you would need to be familiar with the APDL language and/or get support to help. You still need to determine is viscous effects are important. For low velocities the virtual mass would be much more important, so the modal approach should give good results.

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