Hi all,
I was simulating Eigenfrequencies of an sign post 3D object. I obtained many frequency values including pure imaginary,for the first natural frequency.
It shows the pure imaginary frequency for rotation (attached image), but I don't understand the meaning of pure imaginary frequencies.
Would anybody help me to understand this obtained result? Or is this wrong result?
Hi Mojtaba,
that mode looks like a rigid body rotation, i.e. the plate can rotate freely around one axis without performing work. In the undamped case, this eigenfrequency should theoretically turn out to be zero. I assume the small imaginary part is due to material damping or simply an inaccurate solution of the eigenvalue solver. However, the solution is not physically meaningful. You should try and add constraints in a realistic way such that rigid body motions are avoided.
Hi Mojtaba,
Totally Mr. Gravenkamp is right. This is rigid body motion which is occured due to boundary condtions of the structure. Therefore, i can say that you dont have any restraction on the rotational axis of z.
Best Regards
Hi all,
Generally speaking there are some special cases when the natural frequency can be a pure imaginary number, it means system instability, see the attachment.
But I don't think that it is your case. Probably there is a mistake in your modeling. Could you please describe your boundary conditions, and model dimensions in detail.
Hi all,
Thank you. the pole is fixed at the base and pole and plate are fixed together ( it is like a sign post model). the plate is a square and the pole is a cylinder.
plate: 265mm*265mm*3.17mm
pole: length: 339mm and diameter: 6.43mm
density of material : 2850 kg/m3
elastic modulus: 68 GPa
Hi Mojtaba,
Normally it's possible to see this mode shapes, if the mode frequency of this mode shape doesnt equal to zero. To check your results, I also solved your model in ANSYS 17.0. You can find the files, mode frequencies and the mode shapes at the attachment. By the way, this mode shape is also in my simulation. However, it doesnt equal to 0,0052984 Hz. Please check your Connection between plate-pole, rotational boundary conditions and also your matlab code.
Best Regards...
Hi Mojtaba,
I also solved your model in ANSYS, and got the same results as Oguzhan Yalcin. Probably there is a mistake in your solution.
Regards
Hi All,
Thank you for your answers and my apologies for my belated reply. Actually, as you can see in the attached pic, the top of pole is at the center of the plate and it connects to to back of the plate (assuming they are fixed to each other).
Thanks,
Mojtaba
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