joniru
Structural
- May 7, 2015
- 1
Hello everyone,
I'm doing a simple modal analysis. The model consists of a thick pipe and the only active degree of freedom is the torsion. Analytical calculations lead to the following data:
- Torsional stiffness = 5.262e7Nm/rad; Rotatory inertia = 5.09355kg.m^2
I was expecting the torsional natural frequency to be:
Eigenvalue = Stiffness/Inertia = 5.262e7/5.09355 = 1.033e7. Then, Frequency = 3214 rad/s = 511Hz.
The fact is that I simulate the pipe with a beam (B31) element. One side is clamped and the other one is only able to rotate arround its longitudinal axis. The torsional stiffness and the inertia that I obtain match with analytical values. But when I run the modal analysis I get that:
Eigenvalue = 2.02661E+07. Then, Frequency = 4501 rad/s = 716.48Hz
So, the eigenvalue in the FEM is exactly twice the analytical one. ¿What am I doing wrong? ¿Does this make any sense?
I have also tried to model the thick pipe with solid elemenets (C3D20R) and I get the similar result to B31 element.
If anyone could give some tips, I would be glad.
I'm doing a simple modal analysis. The model consists of a thick pipe and the only active degree of freedom is the torsion. Analytical calculations lead to the following data:
- Torsional stiffness = 5.262e7Nm/rad; Rotatory inertia = 5.09355kg.m^2
I was expecting the torsional natural frequency to be:
Eigenvalue = Stiffness/Inertia = 5.262e7/5.09355 = 1.033e7. Then, Frequency = 3214 rad/s = 511Hz.
The fact is that I simulate the pipe with a beam (B31) element. One side is clamped and the other one is only able to rotate arround its longitudinal axis. The torsional stiffness and the inertia that I obtain match with analytical values. But when I run the modal analysis I get that:
Eigenvalue = 2.02661E+07. Then, Frequency = 4501 rad/s = 716.48Hz
So, the eigenvalue in the FEM is exactly twice the analytical one. ¿What am I doing wrong? ¿Does this make any sense?
I have also tried to model the thick pipe with solid elemenets (C3D20R) and I get the similar result to B31 element.
If anyone could give some tips, I would be glad.
