Hello!
I've modeled a simple wing box with some spars and ribs in Ansys APDL. I chose SHELL181 for all the elements. My first natural mode of vibration is bending, but then I get local modes for all the other frequencies instead of the expected torsion and bending modes. I've tried to play with the thickness but I could not improve the results. Does anyone knows why I am getting such results and how to prevent this? Thanks a lot.
Dear Ochana,
Per my knowledge, you will obtain bending modes in flapwise and edgewise directions after that you can get the pure torsional mode or a coupled bending-torsion mode.
The torsion mode is interesting when your wing has big angle of attack (alpha).
Some checks you should make:
-Investigate the connectivity of nodes of the shell elements on different regions such as between spar and wing surface.
-Investigate the material model you should have orthotropic, isotropic, or combination of them in your wing.
-investigate the laminates fiber orientations.
Thank you for your inputs. I know how to conduct a modal analysis in ANSYS. I just don't know why I am getting local modes.
In fact, it is unclear as to what you meant by local modes. Deformations modes for a structure are body functions that span the entire body. To my best knowledge, local modes of vibrations are related to motions of molecules and not structures.
Now, to answer the main question regarding the type of modes, one must note that the deformation modes of a general unsymmetric structure are in general combined modes that could be coupled bending and torsion and/or axial. The simple way to characterize any mode is by examining the corresponding eigenvector to see which nodal coordinates contribute to the specific mode shape. As for not getting torsional or coupled torsional modes, it may be the case with your structural configuration and boundary conditions that torsional modes are at the very high-frequncy range which is not captured by your model. A finer mesh (more degrees of freedom) may reveal such modes. I would also suggest to reexamine what you called local modes; they could be partially torsional modes.
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