In general in finite element analysis modal analysis is conducted in two methods one is free free analysis were first six frequencies will be almost zero from seventh onwards natural frequency value will be obtained in constrained method the first frequency will be having value, in practical in machine tool application eg base of machine tool it is constrained to ground therefore analysis should be with constraints (that is what I feel correct me if I am wrong) then why free free analysis results are required.
One application is Component Mode Synthesis (sometimes referred as 'superstructuring' in FE-software). Depending on the method you will need the one or the other, e.g. Rubin's method or Craig-Bampton.
You can find an overview here:
http://sem-proceedings.com/18i/sem.org-IMAC-XVIII-18th-182501-A-Brief-Tutorial-Substructure-Analysis-Testing.pdf
In free - free Finite element model ,if you have case of rigid body motion then you will get zero natural frequencies for the first three or even sixth modes !
This means rigid body motion NOT vibrating system.
You need free free model in applications such as cars.
I think that Free-Free model is used for planes and ships, since they behave as F-F vibrating bodies.
Thanks for your answers can some of you can share any case study for both the type of analysis will be useful
Regards
Free-free modal analysis is widely used in aircratft, plane and some other space flights. What I am interested is how to deal with the rigid mode in the following free-vibration analysis,that is how to compute the response due to the fluctuated load applied on the flights? Does anyone have the detailed references for providing? thanks very much.
In application level as per my understanding
Free Free Modal- In the design phase of a component/assembly, just perform a free free modal analysis to understand how the body behaves when it is unconstrained. Also it gives idea about where to constrain the design according to get maximum/minimum vibration.
Constrained Modal- To get model to test co-relation, exactly model the physical phenomena by given constraints at the location where its is (eg: fixed support of cantilever). That will give the natural/resonating frequencies of the model based on its mass distribution and fixity. The data (frequencies) so obtained can be used in design. Say the natural frequency of the body is 50Hz, then it should not be used in a environment where the excitation force has a frequency of 50Hz or nearby value.
- Badges
- Science topic