Need help on defining load cases for a Forced Freq Response Analysis 1

[VN1981]

Hi folks,
We are providing design & analysis consulting services to a firm which is performing a conversion of cargo chopper in to VIP one by installing interiors.

One of the concerns was to check resonance freq of the monuments against the external freq of the main rotors. We did this by doing a SOL 103 normal modes analysis on Nastran & comparing the resonant freq with rotors one.

However in future, we want to be more accurate and possibly perform a force freq response analysis.

I am investigating on how to define load cases in Nastran for the above scenario. Lets say that the main rotor freq is around 25Hz. How do I apply this to the FE model?

Most of the examples that I have come across defines a unit load along with a freq domain loading function and solves for displacements, accelerations, stresses etc. Usually the unit load is applied to a node of interest in a particular direction.

In the above case, I am lost on which node & direction to apply the unit load. Lets just take an example of a galley like the one pictured below

I know the above structure is complicated, but we can substitute the above for anything more simpler.

 

[janFEA]

Hello VN1981,

is your question still current?
If I understand you correctly, you don't want to use unit load because you don't know the excitation force, and you are not sure where this force operates.
The rotor is a source of excitation, is it so?

Note that instead of excitation force you can use kinematic excitation - to excite by amplitude of displacement, velocity or acceleration. F.e. in case of combustion engine you can calculate these data from multi-body and implement them to FEM as excitation (loading). If there are more excited regions (f.e. bearings, supports etc.), beside the amplitude don't forget on phase shift if it is relevant.
Instead of entire frequency range you can of course consider only on specific frequency, or set of specific frequencies (if you assume several relevant engine orders) - in your case that 25Hz.

Please check card FREQ in Quick reference guideline, solver SOL 111.

BTW - be honest your structure will not be easy to model properly as a linear dynamic model, I cannot imagine obtaining of good FE results without validation/correlation from testing...how you will define f.e. damping?


Hope this helps,

Jan


With best regards,
Dr. Jan Vojna
Lead Engineer Development

Siemens, s.r.o.