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Spectrum analysis

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03111961

Aerospace
May 29, 2009
2
Who can help? Probably I do not understand exactly how spectrum analysis works. The problem I took is very simple but the results I got are strange.

I took one mass system - link with point mass at one end. The other end of the link is fixed. An axial force
100*SIN(Omega*T) (T-time)is applied to the mass.
Omega = 2*3.141*10 (10 Hz)

The parameters of the link are - length L=3, cross section A = 1E-3, Young's modulus of the material
E = 3E6. The mass is 0.1

The first part (modal analysis) is being done correctly. The frequency of the system is 15.91 Hz

But after I make all other parts of spectrum analysis I got the results of STATIC problem - link, loaded by axial STATIC force of 100.

I do not understand what mistake and where I make.

Thanks in advance who will help
 
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A spectrum analysis expects - by its very name - a spectrum as an input. This is usually an acceleration vs. frequency tabulated load input (see SV and FREQ commands). The modal analysis - prior to the spectrum analysis - will find the eigenvalues of the system and the spectrum values are then input to the system based on the modes (frequencies) and by modal coefficients to give the load on the system via a chosen combination method and a significance factor. The help file explains all of this.

Hence, your static force is meaningless here, unless you convert it into the frequency domain and to units of either acceleration, velocity or displacement (or that required by the SV command based on the SVOPT you give).

Hope this helps.


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Thank you. I understood all that.

After I get results of MODAL analysis (the frequency obtained 15.9 Hz) I set SPECTRUM ANALYSIS, set the given frequency (say 10 Hz, SV and FREQ commands), choose the combination method, run the analysis, but as results I get values which are (as it can be easily checked) static solutions.

I changed the spectrum frequency but again got the same results. That means that probably I make some mistake in input data why ANSYS does not convert the static solution into frequency domain
 
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