# Ansys problem

Discussion in 'Software' started by amateur mariner, Mar 31, 2009.

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### amateur marinerJunior Member

Can anybody tell me about applying various forces of different frequencies in harmonic analysis of a structure

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are you referring to "natural frequencies" of a structure?

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### amateur marinerJunior Member

No,I want to apply different harmonic frequencies to analyze the effcts on the structure?Can i Apply various forces having different frequency ranges on the structure

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Well, that's the problem. One doesn't apply forces.

Different harmonics are all part of "natural frequencies". Once the natural frequency has been calculated, if you have 'instructed' the program to do so, it will then calculate a range of harmonics.

It is not straight forward, you'll need to read the manual thoroughly on how to obtain natural frequencies. Each FE program is slightly different.

You'll need to make sure the structure has a mass for example, then do you want gravity effects, you'll have to look at the eigenvalue for shifting the frequency range...best is to input a very simple structure, calculate it by hand, then run the FE. Check the results. If you can't get the same result, don't do your frequency check, ask someone else to do it for you, otherwise you'll make a pig's ear of it.

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### amateur marinerJunior Member

I think i have not made myself clear.I want to apply various forces on my structure of different harmonicfrequency ranges??

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No, you haven't made yourself clear, doesn't help.

As I've noted above. You'll need to establish these "forces" and apply them as masses, and under gravity loading, to give directionality. Then run the model for natural frequency checks over a range of harmonics.

The applied 'force' acts as increasing the mass of structure locally, hence changing its stiffness, which in turn affects the natural frequency.

It is not a one click operation, it requires a significant degree of understanding of how the FE works (reading the manual), and how to model 'reality' (use simple models to validate hand calc's).

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### formsysformsys

There are two types of dynamic analysis. Natural frequency or harmonic analysis calculates the natural modes of vibration of the structure. For this analysis, no loads are involved, just the mass and stiffness of the structure is required and the results are the mode shapes and frequencies of vibration.
If you are interested in what happens when eg a sinusoidally varying force is applied to the structure over time, then you need to do a time history analysis. As well as setting up the model, you need to specify the time history of the load ie the force value at each time step, and then run the time history analysis.(Ansys calls it Transient dynamic analysis, Multiframe and most other structural programs call it time history analysis)

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As formsys elaborated further, if you are concerned with a sinusoidal varying loads (which again you haven't made clear), you now have the way forward for either case.

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### amateur marinerJunior Member

Yeah, I mean Time History Analysis as told by Formsys.I think it is also a type of harmonic analysis and I need to specify what is the force magnitude in a certain frequency range.I have done analysis considering one force on the structure at one time and several forces of equal magnitudes at different places on the structure.I want to ask that How can i be able to do it for several different types of forces and also having different frequency ranges??

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### alidesignerSenior Member

I think you are talking about forced dynamic response or sometimes called frequency response. Your version of Ansys may not have it as its usually not included in the base versions of FEA programs. Search your manual for frequency response or check with your reseller to see if you have the correct Ansys modules.

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### ilgazcanJunior Member

Hi
Can anybody tell me how can i apply masses of the equipments (Main engines , Generators , weight of tanks etc.) to the mesh model.

thanks.

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ilgazcan

helps to know which software you have!

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### ilgazcanJunior Member

I have been using ANSYS 10.

thanks

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ilgazcan

Im not familair with ansys and its menu layout, perhaps others on here who use it can chip in.

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