Units on Modal Analysis results

[sycip]

In workbench, there are displacement units on the mode shape results.

I'm curious how they decide on the magnitude of these units?

In particular, can the magnitude of these units be directly compared for two similar simulations to determine which has a larger response for a given mode shape and input energy?

Thanks.

 

[cbrn]

Hi,
no, modal eigenresults are normalized to mass or unity. The units are in [mm] only because by default displacements are expressed in [mm]. However, they have no "physical" meaning, dimensionaly speaking. They give you relative information, about what the shape of a natural mode is.
If you want to investigate responses, you have to do a harmonic response analysis.

Regards

 

[meshparts]

Hi cbrn,

I agree with you. But one thing I don't understand: I always thought that the units are the model units. This could be anything (m, mm, µm and so on). Can you explain your statement:

"The units are in [mm] only because by default displacements are expressed in [mm]."

Thanks
Alex

 

[cbrn]

Hi,
ah, yes, I already was aware that my sentence was unprecise.
In fact, "internally", there are NO units at all in the FE world, you only have to provide values which form a coherent set of units. This is what happens in Classical.
For ease of use, Workbench provides some "explicit" sets of units, but these are only "front end" names appended to the input so that you can not fall into confusion.
True, [length] can be expressed in [m], [mm], [micron] etc. I used [mm] "per antonomasia", in fact meaning [length], whatever [length]is in the model.

Regards