Seeking help: Ansys Modal analysis, amount of deformation

[Turbom]

Dear All,

I am doing a forced response analysis on an axial fan rotor. Therefore I carried out a modal analysis to find the natural modes, which I later use to defined the oscillations of the system.

However, I will need to normalise my data with the amplitude of oscillation, which I do not know. In Ansys Workbench the program tells me the nodal deflections, however they are only correct in their proportion, their magnitude is erroneous (8m...). I would like to know if these deflections (being eigenvectors) truly have units or not. The program tells me it is meters but it is nonsense because the deflections this way are about 1000 times reality. Or if they are dimensionless, which would make perfect sense, what is the multiplication factor?

I would like to compare two cases and therefore I need to know at least the proportion of their deflections.

Can anyone help me out with this?

Thank you very much for your help!

 

[yasaraw]

I think there may be a problem with the units that you use in material properties, or may be when defining the gravity, if have defined any.

just make sure all the units are same, I mean for density you have to use kg/m3, not kn/m3. length in m, if you put Exy in kN/m2 then gravity should be 0.00981.

just try to do this as well.

 

[flash3780]

It wouldn't be correct to compare the magnitudes of the deflections of different modes in a modal analysis for a few reasons. You don't know the forcing functions or the damping. Modes may show up in a modal analysis that don't have any energy driving them in the actual device.

 

[kan123]

Just to amplify on flash3780's response, the magnitudes of the deflections listed for the mode shapes (eigenvectors) is arbitrary (usually mass normalized). You can't tell anything by comparing deflections from one mode to another. The values only have relative meaning within the mode. You don't get meaningful displacements until you have applied a load (random vibration, transient impulse, etc.) and the contributions of all selected modes are considered.