Hi everyone,

I'm having some issues with a frequency extraction step, hopefully someone can chime in and help fix the issue. I have a 3D model of a wind turbine blade, which is constrained at one end and free at the other; so similar to a cantilevered beam. I am using SR8 elements, with each of them assigned a certain composite shell section; the sections vary in thickness/materials depending on their location on the blade. For some time I have been working with this model and only extracting 20 modes in the frequency extraction step, but recently I have attempted to extract as many as 100 modes. The issue I'm having is that when I request a number of modes that is not 20, some of the mode shapes are in the opposite direction they would be if I requested 20. For example, let's say the first mode has a frequency of 1 Hz and the tip of the blade is displaced in the positive y direction if I request 20 modes. If I change the number to say 50 modes, the frequency remains at 1 Hz, but the tip of the blade is now displaced in the negative y direction.

I emailed one of the Abaqus technical support centers to ask them for help, and although they don't really offer tech support to academic customers, the person who replied was kind enough to offer this explanation:

let me refer you to section “Output” in “6.3.5 Natural frequency extraction” in the Abaqus Analysis User’s Guide (6.13):

Output variables such as stress, strain, and displacement (which represent mode shapes) are also available for each eigenvalue; these quantities are perturbation values and represent mode shapes, not absolute values.

The direction of the mode shapes you see is controlled by the displacement output.  Since the displacement values are calculated as a linear perturbation to represent shapes, the direction is not consequential.

From that response, what I gather is that it doesn't matter that the mode shapes I see change direction. If I understand that correctly, I find that hard to believe, especially since I have a similar version of this model in ANSYS, and the mode shapes don't change direction regardless of the number of modes I request.

Any help is greatly appreciated!

Javier