## General Mechanical

#### Unrealistically large deformation- modal analysis

• Ofekv
Subscriber

Hi everyone

I am trying to do a modal analysis for a turbine disk held at the center. There is no excitation or outside condition, just a simple analysis.

I set the mesh to fine and ran the solver, but i'm getting some really unrealistic results- while the shape of the modes and deformation looks ok, the values are off the charts.

For a ~20cm body, i'm getting a 3.9m deformation and such.

The frequencies also don't seem right, i don't think they should be close to the level of being 20-30hz apart.

Any idea what went wrong would be much appreciated, thanks.

• peteroznewman
Subscriber

Modal analysis computes the frequency shape of modes and displays a magnitude of displacement that has no meaning for the displacement of a real structure because there is no load applied to the structure in a modal analysis.

To obtain displacement magnitudes of real structures, you have to apply a load and see the response.  You can only apply a load in a Transient Structural or a Harmonic Response analysis where zero load = zero response, in other words, you have to apply a load.  In Modal Analysis, you are not permitted to apply a load.

When you have a rotationally symmetric structure, ANSYS will report two modes at the same frequency, one mode bending around the Y axis and another mode bending in the same way around the Z axis (assuming the axis of rotation is the X axis).  Mathematically, they are the same mode, just with different orientations. You are getting that effect but the frequencies are not quite equal. If you refine the mesh, they may get closer together.

• Ofekv
Subscriber

I guess it was kind of silly of me to forget that to get some kind of deformation response, i need to apply some kind of impact/load....

Thanks for clearing everything up. I'm still new to Ansys, and didn't know it reports the modes around the z and y axis as separate- i created different sections in the analysis to view the shape of the deformation for each axis.

As you say, initially the frequencies were further apart, when i refined the mesh (what you see in the picture) they came closer together.

To view the magnitude of the deformation, do i need to do the analysis in "response spectrum"? as you say no loads can be added in modal analysis

• peteroznewman
Subscriber

Use Harmonic Response if the load on the structure is periodic.

Use Transient Structural if the load is transient.

Response Spectrum is an advanced but approximate method of obtaining the peak response from a transient without simulating the time history.

Find some tutorials on Rotodynamics, since long shafts with masses on them have all sorts of interesting vibration modes tied to the angular velocity of the shaft.

• Ofekv
Subscriber

In this case the above body is going to be fixed in the middle and subjected to an impact, so no rotation.

I understand that the way i need to go about it is with Transient structural then, i'll read and view some tutorials. Thanks!

• peteroznewman
Subscriber

Good idea.  Please mark my post with Is Solution so the topic gets marked as Solved.  You can open a New Discussion if you have questions on the Transient Structural model.

• vibrachid
Subscriber
how we carry out strain mdal analysis using ansys workbench. how i find the strain mode shape.n