ANSYS problem: difference in /post1 and /post26 results

[ceyhunsozbir]

Hi all,

I am doing harmonic analysis in ANSYS 11.0 for a ultrasonic sonotrode we are trying to improve. The sonotrode is bounded only to the transducer that is producing +-0. 6 µm displacement at 60kHz. Tis condition is applied as 1.2 µm displacement boundary condition. (please see problem.doc)
I did modal analysis and then harmonic analysis to see the behavior around a eigen frequency close to 60kHz. I need to read some displacement and stress results at the tip of the sonotrode and on the surface.
After doing harmonic analyses in general postproc (/post1) I looked at the displacement results from results viewer. I have seen that very close to eigenfrequency (60029 Hz), there is a weird movement which is totally different from the eigenmode. But +- 4 Hz from the eigenmode frequency, the movement is similar to the modeshape.

More than that, when I open the timehist postproc (/post26) and track the displacement results at the tip of the sonotrode I saw that result is different from /post1 result. When I read the displacement results at the tip using /post1 prnsol command the tip movement gets very low (due to the weird movement I explained). And if I read displacement using /post26 nsol command the tip displacement is highest at the eigen frequency as expected. (please see frequency response graph in doc file)

My question is:
What may be the reason of the weird movement? Should I read displacement results using /post1 prnsol or /post26 nsol for the analysis I make? I am really confused with the concept and I appreciate any help.

Thanks.
Muharrem Ceyhun SÖzbir
RWTH Aachen

 

[kan123]

I don't think you necessarily have a conflict between /Post1 and /Post26. Your contour plots are of 3 individual mode shapes. All 3 (and any others that are included) will contribute to the measured deflection that results from the applied load. So it looks like the maximum is occurring between the 2 largest contributors and this seems reasonable since they are closely spaced modes. Whether the plot of VALU vs FREQ has one peak or two separate peaks depends on the damping level and how close the 2 dominant modes are.