Learning Forum

[alvincsl]

Good morning all and I hope everything is well,

I have encountered a problem while I'm doing personal study on Harmonic Analysis. I did 2 runs on the same model, with the force applying at the same location (vertex). Fixed support at the bottom surface of the supporting leg, and frictionless support at all 4 sides of the model. 

The pre-stressed/initial conditions for these 2 runs were different.

A: The Harmonic run initial condition is extended from Modal Analysis. Solution method used is under Mode Superposition. Here are the Total Deformation in vertical direction and the frequency response graph of this run in peak velocity. The maximum value is probed at 25.11 (Natural frequency of the pre-stressed model)

B: No initial condition linked to this run. Solution method used is Full Method. Total deformation and frequency graph were probed at 25.11Hz and attached in this post. 

The resulted values from the frequency graphs attached are not the same. Even the total deformation showed had a different contour with one another. Is it due to the solution method used in both runs causes this issue? 

[Chandra Sekaran]

I can see one is mode superposition solution and the other is FULL solution. But you also mention "The pre-stressed/initial conditions for these 2 runs were different.". Is there a static pre-stress condition in one case? I assume not since both have peak at same frequency.

Are you making sure that same level of damping is applied in both simuations? The FULL method seems to have a sharper peak than the MSUP results. Also in the modal analysis how many modes are extracted and what is the highest frequency extracted. You want to extract modes well beyond 27 Hz.