## General Mechanical

#### Resonance on an SDOF model

• 35020306
Subscriber

Hello everyone. I'm currently studying the dynamic phenomenon like resonance on a numerical model. In order to do that, I've made a simple SDOF system which consists of a 1m x 1m x 1m steel cube and a spring connection as shown below. The cube is restrained laterally to prevent rigid body movement.

I did a modal analysis on this model to get the eigenfrequency and the eigenmode. The result was fairly reasonable with the first mode having an eigenfrequency of 1.5 Hz. I later subjected this model to a vertical sinusoidal load with the same frequency as that of the eigenfrequency (1.5 Hz) in order to incite resonance on the model. However, the result I got showed a constant sinusoidal displacement of the steel cube instead of an oscillating graph with exponentially increasing amplitude.

Is there something missing from this model? My guess is in the damping but I've made sure to set every damping value to zero in order to simulate an undamped condition.

• Rahul Kumbhar
Ansys Employee

Can you provide details what type of load are you using?

• 35020306
Subscriber

Sinusoidal with the load frequency being the same value as the eigenfrequency of the first mode. Load was defined using function in Mechanical.

• peteroznewman
Subscriber

The load you show has a frequency of 0.028 Hz.  I count about 6.7 cycles in 240 seconds.

The response of the cube is exacly what I would expect from such a low input frequency.

Try again with the correct input frequency.

• 35020306
Subscriber

Thank you Peter. I calculated the period from the frequency and I found out that it's supposed to be 0.628 second/cycle meaning that the input load frequency was too low. I decided to just make a table of the load value in Excel and input the load as tabular data instead of using the function in Mechanical.

• peteroznewman
Subscriber

In each software you use, you have to know if the trig functions: sine, cosine etc. have inputs in degrees or radians. Using a value in the wrong units is a common mistake.