Learning Forum

[Alvin Evandiaz Audhia]

What is the difference between the modes in modal analysis?, such as in the 4th homework course using modal analysis with 12 modes, whereas in the simulation example where the windmill fan blade only uses 6 modes, will the number of modes used affect the results obtained?

[Raihan Lutfianto]

Hi Alvin. This is actually a very good question. In order to understand the fundamentals of modal analysis, I will give you a simplified explanation.

The basic idea of modal analysis is to obtain two things: natural frequencies and mode shapes. Natural frequency is a frequency in which one system experiences a phenomenon called resonance. To understand mode shapes, try to think of them as "expected" deformation shapes due to excitation at a certain natural frequency. Hence, the deformation shown in the modal analysis is "fake"!

Now consider this figure.

As the number of modes increases, we can simply say that the accuracy of our model is also increasing. To determine whether we already extracted "enough" modes or not, we can evaluate the ratio of effective mass to total mass (should be close to 1, in this case, 0.88). There are some parameters that will not be explained in this reply.

We will continue to learn this exciting concept in 3-4 weeks from now. However, please feel free to ask and discuss this on the forum/group chat.

[Rizqinta Febiantari Purnama]

How to select data for the effective mass to total mass ratio problem, is the data selected from the top 3 and why not the whole?

[Raihan Lutfianto]

Hi Rizqinta. To obtain the "table" from the Modal Analysis output, you can check it here

Please change the "Solution Output" to "Participation Factor Summary". From there, the easiest way to understand the numbers is to take a look at the sum of Ratio of Effective Mass to Total Mass. If the value is somewhat close to 0.9, then you have extracted enough modes. However, if it lies far below 0.9, then you may need to increase the number of extracted modes.