Finding Modes of Vibration in ANSYS

In summary, the important modes of vibration in a structure are typically the lower frequency modes, as it takes more energy to excite higher frequency modes. The "natural frequency" of a structure refers to the frequency associated with each mode of vibration. The lower modes are important in a steady state, while all modes are important in a transient situation. Excitation at a nodal point cannot excite a mode, regardless of frequency alignment.
  • #1
adpr02
8
0
Hi,

I'm playing around with ANSYS to find the modes of vibration of a structure. How do I know which are the most important modes? I understand that there are infinite modes of vibration - getting higher and higher in frequency.

I'm guessing that it has something to do with the effective mass of each mode versus the total mass.

Also, when people say "Natural Frequency," which mode does that frequency belong to?

Cheers
 
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  • #2
There is a "natural frequency" associated with each mode of vibration. For most purposes, the lower frequency modes will be the important modes, because it takes more and more energy to excite the higher frequency modes.
 
  • #3
Ok. How do I know how many of the lower modes are important?

The reason I ask is because the first 9 are well below forcing frequency. 10th mode is pretty much bang on the forcing frequency.
 
  • #4
If your excitation force is a sine wave at a fixed frequency, the important modes are likely to be the ones close to that frequency. It doesn't matter whether that is the first or the 100th mode.

Actually you can improve on that statement by saying the important modes are also those where the structure moves a lot at the point where the force is acting, because theo more the structure can move, the more work force can do to excite that mode (work = force x distance).
 
  • #5
In a transient situation, all of the lower modes are important. In steady state, it is more as AlephZero has described. Remember also that an excitation acting at a nodal point of a particular mode cannot excite that mode, no matter what the extent of frequency alignment.
 

Related to Finding Modes of Vibration in ANSYS

1. How do I define a mode of vibration in ANSYS?

To define a mode of vibration in ANSYS, you will need to first create a finite element model of the structure you want to analyze. Then, you can use the modal analysis tool to calculate the natural frequencies and mode shapes of the structure. The mode shapes represent the different shapes that the structure will vibrate in at each natural frequency.

2. How do I interpret the results of a modal analysis in ANSYS?

The results of a modal analysis in ANSYS include the natural frequencies and mode shapes of the structure. The natural frequencies are the frequencies at which the structure will vibrate when excited by an external force. The mode shapes represent the patterns of displacement and deformation of the structure at each natural frequency. The higher the natural frequency, the more complex the mode shape will be.

3. Can I change the boundary conditions in ANSYS to find different modes of vibration?

Yes, you can change the boundary conditions in ANSYS to find different modes of vibration. By changing the boundary conditions, you can alter the stiffness and mass distribution of the structure, which will affect its natural frequencies and mode shapes. This can be useful in finding the most critical modes of vibration for a given structure.

4. How do I determine the amplitude and phase of each mode shape in ANSYS?

The amplitude and phase of each mode shape in ANSYS can be determined by looking at the displacement plot of the mode shape. The amplitude is represented by the color scale, with higher values indicating larger displacements. The phase can be determined by looking at the direction of the displacement arrows, with the direction indicating the direction of motion at that point in the mode shape.

5. Can I use ANSYS to find the mode shapes of a structure under different loading conditions?

Yes, ANSYS has the capability to perform modal analysis under different loading conditions. This can be useful in determining how the mode shapes of a structure change when subjected to different forces. By analyzing the mode shapes under different loading conditions, engineers can better understand the behavior of the structure and make necessary design modifications to improve its performance.

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