Eigenfrequencies of a multiple DOF system

In summary, the eigenfrequencies of a system depend on the initial conditions given to it, and there is no mathematical relation between these frequencies and the initial conditions. Anthropomorphism is not a useful approach in understanding this concept. In a linear system, the eigenfrequencies will remain constant, but in real systems like musical instruments, the frequency distribution may change over time. An example of this is seen in coupled pendula, where swinging both pendula in the same direction excites one mode, while swinging them in opposite directions excites the other. Ideally, there should be no transfer of energy between modes in this situation.
  • #1
aldo sebastian
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I am confused with this concept. So if a system possesses multiple possible eigenfrequencies (and therefore modes), how does the system "know" which eigenfrequency will it want to vibrate on? Does that depend on the initial condition you give the system? Is there any mathematical relation between the eigenfrequencies of the system and the initial condition that you apply?
 
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  • #2
aldo sebastian said:
will it want
That implies a high degree of intelligence and anthropomorphism is not a useful approach.
If a system has multiple natural (orthogonal) modes and you excite it at one of those modes then it should vibrate at that frequency only. So that's your "initial conditions" idea. If the system is linear then it will stay that way but real systems, like musical instruments will end up changing frequency distribution over time.
 
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  • #3
Second centaur

Simple example: coupled pendula
swing both same way excites one mode,
swing in opposite ways excites the other
 
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  • #4
BvU said:
Second centaur

Simple example: coupled pendula
swing both same way excites one mode,
swing in opposite ways excites the other
And ideally there need be no transfer of energy from one mode to the other. A combination of both modes (say you start just one pendulum off on its own) will result in the classic situation with each pendulum going from maximum amplitude to near zero and back again as the result of the presence of the two modes.
 

Related to Eigenfrequencies of a multiple DOF system

1. What are eigenfrequencies of a multiple DOF system?

Eigenfrequencies refer to the natural frequencies at which a multiple degree of freedom (DOF) system vibrates when it is disturbed from its equilibrium position. These frequencies are characteristic of the system and depend on its mass, stiffness, and damping properties.

2. How are the eigenfrequencies of a multiple DOF system calculated?

The eigenfrequencies of a multiple DOF system can be calculated by solving the eigenvalue problem, which involves finding the roots of the characteristic equation. This equation is derived from the system's equations of motion and represents the relationship between the system's natural frequencies and its mass, stiffness, and damping coefficients.

3. Why are eigenfrequencies important in the study of structural dynamics?

Eigenfrequencies play a crucial role in the study of structural dynamics as they determine the dynamic behavior of a system. Knowing the eigenfrequencies allows engineers to predict how a structure will respond to external forces or disturbances, and to design structures that can withstand these forces without experiencing excessive vibrations or failure.

4. How do eigenfrequencies affect the stability of a multiple DOF system?

The eigenfrequencies of a multiple DOF system can affect its stability by either increasing or decreasing it. If the eigenfrequencies are close to each other, the system can experience resonance, which can lead to excessive vibrations and potential instability. On the other hand, if the eigenfrequencies are well separated, the system is more stable and less prone to resonance.

5. Can the eigenfrequencies of a multiple DOF system be altered?

Yes, the eigenfrequencies of a multiple DOF system can be altered by changing its mass, stiffness, or damping properties. For example, increasing the stiffness of a structure can increase its natural frequencies, while adding damping can decrease them. Altering the mass distribution of the system can also affect its eigenfrequencies.

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