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It is often asked why there is a need for such in-depth knowledge about frequency
or modal tests? How does it affect the design? And in what scenarios does it really play
a role?
It is important to understand the natural frequency of a component under a loading scenario.
A common example observed is when soldiers cross a bridge they break their march. Ever
wondered why is that? Well, if the frequency at which they take steps (loading the bridge)
and the natural frequency of bridge match, it could lead to the catastrophic effect
of bridge failure.
How is it that if the excitation frequency matches the natural frequency it leads to the
failure of the component? This phenomenon is called resonance. It is the tendency of
the system to oscillate at maximum amplitude at a certain frequency.
One familiar example is a playground swing, which is a crude pendulum. When pushing
someone in a swing, pushes that are timed with the correct interval between them,
(the resonant frequency), will make the swing go higher and higher (maximum amplitude),
while attempting to push the swing at a faster or slower rate will result in much smaller arcs.
Now that we understand the need for frequency tests, we now need to understand certain
terms used in the realm of frequency tests. We will understand this using a simple
example. If we take a simple thread in our hands and make sure the two ends are
fixed and cause a small displacement at the center, the string starts oscillating. The
distance by which it moves is called amplitude, the rate at which these oscillations occur
is called time period or frequency. If we further displace the string compared to what
was done earlier (greater amplitude) the string now oscillates assuming a different
shape. This shape is termed as modes of vibration.
Let us try to understand what COSMOS can do for us. Using COSMOS Professional you can
figure out what the natural frequency of a component is under certain loading condition. Let’s
take an example of a tuning fork. Musicians will know that natural frequency of chord A will be
440 Hz. So we fix one end and mention in study properties it is the first 4 modes that we are
interested in. If we use a boundary condition use “Direct Sparse” as the solver. So why will
anyone run this without any Boundary conditions? To find out rigid body modes.
Once the analysis is run, the user can take a look at the displacement or
deformation plot. The legend shown for displacement contour chart has no significance. What
counts is the value for mode shape indicated on the right hand side upper corner. So now that
we can take a look at the modal shapes how do we know if the simulation is good or bad?
Frequency analysis depends on the amount of mass participating in the simulation or the mass
of the component that is oscillated. If you right click on the results folder and plot the mass
participation factor, you can find out the quality of the simulation. The summation of mass in
x, y, z should be at least close to 80% to call it a good simulation.
 
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