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Oscillations: Effect of Damping

CLMECH-AXEVJC

When a simple mass-spring system is excited by a piston that pushes and pulls on the spring in a sinusoidal manner, the oscillatory response of the of the mass-spring system can be described by the equation:

$$\frac{d^2x}{dt^2}+b\,\frac{dx}{dt}+\omega_o\,x=X(t)$$

where $x(t)$ is the position of the mass, $b$ is the damping coefficient, $\omega_o$ is the natural frequency of oscillation and $X(t)$ is the position of the piston that drives the system. Let the motion of the piston be of the form:

$$X(t)=X_o\,cos(\omega\,t)$$

The response of the mass-spring system to the driving force of the piston depends strongly on the frequency of oscillation, $\omega$, of the piston motion, and is also strongly affected by the value of the damping coefficient $b$.

The response can be characterized by a dimensionless parameter $\xi$ equal to the ratio of the amplitude of oscillation, $x_o$ to the amplitude of the piston motion, $X_o$:

$$\xi=\frac{x_o}{X_o}=\frac{\omega_o^2}{\sqrt{(\omega_o^2-\omega^2)^2+b^2\,\omega^2)}}$$

How does the value of the damping coefficient, $b$, effect the magnitude of the response, particularly as seen the parameter $\xi$?

A

Increasing the damping coefficient, $b$, significantly widens the resonance. So, the increase in amplitude is seen for a larger range of driving frequencies $\omega$.

B

The major effect of changing the damping coefficient, $b$ is to change the height of the resonance peak. If $b$ is increased the resonance occurs with a smaller amplitude. If $b$ is decreased the resonance occurs with a larger amplitude. The effect is seen only in the region near the resonance condition i.e. when the driving frequency equals the natural frequency, $\omega=\omega_o$.

C

Decreasing the damping coefficient, $b$, significantly narrows the range of frequencies where the amplitude increases.

D

Increasing the damping coefficient, $b$, will increase the frequency $\omega$ where resonance occurs.