Vibration Reduction at Source

In a large number of practical situations, the vibration can be controlled by reducing the excitation level at the source. This reduction in excitation is possible only after the source has been identified and the nature of excitation clearly understood.
In this module, we shall discuss some examples of forced, self, and parametric excitations where the vibration level can be controlled at the source.

Example 1: Forced Excitation

Let us  consider the vibration of an aircraft due to jet noise. The fuselage and other structures of the aircraft are subjected to intense pressure fluctuations present in the jet noise (which contains of the order of 1% of the total jet energy). This results in the so-called acoustic fatigue of the aircraft structures. Also, due to the fuselage vibration, noise is transmitted within the aircraft cabin. An effective solution to this vibration problem can be obtained by reducing the jet-noise excitation.

Figure 4.1 : Vibration in aircraft due to jet noise

An analysis of the jet flow (Fig.4.1) brings out the following:

1. High shear rate exists in the flow-mixing region in the vicinity of the jet exit. The region is full of small eddies oscillating at high frequencies.
2. Large eddies of low frequencies are present in the region away from the jet exit.
3. The larger the jet diameter, the larger the eddies, implying more of low-frequency noise. Likewise, the high-frequency content in the jet noise is more if the jet diameter is small.