How do we Control?

It is known that atmospheric attenuation of noise is higher for high frequencies than that for low frequencies.

Figure 4.2: Jet noise reduction through modified jet exit

By using multiple number of smaller jets (Fig.4.2a) instead of a big one, the noise at the jet exit can be made to contain predominantly high frequencies. The noise (excitation) impinging on the aircraft structure would then be considerably reduced due to high atmospheric attenuation.
Another method of reducing the jet noise is to provide a serrated jet exit (Fig.4.2b), which improves the mixing of flow in the exit region by making it more gradual. This, in turn, reduces the shear rate and the resulting large-scale turbulences.

Recently, a new technique of impinging microjet (as shown in Figure 4.2c) is being explored for controlling the jet noise. A spining disc containing microjet is introduced around the central nozzle. By varying the disc from 0 to 150 Hz it has been observed that the jet noise could be controlled effectively.

Similar Application

The flow-induced vibration of air duct can always be controlled by providing a tubular flow smoothener at the exit of the fan feeding the duct. The flow smoothener reduces the turbulent pressure fluctuations exciting the duct walls.