CR 02: Flow-generated noise in ventilation systems

When an obstruction is present in a ventilation ductwork, the noise level may be considerably higher than the level measured without the obstruction. This excess noise is due to the interaction of the flow with the element and it is called flow-generated noise in the element. Numerous examples may be found in the literature where this mechanism occurs, such as flow noise generated in duct elbows, dampers, grilles, louvers, duct discontinuities. It reduces the attenuation performance of dissipative silencers and is responsible of the well-known aeolian tones of wires and rods. 
The bandwidth of the flow-generated noise spectrum is more or less important according to the type of component and flow characteristics and the maximum level is observed at a frequency f such as the Strouhal number St= fd/U (where d and U are typical obstruction dimension and mean flow velocity) is constant, the constant depending on the component geometry. Some of these phenomena may generate very narrow bands or even tones, especially when aeroacoustic or vibroacoustic resonances occur. 
Numerous studies have been carried out for a better understanding and prediction of flow-generated noise in ventilation systems. Most of this research rests on empirical approach, which allows to make predictions whose limit of validity depends on the model and the considered application. More recent attempts have been made to predict flow-generated noise levels in obstructions such as orifice plates by an entirely numerical approach, but these attempts remain marginal in industrial applications. The objective of this paper is to make a non-exhaustive review of this research, considering that the obstructions are rigid, i.e. they do not radiate noise because of mechanical excitations by the airflow. Otherwise, a coupling between the flow and the vibrating element should be considered. Such a coupling is not very efficient in air, unlike in water, and may be avoided by stiffening the element. 
Duct elements that are dealt with here are dampers, grids, orifice plates, elbows, takeoffs, duct discontinuities, more or less streamlined spoilers. To simplify the presentation the different references used for this review will be successively considered.