Aeroacoustics of pipe systems with closed branches

Flow induced pulsations in resonant pipe networks with closed branches are considered in this review paper. These pulsations, observed in many technical applications, have been identified as self-sustained aeroacoustic oscillations driven by the instability of the flow along the closed branches. The fundamental aspects of the flow induced pulsations are discussed, with particular attention to the description of the sound sources. A single mode model for the prediction of the self-sustained oscillations is presented, the "energy balance". This model consists of the evaluation of the amplitude of each acoustic mode of the system by means of a balance between the acoustic source power and the acoustic power losses. The main components of this prediction method are discussed; these are the evaluation of the acoustic behavior of a pipe network and the modeling of the sound sources and the acoustic losses. Several field and scale model examples of pipe systems displaying self-sustained oscillations are presented, in order to discuss the parameters influencing the aeroacoustic behavior of pipe networks. Finally some counter-measures for the prevention of self-sustained oscillations are reviewed and perspectives for future work are considered.