The sudden area expansion is a critical element in silencer design since it incorporates a strong coupling between sound and the flow field even at low Mach numbers. A model assuming a non expanding jet with an infinitesimal boundary layer is used and solved with Wiener-Hopf methods. In comparison with experimental results a good agreement is found for relevant Strouhal numbers. In particular, a strong interaction between sound and flow is observed for a Strouhal number near one. This phenomenon is present in experimental data but has not received attention before. The theory predicts, in harmony with experiments, that the absorption due to interaction with flow increases with decreasing Strouhal number. It is proposed to use this effect to attenuate low frequency sound that in general is difficult to attenuate.

The project is carried out in co-operation with Dr. Susann Boij at KTH, Stockholm and Dr. Özge Yanaz Cinar.