Sound Radiation From a Propeller in a Submerged Rib-Stiffened Duct

This paper examines the interaction of nonuniform flows with propeller blades in a submerged elastic duct. The acoustic radiation from the duct is calculated and correlated to the flow nonuniformities and the propeller and duct characteristics. The case of a stiffened duct with ribs is also considered and the dispersion relation of the duct modes is compared with that of a regular duct. The dispersion relation of the stiffened duct has a periodic structure similar to that of connected oscillators with large number of independent modes. Because of our interest in the acoustic radiation of such a system, we focus our attention on the flexure modes. The model is first tested with simple internal forces such as monopoles and dipoles. The results for unstiffened ducts show strong directivity as the dipole radial location moves closer to the duct wall. For stiffened ducts, the magnitude of the acoustic response as well as the directivity vary strongly and show large peaks near the stiffened duct free modes. For a propeller, an Euler code provides the pressure distribution along the blades. This represents the dipole strength distribution. Its radiated sound is calculated by summing up the contribution of the distributed dipoles. In this process, compact source effects are also taken into account.