Beating in propeller singing

The unsteady nature of the shear flow leaving the trailing edge of a propeller blade may give rise to a correlated discrete vortex structure that is also a source of a significant noise, so-called propeller singing. It is well known that beating phenomena occur during the propeller singing. To understand this phenomena, the acoustic field generated by the cavitation-free propeller was investigated. The work started with an analysis of elementary kinematics of sound radiation by the propeller, considering the rotational motion of the blades relative to acoustic media. Then the acoustic spectrum due to the blade force resulting by inhomogeneous inflow was considered. Using the analysis, frequency domain expressions were obtained to examine the beating. Directivity was also calculated to study the nature of the propeller sound field.