Sound generation and radiation from rotor tip-vortex pairing phenomenon

Sound generation and radiation from rotor tip-vortex pairing are calculated numerically. Wake geometries of a two-bladed rotor in axial flights are calculated using a time-marching free-wake method without a nonphysical model of the far wake. The acoustic field can be obtained using acoustic analogy. To show the accuracy of the flow calculation scheme, the tip-vortex geometries of the present calculation are compared with those done in the previous publication of a small-scaled rotor experiment. The accuracy of the vortex sound calculation scheme is validated by comparing its numerical and the analytic solution of circular vortex-ring interaction. The tip-vortex pairing consists of a turn of the tip-vortex from one blade rolling around the tip-vortex from the other blade. The tip-vortex pairing amplifies the noise and generates new dominant frequencies. The tip-vortex pairing noise has a quadrupole directivity pattern, and it is especially dominant in the direction of the rotation axis. The peak components of the noise spectrum represent the pairing period and an instability mode.