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.
展开▼
