Abstract:
Expressions for the edge noise of 2-D moving vortices and 3-D moving dipoles (ringvortices) are presented. The integrals, involving Green's functions in the time domain, are approximated in the far field in terms of the halved derivative of the near-field pseudosound. Attention is paid to the transition from near-field pseudosound to far-field wave behavior, e.g., in the closed form response to a triangular pulse. In 2D the edge sound due to a moving vortex has two components of equal order of magnitude. One is retarded pseudosound from the edge, the other is due to the motion along the trajectory as if the vortex moved in free space. The latter component is deformed as the halfth derivative of free-space pseudosound. In 3D the pressure wave generated by the motion of a dipole near an edge is a factor (r/c)[sup 1/2] times stronger than the pressure of the pseudosound. The free motion of a dipole in 3D generates quadrupole sound, the proximity of an edge amplifies it to dipole sound. The vortex sound in 2D does not show this amplification in the edge effect. The time-domain analysis is efficient and illustrative, and seems more attractive than methods using the frequency domain.