V. F. Kopiev
M. Yu. Zaitsev
Central Aerohydrodynamics Inst. TsAGI), Moscow Branch, Acoust. Div., TsAGI, 17, Radio str., 107005, Moscow, Russia
Experimental investigations of the vortex ring noise carried out recently in an anechoic chamber showed the presence of a narrow peak in averaged spectra near the frequency f=1200 Hz the width of which was 200 Hz. Separate realizations recorded previously by tape recorder were used for averaging. The main questions are: What is the mechanism of narrow peak appearance? What is the reason for such a peak width? In the first part of this work a careful comparison between the experimental data and the simplest theoretical model is conducted. Vortex ring modes that could be responsible for the radiation are obtained. However, the investigation of separate time realization considered in the second part of this work showed that the process of sound radiation by vortex ring is of a more complex nature than it was initially expected. The registered radiation appeared to have the form of short time flashing oscillations (wave trains) not connected in phase with each other. Observed peculiarities of the acoustic signal are inherent to the narrow-band random process, but not the mixture of the harmonic component and broadband noise as it seemed earlier, i.e., the mechanism of vortex ring sound generation is probably connected with an interaction between vortex core modes and the turbulence near the core.