Octupole Noise Radiation of Turbulent Jet and It Connection with Large Scale Structures
V.Kopiev, M.Zaitsev (TsAGI, Moscow, Russia)
The new method of experimental investigation of turbulent jet noise was elaborated. This method is based on azimuthal decomposition of noise with the help of a circular array. The source localization procedure was based on the aeroacoustic theory of noise generation in vortex rings (quadrupole and octupole components) built by authors and with the use of special stroboscope vizualization. It is shown theoretically by the example of vortex ring, that the main difference from quadrupole radiation directivity in the sound field radiated by convecting vortices could be connected not only with a convective factor but with an octupole term as well. It is shown experimentally that it is possible to separate an octupole component from a quadrupole one in the turbulent subsonic jet noise. It is interesting, that just vortex ring octupole term gives the possibility to simulate the measuring forms of azimuthal noise components for the jet. Thus, the acoustic measurements made according to the method proposed turns out to be a powerful means of non-invasive diagnostics of turbulent jet structure. Besides this method is a new means for developing theoretical and numerical models of aerodynamical noise generation, since separate azimuthal noise components offer a more detailed information on the source, than the total isotropic jet noise.
Section : 7