Vortex Ring Input in Subsonic Jet Noise

Abstract

The main goal of the paper is to assess the contribution of large-scale vortices to the noise of the subsonic turbulent jet (M = 0.35, Re = 3.2•10 5) in the selected frequency bands (Sh~0.2-0.45). The mechanism of sound radiation according to which a separate vortex structures itself can be a significant sound source is considered. Vortex core eigen-modes are responsible for noise radiation according to this mechanism. The work is subdivided on three coupling items: (i) theoretical investigation of noise radiation of separate vortex ring (especially little-known octupole contribution to sound radiation), (ii) visualization of large vortices in excited jet and (iii) experimental investigation of turbulent jet noise (subsonic jet with velocity 120m/s excited at 2000Hz) decomposed on the separate azimuthal components with the help of azimuthal decomposition technique (ADT). On the final stage the careful comparison of these results gives the quantitative assessment of the contribution of vortex rings to the jet noise in a tone-excited turbulent jet under consideration.

New correlation model for the cascade of turbulent pulsations as a noise source in jets

“…the same time, similar to the approach based on monopole sources [1], such an approach is also incon sistent with experimental data [8,12], which indicate the quadrupole character of acoustic radiation. At first glance, the quadrupole character of acous tic radiation contradicts the fact that when the integral noise of the turbulent jet is measured by a single microphone (without preliminary harmonic decom position of a signal), sound radiation appears close to isotropic.…”

“…At the same time, the contri bution of the monopole source to jet noise is likely insignificant. This is evident, for example, from the experimental data on jet noise [8,12] obtained by the azimuthal decomposition method, which indicate the quadrupole character of acoustic radiation. Therefore, the monopole source is not considered in this work.…”

“…The characteristic feature of quadrupoles (6)- (8) is the fact that their convolution with Green's function (4) represents separate azimuthal harmonics in a cylindrical system of coordinates with the x axis corresponding to the jet axis ( Fig. 1).…”

“…Therefore, the monopole source is not considered in this work. Thus, it may be accepted that the quadrupole source is characterized by a zero trace; i.e., it represents a com bination of quadrupoles (6)- (8).…”

“…At present, no anonymous opinion exists on how different mechanisms characterizing the dynamics of turbulent pulsations contribute to noise generation. For example, different authors consider different factors as sources of jet noise: small scale tur bulence [1][2][3][4][5][6], instability waves [6,7], and oscillations of large scale turbulent eddies [8].…”