Sergey Chernyshev

Vortex Ring Input in Subsonic Jet Noise

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 = 3.2·105) 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.

Experimental investigation of the role of instability waves in noise radiation by supersonic jets

Existing ideas of instability waves as the main dynamic noise sources in supersonic jets are tested for conformity with the data of acoustic measurements of this noise. Methodologically, the problem consists in the verification of the main principles of Tam’s theory of noise radiation by supersonic jets based on the ideology of instability waves in the shear layer of the jet and their key role in noise generation. Technologically, the study is based on a new technique for measuring the noise, namely, the azimuthal decomposition method developed by the authors. It is shown that on the Strouhal number range from 0.03 to 0.35 the theory satisfactorily describes the radiation pattern of the individual harmonics, while the initial amplitudes of the instability waves are in qualitative agreement with the assumption of their uniform distribution near the nozzle edge.

Correlations of jet noise azimuthal components and their role in source identification

In this paper, we study the subsonic cold jet noise using the azimuthal decomposition technique (ADT). The results of measurement of correlations for jet noise azimuthal components are reported. It is shown that the correlations for tone-excited jet strongly differ from those for unexcited jet; for example, an unexpectedly high value of correlation for tone-excited jet noise azimuthal components has been obtained for observation angles close to 90o to the jet axis. This behavior of the correlation renders it quite promising for the jet noise mechanism identification and source localization. An analytical model based on qudrupole source distribution is proposed for description of the correlations for unexited jet noise azimuthal components and its applicability is validated experimentally. I. Introduction t present the process of noise generation by turbulent subsonic cold jets is thought to be related with different mechanisms: fine-scale turbulence (Refs. 1 and 2), eigen-oscillations of large-scale vortex structures (Ref. 3), instability waves (Refs. 4 and 5) etc. Identification of these mechanisms in the process of jet noise generation and assessment of their contribution to the total noise is an important task. To achieve this on the basis of the measurements of only far-field total noise directivity is difficult, however. The azimuthal decomposition of the far field noise allows us to obtain more detailed characteristics of the sound sources; the previous studies (Refs. 3 and 6) on modeling the azimuthal components show that the observed experimental data for cold subsonic jet noise can be explained if both large-scale vortex structures (vortex rings) and fine-scale turbulence (modeled as moving point quadrupoles) are accounted for as sound sources, the vortex rings’ contribution to the total noise being about 40%. An excellent collapse of the modeling and experimental curves has been observed, which evinces that this is a plausible framework to model turbulent cold subsonic jet noise. To further validate this model and get new insights into the structure of the sound sources, the analysis has been expanded to include correlation characteristics of the azimuthal components. In Ref. 7 the first results of correlation measurements of azimuthal harmonics for unexcited cold subsonic jets (i.e. the correlation between the simultaneous measurements for the same azimuthal mode at two different points) have been reported that experimentally verify the absence of correlation between different modes, which is what should be expected from the theoretical considerations (azimuthal components are orthogonal). A The cross-correlation curve for tone excited jet has characteristic peculiarities, albeit it is generally similar in shape to the unexcited jet curve. These curves demonstrate that the large-scale structure sound radiation is concentrated in the region at the right angle to the jet, because in this region the cross-correlation curve for the tone excited jet is significantly higher than the curve for the unexcited jet. This property of the curve is somewhat unexpected from the general view that large-scale structures in jet radiate in the downstream direction, whereas at the right angle to the jet direction it is so-called fine scale turbulence that radiates. In Ref. 7 the spatial correlations of the azimuthal components have been obtained, but no attempt has been made to propose a theoretical model to explain the observed spatial correlation curves. Such an attempt is made in the present work. The sound sources are modeled as a distribution of moving point quadrupoles. The results of modeling are compared with the results of measurements of cross-correlation function for the far sound field of unexcited cold jet with velocity 120 m/s. The comparison is performed for the zeroth harmonics a0 in two frequency bands 600. II. Experimental Setup

Aeroacoustics of supersonic jet issued from corrugated nozzle: new approach and prospects

It is generally accepted that one of the main sources of acoustic radiation from a supersonic jet is due to spatial instability wave packets propagating downstream within the jet. This approach has enabled us to explain and predict the principal features of sound radiated by a supersonic jet with a circular nozzle. The aim of this present work is to generalize such an approach to jets with a circular nozzle deformed into a weakly corrugated (s-lobed) shape. This leads to the presence of two additional parameters, the lobe number (an integer) characterizing the corrugation wavelength and the corrugation amplitude. Their presence can be shown to lead to a resonant coupling between different instability waves which, in turn, can both intensify and suppress the aerodynamic noise. The assumption of a small corrugation amplitude allows us to consider the problem analytically using disturbance theory methods. The structure of the eigen-oscillations is determined at leading orders, where it is shown that differing azimuthal harmonics are coupled over a wide range of frequencies. The result of such couplings on the radiated sound is significant, even for a small corrugation amplitude. The above suggests that the effect of mode coupling, due to nozzle corrugation, could be used as a tool for noise control in supersonic jets.

Simulation of azimuthal characteristics of turbulent jet noise by correlation model of quadrupole noise sources

In the modeling of the noise sources in turbulent jets two open questions are still remain. The first, what is the multipole type of the sources? The second, what kind of stochastic fields can be used for the noise source description. These issues are analyzed in this work. Two-point cross-correlation of the jet noise is used to determine the multipole type of sources. Modeling of the cross-correlation provides clear evidence in favor of the quadrupole type of the jet noise sources. In the context of the second issue the recently developed two-stage correlation model is tested on the basis of measurements of azimuthally decomposed jet noise. The results show a promising agreement in a wide range of parameters.

Large vortex input in a noise of artificially excited subsonic jet

The main goal of the paper is to assess the contribution of large-scale vortices to the noise of the subsonic turbulent jet 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 2032Hz) decomposed on the separate azimuthal components with the help of azimuthal decomposition technique (ADT). The careful comparison of these results gives the quantitative assessment of the contribution of vortex rings to the real subsonic jet noise and gives the main source localization. The proposed approach for jet noise modelling was additionally checked by measurements with two co-axial microphone arrays. Good agreement of azimuthal noise modelling using the same parameter set for two radial distances was demonstrated.

Octupole radiation of localized vortex

For a weakly compressible inviscid fluid (with the Mach number M<<1), we consider the problem of obtaining the maximum possible number of terms of the asymptotic expansion of a sound field in powers of the Mach number for the aerodynamic sound generated by localized vortices. Using Crows approach and the matching procedure of Van Dyke, quadrupole and octupole terms are obtained in the asymptotic expansion. It is shown that higher moments of the sound field cannot be obtained in such a procedure because of the divergence of the respective integrals in the inner expansion. We discuss the possibility of representing the sound field in terms of expressions for the quadrupole and octupole moments of an incompressible flow with a retarded argument. The theory of vortex ring eigen-oscillations and the formulas for quadrupole and octupole sound obtained previously by the authors on the basis of matching asymptotical expansions are used for predictions of non- axisymmetrical octupole radiation generated by eigen-oscillations of the vortex ring. The result obtained previously for axisymmetric case is elaborated for 3D problem. The calculation of octupole term has shown that for thin vortex rings this component depends on the ratio of two small parameters and may appear to be large.

On the Possibility of Noise Control in Corrugated Supersonic Jets

It is generally accepted that one of the main sources of acoustic radiation from a supersonic jet is due to spatial instability wave packets propagating downstream within the jet. This approach has enabled to explain and predict the principal features of sound radiated by a supersonic jet with a circular nozzle. The aim of this present work is the generalize such an approach to jets with a circular nozzle deformed into a weakly corrugated (slobed) shape. This leads to the presence of two additional parameters, the lobe number (an integer) characterizing the corrugation wavelength and the corrugation amplitude respectively. Their presence can be shown to lead to a resonant coupling between different instability waves which, in turn, can either intensify or suppress the aerodynamic noise. The assumption of a small corrugation amplitude enables us to investigate the problem analytically using disturbance theory methods. The structure of the eigen-oscillations is determined at leading orders, where it is shown that differing azimuthal harmonics are coupled over a wide range of frequencies. The result of such couplings on the radiated sound is significant, even for a small corrugation amplitude. The above suggests that the effect of mode coupling, due to nozzle corrugation, could be used as a tool for noise control in supersonic jets.

Directivity of noise radiated by a vortex ring

It is shown theoretically that the vortex ring noise is to consist of three quadrupoles with axis along the ring velocity vector. These are axisymmetrical modes and two‐mode families with dependency on the azimuthal angle ‘‘fi’’ of exp(i*fi) and exp(2i*fi) kind. Each family consists of an infinite number of closely disposed modes with an identical accumulation point. This fact does not allow the separation of the quadrupoles from the spectrum analysis since all the modes are disposed in a coinciding narrow frequency interval. To reveal the noise directivity, the noise generated by a freely flying vortex ring in the anechoic chamber is measured with the use of an array of six microphones set around the movement axis. A special analysis of six measured signals which account simultaneously for azimuthal and transversal directivity of acoustic signals has allowed the separation of each quadrupole component and has proven that the vortex ring noise really consists of three equally excited quadrupoles perceived in the far field as narrow‐band random noise. [The investigations were partly realized with support of CRDF according to award RE2‐134.]