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Using the example of an integrated airframe, we performed a computational and experimental study of the possibility of using geometric diffraction theory (GDT) to calculate the sound diffraction when the airframe is replaced with simulating flat polygonal screen as pertains to the effectiveness of noise screening of aircraft power plants. We compared (1) impulse responses experimentally measured with maximum length sequences and the reciprocity theorem for both a three-dimensional model of the airframe and its plane model with (2) the responses calculated using GDT; the results showed good qualitative agreement for observation points in the geometric shadow zone and in the illuminated area, as well as quantitative coincidence in the frequency range characteristic of jet noise and the first harmonics of the blade passing frequency of a propeller or fan.

The paper presents the results of a computational–experimental study of jet noise reduction using the screening effect. According to correlation theory, the noise sources of a single circular jet are represented by a set of uncorrelated quadrupoles. Using the basic relations of geometric diffraction theory (GDT), expressions are obtained for the sound field emitted by a point quadrupole source located near an acoustically rigid infinite half-plane. Using the expressions obtained in the framework of the GDT, the correlation model of the jet noise sources was adapted to the calculation of the sound field in the presence of a flat rectangular screen. A comparison of the calculated and experimentally measured spectra of sound pressure levels, performed at a given jet velocity and various screen positions, showed good qualitative, and for certain observation angles, quantitative agreement.

The paper presents the results of an experiment that uses the MLS technique to measure the impulse response when a signal passes through an air jet. The data are compared with the numerical simulation results obtained by solving the equation for sound propagation in a steady flow using a finite-difference method. It is shown that the longitudinal component of the flow causes drift and focusing of the signal.

A method is proposed for decomposing the acoustic field of a compact source into azimuthal components using irregularly arranged microphones.

A qualitative model of the mechanism of intensification of jet noise issuing from a nozzle located near a wing is proposed. A two-dimensional model problem on the diffraction of a plane acoustic wave at the edge of a nozzle nozzle located near a half-plane simulating the wing edge is formulated. It is shown that diffraction on the wing edge of Kelvin-Helmholtz instability waves developing from the edge of the nozzle can lead to the significant intensification of the acoustic energy radiated into the far field.

The possibility of acoustic control of instability waves formed in the mixing layer of a jet is experimentally investigated. The feasibility of suppressing a hydrodynamic instability wave in a subsonic turbulent jet by an external acoustic action is demonstrated. This result can be used in designing active control systems for jet noise suppression.

We study experimentally how plasma actuators operating on the basis of surface barrier high-frequency discharge affect jet noise characteristics. The results of investigations of air jets (100–200 m/s) have demonstrated that the studied plasma actuators have control authority over the noise characteristics of these jets. An actuator’s effect on the jet in the applied configuration is related to acoustic discharge excitation and to a large extent is similar to the well-known Vlasov-Ginevsky effect. It has been shown that jet excitation in the case of St ∼ 0.5 using the barrier-discharge plasma actuator leads to broadband amplification of jet sound radiation. The jet excitation in the case of St > 2 leads to broadband noise reduction if the action is sufficiently intensive.

The mechanisms of noise generation by turbulent jets associated with a cascade of turbulent perturbations are considered from generation of primary eddies to their disintegration. A new correlation model proposed for describing these mechanisms is based on simulation of their generation as a random process with deterministic description of individual eddies. The model makes it possible to analyze available correlation models based on experimental modeling of the correlation function: monopoles, dipoles, or quadrupoles. The main difference between these models is both the different multipole structure of assumed sources and implicit “hard” and “soft” variants of source generation, as well as in the main variable for which the correlation function is developed. This approach is used for developing the correlation model of the noise generation mechanism related to oscillations of surface eddies. The jet noise inferred from the proposed model is well consistent with experimental data on circular jet noise.

The paper considers a model problem of instability wave generation that occurs at a tangential discontinuity leaving the edge of a half-plane because of a specially adjusted external action, which models the basic characteristics of an active action on a jet by means of small-size actuators. The dependence of the instability wave amplitude on both the degree of spatial boundedness of the external action and its position with respect to the edge is investigated.

The turbulent energy dissipation rate time-scale and length-scale has been routinely used for the prediction of noise from turbulent flows, particularly jet streams. However, this is not the only possible choice. In general, scales evolving in a turbulent medium are threefold. First, those associated with the mean flow; second, those attributed to the turbulence and the mean flow interactions; and third, scales related to the turbulence-turbulence interactions. In this paper, special attention will be paid to further study of the underlying physics of aerodynamic noise by examining various time-scales. To do so, three time scales, namely, dissipation, production, and strain rate time scales, are defined and used in the source modelling to emphasis the effect of the turbulence structures at different jet regions on the jet noise production mechanism. The required mean value and turbulence parameters are obtained using a modified k − ∈ turbulence model, and Lighthill’s Acoustic Analogy is used for the prediction of the emanated noise.

The acoustomechanic efficiency of a turbulent jet is determined as the ratio of acoustic radiation power to the kinetic energy flux of the jet. According to the Lighthill acoustic analogy, the acoustomechanic efficiency of axially symmetric jets is proportional to the fifth power of the Mach number. In this paper, on the basis of an experimental study and an analysis of published data, the influence of various factors that can affect the structure of a jet and, consequently, its acoustomechanic efficiency is investigated. In particular, the influence of water injection on the reduction of the jet noise is analyzed. The results of analyzing experimental data show that a systematic deviation from the relation between acoustic and mechanical jet energies obtained from the Lighthill acoustic analogy arises in the case of the outflow of low-velocity jets of low density and with the development of longitudinal vorticity in the flow.

We demonstrate the possibility of lowering the noise level of a subsonic jet using a crimped form of nozzle. The noise reduction is on the order of 2 dB.