Victor F. Kopiev

The Role of Large-Scale Vortex in a Turbulent Jet Noise

In investigations of the role of large vortices in round turbulent Jet noise usually the case in point is mechanisms of radiation produced by collective movement of several vortex rings (pairing, tearing, etc.). The authors suggest to consider another radiation mechanism by which the mdividual vortex ring in a round jet can be an intensive sound source itself. Recent theoretIca and experimental investigations by the authors have shown that the total vortex ring noise consists of three components of similar frequencies but of different directivity. Each component, in its turn, is a stochastic process since it is generated by a large number of oscillations with different (though close) frequencies. So, the principal goal of the work is to “recognize” the individual vortex noise in the total noise of turbulent jet (or to demonstrate ineffectiveness of such a radiation mechanism in a real jet). Now we present the first results m this direction. The measurement procedure developed in the investigations of vortex ring noise and based on synchronous decomposition of signal from hexagon array of microphones with subsequent averaging is used for the analysis of low-speed round jet noise. A microphone array for recording was placed in the far sound field of the jet. Two cases are considered: (i) non-excited jet, (ii) jet excited by a sinusoidal signal. Measurements of nerodynanuc characteristics of jet and visualization were conducted before the acoustic measurements.

Mechanism of sound radiation by turbulence near a rigid body

The problem of aerodynamic sound generation by a transverse turbulent flow past a rigid cylinder is considered. An anomalous displacement of the equivalent dipole source to a region far downstream of the cylinder is found using the method of azimuthal decomposition of the sound field. The effect is explained on the basis of an analysis of the two-dimensional problem of multipole sources near a circle in the incompressible approximation. It is shown that the dipole displacement is a result of the interference of the original quadrupole source and its dipole reflection. It is concluded that in the weakly compressible case the mechanism of dipole noise generation consists in the uncompensated reflection of weak quadrupoles from a curvilinear surface.

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.

Jet noise control using the dielectric barrier discharge plasma actuators

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.

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.

Instability wave control in turbulent jet by plasma actuators

Instability waves in the shear layer of turbulent jets are known to be a significant source of jet noise, which makes their suppression important for the aviation industry. In this study we apply plasma actuators in order to control instability waves in the shear layer of a turbulent air jet at atmospheric pressure. Three types of plasma actuators are studied: high-frequency dielectric barrier discharge, slipping surface discharge, and surface barrier corona discharge. Particle image velocimetry measurements of the shear layer demonstrate that the plasma actuators have control authority over instability waves and effectively suppress the instability waves artificially generated in the shear layer. It makes these actuators promising for application in active control systems for jet noise mitigation.

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

Subsonic Jet Noise Suppression by a Corrugated Nozzle

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.

Acoustic control of instability waves in a turbulent jet

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.

Intensification and suppression of jet noise sources in the vicinity of lifting surfaces

Theoretical and an experimental investigations of jet noise modification are carried out for the jet issuing in the vicinity of the wing. The effect of interaction between instability waves in the jet and flap solid boundaries has been studied. Expermental investigation of jet noise from a dual flow nozzles near a wing provides assessment of several parameters (chevrons on primary and secondary nozzles, distance between the trailing edge of the flap and the nozzle, distance between the plane of the wing to the jet axis, presence of the pylon, etc.) on jet noise installation effect.