# Noise characteristics of twin-square slot jets

TL;DR: In this article, two different twin square jet topologies were evaluated for noise suppression in different edge-vertex orientations and the results indicated that twin square jets effectively suppress noise up to 4 dB and 6 dB in terms of screech tonal SPL compared to an equivalent single circular jet.

Abstract: Acoustic measurements have been undertaken on two different twin square jet topologies. OASPL, spectra, directivity measurement and shock-cell visualization have been conducted to evaluate twin square jets in different edge-vertex orientations. The results indicate that twin square jets effectively suppress noise up to 4 dB in terms of OASPL and 6 dB in terms of screech tonal SPL compared to an equivalent single circular jet. The study promises that twin-jet topology could serve as an effective tool for noise suppression.

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TL;DR: In this article, a review of twin jet exhaust coupling is presented, focusing on the twin jet coupling situation that is rich in fluid flow physics, requires sophisticated analysis tools and is of relevance to aircraft with closely spaced engines and other industrial situations involving multiple jet flows.

Abstract: Interaction of twin supersonic jets can dramatically alter the dynamic pressures in the near-field and cause fatigue of aircraft structures. This review paper focuses on the twin jet coupling situation that is rich in fluid flow physics, requires sophisticated analysis tools and is of relevance to aircraft with closely spaced engines and other industrial situations involving multiple jet flows. The review consists of four sections: (i) a historical survey of twin jet exhaust coupling, (ii) rectangular twin jets with both spanwise uniform and non-uniform geometry, (iii) a description of multi-modes and mode jumps using higher order spectral analysis, and (iv) the suppression of twin jet coupling. The parameters that affect twin jet coupling include nozzle geometry, inter-nozzle spacing and nozzle operating conditions. Certain modes of resonant coupling increase the dynamic pressures drastically in the inter-nozzle region, whereas other coupling modes can suppress unsteady pressure levels. Since coupled res...

34 citations

### Cites background from "Noise characteristics of twin-squar..."

...Sabareesh et al [31] experimentally compared the acoustic characteristics of two different configurations of twin square slot jets with an equivalent single circular jet....

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TL;DR: The results obtained by the computational fluid dynamics (CFD) and three-dimensional background-oriented schlieren (3D-BOS) indicate that the periodic density fluctuation appears in the potential core each nozzle, and the flow structure of the twin jets is quite similar.

Abstract: Three-dimensional density fields of the twin jets were numerically and experimentally investigated. The present study focused on the comparison of the density distribution for the twin jets. The results obtained by the computational fluid dynamics (CFD) and three-dimensional background-oriented schlieren (3D-BOS) indicate that the periodic density fluctuation appears in the potential core each nozzle, and the flow structure of the twin jets is quite similar. The distribution of the normalized density value at the nozzle centerline agrees well with CFD and 3D-BOS. The density value of the shear layer between the nozzles increases as the interaction of the twin jets occurs. The trend of increasing and decreasing the interference between the nozzles was almost the same as each other. On the other hand, the position where the interaction of the twin jets starts and the growth rate of interaction were different. This is probably due to the effect of the laminar-to-turbulent transition occurred in the results of CFD. This result indicates that the laminar-to-turbulent transition can be estimated from the velocity fields obtained by CFD and particle image velocimetry (PIV).

9 citations

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TL;DR: In this paper, a three-dimensional (3D) transient mathematical model is developed to investigate the flow field and acoustic properties of the two-stream jets, which are compared with those of the single-stream jet at Mach number 0.9 and Reynolds number 3600.

Abstract: In order to study the interaction between two independent jets, a three-dimensional (3D) transient mathematical model is developed to investigate the flow field and acoustic properties of the two-stream jets. The results are compared with those of the single-stream jet at Mach number 0.9 and Reynolds number 3600. The large eddy simulation (LES) with dynamic Smagorinsky sub-grid scale (SGS) approach is used to simulate the turbulent jet flow structure. The acoustic field is evaluated by the Ffowcs Williams–Hawkings (FW-H) integral equation. Considering the compressibility of high-speed gas jets, the density-based explicit formulation is adopted to solve the governing equations. Meanwhile, the viscosity is approximated by using the Sutherland kinetic theory. The predicted flow characteristics as well as the acoustic properties show that they are in good agreement with the existing experimental and numerical results under the same flow conditions available in the literature. The results indicate that the merging phenomenon of the dual-jet is triggered by the deflection mechanism of the Coanda effect, which sequentially introduces additional complexity and instability of flow structure. One of the main factors affecting the dual-jet merging is the aperture ratio, which has a direct influence on the potential core and surrounding flow fluctuation. The analysis on the noise pollution reveals that the potential core plays a fundamental role in noise emission while the additional mixing noise makes less contribution than the single jet noise. The overall sound pressure level (OASPL) profiles have a directive property, suggesting an approximate 25° deflection from the streamwise direction, however, shifting toward lateral direction of about 10° to 15° in the dual-jet. The conclusion obtained in this study can provide valuable data to guide the development of manufacturing-green technology in the multi-jet applications.

7 citations

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TL;DR: In this paper, the equal and unequal twin circular slot jets were operated at high, moderate and low levels of underexpanded flow conditions and the results indicated that unequal twin jets are efficient screech tone suppressors at high and moderate levels of unexpanded flow condition but are ineffective at lower levels of underexpansion.

Abstract: Far-field and nearfield acoustic measurements have been conducted on equal and unequal twin circular slot jet for different spacing ratios. The twin jets were operated at high, moderate and low levels of underexpanded flow conditions. Measurement of acoustic power index, OASPL, spectra and shock-cell visualization have been carried out to evaluate equal and unequal twin jets. The results indicate that unequal twin jets are efficient screech tone suppressors at high and moderate levels of underexpanded flow conditions but are ineffective at lower levels of underexpansion. The cross correlation studies reveal an antisymmetric coupling for moderately underexpanded equal twin jet and symmetric coupling for low levels of underexpanded unequal twin jet.

5 citations

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TL;DR: In this paper, a theory for estimating the sound radiated from a fluid flow, with rigid boundaries, which as a result of instability contains regular fluctuations or turbulence is initiated, based on the equations of motion of a gas.

Abstract: A theory is initiated, based on the equations of motion of a gas, for the purpose of estimating the sound radiated from a fluid flow, with rigid boundaries, which as a result of instability contains regular fluctuations or turbulence. The sound field is that which would be produced by a static distribution of acoustic quadrupoles whose instantaneous strength per unit volume is ρv i v j + p ij - a 2 0 ρ δ ij , where ρ is the density, v i the velocity vector, p ij the compressive stress tensor, and a 0 the velocity of sound outside the flow. This quadrupole strength density may be approximated in many cases as ρ 0 v i v j . The radiation field is deduced by means of retarded potential solutions. In it, the intensity depends crucially on the frequency as well as on the strength of the quadrupoles, and as a result increases in proportion to a high power, near the eighth, of a typical velocity U in the flow. Physically, the mechanism of conversion of energy from kinetic to acoustic is based on fluctuations in the flow of momentum across fixed surfaces, and it is explained in § 2 how this accounts both for the relative inefficiency of the process and for the increase of efficiency with U . It is shown in § 7 how the efficiency is also increased, particularly for the sound emitted forwards, in the case of fluctuations convected at a not negligible Mach number.

4,697 citations

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TL;DR: Noncircular jets have been identified as an efficient technique of passive flow control that allows significant improvements of performance in various practical systems at a relatively low cost because noncircular jet rely solely on changes in the geometry of the nozzle as discussed by the authors.

Abstract: Noncircular jets have been the topic of extensive research in the last fifteen years. These jets were identified as an efficient technique of passive flow control that allows significant improvements of performance in various practical systems at a relatively low cost because noncircular jets rely solely on changes in the geometry of the nozzle. The applications of noncircular jets discussed in this review include improved large- and small-scale mixing in low- and high-speed flows, and enhanced combustor performance, by improving combustion efficiency, reducing combustion instabilities and undesired emissions. Additional applications include noise suppression, heat transfer, and thrust vector control (TVC). The flow patterns associated with noncircular jets involve mechanisms of vortex evolution and interaction, flow instabilities, and fine-scale turbulence augmentation. Stability theory identified the effects of initial momentum thickness distribution, aspect ratio, and radius of curvature on the initial flow evolution. Experiments revealed complex vortex evolution and interaction related to selfinduction and interaction between azimuthal and axial vortices, which lead to axis switching in the mean flow field. Numerical simulations described the details and clarified mechanisms of vorticity dynamics and effects of heat release and reaction on noncircular jet behavior.

537 citations

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TL;DR: In this paper, an elementary theory based upon the hypothesis that the acoustic energy originates from the interaction of the stream disturbances with the shock waves of the flow, and connecting the stream disturbance with the radiated sound, is suggested.

Abstract: Schlieren photographs of a small scale approximately two-dimensional air jet in air above the critical pressure have shown the existence of an instability displaying an anti-symmetric pattern, and the associated sound field having several distinguishing characteristics, has been photographed. An elementary theory based upon the hypothesis that the acoustic energy originates from the interaction of the stream disturbances with the shock waves of the flow, and connecting the stream disturbances with the radiated sound, is suggested. This is shown to be consistent with the physical dimensions of the phenomenon and predicts a sound field in reasonable agreement with that observed after certain simplifying assumptions have been made.

195 citations

### "Noise characteristics of twin-squar..." refers background in this paper

...Shock associated noise is further classified into screech [7] and broadband shock associated noise [8]....

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TL;DR: In this article, the authors report results of time-dependent numerical simulation of spatially developing free square jets initialized with a thin square vortex sheet with slightly rounded corner-regions, focusing on the near field of jets with Mach number 03-06 and moderately high Reynolds numbers.

Abstract: We report results of time‐dependent numerical simulation of spatially developing free square jets initialized with a thin square vortex‐sheet with slightly rounded corner‐regions The studies focus on the near field of jets with Mach number 03–06 and moderately high Reynolds numbers A monotonically‐integrated large‐eddy‐simulation approach is used, based on the solution of the unfiltered inviscid equations and appropriate inflow/outflow open boundary conditions The simulations show that the initial development of the square jet is characterized by the dynamics of vortex rings and braid vortices Farther downstream, strong vortex interactions lead to the breakdown of the vortices, and to a more disorganized flow regime characterized by smaller scale elongated vortices and spectral content consistent with that of the Kolmogorov (K41) inertial subrange Entrainment rates significantly larger than those for round jets are directly related to the enhanced fluid and momentum transport between jet and surroundings determined by the vortex dynamics underlying the axis‐rotation of the jet cross‐section The first axis‐rotation of the jet cross‐section can be directly correlated with self‐induced vortex‐ring deformation However, subsequent jet axis‐rotations are the result of strong interactions between ring and braid vortices, rather than being correlated with successive self‐induced vortex‐ring deformations, as previously conjectured based on laboratory observations The interaction between braid and ring vortices has the effect of inhibiting the periodic self‐induced axis‐rotations observed in the case of isolated square vortex rings

177 citations

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TL;DR: In this paper, an experimental and numerical study of a turbulent free jet of air from a sharp-edged square slot is presented. But the experimental results reveal the existence of pronounced mean streamwise velocity off-center peaks in the very near field, where positive mean static pressures were also found.

Abstract: Results of an experimental and numerical study of a turbulent free jet of air from a sharp‐edged square slot are presented. The jet was treated as elliptic in the numerical study that predicted mean streamwise velocity, mean static pressure, and turbulence kinetic energy distributions. The experimental results include the mean velocity, the turbulent normal and shearing stresses obtained with hot‐wire anemometry, and the mean static pressure acquired with a pitot‐static tube in conjunction with a pressure transducer. The experimental results reveal the existence of pronounced mean streamwise velocity off‐center peaks in the very near field, where positive mean static pressures were also found. Furthermore, the square jet is found to spread significantly faster in the near flow field than a jet from a round slot that has the same exit area as the square slot under identical test conditions. The general agreement between experimental results and numerical predictions is relatively good.

165 citations