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The primary objective of this investigation is to determine experimentally the sources of jet mixing noise. In the present study, four different approaches are used. It is reasonable to assume that the characteristics of the noise sources are imprinted on their radiation fields. Under this assumption, it becomes possible to analyse the characterist...
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... Panda et al. (2005); but there are some errors in their figures 12( a ) and 12( b ). The correct data are presented here in figures 25 and 26. In measuring these data, the laser probe was kept fixed at the centreline of the jet at x/D = 12 for the Mach 1.8 jet and x/D = 10 for the Mach 1.4 and Mach 0.95 jets. The far-field microphone was moved at 10 ◦ intervals on a circular arc. Figure 25 shows the directivity of the normalized ρuu correlation, that is ρuu, p max / [( ρuu ) rms ( p ) rms ]. Figure 26 shows the directivity of ρ , p max / ( ρ rms p rms ). Here, a prime represents the deviation from the mean. These directivity data are important. First, they reveal that there are significant correlations between jet turbulence fluctuations at a point inside the jet and the sound field radiated in the downstream direction. This is true regardless of which turbulence related fluctuation is used. Since the Rayleigh scattering measurements are concentrated in a very localized volume in the jet, a 20 % correlation is a huge number. Another significance of the data is that the normalized correlation drops off rapidly as the inlet-angle direction decreases. For angles less than 120 ◦ , the correlation practically diminishes to the noise level. The directivity pattern as given by figures 25 and 26 does not change when the laser probe is moved radially over the half-width of the jet and axially over a few jet diameters. This indicates that the direct correlation function and noise sources are highly directional. It is clear from figures 25 and 26 that the noise source measured by the laser probe exhibits strong directional characteristic. It radiates noise primarily in the downstream direction. There is practically no noise radiation to the sideline and upstream directions. Panda and his coworkers were fully aware of this unusual characteristic. They offered an explanation for the observed directional variation based on the two-noise source model. For the purpose of completeness, we will amplify their reasoning below, using the fine-scale turbulence and large turbulence structure noise source model of figure 4. The two-noise source model of figure 4 suggests that the dominant source of noise radiating to the sideline and upstream directions is the fine-scale turbulence of the jet flow. In measuring the turbulence fluctuations in a jet by the ...
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We present a study of three-stream nozzle concepts with potential to reduce takeoff noise of future commercial supersonic aircraft. The concepts were evaluated at realistic cycle conditions in a subscale acoustic facility. Computations solving the Reynolds-Averaged Navier-Stokes equations provided insight into the changes in the flow field that can...
The noise generation mechanism of screech tone by shock leakage in underexpended round jets is experimentally investigated by means of phase-averaged velocity fields. Two jet flows at Mach numbers 1.10 and 1.15 are measured by a particle image velocimetry apparatus simultaneously with their near acoustic fields and sorted according to their phase w...
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... Finally, to justify the importance of the linear vortex-wave coupling induced by the shear flow non-normality, we present an additional weighty argument. The specificity of the linear generation mechanism of acoustic waves, substantiates the observations of two different sound generating mechanisms, namely omni-directional and highly directional sound emission from small-and large-scale turbulence structures, respectively [60]. These are related to nonlinear and linear mechanisms. ...
This paper aims to establish the pivotal role of the linear non-modal dynamics of perturbations in the aerodynamic sound emission of turbulent temporal shear/mixing layers at the self-similar stage of evolution. This is achieved by comparing the results of direct numerical simulations (DNS) of the mixing layer to the linear non-modal analysis of the flow central/body part model – 3D homentropic, unbounded flow, U0(Ay, 0, 0), with shear rate, A. The non-modal analysis captures the only linear mechanism of acoustic wave generation – the linear vortex-wave mode coupling induced by the non-modal dynamics of perturbations in shear flows. The efficiency of this linear generation is uniquely determined by the mode-dependent Mach number,M= Aλx/cs, related to the convective Mach number viaM= Mcλx/πΔy. Here, λx, cs and Δy denote the streamwise wavelength, speed of sound and shear-layer width, respectively. DNS of compressible turbulent temporal mixing layers were performed for different convective Mach numbers (Mc = 0.3, 0.7, 1.4) and simulation boxes (Lx, Ly, Lz) for reliably identifying the essence of the origin of sound in the turbulent layer. The simulations clearly show the decisive role of the linear non-modal mechanism of sound generation in the flow. This novelty highlights the significance of M in the sound generation, thus ensuring maximum efficiency for harmonics with largest streamwise wavelength, λx = Lx. Accordingly, the aerodynamic sound efficiency is determined by McLx rather than Mc. Overall, our analysis points to the inadequacy of employing the modal approach to describe compressible dynamic processes in the turbulent flow.
... The turbulent mixing noise of the supersonic jet is contributed by two distinct components for jet flow, which are large turbulence structures and fine-scale turbulence. The large turbulent structures of the jet flow are quasi-coherent and quasi-orderly propagated and dominating at the downstream region beginning from the jet exit with tremendous speed relative to ambient sound speed, which means the minimum Mach number should be achieved by 1.0 [12][13][14]. The fine-scale turbulence of supersonic jet flow is radiated omnidirectionally and propagated mainly in the upstream and side-line regions. ...
Noise emission is an essential issue for the aviation industry, as it harms health and induces various physiological responses. The noise generated by supersonic jets is very intense. It will cause fatigue and even damage the human hearing system in the surrounding area of jet operation. Besides, the experimental and prototyping cost for the jet model is prohibitive, and it is a vast project and process that takes a lot of time to run. The purpose of this study is to determine the sound propagation behaviours of a supersonic jet in the far-field region and to analyse the consequences of the velocity of a supersonic jet on sound propagation of supersonic jet by using computational fluid dynamics (CFD) and computational aeroacoustics (CAA) simulations. This study focused on the perspective of observing the distance of the receiver when receiving the sound propagation of a supersonic jet, the observed angle of the receiver when receiving the sound propagation of a supersonic jet, and the velocity of the supersonic jet. The CFD and CAA analyses were performed in transient state simulation and the 2-inch Acoustics Reference Nozzle (ARN2). The result shows that the overall SPL throughout the frequency is proportional to the jet velocity of the supersonic jet. However, the distance and angle of the receiver gave different results in sound propagation behaviour. The results also conclude that as the distance between the receiver and jet nozzle exit increases, the overall SPL trend will decrease throughout the frequency increase. As the vertical distance between the receiver and the axisymmetric line of the jet nozzle increases, the frequency of the receiver starts to observe will decrease.
... An alternative is to model the tensor by a wavepacket [29]. In this case, the jet is treated as a distributed acoustic source [30,31]. The predominant low frequency noise sources are downstream while higher frequencies sources are upstream, near the nozzle [30,31]. ...
... In this case, the jet is treated as a distributed acoustic source [30,31]. The predominant low frequency noise sources are downstream while higher frequencies sources are upstream, near the nozzle [30,31]. Modal decomposition of the jet indicated that the axisymmetric mode (m = 0) is responsible for most of the acoustic energy measured at low angles [24]. ...
In this work, an experimental and numerical analysis of the feasibility of using acoustic liners to reduce the acoustic far-field radiation of a subsonic jet near a flat plate is presented. The Boundary Element Method (BEM) with a wavepacket model of the jet noise and considering an acoustic impedance boundary condition was used to calculate the far-field noise for different conditions. A parametric optimization was performed in order to find an optimum impedance in terms of the Overall Sound Pressure Level reduction. Semi-empirical models of acoustic liners were then used to design a liner with an impedance as close as possible to the optimum impedance. The liners were built using additive manufacturing and tested in terms of their acoustic impedance and the resulting noise reduction when applied to a region of the flat plate. The BEM simulations, considering the impedance, showed good agreement with experimental data. Overall, by means of the BEM model, optimization and liner semi-empirical models, it was possible to design an acoustic liner which significantly reduced the installation noise at the far-field, with the overall noise reduction between 2 dB to 4 dB depending on observation angle.
... To date, a complete dynamic model of a turbulent jet as an object of fluid dynamics has not yet been developed; in particular, there is no commonly accepted point of view on the nature of noise generation by such a flow [5][6][7][8][9][10]. From the viewpoint of the jet near-field fluctuations, the situation is clearer. ...
... In [13,18,19], the possibility of active control of instability waves was studied in order to reduce their intensity in the near field and accordingly attenuate the noise of the interaction of the jet closely integrated with the wing [20,21]. In addition, in high-speed jets, instability waves can themselves become noticeable sound sources even in the absence of nearby scattering surfaces [7,9,10]. ...
... These fluctuations are nonlinear and are described by a complete system of Navier-Stokes equations. In this case, the mean mixing layer is unstable according to the Kelvin-Helmholtz mechanism and causes the development of linear large-scale perturbations -instability waves developing from the nozzle edge downstream in the form of wave packets [7,[9][10][11]. In the mixing layer, these perturbations are small compared to the small-scale vortex fluctuations and can only be detected by special data processing techniques [12,29,30]. However, outside the mixing layer (in the zone of potential mean flow), the intensity of the small-scale fluctuations decreases significantly faster with distance than the intensity of those associated with instability waves (Fig. 1b, zone B). ...
Final paper could be found on https://rdcu.be/dHf4q .
It is shown that the velocity fluctuation spectra measured using a hot-wire in the potential flow region of a turbulent jet near field in the presence of a co-flow can be converted into the spectra of pressure fluctuations. The proposed conversion method is based on the fact that the structure of instability waves, which make a decisive contribution to the jet near-field fluctuations, resembles homogeneous one-dimensional waves, which makes it possible to locally link the pressure fluctuations and the fluctuations of the streamwise velocity component measured by a hot-wire.
... The exit shape of the jet can be changed by mass transport phenomena with the surrounding medium to enhance or retard the mixing process in free-shear layers like jets. The results are in good agreement with Tamet al. 28 examined the effects of mechanical tabs on the nozzle lip on under-expanded supersonic jet mixing. As the NPR increases, the effectiveness of the crosswire in mitigating turbulent mixing noise diminishes in comparison to the baseline nozzle. ...
This experimental investigation is aimed at assessing how the introduction of a cross-wire at the exit of a CD nozzle influences the performance of a supersonic nozzle. The study focuses on cold air jets generated by De Laval nozzles equipped with cross-wires and baseline configurations, particularly at design Mach numbers of 1.5 and 1.75. The investigation involves collecting measurements from the noise field emitted by the cross-wire nozzle with a 2% obstruction at the exit. This passive control approach effectively reduces the occurrence of screech tones in both over-expanded and under-expanded conditions in the azimuthal plane at appropriate operating pressures. Various acoustic parameters, including sound pressure levels (SPL), Strouhal numbers, and the overall sound pressure level spectra (OASPL) are recorded. Schlieren imaging captures images of shock cell patterns, illustrating the impact of shock-associated noise. In comparison to a baseline nozzle, the results demonstrate that a CD nozzle equipped with a cross-wire proves to be a proficient screech tone suppressor, leading to an average reduction of up to 5 dB in OASPL in under-expanded and over-expanded scenarios.
... Thus, using MVGs as a noise reduction approach impacts the development of large turbulent structures and shock cells strength. This approach to jet noise reduction becomes effective because the two largest components of noise sources of supersonic jets, turbulent mixing noise and shock noise, 22,23 are being targeted for high noise reduction gains. ...
The role of micro-vortex generators (MVGs) in supersonic jet noise reduction is investigated. Studies are performed to understand the noise reduction mechanisms of these devices. MVGs are implemented on a scale model representative of GE-F404 nozzles. Configurations consisting of single and dual arrays of MVGs were tested for pressure ratios relevant to takeoff operating conditions. A combination of laboratory measurements and large-eddy simulations are used. Particle image velocimetry reveals that MVGs placed inside the nozzle form oblique shocks that interact with the jet plume's shock cell structure and greatly modify the shock-cell structure issued from the nozzle throat. This modification can weaken the jet plume shock-cell structure and even reduce the jet core velocities. Convective velocities estimated from the high-speed Schlieren imaging showed a significant reduction in magnitude caused by MVGs. When compared to the baseline design, the MVG nozzles showed noise reduction up to 10 dB in the upstream direction and near 5 dB in the peak downstream radiation angle at cold jet conditions. Near-field acoustic measurements showed a significant reduction of low-frequency turbulent mixing noise. Scaling analysis showed that MVGs are capable of delivering the same noise reduction performance across different model scales.
... Sesterhenn, 2017; Tam et al., 2008). Jet noise from volcanoes possesses additional complexities, due to the multiphase nature of the released fluid and to the complexity of crater and conduit morphology, with implications on the directivity of the radiated acoustic signals (e.g., Genco et al., 2014;Matoza et al., 2013;Pena-Fernandez et al., 2020;Taddeucci et al., 2012Taddeucci et al., , 2014. ...
... The first point is explained considering that, with increasing Reynolds number, friction decreases for smooth conduit and increases for rough-walled ones (Busse et al., 2017). The second point is explained by the different sources of jet noise acting in the two regimes and the respective changes in radiation directionality (e.g., Tam et al., 2008). ...
Plain Language Summary
Volcanoes are amongst the most fascinating and mysterious subjects of science, for they allow no direct observation of what is happening within the conduit during eruptive activity. Indirect observations (such as measurements of the sound and vibration accompanying eruptions) are routinely performed for monitoring and research purposes. Laboratory studies mimicking the eruptive processes in small‐scale devices, are of great support for correctly interpreting such data. As such, we investigated the effect of the irregularity of conduit surface, amongst the most relevant and poorly known variables characterizing the eruptive processes, on volcanic jets and on their seismic and acoustic signals. We performed a series of laboratory experiments using conduits with different roughness of the internal surface and various starting pressures. Microphones and accelerometers, capable of measuring conduit sounds and vibration, respectively, in synch with high‐speed camera were used to constrain the characteristics of the generated subsonic and supersonic jets. Results show that conduit roughness controls: (a) the relative amplitude of seismic and acoustic signals; (b) the velocity, turbulence and properties of the sound of these jets. Our results will shed light on the link between observation at the surface and dynamic evolution of conduit geometry at depth.
... D EVELOPMENT of high-speed aircraft has generally focused on increasing thrust. While this has been made possible through the development of high-powered jet engines, a major consequence of the process is production of remarkably high levels of acoustic emission due to large-scale turbulent structures radiated by supersonic jets [1]. The high levels of acoustic emissions lead to harm, both physiological (detriment to quality of life for those in vicinity) and structural [2][3][4][5]. ...
... Shen and Tam [16] found that heating a jet attenuated screech intensity and that a thick nozzle-lip generated higher tonal amplitudes; however, the change in sound pressure levels was not significant in comparison to thin nozzle lips. Tam et al. [1] made use of experimental studies to support the two-noise-source model for supersonic jet flows, i.e., large and fine-scale turbulence form the physical sources of jet noise radiated in downstream and sideline directions, respectively. Theoretical models and experimental measurements support that the mechanism by which large turbulence structures radiate noise is Mach wave radiation [17]. ...
... For a significant period, the two-noise-source model [1] prevailed as a leading model for jet noise understanding and prediction. However, recent efforts that made use of techniques such as proper orthogonal decomposition (POD) and wavelet analysis failed to separate the acoustic field into two statistically independent components [18]. ...
Acoustic emissions from supersonic jets are of interest due to their deleterious effects on nearby personnel and structures. As development of both military and commercial supersonic aircraft continues to grow, it is essential to combat increasing noise pollution. Axisymmetric jets are highly prevalent where their aeroacoustic characteristics have been well-documented; in contrast, relatively few studies have examined non-axisymmetric jets. In the present study, aeroacoustics of supersonic jets produced by a diamond-shaped Mach 2 nozzle are explored across a large operational envelope under different pressure ratios ranging from highly overexpanded to moderately underexpanded. Measurements of the far-field acoustics in an anechoic environment were used to characterize the acoustic flowfield and contrast the distribution of energy in the spectral domain between the characteristic major and minor axes of the nozzle. Overall, the diamond jet exhibits a highly directional acoustic field with a distinct local minimum for sideline noise. Furthermore, screech behavior was only observed in a narrow, overexpanded flow regime; this is in contrast to the documented behavior of axisymmetric and rectangular jets. Over the conditions examined there is no indication of distinct staging. Additionally, there exists a nozzle pressure ratio threshold beyond which peak noise switches axes.
... As a result, the stress tensor becomes where one can note that its simplified version depends only of the velocity fluctuations. Here, the jet is treated as a distributed source and, in the present model, the low frequency contribution to the noise generation is predominant downstream the nozzle (Dougherty and Podboy 2009;Tam et al. 2008). On the other hand, near the jet exit, high-frequency content is more relevant and, in this context, the velocity fluctuations associated with the turbulent structures can be related to this spectral content. ...
A comparative study of the acoustic far-field radiation of a subsonic jet near a folded plate with an opening, intended to represent a flapped wing with thrust gate, is presented in this work. Three openings with different widths were used to evaluate experimentally the influence of the gaps in the far-field noise radiation, for two folding angles. Boundary Element Method simulations with a wavepacket model which represents the jet acoustic source are used to calculate the far-field noise. Numerical simulation results are compared with experimental measurements and show similar trends in terms of acoustic radiation. Through parametric simulations, it was also possible to estimate that opening widths greater than one jet diameter do not contribute significantly to reducing the far-field noise. The results show that even the smallest tested openings were able to reduce the far-field noise for the tested positions.
... From the radiated sound of two coaxial jets (a normal-velocity profile representing a turbofan engine and the invertedvelocity profile configuration), Tanna concluded that the velocity ratio between primary and secondary flow is more dominating than the temperature ratio (Tanna and Morris, 1985). In a joint publication, some of the aforementioned authors point out the sound spectrum of jet noise to be dominated in the low-frequency range by the large-scale turbulent structures and in the high-frequency range by small-scale turbulent structures inside the jet (Tam et al., 2008). ...
As subsonic jets remain one of the major contributions to aircraft noise emissions, near-field flow simulations should be included in aircraft design at an early stage using quantitatively predicted sound pressure levels and the time-domain signal properties of the noise data. In this regard, the interface from the near-field data to the far-field radiation-under consideration of acoustic reflections from objects such as fuselage and wings-remains as bottleneck. This study presents the computation of a spherical equivalent source model of jet noise with minimal complexity by means of spherical harmonic (SH) coefficients. Using spherical Hankel extrapolation of sound pressure data from virtual, concentrical microphone arrays, the results of the determination for the radius, in which all acoustic sources of a flow field are confined, indicate the source radius around the end of the potential core to be equivalent to five times the nozzle diameter. The result of the SH transform shows the dominant energy contribution to be related to nine elementary sources. The resulting equivalent source model of jet noise provides a convenient format for further use in large-scale computational fluid dynamics simulations.
