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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. • The BEM method is used to predict the acoustic field of installed jet noise. • The numerical tool is used to investigate lining the surface close to the jet. • Results suggest that the numerical tool is able to predict the noise attenuation. [ABSTRACT FROM AUTHOR]

Aeroacoustic analyses of jet flows have benefited greatly from a decomposition of turbulent pressure fluctuations into hydrodynamic and acoustic components. This is typically accomplished using signal processing techniques based on phase speeds, coherence properties or spectral analyses. We present an approach, building on the Momentum Potential Theory (MPT) approach of (Doak, 1989), to split pressure fluctuations into their hydrodynamic, acoustic and entropic, collectively designated fluid-thermodynamic (FT), components. Key advantages are that the approach is applicable everywhere in the jet i.e , not restricted to the near-acoustic field, and does not need user-defined thresholds. The effectiveness of the technique is demonstrated by analyzing the flowfields of three simulated jets, to encompass moderate-compressible to supersonic conditions. The statistical properties and wavepacket dynamics of each pressure component, and their relationships with the unsplit pressure are elaborated. The acoustic pressure field has the form of a wavepacket that attenuates downstream and whose modal analysis reveals low-rank behavior. At each Mach number examined, the acoustic pressure also identifies the relative prominence of each of three components: i) waves with upstream propagating energy content (negative group velocity), ii) supersonically traveling radiating downstream waves, and iii) subsonically convected evanescent waves, which follow the convection pattern of hydrodynamic eddies in the turbulent region. With increasing Mach number, the radiating and convected bands of energy move closer to each other. The hydrodynamic pressure also displays a wavepacket structure, but its features are different: it displays large-scale subsonically convected structures even past the core collapse region. Thus, in the turbulent region of the jet, the acoustic pressure displays smaller integral time scales of fluctuations than the hydrodynamic component. The acoustic pressure field, which includes a zero-crossing in its radial profiles, displays larger wavelengths than the hydrodynamic pressure field, correlates better with the near-field pressure signal and captures the radiated component of noise, especially at shallow angles. These properties make it a suitable field for informing pressure-based wavepacket models for jet noise. [ABSTRACT FROM AUTHOR]

Abstract The noise characteristics of a dual-stream jet of bypass ratios greater than 10 were investigated in response to area ratio, nozzle-operating conditions, installation effect, and chevron effect. The key conclusions were as follows. (1) The area ratio impacted the noise by up to 3 dB at aft angles and at mid-frequency for heated conditions. (2) The spectral variation due to the change of primary stream parameters was constant at low Strouhal numbers and diminished at high Strouhal numbers. (3) The impact of the secondary stream pressure was described by the scaling law of single-stream jets for unheated conditions. The flow parameters of other jets were required to describe the spectra of Strouhal numbers between 0.2 and 2.0, based on the secondary jet for heated conditions. (4) The installed jet noise was quantified for various wing configurations and was more pronounced at forward angles. (5) The chevron nozzle reduced overall noise by up to 4 dB, though it became less attractive when installed. The quantified information is expected to improve the estimation of noise from a dual-stream jet of high bypass ratios. [ABSTRACT FROM AUTHOR]

Abstract We examine the three-dimensional energy-exchange mechanisms between stochastic turbulence and an organized radiating wavepacket, and subsequent scattering and transmission of acoustic energy. A validated round-jet large-eddy simulation (LES) is subjected to Kovásznay-type decomposition into hydrodynamic, acoustic and thermal modes, as encapsulated in the generalized framework of the momentum potential theory (MPT). The results are first assimilated with known and postulated aspects of jet-noise dynamics to provide an alternative perspective. Subsequently, new insights are derived on acoustic-energy generation and transport processes. The prolonged axial and azimuthal spatio-temporal coherence of the jittering acoustic mode, which manifests as a wavepacket, and the localization of its energy to the low-order azimuthal modes (m = 0 , 1 and 2) are shown to arise naturally in the decomposed field. The spatial supports of the internal and radiating components of the acoustic-mode are identified with dynamic mode decomposition (DMD). The resulting spectra are observed to be similar to those of the global instability modes of the mean flow. In contrast, the hydrodynamic mode, which is the primary source of energy in this cold jet, has broadband spectral structure and limited spatio-temporal coherence. This knowledge is then leveraged to analyze and quantify sound scattering versus transmission through energy interactions between the relatively chaotic hydrodynamic mode and its stimulation of an ordered response in the acoustic mode. Surface integral analyses reveal the mechanisms by which the omni-directional nature of the turbulent scattering is established. The acoustic mode plays a central role in imparting directionality to the acoustic transmission process, with its axisymmetric component contributing to downstream and shallow angle radiation and higher azimuthal modes contributing increasingly to sideline radiation. Volumetric integral analyses combine these observations to show that the unorganized hydrodynamic mode generates localized noise-producing regions, resulting in a coherent form of sound radiation from the jet. [ABSTRACT FROM AUTHOR]

Abstract Aeroacoustic jet noise theory describes the relationship between acoustic signals and jet operating parameters. Based on empirical aeroacoustic relationships, a link between peak frequency and amplitude and jet diameter and velocity has been used by volcano acousticians to return the operating parameters of column generating volcanic eruptions. There are however significant differences between the nozzle geometries and structures of flows studied in the aeroacoustic literature and flows emitted from volcanic vents. These differences raise questions as to the validity of applying engineering relationships to volcanic eruptions. Combining particle image velocimetry and acoustic data we investigated the flow structures, acoustic signals and dominant source locations of jet flows emitted from three simple nozzle geometries (convergent, straight and divergent). Flows from the straight and divergent nozzles exited in a more developed flow regime than those emitted from convergent nozzles. While for the convergent nozzles dominant sound sources were located within the jet flow, the dominant signals from the straight and divergent nozzles were located in the region of the nozzle exit. Consequently, for the straight and divergent nozzles it was not possible to differentiate between signals generated by the jet flow and those generated by internal sound sources without beamforming analysis. In order to better replicate the volcanic case and in contrast to studies for industrial applications, our experimental test section was not engineered to minimise internal noise. Without acoustic treatment such as screens, choke plates and paddles, jet noise signals were contaminated by signals generated within the test rig and so, even for the convergent nozzles, the aeroacoustic relationships derived from engineering studies could not be used to retrieve the jet operating parameters. Our results indicate that in the volcanic case, the raw acoustic spectra will contain signals generated by both in-conduit and above-vent processes. Therefore inversions for volcanic jet parameters such as mass eruption rate from standard aeroacoustic relationships may be unreliable. Highlights • Nozzle structure controls the flow regime of emitted jets. • Dominant acoustic signals from convergent nozzles are located within the jet flow. • Dominant acoustic signals from divergent and straight nozzles are located in the region of the nozzle exit. [ABSTRACT FROM AUTHOR]

In this work, jet noise reduction using a swirling flow surrounding a circular free jet has been demonstrated. The passive control scheme induces swirl from six flat vanes fixed in an annular passage with vane angles from 0 to 50°, with the corresponding swirl numbers ranging from 0 to 0.91. Noise measurements in terms of overall sound pressure levels, directivity patterns, acoustic spectra; and flow measurements in terms of centerline pitot survey and flow visualization have been carried out to evaluate the efficacy of the passive control scheme. The co-axial swirl jets always reduce the low frequency noise, irrespective of the nozzle pressure ratio. The screech tone is entirely eliminated and broadband shock associated noise mitigated by the co-axial swirl jets. The results of highly under expanded supersonic cases, show that, the weak swirl generates higher noise and high swirl generates lower noise for the same nozzle pressure ratio. The centerline pitot pressure measurements reveal that the co-axial swirl jets decrease the core lengths and the number of shock cells compared to the free jet. The flow visualization study shows that Mach disks are generated at lower pressure ratios for the co-axial swirl jets compared to free jet. The present work proposes swirl as an excellent passive tool for jet noise suppression. [ABSTRACT FROM AUTHOR]

This article explains the flow and acoustic variation due to the jet impinging on inclined plates. There is a misconception that the acoustic radiation reduces by changing the angle of impingement. However, in the present study it is found that the reduction or enhancement of the acoustic radiation depends on the location of measurement and nozzle pressure ratio. A detailed directivity study of jet impinging on the inclined plate is carried out. Further, high speed flow visualization is carried out to compare shock oscillation and tonal frequencies. [ABSTRACT FROM AUTHOR]

The characterization of velocity and acoustic fields from a single-stream free jet operating at subsonic regimes is essential for aeronautical applications. For instance, the investigation of exhaust gases from single or coaxial nozzles or from bleed valve in turbojets and turbofan engines is crucial for understanding the mechanisms of noise generation and propagation and eventually for finding ways to reduce aircraft noise. This article evaluates the velocity and acoustic fields of an isothermal single jet discharging from a circular 38.1 mm conical nozzle at three different Mach and Reynolds numbers of 0.25 ( Re = 2.5 × 10 5 ), 0.50 ( Re = 4.9 × 10 5 ) and 0.75 ( Re = 6.8 × 10 5 ), respectively. Pitot-tube and hot-wire probes were used to identify the mean velocity profiles in longitudinal and transversal directions in the wake of the jet. The hot-wire anemometry system was also used to evaluate the turbulence intensity distribution over eleven axial lines, from the centerline to the edge of the nozzle. The accuracy of hot-wire anemometers for turbulent intensities lower than 15% at low and high subsonic Mach numbers was evaluated by comparing the experimental measurements with available data from the literature. An acoustic investigation was carried out by analyzing the sound pressure level obtained at six positions in the far field, with viewing angles ranging from 40° to 110°. The results were integrated to a database with sound pressure level as a function of Strouhal number, aiming to provide a benchmark for further RANS-based methods applied to aeroacoustic simulations of single jets. [ABSTRACT FROM AUTHOR]

Experimental studies are carried out to reduce the jet noise using co-axial swirlers in the form of curved vanes fixed in an annular passage. The swirl numbers considered for the present work ranged from 0 to 1.31, and the corresponding swirl vane angles ranged from 0 to 60°. The nozzle pressure ratios studied ranged from 1.8 to 6. The acoustic far field study at subsonic conditions revealed the presence of transonic tones for the non-swirl jet. However, swirl eliminates the transonic tones and a weak swirl is most efficient for noise reduction at subsonic conditions. The centerline total pressure measurements indicate the reduced core length for the swirl jets compared to the non-swirl jets. At supersonic conditions, the non-swirl jet emits the highest noise at all the emission angles compared to the swirl jets. The swirl jets are free from screech tones, and have lower amounts of shock associated noise, even at high nozzle pressure ratios. The centerline total pressure measurements and schlieren visualization studies show that shock cell spacing and the number of shock cells are reduced in the swirl jets compared to the non-swirl jet. [ABSTRACT FROM AUTHOR]

Noise measurements have been performed on rectangular jets of aspect ratios ranging from 49 to 987 with the aim of determining the appropriate velocity and length scaling to be used in an empirical noise prediction model. The results have shown that the velocity exponent is a function of the nozzle aspect ratio, decreasing with increasing nozzle aspect ratio. In an effort to establish a general prediction model, the velocity exponent of 7 was chosen as the best compromise to represent all the measured data. The analysis of the noise measurements from high aspect ratio nozzles of varying jet height and width has shown that, for the range of aspect ratios considered, the jet sound power level scales with the nozzle height to the power of 3 and the nozzle width to the power of 1. The derived jet noise scaling has been validated with independent experimental data and shows good agreement. [ABSTRACT FROM AUTHOR]

A subsonic transitional jet is simulated by large eddy simulation, while the flow and acoustic properties are validated against existed experimental results. In the hydrodynamic region, the total disturbance kinetic energy declines as azimuthal wavenumber ( m ) or frequency increases. In the far field, the acoustic field is dominated by the axisymmetric and first helical modes, and the peak of sound spectra at small polar angles to the jet axis are found be located in the range of Strouhal number S t = 0.2 − 0.3 . In order to understand the relationship between instability waves and noise generation, the parabolized stability equation (PSE) is solved for describing the evolution of instability waves. For the acoustic field, the linear model and nonlinear interaction model are built based on PSE solutions in combination with Lilley–Goldstein’s analogy. The beam patterns of above two models are in agreement with experimental results qualitatively, while the nonlinear interaction model predicts more accurate directivity and locations of sound sources. Quantitatively, the linear model underestimates the sound pressure level (SPL) greatly. Nonlinear interaction can enhance the strength of sound sources and raise acoustic efficiency at small polar angles. In some cases, the nonlinear interaction of two instability waves can even increase SPL by nearly 20 dB. However, for m ≥ 2 , the nonlinear interaction model fails to predict reasonable results qualitatively. [ABSTRACT FROM AUTHOR]

The present work experimentally investigates suppression of acoustic fields generated by supersonic jets of the rocket-launch vehicles at the initial period of launch by water injection. Water jets are injected to the combined jet along its perimeter at an angle of 0° and 60°. The solid rocket motor with the rocket-launch vehicles simulator case is used at tests. Effectiveness of reduction of acoustic loads on the rocket-launch vehicles surface by way of creation of water barrier was proved. It was determined that injection angle of 60° has greater effectiveness to reduce pressure pulsation levels. [ABSTRACT FROM AUTHOR]

In the present study, sound field from M40A1, a type of recoilless rifles, was measured using sound analyzers. Contrary to expectations based on the common characteristics of muzzle blast in previous studies, significantly different waveforms compared to those in other directions were observed in each acoustic pressure signal. In order to clarify how the surroundings affected the sound field, the temporal, spectral and directional characteristics were investigated using correlation functions and Fourier transforms. As a result, additional sources near the rear nozzle of the weapon were identified. Under the operating circumstances of recoilless weapons, blast and jet noise sources were found to be consistent with their acoustical characteristics. A hybrid model integrating the influences of blast and jet noise identified from source decomposition was derived according to the ISO standard 17201 and NASA empirical formulae. Since the predicted directivity pattern agrees moderately well with that measured in previous and present studies and the spectral distributions are in accordance with the measured ones, the hybrid model proposed in this study is considered to be appropriate for predicting the sound field from M40A1 recoilless rifle. [ABSTRACT FROM AUTHOR]

We test two methods of 3-D acoustic source localization on volcanic explosions and small-scale jetting events at Karymsky Volcano, Kamchatka, Russia. Recent infrasound studies have provided evidence that volcanic jets produce low-frequency aerodynamic sound (jet noise) similar to that from man-made jet engines. For man-made jet noise, noise sources localize along the turbulent jet flow downstream of the nozzle. Discrimination of jet noise sources along the axis of a volcanic jet requires high resolution in the vertical dimension, which is very difficult to achieve with typical volcano-acoustic network geometries. At Karymsky Volcano, an eroded edifice (Dvor Caldera) adjacent to the active cone provided a platform for the deployment of five infrasound sensors in July 2012 with intra-network relief of ~ 600 m. The network was designed to target large-scale jetting, but unfortunately only small-scale jetting and explosions were recorded during the 12-day experiment. A novel 3-D inverse localization method, srcLoc, is tested and compared against a more common grid-search semblance technique. Simulations using synthetic signals show that srcLoc is capable of determining vertical solutions to within ± 150 m or better (for signal-to-noise ratios ≥ 1) for this network configuration. However, srcLoc locations for explosions and small-scale jetting at Karymsky Volcano show a persistent overestimation of source elevation and underestimation of sound speed. The semblance method provides more realistic source locations, likely because it uses a fixed, realistic sound speed of ~ 340 m/s. Explosion waveforms exhibit amplitude relationships and waveform distortion strikingly similar to those theorized by modeling studies of wave diffraction around the crater rim. We suggest that the delay of acoustic signals and apparent elevated source locations are due to raypaths altered by topography and/or crater diffraction effects, implying that topography in the vent region must be accounted for when attempting 3-D volcano acoustic source localization. Though the data presented here are insufficient to resolve small-scale jet noise sources, similar techniques may be successfully applied to large volcanic jets in the future. [ABSTRACT FROM AUTHOR]

Highlights: [•] We assess a RANS-based method to predict noise of subsonic jet from chevron nozzles. [•] Predictions of noise and flow field were in agreement with experimental data. [•] The method is found as a viable strategy to the analysis of industrial applications. [ABSTRACT FROM AUTHOR]

Turbofan engines are one of the most popular propulsion systems used in commercial aircraft due to their high thrust and good fuel efficiency. To reduce noise generated from turbofan engines-powered aircraft, a number of control approaches have been developed. The dominant noise sources include the fan and the high-speed 'hot' and 'cold' jet. In engineering applications, the noise control approaches include: 1) active control, 2) geometric shape optimization, and 3) passive control (including acoustic boundary control). Because they are considered the most reliable and effective noise reduction methods, the geometric shape optimization and passive control are preferable by the engine manufacturers. In this work, we briefly overview the noise reduction technologies that have great potential to be applied or implemented on turbofan engines. The research and development progress made on the active control, passive control, and geometric shape optimization are reviewed and discussed, aiming to provide an useful guidance on next-generation low-noise turbofan engines. The fundamental noise damping mechanisms of thermos-viscous and vortex shedding are finally overviewed via cases studies. [ABSTRACT FROM AUTHOR]

Abstract: Shockwave structures and overall sound pressure levels from 8×8 arrays of axisymmetric supersonic jets were experimentally characterized for 14 different arrays. The results were also compared to behavior for single jets of similar dimensions. The jet diameters ranged from 3.2 to 6.4mm and the jet spacing to diameter ratio ranged from 1.44 to 3. The arrays were tested at nominal net pressure ratios ranging from 2 to 24. It was found that above a certain net pressure ratio level, the array of jets coalesced into an interacting lattice of oblique shock interactions. These shock structures consisted of oscillatory shocks at lower pressure ratios and then developed into stable shock structures as the pressure ratio increased. The formation of the interacting shock lattices is strongly correlated with the measured sound pressure levels. Initially the arrays emitted low sound pressure levels, similar to that of a single jet within the array. As the shock structure transitioned from weak to strong the sound pressure levels transitioned to a level similar to that of a single nozzle with an exit area equivalent of the entire array. Shadow graph images of the flow in the arrays are used to correlate to the sound pressure levels measured. [Copyright &y& Elsevier]

Abstract: Hybrid large-eddy type simulations for chevron nozzle jet flows are performed at Mach 0.9 and Re ∼ 106. Implicit or numerical large- eddy simulation (ILES or NLES) is employed, namely omitting explicit subgrid scale models. A Reynolds-averaged Navier-Stokes (RANS) solution is blended into the near wall region. This makes an overall hybrid LES-RANS approach. A Hamilton–Jacobi equation is applied to remove the disparate turbulence length scales implied by hybridization. Computations are contrasted for a baseline round nozzle and a highly bended chevron nozzle. The chevron effects are studied by comparing both nozzles using the same mesh resolution and flow conditions. Through the use of RANS simulations the state of the incoming boundary layer from the measurements is explored and the extent of any laminarization. Noise predictions are made using a permeable surface Ffowcs Williams – Hawkings (FWH) method based on the near field LES data. Results are compared with the NASA SHJAR measurement data. Parallel aspects of the flow solver are also explored including an improved data packaging/sending mechanism using scheduling. [Copyright &y& Elsevier]

Abstract: Direct numerical simulations were conducted of a fully turbulent canonical nozzle/jet configuration. For all cases, the target Reynolds number, based on the jet velocity and diameter, was specified as 7500 and the jet Mach number and coflow Mach number were varied. Turbulence statistics at the nozzle exit are shown to collapse with fully developed turbulent pipe flow profiles when using the wall shear-stress, and in the case of higher Mach number cases also the wall density, from the fully developed flow region upstream in the nozzle. Predictions of flow variables in the near-nozzle region obtained from asymptotic theory are found to agree qualitatively with Direct Numerical Simulation data. The data from the different cases are shown to collapse in the potential core region when scaling with the appropriate mixing layer parameter while further downstream the appropriate parameter is the non-dimensional local velocity excess. For all scalings investigated, including virtual-origin correction of the streamwise axis, the case with the highest coflow magnitude did not agree well with the other cases implying that self-similarity of coflowing jets is restricted to low coflow values. Finally, it is shown that the acoustic field is resolved by the simulations making the data suitable for subsequent aeroacoustic analysis. [Copyright &y& Elsevier]

Abstract: There is a large body of experimental evidence which shows that the jet noise spectrum is composed of two sources. Our aim here is to prove, mathematically, that the two-source paradigm can be derived using a minimum number of self-consistent approximations based on our current knowledge of jet turbulence in cold flows. The starting point of the paper is Goldstein’s (2003)  exact re-arrangement of the Navier–Stokes equations, which shows that turbulence enters the acoustic spectrum formula through the Reynolds stress auto-covariance tensor. We extend the shell model of turbulence using a more general symmetry approximation that amounts to assuming that the Reynolds stress auto-covariance is, firstly, axi-symmetric, and secondly is equivalent to the same tensor only after it has been averaged (point-wise) over the azimuthal separation. As a consequence of these two assumptions, the space–time Fourier transform of the Reynolds stress auto-covariance (which we refer to as the spectral tensor) depends on the transverse wave vector only through the square of its magnitude and, moreover, is also an axi-symmetric tensor. This defines the generalized shell model (or GSM) and we apply it to the jet noise problem. The final result shows that the acoustic spectrum can be written as the sum of two groups of terms, one of which corresponds to the peak jet noise in the weakly non-parallel flow limit. [Copyright &y& Elsevier]

Abstract: The current status of a hybrid RANS/CAA approach for the simulation of broadband sound generation is presented. The method rests on the use of steady Reynolds Averaged Navier-Stokes (RANS) simulation to prescribe the time-averaged motion of turbulent flow. By means of synthetic turbulence the steady one-point statistics (e.g. turbulent kinetic energy) and turbulent length- and time-scales of RANS are translated into fluctuations of turbulent velocity (or vorticity), whose statistics very accurately reproduce the spatial target distributions of RANS. The synthetic fluctuations are used to prescribe sound sources which drive linear acoustic perturbation equations. The whole approach represents a methodology to solve statistical noise theories with state-of-the-art Computational Aeroacoustics (CAA) tools in the time-domain. A brief overview of the synthetic turbulence model and its numerical discretization in terms of the Random Particle-Mesh (RPM) and Fast Random Particle-Mesh (FRPM) method is given. Results are presented for trailing edge, slat, jet, and combustion noise. Some problems related to the formulation of vortex sound sources are discussed. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an analysis of data generated by means of Large Eddy Simulation for a single-stream, isothermal Mach 0.9 jet. The acoustic field is decomposed in Fourier modes in the azimuthal direction, and filtered by means of a continuous wavelet transform in the temporal direction. This allows the identification of temporally localised, high-amplitude events in the radiated sound field for each of the azimuthal modes. Once these events have been localised, the flow field is analysed so as to determine their cause. Results show high-amplitude, intermittent sound radiation for azimuthal modes 0 and 1. The mode-0 radiation is found to be an indirect result of the transition from axisymmetric to antisymmetric organisation which occurs towards the end of the potential core: energy is transferred from the axisymmetric mode at a temporal scale corresponding to a frequency to the antisymmetric and higher order modes at a scale corresponding to . The result is a time-varying modulation of both the amplitude and spatial extent of the axisymmetric wavepacket; the strongest axisymmetric propagative disturbances are produced when the wave envelope is truncated. The observed behaviour is modelled using a line-source wave-packet ansatz which includes parameters that account for the said modulation. Inclusion of these parameters, which allow the wavepacket to ‘jitter’ wave in a manner similar to that observed, leads to good quantitative agreement, at low emission angles, with the acoustic field of the LES. This result shows that the said modulations are the salient source features for the low-angle sound emission of the jet considered. [ABSTRACT FROM AUTHOR]

Abstract: Results from recent LES studies of high-speed jet flows and its near and far-field noise (Mendez et al. AIAA2010-271) are reviewed with an emphasis on validation of the LES results. Far-field noise is predicted using data collected on a conical FW-H surface. Very good agreement with the measurement reported by Bridges & Wernet (AIAA-2008-2834) is observed. Data analysis of the flow disturbances within the jet and in the near-field is used to better characterize the sound source processes. We analyze LES data of Bodony and Lele (Phys. Fluids, vol. 46, 2005) using a filter in the wavenumber-frequency domain. The supersonic components of pressure and radial velocity fluctuation show peaks within the jet and show the scaling behavior consistent with the far-field radiated j noise levels. The observed scaling is also consistent with a perturbation expansion of nearly incompressible turbulent fluctuations within the jet flow. Linear global mode analysis of the non-parallel jet mean flow is performed. Features of Mach wave radiation are evident in the global modes for supersonic convection. Large transient growth is found due to convective non-normality. Some implications of this analysis for jet noise modeling are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Experiment and direct numerical simulation provide compelling evidence that nonlinear wave interactions play a significant role in the generation of noise in subsonic jets. A simple ‘difference mode’ approach captures the essential mechanism, which is a much stronger source of large structure sound, particularly at low Mach numbers, than the alternative of direct linear conversion. Significant features of the low frequency end of the jet noise spectrum are predicted correctly, including the shift of the spectral peak to lower frequencies, the directivity pattern and the effect of Mach number. This mechanism distinguishes large structure sound from that associated with the breakdown to small scales. For practical calculations of jet noise a hybrid approach is proposed in which the Reynolds-Averaged Navier-Stokes equations are used to compute a base flow solution and then either a truncated Navier-Stokes approach, reducing to linearised Euler in the far field, or a Parabolised Stability Equations solver coupled to a linearised Euler solver are used to calculate sound sources and propagation. This approach potentially provides a coupling between flow control on the jet nozzle and the radiated sound. [ABSTRACT FROM AUTHOR]

Abstract: models to predict the evolution of low frequencies, large-scale wavepacket structures in turbulent jets and their radiated sound. We consider computations and data corresponding to high subsonic and supersonic jets from circular nozzles. Previous methods are extended to consider nonlinear interactions amongst the waves and use a Kirchhoff-surface type approach to project the near-field wavepacket amplitudes to the far-field. Linear PSE, whose initial conditions are chosen to provide an overall amplitude reference, show excellent agreement for the wavepacket amplitudes and phases with microphone array data just outside the jet shear layers, especially when the microphone data are processed to filter out contributions from uncorrelated fluctuations. Far-field sound predictions based on the linear PSE are also in reasonable agreement with far-field data. In order to investigate nonlinearity, we use an LES database to evaluate initial conditions for the PSE modes, and then compare their later evolution along the jet. Preliminary cases show some sensitivity to the initial amplitudes and their phases, and that nonlinear effects may be important in predicting the far-field sound based on the initial (near-nozzle) spectrum of disturbances. [ABSTRACT FROM AUTHOR]

Abstract: Advanced simulation tools, particularly large-eddy simulation techniques, are becoming capable of making quality predictions of jet noise for realistic nozzle geometries and at engineering relevant flow conditions. Increasing computer resources will be a key factor in improving these predictions still further. Quality prediction, however, is only a necessary condition for the use of such simulations in design optimization. Predictions do not of themselves lead to quieter designs. They must be interpreted or harnessed in some way that leads to design improvements. As yet, such simulations have not yielded any simplifying principals that offer general design guidance. The turbulence mechanisms leading to jet noise remain poorly described in their complexity. In this light, we have implemented and demonstrated an aeroacoustic adjoint-based optimization technique that automatically calculates gradients that point the direction in which to adjust controls in order to improve designs. This is done with only a single flow solutions and a solution of an adjoint system, which is solved at computational cost comparable to that for the flow. Optimization requires iterations, but having the gradient information provided via the adjoint accelerates convergence in a manner that is insensitive to the number of parameters to be optimized. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the importance of initial conditions on subsonic jet noise is emphasized by showing numerical results obtained by large-eddy simulations for initially laminar round jets at Mach number 0.9 and Reynolds number 105. The near and the far sound pressure fields of the jets are found to significantly vary with the flow parameters, namely the boundary-layer thickness and the turbulence levels, at the nozzle exit. With respect to initially turbulent jets, strong additional noise components generated by pairings of coherent vortical structures in the transitional shear layers are in addition observed, in agreement with experiments. [ABSTRACT FROM AUTHOR]

Abstract: The problem of extending the near acoustic field of a high-speed jet to the far field is considered. This is akin to the problem of analytic continuation in complex variable. Analytic continuation is the extension of an analytic function in a limited domain to a larger domain. The general continuation problem involving acoustic sources enclosed by a surface is first analyzed. It is shown that a unique continuation to the far field is possible provided either the pressure or the pressure gradient normal to the surface is given on the surface. In this paper, the continuation is carried out by means of an adjoint Green’s function. One significant advantage of using adjoint Green’s function is that for a given direction of radiation, one is required to solve only a single acoustic scattering problem. Because the present method requires measuring only the fluctuation pressure on the bounding surface whereas other popular methods such as the Kirchhoff integral method or the Ffowcs-Williams and Hawkings equation requires measuring three or more variables, it appears that the present method has definite advantage when it is used to extend an experimentally measured near acoustic field to the far field. [ABSTRACT FROM AUTHOR]

Abstract: The papers presented at a symposium on the topic of ‘Computational Aeroacoustics for Aircraft Noise Prediction’ are reviewed in this article. The background to the symposium and its objectives are presented and the outcomes of the plenary discussions are reported. [ABSTRACT FROM AUTHOR]

Abstract: Strombolian to Plinian activity from Tungurahua Volcano, Ecuador has been recorded by the autonomous infrasound arrays of the Acoustic Surveillance for Hazardous Eruptions (ASHE) project since early 2006. Our studies suggest that acoustic energy release during large eruptions does appear to broadly scale with eruption intensity. This manuscript provides a detailed chronology and characterization of Tungurahua''s eruptive activity between 2006 and 2008 and demonstrates the ability to constrain source parameters of significant eruptions, such as onset, duration, and escalation, at regional distances by combining infrasound and remote sensing techniques. The ASHE system in Ecuador automatically detected over 20,000 volcanic explosions at an array 37km from Tungurahua and was successful at notifying the onset, escalation, and cessation of a hazardous February 2008 eruption with a latency of 5min. Elevated infrasonic energy from sustained and intense Tungurahua eruptions correlates well with ash column heights and their lateral extent during the study period. The spectra of these sustained explosive eruptions appear to be recurrent, readily recognizable, and indicative of volcanic jetting and significant atmospheric ash injection. The paroxysmal Plinian phase of the August 2006 eruption produced an ash cloud that extended well into the stratosphere (>24km), coinciding with a shift of the dominant jetting frequency from 0.25Hz to below 0.1Hz, and radiation of over 5×107 W of acoustic power. Transient explosions were often marked by minor or no ash release and are presumed to be more gas-rich. A change in the acoustic spectrum of volcanic jetting was also detected in the transition from a sustained to collapsed eruption column at the end of the July 14, 2006 eruption. The jetting spectrum at Tungurahua during a period of sustained pyroclastic density current production changes from a typical double-peaked to a single-peaked spectrum, suggesting remote acoustic monitoring can help ascertain the stability and dynamics of an eruptive column. [Copyright &y& Elsevier]

Abstract: This paper presents an analysis of data generated by means of Large Eddy Simulation for a single-stream, isothermal Mach 0.9 jet. The acoustic field is decomposed in Fourier modes in the azimuthal direction, and filtered by means of a continuous wavelet transform in the temporal direction. This allows the identification of temporally localised, high-amplitude events in the radiated sound field for each of the azimuthal modes. Once these events have been localised, the flow field is analysed so as to determine their cause. Results show high-amplitude, intermittent sound radiation for azimuthal modes 0 and 1. The mode-0 radiation is found to be an indirect result of the transition from axisymmetric to antisymmetric organisation which occurs towards the end of the potential core: energy is transferred from the axisymmetric mode at a temporal scale corresponding to a frequency f0 to the antisymmetric and higher order modes at a scale corresponding to f0/2. The result is a time-varying modulation of both the amplitude and spatial extent of the axisymmetric wavepacket; the strongest axisymmetric propagative disturbances are produced when the wave envelope is truncated. The observed behaviour is modelled using a line-source wave-packet ansatz which includes parameters that account for the said modulation. Inclusion of these parameters, which allow the wavepacket to ‘jitter’ in a manner similar to that observed, leads to good quantitative agreement, at low emission angles, with the acoustic field of the LES. This result shows that the said modulations are the salient source features for the low-angle sound emission of the jet considered. [Copyright &y& Elsevier]

Abstract: The current status of a hybrid RANS/CAA approach for the simulation of broadband sound generation is presented. The method rests on the use of steady Reynolds Averaged Navier-Stokes (RANS) simulation to prescribe the time-averaged motion of turbulent flow. By means of synthetic turbulence the steady one-point statistics (e.g. turbulent kinetic energy) and turbulent length- and time-scales of RANS are translated into fluctuations of turbulent velocity (or vorticity), whose statistics very accurately reproduce the spatial target distributions of RANS. The synthetic fluctuations are used to prescribe sound sources which drive linear acoustic perturbation equations. The whole approach represents a methodology to solve statistical noise theories with state-of-the-art Computational Aeroacoustics (CAA) tools in the time-domain. A brief overview of the synthetic turbulence model and its numerical discretization in terms of the Random Particle-Mesh (RPM) and Fast Random Particle-Mesh (FRPM) method is given. Results are presented for trailing edge, slat, jet, and combustion noise. Some problems related to the formulation of vortex sound sources are discussed. [Copyright &y& Elsevier]

Abstract: Results from recent LES studies of high-speed jet flows and its near and far-field noise (Mendez et al. AIAA-2010-271) are reviewed with an emphasis on validation of the LES results. Far-field noise is predicted using data collected on a conical FW-H surface. Very good agreement with the measurement reported by Bridges & Wernet (AIAA-2008-2834) is observed. Data analysis of the flow disturbances within the jet and in the near-field is used to better characterize the sound source processes. We analyze LES data of Bodony and Lele (Phys. Fluids, vol. 46, 2005) using a filter in the wavenumber-frequency domain. The supersonic components of pressure and radial velocity fluctuation show peaks within the jet and show the U8j scaling behavior consistent with the far-field radiated noise levels. The observed scaling is also consistent with a perturbation expansion of nearly incompressible turbulent fluctuations within the jet flow. Linear global mode analysis of the non-parallel jet mean flow is performed. Features of Mach wave radiation are evident in the global modes for supersonic convection. Large transient growth is found due to convective non-normality. Some implications of this analysis for jet noise modeling are discussed. [Copyright &y& Elsevier]

Abstract: Experiment and direct numerical simulation provide compelling evidence that nonlinear wave interactions play a significant role in the generation of noise in subsonic jets. A simple ‘difference mode’ approach captures the essential mechanism, which is a much stronger source of large structure sound, particularly at low Mach numbers, than the alternative of direct linear conversion. Significant features of the low frequency end of the jet noise spectrum are predicted correctly, including the shift of the spectral peak to lower frequencies, the directivity pattern and the effect of Mach number. This mechanism distinguishes large structure sound from that associated with the breakdown to small scales. For practical calculations of jet noise a hybrid approach is proposed in which the Reynolds-Averaged Navier-Stokes equations are used to compute a base flow solution and then either a truncated Navier-Stokes approach, reducing to linearised Euler in the far field, or a Parabolised Stability Equations solver coupled to a linearised Euler solver are used to calculate sound sources and propagation. This approach potentially provides a coupling between flow control on the jet nozzle and the radiated sound. [Copyright &y& Elsevier]

Abstract: We report on the development of parabolized stability equation models to predict the evolution of low frequencies, large-scale wavepacket structures in turbulent jets and their radiated sound. We consider computations and data corresponding to high subsonic and supersonic jets from circular nozzles. Previous methods are extended to consider nonlinear interactions amongst the waves and use a Kirchhoff-surface type approach to project the near-field wavepacket amplitudes to the far-field. Linear PSE, whose initial conditions are chosen to provide an overall amplitude reference, show excellent agreement for the wavepacket amplitudes and phases with microphone array data just outside the jet shear layers, especially when the microphone data are processed to filter out contributions from uncorrelated fluctuations. Far-field sound predictions based on the linear PSE are also in reasonable agreement with far-field data. In order to investigate nonlinearity, we use an LES database to evaluate initial conditions for the PSE modes, and then compare their later evolution along the jet. Preliminary cases show some sensitivity to the initial amplitudes and their phases, and that nonlinear effects may be important in predicting the far-field sound based on the initial (near-nozzle) spectrum of disturbances. [Copyright &y& Elsevier]

Abstract: Advanced simulation tools, particularly large-eddy simulation techniques, are becoming capable of making quality predictions of jet noise for realistic nozzle geometries and at engineering relevant flow conditions. Increasing computer resources will be a key factor in improving these predictions still further. Quality prediction, however, is only a necessary condition for the use of such simulations in design optimization. Predictions do not of themselves lead to quieter designs. They must be interpreted or harnessed in some way that leads to design improvements. As yet, such simulations have not yielded any simplifying principals that offer general design guidance. The turbulence mechanisms leading to jet noise remain poorly described in their complexity. In this light, we have implemented and demonstrated an aeroacoustic adjoint-based optimization technique that automatically calculates gradients that point the direction in which to adjust controls in order to improve designs. This is done with only a single flow solutions and a solution of an adjoint system, which is solved at computational cost comparable to that for the flow. Optimization requires iterations, but having the gradient information provided via the adjoint accelerates convergence in a manner that is insensitive to the number of parameters to be optimized. [Copyright &y& Elsevier]

Abstract: In this paper, the importance of initial conditions on subsonic jet noise is emphasized by showing numerical results obtained by large-eddy simulations for initially laminar round jets at Mach number 0.9 and Reynolds number 105. The near and the far sound pressure fields of the jets are found to significantly vary with the flow parameters, namely the boundary-layer thickness and the turbulence levels, at the nozzle exit. With respect to initially turbulent jets, strong additional noise components generated by pairings of coherent vortical structures in the transitional shear layers are in addition observed, in agreement with experiments. [Copyright &y& Elsevier]

Abstract: The problem of extending the near acoustic field of a high-speed jet to the far field is considered. This is akin to the problem of analytic continuation in complex variable. Analytic continuation is the extension of an analytic function in a limited domain to a larger domain. The general continuation problem involving acoustic sources enclosed by a surface is first analyzed. It is shown that a unique continuation to the far field is possible provided either the pressure or the pressure gradient normal to the surface is given on the surface. In this paper, the continuation is carried out by means of an adjoint Green''s function. One significant advantage of using adjoint Green''s function is that for a given direction of radiation, one is required to solve only a single acoustic scattering problem. Because the present method requires measuring only the fluctuation pressure on the bounding surface whereas other popular methods such as the Kirchhoff integral method or the Ffowcs-Williams and Hawkings equation requires measuring three or more variables, it appears that the present method has definite advantage when it is used to extend an experimentally measured near acoustic field to the far field. [Copyright &y& Elsevier]

Abstract: The papers presented at a symposium on the topic of ‘Computational Aeroacoustics for Aircraft Noise Prediction’ are reviewed in this article. The background to the symposium and its objectives are presented and the outcomes of the plenary discussions are reported. [Copyright &y& Elsevier]

Abstract: Hybrid large-eddy type simulations for chevron nozzle jet flows are performed at Mach 0.9 and Re =1.03×106. Without using any subgrid scale model (SGS), the numerical approach applied in the present study is essentially implicit large-eddy simulation (ILES). However, a Reynolds-averaged Navier–Stokes (RANS) solution is patched into the near wall region. This makes the overall solution strategy hybrid RANS–ILES. The disparate turbulence length scales, implied by these different modeling approaches, are matched using a Hamilton–Jacobi equation. The complex geometry features of the chevron nozzles are fully meshed. With numerical fidelity in mind, high quality, hexahedral multi-block meshes of 12.5×106 cells are used. Despite the modest meshes, the novel RANS–ILES approach shows encouraging performance. Computed mean and second-order fluctuating quantities of the turbulent near field compare favorably with measurements. The radiated far-field sound is predicted using the Ffowcs Williams and Hawkings (FW–H) surface integral method. Encouraging agreement of the predicted far-field sound directivity and spectra with measurements is obtained. [Copyright &y& Elsevier]

Abstract: The concept of effective jet properties introduced by the author (Kandula M. Prediction of turbulent jet mixing noise reduction by water injection. AIAA J 2008;46(11):2714–22) has been extended to the estimation of broadband shock noise reduction by water injection in supersonic jets. Comparison of the predictions with the test data for cold and hot underexpanded supersonic nozzles shows a satisfactory agreement. The results also reveal the range of water mass flow rates over which saturation of mixing noise reduction and existence of parasitic noise are manifest. [Copyright &y& Elsevier]

Abstract: A bibliographical review of the main technologies employed for the mitigation of aircraft noise is presented. According to a component-based approach, analytical and semi-empirical models of the aeroacoustic mechanisms involved in the noise generation from airframe and engine components are presented as a key element of the noise reduction technology. These models, developed in the past to investigate the influence of some design parameters on the overall acoustic levels, are nowadays powerful design tools when employed in a multi-disciplinary optimization framework. In this spirit, the recent achievements in the numerical prediction of complex aeroacoustic phenomena through CFD/CAA techniques, not addressed in this work, provide a complementary approach to experiments to improve the accuracy of the available analytical and semi-empirical models. The bibliographical style of the paper is guaranteed by a qualitative description of the underlying physical mechanisms and their mathematical idealization. The reader is therefore remanded to the cited works for a deeper analysis. [Copyright &y& Elsevier]

Abstract: The flow and sound fields of a heated axisymmetric pulsating jet have been investigated by direct numerical solution of the compressible Navier-Stokes equations in cylindrical coordinates using highly accurate numerical methods. Effects of pulsating frequency and amplitude on the flow structure and sound generation have been examined. The results show that the organized unsteadiness associated with the periodic pulsation leads to a variety of vortical structures in the pulsating flow field. The pulsating frequency and amplitude strongly affect the vortical flow structures and the radiated sound fields. At the higher pulsating frequency, the vortices in the pulsating jet become smaller and the radiated sound field has smaller wavelength. The pressure fluctuation amplitude of the sound field is inversely proportional to the distance from the sound source. Both the larger pulsating frequency and larger pulsating amplitude lead to stronger sound emissions from the pulsating jet. It has been found that the pressure fluctuation amplitude of the sound field is approximately proportional to the pulsating amplitude and frequency. [Copyright &y& Elsevier]

The acoustic post-processing of unsteady aerodynamic jet simulations using Kirchhoff or Ffowcs Williams and Hawkings (FW–H) surface integral methods is investigated from the theoretical and practical points of view. This analysis is carried out for a supersonic hot jet, starting from the flow fields provided by an unsteady aerodynamic simulation whose characteristics are recalled. Both acoustic integral methods are first compared from a theoretical point of view. In particular, the role of the ‘entropic’ sources on the control surface is pointed out. Various acoustic calculations are then carried out for this hot jet to assess the influence of the type of method and of the position, extent and nature (open or closed) of the control surfaces on the noise predictions. This parametric study shows that the Kirchhoff method using density as input data is poorly suited for acoustic predictions of hot jets. The Kirchhoff method using pressure as input data provides results similar to those obtained with the FW–H surface integral but the latter appears to be more reliable. The study also shows that the use of a downstream closed control surface does not present a real interest, whatever the acoustic method is. [Copyright &y& Elsevier]

The influence of swirl on jet flow structure and noise is explored. A novel DES (detached eddy simulation) type procedure is tested for a jet with co-flow case. The approach uses a k–l near wall RANS (Reynolds averaged Navier–Stokes) model. Away from walls MILES (monotone-integrated large eddy simulation) is used. Blending of the RANS and MILES regions is achieved using a Hamilton–Jacobi type equation. The MILES solution content enables correct turbulence development on relatively coarse grids. The new hybrid MILES–RANS approach is validated for a plane channel flow. A good law of the wall is gained. For swirling flows, far-field noise levels are essentially calculated using the surface integral of Ffowcs Williams and Hawkings/Curl. Despite MILES use with mostly fourth-order centred differencing, jet turbulence decays excessively downstream. Streamwise vorticity introduced by swirl (unlike with chevrons) increases noise, slightly biasing it to higher frequencies. Lower swirl levels might bring more success. The hybrid MILES–RANS method appears promising. However, for the jet with co-flow case (due to the low co-flow Reynolds number) it is perhaps only very mildly tested. [Copyright &y& Elsevier]

This paper experimentally investigated characteristics of low-frequency pressure fluctuations in a large-scale open jet wind tunnel using pressure transducers and far-field microphones. Two distinct low-frequency pressure fluctuations have been detected at the range of wind speeds between 50 m/s and 75 m/s. The two peak frequencies vary with the change of wind speed. Analysis results show that both the low-frequency pressure fluctuations are driven by the resonances of the large scale instability waves in the jet shear layer at two different feedback loop paths. One loop is through the entire circuit of the wind tunnel, another exists between the nozzle and the collector, which is termed as edge-tone feedback. A formula for predicting the peak frequency was derived and the prediction agrees well with the measurement. In addition, an effective control strategy to suppress the wind tunnel circuit resonance and the edge-tone feedback resonance were also presented. [ABSTRACT FROM AUTHOR]