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Intense acoustic loads from jet noise cause noise pollution and induce failures, such as the malfunctioning of electronic devices and fatigue failure of internal/external structures. Consequently, the prediction of jet noise characteristics is crucial in the development of high-speed vehicles. This study presents acoustic experiments and predictions for an under-expanded, unheated jet using a small-scale prototype. Outdoor measurements are carried out using a vertical ejection setup. Acoustic characteristics are measured using both linear and circular microphone arrays. Additionally, numerical prediction of the same jet noise is performed using a detached eddy simulation and the permeable Ffowcs-Williams and Hawkings acoustic analogy. The vertical experimental setup exhibits the typical acoustic characteristics of a supersonic jet in terms of directivity and broadband shock-associated noise. Moreover, the numerical prediction exhibits satisfactory accuracy for the jet downstream, where the large-scale turbulence structures of the directivity predominate. However, discrepancy increases in the domain of lower directivity. The presented experiment and prediction will be extended to future studies regarding the noise of various deflector duct configurations impinging on supersonic jets. [ABSTRACT FROM AUTHOR]

Detailed near- and far-field acoustic measurements were conducted for two circular stepped nozzles with 30°deg and 60°deg design inclinations at over- and perfectly-expanded supersonic jet flow conditions and compared to those for a circular nonstepped nozzle. Far-field acoustic results show that stepped nozzles play an insignificant role in altering noise emissions at perfectly expanded condition. At an over-expanded condition, however, the longer stepped nozzle produces significant noise reductions at the sideline and upstream quadrants, while the shorter stepped nozzle does not. Noise spectra analysis and Schlieren visualizations show that noise reduction can be primarily attributed to mitigations in the broadband shock-associated noise (BSAN), due to the ability of the longer stepped nozzle in suppressing shock strengths at downstream region. Near-field acoustic measurements reveal that the source region, as well as the intensity of turbulent and shock noises, are highly sensitive to the stepped nozzle configuration. Furthermore, BSAN seems to be eliminated by the longer stepped nozzle in near-field region due to the shock structure modifications. [ABSTRACT FROM AUTHOR]

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]

The radiation of sound from a perfectly expanded Mach 2.5 cold supersonic jet of 25.4 mm exit diameter flowing through a partially open rigid-walled duct with an upstream f-deflector has been experimentally studied. In the experiments, the nozzle is mounted vertically, with the nozzle exit plane at a height of 73 jet diameters above ground level. Relative to the nozzle exit plane (NEP), the location of the duct inlet is varied at 10, 5, and -1 jet diameters. Far-field sound pressure levels were obtained at 54 jet diameters above ground with the aid of acoustic sensors equally spaced around a circular arc of radius equal to 80 jet diameters from the jet axis. Data on the jet acoustic field for the partially open duct were obtained and compared with those with a free jet and with a closed duct. The results suggest that for the partially open duct the overall sound pressure level (OASPL) decreases as the distance between the NEP and the duct inlet plane decreases, while the opposite trend is observed for the closed duct. It is also concluded that the observed peak frequency in the partially open duct increases above the free jet value as the angle from the duct axis is increased, and as the duct inlet plane becomes closer to the NEP. [ABSTRACT FROM AUTHOR]

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: 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]

Numerical simulations of compressible, turbulent jets at M = 0.75 using the Shear Stress Transport (SST) k-ω model have been carried out for baseline nozzle and chevron nozzles with 4, 6 and 8 lobes and three different penetration angles (0°, 5° and 10°). The predicted far field noise level at several observer locations are compared with experimental data. Overall sound pressure levels at far field observer locations have been calculated using Ffowcs Williams-Hawkings equation. Numerical prediction of aerodynamic quantities like centreline velocity, stagnation pressure, turbulent kinetic energy and axial vorticity are compared among the nozzles. The Unsteady Reynolds Averaged Navier Stokes (URANS) calculations are able to predict the trends in overall SPL, even though the absolute values are slightly under-predicted. [ABSTRACT FROM AUTHOR]