Jet Noise Prediction Research Articles

Overview of the Use of Large-Eddy Simulations in Jet Aeroacoustics

The current state of the use of large-eddy simulations (LESs) in jet aeroacoustics is reviewed. Various computational techniques are discussed, as well as methods to extend the LES results to the acoustic far field. As the computational power increases and computational techniques mature, LES-based calculations are becoming more realistic by including complicated nozzle shapes and by increasing the Reynolds numbers to approach flight conditions. The LES is now being used for evaluation and the understanding of noise mechanisms from heated or unheated, subsonic or supersonic, and single or multiple jets (for example, twin jets). Also, impinging jets, jet blast deflector surfaces, installation effects, and noise reduction strategies (for example, fluidic injection, plasma actuators, chevrons, and beveled nozzles) are currently being studied and evaluated. Thus, the LES is becoming a useful and versatile design tool for the aerospace industry, and it is expected that its use for jet noise prediction will keep increasing.

Jet noise analysis using an efficient LES/high-Order acoustic coupling method

The use of a CFD/CAA method, where fluctuations are extracted on a surface and propagated analytically to the far-field, is becoming a practical approach for industrial jet noise prediction. However, the placement of the surface can be problematic and a source of error, so here an efficient LES/APE coupling method that relies on volumetric sources is utilised. This allows the use of an existing, well validated and robust finite volume LES code to compute the unsteady flow, from which the volumetric sources are extracted, to then compute the propagation of the acoustic waves to the far-field using a high-order finite element APE code with a grid more appropriate for this task. Furthermore, this coupled methodology allows the studying of noise propagation in complex configurations in which the use of surface integral methods could be challenging.In this work, a coupling strategy is used in which all the necessary data is exchanged directly via the high-speed communication network using an open-source library. The efficiency of the parallel-coupling strategy is demonstrated by applying it to a 2D canonical case and comparing it with an existing file-based approach. For the acoustic propagation, the APE solver used is called AcousticSolver, part of the high-order spectral/hp finite element open-source code Nektar++. The present LES/APE framework is fist validated for 3D jet applications by studying the noise propagation of a low-Reynolds number case. Then the method is applied to a more realistic high Reynolds number jet obtaining encouraging results in terms of flow and acoustic predictions.

Modelling of jet noise: a perspective from large-eddy simulations.

In the last decade, many research groups have reported predictions of jet noise using high-fidelity large-eddy simulations (LES) of the turbulent jet flow and these methods are beginning to be used more broadly. A brief overview of the publications since the review by Bodony & Lele (2008, AIAA J. 56, 346–380) is undertaken to assess the progress and overall contributions of LES towards a better understanding of jet noise. In particular, we stress the meshing, numerical and modelling advances which enable detailed geometric representation of nozzle shape variations intended to impact the noise radiation, and sufficiently accurate capturing of the turbulent boundary layer at the nozzle exit. Examples of how LES is currently being used to complement experiments for challenging conditions (such as highly heated pressure-mismatched jets with afterburners) and guide jet modelling efforts are highlighted. Some of the physical insights gained from these numerical studies are discussed, in particular on crackle, screech and shock-associated noise, impingement tones, acoustic analogy models, wavepackets dynamics and resonant acoustic waves within the jet core. We close with some perspectives on the remaining challenges and upcoming opportunities for future applications.This article is part of the theme issue ‘Frontiers of aeroacoustics research: theory, computation and experiment’.

Anisotropic source modelling for turbulent jet noise prediction.

An anisotropic component of the jet noise source model for the Reynolds-averaged Navier–Stokes equation-based jet noise prediction method is proposed. The modelling is based on Goldstein's generalized acoustic analogy, and both the fine-scale and large-scale turbulent noise sources are considered. To model the anisotropic characteristics of jet noise source, the Reynolds stress tensor is used in place of the turbulent kinetic energy. The Launder–Reece–Rodi model (LRR), combined with Menter's ω-equation for the length scale, with modified coefficients developed by the present authors, is used to calculate the mean flow velocities and Reynolds stresses accurately. Comparison between predicted results and acoustic data has been carried out to verify the accuracy of the new anisotropic source model.This article is part of the theme issue ‘Frontiers of aeroacoustics research: theory, computation and experiment’.

Research on Simulation and Performance Optimization of Mach 4 Civil Aircraft Propulsion Concept

Supersonic civil aircraft is of a promising area in the development of future civil transport, and aircraft propulsion system is one of the key issues which determine the success of the aircraft. To get a good conceptual design and performance investigation of the supersonic civil aircraft engine, in this article, a fast, versatile as well as trust-worthy numerical simulation platform was established to analyze the Mach 4 turbine-based combined cycle (TBCC) engine concept so as to be applied to the supersonic civil aircraft. First, a quick and accurate task requirement analysis module was newly established to analyze the mission requirement of the Mach 4 supersonic civil aircraft. Second, the TBCC engine performance simulation model was briefly presented and the number of engines on the supersonic civil aircraft was analyzed, considering single engine inoperative. Third, the Stone model and the DLR method were investigated to estimate the engine jet noise and the NOx emission of the Mach 4 supersonic civil aircraft. Finally, a multiobjective optimization tool made up of a response surface method and a genetic algorithm was developed to optimize the design parameters and the control law of the TBCC engine, in order to make the Mach 4 supersonic civil aircraft engine with better performance, lower noise, and lower emissions. The uniqueness of the developed analysis tool lies in that it affords a numerical simulation platform capable of investigating the task requirement analysis module of the supersonic civil aircraft, engine jet noise prediction model, and the NOx emission prediction model, as well as a multiobjective performance optimization tool, which is beneficial for the conceptual design and performance research of Mach 4 supersonic civil aircraft’s propulsion system.

Nozzles, turbulence, and jet noise prediction

Jet noise prediction is notoriously challenging because only subtle features of the flow turbulence radiate sound. The article by Brès et al. (J. Fluid Mech., vol. 851, 2018, pp. 83–124) shows that a well-constructed modelling procedure for the nozzle turbulence can provide unprecedented sub-dB prediction accuracy with modest-scale large-eddy simulations, as confirmed by detailed comparison with turbulence and sound-field measurements. This both illuminates the essential mechanisms of the flow and facilitates prediction for engineering design.

Parallel computation of aeroacoustics of industrially relevant complex-geometry aeroengine jets

Jet noise is still a distinct noise component when a commercial aircraft is taking off. A parallel high-fidelity simulation framework for industrial jet noise prediction is presented in this paper. This framework includes complex geometry meshing and Ffowcs Williams-Hawkings (FW-H) surface placement during preprocessing, a parallel hybrid RANS-LES flow solver coupled with an FW-H acoustic solver in the simulation and mean and unsteady data processing after the simulation. The use of this framework is demonstrated through two jet noise prediction cases: in-flight heated jets and installed ultra-high bypass-ratio (UHBPR) engines. These simulations can provide more insight than experimental tests into jet flow physics for engineering model improvement. Additional advantages are also shown in the cost and turn-around time. Thus there is great potential for high-fidelity jet noise simulations to partly replace rig tests for industrial use in the future.

Assessment of a Two-Way Coupling Methodology Between a Flow and a High-Order Nonlinear Acoustic Unstructured Solvers

A two-way coupling on unstructured meshes between a flow and a high-order acoustic solvers for jet noise prediction is considered. The flow simulation aims at generating acoustic sources in the near field while the acoustic simulation solves the full Euler equations, thanks to a discontinuous Galerkin method, in order to take into account nonlinear acoustic propagation effects. This methodology is firstly validated on academic cases involving nonlinear sound propagation, shock waves and convection of aerodynamic perturbations. The results are compared to analytical solutions and direct computations. A good behaviour of the coupling is found regarding the targeted space applications. An application on a launch pad model is then simulated to demonstrate the robustness and reliability of the present approach.

Progress in prediction of jet noise and quantification of aircraft/engine noise components

The bypass ratio of newer turbofan engines has been increasing steadily and has reached ∼10; even higher bypass ratios are being considered for improving aircraft fuel efficiency. An accurate method for the prediction of jet noise over a wide range of bypass ratio, from ∼5 to ultra-high bypass ratios (∼20), is required to address current and future needs. The main objective of the current study is the development of a procedure for real-world application that would permit the accurate prediction of jet noise, which in turn would enable the quantification of the non-jet noise component. A new empirical method for prediction of noise from realistic dual-stream jets is developed and validated against an extensive database acquired at model scale; the range of validity covers a velocity ratio ( Vs/Vp) ≥ ∼0.6, as this is the range of interest in real turbofan engines. Accurate absolute spectral predictions at all angles and all cycle conditions are first demonstrated at model scale. The same method is then extended to full-scale predictions, both from static engine tests and airplane flyover tests. The range of Strouhal number of interest in full-scale tests spans ∼0.1 to ∼100. Nearfield effects on the jet noise from dual-stream jets have been quantified and a simple and improved procedure for incorporating spectral effects has been developed. Accurate spectral predictions are obtained for noise from static engine tests and flyover tests, over a wide range of engine operating conditions and bypass ratio. Therefore, the good quality and accuracy of the new prediction method have been confirmed at both model scale and full scale, with and without forward flight. An absolute prediction takes ∼1 s on a workstation, as only a single scaling equation is utilized. The application of the new prediction method in gaining better quantification and understanding of the non-jet noise components, and their reduction, is described.

Rotational Raman-based temperature measurements in a high-velocity, turbulent jet

Spontaneous rotational Raman scattering spectroscopy is used to acquire measurements of the mean and root mean square (rms) temperature fluctuations in turbulent, high-velocity heated jets. Raman spectra in air were obtained across a matrix of radial and axial locations downstream from a 50 mm diameter nozzle operating from subsonic to supersonic conditions over a wide range of temperatures and Mach numbers, in accordance with the Tanna matrix frequently used in jet noise studies. These data were acquired in the hostile, high noise (115 dB) environment of a large scale open air test facility at NASA Glenn Research Center (GRC). Temperature estimates were determined by performing non-linear least squares fitting of the single shot spectra to the theoretical rotational Stokes spectra of N2 and O2. The laser employed in this study was a high energy, long-pulsed, frequency doubled Nd:YAG laser. One thousand single-shot spectra were acquired at each spatial coordinate. Mean temperature and rms temperature variations were calculated at each measurement location. Excellent agreement between the averaged and single-shot temperatures was observed with an accuracy better than 2.5% for temperature, and rms variations in temperature between ±2.2% at 296 K and ±4.5% at 850 K. The mean and normalized rms temperatures measured here were then compared to NASA’s Consensus data set of PIV velocity and turbulence measurements in similar jet flows. The results of this and planned follow-on studies will support NASA GRC’s development of physics-based jet noise prediction, turbulence modeling and aeroacoustic source modeling codes.

Jet noise prediction via coupling large eddy simulation and stochastic modeling

A novel method for efficient and accurate jet noise prediction is developed and tested in the framework of coupled LES and stochastic noise modeling. In the proposed method, the low frequency range of the acoustic spectrum, which corresponds to large turbulent structures, is resolved with an implicit LES model by employing higher-order spatial and temporal discretizations. The high frequency range, which corresponds to the fine-scale turbulence, is modeled via a stochastic broadband noise generation model. The inputs to the stochastic model are represented by statistics from an axisymmetric RANS jet simulation. The smaller synthetic scales are convected by larger scales, accounting for the effects of sweeping from the larger LES-resolved turbulent scales. The farfield acoustic data is obtained using either the Linearized Euler Equations, or an acoustic analogy based on the Ffowcs-Williams Hawkings method. The method is evaluated on cold and heated jets at different Reynolds numbers.

Validation of a hybrid nonlinear approach for jet noise prediction and characterization

In this study, a hybrid approach for non-linear jet noise predictions in complex environments is presented and validated. The method differs from traditional approaches in that interactions of the jet with the surrounding structures as well as non-linear disturbances propagating over large distances are taken into consideration in order to quantify their effects on sound generation and propagation. The noise sources founded in the jet plume and the near-field are first computed using the full Navier-Stokes equations. The variables of interest are then interpolated into a second domain. After penalized, they are used as source terms in the non-linear Euler equations to calculate the sound propagation. The interpolation and penalization steps are performed using a buffer region designed by the principles of sponge layers. The effective one-way communication between the two domains and the capabilities of the buffer region to transfer the data from the NS to the Euler domain without any loss of detail is demonstrated. Results from two- and three-dimensional jets both in free space and interacting with solid obstacles are presented. Comparisons with DNC and experimental data in terms of sound pressure level spectra are in good agreement with the ones calculated by our method.

Similarity scaling of jet noise sources for low-order jet noise modelling based on the Goldstein generalised acoustic analogy

As a first step towards a robust low-order modelling framework that is free from either calibration parameters based on the far-field noise data or any assumptions about the noise source structure, a new low-order noise prediction scheme is implemented. The scheme is based on the Goldstein generalised acoustic analogy and uses the Large Eddy Simulation database of fluctuating Reynolds stress fields from the CABARET MILES solution of Semiletov et al. corresponding to a static isothermal jet from the SILOET experiment for reconstruction of effective noise sources. The sources are scaled in accordance with the physics-based arguments and the corresponding sound meanflow propagation problem is solved using a frequency domain Green’s function method for each jet case. Results of the far-field noise predictions of the new method are validated for the two NASA SHJAR jet cases, sp07 and sp03 from and compared with the reference predictions, which are obtained by applying the Lighthill acoustic analogy scaling for the SILOET far-field measurements and using an empirical jet-noise prediction code, sJet.

Predictive Large Eddy Simulation for Jet Aeroacoustics–Current Approach and Industrial Application

The major techniques for measuring jet noise have significant drawbacks, especially when including engine installation effects such as jet–flap interaction noise. Numerical methods including low order correlations and Reynolds-averaged Navier–Stokes (RANS) are known to be deficient for complex configurations and even simple jet flows. Using high fidelity numerical methods such as large eddy simulation (LES) allows conditions to be carefully controlled and quantified. LES methods are more practical and affordable than experimental campaigns. The potential to use LES methods to predict noise, identify noise risks, and thus modify designs before an engine or aircraft is built is a possibility in the near future. This is particularly true for applications at lower Reynolds numbers such as jet noise of business jets and jet-flap interaction noise for under-wing engine installations. Hence, we introduce our current approaches to predicting jet noise reliably and contrast the cost of RANS–numerical-LES (RANS–NLES) with traditional methods. Our own predictions and existing literature are used to provide a current guide, encompassing numerical aspects, meshing, and acoustics processing. Other approaches are also briefly considered. We also tackle the crucial issues of how codes can be validated and verified for acoustics and how LES-based methods can be introduced into industry. We consider that hybrid RANS–(N)LES is now of use to industry and contrast costs, indicating the clear advantages of eddy resolving methods.

Cost-effective hybrid RANS-LES type method for jet turbulence and noise prediction

Jets at higher Reynolds numbers have a high concentration of energy in small scales in the nozzle vicinity. This is challenging for large-eddy simulation, potentially placing severe demands on grid density. To circumvent this, we propose a novel procedure based on well-known Reynolds number (Re) independent of jets. We reduce the jet Re while rescaling the boundary layer properties to maintain incoming boundary layer thickness consistent with high Re jet. The simulations are carried out using hybrid large-eddy simulation type of approach which is incorporated by using near-wall turbulence model with modified properties. No subgrid scale model is used in these simulations. Hence, they effectively become numerical large-eddy simulation with Reynolds-averaged Navier–Stokes covering the full boundary layer region. The noise post-processing is carried out using the Ffowcs-Williams-Hawking approach. The simulations are made for Mach numbers (M) of 0.75 and 0.875 (cold and hot). The results for the overall sound pressure level are observed to be within 2–3% of the measurements, and directivity of sound is also captured accurately for both the cases. Hence, the low Re simulations can be more beneficial in saving time and cost while providing reasonably accurate results.

Jet-Noise-Prediction Model for Chevrons and Microjets

This study develops a jet-noise-prediction model for chevrons and microjets. A novel equation is proposed to express the amplitude of the fourth-order space-time velocity cross correlations, which represent the sources of noise emanated from unheated jets, in terms of mean-flow parameters and turbulence statistics, such as streamwise circulation, axial velocity, and turbulent kinetic energy. The cross correlations based on a Reynolds-averaged Navier-Stokes flowfield showed a good agreement with those based on a large-eddy-simulation flowfield. With the novel acoustic-source description, there is a good agreement between the model’s jet-noise predictions and the experimental data for unheated jets for a wide range of frequencies and observer angles for both chevrons and microjets. As the model provides quick and accurate jet-noise predictions, a parametric study is performed to understand the impact of chevrons and microjets on jet noise. Chevron penetration is the underpinning factor for jet-noise reduction, and its optimum is found to be around one-seventh of the nozzle diameter. The number of chevrons has a considerable effect on jet noise, and six is found to be an optimum number of chevrons. The injected mass-flow rate of a system of microjets has a noticeable impact on jet noise, and for 18 microjets, its optimum is found to be around 0.0072 of the main-jet mass-flow rate. There is a good agreement between predicted and measured optimum values. This establishes that the model is indeed capable of assessing and optimizing jet-noise-reduction concepts, and could contribute toward the development of quieter nozzles for future aircraft.

Jet noise computation based on enhanced DES formulations accelerating the RANS-to-LES transition in free shear layers

The paper presents jet noise computations performed using the original and some recently proposed modified formulations of detached-eddy simulation, based on alternative definitions of the subgrid length-scale and/or a modified version of the subgrid Spalart-Allmaras model, which involves the Wall-Adapting Local Eddy-viscosity model. The modifications are aimed at the elimination or, at least, a significant weakening of a known flaw of detached-eddy simulation, namely, the severe delay of the Kelvin–Helmholtz instability and transition from fully modeled to mostly resolved turbulence in the free and separated shear layers. This flaw makes the original detached-eddy simulation formulation, in fact, non-applicable to the prediction of the noise of high Reynolds number jet flows when typical grids are used, and has led to the use of Implicit Large-Eddy Simulation instead. Based on examples of application of these modifications to such flows, already available in the literature, they were found capable of resolving the issue and providing quite accurate predictions of the aerodynamic characteristics and turbulent statistics of the high-Re jets. The study performed in the present work allows us to draw a similar conclusion regarding the accuracy of the enhanced detached-eddy simulation formulations in terms of jet-noise prediction. The new formulation gives noise results very close to those of ILES except at the highest frequencies, while being more general and less sensitive to grid resolution. In addition, it removes a (1–3) dB underestimation of the noise spectral maximums for the peak radiation direction, which is typical of ILES on fine grids.

マイクロジェットミキサ付ジェットエンジンの騒音予測

マイクロジェットミキサ付ジェットエンジンの騒音予測

Turbulence Statistics for Jet Noise Source Modeling from Filtered PIV Measurements

Time-resolved, particle image velocimetry was used to measure the velocity fluctuation field at three locations along the lipline of two subsonic jets. The time history data within these fields were filtered in the time domain using empirical mode decomposition to produce two signals from the velocity fluctuation measurements. One signal was for the low-frequency, large-scale turbulent eddy component of the velocity fluctuations and represented the computed results of a large-eddy simulation. The other signal was for the high-frequency, small-scale turbulent eddy component of the velocity fluctuations. This represented the missing scales of a large-eddy simulation. The effect of filtering on the statistical properties of the velocity fluctuations relevant to jet noise prediction modeling were studied. Two-point, space-time, cross correlations were computed for the axial velocity fluctuations. Comparisons were made for the second- and fourth-order two-point correlations, the integral phase velocities, length scales, and time scales, and the frequency dependent phase velocities, length scales, and time scales between the total, large-scale, and small-scale velocity fluctuations.

Investigation of a Jet-Noise-Shielding Methodology

Ongoing research toward the reduction of environmental noise from aircraft is investigating the possible shielding of engine-noise sources by novel airframe configurations. To assess the noise-reduction benefits attainable from such configurations, it is necessary to develop appropriate acoustic evaluation tools. In this paper, a jet-noise-shielding-prediction methodology is described. The Tam–Auriault (“Jet Mixing Noise from Fine-Scale Turbulence,” AIAA Journal, Vol. 37, No. 2, 1999, pp. 145–153) jet-noise model with a Reynolds-averaged Navier–Stokes solution input, together with a Fresnel–Kirchhoff diffraction method (Fundamentals of Physical Acoustics, Wiley-Interscience, New York, 2000, pp. 472–494), is used to make isolated and shielded far-field jet-noise predictions. This methodology is employed as a sensitivity-analysis tool to establish the relative importance of the source location, spatial extent, and directivity in jet-noise-shielding predictions. Predictions have been made for a shielded single-stream Mach 0.9 jet, and compared with experimental data. Good qualitative agreement is observed, and the disagreement in the shielding levels is most likely due to underestimation of the source axial extent by the jet-noise model.