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In the framework of improving aircraft nacelle design to comply international regulation on aircraft noise, a time-domain impedance boundary condition (TDIBC) based on the oscillo-diffusive representation is implemented in an industrial computational fluid dynamics (CFD) solver under a characteristic formalism, in order to take into account acoustic liner attenuation. A validation of such TDIBC is carried out, first on a two-dimensional benchmark case, then on a more realistic three-dimensional cylindrical configuration, with focus on the acoustic modal content and its attenuation with and without flow. Unsteady Reynolds-averaged Navier–Stokes computations are performed, with the grazing flow being considered laminar. Good agreement is found between the CFD results and the references (computational aeroacoustics results and experiments), with differences in the order of [Formula: see text] for most cases. The formulation chosen for the TDIBC proved to have no significant impact on numerical stability or computational time.

This article deals with the zonal detached eddy simulation of the fan module of a modern turbofan engine for the prediction of the broadband noise due to the interaction of the fan turbulent wakes with the stationary outlet guide vanes. The simulation relies on a hybrid RANS/LES approach with a zonal strategy: the core airflow is treated in RANS, while the bypass airflow is solved with the hybrid approach. The simulation was performed during four revolutions, and statistical convergence was reached. Inspections of the flowfields highlight a consistent behavior of the shielding function (border between RANS and LES solving areas) around the blade walls and at the trailing edge for a such complex flow. The fan module was tested in the AneCom facility in which hot wire measurements were made in-between the fan and the outlet guide vanes. The numerical results are compared to this large dataset of measurements. The flow maps are well retrieved by the simulation for both the time-averaged and the turbulent quantities. Comparison of radial profiles shows an excellent agreement for the three root-mean-square (RMS) components of the velocities between zonal detached eddy simulation (ZDES) and the measurements, particularly in the tip gap flow area, in which RANS results do not reproduce correctly the flow quantities. The wake shape, a key feature in the mechanism of generation of rotor–stator interaction noise), is quite well predicted by the ZDES simulation. These numerical results demonstrate the maturity of the approach for the simulation of complex turbomachinery flows.

This paper (Part II) presents the broadband noise predictions of a turbofan stage based on a hybrid method proposed by ONERA in the framework of the European project TurboNoiseBB. The numerical approach relies on a ZDES (Zonal Detached Eddy Simulation) strategy that is applied to a fan module at approach conditions and tested in AneCom facility (Wildau, Germany). The ZDES method with main aerodynamic flow features is detailed in a companion paper (Part I), and this one focuses on acoustic analyses. Acoustic codes based on Amiet's theory and FWH (Ffowcs Williams and Hawkings) analogy, both taking into account for duct propagation effects, are briefly described in the article and chained to the computational fluid dynamics to assess the rotor–stator interaction (RSI) noise (focusing on stator sources). The required inputs are the turbulent wake information (issued from either Reynolds Averaged Navier-Stokes (RANS) or ZDES) in front of the stator and the unsteady pressure on the vane wall, respectively. Turbulent velocity profiles and velocity spectra are compared to hot-wire measurements in the interstage region. A nice agreement is globally observed with a clear improvement compared to RANS solutions. Then, sound power spectra in the intake and bypass duct provided by acoustic codes are discussed and compared to the experiment. Reliable numerical predictions are obtained when undesirable additional sources in the rear-chord region (believed to be caused by local flow detachments from the vane leading edge and near the trailing edge) are removed from the FWH surface integration. ZDES + FWH spectra are found to be not so far from RANS + Amiet ones, with a best fitting to the experimental spectrum shape and predicted levels 3 dB below the measurements.

International audience; The objective of the paper is to assess the capability of the isothermal Lattice Boltzmann Method (LBM) to correctly capture aerodynamic and acoustic features from a low-speed turbofan. The evaluation is done on the Advanced Noise Control Fan (ANCF) model developed by NASA Glenn Research Center. An extensive comparison of the measured and computed aerodynamic results is presented for the first time on this configuration. The trends are well captured but quantitative discrepancies are observed for some variables, leading to lower fan performance values in the simulations. This last point was also observed in the previous related studies but remained unexplained. By validating in parallel the LBM aerodynamic results with a Reynolds-Averaged Navier-Stokes (RANS) simulation provided by the database, a likely uncertainty of the absolute values of some variables in the experimental mean flow data is shown (a result of the intent that experimental data was not acquired with absolute levels as an objective). Direct induct and far-field acoustic results are also investigated, taking advantage of the low-dissipative characteristics of the LBM. A good agreement with the measurements in terms of broadband noise is obtained, but the low frequencies tend to be overestimated. For the tonal noise, if the expected acoustic modes are recovered, quantitative differences are observed. Predictions based on an hybrid method where the acoustic sources computed by the LBM are propagated analytically using Goldstein's analogy are also made and compare well with the direct predictions, thus proving the correct propagation of acoustic waves by the solver.; L'objectif du papier est d'évaluer la capacité de la LBM isothermale à capturer correctement l'aérodynamique et l'acoustique d'une turbomachine basse vitesse. L'évaluation est réalisée sur le modèle ANCF développé par la NASA. Une comparaison approfondie entre les résultats aérodynamiques expérimentaux et numériques est présentée pour la première fois sur cette configuration. Les tendances sont généralement bien capturées mais des différences quantitatives sont observées pour certaines variables, menant à une sous-estimation des coefficients de performance de la soufflante. Ce dernier point a déjà été observé dans d'autres études sur le sujet mais n'avait pas été expliqué. En validant en parallèle les résultats aérodynamiques de la LBM avec ceux d'un calcul RANS donné dans la base de données ANCF, il est montré qu'il y a probablement une incertitude dans les valeurs absolues de certaines variables du champ moyen expérimental (ce qui est lié au fait que les données aérodynamiques n'ont pas été acquises avec l'obtention des niveaux absolus comme objectif). Les résultats acoustiques directs en conduit et en champ lointain ont été analysés, tirant profit de la faible dissipation numérique de la LBM. Un bon accord est obtenu pour le bruit large bande, mais les basses fréquences tendent à être surestimées. Pour le bruit tonal, si les modes acoustiques attendus sont bien retrouvés, des différences quantitatives existent. Des prévisions basées sur une méthode hybride où les sources calculées par la LBM sont propagées analytiquement par l'analogie de Goldstein sont également réalisées et se comparent correctement avec les prévisions directes, validant ainsi la bonne propagation des ondes acoustiques par le solveur.

International audience; The context of this study is the development of innovative stator concepts to reduce noise from rotor/stator interaction in future high bypass engines. Leading edge serrations are examined using a hybrid CFD/CAA method. A stochastic approach is considered for the turbulence generation based on a Fourier modes decomposition. A methodology to obtain a fully 3D turbulence field, while taking into account periodic and wall boundary conditions is developed and a formulation is finally provided to obtain a perfectly incompressible stochastic field. The influence of the turbulence structure on far-field acoustic spectra is first discussed for the baseline computation case. In a second step, numerically assessed noise reductions from the serrated design are favorably compared with an analytical solution and a semi-empirical law. An overall sound power level reduction around 4 to 6 dB is obtained at three acoustics certification points. Finally, the aerodynamic performances of the serrated airfoil are evaluated through RANS computations and an improved variant of the initial design is proposed, allowing for acceptable penalties at the aerodynamic design point.

International audience; Noise pollution is a major concern for aircraft manufacturer during design and certification. Very high sound levels are recorded in take-off phases upstream of the engine fan, where multitonal noise components can appear (so-called Buzz Saw Noise) and contribute in generating a complex acoustics content. Nonlinear waves propagation and modal decomposition are two fundamental aspects that must be taken into account. In this context, Time-Domain Impedance Boundary Conditions (TDIBC) are then necessary when gathering noise source generation, nonlinear propagation and liner attenuation in the same numerical simulation of a nacelle inlet. A first validation of such TDIBC has already been carried out by the authors on a 2D benchmark case. In the present work, the boundary condition will be extended to a more realistic 3D cylindrical configuration, with focus on the acoustic modal content and its attenuation with and without flow.

The present article deals with the Zonal Detached Eddy Simulation of the fan module of a modern turbofan engine. The fan module, tested at the AneCom facility, is equipped with rotating fan blades and stationary outlet guide vanes (OGV). The simulation was performed to capture the interaction of the turbulent fan wakes with the OGV walls. The final goal of this simulation is the prediction of the associated broadband noise, not adressed here. In this paper, only the aerodynamic aspects are treated. The simulation relies on a hybrid RANS/LES approach with a zonal strategy: the core airflow is treated in RANS while the bypass airflow is solved with the hybrid approach. Mesh criteria meeting both RANS/LES and acoustic requirements were fulfilled, leading to a mesh of 380 million cells. The simulation was performed during five revolutions and statistical convergence was reached. Inspections of the flow-fields highlight a consistent behaviour of the shielding function (border between RANS and LES solving areas) around the blade walls, at the trailing-edge and in the tip gap flow areas. Comparisons with performance and hot-wire measurements are also presented. Aerodynamic performance and radial evolution of averaged velocities on a plane in-between the fan and the OGV are well retrieved, both in shape and levels. For the turbulent quantities, the shape of the radial profiles are close to the measurements, with much better accuracy in the upper region compared to the RANS solution.

International audience; This paper presents aeroacoustic investigations on a full-scale Ultra High Bypass Ratio (UHBR) engine with inflow distortion at transonic conditions. Computational Fluid Dynamics (CFD) simulations are first realized to compute the shocks in the vicinity of the fan. The shocks are then radiated outside of the nacelle thanks to Computational AeroAcoustics (CAA) simulations. The same solver is used for both CFD and CAA simulations and the chaining is done by injecting the shocks in terms of usual conservative variables using a non-reflecting boundary condition. The CAA solver is based on the non-linearized Euler equations which allows to define the CFD/CAA interface close to the fan where the propagation of shocks is highly non-linear. Both shock generation and shock propagation mechanisms are investigated and the effects of inflow distortion are highlighted by comparison with a baseline case without distortion. It is shown that the distortion, characterized by an acceleration of the flow at the bottom of the nacelle, is responsible for a modification of the shock amplitudes that depends on the circumferential position. Thus, azimuthal modes appear in addition to the rotor-locked mode present without distortion. The near-field radiation is also highly impacted with most of the noise being directed towards the sky. This is due to the blockage of shocks by a supersonic flow region in the bottom of the nacelle. The radiation is in addition highly heterogeneous in the other directions because of the various angles of the shocks that leave the nacelle.; Ce papier présente des analyses acoustiques d'un moteur à très grand taux de dilution (UHBR) en présence de distorsion de l'écoulement d'entrée et à des conditions d'opération transsoniques. Des simulations des équations de la mécanique des fluides (CFD) sont d'abord réalisées pour déterminer les chocs à proximité de la soufflante. Les chocs sont ensuite rayonnés vers l'extérieur de la nacelle à l'aide de simulations des équations de l'aéroacoustique (CAA). Le même code est utilisé pour les simulations CFD et CAA et le couplage est fait par injection des chocs en termes de variables conservatives en utilisant une condition aux limites non-réfléchissante. Le solveur CAA est basé sur les équations d'Euler non-linéarisées, ce qui permet de définir l'interface CFD/CAA proche de la soufflante où la propagation des chocs est fortement non-linéaire. Les mécanismes de génération et de propagation des chocs sont étudiés et les effets de la distorsion de l'écoulement d'entrée sont mis en évidence en comparant les résultats avec ceux d'un cas de référence sans distorsion. Il est montré que la distorsion, caractérisée par une accélération de l'écoulement dans la partie basse de la nacelle, est responsable d'une modification de l'amplitude des chocs qui dépend de la position azimutale. Ainsi, des modes azimutaux apparaissent en plus du mode attaché qui est le seul présent dans le cas de référence. Le rayonnement en champ proche est aussi fortement impacté avec une directivité fortement orientée vers le ciel. Ceci est dû au blocage des chocs par l'écoulement devenant supersonique dans la partie basse de la nacelle. De plus, le rayonnement est fortement hétérogène dans les autres directions à cause des différences d'angles des chocs qui sortent de la nacelle.

International audience; Modern UHBR (Ultra High Bypass Ratio) aeroengines are characterized by reduced fan speeds and very short asymmetric intakes, so that the mean flow entering into the nacelle is no more uniform along the azimuthal direction (particularly at operating conditions with non-zero angle of attack). Such short inlets and inflow distortions can highly modify the dominant tone noise contributions known from conventional turbofans. This paper presents aeroacoustic investigations applied to a full scale UHBR engine in the framework of the CleanSky2 European project, ASPIRE. Numerical simulations based on (u)RANS calculations are coupled to integral formulations derived from the Ffowcs-Williams and Hawkings analogy and to CAA (Computational AeroAcoustics) based on non-linear disturbance Euler equations, in order to assess the source generation and sound propagation of the fan-OGV stage with and without inflow distortion. Simulations are performed at approach operating condition and CAA is focused on the intake region and forward radiation. It is shown that the inflow distortion effects significantly modify the harmonic loads (on rotor and stator) and so the associated circumferential mode spectrum. Although the blade passing frequency remains cut-off, these effects give rise to an increase of overall power level in the forward direction, and a non expected reduction in the bypass duct too.; Les moteurs modernes à haut taux de dilution (UHBR pour Ultra High Bypass Ratio) sont caractérisés par des vitesses de rotation de soufflante réduites et des entrées d'air asymétriques courtes, induisant en entrée de nacelle des écoulements moyens non uniformes dans la direction azimutale (particulièrement dans les configurations avec incidence). Les entrées d'air courtes et les écoulements avec distorsion peuvent altérer la contribution des bruits tonaux par rapport aux configurations conventionnelles. Ce papier présente l'étude aéroacoustique de ces phénomènes sur un moteur UHBR dans le cadre du projet ASPIRE qui s'inscrit dans le projet européen CleanSky2. Des simulations (u)RANS couplées à des calculs de méthodes intégrales basés sur l'analogie de Ffowcs-Williams et Hawkings, et à des calculs CAA basés sur les équations d'Euler non linéaire en formulation perturbative, sont réalisées afin d'étudier la génération des sources acoustiques par l'étage fan/OGC et leur propagation avec ou sans effet de distorsion. Les calculs sont réalisés dans des conditions d'approche et focalisés sur l'entrée d'air et la propagation acoustique en amont de la nacelle. Il est montré que la distorsion modifie la répartition harmonique des charges (sur le rotor et le stator) et les modes azimutaux associés. Bien que la fréquence associée aux passages des pâles du rotor reste coupée, la distorsion provoque une augmentation significative des niveaux acoustiques globaux dans direction en aval, ainsi que atténuation non attendue dans le conduit en amont.

The interaction of wakes generated by a fan with the outlet guide vanes occurring at the blade-passing frequency and its harmonics is mainly responsible for aeroengine tonal noise emission in approach conditions. Conventional rotor–stator interaction models assume axisymmetric rows and quasi-annular ducts. However, the stator of new engines is characterized by nonidentical vanes (so-called heterogeneous outlet guide vane) and integrates two internal bifurcations up to the nacelle outlet. These new technologies invalidate the existing tools adopted by engine manufacturers at the design stage. For this reason, hybrid methodologies based on a three-dimensional unsteady Reynolds-averaged Navier–Stokes simulation considering the complete geometry of a modern Snecma engine model are investigated in this paper. Unsteady Reynolds-averaged Navier–Stokes simulation output data are postprocessed using different integral methods to assess the impact of the heterogeneity on angular mode distribution (compared to an id...

Whereas the sound radiated from the inlet of turbofans is mainly due to rotor–stator interactions in approach flight, the shock waves (or N-waves) emitted by the rotor at transonic rotation speeds can be a dominant noise source during takeoff and climb. The study of N-waves needs to take account of two main processes: 1) the generation of N-waves for a perfect rotor (in which all blades are identical) and for a real rotor (considering small geometrical blade dispersion), and 2) The N-wave propagation through the inlet duct producing the blade passing harmonics for a perfect rotor, and the multiple pure tones (harmonics of the rotation frequency) for a real rotor. Several approaches have been investigated for the past 40 years. This paper intends to cross-check the main methods by applying them to two modern turbofan demonstrators. Moreover, a prospective way of N-wave generation based on geometrical considerations is investigated thanks to test data related to near-field pressure signatures and blade stag...

Tone noise radiated through the inlet of a turbofan is mainly due to rotor-stator interactions at subsonic regimes (approach flight), and to the shock waves attached to each blade at supersonic helical tip speeds (takeoff). The axial compressor of a helicopter turboshaft engine is transonic as well and can be studied like turbofans at takeoff. The objective of the paper is to predict the sound power at the inlet radiating into the free field, with a focus on transonic conditions because sound levels are much higher. Direct numerical computation of tone acoustic power is based on a RANS (Reynolds averaged Navier–Stokes) solver followed by an integration of acoustic intensity over specified inlet cross-sections, derived from Cantrell and Hart equations (valid in irrotational flows). In transonic regimes, sound power decreases along the intake because of nonlinear propagation, which must be discriminated from numerical dissipation. This is one of the reasons why an analytical approach is also suggested. It is based on three steps: (i) appraisal of the initial pressure jump of the shock waves; (ii) 2D nonlinear propagation model of Morfey and Fisher; (iii) calculation of the sound power of the 3D ducted acoustic field. In this model, all the blades are assumed to be identical such that only the blade passing frequency and its harmonics are predicted (like in the present numerical simulations). However, transfer from blade passing frequency to multiple pure tones can be evaluated in a fourth step through a statistical analysis of irregularities between blades. Interest of the analytical method is to provide a good estimate of nonlinear acoustic propagation in the upstream duct while being easy and fast to compute. The various methods are applied to two turbofan models, respectively in approach (subsonic) and takeoff (transonic) conditions, and to a Turbomeca turboshaft engine (transonic case). The analytical method in transonic appears to be quite reliable by comparison with the numerical solution and with available experimental data.

International audience; Several approaches of N-wave generation and propagation have been studied during the past forty years, but most advanced semi-analytical models still consider geometrical simplifications. In this paper numerical simulations based on the Euler solver of a CFD code dedicated to the capture of shocks generated by turbofans in transonic conditions are investigated. The main objective is to take into account actual inlet geometry and realistic convection flow associated with nonlinear propagation. Regular and non-regular shock waves are directly injected in a plane close to the fan and propagated through the inlet. The initial shock description near the fan is provided either by a steady RANS computation or by experiment, or else from an analytical modeling. Unsteady Euler calculations on suited acoustic grids are favorably compared to analytical solutions on simplified (quasi-2D) cases, and significant 3D effects are highlighted on realistic configurations. First application to a Snecma turbofan demonstrator using both test bed measurements and RANS solution as input data show that the present numerical approach is able to accurately assess the buzz saw noise spectrum measured in the intake.

Turbofan broadband noise prediction is a major concern for industry and probably the most challenging issue for Aeroacoustic community. The dominant source mechanism is attributed to the rotor turbulent wakes impacting on the outlet guide vanes. Unsteady computations of the full rotor-stator stage are generally out of reach so that interaction noise is commonly assessed using analytical models (related to Amiet's theory) and averaged turbulence characteristics issued from a steady RANS calculation and isotropic homogeneous turbulence assumption. However these models are questionable to accurately describe the turbulent rotor wakes characterized by a non-uniform spatial distribution stretched in the flow direction. Thus, a hybrid method based on a 3-D Zonal Detached Eddy Simulation of the flow around a single rotor blade is investigated to provide a full description of the turbulent wakes directly used in the Amiet-based code. The reliability of this approach is checked thanks to the comparisons of turbulent velocity and acoustic spectra respectively with hot-film probe and microphone measurements performed in Snecma engine test rig. A quite good agreement is shown between numerical and experimental turbulent velocity spectra and also the von Karman spectrum model, when mean turbulence characteristics are well adjusted. Moreover, sound pressure and power spectra in the outlet duct predicted by ZDES-Amiet coupling are reasonably close to the available measurements, and not so far from RANS-based ones.

Nowadays, computational aeroacoustics (CAA) is used for simulating wave propagation in ducted turbofans, especially as computational fluid dynamics (CFD) is increasingly employed to model the identified noise sources. An efficient way to match the CFD and CAA domains is to make some assumptions on flow and duct geometry, so that disturbance fields can be expanded over incoming/outgoing acoustic modes. Based on this approach, this paper presents an original matching model in which the outgoing modes are generated by means of equivalent monopole distributions defined as source terms in the equations governing the acoustic propagation, instead of a conventional inflow boundary condition (BC). Advantages and limits of the method are discussed. The process to get back to the sources and its numerical implementing are described. Although initially focused on tones, an extension of the method to broadband noise generation is tackled too. The method then is validated on a simplified turbofan exhaust configuration. Numerical solutions obtained by implementing the source terms in a high-order time-domain Euler code are compared to analytical solutions, either in a uniform or in a radially shear mean flow (provided by RANS). The parallel shear flow solution is obtained by solving the Pridmore–Brown equation. The ability to accurately simulate the standing waves due to acoustic reflections at duct ends is also assessed by comparing the numerical solutions computed using both source-based and BC-based options in the Euler solver.

A source-to-far-field computation procedure aimed at predicting the noise generated by rotor-stator fan interactions is presented here. Interaction noise sources are assessed using a three-dimensional Reynolds-averaged Navier-Stokes (RANS) code. A modal expansion is applied to the computational fluid dynamics (CFD) data, and a source model is used to link the CFD to an Euler code simulating the acoustic propagation. Far-field noise is classically obtained by means of a Kirchhoff integral. The source model is detailed and validated over a selected benchmark. Then the hybrid method is applied to a turbofan model, tested in the German-Dutch Wind Tunnel. Simulations are compared to boundary element method computations previously performed and used here to validate the no-flow solution. Interacting cut-on mode amplitude deduced from CFD output postprocessing is adjusted to fit in-duct measurements on the outer wall. The predicted radiation field is related to the directivity patterns measured at several axial positions upstream of the fan inlet and fairly good agreement is found using the RANS/Euler/Kirchhoff procedure.

International audience; Turbulent-flow interactions with the outlet guide vanes are known to mainly contribute to broadband-noise emission of aeroengines at approach conditions. This paper presents a three-dimensional computational aeroacoustics hybrid method aiming at simulating the aeroacoustic response of an annular cascade impacted by a prescribed homogeneous isotropic turbulent flow. It is based on a time-domain Euler solver coupled to a synthetic turbulence model implemented in the code by means of a suited inflow boundary condition. The fluctuating pressure over the airfoil surface provided by computational aeroacoustics is used as an input to a Ffowcs Williams and Hawkings integral method to calculate the radiated sound field. Euler computations are first validated against an academic computational aeroacoustics benchmark in the case of an harmonic gust interacting with an annular flat-plate cascade. Then, simulations are applied to turbulence–cascade interactions for annular configurations, in uniform and swirling mean flows, and numerical results in terms of sound power spectra in the outlet duct are compared to semi-analytical and numerical solutions, and to an available experiment.

International audience; This paper provides an experimental investigation into the use of leading edge (LE) serrations as a means of reducing the broadband noise generated due to the interaction between the aerofoil’s LE and impinging turbulence. Experiments are performed on a flat plate in an open jet wind tunnel. Grids are used to generate isotropic homogeneous turbulence. The leading edge serrations are in the form of sinusoidal profiles of wavelengths, λ, and amplitudes, 2h. The frequency and amplitude characteristics are studied in detail in order to understand the effect of LE serrations on noise reduction characteristics and are compared with straight edge baseline flat plates. Noise reductions are found to be insignificant at low frequencies but significant in the mid frequency range (500 Hz–8 kHz) for all the cases studied. The flat plate results are also compared to the noise reductions obtained on a serrated NACA-65 aerofoil with the same serration profile. Noise reductions are found to be significantly higher for the flat plates with a maximum noise reduction of around 9 dB compared with about 7 dB for the aerofoil. In general, it is observed that the sound power reduction level (ΔPWL) is sensitive to the amplitude, 2h of the LE serrations but less sensitive to the serration wavelength, λ. Thus, this paper sufficiently demonstrates that the LE amplitude acts as a key parameter for enhancing the noise reduction levels in flat plates and aerofoils.

International audience; Turbulent flow interactions with the outlet guide vanes are known to be mainly contributing to broadband noise emission of aeroengines at approach conditions. This paper presents a 3D CAA hybrid method aiming at simulating the aeroacoustic response of an annular cascade impacted by a prescribed homogeneous isotropic turbulent flow. It is based on a time-domain Euler solver coupled to a synthetic turbulence model implemented in the code by means of a suited inflow boundary condition proposed by Tam. The fluctuating pressure over the airfoil surface provided by CAA is used as an input to a FWH integral to calculate the radiated sound field. The method is first validated against an academic CAA benchmark in the case of a harmonic gust interacting with an annular flat plate cascade. Then, simulations are applied to turbulence-cascade interactions for annular configurations, in uniform and swirling mean flows, and numerical results in terms of sound power spectra in the outlet duct are compared to semi-analytical and numerical solutions, and to an available experiment. Nomenclature B = blade number L = span M x , M θ = axial, azimuthal mean flow Mach number T u = turbulence intensity U 0 , U x = uniform, axial mean flow velocity V = vane number c = chord f = frequency k = wave number p, p ∞ = static pressure, undisturbed static pressure u' = velocity disturbance (x,r,θ) = cylindrical coordinates in the annular duct ∆k, ∆f = wave number and frequency spacing χ = stagger angle φ k x , k r () = 2-wavenumbers turbulent energy spectrum Λ = integral length scale ϕ = random phase ρ, ρ ∞ = density, undisturbed density ω = angular frequency ξ,η, r () = local coordinates attached to the vane

Whereas the sound radiated from the inlet of turbofans is mainly due to rotor-stator interactions in approach flight, the shock waves (or N-waves) emitted by the rotor at transonic rotation speeds can be a dominant noise source during takeoff and climb. The study of N-waves needs to take account of two main processes: (i) The generation of N-waves for a perfect rotor (where all blades are identical) and for a real rotor (considering small geometrical blade dispersion); (ii) The N-wave propagation through the inlet duct producing the blade passing harmonics for a perfect rotor, and the multiple pure tones (harmonics of the rotation frequency) for a real rotor. Several approaches have been investigated for the past forty years. This paper intends to cross-check the main methods by applying them to two modern turbofan demonstrators. Moreover, a prospective way of N-wave generation based on geometrical considerations is investigated thanks to test data related to pressure signal and blade stagger angle measurements during the engine rotation. This prediction overcomes some drawbacks of other methods. Moreover, it appears to be very efficient to build rotor blade orderings that reduce the sound pressure level of the multiple pure tones.

Euler/Kirchhoff methods have been developed at ONERA and DLR to predict the High Speed Impulsive (HSI) noise generated by helicopter rotors in hover or in forward flight. The main purpose of this common work is to validate the aeroacoustic computations with delocalized test cases. In hover, the aerodynamic and acoustic results obtained by each partner are in good agreement with experiment. In forward flight, a first set of computations is performed with three different sized "classical" grids. For both partners, the pressure coefficients on the blade are in better correlation with experiment when the grid is refined. However, the capture of the shock waves propagation beyond the blade is not accurate enough to perform satisfactory Kirchhoff computations. The minimum pressure peak and the recompression slope of the acoustic pressure signatures are not accurately predicted, whatever the grid refinement of the Kirchhoff surface locations may be. An.

The goal of this study was to realize predictions of tonal noise generated by a SNECMA turbofan mock up with a heterogeneous stator and radiated from the inlet using as input a modal decomposition made in the inlet duct wall. A numerical approach based on the CAA (Computational AeroAcoustics) code sAbrinA.V0 with an equivalent source term recently implemented is used as well as an analytical methodology using a Wiener-Hopf technique and an uniform flow assumption. Even if directivities from raw experimental data exhibit significant variations of noise level in the azimuthal direction proving that correlation effect between modes seems to be not negligible for tonal noise, the hypothesis of incoherent modes inducing axisymmetric directivities allow to compare reasonably well with measured data averaged azimuthally. Comparisons between theoretical and numerical predictions confirm the practical interest of using simplified analytical methods even if clear improvement are provided by more complex and time consuming methods like CAA. Especially, for polar angles between 20° and 70°, a very good agreement is obtained with a maximum discrepancy of 0.7 dB on overall sound pressure level for the three first tones.

International audience; Turbulent wakes generated by turbofan blades and interacting with the outlet guide vanes are known to be mainly contributing to broadband noise emission of aero-engines at approach conditions. Analytical approaches, such as the well-known Amiet model can be adopted to estimate the noise generated by turbulent flows impinging thin airfoils, but they are limited by the flat-plate assumptions. The development of numerical methods allowing to consider more complex geometries and realistic flows is required. The method described in the present paper, is based on a CAA code solving the nonlinear Euler equations. The upstream turbulence is synthesized from a stochastic model and injected into the computational domain through an adapted boundary condition. It is first validated in 2D and 3D against academic flat plate configurations by comparison with Amiet solutions (exact in such cases). Then, 3D computations are applied to simulate the effect of a passive treatment (leading edge serrations) aiming at reducing turbulence interaction noise of an isolated airfoil studied in the framework of European project FLOCON. First calculations on baseline conditions are shown to be able to reproduce the measured spectra and far-field directivities, and the acoustic performances of the serrations (3-4 dB PWL reduction) are fairly well assessed too.

In the context of the evaluation and reduction of fan noise in modern turbofan engines, the possibility to compute the acoustic transmission through a rotor-stator system, accounting for the rotation of the rotor, is evaluated. The objective is the development of a numerical method to simulate the propagation of acoustic waves in a global fixed region which includes a partial moving zone, with equations solved in a fixed reference frame. In order to insure a high order resolution these CAA developments and computations were performed using Onera’s CAA solver sAbrinA-V0. Firstly, a full or partial 2D moving zone was employed as a reference test case. It was insured that the moving grid acts only as a resolution support and does not affect the propagation of the waves by its rotation. Monopole and planar sources were considered. In order to connect the two (fixed and rotating) zones, spline cubic interpolation functions were employed in the ghost cells rows. Secondly, a cross-shaped rigid obstacle with zero thickness walls was embedded in the moving grid and the interaction between this obstacle and a fixed monopole was computed. Note that, despite the CAA solver can handle any non-homogeneous mean flow, this case was addressed accounting for a medium at rest. The field of application of the method is restricted to non-viscous interactions between boundary layer and acoustics, so the considered mean flows have to respect this condition. Finally, a 3D test case is presented, based on a simplified rotor with four twisted blades with zero thickness in a perfect annular duct. The acoustic interaction of this rotating blades with an helicoidal modal source injected upstream the rotor within a medium at rest was evaluated. Recent developments in sAbrinA-v0 on improved inlet/outlet acoustic boundary conditions and conclusions of gust-fixed airfoil interactions will allow a proper evaluation of CAA simulations of moving surfaces.

Turbulence-interaction noise due to homogeneous isotropic turbulence impacting on a thick cambered NACA651210 airfoil with a significant spanwise extent (40%) has been simulated for the first time at high Reynolds number (Rec=6.5×105) by a compressible Large Eddy Simulation (LES) accounting for the installation effects. The full wind-tunnel has been simulated by a 2D Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulation that provides the realistic boundary conditions for the LES on a restricted domain embed- ded in the potential core of the wind-tunnel jet. Radiative boundary conditions have been applied all around the LES domain to prevent any reflection or drift of flow parameters at the LES boundaries. The mean URANS flow field was also used for a 2D non-linear Euler computation. The same synthetic turbulence model has been used at the inlet for both methods. Comparison with experimental data show excellent agreement with the Euler results and proper noise level and directivity for the LES when the sources are restricted to the leading edge area. Yet the LES also show some extra noise sources at the trailing edge caused by a large recirculation bubble close to the leading edge yielding some large vorticity up to the trailing edge, and a late transition on the suction side.

A first attempt to numerically simulate the broadband noise transmitted through and radiated from a turbofan bypass duct is presented in this paper. Turbulent source assessment on the fan rows is left out here, and a generic power spectral density of in-duct acoustic pressure is imposed as an input to a computational aeroacoustics simulation performed using a time-domain Euler solver. The acoustic field is expanded over usual Fourier-Bessel modes, and the prescribed spectrum is re-generated by assuming an equi-modal energy distribution. The broadband noise behavior is modeled by introducing a random phase when imposing each acoustic mode and by performing a quadratic averaging of the numerical solution over a few independent runs. This broadband-like source generation approach is firstly validated for a semi-infinite annular duct with uniform flow by comparison with analytical solutions. Decrease of the statistical errors depending on the number of run is checked showing that at least 10 runs have to be considered. The exhaust radiation problem from an idealized nozzle in uniform and shear mean flow is then investigated and validated (for single mode case) by comparison to a Wiener-Hopf based solution. Far-field predictions are obtained by coupling the Euler code with a Kirchhoff integral. Computed results appear to be stable and accurate even in the presence of vorticity waves created in the shear layer. Expected shear effect, leading to a deviation of the directivity lobe from the duct axis, is confirmed for broadband noise. Then, the computation method is applied to a realistic nozzle, including a RANS solution providing the mean flow. The only problem arising is the computational cost resulting in a limitation of high frequency range limited here to 2 3 kHz (with around 600 propagating modes). Anyway, computed far-field directivities relative to idealized and realistic nozzle cases with similar input conditions are found to be rather close up to 90 o of radiation angle.

Aeroacoustic simulation of interaction noise generated by a contrafan model is proposed in this paper, in the framework of European Project VITAL (EnVIronmenTALly friendly aeroengine). Main objective here is to check the ability of advanced computations, including noise generation, propagation and radiation, to give a reliable evaluation of an innovative concept. Numerical simulations are realized by using a hybrid method based on the coupling of CFD (Computational Fluid Dynamics) and CAA (Computational Aero-Acoustics) ONERA codes. The matching between CFD and CAA is based on a Fourier-Bessel mode expansion of the perturbation field along a prescribed cross-section, from which outgoing modes then are traduced, in an original way, as source terms in the CAA. In-duct mode power and spectrum at the outer-wall are analyzed and discussed by comparison with theory and with similar results from project partners. Acoustic far field radiated by inlet and deduced from Kirchhoff integration is addressed in terms of mode spectrum and directivities. These predictions are to be compared to experimental data soon available.

In the context of the on-going European project JTI-SFWA, the modal acoustic content inside a realistic fan duct, designed by Dassault-Aviation, must be characterized experimentally. The objective is to use Onera's CAA solver sAbrinA-V0 to design a non-intrusive sensor distribution inside the nacelle, upstream/downstream the fan plane, accounting for the heterogeneous mean flow and the complex internal geometry. By computing the propagation of several interacting modes in the duct, a region in which the acoustic pattern is close to the infinite duct analytical solution was characterized. Then several 3D distributions of pressure sensors were tested in this zone. Radial and azimuthal sensor arrays were located at the nacelle wall, whereas a radial sensor array was placed along the bifurcation leading edge, without additional intrusivity. The response of the 3D arrays to a given acoustic field, either analytical or propagated through CAA, was projected on an analytical modal basis. For several array configu...

Following large efforts to reduce tone noise during the last decades in modern high‐bypass ratio turbofans, fan broadband noise reduction has become now an industrial priority. A hybrid computational method providing source‐to‐far‐field predictions of broadband noise due to rotor‐stator interaction is presented. The acoustic model is based on the loading term of the FWH (Ffowcs Williams and Hawkings) equation with a modal Green's function valid for an infinite annular duct, and a Kirchhoff approximation for the free‐field radiation. The aerodynamic sources on the airfoils required by the model are expected to be directly issued from a LES (Large Eddy Simulation) computation. The method is applied to a simplified configuration tested in a laboratory rig. The first part of the study is concerned with the assessment of in‐ duct acoustic field. Usual assumptions about coherence and energy distribution between the acoustic modes are analyzed. PSD (Power Spectrum Density) are calculated through several ways. The second part is focused on the ability to generate an equivalent PSD by means of equivalent source distributions. The purpose is to validate a practical way for coupling LES with Computational Aero‐Acoustics Euler solver, in order to include realistic geometry and mean flow effects.

This work reports the noise reduction obtained using simultaneously serrated trailing edges and serrated leading edges in a tandem airfoil con guration. Two types of serrated trailing edge treatments are applied to the upstream airfoil, i.e., a sawtooth serration and a slitted sawtooth serration. The leading edge serrated airfoil was designed and manufactured by ONERA. Broadband noise reductions of between 5 dB and 8.5 dB are reported. It is shown over most of the frequency range that overall noise radiation is dominated by interaction noise, while trailing edge noise dominates the high frequency power spectrum at low mean ow velocity. Therefore, the overall noise reduction is dominated by the reductions in interaction noise, and particularly by the reduction of the downstream airfoil response R(f) due to leading edge serrations. However, additional reductions of the overall noise radiation of up 3.5 dB are provided by the use of the slitted sawtooth trailing edge rather than the sawtooth trailing edge on the upstream airfoil, at the high ow velocity U0 = 80m=s. This is shown to be most likely due to modi cations of the wake parameters, i.e., notably a faster decay of the turbulence intensity behind the slitted sawtooth trailing edge.

International audience; Turbulent RSI (rotor-stator interaction) mechanism is a major broadband source contribution of turbofan noise generation. Acoustic prediction tools used by Industry are based on flat-plate cascade response models with restrictive assumptions on flow and geometry. Due to huge CPU memory and time cost required, Large Eddy Simulations of the complete fan-OGV stage are still out of reach (apart from recent impressive results obtained using the Lattice Boltzmann Method). This paper presents an alternative approach based on the use of a 3-D CAA (Computational Aeroacoustics) code solving the linearized-Euler equations applied to the disturbances and coupled with a synthetic turbulence injection model. The inflow turbulence is synthetized by means of a sum of harmonic gusts with random phases. The Fourier-mode amplitudes are trimmed by a 2 or 3-wave number Von-Karman or Liepmann turbulence spectrum. Swirling convection of the synthetic turbulence is provided by a 3D RANS mean flow solution and interpolated at the nodes of the CAA grid. In this paper, our methodology is first validated on a benchmark case (fully annular duct with swirling flow and a prescribed turbulence) and then applied for the first time to an industrial turbofan in the framework of a European project, TurboNoiseBB. Previous implemented 2D formulation (2-wave number spectrum) for turbulence generation is extended here to 3D (axial, radial, and angular modes) in order to study the sensitivity on cascade effects.

International audience; This work, realized in the framework of the European project TurboNoiseBB, presents an advanced aeroacoustic design methodology of Outlet Guide Vanes (OGVs) with leading edge serrations, including details on their broadband noise and aerodynamic performance. The serrated OGV corresponds to a modified stator from an aero-engine fan stage tested at the AneCom Aerotest's facility (Germany). Sinusoidal leading edge patterns with varying amplitude and wavelength along the span are designed in collaboration with Safran Aircraft Engines. Serrations are adjusted to account for the turbulence characteristics provided by Reynolds-Averaged Navier-Stokes (RANS) calculations. Optimal parameters are found using simple design rules discussed in the paper. Down selection of serrated OGV designs (patent pending) are conducted through a RANS control of aerodynamic performances and in accordance with industrial specifications, ensuring acceptable penalties on the loss coefficient and isentropic efficiency. Broadband noise simulations are performed using a computational aeroacoustic (CAA) code that solves the linearized Euler equations with a synthetic turbulence model. Additionally, the acoustic response of the serrated leading edge airfoils is also estimated using the most relevant analytical formulation in the literature, based on the Wiener-Hopf technique. Numerical predictions at approach conditions are compared to available experimental measurements (on the untreated baseline case) and analytical Amiet-based and Wiener-Hopf solutions, showing a satisfactory agreement in the sound power spectrum in the bypass duct. Finally, the acoustic performances estimated at the design stage are numerically assessed by both CAA and Wiener-Hopf methods, providing around 2.5 dB and 3.5 dB reduction on the overall power level, respectively.