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International audience; A Zonal Detached Eddy Simulation (ZDES) has been performed on the simplified LAGOON nose landing gear geometry using a Navier-Stokes solver on a fully unstructured grid. The attached boundary layers have been finely resolved using Y + values in the order of unity, while the high curvature zones have been intensively meshed in order to accurately solving adverse pressure gradients present in these regions. The mean and fluctuating flow fields have been compared with the experimental results, proving that both the mean flow field and the spectral content recorded at the wall are accurately reproduced. Following these comparisons, a detailed analysis of the topology of the flow has been carried out through the analysis of the skin friction coefficient and friction lines, coupled with three dimensional visualizations of the landing gear wake. The far-field acoustics, computed through the Ffowcs-Williams and Hawkings (FW-H) equation from the computed pressure on the landing gear skin, has been compared with the experimental results, obtaining a very good agreement for the different microphones and directions. Finally, the CFD methodology presented in this study proves to be a moderate cost approach, enabling an accurate flow and noise prediction for bluff bodies such as landing gears.

International audience; In the SWAHILI (SWept Airfoil with HIgh LIft) Onera-DLR Common Research Project (CRP), both research centers are building an experimental database for the validation of CFD/CAA codes applied to the simulation of the unsteady flow and noise generation from a high-lift profile with deployed slat and flap and a global sweep angle of 30°. The project is based on DLR’s model F16, a two-dimensional airfoil (constant section in the span direction), with a 300 mm clean (retracted) chord. This project is a continuation of the LEISA2 (Silent Take-Off and Landing, 2010-2013) project, in which a similar database has been built with the same model, but without sweep angle. Since 2014, the LEISA2 test-case has been included by NASA and AIAA in the Benchmark for Airframe Noise Computations (BANC). In both projects LEISA2 and SWAHILI, the model has been tested in F2, an aerodynamic wind tunnel located in Onera-Le Fauga, with intensive aerodynamic measurements, including steady/unsteady wall pressure sensors, optical devices such as PIV and steady/unsteady LDV and a hot wire probe. Acoustic measurements were also achieved in F2 with a wall microphone array mounted in the windtunnel ceiling. During the SWAHILI project, Dassault-Aviation has partnered with Onera and DLR, contributing to the project with two additional configurations of flap side edge, with the same aerodynamic and acoustic measurements in F2. In LEISA2 the un-swept model has been also tested in AWB, an anechoic open-jet wind tunnel located in DLR-Braunschweig for acoustic data acquisition. In the SWAHILI context, similar tests are under progress with the SWAHILI swept model. The present paper focusses on the influence of the sweep angle and the flap side edge on the characteristics of the unsteady flow and on the radiated noise.

This paper describes some significant steps made towards the numerical simulation of the noise radiated by the high-lift devices of a plane. Since the full numerical simulation of such configuration is still out of reach for present supercomputers, some hybrid strategies have been developed to reduce the overall cost of such simulations. The proposed strategy relies on the coupling of an unsteady nearfield CFD with an acoustic propagation solver based on the resolution of the Euler equations for midfield propagation in an inhomogeneous field, and the use of an integral solver for farfield acoustic predictions.

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.

This study presents numerical simulations of the unsteady flow and noise generation phenomena in the slat cove of a highlift wing profile by mean of a zonal hybrid RANS/LES approach. These computations are part of a joint numerical/experimental aeroacoustics collaborative program, dedicated to slat flow analysis. For that purpose, a dedicated twoelement wing profile (slat+main body) has been designed in order to isolate the slat noise sources from other possible spurious sources (flap for instance), while minimizing mean flow deflection effects in order to improve the reliability of open-jet wind tunnel measurements. The design of this two-element airfoil has been done numerically using an optimization process based on steady RANS calculations. This airfoil has been investigated experimentally in Ecole Centrale de Lyon open jet facility, therefore providing unsteady wall-pressure measurements in the slat cove area and farfield acoustic measurements. Unsteady zonal hybrid RANS/LES simulations have been performed to provide a comprehensive description of the unsteady flow inside the slat cove, with focus being made on the noise generation processes. A fine physical unsteady analysis of the flow inside the slat cove is presented, as well as a comparison of numerical results with available experimental ones. The pressure spectra associated to the slat cove flow appear to be characterized by several tonal peaks emerging from a global broadband content. The existence of such peaks is discussed and attributed to a feedback loop involving the main shear layer inside the slat cove. A theoretical law is proposed and assessed at the end of the paper to predict the associated tonal frequencies.

In the context of the on-going European research project OPENAIR, the acoustic potential of the concept of scarfed nozzles for coaxial turbofan is evaluated. The objective is to reduce the fan noise radiation through the engine nozzle towards the ground, without significant losses in the aerodynamic performances. This evaluation relies on CAA computations achieved with Onera’s CAA solver sAbrinA-V0 . The nozzle configuration is typical of a coaxial turbofan with large bypass ratio, including 3D effects from the internal bifurcation and external pylon. From a reference configuration, a scarfed configuration has been designed by SNECMA with the constraint of minimizing the loss in aerodynamic performance. The acoustic computations rely on the Random Phase Multimodal Injection (RPMI), an innovative technique based on the optimization of the modal phases in order to obtain, with a minimum number of CAA computations, the contribution, in an un-correlated way, of all cut-on modes with evenly distributed acoustic power. The noise propagation accounts for (i) the 3D geometric details of the secondary duct, including the bifurcation and (ii) the refraction effects due to the large velocity gradients in the coaxial nozzle. For this purpose, non-homogeneous RANS mean flows were computed by SNECMA for both the reference and the scarfed configurations, allowing to check their respective aerodynamic performances. The CAA computations provide acoustic fields on a surface enclosing the engine and pylon, then the farfield directivity of the isolated engine is evaluated using a Kirchhoff integral method. Both configurations are compared at large distance in flyover and sideline directions corresponding to certification points. The acoustical benefit of the scarfed nozzle is demonstrated, especially in the flyover direction. Abbreviations

rance This paper presents results of a CFD-CAA coupling approach for the prediction of the aerodynamic noise radiated by a generic landing gear model. These results are compared to experimental aerodynamic/aeroacoustic data gathered in Onera’s aerodynamic and anechoic windtunnels F2 and CEPRA19. This work is achieved in the framework of the LAGOON program ( LA nding Gear NOise database for CAA validatiON ), currently supported by Airbus and involving Onera and DLR (the French and German national aerospace research centres) and Southampton University. In the present work, the simulation of the unsteady flow is achieved with Zonal-DES using Onera’s code elsA . The CFD provides the surface pressure fluctuations which are the entry d ata for a Ffowcs-Williams and Hawkings (FW-H) surface integral solver used to predict the far field noise. Two FW-H formulations are used and compared, based either on the model rigid surface or from a porous surface surrounding the landing gear. Numerical unsteady aerodynamics are presented firstly and compared with experiment results, then acoustic predictions are detailed and compared with measurements from microphones. The noise predicted from the solid surface input is in accurate agreement with the experiment. The acoustic prediction from the porous surface input globally overestimates the experimental noise levels. In general, the agreement with experimental acoustic data depends on the sound direction, upstream or downstream and flyover or sideline. Tonal noise peaks, which may b e attributed to cylindrical cavities located on the inner side of wheels, are well predicted by the simulations.

The main objective of this paper is the characterization of the fan noise radiated in the aft direction for a coaxial turbofan installed in RFN (Rear Fuselage Nacelle) position. The challenge is to validate a numerical hybrid methodology, which has been developed for several years by Onera and Airbus, to provide the acoustic far field in the presence of the aircraft. This activity refers to the NACRE European project, in which a dedicated isolated/installed fan noise experiment was conducted with an aircraft model and a Turbine Powered Simulator (TPS). The numerical approach combines near field CAA computations, achieved by Onera with the sAbrinA-V0 solver, and farfield BEM computations achieved either by Airbus with the ACTIPOLE solver or by Onera with the BEMUSE solver. The fan noise source consists in azimuthal/radial modes of an infinite annular duct with uniform mean flow. All “cut-on” modes are computed individually, and then the un-correlated mode contributions are summed with amplitudes derived from measurements performed inside the by-pass duct with a circular array of pressure sensors. It is shown that the simulation of the internal propagation must take into account the three-dimensional geometry of the bypass duct, including the bifurcation, to explain the strong azimuthal oscillations of the RMS pressure inside and outside the TPS. Onera and Airbus’s BEM solvers are used to compute the installation effects generated by the rear part of the aircraft equipped with an H-tail plane. The agreement between both solvers is excellent, and the comparison with the experiments is satisfying.

The EC research program NACRE (New Aircraft Concepts Research in Europe) studies innovative aircraft concepts, including radical integration of the engines. This paper describes numerical simulations of fan noise installation effect for a turbofan engine with a semi-buried inlet. The main objective is to evaluate the influence of the “offset level” (the vertical distance between the airframe surface and the lowest position of the fan, divided by its diameter). Two different configurations with offset levels of 8 % (Shape 1) and 15 % (Shape 2) are studied. 3D acoustic computations are performed using ONERA's CAA sAbrinA solver based on the non-linear Euler equations in perturbation form, using specific structured multi-block acoustic grids. The entry data are the mean flow RANS simulations computed by ONERA. Acoustic modes (cut-on) generated in the engine fan plane are prescribed by MTU (frequency, azimuthal and radial orders). These acoustic computations account for the acoustic refraction effect through the inlet non-homogeneous mean flow. First results are presented at the BPF for one single particular mode ( m = 0, n = 4), then additional “ broadband ” simulations are achieved for both shapes by combining all prescribed cut-on modes with equal energy. All computations show that noise levels radiated by Shape 2 are globally lower than the noise radiated by Shape 1. However, this difference could be attenuated by an exact estimation of the amplitudes considered for the modes generated by the inflow distorsion.

In the context of the AEROCAV French Research Program, Large-Eddy Simulations were conducted to compute the turbulent structures and the noise generated by the flow over realistic cavities, similar to those present on air craft fuselages and wings. The radiated acoustic field was computed using a Kirchhoff integ ral method. Two geometrical configurations are considered, both with cylindrical shapes but with different Depth/Diameter aspect ratio (H/D = 1 and H/D = 0.5 ) embedded in a subsonic flow ( Mach = 0.2 ) and for a Reynolds number based on the length of the cavity equal to 4.6·10 5 . Dedicated experimental aerodynamic and aeroacoustic measurements databases are used to validate the numerical computations. Experimental and numerical flows are similar except that the boundary layer upstream the cavity is turbulent in the experiments and laminar for the CFD. The aerodynamic fields in the two sets of results are in good agreement, whereas the acoustic field is largely influenced by the install ation effects. These effects are analyzed through computations based on Boundary Element Method and CAA and then corrections factors are provided. The acoustic emission for the H/D = 1 case is characterized by a discrete tonal mode, as a superposition of structur al and Rossiter resonance phenomena, while the H/D = 0.5 case presents a directive and highly dissymmetrica l broadband noise emission. A numerical process is also proposed to c ontrol this dissymmetry.

The present work is devoted to the numerical simulation of acoustic emissions characterizing turbojet engines, a subject that is relevant of the more general purpose of aircraft noise prevision and reduction. More precis ely, we explore here the ability of a structured CAA (Computational AeroAcoustics) method/solver to address complicated problems of engine noise prediction. With that end, and by using the ONERA’s CAA solver sAbrinA.v0 , we conduct realistic calculations of aft fan nois e emission, which involve both a full-3D exhaust geometry (with its pylon / internal bifurcations) and typical fan noise modal contents (high azimuthal order / frequency). The re sults highlight how far the installation / refraction effects induced by the complex geometry / flow of an engine can affect its fan noise emission. Results also tend to demonstrate that bot h the here used CAA method and solver are mature enough to face out industrial-like engin e noise problems.

This paper presents an application of a Computational AeroAcoustics (CAA) hybrid process to the three-dimensional numerical prediction of the downstream fan noise of coaxial engines. This process may include the acoustic refraction effects resulting from the propagation through the highly inhomogeneous (subsonic hot) jet mean flow associated to the nozzle. The CAA hybrid methodology associates two different acoustic methods. Firstly, the near and mid field propagation through the (possibly inhomogeneous) mean flow is computed with an Euler, high order, finite differences solver. Then, the far field noise is calculated by use of a classical Kirchhoff integration. First computations are performed over a classical axi-symmetric coaxial nozzle in a fluid at rest, and are validated against BEM simulations. Then the simulations are performed including the jet viscous mean flow previously computed with a RANS solver. These computations show that the acoustic directivity pattern of fan noise is strongly modified when the jet mean flow is included in the simulation. Finally, a last computation is performed over a modified engine presenting a scarf of it secondary engine, this highlighting the abilities of the CAA process to handle full 3D problems, as well as underlining the potential shielding effect of such a structural modification of the nozzle.

This paper addresses the numerical simulation of the acoustic scattering and propagation through non-uniform mean flows using a high-order, finite-difference, full-conservative Euler solver and, more precisely, the specific problems raised by multi-block structured grids when complex geometries are considered. Two examples of two-dimensional computations of this type are presented. The first case, a benchmark problem from the 4 th CAA Workshop, is the simulation of the acoustic scattering from multiple rigid circular cylinders, without mean flow, of the sound generated by a spatially distributed, axisymmetric source. The proposed solution uses conformal multi-domain grids associating body-fitted grids near solid walls and quasicartesian grids elsewhere. The solutions are favorably compared to analytical data. The second example is the simulation of the acoustic scattering from a high-lift wing section with deployed slat and flap, from a source located in the slat cove region. An acoustic grid is derived from a CFD grid previously used to compute the non-uniform viscous mean flow around the wing. A first acoustic computation, performed with the fluid at rest, is favorably compared to a Boundary Element Method solution. Then a second computation is performed on the non-uniform mean flow. The comparison of both solutions highlights the influence of the flow on the directivity diagram of the sound field.

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...

This study, that is relevant from the turbofan engines noise prediction/reduction, aims at CAA‐computing the aft fan noise propagation/radiation of a realistic full‐3D exhaust (with pylon and internal bifurcations), the latter being affected of (i) typical in‐flight (take‐off) thermodynamic conditions and of (ii) a representative fan noise modal content. As for previous studies conducted over baseline geometries, this CAA computation is conducted following the usual hybrid process, where a preliminary aerodynamic calculation provides a heterogeneous steady mean flow on which an acoustic calculation is then conducted A RANS computation is first performed, delivering the stationary jet mean flow characterizing the 3D exhaust in its typical 'take‐off flight' (M∞ = 0.25). A CAA grid (22 blocks, 28 millions cells) is then derived from the CFD one, before the RANS steady jet mean‐flow is interpolated on it. After what the CAA computation is computed, a fan noise mode (26, 1) being emitted at a reduced frequency of kR = 30 (1 BPF) in the upstream of the engine's secondary exhaust, and numerically propagated along and outside the latter. Finally, a Kirchhoff post‐treatment provides the far‐field radiation characterizing these engine geometry, modal content and thermodynamic conditions.

Cylindrical cavities are mounted at some positions on fuselage and wings of commercial transport aircraft for various service functions. Such cavities were identified as potential airframe noise sources generating high intensity whistles at frequencies situated in the audible range of 0.5‐0.8 kHz. The present paper focuses on the numerical simulation of the far field noise generated by the unsteady flow passing over a cylindrical cavity with an aspect ratio (diameter/height) equal to unity, using a numerical methodology combining a LES and a Kirchhoff integration method. The LES computation was done using ONERA's cell‐center/finite‐volume/CFD code named FLU3M. A laminar boundary layer profile was applied upstream the computational domain, without forcing any turbulence. The LES results were coupled with a 3D Kirchhoff method in the frequency domain. The computed far field noise was compared to dedicated experimental data, showing very good agreement, especially regarding the emission frequency. The final ...