Showing papers in "International Journal of Aeroacoustics in 2014"

Noise of an Overexpanded Mach 3.3 Jet: Non-Linear Propagation Effects and Correlations with Flow

Abstract: The noise emitted by an overexpanded round jet at a Mach number of 3.3 and a Reynolds number of 105, computed in a previous study using large-eddy simulation (LES), is investigated. In a first step, the non-linear sound propagation effects are quantified by performing two far-field wave extrapolations from the LES near-field data. The extrapolations are carried out by solving the linearized Euler equations in one case and the full Euler equations in the other, without atmospheric absorption, up to a distance of 240 radii from the jet nozzle exit. The non-linear effects are shown to be quite significant, resulting in a series of N-shaped waves in the pressure signals, and in weaker mid-frequency components and stronger high-frequency components in the spectra. Close to the peak directivity radiation angle, for instance, they lead to about a 8 dB loss and a 6 dB gain at the Strouhal numbers of 0.2 and 1, respectively. In a second step, noise generation mechanisms are discussed by calculating correlations be...

Efficient Outdoor Sound Propagation Modeling with the Finite-Difference Time-Domain (FDTD) Method: A Review:

Abstract: The finite-difference time-domain (FDTD) method, solving the inhomogeneous, moving medium sound propagation equations, also referred to as the Linearized Euler(ian) Equations (LEE), has become a mature reference outdoor sound propagation model during the last two decades. It combines the ability to account for complex wave effects like reflection, scattering and diffraction near arbitrary objects, and complex medium effects like convection, refraction and (turbulent) scattering. In addition, it has the general advantages of a time-domain method. It is indicated that the numerical discretisation scheme should be chosen depending on the flow speed of the background medium. Perfectly matched layers, applicable to cases in presence of (non-)uniform flow, are state-of-the-art perfectly absorbing boundary conditions that are key in outdoor sound propagation applications, where only a small part of the unbounded atmosphere can be numerically described. Various ways to include outdoor soils are summarized, like t...

Edge-Based Reconstruction Schemes for Prediction of near Field Flow Region in Complex Aeroacoustics Problems

Abstract: The present paper is devoted to the Edge-Based Reconstruction (EBR) schemes for solving Euler equations on unstructured tetrahedral meshes which can be used for prediction of complex turbulent flows The schemes provide lower computational costs and higher accuracy in simulating the near field flow region considered as a distributed acoustic source in aeroacoustic problems In this paper we give a refined formulation of EBR schemes, analyses of their properties and possible extensions The 5th theoretically reachable order EBR scheme is verified on the simulation of monopole acoustic source in a subsonic uniform flow The results of the simulation of turbulent flow around two rectangular cylinders obtained with the use of EBR scheme are shown Both cases were solved within the FP7 VALIANT project

On flow-acoustic resonant interactions in transitional airfoils

Abstract: We discuss our recent experimental and preliminary numerical efforts examining resonant feedback mechanism of flow-acoustic interactions in airfoil transitional boundary layers. Experimentally recorded unsteady responses of loaded, transitional NACA0012 airfoil with tripped suction or pressure sides confirm the presence of shifted ladder-type tonal structures with dual velocity dependence in the acoustic signal previously reported in the literature. The installation of upstream grid generating low-intensity turbulence appears to eliminate the feedback mechanism leaving just a single velocity dependence for the dominant spectral components. Complementary numerical efforts employ a high-order Navier-Stokes solver implementing low-pass filtering of poorly resolved high-frequency solution content to retain numerical accuracy and stability over the range of transitional flow regimes. Within the scope of 2D analysis, the conducted numerical experiments particularly investigate the behavior of the boundary-layer...

Flow simulation and aerodynamic noise prediction for a high-speed train wheelset

Abstract: Aerodynamic noise becomes significant for high-speed trains and its prediction in an industrial context is hard to achieve. The aerodynamic and aeroacoustic behaviour of the flow past a high-speed train wheelset, one of the main components of a bogie, are investigated at a scale 1:10 using a two-stage hybrid method of computational fluid dynamics and acoustic analogy. The near-field unsteady flow is obtained by solving the Navier-Stokes equations numerically through delayed detached-eddy simulations and the results are fed to predict the far-field noise signals using the Ffowcs Williams-Hawkings acoustic analogy. Far-field sound radiated from the scaled model is also measured in a low noise open-jet anechoic wind tunnel. Good agreement is achieved between numerical and experimental results for the dominant frequency of tonal noise and the shape of the spectra. Results show that turbulent flow past the wheelset is characterized by three-dimensional streamwise and spanwise vortices with various scales and o...

CABARET scheme for computational aero acoustics: extension to asynchronous time stepping and 3D flow modelling

Abstract: Explicit time stepping renders many high-resolution schemes in Computational Aeroacoustics to become less efficient when dealing with non-uniform meshes in multiple dimensions. In the present paper, the Compact Accurately Boundary-Adjusting high-REsolution Technique (CABARET) Euler scheme is extended to asynchronous time-stepping which allows it to significantly boost computational performance with non-uniform grids. Numerical examples for 1D and 2D CABARET with asynchronous time stepping are provided. For further demonstration of the CABARET's capabilities in 3D where the asynchronous time stepping is expected to bring a step change in the scheme efficiency, Large Eddy Simulation of subsonic flow over a NACA0012 airfoil at zero angle of attack is conducted. The aerodynamic and acoustic results obtained with a single time stepping CABARET method are compared with the experimental data available.

Simulation of azimuthal characteristics of turbulent jet noise by correlation model of quadrupole noise sources

Abstract: In the modeling of the noise sources in turbulent jets two open questions are still remain. The first, what is the multipole type of the sources? The second, what kind of stochastic fields can be used for the noise source description. These issues are analyzed in this work. Two-point cross-correlation of the jet noise is used to determine the multipole type of sources. Modeling of the cross-correlation provides clear evidence in favor of the quadrupole type of the jet noise sources. In the context of the second issue the recently developed two-stage correlation model is tested on the basis of measurements of azimuthally decomposed jet noise. The results show a promising agreement in a wide range of parameters.

Ground reflection with turbulence induced coherence loss in flyover auralization

Abstract: A method to include the effect of coherence loss due to turbulence is proposed for real-time auralization of aircraft noise. By modifying the direct ray contribution relative to the ground reflected ray, using filters, the coherence loss effect can be included in a propagation scheme. The results of this approach match with the theoretical predictions thereby verifying the ability of the method. Furthermore, the modulation of aircraft tonal components, due to changing ground interference during a flyover, is diminished as a result of the proposed method. Application to an auralization and comparing to a measurement shows that auralizations can benefit from this method. Therefore the current method forms an essential addition to the techniques currently used in the auralization of aircraft noise.

Understanding and reduction of cruise jet noise at aircraft level

Abstract: Within the LINFaN research project, Airbus and Rolls-Royce have jointly investigated the acoustic impact of supersonic jet noise on the fuselage of an Airbus A340 equipped with Rolls-Royce Trent 500 engines in cruise conditions. The main results are presented in this paper. The influence of chevron fan nozzles designed to reduce cruise jet noise on the rear fuselage is investigated. The characterization of both the acoustic field and the aerodynamic flow is carried out. The acoustic data are obtained from a microphone array on the rear fuselage. After specific data de-noising, the acoustic spectra radiated on the fuselage by only the right inner engine are estimated. Beamforming acoustic maps are also computed to localize the jet noise sources. Besides, the supersonic jet flows are characterized by RANS CFD approach. For the baseline round nozzle, the acoustic results show the presence of two distinct broadband shock-associated noise contributions on the fuselage. The first pattern is observed in the far ...

Towards Simulation of Far-Field Aerodynamic Sound from a Circular Cylinder Using OpenFoam

Abstract: The low-Mach number flow-induced noise by the flow past a circular cylinder at sub-critical regime was predicted. First, to assess the accuracy of the numerical methodology, the laminar flow over a circular cylinder at the Reynolds number Re = 140 and Mach number M = 0.2 was calculated by direct solution of the unsteady compressible Navier-Stokes equations. Second, the sound generated by a circular cylinder at the Reynolds number Re = 2.2 × 104 and Mach number M = 0.06 was simulated using a technique of large-eddy simulation. For both cases, the calculated acoustic fields showed a dipole directivity, similar to a natural vortex shedding. The impact of the Doppler effect was investigated and discussed as well. In general, the computed aerodynamic and far-field acoustic results were found to be in good agreement with available experimental measurements and analytical relationships.

Towards Realistic Simulations of Sound Radiation by Moving Sources in Outdoor Environments

Abstract: PACS numbers:43.28.Js, 43.28.En, 43.20. ElA time-domain solver of the linearized Euler equations is used to study outdoor propagation of acoustic waves generated by broadband moving sources. For that, high-order schemes, developed initially in the computational aeroacoustics community, are employed. A time-domain impedance boundary condition recently proposed in the literature is implemented to deal with reflexion of acoustic waves over the ground. In addition, curvilinear coordinates are used to account for topographic effects. First, test cases show that long range sound propagation and diffraction by obstacles in three-dimensional geometries are accurately determined. Simulation of the acoustic radiation by a broadband monopole source moving above a perfectly reflecting plane is then considered. Numerical results are satisfactorily compared to those obtained from an analytical solution. At last, the case of a broadband source moving above a non-flat terrain, with an inhomogeneous impedance ground, is i...

Aeroacoustics of Supersonic Boundary Layers

Abstract: Present paper is devoted to those acoustical problems which are also related to the problem of the turbulence onset in high-speed boundary layers. Basic equations governing linear development of both hydrodynamic and acoustic waves in high-speed boundary layers are presented. Results of the theoretical investigation on external acoustic wave influence on the flat plate boundary layer are described. The intensity of disturbances excited by an acoustic wave inside the boundary layer has been studied. It has been found that interaction of sound with the boundary layer lead to an increase of the amplitude of disturbances inside the boundary. It has been shown that the amplitude of fluctuations inside the boundary layer may exceed the amplitude of the external acoustic field in many times. Besides in the paper nonlinear generation of increasing perturbations by acoustics is studied.

Parallel blade-vortex interaction modelling for helicopter rotor noise control synthesis

Abstract: The present work deals with a methodology for the alleviation of loads and corresponding emitted noise generated by blade-vortex interaction (BVI) phenomena occurring on helicopter main rotors in low-speed, descent flight. It consists of a multi-cyclic, optimal control approach driving higher harmonic blade pitch actuation. First, for the specific flight condition to be examined, the BVI phenomena are simulated as equivalent two-dimensional, multi-vortex, parallel BVI problems and then a local controller methodology is applied for the efficient identification of the closed-loop control algorithm. In order to assess the capability of the proposed control approach to alleviate rotor blade loads and emitted noise, the results of a numerical investigation concerning a realistic helicopter main rotor in descent flight are presented and discussed.

Radiated Sound from a Circular Cylinder in a Turbulent Shear Layer

Abstract: The effect of turbulent approach flow on the radiated sound from a circular cylinder was studied experimentally. The approach flow turbulence was provided by a single stream shear layer produced by an open jet anechoic tunnel facility. An instrumented cylinder was used to measure steady and unsteady surface pressure. The sound radiated from the cylinder placed in both an irrotational approach flow and the highly turbulent approach flow of the shear-layer was measured and compared. The cylinders located within the shear layer produced a less tonal sound with a higher broadband amplitude when compared to that of the free-stream approach flow. Secondly, the radiated sound from cylinders of different diameters was investigated to assess the effect of the ratio of cylinder diameter to approach flow length scale. It was found that as this ratio decreased, the broadband sound levels decreased as well. A simple theoretical model was then used to provide a prediction of the radiated sound for the cylinder within t...

Experimental and numerical investigation of jet flap interaction effects

Abstract: The influence of the wing and flaps geometry on the flow field and acoustic radiation of the jet are experimentally and numerically investigated. The azimuth non-uniformity of jet acoustic field was experimentally defined with a help of far-field microphones, flow field within the jet was measured using PIV technique and numerically calculated with a help high resolution combined RANS/ILES method. The jet flaps interaction (JFI) effects was experimentally and numerically investigated at facility under modeling of real by-pass nozzle configuration and outflow parameters. The comparison between measured flow-field and noise of a jet shows that JFI excess noise is mainly related to jet deformation and increasing of velocity gradients and turbulence within jet which are induced by tip flap vortexes.

Toward hybrid CAA with ground effects

Abstract: CAA based on the Linearised Euler Equations (LEE) is applied to propagate aerodynamic sound over an extended distance including ground effects. The LEE are coupled to data from an LES via an acoustic analogy to follow-up the sound from the source to the extended far field: the complete chain is illustrated on the sound generated by a cylinder in a M ∼ 0.2 and Re ∼ 48000 flow. A very good agreement is found in free field between the approach based on the Ffowcs-Williams & Hawkings (FWH) analogy only and the combined FWH-LEE approach. The ability of the combined approach to handle complex boundary conditions is illustrated on the same data set with a rigid and a grassy ground.

Statistical Analysis of the Pressure Field in the near Region of a M = 0.5 Circular Jet

Abstract: In a recent paper [1] a novel technique providing the separation between hydrodynamic and acoustic components of pressure signals has been presented and applied to near field pressure signals measured in a turbulent jet at M = 0.5. The database is further analyzed herein with the scope of clarifying the physical mechanisms underlying the generation of acoustic pressure fluctuations in the near field. The analysis of cross-correlations between pressure and acceleration and the statistics of the potential core fluctuations, confirm that the region at the end of the potential core plays a fundamental role in this sense. A conditional sampling technique based on the use of wavelet transform identifies ensemble averaged velocity traces of the noise sources. The velocity signatures are interpreted as induced by wavepackets, in the region close to the jet exit, and ring vortices in the far region.

A fast atmospheric sound propagation model for aircraft noise prediction

Abstract: The effect of the atmosphere on sound propagation can impact the perceived noise on the ground significantly and therefore needs to be accounted for in aircraft noise modelling. However, to be practical, propagation modelling cannot impose a limitation on calculation time. The aim is therefore to design a robust and computationally lean atmospheric propagation model for the prediction of aircraft noise. Foreseen applications are the evaluation of en-route noise, noise optimisation studies and the evaluation of noise in airport scenarios.To achieve a computationally low demanding algorithm an analytical approach is pursued. When the wave front orientation is approximated to be perpendicular to the ray path, it follows from the Eikonal equation that a ray obeys to an adapted form of Snell's law. If the atmosphere is then discretized in layers of linearly varying sound speed and wind velocity, it allows the ray path and ray properties to be expressed analytically within a layer. Cross-validation against two ...

A Ray Acoustics Model for the Propagation of Aircraft Noise through the Atmosphere

Abstract: A ray-tracing tool, called RAYTRAC, has been developed to compute the propagation of aircraft noise through a stratified atmosphere. The mathematical model on which RAYTRAC is based is derived from first principles, i.e. the linearized Euler equations for a non-uniform atmosphere. The main result of the model consists of two equations:one for the location of the rays and one for the change in amplitude along each ray. An analysis of these equations is given with respect to their possible solutions under various atmospheric circumstances. Also an efficient iteration procedure is presented that finds the ray connecting the source and a specific observer point. It is shown how multiple rays can be added, with incorporation of a reflecting surface. Finally, the tool is demonstrated by application to two examples that are representative for an aircraft emitting sound to an observer on the ground.

Numerical Investigations of Tonal Noise of Counter-Rotating Open Rotors:

Abstract: In the work are presented the results of a three dimensional computational analysis of tonal noise of two counter-rotating open rotors. The analysis was performed using 3DAS (3 Dimensional Acoustics Solver) CIAM in-house solver, designed for three dimensional calculation of turbomachinery tone noise generation, propagation and radiation in the near and the far acoustic fields. The results of the computations were compared with the results of experiments performed in the wind tunnel of Central Aerohydrodynamic Institute (TsAGI), Russia, within the project DREAM (Validation of radical engine architecture systems) of European Seventh Framework Programme (FP7). The comparison showed satisfactory correspondence between directivity diagrams for the tone noise on combinational harmonics.

On a Generalized Multipole Expansion with Application to Propeller Design Synthesis

Abstract: The radiation of sound by surfaces in motion in a non-uniform flow, including the effects of reflections from obstacles on noise, is specified by an extension of the Kirchhoff integral that leads to a generalized multipole expansion that extends the classical series of spherical harmonics to account for the effects of (i) the moving medium and (ii) the rotating sound sources. The corresponding radiation integrals are evaluated analytically for an arbitrary source distribution along the blades of a propeller at an angular inflow. The effects of the incident flow and propeller rotation on the amplitude and phase of sound, e.g. through the retarded time, lead to an extension of the generating function for Legendre polynomials; this provides the representation of the acoustic field as the generalized multipolar series. Each generalized multipole sound field is shown to consist of a fundamental blade passng frequency (BPF) plus all harmonics, with directivities specified by integrals of Bessel functions, that ...

Tearing Instability of Isolated Compressible Vortices

Abstract: The present paper addresses the instability of a compressible vortex flow. We consider a family of so-called isolated vortices - the circular planar vortices that have zero circulation (net vorticity). The term "isolation" implies the presence of a shield in the vorticity field - a region of vorticity of common sign surrounding the central domain where the vorticity is oppositely signed. To model such a vorticity field, a generalized Taylor-type profile for the swirl velocity in the radial direction is employed, which involves two parameters: intensity μ (proportional to the maximal velocity) and steepness β (characterizing the scale of the shield zone). Vortices are assumed to be compressible and homentropic. The linear-stability analysis is carried out, which shows that isolated vortices can exhibit both stable and unstable behavior depending on the model parameters μ and β. By numerical simulations of the non-linear stage, the unstable normal modes are shown to evolve towards tearing of the basic vorte...

Flow-Sound Interaction Mechanisms in the Wake of Two Side-By-Side Cylinders

Abstract: The phenomenon of flow-excited acoustic resonance, where periodic flow oscillations are enhanced by a resonant sound field, is a design concern in many engineering applications such as in heat exchangers, piping systems, and cavity flows. This study experimentally examines the phenomenon of flow-excited acoustic resonance for two side-by-side cylinders in a duct with cross-flow. This geometry has been investigated for three cylinder spacing ratios, defined as the center-to-center distance between the cylinders normalized by their diameter, of T/D =1.25, 1.46 and 2.5, and for a range of acoustic pressure amplitudes. Intermediate and small spacing ratios have been given special attention, as these cases have been found to exhibit bistable flow in the wake in the absence of acoustic resonance. Phase-locked PIV measurements reveal that the self-excited sound field produces a strong oscillatory flow pattern in the cylinder wakes, which is symmetric for large spacing ratios and high acoustic amplitudes, but rem...

On the effect of atmospheric temperature gradients and ground impedance on sound

Abstract: The acoustic wave equation is solved exactly in a polytropic atmosphere over a flat impedance ground. The effects of atmospheric stratification on sound are presented as a correction to the amplitude and phase of a plane wave compared to a homogeneous medium corresponding to sea level conditions. It is shown that: (i) the direct effect of atmospheric stratification on sound amplitude is very small over the entire audible range 20Hz-20kHz, even for altitude ranges in excess of 1 Km: (ii) the atmospheric stratification causes significant phase changes relative to an homogeneous medium for frequencies above 500Hz even over altitude ranges of less than 100m; (iv) the phase changes due to atmospheric stratification can affect significantly interference between acoustic waves, for example in the case of multiple sound sources or multiple reflections by nearby obstacles. The exact solution of the wave equation for the acoustic potential specifies also the velocity and pressure perturbation that appear in the imp...

Enhanced propagation of aviation noise in complex environments: A hybrid approach

Abstract: Accurate prediction of sound levels around airports and below flight paths can help faithfully represent the impact of aviation noise on communities. However, for large scale assessments, as are often performed by the U.S. Federal Aviation Administrationis environmental models, the accuracy of a model must be weighed against its efficiency. The hybrid propagation model (HPM) is capable of predicting propagation through complex environments. It is a composite of three methods—the generalized terrain parabolic equation (GTPE), fast field program (FFP), and straight ray models—each utilized in a different region of elevation angles from the source. If propagation conditions do not warrant use of the full model, one of the component models with faster runtime can be chosen as a surrogate. Analyses of cases using different source heights and including uneven terrain, refractive atmosphere, and ground type transitions, indicate when a detailed propagation model is needed, or when a simpler model is sufficient.

A comparative study of accuracy of shock capturing schemes for simulation of shock/acoustic wave interactions

Abstract: We simulate transmission of a small-amplitude disturbance wave through a shock wave. Results of our numerical experiments performed with different high-order shock-capturing schemes show that the capability of a scheme to correctly predict the amplification of the disturbances crucially depends on the Riemann solver used in evaluation of numerical fluxes. Incorrectly high amplification rates are produced by the solvers resolving shock waves sharply, with no interior points in numerical profiles of steady shock waves. In particular, both the exact Riemann solver and the popular Roe flux difference splitting demonstrate such unphysical behavior. A possible explanation of such behavior is proposed. More dissipative solvers, such as the global Lax–Friedrichs splitting, produce transmission coefficients close to the predictions of linear theory.

Guest editor biographies

Abstract: Her current scientific interests include high-accuracy numerical methods for hyperbolic problems for unstructured meshes, simulation of turbulent flows and turbulencegenerated noise. She also deals with solving applied problems related to aircraft and helicopter aerodynamics and aeroacoustics and, in particular, acoustic liners, unsteady turbulent flows over airfoils, cylinders, cavities and structural obstacles. She participated in many national and European research projects. In particular, she was a team leader of the FP7 European project VALIANT.