Vincent Fleury

Space-Time Correlations in Two Subsonic Jets Using Dual Particle Image Velocimetry Measurements

Dual particle image velocimetry (dual PIV) measurements have been performed to investigate the space-time correlations in two subsonic isothermal round jets at Mach numbers of 0.6 and 0.9. The correlation scales are analyzed along the centerline and in the shear-layer center over the first 11 jet diameters from the nozzle exit To provide robust results over a wide range of flow conditions, these correlation scales are given in terms of their appropriate quantities, namely, the mean or rms velocity in reference to velocity and the momentum thickness or the half-velocity diameter in reference to length in the shear layer and on the jet axis, respectively. From these results, a discussion on the modeling of turbulence in jets is addressed. The self-similarity of some space correlation functions in the shear layer and on the jet axis is shown. Furthermore, far enough downstream in the shear layer, some of the ratios between the space and time scales are relatively close to the values expected in homogeneous and isotropic turbulence. It is also found that the ratio between the integral length and the time scales in the fixed frame is of the order of the local mean flow velocity. In the convected frame, the appropriate scaling factor is the rms velocity.

Experimental Study of the Spectral Properties of Near-Field and Far-Field Jet Noise:

The near and far pressure fields generated by round, isothermal and cold jets of diameter D = 38 mm with Mach numbers varying over the range 0.6 ≤ Mj ≤ 1.6 are investigated experimentally, and characterized in terms of sound spectra and levels. Properties of near-field jet noise, obtained in particular at 7.5 diameters from the jet centerline, are documented. They differ appreciably from properties of far-field noise, and form a database that can be used for the validation of the acoustic fields determined by compressible Navier-Stokes computations. The near pressure fields originating from simulations can thus be directly compared, without resorting to extrapolation methods which might lead to uncertainties in the far pressure fields. In the present paper, sound source localizations are also carried out from the near-field pressure signals. The experiments provide in addition far-field results evaluated at 52 diameters from the nozzle exit, in good agreement with the data of the literature. The classical dependence of jet noise features with the emission angle is observed. The level and frequency scalings of the pressure spectra obtained for subsonic jets in the sideline and downstream directions are also studied. For small radiation angles, the narrow-banded sound spectra measured are found to scale as the Strouhal number, whereas, as previously noted by Zaman & Yu [1], the one-third octave spectra seem to scale as the Helmholtz number.

Space-time correlations in two subsonic jets using dual-PIV measurements

Dual Particule Image Velocimetry (dual-PIV) measurements have been performed to investigate the space-time correlations in two subsonic isothermal round jets at Mach numbers 0.6 and 0.9. Measurements are obtained along the centerline and the shear-layer region which are closely connected with the noise generation. Integral scales have been calculated with as much as 2000 samples which provides high-quality data. The correlation scales are given in function of appropriate references, namely the local momentum thickness (respectively the jet diameter) for the shear-layer (respectively the jet axis) and the mean or rms local velocity. Far enough downstream in the shear-layer, some of the ratios between the space scales and between the time scales are relatively close to the values expected in isotropic an homogeneous turbulence. Furthermore, the relation between the time and space scales follows well the Taylor’s assumption in the shear-layer.

Extension of deconvolution algorithms for the mapping of moving acoustic sources

Several deconvolution algorithms are commonly used in aeroacoustics to estimate the power level radiated by static sources, for instance, the deconvolution approach for the mapping of acoustic sources (DAMAS), DAMAS2, CLEAN, and the CLEAN based on spatial source coherence algorithm (CLEAN-SC). However, few efficient methodologies are available for moving sources. In this paper, several deconvolution approaches are proposed to estimate the narrow-band spectra of low-Mach number uncorrelated sources. All of them are based on a beamformer output. Due to velocity, the beamformer output is inherently related to the source spectra over the whole frequency range, which makes the deconvolution very complex from a computational point of view. Using the conventional Doppler approximation and for limited time analysis, the problem can be separated into multiple independent problems, each involving a single source frequency, as for static sources. DAMAS, DAMAS2, CLEAN, and CLEAN-SC are then extended to moving sources. These extensions are validated from both synthesized data and real aircraft flyover noise measurements. Comparable performances to those of the corresponding static methodologies are recovered. All these approaches constitute complementary and efficient tools in order to quantify the noise level emitted from moving acoustic sources.

Optimization of Microphone Array Wall Mountings in Closed-Section Wind Tunnels

The objective of the present paper is to improve the implementation of microphone arrays on the wall of closed-section wind tunnels. A test campaign has been conducted in the ONERA F2 1.4 m × 1.8 m wind tunnel to investigate the performance of thin and smooth metallic clothes to protect the microphones from the hydrodynamic fluctuations in the turbulent boundary-layer flow and, still, to preserve the transmitted acoustic signal. Three microphone mountings have been considered : individually recessed microphones in (i) cylindrical and (ii) conical apertures and (iii) a recessed, screened, large-scale microphone mounting plate (microphones flush with the plate). It was found that by covering the top of the cylindrical and conical microphone apertures with a wire-mesh, the strong self-noise due to the interaction of the boundary-layer flow with the cavities can be suppressed. The most significant noise reduction is obtained when the large-scale microphone mounting plate is recessed behind the metallic wire-mesh. A reduction by up to 20 dB is then obtained with respect to a flush-mounted, unscreened, microphone. This microphone mounting out-performs the cone-nose-protected and Kevlar-protected microphone mountings, two conventional low-noise devices. Thanks to an original calibration technique, the acoustic response of the tested microphone mountings could be evaluated in presence of flow (� 80 m/s). The recessed, screened, microphone mounting plate has an oscillating acoustic response (in function of the frequency) by less than 5 dB deviation. This attenuation can be post-corrected by using a very simple acoustic model.

Experimental study of the properties of near-field and far-field jet noise

The near and far pressure fields generated by round, isothermal and cold jets of diameter D = 38 mm with Mach numbers varying over the range 0.6 � Mj � 1.6 are investigated experimentally, and characterized in terms of sound spectra and levels. Properties of near-field jet noise, obtained in particular at 7.5 diameters from the jet centerline, are documented. They differ appreciably from properties of far-field noise, and form a database that can be used for the validation of the acoustic fields determined by compressible Navier-Stokes computations. The near pressure fields originating from simulations can thus be directly compared, without resorting to extrapolation methods which might lead to uncertainties in the far pressure fields. In the present paper, sound sources localizations are also carried out from the near-field pressure signals. The experiments provide in addition far-field results evaluated at 52 diameters from the nozzle exit, in good agreement with the data of the literature. The classical dependence of jet noise features with the emission angle is observed. The level and frequency scalings of the pressure spectra obtained for subsonic jets in the sideline and downstream directions are also studied. For small radiation angles, the narrow-banded sound spectra measured are specially found to scale as the Strouhal number, whereas the one-third octave spectra seem to scale as the Helmholtz number, as previously shown by Zaman & Yu. 1

Understanding and Reduction of Cruise Jet Noise at Model and Full Scale

This paper presents an investigation into supersonic jet noise of commercial aircraft in cruise conditions. It draws on results from the LINFaN research project, which was conducted jointly by Airbus and Rolls-Royce. The main objective of this programme is to study and control the cruise noise levels. It has allowed us to gain a deeper understanding of jet noise sources of modern high-bypass-ratio engine nozzles during cruise, and their impact on the fuselage skin. The study includes dedicated flight tests to measure noise on an Airbus A340 with Rolls-Royce Trent 500 engines, jet noise model-scale tests in a transonic wind tunnel and analytical studies on shock-associated noise using CFD. From the flight tests, jet noise spectra are obtained over the rear part of the fuselage, for different engine conditions and for different fan nozzle chevrons, and acoustic maps of jet noise sources are obtained using a beamforming technique. Model-scale tests have also been performed, which allow us to measure jet noise at representative cruise conditions. Many different nozzle designs can be tested over a full range of operating conditions and flight Mach numbers. RANS CFD methods are used to analyze the supersonic jet behaviour in cruise conditions. The investigation methodology is applied to two distinct dual-stream nozzles, starting with the Rolls-Royce Trent 500 nozzle. In flight, the presence of two distinct broadband shock-associated noise contributions are found on the fuselage, and effects of nozzle operating conditions are highlighted. The flight tests, wind-tunnel tests and flowfield analysis all show that chevrons on the fan nozzle modify significantly the near-field jet noise. For the Trent 500 engine, immersive fan chevrons reduce lowand high-frequency shock noise by as much as 7 dB on the peak level, and also increase shock-associated noise frequencies. The impact of chevrons on shock noise is found to be dependent on the engine

Large-Scale Jet Noise Testing, Reduction and Methods Validation "EXEJET": 5. Analysis of jet-airfoil interaction noise by microphone array techniques

With the constant increase of the size of modern engines to get higher and higher bypassratios, the issue of engine integration is a still more important industrial challenge, especially at take-off and landing. This paper is focused on the identification of jet noise sources and jet-airfoil interaction sources by using microphone array data. The measurements were carried-out in the anechoic, opened test section wind tunnel of Onera, Cepra19. The microphone array technique relies on the Lighthill’s and Ffowcs-Williams & Hawkings’ acoustic analogy equation. The output is the cross-spectrum of the source term of the analogy, defined as the optimal solution to a regularized minimal error equation using the measured microphone cross-spectra as reference. In spite of the heavy computational costs of this technique, a large test matrix could be analyzed. The analysis of isolated jet noise data in subsonic regime shows the contribution of the conventional mixing noise source in the low frequency range, as expected, and of uniformly distributed, decorrelated noise sources in the jet flow at higher frequencies. In underexpanded supersonic regime, a shock-associated noise source is clearly identified, too. An additional contribution is detected in the vicinity of the nozzle exit both in supersonic and subsonic regimes. In presence of an airfoil, the distribution of the noise sources is deeply modified. In particular, a strong noise source is localized on the flap. For high Strouhal numbers, higher than about 2 (based on the jet mixing velocity and diameter), a significant contribution from the jet flow, particularly in the shear-layer near the flap, is observed, too. Indications of acoustic reflection effects on the airfoil are also discerned.

Understanding and reduction of cruise jet noise at aircraft level

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 aft section of the fuselage for Strouhal numbers based on the jet mixing diameter and velocity between St = 4 and 6. The other pattern, around St = 1, radiates preferably forward. In addition, the high frequency source is located between 4 and 5 mixed jet diameters past the secondary nozzle exhaust plane, while the low frequency source is located farther downstream between 7 and 8 mixed jet diameters. Shock-associated noise is usually associated with the interaction between shock cells and turbulent shear-layers. We postulate that the high-frequency noise component results from the interaction between the shock-cells in the secondary flow and the inner shear-layer, while the low-frequency component is due to the interaction between shock cells and the outer shear-layer. This interpretation is consistent with two observations. The first observation is a strong modification of the measured high frequency noise when the engine is pushed to high power. At this regime shock cells are present in the primary flow and the inner shear layer interacts with two supersonic flows, hence modifying emitted noise. The second observation is the reduction of low-and high-frequency noise by the introduction of chevron fan nozzles. The flow field results confirm the strong impact of chevrons on the structure of the shock cells in the fan stream: they are consistent with the experimental observations of the characteristic shock noise frequencies. For the Trent 500 engine, appropriate immersive fan chevrons may bring noise reductions by as much as 6dB SPL integrated over a relevant Strouhal number range [0.7; 2]. However, the reduction level due to the chevrons varies with the engine operating conditions. A given chevron nozzle may be found more efficient at reducing noise levels at low engine operating regime than at high regime.

Estimation of acoustic directivity from microphone array measurements using parametric models

In this paper, we are interested in recovering the far-field acoustic pattern of a directive source from signals recorded in the near-field by an array of microphones with a reduced spatial extent. This question is particularly relevant in small test facilities where far-field acoustic measurements can not be carried out. A two-step approach is suggested. Firstly, the characteristics of sources are estimated from near-field measurements. Secondly, these characteristics are used to estimate the far-field radiation pattern. The main diculty of this problem mainly resides in the first step. Due to the reduced spatial extent of the array, much information is lost about source characteristics, which mathematically leads to solve an ill-posed inverse problem. Our approach consists in using a parametric model based on physical assumptions, which has the virtue of regularizing the estimation problem. The suggested method is firstly evaluated with simulations, and then applied to experimental data recorded during aeroacoustic tests with a subsonic jet in an anechoic wind tunnel. It is shown that comparison between far-field measurements and estimated far-field pattern are in good agreement.