Estelle Laurendeau

Subsonic jet noise reduction by fluidic control : The interaction region and the global effect

A microjet arrangement comprising both penetration (or immersion) and convergence (jets oriented such that two jets of a pair interact with one another) is used to control a subsonic turbulent jet with a view to noise reduction. The acoustic effect of the so-called fluidevron system is comparable to chevrons and nonconverging microjets as far as the noise reduction is concerned. Detailed experimental measurements are performed for a main jet with Mach and Reynolds numbers of 0.3 and 310 000, respectively. A direct numerical simulation study is performed for a model, plane mixing-layer problem using the immersed-boundary method, in order to help understand the topological features of the fluidevron–mixing-layer interaction. In terms of modifications produced in the flow, two relatively distinct regions are identified: the near-nozzle region, 0 1, where the jet recovers many of the uncontrolled-jet flow ch...

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

A study based on the sweeping hypothesis to generate stochastic turbulence

With the development of so-called ’electric’ aircraft, the integration of jet pumps to various systems of the engines could o er signi cant gains (mass, reliability,...) over the technologies currently used. Since jet pumps may directly contribute to ramp noise, an accurate predictive tool is therefore desired to propose adapted noise reduction solutions in an industrial context. A direct computation of the unsteady turbulent ow being too expensive, the main idea is to compute acoustic sources from a stochastic velocity eld and to inject it in Euler’s equations to model neareld acoustic propagation. A Kircho analogy is then used to reduce calculation cost in the far eld. In following previous works by Bailly and Juv e (AIAA Paper 99-1872) and Billson et al. (AIAA paper 2003-3282), a combined approach to generate a stochastic velocity eld is presented and validated in the present work. This methodology is based on the sweeping hypothesis or the fact that small scale turbulent structures are advected by energy containing eddies. The validation study is done on aerodynamic quantities of a cold free jet at Mach number M=0.72. The capability of the method to reproduce space-time velocity correlations in the shear layer is shown. The model is then tested on a cold free jet con guration to predict radiated acoustic levels using a volumic Lighthill solver.

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.

Turbulence Generation from a Sweeping-Based Stochastic Model

Stochastic methods are widely used because they constitute a low-cost computational-fluid-dynamics approach to synthesize a turbulent velocity field from time-averaged variables of a flowfield. A new combined stochastic method based on the sweeping hypothesis is introduced in this paper. This phenomenon, stating that inertial range structures are advected by the energy containing eddies, is known to be an important mechanism of the turbulent velocity field decorrelation process. The proposed method presents the advantage of being easily implementable and applicable to any three-dimensional configuration as long as a steady Reynolds-averaged Navier–Stokes computation of the flow is available and assuming that the considered turbulence physics is compatible with the hypotheses made to build the current numerical model. The developed method is applied on a subsonic round cold free jet. The validation study shows that the synthesized turbulent velocity fields reproduce statistical features of the flow, such a...

Prediction of subsonic jet noise relying on a sweeping based turbulence generation process

Jet pumps, which are considered in the framework of this study as simple jets confined in ducts, could be a significant ally in the mission of the aeroacoustic community to make the aircraft more environmentally friendly. Despite the significant gains that it could bring for various functions of the systems of the engines, such devices could contribute directly to ramp noise. Therefore, a predictive code of the noise radiated by these configurations could be a useful tool to propose noise reduction solutions. A numerical tool, designed for acoustic prediction of confined jets, is presented in this paper. The proposed method is adapted to industrial requirements since it is easily implementable and applicable to any 3D configuration. The acoustic propagation is obtained by computing the response of the linearized Euler equations, enforced by a non-linear source term calculated with a stochastic velocity field synthetized from the sweeping based turbulence generation process proposed by Lafitte et al. [AIAA 2011-2088]. The chosen formulation is validated by computing test cases including simple dipole and quadrupole distributions. The predictive tool is then applied to a cold free jet configuration at Mach number M=0.72. Preliminary computations show that the forcing of the linearized Euler equations by a stochastic source term appears to be a complex operation since far-field acoustic spectra levels are overestimated. An accurate calibration method of the acoustic source term is therefore introduced in the present work.

Nearfield-Farfield Correlations in Subsonic Jets: What Can They Tell Us?

Simultaneous pressure measurements effected in the near and farfield regions of a subsonic jet are used to study (1) the causal relationship between the coherent dynamic of the flow and the sound field it produces, and (2) the localisation and directive character of a broadband high-frequency component of the sound field which is sometimes associated with the fine-scale turbulence. By varying the axial and radial positions of a nearfield microphone, correlations with 10 farfield microphones are used to study the source signatures in three regions: the near-nozzle region (nearfield microphone at x/D = 1), the mixinglayer region (nearfield microphone at x/D = 3), and the end of the potential core (nearfield microphone at x/D = 5). The correlations reveal a number of distinctive signatures, and arguments are presented which relate these to the following kinds of source mechanism: (1) a wavy-wall mechanism which dominates the initial stages of the flow, which comprises strong harmonics with marked directivity patterns in the upstream regions, and which appears to be enhanced by the presence of the nozzle, (2) a more monochromatic wave-like mechanism at the end of the potential core, and (3) a broadband high-frequency production mechanism whose directivity is a strong function of axial position, in contrast with the common assumption that this component radiates omnidirectionally.