Measurement of acoustic velocity components in a turbulent flow using LDV and high-repetition rate PIV

International audienceThe present study provides theoretical details and experimental validation results to the approach proposed by Minotti et al. (Aerosp Sci Technol 12(5):398–407, 2008) for measuring amplitudes and phases of acoustic velocity components (AVC) that are waveform parameters of each component of velocity induced by an acoustic wave, in fully turbulent duct flows carrying multi-tone acoustic waves. Theoretical results support that the turbulence rejection method proposed, based on the estimation of cross power spectra between velocity measurements and a reference signal such as a wall pressure measurement, provides asymptotically efficient estimators with respect to the number of samples. Furthermore, it is shown that the estimator uncertainties can be simply estimated, accounting for the characteristics of the measured flow turbulence spectra. Two laser-based measurement campaigns were conducted in order to validate the acoustic velocity estimation approach and the uncertainty estimates derived. While in previous studies estimates were obtained using laser Doppler velocimetry (LDV), it is demonstrated that high-repetition rate particle image velocimetry (PIV) can also be successfully employed. The two measurement techniques provide very similar acoustic velocity amplitude and phase estimates for the cases investigated, that are of practical interest for acoustic liner studies. In a broader sense, this approach may be beneficial for non-intrusive sound emission studies in wind tunnel testings

Reflection Positivity and Monotonicity

We prove general reflection positivity results for both scalar fields and Dirac fields on a Riemannian manifold, and comment on applications to quantum field theory. As another application, we prove the inequality $C_D \leq C_N$ between Dirichlet and Neumann covariance operators on a manifold with a reflection.Comment: 11 page

Liner impedance eduction under shear grazing flow at a high sound pressure level

International audienceThis paper investigates the combined effects of high sound pressure level and grazing flow on impedance eduction for classical liners. Experiments are conducted in the grazing flow duct at ONERA (B2A). The impedance is then educed with an inverse method adapted to a shear flow. To take into account the effects of incident sound pressure level, a new strategy for impedance eduction is developed, using a space-dependent variable term. The new strategy is applied to different experimental cases and the results are compared to those obtained with the classical method.Cet article étudie les effets combinés d’un niveau de pression acoustique élevé et d'un écoulement sur l'éduction d’impédance pour des liners acoustiques classiques. Des expériences sont menées dans le B2A avec écoulement. L’impédance est ensuite déduite avec une méthode inverse adaptée à un écoulement de cisaillement. Pour prendre en compte les effets du niveau de pression acoustique incidente, une nouvelle condition aux limites d'impédance d’impédance est élaborée à l’aide d’un terme variable dépendant de l’espace. La nouvelle stratégie est appliquée à différents cas expérimentaux et les résultats sont comparés à ceux obtenus avec la méthode classique

Experimental impedance assessment of innovative liner under shear grazing flow

International audienceOn modern aircrafts, passengers and crew breathe a mixture of fresh and recirculated air. This combination allows the regulation of temperature, pressure and humidity. The air is bled from the engines and supplied to air conditioning units. It is then ducted into the cabin, circulated and eventually drawn into the lower fuselage where it is sucked out by the pressurization outow valve for the cycle to begin again. Besides creating a safe and comfortable environment, the aircraft air conditioning systems generate noise. The noise radiated from the aircrafts' air conditioning systems is reduced thanks to acoustic liners. These liners present a major design challenge because of the need to address a wide range of conicting requirements. Acoustic liners must provide high levels of noise reduction over a wide range of operating conditions. They should also be light and exible to meet strict weight and tight space restrictions. Until now, acoustic liners for air conditioning systems are made of porous materials, very efficient for sound absorption in the high-frequency range. Locally-reacting liners made of a multiple layers of a honeycomb core topped by a perforated facesheet are classically used in turbofan engine nacelles for mitigating fan noise. These liners are denoted as SDOF (Single Degree of Freedom) or DDOF (Double Degree of Freedom) liners depending on the number of perforate-over-honeycomb layers. Mid-frequency noise attenuation (in the range of 1-5 kHz) can easily be obtained by tuning the liner acoustic impedance to a target value specific to the duct environment (geometry, ow Mach number, noise source modal content). The impedance achieved by a SDOF or DDOF liner depends on its geometry (porosity and holes diameter of the facesheets, honeycomb depth), on the grazing ow features, and on the noise source level. During the ALIAS project,5 a simulation-based design process was implemented to assess the liners concepts that were the best suited for attenuating the air pump noise in the mid-frequency range. A trade-off between acoustic efficiency, weight and cost manufacturing issues was made before selecting the liners. In the framework of the IDEAS Project funded within the European initiative CleanSky 2 SYSTEM- ITD, ONERA, the French Aerospace Lab, and the SMEs ATECA and Poly-Shape combine their research and technological capabilities to propose new ideas in the domain of acoustic liners and in-duct modal detection for air conditioning systems from Liebherr Aerospace. A compact innovative acoustic liner is designed in order to mitigate this jet pump noise source over all the frequency range, while meeting the strict weight, costs and tight space restrictions. This paper presents two concepts of liner for a large frequency range with high industrial constraints. These liner architectures need to be assessed regarding the ow effect and the incident sound pressure level. The acoustic behavior and the effect of a grazing ow on the impedance of each liner concept are assessed in this paper. This liner concept analysis enables to increase our experimental data base and to identify reliable liner models to optimize the best liner solution regarding industrial cases

Inlet Liner Design for aircraft air conditioning system

International audienceWith regard to the reduction of aircraft noise, so-called acoustic liners are generally based on a (micro-) perforated plate above a honeycomb. They are developed to absorb waves propagated along, for example, engine nacelles or air conditioning systems. Their design must take into account the dimensional and phenomenological characteristics of constituent materials, manufacturing and assembly specifications and industrial requirements involving multiphysical phenomena. Indeed, they are submitted to high sound pressure level, shear turbulent grazing flow and potentially high temperatures. That is the reason why there is a need to design liners thanks to an optimization loop combining numerical and experimental tools. This paper describes an ONERA optimization process (OPAL platform) coupling: firstly, a 1D/2D Linear Euler Equation models evaluates acoustic Transmission Loss along a lined duct in shear and hot flow; secondly, a 1D propagation code determines an impedance surface above porous multilayered structures; thirdly, an evolutionary computation algorithm optimizes the liner impedance; fourthly, an experimental database may provide initial flow characteristics and assess the target impedance. This process is applied to a liner with a perforated plate extended by hollow tubes (LEONAR concept) to absorb low frequency noise generated by a jet pump of an air conditioning system

Experimental assessment of the effect of temperature gradient across an aeroacoustic liner

International audienceThis study investigates the temperature effect on the impedance of conventional single degree-of-freedom liners, both without and with grazing flow. Experiments are performed in a controlled environment, with a detailed monitoring of the temperature all along the liner sample. The liner impedance is either derived from the reflection coefficient measured in a normal impedance tube, or is educed with an inverse method from acoustic velocity or wall pressure fields measured in the ONERA grazing flow duct. The influence of the acoustic source level on the temperature of the sample is also addressed, which enlights strong multiphysics coupling between acoustics, flow and thermal phenomena

Détermination de l'impédance acoustique de matériaux absorbants en écoulement par méthode inverse et mesure LDV

La réduction des nuisances sonores est un enjeu permanent pour les acteurs de l aéronautique. L optimisation de la réduction de bruit apportée par les traitements acoustiques tapissant la nacelle des réacteurs turbofan passe par une caractérisation précise des matériaux employés dans l environnement aéroacoustique d utilisation, qui met en jeu un écoulement rasant de vitesse importante combiné à de forts niveaux sonores. L objectif de cette thèse est de développer une méthode inverse pour la détermination de l impédance acoustique de liners soumis à un écoulement rasant, basée sur des mesures non intrusives du champ de vitesse acoustique au-dessus du matériau par Vélocimétrie Laser Doppler (LDV). L impédance de liner est obtenue par minimisation de l écart entre le champ de vitesse acoustique mesuré et le champ simulé numériquement en résolvant les équations d Euler linéarisées bidimensionnelles harmoniques, discrétisées par un schéma Galerkin discontinu. Le gradient de la fonction objectif minimisée est calculé via la résolution, à chaque itération, des équations directes et adjointes. Une première étape de validation du solveur est effectuée sur des cas-tests académiques, puis sur des cas expérimentaux impliquant des mesures de pression acoustique en paroi rigide opposée au liner. Dans un second temps, la méthode est appliquée à des mesures de vitesse acoustique obtenues par LDV dans le banc B2A de l ONERA en l absence d écoulement. La dernière étape consiste à prendre en compte l effet d un écoulement rasant de profil cisaillé. Les impédances identifiées à partir de mesures LDV en présence d écoulement ont notamment permis de gagner en compréhension sur les phénomènes d absorption intervenant dans le banc B2A.While aircraft noise constraints become increasingly stringent, efficient duct treatment of turbofan engines requires an accurate knowledge of liner impedance with grazing flow at high acoustic levels. This thesis aims at developing an impedance eduction method in the presence of grazing flow. The inverse process is based on acoustic velocity fields acquired by Laser Doppler Velocimetry (LDV) above the liner. The liner acoustic impedance is obtained by minimization of the distance between the measured acoustic velocity field and the simulated one. Computations rely on the resolution of the 2D linearized Euler equations in the harmonic domain, spatially discretized by a discontinuous Galerkin scheme. The gradient of the objective function is achieved by the resolution, at each iteration on the liner impedance, of the direct and adjoint equations. The solver is first validated on academic test cases, then on experimental results of acoustic pressure measurements at the rigid wall opposite the liner. Secondly the method is applied to acoustic velocity measurements obtained by LDV above the liner without flow, in the ONERA B2A test bench. The last step consists in taking into account the effects of a sheared grazing flow. The impedances educed from LDV measurements in the presence of flow namely allowed to gain insight into the absorption phenomena occuring in the B2A test bench.TOULOUSE-INSA-Bib. electronique (315559905) / SudocSudocFranceF

ONERA-NASA Cooperative Effort on Liner Impedance Eduction

As part of a cooperation between ONERA and NASA, the liner impedance eduction methods developed by the two research centers are compared. The NASA technique relies on an objective function built on acoustic pressure measurements located on the wall opposite the test liner, and the propagation code solves the convected Helmholtz equation in uniform ow using a finite element method that implements a continuous Galerkin discretization. The ONERA method uses an objective function based either on wall acoustic pressure or on acoustic velocity acquired above the liner by Laser Doppler Anemometry, and the propagation code solves the linearized Euler equations by a discontinuous Galerkin discretization. Two acoustic liners are tested in both ONERA and NASA ow ducts and the measured data are treated with the corresponding impedance eduction method. The first liner is a wire mesh facesheet mounted onto a honeycomb core, designed to be linear with respect to incident sound pressure level and to grazing ow velocity. The second one is a conventional, nonlinear, perforate-over-honeycomb single layer liner. Configurations without and with ow are considered. For the nonlinear liner, the comparison of liner impedance educed by NASA and ONERA shows a sensitivity to the experimental conditions, namely to the nature of the source and to the sample width

CAPAbility: Comparison of the JOURNEY II Bi-Cruciate Stabilised and GENESIS II total knee arthroplasty in performance and functional ability: protocol of a randomised controlled trial

Background: Osteoarthritis of the knee is a common condition that is expected to rise in the next two decades leading to an associated increase in total knee replacement (TKR) surgery. Although there is little debate regarding the safety and efficacy of modern TKR, up to 20% of patients report poor functional outcomes following surgery. This study will investigate the functional outcome of two TKRs; the JOURNEY II Bi-Cruciate Stabilised knee arthroplasty, a newer knee prosthesis designed to provide guided motion and improve knee kinematics by more closely approximating a normal knee, and the GENESIS II, a proven existing design. Aim: To compare the change in Patient-reported Outcome Measures (PROMs) scores of the JOURNEY II BCS and the GENESIS II from pre-operation to 6 months post operation. Methods: CAPAbility is a pragmatic, blinded, two-arm parallel, randomised controlled trial recruiting patients with primary osteoarthritis due to have unilateral TKR surgery across two UK hospitals. Eligible participants (n = 80) will be randomly allocated to receive either the JOURNEY II or the GENESIS II BCS knee prosthesis. Baseline measures will be taken prior to surgery. Patients will be followed at 1 week, 6 to 8 weeks and 6 months post-operatively. The primary outcome is the Oxford Knee Score (OKS) at 6 months post-operatively. Secondary outcomes include: Other PROMs, biomechanical, radiological (computerised tomography, (CT)), clinical efficacy and safety outcomes. An embedded qualitative study will also investigate patients' perspectives via interview pre and post surgery on variables known to affect the outcome of TKR surgery. A sub-sample (n = 30) will have additional in-depth interviews to explore the themes identified. The surgeons' perspectives on the operation will be investigated by a group interview after all participants have undergone surgery. Discussion: This trial will evaluate two generations of TKR using PROMS, kinematic and radiological analyses and qualitative outcomes from the patient perspective