The present study provides theoretical details and experimental validation results to the approach proposed by Minotti et al. (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 multitone 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 windtunnel testings.

This study investigates the temperature effect on the impedance of conventional singledegree-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.

In the context of noise reduction in diverse applications where a shear grazing flow is present (i.e., engine nacelle, jet pump, landing gear), improved acoustic liner solutions are being sought. This is particularly true in the low-frequency regime, where space constraints currently limit the efficiency of classic liner technology. To perform the required multi-objective optimization of complex meta-surface liner candidates, a software platform called OPAL was developed. Its first goal is to allow the user to assemble a large panel of parallel/serial assembly of unit acoustic elements, including the recent concept of LEONAR materials. Then, the physical properties of this liner can be optimized, relatively to given weighted objectives (noise reduction, total size of the sample, weight), for a given configuration. Alternatively, properties such as the different impedances of liner unit surfaces can be optimized. To accelerate the process, different nested levels of optimization are considered, from 0D analytical coarse designs in order to reduce the parameter space, up to 2D plan or axisymmetric high-order Discontinuous Galerkin resolution of the Linearized Euler Equations. The presentation will focus on the different aspects of liner design considered in OPAL, and present an application on different samples made for a small scale aeroacoustic bench.

This study investigates the temperature effect on the impedance of conventional singledegree-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.

No abstract

Passive acoustic liners, used in aeronautic engine nacelles to reduce radiated fan noise, have a quarter-wavelength behavior. The simplest systems are SDOF-type (single degree of freedom), consisting of a perforated sheet backed with a honeycomb, whose absorption ability is limited to frequencies near the Helmholtz frequency. Thus, to widen the absorption frequency range, manufacturers use a 2DOF (double degree of freedom) system, with an internal layer over another honeycomb (stack of two resonators). However, one constraint is the limited thickness of the overall system, which reduces the space allotted to each honeycomb. A possible approach, based on a previous concept called LEONAR (long elastic open-neck acoustic resonator), could be to link each perforated layer to hollow tubes inserted in each honeycomb layer, in order to shift resonance frequencies to lower frequencies by extending the air column lengths. The presence of an empty chamber on both sides of the internal perforated layer also allows the tube length to be increased through tubes crossing both cavities, preserving the liner thickness. The main aim of this article is to mathematically describe the principle of a 2DOF LEONAR and to show the relevance of the mathematical model through FEM simulations and experiments performed in an impedance tube. Moreover, its behavior is analyzed through a parametric study, in order to explore its potential for an aeronautic application. A remarkable feature of 2DOF LEONAR-type materials with insertion of bottom tubes in the higher cavity is the possibility of maintaining the low frequency band provided by the original LEONAR concept, while adding a second absorption peak at a higher frequency, by the second layer and the accompanying tubes. There is a fundamental difference from classical SDOF/2DOF resonators, for which the thicknesses are obviously different.

This paper focus on the dynamic of a spray issued from the shearing of a liquid jet injected in an air crossflow submitted to high acoustic perturbations. Experimental and numerical approaches are used. Characterization of the liquid jet close to the injection location is obtained from high speed visualizations performed with a back lighting technique. Phase Doppler Anemometry gives useful information on the spray dynamics. The phase-averaged post processing method is chosen to describe the flow oscillations during the excitation cycle. The numerical simulation is performed with the multi-scale LES approach. This method couples a multi-fluid solver for the liquid jet body with a dispersed phase solver dealing with the atomized spray. The experimental results show a swigging phenomenon of the liquid jet and the existence of velocity and concentration waves travelling downstream of the liquid jet. Coupled phenomena between the crossflow, the atomization of the liquid jet and the transport of droplets are observed, revealing different wave transport velocities. The numerical simulation is able to capture the global swinging phenomenon of the liquid jet main body as well. A very good agreement is obtained for the jet trajectories oscillations obtained either by the simulation or from the experiment during the whole excitation cycle.

Nomenclature φ Liner perforated sheet hole diameter, (mm) δ Liner perforated sheet thickness, (mm) POA Percent Open Area L s Thickness of the liner sample ρ 0 Density of the mean flow, (kg/m 3 ) M b Bulk flow Mach number U b Bulk flow velocity h B2A test section height ω = 2πf Angular frequency, (rad/s) Z = R + iX Acoustic impedance β Acoustic reflection coefficient (x, y, z) Axial, transversal and vertical coordinates, (mm) (U, W ) Axial and vertical mean velocity, (m/s) (u, w) Axial and vertical acoustic velocity, (m/s) c 0 Sound celerity, (m/s)