Revealing sub-{\mu}m inhomogeneities and {\mu}m-scale texture in H2O ice at Megabar pressures via sound velocity measurements by time-domain Brillouin scattering

Time-domain Brillouin scattering technique, also known as picosecond ultrasonic interferometry, which provides opportunity to monitor propagation of nanometers to sub-micrometers length coherent acoustic pulses in the samples of sub-micrometers to tens of micrometers dimensions, was applied to depth-profiling of polycrystalline aggregate of ice compressed in a diamond anvil cell to Megabar pressures. The technique allowed examination of characteristic dimensions of elastic inhomogeneities and texturing of polycrystalline ice in the direction normal to the diamond anvil surfaces with sub-micrometer spatial resolution via time-resolved measurements of variations in the propagation velocity of the acoustic pulse traveling in the compressed sample. The achieved two-dimensional imaging of the polycrystalline ice aggregate in-depth and in one of the lateral directions indicates the feasibility of three-dimensional imaging and quantitative characterization of acoustical, optical and acousto-optical properties of transparent polycrystalline aggregates in diamond anvil cell with tens of nanometers in-depth resolution and lateral spatial resolution controlled by pump laser pulses focusing.Comment: 32 pages, 5 figure

Optimized Determination of Elastic Constants of Anisotropic Solids from Wavespeed Measurements

Ultrasonic measurements have been used for a long time to quantitatively determine the elastic properties of solids from wavespeed measurements. The determination is based on measuring the speeds of propagation of various wave modes, propagating in specific directions of the solid. Various methods have been developed for these measurements including both continuous wave and pulsed or burst techniques. A traditional distinction can be done in the way the transducer(s) interact with the solid to be characterized. Contact techniques [1,2] have been supplemented by immersion systems [3,4] which often yield more reproducible results and which permit measurements to be easily made along different directions in the test specimen. For instance, an advanced immersion system using the principle of digital interferometry with the help of a cross-correlation technique and normal mode tracking algorithms was recently designed [51. By using the mode conversion process at a liquid-solid interface, quasi-longitudinal and quasi-transverse bulk modes can be generated in numerous directions in the interior of the solid. The design and implementation of specialized optimization algorithms [6] permit a precise determination of the elastic constants for various synthetic and natural composite materials [7,8]. Nevertheless, a principal limitation of the existing optimization procedures for recovering a material’s elastic constants has been the requirement of wavespeed data measured in principal planes, where some analytical expressions, providing the slowness curves, generally exist [9]

Effet non linéaire d'auto-démodulation d'amplitude dans les milieux granulaires (théories et expériences)

LE MANS-BU Sciences (721812109) / SudocSudocFranceF

Les écueils d'une fiscalité avantageuse

International audienc

WAVESPEEDS MEASUREMENTS IN ANISOTROPIC MATERIALS BY USING LASER GENERATED ULTRASOUND

Wavespeed measurements made on the quasi-longitudinal (qL) mode have been achieved in two composite materials (i.e. a 1-D glass/epoxy and a 2-D SiC/SiC) by using a correlation algorithm between the measurement signal at a given observation angle, and a reference signal recorded at normal incidence. The wavespeed measurement at normal incidence is obtained, when possible, by correlating the direct qL ray with the 3qL multiple reflection. The qL wavespeeds measured by the laser method are then compared to the phase velocities acquired by standard ultrasonics techniques. The agreement is quite acceptable in most cases. When dealing with laser generated ultrasound, the wavefront of any given ray is propagated at the group velocity. The correlation measurements, should provide wavespeeds which tend to be close to such ray surface velocity. However, the difference between the two wavespeeds is small on the qL mode (generally less than 10 %) and can be as a first approximation safely ignored

Conception, réalisation et caractérisation d'un haut-parleur ultra-directif basé sur l'auto-démodulation non linéaire

Ce travail de thèse vise à améliorer les systèmes existants d'antenne paramétrique utilisée en tant que haut parleur directif. Les principes fondamentaux de l'acoustique non linéaire sont rappelés. Ceci permet de conclure sur les propriétés nécessaires du champ d'ondes émis pour cette application. Les distorsions induites par l'auto-démodulation sont également mises en évidence et les traitements de réduction des distorsions sont ensuite détaillés et évalués. Afin d'obtenir une directivité étroite et un fort niveau, un réseau de transducteurs est utilisé. Une modélisation des transducteurs à base de film PVDF couplé à une cavité est proposée. Celle-ci permet de prédire le champ de pression acoustique rayonnée par une cellule élémentaire. L'utilisation des modèles de réseaux de sources permet de définir un réseau de cellules adéquat. Finalement, un prototype de haut-parleur directif est réalisé et caractérisé afin de réaliser une validation des différentes parties de l'étude.This thesis aims at improving the parametric arrays used as directional loudspeaker. Fundamental principles of nonlinear acoustics are reminded in order to understand the self-demodulation phenomenon. This makes possible to conclude on the emitted waves field properties necessary for this application. The distortions induced by the self-demodulation are also highlighted. Then usual distortions reduction processings are detailed and evaluated. In order to obtain a narrow directivity and a strong level, a transducers array is used. A physical model of the PVDF film transducers coupled to a cavity is proposed. This one is used to predict the acoustic pressure field radiated by an elementary cell. Then, array radiation models are used to define a suitable cells array. Finally, a prototype of directive loudspeaker is made to do an experimental assessment of this study.LE MANS-BU Sciences (721812109) / SudocSudocFranceF

Contribution à l'étude des silencieux dissipatifs (effets des vibrations de structure, effets de l'écoulement)

Cette thèse s inscrit dans le cadre du projet européen FLODAC (FLOw Duct ACoustic) et s intéresse à deux aspects des silencieux dissipatifs. La chambre d expansion traitée est couramment utilisée pour atténuer des sons en conduit. On propose ici un modèle approché pour un tel système dans lequel les variations radiales de la pression et de la vitesse dans l air et dans le matériau poreux sont décrites par des polynômes. Ces approximations permettent le calcul des coefficients de transmission et de réflexion qui caractérisent tout à fait les propriétés acoustiques du système à basse fréquence. Des résultats de mesures montrent la validité d une telle approche. La présence d un écoulement uniforme dans le conduit d air est introduite dans le modèle. L accord de celui-ci avec des résultats expérimentaux est satisfaisant pour les coefficients de transmission. L étude montre que pour des nombres de Mach inférieurs à 0.15, l effet de l écoulement est déjà important. On note en particulier une diminution du pouvoir atténuant de la chambre dans le sens de l écoulement. Le second aspect du travail s intéresse à l effet des vibrations de la structure du matériau poreux sur l atténuation d un conduit traité de longueur infinie. Ces travaux s appuient en partie sur ceux de E. Lesquins (SNECMA) et de B. Brouard (LAUM). Le matériaux poreux est décrit par la désormais classique théorie de Biot. Différents modèles sont proposés. La géométrie du conduit peut être bidimensionnelle ou circulaire et deux méthodes sont utilisées pour résoudre le problème, i.e. trouver les constantes de propagation des différents modes. Des simulations numériques montrent que sous certaines conditions les vibrations de structure peuvent provoquer une atténuation importante en basse fréquence. Sous d autres conditions elles peuvent entraîner une diminution de cette dernière. Une première étude expérimentale montre la validité de ces travaux.This thesis is a part of the European project FLODAC (FLOw Duct ACoustic) and is dealing with two aspects of dissipative silencers. The expansion chamber is commonly used as a dissipative silencer. Here we propose an approximate model for such a system, in which the radial variations of the pressure and velocity in the air and in the porous lining are described by polynomials. These approximations allow the calculation of the reflexion and transmission coefficients which completely descibe the acoustic properties of the chamber at low frequency. Experimental results show the validity of such an approach. The presence of uniform flow in the air duct is introduced in the model. Agreement with experimental results is satisfying for the transmission coefficients. The study shows that for Mach numbers lower than 0.15, the effect of flow is already important. In particular, we note that the attenuation is lower in the sense of flow. The second aspect of the work is dealing with the effect of the vibration of the structure of the porous lining on the attenuation of a duet of infinite length. This part is partly using the work of E. Lesquins (SNECMA) and B. Brouard (LAUM). The porous material is described by the classical Biot theory. Several models are proposed. The geometry of the duct can be two-dimensional or circula rand two methods are investigated in order to solve the problem, i.e. determining the propagation constant for the different modes. Numerical simulations show that under certain circumstances structural vibrations can produce an important low frequency attenuation whereas under other circumstances the latter is lower. A first experimental study shows the validity of this work.LE MANS-BU Sciences (721812109) / SudocSudocFranceF

Correlation between tortuosity and transmission coefficient of porous media at high frequency

International audienc