Identification de sources acoustiques sur un moteur essence par méthode de cohérences

L'objet de cette communication est d'illustrer l'applicabilité de certaines méthodes dites ‘de cohérences' au diagnostic vibro-acoustique d‘un moteur d'automobile. Après un rappel des outils d'analyse specrale existant dans la bibliographie, nous présenterons une méthodologie originale permettant d'optimiser le diagnostic lorsque l'objet étudié a un comportement vibro-acoustique complexe d'un point de vue statistique. Le potentiel et l'efficacité de cette approche seront ensuite illustrés par deux applications expérimentales : l'analyse vibro-acoustique d'un moteur essence à l'arrêt sollicité par une source maîtrisée puis l'analyse du même moteur en fonctionnement

Source separation in diesel engines using Wiener filtering: physical interpretations

International audienceThe separation of combustion and mechanical contributions in diesel engine signals can be achieved by using Wiener filtering. The idea is to estimate in operation the transfer functions between cylinder pressure and response signals, accelerometers or microphones. This operation is particularly tricky because of the necessity to observe the response signals on very short time windows (a fraction of cycle) to isolate the contributions of each cylinder. Moreover, several mechanical excitations are occurring at the same time than the combustions, which cannot be removed by using windowing. These mechanical excitations are partly coherent with the combustion, generating a bias on estimated transfer functions. It has been shown in preceding works that a way of lowering significantly this bias is to work on the random parts of signals. The aim and the originality of this contribution is to show that the differences between transfer functions obtained from raw signals and from their random parts can be used for diagnosis purpose. This difference has indeed a physical interpretation, and can be used to improve our understanding concerning the source contributions and their transfer paths. These developments are illustrated by experimental data obtained on several diesel engines

On the use of the Hs estimator for the experimental assessment of transmissibility matrices

International audienceThe experimental estimation of frequency response functions characterizing SISO linear systems is a well established topic. Several estimators are defined in the literature, each estimator being optimal depending upon the assumptions with respect to the balance of noise between the input and output of the system. H1 and H2 have to be used in case of presence of noise on output and input, respectively. The HV or Hs estimator is chosen if input and output are assumed to have equivalent SNR. These estimators are also established for MIMO linear systems, with additional difficulties due to the necessity of inversing cross spectral matrices. A transmissibility function is generally defined as a linear relationship between two outputs of a linear system. For MIMO systems, transmissibilities are not defined by the system only, it also depends on the input quantities. It is however possible to define a transmissibility matrix between two sets of outputs that is, under some assumptions, uniquely defined. This approach is especially the base of Operational Transfer Path analysis, an engineering method benefiting of a strong research effort in the last few years. This paper deals with the use of the application of MIMO system estimators to the experimental assessment of transmissibility matrices. Transmissibility matrices are generally estimated using a H1 like approach in the literature. The possibility of using H2 and Hs is presented in this work, from the theoretical point of view and with numerical and practical illustrations

Contrôle de l'absorption sonore d'un matériau poreux par addition d'une couche résistive

National audienceLes matériaux poreux utilisés pour leur qualité d'absorption sonore sont couramment assemblés avec une couche résistive. Cette couche est parfois ajoutée pour augmenter les performances acoustiques du matériau mais elle est le plus souvent utilisée à des fins de protection ou de décoration. Par exemple les laines de roches, connues comme de bons absorbeurs sonores, sont souvent livrées avec une plaque perforée. Cette plaque permet de conserver une paroi rigide tout en évitant que la laine ne se détériore. Les textiles (tissés ou non tissés) sont également de plus en plus utilisés en amont de matériaux acoustiques. Cette étude montre le rôle que peut avoir une couche résistive en amont d'un complexe poreux. Par exemple, un matériau recyclable de performances médiocres muni d'un film résistif convenablement dimensionné permet de valoriser les matériaux verts. En se basant sur des simulations numériques, ces travaux dégagent les lignes directrices d'utilisation rationnelle de films résistifs pour maximiser le coefficient d'absorption sonore de complexes résistifs. Deux résultats majeurs sont que (1) la couche résistive peut contrôler l'absorption sonore du complexe et rendre négligeable la nature de la seconde couche et (2) la couche peut devenir néfaste à l'absorption sonore du complexe. Une validation expérimentale sur un complexe poreux, contrôlé par un textile tissé, appuie ces résultats

Simulation of the vibro-acoustic response of a truck cabin in low frequency range including the effect of the acoustic treatments; comparison with experiments

International audienceThe low frequency noise in a truck cabin (typically below 300Hz) is a strong component of the interior noise comfort. In the development of a numerical process to predict the interior noise in this frequency range, the simulation of the vibro-acoustic response of the cabin is a step to be developed. This paper summarizes the work done to correlate a numerical method for the vibro-acoustic response of the cabin taking into account the effect of the acoustic treatments. One constraint is to keep a modal scheme although the acoustic treatments are heavily damped and not very suited to be solved with this scheme. The process is based on the modal extraction of structure and cavity modes, followed by the calculation of acoustic treatments impedances which are projected into the modal space. The complete vibro-acoustic response for a given excitation can then be computed with a modal frequency response. The paper presents a measurement campaign performed to get reference measured data and details the different parts of the model. Comparisons between simulated and measured Vibration Transfer Functions and Noise Transfer Functions are presented. The good correlation obtained assesses the validity of the method and the modelling

Vibro-Acoustic Modeling of a Trimmed Truck Cabin in Low Frequency Range to Tackle the Challenge of Weight Reduction

International audienceIn the challenge of reducing the weight of the vehicle structures, a particular focus has to be done on the interior noise. Indeed, the weight reduction of the structure often implies an increase of the noise in the cabin.To maintain a constant acoustic performance, acoustic packages often have to be added, the challenge being that the weight of the acoustic materials added remains lower than the weight saved in the structure.In today’s engineering world, numerical simulation is the primary tool to assess the vibro-acoustic behavior of the vehicle during the design phase. To tackle the challenge of weight reduction, it is necessary to simulate accurately the vibro-acoustic response of the structure including the acoustic treatments.This paper presents the validation of a simulation method for the vibro-acoustic response of a truck cabin, taking into account the effect of acoustic treatments, in the frequency range [0-200 Hz].The method combined a modal scheme for the structure and the cavity with a physical scheme for the acoustic treatment (porous materials).The model consists in a truck cabin with its cavity and five acoustic treatments (three floormats, the headliner and the rear trim panel). A measurement campaign is performed to get reference NTFs, VTFs and local inertances. The model without any treatments is first correlated. The noise reduction given by each treatment alone as well as grouped together is simulated. The comparisons between the simulated and measured results show good agreement both in term of spectrum and amplitude. Although some discrepancies in the low frequency range remain unexplained, the method is considered as validate

Analyse modale expérimentale vibroacoustique : de la préparation des essais à l'identification de modèles réduits

National audienceL'analyse modale expérimentale est largement répandue dans le domaine de la dynamique des structures. Son extension dans le cadre du couplage vibroacoustique pour l'étude des cavités fermées par des structures élastiques est un domaine beaucoup moins mature. Deux points particuliers sont présentés dans ces travaux : - d'une part, l'utilisation d'un modèle numérique permet d'optimiser la préparation des essais, afin de définir au mieux les positions des capteurs (accéléromètres, microphones) et des actionneurs (pot vibrant, marteau de choc, haut-parleur), pour un objectif d'identification donné; - d'autre part, l'exploitation des mesures permet, moyennant des techniques de régularisation à fort sens physique, d'identifier par méthode inverse un modèle réduit vibroacoustique ayant le même comportement que le système réel sur une bande de fréquences d'intérêt. Les méthodologies proposées sur ces deux aspects sont détaillées puis illustrées numériquement et expérimentalement sur un cas-test de couplage vibroacoustique

Vibrational behavior of multi-layer plates in broad-band frequency range: comparisons between experimental and theoretical estimations

International audienceIn the context of aeronautics, automotive and construction applications the design of lightened multilayer plates with optimized vibroacoustical damping and isolation performances remains a major industrial challenge and a hot topic of research. This paper focuses on the vibrational behavior of three-layer composites plates in broad-band frequency range. Several aspects are studied through measurement techniques and analytical modeling of two sandwich systems: symmetrical steel/polymer/steel and nonsymmetrical steel/polymer/aluminum plates. Contactless measurements of the velocity field made using a scanning laser vibrometer allow estimating the equivalent single layer complex rigidity in the mid/high frequency ranges. Coupled with low/mid frequency estimations obtained with a high-resolution modal analysis method, the frequency dependent equivalent Young’s modulus and equivalent loss factors of the two composites plates are finally identified for the whole [0-5 kHz] band. The results are in very good agreement with an equivalent single layer analytical modeling based on wave propagation analysis. The comparison with this model allows identifying the frequency dependent complex modulus of the polymer core layer through inverse resolution. Dynamical mechanical analysis measurements are also performed on the polymer layer alone to compare with the values obtained through inverse method. Again, a good agreement between these two estimations over the broad-band frequency range demonstrates the validity of the approach

Improved acoustical sources location in gear power transmissions

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Source identification on a reduction gearbox using acoustical measurements in a non-anechoic environment

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