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Engines are verified through production tests before delivering them to customers. During those tests, lot of measures are taken on different parts of the engine, considering multiple physical parameters. Unexpected measures can be observed. For this very reason, it is important to assess if these unusual observations are statistically significant. However, anomaly detection is a difficult problem in unsupervised learning. The obvious reason is that, unlike supervised classification, there is no ground truth against which we could evaluate results. Therefore, we propose a methodology based on two independent statistical algorithms to double check our results. One approach is the Isolation Forest (IF) model which is specific to anomaly detection and able to handle a large number of variables. The goal of the algorithm is to find rare items, events or observations which raise suspicions by differing significantly from the majority of the data and, at the same time, it discriminates non-informative variables to improve. One main issue of IF is its lack of interpretability. Within this scope, we extend the shapley values, interpretation indicators, to the unsupervised context to interpret the model outputs. The second approach is the Self-Organizing Map (SOM) model which has nice properties for data mining by providing both clustering and visual representation. The performance of the method and its interpretability depends on the chosen subset of variables. In this respect, we first implement a sparse-weighted K-means to reduce the input space, allowing the SOM to give an interpretable discretized representation. We apply the two methodologies on data on aircraft engines measurements. Both approaches show similar results which are easily interpretable and exploitable by the experts.

Unintentional plastic deformation during production of turbine blades made from Ni-based single-crystal superalloy has a huge risk on its durability. In the present study, a plastic deformation was applied at various temperatures to AM1 alloy and its effect on the alloy’s microstructure and creep properties were analyzed. Microstructure evolution during aging treatment was different for plastic deformation at lower temperature (≤750 °C) and higher temperature (950 °C) because of different deformation mechanisms. AM1 with mild γ′ directional coarsening after plastic deformation at 950 °C and following aging treatments performed well in creep test at 1050 °C/140 MPa, but poorly at 850 °C/500 MPa. Rhenium containing CMSX-4 Plus was tested similarly to AM1. Pre-deformation has huge impact on creep durability of CMSX-4 Plus at 1150 °C/110 MPa; however, the creep life debit at 1050 °C/200 MPa was minor. In order to restore properties of pre-deformed single-crystal superalloys, rejuvenation heat treatment process was added after pre-deformation. Rejuvenation treatment successfully restored microstructure after room-temperature plastic deformation, and creep properties of rejuvenated specimens are equivalent to that of original AM1 and CMSX-4 Plus.

International audience; In this work, we present a new a posteriori error estimator based on the Variational Multiscale method for anisotropic adaptive fluid mechanics problems. The general idea is to combine the large scale error based on the solved part of the solution with the sub-mesh scale error based on the unresolved part of the solution. We compute the latter with two different methods: one using the stabilizing parameters and the other using bubble functions. We propose two different metric tensors H iso and H new aniso. They are both defined by the recovered Hessian matrix of the solution and rely on the new subgrid scale error estimator. Thus, we write a new anisotropic local error indicator and we test it for mesh adaptation on convection-dominated benchmarks in 2D and 3D. The results show that the proposed error estimator lead to enhance and accurate solutions while using a drastically reduced number of elements.

International audience; The mechanical behavior of titanium-based alloy Ti6242S was investigated under uniaxial tensile loading at 723 K (450 °C) and 823 K (550 °C) under air and argon environments. Microtensile specimens ranging from 1 mm to 100 µm in thickness were tested to investigate the influence of the decrease in thickness on mechanical properties. Fractographic analyses were carried out using scanning electron microscopy. At 450 °C and 550 °C, a decrease in yield strength, ultimate tensile strength, and strain-to-failure with decreasing thickness was observed. These drops in the macroscopic tensile properties of the thinnest specimens result from a combination of oxidation, which further impairs the specimens with a high surface-to-volume ratio, and the overall lower number of colonies of α lamellae contained in thinner specimens. Ti6242S exhibited dynamic strain aging at 450 °C, especially in specimens with thickness below 500 µm.

International audience; This article reports a procedure to implement as well as to validate non-reflecting boundary conditions applied for turbomachinery simulations, using Navier-Stokes characteristic boundary conditions in a compressible lattice Boltzmann solver. The implementation of both an inlet condition imposing total pressure, total temperature, and flow angles, as well as an outlet condition imposing a static pressure profile that allows the simulation to reach a simplified radial equilibrium, is described within the context of a lattice Boltzmann approach. The treatment at the boundaries relies on the characteristic methodology to derive conditions which are non-reflecting in terms of acoustics and is also compatible with turbulence injection at the inlet. These properties are evaluated on test cases of increasing complexity, ranging from a simple 2D periodic domain to an S-duct stage with turbulence injection.

This paper is focused on experimental characterization of new active liners technologies in acoustic flow duct facilities (FDF). These measurements have been performed in the NLR (Netherlands Aerospace Center). Different configurations of passive and active treatments have been tested using the same grazing flow conditions (Mach 0.3) in an aeroacoustic test bench: 1 conventional liner (standard SDOF sandwich panels with honeycomb core) used as reference and a 2D active liner based on an array of electroacoustic absorbers. The grazing flow test campaign comprises acoustics and aerodynamics measurements to characterize the aeroacoustics flow conditions, the membrane behavior, the acoustic impedance and the resulting insertion loss.

International audience

International audience; High-dimensional data may often contain both numerical and categorical features, and in some cases features may be available as natural groups (repeated measurements, categories of features, ...). Clustering this kind of data raises several issues: how to simultaneously deal with numerical and categorical features? how to build meaningful clusters of the input entities? how to select the most informative features or groups of features for the clustering? In the k-means framework, one may rely on a penalised version of the between-cluster variance, and find both the best partitioning of the data, and the most informative features or groups of features. The present manuscript illustrates sparse k-means and group sparse k-means for mixed data, using the vimpclust package. The example provided on a small real-life dataset shows how feature selection may be directly combined with clustering, and provide a meaningful selection while preserving the quality of the clustering.

This study focuses on the influence of local moringa oil on the performances of lubricants obtained from a blend of dodecane and pristine or fluorinated carbon nanofibers at ambient temperature. The friction experiments have been evaluated using a reciprocating ball on a plane tribometer. Moringa oil is used as a bio-additive in lubricant formulations. First, the addition of a small amount of moringa oil to dodecane allowed a reduction of about 50% of the base oil friction performances. Second, a comparative study on the friction properties of their blends with pristine carbon nanofibers (CNFs) and fluorinated ones (CF0.85) was investigated, based on which they were intended to be used as solid lubricant additives. An adsorption effect of fatty acid molecules is observed in CNFs particles unlike CF0.85. Whatever was the percentage of moringa oil added, more promising results were obtained with CF0.85. Physicochemical characterizations of the tribofilms evidence the good antiwear properties of the lubricants.

International audience; A compressible Hybrid Lattice Boltzmann Method solver is used to perform a wall-resolved Large eddy simulation of an isothermal axisymmetric jet issuing from a pipe and impinging on a heated flat plate at a Reynolds number of 23 000, a Mach number of 0.1, and an impingement distance of two jet diameters. The jet flow field statistics, Nusselt number profile (including the secondary peak), and shear stress profile were well reproduced. The azimuthal coherence of the primary vortical structures was relatively low, leading to no discernible temporal periodicity of the azimuthally averaged Nusselt number at the location of the secondary peak. While local unsteady near-wall flow separation was observed in the wall jet, this flow separation did not exhibit azimuthal coherence and was not found to be the only cause of the thermal spots blue, which lead to the secondary peak in the Nusselt number, as stream-wise oriented structures also played a significant role in increasing the local heat transfer.

International audience; Abradable coatings are part of dynamic seals used in aircraft engines to reduce inter-stage leakages. However, during running, severe interactions may occur between the rotating blades and the coating of the casing leading to these parts getting damaged. Numerical approaches simulating these interactions could be a way to prevent these contact risks but they require both a characterization of the thermomechanical behavior of the abradable material and of the blade. This paper presents a quasi-static compression behavior study of a heterogeneous and porous AlSi-PE abradable used in the low-pressure compressor of aircraft engines. Combined with the results obtained for tests conducted at high compression strain-rates and extreme temperatures recently published in a previous paper (Skiba et al., in Dyn Behav Mater 6:213–223, 2020. 10.1007/s40870-020-00242-y), a thermoelastoviscoplastic bilinear constitutive law is proposed. This formulation allows the prediction of the macroscopic behavior of a heterogeneous and porous AlSi-PE abradable material from room temperature to 360 °C and with strain-rates ranging from 10–3 to 103 s−1.

My CIFRE thesis is a continuation of the theses conducted by Tsirizo Rabenoro (Rabenoro, 2015), Cynthia Faure (Faure, 2018), Florent Forest (Forest, 2021). The objective of this thesis is to develop a methodology to understand and highlight specific typologies of aircraft engine operation during reception tests carried out on test benches, and to help Safran Aircraft Engines' engineers in the design of their engines.From a theoretical point of view, the main challenge of this thesis is to model and explain the observed and unobserved physical phenomena using statistical methods and to interpret the cause using a (small) part of the explanatory variables. This work is therefore placed in the supervised and unsupervised framework, with a strong constraint, that of constructing interpretable models: we seek to indicate the contribution of each variable to the construction of the model, which is what we call measuring the importance of the variables. In addition, we seek to improve interpretability by constructing parsimonious models, i.e. such that the variables which do not contribute to their construction are excluded. In the unsupervised framework, methods for partitioning groups of observations, or \textit{clustering} methods, will be studied. The solution we are looking for must be able to be used in high dimension, must be interpretable and must be able to take into account the group structure of the variables. This type of method is known as sparse clustering methods. In the supervised framework, we propose methods modelling a phenomenon described by continuous (respectively categorical) variables, i.e. regression (respectively classification) methods. The same constraints are imposed as in the unsupervised case (handling high dimensional data, being sparse, indicating the importance of the variables, taking into account the group structures of the variables). Moreover, let us note that we want to model a physical phenomenon with the aim of explaining its principles and that we are therefore not (directly) interested in the predictive aspects. This point is extremely important because we will see that explaining a phenomenon and explaining the prediction of the associated model are two goals that can be contradictory.A specific feature of sparse algorithms is that they depend on a parameter to be adjusted (in the same way as clustering methods). Different values of this parameter give rise to different models and it is necessary to choose between all these models. Thus, it is essential to have an efficient model selection method. It must be emphasised that model selection is a major challenge in unsupervised clustering. Indeed, there is no universally accepted method to evaluate clustering results for the obvious reason that there is no ground truth against which the results can be compared. This is also the case when one is interested in variable selection (sparse models) and variable importance, even in the supervised framework, as one never has the set of variables and the variable importance defining the underlying phenomenon under study.; Mon travail de thèse CIFRE s’inscrit dans la continuité des thèses menées par Tsirizo Rabenoro (Rabenoro,2015), Cynthia Faure (Faure, 2018), Florent Forest (Forest, 2021). L’objectif de cette thèse est de développer une méthodologie pour comprendre et mettre en évidence des typologies spécifiques du fonctionnement des moteurs d’avion lors de tests de réception effectués sur des bancs d’essai, et d’aider les ingénieurs métier de Safran Aircraft Engines dans l’analyse des résultats.Du point de vue théorique, l'enjeu principal de ce travail de thèse est de modéliser et expliquer les phénomènes physiques observés et non observés à l'aide de méthodes statistiques et d'en interpréter la cause à l'aide d'une (petite) partie des variables explicatives. Ce travail se place donc dans le cadre supervisé et non supervisé, avec une contrainte forte, celle de construire des modèles interprétables: on cherche à indiquer la contribution de chaque variable à la construction du modèle, c'est ce que l'on appelle mesurer l'\emph{importance des variables}. En outre, on cherche à améliorer l'interprétabilité en construisant des modèles parcimonieux ou \emph{sparses} en anglais, c'est-à-dire tels que les variables qui ne contribuent pas à leur construction en soient exclues. Dans le cadre non supervisé, des méthodes de partitionnement de groupes d'observations, ou méthodes de \textit{clustering} en anglais, vont être étudiées. La solution que nous cherchons doit pouvoir s'utiliser en grande dimension, doit être interprétable et elle doit pouvoir tenir compte de la structure de groupes des variables. Ce type de méthode est connu sous le nom de méthodes de clustering sparse. Dans le cadre supervisé, nous proposons des méthodes modélisant un phénomène décrit par des variables continues (respectivement catégorielles), c'est-à-dire des méthodes de régression (respectivement de classification). On impose les mêmes contraintes que dans le cas non supervisé (gérer des données de grande dimension, être sparse, indiquer l'importance des variables, tenir en compte des structures de groupes de variables). Par ailleurs, remarquons que nous voulons modéliser un phénomène physique dans le but d'en expliquer les principes et donc que nous ne sommes pas (directement) intéressés par les aspects prédictifs. Ce point est extrêmement important car nous verrons qu'expliquer un phénomène et expliquer la prédiction du modèle associé sont deux buts qui peuvent être contradictoires.Une caractéristique propre aux algorithmes sparses est qu'ils dépendent d'un paramètre à ajuster (au même titre que les méthodes de clustering). Différentes valeurs de ce paramètre donnent naissance à différents modèles et il est nécessaire de choisir entre tous ces modèles. Ainsi, il est primordial de disposer d'une méthode de sélection de modèle efficace. Il faut insister sur le fait que la sélection de modèle est un défi majeur en clustering non supervisé. En effet, il n'existe pas de méthode universellement admise pour évaluer les résultats du clustering pour la raison évidente qu'il n'y a pas de vérité de terrain par rapport à laquelle les résultats pourraient être comparés. C'est aussi le cas lorsque l'on s'intéresse à la sélection de variables (modèles sparses) et à l'importance de variables, même dans le cadre supervisé, car on ne dispose jamais du \emph{vrai} ensemble de variables et des \emph{vraies} importances de variables définissant le phénomène sous-jacent étudié.

International audience; Aluminum (Al) coatings were deposited on 15CDV6 steel substrates using the electrophoretic deposition (EPD) process and pure propan-2-ol solvent. Key parameters in EPD process are the followings: solvent, additives, deposition conditions, electrical field, cell geometry. Herein, the influence of the applied electric field (0–60 Vcm−1) and the deposition time (0–20 min) was precisely studied as a function of the deposition rate. A uniform and porous Al particle coating was targeted and obtained. Control of the deposit thickness with conservation of the microstructure was possible over a large range of electric fields and deposition times. No thickness limitation was observed in the investigated range of deposition times and electric fields without the need of any added ionic species. Besides, voltage measurements showed that the electric field over the suspension during the EPD process remains high, allowing continuous migration of Al particles to the coated electrode. Overall, the EPD process turned out to be an efficient way to adjust the deposit thicknesses up to 200 μm.

The development of overgrown grains during sub-solvus annealing of a polycrystalline γ − γ ′ nickel-base alloy is investigated, with special emphasis on the primary γ′ precipitates. Quantitative analysis conducted on a large three-dimensional electron back-scattered diffraction dataset reveals a high fraction of γ′ precipitates on the annealing twin boundaries in the large grains. When observed in-situ in a scanning electron microscope, the formation of annealing twins is correlated to the crossing of γ′ precipitates by moving grain boundaries. The role of precipitates in the occurrence of growth accidents is discussed.

A benchmark dedicated to RANS-informed analytical methods for the prediction of turbofan rotor&ndash, stator interaction broadband noise was organised within the framework of the European project TurboNoiseBB. The second part of this benchmark focuses on the impact of the acoustic models. Twelve different approaches implemented in seven different acoustic solvers are compared. Some of the methods resort to the acoustic analogy, while some use a direct approach bypassing the calculation of a source term. Due to differing application objectives, the studied methods vary in terms of complexity to represent the turbulence, to calculate the acoustic response of the stator and to model the boundary and flow conditions for the generation and propagation of the acoustic waves. This diversity of approaches constitutes the unique quality of this work. The overall agreement of the predicted sound power spectra is satisfactory. While the comparison between the models show significant deviations at low frequency, the power levels vary within an interval of &plusmn, 3 dB at mid and high frequencies. The trends predicted by increasing the rotor speed are similar for almost all models. However, most predicted levels are some decibels lower than the experimental results. This comparison is not completely fair&mdash, particularly at low frequency&mdash, because of the presence of noise sources in the experimental results, which were not considered in the simulations.

The direct numerical simulation of a non-equilibrium turbulent heat transfer case is performed in a channel flow, where non-equilibrium is induced by a step change in surface temperature. The domain is thus made of two parts in the streamwise direction. Upstream, the flow is turbulent, homogeneous in temperature and the channel walls are adiabatic. The inflow conditions are extracted from a recycling plane located further downstream so that a fully developed turbulent adiabatic flow reaches the second part. In the domain located downstream, isothermal boundary conditions are prescribed at the walls. The boundary layer, initially at equilibrium, is perturbed by the abrupt change of boundary conditions and a non-equilibrium transient phase is observed until, further downstream, the flow reaches a new equilibrium state presenting a fully developed thermal boundary layer. The study focuses on the spatial transient phase, identifies the main non-equilibrium effects and contrasts these results with usual assumptions of equilibrium turbulent heat transfer. Mean and root-mean-square profiles of temperature and velocity, as well as the respective energy and momentum balances, are presented and discussed along with budgets of second-order moment balance equations for the enthalpy variance and the wall-normal heat flux. For several quantities, an equilibrium near-wall region is identified even near the leading edge while the boundary layer is still developing. Finally, the evolution of the turbulent Prandtl number along the channel flow is investigated and shows that it reaches equilibrium only further downstream.

International audience; Assessing the underlying structure of a dataset is often done by training a clustering procedure on the features describing the data. Inpractice, while the data may be described by a large number of features, only a minority of them may be actually informative with regard to thestructure. Furthermore, redundant features may also bias the clustering, whether one speaks of redundancy in the informative or the uninformativefeatures. The present contribution aims at illustrating two sparse clustering methods designed for mixed data (made of numerical and categoricalfeatures). The proposed methods summarise redundant features into groups, and select the most relevant groups of features only in theclustering procedure. The performances and the interpretability of the sparse methods are illustrated on a real-life data set.

International audience; This paper is a first attempt to make possible the computation of the cycling response of a complex elastoviscoplastic structure up to its possible limit cycle. For this, we try to couple the non-invasive global/local iterative coupling technique with the cycle-jumping method. The different issues regarding the method are discussed on the basis of a 2D examples.

Accurate fatigue lifetime prediction is a central element in many industries. Often, a damage tolerant approach is used to ensure that some given imperfections are safe. In this context, these lifetime predictions are frequently made using simplified crack geometries leading to costly conservatism. In particular for a closed crack front, a circular or elliptical crack enclosing the real defect is generally used leading to an underestimation of the real lifetime. Using an iterative perturbation method to simulate the fatigue growth of many tensile complexly shaped cracks, we show that in the absence of any additional complexity, (i) the front quickly becomes circular; (ii) its evolution with the number of loading cycles can be obtained analytically using a circular crack of same area as the original distorted defect. In practice, it means that replacing a complex crack front shape with this simple, analytically solvable configuration is an effective way to improve predictions and that it is not useful to invoke more complex shapes such as elliptical cracks. These take-home messages are illustrated by an example from the aviation industry.

International audience; The design and the grazing flow aeroacoustical characterization of a 2D active liner based on an array of electroacoustic absorbers are presented in this paper. The strategy stands on a pressure-based, current-driven digital architecture for impedance control with both local and non local architectures. A wind tunnel test rig is used for the in-flow experimental validation. The stability and robustness of the whole system are investigated as a function of flow velocity showing the efficiency of the proposed approached. The air flow slightly reduces the efficiency while maintaining the adaptability and the stability, and better performances are obtained with the non local control strategy.

The noise generated by the interaction of the turbulent wake of the fan with the Outlet Guide Vanes (OGV) is a major component of turbofan engine noise. The development of noise reduction concepts applied to the OGV is an intensive research topic, with some current effort dedicated to an active solution making use of piezoelectric actuators integrated within the OGV airfoils to control the surface impedance. In order to define target values for the impedance that would lead to significant noise reduction over a chosen frequency range, a numerical method is being investigated in the present work. The method relies on solving the linearized Euler’s equations in the frequency domain around an airfoil and introducing a velocity fluctuation representing the fan’s turbulent wake upstream of the airfoil. An impedance boundary condition can be applied to the airfoil surface, and optimal values could be found through a parametric study. This paper presents the first set of 2D simulations that were performed to assess the capability of the method to show the effects of an impedance on the airfoil acoustic response, by comparison with a rigid one.

In the field of composite materials, mesoscale modeling based on X-ray computed tomography are very widespread nowadays. This descriptive method requires image processing to identify the different objects within the material. In the present study, two different instance segmentation approaches are proposed: 1) a method based on Mathematical Morphology and 2) a Deep Learning one. Both methods are applied to determine the yarns paths and their envelopes. We succeed in both tasks on a dry 3D ply-to-ply angle-interlock fabric at low compaction level. In absence of manual labelling of the yarns envelopes, we manage to train a Deep Convolutional Neural Network (DCNN) on the pseudo-labeling provided by the morphological method, improving the latter and showing the potential of deep learning for image segmentation in this context, when yarns cross-sections are labelled by distance functions. At a higher level of compaction, we also manage to recover the yarn paths thanks to deep learning.

International audience; Following an extensive experimental campaign carried out on a cold-rolled Ti-6Al-4V titanium alloy grade within a wide range of (low and high) strain rate, temperature and loading cases (tension, compression, shear), an advanced constitutive model is built within the irreversible thermodynamics framework following the internal variable procedure. Some key aspects such as texture-induced anisotropy (which manifests through orthotropy, kinematic hardening and tension/compression asymmetry), rate dependence, strain hardening and thermal softening are consistently accounted for. Material constants are calibrated using the software Zset and the model has been implemented in the commercial finite element computation code LS-DYNA. The latter has been employed to conduct numerical simulations at the structural scale for evaluating the predictive performances of the numerical version of the model

A composite fan stage representative of a modern UHBR architecture has been investigated experimentally on a novel test facility at Ecole Centrale de Lyon. These measurements show indications for strong overloading of the tip region resulting in extensive blockage of the blade passage. The performance of the fan is analyzed with extensive instrumentation including radial profiles upstream and downstream of the rotor. Unsteady pressure measurements help to interpret the flow structure in the tip region. The results are presented across a range of operating points on the design speedline. At the stability limit, the machine suffers from Non-Synchronous Vibrations which result from small scale aerodynamic disturbances propagating between the leading edges. A detailed analysis on the occurring waveforms is presented for two operating speeds. In order to further analyze the observed phenomena, a numerical study has been conducted using the RANS solver elsA. The results of steady calculations are discussed in comparison with the detailed experiments. Unsteady simulations near the stability limit accurately predict the aerodynamic disturbances observed during NSV. The obtained results are unusual for typical state-of-the-art transonic fans, as they show the same behavior as high-pressure compressor front stages, dominated by blockage caused by tip leakage flow. Even though flutter is not observed, the observed Non-Synchronous Vibration mechanism is a critical aeroelastic phenomenon which is of great interest for future designs of low speed fans.

This article revisits the fundamental problem of parameter selection for Gaussian process interpolation. By choosing the mean and the covariance functions of a Gaussian process within parametric families, the user obtains a family of Bayesian procedures to perform predictions about the unknown function, and must choose a member of the family that will hopefully provide good predictive performances. We base our study on the general concept of scoring rules, which provides an effective framework for building leave-one-out selection and validation criteria, and a notion of extended likelihood criteria based on an idea proposed by Fasshauer and co-authors in 2009, which makes it possible to recover standard selection criteria such as, for instance, the generalized cross-validation criterion. Under this setting, we empirically show on several test problems of the literature that the choice of an appropriate family of models is often more important than the choice of a particular selection criterion (e.g., the likelihood versus a leave-one-out selection criterion). Moreover, our numerical results show that the regularity parameter of a Matérn covariance can be selected effectively by most selection criteria.

National audience; La prédiction de l’usure des composants des turbomachines est un enjeu majeur pour les constructeurs aéronautiques dans une optique d’extension de leur durée de vie. Ce travail s’intéresse aux paliers lisses, ou douilles, situés au cœur du système de calage variable des aubes de redresseur (Variable Stator Vane (VSV)). Ces pièces d’usure fortement sollicitées thermo-mécaniquement assurent un guidage en rotation de l’axe des aubes, ce qui permet d’ajuster en continu l’incidence du flux d’air. Une usure excessive peut entraîner des coûts de maintenance élevés, des baisses de performances du moteur et, dans les cas extrêmes, des risques de pompage du compresseur.Différentes approches numériques de prédiction de l’usure ont été développées antérieurement : un modèle analytique 2D générique capable d’estimer des usures sous des sollicitations cycliques, et des modèles éléments finis 3D [1] permettant d’obtenir les niveaux de pression et d’usure dans les zones de contact. L’approche 2D présente l’avantage d’être rapide et de donner des résultats satisfaisants par rapport à des machines d’essais d’usure, mais nécessite des recalages. L’approche 3D apporte une précision accrue mais est néanmoins actuellement trop coûteuse en vue de simuler des milliers de cycles [2].L’enjeu de ce travail est de développer un modèle numérique de prédiction des efforts de contact vus par les douilles du système VSV en environnement moteur de manière fiable et en des temps de calculs adaptés au pré-dimensionnement en bureau d’études. Le modèle numérique est placé sous l’hypothèse des solides rigides indéformables où seules les zones de contact se déforment de manière élastique selon une loi de comportement définie. Il s’agit de la résolution d’un problème cinématique à forte non-linéarité appliqué à un système multi-corps soumis à des chargements variables et combinés. Ce problème de contact est tout d’abord traité en 2D avec un chargement simplifié de l’aube. Le problème est décrit analytiquement selon une démarche classique de passage local-global et résolu par la méthode de Newton-Raphson. L’écriture analytique du problème permet d’identifier les paramètres importants de modélisation. La maîtrise de la configuration initiale du redresseur en fonction des jeux et des positions relatives des pièces est essentielle à la convergence de l’algorithme de résolution. Afin de prendre en compte de manière plus complète le chargement et les géométries considérées, une extension au cadre tridimensionnel est proposée.La validation des modèles est réalisée par la confrontation des résultats obtenus à l’aide d’un outil de dimensionnement développé par Safran Aircraft Engines permettant d’estimer les pressions de contact maximales. L’objectif sera ensuite d’aboutir à un calcul d’usure sur des cycles de vol complets dans le but d’estimer la durée de vie des douilles du système VSV. La validation de ce modèle en perspective s’appuiera sur des résultats d’essais expérimentaux ainsi que de données de retour de flotte.[1] Y. Xue, J. Chen, S. Guo, Q. Meng, J. Luo (2018) « Finite element simulation and experimental test of the wear behavior for self-lubricating spherical plain bearings ». Friction, 6(3), pp. 297-306.[2] C. Paleczny, A. Selvi, E. Gabellini (2003) « 3D Kinematics Study with SAMCEF MECANO Motion on a nozzle of a turbojet engine - Pre and post- processing in SYNAMEC», SAMCEF User Days. France.

International audience; VHCF properties of the Ni-based single crystal superalloy AM1 with pre-deformation have been investigated at 1000 °C. Pre-deformed material did not present life debit under fully reversed conditions (R = −1). Pre-deformation decreased VHCF lifetime under high mean stress conditions (R = 0.5) with a shift from internal to mode I surface crack initiation. The pre-deformed material has microstructure coarsened bands lying on the former {1 1 1} slip planes, facilitating microstructure shearing during VHCF at R = 0.5. Slip traces increased surface roughness, causing a surface recrystallization and initiation of the fatal crack from the surface. Rejuvenation heat treatment after pre-deformation can restore the microstructure and the fatigue life.

International audience; Fan Blade Flutter is an aeroelastic instability which may occur during the operation of a jet engine, depending on the working conditions of the fan stage. It finds its origins in some various mechanisms, including the impact of the environment of the fan stage, which may play an important role in the stability limits due to acoustic effects. If not properly taken into account, flutter can lead to an anticipated ruin of the fan stage as the fluid keeps on giving energy to the structure. However, nonlinear phenomena may appear at some high vibratory amplitude of the blades, resulting in energy dissipation of the aeroelastic system. Blade roots friction is an example of such a case : by dry-friction dissipation at blade roots, a limitation of the vibratory amplitude may be reached, the so-called Limit Cycle Oscillations (LCO), within the unstable regions of the operating domain predicted with a usual linear structural modelling. By taking these nonlinear effects into account, it is then possible to define more precisely the stability lim-its of the fan stage. In this paper, we describe a methodology to predict LCO induced by blade roots friction, including acoustic effects on stability. First, assuming a linear behaviour of the structure, the stability limits of the fan stage are described using the cyclic symmetry hypothesis and Computational Fluid Dynamics (CFD). Acoustics effects are taken into account by includ-ing the fan inlet environment in the numerical model. Then, a nonlinear structural model of the blade is used to compute nonlinear complex modes in order to check for the presence of a LCO. To do so, a reduced model of the fluid response to the blade movement is used. To sum up, this work intends to establish a methodology to be used in an industrial context for the analysis of the nonlinear stability of the fan stage, including LCO phenomena.; Le flottement de soufflante est une instabilité aéroélastique pouvant apparaître lors du fonctionnement d'un turboréacteur, selon le régime de fonctionnement de celui-ci. De nombreux mécanismes physiques sont à l'origine du flottement, parmi lesquels on peut compter les interactions acoustiques de la soufflante avec son environnement. Ces interactions peuvent jouer un rôle important dans la stabilité aéroélastique du système. En cas de mauvaise prédiction du flottement, celui-ci pourrait conduire à une ruine prématurée de la soufflante, le fluide fournissant de l'énergie à la structure à chaque période de vibration. Toutefois, des phénomènes non-linéaires sources de dissipation énergétique sont susceptibles d'apparaître au-delà d'une certaine amplitude de vibration des aubes. Les frottements entre l'aube et le disque comptent parmi ces phénomènes : en réalisant de la dissipation énergétique par frottement sec dans le contact aube/disque, il est possible d'observer un état de saturation des amplitudes de vibration (appelé cycle limite) à l'intérieur des zones d'instabilité prédites dans le cadre d'une structure linéaire. En prenant en compte ces effets non-linéaires, il est donc possible de définir plus précisément les limites de stabilité de la soufflante. Dans cet article, une méthodologie pour prédire les cycles limites induits par le contact aube/disque est présentée, en considérant les phénomènes d'interaction acoustique entre la soufflante et son environnement. En considérant une structure linéaire dans un premier temps, les limites de stabilité aéroélastique de la soufflante sont déterminées en utilisant une hypothèse de symétrie cyclique et en réalisant un calcul CFD. Les phénomènes acoustiques sont calculés en intégrant dans le modèle utilisé l'entrée d'air de la soufflante. Par la suite, un modèle non-linéaire d'aube est mis en place pour calculer un mode complexe non-linéaire afin de déterminer la présence d'un cycle limite. Un modèle réduit du comportement du fluide est intégré au modèle d'aube non-linéaire. En résumé, cette étude a pour objectif de mettre en place une méthodologie qui sera par la suite appliquée dans un contexte industriel pour l'analyse non-linéaire de la stabilité de la soufflante.

A Large-Eddy Simulation of the tip leakage flow of a single airfoil is carried out. The configuration consists of a non-rotating, isolated airfoil between two horizontal plates with a gap of 10 mm between the tip of the airfoil and the lower plate. The Mach number of the incoming flow is 0.2, and the Reynolds number based on the chord is 9.3 × 105. The objective of the present study is to investigate the best way to compute both the aerodynamics and acoustics of the tip leakage flow. In particular, the importance of the inflow conditions on the prediction of the tip leakage vortex and the airfoil loading is underlined. On the other hand, the complex structure of the tip leakage vortex and its convection along the airfoil was recovered due to the use of a mesh adaptation based on the dissipation of the kinetic energy. Finally, the ability of the wall law to model the flow in the tip leakage flow region was proven in terms of wall pressure fluctuations and acoustics in the far-field.

In the present paper, large eddy simulations are performed to study two different mechanisms of Fan/OGV broadband noise: airfoil self-noise and turbulence interaction noise. Firstly, the current study focuses on the prediction of airfoil self-noise from a thin plate with a sharp trailing edge and a chord-based Reynolds number of the order of 106. The boundary layer is tripped to trigger transition to turbulence, and a parameter study is performed to study the influence of the near-wall modeling, grid topology and refinement in the near-wall and wake regions, the spanwise domain extent, and the tripping method. Empirical and analytical models, as well as available DNS data are used for validation purposes. Secondly, the interaction noise from a thin plate impinged by an incoming synthetic turbulent flow is studied. For both cases, far-field acoustic spectra are compared to Amiet’s models for leading and trailing edge noise showing a good agreement.

International audience; Digital Volume Correlation is an appealing technique for establishing a novel comparison (differentiating) tool based on full field measurements of structured materials such as 3D woven composites. This approach provides a quantifiable description of the weaving distortions on woven composites. It also offers applications ranging from the quantification of the influence of different weaving conditions, up to nondestructive testing of composite parts. The method is validated on real 3D woven composite samples revealing both "metric differ-ences" (deformations of a textile with respect to reference one) and "topological differences" (such as the occurrence of missing yarns).

A methodology to include flow–acoustic interactions in a Helmholtz solver is proposed and used to quantify their impact on the thermoacoustic modes of a real industrial combustor. This methodology ...

International audience; We present an experimental investigation of an original configuration where the laminar flame shape bifurcates when submitted to flowrate modulations. This combustor burns a gaseous methane-air mixture injected vertically in a downward direction. The global equivalence ratio is equal to 0.91, the overall heat release of around 800 W. By increasing the frequency of the mass flow rate modulation from 70Hz to 85Hz, the flame bifurcates from an anchored flame into a lifted flame. The precise switch frequency depends on temperature and flow rates. Some forcing cases lead to bi-stable flames presenting a random transition between the two shapes for a given mass flow rate and modulation frequency. Experimental data stand on high-speed optical diagnostics processed with optical flow and orthogonal decompositions. POD-based analysis reveals different dynamics depending on the flame's average position, particularly a transverse flow motion for detached flames.

International audience; In this paper, experimental results of broadband noise reduction in an acoustic flow duct are presented. An active liner composed of an array of electroacoustic absorbers is used. The control law is based on the pressure-based, current driven digital architecture for impedance control with a local control strategy. A wind tunnel test rig named Caïman has been used for the experimental validation. The results confirm the adaptability and the stability of the whole system with the local control strategy. The air flow slightly reduces the efficiency while maintaining the adaptability and the stability.

In axial compressors, shrouded stator cavity flows are responsible for performance degradation due to their interaction with the power stream. The present paper aims at exploring the possibility of employing a single stage high pressure axial compressor as a test vehicle for cavity flows investigations. In a first step, the robustness of the adopted RANS approach is tested against experimental data on the closed-cavity baseline configuration (i.e. no downstream-to-upstream recirculation). In a second phase, the effect of different hub cavities layouts of different levels of realism is numerically investigated. The focus is set on the representativeness of a closed cavity configuration with injection. The cavity flow topology and impact on the overall performance are considered in the analysis. At its final extent, this paper provides numerical and experimental guidelines for the robust assessment of cavity flows topology and performance effects.

International audience; Lanthanum phosphate, due to its interesting thermal and mechanical properties is a widely studied material for refractory applications. Sintering processes have already been proposed to densify this material and drive its microstructure. Inspired by recent progress on low temperature sintering, we investigate a low temperature Spark Plasma Sintering (LowT-SPS) using hydrated precursor. First, lanthanum phosphate thermal behaviour was studied using TGA/DTA and XRD analysis on various heat-treated powders. Their SPS behaviour were explored by in situ dilatometry measurements. As hydrated precursor showed a low temperature densification, samples were sintered at temperatures from 160 °C to 350 °C under 400 MPa. Even if dense and nano-scaled microstructures were obtained, a residual hydration was observed. Finally, a well densified and fine-grained monazite type lanthanum phosphate was obtained at 550 °C and under 200 MPa. Its mechanical properties are then compared to conventional and Spark Plasma Sintered materials.

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International audience; This research deals with the development of a new semi-analytical model for composite-metal eccentric tensile bolted joints of casing structures. The model is based on a direct stiffness method considering structural elements, where flanges are modeled with axisymmetric shell elements, the bolt with a beam element and contact behavior with a specific number of compressive linear springs, identified by means of convergence tests. A hybrid element is added in order to set the link between flange and bolt. Since the model is intended for the preliminary design of bolted joints including composite materials, we propose an extension of the stiffness matrix formulation of isotropic axisymmetric shell elements to orthotropic ones. This formulation is validated with respect to a 2D FE axisymmetric model. Then, a semi-analytical model is established for a casing assembly approximately corresponding to a real casing of an aircraft engine. Design criteria such as normal stress on the bolt, bending moment and axial load at a specific section of the composite flange, given by the semi-analytical model are compared to a 3D FE computation. Results are in very good agreement and the semi-analytical model offers considerable time saving where the computation time was 120 times faster than the 3D FE model. Moreover, relative error does not exceed 2% for normal stress of the bolt and 9% for the flange bending moment which is satisfactory in a preliminary design approach. However, due to the formulation of hybrid element, less accurate results were observed for axial displacement of the composite flange and it would be advisable to consider a new formulation. This will be detailed in future work. Finally, a parametric study showed the ability of the semi-analytical model to predict the physical behavior of joints and proved its usability for running optimization studies and its reliability as a preliminary design tool.

International audience; The oxidation behavior of a nickel-based superalloy was investigated from 650 to 1,000 °C for up to 1,000 h in air. Samples with thicknesses ranging from 20 to 500 µm were used to document the evolutions of microstructure and oxide scale. Oxidation products and subsurface evolution of the metal microstructure were characterized by XRD and EDS analyses. Local breakaway was observed after 600 h at 800 °C and after 100 h at 900 °C due to the full consumption of Cr from the alloy. Room temperature tensile tests were performed on aged and pre-oxidized specimens with thicknesses ranging from 20 µm to 500 µm, at 800 °C. The results were compared to tensile tests performed on the as-received metallurgical state. Both size effects due to sample thickness reduction and to sample thickness/pre-oxidation width ratio were examined onto the mechanical behavior. Interestingly, the formation of TCP phases, the oxide layer and the subsequent Cr-depleted subsurface region from either aging or oxidation treatments impairs the mechanical integrity. Bulk and subsurface regions are impacted, especially for tens-of-micrometer thin samples.

International audience; Abrasive water jet (AWJ) milling process, though being an effective alternative to conventional machining for difficult-to-machine materials, induces abrasive embedment which is an issue for repair application by structural bonding. In this context, the effectiveness of cleaning Ti6Al4V specimens by Plain Water Jet (PWJ) post AWJ milling is studied. For this, Ti6Al4V specimens are milled by AWJ process with varying parameters to create several levels of surface quality and contamination. Different characterization techniques have been used to perform a multi-scale analysis of the machined surfaces and surface quality has been quantified by an innovative criterion called “crater volume” (Cv). Then the specimens are subjected to PWJ cleaning operation (using a single set of parameters chosen after preliminary study). Finally, surface texture analysis and contamination quantification is performed and compared with the AWJ milled surfaces. The results revealed that PWJ cleaning reduced the surface contamination by 65% without any significant change in Cv, surface texture and topology. However, it was found that it was impossible to dislodge deeply embedded particles. The comparison of pre and post-cleaning contamination levels also revealed that PWJ cleaning process efficiency depends on the AWJ milling parameters (mainly pressure).

An extensive experimental campaign was run to investigate the influence of the loading direction, stress state (triaxiality ratio ranging from −0.5 to 1), and strain rate (from 10−3 to 1.5x103s−1) on the ductile fracture of Ti-6Al-4V titanium alloy. Microscopic and macroscopic observations provided some insight into the shear-driven or micro-voiding-controlled damage mechanisms prevailing at low and high triaxiality ratios, respectively. Numerical simulations were run to determine the local loading paths to fracture in terms of plastic strain as a function of stress triaxiality ratio and Lode parameter. The ductility was found to be anisotropic, but only weakly dependent on the strain rate in the considered range. The anisotropy in ductility was different in tension (maximum along DD) and in compression (maximum along ND). The fracture strain decreased with the absolute value of the triaxiality, with a maximum close to zero. No clear correlation with the Lode parameter was found.

International audience; This paper presents a coupled approach able to describe γ precipitation evolution and associated yield strength after various heat treatments in Inconel 718 alloy. The precipitation state is modeled via the implementation of classical nucleation and growth theories for plate-shaped particles. The precipitation model is validated through small-angle neutron scattering and transmission electron microscopy experiments. The precipitation size distribution serves as an input parameter to model the yield strength using a micromechanical model based on shear and bypass mechanisms accounting for the particular shapes of the precipitates. Results are in good agreement with measured yield stresses for various precipitation states. A complete simulated TTT diagram of the γ phase with the associated yield strength is proposed. The coupled model is finally applied to a series of non-isothermal treatments representative of welding (or additive manufacturing) from the peak aged state.

International audience; Acoustic liners for future aircraft engines will have to achieve contradictory goals: maintaining a strong absorption at low frequencies while reducing the treated surface area and liner thickness. The purpose of this paper is to review several concepts in the development of transducer-based acoustic liners. The use of transducers enables the coupling of several physical processes: acoustics, mechanics and electronics. As shown in previous work, this can lead to highly efficient devices enabling low frequency absorption with thin liners. The present work aims at better understanding the key phenomena governing acoustic absorption and frequency range available for a given liner thickness. This can be done by adding a thin mechanical structure with thin slits. These slits are cut in the structure and form an acoustic short-cut between the front and the back of the device. Coupled with a back- end cavity, it has been shown that the results of adding thin slits could be beneficial in terms of acoustic absorption, bandwidth and frequency shifting. In order to study those phenomena, the Transfer Matrix Method and the Finite Element Method will be used. These prediction methods will be compared and some experimental results will be presented in order to validate the models and better understand the effects of fluid inertia and visco-thermal losses in the slits.

International audience; Homogeneous open-cell materials have been extensively studied for broadband sound absorption applications. However, their absorption is not perfect over the frequency space and their performances in the low and medium frequencies is limited by their intrinsic loss and the treatment?s thickness. Introducing and optimizing a gradient of properties through a porous material?s thickness is a powerful way to increase their absorption over a large frequency range or to shift their low frequency limit of effectiveness. This work introduces a numerical method for the optimization of the properties of any type of porous material to obtain perfect absorption under normal incidence. This design tool is applied, as an example, to a fibrous medium composed of a periodic arrangement of ordered unit cells, that can be 3D printed. The link between the geometry of the porous medium and its effective acoustic properties is made numerically by means of multiscale asymptotic method and experimentally thought an inverse characterization process. A non linear conjugate gradient algorithm creates the best continuous micro-geometric gradient for maximal absorption coefficient over a targeted frequency range. Numerical and experimental results show that optimally graded material has a near perfect absorption over a large frequency range, or that its perfect absorption peak is shifted to lower frequency. The acoustic performances of the optimally graded material are compared to that of homogeneous and monotonically graded material.

In this paper, the contact problem between a cylinder and a half-space with a crystalline anisotropic behavior is solved. The model is based on semi-analytical methods to solve a three dimensional contact problem. A numerical technique based on a Voronoi tessellation is implemented using the Eshelby's equivalent inclusion method to account for the effect of the material microstructure on the contact pressure distribution and subsurface stresses. Fast Fourier Transforms (3D and 2D) are used to reduce the computation cost of the simulations. An application of this method to compute the scatter in fatigue life of rolling element bearings is also presented. Three different critical stresses are used in the Lundberg-Palmgren equation and results are compared in term of Weibull plot slope.

Different polytypes of SiC are described and predicted in literature. Here, we report the first occurrence of an orthorhombic 6O-SiC polytype as rock-forming mineral in the nickel laterite mine of Tiebaghi (New Caledonia). This new class of SiC crystallizes in the space group Cmc21 with 12 atoms per unit cell [a = 3.0778(6) Å, b = 5.335(2) Å, c = 15.1219(6) Å, α = 90°, β = 90°, γ = 120°]. The density of 6O-SiC is about 3.22 g/cm3 and the calculated indirect bandgap at room temperature of 3.56 eV is identical to 6H-SiC. Our results suggest that 6O-SiC is the intermediate state in the wurtzite to rocksalt transformation of 6H-SiC.

A detailed study of ignition mechanisms in aeronautical burners is presented. Even in global lean conditions, liquid fuel injection leads to a strong stratification of the mixture and the whole range of equivalence ratio may be encountered in the burner. The observation of ignition sequences in a representative lab-scale combustion chamber, from both experiment and numerical simulation, reveals a variety of scenarios leading to success or failure. In particular the occurrence of quenching events after the successful creation of a kernel flame is a key mechanism for the outcome of the sequence. A first analysis leads to a classification of ignition/extinction scenarios, based on similar time evolutions and trajectories of the flame kernel. It is found that this classification is much dependent on the sparking location. Then a deeper analysis allows to decompose all scenarios in a succession of more simple, generic mechanisms which are independent of the geometry. This decomposition is a useful tool to describe, understand and predict ignition sequences, being successful or not, in any combustion chamber geometry.

International audience; The present manuscript tackles the issue of variable selection for clustering, in high dimensional data described both by numerical and categorical features. First, we build upon the sparse k-means algorithm with lasso penalty, and introduce the group-L1 penalty-already known in regression-in the unsupervised context. Second, we preprocess mixed data and transform categorical features into groups of dummy variables with appropriate scaling, on which one may then apply the group-sparse clustering procedure. The proposed method performs simultaneously clustering and feature selection, and provides meaningful partitions and meaningful features, numerical and categorical, for describing them.

This paper investigates the use of a Vortex Lattice Method to simulate tip-leakage flow with small computational effort. The module PyLiSuite presented in the present paper has been completely developed from scratch and is validated on two-dimensional and three-dimensional basic cases. The validation lies on the estimate of the lift coefficient computed from the circulation given by the module. The capabilities of PyLiSuite regarding tip-leakage flow are gauged in comparison with novel experimental measurements on a single blade with an adjustable gap. The results show a good prediction of the shape and size of the tip-leakage vortex for large tip gaps. Differences in the position and the deficit of static pressure in the core of the vortex are noted. The future improvements on the module concern the influence of viscosity to be accounted for and the computation time which could be shortened.