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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.

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.

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.

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.

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.

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).

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.

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; 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.

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.

International audience; 2D and 3D numerical permeability of fibrous media are studied based on Stokes and Stokes-Darcy flows simulated by a Finite Element Modelling (FEM). A monolithic approach stabilised by an Algebraic Sub-Grid Scale (ASGS) method and implemented in a FEM software (Z-set) is used. The 2D geometries have been generated according to ideal arrangements and 3D geometry are representative of a highly anisotropic ply-to-ply interlock unit cell. For both geometries, yarns have been first considered as impermeable, and then the same calculations have been conducted with various intra-yarn permeabilities. The main novelty of this work is to address the dual-scale permeability problem with a Stokes-Darcy modelling so as to override numerical strategy robustness issues. An empirical law has been proposed to link the effective permeability to the intra-yarn one : this allows to generalise previously established results to real 3D materials with complex structures. An intra-yarn permeability threshold (asymptotic value) from which yarns can be considered impermeable has also been highlighted. This study underlines the importance of unit cell definition when it comes to computing permeability of 3D structures numerically generated, especially non-periodicity issues. Two methods to decrease the impact of preferential flows are then discussed.

International audience; Adhesive joints are widely developed in aeronautic and automotive design applications. When subjected to impact loading, the rate sensitivity of adhesively bonded joints becomes a subject of interest. In this study, the Mode I fracture behavior of an aeronautic adhesive is characterized. Double Cantilever Beam tests are performed under various displacement rates. A new protocol is developed to accurately measure the fracture toughness. The corrected beam theory with effective crack length is extended to measure the energy release rate during the crack growth stage. The main advantage of this new protocol is to accurately determine the crack speed without a direct experimental error. The validity of the analysis scheme is discussed considering dynamic effects. The crack speed measurement is validated by comparison to a conventional camera monitoring. The crack velocity is found to decrease with the crack length, while the fracture toughness remains unchanged.

International audience; The design of reliable structures and the estimation of the residual fatigue life of industrial parts containing flaws or cracks rely on our ability to predict the propagation of fatigue cracks. Whereas in industrial component cracks might have a complex path due to geometry and loading, lab experiments used for identifying crack propagation law are often in pure mode I. The paper presents a synthesis of an experimental benchmark performed in the context of a French national research network. A sample has been designed to produce mixed-mode crack propagation and variation of small scale yielding conditions. Two geometries and two maximum load levels are defined for the two tested materials: a stainless steel and an aluminum alloy. Around ten participants performed experiments using their usual instrumentation. Among the eight possible parameter sets, three are selected for which detailed results are presented. A satisfying overall agreement is obtained. But, some discrepancies are evidenced due either to limitations of the instrumentation or simply because from one lab to the other the applied load is not exactly the same. It is thus concluded that one of the most important issue is boundary conditions, which is confirmed by numerical simulations.

Mastering physical phenomena taking place in aircraft engines is a key objective for manufacturers. Contact interfaces with dry friction in bladed-disks allows decreasing the dynamic response of the structure. Moreover, friction in the interfaces can induce fretting wear, decreasing even more the dynamic response. Combining harmonic balance method to study the dynamic behaviour, and a semi-analytical contact simulation method, allows to study the dynamic behaviour of the structure considering wear and dry friction in the interfaces. Furthermore, this method provides a computationally efficient tool to study the contact interfaces behaviour with accuracy and the dynamic response of the non-linear system.

International audience; An experimental investigation of the noise radiated by a ducted high pressure flow discharge through diaphragms and perforated plates is carried out for a large range of subsonic and supersonic operating conditions (Nozzle Pressure Ratio (NPR) from 1.2 to 3.6). A parametric study of the geometrical parameters is also conducted to characterize their influence on the acoustic radiation. This covers configurations from single diaphragms to multi-perforated plates with variable hole diameters and arrangements that are placed inside a cylindrical duct. Compared with the free discharge analysed in a first part of the study (perforated plates placed directly at the output of the duct), the discharge into a duct, which is closer to the practical applications, generates strong acoustic modifications. As expected, the broadband noise is disturbed by strong modulations due to acoustic resonances in the output duct (longitudinal resonances and transversal duct modes). However, as in the free configuration, a strong effect of the plate geometries on the mixing noise is observed, allowing to adapt or reduce this source. In particular, the increase of the ratio between the perforation spacing and the perforation diameter allows reducing the maximum amplitude of the mixing noise. Compared to the free-field discharge, the Sound Pressure Level (SPL) in the ducted configuration is on average proportional to the 6-th power of the velocity instead of the 8-th power. Moreover, there are two dominant frequency humps in the sound spectra. The low frequency one is characterized by a constant Helmholtz number, suggesting that the sound is shaped by the duct geometry, whereas the high frequency one is characterized by a constant Strouhal number suggesting that the sound is directly generated by the flow. Finally, for supersonic operating points, the screech radiation appearing with diaphragms in the free configuration is suppressed when the output duct is added but new high amplitude and low frequency tones appear for the largest diaphragms and perforated plates. These lines are due to a coupling between normal shock oscillations and longitudinal resonances.

International audience; Small blade-casing clearances in aircraft engines reduce fuel consumption but increase the risks of contact interactions due to working conditions. These interactions cannot be avoided but their effects can be reduced by using abradable linings thermally sprayed inside the casings. To investigate these interactions and the mechanisms of wear deriving from them, a specific ballistic bench has been developed in order to perform representative tests of the low-pressure compressor environment (up to 270 °C) and enabling only one interaction between an aluminum-based abradable sample and a Ti6Al4V tool. Contact forces and incursion depth were measured for a nominal depth of cut of 100 μm and at a mean velocity of 125 m/s. It has been shown that the temperature affects mainly the abradable behaviour under these high-speed interactions. In addition, the wear that is resulting at high temperature is driven by a process that can be very different from the low temperature case.

International audience; An evaluation of fatigue property determination using numerical simulation is presented for a variant of the staircase procedure with the reuse of unbroken specimens. The simulated staircases are analysed with the Maximum Likelihood Estimation method to obtain the fatigue parameters: the median fatigue limit and the coefficient of variation. For each condition, 1000 staircases are simulated and evaluated to obtain a statistical study. The gain on the fatigue scatter uncertainties with the variant procedure is highlighted. However, an assumption on the damage accumulation must be stated when reloading specimens up to their failure. Thus, staircases are simulated with damage consideration.

International audience; This study presents nozzle and cowl shape optimizations for turbofan separate-jet engines. The main objective of this work is to implement an optimization process including a detailed Computer-Aided Design (CAD) model and improve nozzles propulsive performance, taking into account the complete geometry of the aft-engine body. Integrating detailed CAD models in optimization workflows remains challenging, that is why an original approach is considered. This method uses expert knowledge to reduce dimensionality and enables to compute sensitivities with finite differences. The results illustrate the interest of this innovative industrial optimization approach for the design of turbofan nozzles.

International audience; The goal is to study the influence of abrasive water jet (AWJ) machining parameters (jet pressure, traverse speed and scan step) on the cutting depth of 3D woven Carbon Fibres Reinforced Polymer (CFRP) composite. The original material linked to this non-conventional milling process has not been treated yet. The depths of cut were measured and characterized as a function of the machining parameters. Finally, two prediction models for the cutting depth are proposed and validated experimentally.An increase in cutting depth with the pressure and a decrease as the traverse speed and/or the scan step increase were observed. The first prediction model, based on the pocket depth measurements, has a mean error of 5%. However, the error increases (up to 23%) when the pocket becomes shallow (lesser than 1 mm). The second prediction model, based on the algebraic sum of elementary passes modelled with Gaussian bells, shows at first a mean error of 12%. A correction was performed depending on the erosion regime piloted by the depth of the elementary trench constitutive of the pocket. This enhancement, performed thanks to the primary jet diameters measurements with high speed camera, has improved the second model with a mean error of 5% (error

Tensile and fatigue life variabilities are investigated for new-generation single crystal Ni-based superalloys: the 3rd generation CMSX-4 Plus, the 6th generation TMS-238, and a newer Ni-based superalloy, TROPEA containing Pt. Consistently from the results of previous research, very high cycle fatigue (VHCF) properties at the chosen condition of T = 1000 °C/Re = −1/f = 20 kHz are mainly influenced by the solidification/homogenization pore size and position. TROPEA alloy has the best low cycle fatigue (LCF) life among all tested alloys at 650 °C and Rσ = 0.05/f = 0.5 Hz. To better understand the influence of chemical composition on the LCF endurance, tensile properties were investigated using nine different single crystalline alloys at 650 °C with the strain rate of 5 × 10−4 s−1. The yield stress is directly affected by the chemical composition of the Ni-based superalloys, and alloys with high contents of Ti and Ta have a higher yield stress, due to an increased shearing resistance of γ′ precipitates. Hence, the yield stress is the main control parameter of LCF at the selected condition. No influence of chemical composition on VHCF life durability has been observed, in good agreement with previous studies.

International audience; This study aims at evaluating the feasibility and the accuracy of a Wall-Modeled Large Eddy Simulation of an actual aeronautical multistage axial high-pressure compressor. The computational domain is composed of 37 blades and a geometrically complex recirculat-ing cavity. The numerical method TurboAVBP is able to handle such a technical challenge thanks to its unstructured and massively parallel approach as well as dedicated rotor-stator interface treatments. The influence of grid resolution, from less than 100 millions to more than 1 billion of cells, is particularly evaluated. The intermediate grid correctly predicts the global aerodynamic performances up to the lowest mass flow rate regime. In comparing with time-resolved measurements, the finest grid is shown to accurately predict flow within rotors, especially in their tip regions that are critical for performances and stability of the whole compressor.

International audience

This work deals with sound generation and transmission in a fan stage. The study is done on a subsonic Fan stage and interaction noise between the fan wakes and the Outlet Guide Vanes (OGV) is considered. For this purpose, the Linearized Euler Equations (LEE) are solved with a steady axisymmetric flow. The acoustic sources are modelled by a scattering approach. Numerical simulations are carried out in an unwrapped cylindrical layer using a high-order finite volume solver. In order to explicitly take into account the moving fan blades into the propagation medium, a high-resolution sliding mesh technique is used. The simulation results, which highlight the screen effect of moving fan blades on fan/OGV interaction tones, are consistent with analytical literature. Xesús Nogueira and Luis Ramírez acknowledge the support given by FEDER funds of the European Union, Grants #DPI2015- 68431-R of the Ministerio de Economía y Competitividad and #RTI2018-093366-B-I00 of the Ministerio de Ciencia, Innovación y Universidades of the Spanish Government, and by the Consellería de Cultura, Educación e Ordenación Universitaria of the Xunta de Galicia (program Axudas para potenciación de grupos de investigación do Sistema Universitario de Galicia 2018, grant #ED431C 2018/41).

Fatigue life variability of a newly developed third-generation Ni-based single crystal superalloy resulting from different solution heat treatment procedures has been investigated in low cycle fatigue (T = 650 °C/Rσ = 0.05/f = 0.5 Hz), high cycle fatigue (T = 950 °C/Rσ = 0.05/f = 72 Hz), and very high cycle fatigue (T = 1,000 °C/Re = −1/f = 20 kHz). Special attention has been paid to the amount of γ/γ ′ eutectics, the dendritic chemical homogeneity, the casting pore size and, the presence of incipient melting areas. At fixed pore size, the chemical homogeneity across the dendritic structure is shown to be the main parameter controlling both low and high cycle fatigue through its effect on the yield stress at 650 °C, and oxidation resistance at 950 °C, respectively. While a greater variability is also obtained in very high cycle fatigue with a worse dendritic chemical homogeneity, the casting pore size remains the main parameter controlling fatigue life in the studied conditions. No detrimental impact of incipient melting on the fatigue life has been examined under these three testing conditions.

International audience; The Direct Current Potential Drop method is commonly used to monitor the crack growth during fatigue test when optical access to the crack is not possible. The reliability of this method depends on the calibration curve, which correlates the potential drop measurement with the crack length and shape. Using Finite Element Analysis and experimental approaches with heat tints, a procedure for the optimization of the calibration curve has been proposed. The influence of plastic deformation and of the presence of a secondary crack on the calibration curve has been studied. Finally, a test strategy, using two heat tints, has been developed in order to identify the first order parameters for the calculation of calibration curve. This strategy shows higher accuracy of crack length measurement and permit to balance experimental uncertainties out.

In this study, a comprehensive description of the adjoint formulation based on a systematic use of Lagrange's identity is proposed to compute acoustic propagation effects induced by the presence of a mean flow. The adjoint method is a clever approach introduced by Tam & Auriault (J. Fluid Mech., vol. 370, 1998, pp. 149–174) in aeroacoustics to predict noise of distributed stochastic sources in a complex environment. A clear statement is also provided about the application of the flow reversal theorem, and its restriction to self-adjoint wave equations. As an illustration, sound propagation is computed numerically over a sheared and stratified mean flow for Lilley's and Pierce's wave equations. Acoustic solutions obtained with the adjoint approach are then compared with predictions obtained with the flow reversal theorem. Additionally Pierce's wave equation for potential acoustics is identified as an outstanding candidate to compute accurately acoustic propagation while removing possible instability waves.

International audience; Repair of damaged aircraft structural parts is a regular maintenance activity carried out to preserve airworthiness while being economically viable. Conventional repair techniques used have negative impact on the repair quality and environment due to dust emission. For eliminating aforementioned problem, this study investigates controlled depth abrasive water jet (AWJ) milling for repair procedure of 3D woven CFRP composite structures used in the new aircraft engine developed by Safran Aircraft Engines. The aim is to identify and quantify the damage and surface contamination induced by AWJ milling. For this, the influence of milling parameters on the damage and surface contamination is investigated through full factorial experimental study. Several characterisation techniques like 3D optical profilometry, Scanning Electron Microscopy and X-ray tomography combined with image processing were used for a multi-scale analysis of the machined surface damage and contamination. In fact, the machining quality was quantified using an innovative criterion called “crater volume” (Cv). The obtained results have shown surface contamination by embedded abrasive particles and presence of bare and broken fibres, cracks, craters. The results show that the rate of contamination is influenced by the jet pressure and the scan step and the Cv is influenced by the jet exposure time.

International audience; This paper studies RANS turbulence modelling for a linear compressor cascade corner separation flow, using large-eddy simulation results as reference. The Boussinesq and the quadratic constitutive relations are investigated with two versions of Wilcox's k − ω turbulence model through a priori and a posteriori analyses. In the a priori analysis the quadratic constitutive relation shows improvement on the alignment between the rate-of-strain tensor and the Reynolds stress tensor for the inlet region, compared to the Boussinesq constitutive relation. But the improvement is less significant in the highly vortical region. Using the turbulent kinetic energy and the specific dissipation rate provides a fairly good estimate of the turbulent viscosity. The turbulent kinetic energy budget is also investigated. Large-eddy simulation results present non-equilibrium turbulence, i.e. the production and dissipation are not balanced, whereas the RANS models predicts equilibrium turbulence.

This paper presents the large Eddy simulation (LES) of a propeller representative of the first rotor of a counter rotative open rotor (CROR) configuration based on a multiple frequency phase-lagged approach in conjunction with a proper orthogonal decomposition (POD) data storage. This method enables to perform unsteady simulations on multistage turbomachinery configurations including multiple frequency flows with a reduction of the computational domain composed of one single blade passage for each row. This approach is advantageous when no circumferential periodicity occurs in the blade rows of the configuration and a full 360 deg simulation would be required. The data storage method is based on a POD decomposition replacing the traditional Fourier series decomposition (FSD). The inherent limitation of phase-shifted periodicity assumption remains with POD data storage but this compression method alleviates some issues associated with the Fourier transform, especially spectrum issues. The paper is first dedicated to compare the flow field obtained with the LES with phase-lagged condition against full-matching URANS, LES simulations, and experimental data available around the blade and in the wake of the rotor. The study shows a close agreement of the phase-lagged LES simulation with other simulations performed and a thicker wake compared with the experiments with lower turbulent activity. The analysis of the losses generated in the configuration, based on an entropy formulation and a splitting between boundary layer and secondary flow structures, shows the strong contribution of the blade boundary layer in the losses generated.

A method is presented to predict numerically the homogenized viscoelastic behavior of 3D woven composites using only its constituents’ behavior. It is based on elastic homogenizations applied to the Laplace-Carson transform of the time-dependent viscoelastic behavior of the constituents. Two scale changes are necessary: from micro- to meso-scale to obtain the homogenized behavior of the consolidated yarns, and from meso- to macro-scale. The temperature and cure dependent viscoelastic behavior of the matrix is identified from experimental data, using the time-temperature superposition principle with the cure dependent glass transition temperature as reference temperature. The meso-scale representative unit cell of the composite is extracted from X-ray microtomography images. The homogenized viscoelastic behavior is used to calculate the evolution of the apparent moduli of the composite with temperature between - 50 ° C and 200 ° C. The results are in good agreement with experimental data over the temperature range where the matrix behavior was properly identified.

International audience; Until now, the coupled stress and energy criterion has mainly been used in 2D applications, but it is possible to extend it to a 3D case. Herein the crack initiation in epoxy/aluminum bimaterial specimens under four point bending is predicted through a 3D numerical application of the coupled criterion. The stress and the energy conditions are computed by means of 3D finite element modeling of both undamaged and cracked specimens. The crack initiates at the epoxy/aluminum interface, meshes of the cracked specimens take into account the crack topology which is determined using the interface normal stress isocontours. By indirect confrontation to experimental tests on aluminum/epoxy bimaterial specimens of different width, the proposed approach allows determining the interface strength and fracture energy. The blind application of the proposed method to a crack initiation in aluminum/epoxy/aluminum specimens of different epoxy layer thickness under four point bending leads to a reasonable agreement with experimental data.

Abrasive water jet (AWJ) process has been certified in some aerospace companies for repair applications. However, there is no information in the literature on the influence of the induced damage on the mechanical behaviour of complex materials such as 3D woven CFRP composite. To fill this gap, three texturing levels have been produced and quantified using both surface roughness (Ra) and “crater volume” (Cv) criteria. The specimens have been subjected to tensile static and tension-tension fatigue tests. Contrarily to Ra measurements, a linear correlation was highlighted between the increase in Cv and the degradation of the specimens’ mechanical performances (ultimate tensile stress and fatigue limit). The damage scenario of the textured specimens has been identified via acoustic emission activity, thermal maps and X-ray tomography pictures. Wider cracks and a more intense acoustic activity leading to a greater rise in temperature have been observed when Cv increases (worse quality). However, the small alterations in mechanical properties consecutive to AWJ texturing prove that this technique can be used for surface preparation of 3D woven CFRP parts prior to bounding without damaging them. Moreover, an innovative method, based on the damage accumulation, has been proposed and accurately estimates the endurance limit of the specimens.

International audience; Thermal sprayed abradable materials are frequently used as seal linings between the casing and the blades of aircraft engines to improve safety and reduce fuel consumption. In this paper we report about the enhancements of an experimental bench developed by our group in order to fully characterize the time-evolution of the three-dimensional force occurring during high-speed interactions between an abradable coating and a titanium tip. A new stiffer testbed associated to a three-axis piezoelectric force sensor was designed to ensure a constant incursion depth during the interaction process. The newly designed device was associated with a three-dimensional implementation of the formerly developed dynamic compensation approach. As a validation of the experimental method, a measure of the wear occurring during high-speed contacts have been performed. The results have been correlated to the measured force signals showing the validity of such a full three-dimensional dynamic compensation.

International audience; The aim of the present study is to measure complex shapes of tested objects by using a priori information given by their CAD model via stereocorrelation. To follow a 3D object during its deformation and to determine 3D surface displacement fields, a first measurement of the object shape is necessary. It is achieved by updating the CAD reference via a global approach to stereocorrelation. Once the 3D shape has been determined, the next step is to measure 3D displacement elds during loading. The kinematics of the deformed shape is assumed to be written within the same isogeometric framework. Isogeometric stereocorrelation is applied to analyze a compression test on a ribbed cylinder in two dierent congurations of the stereo rig.

International audience; This paper provides new insight on the simulation of blade-tip/casing rubbing events within aircraft engines accounting for thermomechanical effects within the casing. A multi-physics numerical strategy is presented in order to simulate an interaction experimentally witnessed on a full-scale low-pressure compressor. Experimental data are used for an accurate representation of the blade's incursion depth within the abradable coating. This numerical strategy combines Safran's in-house tool for rotor/stator interaction simulations with a finite element based thermomechanical analysis carried out with Ansys. This work underlines the distinct contributions of both dynamical and thermomechanical phenomena in the simulated interaction. Competition between wear and thermal expansions is investigated as well as their consequences on blade dynamics. The proposed numerical strategy yields an accurate description of the interaction phenomenon as wear patterns, critical speed, amplitude growth rate of the blade vibration and temperature levels may be predicted; Dans cet article, la simulation de contacts entre une aube de moteur d’avion et son carter en vis-à-vis est abordée en tenant compte des effets thermomécaniques du stator. Une stratégie numérique multi-physique est présentée afin de simuler un scénario d’interaction sur la base d’un essai expérimental échelle un, réalisé sur un étage de compresseur basse pression. Les données expérimentales sont utilisées pour une représentation précise de la profondeur initiale d’incursion de l’aube dans le revêtement abradable. Cette stratégie numérique associe un outil métier Safran, pour la simulation d’interaction rotor-stator, à une simulation thermomécanique en éléments finis réalisée avec Ansys. Ce travail souligne les contributions respectives des phénomènes dynamiques et thermomécaniques dans le scénario d’interaction qui est simulé. La compétition entre usure du revêtement et dilatations thermiques du carter ainsi étudiée permet de porter un nouveau regard sur la dynamique de l’aube. La stratégie numérique apporte une description plus fine du phénomène d’interaction avec une prédiction réaliste des profils d’usure, de la vitesse critique, de l’augmentation de l’amplitude de vibration et des niveaux de température.

This paper presents a damage modeling approach to simulate the formation of butterfly wings in rolling contact fatigue coupled with a fast method for solving 3D contact problems with the presence of a spherical heterogeneity. The damage model is implemented in a semi-analytical model using the Eshelby’ s equivalent inclusion method in the contact solver. The proposed technique can easily include the effects of heterogeneous inclusions to the homogeneous solution in the contact algorithm. Contact pressure and subsurface stress field computation time is kept small due to the use of 3D and 2D Fast Fourier Transforms. Cuboidal inclusions with the same size as the discretization of the half-space are superimposed to represent the microstructural alterations during butterfly wings formation.

Shape optimization is a powerful method to design efficient aerodynamic shapes for aircraft and engine configurations at a limited cost. However, performing an optimization on “real-world” problems, including industrial tools and processes, remains challenging. In this paper, an original approach is presented, aiming at integrating expert knowledge and reducing the dimension of the optimization search space. Thanks to this method, it becomes possible to perform gradient-free or gradient-based optimization with an industrial design workflow comprising CAD. When applied to a nozzle shape optimization problem, this approach leads to encouraging performance improvements in both inviscid and viscous cases, for a given level of fuel consumption. Moreover, the reduced number of parameters enables the use of response surfaces and a better understanding of the design space.

International audience; Contacts between titanium alloys may use an anti-fretting coating on one component. However violent events may cause severe sliding.In the high speed and pressure (up to 60 m/s and 280 MPa) sliding of a non-coated on a coated part, we investigate the influence of the coating (CoCrAlYSiBN or CuNiIn) on the friction coefficient, wear, and microstructural affectations. Both coatings did not significantly influence the friction coefficient but reduced the wear.A scenario based on interface-localized heat explaining the generation of a third body and its influence on the tribological behavior was proposed. The dependence of the mechanical and thermal affectations on the coating material was explained based on the kinetic of the α→β phase transformation of the Ti6Al4V alloy.

International audience; Designing a turbofan to operate in distorted inlet conditions is an issue of growing interest. In such conditions however, fan design can be computationally challenging. Indeed, subject to neither axi-symmetrical nor periodic inlet conditions, computations must be carried out throughout the whole circumferential domain i.e. 360 •. Besides, the classical CFD approach implies the use of URANS computations so as to capture the distortion transfer across the fan stage. Eventually, computations are too onerous to be used in design loops. In this context, this paper presents a methodology to effectively assess a fan blade design domain in distorted conditions. This methodology is based on a body-force source term approach formulated in order to accurately recreate deviations, loads and losses across the fan stage. It notably enables to gain two orders in terms of restitution time and thus the possibility to use optimization tools. The design domain of this study is based on variations of profile chord, blade leading and trailing edges angles applied at two different relative heights. A Latin Hypercube Sampling (LHS) associated with a meta-model based on Radial Basis Functions (RBF) enables to assess the impact of geometric variations on performance and operability. Although this study emphasizes that some modeling improvements are still necessary, it also demonstrates the potential of the body-force methodology to conduct fan design when subject to inlet distortion.; La conception de soufflante sous distorsion d'entrée d'air est un problème de plus en plus abordé. Cependant, sous ces conditions, les simulations de soufflante peuvent être complexes. En effet, sous conditions ni périodiques ni axisymétriques, les simulations doivent a priori être exécutée sur l'ensemble de la circonférence. Cela impose aussi l'utilisation de méthodes instationnaires type U-RANS afin de reproduire le transfère de la distorsion à travers l'étage. En partant de ce contexte, cette publication présente une méthodologie efficace pour mener une conception de soufflante sous distorsion d'entrée d'air. Cette méthodologie se base sur l'utilisation de termes sources alias "body-force" afin de correctement reproduire les effets de déviation, de charge et de pertes à travers l'étage de soufflante. Cela permet en outre une diminution du temps de simulation de deux ordres de grandeur et ainsi l'utilisation de méthodologie d'optimisation. Le domaine de conception de cette étude inclut des variations d'angle squelette de bord d'attaque et de bord de fuite ainsi que des variations de corde sur deux hauteurs de contrôle. Une surface de réponse définie par une méthode LHS associée à un méta-modèle basé sur des fonctions RBF ont permis d'évaluer l'impact de variations géométriques sur les performances et l'opérabilité d'une soufflante UHBR sous distorsion de pression d'arrêt. Bien que cette étude conclut sur le fait que de nombreux points sont à perfectionner, elle démontre aussi le fort potentiel de ces méthodes "body-force" au sein des bureau d'étude des concepteurs de soufflante, notamment lorsque les conditions à l'amont ne sont plus homogènes.

International audience; The aim of this work is to develop a semi-empirical model for erosion phenomena under ice crystal condition, which is one of the major phenomena for ice crystal accretion. Such a model would be able to calculate the erosion rate caused by impinging ice crystals on accreted ice layer. This model is based on Finnie [1] and Bitter [2] [3] solid/solid collision theory which assumes that metal erosion due to sand impingement is driven by two phenomena: cutting wear and deformation wear. These two phenomena are strongly dependent on the particle density, velocity and shape, as well as on the surface physical properties such as Young modulus, Poisson ratio, surface yield strength and hardness. Moreover, cutting wear is mostly driven by tangential velocity and is more effective for ductile eroded body, whereas deformation wear is driven by normal velocity and is more effective for brittle eroded body. Several researchers based their erosion modelling on these two phenomena such as Hutchings et al. [4] for deformation erosion, or Huang et al. [5] and Arabnejad et al. [6] for cutting and deformation erosion. The main work of this paper is to develop an erosion model for ice crystal impingement based on these two phenomena, and to show its capability to predict accretion shape by simulating experimental cases from the National Research Council of Canada (NRC). NRC's Currie et al. ice crystal experiments [7] [8] realized in warm aerodynamic conditions, such as the one encountered in high icing severity areas of a turbofan engine, show accretion severity for a large range of liquid water content to total water content. In order to validate the erosion model based on solid/solid collision, this paper presents the simulation of the lower melting rate experiment. Results show fair agreement with experimental data and allow us to propose pertinent further work.

This paper presents a fast method of solving 3D contact problems when one of the mating bodies has an elastic-damageable behavior. The damage model is implemented in a semi-analytical model using Eshelby’ s equivalent inclusion method in the contact solver. The proposed technique can be seen as an enrichment technique for which the effect of heterogeneous inclusions is surimposed on the homogeneous solution in the contact algorithm. Contact pressure and subsurface stress field computation time is kept small due to a massive use of 3D and 2D Fast Fourier Transforms. Cuboidal inclusions with the same size as the discretization of the half-space and with the same elastic properties are surimposed. The damage model affects the elastic properties of the cuboidal inclusions. The emphasis is put on the effects of the fretting regimes on the contact pressure and damage evolution.

International audience; This article proposes a new method for determining the behavior and service life of contact areas of parts subjected to fretting-fatigue. The first objective was to be able to predict the service life of contact parts with various curvature radii from fretting-fatigue tests on laboratory test geometries such as cylinder-plane. The second objective was to be able to predict the service life for variable amplitude and multiaxial loading conditions from laboratory tests in constant amplitude loading conditions.

Purge air is injected in cavities at the hub of axial turbines to prevent hot mainstream gas ingestion into interstage gaps. This process induces additional losses for the turbine due to an interaction between the purge and mainstream flow. This paper investigates the flow in a low-speed linear cascade rig with upstream hub cavity at a Reynolds number commonly observed in modern low-pressure turbine stages by the use of numerical simulation. Numerical predictions are validated by comparing against experimental data available. Three different purge mass flow rates are tested using three different rim seal geometries. Numerical simulations are performed using a large-eddy simulation (LES) solver on structured grids. An investigation of the different mechanisms associated with the turbine flow including cavity and purge air is intended through this simplified configuration. The underlying mechanisms of loss are tracked using an entropy formulation. Once described for a baseline case, the influence of purge flow and rim seal geometry on flow mechanisms and loss generation is described with the emphasis to obtain design parameters for losses reduction. The study quantifies loss generation due to the boundary layer on wetted surfaces and secondary vortices developing in the passage. The analysis shows different paths by which the purge flow and rim seal geometry can change loss generation including a modification of the shear layer between purge and mainstream, interaction with secondary vortices, and a modification of the flow behavior close to hub compared with a smooth configuration. The study shows the influence of purge flow rate and swirl on the strengthening of secondary vortices in the passage and the ability of axial overlapping rim seal to delay the development of secondary vortices compared with simple axial gaps.

International audience; The improvement of efficiency in the design of turbomachines requires a reliable prediction of the vibrating behavior of the whole structure. The simulation of blades vibrations is decisive and this is usually based on elaborated finite element model restricted to the bladed-disk. However, the blades dynamic behavior can be strongly affected by interactions with other parts of the engine. Global dynamic studies that consider these other parts are required but usually come with a high numerical cost. In the case of a birotor architecture, two coaxial rotors with different rotating speeds can be coupled with a bearing system. The mechanical coupling between the shafts generates energy exchange that alters the dynamic behavior of the blades. The equations of motion of the whole structure that take into account the coupling contain periodic time-dependent coefficients due to the difference of rotational speed between both rotors. Equations of this kind, with variable coefficients, are typically difficult to solve. This study presents a preprocessing method to guarantee the elimination of time-dependent coefficients in the birotor equations of motion. This method is tested with a simplified finite element model of two bladed-disks coupled with linear stiffnesses. We obtain accurate results when comparing frequency analysis of preprocessed equations with time-integration resolution of the initial set of equations. The developed methodology also offers a substantial time saving.

International audience; The present study focuses on the impact of the underlying RANS turbulence model in the Zonal Detached Eddy Simulation (ZDES) method when used for secondary flow prediction. This is carried out in light of three issues commonly investigated for hybrid RANS/LES methods (detection and protection of attached boundary layer, emergence, and growth of resolved turbulent fluctuations and accurate prediction of separation front due to progressive adverse pressure gradient). The studied configuration is the first rotor of a high pressure compressor. Three different turbulence modelings (Spalart and Allmaras model (SA), Menter model with (SST) and without (BSL) shear stress correction) are assessed as ZDES underlying turbulence model and also as turbulence model of unsteady RANS simulations. Whatever the underlying turbulence model, the ZDES behaves well with respect to the first two issues as the boundary layers appear effectively shielded and the RANS-to-LES switch is close downstream of trailing edges and separation fronts leading to a quick LES treatment of wakes and shear layers. Both tip leakage and corner flows are strongly influenced by the Navier–Stokes resolution approach (unsteady RANS vs. ZDES) but the underlying turbulence modelling (SA vs. SST vs. BSL) impacts mainly the junction flow near the hub for both approaches. ZDES underlying turbulence model choice appear essential since it leads to quite different corner flow separation topologies and so to inversion of the downstream stagnation pressure radial gradient.; La présente étude s'intéresse à l'impact du modèle de turbulence RANS sous-jacent à l'approche ZDES sur la prévision des écoulements secondaires. Cette étude est menée à la lumière des problématiques rencontrées pour les approches hybrides RANS/LES (détection et protection de la couche limite attachée, apparition et croissance de la turbulence résolue et prévision du front de décollement causé par un gradient de pression adverse). La configuration étudiée est le premier rotor d'un compresseur haute-pression. Trois modèles de turbulence différents sont évalués, à la fois avec les approches ZDES et URANS. Quel que soit le modèle de turbulence sous-jacent, le comportement de la ZDES est correct au regard des deux premières problématiques puisque les couches limites sont efficacement protégées et le passage RANS-LES est proche du bord de fuite et du front de décollement conduisant au traitement LES des sillages et couches de mélange. Les écoulements de jeu et de coin sont tous deux influencés par l'approche de résolution des équations de Navier-Stokes (ZDES vs. URANS) mais le modèle de turbulence sous-jacent impacte principalement l'écoulement de coin proche du moyeu, conduisant à des décollements de coin différents et à une inversion du gradient radial de la pression génératrice.

International audience; This article describes the numerical framework for a reduced order tool which aims at simulating combustion instabilities in liquid rocket engines. The numerical framework relies on the projection of the pressure fluctuations on the eigenmodes of the system. Pressure fluctuations are solutions of the wave equation of the system. After projection on the eigenmodes, the wave equation takes the form of series of second order harmonic equations with source terms that drive combustion instabilities and damping terms that attenuate them. A test rig was developed to study cavity interactions, injector impedance as well as damping effects. Damping rates measured on the test rig show a trend which is consistent with what is observed in liquid rocket engines. On a whole, the test rig can be used to validate simplified models of combustion instabilities. The global framework of the reduced order tool developed to predict combustion instabilities in liquid rocket engines was first validated by comparing the data from simulations against experimental results from the test rig in a series of non-reacting experiments. The tool was then used in a case study of a full-scale rocket engine. This engine, under certain operating conditions, exhibits instabilities. Stable and unstable behaviors have been revealed by the temporal evolution of pressure amplitudes. Nomenclature p = pressure [Pa] x = position vector [m,m,m] t = time [s] * PhD, Ingénieur ISAE-ENSMA, m.

A large variety of different descriptors can be employed for describing 3D woven composites. Each of them is targeted to a specific application, ranging from design, weaving, molding, impregnation down to non-destructive testing, imaging and numerical modeling of the actual micro-structure. In order to relate these different representations, it is proposed herewith to rely on the inherent weaving pattern as an intrinsic common feature, and to resort to images as a common language to guaranty the continuity of information. To connect these 3D images (either “real” or synthesized), Digital Volume Correlation (DVC) is called for in order to exploit the conservation of topology. A complete test scenario is devised in which different manufactured woven samples are compared to the theoretical textile arrangement. The results confirm the effectiveness of the method.