Etude des modes normaux non linéaires d'un arbre rotatif par la méthode des surfaces invariantes

For rotating shaft systems, the mechanism representing the supporting hydraulic bearings is inherently nonlinear even though the deformation of the rotating shaft can be modelled linearly. Complicated behaviors can occur in the whole system and an efficient reduced order model is important for the vibration analysis. In this paper, a rotating shaft supported by two short hydraulic bearings is studied in details. A simplify model of the bearings is applied so that the supporting forces can be expressed analytically. The shaft and the bearings are coupled by Craig-Bampton method to ensure a minimal degrees-of-freedom system model. A very new formulation for the manifolds is applied here to get the nonlinear normal modes of such a system with general damping, gyroscopic and stiffness matrices. Once those nonlinear modes are constructed the motion on the nonlinear manifolds is compared to a direct time simulation.Dans les systèmes intégrant des structures en rotation, les non linéarités proviennent souvent des supports, présents ici sous la forme de paliers lisses. Même si l'hypothèse de déformations linéaires au niveau de l'arbre est permise, le comportement de l'ensemble est non linéaire et un modèle simple est important pour la compréhension des phénomènes vibratoires. Une formule de Reynolds simplifiée pour le comportement hydrodynamique des paliers permet d'exprimer analytiquement les forces issues de la déformation du film d'huile. Les équations du mouvement sont ensuite discrétisées par la méthode de Craig-Bampton pour un nombre minimal de degrés de liberté. Une toute nouvelle formulation des surfaces invariantes permet de générer les modes normaux non linéaires d'un tel système ayant des matrices gyroscopique, d'amortissement et de raideur tout à fait générales et une simulation directe en temps est comparée aux résultats obtenus

Wind and Temperature Networking Applied to Aircraft Trajectory Prediction

International audienceTrajectory prediction estimates the future position of aircraft along their planned trajectories in order to detect potential conflicts and to optimize air space occupancy. This prediction is a critical task in the Air Traffic Control (ATC) process and has been studied for many years. For the future automation processes developed in the SESAR [19], NextGen [15] and CARATS [3] projects, such trajectory prediction will be even more critical. As there is always a deviation between the predicted wind (from the weather forecasts) and the encountered wind, the main longitudinal (along-track) error source between the predicted and the actual trajectory is linked to wind estimation. Even if the main longitudinal (along-track) error source between the predicted and the actual trajectory is linked to wind estimation, temperature wrong estimation may also lead to ETE1 prediction errors. In a previous paper [11] we measured the potential benefit produced by sharing wind measures between aircraft. In the present paper we will try to improve the trajectory prediction by sharing the wind and the temperature information between aircraft. Addressing the temperature came when we noticed that at least the cruising phase of many flight was performed at constant Mach number. Maintaining a given Mach number under changing temperatures equals changing the true air speed. Based on the current performances of Air Traffic Control systems , controllers are able to efficiently detect conflict 20 minutes in advance ; for a larger time horizon (look-ahead time), the induced trajectory prediction uncertainty strongly reduces the reliability of the conflict detection. The goal of this work is to measure the potential benefit produced by sharing wind/temperature measures between aircraft (this concept will be called Wind/Temp Networking (WTN)). To reach this goal, aircraft measure (temperature and pressure) and calculate (wind and density) their local atmospheric data and broadcast them to the other aircraft. Having such distributed weather information, each aircraft is able to compute an enhanced local wind/temperature map as a function of location (3D) and time. These updated wind/temp fields could be shared with other aircraft and/or with ground systems. Using this enhanced weather information, each aircraft is able to improve drastically its own trajectory prediction. This concept has been simulated in the French airspace with 8 000 flights. Comparisons have been investigated on trajectory prediction performances with and without wind/temp networking. Statistics have been conducted in order to measure the benefit of such concept in both time and space dimensions showing higher improvement in high traffic areas, as expected

Etude de l'interaction modale rotor/stator dans un moteur d'avion

International audienceIn modern turbo machines such as aircraft jet engines, contact between the casing and bladed disk may occur through avariety of mechanisms : coincidence of vibration modes, thermal deformation of the casing, rotor imbalance, etc. Thesenonlinear interactions may result in severe damage to both structures and it is important to understand the physicalmechanisms that cause them and the circumstances under which they occur. In this study, we focus on the phenomenonof interaction caused by modal coincidence. A simple two-dimensional model of the casing and bladed disk structures isintroduced in order to predict the occurrence of the interaction phenomenon in terms of the rotation speed of the rotor.Each structure is represented in terms of its two $n_ d$ -nodal diameter vibration modes, which are characteristic of axi-symmetric structures and allow for travelling wave motions that may interact through direct contact. The equations ofmotion are solved first using an explicit time integration scheme in conjunction with the Lagrange multiplier method, andthen by the Harmonic Balance Method. Both methods generally agree well and exhibit two distinct zones of completelydifferent behaviors of the system. Finally, a second planar model is introduced in order to achieve a deeper understandingof the interaction phenomenon.Dans le domaine des turbo-machines comme les moteurs d’avion par exemple, les contacts structurels entre le carter etla roue aubagée peuvent avoir plusieurs origines : coïncidence vibratoire entre des modes, déformation thermique ducarter, apparition d’un balourd au niveau du rotor, etc. Ces interactions non linéaires peuvent fortement endommagerles structures et il est important de comprendre l’origine de ces mécanismes. Dans ce travail, nous nous concentronssur le phénomène d’interaction modale. Un premier modèle planaire du carter et de la roue aubagée est développé pourprédire les vitesses de rotation du moteur pour lesquelles le phénomène d’interaction peut exister. Chaque structure estdiscrétisée sur ses deux modes à $n_ d$ diamètres nodaux, caractéristiques des structures à symétrie cyclique et qui, combinés, permettent de construire des ondes de propagation qui interagissent par l’intermédiaire du contact. Les équations sont résolues grâce à un schéma d’intégration explicite couplé à la méthode des multiplicateurs de Lagrange, puis par la méthode de l’équilibrage harmonique. Les deux méthodes sont en accord et montrent qu’il y a deux zones distinctes de fonctionnement. Finalement, un deuxième modèle planaire est présenté pour comprendre plus en détail le phénomène d’interaction

Forward Error Correction applied to JPEG-XS codestreams

JPEG-XS offers low complexity image compression for applications with constrained but reasonable bit-rate, and low latency. Our paper explores the deployment of JPEG-XS on lossy packet networks. To preserve low latency, Forward Error Correction (FEC) is envisioned as the protection mechanism of interest. Despite the JPEG-XS codestream is not scalable in essence, we observe that the loss of a codestream fraction impacts the decoded image quality differently, depending on whether this codestream fraction corresponds to codestream headers, to coefficients significance information, or to low/high frequency data, respectively. Hence, we propose a rate-distortion optimal unequal error protection scheme that adapts the redundancy level of Reed-Solomon codes according to the rate of channel losses and the type of information protected by the code. Our experiments demonstrate that, at 5% loss rates, it reduces the Mean Squared Error by up to 92% and 65%, compared to a transmission without and with optimal but equal protection, respectively

Nonlinear Normal Modes of a Rotating Shaft Based on the Invariant Manifold Method

International audienceThe nonlinear normal mode methodology is generalized to the study of a rotating shaft supported by two short journal bearings. For rotating shafts, nonlinearities are generated by forces arising from the supporting hydraulic bearings. In this study, the rotating shaft is represented by a linear beam, while a simplified bearing model is employed so that the nonlinear supporting forces can be expressed analytically. The equations of motion of the coupled shaft-bearings system are constructed using the Craig-Bampton method of component mode synthesis, producing a model with as few as six degrees of freedom (d.o.f.). Using an invariant manifold approach, the individual nonlinear normal modes of the shaft-bearings system are then constructed, yielding a single d.o.f. reduced-order model for each nonlinear mode. This requires a generalized formulation for the manifolds, since the system features damping as well as gyroscopic and non-conservative circulatory terms. The nonlinear modes are calculated numerically using a nonlinear Galerkin method that is able to capture large amplitude motions. The shaft response from the nonlinear mode model is shown to match extremely well the simulations from the reference Craig-Bampton model

A reduction technique for mistuned bladed disks with superposition of large geometric mistuning and small model uncertainties

International audienceA new method for vibration analysis of mistuned bladed disks is presented. The method combines two previously reported modeling techniques in order to study the effects of small random parameter variation on geometrically mis-tuned bladed disks. It is based on the observation that the nominal projections usually possess a certain degree of ro-bustness tolerating small perturbations. Hence the subspace spanned by a sufficient number of compensated tuned system normal modes can be used to repeatedly project models with small random parameter variation. The method is validated numerically on an industrial bladed disk model, both free and forced responses are compared with full model finite element analysis. The benchmark cases show acceptable accuracy while retaining low computational cost to build and evaluate obtained ROM

Evaluation of component mode synthesis methods for the detection of modal interaction through rotor/stator contacts

International audienceThe study of interactions through direct contact between blade-tips and outer casings in modern turbomachines may be very time-consuming when the classical finite element method is used. The construction of reduced-order models using component mode synthesis methods generally allows for dramatic increase in computational efficiency and may be used in order to improve the knowledge over these interaction phenomena. Among the available approaches, both a fixed-interface method and a free-interface method are considered here in an original manner to reduce the size of a realistic two-dimensional model. The equations of motion are solved using an explicit time integration scheme with the Lagrange multiplier method where friction is accounted for. This method offers energy momentum conserving which is a critical point to ensure the convergence of the algorithm. Moreover, it is shown that even in a non-linear framework the reduced-order models converge to the finite element solution as the number of modes included in the models increases. Considering the fixed-interface method of Craig-Bampton and the free-interface method of Craig-Chang-Martinez, it is shown that a method with fast displacement convergence may be less efficient in terms of motion convergence

NGAL, biomarker of acute kidney injury

peer reviewedLe diagnostic precoce de l’insufissance renale aigue (IRA) est necessaire et devrait se faire au stade de lesion renale avant meme la degradation du debit de filtration glomerulaire. Plusieurs biomarqueurs d’atteinte renale aigue sont actuellement a l’etude. Parmi ceux-ci, le Neutrophil Gelatinase Associated Lipocalin (NGAL) semble l’un des plus prometteurs et fait l’objet de nombreuses publications. La performance diagnostique de NGAL, dose dans le plasma ou les urines, pour le depistage de l’IRA depend de nombreux facteurs. Bien que les donnees experimentales recentes soient en faveur de l’utilisation preferentielle du dosage urinaire de NGAL, les donnees cliniques issues de nombreuses etudes ne permettent pas de trancher formellement sur la superiorite du dosage urinaire par rapport au dosage plasmatique pour le depistage des atteintes renales aigues. Il n’en reste pas moins que sur le plan analytique, les techniques de dosage du NGAL urinaire sont actuellement plus fiables que celles du dosage plasmatique. La performance diagnostique de NGAL dans un contexte d’IRA est maximale en chirurgie cardiaque pediatrique. Les resultats, chez l’adulte en postoperatoire de chirurgie cardiaque et dans d’autres situations (reanimation, urgences, transplantation), sont moins convaincants. Par ailleurs, il n’est actuellement pas possible d’extrapoler des etudes cliniques une valeur seuil discriminante unique de NGAL, aussi bien dans les urines que dans le plasma. D’autres etudes sont necessaires pour valider definitivement NGAL comme biomarqueur de l’atteinte renale aigue et en preciser les conditions d’utilisation en pratique clinique