La prise en compte des couplages dynamiques entre les différents organes constituant une turbomachine s’inscrit dans le processus d’optimisation des designs moteur. L’amélioration des performances des turboréacteurs passe souvent par l’utilisation d’architectures multi-rotors. Dans le contexte des moteurs avec une architecture bi-rotor, des résultats d’essais expérimentaux montrent qu’il est nécessaire de considérer, dès la conception, l’influence de la dynamique de l’arbre Haute Pression (HP) sur les aubages de l’arbre Basse Pression (BP). Dans ce cadre d’étude, un premier modèle simplifié de bi-rotor aubagé est développé dans le repère tournant lié au rotor BP. Ce modèle est composé de deux rotors modélisés par des équivalents poutres - masses - ressorts et d’une roue aubagée constituée d’aubes souples modélisées par des poutres encastrées sur un disque rigide. Desnon-linéarités de type jeu radial avec contact au niveau des paliers sont également considérées et la réponse des aubes soumises à des excitations multi-fréquentielles de type balourd BP et HP est analysée. La présence de non-linéarités dans le système conduit à mettre en oeuvre des algorithmes adaptés, basés sur des techniques de résolution dans le domaine fréquentiel avec l’évaluation des efforts non-linéaires dans le domaine temporel. Afin d’avoir une meilleure description de la dynamique de la roue aubagée, une méthode spécifique de couplage est proposée, permettant de coupler un modèle réduit de roue aubagée 3D à un modèle simplifié de bi-rotor. Une démarche adaptée à la modélisation de la roue aubagée en symétrie cyclique est implémentée afin de considérer des non-linéarités de type contact en tête d’aube. La méthode de couplage proposée est ensuite illustrée sur un exemple simple puis validée dans un cadre linéaire et non-linéaire. Enfin, cette méthode de couplage est appliquée au cas d’une structure industrielle, constituée d’un modèle d’ensemble simplifié représentatif d’un moteur et d’un modèle éléments finis d’une roue de turbine BP. Les résultats obtenus mettent en évidence le couplage entre la dynamique d’ensemble et la dynamique de la roue aubagée et permettent de prédire la réponse non-linéaire des aubes de turbine BP en présence d’une excitation multi-fréquentielle, dans des configurations de co-rotation et de contra-rotation. / The design and optimization process of high efficiency turbomachinery has become a major challenge and a topical issue at both industrial and research levels. Performance improvement has motivated the use of multi-shaft architecture in engines. In the context of dual-shaft aircraft engines, the interaction between dynamics occurring within shafts and bladed disks seems to play an important role at the design stage. The present research work deals with the coupling of these components involving several unbalances in the dynamic response of blades. Within this framework, a simplified analytical model of a bladed dual-shaft developed in the rotating frame is presented. The dual-shaft is modelled by spring - mass- beam systems and connected to a bladed disk composed of a set of flexible blades modelled by Euler-Bernoulli beams clamped in a rigid disk. Nonlinearities coming from bearings are also considered and modelled as a radial clearance and contact stiffness. Considering nonlinearities requires the implementation of dedicated algorithms and specific resolution techniques in the frequency domain as well as the computation of nonlinear forces in the time domain. The nonlinear response of blades subjected to unbalances excitations is investigated and analysed. To have a finer description of the bladed disk dynamics, a specific coupling method is proposed allowing to connect a bladed disk finite element model with the simplified dual-shaft model. A cyclic symmetry approach well-suited to the nonlinear dynamics of bladed disks is developed in order to consider blade tip contact nonlinearities. Performances of the proposed method are illustrated through an academic example and validated in both linear and nonlinear settings. Eventually, the coupling technique is applied to a complex industrial case involving a classical simplified dual-shaft model and a finite element model of the low pressure turbine bladed disk. Numerical results clearly demonstrate the coupling between dynamics and enable to predict the nonlinear response of low pressure turbine blades to several unbalances, for both co-rotating and counter-rotating engines.

L’objectif de ces travaux est de prédire la stabilité dynamique d’un moteur d’avion de type turbofan, lors de contacts légers entre l’extrémité des aubes et le carter en regard. Une voix d’amélioration des performances des turboréacteurs d’avion consiste, en effet, à réduire les jeux entre les extrémités des aubes en rotation et le carter. Réduction qui s’accompagne d’une augmentation des possibilités de contact entre ces parties tournantes et fixes pouvant conduire à des phénomènes vibratoires instables. La démarche proposée s’appuie sur un modèle hybride qui introduit de manière simplifiée une roue aubagée et un carter flexible à un modèle de ligne d’arbres. Une formulation dite 3D du contact a également été implémentée, elle intègre la cinématique des différents organes du modèle et introduit de manière précise la géométrie locale de la zone de contact. Le comportement du modèle lors de contacts aubes-carter est alors étudié en distinguant les analyses menées en supposant un contact permanent et celles autorisant l’intermittence des contacts. Les résultats obtenus mettent en évidence l’importance des couplages introduits dans l’apparition des phénomènes instables et la pertinence de la formulation 3D dans la prédiction de la stabilité du système. / This work aims to predict the dynamic stability of a turbofan engine submitted to light contacts between blade tips and casing. Reducing the clearance between the rotating blades and the casing improves indeed the performances of turbomachines ; however, it also increases the possible contacts between rotating and stationary parts, which can cause unstable dynamic behavior. The approach is based on a hybrid model which introduces a simplified bladed wheel and a flexible casing to a rotor-shaft model. A 3D contact formulation has also been implemented ; it considers the model kinematic and introduces the local geometry of the contact area. The model behavior with blade-to-casing contacts is analyzed through two approaches : the first one assumes permanent contacts while the other one allows contact intermittence. The results highlight the importance of couplings in the outbreak of unstable phenomena and the relevance of the 3D contact formulation in predicting the stability of the system.

Crack initiation and propagation in the bladed disks of aero-engines caused by high-cycle fatigue under cyclic loads could result in the breakdown of the engines if not detected at an early stage. Although a number of fault detection methods have been reported in the literature, it still remains very challenging to develop a reliable online technique to accurately diagnose defects in bladed disks. One of the main challenges is to characterize signals contaminated by noises. These noises caused by very dynamic engine operation environment. This work presents a new technique for engine bladed disk crack detection, which utilizes advanced analysis of clearance and time-of-arrival signals acquired from blade tip sensors. This technique involves two stages of signal processing: 1) signal pre-processing for noise elimination from predetermined causes; and 2) signal post-processing for characterizing crack initiation and location. Experimental results from the spin rig test were used to validate technique predictions.

The purpose of this master thesis is to provide a reliable methodology to predict the forced response of a monoblock bladed drum from a low pressure compressor. Pre-test forced response calculations have already been made at Techspace Aero in 2013. Now that experimental data are available, the methodology has to be adapted to ensure the best numerical-experimental correlation possible. The final goal is that, at the end of the thesis, engineers at Techspace Aero will be able to launch reliable forced response simulations within a short amount of time. For the sake of confidentiality, some data are not revealed, such as the engine name, some numerical values (forced response, aerodynamic damping, frequency of the mode etc…) and axis scales. In this paper, the study focuses on the forced response of a rotor blade from the first stage under the excitation from the upstream stator. The mode under investigation is the 2S2, the one that responded during the experiment. The TWIN approach is used to compute the forced response of the rotor blade. With this approach, a steady stage computation has first to be carried on as an initialization. Then two unsteady computations are necessary. The first, without blade motion, will provide the excitation aerodynamic forces. The aerodynamic damping will be extracted from the second one, where the motion of the blade is imposed on a given eigenmode. The forced response can then be computed with these two results and some additional structural data.  The results will be compared to the experimental value.

Two state of the art finite element reduction techniquespreviously validated against the direct finite element method,one based on classical modal analysis and another based oncomponent mode synthesis, are applied for efficient mistunedfree vibration and forced response analysis of several bladeddisk geometries. The methods are first applied to two testcases in order to demonstrate the differences in computationalefficiency as well as to validate the methods againstexperimental data. As previous studies have indicated, nonoticeable differences in accuracy are detected for the currentapplications, while the method based on classical modalanalysis is significantly more efficient. Experimental data(mistuned frequencies and mode shapes) available for one of thetwo test cases are compared with numerical predictions, and agood match is obtained, which adds to the previous validationof the methods (against the direct finite element method). The influence of blade-to-blade coupling and rotation speedon the sensitivity of bladed disks to mistuning is thenstudied. A transonic fan is considered with part span shroudsand without shrouds, respectively, constituting a high and alow blade-to-blade coupling case. For both cases, computationsare performed at rest as well as at various rotation speeds.Mistuning sensitivity is modelled as the dependence ofamplitude magnification on the standard deviation of bladestiffnesses. The finite element reduction technique based onclassical modal analysis is employed for the structuralanalysis. This reduced order model is solved for sets of randomblade stiffnesses with various standard deviations, i.e. MonteCarlo simulations. In order to reduce the sample size, thestatistical data is fitted to a Weibull (type III) parametermodel. Three different parameter estimation techniques areapplied and compared. The key role of blade-to-blade coupling,as well as the ratio of mistuning to coupling, is demonstratedfor the two cases. It is observed that mistuning sensitivityvaries significantly with rotation speed for both fans due toan associated variation in blade-to-blade coupling strength.Focusing on the effect of one specific engine order on themistuned response of the first bending modes, it is observedthat the mistuning sensitivity behaviour of the fan withoutshrouds is unaffected by rotation at its resonant condition,due to insignificant changes in coupling strength at thisspeed. The fan with shrouds, on the other hand, shows asignificantly different behaviour at rest and resonant speed,due to increased coupling under rotation. Comparing the twocases at resonant rotor speeds, the fan without shrouds is lessor equally sensitive to mistuning than the fan with shrouds inthe entire range of mistuning strengths considered. This thesis’scientific contribution centres on themistuning sensitivity study, where the effects of shrouds androtation speed are quantified for realistic bladed diskgeometries. However, also the validation of two finite elementreduction techniques against experimental measurementsconstitutes an important contribution. / NR 20140805

No description available.

Sediment is a prevalent non-point source pollutant associated with forest operations. Roads and skid trail surfaces have erosion rates that are greater than the harvest area. Forestry best management practices (BMPs) have been developed to minimize erosion on skid trails, but few projects have compared the effectiveness of different BMPs for bladed skid trails in the mountains. This project evaluated soil erosion rates from bladed skid trails in the Ridge and Valley physiographic region of Virginia following an operational timber harvest. Skid trails were assigned into six blocks where each block had similar slopes and soils. All BMP treatments had waterbars, which are considered the minimum acceptable BMP closure treatment. Each block contained four different skid trail closure BMP treatments (waterbar only (Control), slash-covered (Slash), seeded (Seed), and seeded with fertilizer and mulch (Mulch)). The 24 treatment units were isolated with waterbars and installed following the Virginia Department of Forestry (VDOF) BMP guidelines. The randomized complete block design had three slope class ranges: gentle (0%-10%), moderate (11%-20%), and steep (21%-30%). Stormwater runoff from skid trails was directed at downslope waterbars and eroded material was trapped in silt fences at each treatment area. Depth and area of eroded soil collected in silt fences was measured monthly to quantify total erosion volume for the skid trail area and converted to a per acre basis. Volumes were converted to mass using soil bulk density within the trapped sediment. Control treatments had an average erosion rate of 6.8 tons ac-1 yr-1, with rates up to 73.5 tons ac-1 yr-1 following installation and during extreme rainfall events. Seed treatments recorded an average erosion rate of 2.6 tons ac-1 yr-1, with rates reaching 27.2 tons ac-1 yr-1. Adding grass seed provided ground cover, but not consistently over time. Due to high rates of ground cover, the Mulch treatments averaged 0.5 tons ac-1 yr-1 with an extreme of 3.8 tons ac-1 yr-1. Slash treatments were found to reduce erosion rates to an average of 0.4 tons ac-1 yr-1, with the highest rate being 1.8 tons ac-1 yr-1. Site characteristics on experimental units were collected quarterly in order to model erosion rates with commonly used erosion models for forestland (USLE-Forest, RUSLE2, WEPP:Road). Direct erosion estimates were compared to erosion model predictions produced by USLE-Forest, RUSLE2, and WEPP:Road in order to partially confirm the relationship between sediment trap data and the models. Using multiple analyses it was determined that USLE-Forest and RUSLE2 predicted mean values that are more similar to the actual measured rates, RUSLE2 and WEPP:Road have better linear relationships to the measured rates than does USLE-Forest, and USLE-Forest was the most statistically similar to the measured data using a nonparametric Steel-Dwass Multiple Comparisons Test. All models performed inadequately when attempting to predict Control or Slash treatments; while all models performed the best at predicting Mulch treatments. / Master of Science

Sediment is one of the leading non-point source pollutants in the U.S and has detrimental effects on biological communities such as aquatic communities; human use such as recreation; and natural processes such as flood water storage. For silvicultural operations, the majority of sediment is produced from erosion on highly disturbed areas, such as skid trails, haul roads, and log landings. Erosion from silvicultural activities not only has the potential to introduce sediment into waterways but can also decrease site productivity through the removal of topsoil. In order to minimize erosion from silvicultural operations, forestry Best Management Practices (BMPs) have been developed, but efficacies of various BMP options are not well documented. This study evaluated five closure and cover BMPs for the control of erosion on bladed skid trails through both field based measurements with sediment traps and soil erosion modeling. The erosion models used were the Universal Soil Loss Equation for Forestry (USLE – Forest), the Revised Universal Soil Loss Equation version 2 (RUSLE2), and the Water Erosion Prediction Project for Forest Roads (WEPP – Forest Roads). Erosion model predictions were also regressed against field based results to determine accuracy. The bladed skid trail BMP treatments evaluated were: 1) water bar only (Control); 2) water bar and grass seed (Seed); 3) water bar, grass seed, and straw mulch (Mulch); 4) water bar and piled hardwood slash (Hardwood Slash); and 5) water bar and piled pine slash (Pine Slash). Field based results show that the Control treatment was the most erosive (137.7 tonnes/ha/yr), followed by the Seed treatment (31.5 tonnes/ha/yr), Hardwood Slash treatment (8.9 tonnes/ha/yr), Pine Slash treatment (5.9 tonnes/ha/yr), and finally the Mulch treatment was the most effective erosion control technique (3.0 tonnes/ha/yr). Model accuracy results show that RUSLE2 performed the best overall. Both USLE – Forest and WEPP – Forest Roads under predicted values on the Control treatment, where erosion rates were very high. WEPP – Forest Roads under predicted these values the most. All models generally show that the Control was the most erosive followed by the Seed, Hardwood Slash, Pine Slash, and Mulch treatments. / Master of Science

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ABSTRACT POSSIBILISTIC INTERPRETATION OF MISTUNING IN BLADED DISKS BY FUZZY ALGEBRA Karatas, Hamit &Ccedil / aglar M.S., Department of Mechanical Engineering Supervisor: Prof. Dr. H. Nevzat &Ouml / zg&uuml / ven Co-supervisor: Asst. Prof. Dr. Ender Cigeroglu September 2012, 103 pages This study aims to define the possibilistic interpretation of mistuning and examine the way of determining the worst case situations and assessing reliability value to that case by using possibilistic methods. Furthermore, in this study, benefits of using possibilistic interpretation of mistuning in comparison to probabilistic interpretation of mistuning are investigated. For the possibilistic analysis of mistuned structures, uncertain mistuning parameters are modeled as fuzzy variables possessing possibility distributions. In this study, alpha-cut representations of fuzzy numbers are used which makes fuzzy variables to be represented by interval numbers at each and every confidence level. The solution of fuzzy equations of motion is governed by fuzzy algebra methods. The bounds of the solution of the fuzzy equation of motion, i.e. fuzzy vibration responses of the mistuned structure, are determined by the extension principle of fuzzy functions. The performance of the method for possibilistic interpretation of mistuning is investigated by comparing it to the probabilistic methods both computational and accuracy wise. For the comparison study, two different optimization tools &ndash / genetic algorithm as the global optimization tool and constrained nonlinear minimization method as the gradient based optimization tool- are utilized in possibilistic analysis and they are compared to solutions of probabilistic methods resulted from Monte-Carlo method. The performances of all of the methods are tested on both a cyclically symmetric lumped parameter model and a realistic reduced order finite element model.