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Multi-Fidelity Study of Aerodynamics and Aeroacoustics Characteristics of a Quadrotor Biplane TailsitterHeydari, Morteza 05 1900 (has links)
Recent advances in manufacturing and growing concerns on the sustainability of aviation environment have led to a remarkable interest in electrical unmanned aerial systems (UASs) in the past decade. Among various UAS types, the newly designed quadrotor biplane tailsitter class is capable of delivering a wide range of civilian and military tasks, relying on its Vertical Take-Off and Landing (VTOL) capability as well as great maneuverability. Nevertheless, as such UASs employ rotors to generate thrust, and wings to generate lift, and operate at less-understood low to mid-Reynolds flow regime, they experience complicated flight aerodynamics with a noise generation mechanism which is different from common aircrafts. The present work aims at addressing this knowledge gap by studying the aerodynamics and aeroacoustics of a UAS of this type designed by the Army Research Lab. High-fidelity computational fluid dynamics (CFD) simulations are carried out for a wide range of operating conditions to understand the physics involved in the UAS aerodynamics and characterize its performance. Relying on the CFD results, a physics-informed reduced order model (ROM) is developed based on machine learning algorithms, to predict the propellers effects on the wings and calculate the dominant loads. The results of this study indicate that the UAS aerodynamics is significantly influenced by the propeller-wing interaction, which makes it challenging to estimate the loads by classic methods. The proposed physics-informed ROM shows a promising performance based on its computational cost and accuracy. Additionally, it is found that the aeroacoustics of the UAS is ruled by a two-way mechanism through which the propellers and the structure impose unsteadiness on each other.
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Development and Validation of a Numerical Tool for theAeromechanical Design of TurbomachineryMayorca, María Angélica January 2010 (has links)
<p>In aeromechanical design one of the major rules is to operate under High Cyclic Fatigue (HCF) margins and away from flutter. The level of dynamic excitations and risk of HCF can be estimated by performing forced response analyses from blade row interaction forces or Low Engine Order (LEO) excitation mechanisms. On the other hand, flutter stability prediction can be assessed by calculation of aerodynamic damping forces due to blade motion. In order to include these analyses as regular practices in an industrial aeromechanical design process, interaction between the fields of fluid and structural dynamics must be established in a rather simple yet accurate manner. Effects such as aerodynamic and structural mistuning should also be taken into account where parametric and probabilistic studies take an important role.</p><p>The present work presents the development and validation of a numerical tool for aeromechanical design. The tool aims to integrate in a standard and simple manner regular aeromechanical analysis such as forced response analysis and aerodynamic damping analysis of bladed disks.</p><p>Mistuning influence on forced response and aerodynamic damping is assessed by implementing existing model order reduction techniques in order to decrease the computational effort and assess results in an industrially applicable time frame. The synthesis program solves the interaction of structure and fluid from existing Finite Element Modeling (FEM) and Computational Fluid Dynamics (CFD) solvers inputs by including a mapping program which establishes the fluid and structure mesh compatibility. Blade row interaction harmonic forces and/or blade motion aerodynamic damping forces are inputs from unsteady fluid dynamic solvers whereas the geometry, mass and stiffness matrices of a blade alone or bladed disk sector are inputs from finite element solvers. Structural and aerodynamic damping is also considered.</p><p>Structural mistuning is assessed by importing different sectors and any combinations of the full disk model can be achieved by using Reduced Order Model (ROM) techniques. Aerodynamic mistuning data can also be imported and its effects on the forced response and stability assessed. The tool is developed in such a way to allow iterative analysis in a simple manner, being possible to realize aerodynamically and structurally coupled analyses of industrial bladed disks. A new method for performing aerodynamic coupled forced response and stability analyses considering the interaction of different mode families has also been implemented. The method is based on the determination of the aerodynamic matrices by means of least square approximations and is here referred as the Multimode Least Square (MLS) method.</p><p>The present work includes the program description and its applicability is assessed on a high pressure ratio transonic compressor blade and on a simple blisk.</p> / Turbopower / AROMA
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Réduction de modèles en thermo-mécanique / Reduced order modeling in thermo-mechanicsBenaceur, Amina 21 December 2018 (has links)
Cette thèse propose trois nouveaux développements de la méthode des bases réduites (RB) et de la méthode d'interpolation empirique (EIM) pour des problèmes non-linéaires. La première contribution est une nouvelle méthodologie, la méthode progressive RB-EIM (PREIM) dont l'objectif est de réduire le coût de la phase de construction du modèle réduit tout en maintenant une bonne approximation RB finale. L'idée est d'enrichir progressivement l'approximation EIM et l'espace RB, contrairement à l'approche standard où leurs constructions sont disjointes. La deuxième contribution concerne la RB pour les inéquations variationnelles avec contraintes non-linéaires. Nous proposons une combinaison RB-EIM pour traiter la contrainte. En outre, nous construisons une base réduite pour les multiplicateurs de Lagrange via un algorithme hiérarchique qui conserve la positivité des vecteurs cette base. Nous appliquons cette stratégie aux problèmes de contact élastique sans frottement pour les maillages non-coïncidents. La troisième contribution concerne la réduction de modèles avec assimilation de données. Une méthode dédiée a été introduite dans la littérature pour combiner un modèle numérique avec des mesures expérimentales. Nous élargissons son cadre d'application aux problèmes instationnaires en exploitant la méthode POD-greedy afin de construire des espaces réduits pour tout le transitoire temporel. Enfin, nous proposons un nouvel algorithme qui produit des espaces réduits plus représentatifs de la solution recherchée tout en minimisant le nombre de mesures nécessaires pour le problème réduit final / This thesis introduces three new developments of the reduced basis method (RB) and the empirical interpolation method (EIM) for nonlinear problems. The first contribution is a new methodology, the Progressive RB-EIM (PREIM) which aims at reducing the cost of the phase during which the reduced model is constructed without compromising the accuracy of the final RB approximation. The idea is to gradually enrich the EIM approximation and the RB space, in contrast to the standard approach where both constructions are separate. The second contribution is related to the RB for variational inequalities with nonlinear constraints. We employ an RB-EIM combination to treat the nonlinear constraint. Also, we build a reduced basis for the Lagrange multipliers via a hierarchical algorithm that preserves the non-negativity of the basis vectors. We apply this strategy to elastic frictionless contact for non-matching meshes. Finally, the third contribution focuses on model reduction with data assimilation. A dedicated method has been introduced in the literature so as to combine numerical models with experimental measurements. We extend the method to a time-dependent framework using a POD-greedy algorithm in order to build accurate reduced spaces for all the time steps. Besides, we devise a new algorithm that produces better reduced spaces while minimizing the number of measurements required for the final reduced problem
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EXPERIMENTALLY VALIDATED CRYSTAL PLASTICITY MODELING OF TITANIUM ALLOYS AT MULTIPLE LENGTH-SCALES BASED ON MATERIAL CHARACTERIZATION, ACCOUNTING FOR RESIDUAL STRESSESKartik Kapoor (7543412) 30 October 2019 (has links)
<p>There is a growing need to understand the
deformation mechanisms in titanium alloys due to their widespread use in the
aerospace industry (especially within gas turbine engines), variation in their
properties and performance based on their microstructure, and their tendency to
undergo premature failure due to dwell and high cycle fatigue well below their
yield strength. Crystal plasticity finite element (CPFE) modeling is a popular
computational tool used to understand deformation in these polycrystalline alloys.
With the advancement in experimental techniques such as electron backscatter
diffraction, digital image correlation (DIC) and high-energy x-ray diffraction,
more insights into the microstructure of the material and its deformation
process can be attained. This research leverages data from a number of
experimental techniques to develop well-informed and calibrated CPFE models for
titanium alloys at multiple length-scales and use them to further understand
the deformation in these alloys.</p>
<p>The first part of the research utilizes
experimental data from high-energy x-ray diffraction microscopy to initialize
grain-level residual stresses and capture the correct grain morphology within
CPFE simulations. Further, another method to incorporate the effect of grain-level
residual stresses via geometrically necessary dislocations obtained from 2D
material characterization is developed and implemented within the CPFE
framework. Using this approach, grain level information about residual stresses
obtained spatially over the region of interest, directly from the EBSD and
high-energy x-ray diffraction microscopy, is utilized as an input to the model.</p>
<p>The second part of this research involves
calibrating the CPFE model based upon a systematic and detailed optimization routine
utilizing experimental data in the form of macroscopic stress-strain curves
coupled with lattice strains on different crystallographic planes for the α and
β phases, obtained from high energy X-ray diffraction experiments for multiple
material pedigrees with varying β volume fractions. This fully calibrated CPFE
model is then used to gain a comprehensive understanding of deformation
behavior of Ti-6Al-4V, specifically the effect of the relative orientation of
the α and β phases within the microstructure.</p>
<p>In the final part of this work, large and highly
textured regions, referred to as macrozones or microtextured regions (MTRs),
with sizes up to several orders of magnitude larger than that of the individual
grains, found in dual phase Titanium alloys are modeled using a reduced order
simulation strategy. This is done to overcome the computational challenges
associated with modeling macrozones. The reduced order model is then used to
investigate the strain localization within the microstructure and the effect of
varying the misorientation tolerance on the localization of plastic strain
within the macrozones.</p>
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Multi-Scale Thermal Modeling Methodology for High Power-Electronic CabinetsBurton, Ludovic Nicolas 24 August 2007 (has links)
Future generation of all-electric ships will be highly dependent on electric power, since every single system aboard such as the drive propulsion, the weapon system, the communication and navigation systems will be electrically powered. Power conversion modules (PCM) will be used to transform and distribute the power as desired in various zone within the ships. As power densities increase at both components and systems-levels, high-fidelity thermal models of those PCMs are indispensable to reach high performance and energy efficient designs. Efficient systems-level thermal management requires modeling and analysis of complex turbulent fluid flow and heat transfer processes across several decades of length scales.
In this thesis, a methodology for thermal modeling of complex PCM cabinets used in naval applications is offered. High fidelity computational fluid dynamics and heat transfer (CFD/HT) models are created in order to analyze the heat dissipation from the chip to the multi-cabinet level and optimize turbulent convection cooling inside the cabinet enclosure. Conventional CFD/HT modeling techniques for such complex and multi-scale systems are severely limited as a design or optimization tool. The large size of such models and the complex physics involved result in extremely slow processing time. A multi-scale approach has been developed to predict accurately the overall airflow conditions at the cabinet level as well as the airflow around components which dictates the chip temperature in details. Various models of different length scales are linked together by matching the boundary conditions. The advantage is that it allows high fidelity models at each length scale and more detailed simulations are obtained than what could have been accomplished with a single model methodology.
It was found that the power cabinets under the prescribed design parameters, experience operating point airflow rates that are much lower than the design requirements. The flow is unevenly distributed through the various bays. Approximately 90 % of the cold plenum inlet flow rate goes exclusively through Bay 1 and Bay 2. Re-circulation and reverse flow are observed in regions experiencing a lack of flow motion. As a result high temperature of the air flow and consequently high component temperatures are also experienced in the upper bays of the cabinet.
A proper orthogonal decomposition (POD) methodology has been performed to develop reduced-order compact models of the PCM cabinets. The reduced-order modeling approach based on POD reduces the numerical models containing 35 x 109 DOF down to less than 20 DOF, while still retaining a great accuracy. The reduced-order models developed yields prediction of the full-field 3-D cabinet within 30 seconds as opposed to the CFD/HT simulations that take more than 3 hours using a high power computer cluster. The reduced-order modeling methodology developed could be a useful tool to quickly and accurately characterize the thermal behavior of any electronics system and provides a good basis for thermal design and optimization purposes.
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Response mechanisms of attached premixed flames to harmonic forcingShreekrishna 26 August 2011 (has links)
The persistent thrust for a cleaner, greener environment has prompted air pollution regulations to be enforced with increased stringency by environmental protection bodies all over the world. This has prompted gas turbine manufacturers to move from non-premixed combustion to lean, premixed combustion. These lean premixed combustors operate quite fuel-lean compared to the stochiometric, in order to minimize CO and NOx productions, and are very susceptible to oscillations in any of the upstream flow variables. These oscillations cause the heat release rate of the flame to oscillate, which can engage one or more acoustic modes of the combustor or gas turbine components, and under certain conditions, lead to limit cycle oscillations. This phenomenon, called thermoacoustic instabilities, is characterized by very high pressure oscillations and increased heat fluxes at system walls, and can cause significant problems in the routine operability of these combustors, not to mention the occasional hardware damages that could occur, all of which cumulatively cost several millions of dollars.
In a bid towards understanding this flow-flame interaction, this research works studies the heat release response of premixed flames to oscillations in reactant equivalence ratio, reactant velocity and pressure, under conditions where the flame preheat zone is convectively compact to these disturbances, using the G-equation. The heat release response is quantified by means of the flame transfer function and together with combustor acoustics, forms a critical component of the analytical models that can predict combustor dynamics.
To this end, low excitation amplitude (linear) and high excitation amplitude (nonlinear) responses of the flame are studied in this work.
The linear heat release response of lean, premixed flames are seen to be dominated by responses to velocity and equivalence ratio fluctuations at low frequencies, and to pressure fluctuations at high frequencies which are in the vicinity of typical screech frequencies in gas turbine combustors.
The nonlinear response problem is exclusively studied in the case of equivalence ratio coupling. Various nonlinearity mechanisms are identified, amongst which the crossover mechanisms, viz., stoichiometric and flammability crossovers, are seen to be responsible in causing saturation in the overall heat release magnitude of the flame. The response physics remain the same across various preheat temperatures and reactant pressures.
Finally, comparisons between the chemiluminescence transfer function obtained experimentally and the heat release transfer functions obtained from the reduced order model (ROM) are performed for lean, CH4/Air swirl-stabilized, axisymmetric V-flames. While the comparison between the phases of the experimental and theoretical transfer functions are encouraging, their magnitudes show disagreement at lower Strouhal number gains show disagreement.
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Ordnungsreduktion in der MikrosystemtechnikGugel, Denis 19 July 2010 (has links) (PDF)
Die vorliegende Arbeit befasst sich mit der Methode der modalen Superposition als Ordnungsreduktionsverfahren in der Mikrosystemtechnik. Typische Anwendungsgebiete sind Inertialsensoren und dabei im Besonderen Drehratensensoren, für die die Simulation von zeitabhängigen Phänomenen von entscheidender Bedeutung ist.
Im Rahmen der Weiterentwicklung der Ordnungsreduktion nach der Methode der modalen Superposition ist es gelungen für typische lineare Kräfte eine auf analytischen Gleichungen basierende Beschreibung im reduzierten Raum zu finden. Für die Beschreibung von nichtlinearen Kräften ist im Rahmen dieser Arbeit ein Verfahren entwickelt worden, das es erlaubt, bestehende Modelle im Finite-Elemente-Raum in der modalen
Beschreibung zu nutzen.
In dieser Arbeit werden die theoretischen Grundlagen zur Berücksichtigung von Einflüssen der Aufbau- und Verbindungstechnik in ordnungsreduzierten Modellen dargestellt. Neben der Einkopplung äußerer Kräfte und der Veränderung der mechanischen Randbedingungen wird auch der Einfluss der Aufbau- und Verbindungstechnik auf die elektrostatischen Eigenschaften untersucht.
Die Parametrisierung des Verfahrens der modalen Superposition über Fit- und Interpolationsverfahren erlaubt es, parametrisierte ordnungsreduzierte Modelle für die zeitabhängige Systemsimulation zu generieren. Damit wird die Durchführung von Designoptimierung und die Berücksichtigung von Fertigungs- und Prozessschwankungen in ordnungsreduzierten Modellen auf Systemebene möglich.
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Ordnungsreduktion von elektrostatisch-mechanischen Finite Elemente Modellen für die Mikrosystemtechnik: Ordnungsreduktion von elektrostatisch-mechanischen FiniteElemente Modellen für die MikrosystemtechnikBennini, Fouad 25 January 2005 (has links)
In der vorliegenden Arbeit wird eine Prozedur zur Ordnungsreduktion von Finite Elemente
Modellen mikromechanischer Struktur mit elektrostatischem Wirkprinzip entwickelt und
analysiert. Hintergrund der Ordnungsreduktion ist eine Koordinatentransformation von
lokalen Finite Elemente Koordinaten in globale Koordinaten. Die globalen Koordinaten des
reduzierten Modells werden durch einige wenige Formfunktionen beschrieben. Damit wird
das Makromodell nicht mehr durch lokale Knotenverschiebungen beschrieben, sondern durch
globale Formfunktionen, welche die gesamte Deformation der Struktur beeinflussen. Es wird
gezeigt, dass Eigenvektoren der linearisierten mechanischen Struktur einfache und effiziente
Formfunktionen darstellen. Weiterhin kann diese Methode für bestimmte Nichtlinearitäten
und für verschiedene in Mikrosystemen auftretende Lasten angewendet werden. Das Ergebnis
sind Makromodelle, die über Klemmen in Systemsimulatoren eingebunden werden können,
die Genauigkeiten einer Finite Elemente Analyse erreichen und für Systemsimulationen
typische Laufzeitverhalten besitzen.
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Instabilités thermoacoustiques dans les moteurs à propergol solide / Thermo-acoustic instabilities in solid rocket motorsGenot, Aurélien 21 June 2019 (has links)
Dans un moteur à propergol solide, des instabilités thermoacoustiques auto-entretenues, induites par le couplage de la dynamique de la combustion des gouttes d’aluminium, libérées par la combustion du propergol, avec le champ acoustique peuvent induire des oscillations de pression.L’analyse menée tout au long de ce manuscrit repose sur un ensemble d’hypothèses simplificatrices: (i) la réponse de la combustion de gouttes d’aluminium aux perturbations acoustiques est contrôlée par l’écoulement local autour de la goutte, (ii) le processus de combustion peut être supposé quasi stationnaire pour la gamme de fréquences et les amplitudes acoustiques étudiées et (iii) la combustion de l’aluminium est brusquement arrêtée lorsque le diamètre de la goutte d’aluminium diminue en dessous d’un diamètre résiduel.L’instabilité thermoacoustique est étudiée au moyen de simulations numériques de l’écoulement dans un moteur générique et d’analyses théoriques. Le diamètre résiduel des gouttes d’aluminium après la combustion, l’amplitude de la perturbation acoustique et la durée de la combustion des gouttes d’aluminium figurent parmi les principaux paramètres modifiant l’instabilité. En outre, trois comportements de réponse de la combustion à l’acoustique sont identifiés : un comportement linéaire pour les faibles niveaux de pression acoustique puis un comportement quadratique (faiblement non-linéaire) et enfin un comportement fortement non-linéaire quand l’amplitude des oscillations augmente.Ensuite, deux aspects importants de la réponse des gouttes d’aluminium sont identifiés. Ils sont associés aux oscillations de la durée du temps de combustion des gouttes, identifiables à la frontière du nuage de gouttes, et aux fluctuations du taux d’évaporation contrôlées par la convection de l’écoulement gazeux autour de chaque goutte. Tenant compte de ces dynamiques,des expressions analytiques sont obtenues permettant de reproduire avec précision les résultats numériques des simulations de l’écoulement. Quatre nombres sans dimension qui régissent la dynamique de ces instabilités sont également identifiés. Inspiré de l’analyse théorique précédente, un modèle numérique d’ordre réduit faiblement non linéaire est finalement développé pour prédire des cycles limites. / In a solid rocket motor, self-sustained thermo-acoustic instabilities, induced by the coupling of the combustion dynamics of aluminum droplets released by the burning propellant with the acoustic field can induce pressure oscillations.The analysis conducted throughout this manuscript relies thus on a set of simplifying hypothesis by assuming (i) that the response of the combustion of aluminum droplets to acoustic perturbations is controlled by the oscillating drag exerted by the local flow around the droplet, (ii) that this unsteady combustion process can be assumed quasi-steady for the range of frequencies and acoustic amplitudes studied and (iii) that aluminum combustion is abruptly quenched when the aluminum droplet diameter falls below a residual diameter.The thermo-acoustic instability is studied first by numerical flow simulations in a generic solid rocket motor and theoretical analyses. The post-combustion residual diameter of the aluminum particles, the amplitude of acoustic perturbation and the lifetime of the burning aluminum droplets are among the main parameters altering the instability. Also, three combustion response behaviors to acoustics are identified : a linear behavior for small acoustic pressure levels followed by a quadratic behavior then a highly non-linear behavior when the pressure amplitude increases in the motor chamber. Moreover, two important features of the response of aluminum droplets are identified. They are associated to oscillations of the droplet lifetime at the boundary of the droplet cloud and to fluctuations of the droplet evaporation rate, controlled by convection. The dynamics of the droplets highly depends on gas and droplet velocity fields and on droplet diameter. Taking these features into account, yields analytical expressions that allow to reproduce with accuracy the numerical results from the flow simulations. Four dimension less numbers are then identified. They govern the dynamics of these instabilities. Inspired from the previous theoretical analysis, a weakly nonlinear low-order numerical model is finally developed to predict limit cycles.
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Theory and Application of Damping in Jointed StructuresMathis, Allen, MATHIS 28 June 2019 (has links)
No description available.
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