The main objective of this thesis is to investigate the effects of turbulent flow on the random dynamics of structurally nonlinear airfoil. A secondary objective is to articulate a more comprehensive picture of the contribution of the longitudinal component of turbulence, as experienced by the airfoil, be it linear or nonlinear. In this regard, a systematic and detailed numerical analysis of the airfoil experiencing random flutter/Hopf bifurcation is presented. Some aspects of the divergence/pitchfork problem are also discussed. / The airfoil is modelled as a flexibly mounted rigid flat plate with degrees-of-freedom in pitch and heave. The principal nonlinearity considered is a hardening cubic torsional spring. The aerodynamics is incompressible and linear. Unsteady aerodynamic effects due to arbitrary motion and turbulence are modelled. Both longitudinal and vertical components of the Gaussian turbulence are considered. Longitudinal turbulence acts as a parametric excitation, whereas the latter represents an external forcing. / A Monte Carlo simulation is performed to solve numerically the system of random differential equations. The time history solutions are then studied in terms of their mean-square, probability density function and power spectral density. The largest Lyapunov exponent is also calculated. / The bifurcation, stability and response characteristics of the airfoil are examined. For the linear airfoil, it is found that the coalescence flutter speed is always advanced by the longitudinal component of turbulence, and generally dominated by the very low frequency range of the excitation. Divergence can be either advanced or postponed, but the magnitude of the shift is not significant compared with flutter. Furthermore, it is shown that in general longitudinal turbulence decreases the overall stability of the airfoil, be it linear or nonlinear. / For the nonlinear airfoil, it is the vertical component of turbulence that determines the essential features of the stochastic bifurcation and the qualitative characteristics of the response. The interplay between turbulence and nonlinear stiffness has a significant impact on the probability structure of the aeroelastic response. Uni-, bi- and double bi-modal distributions are observed, and found to occur at different airspeeds depending on which state variable is considered. Furthermore, the spectral content displays noise-controlled, and noise-induced, time scales.

The near-field flow structure of the tip vortex generated by a NACA 0015 wing oscillating though the attached-flow, light-stall and deep-stall regimes was investigated at Rec = 1.86 x 105. Particular emphasis was placed on the effects of oscillation frequency and mean incidence upon the spatial and temporal evolution of the unsteady vortex structure. Phase-locked, ensemble-averaged cross-flow and axial velocity fields, vorticity distributions, and turbulence structures over a full cycle of oscillation were compared to static wing-tip vortex results, and the dynamic effects upon the vortex strength, size, trajectory and associated induced drag were examined. Through the attached-flow and light-stall oscillations, most vortex properties were qualitatively similar to the static cases, though a small degree of hysteresis between the pitch-up and pitch-down phases of motion was observed. The radial distributions of circulation within the inner region of the vortex were self-similar, and showed only small variations from the static case. When the wing was oscillated through the deep-stall regime, a dramatic decrease in tip vortex strength and concentration was observed at the end of the upstroke, as a result of the growth of the leading-edge vortex and subsequent catastrophic flow separation. The use of passive spoilers and active flaps to control the strength and trajectory of the tip vortex was also investigated.

A methodology is presented to calculate the response of a wing as well as the aerodynamic forces on the wing, due to forced aileron oscillations. The purpose of oscillating the ailerons in flight is to excite the vibration modes of the wing. From vibration recordings and knowledge of the aerodynamic forces induced by the aileron oscillation, the vibration modes of the wing can be identified. Identified modes may then be compared to wing vibration modes which are calculated through aeroelastic analyses of the wing. In particular, the flutter characteristics of the wing, predicted through aeroelastic analyses, can be verified. / The flutter and forced aileron oscillation analyses are performed on the aeroelastic equations built for a finite-element model of the wing. An existing finite-element model is used. The aerodynamic forces induced by wing vibrations are calculated through the doublet-lattice method, using existing software. The flutter and forced aileron oscillation analyses are based on the modal form of the aeroelastic equations. The modal transformation and the resulting equations are rigourously outlined. A physical interpretation of the mathematical formulae is given. (Abstract shortened by UMI.)

This thesis presents a method based on velocity singularities applied to the analysis of steady flows over symmetrical thickness airfoils. The method of velocity singularities has been originally developed by Mateescu for the analysis of steady flows past thin airfoils and makes use of special singularities associated to the leading edge and ridges of the airfoil to represent the complex perturbation velocity. The application of the linear method to symmetrical thickness airfoils presents inaccuracies near the leading and trailing edges of the airfoils (namely, the magnitude of the perturbation velocity in the vicinity of these points is infinite). The aim of the present thesis is to provide a non-linear extension of the velocity singularities method, in order to eliminate these inaccuracies. / Closed form solutions were derived for the perturbation velocity for various airfoils with pointed and rounded leading edges. The non-linear boundary conditions on the airfoil have been vigorously implemented in the present analytical formulation. / The present method has been validated by comparison with the exact solutions obtained by conformal transformation for the lenticular circular-arc airfoils and for Karman-Trefftz airfoils, and by experimental results for NACA airfoils. The present solution was found in very good agreement with these previous results.

A recent series of high altitude turbofan engine malfunctions, characterized by flameout and sudden power losses have been reported in recent years. The source of these incidents has been hypothesized to be due to the presence of ice crystals at high altitudes. Ice crystals have been shown to have ballistic trajectories and consequently enter the core engine flow, without getting centrifuged out towards the engine bypass as droplets do. The crystals may melt as they move downstream to higher temperatures in successive stages, or hit a heated surface. The wetted surface may then act as an interface for further crystal impingement, which locally reduces the temperature and could lead to an ice accretion on the components. Ice can accrete to dangerously high levels, causing compressor surge due to blockage of the primary flowpath, vibrational instabilities due to load imbalances of ice on rotating components, mechanical damage of components downstream due to large shed ice fragments, or performance losses if ice enters the combustor, causing a decreased burner efficiency and an eventual flame-out. / In order to provide a numerical tool to analyze such situations, FENSAP-ICE has been extended to model mixed-phase flows that combine air, water and ice crystals, and the related ice accretion. DROP3D has been generalized to calculate particle impingement, concentration, and field velocities in an uncoupled approach that neglects any phase change by assuming both ice crystals and supercooled droplets are in thermodynamic equilibrium. ICE3D then accounts for the contribution of ice crystals that stick and melt on an existing water-film and promote ice accretion. / The extended ice crystal impingement and ice accretion model has been validated against test data from Cox and Co. and National Research Council icing tests conducted on a NACA0012 airfoil and unheated non-rotating cylinder respectively. The tests show a consistent agreement with respect to experimental profiles in terms of capturing the overall shape, although some of the ice profiles were conservative since they over-predicted the amount of ice accreted. The experimental observations suggest that ice crystals cause splashing of an existing film, and erosion effects when they impact an iced surface, and cause an overall loss in the amount of ice, as well as a general streamlining of the ice profile. This has not been taken into account in the present numerical model. The overall predictions in comparison with other numerical models, however, have improved and are a promising step towards simulating ice-shedding characteristics in a turbomachine. / De nombreux incidents liés à des problèmes de fonctionnement de moteurs d'avions ont été observés ces dernières années, tous caractérisés par l'extinction du moteur ou une perte soudaine de sa puissance. Ces incidents à haute altitude pourraient être causés par des cristaux de glace qui, de par leur trajectoire balistique, entrent directement dans le coeur du moteur sans être déviés par la force centrifuge vers le pontage, comme pour les gouttelettes d'eau. Les cristaux peuvent alors fondre lorsqu'ils rencontrent des températures plus élevées dans le moteur ou lorsqu'ils heurtent une surface chaude. Une telle surface humide pourrait devenir un noyau de cristallisation en réduisant localement la température, favorisant ainsi la formation de glace sur les composants internes du moteur. Cette accumulation présente un danger lorsqu'elle réduit l'espace libre pour l'écoulement d'air, engendrant un phénomène de pompage du compresseur. Elle peut aussi causer des instabilités vibrationnelles lorsqu'elle n'est pas uniforme sur les composantes rotatives, causant ainsi un débalancement de charge. De l'impact des tessons de glace qui se décollent de la surface peut endommager l'équipement mécanique en aval, et causer des pertes de performance liées à la présence de glace dans la chambre de combustion, engendrant une chute de l'efficacité du brûleur et éventuellement l'extinction de la flamme. / Afin de fournir un outil numérique pour l'analyse de telles situations, des modifications ont été apportées à FENSAP-ICE pour lui permettre de simuler l'écoulement de phases hétérogènes (air, eau, cristaux de glace) et l'accumulation de glace sur la surface. DROP3D a été généralisé afin de calculer la concentration et les champs de vitesses d'une particule de façon non couplée, en supposant que les cristaux de glace et les gouttelettes d'eau surgelées coexistent en équilibre thermodynamique. ICE3D à été modifié pour tenir compte des cristaux de glace qui se collent sur une couche d'eau existante et qui fondent, favorisant le phénomène d'accrétion de glace. / Les modifications au modèle de simulation d'accrétion de glace pour les cristaux de glace ont été validées à l'aide des données de Cox & Co., ainsi que des essais du Conseil National de Recherche du Canada, portant sur un profil NACA0012 et un cylindre sans rotation ni chauffage. Les résultats de ces essais démontrent que le modèle de simulation est généralement capable de prédire les formes de glace, sauf pour quelques profils qui ont donné des résultats conservateurs avec une plus importante accumulation de glace. Cet excès de glace peut être expliqué par des observations expérimentales qui suggèrent que l'impact des cristaux de glace incidents avec la surface cause des éclaboussures dans la couche d'eau existante, et que l'écoulement autour de la glace provoque l'érosion de celle-ci, produisant ainsi une surface plus lisse et plus réfractaire à l'accumulation. Ces effets n'ont pas été considérés dans ce modèle de simulation numérique. En général, les prédictions s'améliorent lorsqu'on les compare avec d'autres modèles et représentent un résultat prometteur pour la simulation des caractéristiques de délestage de glace dans un turboréacteur.

The space elevator offers an alternate and very efficient method for space travel. It will have two main components. The first component is the tether (or the ribbon), which extends from the Earth to an equatorial satellite at an altitude of about 100,000 kilometres, and is fixed to a base on the surface of the Earth at its lower end. The second component is the climber, which scales the ribbon, transporting payloads to space. An important issue for effective operation of the space elevator will be to understand its dynamics. This thesis attempts to develop a realistic and yet simple planar model for this. Both rigid and elastic ribbon models are considered. Their response to ascending climbers and to aerodynamic loads is studied. Specific climbing procedures are devised based on these results. The effect of the space elevator's motion on the orbit of a launched satellite is also examined.

The problem of flutter is first introduced. The equations of motion of an airfoil with two degrees of freedom, in pitch and plunge, are obtained. Then, the unsteady aerodynamic theories for different flow regimes are presented. The traditional solutions to the flutter problem, namely, the p-k and U-g methods, are formulated, and the Laplace transformation method for flutter analysis is also introduced. Then, the effect of different design parameters of an airfoil on the flutter speed is analyzed, both in the incompressible and transonic regimes. Furthermore, the effect of the relative values of the design parameters on the occurrence of flutter is investigated. Finally, some general conclusions regarding the above-mentioned phenomena are derived. The goal of this work is the fact that, the unsteady aerodynamic data has been used, both in the incompressible and transonic regimes, and, therefore, the obtained results are fairly precise.

In this thesis, spacecraft rendezvous and spacecraft formation flying were examined in context of the TECSAS mission. Three terminal rendezvous trajectories, one V-bar approach and two R-bar approaches, are compared in terms of DeltaV fuel usage and time of flight (TOF). Results showed that the lidar-based V-bar approach trajectory with a 90 m straight-line approach distance is the optimal scenario for the given TECSAS mission guidelines. Four formation flying scenarios are examined: two projected circular formations and two in-track formations. The effects of the J2 and atmospheric drag perturbations on these formations are studied for several time spans. Results showed that the projected circular formations are disrupted by the J2 perturbations after a short time span, and that atmospheric drag perturbations caused significant in-track and radial drifts for formations where the two spacecraft are not identical. Finally, control force requirements are much higher for both formations when the two spacecraft are not identical.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 68-11, Section: B, page: 7462. Adviser: Michael B. Bragg. Includes bibliographical references (leaves 409-415) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--California Institute of Technology, 2006. / (UMI)AAI3235580. Adviser: Hans G. Hornung. Source: Dissertation Abstracts International, Volume: 67-09, Section: B, page: 5221.