Aeroelastic behavior prediction is often confined to analytical or highly computational methods, so I developed a low degree of freedom computational method using structural finite elements and unsteady loading to cover a gap in the literature. Finite elements are readily suitable for determination of the free vibration characteristics of eccentric, elastic structures, and the free vibration characteristics fundamentally determine the aeroelastic behavior. I used Theodorsen’s unsteady strip loading formulation to model the aerodynamic loading on linear elastic structures assuming harmonic motion. I applied Hassig’s ‘p-k’ method to predict the flutter boundary of nonsymmetric, aeroelastic systems. I investigated the application of a quintic interpolation assumed displacement shape to accurately predict higher order characteristic effects compared to linear analytical results. I show that quintic interpolation is especially accurate over cubic interpolation when multi-modal interactions are considered in low degree of freedom flutter behavior for high aspect ratio HALE aircraft wings.

The unsteady interaction between trailing edge aerodynamic flow control and airfoil motion in pitch and plunge is investigated in wind tunnel experiments using a two degree-of-freedom traverse which enables application of time-dependent external torque and forces by servo motors. The global aerodynamic forces and moments are regulated by controlling vorticity generation and accumulation near the trailing edge of the airfoil using hybrid synthetic jet actuators. The dynamic coupling between the actuation and the time-dependent flow field is characterized using simultaneous force and particle image velocimetry (PIV) measurements that are taken phase-locked to the commanded actuation waveform. The effect of the unsteady motion on the model-embedded flow control is assessed in both trajectory tracking and disturbance rejection maneuvers. The time-varying aerodynamic lift and pitching moment are estimated from a PIV wake survey using a reduced order model based on classical unsteady aerodynamic theory. These measurements suggest that the entire flow over the airfoil readjusts within 2-3 convective time scales, which is about two orders of magnitude shorter than the characteristic time associated with the controlled maneuver of the wind tunnel model. This illustrates that flow-control actuation can be typically effected on time scales that are commensurate with the flow's convective time scale, and that the maneuver response is primarily limited by the inertia of the platform.

This thesis addresses flight control of the perch landing maneuver for micro-aerial vehicles. A longitudinal flight model is constructed for a pigeon-sized aircraft. In addition to a standard elevator control surface, wing-rotation also considered as a non-standard actuator for increasing low-speed aerodynamic braking. Optimal state and control trajectories for the perch landing maneuver are computed using commercial software. A neighboring optimal control law is then developed and implemented in a set of flight simulations. Simulations are run with both a quasisteady and an unsteady aerodynamic model. The effectiveness of wing rotation and of the neighboring optimal control law is discussed, as is the importance of unsteady aerodynamics during the maneuver. Wing rotation was found to be minimally effective in this case, but it showed potential to be more effective in further research. The unsteady aerodynamic model has significant influence over the success or failure of the maneuver.

Ziel dieser Arbeit war, die Auswirkungen von Clocking der Leitreihen eines mehrstufigen Axialverdichters auf Potentiale hinsichtlich der Beeinflussung instationärer und stationärer Effekte zu untersuchen und zum grundlegenden Verständnis der instationären Schaufelinteraktion beizutragen. Dazu wurden über 2000 Leitgitterkonfigurationen vermessen, so dass der Einfluss von Clocking auf den Wirkungsgrad entlang der Kennlinie bei Auslegungsdrehzahl, auf die Schaufelgrenzschichten und auf die Betriebsgrenzen untersucht und dokumentiert werden konnte. Vor allem wurde so eine erhebliche Beeinflussung der Pumpgrenze gefunden, während das Grenzschichtverhalten auf den Schaufeln und der Wirkungsgrad im praktisch relevanten Bereich der Kennlinie kaum verändert wurden. Hauptgegenstand der Untersuchungen war aber der Einfluss von Stator-Clocking auf die instationären Druckverteilungen und die resultierenden instationären Erregerkräfte an den Lauf- und Leitschaufeln. Die Vermessung der Auswirkungen der Positionierung jedes einzelnen Leitgitters wurde genutzt, um durch eine einfache Optimierung zwei geometrische Konfigurationen aller Leitgitter zu entwickeln. Die eine Konfiguration führte zu geringen aerodynamischen Erregerkräften an den Laufschaufeln aller Stufen, während die andere Konfiguration eine gleichmäßig hohe instationäre Anregung zur Folge hatte. Die Unterschiede der instationären Erregerkräfte zwischen den Konfigurationen waren erheblich und über weite Bereiche der Kennlinie unabhängig vom Betriebspunkt, ohne das die Konfiguration der Leitgitter geändert wurde. Für eine umfassende Analyse der periodisch instationären, aerodynamischen Schaufelinteraktion wurden sowohl die Schaufeldruckverteilungen, als auch das Strömungsfeld in den axialen Schaufelzwischenräumen im Mittelschnitt der Beschaufelung für beide Clocking-Konfigurationen zeitgenau vermessen und vergleichend ausgewertet. Aus diesen Analysen konnte mithilfe der Wellenmechanik eine einfache analytische Beschreibung der instationären Interaktion der Potentialfelder der Beschaufelung entwickelt werden. Für eine einzelne Stufe wurde mit diesem Modell die experimentell bestimmte Phasendifferenz der Druckschwankungen auf Druck- und Saugseite auf sehr einfache Weise nachgewiesen. Damit liegt ein einfaches, analytisches Modell für die Beschreibung der komplexen Überlagerung der sich relativ zueinander bewegenden Druckfelder der Beschaufelung axialer Turbomaschinen vor, das für das physikalische Verständnis der instationären Schaufelinteraktion einen wertvollen Beitrag liefert.

Birds and bats employ a variety of advanced wing motions in the efficient production of thrust. The purpose of this thesis is to quantify the benefit of these advanced wing motions, determine the optimal theoretical wing kinematics for a given flight condition, and to develop a methodology for applying the results in the optimal design of flapping-wing aircraft (ornithopters). To this end, a medium-fidelity, combined aero-structural model has been developed that is capable of simulating the advanced kinematics seen in bird flight, as well as the highly non-linear structural deformations typical of high-aspect ratio wings. Five unique methods of thrust production observed in natural species have been isolated, quantified and thoroughly investigated for their dependence on Reynolds number, airfoil selection, frequency, amplitude and relative phasing. A gradient-based optimization algorithm has been employed to determined the wing kinematics that result in the minimum required power for a generalized aircraft or species in any given flight condition. In addition to the theoretical work, with the help of an extended team, the methodology was applied to the design and construction of the world's first successful human-powered ornithopter. The Snowbird Human-Powered Ornithopter, is used as an example aircraft to show how additional design constraints can pose limits on the optimal kinematics. The results show significant trends that give insight into the kinematic operation of natural species. The general result is that additional complexity, whether it be larger twisting deformations or advanced wing-folding mechanisms, allows for the possibility of more efficient flight. At its theoretical optimum, the efficiency of flapping-wings exceeds that of current rotors and propellers, although these efficiencies are quite difficult to achieve in practice.

Birds and bats employ a variety of advanced wing motions in the efficient production of thrust. The purpose of this thesis is to quantify the benefit of these advanced wing motions, determine the optimal theoretical wing kinematics for a given flight condition, and to develop a methodology for applying the results in the optimal design of flapping-wing aircraft (ornithopters). To this end, a medium-fidelity, combined aero-structural model has been developed that is capable of simulating the advanced kinematics seen in bird flight, as well as the highly non-linear structural deformations typical of high-aspect ratio wings. Five unique methods of thrust production observed in natural species have been isolated, quantified and thoroughly investigated for their dependence on Reynolds number, airfoil selection, frequency, amplitude and relative phasing. A gradient-based optimization algorithm has been employed to determined the wing kinematics that result in the minimum required power for a generalized aircraft or species in any given flight condition. In addition to the theoretical work, with the help of an extended team, the methodology was applied to the design and construction of the world's first successful human-powered ornithopter. The Snowbird Human-Powered Ornithopter, is used as an example aircraft to show how additional design constraints can pose limits on the optimal kinematics. The results show significant trends that give insight into the kinematic operation of natural species. The general result is that additional complexity, whether it be larger twisting deformations or advanced wing-folding mechanisms, allows for the possibility of more efficient flight. At its theoretical optimum, the efficiency of flapping-wings exceeds that of current rotors and propellers, although these efficiencies are quite difficult to achieve in practice.

A three-dimensional, parallel, finite volume solver which uses Roe&#039 / s upwind flux differencing scheme for spatial and Runge-Kutta explicit multistage time stepping scheme for temporal discretization on unstructured meshes is developed for the unsteady solution of external viscous flow around rotating bodies. The main aim of this study is to evaluate the aerodynamic dynamic stability derivative coefficients for rotating missile configurations. Arbitrary Lagrangian Eulerian (ALE) formulation is adapted to the solver for the simulation of the rotation of the body. Eigenvalues of the Euler equations in ALE form has been derived. Body rotation is simply performed by rotating the entire computational domain including the body of the projectile by means of rotation matrices. Spalart-Allmaras one-euqation turbulence model is implemented to the solver. The solver developed is first verified in 3-D for inviscid flow over two missile configurations. Then inviscid flow over a rotating missile is tested. Viscous flux computation algorithms and Spalarat-Allmaras turbulence model implementation are validated in 2-D by performing calculations for viscous flow over flat plate, NACA0012 airfoil and NLR 7301 airfoil with trailing edge flap. The ALE formulation is validated in 2-D on a rapidly pitching NACA0012 airfoil. Afterwards three-dimensional validation studies for viscous, laminar and turbulent flow calculations are performed on 3-D flat plate problem. At last, as a validation test case, unsteady laminar and turbulent viscous flow calculations over a spinning M910 projectile configuration are performed. Results are qualitatively in agreement with the analytical solutions, experimental measurements and previous studies for steady and unsteady flow calculations.

The aim of this study is to simulate the unsteady flow around rotating or oscillating airfoils. This will help to understand the rotor aerodynamics, which is essential in turbines and propellers. In this study, a pre-existing Euler solver with finite volume method that is developed in the Mechanical Engineering Department of Middle East Technical University (METU) is improved. This structured pre-existing code was developed for 2-D internal flows with Lax-Wendroff scheme. The improvement consist of firstly, the generalization of the code to external flow / secondly, implementation of first order Roe&rsquo / s flux splitting scheme and lastly, the implementation of rotation with the help of Arbitrary Lagrangian Eulerian (ALE) method. For the verification of steady and unsteady results of the code, the experimental and computational results from literature are utilized. For steady conditions, subsonic and transonic cases are investigated with different angle of attacks. For the verification of unsteady results of the code, oscillating airfoil case is used. The flow is assumed as inviscid, unsteady, adiabatic and two dimensional. The gravity is neglected and the air is taken as ideal gas. The developed code is run on computers housed in METU Mechanical Engineering Department Computational Fluid Dynamics High Performance Computing (CFD-HPC) Laboratory.

This thesis aims to explore the potential for using large eddy simulation (LES) as a predictive tool for gas-turbine compressor flows. Compressors present a significant challenge for the Reynolds Averaged Navier-Stokes (RANS) based CFD methods commonly used in industry. RANS models require extensive calibration to experimental data, and thus cannot be used predictively. This thesis explores how LES can offer a more predictive alternative, by exploring the sensitivity of LES to sources of uncertainty. Specifically, the importance of the numerical scheme, the Sub-Grid Scale (SGS) model, and the correct specification of inflow turbulence is examined. The sensitivity of LES to the numerical scheme is explored using the Taylor-Green vortex test case. The numerical smoothing, controlled by a user defined smoothing constant, is found to be important. To avoid tuning the numerical scheme, a locally adaptive smoothing (LAS) scheme is implemented. But, this is found to perform poorly in a forced isotropic turbulence test case, due to the intermittency of the dispersive error. A novel scheme, the LAS with windowing (LASW) scheme, is thus introduced. The LASW scheme is shown to be more suitable for predictive LES, as it does not require tuning to a known solution. The LASW scheme is used to perform LES on a compressor cascade, and results are found to be in close agreement with direct numerical simulations. Complex transition mechanisms, combining characteristics of both natural and bypass modes, are observed on the pressure surface. These mechanisms are found to be sensitive to numerical smoothing, emphasising the importance of the LASW scheme, which returns only the minimum smoothing required to prevent dispersion. On the suction surface, separation induced transition occurs. The flow here is seen to be relatively insensitive to numerical smoothing and the choice of SGS model, as long as the Smagorinsky-Lilly SGS model is not used. These findings are encouraging, as they show that, with the LASW scheme and a suitable SGS model, LES can be used predictively in compressor flows. In order to be predictive, the accurate specification of inflow conditions was shown to be just as important as the numerics. RANS models are shown to over-predict the extent of the three dimensional separation in the endwall - suction surface corner. LES is used to examine the challenges for RANS in this region. The LES shows that it is important to accurately capture the suction surface transition location, with early transition leading to a larger endwall separation. Large scale aperiodic unsteadiness is also observed in the endwall region. Additionally, turbulent anisotropy in the endwall - suction surface corner is found to be important. Adding a non-linear term to the RANS model leads to turbulent stresses that are in better agreement with the LES. This results in a stronger corner vortex which is thought to delay the corner separation. The addition of a corner fillet reduces the importance of anisotropy, thereby reducing the uncertainty in the RANS prediction.

L’introduction de fuselages et de pales de plus en plus légers durant le développement des nouveaux hélicoptères, combinée à une puissance disponible augmentée peut donner lieu à des couplages rotor/structure d’un nouveau genre. Ces instabilités complexes apparaissent à des fréquences plus élevées que les couplages connus et étudiés tels que les résonances sol et air, et impliquent des modes de pale souple, des modes de structure, et des phénomènes aérodynamiques. Des codes de calcul multi-corps aéromécaniques tels que HOST sont capables de déterminer la stabilité de l’hélicoptère, mais sont difficilement modifiables et manipulables. Des modèles analytiques existent également pour les instabilités maîtrisées citées précédemment, mais n’ont pas les capacités de modélisation nécessaires à la prédiction de ces couplages haute fréquence. Ce travail de thèse se concentre sur le développement d’un modèle semi-analytique, capable de prédire la stabilité de l’hélicoptère vis-à-vis de ces phénomènes. Cette approche est différente de l’approche multi-corps et a un double avantage car elle permet des études paramétriques rapides et une analyse terme à terme des équations de la dynamique de l’hélicoptère. Ce modèle a été validé à l’aide de HOST et le mécanisme de l'instabilité a été détaillé. Enfin, l’influence des paramètres de rotor, de structure, et de vol a été évaluée et les considérations architecturales pour éviter l'apparition de tels phénomènes sont présentées. / The introduction of lightweight fuselages and blades during new developments, combined with an increased available power, may lead to the triggering of a new kind of rotor/structure coupling. These complex instabilities appear at higher frequencies than known and studied couplings, such as ground and air resonance, and involve elastic blade modes, structure modes, and aerodynamic phenomena. Comprehensive analysis codes, like HOST, are able to determine the helicopter stability but can hardly be tweaked and handled. Rotor/structure coupling analytical models also exist for ground and air resonance, but do not have the modeling capabilities required to predict these high frequency couplings. This research work focuses on the development of a semi-analytical model, able to predict the helicopter stability with respect to these phenomena. This approach has a two-fold advantage since fast parametric studies can be carried out and a term-by-term analysis of the helicopter stability equations can be performed. This model has been validated with HOST and the triggering mechanism has been detailed. Finally, the influence of rotor, structure, and flight parameters has been evaluated and architectural considerations to avoid the appearance of such couplings are presented.