Nonlinear Aeroelasticity and Active Control of Airfoils Subjected to Gusts

In this thesis, the coupling effects of structural nonlinearities and a gust input on the aeroelastic behaviour of an airfoil are studied, and an adaptive controller which is effective for suppressing limit-cycle oscillations (LCOs) is designed. The dynamics of the airfoil are approximated via two- (pitch and plunge) and three-degree-of-freedom (pitch, plunge and flap) models. Different types of structural nonlinearities, such as free-play and hysteresis are considered in the modelling. The nonlinear dynamics is analyzed based on time history, power spectral density (PSD), phase-plane, and Poincar\'{e} section plots, along with the estimation of the dominant Lyapunov exponent for the chaotic-like motion. It is found that free-play and hysteresis nonlinearities may considerably reduce the critical flow velocity compared to the linear system. The dynamic responses of the nonlinear system to sharp-edged and 1-cosine gust profiles are obtained at different flow velocities and compared to those of the system with no gust input. In addition, basin of attraction is plotted to show the stability boundary of the system subjected to a sharp-edged gust with various amplitudes. It is discussed that as the gust becomes stronger, the likelihood of the occurrence of LCO increases. Based on the nonlinear model with a control surface, the suppression of LCO is studied. Without uncertainties, a PD controller together with a partial feedback linearized controller can effectively alleviate oscillations due to gusts and structural nonlinearities. Considering some uncertain structural parameters, an adaptive controller with estimation parameter update law is further designed to stabilize the system. A Lyapunov function is constructed and utilized to prove the stability of the system

Computational Fluid Dynamics Simulations of Oscillating Wings and Comparison to Lifting-Line Theory

Computational fluid dynamics (CFD) analysis was performed in order to compare the solutions of oscillating wings with Prandtl’s lifting-line theory. Quasi-steady and steady-periodic simulations were completed using the CFD software Star-CCM+. The simulations were performed for a number of frequencies in a pure plunging setup. Additional simulations were then completed using a setup of combined pitching and plunging at multiple frequencies. Results from the CFD simulations were compared to the quasi-steady lifting-line solution in the form of the axial-force, normal-force, power, and thrust coefficients, as well as the efficiency obtained for each simulation. The mean values were evaluated for each simulation and compared to the quasi-steady lifting-line solution. It was found that as the frequency of oscillation increased, the quasi-steady lifting-line solution was decreasingly accurate in predicting solutions

CEAS/AIAA/ICASE/NASA Langley International Forum on Aeroelasticity and Structural Dynamics 1999

These proceedings represent a collection of the latest advances in aeroelasticity and structural dynamics from the world community. Research in the areas of unsteady aerodynamics and aeroelasticity, structural modeling and optimization, active control and adaptive structures, landing dynamics, certification and qualification, and validation testing are highlighted in the collection of papers. The wide range of results will lead to advances in the prediction and control of the structural response of aircraft and spacecraft

Analysis of non-linear aeroelastic systems using numerical continuation

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Aeroelastic study of a multi-hinged wing

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2003.Includes bibliographical references (p. 109-112).Dynamic aeroelastic response of multi-segmented hinged wings is studied theoretically and experimentally in this thesis. For the theoretical study, a method of modeling the aeroelastic characteristics of multi-hinged wings is proposed. The method employs the Runge-Kutta scheme to solve the governing equations of a flexible multibody dynamic system. The Henon method is used to switch between bilinear stiffness states of the wing in bending. Experimental wind tunnel tests of one- and five-hinged wings were conducted for better insight into the mechanics of the motion. Correlation between the experimental and theoretical results is presented. The theoretical model is found to capture both the linear and nonlinear aeroelastic behavior of a hinged wing. Adding hinges to a wing is found to significantly alter the speed at which an instability will occur. The stiffness of the hinges is found to play a major role in the determination of flutter speeds with a reduction in hinge stiffness nominally leading to an increase in first bending / first torsion instability speeds. However, for low hinge stiffness, hinged wings were also found to have the possibility of a second bending / first torsion instability at speeds far below the first bending instability. The hinged wing is found to enter into chaotic or limit cycle motion at speeds at, near, or above flutter speeds. The bi-linear nature of a hinge is found to cause a disruption in the coalescence of modes. This limits the energy added to the system while it is in an unstable state. The hinges allow the wing to "fold" under low net loads. The theoretical model can be used for aeroelastic design of future hinged wings for remotely deployable vehicles.by Torrey Owen Radcliffe.Ph.D

CEAS/AIAA/ICASE/NASA Langley International Forum on Aeroelasticity and Structural Dynamics 1999

The proceedings of a workshop sponsored by the Confederation of European Aerospace Societies (CEAS), the American Institute of Aeronautics and Astronautics (AIAA), the National Aeronautics and Space Administration (NASA), Washington, D.C., and the Institute for Computer Applications in Science and Engineering (ICASE), Hampton, Virginia, and held in Williamsburg, Virginia June 22-25, 1999 represent a collection of the latest advances in aeroelasticity and structural dynamics from the world community. Research in the areas of unsteady aerodynamics and aeroelasticity, structural modeling and optimization, active control and adaptive structures, landing dynamics, certification and qualification, and validation testing are highlighted in the collection of papers. The wide range of results will lead to advances in the prediction and control of the structural response of aircraft and spacecraft

Numerical study of self-sustained oscillations in transitional flows

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2012-2013.Ce mémoire présente une étude numérique du phénomène d'oscillations auto-induites d'une aile rigide montée sur un support élastique. Ces oscillations ont été rapportées expérimentalement au Collège Militaire Royal du Canada par l'équipe du professeur Poirel. Ils ont montré que le phénomène a lieu dans une plage de nombres de Reynolds spécifique où la transition de la couche limite peut survenir : 5 x 10⁴ < Rec < 1.3 x 10⁵. Des oscillations en tangage seulement ainsi qu'en tangage et pilonnement ont été observées. Les oscillations en tangage seulement ont une amplitude d'environ 5 degrés et une fréquence aux alentours de 3 Hz. Les oscillations en tangage et pilonnement ont des amplitudes de tangage pouvant atteindre 65 degrés selon la rigidité structurale et des fréquences allant de 3 à 5 Hz. Le phénomène a été étudié ici par la mécanique des fluides numérique. Le code libre OpenFOAM utilisant la méthode des volumes finis a été utilisé pour simuler le problème aéroélastique. Dans le cas des oscillations en tangage, une très bonne comparaison entre les résultats numériques et expérimentaux a été obtenue. L'utilisation d'un modèle de transition a entraîné une amélioration par rapport aux simulations numériques réalisées dans le passé et a contribué à mieux élucider la physique en jeu. La séparation de la couche limite laminaire étant le mécanisme déclencheur du phénomène, ces oscillations sont appelées flottement de séparation laminaire. L'impact de 1' intensité turbulente de 1' écoulement sur les oscillations a été étudié et s'est révélé jouer un rôle très important: un haut niveau empêchant l'apparition des oscillations. Le caractère secondaire du rôle joué par les structures d'écoulement à haute fréquence a été démontré ainsi que les différents mécanismes de dissipation d'énergie en jeu. Les oscillations auto-induites en tangage et pilonnement combinés ont également été simulées. La comparaison entre les résultats numériques et expérimentaux n'est pas aussi bonne que dans le cas de oscillations en tangage, mais des tendances similaires sont tout de même observées. Lorsque la rigidité structurale en pilonnement est petite, des oscillations de faibles amplitudes en tangage et pilonnement sont obtenues, tel que dans le cas en tangage pure. Lorsque la rigidité structurale est grande, d' importantes amplitudes de tangage sont obtenues qui s'avèrent du même ordre de grandeur que celles observées en expérimental. Ces oscillations diffèrent du cas en tangage puisqu'elles sont caractérisées par un flottement de coalescence plutôt qu'un flottement de séparation laminaire

EXPERIMENTAL AND COUPLED CFD/CSD INVESTIGATION OF FLEXIBLE MAV-SCALE FLAPPING WINGS IN HOVER

Due to their potential to expand our sensing and mission capabilities in both military and civilian applications, micro air vehicles (MAVs) have recently gained increased recognition. However, man-made MAVs have struggled to meet the aerodynamic performance and maneuvering capabilities of biological flapping wing flyers (small birds and insects) which operate at MAV-scales (Reynolds numbers on the order of 103–104). Several past studies have focused on developing and analyzing flapping-wing MAV designs due to the possibility of achieving the increased lift, performance and flight capabilities seen in biological flapping wing flyers. However, there are still a lack of baseline design principles to follow when constructing a flexible flapping wing for a given set of wing kinematics, target lift values, mission capabilities, etc. This is due to the limited understanding of the complex, unsteady flow and aeroelastic effects intrinsic to flexible flapping wings. In the current research, a computational fluid dynamics (CFD) solver was coupled with a computational structural dynamics (CSD) solver to simulate the aerodynamics and inherent aeroelastic effects of a flexible flapping wing in hover. The coupled aeroelastic solver was validated against experimental test data to assess the predictive capability of the coupled solver. The predicted and experimental results showed good correlation over several different test cases. Experimental tests included particle image velocimetry (PIV) measurements, instantaneous aerodynamic force measurements and dynamic wing deformation recordings via a motion capture system. The aeroelastic solver was able to adequately predict the process of leading edge vortex (LEV) formation and shedding observed during experimentation. Additionally, the instantaneous lift and drag force-time histories as well as passive wing deformations agreed satisfactorily with the experimental measurements. The coupled CFD/CSD solver was used to determine how varied wing structural compliance influences aerodynamic force production, temporal and spatial evolution of the flowfield and overall wing performance. Results showed that for the wings tested, decreasing wing stiffness, especially toward the wing root, increased the time-averaged aerodynamic lift with minimal effect on drag. This is primarily due to prolonged sustainment of the LEVs produced during flapping and suggests that aeroelastic tailoring of flapping wings could improve performance