Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Investigation of Aerodynamics of Flapping Wings for Miro Air Vehicle Applications(2013) Malhan, Ria Pavnish; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A coupled CFD-CSD solver was used to simulate the aerodynamics of a flexible flapping wing. The CFD solver is a compressible RANS (Reynolds Averaged Navier Stokes) solver. Multibody dynamics solver `MBDyn', was used as the structural solver to take into account non linear shell straining, making it possible to analyze low aspect ratio wings with large deformations. Validation of the two codes was carried out independently. The solvers were then coupled using python and validated against prior experiments and analysis on spanwise and chordwise flexible wings. As realistic MAV wings are extremely flexible and lightweight, under the effect of high inertial and aerodynamic forces, they undergo large non linear deformations over a flap cycle. However, there is a dearth of experimental data on well characterized flapping wings (with known structural and mass properties) at MAV-scale Reynolds numbers. Systematic experiments were carried out on rigid and flexible flapping wings in an open jet wind tunnel and forces were measured using a test bed. Pure flapping of rigid wings did not generate sufficient propulsive force and may not be a viable configuration. Passive pitching of rigid wing generated both, target vertical and propulsive forces. Dynamic wing twist was then incorporated using flexible wings. A flexible wing was fabricated using a combination of unidirectional carbon fiber strips (chordwise ribs), carbon rod (leading edge spar) and mylar film (membrane). Structural model of the wing (combination of beam and shell elements) was developed and then coupled to the CFD model. CFD-CSD analysis of flexible wing was carried out and good correlation was obtained for all the configurations. This comprehensive experimental data set can also be used to validate other aeroelastic analyses of the future. Further, the analysis was used to gain more insights into flow physics. It was observed that as a result of flexibility, by taking advantage of unsteady flow features, a lighter, simpler mechanism could be used to produce larger forces than a rigid wing. The validated, comprehensive analysis developed in this work may serve as a design tool for deciding configurations and wing kinematics of next generation MAVs.Item An Analytical Investigation of Flapping Wing Structures for Micro Air Vehicles(2011) Rosenfeld, Nicholas Charles; Wereley, Norman W.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An analytical model of flapping wing structures for bio-inspired micro air vehicles is presented in this dissertation. Bio-inspired micro air vehicles (MAVs) are based on insects and hummingbirds. These animals have lightweight, flexible wings that undergo large deformations while flapping. Engineering studies have confirmed that deformations can increase the lift of flapping wings. Wing flexibility has been studied through experimental construction-and-evaluation methods and through computational numerical models. Between experimental and numerical methods there is a need for a simple method to model and evaluate the structural dynamics of flexible flapping wings. This dissertation's analytical model addresses this need. A time-periodic assumed-modes beam analysis of a flapping, flexible wing undergoing linear deformations is developed from a beam analysis of a helicopter blade. The resultant structural model includes bending and torsion degrees of freedom. The model is non-dimensionalized. The ratio of the system's structural natural frequency to wingbeat frequency characterizes its constant stiffness, and the amplitude of flapping motion characterizes its time-periodic stiffness. Current flapping mechanisms and MAVs are compared to biological fliers on the basis of the characteristic parameters. The beam analysis is extended to develop an plate model of a flapping wing. The time-periodic stability of the flapping wing model is assessed with Floquet analysis. A flapping-wing stability diagram is developed as a function of the characteristic parameters. The analysis indicates that time-periodic instabilities are more likely for large-amplitude, high-frequency flapping motion. Instabilities associated with the first bending mode dominate the stability diagram. Due to current limitations of flapping mechanisms, instabilities are not likely in current experiments but become more likely at the operating conditions of biological fliers. The effect of structural design parameters, including wing planform and material stiffness, are assessed with an assumed-modes aeroelastic model. Wing planforms are developed from an empirical model of biological planforms. Non-linearities are described in the effect of membrane thickness on lift generation. Structural couplings due to time-periodic stiffness are identified that can decrease lift generation at certain wingbeat frequencies.