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
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Unsteady force production on a flat plate wing by large transverse gusts and plunging maneuvers(2017) Perrotta, Gino; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The transient forces produced by large-amplitude transverse gust encounters and plunge maneuvers were studied experimentally in a water-filled towing tank. Forces were measured as a flat plate wing with an aspect ratio of four was towed through a fluid gust and as the same wing performed plunge maneuvers which matched the shape of the gust velocity profile. The transient velocity in each case conformed to the sine-squared profile, and the peak transient velocities were of the same order of magnitude as the steady towing velocities. In most cases, the wing pitch angle was high enough to cause constant flow separation. Even at low wing pitch angles, the increase in flow incidence angle by the transverse gust or plunge velocity was enough to cause flow separation. Transient force magnitudes were shown to increase with increasing stream-normal velocity for both the gust encounters and plunge maneuvers. Transient forces varied with increasing wing pitch angle during gust encounters but not during plunge maneuvers. Force histories in each case were compared to predictions made by existing small-perturbation force models, and adaptations were made to those models based on physical interpretation of the observed characteristics. Measured forces in both the gust encounters and the plunge maneuvers were found to correspond more closely to predictions made based on attached flow than on separated flow, which supports the suggestion that the presence of a leading edge vortex significantly augments the transient lift. Additionally, a large trailing edge vortex forms at the end of the gust encounter which temporarily reduces the force production below the steady-state values. This was not observed in the plunge maneuver force histories, which were much closer to quasi-steady than were the gust encounter force histories. This analysis contributes to the understanding of unsteady force production in large-amplitude events, and in particular in conditions with separated flow, the behaviors of which are not adequately captured by existing small-perturbation models.Item Photogrammetric Reconstruction of Tandem-Wing Kinematics for Free-Flying Dragonflies Undergoing a Range of Flight Maneuvers(2017) Gabryszuk, Mateusz; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Photogrammetric methods are used to reconstruct the body and wing kinematics of free-flying dragonflies. A novel experimental setup was designed and constructed to allow for repeated untethered flights in a constrained flight arena. Kinematic data are presented for twelve individual flights and a total of 23 complete wing strokes, including unaccelerating, accelerating, climbing, and turning flight. High variability is observed in the wing motions employed by individual dragonflies, particularly in terms of stroke amplitude, pitch angle, and wingbeat frequency. Forewing and hindwing flapping is found to be neither in phase nor fully out of phase across all cases, with the forewings lagging the hindwings by an average of 90 degrees. Downstroke durations are observed to be shorter than upstroke durations except in highly accelerating flights. Migratory dragonflies are found to exhibit notably different wing kinematics than non-migratory species.Item DESIGN, FABRICATION, AND PERFORMANCE CHARACTERIZATION OF MULTIFUNCTIONAL STRUCTURES TO HARVEST SOLAR ENERGY FOR FLAPPING WING AERIAL VEHICLES(2016) Perez-Rosado, Ariel; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flapping Wing Aerial Vehicles (FWAVs) have the capability to combine the benefits of both fixed wing vehicles and rotary vehicles. However, flight time is limited due to limited on-board energy storage capacity. For most Unmanned Aerial Vehicle (UAV) operators, frequent recharging of the batteries is not ideal due to lack of nearby electrical outlets. This imposes serious limitations on FWAV flights. The approach taken to extend the flight time of UAVs was to integrate photovoltaic solar cells onto different structures of the vehicle to harvest and use energy from the sun. Integration of the solar cells can greatly improve the energy capacity of an UAV; however, this integration does effect the performance of the UAV and especially FWAVs. The integration of solar cells affects the ability of the vehicle to produce the aerodynamic forces necessary to maintain flight. This PhD dissertation characterizes the effects of solar cell integration on the performance of a FWAV. Robo Raven, a recently developed FWAV, is used as the platform for this work. An additive manufacturing technique was developed to integrate photovoltaic solar cells into the wing and tail structures of the vehicle. An approach to characterizing the effects of solar cell integration to the wings, tail, and body of the UAV is also described. This approach includes measurement of aerodynamic forces generated by the vehicle and measurements of the wing shape during the flapping cycle using Digital Image Correlation. Various changes to wing, body, and tail design are investigated and changes in performance for each design are measured. The electrical performance from the solar cells is also characterized. A new multifunctional performance model was formulated that describes how integration of solar cells influences the flight performance. Aerodynamic models were developed to describe effects of solar cell integration force production and performance of the FWAV. Thus, performance changes can be predicted depending on changes in design. Sensing capabilities of the solar cells were also discovered and correlated to the deformation of the wing. This demonstrated that the solar cells were capable of: (1) Lightweight and flexible structure to generate aerodynamic forces, (2) Energy harvesting to extend operational time and autonomy, (3) Sensing of an aerodynamic force associated with wing deformation. Finally, different flexible photovoltaic materials with higher efficiencies are investigated, which enable the multifunctional wings to provide enough solar power to keep the FWAV aloft without batteries as long as there is enough sunlight to power the vehicle.Item AN INVESTIGATION OF A MAV-SCALE FLEXIBLE FLAPPING WING IN FORWARD FLIGHT: FLOW FIELD AND AIRLOADS EXPERIMENTS WITH COUPLED CFD-CSD(2014) Mayo, David Benjamin; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Experiments were systematically executed with a coupled computational fluid/structural dynamics aeroelastic analysis for a micro air vehicle scale flexible flapping-wing undergoing pure flap wing kinematics in forward flight. 2-D time-resolved particle image velocimetry and force measurements were performed in a wind tunnel where the free stream velocity was set to 3 m/s, Re = 15,000. Chordwise velocity fields were obtained at equally spaced spanwise sections along the wing (30% to 90% span) at the mid-downstroke and mid-upstroke of the flap cycle. The flowfield measurements and averaged force measurements were used for the validation of the 3-D aeroelastic model. The computational fluid/structural dynamics analysis combined a compressible Reynolds Averaged Navier Stokes solver with a multi-body structural solver. The objective of the combined efforts was to understand the force production of a flexible wing undergoing an avian-type flapping motion. The temporal and spanwise variation of wing pitch angle affected lift and drag, and primarily aided in producing positive thrust during both upstroke and downstroke.Item The Fluid Dynamics of Mayfly Naiads(2014) Abdelziz, Khaled Mohamed-Refaat; Kiger, Kenneth T; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)During their aquatic phase, mayflies Centroptilum triangulifer use a series of tracheal gills to facilitate gas exchange. Recent experimental studies on nymphal mayflies have identified two coupled features associated with the ontogenic progression of their ventilatory kinematics: 1) there is an abrupt shift from a rowing mechanism in small instars to a flapping mechanism in larger instars, and 2) the flapping mechanism is associated with the development of a flexural hinge that permits the passive movement of a distal flap. The primary role of the tracheal gills is tied to ventilation rather than locomotion. As such, it is not yet understood why such a transition happens and which performance metric is improved, if any. Hence, the goal of the current research is to investigate both features using numerical simulations. First, a computational model of the mayfly is built from a dissected animal. Then, a 3-level prescribed kinematic chain is introduced to a previously in-house developed and in-house validated explicit parallel Navier-Stokes solver where both the advective and diffusive terms are advanced explicitly using a third-order, low-storage, Runge-Kutta scheme. Finally, an immersed boundary method based on a moving least squares reconstruction is implemented to enforce the correct moving boundary conditions. Two different parametric spaces are constructed. The first one investigates the transition from rowing to flapping kinematics, the morphological effects on the flow field and on the proposed performance parameters while the second aims to provide an explanation for the hinge development. Two metrics based on control volume analysis are proposed to quantify the performance of each numerical case. The first metric is simply the mechanical efficiency of the energy transfer from the moving gills to the surrounding flow field while the second incorporates the mass flow rate across the control surface. The second metric is promising because it is able to provide a plausible explanation of both features by showing that the rate of work done by the mayfly is diminished throughout ontogeny with respect to the induced mass flow rate.Item Nonlinear Fluid-Structure Interactions in Flapping Wing Systems(2013) Fitzgerald, Timothy; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work relates to fluid-structure interactions in the context of flapping wing systems. System models of flapping flight are explored by using a coupling scheme to provide communication between a fluid model and a structural model describing a flexible wing. The constructed computational models serve as a tool for investigating complex fluid-structure interactions and characterizing them. Primary goals of this work are construction of models to understand nonlinear phenomena associated with the flexible flapping wing systems, and explore means and methods to enhance their performance characteristics. Several system analysis tools are employed to characterize the coupled fluid-structure system dynamics, including proper orthogonal decomposition, dimension calculations, time histories, and frequency spectra. Results obtained from two-dimensional simulations conducted for a combination of a two-link structural system and a fluid system are presented and discussed. Comparisons are made between the use of direct numerical simulation and the unsteady vortex lattice method as the fluid model in this coupled dynamical system. To enable three-dimensional studies, a novel solid model is formulated from continuum mechanics for geometrically exact finite elements. A new partitioned fluid-structure interaction algorithm based on the Generalized-α method is formulated and implemented in a large scale fluids solver inside the FLASH framework. Consistent boundary conditions are also formulated by using Lagrangian particles. Several examples demonstrating the effectiveness of the methods and implementation are shown, in particular, for flapping flight at low Reynolds numbers. Unique experiments have also been undertaken to determine the first few natural frequencies and mode shapes associated with hawkmoth wings. The computational framework developed in this dissertation and the research findings can be used as a basis to understand the role of flexibility in flapping wing systems, further explore the complex dynamics of flapping wing systems, and also develop design schemes that might make use of nonlinear phenomena for performance enhancement.Item PARAMETRIC INVESTIGATIONS INTO FLUID-STRUCTURE INTERACTIONS IN HOVERING FLAPPING FLIGHT(2013) Maxwell, Jesse R.; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A parametric investigation into flapping flight is presented. For a Reynolds number of 75, harmonically forced flapping dynamics is studied. A wing section is modeled as two rigid links connected by a hinge with a torsion spring-damper combination. This section is wrapped in a smooth aerodynamic surface for immersion in the fluid domain. An immersed boundary method is employed on a two-dimensional structured Cartesian grid to solve the incompressible form of the Navier-Stokes equations for low Reynolds numbers by using a finite difference method. Fully coupled fluid-structure interactions are considered. Performance metrics, which include cycle-averaged lift, drag, power, and their ratios, are used to characterize the effects of different parameters and kinematics. Principal components of flow-field structures are quantified, and the system's response is correlated to performance. The thesis findings can serve as a basis to understand and identify flapping frequencies that provide high performance.