Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item EXPERIMENTAL AND COUPLED CFD/CSD INVESTIGATION OF FLEXIBLE MAV-SCALE FLAPPING WINGS IN HOVER(2018) Lankford, James; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.Item UNDERSTANDING OF LOW REYNOLDS NUMBER AERODYNAMICS AND DESIGN OF MICRO ROTARY-WING AIR VEHICLES(2016) Winslow, Justin Michael; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The goal of the present research is to understand aerodynamics at low Reynolds numbers and synthesize rules towards the development of hovering micro rotary-wing air vehicles (MRAVs). This entailed the rigorous study of airfoil characteristics at low Reynolds numbers through available experimental results as well as the use of an unsteady Reynolds-Averaged Navier-Stokes solver. A systematic, experimental, variation of parameters approach with physical rotors was carried out to design and develop a micro air vehicle-scale rotor which maximizes the hover Figure of Merit. The insights gained in low Reynolds number aerodynamics have been utilized in the systematic design of a high endurance micro-quadrotor. Based on available characteristics, the physical relations governing electric propulsion system and structural weights have been derived towards a sizing methodology for small-scale rotary-wing vehicles.Item A Computational Study of the Force Generation Mechanisms in Flapping-Wing Hovering Flight(2013) Bush, Brandon Lamar; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An incompressible Navier-Stokes computational fluid dynamics (CFD) solver is developed for simulating flapping wings at Reynolds numbers (Re) of approximately 102 - 103 in which the governing equations are solved in an immersed boundary framework on fixed Cartesian meshes. The dissertation work is divided into two portions: (1) Implementation of the immersed boundary method for incompressible low-Re flowfields. The applicability and robustness of various solution schemes are studied, with specific applicability to low Re biological flows (staggered variable formulations versus collocated implementations, upwind schemes as applied to incompressible flows, ray-tracing and geometric optimization of immersed boundary determination, Large Eddy Simulation (LES) model implementations). (2) The extension and application of the flow solver (IBINS) to model flapping-wing kinematics, and the analysis of the influence of kinematics and flow parameters on the force production for idealized flapping strokes. A representative Drosophila wing is simulated undergoing an idealized periodic flapping stroke. A detailed characterization of the vortical structures that develop in the near and far wake, along with their correlation with the force and power time histories, is given for simulations of various stroke kinematics at Re = 147 and Re = 1400.Item Optimal Propulsion System Design for a Micro Quad Rotor(2011) Harrington, Aaron Michael; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Currently a 50 gram micro quad rotor vehicle is being developed in collaboration with Daedalus Flight Systems. Optimization of the design at this scale requires a systematic study to be carried out to investigate the factors that affect the vehicles performance. Endurance of hovering vehicles at this scale is severely limited by the low efficiencies of their propulsion systems and rotor design and optimization has been performed in the past in an attempt to increase endurance, but proper coupling of the rotor with the motor has been lacking. The current study chose to investigate the factors that had the greatest effect on the vehicle's endurance through analysis of the propulsion system. Therefore, a coupled aerodynamic and structural analysis was carried out that incorporated low Reynolds number airfoil table lookup in order to predict micro rotor performance. A parametric study on rotor design was performed further determine the effect of different rotor designs on hover performance. The experiments performed showed that airfoil camber had the biggest impact on rotor efficiency and other factors such as leading edge shape, number of blades, max camber location, and blade planform taper only had negligible influence on performance. Systematic studies of the interactions between micro rotor blades operating in close proximity to each other were performed in order to determine the changes in rotor efficiency that might occur in a compact quad rotor design. Tests done on the effect of rotor separation demonstrated that there is a negligible interaction between rotors operating near each other. Brushless motors were also tested systematically and characterized by their torque, rpm, and efficiency. It was found that the maximum efficiency of the motors tested was only 60%, which has significant effects on the efficiency of the coupled system. A method for rotor and motor coupling was also established that utilized the motor efficiency curves and the known torque and rotational speed of the rotors at their operating thrust. Through this, it was found that propulsion system efficiency could be increased by 10% by simply using the proper motor and rotor combination. Further, coupled design would have additional benefits and could increase vehicle efficiency further.