Aerospace Engineering Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2737

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Subject: search.filters.subject.Micro Air Vehicle

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    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.
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    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.
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    An Experimental Investigation of a Micro Air Vehicle-Scale Cycloidal Rotor in Forward Flight
    (2013) Jarugumilli, Tejaswi; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The current research aims to explore the forward flight capability of an unconventional rotary-wing concept for micro air vehicle (MAV) applications, known as the cycloidal rotor (or cyclorotor). Two approaches are undertaken to fulfill this objective: 1) performance studies to examine the time-averaged forces produced by the cyclorotor, and 2) flow field studies to investigate the underlying physics of force production. In the performance studies, the dependence of time-averaged lift, propulsive force and power on blade pitching kinematics, rotor geometry and forward flight operating conditions (i.e. advance ratio) were first examined through independent parametric studies. Next, the performance results were interpolated to determine the steady level flight characteristics of the cyclorotor, specifically the power consumption, lift-to-drag ratio and control input requirements at various forward speeds. The baseline values of lift and rotational speed for these trimmed flight studies were determined based on an existing twin-cyclorotor MAV. These studies showed the cyclorotor to be capable of achieving relatively high advance ratios (up to 0.94), with significant reductions in power consumption. In the second research approach, flow visualization experiments and time-resolved, planar particle image velocimetry (PIV) measurements were performed to gain a qualitative and quantitative understanding of the flow field. The time-averaged and phase-averaged flow fields of a 2-bladed cyclorotor were examined at different advance ratios. The PIV measurements were then correlated with previous computational fluid dynamics (CFD) simulations to help explain the distribution of forces along the rotor azimuth. The flow field studies revealed that the cyclorotor needs to operate in the counter-clockwise direction (freestream velocity from left to right) in order to produce the necessary lift force in high-speed forward flight, the primary lift and propulsive force producing regions of the cyclorotor are located in the lower-rear half of the rotor azimuth (symmetric pitching kinematics), and that unsteady aerodynamics (e.g. blade-vortex interactions) plays an important role in the generation of lift and propulsive blade force production.
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    An Observer for Estimating Translational Velocity from Optic Flow and Radar
    (2011) Gerardi, Steven Anthony; Humbert, James S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis presents the development of a discrete time observer for estimating state information from optic flow and radar measurements. It is shown that estimates of translational and rotational speed can be extracted using a least squares inversion for wide fields of view or, with the addition of a Kalman Filter, for small fields of view. The approach is demonstrated in a simulated three dimensional urban environment on an autonomous quadrotor micro-air-vehicle (MAV). A state feedback control scheme is designed, whereby the gains are found via static H∞, and implemented to allow trajectory following. The proposed state estimation scheme and feedback method are shown to be sufficient for enabling autonomous navigation of an MAV. The resulting methodology has the advantages of computational speed and simplicity, both of which are imperative for implementation on MAVs due to stringent size, weight, and power requirements.
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    Computational Investigation of Micro-Scale Coaxial Rotor Aerodynamics in Hover
    (2009) Lakshminarayan, Vinod K.; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this work, a compressible Reynolds-Averaged Navier Stokes (RANS) solver is extended to investigate the aerodynamics of a micro-scale coaxial rotor configuration in hover. This required the following modifications to the solver: implementation of a time-accurate low Mach preconditioner, implementation of a sliding mesh interface boundary condition, improvements in the grid connectivity and parallelization of the code. First, an extensive validation study on the prediction capability of the solver is performed on a hovering micro-scale single rotor, for which performance data and wake characteristics have been measured experimentally. The thrust and power are reasonably well predicted for different leading and trailing geometries. Blunt leading edge geometries show poorer performance compared to the sharp leading edge geometries; the simulations show that this is mainly because of the large pressure drag acting at the blunt front. The tip vortex trajectory and velocity profiles are also well captured. The predicted swirl velocities in the wake for the micro-rotor are found to be significantly larger as compared to those for a full-scale rotor, which could be one of the reasons for additional power loss in the smaller scale rotors. The use of twist and taper is studied computationally and is seen to improve the performance of micro-rotor blades. Next, the solver is applied to simulate the aerodynamics of a full-scale coaxial rotor configuration in hover, for which performance data is available from experiments. The global quantities such as thrust and power are predicted reasonably well. In the torque trimmed situation, the top rotor shares significant percentage of the total thrust at lower thrust levels, which decreases to about 55% of the total thrust at higher thrust values. The simulations reveal that the interaction between the rotor systems is seen to generate significant impulses in the instantaneous thrust and power. The characteristic signature of this impulse is explained in terms of the blade thickness effect and loading effect, as well as blade-vortex interactions for the bottom rotor (wake effect). Finally, the RANS solver is applied to investigate the aerodynamics of a micro-scale coaxial rotor configuration in hover. The overall performance is well predicted. The interaction between the rotor systems is again seen to generate 3­8% fluctuation in the instantaneous thrust and power. The wake effect in the simulation is seen to be very prominent and the phasing of the impingement of the tip vortex from the top rotor upon the bottom rotor plays a significant role in the amount of unsteadiness on the bottom rotor. Interaction of the top rotor vortex and inboard sheet with the bottom rotor results in significant shedding on the bottom rotor blade, and this is believed to be caused by the of sharp leading edge geometry. Significant blade-vortex and vortex-vortex interactions are observed for coaxial systems.