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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Control of Boundary Layer Separation and the Wake of an Airfoil using ns-DBD Plasma Actuators

Ashcraft, Timothy Allen January 2016 (has links)
The efficacy of nanosecond pulse driven dielectric barrier discharge (ns-DBD) plasma actuators for boundary layer separation and wake control is investigated experimentally. A single ns-DBD plasma actuator is placed at the leading edge of a NACA 0012 airfoil model. Both baseline and controlled flow fields are studied using static pressure measurements, Particle Image Velocimetry (PIV) and Constant Temperature Anemometry (CTA). Experiments are primarily performed at Re = 0.74 x 10⁶ and α = 18°. CP, PIV and CTA data show that a forcing frequency of F⁺ = 1.14 is optimal for separation control. CTA surveys of the wake at x/c = 7 indicate three approximate regimes of behavior. Forcing in the range 0.92 < F⁺ < 1.52 results in the best conditions for separation control over the airfoil, but has no dominant signature in the wake at x/c = 7. Excitation in the range of 0.23 < F⁺ < 0.92 produces a single dominant frequency in the wake while F⁺ < 0.23 shows behavior representing a possible impulse response or nonlinear effects. PIV data confirm these observations in all three regimes. Cross-correlations of CTA data are also employed to evaluate the two-dimensionality of the excited wake. The initial results presented here are part of an ongoing effort to use active flow control (AFC), in the form of ns-DBDs, as an enabling technology for the study of unsteady aerodynamics and vortex-body interactions.
2

Numerical investigations of flow characteristics and aerodynamic parameters of an airfoil with a trailing edge rotary cylinder

Chan, Preston C. T. 13 April 2016 (has links)
<p> The objective of this thesis was to study flow characteristics and aerodynamic parameters of an optimized low Reynolds number airfoil with a rotating cylinder at its trailing edge for application in Micro Aerial Vehicles (MAVs). The airfoil had a chord length, a width, and a thickness of 15.24 cm, 30.48 cm, and 1.6 mm, respectively. Investigations of flow characteristics and aerodynamic parameters were performed using the CFD software, STAR-CCM+, by CD-Adapco. The freestream mean velocity was 10 m/sec. which corresponds to a chord length Reynolds number of 104,400. The simulations were performed at five different Angles of Attack (AOA), 0, 5, 10, 15 and 30 degrees, with and without cylinder rotation. Results indicate 53% improvement in lift to drag ratio of the airfoil at a velocity ratio of 1.66 with rotation, as compared to corresponding condition without cylinder&rsquo;s rotation. Further analyses were conducted with velocity ratios of 1 and 0.53 and the results indicate increased lift to drag ratio in both conditions but not as significant when the velocity ratiofs was at 1.66.</p>
3

Payload mass improvements of supersonic retropropulsive flight for human class missions to Mars

Fagin, Maxwell H. 29 March 2016 (has links)
<p> Supersonic retropropulsion (SRP) is the use of retrorockets to decelerate during atmospheric flight while the vehicle is still traveling in the supersonic/hypersonic flight regime. In the context of Mars exploration, <i>subsonic</i> retropropulsion has a robust flight heritage for terminal landing guidance and control, but all <i>supersonic</i> deceleration has, to date, been performed by non-propulsive (i.e. purely aerodynamic) methods, such as aeroshells and parachutes.</p><p> Extending the use of retropropulsion from the subsonic to the supersonic regime has been identified as an enabling technology for high mass humans-to-Mars architectures. However, supersonic retropropulsion still poses significant design and control challenges, stemming mainly from the complex interactions between the hypersonic engine plumes, the oncoming air flow, and the vehicle&rsquo;s exterior surface. These interactions lead to flow fields that are difficult to model and produce counter intuitive behaviors that are not present in purely propulsive or purely aerodynamic flight.</p><p> This study will provide an overview of the work done in the design of SRP systems. Optimal throttle laws for certain trajectories will be derived that leverage aero/propulsive effects to decrease propellant requirements and increase total useful landing mass. A study of the mass savings will be made for a 10 mT reference vehicle based on a propulsive version of the Orion capsule, followed by the 100 mT ellipsoid vehicle assumed by NASA&rsquo;s Mars Design Reference Architecture.</p>
4

Application of X-ray pulsar navigation| A characterization of the Earth orbit trade space

Yu, Wayne Hong 15 April 2016 (has links)
<p> The potential for pulsars as a navigation source has been studied since their discovery in 1967. X-ray pulsar navigation (XNAV) is a celestial navigation system that uses the consistent timing nature of X-ray photons from milli-second pulsars (MSP) to perform space navigation. Much of the challenge of XNAV comes from the faint signal, availability, and distant nature of pulsars. This thesis is the study of pulsar XNAV measurements for extended Kalman filter (EKF) tracking performance within a wide trade space of bounded Earth orbits, using a simulation of existing X-ray detector space hardware. An example of an X-ray detector for XNAV is the NASA Station Explorer for X-ray Timing and Navigation (SEXTANT) mission, a technology demonstration of XNAV set to perform on the International Space Station (ISS) in 2016. </p><p> The study shows that the closed Earth orbit for XNAV performance is reliant on the orbit semi-major axis and eccentricity as well as orbit inclination. These parameters are the primary drivers of pulsar measurement availability and significantly influence the natural spacecraft orbit dynamics. Sensitivity to initial orbit determination error growth due to the scarcity of XNAV measurements within an orbital period require appropriate timing of initial XNAV measurements. The orbit angles of argument of perigee and right ascension of the ascending node, alongside the other orbit parameters, complete the initial cadence of XNAV measurements. The performance of initial XNAV measurements then propagates throughout the experimental period.</p>
5

Quaternion-Based Aircraft Attitude Estimation

Filarsky, Brian Michael 09 July 2016 (has links)
<p> Aircraft attitude estimation requires fusing several sensors in order to recover both high and low frequency information in an observable manner. This thesis explores the fusion of gyroscope integration, gravity vector estimation, and magnetic field vector estimation using a complementary filter and an extended Kalman filter (EKF), both of which use a unit quaternion to represent the attitude portion of the state. </p><p> First, a set of models, which contain bias, scale factor errors, alignment errors, and Gaussian white noise, is introduced to govern the available sensors. The gyroscope bias is modeled as a random walk. A calibration routine is then established to minimize scale factor and bias errors. After some definitions and derivations for quaternion algebra are established, the attitude solution is then estimated using the complementary filter. Then the EKF is introduced and used to estimate both the quaternion state and gyroscope bias. </p><p> The thesis is concluded with a Monte Carlo run to compare the complementary filter with the EKF. Due in large part to the estimation of gyroscope bias in the EKF, this filter is shown to give a significantly more accurate state estimate. The robustness is also evaluated, with both filters initialized with the incorrect initial quaternion and gyroscope bias estimate. The EKF is shown to converge relatively quickly, while the complementary filter does not reliably converge due to the lack of gyroscope bias estimation.</p>
6

Characterization of dynamic thermal control schemes and heat transfer pathways for incorporating variable emissivity electrochromic materials into a space suit heat rejection system

Massina, Christopher James 03 June 2016 (has links)
<p> The feasibility of conducting long duration human spaceflight missions is largely dependent on the provision of consumables such as oxygen, water, and food. In addition to meeting crew metabolic needs, water sublimation has long served as the primary heat rejection mechanism in space suits during extravehicular activity (EVA). During a single eight hour EVA, approximately 3.6 kg (8 lbm) of water is lost from the current suit. Reducing the amount of expended water during EVA is a long standing goal of space suit life support systems designers; but to date, no alternate thermal control mechanism has demonstrated the ability to completely eliminate the loss. One proposed concept is to convert the majority of a space suit&rsquo;s surface area into a radiator such that the local environment can be used as a radiative thermal sink for rejecting heat without mass loss. Due to natural variations in both internal (metabolic) loads and external (environmental) sink temperatures, radiative transport must be actively modulated in order to maintain an acceptable thermal balance. Here, variable emissivity electrochromic devices are examined as the primary mechanism for enabling variable heat rejection. This dissertation focuses on theoretical and empirical evaluations performed to determine the feasibility of using a full suit, variable emissivity radiator architecture for space suit thermal control. Operational envelopes are described that show where a given environment and/or metabolic load combination may or may not be supported by the evaluated thermal architecture. Key integration considerations and guidelines include determining allowable thermal environments, defining skin-to-radiator heat transfer properties, and evaluating required electrochromic performance properties. Analysis also considered the impacts of dynamic environmental changes and the architecture&rsquo;s extensibility to EVA on the Martian surface. At the conclusion of this work, the full suit, variable emissivity radiator architecture is considered to be at a technology readiness level of 3/4, indicating that analytical proof-of-concept and component level validation in a laboratory environment have been completed. While this is not a numeric increase from previous investigations, these contributions are a significant iteration within those levels. These results improve the understanding of the capabilities provided by the full suit, variable emissivity architecture.</p>
7

Reducing drag of a commuter train, using engine exhaust momentum

Ha, Dong Keun 01 June 2016 (has links)
<p> The objective of this thesis was to perform numerical investigations of two different methods of injecting fluid momentum into the air flow above a commuter train to reduce its drag. Based on previous aerodynamic modifications of heavy duty trucks in improving fuel efficiency, two structural modifications were designed and applied to a Metrolink Services commuter train in the Los Angeles (LA) County area to reduce its drag and subsequently improve fuel efficiency. The first modification was an L-shaped channel, added to the exhaust cooling fan above the locomotive roof to divert and align the exhaust gases in the axial direction. The second modification was adding an airfoil shaped lid over the L-shape channel, to minimize the drag of the perturbed structure, and thus reduce the overall drag. </p><p> The computational fluid dynamic (CFD) software CCM+ from CD-Adapco with the ?-? turbulence model was used for the simulations. A single train set which consists of three vehicles: one locomotive, one trailer car and one cab car were used. All the vehicles were modeled based on the standard Metrolink fleet train size. The wind speed was at 90 miles per hour (mph), which is the maximum speed for the Orange County Metrolink line. Air was used as the exhaust gas in the simulation. The temperature of the exhausting air emitting out of the cooling fan on the roof was 150 F and the average fan speed was 120 mph. </p><p> Results showed that with the addition of the lid, momentum injection results in reduced flow separation and pressure recovery behind the locomotive, which reduces the overall drag by at least 30%.</p>
8

Numerical prediction of the spatial and temporal characteristics of the aero-optical disturbance produced by a helicopter in hover

Kelly, Ryan T. 11 February 2016 (has links)
<p> Aero-optical disturbances produced from turbulent compressible flow-fields can seriously degrade the performance of an optical signal. At compressible flight speeds these disturbances stem from the density variations present in turbulent boundary layers and free shear layers; however helicopters typically operate at incompressible speeds, which nearly eliminates the aberrating effect of these flows. For helicopter platforms the sources of aberration originate from the high subsonic flow-field near the rotor blade tips in the form of rotor-tip vortices and from the high temperatures of the engine effluence. During hover the shed rotor-tip vortices and engine effluence convect with the rotor wake encircling the airframe and subsequently a helicopter mounted optical system.</p><p> The aero-optical effects of the wake beneath a hovering helicopter were analyzed using a combination of Unsteady RANS (URANS) and Large-Eddy Simulations (LES). The spatial and temporal characteristics of the numerical optical wavefronts were compared to full-scale aero-optic experimental measurements. The results indicate that the turbulence of the rotor-tip vortices contributes to the higher order aberrations measured experimentally and that the thermal exhaust plumes effectively limit the optical field-of-regard to forward- and side-looking beam directions. This information along with the computed optical aberrations of the wake can be used to guide the development of adaptive-optic systems or other beam-control approaches.</p>
9

Improved Collocation Methods to Optimize Low-Thrust, Low-Energy Transfers in the Earth-Moon System

Herman, Jonathan F. C. 31 December 2015 (has links)
<p> Modern and near-future Solar Electric Propulsion capabilities enable many new missions that were inconceivable using chemical propulsion systems. Many of these involve highly complex trajectories that are very challenging to design. New tools are needed that effectively utilize the rapidly growing parallel processing capabilities of modern computers. This research improves Gauss-Lobatto collocation methods, which are known to perform very well for low-thrust trajectory optimization, by formulating them as massively parallel processes. The parallelized elements of the problem formulation execute up to 11 times faster, depending on what force model is used and when evaluated by themselves. When accounting for the operations of the nonlinear programming solver, this translates to up to 3.7 times faster performance for solving a complete trajectory optimization problem, again depending on the force model that is used. The remaining barriers to further performance improvements, and the conditions upon which these depend, are clearly identified.</p><p> The implemented methods are combined into an optimization tool named <i> Maverick.</i> More general improvements to the formulation of the Gauss-Lobatto collocation methods are also developed and included in <i>Maverick,</i> which permit a more flexible use of these optimization schemes and enable them to find more complex solutions. One example of this is <i>Maverick's </i> ability to autonomously introduce gravity assists into trajectories, which greatly increases the utility and convergence radius of these methods.</p><p> In order to demonstrate the benefit of this work, three applications are studied. The first are transfers between halo-like orbits in the Earth-Moon system, which shows this is likely an unattractive region for missions like the New Worlds Observer. The second application investigates stabilization maneuvers in lunar distant retrograde orbits. This work demonstrates the feasibility of these stabilization transfers for a variety of sample return missions, such as the upcoming Asteroid Redirect Mission. The final application discussed is a series of multi-body low-thrust transfers from the Earth to the Moon that efficiently utilize highly variable dynamics to reduce propellant consumption, which is relevant for a variety of future mission concepts. These are computed for a wide range of flight times, showing that reductions up to 45% of the transfer time can be achieved with a propellant consumption as little as 0.5% of the total spacecraft mass. Up to 90% of the flight time can be eliminated for a propellant cost of 4% of the total spacecraft mass, or up to 83% for a propellant cost of less than 2%. The developed algorithm seamlessly transitions its solutions from full low-thrust, low-energy trajectories to the 'pure' low-thrust trajectories that define the shortest transfer trajectories, validating its robust performance. Beyond these quantifiable results, these examples illustrate the complexity of the solutions that can be identified with these improved implementations of Gauss-Lobatto collocation methods, with many instances where the optimization method autonomously introduces powered gravity assists, an unusual capability that has the potential for useful application to many other trajectory optimization problems.</p>
10

Combustion instability and active control| Alternative fuels, augmentors, and modeling heat release

Park, Sammy Ace 29 June 2016 (has links)
<p> Experimental and analytical studies were conducted to explore thermo-acoustic coupling during the onset of combustion instability in various air-breathing combustor configurations. These include a laboratory-scale 200-kW dump combustor and a 100-kW augmentor featuring a v-gutter flame holder. They were used to simulate main combustion chambers and afterburners in aero engines, respectively. The three primary themes of this work includes: 1) modeling heat release fluctuations for stability analysis, 2) conducting active combustion control with alternative fuels, and 3) demonstrating practical active control for augmentor instability suppression. </p><p> The phenomenon of combustion instabilities remains an unsolved problem in propulsion engines, mainly because of the difficulty in predicting the fluctuating component of heat release without extensive testing. A hybrid model was developed to describe both the temporal and spatial variations in dynamic heat release, using a separation of variables approach that requires only a limited amount of experimental data. The use of sinusoidal basis functions further reduced the amount of data required. When the mean heat release behavior is known, the only experimental data needed for detailed stability analysis is one instantaneous picture of heat release at the peak pressure phase. This model was successfully tested in the dump combustor experiments, reproducing the correct sign of the overall Rayleigh index as well as the remarkably accurate spatial distribution pattern of fluctuating heat release. </p><p> Active combustion control was explored for fuel-flexible combustor operation using twelve different jet fuels including bio-synthetic and Fischer-Tropsch types. Analysis done using an actuated spray combustion model revealed that the combustion response times of these fuels were similar. Combined with experimental spray characterizations, this suggested that controller performance should remain effective with various alternative fuels. Active control experiments validated this analysis while demonstrating 50-70\% reduction in the peak spectral amplitude. A new model augmentor was built and tested for combustion dynamics using schlieren and chemiluminescence techniques. Novel active control techniques including pulsed air injection were implemented and the results were compared with the pulsed fuel injection approach. The pulsed injection of secondary air worked just as effectively for suppressing the augmentor instability, setting up the possibility of more efficient actuation strategy. </p>

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