published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

The structure of direct-current microdischarges is investigated using a detailed two-dimensional multi-species continuum model. Microdischarges are directcurrent discharges that operate at a relatively high pressure of about 100 Torr and geometric dimensions in the 10-100 micrometer range. Our motivation for the study of microdischarges comes from a potential application of these devices in microthrusters for small satellite propulsion. The Micro Plasma Thruster (MPT) concept consists of a direct-current microdischarge in a geometry comprising a constant area flow section followed by a diverging exit nozzle. A detailed description of the plasma dynamics inside the MPT including power deposition, ionization, coupling of the plasma phenomena with high-speed flow, and propulsion system performance is reported in this study. A two-dimensional model is developed as part of this study. The model consists of a plasma module coupled to a flow module and is solved on a hybrid unstructured mesh framework. The plasma module provides a self-consistent, multispecies, multi-temperature description of the microdischarge phenomena while the flow module provides a description of the low Reynolds number compressible flow through the system. The plasma module solves conservation equations for plasma species continuity and electron energy, and Poisson’s equation for the self-consistent electric field. The flow module solves mass, bulk gas momentum and energy equations. The coupling of energy from the electrostatic field to the plasma species is modeled by the Joule heating term which appears in the electron and heavy species energy equations. Discretization of the Joule heating term on unstructured meshes requires special attention. We propose a new robust method for the numerical discretization of the Joule heating term on such meshes using a cell-centered, finite volume approach. A prototypical microhollow cathode discharge (MHCD) is studied to guide and validate the modeling effort for theMPT. Computational results for the impedance characteristics as well as electrodynamic and chemical features of the discharge are reported and compared to experimental results. At low current (< 0.1 mA), the plasma activity is localized inside the cylindrical hollow region of the discharge operating in the so-called “abnormal regime”. For larger currents, the discharge expands over the outer flat surface of the cathode and operates in the “normal regime”. Transient relaxation oscillations are predicted in the plasma properties for intermediate discharge currents ranging from 0.1 mA to 0.3 mA; a phenomenon that is reported in experiments. The MPT, in its present configuration, is found to operate as an electrothermal, rather than as an electrostatic thruster. A significant increase in specific impulse, compared to the cold gas micronozzle, is obtained from the power deposition into the expanding gas. For a discharge voltage of 750 V, a power input of 650 mW, and an argon mass flow rate of 5 sccm, the specific impulse of the device is increased by a factor of 1.5 to a value of 74 s. The microdischarge remains mostly confined inside the micronozzle and operates in an abnormal regime. Gas heating, primarily due to ion Joule heating, is found to have a strong influence on the overall discharge behavior. The study provides crucial understanding to aid in the design of direct-current microdischarge based thrusters. / text

I ett marint framdrivningssystem byggt av Rolls-Royce Marine i Kristinehamn, kallat POD Propulsion - MermaidTM, kyls systemets yttre delar med förbiströmmande vatten. Det kylande vattnet kompletteras av ett internt luftkylningssystem vilket, till följd av en lägre kylningskapacitet än för det externa vattnet, förorsakar en otillräcklig kylning i den sektor av elmotorns stator vars periferi kyls av kylluftsströmmen. En andel av statorn är således enbart luftkyld vilket medför att den axiella temperaturen i denna sektor uppnår ett lokalt maximum, kallad ”hotspot”, vilken är högre än för resten av elmotorns stator. Avsaknaden av en tangentiellt uniform temperatur i statorn medför begränsningar vid dimensionering av framdrivningssystemet. Begränsningar som kan få till följd att motorn i en POD överdimensioneras, gentemot fartygets effektbehov, sådan att elmotorn vid drift inte ska generera lika mycket värme och därmed undvika de höga statortemperaturerna. En effektiv kylning av hotspoten och därigenom en tangentiellt mer uniform temperatur skulle därmed medföra att elmotorn kan; dimensioneras mer effektivt och därigenom ge en lägre installationskostnad, uppnå en högre utnyttjandegrad samt leda till en förbättrad hydrodynamisk verkningsgrad. Rapporten ämnar genom både analytiska och numeriska simuleringar utvärdera ett flertal olika passiva kylningsmetoder som kan appliceras över POD-husets hotspot. De tilltänkta kylningsmetoderna baseras i grunden på två olika principer. En av principerna, kallad utvidgade ytor, avser kyla hotspoten genom att underlätta överföringen av värme från hotspoten till en förbipasserande fluid där den andra, genom tillämpning av ett ledande material, leder värme från hotspoten till en omgivande kallare yta. Resultaten visar att alla de tilltänkta modifikationerna, i varierande grad, möjliggör en temperatursänkning av det vertikala hotspotområdet, vilken är belägen intill luftkylningskanalen. De modifikationer som presterar bäst är modifikationerna som tillämpar en heatpipe som ledande material och modifikationen utvidgad solid gjort av koppar. Dessa modeller visar en 60 procentig temperatursänkning av den vertikala hotspoten relativt referensmodellen. Rapportens resultat kan även visa en temperatursänkning av luften då modifikationen ledande stag tillämpas, vilket är gynnsamt för fartygets luftkylningsprocess.

As the United States Navy moves toward the all-electric ship, the need for a robust, high fidelity inverter for propulsion motors becomes mandatory. Military vessels require high power converters capable of producing nearly sinusoidal outputs to prevent torque pulsations and electrical noise that can compromise the mission location. This thesis presents a hybrid pulse-width-modulated controller for a 3x3 Cascaded Multi-Level Converter (CMLC). Ancillary results include a simple technique for extracting the reference sine wave from an independent bulk converter and implementing a synchronization technique that coordinates a space vector modulation controller with the switching pattern of a bulk inverter. The algorithms were tested on CMLC hardware that resides in the Naval Postgraduate School Power Systems Laboratory, and the results were compared with a sine-triangle pulse width modulation algorithm. The controller and converter were used to power a quarter-horsepower three-phase induction motor.

Elimination or reduction of auxiliary oxygen use in Pulse Detonation Engines (PDEs) is necessary if the technology is to compete with existing Ramjet systems. This thesis investigated a Transient Plasma Ignition (TPI) system and found that the technology can at least reduce and may be able to completely remove the auxiliary oxygen requirement of current PDE systems. TPI was tested and compared with a traditional capacitive discharge spark plug system in a dynamic flow, ethylene/air mixture combustor. Ignition delay time, Deflagration-to-Detonation transition (DDT) distance and time, detonation wave speed and fire success rate performance were analyzed for various mass flow rates and stoichiometric ratios. A transient plasma dualelectrode concept was also employed and analyzed. Results show that TPI is more effective and reliable than the spark plug ignition with considerable improvements to DDT performance. The TPI dual-electrode concept was proven to be the most effective configuration with average reductions in DDT distance and time of 17% and 41% respectively when compared to the capacitive discharge spark plug system configuration.

We theoretically investigate the fluid mechanics of self-propelled (or swimming) bodies. An important factor concerning the hydrodynamics of locomotion concerns the relative strength of inertial to viscous forces experienced by the swimmer, the ratio of which is quantified by the Reynolds number, Re. Particular attention is given to the regime where Re is intermediate, where viscous and inertial forces are both relevant to fluid motion. We study two broad classes of swimmers: ‘pushers’ and ‘pullers’. Pushers produce thrust from the rear of their body, while pullers generate thrust from the front. We first investigate the near-field flow due to pushers and pullers by numerically solving the Navier-Stokes equations for Re of 0.01–1000. We show that, although the locomotion of pushers and pullers is similar at small Re, drastic differences due to fluid inertia arise as Re is increased. Most remarkably, flow instabilities develop at much smaller Re for a puller than a pusher. Further, we investigate the large scale fluid transport induced by a swimmer as a function of Re in the context of the induced ‘drift volume’. The drift volume quantifies the volume of fluid swept out by a ‘dyed’ fluid plane that is initially perpendicular to the body’s path. However, we first address the previously unsolved problem of the drift volume due to a body that is towed by an external force at finite Re. While the drift volume is comparable to the body volume in inviscid flow (Re ! 1), it is much larger when Re is finite due to viscous effects. The drift volume due to a swimmer is smaller than that due to a towed body because swimmers generate a weaker far-field flow. However, it is still potentially large compared to the volume of the swimmer’s body in the viscously dominated small-Re regime. However, the drift volume of a swimmer quickly diminishes as Re is increased.

This thesis describes a novel approach to augmenting the combustion performance of hybrid rocket fuels, specifically in terms of regression rate and combustion efficiency. Liquid additives are emulsified into molten paraffin wax using nonionic surfactants and cured to form cylindrical fuel grains. Fuel grains were tested in a lab scale, optically accessible hybrid rocket motor and compared to the performance of neat paraffin fuel grains.

<div>A sharp, circular 7° half-angle cone was tested in the Boeing/AFOSR Mach-6 Quiet Tunnel</div><div>at 6° angle of attack, extending several previous experiments on the growth and breakdown of</div><div>stationary crossflow instabilities in the boundary layer. </div><div><br></div><div>Measurements were made using infrared</div><div>imaging and surface pressure sensors. Detailed measurements of the stationary and traveling</div><div>crossflow vortices, as well as various secondary instability modes, were collected over a large</div><div>region of the cone.</div><div><br></div><div>The Rod Insertion Method (RIM) roughness, first developed for use on a flared cone, was</div><div>adapted for application to crossflow work. It was demonstrated that the roughness elements were</div><div>the primary factor responsible for the appearance of the specific pattern of stationary streaks</div><div>downstream, which are the footprints of the stationary crossflow vortices. In addition, a roughness</div><div>insert was created with a high RMS level of normally-distributed roughness to excite the naturally</div><div>most-amplified stationary mode.</div><div><br></div><div>The nonlinear breakdown mechanism induced by each type of roughness appears to be</div><div>different. When using the discrete RIM roughness, the dominant mechanism seems to be the</div><div>modulated second mode, which is significantly destabilized by the large stationary vortices. This</div><div>is consistent with recent computations. There is no evidence of the presence of traveling crossflow</div><div>when using the RIM roughness, though surface measurements cannot provide a complete picture.</div><div>The modulated second mode shows strong nonlinearity and harmonic development just prior</div><div>to breakdown. In addition, pairs of hot streaks merge together within a constant azimuthal</div><div>band, leading to a peak in the heating simultaneously with the peak amplitude of the measured</div><div>secondary instability. The heating then decays before rising again to turbulent levels. This nonmonotonic</div><div>heating pattern is reminiscent of experiments on a flared cone and earlier computations</div><div>of crossflow on an elliptic cone.</div><div><br></div><div>When using the distributed roughness there are several differences in the nonlinear breakdown</div><div>behavior. The hot streaks appear to be much more uniform and form at a higher wavenumber,</div><div>which is expected given computational results. Furthermore, the traveling crossflow waves become</div><div>very prominent in the surface pressure fluctuations and weakly nonlinear. In addition there</div><div>appears in the spectra a higher-frequency peak which is hypothesized to be a type-I secondary instability</div><div>under the upwelling of the stationary vortices. The traveling crossflow and the secondary</div><div>instability interact nonlinearly prior to breakdown.</div>

<p>This thesis focuses on the development of reduced pressure testing capabilities at Zucrow Laboratories. A two-stage ejector on loan from NASA Marshall is used in series with a supersonic diffuser to allow for the testing of up to100 lb_f rocket engines at equivalent altitudes of up to 100,000 ft. The objective of this research is to implement a one-dimensional (1-D) model which accurately predicts the performance of the two-stage ejector in real time, informing the maximum thrust and simulated altitude capabilities within the altitude chamber located in room 134A of ZL3 during experimental testing.</p>

Undulatory ribbon- n-based propulsion is an appealing propulsion mechanism due to its rich locomotor capabilities that can improve the propulsive performance and maneuverability of underwater vehicles. For instance, the swimming mechanics of weakly electric black ghost knife sh (Apteronotus albifrons) is of great interest to study because of their high swimming e ciency at low speeds and extraordinary agility such as rapid reversal swimming, hovering in presence of water disturbance, rolling and vertical swimming. In this thesis work, to facilitate our understanding on the exible undulatory ribbon n propulsion, we have four research motivations. The rst objective is to study how the use of exible rays and di erent n morphology can in uence the propulsive performance of ribbon- n propulsion. It is possible that natural swimmers using this locomotion method could take advantage of passive n motion based on the coupling of uid-structure interaction and the elasto-mechanical responses of the undulating n. Therefore, the second objective is to understand how an under-actuated undulating n can take advantage of natural dynamics of the uid-structure interaction for the propulsive force generation. In addition to the impressive propulsive performance of the undulatory n propulsion, the exceptional maneuverability of knife sh is also a key motivation that drives this thesis work. Thus, we dedicate to investigate how traveling wave shapes and actuation parameters (frequency, wavelength) can manipulate the maneuvering behaviors of a swimmer propelled by an undulating ribbon n. Lastly, we aim to uncover the e ect of varying traveling wave amplitudes and pectoral ns on its maneuvering performances. Two robotic devices were developed to study the propulsive performance of both fullyactuated and under-actuated ribbon n propulsion and investigate the maneuver control of a free-swimming underwater robot propelled by an undulatory n. For the rst research aim, we study the e ect of exible rays and di erent n morphology on the propulsive performance of ribbon- n propulsion. A physical model composed of fteen rays interconnected with an elastic membrane was used to test four di erent ray exural sti ness and four aspect ratios. Our results show that exible rays can improve the propulsive e ciency compared to a rigid counterpart. In addition, the morphology of the ribbon n a ects its propulsive performance as well, and there could exist an optimal n morphology. To understand how an underactuated undulating n can modify its active and passive n motion to e ectively control the hydrodynamic force and propulsive e ciency. We did a series of experiments using the same robotic n model but with some structural modi cations and we measured n kinematics, net surge force and power consumption. We nd that the under-actuated n can keep the equivalent propulsive e ciency as the fully-actuated counterpart within our experimental parameter range. Moreover, our results demonstrate that the thrust force and power consumption of an under-actuated n follow the same scaling laws as the fully-actuated n. To conduct the free-swimming maneuver study, we developed a self-contained, free-swimming robot propelled by an undulatory n, which is able to perform the following maneuvers: forward, reversed swimming and hovering motion. We also performed V3V PIV experiments to capture the ow structures generated by the robotic device. Our results show that the robot can reach higher swimming e ciency at low frequencies. As the number of traveling waves increases, the robot swims more stably in roll, pitch and yaw motions. For cases with varying wave amplitudes, traveling wave with incremental wave amplitude can achieve free-swimming velocity higher than that of decremental wave amplitude. However, the latter case can generate higher pitch angles. For the robot with slightly negative-pitched pectoral ns, it can perform slow diving maneuvers. These ndings demonstrate that we can take advantage of the undulating ribbon n propulsion to achieve high maneuverability for the future underwater vehicles in complex environment. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection