Theses and Dissertations

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Open Access
A novel smoother-based data assimilation method for complex CFD
(Colorado State University. Libraries, 2024) Hurst, Christopher L., author; Gao, Xinfeng, advisor; Guzik, Stephen, advisor; Troxell, Wade, committee member; van Leeuwen, Peter Jan, committee member
Accurate computational fluid dynamics (CFD) modeling of turbulent flows is necessary for improving fluid-driven engineering designs. Traditional CFD often falls short of providing truly accurate solutions due to inherent uncertainties stemming from modeling assumptions and the chaotic nature of fluid flow. To overcome these limitations, we propose the integration of data assimilation (DA) techniques into CFD simulations. DA, which incorporates observational data into numerical models, offers a promising avenue to enhance predictability by reducing uncertainties associated with initial conditions and model parameters. This research aims to advance our understanding and application of DA for CFD modeling of highly chaotic dynamical systems. This dissertation makes several novel contributions in DA and CFD: i) A novel DA algorithm, the maximum likelihood ensemble smoother (MLES), has been developed and implemented to provide better model parameter estimation and assimilate time-integrated observations while addressing nonlinearity, ii) Multigrid-in-time techniques are applied to enhance the computational efficiency of the MLES by improving the optimization processes, and iii) The MLES+CFD framework has been validated by classical test problems such as the Lorenz 96 model and the Kuramoto-Sivashinsky equation. The effectiveness of the MLES has been demonstrated through a few test problems featuring chaos, discontinuity, or high dimensionality.
Embargo
Thermally-assisted frontal polymerization for rapid curing of fiber-reinforced polymer composites
(Colorado State University. Libraries, 2024) Naseri, Iman, author; James, Susan, advisor; Bailey, Travis, committee member; Herrera-Alonso, Margarita, committee member; Ma, Kaka, committee member
Fiber-reinforced polymer composites (FRPCs) are widely used in a variety of applications owing to their excellent specific mechanical properties, chemical stability, and fatigue resistance. However, the state-of-the-art technologies for manufacturing FRPCs are intensive in terms of time and energy, generate a significant carbon footprint, and require costly resources. In addition, FRPCs lack key non-structural functionalities (e.g., de-icing, damage sensing) required for many applications. Despite the enormous efforts made to improve the manufacturability of FRPCs and address the shortcomings associated with the performance of FRPCs, there is still a pressing need for alternative manufacturing technologies to enable the rapid, energy-efficient, and low-cost manufacturing of multifunctional fiber-reinforced polymer composites. In this dissertation, a novel technique for rapid and cost-effective manufacturing of multifunctional fiber-reinforced polymer composites is developed by exploiting the frontal polymerization concept and joule heating of nanostructured materials. A nanostructured paper or fabric is integrated into the composite layup to supply the energy required to trigger frontal polymerization via the Joule heating effect. In addition, the nanostructured paper remains advantageous in in-service conditions and imparts new functionalities to the host composite structure. In the first chapter, the recent developments in material systems, as well as heating techniques reported for improving the manufacturability of FRPCs, are reviewed, and frontal polymerization (FP) as a rapid and energy-efficient technique for curing thermoset matrix composites is introduced. In the second chapter, frontal curing of multifunctional composites via a commercial nanostructured heater (buckypaper) is demonstrated, and the curing behavior of composite laminate is studied under various layup conditions. It is demonstrated that the through-thickness FP manufacturing strategy using an embedded buckypaper surface heater allows for rapid and energy-efficient manufacturing of fully cured composite panels using the conventional tooling materials utilized in the composite industry. However, the temperature profiles developed during the cure cycle, as well as the degree of cure of resin in produced composites, are greatly affected by the thermal properties of the tooling materials, where lower front temperatures and degree of cure are measured for composite panels manufactured using thermally conductive tooling materials such as aluminum. This issue can be effectively addressed by preheating the dry composite layup for a few minutes. Despite the relatively uniform heat generation in nanostructured buckypaper heaters, the infrared thermal imaging of the curing process reveals that the front initiates from multiple locations and propagates in both the through-thickness and in-plane directions. In addition, the de-icing functionality is demonstrated in the cured composite as one of the several possible functionalities imparted to composite structures due to the presence of a buckypaper layer. In the third chapter, a fabric heater is developed by writing laser-induced graphene on aramid fabric using a CO2 laser and used as an integrated heater for manufacturing FRPCs via the through-thickness FP manufacturing technique. A 10 cm × 10 cm composite panel is successfully cured within only 1 minute with a total energy consumption of 4.13 KJ, which is comparable to the time and energy required for producing a similar composite panel using a buckypaper heater. In addition to composite manufacturing, flexible heaters are prepared with the addition of silicone rubber to fabric heaters. Although the addition of electrically insulating rubber negatively affects the electrothermal performance of fabric heaters, it greatly improves the durability of fabric heaters. In the fourth chapter, a facile and rapid technique for the preparation of mechanically robust nanocomposite film heaters is developed based on a frontally polymerizable resin system. The mechanical and electrothermal properties of the nanocomposite film heaters are characterized, and the produced heaters are used for out-of-oven manufacturing composite laminates. In the final chapter, the main research findings are summarized, and the recommendations for future studies are presented.
Open Access
Multi-scale & multi-resolution experimental and analytical methods for mitigating blast risk with barrier walls
(Colorado State University. Libraries, 2024) Sullivan, Kellan M., author; Mahmoud, Hussam, advisor; Puttlitz, Christian, advisor; Gadomski, Benjamin, committee member; Jia, Gaofeng, committee member; Stephens, Catherine, committee member; Pezzola, Genevieve, committee member
Over the last decade, interest in blast resistance and protection has increased as a result of the perpetual threat of terrorist groups around the world. In evaluating the Department of State (DOS) reports on terrorism since 2007, an estimated 330,000 fatalities and 430,000 injuries have been caused by terrorist attacks worldwide (2022). In the United States, various large scale explosive attacks have occurred over the years including the World Trade Center bombings in 1993, the Alfred Murrah Federal Building bombing in 1995, and the coordinated September 11th attacks in 2001. More recently, there has been a shift in the tactics of terrorist groups to use improvised explosive devices (IEDs) to target civilians due to regulations put in place after the September 11th, 2001, attacks that made it difficult for them to obtain a large amount of explosive material among other factors contributing the rise of terrorist activity. Attacks such as the Boston Marathon bombing in 2013 and the Madrid train bombing in 2004 demonstrate this shift in tactics. The upward trend of the use of IEDs around the globe since the September 11th, 2001, attacks presents a catalyst for a shift in research methods for blast mitigation techniques to provide protection to people rather than just structures. Therefore, developing methods to provide protection for people from blast effects is necessary to minimize the impact these terrorist groups have on our communities. Of the existing blast mitigation strategies, perimeter walls or barriers are specifically advantageous in that they increase standoff distances and provide an obstacle to the propagation path of the blast wave as well as primary fragmentation. The use of perimeter walls or barriers to protect structures has been well established in literature, however the use of barriers to protect people has not. The ability to predict airblast effects accurately and efficiently over a large variation in scaled ranges, within a complex environment, is important to characterize the potential severity of damage to structures and casualties among personnel in both military and civilian settings. Many different techniques have been used over the years to perform blast prediction of various airblast parameters such as pressure and impulse and blast resistant design research. While experimentation remains a valuable and powerful tool, in recent years, computational and numerical models have grown in popularity for their accurate evaluation capabilities. Advanced numerical software such as hydrocodes and computational fluid dynamic programs are often used to model airblast propagation and its impact on structures. However, in more complex environments, where blast loading in large areas of interest may occur, using high-fidelity computational modeling software could be inefficient due to the computing power required. The goal of this dissertation was to develop a performance-based design framework for predicting the probability of survivability of a double-barrier system under blast loading, and the probability of different bodily injuries for personnel from the blast wave itself. In this dissertation, the gaps in research for protecting civilians from IED attacks in large open areas, understanding the impact of multiple barriers on the blast shockwave and pressures around the barriers, and investigating an absorption focused barrier were addressed. A combination of analytical, numerical, and experimental methods at multiple scales was used to develop and validate the various elements needed to conduct the performance-based design. This dissertation developed rapid computational models to predict the pressure field around a double-barrier system, analyzed a new barrier design that focuses on reducing the energy of the shockwave in order to protect people, and accounted for the uncertainty and variability in multiple parameters to establish potential risk for various scenarios for both the barrier and for people. The analyses combined numerical, analytical, and experimental methods at multiple scales, to create models to predict and assess the pressures associated with person-borne-improvised-explosive-devices (PBIEDS). The developed models used to predict and quantify the pressures around a rigid double-barrier system and the response of the wood barrier to blast loading were coupled with small- and full-scale experimental testing to validate and assess the accuracy and efficiency of the models. From the results of dissertation, it can be observed how the implementation of a double-barrier system can significantly reduce the pressures experienced around the barriers, which can lead to less potential for serious injury or damage from blast events. Additionally, it showed that the distance between the barriers plays a critical role in the pressures and therefore the potential for injury between the barriers. In addition, adopting an innovative approach to blast barrier design to consider the use of more lightweight, commonly available, non-rigid materials to increase the energy absorption to attenuate the blast shockwave rather than just reflect was proven to be beneficial.
Open Access
Numerical algorithms for two-fluid, weakly-compressible flows
(Colorado State University. Libraries, 2024) Brodin, Erik, author; Guzik, Stephen, advisor; Colella, Phillip, advisor; Gao, Xinfeng, committee member; Troxell, Wade, committee member; Bangerth, Wolfgang, committee member
A multifluid numerical method is developed for flows of two fluids in a single domain at low Mach numbers. An all-speed formulation of the Navier-Stokes equations governs the dynamics of both fluids and the level-set method defines the interface between them and the domain of each fluid. The algorithm represents velocity and pressure as single valued throughout the whole domain, and fluid dependent variables, density and bulk modulus, only in the domain of their respective fluid. The all-speed equations are not subject to the divergence-free velocity constraint through use of a redundant velocity equation, and are evolved in time using an implicit-explicit additive Runge-Kutta method resulting in a time step constrained only by the bulk fluid velocity. Each fluid is evolved conservatively with respect to the moving interface between them. Due to errors in the evolution in the interface, perturbations in the volume of each fluid, and thereby the density, can develop. A thermodynamically consistent correction is made to the position of the interface to reduce these unphysical perturbations. The algorithm developed here includes three novel contributions: (i) the use of a multifluid all-speed algorithm with a level-set method for evolution of the solution in time, (ii) a multifluid algorithm using the level-set to capture the interface in the weakly compressible regime that is thermodynamically consistent, and (iii) an initialization method for sharp corners in the level-set. Numerical tests have demonstrated that the algorithm exhibits the expected low Mach number behavior, achieves second order-accuracy, and ensures fluid volumes are bounded and convergent.
Open Access
Analysis and refinement of Methane Number test procedure for gaseous fuels
(Colorado State University. Libraries, 2024) Baucke, Dawson, author; Olsen, Daniel B., advisor; Wise, Daniel M., committee member; Daily, Jeremy S., committee member
Methane Number (MN) is an experimentally determined parameter for quantifying the resistance of gaseous fuels to End Gas Auto Ignition (EGAI). Originating from Leiker et al. in Graz, Austria, MN was introduced as an alternative to traditional gasoline rating techniques due to limitations on maximum obtainable values without extrapolative methods. Through funding provided to AVL, Leiker, et al. explored the impact of gas composition on fuel reactivity, although the specific details of their testing method remain unpublished. Subsequently, refinements to Leiker's proposed analytical method were made by AVL and MWM including digitizing of the AVL experimental data and the use of a computer program. The American Society of Testing Materials (ASTM) developed a standard for calculating a methane number (MNC)based on the gaseous fuel composition using the latest MWM methodology and experimental data. Amidst growing interest in renewable and hydrogen-blended natural gas, uncertainties within the experimental data used in the MNC method have spurred re-evaluation of the MN testing method. The purpose of this research is to create a repeatable method for determining the knock resistance of gaseous fuels analogous to the methods used for gasoline utilizing reference fuel blends of methane, hydrogen, and carbon dioxide. While Leiker, et al. did not disclose details of their MN quantification testing method, numerous research groups have developed their own methods, often without divulging test specifics or operating conditions. Presently, there is no standardized method for experimentally determining the MN of a gaseous fuel. This study aims to establish and share a repeatable method for MN determination using a modified Cooperative Fuel Research Engine (CFR). The investigation includes justification of allowable environmental parameters and operating variation limits, as well as exploring potential adaptations to the original proposed method. A pivotal aspect of the MN method involves identifying and quantifying a Knock Index (KI) parameter during engine operation, a challenge tackled through various approaches. CFR engines, originally designed for gasoline EGAI testing, come equipped with their own knock detection measurement systems. CSU has devised its method for determining a KI, and a comparison between the two systems was conducted to facilitate the publication of a standardized MN testing protocol.
Open Access
Design, fabrication, and characterization of 3D printed ceramic scaffolds for bone regeneration
(Colorado State University. Libraries, 2024) Baumer, Vail Olin, author; Prawel, David, advisor; McGilvray, Kirk, committee member; Heyliger, Paul, committee member
Synthetic bone tissue scaffolds are a promising alternative to current clinical techniques for treating critically large bone defects. Scaffolds provide a three-dimensional (3D) environment that mimics the properties of bone to accelerate bone regeneration. Optimal scaffolds should match the mechanical properties of the implantation site, feature a highly porous network of interconnected channels to facilitate mass transport, and exhibit surface properties for the attachment, proliferation, and differentiation of bone cell lineages. 3D printing has enabled the manufacture of complex scaffold topologies that meet these requirements in a variety of biomaterials which has led to rapidly expanding research. Structural innovations such as triply periodic minimal surfaces (TPMS) are enabling the production of scaffolds that are stiffer and stronger than traditional rectilinear topologies. TPMS are proving to be ideal candidates for bone tissue engineering (BTE) due to their relatively high mechanical energy absorption and robustness, interconnected internal porous structure, scalable unit cell topology, and smooth internal surfaces with relatively high surface area per volume. Among the material options, calcium phosphate-based ceramics, such as hydroxyapatite and tricalcium phosphate, are popular for BTE due to their high levels of bioactivity (osteoconductivity, osteoinductivity and osteointegration), compositional similarities to human bone mineral, non-immunogenicity, tunable degradation rates, and promising drug delivery capabilities. Despite the potential for TPMS ceramic scaffolds in BTE, few studies have explored beyond the popular Gyroid topology. Of the many TPMS options, the Fischer Koch S (FKS) has been simulated to be stronger, be more isotropic, have higher surface area, and absorb more energy than Gyroid at high porosities. In this report, we present a method for photocasting any TPMS in hydroxyapatite which is used to 3D print the first FKS ceramic scaffold. Results indicated that the resolution and accuracy of the process is suitable for BTE, and the custom software for producing the scaffolds was made available to the open-source community. Then, FKS and Gyroid scaffolds were designed to match the properties of trabecular bone using this method for use in critical bone defect repair. The scaffolds were printed and characterized using compressive and flow-based testing to reveal that, while both designs could mimic the low end of natural bone performance, the FKS were 32% stronger and only 11% less permeable than Gyroid. These findings emphasized the need for further characterization of these scaffolds beyond mechanical analysis and into studies of cell growth. To accomplish this, a custom multi-channel perfusion bioreactor was designed to culture cells on these scaffolds to investigate differences in cell behavior with higher efficiency than current designs. The design, capable of culturing many samples simultaneously, was validated using computational fluid dynamics and cell growth assays to demonstrate osteogenic effects and repeatability. In this work, novel TPMS scaffolds were fabricated from hydroxyapatite with sufficient accuracy and quality for large defects, testing of these scaffolds matched trabecular bone performance and suggested that FKS may be superior to Gyroid, and lastly, a four-channel bioreactor system was designed and validated to enable researchers to further characterize scaffolds for BTE.
Open Access
Direct digital manufacture of continuous fiber reinforced thermoplastic high aspect ratio composite grid stiffeners and grid stiffener intersections with radically reduced tooling
(Colorado State University. Libraries, 2024) Hogan, Steven J., author; Radford, Donald W., advisor; Heyliger, Paul, committee member; Yourdkhani, Mostafa, committee member
Grid stiffened structures are widely used in the aerospace industry due to their high strength and stiffness to weight ratio and impact damage tolerance. These structures consist of a lattice pattern of stiffening ribs bonded to a thin shell structure, where the stiffening ribs commonly act as the main load bearing members, and the shell acts to cover the ribs and transfer loads through membrane action. These structures offer a variety of beneficial structural properties including high specific strength and stiffness, high impact resistance, high compressive resistance, and high energy absorption. However, the complexity of a grid pattern can lead to excessive manufacturing times, especially for simple constructions such as flat plates. A more promising alternative for manufacturing grid stiffened structures is the use of automated manufacturing methods including ATL, AFP, and filament winding. Because composite grid stiffened structures can be composed entirely of the same composite material, the manufacturing process with these methods can be almost entirely automated, saving time and money. However, the traditional and automated methods of producing composite grid stiffened structures require the fabrication of complex tooling to develop the geometry of stiffening ribs. In addition, all composite grid stiffened structures suffer from the same manufacturing difficulty: for all of the fibers to be continuous through an intersection node, there must be twice as much material at each intersection than in each rib, making intersection compaction extremely difficult. A more recently developed composite manufacturing method is additive manufacturing (AM) in the form of composite 3D printing, which offers a much higher degree of geometric freedom than other autonomous manufacturing methods and does not require tooling. However, composite 3D printing is generally limited to low fiber volume fractions. A manufacturing method with the ability to make high quality, high fiber volume fraction continuous fiber grid stiffened structures without the need for tooling could significantly increase the efficiency and decrease the cost to produce these structures. The current study proposes the use of a novel additive manufacturing method which uses a commingled feedstock and features in situ consolidation to produce grid stiffened structures without the need for tooling. Several stiffener ribs and stiffener rib intersections were produced and tested for composite quality. The fiber volume fraction and void volume fraction through the height and length of printed stiffener ribs and intersections was analyzed to determine if the quality was consistent. A micrograph evaluation was performed on the high aspect ratio stiffener rib and intersection composites to qualitatively evaluate the reinforcement distribution, determine the void locations, and to support the constituent material concentration measurements. The consolidation force was measured during the manufacturing of the samples to better understand the forces experienced during printing and to form a relationship between the consolidation force experienced and the constituent volume fraction of the samples. The results of this study suggest that the application of direct digital manufacture to the placement and consolidation of commingled tow for the fabrication of high aspect ratio grid stiffeners and intersections, without the need for tooling, can readily achieve fiber volume fractions greater than 50% and void fractions as low as 5%. Volume fraction analysis results show that manufactured stiffener ribs and stiffener grid intersections exhibit high fiber volume fractions and low void volume fractions which remain consistent through the height of the samples. Consolidation force measurement results show that a significant decrease in force is experienced between print layers. Microscopic analysis results show that the majority of voids collect at the edges of print layers leading to an increase in void content at the intersection node and potentially masking any quality gradient through the height of samples that may exist. The results of this study show the high potential for the manufacturing of high quality high aspect ratio continuous fiber composite grid stiffener structures through direct digital manufacturing technologies without the need for tooling.
Open Access
Computational modeling of the lower cervical spine: facet cartilage distribution and disc replacement
(Colorado State University. Libraries, 2009) Womack, Wesley J., author; Puttlitz, Christian, advisor
Anterior cervical fusion has been the standard treatment following anterior cervical discectomy and provides sufficient short-term symptomatic relief, but growing evidence suggests that fusion contributes to adjacent-segment degeneration. Motion-sparing disc replacement implants are believed to reduce adjacent-segment degeneration by preserving motion at the treated level. Such implants have been shown to maintain the mobility of the intact spine, but the effects on load transfer between the anterior and posterior elements remain poorly understood. In order to investigate the effects of disc replacement on load transfer in the lower cervical spine, a finite element model was generated using cadaver-based Computed Tomography (CT) imagery. The thickness distribution of the cartilage on the articular facets was measured experimentally, and material properties were taken from the literature. Mesh resolution was varied in order to establish model convergence, and cadaveric testing was undertaken to validate model predictions. The validated model was altered to include a disc replacement prosthesis at the C4/C5 level. The effect of disc-replacement on range of motion, antero-posterior load distribution, total contact forces in the facets, as well as the distribution of contact pressure on the facets were examined, and the effect of different facet cartilage thickness models on load sharing and contact pressure distribution predictions were examined. Model predictions indicate that the properly-sized implant retains the mobility, load sharing, and contact force magnitude and distribution of the intact case. Mobility, load sharing, nuclear pressures, and contact pressures at the adjacent motion segments were not strongly affected by the presence of the implant, indicating that disc replacement may not be a significant cause of post-operative adjacent-level degeneration. Variation in articular cartilage distribution did not substantially affect mobility, contact forces, or load sharing. However, mean and peak contact pressure, contact area, and center of pressure predictions were strongly affected by the cartilage distribution used in the model. These results indicate that oversimplification of the cartilage thickness distribution will negatively affect the ability of the model to predict facet contact pressures, and thus subsequent cartilage degeneration.
Open Access
Development of a hyaluronan-polyethylene copolymer for use in articular cartilage repair
(Colorado State University. Libraries, 2009) Oldinski, Rachael, author; James, Susan P., advisor
Articular cartilage is the connective tissue which covers the ends of long bones, providing a lubricious, hydrodynamic surface for articulation and energy dissipation. Articular cartilage has a limited ability to repair itself; once the native tissue has become damaged, either from injury or disease (e.g., arthritis), it is irreversible and the tissue will degrade with time resulting in joint pain. The goal of this research was to develop a permanent (i.e., non biodegradable/bioerodible) bioactive material and assess its applicability for articular cartilage repair and/or replacement. Utilizing two constituents, polyethylene (the 'gold standard' bearing material for total joint replacements) and hyaluronan (HA, a native component of articular cartilage), a hyaluronan-polyethylene graft copolymer (HA-co-HDPE) was developed. The novel HA- co-HDPE material was successfully synthesized using an interfacial polymerization reaction in a non-aqueous environment. Although the material has limited melt-processability, it is more processable than HA and was successfully compression molded into samples for physical, mechanical and in vitro biological characterization (e.g., swell ratio, dynamic mechanical analysis). HA-co-HDPE exploits the strength, rigidity and melt-processability associated with HDPE, and achieves increased osteogenic potential by incorporating the highly hydrophilic biopolymer HA. In conclusion, the swelling, mechanical and degradation properties of the copolymer can be custom-optimized for biomedical applications by tailoring chemical or physical crosslinking strategies and varying the amount and molecular weights of HA and HDPE incorporated into the copolymer.
Open Access
A biomechanical analysis of venous tissue in its normal, post-phlebitic, and genetically altered conditions
(Colorado State University. Libraries, 2009) McGilvray, Kirk Cameron, author; Puttlitz, Christian M., advisor
The incidence of vein disease is very high, affecting more than 2% of the hospitalized patients in the United States; a number that is expected to increase. Post phlebitic veins, the result of chronic deep vein thrombosis, is considered to be one of the most important venous disease pathologies. Unfortunately, little information is currently available on the biomechanical effects of thrombus resolution in the deep veins. The aim of this research was to characterize the biomechanical response of both healthy and diseased venous tissue using a murine model. It was hypothesized that biomechanical response parameters derived from healthy and diseased tissue would give insight into the resultant clinical complications observed in patients following thrombus resolution. Biomechanical analysis revealed that statistically significant deleterious changes in vein wall compliance were observed following thrombus resolution. Data also revealed that matrix metallopeptidase 9 expression has a statistically significant effect on the biomechanical response of the tissue. These results indicate that clinical complications following deep venous thrombosis manifest from significant decreases in the compliance of the vein wall. Finite element analyses were also performed. Biomechanical data served as input material parameters for modeling. Finite element modeling was used to evaluate the response of the inferior vena cava under physiologic loads. The results indicate that peak stresses are generated in the circumferential direction of loading during luminal pressurization. Decreased dilatation was observed following thrombus resolution. The data indicates that deep venous thrombosis lead to increased vein wall stress in correlation with decreased luminal distensability.
Open Access
Development and optimization of a stove-powered thermoelectric generator
(Colorado State University. Libraries, 2008) Mastbergen, Dan, author; Willson, Bryan, advisor
Almost a third of the world's population still lacks access to electricity. Most of these people use biomass stoves for cooking which produce significant amounts of wasted thermal energy, but no electricity. Less than 1% of this energy in the form of electricity would be adequate for basic tasks such as lighting and communications. However, an affordable and reliable means of accomplishing this is currently nonexistent. The goal of this work is to develop a thermoelectric generator to convert a small amount of wasted heat into electricity. Although this concept has been around for decades, previous attempts have failed due to insufficient analysis of the system as a whole, leading to ineffective and costly designs. In this work, a complete design process is undertaken including concept generation, prototype testing, field testing, and redesign/optimization. Detailed component models are constructed and integrated to create a full system model. The model encompasses the stove operation, thermoelectric module, heat sinks, charging system and battery. A 3000 cycle endurance test was also conducted to evaluate the effects of operating temperature, module quality, and thermal interface quality on the generator's reliability, lifetime and cost effectiveness. The results from this testing are integrated into the system model to determine the lowest system cost in $/Watt over a five year period. Through this work the concept of a stove-based thermoelectric generator is shown to be technologically and economically feasible. In addition, a methodology is developed for optimizing the system for specific regional stove usage habits.
Open Access
Interaction space abstractions: design methodologies and tools for autonomous robot design and modeling
(Colorado State University. Libraries, 2009) Kaiser, Carl L., author; Troxell, Wade O., advisor
Current abstractions, design methodologies, and design tools are useful but inadequate for modern mobile robot design. By viewing robotics systems as an interactive and reactive agent and environment combination, and focusing on the interactions between the two, particularly those interactions that result in task accomplishment, one arrives at the interaction space abstraction. The role of abstractions, formalisms and models are discussed, with emphasis on several specific abstractions used for robotics as well as the strengths and shortcomings of each. The role of design methodologies is also discussed, again with emphasis on several currently used in robotics. Finally, design tools and the use thereof are briefly discussed. The concept of interaction spaces as an abstraction and a formalism is developed specifically for use in robot design. Types of elements within this formalism are developed, defined, and described. A formal nomenclature is introduced for these elements based on Simulink blocks. This nomenclature is used for descriptive models and the Simulink blocks are used for predictive models. The interaction space abstraction is combined with the concept of exploration-based design to create a design methodology specifically adapted for use in descriptive modeling of autonomous robots. This process is initially developed around a simple wall-following robot, then is expanded around a multi-agent foraging system and an urban search and rescue robot model, each of which demonstrates different aspects and capabilities of interaction space modeling as a design methodology. A design tool based on iterative simulation is developed. The three specific examples above are used to perform quantitative simulation and the results are discussed with emphasis on determination and quantification of factors necessary for task accomplishment. These simulations are used to illustrate how to explore the design space and evaluate trade offs between design parameters in a system.
Open Access
Fiber delivery and diagnostics of laser spark ignition for natural gas engines
(Colorado State University. Libraries, 2008) Joshi, Sachin, author; Yalin, Azer, advisor; Willson, Bryan, advisor
Laser ignition via fiber optic delivery is challenging because of the need to deliver pulsed laser beam with relatively high energy and sufficient beam quality to refocus the light to the intensity required for creating spark. This dissertation presents work undertaken towards the development of a multiplexed fiber delivered laser ignition system for advanced lean-burn natural gas engines. It also describes the use of laser ignition system to perform in-cylinder optical diagnostics in gas engines. Key elements of the dissertation includes: (i) time resolved emission spectroscopy (TRES) of laser sparks in air to investigate the dependence of spark temperatures and electron number densities on ambient gas pressures, (ii) optical characterization of hollow core fibers, step-index silica fibers, photonic crystal fibers (PCFs) and fiber lasers, (iii) development and on-engine demonstration of a multiplexer to deliver the laser beam from a single laser source to two engine cylinders via optical fibers, and (iv) demonstration of simultaneous use of laser sparks for ignition and Laser Induced Breakdown Spectroscopy (LIBS) to measure in-cylinder equivalence ratios in a Cooperative Fuel Research (CFR) engine. For TRES of laser sparks, the ambient gas pressure is varied from 0.85 bar to 48.3 bar (high pressures to simulate elevated motored in-cylinder pressures at time of ignition in advanced gas engines). At later stages (~1μs) of spark evolution, spark temperatures become comparable at all pressures. Electron number densities increase initially with increasing ambient gas pressure but become comparable at pressures greater than ~20 bar. The effects of launch conditions and bending for 2-m long hollow core fibers are studied and an optimum launch f/# of ~55 is shown to form spark in atmospheric pressure air. Spark formation using the output of a pulsed fiber laser is shown and delivery of 0.55 mJ nanosecond pulses through PCFs is achieved. Successful multiplexed laser ignition of a CAT G3516C gas engine via hollow core fibers is shown. LIBS analysis conducted at equivalence ratios from 0.6 to 0.95 in the CFR engine show a linear variation and linear correlation (R2 > 0.99) of line intensity ratio (Hα/O777 and Hα/Ntot) with equivalence ratio.
Open Access
Plasma flow field measurements downstream of a hollow cathode
(Colorado State University. Libraries, 2007) Farnell, Casey Coffman, author; Williams, John D., advisor; Wilbur, Paul J., advisor
The focus of the research described herein is to investigate and characterize the plasma produced downstream of a hollow cathode with the goal of identifying groups of ions and possible mechanisms of their formation within a plasma discharge that might cause erosion, especially with respect to the hollow cathode assembly. In space applications, hollow cathodes are used in electrostatic propulsion devices, especially in ion thrusters and Hall thrusters, to provide electrons to sustain the plasma discharge and neutralize the ion beam. This research is considered important based upon previous thruster life tests that have found erosion occurring on hollow cathode, keeper, and ion optics surfaces exposed to the discharge plasma. This erosion has the potential to limit the life of the thruster, especially during ambitious missions that require ultra long periods of thruster operation. Results are presented from two discharge chamber configurations that produced very different plasma environments. Four types of diagnostics are described that were used to probe the plasma including an emissive probe, a triple Langmuir probe, a remotely located electrostatic analyzer (ESA), and an ExB probe attached to the ESA. In addition, a simulation model was created that correlates the measurements from the direct and remotely located probes.
Open Access
Performance and lifetime simulation of ion thruster optics
(Colorado State University. Libraries, 2007) Farnell, Cody Coffman, author; Williams, John D., advisor; Wilbur, Paul J., advisor
A simulation code, ffx, was developed to study various aspects of ion thruster optics. Information concerning sheaths, impingement limits, perveance, electric potential, charge exchange, and sputtering is covered. Electron backstreaming and pit and groove wear are discussed in detail as two grid failure mechanisms. The code was used to study the effects of several parameters on grid performance and lifetime, including grid spacing, aperture diameter, and grid thickness. An evolutionary algorithm was used with the ffx code to design grid sets, utilizing net accelerating voltage and current density as primary input parameters. Validation of the ffx code was accomplished through comparison to other ion optics codes and to experimental data obtained from both gridlet and full thruster testing. Gridlet test comparisons included simulations of finite aperture grid sets. The NSTAR thruster was studied in detail with regard to lifetime. The methods used for accurate and efficient optics simulation are discussed, including the multigrid method for solving for electric potential.
Open Access
Laser ignition for internal combustion engines via fiber optic delivery
(Colorado State University. Libraries, 2009) DeFoort, Morgan, author; Yalin, Azer, advisor; Willson, Bryan, advisor
In the effort to reduce emissions and improve the efficiency of Otto cycle engines, the ignition system is often a limiting factor. Many "high energy" ignition systems have been developed, but almost all of these are based on traditional electric arc spark plugs. Laser ignition represents a fundamentally different approach to igniting gas mixtures and opens the door to improvements in fuel-lean engine operation and high-pressure combustion environments. Yet the promise of laser ignition remains unexploited, as practical systems have not been developed. In this contribution, we work towards the goal of developing a practical laser ignition system for stationary natural gas engines. Specifically, we focus on fiber optic delivery of the laser beam to the engine, thereby making a significant advance relative to past open-air (free-space) configurations. A combination of modeling and experimentation has been used to develop the needed fiber optic delivery systems, culminating in the first demonstration of fiber-optically delivered laser ignition on an engine.
Open Access
Extending the performance of net shape molded fiber reinforced polymer composite valves for use in internal combustion engines
(Colorado State University. Libraries, 2007) Buckley, Richard Theodore, author; Stanglmaier, Rudolf, advisor; Radford, Donald, advisor
Fiber Reinforced Composite (FRC) materials offer the possibility of reduced mass and increased structural performance over conventional metals. When used in reciprocating components of internal combustion engines, this may enable increased power and mechanical efficiency. Previously published work on FRC engine valves has both shown structural and thermal limitations.
Open Access
Expanding the knock/emissions limits for the realization of ultra-low emissions, high-efficiency heavy-duty natural gas engines
(Colorado State University. Libraries, 2023) Rodriguez Rueda, Juan Felipe, author; Olsen, Daniel B., advisor; Windom, Bret, committee member; Baker, Daniel, committee member; Quinn, Jason, committee member
Heavy-duty on-highway natural gas (NG) engines are a promising alternative to diesel engines to reduce greenhouse gas and harmful pollutant emissions if the limitations (knock and misfire) for achieving diesel-like efficiencies are addressed. This study shows innovative technologies for developing high-efficiency stoichiometric, spark-ignited (SI) natural gas engines. To develop the base knowledge required to reach the desired efficiency, a Single Cylinder Engine (SCE) is the most effective platform for acquiring reliable and repeatable data. An SCE test cell was developed using a Cummins 15-liter six-cylinder heavy-duty engine block modified to fire one cylinder (2.5-liter displacement). A Woodward Large Engine Control Module (LECM) is integrated to permit real-time advanced combustion control implementation. Fixed location of 50% burn and Controlled End Gas Auto-Ignition (C-EGAI) were used to define the ignition timing. C-EGAI allows operation with an optimized fraction of end gas auto-ignition combustion. Intake and exhaust characteristics, fuel composition, and exhaust gas recirculated substitution rate (EGR) are fully adjustable. A high-speed data acquisition system acquires in-cylinder, intake, and exhaust pressure for combustion analysis. Further development includes advanced control methodologies to maintain stable operation and higher dilution tolerance. Controlled end-gas autoignition (C-EGAI) is used as a combustion control strategy to improve efficiency. A Combustion Intensity Metric (CIM) is used for ignition control while operating the engine under C-EGAI. During the baseline testing of the developed SCE test cell, effective control of intake manifold pressure, exhaust manifold pressure, engine equivalence ratio, speed, torque, jacket water temperature, and oil temperature was demonstrated. The baseline testing shows reliable and consistent results for engine thermal efficiency, indicated mean effective pressure (IMEP), and coefficient of variance of the IMEP over a wide range of operating conditions. High Brake Thermal Efficiency (BTE) was achieved using improved hardware and a high EGR rate. Due to the correlation of CIM to the fraction of EGAI (f-EGAI), CIM was used as the reference variable to implement C-EGAI. Achieving conditions of C-EGAI allowed for the utilization of high EGR at high IMEP without inducing knock. The operation of the engine under these conditions showed peak brake thermal efficiency above 46% using an EGR ratio of 30% The work described proves the concept of using new and innovative control algorithms and CFD-optimized combustion chamber designs, allowing ultra-high efficiency and low emissions for NG ICE's heavy-duty on-road applications.
Open Access
Modeling deformation twinning in BCC transition metals
(Colorado State University. Libraries, 2023) Faisal, Anik H. M., author; Weinberger, Christopher, advisor; Radford, Donald, committee member; Ma, Kaka, committee member; Heyliger, Paul, committee member
Deformation twinning is one of the important deformation mechanisms in body centered cubic (BCC) transition metals, especially under low temperature and high strain rate conditions. Plastic deformation via deformation twinning has been studied for decades both experimentally and computationally however, atomic level insights such as critical nuclei size, their local atomic structures and energetics which are important parameters in modeling twin nucleation has been lacking. In this work, deformation twins in BCC transition metals and their atomic level structures and energetics have been rigorously studied to reveal the full atomic level details of twin nucleation and propagation. As such, critical thickness of deformation twins in BCC transition metals have been a topic of debate with many computational and experimental studies accepting a three-layer twin thickness based on nucleation from a screw dislocation without proof whereas recent in-situ experiments suggest six-layer thick twin nuclei observed via High resolution transmission electron microscopy (HRTEM). In this study, we have determined the critical twin nuclei thickness in these metals using atomistic simulations to examine atomic structure and energetics of deformation twins under both zero and nonzero finite pure shear stresses. Our study reveals that twins in group VB BCC transition metals nucleate as two-layer thick nuclei under stress as opposed to the three-layer thick twin nuclei under zero stress. For group VIB BCC transition metals, for both zero and nonzero stresses, the critical twin nuclei thickness is two layer near reflection. As the twins grow and stress is relieved, twins under finite stresses adopt configurations that are much closer to the zero stress stability predictions. In addition to nucleation, growth of mechanisms of twins are explored and computational insights into the growth of twins in Tungsten bicrystals explaining multi-layer growth as opposed to layer-by-layer growth associated with small barriers. Free-end string simulations were used to investigate energy barrier associated with homogeneous twin nucleation using embedded atom method (EAM) potentials. Since homogeneous twin nucleation occurs near the ideal strengths of the material described by the potentials, energy barrier calculations were not possible for all BCC transition metals as some available potentials break down under large stresses. Moreover, density functional theory (DFT) simulations are known to be more accurate in describing atomic bonding but direct nucleation simulations in bulk crystals is prohibitively expensive. Hence, existing dislocation nucleation models are thoroughly analyzed to examine the behavior of these models near ideal strength of the material because spontaneous nucleation of dislocations occurs at high stresses. From there, a robust homogeneous twin nucleation model that includes elastic interaction among the twinning dislocation loops is developed which is able to replicate energy barrier data from free-end string simulations for multiple interatomic potentials. This model takes atomistic simulation inputs such as the concurrent twinning generalized stacking fault (GSF) energy curves and corresponding burgers vector of the twinning dislocations to compute the energy barriers as a function of applied stress. This model can be useful in modeling homogeneous twin nucleation all BCC transition metals and has the potential advantage of using DFT simulation inputs for accurate description of atomic bonding within the twin nuclei. Finally, nucleation stresses for twinning in bulk crystals have been studied to investigate whether the formation of twinning in experimental studies were initiated by homogeneous nucleation. Upper and lower bounds of stress values required for homogeneous twin nucleation has been computed and a semi-empirical model has been developed to predict homogeneous twin nucleation stresses as a function of temperature and strain rate. This analysis shows that reported critical resolved shear stress (CRSS) values in experimental studies are not associated with homogeneous twin nucleation despite some modeling studies claiming otherwise.
Open Access
Diagnostics and characterization of direct injection of liquified petroleum gas for development of spray models at engine-like conditions
(Colorado State University. Libraries, 2023) Sharma, Manav, author; Windom, Bret, advisor; Yalin, Azer, committee member; Yost, Dylan, committee member
Research within the realm of internal combustion (IC) engines is concentrated on enhancing fuel efficiency and curbing tailpipe emissions, particularly CO2 and regulated pollutants. Promising solutions encompass the utilization of direct injection (DI) and alternative fuels, with liquefied petroleum gas (LPG) standing out as a notable candidate. LPG presents a pragmatic and economical option for fueling the heavy-duty transportation sector in the United States. However, widespread adoption hinges on achieving energy conversion efficiencies in LPG engines comparable to those in diesel engine platforms. The overarching goal of this research is to address fundamental limitations to achieving or surpassing near-diesel efficiencies in heavy-duty on-road liquefied petroleum gas engines. Owing to substantial differences in physical properties compared to traditional fuels, an enhanced understanding and modeling of LPG sprays become imperative. This work conducts an experimental and numerical analysis of direct-injected propane and iso-octane, serving as surrogates for LPG and gasoline, respectively, under diverse engine-like conditions. The overall objective is to establish a baseline for the fuel delivery system required in future high-efficiency DI-LPG heavy-duty engines. Propane, emulating LPG, undergoes injection across various engine-like conditions, encompassing early and late injections, as well as boosted engines, using a range of direct injectors available in both research and commercial domains. Optical diagnostics, including high-speed schlieren and planar Mie scattering imaging, were performed to study the spray penetration, liquid and vapor phase regions, and mixing of propane and to characterize bulk and the plume-specific spray behavior of propane. The study also investigates the influence of injector geometry on spray performance. Iso-octane was used as a surrogate for gasoline, and propane was used to compare LPG's behavior with more conventional DI fuel. The experimental results and high-fidelity internal nozzle-flow simulations were then used to define best practices in computational fluid dynamics (CFD) Lagrangian spray models. Optical imaging revealed that, unlike iso-octane, propane's spray propagation was fed by its flash boiling, spray collapse, and a high degree of vaporization, resulting in a direct proportionality of propane's penetration length to temperature. These unique attributes categorize propane as an unconventional spray, necessitating corrections to injection and breakup models to replicate under-expanded jet dynamics and emulate flash boiling-driven spray development across various research and commercial injectors.