Theses and Dissertations

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Open Access
Optical performance of cylindrical absorber collectors with and without reflectors
(Colorado State University. Libraries, 1994) Menon, Arun B., author; Duff, William, advisor; Burns, Patrick J., committee member; Zachmann, David W., committee member
The optical efficiency of a solar collector, which depends on the collector geometry and material properties (i.e., geometry and radiative properties of the cover, absorber and any reflector), contributes significantly towards its overall performance. This optical efficiency is directly proportional to the transmittance-absorptance or τα product for all possible angles of incidence. A 3-D Monte Carlo ray tracing technique is used to determine this τα product for evacuated tubular collectors (ETCs) with cylindrical absorbers in an effort to identify the most efficient optical design parameters. These collectors are asymmetric with respect to the incident solar radiation and their optical efficiencies are therefore difficult to estimate using any other method. The collector geometry is modeled using constructive solid geometry (CSG). CSG allows the generation of complex collector shapes by combining simple primitive objects. The ray tracing algorithm tracks individual photons through the collector geometry to provide a means of obtaining the absorbed fraction for a particular angle of radiation incident on the collector plane. Incidence angle modifiers (IAMs), the ratio of the τα product at a particular set of longitudinal and transverse radiation incidence angles to the τα product at normal incidence are thereby obtained. IAMs are calculated for variations in five different design parameters to determine the most advantageous geometries. It is found that diffusely reflecting back planes significantly enhance optical performance of tubular collectors. Verification of the ray trace calculations is made by comparing with experimental results from the indoor solar simulator at CSU. TRNSYS predicted values of τα are within 1% of the ray trace results for normal incidence tests and within 7% for off-normal tests. Inaccuracies resulting from the use of a multiplicative technique wherein off-axis IAMs are obtained by a multiplicative combination of the biaxial IAMs are also addressed. The multiplicative approach is found to be very inaccurate for angles of incidence greater than 40°. To further assess the relative advantages of tubular collectors over flat plate collectors and whether a reflective back plane is really necessary, the two types of collectors are modeled in a simple fashion and the amount of radiation that is available for collection by each is determined. Calculations show that reflectors would probably not be required for collector slopes in excess of 50°. However, for slope angles less than 50°, a reflector placed behind the tubes is beneficial.
Embargo
Computational methods for the analysis of cell migration and motility
(Colorado State University. Libraries, 2024) Havenhill, Eric Colton, author; Ghosh, Soham, advisor; Heyliger, Paul, committee member; McGilvray, Kirk, committee member; Zhao, Jianguo, committee member
Collective cell migration (CCM) is necessary for many biological processes, such as in the formation or regeneration of tissue, fibroblast movement in wound healing, and the movement of macrophages and neutrophils in the body's immune response, to name a few. CCM is commonly modeled with PDEs, however these equations usually model the population density, rather than the displacement field describing the movement of any arbitrary cell. One unknown aspect of this movement is the various methods that cells use to facilitate communication to each other. Chemical communication plays a substantial role in directed cell movement, however, other mechanical methods, such as the propagation of stresses through a shared substrate to neighboring cells and cell behavior in a crowded environment, also play an important role which is less understood. The quantification of the kinematic and dynamic characteristics in CCM would present several novel advancements in understanding the collective cell behavior. First, the dynamic mode decomposition (DMD) framework is utilized. DMD allows for the recovery of a dynamic system, in the form of an ODE or PDE, by sampling the states of a system. In the context of the cell migration, the displacements of fibroblasts during a scratch-wound assay are obtained, which result in a governing PDE through the DMD process. This PDE is used in conjunction with modern optimal control theory to develop a 2D and 3D trajectory for the migration of controllable cells to a target. On an individual level, with the hybrid use of modern static structural optimization and simple non-linear control, a cell's cytoskeleton during migration can be studied, providing for the quantification of the traction force exerted on the substrate. The results of this analysis are compared with stress and structural optimization models in ANSYS and FEBio, which uses the finite element method, so that a reasonable range of these stresses during CCM can be provided. To further study the individual mechanics of cell migration, the proposed hybrid model is extended to a fully dynamic model which predicts the cytoskeletal stress fiber formations over time that require the minimal amount of material with the use of optimal control theory. The results of this research could provide useful applications in many real-world situations, from the generating of a trajectory for microrobots during drug delivery to the study of the collective migration of organisms including cells.
Open Access
Prediction and mitigation strategies for the transient thermal performance of low thermal resistance microchannel evaporators
(Colorado State University. Libraries, 2024) Anderson, Caleb Del, author; Bandhauer, Todd M., advisor; Venayagamoorthy, Karan, committee member; Windom, Bret, committee member; Wise, Daniel, committee member
Microchannel flow boiling heat transfer offers an effective thermal management solution for high heat flux microelectronic devices such as laser diodes. The high heat transfer rates, nearly isothermal flow conditions, high surface area-to-volume ratios, and lower required pumping powers facilitate smaller component systems while more efficiently cooling devices and reducing packaging stresses associated with thermal expansion when compared with single-phase cooling systems. Although much study has been dedicated to optimizing steady state flow boiling performance, the typically highly transient operation of these microelectronic devices leads to unsteady spikes in heat flux and, subsequently, in device temperatures and may potentially exacerbate flow instabilities present at steady state. The low thermal capacitance of the package that often accompanies the low thermal resistance of microchannel evaporators increases the potential for device damage and failure since large temperature swings are more likely. Predicting and mitigating the transient response of a low thermal resistance microchannel evaporator is paramount to practical application as a thermal management technique. In this work, temperature, pressure, and flow visualization measurements during stepped heat loads on two, low thermal resistance, microchannel evaporators revealed the presence of severe vapor backflow, large temperature overshoots, and impacted flow dynamics at the onset of nucleate boiling (ONB) despite the stability and high performance of the device under steady state heating conditions. These overshoots were exacerbated with higher heating rates and reduced subcooling but were generally improved with higher flow rates. Applying a slower heating rate greatly improved the transient thermal response, reducing both peak temperature and vapor backflow. Channel and inlet orifice geometry were found to greatly impact the performance, with smaller channels and smaller orifice-to-channel restriction ratios resulting in intensified vapor backflow and temperature spikes, despite offering improved steady state performance. A computational model embedded in a reduced order design tool was created and validated with the experiments. Two separate models were created due to the different transient conditions observed between the two tested microchannel evaporators. The models allow predictive modeling of these evaporators to determine the impact of the transient heating behavior on microchannel evaporator devices. The effect of incorporating gallium-based, solid-liquid Phase Change Materials (PCMs) was studied semi-empirically by simulating the performance of a virtual test section with predicted properties of a microchannel evaporator combined with gallium and gallium-composite foam PCMs. Properties of the PCMs were estimated and used to predict the test section thermal response under a range of PCM volumes. Models assuming single phase performance were conducted initially and the resulting predicted heat rate to the fluid applied experimentally to the test section heater to determine the temperature response. It was found that the simulated addition of the PCM slightly reduced the ONB temperatures but did not affect the peak temperature experienced by the device. The applied heating rate, however, did not consider the increased thermal resistance to the refrigerant fluid during the transient vapor backflow regime. The effect was most pronounced in the PCMs with the largest exposed surface area and with thermal conductivity-enhanced PCM composites comprised of gallium infiltrated in a copper foam matrix. Additional PCM models utilizing the transient flow boiling model were subsequently run on a series of representative heat load test cases comparing the performance of a gallium-nickel and gallium-copper composite with similar dimensions to the earlier simulations. Key assumptions included the same ONB temperatures and vapor backflow conditions as the baseline cases without PCMs. The models predicted significantly lowered peak device temperatures due to the heat absorption into the PCM during the transient vapor backflow phase. The effect was dependent on the PCM thickness, latent heat, and thermal conductivity, reflecting trade-offs in material. In addition, peak temperature variability observed experimentally across multiple trials at the same nominal testing conditions was greatly reduced with the inclusion of a PCM.
Embargo
Towards automated manufacturing of composites via thermally assisted frontal polymerization
(Colorado State University. Libraries, 2024) Jordan, Walter Patrick, author; Yourdkhani, Mostafa, advisor; Zhao, Jianguo, committee member; Simske, Steve, committee member
Current methods for the manufacturing and repair of fiber-reinforced thermoset composites are energy-intensive, slow, and costly due to extensive processing steps and expensive equipment required to achieve complete cure. This is especially true for large, complex geometries that require autoclaves and prolonged cure times. As a result, there is a need to develop faster, cost-effective, energy-efficient processes. With the implementation of rapid curing thermoset resins, the cure cycle can be reduced from hours to minutes. This research focuses on the development, implementation, and testing of these resin systems in the established fields of mobile additive manufacturing and filament winding to demonstrate unprecedented, rapid manufacturing of composite parts. Additive manufacturing of fiber-reinforced thermoset composites is desirable due to its inherent ability to produce custom, complex parts quickly, with minimal required tooling. By printing and simultaneously curing the composite as it is deposited, freeform unsupported structures with high mechanical properties can be created. One limitation of current additive manufacturing methods is the print volume associated with traditional gantry style additive manufacturing systems. By combining the highly desirable properties of additive manufacturing using rapid, thermally curable resin systems with the mobility of a mobile additive manufacturing system, large, mechanically sound structures with virtually no limitations on print volume can be created. Moreover, rapid curing thermoset resin systems have the potential to revolutionize traditional composite manufacturing processes. Due to its wide range of applications and its ubiquitous nature, filament winding serves as a natural starting point to do so. Traditional filament winding is typically a two-step manufacturing process, where the composite part is first wound on a rotating mandrel and then cured using autoclaves or ovens. By combining these processes on the winding machine, the labor involved in manufacturing, the energy required for curing, and the overall production time are significantly reduced. In this research, a mobile additive manufacturing robot is designed, validated, and optimized for accurate locomotion and fast, dimensionally accurate printing of composite structures with high fiber alignment and degree of cure. The capabilities of this system are exhibited throughout several demonstrations that involve printing unsupported structures upside-down, the manufacturing of a bridge strong enough for the robot to pass over, and bridging the material across a 60 cm gap. Additionally, a pre-existing filament winding machine is optimized for the manufacturing of large, geometrically unconstrained composite structures. Improvements in fiber volume fraction are achieved through processing changes and a thermal profile for dry fibers is established to facilitate identification of frontal polymerization.
Open Access
Direct digital manufacturing of uniform thickness continuous fiber grid stiffened composites through tow spreading via roller based deposition
(Colorado State University. Libraries, 2024) Ratkai, Harry, author; Radford, Donald, advisor; Yourdkhani, Mostafa, committee member; Heyliger, Paul, committee member
Grid stiffened structures are an effective method for lightweighting designs. While continuous fiber composites are attractive materials for creating grid stiffened structures, there are two major impediments to the wider acceptance of such structures: the high capital costs for manufacturing and the material buildup at the crossover points. The high capital costs not only come from the complex tooling but also from the need to cure the parts after deposition. The material buildup at the crossover points is not only geometrically undesirable but can reduce the mechanical performance of the part. Many options to overcome this additional thickness have been implemented, but the majority cut the continuous fiber at the crossover, further reducing the performance. Previous work at Colorado State University has demonstrated that crossovers can be manufactured using a nozzle-based gantry printer and continuous glass fiber/PET commingled tow with a minimal thickness buildup at the crossover, all with radically reduced tooling, without compromising the structural performance. Unfortunately, the direct digital manufacturing system used did not utilize a cut and refeed system for the commingled tow; thus requiring the part to be made using continuous pathing or for a person to manually stop, cut and restart the tow at the end/beginning of each discrete path. These shortfalls of the nozzle-based printer make this technology, in its current form, impractical for adoption by industry. This work details the development of a robotic end effector for a new manufacturing method utilizing a heated roller for deposition and a programmable cut and refeed system. Initially, a comparison of the two methods of deposition, nozzle and roller, was done; both systems made crossover samples where part thickness and void and fiber volume fractions were measured. Next, an optimization of process parameters was performed on the beam and crossover sections, separately, for the roller-based end effector. Both the beams and crossovers were evaluated using thickness measurements, void and fiber volume fraction measurements and microscope imaging. Finally, a molding shoe was attached to the end effector to determine the effectiveness of molding the beam side walls, in-situ. It was demonstrated that the roller-based system can manufacture grid stiffened parts with less thickness deviations and fewer voids then the nozzle-based system. Additionally, optimized processing parameters were found for beams at three different deposition speeds, 450mm/min, 600mm/min and 750mm/min. Under the best conditions. The system is capable of direct digital manufacture of continuous fiber reinforced composite grids with under 2% void content. By slowing the deposition speed and increasing the consolidation force at the crossover points, the system is able to spread and thin the tow, thus, minimizing the thickness buildup at the crossover points. Using the understanding developed in determining optimized parameter two additional demonstrations of the capabilities of the system were completed: a preliminary example of full molding of the grid cross-section and the manufacture of curvilinear grids via in-plane steering. Combined, the outcomes demonstrate that a roller-based system with cut and refeed can produce grid stiffened structures with discrete fiber paths, that have crossovers of uniform thickness, at higher deposition rates than previous nozzle-based technology.
Open Access
Arsenic doping, kinetic behavior and oxide formation for polycrystalline CdTe thin films photovoltaics
(Colorado State University. Libraries, 2024) Mate, Mayank N., author; Sampath, Walajabad, advisor; Munshi, Amit, advisor; Sites, James, committee member
CdTe thin-film photovoltaics (PV) is one of the most promising renewable energy technologies currently in the market. Since the inception of CdTe PV in the 1950s, the technology has come a long way to be one of the most efficient solar cell technologies on the planet. However, there are still challenges associated with this technology which limit the power conversion efficiency. Currently, the most efficient CdTe solar cell to have been fabricated is still only at ~69% of the maximum possible efficiency, given by the Shockley-Queisser (SQ) limit. Performance loss analysis has suggested that the limiting electrical parameter is the open-circuit voltage (Voc) and detailed investigations have suggested that a defect state within the bandgap of Se allowed CdTe contributes significantly to this loss of Voc, which can be observed by Photoluminescence (PL). For thin-film CdTe PV to realize its full potential, research has been ongoing to improve the Voc by doping CdTe with a group V dopant, as well as passivating the surface of CdTe which causes significant recombination of charged carriers and may inhibit voltage even further. The research in this thesis focuses on group-V doping using As as the dopant and Cd3As2 as the doping material, and the formation of TeO2 as the metal oxide on the surface of CdTe. The goal of this research was to observe any improvements in Voc and overall device performance with As doping, TeO2, and a combination of both, and whether As doping mitigates the primary defect state within the bandgap of Se alloyed CdTe. To investigate the effects of these doping and surface treatments, three key hypotheses statements were formed. The first hypothesis is that As can be incorporated into CdTe using Cd3As2 without depositing it as a separate layer. This distinction of not depositing a film of Cd3As2 is important, and it aims to avoid any complexes caused by depositing Cd3As2 on the CdTe surface, creating another interface and potentially limiting the device performance, particularly due to Cd3As2 being a semi-metal. Secondary-Ion Mass Spectroscopy (SIMS) and X-Ray Photoelectron Spectroscopy (XPS) were used as the characterization methods to confirm the incorporation of As into the CdTe film by dissociation and diffusion, without depositing a layer of Cd3As2. The second hypothesis is that exposing a CdTe film to ambient air at high temperatures can lead to the enhanced formation of TeO2. Metal oxides such as Al2O3 have long been investigated to passivate the surface of CdTe, improving carrier lifetime and Voc. Traditionally, these metal oxides have been sputtered onto the surface, while native oxides on the surface of CdTe take a long time to form, on the timescale of days to weeks. However, this hypothesis investigates a novel method of forming a native layer of TeO2 by annealing a CdTe film in vacuum at elevated temperatures (~500℃) and rapidly exposing it to atmospheric oxygen while the high temperature is maintained. XPS and PL characterization was performed on the films that had undergone this 'rapid oxidation', and the formation of TeO2 was confirmed. The effect of TeO2 was combined with As doping, as well as compared in an undoped film, and it demonstrated a significant improvement in Voc when compared to control samples that were Cu doped and undoped with no intentional formation of TeO2. The third and final hypothesis is that As doping using Cd3As2 mitigates the sub-band-gap emission in photoluminescence originating from Se alloyed CdTe. To confirm or deny this theory, absolute PL was measured with an InGaAs detector, for samples that were doped with As and for samples that had a combination As doping treatments and/or TeO2 formed by using the method described for hypothesis two. The photoluminescence results showed no mitigation of the sub-band-gap emission in Se alloyed CdTe films that were doped with As. However, the addition of TeO2 demonstrated a lowering of the sub-bandgap emission intensity and overall improvement to the band-gap PL emission. Additionally, electrical parameters of six distinct device structures were measured and compared, with Cu-doped and undoped Se-alloyed CdTe being the reference/control samples. As a result of this research, a cell measuring close to 18% power conversion efficiency was measured with As doping, the native TeO2 layer, and one third of the Cu doping that went into reference samples. Finally, this research also provides some insight into the effects of high O2 presence during the As doping process using Cd3As2, and how it inhibits the ionization of As a small P-type dopant.
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
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
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
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
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
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
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.