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.
Open Access
Performance and accuracy enhancements of radiative heat transfer modeling via Monte Carlo
(Colorado State University. Libraries, 2002) Zeeb, Charles Nelson, author
Two ways to reduce the computational requirements of radiative heat transfer Monte Carlo simulation are explored. First, an efficient algorithm for tracing particles in large, arbitrarily complex, planar geometries containing nonparticipating media is presented. For arbitrary triangles and/or convex planar quadrilaterals, an efficient intersection algorithm is discussed in detail. After surveying several techniques used in ray tracing to limit the number of surfaces tested, the method of Uniform Spatial Division (USD) is implemented. The efficiency of the intersection algorithm and USD are demonstrated by timing results. Second, improving the accuracy of the Monte Carlo results by applying reciprocity and closure is explored. Statistical theory is applied to the reciprocity estimation smoothing (RES) technique which combines reciprocity enforcement through estimation and closure enforcement through the technique of least-squares smoothing. By examining a large number of runs of two large geometries, several RES methods are compared to find the best method. The effects of the RES method on surfaces and individual results between surfaces are also explored. Estimates of the improvements caused by the RES method that can be calculated from the results of a single run are also derived.
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
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
Experimental and CFD investigation of re-agent mixing in an SCR system
(Colorado State University. Libraries, 2007) Ivaturi, Krishna, author; Olsen, Daniel B., advisor; Mitchell, Charles E., committee member; Meroney, Robert, committee member
Nitrogen oxides (NOx) cause a gamut of problems such as harmful particulate matter, ground level ozone (smog) and acid rain. Currently, a significant capital is being invested researching new techniques to control NOx emissions. One of the best ways to breakdown NOx is the Selective Catalytic Reduction (SCR) after-treatment method. A reducing agent (re-agent) is injected into exhaust gases and passed through a catalyst that facilitates NOx breakdown into Nitrogen and Water. To ensure effective NOx conversion, there must be uniform mixing between re-agent and exhaust gas upstream of the catalyst blocks. The current thesis focuses on investigating the mixing quality for an SCR test system employed for a 2-stroke lean-bum natural gas engine. CFD investigations were conducted to simulate the physical flow process. The mixing quality for different injector locations and the effect of utilizing a downstream in-line mixer was investigated. The CFD simulations were compared to experimental results. To measure ammonia concentrations experimentally, a traversing probe was designed and built. Re-agent concentrations were measured at various locations on a plane slightly upstream of the catalyst substrate. Detailed discussion is presented on different cases of CFD analysis. Experiments were conducted for the best and worst case of mixing based on CFD computation. Results suggest that a mixer plays a vita1 role in improving the mixing.
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
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
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
Characterization of gaseous and particulate emissions from combustion of algae based methyl ester biodiesel
(Colorado State University. Libraries, 2009) Fisher, Bethany, author; Marchese, Anthony John, 1967-, advisor; Olsen, Daniel B., committee member; Volckens, John, committee member
The advantages to using biodiesel in place of petroleum diesel are also accompanied by disadvantages. Biodiesel is usually made from crops that are also used to produce food. The land and water use impacts would be profound if current biodiesel feedstocks were used to displace a significant portion of current global petroleum diesel consumption. Oil-producing algae is a favorable alternative to the more common biodiesel feedstocks (soy, canola, etc.) because it does not compete with food sources, does not require arable land to grow and has the potential to produce significantly more oil per area per year than any other oil crops. However, the fatty acid composition of the oil produced by algal species currently under consideration for fuel production differs from that of the more common vegetable oils in that it often includes high quantities of long chain and highly unsaturated fatty acids. When transesterified into fatty acid methyl esters (FAME) biodiesel, the unique fatty acid composition could have a substantial impact on emissions such as Nitrogen Oxides (NOx) and particulate matter (PM). Accordingly, the goal of this study was to examine the effect of the chemical structure of algal methyl esters on pollutant emissions from a diesel engine operating on algae-based FAME biodiesel. Tests were performed on a 2.4 L, 39 kW John Deere 4024T, off-road diesel engine meeting USEPA Tier 2 emissions regulations. The engine was fitted with a unique, low-volume fuel system that enabled emissions tests to be conducted with small specialty fuel samples. Tests were performed on 9 different fuel blends at 2 different engine loading conditions. Exhaust gas measurements were made using a 5-gas emissions analysis system that includes chemiluminescence measurement of NOx, flame ionization detection of total hydrocarbons, paramagnetic detection of oxygen and non-dispersive infrared detection of CO and CO2. Particulate matter was characterized using an Aerosol Mass Spectrometer (AMS), which is capable of direct measurement of particle composition. The PM size distributions (between 10 to 1000 nm) were measured using a Sequential Mobility Particle Sizer. Total PM mass emissions were measured using gravimetric analysis of Teflon filters and the ratio of elemental carbon to organic was measured using thermo-optical analysis of quartz filters. Experiments were performed with ultra-low sulfur diesel, soy biodiesel (both pure biodiesel, B100, and a blend of 20% biodiesel and 80% diesel, B20), canola biodiesel (B20 and B100), and two synthetic algal methyl ester formulations (B20 and B100 for each). Combustion of algal methyl esters resulted in decreased NOx relative to both canola and soy biodiesel and ULSD, in contrast to previous research that examined the effect of fatty acid saturation and chain length on NOx emissions. A correlation was found between NOx emissions and premixed burn fraction, which provides an explanation for these results. Emissions of formaldehyde and organic PM were found to be slightly elevated with the two algal fuels in comparison with the traditional feedstocks. Particle size distribution, total PM mass, total hydrocarbons, CO and acetaldehyde emissions were similar between the different types of biodiesel.
Open Access
On the energy sources of Mozambican households and the demand-supply curves for domestic electricity in the northern electrical grid in Mozambique
(Colorado State University. Libraries, 2009) Arthur, Maria de Fatima Serra Ribeiro, author; Willson, Bryan D., advisor; Bond, Craig A., advisor; Duff, William S., committee member; Troxell, Wade O., committee member; Kling, Robert W., committee member
The development of electrical infrastructure to supply rural households is considered economically unfeasible because of the high cost of capital investment required to expand the distribution grids. Although domestic electricity consumption in many developing regions is small when compared to the requirements of some emerging agroindustries, the social benefits are significant, such that many donor agencies agree to finance grid extensions based on poorly projected social benefits of electrification. However, there is evidence that households with electrical connections do not increase their electricity consumption above the bare minimum, allegedly because electricity is more expensive and possibly because of insufficient funds to invest in electrical appliances. The controversy is then whether or not electrification can support household development (and poverty alleviation) and vice-versa, can domestic consumption support the costs of electrification investments. The current work is composed of a theoretical model and two empirical models, developed in order to answer the following specific questions: 1) To what extent the ownership of assets is determinant to the adoption of high-grade energy sources in the domestic settings of poor families? 2) What is the price of electricity that sustains the supply costs and still promotes increased energy consumption in Mozambican households? To answer these questions the study formulated an inter-temporal utility maximization problem by which households can determine the limits of investment for energy consumption and for income generation that is required to evolve out of poverty in a sustainable manner. Next, the study calculated the elasticities of demand for the various domestic sources used by Mozambican households, surveyed in 2002/3 at the national level, enabling the construction of demand curves for these sources. The study also derived empirical loss equations for the northern transmission electrical grid (Linha Centro-Norte, LCN) in Mozambique, and constructed the supply curves for the distribution networks connected to the substations of the system. Based on the household data, the likelihood of adopting electricity as a domestic source was analyzed and results show that wealth is a major determining factor, confirming the findings of the theoretical and empirical household models. Finally, the study constructed the supply and demand plots, from which the sustainable price of electricity supplied to domestic consumers can be estimated and welfare evaluations made. Results indicate that households can evolve consuming electricity if credit for investment is made available and the income base is enlarged. Furthermore, it is demonstrated that current electricity prices are within budget of households and that electricity is competitive with biomass sources and kerosene in the domestic setting.
Open Access
A simulation method and laboratory brake friction dynamometer for tribology studies
(Colorado State University. Libraries, 2009) Nivala, Peter Thompson, author; Radford, Donald W., advisor; Sakurai, Hiroshi, committee member; Heyliger, Paul Roy, 1958-, committee member
Two of the most important parameters of brake system design are the frictional and wear capabilities of the rotor and pad materials. These parameters must meet minimum design requirements in an effort to enhance friction and reduce wear to improve the performance and life of brake system components. The frictional and wear performance of the rotor and pad materials can be assessed through laboratory brake dynamometer testing and evaluation. In the current study, a wear testing simulation and an inertia laboratory brake dynamometer were developed to resolve differences in wear rates of brake materials. Dynamometer testing was conducted to verify the logic of the simulation and the functionality of the dynamometer by measuring wear rates of brake rotor material samples, some of which were subjected to cryogenic heat treatment to modify their wear rates, at varying brake application pressures. Dynamometer testing established that the wear simulation and inertia laboratory brake dynamometer developed during the current study could function together as a suitable tribological experimental apparatus. Specifically, dynamometer testing demonstrated the ability of the experimental apparatus to resolve differences in wear rates of brake materials due to variations in brake application pressure at relatively short test durations; however, dynamometer test results did not show conclusive evidence to suggest an advantage in subjecting the rotor materials used in the current study to cryogenic treatment to lower the rotor or pad wear rates.
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
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
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
Selective catalytic reduction: testing, numeric modeling, and control strategies
(Colorado State University. Libraries, 2010) Schmitt, Joshua C., author; Olsen, Daniel B., advisor; Marchese, Anthony John, 1967-, committee member; Young, Peter M., committee member
Selective Catalytic Reduction (SCR) catalysts respond slowly to transient inputs, which is troublesome when designing ammonia feed controllers. An experimental SCR test apparatus was installed on a Cooper Bessemer GMV-4 natural gas engine. Transient data was taken of commercially available SCR Catalysts. These transient tests are used to quantify SCR catalyst response. Space velocity, catalyst temperature, inlet NOx concentration, and ammonia to NOx molar feed ratio were varied. A Simulink numeric model was created to examine the SCR transient phenomena. The Simulink numeric model showed in-catalyst ammonia and NOx concentration as a function of length in the direction of exhaust flow. This helped explain the SCR transient results. Transient testing showed a fifteen minute delayed response in NOx reduction from ammonia transitions. Ammonia slip succeeded ammonia transitions by thirty minutes. Simulink modeling revealed that these delays are caused by large quantities of ammonia stored in the catalyst. Due to ammonia storage, ammonia waves propagate through the catalyst, front to back. Emission of these constituents through the catalyst is delayed because the wave takes time to propagate through the entire catalyst length. Ammonia feed rate control testing was done on the experimental setup to improve ammonia and NOx emissions from the catalyst. Three control algorithms were used: feed forward control, using a pre ammonia injection ceramic NOx sensor; a feed forward plus feedback control, using a pre ammonia injection ceramic NOx sensor and post catalyst ceramic NOx sensor to generate feed signals; and a feed forward plus feedback algorithm that used a pre ammonia injection ceramic NOx sensor and a mid catalyst ceramic NOx sensor to generate feed forward and feedback signals. The feed forward controller used molar ratio as the control variable, and the feedback system used a technique that minimized the post catalyst ceramic NOx sensor signal. Ammonia to NOx molar ratio was stepped every five or fifteen minutes, and the algorithm made decisions, based on the catalyst response to the step. The decisions were made to minimize the post catalyst ceramic NOx sensor. Feed forward testing revealed that the lack of pressure compensation on ceramic NOx sensors causes errors in feed forward NOx readings, and sub optimal ammonia feed. Feedback testing revealed that a minimization technique can be used successfully with a feedback step rate of one step per fifteen minutes, and a step size of 5% ammonia to NOx molar ratio. The feedback algorithm, with the feedback ceramic NOx sensor located one third the way through the catalyst length, worked poorly. The technique approached a lean ammonia to NOx molar ratio, and stabilized slower than the post catalyst feedback ceramic NOx sensor technique. These phenomena are explained with the Simulink numeric model.
Open Access
Dynamic model of a spherical robot from first principles
(Colorado State University. Libraries, 2010) Schroll, Gregory C., author; Alciatore, David G., advisor; Bradley, Thomas H., advisor; Young, Peter M., committee member
A prototype of a pendulum driven spherical robot has been developed during previous research and shown to exhibit unique dynamic behavior. Starting from first principles, a mathematical model for this spherical robot rolling on flat ground was developed in order to determine if this unique behavior was inherent to spherical robots in general or simply peculiar to this prototype. The complete equations of motion were found using Lagrangian methods, and numerically integrated using computer tools. A 3D simulation program was written to animate the results of integrating the equations. The dynamics apparent in the simulations were found to closely match the observed dynamics of the physical prototype.
Open Access
Comprehensive viscoelastic characterization of human lower cervical spine ligaments
(Colorado State University. Libraries, 2010) Troyer, Kevin Levi, author; Puttlitz, Christian Matthew, advisor; Sakurai, Hiroshi, committee member; Heyliger, Paul Roy, 1958-, committee member
Accurate definition of cervical spine ligament mechanical properties is requisite to understand and model global cervical spine biomechanics. These ligaments have been shown to exhibit complex nonlinear elastic behavior. In addition, ligamentous mechanical behavior is highly time-dependent (viscoelastic). Previous investigators have reported the viscoelastic stress relaxation behavior of the anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL), and ligamentum flavum (LF) of the lower cervical spine using quasi-linear viscoelastic (QLV) theory. However, QLV theory assumes that the viscoelastic behavior is independent of the applied strain magnitude. Cervical spine ligaments are subjected to multiple strain magnitudes and loading rates during physiologic loading regimes. Thus, in order to characterize the comprehensive viscoelastic behavior of cervical spine ligaments within their physiological range, and to test the validity of the use of QLV theory to model this behavior, the mechanical response of human lower cervical spine ALL, PLL, and LF was recorded from stress relaxation experiments at multiple strain magnitudes and from cyclic experiments at multiple strain amplitudes and frequencies. The ALL, PLL, and LF were dissected from the C5-C6 level of human cadaveric cervical spines. Each ligament was isolated into a bone-ligament-bone (B-L-B) preparation by removing all surrounding non-osteoligamentous tissue. Each B-L-B preparation was placed in an environmental chamber, submerged in warmed saline (37 °C), and mounted to a servo-hydraulic materials testing machine. Ligaments were subjected to a uniaxial cyclic testing protocol at multiple strain amplitudes and frequencies, as well as a stress relaxation protocol at multiple strain magnitudes. Dynamic material properties (phase shift, storage modulus, and loss modulus) were determined from the resulting load displacement data via transformation into the stress-strain space. Stress relaxation data were fitted to QLV theory and a power law formulation in order to characterize the appropriate analytic function that best described the ligament relaxation behavior. Experimental results indicated that the dynamic material properties of the ALL, PLL, and LF were dependent upon both strain amplitude and frequency. In general, the dynamic material properties of the ALL and the PLL were not statistically different, but both were statistically different form the LF. The stress relaxation data was strongly dependent on the applied strain magnitude. Also, the relaxation rate of the ALL and PLL exhibited a converging trend as strain magnitude increased, while the relaxation rate of the LF diverged with increasing strain magnitude. The different strain-dependent relaxation rate behavior of the longitudinal ligaments and the LF is possibly a result of the compositional and microstructural differences between the two ligament types. Results from both the cyclic and stress relaxation experiments indicated that QLV theory cannot adequately describe the comprehensive viscoelastic behavior of these ligaments within the physiologic loading range. Therefore, a more rigorous, fully nonlinear, viscoelastic formulation is required to model the comprehensive viscoelastic behavior of the ALL, PLL, and LF in the human lower cervical spine.
Open Access
Sputter deposited hydroxyapatite thin films to enhance osseointegration
(Colorado State University. Libraries, 2010) Riedel, Nicholas Alfred, author; Williams, John D., advisor; Popat, Ketul, advisor; Prieto, Amy L. (Amy Lucia), committee member
As the demand for hip and knee replacements continues to grow, researchers look to increase the operational lifetimes of these implants. Many of these implants fail as a result of aseptic loosening caused from repeated loading of these joints. It is thought that implant life could be extended by improving the interface between the implant and natural tissue. To this effect, hydroxyapatite coatings have been demonstrated to improve implant to bone bonding and allow a more natural integration of the metallic substrates. This work explores the potential of using ion beam etching and sputter deposition to produce a hydroxyapatite thin film with a unique surface topography that would potentially enhance osseointegration. First, the effects of ion etching bare titanium were evaluated. Three ion energies (300 eV, 700 eV, and 1100 eV) were used to etch either as-received or polished substrates. Topographical changes were examined by scanning electron microscopy. Rat mesenchymal stem cells were differentiated to osteoblasts to test the biocompatibility of the surfaces with bone cells. It was found that ion etching the titanium increases cellular activity, and an ion energy of 700 eV appears to create the most beneficial topography. Hydroxyapatite thin films were then sputter deposited on titanium substrates etched at 700 eV. After the coatings were deposited, some of the hydroxyapatite films were re-etched in efforts to induce a unique topography. It was found that the hydroxyapatite coatings improved short term cell response but degraded over the course of the culture. Further investigation showed the as-sputtered coatings were amorphous. To prevent degradation of the coatings, annealed films were then prepared by heat treating at 600 °C for 2 hours. X-ray diffraction was used to confirm the presence of a crystalline hydroxyapatite phase. Films were immersed in culture media for four weeks, showing no signs of degradation. Ion etching performed on the substrates post annealing yielded a unique topography in the hydroxyapatite film. A final study was conducted evaluating the MSC response to the annealed and post-anneal etched films. It was found that the post-anneal etched hydroxyapatite coating had the highest cellular activity, indicating that this preparation may be an effective means to enhance osseointegration on medical implants.