Theses and Dissertations

Permanent URI for this collectionhttps://hdl.handle.net/10217/100472

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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
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
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
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
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
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
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
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
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 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 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
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
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
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 model for baffle energy dissipation in liquid fueled rocket engines
(Colorado State University. Libraries, 2010) Miller, Nathan A., author; Kirkpatrick, Allan T., advisor; Duflot, Jeanne, committee member; Williams, John D., committee member
In this thesis the energy dissipation from a combined hub and blade baffle structure in a combustion chamber of a liquid-fueled rocket engine is modeled and computed. An analytical model of the flow stabilization due to the effect of combined radial and hub blades was developed. The rate of energy dissipation of the baffle blades was computed using a corner-flow model that included unsteady flow separation and turbulence effects. For the inviscid portion of the flow field, a solution methodology was formulated using an eigenfunction expansion and a velocity potential matching technique. Parameters such as local velocity, elemental path length, effective viscosity, and local energy dissipation rate were computed as a function of the local angle α for a representative baffle blade, and compared to results predicted by the Baer-Mitchell blade dissipation model. The sensitivity of the model to the overall engine acoustic oscillation mode, blade length, and thickness was also computed and compared to previous results. Additional studies were performed to determine the sensitivity to input parameters such as the dimensionless turbulence coefficient, the location of the potential difference in the generation of the dividing streamline, the number of baffle blades and the size of the central hub. Stability computations of a test engine indicated that when the baffle length is increased, the baffles provide increased stabilization effects. The model predicts greatest dissipation for radial modes with a hub radius at approximately half the chamber's radius.
Open Access
Alterations in lumbar spine mechanics due to degenerative disc disease
(Colorado State University. Libraries, 2010) Ayturk, Ugur Murat, author; Puttlitz, Christian Matthew, advisor; James, Susan P., committee member; Qian, Xianghong, committee member; Heyliger, Paul Roy, 1958-, committee member
Degenerative disc disease is a major source of low back pain. It is hypothesized to significantly alter the biomechanics of the lumbar spine both at the tissue and motion segment (multi-vertebral) levels. However, explicit correlations between the former and the latter has not been established, and this critical link is only possible through modeling the intervertebral disc tissue behavior within a constitutive framework and implementing it in a finite element model of the lumbar spine. In order to develop a better appreciation of the biomechanics of disc degeneration, the main objectives of this dissertation work were to investigate the degenerative disease related mechanical alterations on lumbar spine through finite element modeling and experimentation, and evaluate the contemporary treatment strategies. To meet this objective, a finite element model of the healthy human lumbar spine was generated based on computed tomography (CT) imagery. Mesh convergence was verified based on strain energy density predictions. Kinematic and mechanical predictions of clinical interest, including range of motion and intradiscal nuclear pressure, were validated under pure moment loading. The mechanical properties of healthy and degenerated annulus fibrosus tissue were quantified using an orthotropic continuum model, with empirical determination of the requisite material coefficients derived from biaxial and uniaxial tension tests. The resultant material models were implemented into the validated finite element model in order to simulate disc degeneration at the L3-L4 level. At the tissue level, degeneration was found to significantly increase the dispersion in the collagen fiber orientation and the nonlinearity of the fiber mechanical behavior. At the motion segment level, degeneration increased the mobility of the spine, with concomitant increases in the local stress predictions in the annulus and facet force transmission. Our results were in good agreement with the clinical findings of instability and injury to the intervertebral disc due to degeneration. Total disc replacement was also considered as a treatment option within the aforementioned finite element framework. The model predictions indicated that single and two-level disc replacement restored motion at the treated levels, while linearizing the kinematic response and increasing the facet force transmission. The data reflect that the successful surgical outcome is most likely obtained when maximum preservation of native disc tissue is achieved during implantation of the prosthetic device.
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
A simplified model for understanding natural convection driven biomass cooking stoves
(Colorado State University. Libraries, 2010) Agenbroad, Joshua Nicholas, author; DeFoort, Morgan W., advisor; Willson, Bryan D., advisor; Kirkpatrick, Allan Thomson, committee member; Maloney, Eric D., committee member
It is estimated that half the world's population cooks over an open biomass fire; improved biomass cooking stove programs have the potential to impact indoor air quality, deforestation, climate change, and quality of life on a global scale. The majority of these cooking stoves operate in a natural convection mode (being driven by chimney effect buoyant fluid forces). Simplified theories for understanding the behavior of this unexpectedly complex combustion system, along with practical engineering tools to inform its design are markedly lacking. A simplified model of the fundamental stove flow physics is developed for predicting bulk flow rate, temperature, and excess air ratio based on stove geometry (chimney height, chimney area, viscous and heat release losses) and the firepower (as established by the stove operator). These parameters are intended to be fundamental inputs for future work understanding and improving biomass cook stove emissions and heat transfer. Experimental validation is performed and the simplified model is shown to be both accurate and applicable to typical stove operation. Carbon monoxide and particulate matter emissions data has been recorded in conjunction with the validation data. The initial results are presented and indicate that the excess air ratio may be a promising tool for reducing carbon monoxide emissions. A dimensionless form of the simplified stove flow model is then developed. This form offers several advantages, including scale similarity and a reduction of independent experimental parameters. Plotting with dimensionless parameters, various stove configurations can be plotted concurrently, and general stove flow behavior common to all natural convection stoves is observed. With a dimensionless firepower axis, emissions trends for both carbon monoxide and particulate matter become apparent, and a region of improved combustion efficiency and lowered emissions is identified.