Mountain Scholar :: Browsing by Author "Sakurai, Hiroshi, committee member"

Browsing by Author "Sakurai, Hiroshi, committee member"

Now showing 1 - 20 of 26

Open Access
A fine resolution CDF simulation approach for biomass cook stove development
(Colorado State University. Libraries, 2011) Miller-Lionberg, Daniel David, author; Willson, Bryan, advisor; DeFoort, Morgan, committee member; Sakurai, Hiroshi, committee member; Volckens, John, committee member
More than half of the world's population meets cooking and heating needs through small-scale biomass combustion. Emissions from these combustion processes are a major health hazard and air pollution concern. Simple improvements over traditional cooking fires have been shown to increase combustion and heat transfer efficiency while reducing physically harmful gaseous and particulate matter (PM) emissions. Over approximately 30 years of modern stove development history, designs have largely been based on empirical guidelines, and attempts at improvements have been made through an iterative, trial-and-error approach. Feedback in this design process is typically attained through bulk measurements made during experimental testing of prototypes. While important for assessing the performance of a stove, such testing offers no information on the fine spatial or temporal scales of phenomena within the stove, leaving it a "black box" in the view of the designer. Without higher resolution information, the rate and ultimate level of design improvement may be limited. In response, a computational fluid dynamic (CFD) simulation of a common, production cook stove is conducted using ANSYS FLUENT 13.0 software. Aspects critical to achieving high spatial and temporal resolution flow and temperature field results are included, enabled by necessary simplifications to less important elements. A model for the steady, time-averaged drying and pyrolysis of wood stick fuel is used in conjunction with a consideration for the simultaneous oxidation of the resulting char, to generate gas-phase fuel boundary conditions for the simulation. Fine spatial and temporal resolution are simultaneously possible in an unsteady formulation with the use of the simplified fuel condition, reduced-mass solid boundaries, and abbreviated runtimes. Employment of a large eddy simulation (LES) turbulence model is proposed as necessary to realistically consider the larger scales of gas mixing. Combustion heat release is approximated by reactions dictated by a mixture fraction formulation, assuming equilibrium conditions in a non-adiabatic system, affected by turbulent fluctuations through a probability density function (PDF). Sensitivity studies are conducted on grid parameters, boundary condition assumptions, and the duration of simulation runtime necessary to achieve result significance. A model for particulate emission formation is secondarily explored. A thermocouple-instrumented stove is used in an experiment to generate internal gas temperature profiles for the validation of the CFD simulation through comparable results. Likewise, a heat-exchanger integrated into a cooking pot is employed with the instrumented stove to measure short time-scale heat transfer values that are compared to the CFD simulation results, as well as to benchmark test data from the production stove. Recommendations for future efforts in stove simulation are made.
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
Analysis of advanced vapor source for cadmium telluride solar cell manufacturing
(Colorado State University. Libraries, 2013) Khetani, Tejas Harshadkumar, author; Sampath, Walajabad, advisor; Sakurai, Hiroshi, committee member; Sites, James, committee member
A thin film CdS/CdTe solar cell manufacturing line has been developed in the Materials Engineering Laboratory at Colorado State University. The original design incorporated infrared lamps for heating the vapor source. This system has been redesigned to improve the energy efficiency of the system, allow co-sublimation and allow longer run time before the sources have to be replenished. The advanced vapor source incorporates conduction heating with heating elements embedded in graphite. The advanced vapor source was modeled by computational fluid dynamics (CFD). From these models, the required maximum operating temperature of the element was determined to be 720 C for the processing of CdS/CdTe solar cells. Nichrome and Kanthal A1 were primarily selected for this application at temperature of 720 °C in vacuum with oxygen partial pressure. Research on oxidation effects and life due to oxidation as well as creep deformation was done, and Nichrome was found more suitable for this application. A study of the life of the Nichrome heating elements in this application was conducted and the estimate of life is approximately 1900 years for repeated on-off application. This is many orders of magnitude higher than the life of infrared heat lamps. Ceramic cement based on aluminum oxide (Resbond 920) is used for bonding the elements to the graphite. Thermodynamic calculations showed that this cement is inert to the heating element. An earlier design of the advanced source encountered failure of the element. The failed element was studies by scanning electron microscopy and the failure was attributed to loss of adhesion between the graphite and the ceramic element. The design has been modified and the advanced vapor source is currently in operation.
Open Access
Analysis of CuCl thin-film deposition and growth by close-space sublimation
(Colorado State University. Libraries, 2016) Nicholson, Anthony, author; Sampath, Walajabad, advisor; Sakurai, Hiroshi, committee member; Sites, James, committee member
There is a growing need to implement high fidelity, scalable computational models to various thin-film photovoltaic industries. Developing accurate simulations that govern the thermal and species-transport diffusion characteristics within thin-film manufacturing processes will lead to better predictions of thin-film uniformity at varied deposition conditions that ultimately save time, money, and resources. Thin-film deposition and growth of Copper I Chloride (CuCl) by the Close-Space Sublimation (CSS) process was investigated in an extensive range of operating and thermal conditions. A simulation model based on the ANSYS FLUENT® solver platform was developed to encompass the kinetic behavior of the CuCl species within the CSS domain while predicting the growth rate at varied system parameters. Surface physics associated with the process, notably sublimation and adsorption, were studied, quantified, and applied to the continuum-based thin-film deposition model. Experimentation of CuCl thin-film growth was performed across a range of substrate and source temperatures for verification of the model’s computational accuracy. Furthermore, characterization of the inherent growth mode exhibited by CuCl was studied in conjunction with simulation and experimental tasks. It was concluded that the simulation model provided predictions for the CuCl thickness as a function of temperature within the range of typical CSS conditions. Equally important was the elucidation of the CuCl growth mechanism, which displays a Volmer-Weber growth mode on the Fluorine-doped Tin Oxide coated layer of the substrate. Such knowledge along with the current modeling capabilities will be useful in extending the computational method to predicting the non-uniformities present in CuCl and other thin-film depositions.
Open Access
Analysis of multi-channel wind loading using proper orthogonal decomposition
(Colorado State University. Libraries, 2014) Adhikari, Rajendra, author; Bienkiewicz, Bogusz, advisor; Chen, Suren, committee member; Sakurai, Hiroshi, committee member
Wind tunnel testing utilizing multi-channel pressure measurement system leads to large volume of the acquired wind pressure data. In the presented research, use of Proper Orthogonal Decomposition (POD), to analyze such data, is described. Wind pressure time series acquired for a generic low-rise building were used in the analysis. First, the pressure covariance matrices were calculated. They were subsequently used to determine the pressure eigenvalues and the eigenfunctions. These quantities were next employed to calculate the POD principal coordinates. Finally, the eigenvectors and the principal coordinates were used to reconstruct the pressure time series. This analysis was carried out for pressures exerted on the whole building and on its distinct surfaces - side walls and roof. The convergence of the pressure time series reconstruction was inspected. The mean, standard deviation and the peak values of the reconstructed pressure were evaluated. The effects of wind direction on the original and reconstructed pressures were investigated. The POD modal contributions and the convergence of the pressure reconstruction were quantified. Overall, the obtained results were found to be consistent with findings of related POD studies reported by other researchers. High spatial and temporal resolutions of the wind loading data used in the present research made possible refined quantification of the effects of the studied parameters.
Open Access
Application of distributed DC/DC electronics in photovoltaic systems
(Colorado State University. Libraries, 2017) Kabala, Michael, author; Collins, George, advisor; Sakurai, Hiroshi, committee member; Siegel, H. J., committee member; Young, Peter, committee member
In a typical residential, commercial or utility grade photovoltaic (PV) system, PV modules are connected in series and in parallel to form an array that is connected to a standard DC/AC inverter, which is then connected directly to the grid. This type of standard installation; however, does very little to maximize the energy output of the solar array if certain conditions exist. These conditions could include age, temperature, irradiance and other factors that can cause mismatch between PV modules in an array that severely cripple the output power of the system. Since PV modules are typically connected in series to form a string, the output of the entire string is limited by the efficiency of the weakest module. With PV module efficiencies already relatively low, it is critical to extract the maximum power out of each module in order to make solar energy an economically viable competitor to oil and gas. Module level DC/DC electronics with maximum power point (MPP) tracking solves this issue by decoupling each module from the string in order for the module to operate independently of the geometry and complexity of the surrounding system. This allows each PV module to work at its maximum power point by transferring the maximum power the module is able to deliver directly to the load by either boosting (stepping up) the voltage or bucking (stepping down) the voltage. The goal of this thesis is to discuss the development of a per-module DC/DC converter in order to maximize the energy output of a PV module and reduce the overall cost of the system by increasing the energy harvest.
Open Access
Application of model reduction tools in analysis of wind-induced pressures on low-rise buildings
(Colorado State University. Libraries, 2012) Wu, Chieh-hsun, author; Bienkiewicz, Bogusz, advisor; Criswell, Marvin E., committee member; Sakurai, Hiroshi, committee member
Recent advances in laboratory and field measurement techniques and numerical simulations of wind-induced loading on buildings and structures made possible generation of large data sets, suitable for database-assisted wind-resistant design. In parallel, data reduction tools have been developed to aid storage, management and accessibility issues associated with large datasets/databases. In the presented research, application of such tools in analysis of stationary and non-stationary wind-induced pressures on a generic low-rise building is discussed. Both stationary and non-stationary cases are addressed. In stationary analyses, Proper Orthogonal Decomposition (POD) and Method of Snapshot (SPOD) were used to identify the most energetic spatio-temporal structures of the pressures. Linear Stochastic Estimation (LSE) and Gappy POD (GPOD) were employed to generate the pressures at specified target locations via extrapolation of the pressures provided at chosen reference locations. Optimized reference positions were determined using algorithm-based and empirical approaches. In non-stationary analyses, Wavelet De-noising and Two-Stage-Moving-Averaging were applied to decompose the non-stationary pressure into time-varying mean, standard deviation and normalized fluctuation. The techniques developed for stationary pressures were adapted for non-stationary cases. In analysis of the stationary data, the extrapolation techniques (GPOD and LSE) were found to reduce the data more efficiently than the modal reduction tools (POD and SPOD). In pressure extrapolation, LSE provided more accurate pressure predictions than GPOD. A hybrid approach combining the use of GPOD, with algorithm-based reference positions selection, and LSE extrapolation enabled the most efficient capturing of the primary and secondary spatio-temporal features of the pressure. This technique is recommended for analyses focused on development of reduced models of wind pressures induced on low-rise buildings. In the non-stationary investigations, the hybrid GPOD-LSE technique, developed in analysis of the stationary pressures and modified for the non-stationary cases, led to accurate pressure predictions and model reductions. This methodology appears to be a suitable tool for similar analyses of non-stationary wind-induced pressures on low-rise buildings. Follow-up investigations of stationary and non-stationary cases are recommended to assess potential for further optimization of the developed techniques and their application in analyses of wind-induced loading on other buildings and structures.
Open Access
Assessment of the effects of ligamentous injury in the human cervical spine
(Colorado State University. Libraries, 2012) Leahy, Patrick Devin, author; Puttlitz, Christian, advisor; Heyliger, Paul, committee member; Sakurai, Hiroshi, committee member; Santoni, Brandon, committee member
Ligamentous support is critical to constraining motion of the cervical spine. Injuries to the ligamentous structure can allow hypermobility of the spine, which may cause deleterious pressures to be applied to the enveloped neural tissues. These injuries are a common result of head trauma and automobile accidents, particularly those involving whiplash-provoking impacts. The injuries are typically relegated to the facet capsule (FC) and anterior longitudinal (ALL) ligaments following cervical hyperextension trauma, or the flaval (LF) and interspinous (ISL) ligaments following hyperflexion. Impacts sustained with the head turned typically injure the alar ligament. The biomechanical sequelae resulting from each of these specific injuries are currently ill-defined, confounding the treatment process. Furthermore, clinical diagnosis of ligamentous injuries is often accomplished by measuring the range of motion (ROM) of the vertebrae, where current methods have difficulty differentiating between each type of ligamentous injury. Pursuant to enhancing treatment and diagnosis of ligamentous injuries, a finite element (FE) model of the intact human full-cervical (C0-C7) spine was generated from computed tomography (CT) scans of cadaveric human spines. The model enables the quantification of ROM, stresses, and strains, and can be modified to reflect ligamentous injury. In order to validate the model, six human, cadaveric, full-cervical spines were tested under pure ±1.5 Nm moment loadings in the axial rotation, lateral bending, flexion, and extension directions. ROM for each vertebra, facet contact pressures, and cortical strains were experimentally measured. To evaluate injured ligament mechanical properties, a novel methodology was developed where seven alar, fourteen ALL, and twelve LF cadaveric bone-ligament-bone preparations were subjected to a partial-injury inducing, high-speed (50 mm/s) tensile loading. Post-injury stiffnesses and toe region lengths were compared to the pre-injury state for these specimens. These experimental data were incorporated into the FE model to analyze the kinematic and kinetic effects of partial ligamentous injury. For comparison, the model was also adapted to reflect fully injured (transected) ligaments. Injuries simulated at the C5-C6 level included: 1) partial FC injury, 2) full FC injury, 3) partial FC and ALL injury, 4) full FC and ALL injury, 5) partial LF and full ISL jury, 6) full LF and ISL injury, 7) partial FC, ALL, LF, and full ISL injury, and 8) full FC, ALL, LF, and ISL injury. The model was also modified to replicate injury to the right alar ligament. Five cadaveric cervical spines were tested under pure moment conditions with scalpel-sectioning of these ligaments for validation of the full-injury models. Comparisons between the intact and various injury cases were made to determine the biomechanical alterations experienced by the cervical spine due to the specific ligamentous injuries. Variances in ROM and potential impingement on the neural tissues were focused upon. The overarching goals of the study were to identify a unique kinematic response for each specific ligamentous injury to allow for more accurate clinical diagnosis, and to enhance the understanding of the post-injury biomechanical sequelae.
Open Access
Clamping devices to enable concurrent mechanical and electrical connections of a power semiconductor
(Colorado State University. Libraries, 2011) Shover, Michael Andrew, author; Collins, George J., advisor; Reising, Steven C., committee member; Chen, Thomas Wei, committee member; Sakurai, Hiroshi, committee member
In response to the restrictions of lead bearing solders in the European Union Restriction of Hazardous Substances Directive of 2002, new strategies for solderless electrical connections are desired. In this work, such concepts are used in the simultaneous electrical connection of power semiconductor leads to a PCBA and mechanical attachment of the device package to a heat sink. These concepts are specifically designed for use in an industrial high power (kW to tens of kW) radio frequency generator. The many constraints of such a system, some of which are directly contradictory to each other, are considered throughout, including manufacturability, mechanical tolerances, system reliability, and cost. Theoretical models predict that in the expected usage environment, the transistor leads in the clamped connection under consideration will move 5.1 micrometers for a thermal excursion of 50°C. SEM micrographs showing that the size of z-axis asperities is on the order of 1 micrometers and calculations estimating adhesive junctions with surface energies on the order of 100 N/mm2 demonstrate that contact wear is likely. A survey of available materials has been conducted, with beryllium copper and polyetherimide being the favored options for clamp construction. Four concepts are modeled, noting the benefits and drawbacks of each. The preferred embodiment is found to be a clamping mechanism fabricated from electrically insulating material, specifically injection molded 30% fiberglass filled polyetherimide, incorporating cantilevered beams which deflect upon installment into the system with fasteners, thus forming the electrical connections. Test regimens have been performed, including room temperature aging, elevated temperature aging at 50°C and 80°C, thermal cycling, highly accelerated life testing, and thermal analysis in both the steady state and transient regimes. The results of the experiments show this clamping system to have a useful life in the intended environment of multiple years.
Open Access
Comparative study between vector control and direct torque control of induction motor using MATLAB Simulink
(Colorado State University. Libraries, 2012) Eldali, Fathalla, author; Collins, George J., advisor; Suryanarayanan, Siddharth, committee member; Sakurai, Hiroshi, committee member
This thesis project studies and compares two of the most commonly used electric driving methods of induction motors (IM). These methods, which have been used for three decayed are Field Orientation Control (FOC) and Direct Torque Control (DTC). Theoretical background for both methods is explained. Due to its simplicity of use, MATLAB/ SIMULINK is used to simulate the dynamic model of (IM) and applying both techniques on it. The comparative study of speed, torque, and flux is performed under two cases which are the normal operation and in the presence of voltage-sag and short interruption.
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
Design and implementation of a compact highly efficient 472kHz radio frequency generator for electrosurgery
(Colorado State University. Libraries, 2011) Eberhardt, Gerald M., author; Collins, George J., advisor; Siegel, Howard Jay, committee member; Chen, Thomas Wei, committee member; Sakurai, Hiroshi, committee member
This thesis explores the utilization of modern design practices and advance technologies to reduce the size of traditional 472kHz radio frequency generators used for electrosurgery. Achieving the reduced size requires an innovative approach to increase the overall efficiency to lower the internal heat dissipation allowing the overall package size to shrink. This thesis covers the selection and design process to achieving the final topology of an innovative approach utilizing a variation of the Class-D amplifier to produce a resonance type power saturation amplifier. While using a high-efficiency power source to control the amplifier voltage rails, and to control the amplitude of the output signal will produce a sinusoidal power source capable of driving a radio frequency surgical scalpel.
Open Access
Dynamic assessment of the long-span cable-stayed bridge and traffic system subjected to multiple hazards
(Colorado State University. Libraries, 2016) Zhou, Yufen, author; Chen, Suren, advisor; Ellingwood, Bruce R., committee member; Mahmoud, Hussam N., committee member; Sakurai, Hiroshi, committee member
Critical infrastructure systems, such as long-span bridges, offer the underlying foundation for many aspects of modern society, such as national security, quality of life and economy. Although the total number of long-span bridges is relatively small compared to short-span and medium-span bridges, long-span bridges often serve as backbones for critical interstate transportation corridors and also evacuation routes. Any traffic disruption due to bridge damage, failure, retrofitting or even major traffic accidents following some hazards can become disastrous to local community and emergency response efforts, underscoring the importance of the continued integrity, functionality and resilience following hazardous conditions. Wind and traffic are the major service loads for long-span bridges. The extreme loads may include those caused by various natural or man-made hazards, such as earthquake, hazardous winds (hurricane, tornado), fire, blast, vehicle and barge collision etc. Compared to other hazards, hazardous wind and earthquake are particularly critical for long-span bridges, primarily due to their significant threats to the global structure performance and challenges of appropriately modeling the dynamic coupling effects between the bridge, traffic and hazards. In addition, there is another disastrous event: cable loss, which is very unique and critical for cable-supported bridges and could be caused by various natural and man-made hazards. There exist major challenges in the current state of the art on rationally predicting the long-span bridge performance subjected to multiple service and extreme loads. These challenges include realistic load characterization, methodological limitations and considerations of uncertainties. A suite of holistic analytical frameworks of long-span cable-stayed bridges subjected to various service and hazardous loads are developed, with which insightful numerical analyses of the bridge performance subjected to these loads are carried out in this dissertation. Firstly, two general dynamic assessment frameworks are developed based on the mode superposition and finite element methods respectively for a long-span cable-stayed bridge and traffic system subjected to multiple threats, such as stochastic traffic, wind and some hazardous loads. Although developed based on a long-span cable-stayed bridge, the frameworks can be readily applied to long-span suspension bridges as well as bridges with shorter spans. In both simulation platforms, the bridge model and all individual moving vehicles in the stochastic traffic flow are directly coupled under multiple excitations from bridge deck roughness and other external dynamic loads. Through the established simulation platforms, the global dynamic responses of the bridge and each individual vehicle subjected to various service and extreme loads can be rationally predicted in the time domain. Secondly, built on the proposed general simulation platforms, a novel dynamic safety assessment model and a vehicle ride comfort evaluation model for the bridge-traffic system are further developed. Thirdly, also extended from the proposed simulation platforms, both deterministic and reliability-based assessment frameworks for long-span cable-stayed bridges subjected to breakage of stay cables are established by considering more rational service load conditions as well as cable-breakage characterizations. Lastly, in addition to the in-house programs focusing on research purposes, a hybrid simulation strategy for the bridge under traffic and seismic excitations and a time-progressive simulation methodology for cable breakage events are also developed by taking advantage of the strength offered by commercial finite element software, e.g., SAP2000. These SAP2000-based strategies are expected to facilitate design engineers to more easily understand and conduct the related analyses in future engineering practices.
Open Access
Effects of structure on flow mechanics in the human left ventricle and respiratory tract
(Colorado State University. Libraries, 2011) Moore, Brandon L., author; Dasi, Lakshmi Prasad, advisor; Orton, Christopher, committee member; Sakurai, Hiroshi, committee member
Cardiac and respiratory dysfunctions represent a large portion of healthcare problems in the United States. Many of these problems are caused by abnormal flow mechanics due to altered anatomical structure. This structure in the human body is very complex and ranges over many different scales. At relatively small scales, one facet that is still not well understood is the role of trabeculae on the biomechanics of the left ventricle. Similarly, large-scale airflow through the respiratory tract has not been fully investigated as a function of age or mechanical ventilation. This research has revealed some of the flow patterns caused by these different scale structures. Fractal geometry was used to help characterize the inner surface of the left ventricle at different times during the cardiac cycle. The fractal dimension of the ventricle was determined using a custom box-counting algorithm developed in MATLAB, and it was shown that trabeculae do indeed play a role in the biomechanics of heart pumping. Computational fluid dynamics (CFD) was also run on the respiratory tracts of three different patients to determine airflow effects due to age and intubation. Three dimensional models were constructed from computed tomography (CT) scans and simulations were run in ANSYS Fluent. Results of the study were validated through grid and time step sensitivity studies as well as comparison to previous studies. It was shown that flow mechanics in the airways of children change with age as well as with the introduction of an intubation tube.
Open Access
Experimental investigation of wind effects on long-span slender bridges with stochastic traffic flow
(Colorado State University. Libraries, 2011) Nelson, Ryan Scott, author; Chen, Suren, 1973-, advisor; Bienkiewicz, Bogusz, committee member; Sakurai, Hiroshi, committee member
The aeroelastic and aerodynamic effects on long-span slender bridges due to traffic has traditionally been neglected as it is assumed that the bridges will be closed to traffic under strong winds. However, with ever changing weather, natural disasters, and important roles of many long-span bridges throughout the United States, the reality is that these long-span bridges are often not closed and there are still many vehicles on the bridges even when considerably strong winds exist. Therefore, to rationally evaluate the aerodynamic performance of a bridge deck, the impacts from stochastic traffic should be appropriately considered as a key part toward any safety or serviceability study. The present study discusses the wind tunnel experimental tests of a long-span bridge section with stochastic traffic. The details of the experimental investigations are reported, including the design and construction of a bridge section model, two-degree-of-freedom testing frame and vehicle models representing stochastic traffic. Several tests were performed to determine a baseline for the bridge section without traffic, under different wind speeds and attack angles. The bridge section was then re-tested with many scenarios representing stochastic and extreme traffic conditions. The aeroelastic flutter derivative coefficients were extracted using the iterative mean square method and the values plotted and compared with the baseline results. Under the given reduced velocity range being tested, it is observed that several traffic scenarios increase the aeroelastic and aerodynamic effects as the bridge section becomes more susceptible to flutter and vortex shedding. Finally, the statistical descriptions of the flutter derivatives with the presence of traffic on the bridge section model are also made.
Open Access
Flow dynamics and scalar transport in drinking water contact tanks
(Colorado State University. Libraries, 2013) Barnett, Taylor C., author; Venayagamoorthy, Subhas Karan, advisor; Julien, Pierre Y., committee member; Sakurai, Hiroshi, committee member
The research and studies presented in this thesis focus on ways to improve the internal hydraulics of chlorine contact tanks used in drinking water disinfection systems. This was accomplished through the use of computational fluid dynamics (CFD) and physical tracer studies of a number of different systems. Three primary tank modifications were investigated in these studies: internal baffling; inlet modifications; and random packing material. The findings from these studies were then applied in a case study of the Jamestown chlorine contact tank. All of the studies presented in this thesis use the baffle factor (BF) designation as defined by the United States Environmental Protection Agency as the primary indicator of a system's disinfection efficiency. The CFD models used for the internal baffling study were first validated using a laboratory scale study of the Embsay chlorine contact tank in Yorkshire, England. This tank footprint was then modified to replicate a precast concrete tank that was installed in the Hydraulics Laboratory located at the Engineering Research Center (ERC) at Colorado State University. This concrete tank was used as the footprint for a parametric study in which the number and length of internal baffles were modeled in various configurations. The internal hydraulics of this baffle tank were optimized using only two dimensionless relationships namely: the baffle opening ratio L* and the baffle opening to channel width ratio Lbo/Wch. The resulting tank geometry from these two relationships yielded a BF of 0.80 and also maximized the length to width ratio of each channel within the concrete tank. The inlet modification study was performed to investigate how the BF of a 400-gallon doorway storage tank could be improved. Three different inlet types with two inlet sizes were modeled and simulated for six different flow rates. Three of these CFD simulations were then physically tested using both saline and lithium tracers to validate the computer models. Key findings from this study show that the size of the inlet and its orientation play a dominant role in the internal hydraulics of the system. For the random packing material study, three different packing material sizes, two tank sizes, and two different flow rates were tested. CFD models were not feasible due to the randomness of how the packing material would settle in these contact tanks. Over 64 saline tracer studies and 6 lithium tracer studies were conducted to complete this study. Key findings show that the initial BF of the system and the volume of the tank filled with the packing material were the dominant variables in the study. The tank size, flow rate, and packing material size had little to no impact on the performance. The Jamestown case study presented in this thesis used findings from both the internal baffle study and the inlet modification study. The BF of the contact tank would fluctuate annually between 0.52 and 0.63 due to a shift in flow regimes caused by a change in the system's flow rate. This turbulent to laminar flow regime change was validated with the use of CFD models coupled with physical tracer studies. Several inlet modifications were investigated using CFD to determine what modifications, if any, the plant operators should implement. Key findings from the CFD models showed that with the proper inlet modification, the BF of the system could be stabilized at 0.63 during both the high flow summer months and low flow winter months.
Open Access
Impact of geometric design of hydraulic contact tanks on residence time distributions
(Colorado State University. Libraries, 2015) Carlston, Jeremy S., author; Venayagamoorthy, S. Karan, advisor; Ramirez, Jorge A., committee member; Sakurai, Hiroshi, committee member
The research outlined in this thesis investigates how geometric design affects the mixing efficiency of contact tanks used for drinking water disinfection. In particular the configuration of baffles and inlets are assessed in depth using both physical tracer studies and computational fluid dynamics (CFD) simulations. Two rectangular contact tanks were used, both of which are assumed to be representative of disinfection tanks for small municipalities throughout the United States. System performance is analyzed by means of residence time distributions from which the baffling factor (BF) and Morrill index (MI) performance indicators can be calculated. First, a parametric study of a 300 gallon tank used for previous research was undertaken. After model validation, 30 CFD simulations were conducted in order to determine optimal baffle configuration by altering channel width, baffle length, and flow orientation. It was found that the baffle lengths should be prescribed such that their openings are roughly equal to the width of the tank's channels. In this manner excessive constrictions or expansions, which lead to undesirable flow separation, can be prevented. It was also confirmed that high length-to-width ratios in the flow lead to a better emulation of ideal plug flow by promoting advective transport of both the water and disinfectant. Second, a 1,500 gallon, two-baffled tank housed at Colorado State University's hydraulic laboratory in the Engineering Research Center was used to investigate "sharp" inlets, which lead to poor mixing conditions. Unfortunately operating budgets prohibit many small disinfection facilities from upgrading these preexisting sharp inlets. In an attempt to provide an inexpensive solution, seven attachments to the inlet were tested for potential improvements to contact tank performance. Physical tracer studies were conducted on the prototype in order to obtain RTD curves resulting from each modification. The horizontal T-shaped attachment performed best, attaining a BF of 0.59 at a flow rate of 40 gallons per minute (gpm). This is a 74% gain over the unmodified inlet's BF of 0.34. From a hydrodynamic analysis of the flow fields obtained from CFD simulations, it was concluded that any inlet configuration leading to a quicker homogenization of longitudinal velocities (especially in the first channel of the tank) and hence better approximation of plug flow will improve system performance. Finally, seven different inlet modifications to the un-baffled counterpart of the 1,500 gallon tank were investigated. Without any alterations the un-baffled sharp inlet tank provides extremely poor disinfection as evidenced by its baffling factor of 0.05. Due to good agreement between physical and numerical tests for the baffled tank, the un-baffled system was only researched with CFD. Four equally-spaced orifices deflecting flow to the back wall of the tank resulted in the largest gain in performance with an estimated BF of 0.27. A hydrodynamic analysis again confirmed that despite lower length-to-width ratios leading to a lower ceiling of mixing performance, better imitation of plug flow results in larger baffling factors and lower Morrill indices.
Open Access
Mixing of scalars in turbulent flows using direct numerical simulations
(Colorado State University. Libraries, 2015) Nithianantham, Ajithshanthar, author; Venayagamoorthy, S. Karan, advisor; Julien, Pierre, committee member; Sakurai, Hiroshi, committee member
The research presented in this thesis focuses on scalar mixing in unstratified (neutral) flows and stably stratified flows using Direct Numerical Simulations (DNS). Such flows are ubiquitous in natural flows such as rivers, estuaries, oceans and the atmosphere. First, a detailed study was performed to investigate the effect of varying Schmidt numbers (Sc) on turbulent mixing of a passive scalar in a stationary homogeneous unstratified flow using forced DNS. A total of 6 simulations were performed for 0.1 ≤ Sc < 3. Qualitative and quantitative results of the flow field and the passive scalar fields are presented and discussed. The effect of the Schmidt number on the turbulent mixing was found to be negligible and becomes important (as it should) only when mixing occurs under laminar flow conditions. Using a model proposed by Venayagamoorthy and Stretch in 2006 for the turbulent diascalar diffusivity as a basis, a practical (and new) model for quantifying the turbulent diascalar diffusivity is proposed asKS = 1.1 γ' LT k1/2, where LT is defined as the Thorpe length scale, k is the turbulent kinetic energy and γ' is one-half of the mechanical to scalar time scale ratio, which was shown by previous researchers to be approximately 0.7. The novelty of the proposed model lies in the use of LT, which is a widely used length scale in stably stratified flows (almost exclusively used in oceanography), for quantifying turbulent mixing in unstratified flows. LT can be readily obtained in the field using a Conductivity, Temperature and Depth (CTD) profiler or obtained from density fields in a numerical model. The turbulent kinetic energy is mostly contained in the large scales of the flow field and hence can be measured in the field using devices such as an Acoustic Doppler Current Profiler (ADCP) or modeled in numerical simulations. Comparisons using DNS data show remarkably good agreement between the predicted and exact diffusivities. Finally, the suitability of the proposed model for stably stratified flows was explored for varying degrees of stratification ranging from mildly stable flow conditions to strongly stable conditions. In stably stratified flows, density variations of the fluid dynamically affect the flow field and hence the density acts as what is widely known as an active scalar. Under strongly stable conditions, the DNS results indicate an inverse relationship between the Thorpe scale LT and kinetic energy length scale Lkε, which is different to the direct (almost one to one correspondence) relationship that was found for unstratified flows. Hence, in order to account for this difference, a modified turbulent diascalar diffusivity model was proposed as Kd = 13 γ' LT3 k1/2. It must be noted that this modified model while dimensionally inconsistent (due to the inverse relationship between the length scales), provides reasonable quantitative estimates of the diffusivity under stably stratified flow conditions. The models proposed in this study require further (extensive) testing under higher Reynolds number flow conditions. If shown to be valid, they would be widely useful for quantifying turbulent mixing using field measurements of large scale quantities (i.e. LT and k) as well as a simple and improved turbulence closure scheme.
Open Access
Modeling and design of a current mode control boost converter
(Colorado State University. Libraries, 2012) Yao, Hong, author; Collins, George J., advisor; Chen, Tom W., committee member; Suryanarayanan, Siddharth, committee member; Sakurai, Hiroshi, committee member
The boost or step up converter produces an undesirable Right-Half Plane Zero (RHPZ) in the small signal analysis of the "Duty Cycle Control to Output Voltage" transfer function. It is well documented that the boost converter has the reputation of low-performance and stability is complicated due to the RHPZ which makes Voltage Mode Control (VMC) very hard to implement. Even when Current Mode Control (CMC) is applied in the topology of converters operating in Continuous Conduction Mode (CCM), the current feedback loop becomes unstable if the duty cycle exceeds 0.5---the instability is known as Subharmonic Oscillation.
Open Access
Offline detection of broken rotor bars in AC induction motors
(Colorado State University. Libraries, 2015) Powers, Craig Stephen, author; Collins, George J., advisor; Reising, Steven C., committee member; Sakurai, Hiroshi, committee member
The detection of the broken rotor bar defect in medium- and large-sized AC induction machines is currently one of the most difficult tasks for the motor condition and monitoring industry. If a broken rotor bar defect goes undetected, it can cause a catastrophic failure of an expensive machine. If a broken rotor bar defect is falsely determined, it wastes time and money to physically tear down and inspect the machine only to find an incorrect diagnosis. Previous work in 2009 at Baker/SKF-USA in collaboration with the Korea University has developed a prototype instrument that has been highly successful in correctly detecting the broken rotor bar defect in ACIMs where other methods have failed. Dr. Sang Bin and his students at the Korea University have been using this prototype instrument to help the industry save money in the successful detection of the BRB defect. A review of the current state of motor conditioning and monitoring technology for detecting the broken rotor bar defect in ACIMs shows improved detection of this fault is still relevant. An analysis of previous work in the creation of this prototype instrument leads into the refactoring of the software and hardware into something more deployable, cost effective and commercially viable.