Browsing by Subject "Model"

Now showing 1 - 13 of 13
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    A Comparison of the Genetic Algorithm and the Mixing Genetic Algorithm
    (2020-07) Gulfam, Muhammad
    Genetic Algorithms (GAs) are optimization techniques inspired by the idea of evolution. They can sometimes take a long time to find the solution to a problem, but it is not always obvious when, or how to configure their various parameters. Recently, a new GA was introduced [8] that has a lot of potential for parallelization. This algorithm, called the Mixing Genetic Algorithm, has shown promising results on the well-known Traveling Salesman Problem. In this work, we have compared the effectiveness of the Mixing GA over a traditional GA on three discrete optimization problems: the OneMax problem and two topologies of the Ising Model (Ising Model on Tree and Ising Model on Ring). The comparison has been done for the success rate at the given time, for the given problem size and size of population. The comparison has been done for, both, serial and parallel implementations. Overall, the success rate for the Mixing GA is better than the traditional GA. We have also compared two population selection methods, namely, tournament selection and generational population selection. The tournament selection outperformed generational population selection for all the problems and problem sizes that we experimented with.
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    Creating and applying a cognitive change model: a transdisciplinary (education, cognitive psychology, neuroscience) approach
    (2013-05) Meyer, John Edward
    This study uses qualitative data and literature from various disciplines to shed light on the complex phenomenon of cognitive change, especially as it occurs within educators. The resulting understandings are used to develop both verbal and visual models to illustrate the dynamics of such a transformative mental change. The qualitative data represents reflections of individual participants in a collaborative leadership problem-solving virtual environment designed to elicit cognitive conflict and potentially resulting in new understandings about power. This data was analyzed iteratively with research and literature from education, cognitive psychology, and neuroscience to gain a complete picture of cognitive change from an individual perspective. The study tests the cognitive change model's usefulness by applying it to individual participants' experiences.
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    Developing a Predictive Model and Novel Imaging Technique for the Failure of Polyethylene Insulators
    (2019-08) Zoltek, Daniel
    Polyethylene is the most widespread polymer used in insulative cable housings due to its low cost, high chemical resistivity and low permeability to liquids and gases. This does not mean, however, that the material is not susceptible to failure under environmental working conditions. Many techniques for monitoring both chemical and physical changes have been developed, though no attempts have been made to integrate these findings. Here, we put forth a model for the failure of polyethylene cable housings under thermooxidative conditions. This model revealed an absence of data on the monitoring of polyethylene crystalline structure during the aging process, which in part controls the insulative properties of the polyethylene. Polyethylene films (30 µm) were aged at 110°C for 24-hour periods in an oven and carbonyl content, a common aging indicator, was monitored. An ATR-FTIR crystallinity monitoring technique was developed and revealed a 3-phase change of crystalline structure upon thermal aging. To better visualize the hypothesized pore formation in the polyethylene, which follows aging, EIS was used to saturate pores with gold nanoparticles before elemental analysis and imaging with SEM. Results suggest the existence of these pores and the ability for ions to penetrate the aged films.
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    Enabling Increased Variable Renewable Energy Penetration via Thermal Energy Storage Coupled with Nuclear Power
    (2021-04) Carlson, Fletcher
    Thermal energy storage (TES) used with baseload nuclear power plants to provide low-carbon flexible electricity and to support the expansion of variable renewable energy sources is analyzed. A thermodynamic model quantifies the impact of options for integration of TES into the Rankine power cycle on cycle behavior and capacity factor. The diurnal energy production ratio, equivalent to a relative capacity factor, is compared in a parametric study of operating conditions, including discharge power (up to 2 times the baseload power, charge duration (2-10 hours), discharge power, discharge duration and the round-trip efficiency of the TES (0.7 to 1), for three configurations. Configuration I charges the TES using high-pressure steam and discharges steam to the low-pressure turbine. Configuration II charges the TES in the same manner and discharges preheated condensate to the steam generator. Configuration III charges the TES using low-pressure steam and discharges the TES to a secondary cycle. Conceptual designs of sensible and latent heat storage devices are discussed with estimates of volume, mass and cost of the storage material. Configuration III has the highest energy production ratio over the entire parameter range. Use of a secondary cycle eliminates any penalty on baseload operation and reduces the penalty on turbines compared to discharge to the primary cycle. At a discharge power of 1.2 times baseload power, charge and discharge durations of 4 and 3 hours, respectively and a TES round-trip efficiency of 0.9, the energy production ratio is 0.99. Discharge powers up to approximately 1.6 times baseload power are achievable for these parameters. Configuration I can also reach high discharge power but at a lower energy production ratio. Configuration II is restricted to a discharge power of 1.1 times the baseload. The energy production ratio of all configurations decreases with increasing discharge duration and discharge power and increases with increasing TES round trip efficiency. Increased discharge power can be achieved through an increase in charging duration and TES round-trip efficiency and a decrease in discharge duration. Sensible heat storage is favorable with estimated costs of material from $5-20 per kWhe compared to $20-40 kWhe for latent heat storage. The narrow temperature range restricts latent heat storage materials to expensive hydroxide-based salts. Configuration II, although limited in discharge power and energy production ratio, requires the lowest thermal capacity, 950 MWhth to provide a discharge power of 1.1 times baseload power with an estimated cost of $4.87 per kWhe for a discharge duration of 3 hours, and round-trip efficiency of 0.9. At the same operating parameters, configurations I and III require a storage capacity of approximately 1500 MWhth at cost $9 and $21 per kWhe. The use of TES can be a transformative technology in the ability to convert baseload nuclear power plants to flexible generation sources for the support of renewable energy. All configuration of TES presented in this work allow for some degree of flexibility. This work demonstrates the importance of how the TES is integrated into the cycle on the energy production of the system, a key indicator of economic viability, the maximum discharge power and the required storage capacity.
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    A grounded theory study of sociocultural and psychosocial factors that influence the construction and deconstruction of heteronormative and heterosexist worldviews.
    (2009-05) Howe, L. Andrew
    The purpose of this study is to investigate the psychological developmental process of learning and unlearning heteronormative and heterosexist worldviews and the sociocultural influences that encourage or discourage transition to more affirming beliefs toward same-sex sexual orientation. Given the purpose, a sample of participants who could speak to their experiences with learning and unlearning anti-gay biases was needed. Thus, professionals working in higher education institutions who were members of divisions of professional associations with an emphasis in social justice, diversity, or multiculturalism were invited to participate. The sample was developed from one or more of the following professional organizations: American Psychological Association (APA), American Counseling Association (ACA), and American College Personnel Association (ACPA). This grounded theory study consisted of two phases: (1) questionnaires and (2) interviews. The questionnaire provided a breadth of common experiences among the participants, while the interviews provided a deeper understanding of these experiences. Two models emerged from structured coding procedures: (1) Psychosocial Interaction Model of Heterosexism (PI) and (2) Personal Development Model of Heterosexism (PD). The PI model illustrates interaction processes among significant associations (i.e., individuals, social groups, and institutions), life experiences, and personal meaning making that were influential when developing and changing core beliefs about same-sex sexual orientation. The PD model outlines psychological stages of personal development when learning and unlearning heteronormative and heterosexist worldviews from early childhood to adulthood.
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    Mathematical models of bacterial chemotaxis
    (2010-01) Xin, Xiangrong
    In response to environmental signals such as light, temperature or chemicals, motile organisms can change their behavior by directed movement toward or away from the signal, by changing their speed of movement and/or frequency of turning. The process is called chemotaxis. E. coli employs chemotaxis to move toward favorable locations. E. coli chemotaxis is a widely studied system. The recent research mainly concentrates on receptor clustering, which is established to account for the dramatic features of the system such as high sensitivity, precise adaptation, and robustness. There are multiple levels of organization of a receptor cluster, and researchers are gaining insights into its structure-function relationship. We hypothesize that multiple-level molecular interactions exist in the receptor cluster, and each of them contributes specific functions to the high-performance signaling. To test it, we first develop a model based on the experimental observation that the most permanent clusters of receptor homodimers are trimers of dimers. We only consider the interactions among dimers within a trimer, called intratrimer interactions. We show that the model can reproduce most of the experimentally-observed behaviors, including excitation, adaptation, high sensitivity, and robustness to parameter variations. In addition, the model makes a number of new predictions as to how the adaptation time varies with the expression level of proteins. Second, we use the approaches of multi-time-scale analysis and mean-field theory to perform model reduction, and obtain two low-dimension models. They successfully capture the output of the original model. Third, we develop a free-energy-based model for a cluster of coupled trimers, emphasizing the interactions among trimers, called intertrimer interactions. We use the model to explain high cooperativity in kinase activity responses by the cheRcheB mutants with overexpression of Tar or Tsr. Last, we develop a stochastic model of adaptation with the mobile CheR and CheB and show some preliminary results.
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    A mechanistic-empirical model of central metabolism, signaling, and the reactor environment for bioprocesses
    (2020-10-07) O'Brien, Conor M; Hu, Wei-Shou; acre@umn.edu; Hu, Wei-Shou
    This model was built and optimized to reproduce the variability inherent to many industrial cell-culture processes. Classically, fed-batch Chinese Hamster Ovary (CHO) cell cultures will initially produce lactate in the early phase of culture before switching to lactate consumption. However, some processes may revert to lactate production in the late stage of culture, driving up osmolarity while reducing viable cell density, and ultimately lowering process performance. This phenomenon may occur in only some runs of a manufacturing processes and even may differ among runs with similar initial conditions and trajectories, leading to longstanding questions about the mechanisms driving this switch. By simulating cultures which were exposed to different amounts of stress before the production bioreactor we show that similar starting conditions in the bioreactor environment can lead to variability in metabolic shift. We provide this model as a tool to demonstrate this metabolic variability and provide a platform for hypothesis testing, in silico bioprocess optimization, and simulation of reactor scale-up and scale-down.
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    Modeling Regional Variation of Cortical Spreading Depression: A Computational Study
    (2019-05) Tuttle, Austin
    Cortical Spreading Depression(CSD) is a pathological phenomenon in the central nervous system in which normal cellular function is disrupted by a prolonged depolarization due to massive ionic fluxes. This spreads at a rate of millimeters per minute and is connected to with several medical conditions: migraine aura, stroke, traumatic brain injury, etc. In this thesis we present a multi-phasic continuum electrodiffusion model of spreading depression. The main result of this work is the efficient numerical simulation of 2D and 3D versions of this model. We make use of these simulations by focusing on the introduction of NMDA receptors and their effects on previous findings. From there, we investigate spatial variance of CSD in two ways. First, the natural occurrence of spiral wave patterns in a homogeneous domain. Second, we introduce spatial dependence of parameters to investigate how the varied structure of the hippocampus can impact CSD.
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    Modeling Stream Thermal Dynamics: The Influence of Beaver Dams in a Minnesota Watershed
    (2020-08) Behar, Hannah
    Beaver dams are known to alter the thermal regime of ponds, streams, and adjacent subsurface waters. Downstream of a dam, stream temperature is influenced by increased exchange with the hyporheic zone, which may cool and buffer the stream’s diel temperature cycles. Concurrently, reduced shading in the beaver forage zone is likely to increase heat flux at the stream-atmosphere boundary. The dynamics of these processes can be analyzed to understand how stream temperature is affected on diel time scales, as well as longitudinally at distances downstream from the dam. At two beaver dam-impacted stream sites in the Knife River Watershed in Minnesota, USA, I monitored in-stream and shallow subsurface flow and temperature during low-flow summer conditions. I used a dye tracer test, vertical heat transport modelling, and soil characterization to estimate flux through the streambed at multiple locations. Temperature, stream flow, and atmospheric data were also collected throughout the summer from the two sites. A one-dimensional model of longitudinal stream temperature, calibrated to in-stream temperature measurements, was developed to determine which physical parameters and heat flux components have the greatest influence on stream temperature. The model was then used to demonstrate how these changes persist downstream, as well as to simulate stream temperature under potential future site conditions. These findings increase scientific understanding of stream temperature regime in the context of beaver dam-altered watersheds.
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    Temperature and Rate Effects on the Mechanical Behavior of Tungsten and Its Nanocomposites
    (2022-05) Schmalbach, Kevin
    Methods to predict material failure frequently rely on large experimental datasets tuned to the properties of one material or are based on computationally expensive modeling. Both approaches are slow and do not adapt well to changes in material chemistry or processing. Development of analytical models with easily measured, physically meaningful parameters are key to alleviating bottlenecks in new materials development. One such model is presented in this work, which relies on easily measured physical parameters, effective stress and activation volume, referred to as activation parameters. This approach requires knowledge of both the temperature and strain rate effects on mechanical properties, which is the primary focus of this dissertation.Nanoindentation is an ideal way to measure these physical parameters due to its ability to measure properties at a length scale appropriate for grain- or phase-dependent deformation mechanisms and the small quantities of material required for testing (~mm3). Newer nanoindentation tests, such as strain rate jump tests, can quickly determine activation parameters, but lack standardized protocols for experiments and analysis. For this work, tungsten (W) was chosen as a model system in which to understand temperature and strain rate effects. This is in part due to its elastic isotropy, which simplifies data analysis, and its relatively experimentally accessible brittle-ductile transition temperature. In the first half of this dissertation, I will describe the use of nanoindentation strain rate jump tests to predict the fracture behavior of macroscale tungsten single crystals. I begin by presenting necessary protocol development for strain rate jump testing and analysis using a new, freely available software package. Included in the software package are a Python-based load function generator and a series of Matlab functions for data analysis. These tools were validated on single crystal tungsten, yielding a strain rate sensitivity nearly identical to that reported in the literature. The techniques were then applied at low temperature (-100 °C) and high temperature (50-300 °C) to measure activation parameters, effective stress and activation volume, as a function of temperature. The activation parameters, in combination with an analytical model for the strain energy release rate, accurately predict the brittle-ductile transition temperature along particular fracture systems in single crystal tungsten. Activation parameters measured from indentation of the (100) surface of single crystal tungsten accurately predict the brittle-ductile transition and fracture toughness along the {100}<011> fracture system of macroscale tungsten single crystals. Use of data from bulk tension of single crystal tungsten from the literature accurately predicts the fracture toughness in the {110}<110> fracture system of macroscale tungsten single crystals. In the second half of this dissertation, I will describe the synthesis and mechanical properties of 3-dimensionally ordered macroporous (3DOM) tungsten and a nanocomposite based on the porous framework. 3DOM tungsten was made with 35-40 nm wide ligaments, which exploit material size effect to have a ligament yield strength of 6.1 GPa at room temperature, approaching the ideal strength of tungsten. Considerable plasticity was observed above 125 °C, implying a brittle-ductile transition around this temperature, consistent with the predictions based on the earlier-presented model. Filling of the 3DOM tungsten framework with a silicon oxycarbide (SiOC) reinforcing phase resulted in a heterogeneous structure containing tungsten, silicon oxycarbide glass, and small domains of free carbon. The failure strength of 3DOM W-SiOC was 1.1 GPa, a factor of 22 greater than the 3DOM W framework. Although an increase in deformability was observed at 225 °C, pillars still failed by abrupt crack propagation. Fracture was only prevented at the next testing temperature of 425 °C. This places the brittle-ductile transition temperature of the W-SiOC composite significantly higher than that of pure tungsten. Additionally, the composite retains high strength even to 425 °C, achieving a yield strength of 400 MPa.
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    A three-dimensional Mathematical model of directional drilling
    (2013-01) Perneder, Luc
    The dynamical model governing the 3D kinematics of a drill bit is constructed for rotary drilling applications for which the bit is guided by a push-the-bit rotary steerable system. The evolution of the bit trajectory, and thus of the borehole geometry, is a consequence of the interaction between the borehole, a geometric object, and the drilling structure, a mechanical object. In this respect, the model describing this evolution consists of the association between (i) a model of the near-bit region of the drillstring, (ii) a model of the bit/rock interaction, and (iii) kinematic relationships relating the motion of the bit into the rock to geometric variables for the borehole evolution. The mathematical formulation of these three elements yields a set of functional differential equations with secular terms accounting for a delayed influence of the borehole geometry on the bit trajectory. The parameters entering these relations account for the loads and properties of the drilling structure and for the properties of the bit and rock formation. Three length scales are identified in the response of the directional drilling system; they correspond to short-, intermediate-, and long-range behaviors. The short-range response is associated with the dimensions of the bit, the small scale of the problem. It corresponds to fast variations of the bit orientation. On the intermediate-range, the wellbore trajectory converges to a quasi-constant curvature solution, if it is stable. On the long-range, the borehole curvature slowly varies and for appropriate set of drilling parameters the borehole converges toward a stationary helical path. Finally, the stability and rate of convergence on the intermediate and long range are investigated.
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    Time Dependent Effects of Contemporary Irrigants on a Polymicrobial Biofilm
    (2014-03) Barsness, Brian
    INTRODUCTION: Removal of contaminated canal contents and biofilms by mechanical preparation alone are limited. Irregular cross sections, lateral canals, and apical deltas are mostly inaccessible to mechanical preparation (Siqueira et al., 1997). Therefore, the use of chemical dissolution and disinfection of these regions is necessary. Many studies have demonstrated the efficacy of sodium hypochlorite in providing these functions (Hand et al., 1978; Rosenfeld et al., 1978). However, adverse cytotoxic outcomes have been reported with its use (Joffe et al., 1991), as well as reports of allergic hypersensitivity (Kaufman et al., 1989). The ideal irrigant has attributes of; broad spectrum antimicrobial activity, ability to dissolve pulp tissue remnants, inactivates endotoxin, removes the smear layer components, and is systemically nontoxic to periodontal tissues. Such an irrigant does not yet exist (Zehnder, 2006). MTAD, a final irrigation solution containing 3% doxycycline, 4.25% citric acid, and 0.5% Tween-80 has been shown to remove smear layer on extracted human teeth and provides broad spectrum antimicrobial activity (Torbinejad et al., 2003). QMiX, a final irrigation solution containing; bisbiguanide, calcium chelating agent, cetrimide surfactant, and saline has demonstrated efficacy at removing smear layer and disinfecting dentin tubules (Wang et al., 2013). SmearClear, a smear layer removing irrigant containing EDTA, cetrimide and water, has been shown to remove smear layer efficiently in an ex-vivo, split-tooth model (Andrabi et al., 2013). The aims of this study are to; demonstrate the use of a novel polymicrobial biofilm model to test endodontic irrigants, measure the susceptibility of a biofilm to individual irrigant treatment groups, and to characterize the responses visually through scanning electron microscopy (SEM) and laser confocal microscopy. METHODS: Clinical endodontic microbial samples were collected from six adult patients that presented to the graduate endodontics clinic at the University of Minnesota School of Dentistry with a diagnosis of pulpal necrosis. For each of the 3 experimental runs a pool of one to three patients were combined to ensure a collection of heterogeneous microbial species. A series of baffled 500 mL Erlenmeyer culture flasks were used as a “bioreactor” system to facilitate biofilm formation on hydroxyapatite discs. A total of six discs were contained per flask (2 discs per cell strainer basket). The flasks were covered with a sterile, vented flask cap and placed within the anaerobic chamber for incubation at 37°C and 20 RPM. This allowed for biofilm formation on the hydroxyapatite discs under shear force conditions. The flasks were allowed to incubate according to the conditions described above for approximately 48 total hours. Following incubation, discs were randomly selected for placement into a sterile, flat-bottomed, 12-well polystyrene culture plate (Sigma-Aldrich) for treatment according to the irrigant protocol.Four irrigation solutions were independently used for testing on the 48 hour biofilms. Sterile phosphate buffered saline (PBS) was used as a negative control treatment group. MTAD (DENTSPLY, Int.), QMiX (DENTSPLY, Int.), and Smearclear (SybronEndo, Orange, CA) were used as challenge treatment groups. Groups included; 1X PBS at 1 minute exposure, MTAD at 1 minute exposure,MTAD at 20 minutes exposure, QMiX at 1 minute exposure,QMiX at 20 minutes exposure,SmearClear at 1 minute exposure, and SmearClear at 20 minutes exposure. Evaluation of biofilm disruption was determined by; cell viability staining, crystal violet biomass staining, SEM, and confocal laser microscopy. Microbial speciation was performed by the Forsyth Institute (Cambridge, MA) by HOMIM 16sRNA analysis. RESULTS: The flask bioreactor, as previously described, can be utilized for growing a polymicrobial biofilm suitable for testing the efficacy of antimicrobial and biofilm removing endodontic irrigants. When compared as a group, the endodontic irrigants in this study achieved a statistically significant reduction in cell viability as time of exposure increased. In a pairwise comparison between irrigants, the mean cell viability was lowest for QMiX. No statistically significant difference in the reduction of biomass between endodontic irrigants was observed. Comparing endodontic irrigants individually by time did not result in a statistically significant difference in reducing cell viability or biomass. None of the endodontic irrigants completely removed the biofilm, as observed by SEM and confocal laser microscopy. 6 of 12 microbial species recovered were found to be in common between the patient pools when analyzed by HOMIM 16sRNA identification. This study presents a simple model for growing endodontic biofilms under anaerobic conditions. CONCLUSIONS: Findings from this study support the inefficient removal of biofilm by the irrigants included in the study. It was determined that QMiX achieved the lowest cell viability over the 20 minute treatment exposure. As a group, the irrigants were statistically significant in reducing cell viability and reducing biomass. However, it could be concluded that they should not be utilized as a single irrigant to disinfect or remove biofilm.