Browsing by Subject "Simulations"
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Item Burning season effect on four southern Chihuahuan desert plants(Texas Tech University, 2009-05) Luna, Miguel; Britton, Carlton M.Use of prescribed fire to manage undesirable vegetation in the Chihuahuan Desert of Mexico, promises acceptable results, but information on plants responses to different weather conditions and fuel load availabilities is lacking in Mexico. This study investigated the effect of three burning seasons with two fuel load simulations and two plants size on plant mortality and changes in basal area of four native species of southern Chihuahuan Desert. The study was conducted in the Mexican High Plateau in Jalisco, Mexico during two consecutive years, 2005 and 2006, in a shortgrass prairie of blue grama (Bouteloua gracilis) with problematical populations of broomweed (Isocoma venetus), brickellbush (Brickellia spinulosa) and broomgrass (Muhlembergia rigida). Fire environment was simulated using a portable propane burner calibrated to simulate time-temperature reached with 1,700 and 2,800 kg/ha fine fuel load. Size of plant was determined by height and canopy diameter in shrubs, and indicial area diameter in grasses. Shrubs with a minimum height of 25 cm and canopy diameter of 31 cm were classed as large plants. Grasses with a basal area diameter of 10 cm were classed as large plants. Plants with measurements less than these were placed in the small plant group. For each species, 50 plants were randomly treated each season at each fuel load and plant size. Plants were identified with numbered tags to evaluate mortality and basal area changes in subsequent growing season. As control, 50 plant of each species and size received no fire treatment. Plants with no live tillers one year after treatment were assumed to be dead. Basal area change was estimated based in number of pixels/cm2, using vertical photos and the Adobe Photoshop Software. Statistical analysis was performed with the GLM procedure in SAS. Mortality of blue grama plants was affected significantly by season of burning, fuel load simulation, and plant size (P<0.05). Small blue grama plants were significantly (P<0.05) more affected than large plants, mainly after spring and summer burns. Mortality of broomgrass muhly was slightly affected by burning treatments in either evaluation year, with average mortality of 1% and 2% for 2005 and 2006. Brickellbush mortality was significantly affected by burning season (P<0.05), and plant size (P<0.05). Spring and winter burns were the most detrimental seasons for small brickellbush plants. Broomweed plants were severely affected (P<0.05) by all burn treatments, regardless of fuel load simulation and plant size, with 82% average mortality. Spring and summer burning with high fuel load simulations reduced basal area of large blue grama plants. However these changes were similar (P>0.05) to unburned plants that decreased 8%. Winter burning with high fuel load had a positive effect (P<0.05) on basal area of large blue grama plants, with an increase of 37%. Small blue grama plants had a positive response to all burning treatments regardless of fuel load simulations, with average increases of 80%, 41%, and 31% for spring, summer, and winter burns respectively. Nevertheless, these changes were similar (P>0.05) with unburned plants that showed increased basal area of 41%. Basal area of muhly plants was generally reduced by all burn treatments. Basal area of large muhly plants had average reductions of 24% and 48% with summer and winter burns, while spring, summer, and winter burns reduced average basal area of small muhly plants by 2%, 4%, and 47% respectively.Item Connecting the dots : tracking galaxy evolution using constant cumulative number density at 3(2015-12) Jaacks, Jason Dale; Finkelstein, Steven L.; Bromm, VolkerUsing the cosmological smoothed particle hydrodynamical code GADGET-3 we make a realistic assessment of the technique of using constant cumulative number density as a tracer of galaxy evolution. We find that over a redshift range of 3Item Effects of Presentation Modality and Practice Strategy in Animated Demonstrations of Software on Achievement of Undergraduate Students(Texas Tech University, 2008-05) Ou, Chaohua; Maushak, Nancy; Crooks, Steven M.; Price, RobertComputer-based or web-based software training has become an increasingly popular training model in which animated demonstrations are used for instructions and simulations for practices. However, research studies on their effectiveness are very limited and the findings are inconsistent. This research study investigated the effects of presentation modality of animated demonstrations and practice strategy on undergraduate students’ achievement in learning web design with Microsoft FrontPage 2003 in a computerized environment. A 3 x 2 factorial experimental design was used in this study, with presentation modality and practice strategy as the two between-subjects independent variables. The instructions in animated demonstrations and practice simulations were presented in three modalities: 1) audio, 2) text, and 3) dual (a combination of text and audio). Two practice strategies were designed to help students practice what they had learned from the animated demonstrations: practice with program-controlled help, in which instructional hints were automatically provided, and practice with user-controlled help, in which no instructional hints were provided unless students seek assistance by clicking a help button. The dependent variables consisted of a retention test immediately after the training and a transfer test two days after the training. Participants were 123 undergraduate students from a large public research university in the southwest of the United States. They were randomly assigned to one of the six experimental treatment groups. The results of the experimental study revealed significant difference in the practice strategy factor. Students who practiced with user-controlled help had significantly better transfer performance than those who practiced with program-controlled help. However, there was no significant difference in the retention performance. No main effect was observed for the presentation modality factor and no significant interaction was detected between the two factors.Item The macro- and micro-instabilities in the pedestal region of the Tokamak(2015-05) Ma, Jingfei; Morrison, Philip J.; Horton, C. W. (Claude Wendell), 1942-; Berk, Herbert; Fitzpatrick, Richard; Hallock, GaryIn this paper, we present the theoretical and numerical studies of the linear characteristics and nonlinear transport features of the instabilities driven by the steep profile gradient and edge current in the pedestal region of the tokamak. Two important instabilities, the peeling-ballooning (P-B) modes (macro-instability) and the drift-Alfven modes (micro-instability), are studied using the fluid analysis and the BOUT++ codes. In particular, the edge-localized modes (ELMs), which appear to be the energy burst in the nonlinear stage of the peeling-ballooning mode, are numerically studied and the results are compared with the experimental measurement. In addition, the features of the impurity transport in the edge region of the tokamak are theoretically analyzed. Firstly, we explore the fundamental characteristics of the P-B modes and the ELM bursts numerically using the three-field reduced MHD model under the BOUT++ framework, in the shifted-circular geometry, i.e. the limiter tokamak geometry. In the linear simulations, the growth rate and real frequency and the mode structure versus the toroidal mode number (n) are shown. The features of the ELM bursts are shown in the nonlinear simulations, including the time evolution of the relative energy loss (ELM size) and the pedestal profile. Secondly, two original research projects related to the P-B modes and the ELM burst are described. One is the study of the scaling law between the relative energy loss of ELMs and the edge collisionality. We generate a sequence of shifted-circular equilibria with different edge collisionality varying over four orders of magnitude using EFIT. The simulation results are in good agreement with the multi-tokamak experimental data. Another is the study of the differences of the linear behaviors of the P-B modes between the standard and snowflake divertor configurations. Using DIII-D H-mode ElMing equilibria, we found that the differences are due to the local magnetic shear change at the outboard midplane, which is the result of the realization of the snowflake configuration. Finally, the micro-instability, the drift-Alfven instability in the pedestal region of the DIII-D tokamak is studied. A modified six-field Landau fluid model under BOUT++ framework is used to study the linear characteristics and transport features of the drift-Alfven modes. Based on the DIII-D H-mode discharge, a sequence of divertor tokamak equilibria with different pedestal height is generated by the ’VARYPED’ tool for our studies. Qualitative agreement is obtained between theoretical analysis and the simulation results in the linear regime. Moreover, the heat transport induced by the drift-Alfven turbulence is explored and the convection level is estimated for both ions and electrons.Item Monte-Carlo simulations of a comptonization model for the photospheric process(2015-08) Hernandez, Roberto Amilcar; Dicus, Duane A.; Kumar, PawanThis thesis presents the results of numerical simulations of an InverseCompton scattering model for the photospheric process. We use a Monte-Carlo method to simulate the processing and broadening of Planckian radiation below the Thomson photosphere of hot relativistic outflows. A new numerical code was developed and allowed us to explore a completely new region of the parameter space, in particular a higher and more realistic photon-to-electron ratio. The results may be relevant to the prompt emission of Gamma-ray Bursts (GRBs), Tidal Disruption Events (TDEs), and other high-energy transients where optically thick outflows are present.Item Physics-based material constitutive models for the simulation of high-temperature forming of magnesium alloy AZ31(2012-08) Carpenter, Alexander James; Taleff, Eric M.; Bourell, David L.; Kovar, Desiderio; Seepersad, Carolyn C.; Engelhardt, Michael D.Magnesium sheet alloys, such as wrought AZ31, have material properties that make them an attractive option for use in automotive and aircraft components. However, the low ductility of magnesium alloys at room temperature necessitates the use of high-temperature forming to manufacture complex components. Finite-element-method (FEM) simulations can assist in determining the optimum processing parameters for high-temperature forming, but only if an accurate material constitutive model is used. New material constitutive models describing the deformation behavior of AZ31 sheet at 450°C are proposed. These models account for both active deformation mechanisms at this temperature: grain-boundary-sliding creep and five-power dislocation-climb creep. Phenomena affecting these deformation mechanisms, such as material anisotropy and grain growth, are also investigated. This physics-based approach represents an improvement over previous material models, which require nonphysical parameters and can only predict forming for a limited range of conditions. Tensile tests are conducted to obtain data used in fitting constitutive models. New models are used in FEM simulations of both tensile tests and biaxial gas-pressure bulge tests. Simulation results are compared to experimental data for validation and determination of model accuracy.Item Prospects for directly detecting the first supernovae, and their impact on early star formation(2016-05) Hummel, Jacob Alexander; Bromm, Volker; Milosavljevic, Milos; Wheeler, J. Craig; Finkelstein, Steven; Yoshida, NaokiThe formation of the first stars in the Universe marked a pivotal moment in cosmic history, initiating the transition from the simple initial conditions of the big bang to the complex structures we see today. Ionizing radiation produced by these so-called Population III stars began the process of reionization, and the supernovae marking their deaths initiated the process of chemical enrichment. We assess the prospects for direct detection of the first supernovae should they happen to end their lives as extremely energetic pair-instability supernovae, which should be within the detection limits of the upcoming James Webb Space Telescope. Using a combination of semi-analytic models and cosmological simulations to estimate their source density, we find that the primary obstacle to observing such events is their scarcity, not their faintness. The first supernovae and the compact remnants they leave behind also produce significant amounts of high-energy X-rays and cosmic rays able to travel through the predominantly neutral intergalactic medium and build up a cosmic background. To better understand how these violent explosions impact subsequent episodes of metal-free star formation, we employ ab-initio, cosmological hydrodynamics simulations to model the formation of stars in a minihalo at z = 20-30 under the influence of both an X-ray and cosmic ray background. The presence of an ionizing background---whether X-rays or cosmic rays---serves to expedite the collapse of gas to high densities by enhancing molecular hydrogen cooling, thus allowing stars to form at substantially earlier epochs in strongly irradiated minihalos. The mass of the stars thus formed however appears to be quite robust, maintaining a characteristic mass of order a few tens of solar masses even as the strength of the ionizing background varies by several orders of magnitude. Finally, we describe the novel software developed to enable this research. These tools for manipulating and analyzing simulation data have been released as the open-source GAdget DataFrame Library: gadfly.Item Quantification of the confidence that can be placed in land-surface model predictions : applications to vegetation and hydrologic processes(2009-08) Gulden, Lindsey Elizabeth; Yang, Zong-liangThe research presented here informs the confidence that can be placed in the simulations of land-surface models (LSMs). After introducing a method for simplifying a complex, heterogeneous land-cover dataset for use in LSMs, I show that LSMs can realistically represent the spatial distribution of heterogeneous land-cover processes (e.g., biogenic emission of volatile organic compounds) in Texas. LSM-derived estimates of biogenic emissions are sensitive (varying up to a factor of 3) to land-cover data, which is not well constrained by observations. Simulated emissions are most sensitive to land-cover data in eastern and central Texas, where tropospheric ozone pollution is a concern. I further demonstrate that interannual variation in leaf mass is at least as important to variation in biogenic emissions as is interannual variation in shortwave radiation and temperature. Model estimates show that more-humid regions with less year-to-year variation in precipitation have lower year-to-year variation in biogenic emissions: as modeled mean emissions increase, their mean-normalized standard deviation decreases. I evaluate three parameterizations of subsurface hydrology in LSMs (with (1) a shallow, 10-layer soil; (2) a deeper, many-layered soil; and (3) a lumped aquifer model) under increasing parameter uncertainty. When given their optimal parameter sets, all three versions perform equivalently well when simulating monthly change in terrestrial water storage. The most conceptually realistic model is least sensitive to errant parameter values. However, even when using the most conceptually realistic model, parameter interaction ensures that knowing ranges for individual parameters is insufficient to guarantee realistic simulation. LSMs are often developed and evaluated at data-rich sites but are then applied in regions where data are sparse or unavailable. I present a framework for model evaluation that explicitly acknowledges perennial sources of uncertainty in LSM simulations (e.g., parameter uncertainty, meteorological forcing-data uncertainty, evaluation-data uncertainty) and that evaluates LSMs in a way that is consistent with models’ typical application. The model performance score quantifies the likelihood that a representative ensemble of model performance will bracket observations with high skill and low spread. The robustness score quantifies the sensitivity of model performance to parameter error or data error. The fitness score ranks models’ suitability for broad application.Item A simulation approach to studying the relationship between landscape features and social system on the genetic structure of a tamarin primate population(2013-05) Valencia Rodriguez, Lina Maria; Di Fiore, Anthony, Ph. D.Landscape genetics is an emerging field that seeks to understand how specific landscape features and microevolutionary processes such as gene flow, genetic drift, and selection interact to shape the amount and spatial distribution of genetic variation. This study explores, through agent based simulations, how the specific mating and social system of tamarin primates (genus Saguinus) influences population genetic structure and patterns of relatedness within and among groups of this primate species, which might affect the ability of landscape genetic studies to detect the effects of fragmentation on gene flow. I use a spatially-explicit agent-based population genetics simulation model (GENESYS) configured to reflect the particular social system of tamarin monkeys (i.e. small group size, limited numbers of breeders per group, frequent twin births, and short dispersal distances) to assess whether the isolation by distance model of genetic differentiation expected in an unfragmented landscape can be distinguished from the isolation by barrier model expected in a fragmented landscape. GENESYS allows a user to explore the effects of social structure and landscape features on the population genetic structure of social animals, such as primates. I simulated two different landscapes containing an otherwise equivalent population of tamarins. In the first setup I simulated a homogeneous landscape unconstrained by any barriers to gene flow, while for the second setup, a barrier to gene flow restricted dispersal from one half of the landscape to the other. I found that the particular mating system of tamarin results in the rapid genetic differentiation of its social groups and consequently its populations. Social groups in the continuous landscape indeed revealed an isolation by distance pattern, while social groups on the fragmented landscape yielded instead an isolation by barrier model, where the barrier rather than geographic distance per se influenced the spatial genetic structure of the population. The results from this study suggest that features of the tamarin social system influence population genetic structure, which could affect the ability of landscape genetic studies to detect the effects of fragmentation on gene flow. To more fully address that issue, future studies should focus on a range of different primate social systems.