Assessing the Roles of Fire Frequency and Precipitation in Determining Woody Plant Expansion in Central U.S. Grasslands

Woody plant expansion into grasslands and savannas is occurring and accelerating worldwide and often impacts ecosystem processes. Understanding and predicting the environmental and ecological impacts of encroachment has led to a variety of methodologies for assessing its onset, transition, and stability, generally relying on dynamical systems approaches. Here we continue this general line of investigation to facilitate the understanding of the roles of precipitation frequency and intensity and fire frequency on the conversion of grasslands to woody‐dominated systems focusing on the central United States. A low‐dimensional model with stochastic precipitation and fire disturbance is introduced to examine the complex interactions between precipitation and fire as mechanisms that may suppress or facilitate increases in woody cover. By using Lyapunov exponents, we are able to ascertain the relative control exerted on woody encroachment through these mechanisms. Our results indicate that precipitation frequency is a more important control on woody encroachment than the intensity of individual precipitation events. Fire, however, exerts a much more dominant impact on the limitation of encroachment over the range of precipitation variability considered here. These results indicate that fire management may be an effective strategy to slow the onset of woody species into grasslands. While climate change might predict a reduced potential for woody encroachment in the near future, these results indicate a reduction in woody fraction may be unlikely when considering anthropogenic fire suppression

Numerical Studies of Nuclear Containment Spray Process by Stochastic Field Method and CGCFD approach

Die Kernenergie ist eine der grundlegenden Technologien zur Deckung des weltweiten Stromversorgungsbedarfs. Aufgrund des geringen CO2-Ausstoßes, welcher durch Kernenergie verursacht wird, stellt die Kernenergie in den Augen der Befürworter eine der bedeutendsten Optionen im neuen Jahrhundert dar. Die nukleare Sicherheit ist das entscheidende Thema beim Betrieb von Kernreaktoren und bei der (Weiter-) Entwicklung von Reaktorkonzepten, da die Zerfallsprodukte der Kernspaltungsreaktion eine hohe Radiotoxizität aufweisen. Unter allen Fragestellungen der nuklearen Sicherheitsforschung bleibt die Vorhersagegenauigkeit von nuklearen Sicherheitsanalysen eines der wichtigsten Themen. In den letzten 60 Jahren ereigneten sich drei schwere nukleare Unfälle, die enormen Schaden verursacht haben. Betroffen war nicht nur das Leben einzelner, sondern der Zusammenhalt der ganzen Gesellschaft. Bei der jüngsten nuklearen Katastrophe von Fukushima Daiichi wird fest davon ausgegangen, dass die im Sicherheitsbehälter aufgetretene Wasserstoffexplosion der ursächliche Auslöser für die schlussendliche Freisetzung von Radioaktivität an die Umgebung war. Im Falle schwerer Störfälle stellt das nukleare Containment die letzte Schutzbarriere des Kernkraftwerks gegenüber der Umgebung dar. Dieses Containment ist einer lang anhaltenden Erwärmung und hohen Drücken ausgesetzt. Um zu verhindern, dass der Sicherheitsbehälter während eines nuklearen Unfalls versagt, werden Sprühsysteme eingesetzt, welche die Atmosphäre im Sicherheitsbehälter kühlen. Aus diesem Grund betrachtet diese vorgelegte Dissertationsschrift den Themenkomplex der Sprühkühlungsmodellierung zur Eindämmung der Folgen schwerer nuklearer Störfälle. Bei der Analyse der Vorgänge während eines Störfalls stellt schon alleine die ausgedehnte räumliche Dimension eines Kernkraftwerks und dessen Containments den Wissenschaftler vor Herausforderungen. Traditionell wird auf einen experimentellen Ansatz zurückgegriffen, wenn Sprühprozesse im Containment untersucht werden. Allerdings treiben die hohen Kosten, für den Bau geeigneter Versuchsanlagen und die Durchführung von Messkampagnen, die Projektkosten von Kernkraftwerksneubauprojekten erheblich in die Höhe. Andererseits, mit der stetig wachsenden Leistungsfähigkeit von Computern, wird die numerische Berechnung mittels der Computational Fluid Dynamics (CFD) im Konstruktionsprozess immer beliebter. Das Fehlen hinreichend zuverlässiger physikalischer Sprühstrahlmodelle und die nach wie vor unbefriedigende Effizienz bei der Vorhersage von großräumigen Problemen, schränken jedoch die Anwendung von CFD in Sprühkühlungsstudien zur Begrenzung der Unfallfolgen bei schweren nuklearen Störfällen ein. In dieser Arbeit wird die Analyse von Spühstrahlen im Containment eines Kernreaktors unter besonderer Berücksichtigung zweier Aspekte betrachtet: Zuverlässigkeit und Geschwindigkeit. Für die Mission „Zuverlässigkeit“ wird ein geeigneter Modellierungsansatz für Sprühnebel, welcher auch auf bei räumlich ausgedehnten Problemen anwendbar ist, abgeleitet. Darüber hinaus wird diskutiert wie die Turbulenz-Tröpfchen-Wechselwirkungen beschrieben werden kann. Der Lagrange-Ansatz, der Mischungsansatz und der Euler-Ansatz zur Modellierung der diskreten Flüssigkeitströpfchen werden auf drei verschiedene Arten diskutiert und verglichen, um eine geeignete Modellwahl zu treffen. Die Modellierung von Austauschtermen für Masse-, Impuls- und Energie wird vorgestellt und diskutiert. Eine geeignete thermophysikalische Modellierung, einschließlich eines neuen Gasfilm-basierten Verdampfungsmodells, wird vorgestellt. Nachdem der geeignetste Modellierungsansatz für den Sprühprozess abgeleitet wurde, wird der Umgang mit den nichtlinearen Eigenschaften des turbulenten Transports von Sprühtröpfchen weiter diskutiert. Die PDF-Methode wird detailliert vorgestellt. Für großskalige Sprühkühlungsprobleme wird die Anwendung einer Euler‘schen Realisierung, der stochastischen Feldmethode, ausführlich diskutiert. Dabei gilt der Handhabung von Stoff- und Wärmeübertragung ein besonderes Augenmerk. Für die Geschwindigkeitsmission wird eine kürzlich entwickelte Coarse Grid CFD-Methode vorgestellt und diskutiert. Mit dieser lassen sich Probleme effizient vorhersagen, indem vorausgehende detaillierte Simulationsdaten gut genutzt werden, um insbesondere Grobstrukturmodelle für die Flussterme in den Gleichungen abzuleiten. Eine neue physikalisch basierte Flusskorrektur wird eingeführt und mit dem ursprünglichen geometrischen Flusskorrekturansatz verglichen. Einige Techniken zur Behandlung singulärer Probleme in der Coarse Grid CFD-Simulation werden ebenfalls diskutiert. Im Fallstudienteil wird zunächst der THAI-Benchmark zur Validierung des Sprühmodellierungsansatzes verwendet. Zusätzlich wird ein Trocknergehäuse mit ähnlichen geometrischen und physikalischen Eigenschaften betrachtet. Bewertet wird die stochastische Feldmethode zur Berücksichtigung turbulenter Wechselwirkungen mit Sprühtröpfchen. Zuletzt wird die Coarse Grid CFD mit einem physikalisch basierten Flusskorrekturansatz validiert. Betrachtet wird als kerntechnische Anwendung ein prototypisches Brennelementbündel mit Drahtwendelabstandshalter. Im Ausblick wird der Trocknerfall erneut untersucht, um die Kombination aller oben diskutierter Ansätze für die Sprühstrahlsimulation bei typischen Betriebsparametern zu testen. Zusammenfassend kann festgestellt werden, dass die Arbeit einerseits die Praktikabilität eines Euler‘schen Ansatzes in Form der Stochastischen Feldmethode zur Bereitstellung zuverlässiger Simulationsergebnisse demonstriert und andererseits die Eignung der Coarse Grid CFD für schnelle Vorhersagen bei großen Problemen zeigt

On the Nature and Shape of Tubulin Trails: Implications on Microtubule Self-Organization

Microtubules, major elements of the cell skeleton are, most of the time, well organized in vivo, but they can also show self-organizing behaviors in time and/or space in purified solutions in vitro. Theoretical studies and models based on the concepts of collective dynamics in complex systems, reaction-diffusion processes and emergent phenomena were proposed to explain some of these behaviors. In the particular case of microtubule spatial self-organization, it has been advanced that microtubules could behave like ants, self-organizing by 'talking to each other' by way of hypothetic (because never observed) concentrated chemical trails of tubulin that are expected to be released by their disassembling ends. Deterministic models based on this idea yielded indeed like-looking spatio-temporal self-organizing behaviors. Nevertheless the question remains of whether microscopic tubulin trails produced by individual or bundles of several microtubules are intense enough to allow microtubule self-organization at a macroscopic level. In the present work, by simulating the diffusion of tubulin in microtubule solutions at the microscopic scale, we measure the shape and intensity of tubulin trails and discuss about the assumption of microtubule self-organization due to the production of chemical trails by disassembling microtubules. We show that the tubulin trails produced by individual microtubules or small microtubule arrays are very weak and not elongated even at very high reactive rates. Although the variations of concentration due to such trails are not significant compared to natural fluctuations of the concentration of tubuline in the chemical environment, the study shows that heterogeneities of biochemical composition can form due to microtubule disassembly. They could become significant when produced by numerous microtubule ends located in the same place. Their possible formation could play a role in certain conditions of reaction. In particular, it gives a mesoscopic basis to explain the collective dynamics observed in excitable microtubule solutions showing the propagation of concentration waves of microtubules at the millimeter scale, although we doubt that individual microtubules or bundles can behave like molecular ants

Analysis of multi-phase systems relevant to bioengineering and materials science

In the thesis the focus is on the analysis of two multiphase systems related to biomedical engineering and materials science. The first part of the thesis is related to the long-term preservation of human Mesenchymal Stem Cells (hMSCs) from Umbilical Cord Blood (UCB) by means of cryopreservation, i.e. freezing the bio-specimens to cryogenic temperature, and drypreservation, i.e. air-drying at room temperature and atmospheric pressure. First, the peculiar osmotic behavior of hMSCs from UCB is analyzed by developing a novel mathematical model. Then, the addition and removal phases of a permeant Cryo-Protectant Agent (CPA) like dymethil-sulfoxide (DMSO) during a cryopreservation protocol is studied taking into account both the cytotoxic effect and osmotic injury (i.e. expansion lysis). Finally, a mathematical model is proposed to guide and support the development of a long-term preservation of hMSCs through air-drying. The second part of the thesis is related to the modeling of mechanical processing of powders by Ball Milling (BM). First the propagation of mechanically activated self-sustaining reactions during BM is investigated by numerical simulations. Then, a statistical description of the kinetics of processes activated by BM coupled with the mathematical description of advection cycles induced by plastic deformation is developed to model the refinement of the lamellar structure in composite Ag-Cu particles during the early stages of mechanical processing. Finally, considering the intrinsic statistical nature of this mechanical process, a kinetic model that combines a phenomenological description of the rheological behavior of molecular solids with the chemistry of interface reactions is developed

NASA/ASEE Summer Faculty Fellowship Program, 1990, Volume 1

The 1990 Johnson Space Center (JSC) NASA/American Society for Engineering Education (ASEE) Summer Faculty Fellowship Program was conducted by the University of Houston-University Park and JSC. A compilation of the final reports on the research projects are presented. The topics covered include: the Space Station; the Space Shuttle; exobiology; cell biology; culture techniques; control systems design; laser induced fluorescence; spacecraft reliability analysis; reduced gravity; biotechnology; microgravity applications; regenerative life support systems; imaging techniques; cardiovascular system; physiological effects; extravehicular mobility units; mathematical models; bioreactors; computerized simulation; microgravity simulation; and dynamic structural analysis

Modern Climatology - Full Text

Climatology, the study of climate, is no longer regarded as a single discipline that treats climate as something that fluctuates only within the unchanging boundaries described by historical statistics. The field has recognized that climate is something that changes continually under the influence of physical and biological forces and so, cannot be understood in isolation but rather, is one that includes diverse scientific disciplines that play their role in understanding a highly complex coupled “whole system” that is the Earth’s climate. The modern era of climatology is echoed in this book. On the one hand it offers a broad synoptic perspective but also considers the regional standpoint as it is this that affects what people need from climatology, albeit water resource managers or engineers etc. Aspects on the topic of climate change – what is often considered a contradiction in terms – is also addressed. It is all too evident these days that what recent work in climatology has revealed carries profound implications for economic and social policy; it is with these in mind that the final chapters consider acumens as to the application of what has been learned to date. This book is divided into four sections that cover sub-disciplines in climatology. The first section contains four chapters that pertain to synoptic climatology, i.e., the study of weather disturbances including hurricanes, monsoon depressions, synoptic waves, and severe thunderstorms; these weather systems directly impact humanity. The second section on regional climatology has four chapters that describe the climate features within physiographically defined areas. The third section is on climate change which involves both past (paleoclimate) and future climate: The first two chapters cover certain facets of paleoclimate while the third is centered towards the signals (observed or otherwise) of climate change. The fourth and final section broaches the sub-discipline that is often referred to as applied climatology; this represents the important goal of all studies in climatology–one that affects modes of living. Here, three chapters are devoted towards the application of climatological research that might have useful application for operational purposes in industrial, manufacturing, agricultural, technological and environmental affairs. Please click here to explore the components of this work.https://digitalcommons.usu.edu/modern_climatology/1014/thumbnail.jp

Metapopulation Modelling and Spatial Analysis for HEG Technology in the Control of Malaria

The success of any vector control strategy can be enhanced by onsite analysis and investigation. Combatting malaria, a global disease carried by the vector Anopheles gambiae, has led to the development of novel genetic technologies such as the use of HEG; homing endonuclease genes. This thesis explored the age and stage elements of the vector, building upon current biological understanding and using fitting algorithms with metapopulation matrices to create cohort orientated survival and transition. The environmental forces were analysed alongside this with emphasis on sub-model creation and tool design, employing an array of methods from RBF to satellite classification to couple the local environment and vector. When added, the four potential genetic strategies all demonstrated the ability to suppress a wild type population and even eradicate it, although reinvasion and hotspot population phenomena were reoccurring observations. The movement of the vector was an important factor in control efficiency, which was investigated as a series of different assumptions using wind driven movement and host attraction. Lastly, practical factors such as monitoring and resource distribution within a control project were assessed, which required routing solutions and landscape trapping assessments. This was explored within a framework of Mark-Release-Recapture experiment design that could provide critical information for efficient HEG release strategies.Open Acces

Lagrangian studies, circulation and mixing in the Southern Ocean

Oceans play a vital role as one of the major components of Earth's climate system. The study of oceanic processes and the complexity inherent in dynamic ows is essential for understanding their regulatory character on the climate's variability. A key region for the study of such intrinsic oceanic variability is the Southern Ocean. In the form of a wind-driven, zonally unbounded, strong eastward ow, the Antarctic Circumpolar Current (ACC) circumnavigates the Antarctic continent connecting each of the ocean basins. The dynamics of the ACC, which is characterised by the absence of land barriers, apart from when crossing Drake Passage, have long been a topic of debate [Rintoul et al., 2001]. The main interests of this study focus on inferring and mapping the dynamic variability the ACC exhibits by means of transient disturbances [Hughes, 2005] (such as mesoscale eddies) and subsequent mixing from Lagrangian trajectories. The distribution of eddy transport and intensity, the mixing of conservative quantities and ow dynamics through to the interaction of eddy kinetic energy, mean ow and topography are examined. The sparseness of observations in the Southern Ocean and the necessity to understand the role of the oceanic circulation in the climate by a holistic approach highlights computational ocean circulation models as indispensable. In the context of this study, output from the run401 of the Ocean Circulation and Climate Advance Model (OCCAM) 1/12� ocean model, developed at the U.K. National Oceanography Centre, is utilised. In order to deduce the temporal and spatial variability of the ow dynamics, as well as its vertical distribution, simulation of monthly releases of passive particles using di�erent schemes (i.e. cluster or linear alignment) on isobaric and isoneutral surfaces was conducted. An analysis of the Lagrangian trajectories reveals the characteristics of the dynamics that control the ow and depict regions of enhanced eddy activity and mixing. The model's ability to simulate real oceanic ows is established through comparison with a purposeful release of the tracer CF3SF5, which is conducted as part of the DIMES experiment (http://dimes.ucsd.edu/). We �nd that topography plays a fundamental role in the context of Southern Ocean mixing through the association of high EKE regions, where the interaction of vortical elements and multi �lamented jets in non-parallel ows supports an e�ective mechanism for eddy stirring, resulting in the enhanced dispersion of particles. Suppression of mixingin regions where the ow is delineated by intensi�ed and coherent, both in space and time, jets (strong PV gradients) signifying the separation of the ow in di�erentiated kinematic environments, is illustrated. The importance of a local approximation to mixing instead of the construction of zonal averages is presented. We present the caveats of classical di�usion theory in the presence of persistent structures and �nd that values of 1000-2000 m2

Proceedings Of The 18th Annual Meeting Of The Asia Oceania Geosciences Society (Aogs 2021)

The 18th Annual Meeting of the Asia Oceania Geosciences Society (AOGS 2021) was held from 1st to 6th August 2021. This proceedings volume includes selected extended abstracts from a challenging array of presentations at this conference. The AOGS Annual Meeting is a leading venue for professional interaction among researchers and practitioners, covering diverse disciplines of geosciences