Fresno City College

This work presents Stage I of a multi-phase research effort focused on the analytical and computational development of ceramic-matrix composite (CMC) rocket nozzle components, with consideration given to feasible manufacturing approaches for future prototype fabrication. High-purity Alumina 4N, Calcined Alumina, and Nextel™ 720 or 610 fibers were evaluated as candidate materials for high-temperature propulsion environments, with additive manufacturing and powder-sintered composite processing identified as viable fabrication pathways for subsequent project phases. A combined analytical framework incorporating micromechanics-based lamina modeling, Classical Laminate Theory, and spatially varying pressure and thermal loading derived from quasi-one-dimensional compressible-flow analysis was developed to predict laminate stresses and strain margins along the nozzle contour. Ply-level composite behavior was evaluated using ANSYS ACP structural simulations, while a steady-state thermal model employing a homogenized orthotropic composite representation was used to resolve temperature gradients and thermally induced deformation. Together, these numerical analyses define critical loading regions, quantify structural safety margins, and establish preliminary material selection, geometry, and thermo-mechanical design requirements that will guide future laminate fabrication efforts and experimental hot-fire testing of CMC nozzle prototypes.

Ancient Greek accent involves both stress and tone (Allen 1973, Steriade 1988). We present here a new piece of evidence for the details of the tonal part, based on a lowering process whereby a H tone on the final TBU of a word lowers if the word is followed by another tonic word: H...H → L...H. This is essentially what is reported for Rimi, a Tanzanian Bantu language, where HH loses the first of its H tones (Olson 1964, ‘Anti-Meeussen’s Rule’). If correct, this motivates three tonal classes in Ancient Greek: HL* in words like basiléˈàà ‘king.acc’ (Sauzet 1989, Golston 1990) and two additional classes, H*L in basiˈléù ‘king.voc’ and H in basiˈleús ‘king.nom’.

Hybrid zones provide valuable opportunities to understand the genomic mechanisms that promote speciation by providing insight into factors involved in intermediate stages of speciation. Here, we investigate introgression in a hybrid zone between two rattlesnake species (Crotalus viridis and Crotalus oreganus concolor) that have undergone historical allopatric divergence and recent range expansion and secondary contact. We use Bayesian genomic cline models to characterize genomic patterns of introgression between these lineages and identify loci potentially subject to selection in hybrids. We find evidence for a large number of genomic regions with biased ancestry that deviate from the genomic background in hybrids (i.e., excess ancestry loci), which tend to be associated with genomic regions with higher recombination rates. We also identify suites of excess ancestry loci that show highly correlated allele frequencies (including conspecific and heterospecific combinations) across physically unlinked genomic regions in hybrids. Our findings provide evidence for multiple multilocus evolutionary processes impacting hybrid fitness in this system.

The origin of snake venom involved duplication and recruitment of non-venom genes into venom systems. Several studies have predicted that directional positive selection has governed this process. Venom composition varies substantially across snake species and venom phenotypes are locally adapted to prey, leading to coevolutionary interactions between predator and prey. Venom origins and contemporary snake venom evolution may therefore be driven by fundamentally different selection regimes, yet investigations of population-level patterns of selection have been limited. Here, we use whole-genome data from 68 rattlesnakes to test hypotheses about the factors that drive genomic diversity and differentiation in major venom gene regions. We show that selection has resulted in long-term maintenance of genetic diversity within and between species in multiple venom gene families. Our findings are inconsistent with a dominant role of directional positive selection and instead support a role of long-term balancing selection in shaping venom evolution. We also detect rapid decay of linkage disequilibrium due to high recombination rates in venom regions, suggesting that venom genes have reduced selective interference with nearby loci, including other venom paralogues. Our results provide an example of long-term balancing selection that drives trans-species polymorphism and help to explain how snake venom keeps pace with prey resistance.

This dissertation analyzes the differences among homeless rates in urban and suburban “continuums of care” (service areas for homelessness in the United States) over the period of 2014-2018. The purpose is to determine which variables are useful to predict the rates of two definitions of homelessness: the more extreme “Category One” homelessness as defined by the U.S. Department of Housing and Urban Development (HUD): those unsheltered or living in homeless shelters; and the broader Department of Education definition of homelessness: families with children that are homeless, including those in Category One but also those living in hotels, staying temporarily with other families, or in other inadequate housing that is not their own. Comparing these two forms of homelessness helps to provide insight into the overall spectrum of homelessness in U.S. cities. This study finds that housing affordability is a significant predictor of both Category One and school-reported homelessness. A comparison of the data for both forms of homelessness indicates that less affordable communities tend to have higher ratios of Category One homelessness compared to school-reported homelessness. The model for Category One homelessness also suggests that continuums of care networks have lower rates of homelessness when they devote a greater share of resources to rapid rehousing programs. The findings of this study do not support the popular belief that the homeless tend to migrate to areas that are warmer or have better homeless services.

We show that a massless scalar field in a gravitational wave background can develop a non-zero vacuum expectation value. We draw comparisons to the generation of a non-zero vacuum expectation value for a scalar field in the Higgs mechanism and with the dynamical Casimir vacuum. We propose that this vacuum expectation value, generated by a gravitational wave, can be connected with particle production from gravitational waves and may have consequences for the early Universe where scalar fields are thought to play an important role.

The tardigrade class Mesotardigrada was erected on the basis of the description of Thermozodium esakii by Gilbert Rahm in 1937. In some characteristics, T. esakii is intermediate between members of the classes Eutardigrada and Heterotardigrada. The class Mesotardigrada is known only from Rahm's published drawings of T. esakii; no voucher specimens are known, and subsequent attempts to collect it at the locus typicus have been unsuccessful. Among the possible explanations for this situation are that Rahm may have collected specimens of a more typical tardigrade, but misinterpreted what he saw. Alternatively, changes in habitat in the area may have led to the tardigrade's extirpation. Perhaps T. esakii is a rare species, such that recent sampling efforts have been insufficient to rediscover it. Finally, Rahm's 1937 description may be an attempt at deception. Until physical evidence of T. esakii is found, the species, and by extension the class Mesotardigrada, should be considered nomen dubium.