Thesis (Ph. D.)--University of Hong Kong, 1980. / Typescript.

Thesis (M.S. in Electrical Engineering)--Naval Postgraduate School, June 2003. / Thesis advisor(s): Robert W. Ashton, John G. Ciezki, Douglas J. Fouts. Includes bibliographical references (p. 125-126). Also available online.

Snow glide is the translational slip of the entire snow pack over a sloping ground surface. It is thought that rapid rates of snow glide precede the release of full-depth avalanches. The nature of avalanches that release at the ground makes them difficult to predict and difficult to control using explosives. The aim of this research is to determine the relationship between rapid snow glide and full-depth avalanche occurrence and to examine climate factors affecting both processes. Data collected from an instrumented site along the Coquihalla Highway in the Cascade Mountains of British Columbia were used for analysis during two winter seasons (1992-93, 1993-94). Glide is influenced by the nature of the interaction between the roughness of the ground and the snow pack, and by the distribution of water at the interface. The presence of water at the interface affects the material properties of snow and the friction conditions. The impact of freewater on glide is influenced by the volume and rates of water input. Higher glide rates and fulldepth avalanche release are the almost immediate responses to contributions of free-water. The data show that the most significant contributor is rainfall, which is common in the study area throughout the winter season. The supply of free-water from snow melt due to radiative and thermal sources of energy become more significant in the spring. Water inputs increase the thickness of the saturated layer at the base of the snow pack, allowing greater amplitudes of roughness to be overcome. By drowning or partially drowning the roughness elements, a thin film of water reduces the shear resistance of the snow pack to downslope movement. Inputs of water at rates higher than transmissions rates will increase pore pressures and decrease shear stress encouraging further downslope movement. Failure of the snow pack at the ground is translational, most often occurring 12-24 hours after a rainfall event, but sometimes much later when avalanche release would not be expected. Although there is no threshold glide velocity associated with avalanche release, it can be concluded that snow glide is a good indicator of active periods of full-depth avalanche occurrence. However, results from this study show that rainfall rates and snow melt rates may be more accurate predictors of avalanche occurrence in the study area.

Terahertz (THz) radiation has become increasingly important in many scientific and commercial fields in recent years. It possesses many remarkable features resulting in an increased use of THz radiation for various applications, like biomedical imaging, security screening, and industrial quality control. The capability of these applications depends directly on the availability of powerful THz sources and high-responsivity, fast THz detectors. Current commercial products used to detect THz radiation, like Golay cells and pyroelectric detectors, have only slow detection rates and poor sensitivities. Other commercial THz detectors, like bolometers, are more sensitive but require liquid helium cooling. In this thesis, two types of room-temperature high-responsivity graphene-based THz detectors are presented, relying on the unique properties of graphene and the function of plasmonic antenna arrays which boost the interaction between THz waves and graphene. Graphene has been demonstrated as a promising material for THz detection. However, the challenge is its insufficient light absorption that largely limits the responsivity. The first design is based on an array of planar antennas arranged in series and shorted by graphene squares. Highly efficient photodetection can be achieved by using the metallic antenna to simultaneously improve both light absorption, as resonant elements, and photocarrier collection, as electrodes. The device has been characterized with quantum cascade lasers, yielding a maximum responsivity of ~2 mA/W at 2 THz. The second detector is based on an array of interdigitated bow-tie antennas connected in parallel and shunted by graphene squares. The arms of the bow-tie antennas were made of two metals with different work functions to create a built-in electric field and improve the responsivity. The device has been characterized and yields a maximum responsivity of ∼34 μA/W at 2 THz. Efficient THz imaging is presented by integrating the detector in a QCL-based THz imaging system.

Snow glide is the translational slip of the entire snow pack over a sloping ground surface. It is thought that rapid rates of snow glide precede the release of full-depth avalanches. The nature of avalanches that release at the ground makes them difficult to predict and difficult to control using explosives. The aim of this research is to determine the relationship between rapid snow glide and full-depth avalanche occurrence and to examine climate factors affecting both processes. Data collected from an instrumented site along the Coquihalla Highway in the Cascade Mountains of British Columbia were used for analysis during two winter seasons (1992-93, 1993-94). Glide is influenced by the nature of the interaction between the roughness of the ground and the snow pack, and by the distribution of water at the interface. The presence of water at the interface affects the material properties of snow and the friction conditions. The impact of freewater on glide is influenced by the volume and rates of water input. Higher glide rates and fulldepth avalanche release are the almost immediate responses to contributions of free-water. The data show that the most significant contributor is rainfall, which is common in the study area throughout the winter season. The supply of free-water from snow melt due to radiative and thermal sources of energy become more significant in the spring. Water inputs increase the thickness of the saturated layer at the base of the snow pack, allowing greater amplitudes of roughness to be overcome. By drowning or partially drowning the roughness elements, a thin film of water reduces the shear resistance of the snow pack to downslope movement. Inputs of water at rates higher than transmissions rates will increase pore pressures and decrease shear stress encouraging further downslope movement. Failure of the snow pack at the ground is translational, most often occurring 12-24 hours after a rainfall event, but sometimes much later when avalanche release would not be expected. Although there is no threshold glide velocity associated with avalanche release, it can be concluded that snow glide is a good indicator of active periods of full-depth avalanche occurrence. However, results from this study show that rainfall rates and snow melt rates may be more accurate predictors of avalanche occurrence in the study area. / Arts, Faculty of / Geography, Department of / Graduate

Measurements of the turbulent flow field through a highly staggered cascade propulsor configuration in the Virginia Tech cascade wind tunnel have been taken. Predictions of the same flow using Rapid Distortion Theory (RDT) were performed. Measurements and predictions were compared. The comparison was oriented to check the aptness of RDT in describing this kind of flow. Since this study represents the initial steps of a major project, the RDT model was kept simple. The non-penetration condition (blade blocking effect) was not modeled and the viscous effects were roughly accounted for. This work reveals the capabilities of RDT in predicting the development of turbulence convected through a highly staggered cascade propulsor configuration formed by non-symmetrical airfoils. This present study was possible thank to the support from the Office of Naval Research, in particular Candace Wark and Pat Purtell, under grant number NAG 00014-99-1-0230. / Master of Science

Thesis advisor: Xiao-Xiang Zhang / Thesis advisor: James Morken / This dissertation will present three projects focusing on the development of stereoselective radical cascade reactions via metalloradical catalysis (MRC) using Co(II) D2-symmetric chiral amidoporphyrins [Co(D2-Por*)] as the catalyst. The first project demonstrated the feasibility of applying MRC for asymmetric radical cascade processes by achieving an enantioselective radical bicyclization of 1,6-enynes with diazo compounds, which constructed multi-substituted cyclopropane-fused tetrahydrofurans bearing three contiguous stereogenic centers and one trisubstituted alkene. Detailed mechanistic studies including EPR studies and DFT calculation unveiled a radical-based stepwise mechanism. The synthetic utility of this reaction was demonstrated by a series of diastereoselective transformations of the bicyclic products. In the second project, this strategy was expanded to the application of Co(II)-based MRC to catalyze radical cascade reactions involving hydrogen-atom abstraction (HAA) process. A broad array of homopropargyl ethers reacted with diazo compounds to generate enantiomerically enriched ɑ,β-disubstituted tetrahydrofurans in good yields with high diastereoselectivities and enantioselectivities. The third project explored the utilization of the established strategy to accomplish more challenging bicyclization of 1,6,8-dienynes for the construction of cycloheptadiene-fused tetrahydrofurans in regio- and diastereoselective fashions. Such 5,7-fused ring system has been widely found in natural products and bioactive species. / Thesis (PhD) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

This paper investigates the effect of blowing ratio on the film cooling performance of a showerhead film cooled first stage turbine blade. The blade was instrumented with double-sided thin film heat flux gages to experimentally characterize the Nusselt number and film cooling effectiveness distributions over the surface of the blade. The blade was arranged in a two-dimensional, linear cascade within a transonic, blowdown type wind tunnel. The wind tunnel freestream conditions were varied over two exit Mach numbers, Me=0.78 and Me=1.01, with an inlet freestream turbulence intensity of 12% , with an integral length scale normalized by blade chord of 0.26 generated by a passive, mesh turbulence grid. The coolant conditions were varied by changing the ratio of coolant to freestream mass flux, blowing ratio, over three values, BR=0.60, 1.0, and 1.5 while keeping a density ratio of 1.7. Experimental results show that ingestion of freestream flow into the showerhead cooling plenum can occur below a blowing ratio of 0.6. Film cooling increases Nusselt number over the uncooled case and increasing the blowing ratio also increases Nusselt number. At a blowing ratio of 1.5 and Me=1.01 a large drop in effectiveness just downstream of injection on both the pressure and suction surfaces is evidence of jet liftoff. The blowing ratio of 1.0 was found to have superior heat load reduction over the blade surface at both freestream conditions tested. The blowing ratio of 1.0 reduced the heat load by as much as 39% and 32% at Me=0.78 and 1.01, respectively. / Master of Science

Typescript. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1986. / Bibliography: leaves 361-395. / Photocopy. / xviii, 395 leaves, bound ill. 29 cm

Montane meadows comprise less than 5% of the landscape of the western Cascades of Oregon, but they provide habitat for diverse species of plants and pollinators. Little is known about plant-pollinator network structure at these sites. This study quantified plant-pollinator interactions over the summer of 2011, based on six observations of 10 permanent subplots in 15 meadows, stratified by size and isolation. The study examined (1) relationships between richness and abundance of flowers, pollinators, and interactions; (2) distribution of abundance and richness of flowers, pollinators, and interactions with regards to surrounding meadow habitat; (3) change in flower and pollinator abundance over the season; (4) factors associated with the presence of various guilds of pollinators; and (5) the structure of plant-pollinator networks. The study showed that (1) richness of pollinators increased 2x faster than richness of flowers with increased abundance; (2) density of flowers and interactions was positively correlated with meadow size and diversity of pollinators and interactions were both correlated with surrounding habitat at two spatial scales; (3) peak flower abundance coincided with or preceded peaks in pollinator populations; (4) abundance of three guilds of bees exhibited different patterns of association to surrounding habitat and meadow soil moisture corresponding to various dispersal potential and phenology of guild species; and (5) the number of network pairings for plants and pollinators increased with increasing species richness of potential interaction partners and all networks were found to be significantly nested. Results of this study indicate that plant-pollinator networks are complex assemblages of species, in which spatial and temporal patterns of habitat affect species composition and network structure. In particular, flower and pollinator abundance and richness are depressed in small and isolated meadows. Significant nestedness emerged as a pattern of network level organization across the study meadows. / Graduation date: 2013