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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenFri, 12 Apr 2024 15:23:03 +0000Investigations into Electronic Stopping Regime Sputtering of Uranium Tetrafluoride
https://resolver.caltech.edu/CaltechTHESIS:10282015-112437158
Authors: {'items': [{'id': 'Meins-Charles-Kenneth-Jr', 'name': {'family': 'Meins', 'given': 'Charles Kenneth, Jr.'}}]}
Year: 1982
DOI: 10.7907/vsb2-2g34
<p>Yields were measured for <sup>235</sup>U sputtered from UF<sub>4</sub> by <sup>16</sup>O, <sup>19</sup>F, and <sup>35</sup>Cl over the energy range ~.12 to 1.5 MeV/ amu sing a charge equilibrated beam in the stripped beam arrangement for all the incident ions and in the transmission arrangement for <sup>19</sup>F and <sup>35</sup>Cl. In addition, yields were measured for <sup>19</sup>F incident in a wide range of discrete charge states. The angular dependence of all the measured yields were consistent with cosʋ. The stripped beam and transmission data were well fit by the form (Az<sup>2</sup>eqln(BƐ)/Ɛ)<sup>4</sup> (where Ɛ was the ion energy in MeV/amu and z<sub>eq</sub>(Ɛ) was taken from Zeigler (80). The fitted values of B for the various sets of data were consistent with a constant B<sub>0</sub>, equal to 36.3 ± 2.7, independent of incident ion. The fitted values of A show no consistent variation with incident ion although a difference can be noted between the stripped beam and transmission values, the transmission values being higher.</p>
<p>The incident charge data were well fit by the assumptions that the sputtering yield depended locally on a power of the incident ion charge and that the sputtering from the surface is exponentially correlated to conditions in the bulk. The equilibrated sputtering yields derived from these data are in agreement with the stripped beam yields.</p>
<p>In addition, to aid in the understanding of these data, the data of Hakansson (80,81a,81b) were examined and contrasted with the UF<sub>4</sub> results. The thermal models of Seiberling (80) and Watson (81) were discussed and compared to the data.</p>
https://thesis.library.caltech.edu/id/eprint/9254A Physical Model of Wind-Blown Sand Transport
https://resolver.caltech.edu/CaltechETD:etd-08042008-114600
Authors: {'items': [{'id': 'Werner-Bradley-T', 'name': {'family': 'Werner', 'given': 'Bradley T.'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/6cbp-es88
<p>Eolian saltation, the transport of sand by the wind, involves a variety of physical processes. A fundamental understanding of saltation requires an analysis starting from the level of the individual sand grain. The complexity of this nonlinear dynamical system compels us to divide the problem into more easily handled decoupled components: the saltating grain-bed impact process, the force of the wind on individual grains, the determination of the wind profile from the spatially averaged force of the moving grains on the air, and the formation of small-scale bedforms: ripples.</p>
<p>The impact of a moving sand grain with a bed of sand is studied with two-dimensional dynamical computer simulations and an experiment propelling single grains onto a sand bed. We find that the result of the impact may be described in terms of the rebound of the incident particle and the ejection of bed grains. The bed grain ejections originate from a localized area around the impact point, and at steps in the surface (elevation changes of one grain diameter) which are more widely distributed; these surface steps we term brinks (downstream-facing) and anti-brinks (upstream-facing).</p>
<p>A model for steady-state saltation is proposed which incorporates both aerodynamics and the mechanics of the grain-bed impacts, and balances the losses of saltating particles on impact with the bed by gains due to impact generated bed grain ejections. This model does not require data on blowing sand. Results are obtained which qualitatively agree with existing data. Quantitative tests will require new experiments. We argue that grain-bed impacts, not fluid stresses, are the means for entraining grains in steady-state eolian saltation.</p>
<p>The development of sand surface topography is viewed as a result of surface grain transport (reptation) driven by the impact of high-energy saltating grains onto the bed. The collision and merger of small collections of sand, proto-ripples, lead to the asymptotic development of uniform ripples from an initially smoothed surface. The limiting wavelength is pictured as being determined by statistical fluctuations in the saltating impact flux and/or the shortening of the saltation shadow zone below the mean reptation length during a collision between two ripples. Field observations of developing ripple cross-sectional shapes confirm these ideas qualitatively, and rough calculations of limiting wavelengths agree with existing data.</p>
https://thesis.library.caltech.edu/id/eprint/3014The Physics of Granular Systems
https://resolver.caltech.edu/CaltechETD:etd-05302007-081951
Authors: {'items': [{'email': 'Gary.M.Gutt@jpl.nasa.gov', 'id': 'Gutt-Gary-Michael', 'name': {'family': 'Gutt', 'given': 'Gary Michael'}, 'show_email': 'NO'}]}
Year: 1989
DOI: 10.7907/GYAT-RE38
<p>To gain an improved understanding of the physics of granular systems, investigations of two aspects of continuum theories of granular flows and a new paradigm for modelling granular systems are presented.</p>
<p>The analogy between a flowing granular system and a molecular gas allows the adaptation of elements of the kinetic theory of gases to a continuum theory of granular flow. Two significant areas of difference between gases and granular materials are the additional degrees of freedom due to the spin and surface roughness of the grains and the boundary conditions between a granular flow and a bounding surface. Using techniques from the field of nonequilibrium thermodynamics, equations of motion and constitutive relations are obtained which include the effects of the spin and surface roughness of the grains. Also, boundary conditions on continuum theories for flows of smooth grains are presented, emphasizing the need to allow for several "slip" degrees of freedom.</p>
<p>In order to obtain a more general description of large scale effects in granular systems involving both flowing and static assemblies of grains, a model is developed (the lattice grain dynamics paradigm) for simulating systems containing large numbers of grains. This model is based on concepts from the fields of cellular automata, lattice gases, and particle dynamics simulations.</p>https://thesis.library.caltech.edu/id/eprint/2307