12023 research outputs found
Sort by
Recollected Sanity
No full textJohn Clare’s madness later in life is reflected in his poetry. In particular, his nature poems burst with overwhelming and disorganized sense-impressions that do not seem conventionally “romantic.” To get a more precise sense of what makes Clare’s later nature poems so disconcerting, it is useful to examine “Remembrances” as a nature poem that predates his insanity. In contrast with the bewilderment of “Snow Storm,” “Autumn,” and “The Flood,” “Remembrances” demonstrates a mastery over the speaker’s environment through a sense of narrative continuity and cognitive order. Through the conspicuous absence of these elements, the later nature poems reflect the state of mind of an untethered individual who has not only become estranged from the past but has also lost the ability to make sense of the present
When I Learned to be Meek
No full textWe were playing
While it is tempting to conclude that memory distortions point to
just the three of us, under the slide, when Damara was
leaving out details and introducing mistakes
a meanie to Franchesca, and I pulled her shirt to get her to apologize.</p
Large-Eddy Simulation of Turbulent Boundary Layers with Spatially Varying Roughness
Get PDFThis dissertation addresses high Reynolds number turbulent boundary layers flows with different inhomogeneous surface roughness distributions using large eddy simulations. The stretched vortex subgrid scale model for the outer flow LES is coupled with a virtual-wall model for the friction velocity with a correction accounting for local roughness effects.
A semi-empirical model that describes a fully developed rough-walled turbulent boundary layer with sand-grain roughness length-scale ks = αx that varies linearly with streamwise distance is first developed, with α a dimensionless constant. For large Rex and a free-stream velocity U∞ ~ xm, a simple log-wake model of the local turbulent mean-velocity profile is used that contains a standard mean-velocity correction for the asymptotic, fully rough regime. A two parameter (α; m) family of solutions is obtained for which U∞+ (or equivalently Cf) and boundary-layer measures can be calculated. These correspond to perfectly self-similar boundary-layer growth in the streamwise direction with similarity variable z/ks where z is the wall-normal co-ordinate. Results over a range of α are discussed for cases including the zero-pressure gradient (m = 0) and sink-flow (m = -1) boundary layers. Model trends are supported by high Re wall-modeled LES. Linear streamwise growth of boundary layer measures is confirmed, while for each α, mean-velocity profiles and streamwise turbulent stresses are shown to collapse against z/(αx). Inner scaled velocity defects are shown to collapse against z/Δ, where Δ is the Rotta-Clauser parameter. The present results suggest that these flows may be interpreted as the fully-rough limit for boundary layers in the presence of small-scale, linear roughness.
Next, an LES study of a flat-plate turbulent boundary layer at high Re under nonequilibrium flow conditions due to the presence of abrupt changes in surface roughness is presented. Two specific cases, smooth-rough (SR) and rough-smooth (RS) transition are examined in detail. Streamwise developing velocity and turbulent stress profiles are considered and sharp departures from equilibrium flow properties with subsequent relaxation are shown downstream. Relaxation trends are studied using integral parameters and higher-order mean flow statistics with emphasis on Reτ and ks+ dependence. Results are compared with RS experiments at matched Reτ, and show good agreement in terms of recovery rates.
Finally, the case of static, impulsive wall-roughness in flows at high Re is addressed using the same LES framework. The initial perturbation from smooth-to-rough appears to dominate the flow behaviour with the length of the impulsive patch showing little effect on recovery rates at matched Reτ and ks+. The resulting trends show good agreement with low Re experiments and support the wall-modeled LES framework as a suitable method for analysing high Re flows in practical applications.</p
Equations of State, Sound Velocities, and Thermoelasticity of Iron-Nickel-Silicon Alloys in the Earth’s Inner Core
Get PDFThe core of the Earth is predominately iron alloyed with approximately 5 wt% nickel along with some amount of light elements, e.g., Si, O, S, C, H, Mg. Mineral physics studies, in conjunction with seismological and cosmochemical observations, provide an opportunity to improve constraints on the composition of the core. In this thesis, we investigate the thermoelastic and vibrational properties of bcc- and hcp-structured Fe0.91Ni0.09 and Fe0.8Ni0.1Si0.1 (atomic percent) at high pressures.
We present powder x-ray diffraction data on bcc- and hcp-structured Fe0.91Ni0.09 and Fe0.8Ni0.1Si0.1 at 300 K up to 167 GPa and 175 GPa, respectively. The alloys were compressed in diamond anvil cells, and their equations of state and axial ratios were measured with high statistical quality. These equations of state are combined with thermal parameters from previous reports to improve the extrapolation of the density, adiabatic bulk modulus, and bulk sound speed to the pressures and temperatures of Earth’s inner core. We place constraints on the composition of Earth’s inner core by combining these results with seismic observations and available data on other light-element alloys of iron. We find the addition of 4.3 to 5.3 wt% silicon to Fe0.95Ni0.05 alone can explain geophysical observations of density, adiabatic bulk modulus, and bulk sound speed at the inner core boundary, as can up to 7.5 wt% sulfur with negligible amounts of silicon and oxygen. Our findings favor an inner core with less than ∼2 wt% oxygen and less than ∼1 wt% carbon, although uncertainties in electronic and anharmonic contributions to the equations of state may shift these values.
Seismic studies provide evidence for an anisotropic inner core, which is suggested to be related to the ratio of the c- to a-unit cell parameters of hcp-structured materials. We demonstrate hcp-Fe0.91Ni0.09 and Fe0.8Ni0.1Si0.1 have measurably greater c/a axial ratios than those of hcp-Fe over the measured pressure range. We further investigate the relationship between the axial ratios, their pressure derivatives, and elastic anisotropy of hcp-structured materials.
Next, we present high pressure NRIXS data on bcc- and hcp-Fe0.91Ni0.09 and Fe0.8Ni0.1Si0.1 at 300 K with in situ x-ray diffraction. From these data, we determine the partial phonon density of states for each composition, and we systematically compare our results to iron. We constrain the Debye sound velocity from the low energy region of the phonon density of states. Using our previously determined equations of state for the same compositions, we constrain the compressional and shear sound velocities and shear moduli. At 300 K, we find that 9 at% nickel decreases the shear velocity of hcp-iron by ∼6% and that silicon has a minimal effect on the shear velocity of hcp-Fe0.91Ni0.09. Thermal effects likely play a large role in the sound velocities of iron alloys at core conditions, so constraining these effects is critical to further constrain the composition of the core.
From the volume scaling of the phonon DOS, we find the 300 K vibrational components of the Grüneisen parameter for hcp-Fe0.91Ni0.09 and Fe0.8Ni0.1Si0.1 are very similar to that of hcp-Fe within uncertainties. We also constrain vibrational thermal pressure from the volume dependence of vibrational free energy, and we find negligible differences within uncertainty between the vibrational thermal pressures of hcp-Fe, Fe0.91Ni0.09, and Fe0.8Ni0.1Si0.1. By combining the vibrational component of thermal pressure with theoretical estimates of the anharmonic and electronic contributions, we provide an estimate for the total thermal pressure. We constrain a variety of additional parameters from the NRIXS data and phonon density of states, including the vibrational component of entropy, the vibrational thermal expansion, the vibrational kinetic energy, the Lamb-Mössbauer factor, and the vibrational specific heat.</p
Small-Scale Deformation and Fracture of Hard Biomaterials
Get PDFStructural materials engineering often aims to realize materials that are simultaneously strong, tough, and lightweight — a combination classically considered mutually exclusive. Natural composite materials such as bone exhibit a combination of these properties far exceeding that of their constituents, a feat generally credited to their hierarchical structure — all the way down the nanoscale. To date, a quantitative description of how this property combination arises in such microstructurally complex materials has remained elusive due to challenges in experimentally isolating and probing the salient deformation and toughening mechanisms at the micro and nanometer scales — length scales on the order the constituents of many natural composites.
In this thesis, we first investigate the site-specific nanoscale structure of human bone using transmission electron microscopy. We show the presence of previously undiscovered disordered arrangement of collagen and mineral — alongside a well known ordered structure — within the trabecular architecture of bone. We perform micro- and nano-mechanical compression experiments to probe strength and deformation of each of these microstructures, revealing a size-dependent strength of bone attributed to the limited number of failure-initiating critical defects (e.g pores) in the small-scale samples relative to macro-scale tissue.
Unlike experiments for investigating strength at small-scales, fracture experiments are standardized for the macroscale. To address this, we developed an in situ SEM/nanoindenter methodology that enables 3-point bending fracture experiments with observation and measurement of crack growth and toughening behavior at nano and micrometer scales. Using this technique, we discuss the crack initiation and growth toughness arising primarily from the underlying fibril microstructure in bone. In the context of a crack growth resistance, we describe a transition in the toughening behavior of bone originating from different levels of hierarchy. Given its versatility, this experimental technique establishes a platform for understanding the coupling between structure and fracture behavior of micron-sized materials.</p
Architecture, Design, and Tradeoffs in Biomolecular Feedback Systems
Get PDFA core pursuit in systems and synthetic biology is the analysis of the connection between the low-level structure and parameters of a biomolecular network and its high-level function and performance. Elucidating this mapping has become increasingly feasible as precise measurements of both input parameters and output dynamics become abundant. At the same time, cross-pollination between biology and engineering has led to the realization that many of the mathematical tools from control theory are well-suited to analyze biological processes.
The goal of this thesis is to use tools from control theory to analyze a variety of biomolecular systems from both natural and synthetic settings, and subsequently yield insight into the architecture, tradeoffs, and limitations of biological network. In Chapter 2, I demonstrate how allosteric proteins can be used to respond logarithmically to changes in signal. In Chapter 3, I show how control theoretic techniques can be used to inform the design of synthetic integral feedback networks that implement feedback with a sequestration mechanism. Finally, in Chapter 4 I present a novel simplified model of the E. coli heat shock response system and show how the the mapping of circuit parameters to function depends on the network's architecture.
The unifying theme of this research is that the conceptual framework used to study engineered systems is remarkably well-suited to biology. That being said, it is important to apply these tools in a way that is informed by the molecular details of biological processes. By combining structural and biochemical data with the functional perspective of engineering, it is possible to understand the architectural principles that underlie living systems.</p
Tectonics in Nevada and Southern California: Subsidence of the Ediacaran Johnnie Formation, Cumulative Offset Along the Lavic Lake Fault, and Geomorphic Surface Development Along the Southern San Andreas Fault
Get PDFWhile we know the ages and tectonic histories of many critical geologic events in the history of the Earth, there are still questions regarding the timing of key events and structures that have and continue to influence life on this planet. This thesis includes three separate studies in Nevada and southern California: two potential new methods for measuring/organizing geologic time, and also an analysis of the long-term displacement along an active fault in the eastern California shear zone. In Chapter II, we used tectonic subsidence modeling to find that the Shuram carbon isotopic excursion in the Ediacaran Johnnie Formation likely occurred from 585-579 Ma, and that incision of the Rainstorm Member shelf occurred during the 579 Ma Gaskiers glaciation. The pre-Shuram-excursion chemostratigraphic carbon isotope profiles from the Khufai Formation in Oman and the type locality of the Johnnie Formation in Nevada are both generally positive and therefore possibly correlative. In Chapter III, we determined the cumulative tectonic offset along the Lavic Lake fault, an active structure that ruptured with >5 m of coseismic slip in the 1999 Mw 7.1 Hector Mine earthquake. We calculated a net slip of 960 +70/-40 m, based on the slip vector formed by a vertically separated lithologic contact and a horizontally separated older cross fault. The net slip we calculated is significantly less than a previous estimate that was based on an offset magnetic gradient, a disparity that may be explained by considering off-fault deformation, as well as the unknown depth and nature of the source of the magnetic contrast. In Chapter IV, we explored using a new method for the relative dating of Quaternary geomorphic surfaces, which is based on the positive correlation between increased spectral contrast in thermal hyperspectral airborne imagery and surface age. With field data, we found that desert varnish scores, desert pavement scores, and vegetation spacing estimates also correlate positively with surface age, implying that these factors could contribute to the increased spectral contrast in airborne remote sensing spectra. Additionally, the general increase in the band depth of airborne spectra at 9.16 μm could be due to increasing clay mineral abundance in progressively heavier desert varnish coatings on older surfaces. The positive correlation observed in this study between surface age and spectral contrast in airborne spectra can perhaps be used to develop a method for relative dating of varnished geomorphic surfaces elsewhere. All of the chapters in this thesis are broadly related by the concepts of geologic time and tectonic activity, which are two aspects of modern geology that are intrinsic to the science as a whole.</p
The Changing Mouse Embryo Transcriptome at Whole Tissue and Single-Cell Resolution
Get PDFMammalian histogenesis is a sophisticated process of coordinated changes of cellular composition governed by selective gene expression. This thesis focuses on the systematic application of modern RNA-seq methods to histogenesis processes in developing mouse embryos. Most of the work presented here is conducted as part of the ENCODE (ENCyclopedia Of DNA Elements) Project. Chapter 1 introduces the current advances of transcriptome studies on tissue development. Chapter 2 discusses a large-scale study on the whole-tissue transcriptome of 12 embryonic tissues at up to 8 time points and 5 additional perinatal tissues. Coherent themes of biological function and underlying regulatory mechanisms are revealed from the large-scale analysis. Chapter 3 presents a high-resolution single-cell RNAseq study focused on the developing forelimb of the mouse embryo. This approach enables the assignment of differential genes to corresponding lineages and provides an even more accurate picture of RNA level patterns and regulatory modes. Finally, whole-tissue and single-cell methods are compared, contrasted, and integrated in Chapter 4 to extrapolate from the main discoveries of this thesis.</p
Linear and Non-linear Interactions in a Rough-Wall Turbulent Boundary Layer
Get PDFThis thesis explores the linear and non-linear interactions which take place in a rough-wall turbulent boundary through experiments and modeling. In order to derive physics-based models for the relation between roughness geometry and flow physics, two very simple periodic roughnesses were 3D printed and placed in a boundary layer wind tunnel for separate experiments. Hot-wire measurements were taken at a grid of points within a single period of the roughness in order to map the spatial variation of important flow statistics in way that allows correlation back to the roughness geometry. Time averaged streamwise velocity and the power spectrum of instantaneous streamwise velocity were both found to vary coherently with the roughness. The spatial variation of the time averaged velocity was identified as the linear result of the roughness, as it has identical wavenumber and frequency to the static roughness geometry. Modeling the time-averaged velocity field as a response mode of the linear resolvent operator was found to be reasonable for certain wavenumbers. The spatial distribution of the power spectrum was shown to be a non-linear effect of the roughness; the power spectrum only measures the energy of convecting modes, which necessarily have non-zero frequency and cannot correlate linearly to the static roughness. The spatial modulation of the power spectrum was found to be indicative of non-linear triadic interactions between the static velocity Fourier modes and pairs of convecting modes, as allowed by the Navier-Stokes equations. A low-order model for these interactions, and their effect on the power spectrum, was constructed using resolvent response modes to represent all velocity Fourier modes. The model was found to qualitatively predict the modulation of the power spectrum for several sets of wavenumbers. The success of such a simple model suggests that it presents a useful low-order understanding of non-linear forcing between scales in rough-wall boundary layers.</p
Metasurfaces: Beyond Diffractive and Refractive Optics
Get PDFOptical metasurfaces are a category of thin diffractive optical elements, fabricated using the standard micro- and nano-fabrication techniques. They provide new ways of controlling the flow of light based on various properties such as polarization, wavelength, and propagation direction. In addition, their compatibility with standard micro-fabrication techniques and compact form factor allows for the development of several novel platforms for the design and implementation of various complicated optical elements and systems. In this thesis, I first give a short overview and a brief history of the works on optical metasurfaces. Then I discuss the capabilities of metasurfaces in controlling the polarization and phase of light, and showcase their potential applications through the cases of polarimetric imaging and vectorial holography. Then, a discussion of the chromatic dispersion in optical metasurfaces is given, followed by three methods that can be utilized to design metasurfaces working at multiple discrete wavelengths. As a potential application of such metasurfaces, I present results of using them as objective lenses in two-photon microscopy. In addition, I discuss how metasurfaces enable the at-will control of chromatic dispersion in diffractive optical elements, demonstrate metasurfaces with controlled dispersion, and provide a discussion of their limitations. Integration of multiple metasurfaces into metasystems allows for implementation of complicated optical functions such as imaging and spectrometry. In this regard, I present several examples of how such metasystems can be designed, fabricated, and utilized to provide wide field of view imaging and projection, microelectromechanically tunable lenses, optical spectrometers, and retroreflectors. I conclude with an outlook on where metasurfaces can be most useful, and what limitations should be overcome before they can find wide-spread application.</p