No description available.

With the increasing developments of exploiting oil and natural gas in deep water and harnessing renewable (wave and wind) energy in the sea, mooring lines and risers are widely deployed to position the related floating structures. Subject to environmental loads, a mooring line or riser connected to floating structure, moves up and down, back and forth, and sometimes from the left to the right. In computation of the dynamics of a mooring line or riser, it is often modeled as a flexible slender rod. While the bending moment of a chain or a rope is neglected, that of a riser is considered and specified by characteristics of the riser. Existing numerical codes for simulating the dynamics of a slender rod, such as CABLE3D, allow for the vertical support force and longitudinal (along the direction of the rod) friction from soils of the seabed while the transverse (in the direction transverse to the slender rod) friction between the rod and the seabed soils is not considered. In this study, we extend the current version of CABLE3D to allow for the transverse friction applied on the portion of a slender rod contacting the seabed soil, which is time-varying when it is moving. The friction between a slender rod and the seabed soil is computed based upon a Coulomb model originally developed for the simulation of the friction in all dry contact mechanical systems. In applying the Coulomb model, the transverse friction depends on the transverse displacement and/or velocity of a slender rod contacting the seabed. In addition, vertical bottom support of the seabed soil is calculated based on the shear stress of the seabed soil. The simulations of the dynamics of a few typical mooring lines are made given their motions at their fairleads and the results are compared with the corresponding results obtained using Orcaflex, a commercial code, and the existing version of CABLE3D.

Nanocrystalline materials could exhibit high mechanical yield strength. Nevertheless, with a high volume fraction in nanocrystalline material, grain boundaries and triple junctions which store a relatively high free energy, are thermally instable which potentially contribute to grain growth. On the other hand, since both grain boundaries and triple junctions are prior sites of impurity enrichment which could in return reduce the triple junction energy, alloys with impurity enriched in grain boundaries and triple junctions are widely applied to stabilize the nanostructures. However, past studies mainly focused on grain boundaries and the kinetic properties of triple junctions and their influences on the thermal stability of nanocrystalline metals is less studied. In this work, triple junction mobility and impurity diffusivity in triple junction are studied by molecular dynamics simulations. Specifically, interface random walk method due to thermal fluctuation which has been widely applied to extract grain boundary mobility is extended to study triple junction motion.

No description available.

We use molecular dynamics to simulate the rapid directional solidification of binary alloy solid-liquid interface in the non-equilibrium state. In the pulling fixed velocities, we report the temperature, density, and diffusion coefficient of the interface. In cooling fast, controlling the velocities of solidification for the important parameter of this text¡Ait produces different changes that velocity value will be affected by atom potential energy and system temperature and density¡Athough the system is pulling a fixed velocities, that the speed of every atom of the system is all not constant .The velocity will be changed into the driving force that the solute will be separated and trapped. In the segregation regime, we recover the exponential form of the concentration profile within the liquid phase. Solute trapping is shown to settle in progressively as V is increased or reduction and our results are in good agreement with the theoretical predictions of Aziz.

A recently developed chemical vapor deposition (CVD) synthesis process called CoMoCAT yields single-wall carbon nanotubes (SWCNT)s of controlled diameter and chirality, making them extremely attractive for technological applications. In this dissertation, we use molecular dynamics simulations and density functional theory to study the selective growth mechanisms. In the CoMoCAT process, growth of SWCNTs happens on Co clusters with diameters of about 1 �. Effective force fields for Ni-C interactions developed by Yamaguchi and Maruyama for the formation of metallofullerenes and the reactive empirical bond order Brenner potential for C-C interactions are modified to describe interactions in such system. Classical molecular dynamics (MD) simulations using this force field are carried out to study the growth of SWCNT on floating and supported metal clusters. The effect of metal-cluster interactions on the growth process is discussed. The energy of forming one more ring at the open end of one-end-closed nanotubes with different chiralities, which is believed to be the basic step of nanotube elongation, are studied as a function of tube length. The energy and shape of the frontier highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of armchair nanotubes are studied and used to explain the change of reaction energy with tube length. Another property, the aromaticity of the rings forming a tube is also studied using Nucleus Independent Chemical Shift (NICS) as probe. NICS of rings in one-end-closed nanotubes with different chirality are studied as a function of tube length. NICS behavior of one-end-closed nanotube is compared with that of two-ends-open nanotube with the same chirality for nanotubes (6, 5) and (9, 1). Also (8, 3) nanotubes with one end open and the other end bonded to three different kinds of cap structures are compared. Since from both experimental observation and from our MD simulation results, the growth process of SWCNT can be affected by the interaction between Co clusters and their substrate, the performance of a series of CoN Clusters (N=1-4, 7, 10, 14, 15) adsorbed on MoC surface are studied with density functional theory.

The aim of DNA sequencing is to obtain the order of DNA composition comprising the base pairs A (adenine) T (thymine), and C (cytosine) G (guanine). The fast development of DNA sequencing technology allows us to better understand the relationships among diseases, inheritance, and individuality. Solid state nanopores have been recommended as the next generation platform for DNA sequencing due to its low-cost and high-throughput. In particular, nanopores fabricated from graphene sheets are extremely thin and structurally robust and have been extensively used in DNA detection in recent years. In DNA sequencing, the translocation of a DNA molecule through a nanopore is known to be a very complicated issue and is affected by many factors, such as ion concentration, thickness of the nanopore, and the nanopore diameter. The technique of molecular dynamic simulations has been a complementary tool to study DNA translocation through nanopores.       In this thesis, I summarize my work of computational studies of DNA sequencing using graphene nanopores. These studies include: DNA translocation through single-layer graphene nanopores of different diameters under conditions of various ion concentrations and applied voltages; DNA translocation through multilayer graphene nanopores varied from a single to a few layers; pulling out single strand DNA molecules from small graphene nanopores of different geometries. The major contributions of this work include: 1. Effects of bias voltage on DNA translocation time were investigated leading to the insight that lower applied voltages can extend the time of DNA translocation through monolayer graphene nanopores. The effect of salt concentration on the corresponding ionic current was studied. At a low ionic concentration (&lt; 0.3M), the current increases as DNA translocates through a nanopore. However, at a high ionic concentration (&gt;0.5M), the current decreases as DNA translocates through the nanopore. A theoretical model was proposed to explore the relationship between the current and the occupied nanopore area. We demonstrated that the DNA translocation time can be prolonged by narrowing the diameter of a nanopore properly and the reduction of the blockade current depends on the ratio of the unoccupied nanopore area to the total nanopore area. 2.  DNA translocation through multilayer graphene nanopores was studied by molecular dynamics simulations with the aim to achieve single-base resolution. We show that the DNA translocation time can be extended by increasing the graphene layers up to a moderate number (7) and that the current in DNA translocation undergoes a stepwise change upon DNA going through an multi-layer graphene (MLG) nanopore. A model was built to account for the relationship between the current change and the unoccupied volume of the MLG nanopore. We demonstrate that the blockade current is closely related to the unoccupied volume. The dynamics of DNA translocation depends specifically on the interaction of nucleotides with the graphene sheet. Thus, our study indicates that the resolution of DNA detection can be improved by increasing the number of graphene layers in a certain range and by modifying the surface of graphene nanopores. 3. The effect of graphene nanopore geometry on DNA sequencing has been assessed by steered molecular dynamics simulations. DNA fragments including A, T, C, G and 5-methylcytosine (MC) were pulled through graphene nanopores of different geometries with diameters down to ~1nm by steered molecular dynamics simulations. We demonstrated that the bases (A, T, C, G, and MC) can be indentified in single-base resolution by the characteristic force peak values in a circular graphene nanopore but not in graphene nanopores of other geometries. Symmetric nanopores are thus better suited to DNA sequence detection via force curves than asymmetric nanopores. This implies that the graphene nanopore surface should be modified as symmetric as possible to sequence DNA by an atomic force microscope or optical tweezers. This helps us to understand low-cost and time-efficient DNA sequencing in narrow nanopores. 4. The translocation time for different nucleotides to pass through graphene nanopores with certain diameters was investigated. It was found that the translocation times are different for different bases under a low electric field. The results indicate that DNA can be sequenced by the translocation time to pass through a graphene nanopore. 5. Inspired by the structure of K+ channel proteins, a series of oxygen doped graphene nanopores of different size were designed to discriminate the transport of K+ and Na+ ions. The results indicate that the ion selectivity of such biomimetic graphene nanopores can be simply controlled by the size of the nanopore.  Compared to K+, the smaller radius of Na+ leads to a much higher free energy barrier in the nanopore of a certain size. / <p>QC 20141212</p>

The continuously variable transmission (CVT), which has been in use in some of the vehicles in the market today, presents the possibility of decoupling the engine speed and the vehicle speed. By this way, it is now possible to operate the engine at its maximum efficient or performance point and fix it at that operating point without losing from the vehicle speed. Instead of using gears, which are the main transmission elements of conventional transmission, CVT uses two pulleys and a belt. By changing the pulley diameters, a continuously variable transmission ratio is obtained. Besides all its advantages, it has some big drawbacks like low efficiency, torque transmission ability and limited speed range. With developing technology, however, new solutions are developed to eliminate these drawbacks. In this study simulation models for the performance and fuel consumption of different types and arrangements of continuously variable transmission (CVT) systems are developed. Vehicles, which are equipped with two different arrangements of CVT and an automatic transmission, are modelled by using Matlab&amp / #8217 / s simulation toolbox Simulink. By defining the required operating points for better acceleration performance and fuel consumption, and operating the engine at these points, performance optimization is satisfied. These transmissions are compared with each other according to their &amp / #8216 / 0-100 kph&amp / #8217 / acceleration performances, maximum speeds, required time to travel 1000 m. and fuel consumptions for European driving cycles ECE and EUDC. These comparisons show that CVT systems are superior to automatic transmission, according to their acceleration and fuel consumption performances. CVTs also provide smoother driving, while they can eliminate jerks at gear shifting points.

This thesis focuses on the development of a short-term, operationally stable finite element-based simulation of a mechanical watch lever escapement. This was accomplished in four steps: by choosing a reference escapement based on the needs of the study, by executing a reverse engineering methodology to create a lever escapement in computer-aided design (CAD) software, by capturing experimental data from the reference escapement via custom- built apparatus and then reconciling this data with an analytical model, and by using the knowledge gained from these efforts to develop an implicit dynamic simulation of a lever escapement that aimed to achieve performance metrics defined by watchmaking sources. The final version of the simulated lever escapement was able to meet two of the three performance goals defined for the study. The simulation met the primary performance goal by achieving stable operation for two seconds. During this window of stability, the simulated lever escapement met the secondary performance goal of the study by achieving timing performance metrics defined by watchmaking sources. Unfortunately, the tertiary performance goal was not met as the balance amplitude of the final simulation was outside of the target range by 5.23% when compared against the lower bound. Although the balance amplitude error of the simulated escapement would be indicative of a mechanism that needs servicing, its performance during the stability period was assessed to be representative of a functional lever escapement and therefore, its dynamics and sensitivities were explored and presented. / Master of Science / Mechanical watches rely on physics to keep accurate time. The time regulation mechanism within a mechanical watch is called an escapement, and the most widely used escapement design adopted by watchmakers is the lever escapement. While prior attempts have been made to simulate the physics that these mechanisms use to keep accurate time, achieving stable operating performance in a complete lever escapement simulation remains elusive in published studies. The examination of a stable, simulated lever escapement could reveal new insights into these mechanisms by reducing the impact of transient phenomena. This thesis focuses on the development of a short-term, operationally stable simulation of a lever escapement mechanism. This was achieved by developing a model of a real-world lever escapement, by capturing experimental data to improve the model, and then by applying the knowledge gained from these efforts to create a dynamic simulation in Abaqus/CAE. The final simulation was able to meet two of the three performance goals defined for the study, which proved that it is possible to create a simulation of a lever escapement. Furthermore, the study revealed unexpected phenomena that may be present in real-world lever escapements and may affect their performance.

Polymer Derived Ceramics are a promising class of Materials that allow for higher levels of tunability and shaping that traditional sintering methods do not allow for. Polysiloxanes are commonly used as a precursor for these types of material because of their highly tunable microstructures by adjusting the side groups on the initial polymer. These Polymers are generally cross linked and pyrolyzed in inert atmospheres to form the final polymer. The microstructures of Polymer Derived Ceramics is complex and hard to observe due to the size of each microstructure region and the proximity in the periodic table that the elements present have. The process of forming phases such as Graphitic Carbon, Amorphous Carbon, Silicon Carbide. Silicon Oxide, and SiliconOxycarbide are not well understood. Simulation provides a route to understanding the phenomenon behind these phase formations. Specifically, Molecular dynamics simulation paired with the Reaxff forcefield provides a framework to simulate the complex processes involved in pyrolysis such as chemical reactions and a combination of thermodynamic and kinetic interactions. This Thesis examines firstly the size effect that a system can have on phase separation and the change in composition. Showing that size plays a major role in how the system develops and limits the occurrence of specific reactions. Secondly, this thesis shows that using polymer precursors with different initial polymer components leads to vastly different microstructures and yield. This provides insights into how the transition from polymer to ceramic takes place on a molecular level. / Master of Science / Ceramics and Polymers are seen all around the world. Polymers are used in many things from grocery bags to high performance panels on airplanes. Polymers are generally cheap to produce and can be molded into a variety of shapes. Ceramics are generally hard materials and are also used in a wide variety of situations from the concrete in buildings to coatings that protect turbine blades. Ceramics tend to be harder to form specific shapes and more costly to machine. Polymer derived polysiloxanes address this problem by being formed in the polymer state and then transformed into a ceramic by being heated in inert atmospheres. The process of the heating is very complex and the effect that different polymers have on the atomic level is not well understood. This thesis works to address this by using simulation to see what cannot be seen through experimentation alone.