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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

An Investigation of Chaotic Diffusion in a Family of Hamiltonian Mappings Whose Angles Diverge in the Limit of Vanishingly Action

The chaotic diffusion for a family of Hamiltonian mappings whose angles diverge in the limit of vanishingly action is investigated by using the solution of the diffusion equation. The system is described by a two-dimensional mapping for the variables action, I, and angle, θ and controlled by two control parameters: (i) ϵ, controlling the nonlinearity of the system, particularly a transition from integrable for ϵ=0 to non-integrable for ϵ≠0 and; (ii) γ denoting the power of the action in the equation defining the angle. For ϵ≠0 the phase space is mixed and chaos is present in the system leading to a finite diffusion in the action characterized by the solution of the diffusion equation. The analytical solution is then compared to the numerical simulations showing a remarkable agreement between the two procedures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

A Simple Kinetic Model for the Phase Transition of the van der Waals Fluid

A simple kinetic model, which is presumably minimum, for the phase transition of the van der Waals fluid is presented. In the model, intermolecular collisions for a dense gas has not been treated faithfully. Instead, the expected interactions as the non-ideal gas effect are confined in a self-consistent force term. Collision term plays just a role of thermal bath. Accordingly, it conserves neither momentum nor energy, even globally. It is demonstrated that (i) by a natural separation of the mean-field self-consistent potential, the potential for the non-ideal gas effect is determined from the equation of state for the van der Waals fluid, with the aid of the balance equation of momentum, (ii) a functional which monotonically decreases in time is identified by the H theorem and is found to have a close relation to the Helmholtz free energy in thermodynamics, and (iii) the Cahn–Hilliard type equation is obtained in the continuum limit of the present kinetic model. Numerical simulations based on the Cahn–Hilliard type equation are also performed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Accounting for Melt Flow Pattern and Solid Fraction Evolution in DC Casting of Al-Cu Alloy Using v{sup 2}−f Turbulence Model

Experimental techniques remain reliable approaches for investigating the effects of parameters of the direct chill (DC) casting process on macrosegregation, grain structure formation, and surface quality. However, experimental analysis methods of these effects are expensive and can often prove difficult to implement. Understanding of the complex physics of the solidification process remains limited, and this makes it indispensable to use accurate numerical modeling tools as a complement to experiments. Numerical modeling without consideration of turbulent flow is unsuitable for application to solidification occurring during DC casting, since significant turbulent flow exists in the upper section of the cast, where the liquid pool exists. The present work employed a low-Re v{sup 2}−f turbulence model and a dual-zone solidification flow model to predict the melt flow pattern, turbulence level, sump depth, transition zone thickness, and mushy zone thickness in the DC casting of Al-Cu alloy. The obtained results provided evidence of significant damping of the flow in the slurry zone. Furthermore, results of different simulation cases revealed a narrower transition zone and shallower sump.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Numerical Simulation of V-shaped Segregation in Continuous Casting Blooms Based on a Microsegregation Model

This study establishes a volume-averaged model for transport phenomena to study V-shaped segregation in continuous casting blooms with different superheating and cooling rates by coupling fluid flow, macro-scale heat transfer, and solute transport. The results demonstrate that the modified Won–Thomas model considering back diffusion in the solid phase corresponds well with the measured carbon segregation. V-shaped segregation decreases with decreasing superheating and increases slightly with decreases in the cooling rate. The results of original position statistical distribution analysis correspond well with the simulation results. This phenomenon can be exploited to improve the quality of bloom.

42 ENGINEERING↗
DOE OSTI2018

Processing Parameter Effects on Residual Stress and Mechanical Properties of Selective Laser Melted Ti6Al4V

Selective laser melting (SLM) process is characterized by large temperature gradients resulting in high levels of residual stress within the additively manufactured metallic structure. SLM-processed Ti6Al4V yields a martensitic microstructure due to the rapid solidification and results in a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. Residual stress buildup and the mechanical properties of SLM parts can be controlled by varying the SLM process parameters. This investigation studies the effect of layer thickness on residual stress and mechanical properties of SLM Ti6Al4V parts. This is the first-of-its kind study on the effect of varying power and exposure in conjunction with keeping the energy density constant on residual stress and mechanical properties of SLM Ti6Al4V components. It was found that decreasing power and increasing exposure for the same energy density lowered the residual stress and improved the % elongation of SLM Ti6Al4V parts. Increasing layer thickness resulted in lowering the residual stress at the detriment of mechanical properties. The study is based on detailed experimental analysis along with finite element simulation of the process using ABAQUS to understand the underlying physics of the process.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Tempering and Rolling on Fatigue Crack Growth Behavior of Modified 9Cr-1Mo Steel

In the present work, fatigue crack growth (FCG) behavior of modified 9Cr-1Mo steel has been investigated near fatigue threshold and Paris regime at room temperature (RT) and 650 °C. The modified 9Cr-1Mo steel was subjected to normalizing at 1050 °C followed by tempering (650-750 °C) and warm rolling at 550 °C followed by tempering in order to improve its precipitation state and refine microstructure. The microstructural parameters such as grain size, block size, lath width and precipitate size were examined and correlated with FCG and tensile properties of investigated samples. The results showed that decreasing tempering temperature significantly reduces the fatigue threshold at RT while increases the yield strength at elevated temperature (i.e., 650 °C). Increase in fatigue threshold is found to increase with the increase in block size, whereas high density of fine precipitates contributed the high temperature strength. The FCG rate in Paris regime was less affected by heat treatment and rolling process at RT. However, rolled sample tested at 650 °C shows a decrease in FCG rate as compared to heat-treated samples. In terms of higher FCG resistance and yield strength, warm rolling followed by tempering at low temperature (i.e., 700 °C) can be considered optimum. For the numerical simulation of FCG behavior, extended finite element method (XFEM) along with Paris law has been implemented in ABAQUS using python script. The simulated FCG behavior is found in good agreement with the experimental results.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Effective Coulomb logarithm for one component plasma

An expression for the effective Coulomb logarithm in one-component-plasma is proposed, which allows to extend the applicability of the classical formula for the self-diffusion coefficient to the strongly coupled regime. The proposed analytical approximation demonstrates reasonable agreement with previous numerical simulation results. Relevance to weakly screened Yukawa systems (and, in particular, complex plasmas) is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Dynamic and impact contact mechanics of geologic materials: Grain-scale experiments and modeling

High fidelity treatments of the generation and propagation of seismic waves in naturally occurring granular materials is becoming more practical given recent advancements in our ability to model complex particle shapes and their mechanical interaction. Of particular interest are the grain-scale processes that are activated by impact events and the characteristics of force transmission through grain contacts. To address this issue, we have developed a physics based approach that involves laboratory experiments to quantify the dynamic contact and impact behavior of granular materials and incorporation of the observed behavior indiscrete element models. The dynamic experiments do not involve particle damage and emphasis is placed on measured values of contact stiffness and frictional loss. The normal stiffness observed in dynamic contact experiments at low frequencies (e.g., 10 Hz) are shown to be in good agreement with quasistatic experiments on quartz sand. The results of impact experiments - which involve moderate to extensive levels of particle damage - are presented for several types of naturally occurring granular materials (several quartz sands, magnesite and calcium carbonate ooids). Implementation of the experimental findings in discrete element models is discussed and the results of impact simulations involving up to 5 Multiplication-Sign 105 grains are presented.

58 GEOSCIENCES↗
DOE OSTI2013

Experimentally observed field–gas interaction in intense optical lattices

When a gas perturbed by a laser interference pattern, an optical lattice, exhibits a periodic modulation of its refractive index, strong Bragg diffraction of the perturbing light can occur. This scattering reduces the field's ability to further manipulate the gas. Experimental observations of Bragg scattering, evidence of a two-way coupling, are compared to the evolution of the light fields calculated by solutions to the wave equation. Comparison indicates momentum deposition as a prime contributor to the shape of the scattering function vs. lattice velocity, a rationale further supported through additional direct simulation Monte Carlo simulation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Cluster formation in fluids with competing short-range and long-range interactions

We investigate the low density behaviour of fluids that interact through a short-ranged attraction together with a long-ranged repulsion (SALR potential) by developing a molecular thermodynamic model. The SALR potential is a model of effective solute interactions where the solvent degrees of freedom are integrated-out. For this system, we find that clusters form for a range of interaction parameters where attractive and repulsive interactions nearly balance, similar to micelle formation in aqueous surfactant solutions. We focus on systems for which equilibrium behaviour and liquid-like clusters (i.e., droplets) are expected, and find in addition a novel coexistence between a low density cluster phase and a high density cluster phase within a very narrow range of parameters. Moreover, a simple formula for the average cluster size is developed. Based on this formula, we propose a non-classical crystal nucleation pathway whereby macroscopic crystals are formed via crystal nucleation within microscopic precursor droplets. We also perform large-scale Monte Carlo simulations, which demonstrate that the cluster fluid phase is thermodynamically stable for this system.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

On the accuracy of coherent modified Redfield theory in simulating excitation energy transfer dynamics

In this study, we investigate the accuracy of a recently developed coherent modified Redfield theory (CMRT) in simulating excitation energy transfer (EET) dynamics. The CMRT is a secular non-Markovian quantum master equation that is derived by extending the modified Redfield theory to treat coherence dynamics in molecular excitonic systems. Herein, we systematically survey the applicability of the CMRT in a large EET parameter space through the comparisons of the CMRT EET dynamics in a dimer system with the numerically exact results. The results confirm that the CMRT exhibits a broad applicable range and allow us to locate the specific parameter regimes where CMRT fails to provide adequate results. Moreover, we propose an accuracy criterion based on the magnitude of second-order perturbation to characterize the applicability of CMRT and show that the criterion summarizes all the benchmark results and the physics described by CMRT. Finally, we employ the accuracy criterion to quantitatively compare the performance of CMRT to that of a small polaron quantum master equation approach. The comparison demonstrates the complementary nature of these two methods, and as a result, the combination of the two methods provides accurate simulations of EET dynamics for the full parameter space investigated in this study. Our results not only delicately evaluate the applicability of the CMRT but also reveal new physical insights for factors controlling the dynamics of EET that should be useful for developing more accurate and efficient methods for simulations of EET dynamics in molecular aggregate systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Unidirectional light emission from low-index polymer microlasers

We report on experiments with deformed polymer microlasers that have a low refractive index and exhibit unidirectional light emission. We demonstrate that the highly directional emission is due to transport of light rays along the unstable manifold of the chaotic saddle in phase space. Experiments, ray-tracing simulations, and mode calculations show very good agreement.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Time-delayed feedback control of coherence resonance near subcritical Hopf bifurcation: Theory versus experiment

Using the model of a generalized Van der Pol oscillator in the regime of subcritical Hopf bifurcation, we investigate the influence of time delay on noise-induced oscillations. It is shown that for appropriate choices of time delay, either suppression or enhancement of coherence resonance can be achieved. Analytical calculations are combined with numerical simulations and experiments on an electronic circuit.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Growth of bi- and tri-layered graphene on silicon carbide substrate via molecular dynamics simulation

Molecular dynamics (MD) simulation with simulated annealing method is used to study the growth process of bi- and tri-layered graphene on a 6H-SiC (0001) substrate via molecular dynamics simulation. Tersoff-Albe-Erhart (TEA) potential is used to describe the inter-atomic interactions among the atoms in the system. The formation temperature, averaged carbon-carbon bond length, pair correlation function, binding energy and the distance between the graphene formed and the SiC substrate are quantified. The growth mechanism, graphitization of graphene on the SiC substrate and characteristics of the surface morphology of the graphene sheet obtained in our MD simulation compare well to that observed in epitaxially grown graphene experiments and other simulation works.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Second order classical perturbation theory for the sticking probability of heavy atoms scattered on surfaces

A second order classical perturbation theory is developed to calculate the sticking probability of a particle scattered from an uncorrugated thermal surface. An analytic expression for the temperature dependent energy loss of the particle to the surface is derived by employing a one-dimensional generalized Langevin equation. The surface temperature reduces the energy loss, since the thermal surface transfers energy to the particle. Using a Gaussian energy loss kernel and the multiple collision theory of Fan and Manson [J. Chem. Phys. 130, 064703 (2009)], enables the determination of the fraction of particles trapped on the surface after subsequent momentum reversals of the colliding particle. This then leads to an estimate of the trapping probability. The theory is tested for the model scattering of Ar on a LiF(100) surface. Comparison with numerical simulations shows excellent agreement of the analytical theory with simulations, provided that the energy loss is determined by the second order perturbation theory.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Experimental validation of a transformation optics based lens for beam steering

A transformation optics based lens for beam control is experimentally realized and measured at microwave frequencies. Laplace's equation is adopted to construct the mapping between the virtual and physical spaces. The metamaterial-based lens prototype is designed using electric LC resonators. A planar microstrip antenna source is used as transverse electric polarized wave launcher for the lens. Both the far field radiation patterns and the near-field distributions have been measured to experimentally demonstrate the beam steering properties. Measurements agree quantitatively and qualitatively with numerical simulations, and a non-narrow frequency bandwidth operation is observed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Two-color QCD at high density

QCD at high chemical potential has interesting properties such as deconfinement of quarks. Two-color QCD, which enables numerical simulations on the lattice, constitutes a laboratory to study QCD at high chemical potential. Among the interesting properties of two-color QCD at high density is the diquark condensation, for which we present recent results obtained on a finer lattice compared to previous studies. The quark propagator in two-color QCD at non-zero chemical potential is referred to as the Gor’kov propagator. We express the Gor’kov propagator in terms of form factors and present recent lattice simulation results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Design of a multi beam klystron cavity from its single beam parameters

The klystron is a well-known microwave amplifier which uses kinetic energy of an electron beam for amplification of the RF signal. There are some limitations of conventional single beam klystron such as high operating voltage, low efficiency and bulky size at higher power levels, which are very effectively handled in Multi Beam Klystron (MBK) that uses multiple low purveyance electron beams for RF interaction. Each beam propagates along its individual transit path through a resonant cavity structure. Multi-Beam klystron cavity design is a critical task due to asymmetric cavity structure and can be simulated by 3D code only. The present paper shall discuss the design of multi beam RF cavities for klystrons operating at 2856 MHz (S-band) and 5 GHz (C-band) respectively. The design approach uses some scaling laws for finding the electron beam parameters of the multi beam device from their single beam counter parts. The scaled beam parameters are then used for finding the design parameters of the multi beam cavities. Design of the desired multi beam cavity can be optimized through iterative simulations in CST Microwave Studio.

42 ENGINEERING↗