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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 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
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 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↗
DOE OSTI2016

An alternative realization of the exact non-Markovian stochastic Schrödinger equation

Based on the path integral approach, we derive a new realization of the exact non-Markovian stochastic Schrödinger equation (SSE). The main difference from the previous non-Markovian quantum state diffusion (NMQSD) method is that the complex Gaussian stochastic process used for the forward propagation of the wave function is correlated, which may be used to reduce the amplitude of the non-Markovian memory term at high temperatures. The new SSE is then written into the recently developed hierarchy of pure states scheme, in a form that is more closely related to the hierarchical equation of motion approach. Numerical simulations are then performed to demonstrate the efficiency of the new method.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Generation of attosecond electron bunches upon laser pulse propagation through a sharp plasma boundary

The formation of an electron bunch produced due to electron self-injection into a wake wave that is generated by a relativistic-intensity laser pulse propagating through a sharp boundary of semi-bounded plasma is studied in one-dimensional geometry. Analytical expressions are obtained for estimating the bunch length and the energy spread of electrons in the bunch. Numerical simulation is performed which confirms the results of analytical consideration. (paper)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

HYSTERESIS BETWEEN DISTINCT MODES OF TURBULENT DYNAMOS

Nonlinear mean-field models of the solar dynamo show long-term variability, which may be relevant to different states of activity inferred from long-term radiocarbon data. This paper is aimed at probing the dynamo hysteresis predicted by the recent mean-field models of Kitchatinov and Olemskoy with direct numerical simulations. We perform three-dimensional (3D) simulations of large-scale dynamos in a shearing box with helically forced turbulence. As an initial condition, we either take a weak random magnetic field or we start from a snapshot of an earlier simulation. Two quasi-stable states are found to coexist in a certain range of parameters close to the onset of the large-scale dynamo. The simulations converge to one of these states depending on the initial conditions. When either the fractional helicity or the magnetic Prandtl number is increased between successive runs above the critical value for onset of the dynamo, the field strength jumps to a finite value. However, when the fractional helicity or the magnetic Prandtl number is then decreased again, the field strength stays at a similar value (strong field branch) even below the original onset. We also observe intermittent decaying phases away from the strong field branch close to the point where large-scale dynamo action is just possible. The dynamo hysteresis seen previously in mean-field models is thus reproduced by 3D simulations. Its possible relation to distinct modes of solar activity such as grand minima is discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Multiple lensing of the cosmic microwave background anisotropies

We study the gravitational lensing effect on the Cosmic Microwave Background (CMB) anisotropies performing a ray-tracing of the primordial CMB photons through intervening large-scale structures (LSS) distribution predicted by N-Body numerical simulations with a particular focus on the precise recovery of the lens-induced polarized counterpart of the source plane. We apply both a multiple plane ray-tracing and an effective deflection approach based on the Born approximation to deflect the CMB photons trajectories through the simulated lightcone. We discuss the results obtained with both these methods together with the impact of LSS non-linear evolution on the CMB temperature and polarization power spectra. We compare our results with semi-analytical approximations implemented in Boltzmann codes like, e.g., CAMB. We show that, with our current N-body setup, the predicted lensing power is recovered with good accuracy in a wide range of multipoles while excess power with respect to semi-analytic prescriptions is observed in the lensing potential on scales ℓ ∼> 3000. We quantify the impact of the numerical effects connected to the resolution in the N-Body simulation together with the resolution and band-limit chosen to synthesise the CMB source plane. We found these quantities to be particularly important for the simulation of B-mode polarization power spectrum.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

How to simulate global cosmic strings with large string tension

Global string networks may be relevant in axion production in the early Universe, as well as other cosmological scenarios. Such networks contain a large hierarchy of scales between the string core scale and the Hubble scale, ln( f {sub a} / H ) ∼ 70, which influences the network dynamics by giving the strings large tensions T ≅ π f {sub a} {sup 2} ln( f {sub a} / H ). We present a new numerical approach to simulate such global string networks, capturing the tension without an exponentially large lattice.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

SCALING PROPERTIES OF SMALL-SCALE FLUCTUATIONS IN MAGNETOHYDRODYNAMIC TURBULENCE

Magnetohydrodynamic (MHD) turbulence in the majority of natural systems, including the interstellar medium, the solar corona, and the solar wind, has Reynolds numbers far exceeding the Reynolds numbers achievable in numerical experiments. Much attention is therefore drawn to the universal scaling properties of small-scale fluctuations, which can be reliably measured in the simulations and then extrapolated to astrophysical scales. However, in contrast with hydrodynamic turbulence, where the universal structure of the inertial and dissipation intervals is described by the Kolmogorov self-similarity, the scaling for MHD turbulence cannot be established based solely on dimensional arguments due to the presence of an intrinsic velocity scale—the Alfvén velocity. In this Letter, we demonstrate that the Kolmogorov first self-similarity hypothesis cannot be formulated for MHD turbulence in the same way it is formulated for the hydrodynamic case. Besides profound consequences for the analytical consideration, this also imposes stringent conditions on numerical studies of MHD turbulence. In contrast with the hydrodynamic case, the discretization scale in numerical simulations of MHD turbulence should decrease faster than the dissipation scale, in order for the simulations to remain resolved as the Reynolds number increases.

79 ASTRONOMY AND ASTROPHYSICS↗