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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Bulk and surface diffusion of neodymium in alpha-uranium: Ab initio calculations and kinetic Monte Carlo simulations

A fundamental understanding of lanthanide transport in metallic fuels is critical for high fidelity modeling of the fuel-cladding chemical interaction (FCCI) phenomenon, which can lead to the formation of brittle intermetallic compounds and premature failure of the cladding. Here we report a combined ab initio density functional theory (DFT) and kinetic Monte Carlo (KMC) study of the bulk diffusivity of Nd in α-U, fully taking into account the effect of radiation enhanced diffusion. In this work, the vacancy mechanism is considered to be the dominant mechanism for the bulk diffusion of Nd since a Nd interstitial is found to be intrinsically unstable in α-U. The surface diffusivity of a Nd adatom on α-U (001) surface has been further predicted using KMC simulations parameterized by DFT calculations. The present study suggests that Nd transport via the surface diffusion mechanism can be many orders of magnitude faster than bulk diffusion. Furthermore, the results from the present lower length scale study can be used to inform mesoscale phase-field simulations to determine the effective diffusion coefficient of Nd through α-U with a porous microstructure.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Code Coverage Status of the ARC Code PERSENT

The Argonne Reactor Code (ARC) software system supports users in their fast reactor design goals by providing neutronic, thermal-hydraulic, and structural analysis capabilities. PERSENT fulfills the role of generating reactivity coefficients for a given time point of a REBUS calculation usable in a point kinetics based safety analysis capability. PERSENT also provides a sensitivity coefficient capability on eigenvalue, reactivity worth, and several other key coefficients that are used in the follow-on safety analysis. Given a co-variance matrix, PERSENT can carry out the uncertainty quantification to indicate the amount of error in the reactivity coefficients derived from the errors in the cross section measurements. With continued improvement of computational resources, many of the geometry modeling capabilities in DIF3D that were primarily used in low order schemes are not really needed anymore. Today, the diffusion and transport capabilities of DIF3D-VARIANT are primarily used in the reactor design process with some scattered usage of DIF3D-FD and DIF3D-Nodal. PERSENT is part of the ARC code system and is built around DIF3D-VARIANT and the flux solution it provides. The purpose of the present work is to identify a set of test problems for PERSENT and assess the code coverage of PERSENT for those test problems. PERSENT treats the DIF3D executable as an external executable and thus the code coverage considerations only need to focus on the PERSENT source code and only a fraction of the connected modules in the existing ARC software library. The goal is to document what parts of the existing PERSENT code are touched by the set of test problems and which are not. Because the verification work done on PERSENT was focused on the most common uses of PERSENT for fast reactor analysis, the code coverage assessment of those capabilities is the highest priority. This will ensure that nothing is being missed by the existing verification test problems that users of PERSENT rely upon. The code coverage analysis of PERSENT was performed with the Code Coverage Tool of the Intel Fortran compiler which requires modifications to the compilation of PERSENT. The detailed coverage tables are given for each submodule of PERSENT. Most of the uncovered parts/files could be easily ignored because they are either for error message and debugging output or not needed by PERSENT today. Only a few uncovered parts of PERSENT deserve extending the verification test suite.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
NASA NTRS1972

The coronal transport of the flare associated scatter free electrons

A total of 11 scatter-free electron events from McMath plage region 8905 were observed by IMP 4 during the period from 29 July to 3 August 1967. The transit times and duration of these electron events are examined in detail. The duration of 170- to 1000-keV electrons shows a strong dependence upon the heliolongitude of the associated flare. Typical values of the duration full width at half maximum vary from 4 min at 50 deg W to 12 min at 10 deg and 90 deg W heliolongitudes. In addition, the difference in times at which the maximum intensities of the 22- to 45-keV and 170- to 1000-keV electrons occur is observed to change from 18 min near 50 deg W to 13 min at 10 deg and 90 deg W heliolongitudes. An idealized two-dimensional diffusion model for the transport of these electrons in the solar corona is proposed. The 22- to 45-keV electrons escape promptly from flare site to the feet of the interplanetary magnetic field lines, whereas 170- to 1000-keV electrons suffer slight scattering. The diffusion coefficient for 170- to 1000-keV electrons is estimated to be 3 X 10 to the 18th power sq cm/sec. Finally, a possible relation between the scatter-free and the classical diffusive-type electron events is discussed.

Wang, J. R.↗
NASA NTRS1974

A wind tunnel investigation of the wake near the trailing edge of a deflected externally blown flap

The model tested was a general research model of a swept-wing, jet-powered STOL transport with externally blown flaps. The model was tested with four engine simulators mounted on pylons under the wing. Tests were conducted in the V/STOL tunnel over an angle of attack range of 0 deg to 16 deg and a thrust coefficient range from 0 to approximately 4 at a Reynolds number of 0.461 x 1 million based on the wing reference chord. The results of this investigation are presented primarily as plots of the individual velocity vectors obtained from the wake survey. These data are used to extend an earlier analysis to isolate the effects of the engine thrust on the behavior of the flow at the flap trailing edge. Results of a comparison with a jet-flap theory are also shown.

Johnson, W. G., Jr.↗
NASA NTRS2007

Robust Gain-Scheduled Fault Tolerant Control for a Transport Aircraft

This paper presents an application of robust gain-scheduled control concepts using a linear parameter-varying (LPV) control synthesis method to design fault tolerant controllers for a civil transport aircraft. To apply the robust LPV control synthesis method, the nonlinear dynamics must be represented by an LPV model, which is developed using the function substitution method over the entire flight envelope. The developed LPV model associated with the aerodynamic coefficient uncertainties represents nonlinear dynamics including those outside the equilibrium manifold. Passive and active fault tolerant controllers (FTC) are designed for the longitudinal dynamics of the Boeing 747-100/200 aircraft in the presence of elevator failure. Both FTC laws are evaluated in the full nonlinear aircraft simulation in the presence of the elevator fault and the results are compared to show pros and cons of each control law.

Shin, Jong-Yeob↗
DOE OSTI2022

Development of an Integrated Multidiagnostic to Assess the High-Z Impurity Fluxes in the Metallic Environment of WEST Using IMAS

WEST is an actively cooled, long-pulse tokamak with nearly all plasma-facing components (PFC) made of tungsten. One of the aims of WEST is to study plasma operations with tungsten PFCs in preparation for long-pulse operations on high-Z divertor devices, such as ITER. For long-pulse operation, the high-Z impurity content and transport to the core plasma are critical concerns that require further measurement and interpretation to improve plasma performance and PFC durability. This work details the impurity influxes in WEST during a series of discharges in which the lower hybrid (LH) injected power was incrementally increased. An analysis has been performed of measurements collected from an array of edge diagnostics. Visible spectroscopy was utilized to measure the spectral radiances generated by fuel particles (D) and impurities (W, O, and C) at the divertor and at the antennas with a newly developed spectral peak-fitting tool used to analyze the data in WEST. The scrape-off layer (SOL) plasma conditions (n e and T e ) measured at the divertor target with flush Langmuir probes and near the outer mid-plane (OMP) using reciprocating Langmuir probes (RCPs) are used to evaluate the number of ionizations per photons (S/XB) coefficients required to estimate the impurity fluxes obtained with the collisional-radiative model ColRadPy. Here, the array of edge diagnostics discussed in this work, coupled with SOL plasma modeling tools, represents a multidiagnostic interpretative modeling workflow that will continue to be applied to upcoming experimental campaigns on the WEST experiment to assess high-Z impurity transport.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2019

Toward Improving Short-Term Predictions of Fine Particulate Matter Over the United States Via Assimilation of Satellite Aerosol Optical Depth Retrievals

This study develops a new approach to improve simulations of the particulate matter of aerodynamic diameter smaller than 2.5μm (PM2.5) in the Community Multiscale Air Quality (CMAQ) model via assimilation of Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth(AOD) retrievals using the Grid point Statistical Interpolation (GSI) system. In contrast to previous studies that only consider errors due to transport, our computation of the background error covariance matrix incorporates uncertainties in anthropogenic emissions. To understand the impact of this approach, three experiments (one background and two assimilations) are performed over the contiguous United States(CONUS) from 15 July to 14 August 2014. The background CMAQ experiment significantly underestimates both the MODIS AOD and surface PM2.5levels. MODIS AOD assimilation pushes both the CMAQ AOD and surface PM2.5distributions toward the observed distributions, but CMAQ still underestimates the observations. Averaged over CONUS, the two assimilation experiments with and without including the anthropogenic emission uncertainties improve the correlation coefficient between the model and independent observations of PM2.5 by ~67% and ~48%, respectively, and reduces the mean bias by ~38% and~10%, respectively. The assimilation improves the model performance everywhere over CONUS, except the New York and Wisconsin, where CMAQ overestimates the observed PM2.5during nighttime after assimilation likely because of overcorrection of aerosol mass concentrations by the AOD assimilation. Future work should incorporate uncertainties in other processes (biomass burning and biogenic emissions, deposition, chemistry, transport, and boundary conditions) to further enhance the value of assimilating spaceborne AOD retrievals.

Assimilation↗
DOE OSTI2024

Hydrogen Diffusion in Slit Pores: Role of Temperature, Pressure, Confinement, and Roughness

Diffusion of hydrogen (H 2 ) is important to understand the leakage risk and transport behavior for H 2 geologic storage. We applied molecular dynamics simulations to investigate the influencing factors of H2 diffusion in the slit pores of calcite, hematite, and quartz, owing to their abundance. It is revealed that the H2 self-diffusion coefficient increases with the temperature, regardless of the type of pore minerals. The diffusion of H 2 in the 20 nm slit pores falls into the bulk diffusion regime when the pressure is 10 MPa. The self-diffusion of H 2 decreases with pressure in all three types of slit pores, following a power law model with the exponents ranging from -0.825 to -0.964. Furthermore, the impact of confinement on H 2 diffusion is more pronounced for the slit pores with stronger interactions with H 2 -like calcite. The role of surface roughness in H 2 diffusion depends on the slit aperture. The rough surface enhances H 2 diffusion in the larger slit pores due to the enlarged effective pore space, whereas it weakens H 2 diffusion in the small slit pores due to stronger adsorption. These findings will fill the knowledge gap on the coupling effect of different factors influencing H 2 diffusion.

08 HYDROGEN↗
DOE OSTI2020

Pressure tuning of the charge density wave and superconductivity in 2 H -TaS 2

Both the vibrational and electrical transport properties of 2 H -TaS 2 have been investigated at high pressures and low temperatures. In this work, the collapse of the charge-density-wave order at pressures above 7.3 GPa has been verified by Raman scattering, resistivity, and Hall coefficient measurements. For pressures above the critical pressure of 7.3 GPa, the superconducting transition temperature continues to increase and reaches its maximum value at 11.5 GPa, suggesting that it is not a simple competition between the charge-density-wave order and superconductivity. Through the standard resistivity fit in the normal state, the decline of the superconducting transition temperature with increasing pressure up to 47.0 GPa is due to the decrease of interaction strength and the increase of the impurity scattering. These results are very important in understanding the superconducting mechanism of transition-metal dichalcogenides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Two self-similar Reynolds-stress transport models with anisotropic eddy viscosity

Two Reynolds-averaged Navier-Stokes models with full Reynolds-stress transport (RST) and tensor eddy viscosity are presented. These new models represent RST extensions of the $k−2L−a−\mathscr{C}$ and $k−ϕ−L−a−\mathscr{C}$ models by Morgan. Self-similarity analysis is used to derive constraints on model coefficients required to reproduce expected growth parameters for a variety of canonical flows, including Rayleigh-Taylor (RT) and Kelvin-Helmholtz (KH) mixing layers. Both models are then applied in one-dimensional simulation of RT and KH mixing layers, and the expected self-similar growth rates and anisotropy are obtained. Next, models are applied in two-dimensional simulation of the so-called “tilted rocket rig” inclined RT experiment and in simulation of a shock-accelerated localized patch of turbulence. Here it is found that RST is required to capture the qualitative growth of the shock-accelerated patch, and an anisotropic eddy viscosity provides substantial improvement over a Boussinesq treatment for the tilted rocket rig problem.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1974

Free-flight wind tunnel investigation of a four-engine sweptwing upper-surface blown transport configuration

The dynamic stability characteristics of a four-engine turbofan transport model having an upper-surface blown-jet flap have been investigated by means of the free-flight technique in the Langley full-scale tunnel. The flight characteristics of the model were investigated through a range of lift coefficients from 3 to 8 with all four engines operating and with one outboard engine not operating. Static characteristics were investigated by conventional power-on force tests over the flight-test angle-of-attack range and through the stall.

Parlett, L. P.↗
NASA NTRS1977

Re-entrant groove heat pipe

This paper describes theoretical and experimentally verified heat pipe characteristics of an axially grooved aluminum extrusion with a re-entrant groove profile. The extrusion is 13 mm diameter with 20 axial grooves, each groove consisting of a nominal .8 mm diameter channel with a .2 mm wide passageway connecting the channel to the hollow core. A computer program was written to compute the zero gravity heat transport capability of the extrusion. A heat pipe was fabricated and its performance characteristics measured. The characteristics of the pipe with ammonia at 20 C are: zero gravity pumping limit 143 w-meters; static wicking height 21.5 mm; evaporator and condenser coefficients 7300 and 20,500 watt/sq m C, respectively.

Harwell, W.↗
NASA NTRS1990

Nongeostrophic theory of zonally averaged circulation. II - Eliassen-Palm flux divergence and isentropic mixing coefficient

A diagnostic study of Eliassen-Palm flux divergence and isentropic mixing coefficient (Kyy) in the stratosphere is performed for different seasons and for both hemispheres, based on the nongeostrophic theory of zonally averaged circulation of Tung (1986). An easily implementable procedure is described which yields a Kyy field that is consistent with advective transport. It is shown that the resultant time- and space-varying structure of Kyy leads to improved simulation of stratospheric species whose distributions are affected by dynamical transport. In particular, it is shown that the simulation of nitric acid is more satisfactory than previous model calculations.

Yang, H.↗
NASA NTRS1993

Large-scale isentropic mixing properties of the Antarctic polar vortex from analyzed winds

Winds derived from analyzed geopotential height fields are used to study quasi-horizontal mixing by the large-scale flow in the lower stratosphere during austral spring. This is the period when the Antarctic ozone hole appears and disappears. Trajectories are computed for large ensembles of particles initially inside and outside the main polar vortex. Mixing and transport are diagnosed through estimates of finite time Lyapunov exponents and Lagrangian dispersion statistics of the tracer trajectories. At 450 K and above prior to the vortex breakdown: Lyapunov exponents are a factor of 2 smaller inside the vortex than outside; diffusion coefficients are an order of magnitude smaller inside than outside the vortex; and the trajectories reveal little exchange of air across the vortex boundary. At lower levels (425 and 400 K) mixing is greater, and there is substantial exchange of air across the vortex boundary. In some years there are large wave events that expel small amounts of vortex air into the mid-latitudes. At the end of the spring season during the vortex breakdown there is rapid mixing of air across the vortex boundary, which is evident in the mixing diagnostics and the tracer trajectories.

Bowman, Kenneth P.↗
NASA NTRS2016

Filled Nd(z)Fe(x)Co(4-x)Sb(12-y)Ge(y) Skutterudites: Processing and Thermoelectric Properties

Skutterudites have proven to be a useful thermoelectric system as a result of their enhanced figure of merit (ZT1), cheap material cost, favorable mechanical properties, and good thermal stability. The majority of skutterudite interest in recent years has been focused on binary skutterudites like CoSb3. Binary skutterudites are often double and triple filled, with a range of elements from the lanthanide series, in order to reduce the lattice component of thermal conductivity. Ternary and quaternary skutterudites, such as Co4Ge6Se6 or Ni4Sb8Sn4, provide additional paths to tune the electronic structure. The thermal conductivity can further be improved in these complex skutterudites by the introduction of fillers. The Nd(z)Fe(x)Co(4-x)Sb(12-y)Ge(y) system has been investigated as a p-type thermoelectric material, and is stable up to 600 C. The influence of Fe and Ge content, along with filler Nd, was investigated on thermoelectric transport properties. In addition to the chemical influence on properties, some processing details of the system will also be addressed.

Thermoelectric↗
NASA NTRS1985

Constraints of solar flare particle transport models from anisotropy observations at Voyager 1

In general a particle transport model for energetic solar flare particles contains a number of free parameters which are determined by fitting various features of observed particle events. Frequently the parameter values are not uniquely determined. In order to place tighter constraints on the models, the anisotropy of 1 and 25 MeV/nuc protons and helium nuclei were examined during the 22 November 1977 solar particle event using data from the LECP experiment on Voyager 1 at 1.6 AU. These observations were combined with the time intensity profiles at Voyager 1 and at 1 AU from ISEE-1 and IMP-8 to determine the magnitude and radial dependence of the interplanetary diffusion coefficient and the required injection duration at the sun. The first order anisotropy amplitudes for both 1 MeV and 25 MeV protons are observed to decrease from maximum values (approx. 1) during the event onset at Voyager 1 to values consistent with convection in the solar wind at about 3 days into the event decay phase. The intensity and anisotropy profiles at 1.6 AU are consistent with predictions of diffusive transport with a modest mean free path (lambda = approx. 0.1 AU).

Hamilton, D. C.↗
DOE OSTI2023

Turbulent entrainment in finite-length wind farms

In this article, we present an entrainment-based model for predicting the flow and power output of finite-length wind farms. The model is an extension of the three-layer approach of Luzzatto-Fegiz & Caulfield ( Phys. Rev. Fluids , vol. 3, 2018, 093802) for wind farms of infinite length, and assumes dependence of key flow quantities, such as the wind farm bulk velocity, on the streamwise distance from the farm entrance. To assist our analysis and validate the proposed model, we undertake a series of large-eddy simulations with different turbine spacing arrangements and layouts. Comparisons are also made with the top-down model with entrance effects of Meneveau ( J. Turbul. , vol. 13, 2012, N7) and data from the literature. The finite-length entrainment model is shown to be capable of capturing the power drop between contiguous rows of turbines as well as describing the advection and turbulent transport of kinetic energy in both the entrance and fully developed regions. The fully developed regime is approximated only deep in the wind farm, after approximately 15 rows of turbines. Our data suggest that for the cases considered in this study, the empirical coefficients that can be used to describe turbulent entrainment and transfers above the wind farm exhibit little dependence on the farm layout and may be considered constant for modelling purposes. However, the flow field within the wind farm layer can be strongly modulated by the turbine density (spacing) as well as the array layout, and to that extent it can be argued that they are both primary factors determining the wind farm power output.

17 WIND ENERGY↗