Dynamic diffusion processes in the exosphere.
Proton diffusion processes in exosphere, noting importance in determining ion scale height above f-2 peak
GREENHOUSE EFFECT↗
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Results for “effective diffusivity”
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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Synthesis of Fullerenes in Low Pressure Benzene/Oxygen Diffusion Flames
The interest in fullerenes is strongly increasing since their discovery by Kroto et al. in 1985 as products of the evaporation of carbon into inert gas at low pressure. Due to their all carbon closed-shell structure, fullerenes have many exceptional physical and chemical properties and a large potential for applications such as superconductors, sensors, catalysts, optical and electronic devices, polymers, high energy fuels, and biological and medical materials. This list is still growing, because the research on fullerenes is still at an early stage. Fullerenes can be formed not only in a system containing only carbon and an inert gas, but also in premixed hydrocarbon flames under reduced pressure and fuel rich conditions. The highest yields of fullerenes in flames are obtained under conditions of substantial soot formation. There is a need for more information on the yields of fullerenes under different conditions in order to understand the mechanisms of their formation and to enable the design of practical combustion systems for large-scale fullerene production. Little work has been reported on the formation of fullerenes in diffusion flames. In order to explore the yields of fullerenes and the effect of low pressure in diffusion flames, therefore we constructed and used a low pressure diffusion flame burner in this study.
Hebgen, Peter↗
Modulation and diffusion theory of cosmic rays
Papers given on the modulation of galactic cosmic rays by the solar wind at the 14th International Cosmic Ray Conference are reviewed. Some of the topics treated in this review are Pioneer and Helios radial gradient measurement, heliolatitude effects in modulation models, diffusion (scattering) theory - including pitch angle diffusion diagrams - solar flare particle transport theory and cosmic ray fluctuations at neutron monitor energies.
Forman, M. A.↗
The effects of thermospheric winds and chemistry in the diurnal variations of thermospheric species
The reported investigation considers on the basis of a theoretical model, the diurnal variations of the thermospheric composition (H, He, O, O2, and Ar) in terms of thermal expansion with diffusive equilibrium and transport effects associated with thermospheric winds, chemistry, and exospheric flow. The theoretical results are compared with satellite composition data which indicate that the fundamental diurnal tide can be reasonably well understood. It is found that winds are only important for molecular oxygen below 180 km, while thermal expansion due to the larger mass is relatively more important for O2 than for O. Distinct from O, photodissociation and in particular photoionization of O2 are very significant for molecular oxygen.
Mayr, H. G.↗
First principles density functional theory study of tritium species adsorption on Ni(111) surface and diffusion in nickel-sublayer for tritium storage
The nickel-plated zircaloy-4 is used as a tritium ( 3 H) getter in the tritium-producing burnable absorber rods (TPBARs) to capture 3 H produced in the 6 Li-riched annular γ-LiAlO 2 pellet under neutron irradiation. The experimental data and our previous theoretical results showed that the 3 H species produced from the γ-LiAlO 2 pellet were mainly 3 H 2 and 3 H 2 O. These 3 H species diffuse from the surface of the LiAlO 2 pellet across vacuum to the nickel-plated zircaloy-4 getter and then further diffuse into the getter to chemically form metal hydrides. While a number of studies show that oxygen binds strongly as compared to 3 H on the nickel (Ni) layer, the detailed mechanism of 3 H species absorption and diffusion across the Ni plate and Ni/Zr interface are still unclear. By employing density functional theory calculations, here we explored the 3 H 2 and 3 H 2 O species adsorption and dissociation on the Ni(111) surface and diffusion into the Ni sublayer. Our results indicated that the 3 H 2 and 3 H 2 O dissociate on the Ni(111) surface. The NiO x and Ni(O 3 H) x could be formed in the Ni layer due to the higher oxygen (O) diffusion energy barrier and formation of Ni vacancy defects. The oxygen was found to be retained in the Ni layer from diffusing across the Ni–Zr interface. This was revealed by comparing the diffusion barriers for 3 H with O. 3 H was found to have nearly three times smaller diffusion barrier than for O, making 3 H comparatively easier to diffuse through the Ni layer. In conclusion, the obtained results provide guidelines for experimental measurements on 3 H retention behavior in TPBARs and may open further avenues to explore the impurity effects on 3 H diffusion and storage at the Ni/zircaloy interfaces.
Tafen, De Nyago [National Energy Technology Lab. (↗
Parametric evaluation of the aerodynamic performance of an annular combustor-diffuser system
This study experimentally determined the effects of dominant geometric and flow parameters on the aerodynamic performance of an annular combustor-diffuser system. For each parametric configuration, data were obtained at 13 different flow rates through the inner annulus, outer annulus, and combustor dome. The parametric effects investigated included the dump gap dimension, flow splits, prediffuser inlet velocity profile, and combustor channel height. During these measurements, the average prediffuser inlet Mach number was 0.305. The measured static pressure recovery coefficient and the total pressure loss coefficient in the prediffuser as well as the overall system are presented in this paper. The study found that the performance of the combustor-diffuser system improves with increasing dome flow rate or dump gap dimension.
Srinivasan, R.↗
Numerical validation of scaling laws for stratified turbulence
Recent theoretical progress using multiscale asymptotic analysis has revealed various possible regimes of stratified turbulence. Notably, buoyancy transport can either be dominated by advection or diffusion, depending on the effective Péclet number of the flow. Two types of asymptotic models have been proposed, which yield measurably different predictions for the characteristic vertical velocity and length scale of the turbulent eddies in both diffusive and non-diffusive regimes. The first, termed a ‘single-scale model’, is designed to describe flow structures having large horizontal and small vertical scales, while the second, termed a ‘multiscale model’, additionally incorporates flow features with small horizontal scales, and reduces to the single-scale model in their absence. By comparing predicted vertical velocity scaling laws with direct numerical simulation data, we show that the multiscale model correctly captures the properties of strongly stratified turbulence within regions dominated by small-scale isotropic motions, whose volume fraction decreases as the stratification increases. Meanwhile its single-scale reduction accurately describes the more orderly, layer-like, quiescent flow outside those regions.
Mechanics↗
Evaluation of Tungsten—Steel Solid-State Bonding: Options and the Role of CALPHAD to Screen Diffusion Bonding Interlayers
Critical aspects of innovative design in engineering disciplines like infrastructure, transportation, and medical applications require the joining of dissimilar materials. This study investigates the literature on solid-state bonding techniques, with a particular focus on diffusion bonding, as an effective method for establishing engineering bonds. Welding and brazing, while widely used, may pose challenges when joining materials with large differences in melting temperature and can lead to mechanical property degradation. In contrast, diffusion bonding offers a lower temperature process that relies on solid-state interactions to develop bond strength. The joining of tungsten and steel, especially for fusion reactors, presents a unique challenge due to the significant disparity in melting temperatures and the propensity to form brittle intermetallics. Here, diffusion characteristics of tungsten–steel interfaces are examined and the influence of bonding parameters on mechanical properties are investigated. Additionally, CALPHAD modeling is employed to explore joining parameters, thermal stability, and diffusion kinetics. The insights from this research can be extended to join numerous dissimilar materials for specific applications such as aerospace, automobile industry, power plants, etc., enabling advanced and robust design with high efficiency.
36 MATERIALS SCIENCE↗
The role of the turbulent Prandtl number in turbine blade heat transfer prediction
Improvement of the prediction of external (gas-to-blade) heat transfer coefficients in gas turbine engines was undertaken. The effects of improved eddy diffusivity of heat modeling in the turbulence model was specifically investigated. A two-dimensional boundary STAN5, was selected and modified by incorporating several different turbulent Prandtl number models. Results indicated that slight effects were attributable to the modified turbulence model. Boundary layer character appeared to be much more significant.
Whitaker, Kevin W.↗
The role of the turbulent Prandtl number in turbine blade heat transfer prediction
Improvement of the prediction of external (gas-to-blade) heat transfer coefficients in gas turbine engines was undertaken. The effects of improved eddy diffusivity of heat modeling in the turbulence model was specifically investigated. A two-dimensional boundary STAN5, was selected and modified by incorporating several different turbulent Prandtl number models. Results indicated that slight effects were attributable to the modified turbulence model. Boundary layer character appeared to be much more significant.
Whitaker, Kevin W.↗
Techniques for improved statistical convergence in quantification of eddy diffusivity moments
While recent approaches, such as the macroscopic forcing method (MFM) or Green's function-based approaches, can be used to compute Reynolds-averaged Navier-Stokes closure operators using forced direct numerical simulations, MFM can also be used to directly compute moments of the effective nonlocal and anisotropic eddy diffusivities. The low-order spatial and temporal moments contain limited information about the eddy diffusivity but are often sufficient for quantification and modeling of nonlocal and anisotropic effects. However, when using MFM to compute eddy diffusivity moments, the statistical convergence can be slow for higher-order moments. In this work, we demonstrate that using the same direct numerical simulation (DNS) for all forced MFM simulations improves statistical convergence of the eddy diffusivity moments. We present its implementation in conjunction with a decomposition method that handles the MFM forcing semianalytically and allows for consistent boundary condition treatment, which we develop for both scalar and momentum transport. We demonstrate that for a two-dimensional Rayleigh-Taylor instability case study, using the same DNS for all forced MFM simulations results in convergence with 𝒪(100) simulations rather than 𝒪(1000) simulations. In conclusion, we then demonstrate the impacts of improved convergence on the quantification of the eddy diffusivity.
general physics↗
Structural and metabolic responses of Staphylococcus aureus biofilms to hyperosmotic and antibiotic stress
Biofilms alter their metabolism in response to environmental stress. This study explores the effect of a hyperosmotic agent–antibiotic treatment on the metabolism of Staphylococcus aureus biofilms through the use of nuclear magnetic resonance (NMR) techniques. To determine the metabolic activity of S. aureus, we quantified the concentrations of metabolites in spent medium using high-resolution NMR spectroscopy. Biofilm porosity, thickness, biovolume, and relative diffusion coefficient depth profiles were obtained using NMR microimaging. Dissolved oxygen (DO) concentration was measured to determine the availability of oxygen within the biofilm. Under vancomycin-only treatment, the biofilm communities switched to anaerobic fermentation, as evidenced by high concentrations of formate, acetate, and lactate, and there was no detectable dissolved oxygen in the biofilm. Anaerobic conditions such as fermentation can signify that biofilm is combating antibiotic stress by developing resistance. In addition, we observed the highest consumption of pyruvate, the sole carbon source, under the vancomycin-only treatment. On the other hand, relative effective diffusion coefficients increased under vancomycin-only treatment but decreased under maltodextrin-only and combined treatments. No change was observed in either biofilm thickness or biovolume for biofilms treated with maltodextrin-only or in combination with vancomycin. This indicates that biofilm growth was halted during maltodextrin-only and combined treatments. Overall, we demonstrated that the metabolic activity of S. aureus biofilm is affected by hyperosmotic and antibiotic stress.
biofilm↗
Kinetic Theory of Spin Diffusion and Superdiffusion in X X Z Spin Chains
We address the nature of spin transport in the integrable XXZ spin chain, focusing on the isotropic Heisenberg limit. We calculate the diffusion constant using a kinetic picture based on generalized hydrodynamics combined with Gaussian fluctuations: we find that it diverges, and show that a self-consistent treatment of this divergence gives superdiffusion, with an effective time-dependent diffusion constant that scales as $D(t) \sim t^{1/3}$. This exponent had previously been observed in large-scale numerical simulations, but had not been theoretically explained. We briefly discuss XXZ models with easy-axis anisotropy $\Delta > 1$. Our method gives closed-form expressions for the diffusion constant $D$ in the infinite-temperature limit for all $\Delta > 1$. We find that $D$ saturates at large anisotropy, and diverges as the Heisenberg limit is approached, as $D \sim (\Delta - 1)^{-1/2}$.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Simulation of the thermospheric tides by use of the NCAR thermospheric general circulation model
Numerical calculations of the thermospheric tidal winds and temperatures at equinox are presented. The calculations were made using the National Center for Atmospheric Research Thermospheric General Circulation Model (TGCM), which includes the effects of viscosity, conductivity, diffusion, ion drag, winds, and temperature gradients. The thermospheric diurnal and semidiurnal tides are excited in situ by solar heating and by ion-neutral momentum coupling. The semidiurnal tidal calculations also include the effects of upward propagating waves generated by heating in the lower atmosphere. This semidiurnal propagating component is modeled by use of the classical tidal perturbations as lower boundary conditions. The model is tuned by adjusting the propagating tidal forcing term until calculated semidiurnal wind and temperature fields best approximate incoherent scatter observations. The tidal TGCM results are consistent with previous theoretical work and successfully reproduce high altitude temperature and meridional velocity data, but they give significantly lower magnitudes for velocities and temperatures near 160 km than are seen by observations.
Fensen, C. G.↗
Diffusive mass transport in agglomerated glassy fallout from a near-surface nuclear test
Aerodynamically-shaped glassy fallout is formed when vapor phase constituents from the nuclear device are incorporated into molten carriers (i.e. fallout precursor materials derived from soil or other near-field environmental debris). The effects of speciation and diffusive transport of condensing constituents are not well defined in models of fallout formation. Previously we reported observations of diffuse micrometer scale layers enriched in Na, Fe, Ca, and 235 U, and depleted in Al and Ti, at the interfaces of agglomerated fallout objects. Here in this paper, we derive the timescales of uranium mass transport in such fallout as it cools from 2500 K to 1500 K by applying a 1-dimensional planar diffusion model to the observed 235 U/ 30 Si variation at the interfaces. By modeling the thermal transport between the fireball and the carrier materials, the time of mass transport is calculated to be <0.6 s, <1 s, <2 s, and <3.5 s for fireball yields of 0.1 kt, 1 kt, 10 kt, and 100 kt respectively. Based on the calculated times of mass transport, a maximum temperature of deposition of uranium onto the carrier material of ~2200 K is inferred (1σ uncertainty of ~200 K). We also determine that the occurrence of micrometer scale layers of material enriched in relatively volatile Na-species as well as more refractory Ca-species provides evidence for an oxygen-rich fireball based on the vapor pressure of the two species under oxidizing conditions. These results represent the first application of diffusion-based modeling to derive material transport, thermal environments, and oxidation-speciation in near-surface nuclear detonation environments.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Experimental Design for Voltage Driven Tracer Incorporation and Diffusion Studies on Oxide Thin Film Electrodes
The effect of an applied overpotential on oxygen isotope incorporation and diffusion in oxide thin film electrodes is investigated by a novel experimental approach. A special electrode geometry leads to in-plane electron flow, perpendicular oxide ion flow and a well-defined laterally varying driving force. This design allows one to obtain a series of tracer depth profiles induced by a range of overpotentials on one and the same thin film. The approach was applied to La 0.8 Sr 0.2 MnO 3 (LSM) thin films deposited by pulsed laser deposition (PLD) on yttria stabilized zirconia (YSZ) single crystals. Tracer depth profiles were measured by secondary ion mass spectrometry (SIMS). These depth profiles include examples of pronounced apparent uphill diffusion that can be explained by considering an interplay of polarization-induced changes in stoichiometry within the LSM grains combined with fast oxygen transport along the grain boundaries.
25 ENERGY STORAGE↗
Low-energy pathways lead to self-healing defects in CsPbBr 3
Self-regulation of free charge carriers in perovskites via Schottky defect formation has been posited as the origin of the well-known defect-tolerance of metal halide perovskite materials. Understanding the mechanisms of self-regulation promises to lead to the fabrication of better performing solar cell materials with higher efficiencies. We investigate many such mechanisms here for CsPbBr 3 , a popular representative of a more commercially viable all-inorganic metal halide perovskite. We investigate different atomic-level mechanisms and pathways of the diffusion and recombination of neutral and charged interstitials and vacancies (Schottky pairs) in CsPbBr 3 . We use nudged elastic band calculations and ab initio-derived pseudopotentials within quantum ESPRESSO to determine energies of formation and migration and hence the activation energies for these defects. While halide vacancies are known to exhibit low formation energies, the migration of interstitials is less studied. Our calculations uncover interstitial defect pathways capable of producing an activation energy at, or below, the single experimental value of 0.53 eV observed for the slow, temperature-dependent recovery of light-induced conductivity in bulk CsPbBr 3 . Our work reveals the existence of a low-energy diffusion pathway involving a concerted “domino effect” of interstitials, with the net result that interstitials can diffuse more readily over longer distances than expected. This observation suggests that defect self-healing can be promoted if the “domino effect” strategy can be engaged.
14 SOLAR ENERGY↗
Perpendicular Diffusion of Solar Energetic Particles: Model Results and Implications for Electrons
The processes responsible for the effective longitudinal transport of solar energetic particles (SEPs) are still not completely understood. We address this issue by simulating SEP electron propagation using a spatially 2D transport model that includes perpendicular diffusion. By implementing, as far as possible, the most reasonable estimates of the transport (diffusion) coefficients, we compare our results, in a qualitative manner, to recent observations at energies of 55–105 keV, focusing on the longitudinal distribution of the peak intensity, the maximum anisotropy, and the onset time. By using transport coefficients that are derived from first principles, we limit the number of free parameters in the model to (i) the probability of SEPs following diffusing magnetic field lines, quantified by a∈[0,1], and (ii) the broadness of the Gaussian injection function. It is found that the model solutions are extremely sensitive to the magnitude of the perpendicular diffusion coefficient and relatively insensitive to the form of the injection function as long as a reasonable value of a = 0.2 is used. We illustrate the effects of perpendicular diffusion on the model solutions and discuss the viability of this process as a dominant mechanism by which SEPs are transported in longitude. Lastly, we try to quantity the effectiveness of perpendicular diffusion as an interplay between the magnitude of the relevant diffusion coefficient and the SEP intensity gradient driving the diffusion process. It follows that perpendicular diffusion is extremely effective early in an SEP event when large intensity gradients are present, while the effectiveness quickly decreases with time thereafter.
79 ASTRONOMY AND ASTROPHYSICS↗