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Results for “fractional diffusion”

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 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Cervical Gross Tumor Volume Dose Predicts Local Control Using Magnetic Resonance Imaging/Diffusion-Weighted Imaging—Guided High-Dose-Rate and Positron Emission Tomography/Computed Tomography—Guided Intensity Modulated Radiation Therapy

Purpose: Magnetic resonance imaging/diffusion weighted-imaging (MRI/DWI)-guided high-dose-rate (HDR) brachytherapy and {sup 18}F-fluorodeoxyglucose (FDG) — positron emission tomography/computed tomography (PET/CT)-guided intensity modulated radiation therapy (IMRT) for the definitive treatment of cervical cancer is a novel treatment technique. The purpose of this study was to report our analysis of dose-volume parameters predicting gross tumor volume (GTV) control. Methods and Materials: We analyzed the records of 134 patients with International Federation of Gynecology and Obstetrics stages IB1-IVB cervical cancer treated with combined MRI-guided HDR and IMRT from July 2009 to July 2011. IMRT was targeted to the metabolic tumor volume and lymph nodes by use of FDG-PET/CT simulation. The GTV for each HDR fraction was delineated by use of T2-weighted or apparent diffusion coefficient maps from diffusion-weighted sequences. The D100, D90, and Dmean delivered to the GTV from HDR and IMRT were summed to EQD2. Results: One hundred twenty-five patients received all irradiation treatment as planned, and 9 did not complete treatment. All 134 patients are included in this analysis. Treatment failure in the cervix occurred in 24 patients (18.0%). Patients with cervix failures had a lower D100, D90, and Dmean than those who did not experience failure in the cervix. The respective doses to the GTV were 41, 58, and 136 Gy for failures compared with 67, 99, and 236 Gy for those who did not experience failure (P<.001). Probit analysis estimated the minimum D100, D90, and Dmean doses required for ≥90% local control to be 69, 98, and 260 Gy (P<.001). Conclusions: Total dose delivered to the GTV from combined MRI-guided HDR and PET/CT-guided IMRT is highly correlated with local tumor control. The findings can be directly applied in the clinic for dose adaptation to maximize local control.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2016

Quantified Pore-Scale Nanoparticle Transport in Porous Media and the Implications for Colloid Filtration Theory

This study evaluates the pore-scale distribution of silver nanoparticles during transport through a sandy porous medium via quantitative synchrotron X-ray computed microtomography (qSXCMT). The associated distributions of nanoparticle flow velocities and mass flow rates were obtained by coupling these images with computational fluid dynamic (CFD) simulations. This allowed, for the first time, the comparison of nanoparticle mass flow with that assumed by the standard colloid filtration theory (CFT) modeling approach. It was found that (i) 25% of the pore space was further from the grain than assumed by the CFT model; (ii) the average pore velocity agreed well between results of the coupled qSXCMT/CFD approach and the CFT model within the model fluid envelope, although the former were 2 times larger than the latter in the centers of the larger pores and individual velocities were upwards of 20 times those in the CFT model at identical distances from grain surfaces ; and (iii) approximately 30% of all nanoparticle mass and 38% of all nanoparticle mass flow occurred further away from the grain surface than expected by the CFT model. Furthermore, this work suggests that a significantly smaller fraction of nanoparticles than expected will contact a grain surface by diffusion via CFT models, likely contributing to inadequate CFT model nanoparticle transport predictions.

42 ENGINEERING↗
DOE OSTI2016

Calculation of prompt loss and toroidal field ripple loss under neutral beam injection on EAST

Here, neutral beam injection is a major auxiliary heating method in the EAST experimental campaign. This paper gives detailed calculations of beam loss with different plasma equilibria using the guiding center code ORBIT and NUBEAM/TRANSP. Increasing plasma current can dramatically lower the beam ion prompt loss and ripple loss. Countercurrent beam injection gives a much larger prompt loss fraction than co-injection, and ripple-induced collisionless stochastic diffusion is the dominant loss channel.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1988

Determination of the specific area of liquid gas and the velocity of weak pressure waves in aqueous foams

The specific area of an aqueous foam contained in a tube is obtained by determining the fraction of the quantity of light emitted by a source and diffused by the specific area of the column of the foam. The velocity of the waves of weak pressure (which propagate in the air with the velocity of sound) is measured by noticing the moment when the wave penetrates the column and the moment when, having reached the opposite side, it determines a variation of the light diffused by the area of the latter.

Saint-Cloud, Jean↗
NASA NTRS1999

Effects of Gravity on Sheared Turbulence Laden with Bubbles or Droplets

The objective of this numerical/experimental study is to improve the understanding of the effects of gravity on the two-way interaction between dispersed particles (bubbles or liquid droplets) and the carrier turbulent flow. The first phase of the project considers isotropic turbulence. Turbulent homogeneous shear flows laden with droplets/bubbles will be studied in the next phase. The experiments reported here are concerned with the dispersion of liquid droplets by homogeneous turbulence under various gravitational conditions and the effect of these droplets on the evolution of the turbulence of the carrier fluid (air). Direct numerical simulations (DNS) of bubble - laden isotropic decaying turbulence are performed using the two-fluid approach (TF) instead of the Eulerian-Lagrangian approach (EL). The motivation for using the TF formulation is that EL requires considerable computational resources especially for the case of two-way coupling where the instantaneous trajectories of a large number of individual bubbles need to be computed. The TF formulation is developed by spatially averaging the instantaneous equations of the carrier flow and bubble phase over a scale of the order of the Kolmogorov length scale which, in our case, is much larger than the bubble diameter. On that scale, the bubbles are treated as a continuum (without molecular diffusivity) characterized by the bubble phase velocity field and concentration (volume fraction). The bubble concentration, C, is assumed small enough to neglect the bubble-bubble interactions.

Elghobashi, Said↗
DOE OSTI2018

Theoretical analysis of mixing in liquid clouds – Part IV: DSD evolution and mixing diagrams

Evolution of droplet size distribution (DSD) due to mixing between cloudy and dry volumes is investigated for different values of the cloud fraction and for different initial DSD shapes. The analysis is performed using a diffusion–evaporation model which describes time-dependent processes of turbulent diffusion and droplet evaporation within a mixing volume. Time evolution of the DSD characteristics such as droplet concentration, LWC and mean volume radii is analyzed. The mixing diagrams are plotted for the final mixing stages. It is shown that the difference between the mixing diagrams for homogeneous and inhomogeneous mixing is insignificant and decreases with an increase in the DSD width. The dependencies of the normalized cube of the mean volume radius on the cloud fraction were compared with those on normalized droplet concentration and found to be quite different. If the normalized droplet concentration is used, mixing diagrams do not show any significant dependence on relative humidity in the dry volume. The main conclusion of the study is that traditional mixing diagrams cannot serve as a reliable tool for analysis of mixing type.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1993

Mars atmosphere loss and isotopic fractionation by solar-wind-induced sputtering and photochemical escape

The effects of loss of Mars atmospheric constituents by solar-wind-induced sputtering and by photochemical escape during the last 3.8 b.y. were examined. Sputtering is capable of efficiently removing all species from the upper atmosphere, including the light noble gases; N also is removed by photochemical processes. Due to the diffusive separation by mass above the homopause, removal from the top of the atmosphere will fractionate the isotopes of each species, with the lighter mass being preferentially lost. For C and O, this allows us to determine the size of nonatmospheric reservoirs that mix with the atmosphere; these reserviors can be accounted for by exchange with CO2 adsorbed in the regolith and with H2O in the polar ice deposits. Both simple analytical models and time-dependent models of the loss of volatiles from and supply to the Martian atmosphere were constructed. Both Ar and Ne require continued replenishment from outgassing over geologic time.

Jakosky, B. M.↗
NASA NTRS1993

Mars atmospheric loss and isotopic fractionation by solar-wind-induced sputtering and photochemical escape

We examine the effects of loss of Mars atmospheric constituents by solar-wind-induced sputtering and by photochemical escape during the last 3.8 b.y. Sputtering is capable of efficiently removing all species from the upper atmosphere including the light noble gases; N is removed by photochemical processes as well. Due to diffusive separation (by mass) above the homopause, removal from the top of the atmosphere will fractionate the isotopes of each species with the lighter mass being preferentially lost. For C and O, this allows us to determine the size of nonatmospheric reservoirs that mix with the atmosphere; these reservoirs can be CO2 adsorbed in the regolith or H2O in the polar ice caps. We have constructed both simple analytical models and time-dependent models of the loss from and supply of volatiles to the Martian atmosphere.

Jakosky, B. M.↗
NASA NTRS1994

Mars atmospheric loss and isotopic fractionation by solar-wind-induced sputtering and photochemical escape

We examine the effects of the loss of Mars atmospheric constituents by solar-wind-induced sputtering and by photochemical escape during the past 3.8 billion years. Sputtering is capable of efficiently removing species from the upper atmosphere, including the light noble gases; nitrogen and oxygen are removed by photochemical processes as well. Due to diffusive separation (by mass) above the homopause, removal from the top of the atmosphere will fractionate the isotopes of each species, with the lighter mass being preferentially lost. For carbon and oxygen, this allows us to determine the size of nonatmospheric reservoirs which mix with the atmosphere; these reservoirs can be CO2 adsorbed in the regolith and H2O in the polar ice caps. We have constructed both simple analytical models and time-dependent models of the loss of volatiles from and supply to the martian atmosphere. Both argon and neon require continued replenishment from outgassing over geologic time. For argon, sputtering loss explains the fractionation of (Ar-36)/(Ar-38) without requiring a distinct epoch of hydrodynamic escape (although fractionation of Xe isotopes still requires very early hydrodynamic loss). For neon, the current (Ne-22)/(Ne-20) ratio represents a balance between loss to space and continued resupply from the interior; the similarity of the ratio to the terrestrial value is coincidental. For nitrogen, the loss by both sputtering and photochemical escape would produce a fractionation of (N-15)/(N-14) larger than observed; an early, thicker carbon dioxide atmosphere could mitigate the nitrogen loss and produce the observed fractionation, as could continued outgassing of juvenile nitorgen. Based on the isotopic constraints, the total amount of carbon dioxide lost over geologic time is probably on the order of tens of millibars rather than a substantial fraction of a bar. The total loss from solar-wind-induced sputtering and photochemical escape, therefore, does not seem able to explain the loss of a putative thick, early atmosphere withput requiring formation of extensive surface carbonate deposits or other nonatmospheric reservoirs for CO2.

Jakosky, Bruce M.↗
DOE OSTI2018

Siderophile element constraints on the thermal history of the H chondrite parent body

The abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Pd), as well as 187 Re- 187 Os and 182 Hf- 182 W isotopic systematics were determined for separated metal, slightly magnetic, and nonmagnetic fractions from seven H4 to H6 ordinary chondrites. The HSE are too abundant in nonmagnetic fractions to reflect metal-silicate equilibration. The disequilibrium was likely a primary feature, as 187 Re- 187 Os data indicate only minor open-system behavior of the HSE in the slightly and non-magnetic fractions. 182 Hf- 182 W data for slightly magnetic and nonmagnetic fractions define precise isochrons for most meteorites that range from 5.2 ± 1.6 Ma to 15.2 ± 1.0 Ma after calcium aluminum inclusion (CAI) formation. By contrast, 182W model ages for the metal fractions are typically 2–5 Ma older than the slope-derived isochron ages for their respective, slightly magnetic and nonmagnetic fractions, with model ages ranging from 1.4 ± 0.8 Ma to 12.6 ± 0.9 Ma after CAI formation. This indicates that the W present in the silicates and oxides was not fully equilibrated with the metal when diffusive transport among components ceased, consistent with the HSE data. Further, the W isotopic compositions of size-sorted metal fractions from some of the H chondrites also differ, indicating disequilibrium among some metal grains. The chemical/isotopic disequilibrium of siderophile elements among H chondrite components is likely the result of inefficient diffusion of siderophile elements from silicates and oxides to some metal and/or localized equilibration as H chondrites cooled towards their respective Hf-W closure temperatures. The tendency of 182 Hf- 182 W isochron ages to young from H5 to H6 chondrites may indicate derivation of these meteorites from a slowly cooled, undisturbed, concentrically-zoned parent body, consistent with models that have been commonly invoked for H chondrites. Overlap of isochron ages for H4 and H5 chondrites, by contrast, appear to be more consistent with shallow impact disruption models. The W isotopic composition of metal from one CR chondrite was examined to compare with H chondrite metals. In contrast to the H chondrites, the CR chondrite metal is characterized by an enrichment in 183 W that is consistent with nucleosynthetic s-process depletion. Once corrected for the correlative nucleosynthetic effect on 182 W, the 182 W model age for this meteorite of 7.0 ± 3.6 Ma is within the range of model ages of most metal fractions from H chondrites. Finally, the metal is therefore too young to be a direct nebular condensate, as proposed by some prior studies.

58 GEOSCIENCES↗
DOE OSTI2016

Geometric consideration of nanostructures for energy storage systems

Battery performance and its fade are determined by various aspects such as the transport of ions and electrons through heterogeneous internal structures; kinetic reactions at the interfaces; and the corresponding interplay between mechanical, chemical, and thermal responses. The fundamental factor determining this complex multiscale and multiphysical nature of a battery is the geometry of active materials. In this work, we systematically consider the tradeoffs among a selection of limiting geometries of media designed to store ions or other species via a diffusion process. Specifically, we begin the investigation by considering diffusion in spheres, rods, and plates at the particle level, in order to assess the effects of geometry, diffusivity, and rate on capacity. Then, the study is extended to considering of the volume fraction and particle network, as well as kinetics at the interface with electrolyte. Our study suggests that, in terms of overall bulk level material performance, thin film batteries may generate the highest energy density with high power capability when they are implemented at nanoscales or with highly diffusion materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Experimental Studies and Model Analysis of Noble Gas Fractionation in Porous Media

Core Ideas Ambient noble gases in porous media fractionate from through‐flow of other gases. CO 2 fluxes and noble‐gas fractionation are measured in simple sandbox experiments. Modeled fractionation provides indirect proxy, e.g., of steady CO 2 leakage fluxes. The noble gases, which are chemically inert under normal terrestrial conditions but vary systematically across a wide range of atomic mass and diffusivity, offer a multicomponent approach to investigating gas dynamics in unsaturated soil horizons, including transfer of gas between saturated zones, unsaturated zones, and the atmosphere. To evaluate the degree to which fractionation of noble gases in the presence of an advective–diffusive flux agrees with existing theory, a simple laboratory sand column experiment was conducted. Pure CO 2 was injected at the base of the column, providing a series of constant CO 2 fluxes through the column. At five fixed sampling depths within the system, samples were collected for CO 2 and noble gas analyses, and ambient pressures were measured. Both the advection–diffusion and dusty gas models were used to simulate the behavior of CO 2 and noble gases under the experimental conditions, and the simulations were compared with the measured depth‐dependent concentration profiles of the gases. Given the relatively high permeability of the sand column (5 × 10 −11 m 2 ), Knudsen diffusion terms were small, and both the dusty gas model and the advection–diffusion model accurately predicted the concentration profiles of the CO 2 and atmospheric noble gases across a range of CO 2 flux from ∼700 to 10,000 g m −2 d −1 . The agreement between predicted and measured gas concentrations demonstrated that, when applied to natural systems, the multi‐component capability provided by the noble gases can be exploited to constrain component and total gas fluxes of non‐conserved (CO 2 ) and conserved (noble gas) species or attributes of the soil column relevant to gas transport, such as porosity, tortuosity, and gas saturation.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRSDate not supplied

Surface Radiation Budget (SRB) Release 2 Shortwave 3 hourly Monthly Data in Native Format (SRB_REL2_SW_3HRLY_MONTHLY)

The SRB data include the average upward and downward fluxes, photosynthetically active radiative flux, aerosol and cloud optical depth, cloud fraction, and solar zenith angle at three hourly intervals for each day for the entire globe between 07/01/1983 and 10/31/1995. These parameters were derived with the Shortwave algorithm of the NASA World Climate Research Programme/Global Energy and Water-Cycle Experiment (WCRP/GEWEX) Surface Radiation Budget (SRB) Project. [Location=GLOBAL] [Temporal_Coverage: Start_Date=1983-07-01; Stop_Date=1998-07-26] [Spatial_Coverage: Southernmost_Latitude=-90; Northernmost_Latitude=90; Westernmost_Longitude=-180; Easternmost_Longitude=180] [Data_Resolution: Latitude_Resolution=1 degree; Longitude_Resolution=Ranges from 1 degree (tropics and subtropics 120 degrees (the poles); Temporal_Resolution=3 hourly; Temporal_Resolution_Range=3 hourly].

AEROSOL OPTICAL DEPTH↗
NASA NTRS1993

Alveolar ventilation to perfusion heterogeneity and diffusion impairment in a mathematical model of gas exchange

This study describes a two-compartment model of pulmonary gas exchange in which alveolar ventilation to perfusion (VA/Q) heterogeneity and impairment of pulmonary diffusing capacity (D) are simultaneously taken into account. The mathematical model uses as input data measurements usually obtained in the lung function laboratory. It consists of two compartments and an anatomical shunt. Each compartment receives fractions of alveolar ventilation and blood flow. Mass balance equations and integration of Fick's law of diffusion are used to compute alveolar and blood O2 and CO2 values compatible with input O2 uptake and CO2 elimination. Two applications are presented. The first is a method to partition O2 and CO2 alveolar-arterial gradients into VA/Q and D components. The technique is evaluated in data of patients with chronic obstructive pulmonary disease (COPD). The second is a theoretical analysis of the effects of blood flow variation in alveolar and blood O2 partial pressures. The results show the importance of simultaneous consideration of D to estimate VA/Q heterogeneity in patients with diffusion impairment. This factor plays an increasing role in gas alveolar-arterial gradients as severity of COPD increases. Association of VA/Q heterogeneity and D may produce an increase of O2 arterial pressure with decreasing QT which would not be observed if only D were considered. We conclude that the presented computer model is a useful tool for description and interpretation of data from COPD patients and for performing theoretical analysis of variables involved in the gas exchange process.

NASA Discipline Environmental Health↗
NASA NTRS2014

Precision Control of Thermal Transport in Cryogenic Single-Crystal Silicon Devices

We report on the diffusive-ballistic thermal conductance of multi-moded single-crystal silicon beams measured below 1 K. It is shown that the phonon mean-free-path is a strong function of the surface roughness characteristics of the beams. This effect is enhanced in diffuse beams with lengths much larger than, even when the surface is fairly smooth, 510 nm rms, and the peak thermal wavelength is 0.6 microns. Resonant phonon scattering has been observed in beams with a pitted surface morphology and characteristic pit depth of 30 nm. Hence, if the surface roughness is not adequately controlled, the thermal conductance can vary significantly for diffuse beams fabricated across a wafer. In contrast, when the beam length is of order, the conductance is dominated by ballistic transport and is effectively set by the beam cross-sectional area. We have demonstrated a uniformity of +/-8% in fractional deviation for ballistic beams, and this deviation is largely set by the thermal conductance of diffuse beams that support the micro-electro-mechanical device and electrical leads. In addition, we have found no evidence for excess specific heat in single-crystal silicon membranes. This allows for the precise control of the device heat capacity with normal metal films. We discuss the results in the context of the design and fabrication of large-format arrays of far-infrared and millimeter wavelength cryogenic detectors.

multi-moded single-crystal silicon beams↗
DOE OSTI2023

Ensemble flow reconstruction in the atmospheric boundary layer from spatially limited measurements through latent diffusion models

Due to costs and practical constraints, field campaigns in the atmospheric boundary layer typically only measure a fraction of the atmospheric volume of interest. Machine learning techniques have previously successfully reconstructed unobserved regions of flow in canonical fluid mechanics problems and two-dimensional geophysical flows, but these techniques have not yet been demonstrated in the three-dimensional atmospheric boundary layer. Here, we conduct a numerical analogue of a field campaign with spatially limited measurements using large-eddy simulation. We pose flow reconstruction as an inpainting problem, and reconstruct realistic samples of turbulent, three-dimensional flow with the use of a latent diffusion model. The diffusion model generates physically plausible turbulent structures on larger spatial scales, even when input observations cover less than 1% of the volume. Through a combination of qualitative visualization and quantitative assessment, we demonstrate that the diffusion model generates meaningfully diverse samples when conditioned on just one observation. These samples successfully serve as initial conditions for a large-eddy simulation code. We find that diffusion models show promise and potential for other applications for other turbulent flow reconstruction problems.

17 WIND ENERGY↗
DOE OSTI2014

Precision control of thermal transport in cryogenic single-crystal silicon devices

We report on the diffusive-ballistic thermal conductance of multi-moded single-crystal silicon beams measured below 1 K. It is shown that the phonon mean-free-path ℓ is a strong function of the surface roughness characteristics of the beams. This effect is enhanced in diffuse beams with lengths much larger than ℓ, even when the surface is fairly smooth, 5–10 nm rms, and the peak thermal wavelength is 0.6 μm. Resonant phonon scattering has been observed in beams with a pitted surface morphology and characteristic pit depth of 30 nm. Hence, if the surface roughness is not adequately controlled, the thermal conductance can vary significantly for diffuse beams fabricated across a wafer. In contrast, when the beam length is of order ℓ, the conductance is dominated by ballistic transport and is effectively set by the beam cross-sectional area. We have demonstrated a uniformity of ±8% in fractional deviation for ballistic beams, and this deviation is largely set by the thermal conductance of diffuse beams that support the micro-electro-mechanical device and electrical leads. In addition, we have found no evidence for excess specific heat in single-crystal silicon membranes. This allows for the precise control of the device heat capacity with normal metal films. We discuss the results in the context of the design and fabrication of large-format arrays of far-infrared and millimeter wavelength cryogenic detectors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗