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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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

The theory of Langmuir probes in strong electrostatic potential structures

The operation of collecting and emitting Langmuir probes and double probes within time-stationary strong electrostatic potential structures is analyzed. The cross sections of spherical and cylindrical probes to charged particles within the structures are presented and used to obtain the current-voltage characteristics of idealized probes. The acquisition of plasma parameters from these characteristics is outlined, and the operation of idealized floating double-probe systems is analyzed. Probe surface effects are added to the idealized theory, and some surface effects pertinent to spacecraft probes are quantified. Magnetic field effects on idealized probes are examined, and the time required for floating probes to change their potentials by collecting charge and by emitting photoelectrons is discussed. Calculations on the space-charge effects of probe-perturbed beams and on the space-charge limiting of electron emission are given in an appendix.

Borovsky, J. E.↗
NASA NTRS2011

The Effects of Surface Roughness on the NEAR XRS Elemental Results: Monte-Carlo Modeling

The objective of the NEAR-Shoemaker X-ray Gamma-Ray Spec1roscopy ("XGRS") investigation was to determine the elemental composition of the near-Earth asteroid 433 Eros. The X-ray Spectrometer (XRS) system measured the characteristic fluorescence of six major elements (Mg, Al, Si, S, Ca, Fe) in the 1-10 keV energy range excited by the interaction of solar X-rays with the upper 100 microns of the surface of 433 Eros. Various investigators, using both laboratory experiments and computer simulations have established that X-ray fluorescent line ratios can be influenced by small-scale surface roughness at high incidence or emission angles. The effect on the line ratio is specific to the geometry, excitation spectrum, and composition involved, In general, however, the effect is only substantial for ratios of lines with a significant energy difference between them: Fe/Si and Ca/Si are much more likely to be affected than AI/Si or Mg/Si. We apply a Monte-Carlo code to the specific geometry and spectrum of a major NEAR XRS solar flare observation, using an H chondrite composition as the substrate. The seventeen most abundant elements were included in the composition model, from oxygen to titanium.

Lin, Lucy F.↗
DOE OSTI2017

Effect of surface loading and bulk doping of La{sup 3+} on the thermal stability and photocatalytic activity of Bi{sub 2}O{sub 2}CO{sub 3}

Graphical abstract: The Bi{sub 2}O{sub 2}O{sub 3} modified La{sup 3+} was synthesized using two methods of co-precipitation (La-Bi{sub 2}O{sub 2}O{sub 3}) and wet impregnation (La/Bi{sub 2}O{sub 2}O{sub 3}). As compared with La-Bi{sub 2}O{sub 2}O{sub 3}, Bi{sub 2}O{sub 2}CO{sub 3} can be effectively stabilized by impregnation method, as La{sup 3+} is mainly dispersed onto the surface of Bi{sub 2}O{sub 2}O{sub 3}. In photocatalytic degradation of MO, it is found that the surface modification with La{sup 3+} can enhance the photocatalytic efficiency of Bi{sub 2}O{sub 2}O{sub 3}. The surface loading La{sup 3+} could act as an effective electron scavenger to trap the CB electrons of Bi{sub 2}O{sub 2}CO{sub 3} to reduce the recombination of electrons and holes in the surface. - Highlights: • The effect of La{sup 3+} on the thermal stability of Bi{sub 2}O{sub 2}CO{sub 3} is different for the impregnation method and coprecipitation method. • Bi2O2CO3 can be effectively stabilized by impregnation method, as La3+ is mainly dispersed onto the surface of Bi2O2O3. • Surface loading La3+ is benefit for the improvement in the photocatalytic activity of Bi2O2CO3. • Surface modification of La3+ greatly suppresses the charge recombination of photogenerated electron–hole pairs in Bi2O2CO3. - Abstract: The Bi{sub 2}O{sub 2}O{sub 3} modified La{sup 3+} was synthesized using two methods of co-precipitation (La-Bi{sub 2}O{sub 2}O{sub 3}) and wet impregnation (La/Bi{sub 2}O{sub 2}O{sub 3}). The effects of the above two modification methods on the stability of Bi{sub 2}O{sub 2}CO{sub 3} after calcination and the physicochemical properties of Bi{sub 2}O{sub 2}CO{sub 3} were investigated by XRD, SEM, BET, and XPS. As compared with La-Bi{sub 2}O{sub 2}O{sub 3}, it is observed that Bi{sub 2}O{sub 2}CO{sub 3} can be effectively stabilized by impregnation method, as La{sup 3+} is mainly dispersed onto the surface of Bi{sub 2}O{sub 2}O{sub 3}. The photocatalytic activities for the methyl orange degradation for the La/Bi{sub 2}O{sub 2}CO{sub 3} and the La-Bi{sub 2}O{sub 2}CO{sub 3} samples were tested, and the results showed that surface doping (impregnation method) is benefit for the improvement in the photocatalytic activity, as compared with bulk doping (coprecipitation method). PL characterization revealed the differences in the photocatalytic process for surface doping and bulk doping. The surface modification of La{sup 3+} greatly suppresses the charge recombination of photogenerated electron−hole pairs in Bi{sub 2}O{sub 2}CO{sub 3}, which would be beneficial to improve their photocatalytic activity.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effects of Surface and Top Wind Shear on the Spatial Organization of Marine Stratocumulus-Topped Boundary Layers

Typically, the convective nature of Stratocumulus topped boundary layers (STBL) involves the motion of updrafts and downdrafts, driven by surface fluxes and radiative cooling, respectively. The balance between shear and buoyant forcings at the surface can determine the organization of updrafts between cellular and roll structures. We investigate the effect of varying shear at the surface and top of the STBL using Large Eddy Simulations, taking DYCOMS II RF01 as a base case. We focus on spatial identification of the following features: coherent updrafts and downdrafts, and observe how they are affected by varying shear. Stronger surface shear organizes the updrafts in rolls, causes less well-mixed thermodynamic profiles, and decreases cloud fraction and liquid water path (LWP). Stronger top shear also decreases cloud fraction and LWP more than surface shear, by thinning the cloud from the top. Features with stronger top than surface shear are associated with a net downward momentum transport and show early signs of decoupling. Classifying updrafts and downdrafts based on their vertical span and horizontal size confirms the dominance of tall objects spanning the whole STBL. Tall objects occupy 30% of the volume in the STBL, while short ones occupy less than 1%. For updraft and downdraft fluxes, these tall objects explain 65% of the vertical velocity variance and 83% of the buoyancy flux, on average. Stronger top shear also weakens the contribution of downdrafts to the turbulent fluxes and tilts the otherwise vertical development of updrafts.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1992

Surface tension effects on the onset of double-diffusive convection

Experiments have been carried out to determine the critical thermal Rayleigh number for onset of convection in a horizontal layer of density-stratified fluid with a free surface when heated from below. Three different aqueous solutions were used: salt, glycerol, and acetic acid. The rates of change in surface tension with concentration for these three solutions are positive, nearly zero, and negative, respectively. Compared to the rigid-rigid boundaries, the critical thermal Rayleigh number was found to be larger by 11.2 percent for the salt solution and smaller by 10.0 percent for the glycerol solution. With the acetic acid solution, however, the effect of the free surface was found to be negligible.

Chen, C. F.↗
NASA NTRS1939

The Effects of Surface Waviness and of Rib Stitching on Wing Drag

Surface waviness and rib stitching have been investigated as part of a series of tests to determine the effects on wing drag of common surface irregularities. The tests were made in the N.A.C.A. 8-foot high-speed wind tunnel at Reynolds Numbers up to 17,000,000. The results of the tests showed that the waviness common to airplane wings will cause no serious increase in drag unless the waviness exists on the forward part of the wing, where it may cause premature transition or premature compressibility effects. Waves 3 inches wide and 0.048 inch high, for example, increased the drag 1 percent when they covered the rear 67 percent of both surfaces and 10 percent when they covered the rear 92 percent. A single wave 3 inches wide and only 0.020 inch high at the 10.5-percent-chord point on the upper surface caused transition to occur on the wave and increased the drag 6 percent. Rib stitching increased the drag 7 percent when the rib spacing was 6 inches; the drag increment was proportional to the number of ribs for wider spacings. About one-third of the increase was due to premature transition at the forward ends of the stitching.

Hood, Manley J↗
DOE OSTI2018

Effect of Surface Heterogeneity on the Boundary-Layer Height: A Case Study at a Semi-Arid Forest

Here, we investigate the effects of an isolated meso-$\gamma$-scale surface heterogeneity for roughness and albedo on the atmospheric boundary-layer (ABL) height, with a case study at a semi-arid forest surrounded by sparse shrubland (forest area: 28 km 2 , forest length in the main wind direction: 7 km). Doppler lidar and ceilometer measurements at this semi-arid forest show an increase in the ABL height over the forest compared with the shrubland on four out of eight days. The differences in the ABL height between shrubland and forest are explained for all days with a model that assumes a linear growth of the internal boundary layer of the forest through the convective ABL upwind of the forest followed by a square-root growth into the stable free atmosphere. For the environmental conditions that existed during our measurements, the increase in ABL height due to large sensible heat fluxes from the forest (600 W~m -2 in summer) is subdued by stable stratification in the free atmosphere above the ABL, or reduced by high wind speeds in the mixed layer.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1971

Effects of surface roughness on the photometric properties of Mars

Starting with a macroscopically flat surface, which at each point scatters according to the Minnaert law with exponent k, the photometric effects of increasing the large-scale roughness of the Martian surface were investigated. The photometric properties of macroscopically rough surfaces can still be described by the Minnaert law, but with an exponent k star, in general not equal to k. Therefore observed values of the Minnaert exponent cannot be used to infer the small-scale surface properties of Mars (or of any planet) unless proper allowance is made for the photometric effects of large-scale roughness. The azimuthal dependence of k star provides a sensitive test for the importance of these effects.

Veverka, J.↗
NASA NTRS1983

Surface roughness effects with solid lubricants dispersed in mineral oils

The lubricating effectiveness of solid-lubricant dispersions are investigated in both point and line contacts using surfaces with both random and directional roughness characteristics. Friction and wear data obtained at relatively low speeds and at room temperature, indicate that the existence of solid lubricants such as graphite, MoS2, and PTFE in a plain mineral oil generally will not improve the effectiveness of the oil as a lubricant for such surfaces. Under boundary lubrication conditions, the friction force, as a function of time, initially depends upon the directional roughness properties of the contacting surfaces irrespective of whether the base oil or dispersions are used as lubricants.

Cusano, C.↗
NASA NTRS2003

Dwell Time and Surface Parameter Effects on Removal of Silicone Oil From D6ac Steel Using TCA

This study was conducted to evaluate the impact of dwell time, surface roughness, and the surface activation state on 1,1,1-trichloroethane's (TCA's) effectiveness for removing silicone oil from D6ac steel. Silicone-contaminated test articles were washed with TCA solvent, and then the surfaces were analyzed for residue, using Fourier transform infrared spectroscopy. The predominant factor affecting the ability to remove the silicone oil was surface roughness.

Boothe, R. E.↗
NASA NTRS1972

Effects of surface roughness on the photometric properties of Mars.

Starting with a macroscopically flat surface which at each point scatters according to the Minnaert law with exponent k, we investigate the photometric effects of increasing the large scale roughness of the surface. It is found that the photometric properties of macroscopically rough surfaces can still be described by the Minnaert law, but with an exponent k1, in general not equal to k. In fact, examples are given where k1 differs considerably from k. Therefore observed values of the Minnaert exponent cannot be used to infer the small scale surface properties of Mars (or of any planet) unless proper allowance is made for the photometric effects of large scale roughness. The azimuthal dependence of k1 provides a sensitive test for the importance of these effects.

Veverka, J.↗
DOE OSTI2023

Effect of surface roughness on phase transition timing in megaampere pulsed-power–driven exploding conductors

An understanding of material phase transitions in megaampere pulsed-power–driven exploding conductors is important for predicting the growth of hydrodynamic instabilities in magneto-inertial fusion concepts. Here, this study analyzes phase transitions in electrical conductor explosions using 1D Lagrangian and 2D arbitrary Lagrangian–Eulerian resistive magnetohydrodynamic simulations to show that micrometer-scale surface roughness can lead to the electrothermal instability (ETI), a feedback effect that concentrates resistive heating and leads to early melting and ablation. Simulations of the Mykonos electrothermal instability II (METI-II) experiment show melting begins 19% sooner for machined rods with micrometer-scale surface roughness than for rods without these features. The surface magnetic field is 41 T around the initial region of melt, representing a lower magnitude than both the 86 T from 1D simulations and the 85 T threshold reported elsewhere. In 2D simulations with micrometer-scale surface roughness, temperature measurements indicate the critical point temperature of aluminum is reached 17% faster in comparison with 1D simulations. Values from 2D simulations with surface roughness align with predictions from ETI theory, and the observed temperature redistribution further supports the ETI as an underlying mechanism. Simulation results are validated against experimental photonic Doppler velocimetry data. This study shows 1D simulations are adequate to model conductors with sub-micrometer-scale surface roughness in this high-energy-density regime; however, 2D or 3D simulations are required to capture the full range of physics for accurately describing phase transitions in conductors with micrometer-scale or larger surface roughness.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1976

Determination of surface recombination velocity in heavily doped silicon

A method was developed and successfully tested for the determination of the effective surface recombination velocity of silicon layers doped by diffusion of phosphorus to a level of 10 to the 19th to 10 to the 21st per cu cm. The effective recombination velocity was obtained from the dependence of the electron-beam-induced current on the penetration of the electron beam of a scanning electron microscope. A special silicon diode was constructed which permitted the collection at the p-n junction of the carriers excited by the electron beam. This diode also permitted the study of the effects of surface preparation on the effective surface recombination velocity.

Watanabe, M.↗
NASA NTRS1985

Analysis of a two row hydrostatic journal bearing with variable properties, inertia effects and surface roughness

A computer algorithm for simulation of hydrostatic journal bearing pressure-flow behavior has been generated. The effects taken into account are inertia, cavitation, variable properties (isothermal bearing) and roughness. The program has been specifically tailored for simulation of the hybrid bearing of the cryogenic turbopumps of the main shuttle engine. Due to the high pressure (515 psia) of the supply line no cavitation has been found. The influence of the roughness effects have been found to become important only when the surface-roughness order of magnitude is comparable with that of the bearing clearance itself. Pocket edge inertia and variable properties have been found to have quite an important influence upon the pocket pressure, field pressure distribution and lubricant mass flow.

Braun, M. J.↗
NASA NTRS1984

The effect of surface contamination on thermocapillary flow in a two-dimensional slot

In an analysis of the effect of insoluble surfactants on the steady capillary flow in a shallow slot, the problem is formulated as a two-dimensional channel heated differentially from the side, with a free surface on which an insoluble surface species is adsorbed. The vertical boundaries are considered to be isothermal and impenetrable and the lower horizontal boundary to be insulated and impenetrable. Lubrication theory and matched asymptotic expansions are used to reduce the problem to a single nonlinear integral-algebraic equation. For small elasticity E, the surface velocity is nearly constant, with the result that, for small Pe, the surface concentration is nearly uniform while, for large Pe, it develops a boundary layer near the stagnation point at the cold wall, resulting in high local surface concentrations. Condensed surfactant layers will form only with E much less than one and Pe much greater than one.

Homsy, G. M.↗
NASA NTRS1991

Titanium aluminides - Surface composition effects as a function of temperature

Results are presented from an AES investigation of the surface characteristics of pure Ti, Ti-14Al-21Nb (alpha-2 aluminide), and Ti-33Al-6Nb-1.4Ta (gamma aluminide) under ultrahigh vacuum conditions over the 20-1000 C temperature range. The observed variations in surface composition are noted to be dramatic and indicative of a direct relationship between the amount of retained oxygen and carbon on the surface and the quantity of Al in that surface's vicinity. While TiO2 was the main surface oxide for pure Ti and the alpha-2 and gamma aluminides, only metallic peaks were observable at 600 C; at 1000 C, metallic Al and Ti were observed.

Outlaw, R. A.↗
DOE OSTI2017

Modeling quantum yield, emittance, and surface roughness effects from metallic photocathodes

Here, detailed measurements of momentum distributions of emitted electrons have allowed the investigation of the thermal limit of the transverse emittance from metal photocathodes. Furthermore, recent developments in material design and growth have resulted in photocathodes that can deliver high quantum efficiency and are sufficiently robust to use in high electric field gradient photoinjectors and free electron lasers. The growth process usually produces photoemissive material layers with rough surface profiles that lead to transverse accelerating fields and possible work function variations, resulting in emittance growth. To better understand the effects of temperature, density of states, and surface roughness on the properties of emitted electrons, we have developed realistic three-dimensional models for photocathode materials with grated surface structures. They include general modeling of electron excitation due to photon absorption, charge transport, and emission from flat and rough metallic surfaces. The models also include image charge and field enhancement effects. We report results from simulations with flat and rough surfaces to investigate how electron scattering, controlled roughness, work function variation, and field enhancement affect emission properties. Comparison of simulation results with measurements of the quantum yield and transverse emittance from flat Sb emission surfaces shows the importance of including efficient modeling of photon absorption, temperature effects, and the material density of states to achieve agreement with the experimental data.

36 MATERIALS SCIENCE↗