Search NASA⌕ Search

SEARCH · Search NASA

Results for “Stable 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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Observations of the structure and evolution of solar flares with a soft X-ray telescope

Soft X ray flare events were observed with the S-056 X-ray telescope that was part of the ATM complement of instruments aboard SKYLAB. Analyses of these data are reported. The observations are summarized and a detailed discussion of the X-ray flare structures is presented. The data indicated that soft X-ray emitted by a flare come primarily from an intense well-defined core surrounded by a region of fainter, more diffuse emission. An analysis of flare evolution indicates evidence for preliminary heating and energy release prior to the main phase of the flare. Core features are found to be remarkably stable and retain their shape throughout a flare. Most changes in the overall configuration seem to be result of the appearance, disappearance or change in brightness of individual features, rather than the restructuring or reorientation of these features. Brief comparisons with several theories are presented.

Vorpahl, J. A.↗
NASA NTRS1985

Short-lived nuclides in the early solar system and general isotopic anomalies in meteorites

Evidence for short-lived nuclides in the early solar system is reviewed. The presence of these nuclides requires the addition to the interstellar medium, from which the solar system formed, of freshly synthesized materials just prior to the formation of the solar system. The best evidence is obtained for Al26. The time scale implied by the Al26 effects is equated with the free-fall time for the collapse of the protosolar cloud. From the observation of ratios of short-lived parent to stable isotopes for nuclides with different half-lives, it is apparent that addition of exotic material to the solar system occurred just prior to its formation. Evidence for a diffuse galactic gamma-ray line emission attributed to Al26 provides support for the existence of Al26 in the protosolar cloud. The time scale for isolation of the protosolar cloud would still be restricted on a timescale comparable to tau 1/2 (Al26).

Papanastassiou, D. A.↗
NASA NTRS2003

Finite Element Method for Capturing Ultra-relativistic Shocks

While finite element methods are used extensively by researchers solving computational fluid dynamics in fields other than astrophysics, their use in astrophysical fluid simulations has been predominantly overlooked. Current simulations using other methods such as finite difference and finite volume (based on finite difference) have shown remarkable results, but these methods are limited by their fundamental properties in aspects that are important for simulations with complex geometries and widely varying spatial and temporal scale differences. We have explored the use of finite element methods for astrophysical fluids in order to establish the validity of using such methods in astrophysical environments. We present our numerical technique applied to solving ultra-relativistic (Lorentz Factor Gamma >> 1) shocks which are prevalent in astrophysical studies including relativistic jets and gamma-ray burst studies. We show our finite element formulation applied to simulations where the Lorentz factor ranges up to 2236 and demonstrate its stability in solving ultra-relativistic flows. Our numerical method is based on the Flowfield Dependent Variation (FDV) Method, unique in that numerical diffusion is derived from physical parameters rather than traditional artificial viscosity methods. Numerical instabilities account for most of the difficulties when capturing shocks in this regime. Our method results in stable solutions and accurate results as compared with other methods.

Richardson, G. A.↗
NASA NTRS2018

Demonstration of Subscale Cermet Fuel Specimen Fabrication Approach Using Spark Plasma Sintering and Diffusion Bonding

Nuclear thermal propulsion (NTP) has the potential to expand the limits of human space exploration by enabling crewed missions to Mars and beyond. The viability of NTP hinges on the development of a robust nuclear fuel material that can perform in the harsh operating environment (> or = 2500K, reactive hydrogen) of a nuclear thermal rocket (NTR) engine. Efforts are ongoing to develop fuel material and to assemble fuel elements that will be stable during the service life of an NTR. Ceramic-metal (cermet) fuels are being actively pursued by NASA Marshall Space Flight Center (MSFC) due to their demonstrated high-temperature stability and hydrogen compatibility. Building on past cermet fuel development research, experiments were conducted to investigate a modern fabrication approach for cermet fuel elements. The experiments used consolidated tungsten (W)-60vol%zirconia (ZrO2) compacts that were formed via spark plasma sintering (SPS). The consolidated compacts were stacked and diffusion bonded to assess the integrity of the bond lines and internal cooling channel cladding. The assessment included hot hydrogen testing of the manufactured surrogate fuel and pure W for 45 minutes at 2500 K in the compact fuel element environmental test (CFEET) system. Performance of bonded W-ZrO2 rods was compared to bonded pure W rods to access bond line integrity and composite stability. Bonded surrogate fuels retained structural integrity throughout testing and incurred minimal mass loss.

Barnes, Marvin W.↗
NASA NTRS2021

On-Orbit Calibration and Performance of NOAA-20 VIIRS Reflective Solar Bands

The NOAA-20 (N20) satellite was launched on November 18, 2017 carrying the second Visible Infrared Imaging Radiometer Suite (VIIRS) instrument. Immediately following the launch, the VIIRS passed a series of intensive calibration and validation tests, after which regular calibration and operation activities have continued successfully for more than three years. The production of NASA Collection 2 Level 1B (C2 L1B) for N20 VIIRS began in summer 2019. In this article, we evaluate the early mission performance of the N20 VIIRS reflective solar bands (RSB) covering the first three full years of operation. The calibrated RSB gains are calculated primarily from the onboard solar diffuser (SD) and used in generating the C2 L1B reflectance and radiance products. We also show the on-orbit performance of the instrument noise, signal-to-noise ratio (SNR), and a reflectance uncertainty assessment. Comparisons are made to the first three years of operation of the first VIIRS instrument, aboard the Suomi National Polar-orbiting Partnership (SNPP) satellite. We evaluate the long-term stability of the calibrated N20 RSB reflectance product by looking at the long-term trends of lunar observations and data from the pseudo-invariant Libya 4 desert site. The N20 RSB have had excellent early mission performance, with changes in the gain of less than 0.5% in the first three years across all detectors, stable L1B reflectance, and very stable values of detector SNR and reflectance uncertainty.

calibration↗
NASA NTRS1990

ULTRA-SHARP nonoscillatory convection schemes for high-speed steady multidimensional flow

For convection-dominated flows, classical second-order methods are notoriously oscillatory and often unstable. For this reason, many computational fluid dynamicists have adopted various forms of (inherently stable) first-order upwinding over the past few decades. Although it is now well known that first-order convection schemes suffer from serious inaccuracies attributable to artificial viscosity or numerical diffusion under high convection conditions, these methods continue to enjoy widespread popularity for numerical heat transfer calculations, apparently due to a perceived lack of viable high accuracy alternatives. But alternatives are available. For example, nonoscillatory methods used in gasdynamics, including currently popular TVD schemes, can be easily adapted to multidimensional incompressible flow and convective transport. This, in itself, would be a major advance for numerical convective heat transfer, for example. But, as is shown, second-order TVD schemes form only a small, overly restrictive, subclass of a much more universal, and extremely simple, nonoscillatory flux-limiting strategy which can be applied to convection schemes of arbitrarily high order accuracy, while requiring only a simple tridiagonal ADI line-solver, as used in the majority of general purpose iterative codes for incompressible flow and numerical heat transfer. The new universal limiter and associated solution procedures form the so-called ULTRA-SHARP alternative for high resolution nonoscillatory multidimensional steady state high speed convective modelling.

Leonard, B. P.↗
NASA NTRS2017

A Consistent EPIC Visible Channel Calibration using VIIRS and MODIS as a Reference

The Earth Polychromatic Imaging Camera (EPIC) aboard the Deep Space Climate Observatory (DSCOVR) satellite constantly images the sunlit disk of Earth from the Lagrange-1 (L1) point in 10 spectral channels spanning the UV, VIS, and NIR spectrums. Recently, the DSCOVR EPIC team has publicly released version 2 dataset, which has implemented improved navigation, stray-light correction, and flat-fielding of the CCD array. The EPIC 2-year data record must be well-calibrated for consistent cloud, aerosol, trace gas, land use and other retrievals. Because EPIC lacks onboard calibrators, the observations made by EPIC channels must be calibrated vicariously using the coincident measurements from radiometrically stable instruments that have onboard calibration systems. MODIS and VIIRS are best-suited instruments for this task as they contain similar spectral bands that are well-calibrated onboard using solar diffusers and lunar tracking. We have previously calibrated the EPIC version 1 dataset by using EPIC and VIIRS angularly matched radiance pairs over both all-sky ocean and deep convective clouds (DCC). We noted that the EPIC image required navigations adjustments, and that the EPIC stray-light correction provided an offset term closer to zero based on the linear regression of the EPIC and VIIRS ray-matched radiance pairs. We will evaluate the EPIC version 2 navigation and stray-light improvements using the same techniques. In addition, we will monitor the EPIC channel calibration over the two years for any temporal degradation or anomalous behavior. These two calibration methods will be further validated using desert and DCC invariant Earth targets. The radiometric characterization of the selected invariant targets is performed using multiple years of MODIS and VIIRS measurements. Results of these studies will be shown at the conference.

Haney, C. O.↗
NASA NTRS1992

The neutral circulation in the vicinity of a stable auroral arc

The effects of asymmetric potential distributions, arc-neutral feedback, and cross-arc winds on the neutral circulation near auroral arc were examined using a sophisticated two-dimensional high-resolution model of the neutral atmosphere which was preconditioned by allowing weak diffuse aurora to exist prior to the formation of an arc. Simulations were performed with asymmetric potential structures and with an initial state which was warmed and accelerated due to a preexisting diffuse aurora, as well as with arc-zonal wind feedback and with large-scale cross-arc flow. It is shown that, with a preexisting zonal flow and asymmetrical arc structures, strong zonal winds are generated within the arc. Simulations with the arc-zonal feedback indicate that the effect of feedback is not large because of the limited duration of an arc. Simulations with a strong large-scale cross-arc wind indicate that the zonal wind is not strongly sensitive to the large-scale wind.

Walterscheid, R. L.↗
NASA NTRS1990

Stability of dendritic arrays

An approximate method for studying steady-state properties and linear stability of the dendritic arrays that are formed in directional solidification of alloys is proposed. This analysis is valid at high growth rates where the primary spacing between dendrites is larger than the velocity-dependent solutal diffusion length. A neutral stability boundary is computed and it is found that, in the situations where the results should be valid, the experimental data of Somboonsuk, et al. (1984) lie in the stable region, well away from the boundary.

Warren, James A.↗
NASA NTRS2000

Gasdynamic Mirror Fusion Propulsion Experiment

A gasdynamic mirror (GDM) fusion propulsion experiment is currently being constructed at the NASA Marshall Space Flight Center (MSFC) to test the feasibility of this particular type of fusion device. Because of the open magnetic field line configuration of mirror fusion devices, they are particularly well suited for propulsion system applications since they allow for the easy ejection of thrust producing plasma. Currently, the MSFC GDM is constructed in three segments. The vacuum chamber mirror segment, the plasma injector mirror segment, and the main plasma chamber segment. Enough magnets are currently available to construct up to three main plasma chamber segments. The mirror segments are also segmented such that they can be expanded to accommodate new end plugging strategies with out requiring the disassembly of the entire mirror segment. The plasma for the experiment is generated in a microwave cavity located between the main magnets and the mirror magnets. Ion heating is accomplished through ambipolar diffusion. The objective of the experiment is to investigate the stability characteristics of the gasdynamic mirror and to map a region of parameter space within which the plasma can be confined in a stable steady state configuration. The mirror ratio, plasma density, and plasma "b" will be varied over a range of values and measurements subsequently taken to determine the degree of plasma stability.

Emrich, Bill↗
NASA NTRS2010

Electrochemical Hydrogen Peroxide Generator

Two-electron reduction of oxygen to produce hydrogen peroxide is a much researched topic. Most of the work has been done in the production of hydrogen peroxide in basic media, in order to address the needs of the pulp and paper industry. However, peroxides under alkaline conditions show poor stabilities and are not useful in disinfection applications. There is a need to design electrocatalysts that are stable and provide good current and energy efficiencies to produce hydrogen peroxide under acidic conditions. The innovation focuses on the in situ generation of hydrogen peroxide using an electrochemical cell having a gas diffusion electrode as the cathode (electrode connected to the negative pole of the power supply) and a platinized titanium anode. The cathode and anode compartments are separated by a readily available cation-exchange membrane (Nafion 117). The anode compartment is fed with deionized water. Generation of oxygen is the anode reaction. Protons from the anode compartment are transferred across the cation-exchange membrane to the cathode compartment by electrostatic attraction towards the negatively charged electrode. The cathode compartment is fed with oxygen. Here, hydrogen peroxide is generated by the reduction of oxygen. Water may also be generated in the cathode. A small amount of water is also transported across the membrane along with hydrated protons transported across the membrane. Generally, each proton is hydrated with 3-5 molecules. The process is unique because hydrogen peroxide is formed as a high-purity aqueous solution. Since there are no hazardous chemicals or liquids used in the process, the disinfection product can be applied directly to water, before entering a water filtration unit to disinfect the incoming water and to prevent the build up of heterotrophic bacteria, for example, in carbon based filters. The competitive advantages of this process are: 1. No consumable chemicals are needed in the process. The only raw materials needed are water and oxygen or air. 2. The product is pure and can therefore be used in disinfection applications directly or after proper dilution with water. 3. Oxygen generated in the anode compartment is used in the electrochemical reduction process; in addition, external oxygen is used to establish a high flow rate in the cathode compartment to remove the desired product efficiently. Exiting oxygen can be recycled after separation of liquid hydrogen peroxide product, if so desired. 4. The process can be designed for peroxide generation under microgravity conditions. 5. High concentrations of the order of 6-7 wt% can be generated by this method. This method at the time of this reporting is superior to what other researchers have reported. 6. The cell design allows for stacking of cells to increase the hydrogen peroxide production. 7. The catalyst mix containing a diquaternary ammonium compound enabled not only higher concentration of hydrogen peroxide but also higher current efficiency, improved energy efficiency, and catalyst stability. 8. The activity of the catalyst is maintained even after repeated periods of system shutdown. 9. The catalyst system can be extended for fuel-cell cathodes with suitable modifications.

Tennakoon, Charles L. K.↗
NASA NTRS1979

Phase 2 of the array automated assembly task for the low cost solar array project

The process sequence for the fabrication of dendritic web silicon into solar panels was modified to include aluminum back surface field formation. Plasma etching was found to be a feasible technique for pre-diffusion cleaning of the web. Several contacting systems were studied. The total plated Pd-Ni system was not compatible with the process sequence; however, the evaporated TiPd-electroplated Cu system was shown stable under life testing. Ultrasonic bonding parameters were determined for various interconnect and contact metals but the yield of the process was not sufficiently high to use for module fabrication at this time. Over 400 solar cells were fabricated according to the modified sequence. No sub-process incompatibility was seen. These cells were used to fabricate four demonstration modules. A cost analysis of the modified process sequence resulted in a selling price of $0.75/peak watt.

Campbell, R. B.↗
NASA NTRS1994

Features of steady magnetospheric convection

The large-scale patterns of ionospheric convection and particle precipitation are described during two intervals of steady magnetospheric convection (SMC) on November 24, 1981. The unique data set used in the analysis includes recordings from the worldwide network of magnetometers and all-sky cameras, global auroral images from the Dynamics Explorer (DE) 1 spacecraft, and particle precipitation data from low-altitude National Oceanic and Atmospheric Administration (NOAA) 6 and NOAA 7 spacecraft. The data show that intense magnetospheric convection continued during more than 10 hours under the steady southward interplanetary magnetic field without any distinct substorm signatures. All data sets available confirmed the stable character of the large-scale magnetospheric configuration during this period. In particular, the magnetic flux threading the polar cap was stable (within 10%) during 3.5 hours of continued DE 1 observations. The dayside cusp was located at an unusually low latitude (70 deg CGL). The nightside auroral pattern consisted of two distinct regions. The diffuse aurora in the equatorward half of the expanded (10 deg wide) auroral oval was well-separated from the bright, active auroral forms found in the vicinity of the poleward boundary of the oval. The twin-vortex convection pattern had no signature of the Harang discontinuity; its nightside 'convection throat' was spatially coincident with the poleward active auroras. This region of the auroral oval was identified as the primary site of the short-lived transient activations during the SMC intervals. The energetic particle observations show that the auroral precipitation up to its high-latitude limit is on closed field lines and that particle acceleration up to greater than 30-keV energy starts close to this limit. The isotropic boundaries of the greater than 30-keV protons and electrons were found close to each other, separating regions of discrete and diffuse precipitation. This suggests that these precipitation types originate on the very taillike and very dipolelike field lines, respectively.

Yahnin, A.↗
NASA NTRS2000

Soot Reaction Properties (Ground-Based Study)

Three major soot reaction processes are needed to predict soot properties in flame environments: soot growth, or the formation of soot on soot nuclei and soot particles; soot oxidation, or the reaction of soot with oxidizing species to yield the combustion products of soot oxidation; and soot nucleation, or the formation of soot nuclei from soot precursors having large molecular weights (generally thought to be large and particularly stable PAH molecules in flame environments, called stabilomers). These processes are addressed in the following, considering soot growth, oxidation and nucleation, in turn, by exploiting the soot and flame structure results for premixed and diffusion flames already discussed in Section 2.

Dai, Z.↗
NASA NTRS2017

The History of Venting (part I)

Venting techniques and design are an important implementation strategy for observatory and payload contamination control, and yet venting analysis has seen a topsey turvey history, at lease from the perspective of the simple Layman trying to design a black box. Additionally, designing the vent has competing controls from Safety and EMIEMC. In the days of Shuttle, Safety placed liens against the vents of blankets, boxes, and large structural items principally to protect cargo bay vents but also from a flammability perspective. What continues to elude the Designer Community is a stable, simple way of designing vents for black boxes that satisfies everybody. But we continue to try.

Contamination Control↗
NASA NTRS1975

Steady-state solutions of a diffusive energy-balance climate model and their stability

A diffusive energy-balance climate model, governed by a nonlinear parabolic partial differential equation, was studied. Three positive steady-state solutions of this equation are found; they correspond to three possible climates of our planet: an interglacial (nearly identical to the present climate), a glacial, and a completely ice-covered earth. Models similar to the main one are considered, and the number of their steady states was determined. All the models have albedo continuously varying with latitude and temperature, and entirely diffusive horizontal heat transfer. The stability under small perturbations of the main model's climates was investigated. A stability criterion is derived, and its application shows that the present climate and the deep freeze are stable, whereas the model's glacial is unstable. The dependence was examined of the number of steady states and of their stability on the average solar radiation.

Ghil, M.↗
NASA NTRS1994

Second-order closure PBL model with new third-order moments: Comparison with LES data

This paper contains two parts. In the first part, a new set of diagnostic equations is derived for the third-order moments for a buoyancy-driven flow, by exact inversion of the prognostic equations for the third-order moment equations in the stationary case. The third-order moments exhibit a universal structure: they all are a linear combination of the derivatives of all the second-order moments, bar-w(exp 2), bar-w theta, bar-theta(exp 2), and bar-q(exp 2). Each term of the sum contains a turbulent diffusivity D(sub t), which also exhibits a universal structure of the form D(sub t) = a nu(sub t) + b bar-w theta. Since the sign of the convective flux changes depending on stable or unstable stratification, D(sub t) varies according to the type of stratification. Here nu(sub t) approximately equal to wl (l is a mixing length and w is an rms velocity) represents the 'mechanical' part, while the 'buoyancy' part is represented by the convective flux bar-w theta. The quantities a and b are functions of the variable N(sub tau)(exp 2), where N(exp 2) = g alpha derivative of Theta with respect to z and tau is the turbulence time scale. The new expressions for the third-order moments generalize those of Zeman and Lumley, which were subsequently adopted by Sun and Ogura, Chen and Cotton, and Finger and Schmidt in their treatments of the convective boundary layer. In the second part, the new expressions for the third-order moments are used to solve the ensemble average equations describing a purely convective boundary laye r heated from below at a constant rate. The computed second- and third-order moments are then compared with the corresponding Large Eddy Simulation (LES) results, most of which are obtained by running a new LES code, and part of which are taken from published results. The ensemble average results compare favorably with the LES data.

Canuto, V. M.↗
NASA NTRS1987

Combustion rate limits of hydrogen plus hydrocarbon fuel: Air diffusion flames from an opposed jet burner technique

Combustion of H2/hydrocarbon (HC) fuel mixtures may be considered in certain volume-limited supersonic airbreathing propulsion applications. Effects of HC addition to H2 were evaluated, using a recent argon-bathed, coaxial, tubular opposed jet burner (OJB) technique to measure the extinction limits of counterflow diffusion flames. The OJB flames were formed by a laminar jet of (N2 and/or HC)-diluted H2 mixture opposed by a similar jet of air at ambient conditions. The OJB data, derived from respective binary mixtures of H2 and methane, ethylene, or propane HCs, were used to characterize BLOWOFF and RESTORE. BLOWOFF is a sudden breaking of the dish-shaped OJB flame to a stable torus or ring shape, and RESTORE marks sudden restoration of the central flame by radial inward flame propagation. BLOWOFF is a measure of kinetically-limited flame reactivity/speed under highly stretched, but relatively ideal impingement flow conditions. RESTORE measures inward radial flame propagation rate, which is sensitive to ignition processes in the cool central core. It is concluded that relatively small molar amounts of added HC greatly reduce the reactivity characteristics of counterflow hydrogen-air diffusion flames, for ambient initial conditions.

Pellett, Gerald L.↗