SEARCH · Search NASA

Results for “Importance sampling”

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.

SourceResearch connectionFrom yearThrough year

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

At least 973 records · Page 54

All sources NASA NTRS DOE OSTI

DOE OSTI2019

Using machine learning to predict antimicrobial MICs and associated genomic features for nontyphoidal Salmonella

Nontyphoidal Salmonella species are the leading bacterial cause of food-borne disease in the United States. Whole genome sequences and paired antimicrobial susceptibility data are available for Salmonella strains because of surveillance efforts from public health agencies. In this study, a collection of 5,278 nontyphoidal Salmonella genomes, collected over 15 years in the United States, were used to generate XGBoost-based machine learning models for predicting minimum inhibitory concentrations (MICs) for 15 antibiotics. The MIC prediction models have an overall average accuracy of 95% within ± 1 two-fold dilution step (confidence interval of 95-95%), an average very major error rate of 2.7% (confidence interval of 2.4-3.0%) and an average major error rate of 0.1% (confidence interval of 0.1-0.2%). The model predicts MICs with no a priori information about the underlying gene content or resistance phenotypes of the strains. By selecting diverse genomes for training sets, we show that highly accurate MIC prediction models can be generated with fewer than 500 genomes. We also show that our approach for predicting MICs is stable over time despite annual fluctuations in antimicrobial resistance gene content in the sampled genomes. Finally, using feature selection, we explore the important genomic regions identified by the models for predicting MICs. To date, this is one of the largest MIC modeling studies to be published. Furthermore, our strategy for developing whole genome sequence-based models for surveillance and clinical diagnostics can be readily applied to other important human pathogens.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2015

Computer simulation of the CSPAD, ePix10k, and RayonixMX170HS X-ray detectors

The invention of free-electron lasers (FELs) has opened a door to an entirely new level of scientific research. The Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory is an X-ray FEL that houses several instruments, each with its own unique X-ray applications. This light source is revolutionary in that while its properties allow for a whole new range of scientific opportunities, it also poses numerous challenges. For example, the intensity of a focused X-ray beam is enough to damage a sample in one mere pulse; however, the pulse speed and extreme brightness of the source together are enough to obtain enough information about that sample, so that no further measurements are necessary. An important device in the radiation detection process, particularly for X-ray imaging, is the detector. The power of the LCLS X-rays has instigated a need for better performing detectors. The research conducted for this project consisted of the study of X-ray detectors to imitate their behaviors in a computer program. The analysis of the Rayonix MX170-HS, CSPAD, and ePix10k in particular helped to understand their properties. This program simulated the interaction of X-ray photons with these detectors to discern the patterns of their responses. A scientist’s selection process of a detector for a specific experiment is simplified from the characterization of the detectors in the program.

43 PARTICLE ACCELERATORS↗

DOE OSTI2024

Power Production, Inter- and Intra-Array Wake Losses from the U.S. East Coast Offshore Wind Energy Lease Areas

There is an urgent need to develop accurate predictions of power production, wake losses and array–array interactions from multi-GW offshore wind farms in order to enable developments that maximize power benefits, minimize levelized cost of energy and reduce investment uncertainty. New, climatologically representative simulations with the Weather Research and Forecasting (WRF) model are presented and analyzed to address these research needs with a specific focus on offshore wind energy lease areas along the U.S. east coast. These, uniquely detailed, simulations are designed to quantify important sources of wake-loss projection uncertainty. They sample across different wind turbine deployment scenarios and thus span the range of plausible installed capacity densities (ICDs) and also include two wind farm parameterizations (WFPs; Fitch and explicit wake parameterization (EWP)) and consider the precise WRF model release used. System-wide mean capacity factors for ICDs of 3.5 to 6.0 MWkm−2 range from 39 to 45% based on output from Fitch and 50 to 55% from EWP. Wake losses are 27–37% (Fitch) and 11–19% (EWP). The discrepancy in CF and wake losses from the two WFPs derives from two linked effects. First, EWP generates a weaker ‘deep array effect’ within the largest wind farm cluster (area of 3675 km2), though both parameterizations indicate substantial within-array wake losses. If 15 MW wind turbines are deployed at an ICD of 6 MWkm−2 the most heavily waked wind turbines generate an average of only 32–35% of the power of those that experience the freestream (undisturbed) flow. Nevertheless, there is no evidence for saturation of the resource. The wind power density (electrical power generation per unit of surface area) increases with ICD and lies between 2 and 3 Wm−2. Second, EWP also systematically generates smaller whole wind farm wakes. Sampling across all offshore wind energy lease areas and the range of ICD considered, the whole wind farm wake extent for a velocity deficit of 5% is 1.18 to 1.38 times larger in simulations with Fitch. Over three-quarters of the variability in normalized wake extents is attributable to variations in freestream wind speeds, turbulent kinetic energy and boundary layer depth. These dependencies on meteorological parameters allow for the development of computationally efficient emulators of wake extents from Fitch and EWP.

17 WIND ENERGY↗

DOE OSTI2023

Reconstructing Ly$α$ Fields from Low-resolution Hydrodynamical Simulations with Deep Learning

Hydrodynamical cosmological simulations are a powerful tool for accurately predicting the properties of the intergalactic medium (IGM) and for producing mock skies that can be compared against observational data. However, the need to resolve density fluctuation in the IGM puts a stringent requirement on the resolution of such simulations, which in turn limits the volumes that can be modeled, even on the most powerful supercomputers. In this work, we present a novel modeling method that combines physics-driven simulations with data-driven generative neural networks to produce outputs that are qualitatively and statistically close to the outputs of hydrodynamical simulations employing eight times higher resolution. We show that the Ly$α$ flux field, as well as the underlying hydrodynamic fields, have greatly improved statistical fidelity over a low-resolution simulation. Importantly, the design of our neural network allows for sampling multiple realizations from a given input, enabling us to quantify the model uncertainty. Using test data, we demonstrate that this model uncertainty correlates well with the true error of the Ly$α$ flux prediction. Ultimately, our approach allows for training on small simulation volumes and applying it to much larger ones, opening the door to producing accurate Ly$α$ mock skies in volumes of Hubble size, as will be probed with DESI and future spectroscopic sky surveys.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1971

Some physical properties of Apollo 12 lunar samples

The size distribution of the lunar fines is measured, and small but significant differences are found between the Apollo 11 and 12 samples as well as among the Apollo 12 core samples. The observed differences in grain size distribtuion in the core samples are related to surface transportation processes, and the importance of a sedimentation process versus meteoritic impact gardening of the mare grounds is discussed. The optical and the radio frequency electrical properties are measured and are also found to differ only slightly from Apollo 11 results.

Gold, T.↗

NASA NTRS1973

Carbon chemistry of the Apollo 15 and 16 deep drill cores

The carbon chemistry of the Apollo 15 and 16 deep drill cores is a function of the surface exposure plus the chemical and mineralogical composition of the individual samples. The depth profiles of carbide and methane yields in the Apollo 15 core show a general decline with depth and correlate with the solar wind noble gas content, percentage agglutinates, track densities, and metallic iron. All horizons examined were exposed for a considerable time on the lunar surface. The Apollo 16 core samples show that chemical and mineralogical composition plays an important role in determining the nature of carbide-like material present in the fines. The higher aluminum and calcium contents and lower iron contents of highlands material result in carbide-like material yielding less CD4 and more C2D2 (deuteroacetylene) upon DF acid dissolution.

Wszolek, P. C.↗

NASA NTRS1975

Technology for return of planetary samples

The problem of returning a Mars sample to Earth was considered. The model ecosystem concept was advanced as the most reliable, sensitive method for assessing the biohazard from the Mars sample before it is permitted on Earth. Two approaches to ecosystem development were studied. In the first approach, the Mars sample would be introduced into the ecosystem and exposed to conditions which are as similar to the Martian environment as the constitutent terrestrial organisms can tolerate. In the second approach, the Mars sample would be tested to determine its effects on important terrestrial cellular functions. In addition, efforts were directed toward establishing design considerations for a Mars Planetary Receiving Laboratory. The problems encountered with the Lunar Receiving Laboratory were evaluated in this context. A questionnaire was developed to obtain information regarding important experiments to be conducted in the Planetary Receiving Laboratory.

Source record↗

NASA NTRS1982

Glasses of impact origin from Apollo 11, 12, 15, and 16 - Evidence for fractional vaporization and mare/highland mixing

Electron microprobe analyses have been performed on glasses of impact origin in Apollo 11 breccias (10059, 10060, 10061), Apollo 12 soil (12070), and Apollo 15 breccias (15318, 15425, 15426, 15427). These glasses were produced by shock melting of regolith, rather than of rock. Simple concepts for better understanding and interpreting the chemical data from impact glasses have been developed. These concepts are a significant improvement on earlier strategies, which centered principally on cluster analysis. Using ratios of refractory lithophile elements, the compositional effects of fractional vaporization often associated with impact melting to obtain chemical information about the mare and highland components in the regoliths parental to the glasses have been 'seen through'. This method is also applied to the mare-derived impact glasses from Apollo 16 in order to place constraints on the types of volcanic components occurring in Mare Nectaris. Since impact glasses can be used to derive chemical constraints on the indigenous lithologies comprising multi-component regoliths; the frequent occurrence of these glasses, as well as their low masses, should make them critically important for study when small quantities of grab-samples are returned by future unmanned spacecraft from planets, satellites, and asteroids where regoliths are present.

Delano, J. W.↗

NASA NTRS1983

Analysis of defect structure in silicon. Characterization of SEMIX material. Silicon sheet growth development for the large area silicon sheet task of the low-cost solar array project

Statistically significant quantitative structural imperfection measurements were made on samples from ubiquitous crystalline process (UCP) Ingot 5848 - 13C. Important correlation was obtained between defect densities, cell efficiency, and diffusion length. Grain boundary substructure displayed a strong influence on the conversion efficiency of solar cells from Semix material. Quantitative microscopy measurements gave statistically significant information compared to other microanalytical techniques. A surface preparation technique to obtain proper contrast of structural defects suitable for quantimet quantitative image analyzer (QTM) analysis was perfected and is used routinely. The relationships between hole mobility and grain boundary density was determined. Mobility was measured using the van der Pauw technique, and grain boundary density was measured using quantitative microscopy technique. Mobility was found to decrease with increasing grain boundary density.

Natesh, R.↗

NASA NTRS1984

Sensitivity of bone cell populations to weightlessness and simulated weightlessness

A rat suspension model for simulating certain aspects of weightlessness is discussed. Perturbations in physiological systems induced by this head down suspension model are verified by flight data. Findings of a suppression of osteoblast differentiation help explain the inhibition of bone formation inflight and during Earth-bound simulations. Since the anatomical site for these studies was in the maxilla, which is gravity loaded but non weightbearing in ground-based simulations, the similarity of bone cell kinetic changes, both inflight and in the ground-based model, suggest that fluid shifts rather than unloading may play an important role in bone alterations, at least at this sampling site.

Roberts, W. E.↗

NASA NTRS1984

Radiosonde pressure sensor performance - Evaluation using tracking radars

The standard balloon-borne radiosonde employed for synoptic meteorology provides vertical profiles of temperature, pressure, and humidity as a function of elapsed time. These parameters are used in the hypsometric equation to calculate the geopotential altitude at each sampling point during the balloon's flight. It is important that the vertical location information be accurate. The present investigation was conducted with the objective to evaluate the altitude determination accuracy of the standard radiosonde throughout the entire balloon profile. The tests included two other commercially available pressure sensors to see if they could provide improved accuracy in the stratosphere. The pressure-measuring performance of standard baroswitches, premium baroswitches, and hypsometers in balloon-borne sondes was correlated with tracking radars. It was found that the standard and premium baroswitches perform well up to about 25 km altitude, while hypsometers provide more reliable data above 25 km.

Parsons, C. L.↗

NASA NTRS1986

The Earth's magnetosphere as a sample of the plasma universe

Plasma processes in the Earth's neighborhood determine the environmental conditions under which space-based equipment for science or technology must operate. These processes are peculiar to a state of matter that is rare on Earth but dominates the universe as whole. The physical, and especially the electrodynamic, properties of this state of matter is still far from well understood. By fortunate circumstances, the magnetosphere-ionosphere system of the Earth provides a rich sample of widely different plasma populations, and, even more importantly, it is the site of a remarkable variety of plasma processes. In different combinations such processes must be important throughout the universe, which is overwhelmingly dominated by matter in the plasma state. Therefore, observations and experiments in the near-Earth plasma serve a multitude of purposes. They will not only (1) clarify the dynamics of the space environment but also (2) widen the understanding of matter, (3) form a basis for interpretating remote observations of astrophysical objects, thereby even (4) help to reconstruct events that led to the evolution of the solar system. Last but not least they will (5) provide know-how required for adapting space-based technology to the plasma environment. Such observations and experiments will require a close mutual interplay between science and technology.

Faelthammar, Carl-Gunne↗

NASA NTRS1990

Filling in the retinal image

The optics of the eye form an image on a surface at the back of the eyeball called the retina. The retina contains the photoreceptors that sample the image and convert it into a neural signal. The spacing of the photoreceptors in the retina is not uniform and varies with retinal locus. The central retinal field, called the macula, is densely packed with photoreceptors. The packing density falls off rapidly as a function of retinal eccentricity with respect to the macular region and there are regions in which there are no photoreceptors at all. The retinal regions without photoreceptors are called blind spots or scotomas. The neural transformations which convert retinal image signals into percepts fills in the gaps and regularizes the inhomogeneities of the retinal photoreceptor sampling mosaic. The filling-in mechamism plays an important role in understanding visual performance. The filling-in mechanism is not well understood. A systematic collaborative research program at the Ames Research Center and SRI in Menlo Park, California, was designed to explore this mechanism. It was shown that the perceived fields which are in fact different from the image on the retina due to filling-in, control some aspects of performance and not others. Researchers have linked these mechanisms to putative mechanisms of color coding and color constancy.

Larimer, James↗

NASA NTRS1989

Surface composition of solid-rocket exhausted aluminum oxide particles

Particulate samples of aluminum oxide were collected on Teflon filters from the exhaust plume of the Space Shuttle (STS-61A, October 30, 1985) over the altitude interval 4.6-7.6 km immediately after launch. These particles were analyzed using SEM, energy-dispersive X-ray analysis, electron spectroscopy for chemical analysis, X-ray fluorescent spectroscopy, and conventional wet-chemical techniques. The samples were 0.6-1.0 percent surface-chlorided (chlorided meaning predominantly aluminum chlorides and oxychlorides, possibly including other adsorbed forms of chloride) by weight. This level of chloriding is about one-third of the amount determined previously from laboratory-prepared alumina and surface site samples of solid-rocket-produced alumina (SRPA) after both had been exposed to moist HCl vapor at temperatures down to ambient. This level is equivalent to previous laboratory results with samples exposed to moist HCl at temperatures above the boiling point of water. It is suggested that the present lower chloriding levels, determined for samples from a 'dry' Shuttle exhaust cloud, underscore the importance of a liquid water/hydrochloric acid phase in governing the extent of surface chloriding of SRPA. The reduced chloriding is not trivial with respect to potential physical/chemical modification of the SRPA particle surfaces and their corresponding interaction with the atmosphere.

Cofer, Wesley R., III↗

NASA NTRS1993

Filtering Interpolators for Image Comparison Algorithms

Comparing two or more images, either by differencing or ratioing, is important to many remote sensing problems. Because the pixel sample points for the images are (almost) always separated by some nonzero shift, a resampling, or interpolation, process must be performed if one image is to be accurately compared to another. Considered in Fourier space, an interpolator acts as a filter that attenuates some frequencies (usually high) of the image. Thus, when the shifted and unshifted images are compared, the former has been filtered, while the latter has not; the effect of this difference is called interpolation error. The key idea of this paper is to apply a filter to the unshifted image that matches the filtering effect of applying the interpolator to the shifted image, thereby drastically reducing interpolation error. The resulting interpolators, called filtering interpolators, are derived and discussed in detail elsewhere. Basic results will be given in this presentation.

Lucke, R. L.↗

NASA NTRS1993

Energy loss partitioning during ballistic impact of polymer composites

The objective of this study is to determine the energy dissipation processes in polymer-matrix composites during impact of ballistic projectiles. These processes include heat, fiber deformation and breakage, matrix deformation and fracture, and interfacial delamination. In this study, experimental measurements were made, using specialized specimen designs and test methods, to isolate the energy consumed by each of these processes during impact in the ballistic range. Using these experiments, relationships between material parameters and energy dissipation were examined. Composites with the same matrix but reinforced with Kevlar, PE, and graphite fabric were included in this study. These fibers were selected based on the differences in their intrinsic properties. Matrix cracking was found to be one of the most important energy absorption mechanisms during impact, especially in ductile samples such as Spectra-900 PE and Kevlar-49 reinforced polymer. On the contrary, delamination dominated the energy dissipation in brittle composites such as graphite reinforced materials. The contribution from frictional forces was also investigated and the energy partitioning among the different processes evaluated.

Zee, Ralph H.↗

NASA NTRS1994

Completing below-ground carbon budgets for pastures, recovering forests, and mature forests of Amazonia

The objective of this grant was to complete below-ground carbon budgets for pastures and forest soils in the Amazon. Profiles of radon and carbon dioxide were used to estimate depth distribution of CO2 production in soil. This information is necessary for determining the importance of deep roots as sources of carbon inputs. Samples were collected for measuring root biomass from new research sites at Santana de Araguaia and Trombetas. Soil gases will be analyzed for CO2 and (14)CO2, and soil organic matter will be analyzed for C-14. Estimates of soil texture from the RADAMBRASIL database were merged with climate data to calculate soil water extraction by forest canopies during the dry season. In addition, a preliminary map of areas where deep roots are needed for deep soil water was produced. A list of manuscripts and papers prepared during the reporting periods is given.

Davidson, Eric A.↗

NASA NTRS1995

Tests and consequences of disk plus halo models of gamma-ray burst sources

The gamma-ray burst observations made by the Burst and Transient Source Experiment (BATSE) and by previous experiments are still consistent with a combined Galactic disk (or Galactic spiral arm) plus extended Galactic halo model. Testable predictions and consequences of the disk plus halo model are discussed here; tests performed on the expanded BATSE database in the future will constrain the allowed model parameters and may eventually rule out the disk plus halo model. Using examples, it is shown that if the halo has an appropriate edge, BATSE will never detect an anisotropic signal from the halo of the Andromeda galaxy. A prediction of the disk plus halo model is that the fraction of the bursts observed to be in the 'disk' population rises as the detector sensitivity improves. A careful reexamination of the numbers of bursts in the two populations for the pre-BATSE databases could rule out this class of models. Similarly, it is predicted that different satellites will observe different relative numbers of bursts in the two classes for any model in which there are two different spatial distribiutions of the sources, or for models in which there is one spatial distribution of the sources that is sampled to different depths for the two classes. An important consequence of the disk plus halo model is that for the birthrate of the halo sources to be small compared to the birthrate of the disk sources, it is necessary for the halo sources to release many orders of magnitude more energy over their bursting lifetime than the disk sources. The halo bursts must also be much more luminous than the disk bursts; if this disk-halo model is correct, it is necessary to explain why the disk sources do not produce halo-type bursts.

Smith, I. A.↗