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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 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

On twistors and conformal field theories from six dimensions

We discuss chiral zero-rest-mass field equations on six-dimensional space-time from a twistorial point of view. Specifically, we present a detailed cohomological analysis, develop both Penrose and Penrose-Ward transforms, and analyse the corresponding contour integral formulae. We also give twistor space action principles. We then dimensionally reduce the twistor space of six-dimensional space-time to obtain twistor formulations of various theories in lower dimensions. Besides well-known twistor spaces, we also find a novel twistor space amongst these reductions, which turns out to be suitable for a twistorial description of self-dual strings. For these reduced twistor spaces, we explain the Penrose and Penrose-Ward transforms as well as contour integral formulae.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Facile synthesis of highly active reduced graphene oxide-CuI catalyst through a simple combustion method for photocatalytic reduction of CO{sub 2} to methanol

We report a facile combustion method synthesis of reduced graphene oxide/CuI composites as a photocatalyst, in which CuI nanoparticles were homogeneously distributed on the surface of reduced graphene oxide (rGO), showing a good visible light response. The rGO-supported and unsupported CuI hybrids were tested over the photocatalytic reduction of CO{sub 2} for methanol evolution in visible light. In the current study rGO-CuI composites have shown excellent yields (19.91 μmol g-cat{sup −1}). rGO provides a light-weight, charge complementary and two-dimensional material that interacts effectively with the CuI nanoparticles. - Graphical abstract: A facile reduced graphene oxide（rGO）/ CuI nanoparticles complexes hybrid material was prepared by a naked flame. This method can be simple and rapid preparation of composite materials which exhibit excellent catalytic performance. It is not only can improve the light absorption but also can be highly efficient reduction of CO{sub 2} to methanol in visible light. So the rGO-CuI composite will find potential applications as catalysts in organic photosynthetic reaction. Display Omitted - Highlights: • Reduced graphene oxide-CuI composites have been prepared by a facile method. • This method can produce reduced graphene oxide from graphene oxide without any reducing agent. • The absorption efficiency of rGO-CuI composites is significantly higher than rGO and CuI.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

A strategy for improved computational efficiency of the method of anchored distributions

This paper proposes a strategy for improving the computational efficiency of model inversion using the method of anchored distributions by “bundling” similar model parametrizations in the likelihood function. Inferring the likelihood function typically requires a large number of forward model simulations for each possible model parametrization; as a result, the process is quite expensive. To ease this prohibitive cost, we present an approximation for the likelihood function – called “bundling” – that relaxes the requirement for high quantities of forward model simulations. This approximation redefines the conditional statement of the likelihood function as the probability of sets of similar model parametrizations – “bundles” – replicating field measurements. To evaluate the effectiveness of these modifications, we compare the quality of predictions and computational cost of bundling relative to a baseline method of anchored distributions inversion of three-dimensional flow and transport model parameters. For our synthetic experiment, bundling achieved a 35% reduction in overall computational cost and had a limited negative on predicted probability distributions of the model parameters. Strategies for minimizing error in the bundling approximation, for enforcing similarity amongst the sets of model parametrizations, and for identifying convergence of the likelihood function are also presented.

Bayesian Inversion, SVD, computational efficiency↗
DOE OSTI2017

A low-rank control variate for multilevel Monte Carlo simulation of high-dimensional uncertain systems

Multilevel Monte Carlo (MLMC) is a recently proposed variation of Monte Carlo (MC) simulation that achieves variance reduction by simulating the governing equations on a series of spatial (or temporal) grids with increasing resolution. Instead of directly employing the fine grid solutions, MLMC estimates the expectation of the quantity of interest from the coarsest grid solutions as well as differences between each two consecutive grid solutions. When the differences corresponding to finer grids become smaller, hence less variable, fewer MC realizations of finer grid solutions are needed to compute the difference expectations, thus leading to a reduction in the overall work. Here this paper presents an extension of MLMC, referred to as multilevel control variates (MLCV), where a low-rank approximation to the solution on each grid, obtained primarily based on coarser grid solutions, is used as a control variate for estimating the expectations involved in MLMC. Cost estimates as well as numerical examples are presented to demonstrate the advantage of this new MLCV approach over the standard MLMC when the solution of interest admits a low-rank approximation and the cost of simulating finer grids grows fast.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2022

A general synthesis of single atom catalysts with controllable atomic and mesoporous structures

The control of single metal atomic sites has been extensively studied in the field of single atom catalysts. By contrast, the precise control of the mesoporous structure in the matrix material, which directly correlates with mass diffusions and may play a dominant role in delivering industrially relevant reaction rates, has been overlooked. In this work, we report a general method for the synthesis of a single atom catalyst with control of the atomic structure of the single atomic site as well as the mesoporous structure of the carbon support for optimized catalytic performance. Various combinations of metal centres (Ni, Co, Mn, Zn, Cu, Sc and Fe) and mass diffusion channels in two dimensions and three dimensions were achieved. Using CO 2 reduction to CO as an example, our Ni single atom catalyst with three-dimensional diffusion channels delivered a practical current of 350 mA cm –2 while maintaining a 93% CO Faradaic efficiency, representing a sixfold improvement in turnover frequency compared to two-dimensional counterparts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

A Novel In Situ Flamelet Tabulation Methodology for the Representative Interactive Flamelet Model

A priori flamelet tabulation methods have been used extensively to model premixed and non-premixed combustion in Reynolds Averaged Navier-Stokes (RANS) and Large Eddy Simulations (LES). Pre-tabulation of flamelet libraries have led to significant cost reduction of CFD simulations. However, these tabulation methods rely on certain simplifying assumptions. The dimensionality of the flamelet manifolds and size of flamelet libraries are also constrained due to memory and computational limitations. This limits the applicability and predictive capability of flamelet models. The traditional representative interactive flamelet (RIF) approach with multiple flamelets on the other hand involves online solution of flamelet equations and presumed PDF integration at each computational cell making it attractive but computationally expensive and not suitable for LES with large cell counts. The current work discusses the development of a novel methodology that combines the advantages of an online flamelet solver along with the computational efficacy of tabulated models. The novel in situ tabulation approach consists of creating a flamelet table at each computational time step using the unsteady flamelet equations. The flamelets are then integrated in real time based on their respective scalar dissipation rate histories. This approach is capable of including history effects and unsteady chemical kinetic effects as it involves the online solution of unsteady flamelets. Implementation and comprehensive validation of the new framework is carried out against gas jet flames as well as spray flames at high pressures. The approach is first validated for a partially premixed methane gas-jet flame in a RANS framework. The temperature and species mass fractions are compared against experimental results at different locations of a lifted flame. The approach is able to capture the transient flame characteristics. The model is then validated against an n -dodecane spray flame at high pressures in a LES framework with a 103 species n -dodecane chemistry mechanism. The model is able to capture ignition delays and flame liftoff of the n -dodecane spray over varying ambient temperature conditions. Due to the offsetting of presumed PDF integration cost, the proposed framework is shown to be faster than the traditional RIF approach and the speedup over RIF increases with an increase in the cell count. Furthermore, this framework enables use of the RIF model for large cell count LES simulations.

42 ENGINEERING↗
DOE OSTI2025

Quantum learning advantage on a scalable photonic platform

Recent advances in quantum technologies have demonstrated that quantum systems can outperform classical ones in specific tasks, a concept known as quantum advantage. Although previous efforts have focused on computational speedups, a definitive and provable quantum advantage that is unattainable by any classical system has remained elusive. Here, in this work, we demonstrate a provable photonic quantum advantage by implementing a quantum-enhanced protocol for learning a high-dimensional physical process. Using imperfect Einstein–Podolsky–Rosen entanglement, we achieve a sample complexity reduction of 11.8 orders of magnitude compared to classical methods without entanglement. These results show that large-scale, provable quantum advantage is achievable with current photonic technology and represent a key step toward practical quantum-enhanced learning protocols in quantum metrology and machine learning.

Liu, Zheng-Hao [Technical Univ. of Denmark, Lyngby↗
NASA NTRS1975

Marine geodetic control for geoidal profile mapping across the Puerto Rican Trench

A marine geodetic control was established for the northern end of the geoidal profile mapping experiment across the Puerto Rican Trench by determining the three-dimensional geodetic coordinates of the four ocean-bottom mounted acoustic transponders. The data reduction techniques employed and analytical processes involved are described. Before applying the analytical techniques to the field data, they were tested with simulated data and proven to be effective in theory as well as in practice.

Fubara, D. M.↗
NASA NTRS1980

Reduced order adaptive controller studies

The use of a reduced-order adaptive controller, can arise from a desire to reduce control complexity with a commensurate reduction in controller sensitivity or from necessity in an attempt to use a finite dimensional controller on an infinite dimensional system. An interest in developing adaptive controllers for flexible structures by application of existing lumped-parameter system (LPS) adaptive controller strategies to truncated expansion descriptions of the distributed parameter system (DPS) behavior of flexible structures has led to two qualitative descriptions of the misbehavior of reduced-order adaptive controllers. A summary is provided of these interpretations of the additional difficulties facing reduced-order adaptive controllers, which are bypassed by exact-order adaptive controllers. A test problem, which initializes attempts to quantify the qualitative insights, is also formulated.

Johnson, C. R., Jr.↗
NASA NTRS1982

On making large nonlinear problems small

Reduction methods for solving large-scale nonlinear problems are considered. Attention is given to: (1) the selection of basis vectors for steady-state problems; (2) the identification and determination of bifurcation and limit points, including tracing post-limit-point and post-bifurcation-point paths using reduction methods; (3) the application of reduction methods to nonlinear problems with prescribed nonzero values of the fundamental unknowns; and (4) the use of reduction methods in conjunction with multifield (mixed) finite element models. The effectiveness of using reduction methods is demonstrated on the basis of several numerical examples including two-dimensional steady conduction in a square plate with temperature dependent thermal conductivity and a shallow spherical cap subjected to a central-point load and a ring load.

Noor, A. K.↗
NASA NTRS1986

A data reduction package for multiple object spectroscopy

Experience with fiber-optic spectrometers has demonstrated improvements in observing efficiency for clusters of 30 or more objects that must in turn be matched by data reduction capability increases. The Medusa Automatic Reduction System reduces data generated by multiobject spectrometers in the form of two-dimensional images containing 44 to 66 individual spectra, using both software and hardware improvements to efficiently extract the one-dimensional spectra. Attention is given to the ridge-finding algorithm for automatic location of the spectra in the CCD frame. A simultaneous extraction of calibration frames allows an automatic wavelength calibration routine to determine dispersion curves, and both line measurements and cross-correlation techniques are used to determine galaxy redshifts.

Hill, J. M.↗
NASA NTRS1993

Calculations of low Reynolds number rocket nozzles

The performance of low-thrust rocket nozzles was studied with a full Navier-Stokes code. The effect of the reduction of the nozzle length on the viscous loss and on the two-dimensional loss due to the increase in the nozzle exit angle was examined by calculating the flowfield and performance values of hydrogen resistojet nozzle with various lengths and shapes (such as 20-deg or 30-deg conical nozzles and a nozzle whose wall contour is given by the Rao nozzle optimization code). It was found that the vacuum specific impulse value of the 30-deg conical nozzle was the highest and that of the contoured nozzle was the lowest among the three nozzles, whose throat Reynolds number and area ratio were 1150 and 82, respectively.

Kim, Suk C.↗
NASA NTRS1955

On the Kernel function of the integral equation relating the lift and downwash distributions of oscillating finite wings in subsonic flow

This report treats the Kernel function of an integral equation that relates a known prescribed downwash distribution to an unknown lift distribution for a harmonically oscillating finite wing in compressible subsonic flow. The Kernel function is reduced to a form that can be accurately evaluated by separating the Kernel function into two parts: a part in which the singularities are isolated and analytically expressed and a nonsingular part which may be tabulated. The form of the Kernel function for the sonic case (Mach number 1) is treated separately. In addition, results for the special cases of Mach number of 0 (incompressible case) and frequency of 0 (steady case) are given. The derivation of the integral equation which involves this Kernel function is reproduced as an appendix. Another appendix gives the reduction of the form of the Kernel function obtained herein for the three-dimensional case to a known result of Possio for two-dimensional flow. A third appendix contains some remarks on the evaluation of the Kernel function, and a fourth appendix presents an alternate form of expression for the Kernel function.

Watkins, Charles E↗
NASA NTRS1994

Metal-Ion Additives Reduce Thermal Expansion Of Polyimides

Polyimides widely used as high-performance polymers because of their excellent thermal stability and toughness. However, their coefficients of thermal expansion (CTE's) greater than those of metals, ceramics, and glasses. Decreasing CTE's of polyimides increase usefulness for aerospace and electronics applications in which dimensional stability required. Additives containing metal ions reduce coefficients of thermal expansion of polyimides. Reductions range from 11 to over 100 percent.

Stoakley, Diane M.↗
NASA NTRS1994

Ozone response to enhanced heterogeneous processing after the eruption of Mt. Pinatubo

Increases in aerosol loading after the Pinatubo eruption are expected to cause additional ozone depletion. Even though aerosol loadings were highest in the winter of 1991-1992, recent analyses of satellite and ground-based ozone measurements indicate that ozone levels in the winter of 1992-1993 are the lowest recorded in recent years, raising the question of the mechanisms responsible for such behavior. We have incorporated aerosol surface areas derived from the Stratospheric Aerosol and Gas Experiment II (SAGE-II) measurements into our two-dimensional model. Inclusion of heterogeneous chemsitry on these enhanced aerosol surfaces yields maximum ozone reductions during the winter of 1992-1993 in the Northern Hemisphere, consistent with those derived from observations. This delayed behavior is due to the combination of the non-linear nature of the impact of heterogeneous reactions as a function of aerosol surface area, and the long time constants for ozone in the lower stratosphere. If heterogeneous mechanisms are primarily responsible for the low 1992-1993 ozone levels, we expect ozone concentrations to start recovering in 1994.

Rodriguez, Jose M.↗
DOE OSTI2015

On nonlinear evolution of low-frequency Alfvén waves in weakly-expanding solar wind plasmas

A multi-dimensional nonlinear evolution equation for Alfvén waves in weakly-expanding solar wind plasmas is derived by using the reductive perturbation method. The expansion of solar wind plasma parcels is modeled by an expanding box model, which includes the accelerating expansion. It is shown that the resultant equation agrees with the Wentzel-Kramers-Brillouin prediction of the low-frequency Alfvén waves in the linear limit. In the cold and one-dimensional limit, a modified derivative nonlinear Schrodinger equation is obtained. Direct numerical simulations are carried out to discuss the effect of the expansion on the modulational instability of monochromatic Alfvén waves and the propagation of Alfvén solitons. By using the instantaneous frequency, it is quantitatively shown that as far as the expansion rate is much smaller than wave frequencies, effects of the expansion are almost adiabatic. It is also confirmed that while shapes of Alfvén solitons temporally change due to the expansion, some of them can stably propagate after their collision in weakly-expanding plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Phase Space Dynamics of a Plasma Wakefield Dechirper for Energy Spread Reduction

Plasma-based accelerators have made impressive progress in recent years. However, the beam energy spread obtained in these accelerators is still at the ~1% level, nearly one order of magnitude larger than what is needed for challenging applications like coherent light sources or colliders. In plasma accelerators, the beam energy spread is mainly dominated by its energy chirp (longitudinally correlated energy spread). Furthermore, we demonstrate that when an initially chirped electron beam from a linac with a proper current profile is sent through a low-density plasma structure, the self-wake of the beam can significantly reduce its energy chirp and the overall energy spread. The resolution-limited energy spectrum measurements show at least a threefold reduction of the beam energy spread from 1.28% to 0.41% FWHM with a dechirping strength of ~1 (MV/m)/(mm pC). Refined time-resolved phase space measurements, combined with high-fidelity three-dimensional particle-in-cell simulations, further indicate the real energy spread after the dechirper is only about 0.13% (FWHM), a factor of 10 reduction of the initial energy spread.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1988

Anharmonic and screening corrections to the melting of a two-dimensional charged colloidal crystal

The Kosterlitz-Thouless-Halperin-Nelson-Young theory of melting is applied to a two-dimensional charged colloidal crystal. The renormalization of elastic constants due to anharmonic and dislocation pair-screening effects is calculated. For colloidal spheres ('polyballs') suspended in a thin film of water confined between two glass plates whose surfaces are highly charged, as in recent experimental configurations, the corrections to the harmonic results simply require an altered effective polyball charge to reproduce virtually the identical melting curve of the harmonic approximation (which agrees well with experiment). The effect on the melting curve of added salt is also calculated; the corresponding experiments have yet to be performed. For a film between uncharged surfaces the anharmonic correction again softens the lattice significantly. Dislocation pair screening causes an additional but minor reduction of the elastic constants. Neither correction eliminates a nonmelting behavior characteristic of the two-dimensional screened Coulomb system in this configuration.

Chang, Enson↗