Search NASA⌕ Search

SEARCH · Search NASA

Results for “Dynamical cluster approximation”

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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

The orbital eccentricities of binary millisecond pulsars in globular clusters

Low-mass binary millisecond pulsars (LMBPs) are born with very small orbital eccentricities, typically of order e(sub i) approximately 10(exp -6) to 10(exp -3). In globular clusters, however, higher eccentricities e(sub f) much greater than e(sub i) can be induced by dynamical interactions with passing stars. Here we show that the cross section for this process is much larger than previously estimated. This is becuse, even for initially circular binaries, the induced eccentricity e(sub f) for an encounter with pericenter separation r(sub p) beyond a few times the binary semimajor axis a declines only as a power law (e(sub f) varies as (r(sub p)/a)(exp -5/2), and not as an exponential. We find that all currently known LMBPs in clusters were probably affected by interactions, with their current eccentricities typically greater than at birth by an order of magnitude or more.

Rasio, Frederic A.↗
NASA NTRS1998

Critical Slowing Down in Time-to-Extinction: An Example of Critical Phenomena in Ecology

We study a model for two competing species that explicitly accounts for effects due to discreteness, stochasticity and spatial extension of populations. The two species are equally preferred by the environment and do better when surrounded by others of the same species. We observe that the final outcome depends on the initial densities (uniformly distributed in space) of the two species. The observed phase transition is a continuous one and key macroscopic quantities like the correlation length of clusters and the time-to-extinction diverge at a critical point. Away from the critical point, the dynamics can be described by a mean-field approximation. Close to the critical point, however, there is a crossover to power-law behavior because of the gross mismatch between the largest and smallest scales in the system. We have developed a theory based on surface effects, which is in good agreement with the observed behavior. The course-grained reaction-diffusion system obtained from the mean-field dynamics agrees well with the particle system.

Gandhi, Amar↗
NASA NTRS1996

Effect of Dust Coagulation Dynamics on the Geometry of Aggregates

Master equation gives a more fundamental description of stochastic coagulation processes rather than popular Smoluchowski's equation. In order to examine the effect of the dynamics on the geometry of resulting aggregates, we study Master equation with a rigorous Monte Carlo algorithm. It is found that Cluster-Cluster aggregation model is a good approximation of orderly growth and the aggregates have fluffy structures with a fractal dimension approx. 2. A scaling analysis of Smoluchowski's equation also supports this conclusion.

Nakamura, R.↗
NASA NTRS2004

Mass and Dynamical Structures of the Lensing Clusters CL0024+17 and CL2244+02

We present a detailed gravitational mass measurement based on the XMM-Newton imaging spectroscopy analysis of the lensing cluster of galaxies CL0024+17 at $z = 0.395$. The emission appears approximately symmetric. However, on the scale of $r\sim3.3'$, some indication of elongation is visible in the northwest-southeast direction from the hardness ratio map. Within $3'$, we measure a global gas temperature of $3.52\pm0.17$ keV, metallicity of $0.22\pm0.07$, and a bolometric luminosity of $2.9\pm0. l\times10(exp 44)$ erg/s. We derive a temperature distribution with an isothermal temperature of 3.9 keV up to a radius of $1.5'$ and a strong temperature gradient in the outskirts ($1.3' less than r less than 3.3'$). Under the assumption of hydrostatic equilibrium, we measure the gravitational mass and gas mass fraction to be $M-{200} = 2.0\pm0.3\times 10(exp 14)$ solar masses and $f-{gas} = 0.20\pm0.03$ at $r-{200} = 1.05$ Mpc (all for a Hubble constant of 70 km/sec/Mpc) using the observed gas temperature profile. The complex core structure is the key to explaining the discrepancy between the gravitational mass determined from the XMM-Newton observations and HST optical lensing measurements.

Forman, William↗
NASA NTRS1994

The giant molecular cloud Monoceros R2. 1: Shell structure

We have obtained a 45 sec resolution, Nyquist-sampled map in CO J = 1-0 covering approximately a 3 deg x 3 deg region of the giant molecular cloud Monoceros R2. The map consists of 167,000 spectra observed with the 15 element focal-plane array system on the FCRAO 14 m telescope. The data reveal that the large-scale structure of Mon R2 is dominated by a is approximately 30 pc diameter largely hemispherical shell containing approximately 4 x 10(exp 4) solar mass of molecular material and expanding at approximately 3-4 km s(exp -1) with symmetric axis roughly along the line of sight. The dynamical timescale of the shell is estimated to be approximately 4 x 10(exp 6) yr, which is consistent with the age of main-sequence stars powering the clusters of reflection nebulea in this region. There is no evidence for a redshifted shell on the far side of the interior 'bubble,' which is largely devoid of molecular material. Distortions of the shell are obvious, suggesting inhomogeneity of the cloud and possible presence of a magnetic field prior to its formation. Dense clumps in Mon R2, including the main core and the GGD 12-15 core, appear to be condensations located on the large shell. The reflection nebulea with their illuminating stars as well as embedded IRAS sources suggest that triggered star formation has taken place over a large part of the Mon R2 shell.

Xie, Taoling↗
DOE OSTI2025

The importance of accounting for the Tolman correction to surface tension for nucleation and growth modeling of Fe clusters

Gibbs free energies of clusters are required for predictive modeling of cluster growth during condensation of a cooling vapor. Here, we present a straightforward method of calculating free energies of cluster formation using the data from molecular dynamics (MD) simulations. We apply this method to iron clusters having from 2 to 100 atoms. The energies obtained are verified by comparing to an MD-simulated equilibrium cluster size distribution in a sub-saturated vapor. We show that these free energies differ significantly from those obtained with a commonly used spherical cluster approximation, which relies on a surface tension coefficient of a flat surface, as it is used in the classical nucleation theory (CNT). We show that the spherical cluster approximation in CNT can be improved by using a cluster-size-dependent Tolman correction for the surface tension. The Tolman length and effective surface tension values were derived for iron clusters, and they significantly differ from the commonly used experimentally measured values. This improved approximation does not account for geometric magic number effects responsible for spikes and troughs in densities of neighbor cluster sizes. Nonetheless, it allows to more accurately model cluster formation from a cooling vapor. It better reproduces the condensation timeline, overall shape of the cluster size distribution, average cluster size, and the distribution width. In contrast, using a constant surface tension coefficient (as done in CNT) resulted in incorrect condensation dynamics and cluster size distributions. The analytical expression for cluster nucleation rate from CNT was updated to account for the size-dependence of cluster surface tension.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Coupled cluster evaluation of the frequency dispersion of the first and second hyperpolarizabilities of water, methanol, and dimethyl ether

The static and dynamic first (β{sub ‖}) and second (γ{sub ‖}) hyperpolarizabilities of water, methanol, and dimethyl ether have been evaluated within the response function approach using a hierarchy of coupled cluster levels of approximation and doubly augmented correlation consistent atomic basis sets. For the three compounds, the electronic β{sub ‖} and γ{sub ‖} values calculated at the CCSD and CC3 levels are in good agreement with gas phase electric field-induced second harmonic generation (EFISHG) measurements. In addition, for dimethyl ether, the frequency dispersion of both properties follows closely recent experimental values [V. W. Couling and D. P. Shelton, J. Chem. Phys. 143, 224307 (2015)] demonstrating the reliability of these methods and levels of approximation. This also suggests that the vibrational contributions to the EFISHG responses of these molecules are small.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Quantum spin ice response to a magnetic field in the dipole-octupole pyrochlore Ce 2 Zr 2 O 7

The pyrochlore magnet Ce 2 Zr 2 O 7 has attracted much attention as a quantum spin ice candidate whose novelty derives in part from the dipolar-octupolar nature of the Ce 3+ pseudospin-1/2 degrees of freedom it possesses. We report heat capacity measurements on single crystal samples of Ce 2 Zr 2 O 7 down to T~0.1K in a magnetic field along the [$1,\bar{1},0$] direction. These measurements show that the broad hump in the zero-field heat capacity moves higher in temperature with increasing field strength and is split into two separate humps by the [$1,\bar{1},0$] magnetic field at ~2T. These separate features are due to the decomposition of the pyrochlore lattice into effectively decoupled chains for fields in this direction: One set of chains (α chains) is polarized by the field while the other (β chains) remains free. This situation is similar to that observed in the classical spin ices Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 , but with the twist that here the strong transverse exchange interactions produce substantial quantum effects. Our theoretical modeling suggests that the β chains are close to a critical state, with nearly-gapless excitations. We also report elastic and inelastic neutron scattering measurements on single crystal Ce 2 Zr 2 O 7 in [$1,\bar{1},0$] and [0,0,1] magnetic fields at temperatures down to T=0.03K. The elastic scattering behaves consistently with the formation of independent chains for a [$1,\bar{1},0$] field, while the [0,0,1] field produces a single field-induced elastic magnetic Bragg peak at (0,2,0) and equivalent wavevectors, indicating a polarized spin ice state for fields above ~3T. For both [$1,\bar{1},0$] and [0,0,1] magnetic fields, our inelastic neutron scattering results show an approximately dispersionless continuum of scattering that increases in both energy and intensity with increasing field strength. By modeling the complete set of experimental data using numerical linked cluster and semiclassical molecular dynamics calculations, we demonstrate the dominantly multipolar nature of the exchange interactions in Ce 2 Zr 2 O 7 and the smallness of the parameter θ, which controls the mixing between dipolar and octupolar degrees of freedom. Finally, these results support previous estimates of the microscopic exchange parameters and place strong constraints on the theoretical description of this prominent spin ice candidate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1994

UBV stellar photometry of the 30 Doradus region of the large Magellanic Cloud with the Hubble Space Telescope

We report on Planetary Camera observations of the central region of 30 Doradus in the Large Magellanic Cloud (LMC). These images of 30 Doradus are the first `deep' Hubble Space Telescope (HST) exposures that have appropriate photometric calibration. The B band (F439W) image, which shows R136a at the center of the PC6 charge coupled device (CCD) chip, reveals over 200 stars within 3 sec of the center of R13a, and over 800 stars in a 35 sec x 35 sec area. We used Malumuth et al.'s (1991) Point Spread Function (PSF)-fitting method to measure the magnitudes of all stars on the PC6 chip. These new B magnitudes, along with U and V magnitudes from archival PC images, yield a luminosity function, mass density profile, and initial mass function of the 30 Doradus ionizing cluster. The mass distribution is well fit by a King model with a core radius, R(sub c) = 0.96 sec (0.24 pc), a tidal radius, R(sub t) = 110 sec (28 pc), and a total mass, Mass = 16,800 solar mass. Both the luminosity function and initial mass function show evidence for mass segregation, in the sense that the central region has a higher fraction of massive stars than the outer regions. This is the first observational evidence for mass segregation in a very young cluster (age approximately 3 million years). The observations admit the hypothesis that the mass segregation occurred in the process of star formation and/or that the mass segregation is the result of dynamical evolution.

Malumuth, Eliot M.↗
NASA NTRS1995

Very low mass stars and white dwarfs in NGC 6397

Deep Wide Field/Planetary Camera 2 (WFPC2) images in wide bands centered at 606 and 802 nm were taken with the Hubble Space Telescope (HST) 4.6 min from the center of the galactic globular cluster NGC 6397. The images were used to accurately position approximately 2120 stars detected in the field on a color magnitude diagram down to a limiting magnitude m(sub 814) approximately = m(sub I) approximately = 26 determined reliably and solely by counting statistics. A white dwarf sequence and a rich, narrow cluster main sequence are detected for the first time, the latter stretching from m(sub 814) = 18.5 to m(sub 814) = 24.0 where it becomes indistinguishable from the field population. Two changes of slope of the main sequence at m(sub 814) approximately = 20 and m(sub 814) approximately = 22.5 are evident. The corresponding luminosity function increases slowly from M(sub 814) approximately = 6.5 to 8.5 are expected from ground-based observations but then drops sharply from there dwon to the measurement limit. The corresponding mass function obtained bu using the only presently available mass-luminosity function for the cluster's metallicity rises to a plateau between approximately 0.25 and approximately 0.15 solar mass, but drops toward the expected mass limit of the normal hydrogen burning main sequence at approximately 0.1 solar mass. This result is in clear contrast to that obtained from the ground and implies either a substantial modification of the cluster's initial mass function due to dynamical evolution in its lifetime, or that very low mass stars are not produced in any dynamically significant amount by clusters of this type. The white dwarf sequence is in reasonable agreement with a cooling sequence of models of mass 0.5 solar mass at the canonical distance of NGC 6397 with a scatter that is most likely due to photometric errors, but may also reflect real differences in mass or chemical composition. Contamination from unresolved galaxies, which cannot be reliably identified with our filters, makes it difficult to meaningfully compare the observed white dwarf luminosity function with its theoretical counterpart.

Paresce, Francesco↗
DOE OSTI2015

Incorporation of memory effects in coarse-grained modeling via the Mori-Zwanzig formalism

The Mori-Zwanzig formalism for coarse-graining a complex dynamical system typically introduces memory effects. The Markovian assumption of delta-correlated fluctuating forces is often employed to simplify the formulation of coarse-grained (CG) models and numerical implementations. However, when the time scales of a system are not clearly separated, the memory effects become strong and the Markovian assumption becomes inaccurate. To this end, we incorporate memory effects into CG modeling by preserving non-Markovian interactions between CG variables, and the memory kernel is evaluated directly from microscopic dynamics. For a specific example, molecular dynamics (MD) simulations of star polymer melts are performed while the corresponding CG system is defined by grouping many bonded atoms into single clusters. Then, the effective interactions between CG clusters as well as the memory kernel are obtained from the MD simulations. The constructed CG force field with a memory kernel leads to a non-Markovian dissipative particle dynamics (NM-DPD). Quantitative comparisons between the CG models with Markovian and non-Markovian approximations indicate that including the memory effects using NM-DPD yields similar results as the Markovian-based DPD if the system has clear time scale separation. However, for systems with small separation of time scales, NM-DPD can reproduce correct short-time properties that are related to how the system responds to high-frequency disturbances, which cannot be captured by the Markovian-based DPD model.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1995

The spatial distribution, kinematics, and dynamics of the galaxies in the region of Abell 2634 and 2666

A total of 663 galaxies with known redshifts in a 12 deg x 12 deg field centered on A2634, including 211 new measurements, are used to study in detail the structure of the region. In it we find six main galaxy concentrations: the nearby clusters A2634 and A2666, two groups in the vicinity of A2634, and two distant clusters at approximately 18,000 (A2622) and approximately 37,000 km/s seen in projection near the core of A2634. For A2634, the most richly sampled of those concentrations, we are able to apply strict cluster membership criteria. Two samples - one containing 200 galaxies within 2 deg from the cluster center and a second, magnitude-limited, of 118 galaxies within the central half degree - are used to examine the structure, kinematics, dynamics, and morphological segregation of the cluster. We show that early type galaxies appear to be a relaxed system, while the spiral population eschews the center of the cluster and exhibits both a multimodal velocity distribution and a much larger velocity dispersion that the ellipticals. We propose that the spiral galaxies of A2634 represent a dynamically young cluster population. For the galaxy component of A2634, we find no evidence of significant substructure in the central regions. We also conclude that the adoption of lenient membership criteria that ignore the dynamical complexity of A2634 are unlikely to be responsible for the conflicting results reported on the motion of this cluster with respect ot the CMB. The kinematical and dynamical analysis is extended to A2634's close companion, A2666, and the two distant background clusters.

Scodeggio, Marco↗
DOE OSTI2017

ULTRAVIOLET ESCAPE FRACTIONS FROM GIANT MOLECULAR CLOUDS DURING EARLY CLUSTER FORMATION

The UV photon escape fraction from molecular clouds is a key parameter for understanding the ionization of the interstellar medium and extragalactic processes such as cosmic reionization. We present the ionizing photon flux and the corresponding photon escape fraction ( f {sub esc}) arising as a consequence of star cluster formation in a turbulent, 10{sup 6} M {sub ⊙} giant molecular cloud, simulated using the code FLASH. We make use of sink particles to represent young, star-forming clusters coupled with a radiative transfer scheme to calculate the emergent UV flux. We find that the ionizing photon flux across the cloud boundary is highly variable in time and space due to the turbulent nature of the intervening gas. The escaping photon fraction remains at ∼5% for the first 2.5 Myr, followed by two pronounced peaks at 3.25 and 3.8 Myr with a maximum f {sub esc} of 30% and 37%, respectively. These peaks are due to the formation of large H ii regions that expand into regions of lower density, some of which reaching the cloud surface. However, these phases are short-lived, and f {sub esc} drops sharply as the H ii regions are quenched by the central cluster passing through high-density material due to the turbulent nature of the cloud. We find an average f {sub esc} of 15% with factor of two variations over 1 Myr timescales. Our results suggest that assuming a single value for f {sub esc} from a molecular cloud is in general a poor approximation, and that the dynamical evolution of the system leads to large temporal variation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Physics-Informed Sparse Gaussian Process for Probabilistic Stability Analysis of Large-Scale Power System with Dynamic PVs and Loads

This work proposes a physics-informed sparse Gaussian process (SGP) for probabilistic stability assessment of large-scale power systems in the presence of uncertain dynamic PVs and loads. The differential and algebraic equations considering uncertainties from dynamic PVs and loads are reformulated to a nonlinear mapping relationship that allows the application of SGP. Thanks to the nonparametric characteristic of Gaussian process, the proposed framework does not require distributions of uncertain inputs and this distinguishes it from existing approaches. As the original Gaussian process is not scalable to large-scale systems with high dimensional uncertain inputs, this paper develops the SGP with a stochastic variational inference technique. It leads to approximately two orders of complex reduction. A data pre-processing step is also introduced to tackle the coexistence of stable and unstable cases by sample clustering and constructing separate SGPs. The probabilistic transient stability index is analyzed to assess system stability under different uncertain dynamics loads and PVs. Comparisons are performed with the sampling-based, the polynomial chaos expansion-based, and traditional Gaussian process-based methods on the modified IEEE 118-bus and Texas 2000-bus systems under various scenarios, including different levels of uncertainties and the existence of nonlinear correlations among dynamic PVs. The impacts of data quality and quantity issues are also investigated. It is shown that the proposed SGP achieves significantly improved computational efficiency while maintaining high accuracy with a limited number of data.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2021

Investigation of the isoscalar response of 24 Mg to 6 Li scattering

24 Mg is a strongly deformed nucleus in the ground state. Deformation effects can be observed in the structure of the isoscalar giant monopole and quadrupole resonances. 24 Mg is also a nucleus that is well known to present different types of cluster-oscillation modes. Both giant resonances and cluster states are strongly populated by isoscalar transitions. To extract the E0, E1, and E2 transition strengths via 6 Li scattering. The 6 Li probe is a powerful tool for investigating the isoscalar nuclear response with a very favorable ratio of resonance-to-continuum background. Double-differential cross sections of 6 Li inelastic scattering, at the beam energy of 100 MeV/u, were measured in the excitation-energy range 10 – 40 MeV and scattering angles 0 - 3°. A multipole-decomposition analysis was performed for extracting the isoscalar E0 , E1, and E2 strength distributions. Results: The extracted multipole strengths were compared with predictions from consistent quasiparticle random phase approximation calculations. The theoretical predictions are in fair agreement with the experimental data. The E0 strength was also compared with results from antisymmetrized molecular dynamics calculations found in the literature. A few peaks in the experimental data might be associated with clustering in 24 Mg. Ground-state deformation effects were observed in the isoscalar giant monopole resonance (ISGMR) and isoscalar giant quadrupole resonance (ISGQR) distributions. The ISGMR strength is split in two peaks around 19 and 28 MeV. The ISGQR exhibits a pronounced peak at 20 MeV with a broadening at the low-energy region, similar to predictions from microscopic calculations. Signatures of excitation of cluster states were observed in the E0 response. Further studies including particle-decay measurements will be required to confirm the nature of the observed peaks.

20 ≤ A ≤ 38↗
DOE OSTI2021

Origins of Clustering of Metalate–Extractant Complexes in Liquid–Liquid Extraction

Effective and energy-efficient separation of precious and rare metals is very important for a variety of advanced technologies. Liquid-liquid extraction (LLE) is a relatively less energy intensive separation technique, widely used in separation of lanthanides, actinides, and platinum group metals (PGMs). In LLE, the distribution of an ion between an aqueous phase and an organic phase is determined by enthalpic (coordination interactions) and entropic (fluid reorganization) contributions. The molecular scale details of these contributions are not well understood. Preferential extraction of an ion from the aqueous phase is usually correlated with the resulting fluid organization in the organic phase, as the longer-range organization increases with metal loading. However, it is difficult to determine the extent to which organic phase fluid organization causes, or is caused by, metal loading. In this study, we demonstrate that two systems with the same metal loading may impart very different organic phase organizations and investigate the underlying molecular scale mechanism. Small-angle X-ray scattering shows that the structure of a quaternary ammonium extractant solution in toluene is affected differently by the extraction of two metalates (octahedral PtCl 6 2- and square-planar PtCl 6 2- ), although both are completely transferred into the organic phase. The aggregates formed by the metalate-extractant complexes (approximated as reverse micelles) exhibit a more long-range order (clustering) with PtCl 6 2- compared to that with PtCl 6 2- . Vibrational sum frequency generation spectroscopy and complementary atomistic molecular dynamics simulations on model Langmuir monolayers indicate that the two metalates affect the interfacial hydration structures differently. Furthermore, the interfacial hydration is correlated with water extraction into the organic phase. Overall, these results support a strong relationship between the organic phase organizational structure and the different local hydration present within the aggregates of metalate-extractant complexes, which is independent of metalate concentration.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗