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Results for “PHASE VELOCITY”

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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

A TWO-PHASE LOW-VELOCITY OUTFLOW IN THE SEYFERT 1 GALAXY Ark 564

The Seyfert 1 galaxy Ark 564 was observed with Chandra high-energy transmission gratings for 250 ks. We present the high-resolution X-ray spectrum that shows several associated absorption lines. The photoionization model requires two warm absorbers (WAs) with two different ionization states (log U = 0.39 {+-} 0.03 and log U = -0.99 {+-} 0.13), both with moderate outflow velocities ({approx}100 km s{sup -1}) and relatively low line of sight column densities (log N{sub H} = 20.94 and 20.11 cm{sup -2}). The high-ionization phase produces absorption lines of O VII, O VIII, Ne IX, Ne X, Mg XI, Fe XVII, and Fe XVIII, while the low-ionization phase produces lines at lower energies (O VIand O VII). The pressure-temperature equilibrium curve for the Ark 564 absorber does not have the typical ''S'' shape, even if the metallicity is super-solar; as a result, the two WA phases do not appear to be in pressure balance. This suggests that the continuum incident on the absorbing gas is perhaps different from the observed continuum. We also estimated the mass outflow rate and the associated kinetic energy and find it to be at most 0.009% of the bolometric luminosity of Ark 564. Thus, it is highly unlikely that these outflows provide significant feedback required by the galaxy formation models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Some issues in the simulation of two-phase flows: The relative velocity

In this paper we compare numerical approximations for solving the Riemann problem for a hyperbolic two-phase flow model in two-dimensional space. The model is based on mixture parameters of state where the relative velocity between the two-phase systems is taken into account. This relative velocity appears as a main discontinuous flow variable through the complete wave structure and cannot be recovered correctly by some numerical techniques when simulating the associated Riemann problem. Simulations are validated by comparing the results of the numerical calculation qualitatively with OpenFOAM software. Simulations also indicate that OpenFOAM is unable to resolve the relative velocity associated with the Riemann problem.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Machine Learning the Sixth Dimension: Stellar Radial Velocities from 5D Phase-space Correlations

The Gaia satellite will observe the positions and velocities of over a billion Milky Way stars. In the early data releases, the majority of observed stars do not have complete 6D phase-space information. In this Letter, we demonstrate the ability to infer the missing line-of-sight velocities until more spectroscopic observations become available. Here, we utilize a novel neural network architecture that, after being trained on a subset of data with complete phase-space information, takes in a star's 5D astrometry (angular coordinates, proper motions, and parallax) and outputs a predicted line-of-sight velocity with an associated uncertainty. Working with a mock Gaia catalog, we show that the network can successfully recover the distributions and correlations of each velocity component for stars that fall within ~5 kpc of the Sun. We also demonstrate that the network can accurately reconstruct the velocity distribution of a kinematic substructure in the stellar halo that is spatially uniform, even when it comprises a small fraction of the total star count.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Vibrational circular dichroism from ab initio molecular dynamics and nuclear velocity perturbation theory in the liquid phase

We report the first fully ab initio calculation of dynamical vibrational circular dichroism spectra in the liquid phase using nuclear velocity perturbation theory (NVPT) derived electronic currents. Our approach is rigorous and general and thus capable of treating weak interactions of chiral molecules as, e.g., chirality transfer from a chiral molecule to an achiral solvent. We use an implementation of the NVPT that is projected along the dynamics to obtain the current and magnetic dipole moments required for accurate intensities. The gauge problem in the liquid phase is resolved in a twofold approach. The electronic expectation values are evaluated in a distributed origin gauge, employing maximally localized Wannier orbitals. In a second step, the gauge invariant spectrum is obtained in terms of a scaled molecular moments, which allows to systematically include solvent effects while keeping a significant signal-to-noise ratio. We give a thorough analysis and discussion of this choice of gauge for the liquid phase. At low temperatures, we recover the established double harmonic approximation. The methodology is applied to chiral molecules ((S)-d{sub 2}-oxirane and (R)-propylene-oxide) in the gas phase and in solution. We find an excellent agreement with the theoretical and experimental references, including the emergence of signals due to chirality transfer from the solute to the (achiral) solvent.

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

THE LOCATION OF NON-THERMAL VELOCITY IN THE EARLY PHASES OF LARGE FLARES-REVEALING PRE-ERUPTION FLUX ROPES

Non-thermal velocity measurements of the solar atmosphere, particularly from UV and X-ray emission lines have demonstrated over the decades that this parameter is important in understanding the triggering of solar flares. Enhancements have often been observed before intensity enhancements are seen. However, until the launch of Hinode, it has been difficult to determine the spatial location of the enhancements to better understand the source region. The Hinode EUV Imaging Spectrometer has the spectral and spatial resolution to allow us to probe the early stages of flares in detail. We analyze four events, all of which are GOES M- or X-classification flares, and all are located toward the limb for ease of flare geometry interpretation. Three of the flares were eruptive and one was confined. In all events, pre-flare enhancement in non-thermal velocity at the base of the active region and its surroundings has been found. These enhancements seem to be consistent with the footpoints of the dimming regions, and hence may be highlighting the activation of a coronal flux rope for the three eruptive events. In addition, pre-flare enhancements in non-thermal velocity were found above the looptops for the three eruptive events.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Phase control of group velocity of light in an InGaN/GaN quantum dot nanostructure

By solving self-consistently Schrödinger–Poisson equations for a carrier in the conduction band of an InGaN/GaN quantum dot, a four-level quantum system is described. It is found that in the presence of terahertz signal radiation, the medium becomes phase dependent, which ensures the phase control of the group velocity of a weak probe pulse from slow to fast light. (nonlinear optical phenomena)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Complementary Probes of Dark Energy using Galaxy Clusters

Here, we have built a framework to constrain Dark Energy (DE) using the spatial distributions, number counts, and escape velocities of galaxy clusters in the context of next generation spectroscopic surveys. In the ΛCDM paradigm, galaxy clusters trace the expansion of the Universe in three different ways. (1.) Spatial distribution:The baryon acoustic oscillations (BAO) formed during recombination are imprinted into the spatial distribution of clusters. The locations of the BAO are standard-rulers which map the length-scale of the sound horizon. Statistical measures like the two-point correlation function and the power spectrum can be used to trace the scale of the BAO and thus the expansion history of the Universe.(2.) Number counts: Galaxy clusters are identified as peaks above a threshold in the underlying matter density field. Galaxies attract onto these peaks over time until gravity loses the battle against the accelerating expansion of space. The number and mass of galaxy clusters over time is sensitive to the strength of the DE and the amount of matter in the Universe.(3.) Escape velocity:The radius-velocity phase-space of galaxies within clusters has a sharp edge which traces the escape velocity of galaxies. This escape velocity is a combination of the local Newtonian gravitational potential and the acceleration of the expansion of the Universe. Weak lensing mass profiles of clusters can be used to predict the escape velocity profile from just the gravitational Newtonian component. In an expanding or contracting Universe, the observed phase-space escape velocity surface would be modified from this prediction.We built the large-scale structure covariances from mock catalogs which we can now use in the likelihood analyses to constrain Dark Energy models. We have developed the analytical framework to conduct the joint likelihood analysis of the bias function and the abundance function in order to constrain DE through their redshift evolution.We have investigated a new Dark Energy test that uses the weak lensing prediction of the grav-itational potential compared to the observed escape velocity profile from galaxy clusters. We have made predictions on the improvement in the Dark Energy figure-of-merit from this new test. We have also investigated the predicted enhancements of the cluster escape velocity profiles from modifi-cations to gravity. This effort can help rule out model classes of modified gravity as alternatives to Dark Energy.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Maximal energy extraction under discrete diffusive exchange

Waves propagating through a bounded plasma can rearrange the densities of states in the six-dimensional velocity-configuration phase space. Depending on the rearrangement, the wave energy can either increase or decrease, with the difference taken up by the total plasma energy. In the case where the rearrangement is diffusive, only certain plasma states can be reached. It turns out that the set of reachable states through such diffusive rearrangements has been described in very different contexts. Building upon those descriptions, and making use of the fact that the plasma energy is a linear functional of the state densities, the maximal extractable energy under diffusive rearrangement can then be addressed through linear programming.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Stereoscopy of dust density waves under microgravity: Velocity distributions and phase-resolved single-particle analysis

Experiments on dust-density waves have been performed in dusty plasmas under the microgravity conditions of parabolic flights. Three-dimensional measurements of a dust density wave on a single particle level are presented. The dust particles have been tracked for many oscillation periods. A Hilbert analysis is applied to obtain trajectory parameters such as oscillation amplitude and three-dimensional velocity amplitude. While the transverse motion is found to be thermal, the velocity distribution in wave propagation direction can be explained by harmonic oscillations with added Gaussian (thermal) noise. Additionally, it is shown that the wave properties can be reconstructed by means of a pseudo-stroboscopic approach. Finally, the energy dissipation mechanism from the kinetic oscillation energy to thermal motion is discussed and presented using phase-resolved analysis.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Chorus Waves Modulation of Langmuir Waves in the Radiation Belts

Using high-resolution waveforms measured by the Van Allen Probes, we report a novel observation in the radiation belts. Namely, we show that multiband, discrete, rising-tone whistler-mode chorus emissions exhibit a one-to-one correlation with Langmuir wave bursts. Moreover, the periodic Langmuir wave bursts are generally observed at the phase location where the chorus wave E || component is oriented opposite to its propagation direction. The electron measurements show a beam in phase space density at the particle velocity that matches the parallel phase velocity of the chorus waves. Based on this evidence, we conclude that the chorus waves accelerate the suprathermal electrons via Landau resonance, and generate a localized electron beam in phase space density. Consequently, the Langmuir waves are excited locally and are modulated by the chorus wave phase. As a result, this microscale interaction between chorus waves and high frequency electrostatic waves provides a new insight into the nonlinear wave-particle interaction process.

58 GEOSCIENCES↗
DOE OSTI2015

The silicon and calcium high-velocity features in type Ia supernovae from early to maximum phases

The high-velocity features (HVFs) in optical spectra of type Ia supernovae (SNe Ia) are examined with a large sample including very early-time spectra (e.g., t < −7 days). Multiple Gaussian fits are applied to examine the HVFs and their evolutions, using constraints on expansion velocities for the same species (i.e., Si ii 5972 and Si ii 6355). We find that strong HVFs tend to appear in SNe Ia with smaller decline rates (e.g., Δm{sub 15}(B) ≲ 1.4 mag), clarifying that the finding by Childress et al. for the Ca-HVFs in near-maximum-light spectra applies both to the Si-HVFs and Ca-HVFs in the earlier phase. The Si-HVFs seem to be more common in rapidly expanding SNe Ia, which is different from the earlier result that Ca-HVFs are associated with SNe Ia that have slower Si ii 6355 velocities at maximum light (i.e., V{sup Si}{sub max}). Moreover, SNe Ia with both stronger HVFs at early phases and larger V{sup Si}{sub max} are found to have noticeably redder B−V colors and to occur preferentially in the inner regions of their host galaxies, while those with stronger HVFs but smaller V{sup Si}{sub max} show opposite tendencies, suggesting that these two subclasses have different explosion environments and their HVFs may have different origins. We further examine the relationships between the absorption features of Si ii 6355 and Ca ii IR lines, and find that their photospheric components are well correlated in velocity and strength but that the corresponding HVFs show larger scatter. These results cannot be explained with ionization and/or thermal processes alone, and different mechanisms are required for the creation of HVF-forming regions in SNe Ia.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

An approach to improving transporting velocity in the long-range ultrasonic transportation of micro-particles

In existing ultrasonic transportation methods, the long-range transportation of micro-particles is always realized in step-by-step way. Due to the substantial decrease of the driving force in each step, the transportation is lower-speed and stair-stepping. To improve the transporting velocity, a non-stepping ultrasonic transportation approach is proposed. By quantitatively analyzing the acoustic potential well, an optimal region is defined as the position, where the largest driving force is provided under the condition that the driving force is simultaneously the major component of an acoustic radiation force. To keep the micro-particle trapped in the optimal region during the whole transportation process, an approach of optimizing the phase-shifting velocity and phase-shifting step is adopted. Due to the stable and large driving force, the displacement of the micro-particle is an approximately linear function of time, instead of a stair-stepping function of time as in the existing step-by-step methods. An experimental setup is also developed to validate this approach. Long-range ultrasonic transportations of zirconium beads with high transporting velocity were realized. The experimental results demonstrated that this approach is an effective way to improve transporting velocity in the long-range ultrasonic transportation of micro-particles.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Collective modes in excitonic insulators: Effects of electron-phonon coupling and signatures in the optical response

Here, we consider a two-band spinless model describing an excitonic insulator (EI) on the two-dimensional square lattice with anisotropic hopping parameters and electron-phonon (el-ph) coupling, inspired by the EI candidate Ta 2 NiSe 5 . We systematically study the nature of the collective excitations in the ordered phase which originates from the interband Coulomb interaction and the el-ph coupling. When the ordered phase is stabilized only by the Coulomb interaction (pure EI phase), its collective response exhibits a massless phase mode in addition to the amplitude mode. We show that in the BEC regime, the signal of the amplitude mode becomes less prominent and that the anisotropy in the phase-mode velocities is reduced. Through coupling to the lattice, the phase mode acquires a mass and the signal of the amplitude mode becomes less prominent. Importantly, the character of the softening mode at the boundary between the normal semiconductor phase and the ordered phase depends on the parameters. In particular, we point out that even for el-ph coupling smaller than the Coulomb interaction, the mode that softens to zero at the boundary can have a phonon character. We also discuss how the collective modes can be observed in the optical conductivity. Furthermore, we study the effects of nonlocal interactions on the collective modes and show the possibility of realizing a coexistence of an in-gap mode and an above-gap mode split off from the single-amplitude mode in the system with local interactions only.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Computational Analysis of the Impact of Boundary Conditions on a Particle-Laden Flow: A Case Study in a Pressurized Oxy-Coal Combustor

Designing an effective burner is vital for the development of coal combustion technologies. Because of high pressure, the volumetric fraction of the coal particles in the injected fuel in a pressurized oxy-combustion (POC) burner approaches or even exceeds the limitations allowed by the commercial computational fluid dynamics codes (e.g., Ansys Fluent). Consequently, for such high particle volumetric fractions, the interplay between the particles, the fluid flow, and the burner wall needs to be re-evaluated. The present computational work is a first step in a systematic analysis of the roles of various characteristics involved in the POC process, such as the method of particle release, its location, and the particle size. Specifically, pulverized coal is burned under an elevated pressure of 15 bar in an O 2 /CO 2 environment. A 100 kW, a POC combustor, is modeled with Ansys Fluent using the Reynolds-averaged Navier−Stokes approach. It is revealed that for this pilot-scale, pressurized burner, the gas phase flow velocity in the near-wall region exhibits anomalies. With the major focus on POC, this work aims to eliminate/reduce the impact of high particle loading on the gas-phase flow. To scrutinize the role of particle loading in the near-wall region and eliminate the impact of this velocity on POC downstream, the particle−gas interplay in the boundary layer is investigated by means of the computational simulations incorporating the coupling between the turbulent flow and the particles. It is found that the tuning of the particle release location makes the gas-phase flow velocity in the presence of particles consistent with the pure gas flow velocity profile. The particles size is also found to have a significant impact on the particle trajectory.

CFD↗