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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 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

An absolute instrument for determination of the speed of sound in water

An apparatus for the absolute determination of the sound speed in water based on the time-of-flight technique is described. The time measurement is realized by hardware circuits and the distance measurement by a double-beam plane-mirror interferometer. A highly accurate time chip, with a resolution of approximately 90 ps, is employed for time measurements. The acoustic path length is adjustable and can be measured directly. Two transducers are used for transmitting and receiving ultrasonic signals without reflection. The transducers are immersed in a thermostatic vessel that maintains bath temperature with high stability. The speed of sound in pure water was measured at ambient pressure and at the temperatures 308 K, 303 K, 298 K, and 293 K. The achieved measurement uncertainties are 2 mK for temperature and 0.045 m/s for speed of sound. The results are compared to data from the literature, equation of state models, and measurements by two commercial sensors in the same experiment, showing excellent agreement among them.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

High speed X-ray phase contrast imaging of energetic composites under dynamic compression

Fracture of crystals and subsequent frictional heating are associated with formation of hot spots in energetic composites such as polymer bonded explosives (PBXs). Traditional high speed optical imaging methods cannot be used to study the dynamic sub-surface deformation and fracture behavior of such materials due to their opaque nature. In this study, high speed synchrotron X-ray experiments are conducted to visualize the in situ deformation and fracture mechanisms in PBXs manufactured using octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals and hydroxyl-terminated polybutadiene (HTPB) binder. A modified Kolsky bar apparatus was used to apply controlled dynamic compression on the PBX specimens, and a high speed synchrotron X-ray phase contrast imaging (PCI) setup was used to record the in situ deformation and failure in the specimens. The experiments show that synchrotron X-ray PCI provides a sufficient contrast between the HMX crystals and the doped binder, even at ultrafast recording rates. Under dynamic compression, most of the cracking in the crystals was observed to be due to the tensile stress generated by the diametral compression applied from the contacts between the crystals. Tensile stress driven cracking was also observed for some of the crystals due to the transverse deformation of the binder and superior bonding between the crystal and the binder. In conclusion, the obtained results are vital to develop improved understanding and to validate the macroscopic and mesoscopic numerical models for energetic composites so that eventually hot spot formation can be predicted.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Computing the Paschen curve for argon with speed-limited particle-in-cell simulation

Upon inclusion of collisions, the speed-limited particle-in-cell (SLPIC) simulation method successfully computed the Paschen curve for argon. The 1D3V simulations modeled an electron cascade across an argon-filled capacitor, including electron-neutral ionization, electron-neutral elastic collisions, electron-neutral excitation, and ion-induced secondary electron emission. In electrical breakdown, the timescale difference between ion and electron motion makes traditional PIC methods computationally slow. To decrease this timescale difference and speed up computation, here we used SLPIC, a time-domain algorithm that limits the speed of the fastest electrons in the simulation. The SLPIC algorithm facilitates a straightforward, fully kinetic treatment of dynamics and collisions. SLPIC was as accurate as PIC, but ran up to 200 times faster. SLPIC accurately computed the Paschen curve for argon over three orders of magnitude in pressure.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Transport physics dependence of Bohm speed in presheath–sheath transition

We report the ion exit flow speed at the sheath entrance is constrained by the Bohm criterion, which is used as a boundary condition for simulations that do not resolve the sheath region. Traditional Bohm criterion analysis invokes the equation of state and, thus, ignores transport physics in the sheath transition problem. An expression for the Bohm speed away from the asymptotic limit is derived from a set of anisotropic plasma transport equations. The thermal force, collisional temperature isotropization, and heat flux enter into the evaluation of the Bohm speed. By comparison with kinetic simulation results, this expression is shown to be accurate in the presheath–sheath transition region rather than a single point at the sheath entrance over a broad range of collisionality.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Initial speed of knots in the plasma tail of C/2013 R1(Lovejoy)

We report short-time variations in the plasma tail of C/2013 R1(Lovejoy). A series of short (2–3 minutes) exposure images with the 8.2 m Subaru telescope shows faint details of filaments and their motions over a 24 minute observing duration. We identified rapid movements of two knots in the plasma tail near the nucleus (∼3×10{sup 5} km). Their speeds are 20 and 25 km s{sup −1} along the tail and 3.8 and 2.2 km s{sup −1} across it, respectively. These measurements set a constraint on an acceleration model of plasma tail and knots as they set the initial speed just after their formation. We also found a rapid narrowing of the tail. After correcting the motion along the tail, the narrowing speed is estimated to be ∼8 km s{sup −1}. These rapid motions suggest the need for high time-resolution studies of comet plasma tails with a large telescope.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

HELIOSHEATH MAGNETIC FIELDS BETWEEN 104 AND 113 AU IN A REGION OF DECLINING SPEEDS AND A STAGNATION REGION

We examine the relationships between the magnetic field and the radial velocity component V{sub R} observed in the heliosheath by instruments on Voyager 1 (V1). No increase in the magnetic field strength B was observed in a region where V{sub R} decreased linearly from 70 km s{sup -1} to 0 km s{sup -1} as plasma moved outward past V1. An unusually broad transition from positive to negative polarity was observed during a Almost-Equal-To 26 day interval when the heliospheric current sheet (HCS) moved below the latitude of V1 and the speed of V1 was comparable to the radial speed of the heliosheath flow. When V1 moved through a region where V{sub R} Almost-Equal-To 0 (the 'stagnation region'), B increased linearly with time by a factor of two, and the average of B was 0.14 nT. Nothing comparable to this was observed previously. The magnetic polarity was negative throughout the stagnation region for Almost-Equal-To 580 days until 2011 DOY 235, indicating that the HCS was below the latitude of V1. The average passage times of the magnetic holes and proton boundary layers were the same during 2009 and 2011, because the plasma moved past V1 during 2009 at the same speed that V1 moved through the stagnation region during 2011. The microscale fluctuations of B in the stagnation region during 2011 are qualitatively the same as those observed in the heliosheath during 2009. These results suggest that the stagnation region is a part of the heliosheath, rather than a 'transition region' associated with the heliopause.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE OCCURRENCE AND SPEED OF CMEs RELATED TO TWO CHARACTERISTIC EVOLUTION PATTERNS OF HELICITY INJECTION IN THEIR SOLAR SOURCE REGIONS

Long-term (a few days) variation of magnetic helicity injection was calculated for 28 solar active regions that produced 47 coronal mass ejections (CMEs) to find its relationship to the CME occurrence and speed using SOHO/MDI line-of-sight magnetograms. As a result, we found that the 47 CMEs can be categorized into two different groups by two characteristic evolution patterns of helicity injection in their source active regions, which appeared for {approx}0.5-4.5 days before their occurrence: (1) a monotonically increasing pattern with one sign of helicity (Group A; 30 CMEs in 23 active regions) and (2) a pattern of significant helicity injection followed by its sign reversal (Group B; 17 CMEs in 5 active regions). We also found that CME speed has a correlation with average helicity injection rate with linear correlation coefficients of 0.85 and 0.63 for Group A and Group B, respectively. In addition, these two CME groups show different characteristics as follows: (1) the average CME speed of Group B (1330 km s{sup -1}) is much faster than that of Group A (870 km s{sup -1}), (2) the CMEs in Group A tend to be single events whereas those in Group B mainly consist of successive events, and (3) flares related to the CMEs in Group B are relatively more energetic and impulsive than those in Group A. Our findings therefore suggest that the two CME groups have different pre-CME conditions in their source active regions and different CME characteristics.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ON THE ORIGIN OF THE SLOW SPEED SOLAR WIND: HELIUM ABUNDANCE VARIATIONS

The first ionization potential (FIP) effect is the by now well-known enhancement in abundance over photospheric values of Fe and other elements with FIP below about 10 eV observed in the solar corona and slow speed solar wind. In our model, this fractionation is achieved by means of the ponderomotive force, arising as Alfven waves propagate through or reflect from steep density gradients in the solar chromosphere. This is also the region where low FIP elements are ionized, and high FIP elements are largely neutral leading to the fractionation as ions interact with the waves but neutrals do not. Helium, the element with the highest FIP and consequently the last to remain neutral as one moves upward, can be depleted in such models. Here, we investigate this depletion for varying loop lengths and magnetic field strengths. Variations in this depletion arise as the concentration of the ponderomotive force at the top of the chromosphere varies in response to Alfven wave frequency with respect to the resonant frequency of the overlying coronal loop, the magnetic field, and possibly also the loop length. We find that stronger depletions of He are obtained for weaker magnetic field, at frequencies close to or just above the loop resonance. These results may have relevance to observed variations of the slow wind solar He abundance with wind speed, with slower slow speed solar wind having a stronger depletion of He.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Speeding up low-mass planetary microlensing simulations and modeling: The caustic region of influence

Extensive simulations of planetary microlensing are necessary both before and after a survey is conducted: before to design and optimize the survey and after to understand its detection efficiency. The major bottleneck in such computations is the computation of light curves. However, for low-mass planets, most of these computations are wasteful, as most light curves do not contain detectable planetary signatures. In this paper, I develop a parameterization of the binary microlens that is conducive to avoiding light curve computations. I empirically find analytic expressions describing the limits of the parameter space that contain the vast majority of low-mass planet detections. Through a large-scale simulation, I measure the (in)completeness of the parameterization and the speed-up it is possible to achieve. For Earth-mass planets in a wide range of orbits, it is possible to speed up simulations by a factor of ∼30-125 (depending on the survey's annual duty-cycle) at the cost of missing ∼1% of detections (which is actually a smaller loss than for the arbitrary parameter limits typically applied in microlensing simulations). The benefits of the parameterization probably outweigh the costs for planets below 100 M{sub ⊕}. For planets at the sensitivity limit of AFTA-WFIRST, simulation speed-ups of a factor ∼1000 or more are possible.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

OCCURRENCE OF HIGH-SPEED SOLAR WIND STREAMS OVER THE GRAND MODERN MAXIMUM

In the declining phase of the solar cycle (SC), when the new-polarity fields of the solar poles are strengthened by the transport of same-signed magnetic flux from lower latitudes, the polar coronal holes expand and form non-axisymmetric extensions toward the solar equator. These extensions enhance the occurrence of high-speed solar wind (SW) streams (HSS) and related co-rotating interaction regions in the low-latitude heliosphere, and cause moderate, recurrent geomagnetic activity (GA) in the near-Earth space. Here, using a novel definition of GA at high (polar cap) latitudes and the longest record of magnetic observations at a polar cap station, we calculate the annually averaged SW speeds as proxies for the effective annual occurrence of HSS over the whole Grand Modern Maximum (GMM) from 1920s onward. We find that a period of high annual speeds (frequent occurrence of HSS) occurs in the declining phase of each of SCs 16-23. For most cycles the HSS activity clearly reaches a maximum in one year, suggesting that typically only one strong activation leading to a coronal hole extension is responsible for the HSS maximum. We find that the most persistent HSS activity occurred in the declining phase of SC 18. This suggests that cycle 19, which marks the sunspot maximum period of the GMM, was preceded by exceptionally strong polar fields during the previous sunspot minimum. This gives interesting support for the validity of solar dynamo theory during this dramatic period of solar magnetism.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Local non-Gaussianity from rapidly varying sound speeds

We study the effect of non-trivial sound speeds on local-type non-Gaussianity during multiple-field inflation. To this end, we consider a multiple-DBI model and use the δN formalism to track the super-horizon evolution of perturbations. By adopting a sum separable Hubble parameter we derive analytic expressions for the relevant quantities in the two-field case, valid beyond slow variation. We find that non-trivial sound speeds can, in principle, curve the trajectory in such a way that significant local-type non-Gaussianity is produced. Deviations from slow variation, such as rapidly varying sound speeds, enhance this effect. To illustrate our results we consider two-field inflation in the tip regions of two warped throats and find large local-type non-Gaussianity produced towards the end of the inflationary process.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Scale-dependent CMB power asymmetry from primordial speed of sound and a generalized δ N formalism

We explore a plausible mechanism that the hemispherical power asymmetry in the CMB is produced by the spatial variation of the primordial sound speed parameter. We suggest that in a generalized approach of the δ N formalism the local e-folding number may depend on some other primordial parameters besides the initial values of inflaton. Here the δ N formalism is extended by considering the effects of a spatially varying sound speed parameter caused by a super-Hubble perturbation of a light field. Using this generalized δ N formalism, we systematically calculate the asymmetric primordial spectrum in the model of multi-speed inflation by taking into account the constraints of primordial non-Gaussianities. We further discuss specific model constraints, and the corresponding asymmetry amplitudes are found to be scale-dependent, which can accommodate current observations of the power asymmetry at different length scales.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

A semi-analytic estimate for the effective sound speed counterterm in the EFTofLSS

The Effective Field Theory of Large Scale Structure (EFTofLSS) has found tremendous success as a perturbative framework for the evolution of large scale structure, and it is now routinely used to compare theoretical predictions against cosmological observations. The model for the total matter field includes one nuisance parameter at 1-loop order, the effective sound speed, which can be extracted by matching the EFT to full N-body simulations. In this work we first leverage the Layzer-Irvine cosmic energy equation to show that the equation of state can be exactly computed with knowledge of the fully nonlinear power spectrum. When augmented with separate universe methods, we show one can estimate the effective sound speed. This estimate is in good agreement with simulation results, with errors at the few tens of percent level. Here, we apply our method to investigate the cosmology dependence of the effective sound speed and to shed light on what cosmic structures shape its value.

Cosmological perturbation theory in GR and beyond↗
DOE OSTI2012

SOUND-SPEED INVERSION OF THE SUN USING A NONLOCAL STATISTICAL CONVECTION THEORY

Helioseismic inversions reveal a major discrepancy in sound speed between the Sun and the standard solar model just below the base of the solar convection zone. We demonstrate that this discrepancy is caused by the inherent shortcomings of the local mixing-length theory adopted in the standard solar model. Using a self-consistent nonlocal convection theory, we construct an envelope model of the Sun for sound-speed inversion. Our solar model has a very smooth transition from the convective envelope to the radiative interior, and the convective energy flux changes sign crossing the boundaries of the convection zone. It shows evident improvement over the standard solar model, with a significant reduction in the discrepancy in sound speed between the Sun and local convection models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

A High-Speed Rotational Diamond Anvil Cell for In Situ Analysis of Hierarchical Microstructural Evolution of Metallic Alloys during Extreme Shear Deformation

High speed shear deformation is ubiquitous in engineering applications, ranging from material processing methods such as friction stir processing/extrusion and in tribological contacts. However, analyzing the microstructural evolution of materials while they are undergoing high speed shear deformation have been a long-standing challenge. This led to predominant reliance on ex situ microscopy before and after shear deformation. But ex situ microscopy lacks the ability to analyze dynamic and transient hierarchical microstructural evolution mechanisms that could occur during shear deformation of materials. Therefore, to better understand the dynamic mechanisms of mass and energy transfer in materials under shear deformation, we developed a first of its kind high-speed rotational diamond anvil cell (HS-RDAC) for synchrotron-based in situ high-energy x-ray diffraction (XRD). We studied the time resolved lattice strain evolution, XRD peak broadening and changes in spatial variation of shear deformation induced alloying in pure metal and metal alloy sheets and powder mixture using the HS-RDAC. These in situ results were combined with detailed ex situ microstructural characterization before and after the shear deformation using transmission electron microscopy and atom probe tomography, which revealed the different stages of evolution of a shear deformation induced hierarchical nanostructure. Multiscale computational simulations including computational fluid dynamics, crystal plasticity, molecular dynamic simulation and density functional theory uncovered the mechanisms behind morphological changes, evolution of defect structures and changes in driving force for shear deformation induced intermixing. In conclusion, this in situ HS-RDAC capability, in combination with ex situ microstructural characterization and computational simulations, can provide new insights into the hierarchical microstructural evolution pathway during shear deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Feasibility of measuring the speed of sound of the quark-gluon plasma from the multiplicity and mean 𝑝 𝑇 of ultracentral heavy-ion collisions

The mean transverse momentum ⟨𝑝 𝑇 ⟩ of hadrons has been observed experimentally and in numerical simulations to have a power-law dependence on the hadronic multiplicity 𝑁 in ultracentral relativistic heavy-ion collisions: ⟨𝑝 𝑇 ⟩∝𝑁 𝑏 UC . It has been put forward that this exponent 𝑏 UC is the speed of sound of quark-gluon plasma measured at a temperature determined from ⟨𝑝 𝑇 ⟩. We study step by step the connection between (i) the energy and entropy of hydrodynamic simulations and (ii) experimentally measurable observables. We show that an argument based on energy and entropy should yield an exponent equal to the pressure over energy density 𝑃/ɛ, rather than the speed of sound 𝑐$_s^2$; however, we also observe that ⟨𝑝 𝑇 ⟩ and 𝑁 are not sufficiently accurate proxies for the energy and entropy to make this possible in practice. From simulations, we find that the exponent 𝑏 UC is significantly different whether the “effective volume” is strictly constant or not, a condition that cannot be enforced experimentally. Additional tests using a modified equation of state find that the exponent 𝑏 UC exhibits a variable degree of correlations with the speed of sound and with 𝑃/ɛ, but is not an accurate measurement of either quantity in general.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Approach to accurately measuring the speed of optical precursors

Precursors can serve as a bound on the speed of information with dispersive medium. We propose a method to identify the speed of optical wave fronts using polarization-based interference in a solid-state device, which can bound the accuracy of the speed of wave fronts to less than 10{sup -4} with conventional experimental conditions. Our proposal may have important implications for optical communications and fast information processing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Analytical insights into the interplay of momentum, multiplicity, and the speed of sound in heavy-ion collisions

We introduce a minimal model of ultracentral heavy-ion collisions to study the relation between the speed of sound of the produced plasma and the final particles' energy and multiplicity. We discuss how the particles' multiplicity N tot and average energy E tot / N tot is related to the speed of sound c s by c s 2 = d ln ( E tot / N tot ) / d ln N tot if the fluid is inviscid, its speed of sound is constant and all final particles can be measured. We show that finite rapidity cuts on the particles' multiplicity N and energy E introduce corrections between c s 2 and d ln ( E / N ) / d ln N that depend on the system's lifetime. We study analytically these deviations with the Gubser hydrodynamic solution, finding that, for ultrarelativistic bosons, they scale as the ratio of the freeze-out temperature T FO over the maximum initial temperature of the fluid T 0 ; the nonthermodynamic aspect of these corrections is highlighted through their dependence on the system's initial conditions. Published by the American Physical Society 2024

Physics↗