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All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Time-resolved velocity and ion sound speed measurements from simultaneous bow shock imaging and inductive probe measurements

We present a technique to measure the time-resolved velocity and ion sound speed in magnetized, supersonic high-energy-density plasmas. We place an inductive (“b-dot”) probe in a supersonic pulsed-power-driven plasma flow and measure the magnetic field advected by the plasma. As the magnetic Reynolds number is large (R M > 10), the plasma flow advects a magnetic field proportional to the current at the load. This enables us to estimate the flow velocity as a function of time from the delay between the current at the load and the signal at the probe. The supersonic flow also generates a hydrodynamic bow shock around the probe, the structure of which depends on the upstream sonic Mach number. By imaging the shock around the probe with a Mach–Zehnder interferometer, we determine the upstream Mach number from the shock Mach angle, which we then use to determine the ion sound speed from the known upstream velocity. We use the sound speed to infer the value of Z̄T e , where Z̄ is the average ionization and T e is the electron temperature. We use this diagnostic to measure the time-resolved velocity and sound speed of a supersonic (M S ~ 8), super-Alfvénic (M A ~ 2) aluminum plasma generated during the ablation stage of an exploding wire array on the Magpie generator (1.4 MA, 250 ns). The velocity and Z̄T e measurements agree well with the optical Thompson scattering measurements reported in the literature and with 3D resistive magnetohydrodynamic simulations in GORGON.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Characterization of wind speed and directional shear at the AWAKEN field campaign site

The American wake experiment (AWAKEN) is taking place in northern Oklahoma, USA, close to the Atmospheric Radiation Measurement Southern Great Plains (ARM SGP) atmospheric observatory. The planning for the deployment of the instruments in this observational field campaign required an assessment of the wind characteristics of the site. This paper analyzes long-term data collected by instruments at the ARM SGP observatory to characterize the winds near the AWAKEN site. The analysis shows that this site experiences high wind shear and veer events with a large number of nocturnal low-level jets. A total of 7086 low-level jet wind profiles over 6 years are examined and found to be dominant from the south and southeast. Significant nocturnal wind veer is observed, which causes southerly wind near the surface to become westerly wind aloft. By identifying a strong relationship between atmospheric stability and wind shear, the wind shear at the site is predicted using the Monin–Obukhov similarity theory (MOST) and validated with the observational data collected by a scanning Doppler lidar. The results show that wind speed at a height of 91 m, a proxy hub height for wind turbines in this area, can be predicted from data collected at a height of 10 m with a bias of −0.35 and 0.65 m s−1 in unstable and stable atmospheric boundary layers, respectively. The bias of the predicted wind speed is mostly in the region of low wind speed, and wind speed above 5 m s−1 at a height of 91 m can be predicted with a bias of less than 0.2 m s−1, and the limitations of the MOST in predicting winds during the stably stratified boundary layer is well-observed.

17 WIND ENERGY↗
DOE OSTI2012

EVOLUTION OF THE RELATIONSHIPS BETWEEN HELIUM ABUNDANCE, MINOR ION CHARGE STATE, AND SOLAR WIND SPEED OVER THE SOLAR CYCLE

The changing relationships between solar wind speed, helium abundance, and minor ion charge state are examined over solar cycle 23. Observations of the abundance of helium relative to hydrogen (A{sub He} {identical_to} 100 Multiplication-Sign n{sub He}/n{sub H}) by the Wind spacecraft are used to examine the dependence of A{sub He} on solar wind speed and solar activity between 1994 and 2010. This work updates an earlier study of A{sub He} from 1994 to 2004 to include the recent extreme solar minimum and broadly confirms our previous result that A{sub He} in slow wind is strongly correlated with sunspot number, reaching its lowest values in each solar minima. During the last minimum, as sunspot numbers reached their lowest levels in recent history, A{sub He} continued to decrease, falling to half the levels observed in slow wind during the previous minimum and, for the first time observed, decreasing even in the fastest solar wind. We have also extended our previous analysis by adding measurements of the mean carbon and oxygen charge states observed with the Advanced Composition Explorer spacecraft since 1998. We find that as solar activity decreased, the mean charge states of oxygen and carbon for solar wind of a given speed also fell, implying that the wind was formed in cooler regions in the corona during the recent solar minimum. The physical processes in the coronal responsible for establishing the mean charge state and speed of the solar wind have evolved with solar activity and time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SOLAR-CYCLE VARIATION OF SOUND SPEED NEAR THE SOLAR SURFACE

We present evidence that the sound-speed variation with solar activity has a two-layer configuration, similar to the one observed below an active region, which consists of a negative layer near the solar surface and a positive one in the layer immediately below the first one. Frequency differences between the activity minimum and maximum of solar cycle 23, obtained applying global helioseismology to the Michelson Doppler Imager on board the Solar and Heliospheric Observatory, is used to determine the sound-speed variation from below the base of the convection zone to a few Mm below the solar surface. We find that the sound speed at solar maximum is smaller than at solar minimum at the limit of our determination (5.5 Mm). The min-to-max difference decreases in absolute values until {approx}7 Mm. At larger depths, the sound speed at solar maximum is larger than at solar minimum and the difference increases with depth until {approx}10 Mm. At this depth, the relative difference ({delta}c{sup 2}/c{sup 2}) is less than half of the value observed at the lowest depth determination. At deeper layers, it slowly decreases with depth until there is no difference between maximum and minimum activity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

CORONAL ALFVEN SPEED DETERMINATION: CONSISTENCY BETWEEN SEISMOLOGY USING AIA/SDO TRANSVERSE LOOP OSCILLATIONS AND MAGNETIC EXTRAPOLATION

Two transversely oscillating coronal loops are investigated in detail during a flare on the 2011 September 6 using data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. We compare two independent methods to determine the Alfven speed inside these loops. Through the period of oscillation and loop length, information about the Alfven speed inside each loop is deduced seismologically. This is compared with the Alfven speed profiles deduced from magnetic extrapolation and spectral methods using AIA bandpass. We find that for both loops the two methods are consistent. Also, we find that the average Alfven speed based on loop travel time is not necessarily a good measure to compare with the seismological result, which explains earlier reported discrepancies. Instead, the effect of density and magnetic stratification on the wave mode has to be taken into account. We discuss the implications of combining seismological, extrapolation, and spectral methods in deducing the physical properties of coronal loops.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Connecting speeds, directions and arrival times of 22 coronal mass ejections from the sun to 1 AU

Forecasting the in situ properties of coronal mass ejections (CMEs) from remote images is expected to strongly enhance predictions of space weather and is of general interest for studying the interaction of CMEs with planetary environments. We study the feasibility of using a single heliospheric imager (HI) instrument, imaging the solar wind density from the Sun to 1 AU, for connecting remote images to in situ observations of CMEs. We compare the predictions of speed and arrival time for 22 CMEs (in 2008-2012) to the corresponding interplanetary coronal mass ejection (ICME) parameters at in situ observatories (STEREO PLASTIC/IMPACT, Wind SWE/MFI). The list consists of front- and backsided, slow and fast CMEs (up to 2700 km s{sup –1}). We track the CMEs to 34.9 ± 7.1 deg elongation from the Sun with J maps constructed using the SATPLOT tool, resulting in prediction lead times of –26.4 ± 15.3 hr. The geometrical models we use assume different CME front shapes (fixed-Φ, harmonic mean, self-similar expansion) and constant CME speed and direction. We find no significant superiority in the predictive capability of any of the three methods. The absolute difference between predicted and observed ICME arrival times is 8.1 ± 6.3 hr (rms value of 10.9 hr). Speeds are consistent to within 284 ± 288 km s{sup –1}. Empirical corrections to the predictions enhance their performance for the arrival times to 6.1 ± 5.0 hr (rms value of 7.9 hr), and for the speeds to 53 ± 50 km s{sup –1}. These results are important for Solar Orbiter and a space weather mission positioned away from the Sun-Earth line.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Extracting the speed of sound in quark–gluon plasma with ultrarelativistic lead–lead collisions at the LHC

Abstract Ultrarelativistic nuclear collisions create a strongly interacting state of hot and dense quark–gluon matter that exhibits a remarkable collective flow behavior with minimal viscous dissipation. To gain deeper insights into its intrinsic nature and fundamental degrees of freedom, we determine the speed of sound in an extended volume of quark–gluon plasma using lead–lead (PbPb) collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The data were recorded by the CMS experiment at the CERN LHC and correspond to an integrated luminosity of 0.607 nb −1 . The measurement is performed by studying the multiplicity dependence of the average transverse momentum of charged particles emitted in head-on PbPb collisions. Our findings reveal that the speed of sound in this matter is nearly half the speed of light, with a squared value of 0.241 ± 0.002 ( stat ) ± 0.016 ( syst ) in natural units. The effective medium temperature, estimated using the mean transverse momentum, is 219 ± 8 ( syst ) MeV . The measured squared speed of sound at this temperature aligns precisely with predictions from lattice quantum chromodynamic (QCD) calculations. This result provides a stringent constraint on the equation of state of the created medium and direct evidence for a deconfined QCD phase being attained in relativistic nuclear collisions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

The speed of Galileon gravity

We analyse the speed of gravitational waves in coupled Galileon models with an equation of state ω{sub φ}=−1 now and a ghost-free Minkowski limit. We find that the gravitational waves propagate much faster than the speed of light unless these models are small perturbations of cubic Galileons and the Galileon energy density is sub-dominant to a dominant cosmological constant. In this case, the binary pulsar bounds on the speed of gravitational waves can be satisfied and the equation of state can be close to -1 when the coupling to matter and the coefficient of the cubic term of the Galileon Lagrangian are related. This severely restricts the allowed cosmological behaviour of Galileon models and we are forced to conclude that Galileons with a stable Minkowski limit cannot account for the observed acceleration of the expansion of the universe on their own. Moreover any sub-dominant Galileon component of our universe must be dominated by the cubic term. For such models with gravitons propagating faster than the speed of light, the gravitons become potentially unstable and could decay into photon pairs. They could also emit photons by Cerenkov radiation. We show that the decay rate of such speedy gravitons into photons and the Cerenkov radiation are in fact negligible. Moreover the time delay between the gravitational signal and light emitted by explosive astrophysical events could serve as a confirmation that a modification of gravity acts on the largest scales of the Universe.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

The pattern speed of the Milky Way bar from transverse velocities

ABSTRACT We use the continuity equation to derive a method for measuring the pattern speed of the Milky Way’s bar/bulge from proper motion data. The method has minimal assumptions but requires complete coverage of the non-axisymmetric component in two of the three Galactic coordinates. We apply our method to the proper motion data from a combination of Gaia DR2 and VISTA Variables in the Via Lactea (VVV) to measure the pattern speed of the bar as $\Omega _\mathrm{p}=(41\pm 3)\, \mathrm{km\, s^{-1}\, kpc^{-1}}$ (where the error is statistical). This puts the corotation radius at $(5.7\pm 0.4)\, \mathrm{kpc}$, under the assumptions of the standard peculiar motion of the Sun and the absence of non-axisymmetric streaming in the Solar neighbourhood. The obtained result uses only data on the near side of the bar which produces consistent measurements of the distance and velocity of the centre of the Galaxy. Addition of the data on the far side of the bar pulls the pattern speed down to $\Omega _\mathrm{p}=(31\pm 1)\, \mathrm{km\, s^{-1}\, kpc^{-1}}$ but requires a lower transverse velocity for the Galactic centre than observed. This suggests systematics of $5-10\, \mathrm{km\, s^{-1}kpc^{-1}}$ dominate the uncertainty. We demonstrate using a dynamically formed bar/bulge simulation that even with the limited field of view of the VVV survey our method robustly recovers the pattern speed.

Sanders, Jason L.↗
DOE OSTI2021

Signature of transition to supershear rupture speed in the coseismic off-fault damage zone

Most earthquake ruptures propagate at speeds below the shear wave velocity within the crust, but in some rare cases, ruptures reach supershear speeds. The physics underlying the transition of natural subshear earthquakes to supershear ones is currently not fully understood. Most observational studies of supershear earthquakes have focused on determining which fault segments sustain fully grown supershear ruptures. Experimentally cross-validated numerical models have identified some of the key ingredients required to trigger a transition to supershear speed. However, the conditions for such a transition in nature are still unclear, including the precise location of this transition. In this work, we provide theoretical and numerical insights to identify the precise location of such a transition in nature. We use fracture mechanics arguments with multiple numerical models to identify the signature of supershear transition in coseismic off-fault damage. Furthermore, we then cross-validate this signature with high-resolution observations of fault zone width and early aftershock distributions. We confirm that the location of the transition from subshear to supershear speed is characterized by a decrease in the width of the coseismic off-fault damage zone. We thus help refine the precise location of such a transition for natural supershear earthquakes.

42 ENGINEERING↗
DOE OSTI2023

Shock-induced melting of [100] lithium fluoride: Sound speed and Hugoniot measurements to 230 GPa

Although [100] lithium flouride (LiF) is the most widely used optical window material in dynamic compression experiments, its high stress (>100 GPa) shock compression response, including melting, is not well understood. To address this need, we measured wave profiles in plate impact experiments to determine the Hugoniot states and longitudinal sound speeds in [100] LiF crystals shock compressed to 231 GPa. The measured peak states are fitted well by a linear shock velocity-particle velocity relation, providing an accurate determination of the LiF Hugoniot curve to significantly higher stresses than previous experiments. The longitudinal sound speeds show a near linear increase with density compression to 182 GPa. Between 182 GPa and 195 GPa, the sound speed and the longitudinal modulus decrease abruptly, due to shock-induced melting. The increasing sound speeds and moduli at higher stresses suggest that shock compressed LiF is fully liquid at 195 GPa and above, allowing determination of the Gruneisen parameter for liquid LiF. Here, the melt stress determined here differs from that predicted by current multiphase equations of state for LiF. Our results provide important insight into the high stress solid and liquid states of shock compressed LiF and point to the need for an improved multiphase equation of state at high pressures and high temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Sound speed and Grüneisen parameter up to three terapascal in shock-compressed iron

This paper presents the first sound speed and Grüneisen parameter data for fluid iron compressed to 3 TPa (30 million atmospheres) and 20 g/cm 3 on the Hugoniot. Both the sound speed and Grüneisen parameter are derivatives of the equation of state (EOS), and thus tightly constrain the contours of the EOS surface. The sound speed data are systematically lower than expected from a simple extrapolation of previous data. The Grüneisen parameter shows a 30% drop at pressures and temperatures above the melt transition. Furthermore, while some models compare well with either the sound speed or Grüneisen parameter, none of today’s state-of-the-art models can explain both sets of data. Furthermore these new data will provide pivotal benchmarks for both future theoretical EOSs of warm dense iron and modeling planetary states and processes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Strong coupling problem with time-varying sound speed

For a single scalar field with unit sound speed minimally coupled to Einstein gravity, there are exactly three distinct cosmological solutions which produce a scale invariant spectrum of curvature perturbations in a dynamical attractor background, assuming vacuum initial conditions: slow-roll inflation; a slowly contracting adiabatic ekpyrotic phase, described by a rapidly-varying equation of state; and an adiabatic ekpyrotic phase on a slowly expanding background. Of these three, only inflation remains weakly coupled over a wide range of modes, while the other scenarios can produce at most 12 e-folds of scale invariant and Gaussian modes. In this paper, we investigate how allowing the speed of sound of fluctuations to evolve in time affects this classification. While in the presence of a variable sound speed there are many more scenarios which are scale invariant at the level of the two-point function, they generically suffer from strong coupling problems similar to those in the canonical case. There is, however, an exceptional case with superluminal sound speed, which suppresses non-Gaussianities and somewhat alleviates strong coupling issues. We focus on a particular realization of this limit and show these scenarios are constrained and only able to produce at most 28 e-folds of scale invariant and Gaussian perturbations. A similar bound should hold more generally - the condition results from the combined requirements of matching the observed amplitude of curvature perturbations, demanding that the Hubble parameter remain sub-Planckian and keeping non-Gaussianities under control. We therefore conclude that inflation remains the unique cosmological scenario, assuming a single degree of freedom on an attractor background, capable of producing arbitrarily many scale invariant modes while remaining weakly coupled. Alternative mechanisms must inevitably be unstable or rely on multiple degrees of freedom.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Frequency Support Enhancement of a Permanent Magnet-Based Adjustable-Speed Pumped Hydropower Plant

When a permanent magnet (PM) -based adjustable-speed pumped storage hydropower (AS-PSH) plant delivers frequency response via the full-scaled power converter, there is power imbalance at the PM generator during the initial stage of the frequency response. This is because the AS-PSH has the different time constants between the power converter and wicket gate in the governor valve. Thus, this imbalance causes a sudden reduction of the rotational speed of the machine during the frequency response. As a result, a voltage deficit arises at the generator terminal in proportion to the decrement in the rotational speed, and thereby it is promising that the current flow at the converter will reach its limit when attempting to inject the commanded active power at a reduced voltage level. Thus, this causes the degradation of the frequency response of the AS-PSH due to the self-demagnetization at a reduced rotational speed. This paper proposes a frequency support scheme of a PM-based AS-PSH to enhance the frequency response by bridging the gap between the different time constants. To achieve this, the frequency support scheme includes a gate opening adjustment in the governor and a dynamic ramp-down in converter controller of the AS-PSH.

13 HYDRO ENERGY↗
DOE OSTI2020

Design of a 100 kW Surface Permanent Magnet Machine with Wide Constant Power Speed Ratio for Traction Applications

Surface permanent magnet (SPM) machines are considered to be strong candidates for traction applications due to their high power density and high efficiency. By adopting fractional-slot concentrated windings, SPM machines can achieve a wide constant power speed range. A carbon fiber sleeve is applied to ensure the retainment of the magnets at high speed operating conditions. The purpose of this paper is to present the design of a high speed SPM machine for traction applications with 25 kW/Liter active power density and a wide constant-power speed range that is consistent with guidelines and targets presented in the Electrical and Electronics Technical Team Roadmap published by the US Department of Energy. The predicted electromagnetic performance and structural characteristics are presented based on both analytical and FEA results.

42 ENGINEERING↗
DOE OSTI2024

Crack Propagation Modeling of a High-Speed Outer Rotor Motor

This study aims to investigate the propagation of initial cracks in electric motors and their rate of spread through the rotor structure under various operating conditions. Traction motors are subjected to high centrifugal load owing to their speed and rapid acceleration and deceleration. In the manufacturing process, some initial imperfections or cracks in the rotor components are likely to occur. Under the influence of loading, these cracks will grow, and factors such as loading frequency, material strength, and location of material voids will affect this threshold speed. In the current study, a fracture mechanics–based approach has been used to investigate the crack growth rate for a 100 kW outer rotor motor with a rated speed of 6,000 rpm and a peak speed of 20,000 rpm for an integrated drive application.

Barua, Himel↗
DOE OSTI2025

Improving Dynamic Wireless Charging System Performance For Electric Vehicles Through Variable Speed Limit Control Integration

Electric Vehicle (EV) charging has been a significant barrier to the widespread use of EVs. Traditional EV charging methods depend on cables, and there are concerns about safety, accessibility, convenience, and weather. A recent development, dynamic (or in-motion) wireless charging, enables EVs to charge wirelessly by incorporating charging infrastructure into roadways, allowing EVs to charge while moving. However, the energy transferred relies heavily on vehicle speed and time spent in the charging lane. This paper proposes an innovative solution that combines dynamic wire-less charging with Variable Speed Limit (VSL) control. This dynamic traffic control strategy adjusts speed limits based on real-time traffic, weather, and incidents. This integration of dynamic wireless charging and VSL has two potential benefits. First, it can motivate driver compliance with VSL through the incentive of charging. Second, it can promote smoother traffic flow and improve traffic safety by implementing lower speed limits at certain times. To verify these benefits, microscopic traffic simulations in SUMO were conducted under different EV penetration rates and VSL compliance rates. Simulation results reveal that the proposed approach can enhance dynamic wireless charging system performance while improving traffic flow and safety.

Xu, Guanhao [ORNL] (ORCID:0000000214326357)↗
DOE OSTI2020

Leveraging Multiple Connected Traffic Light Signals in an Energy-Efficient Speed Planner

Connecting automated vehicles to traffic lights can lead to significant energy savings by enabling them to pass through intersections in an energy-efficient way without unnecessary stops. A cellular-based communication system connecting multiple traffic lights can help realize the full potential of energy-efficient driving at intersections. Thus, we propose a hierarchical speed planner that can leverage information from multiple connected traffic lights. The proposed speed planner consists of two modules: a green window selector and a reference trajectory generator. The green window selector, based on Dijkstra’s algorithm, finds a series of "green windows" for connected traffic lights that builds an energy-optimal path for vehicles to follow. The reference trajectory generator finds optimal entering times, based on the selected green window at each intersection, and then computes reference trajectories. Deriving and using analytical optimal entering speeds as a function of entering times allows us to guarantee the computational simplicity suitable for real-time implementation. We also demonstrate how to balance energy and traffic flow perspectives in the reference trajectory generator. Lastly, a high-fidelity simulation framework is used to evaluate the proposed speed planner and quantify the extent to which it can save energy in various real-world urban route scenarios.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗