Transverse Expansion Velocity Research Articles

Ballistic impact wave and back bulge propagation mechanism and blunt trauma assessment of ceramic/UHMWPE composite body armor

With increasing local conflicts worldwide, studying the impact response of body armor is crucial for enhancing soldier survival rates. However, there is still insufficient understanding of the back bulge expansion patterns, the impact waves transmission features, and blunt injury assessing method of body armor. To investigate these problems deeply, the three-dimensional digital image correlation method and numerical models were employed to reveal the ballistic response mechanism of ceramic/ultra-high-molecular-weight polyethylene (UHMWPE) composite body armor impacted by 7.62 mm rifle bullet. This study built theoretical function for the bulge expansion, revealed the wave transmission mechanism and fractures in both materials and their interaction. Firstly, the experiment revealed that the back-face bulge transverse expansion velocity exhibits a double exponential decrease. Secondly, the contour of back-face bulges consistently demonstrated excellent hyperbolic characteristics at various moments. Thirdly, the average Viscous Criterion (VC) and Blunt Criterion (BC) values were 5.37 and 0.95, corresponding to Abbreviated Injury Scale (AIS) values of 6 and 4. Fourthly, the simulation revealed that the ceramics transverse and radial cracks seems primarily induced by shear stress xy; ceramics conical cracks occurring at the boundaries of equivalent stress seems primarily induced by stress in x and y directions. Fifthly, numerical results revealed that the velocity of compression waves within the laminate increases over time in the impact direction and consistent with ceramic velocity impacted on the laminate. These findings lead to experimental and theoretical advances in the impact response mechanism of composite body armor and provide index for improving the protective performance.

Thermodynamic and hydrodynamic characteristics of interacting system formed in relativistic heavy ion collisions

To study the energy-dependent characteristics of thermodynamic and hydrodynamic parameters, based on the framework of a multi-source thermal model, we analyze the soft transverse momentum ([Formula: see text]) spectra of the charged particles ([Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and p) produced in gold–gold (Au–Au) collisions at the center-of-mass energies [Formula: see text], 11.5, 14.5, 19.6, 27, 39, 62.4, and 200[Formula: see text]GeV from the STAR Collaboration and in lead–lead (Pb–Pb) collisions at [Formula: see text] and 5.02[Formula: see text]TeV from the ALICE Collaboration. In the rest framework of emission source, the probability density function obeyed by meson momenta satisfies the Bose–Einstein distribution, and that obeyed by baryon momenta satisfies the Fermi–Dirac distribution. To simulate the [Formula: see text] of the charged particles, the kinetic freeze-out temperature T and transverse expansion velocity [Formula: see text] of emission source are introduced into the relativistic ideal gas model. Our results, based on the Monte Carlo method for numerical calculation, show a good agreement with the experimental data. The excitation functions of thermodynamic parameter T and hydrodynamic parameter [Formula: see text] are then obtained from the analyses, which shows an increasing tendency from 7.7[Formula: see text]GeV to 5.02[Formula: see text]TeV in collisions with different centralities.

Average transverse expansion velocities and global freeze-out temperatures in central Cu + Cu, Au + Au, and Pb + Pb collisions at high energies at RHIC and LHC

The experimental invariant transverse momentum [Formula: see text] spectra of the charged pions and kaons, protons and antiprotons, produced at midrapidity in central (0–10%) Au[Formula: see text]Au collisions at [Formula: see text], central (0–10%) Cu[Formula: see text]Cu collisions at [Formula: see text], central (0–10%) Au[Formula: see text]Au collisions at [Formula: see text], and central (0–5%) Pb[Formula: see text]Pb collisions at [Formula: see text], measured by BRAHMS, STAR and ALICE collaborations, were analyzed using three different transverse expansion (blast-wave) models: Siemens–Rasmussen blast-wave model, Simple transverse flow model, and Simplified (hydro-inspired) blast-wave model of Schnedermann et al. Combined (simultaneous) minimum [Formula: see text] fits of the experimental invariant [Formula: see text] spectra of the charged pions and kaons, protons and antiprotons with the above three model functions were conducted, using the identical selected optimal fitting ranges in [Formula: see text] in each studied collision system, and the values of the average transverse expansion velocity [Formula: see text] and global kinetic freeze-out temperature [Formula: see text] and their dependencies on the collision system [Formula: see text] and [Formula: see text] were extracted. The combined (simultaneous) fits using Hagedorn formula with the (embedded) simple transverse flow describe well the experimental invariant [Formula: see text] spectra of the charged pions, kaons, protons and antiprotons in the whole measured range in region [Formula: see text] in the analyzed central heavy ion collisions at RHIC and LHC, reproducing qualitatively well all the established dependencies of the parameters [Formula: see text] and [Formula: see text] on the collision system [Formula: see text] and [Formula: see text]. The obtained results were compared with those of the previous analyses of high energy heavy ion collisions.

Kinetic freeze-out in central heavy-ion collisions between 7.7 and 2760 GeV per nucleon pair

We fit the single-particle pt spectra of identified pions, kaons, and (anti)protons from central collisions of gold or lead nuclei at energies between 7.7 and 2760 GeV per nucleon pair. The blast wave model with included resonance production and with an assumption of partial chemical equilibrium is used and the fits are performed with the help of a Gaussian emulator process. A kinetic freeze-out temperature is found about 100 MeV for the lowest collision energies and 80 MeV at the LHC. The average transverse expansion velocity grows with increasing from 0.45 to 0.65. Due to partial chemical equilibrium, the influence of resonance decays on the shape of the pt spectra for above 27 GeV is small.

Expansion and Fragmentation of a Liquid-Metal Droplet by a Short Laser Pulse

We report an experimental and numerical investigation of the fragmentation mechanisms of micrometer-sized metal droplet irradiated by ultrashort laser pulses. The results of the experiment show that the fast one-side heating of such a droplet may lead to either symmetric or asymmetric expansion followed by different fragmentation scenarios. To unveil the underlying processes leading to fragmentation we perform simulation of liquid-tin droplet expansion produced by the initial conditions similar to those in experiment using the smoothed particle hydrodynamics (SPH) method. Simulation demonstrates that a thin heated surface layer generates a ultrashort shock wave propagating from the frontal side to rear side of the droplet. Convergence of such shock wave followed by a rarefaction tale to the droplet center results in the cavitation of material inside the central region by the strong tensile stress. Reflection of the shock wave from the rear side of droplet produces another region of highly stretched material where the spallation may occur producing a thin spallation layer moving with a velocity higher than expansion of the central shell after cavitation. It is shown both experimentally and numerically that the threshold laser intensity necessary for the spallation is higher than the threshold required to induce cavitation in the central region of droplet. Thus, the regime of asymmetrical expansion is realized if the laser intensity exceeds the spallation threshold. The transverse and longitudinal expansion velocities obtained in SPH simulations of different regimes of expansion are agreed well with our experimental data.

Higher-order anisotropies in the blast-wave model: Disentangling flow and density field anisotropies

We formulate a generalisation of the blast-wave model which is suitable for the description of higher order azimuthal anisotropies of the hadron production. The model includes anisotropy in the density profile as well as an anisotropy in the transverse expansion velocity field. We then study how these two kinds of anisotropies influence the single-particle distributions and the correlation radii of two-particle correlation functions. Particularly we focus on the third-order anisotropy and consideration is given to the averaging over different orientation of the event plane.

DRoplet and hAdron generator for nuclear collisions: An update

DRoplet and hAdron generator for nuclear collisions: An update

Reconstructing the final state of Pb+Pb collisions at TeV

We fit the single-hadron transverse-momentum spectra measured in Pb+Pb collisions at TeV with the blast-wave model that includes production via resonance decays. Common fit to pions, kaons, (anti)protons, and lambdas yields centrality dependence of the freeze-out temperature and transverse expansion velocity. In the most central collisions we see T = 98 MeV and The resonance fits into this picture but the ϕ meson might freeze-out a little earlier. Multi-strange baryons seem to decouple at higher temperature and weaker transverse flow. Within our model we observe hints of chemical potential for the charged pions.

Linear discrete diffraction and transverse localization of light in two-dimensional backbone lattices

We study the linear discrete diffraction characteristics of light in two-dimensional backbone lattices. It is found that, as the refractive index modulation depth of the backbone lattice increases, high-order band gaps become open and broad in sequence, and the allowed band curves of the Floquet-Bloch modes become flat gradually. As a result, the diffraction pattern at the exit face converges gradually for both the on-site and off-site excitation cases. Particularly, when the refractive index modulation depth of the backbone lattice is high enough, for example, on the order of 0.01 for a square lattice, the light wave propagating in the backbone lattice will be localized in transverse dimension for both the on-site and off-site excitation cases. This is because only the first several allowed bands with nearly flat band curves are excited in the lattice, and the transverse expansion velocities of the Floquet-Bloch modes in these flat allowed bands approach to zero. Such a linear transverse localization of light may have potential applications in navigating light propagation dynamics and optical signal processing.

A Catalog of Outer Ejecta Knots in the Cassiopeia A Supernova Remnant

Hubble Space Telescope images of the core-collapse supernova remnant Cassiopeia A are used to identify high-velocity knots of ejecta located outside the remnant's main emission shell of expanding debris. These ejecta fragments are found near or ahead of the remnant's forward shock front and mostly lie from 12000 to 30000 in radial distance from the remnant's center of expansion. Filter flux ratios when correlated with published spectra show that these knots can be divided into three emission classes: (1) knots dominated by [N II] lambda lambda 6548, 6583 emissions, ( 2) knots dominated by [O II] kk7319, 7330 emissions, and (3) knots displaying filter flux ratios suggestive of [ S II], [ O II], and [Ar III] lambda 7135 emission line strengths similar to the fast-moving knots'' (FMKs) found in the remnant's bright main shell. Of 1825 knots identified, 444 are strong [N II] emission knots, 192 are strong [O II] emission knots, and 1189 are FMK-like knots. In terms of location around the remnant, 972, 207, and 646 knots are found in the remnant's northeast jet, southwest jet, and non-jet regions, respectively. Assuming a distance of 3.4 kpc, derived knot transverse velocities based on proper motion measurements spanning a 9 month interval indicate maximum transverse expansion velocities for these three knot classes of 14,500, 13,500, and 11,500 km s(-1), respectively. We present a catalog of these outlying ejecta clumps comprising finding charts, epoch 2004.2 knot positions, proper motions, photometric filter fluxes, and estimated knot emission type, along with cross-references to previous knot identifications and data. This compilation represents a nearly tenfold increase in the number of outlying, high-velocity ejecta knots identified around the Cassiopeia A remnant.

Ultracold Plasma Expansion in a Magnetic Field

We measure the expansion of an ultracold plasma across the field lines of a uniform magnetic field. We image the ion distribution by extracting the ions with a high-voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma (<20 micros), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of the magnetic field (up to 70 G). We observe that the expansion velocity scales as B(-1/2), explained by a nonlinear ambipolar diffusion model with anisotropic diffusion in two different directions.

The Expansion Asymmetry and Age of the Cassiopeia A Supernova Remnant

HST images of the young supernova remnant Cas A are used to explore the expansion and spatial distribution of its highest velocity debris. ACS WFC images taken in 2004 March and December with Sloan F625W, F775W, and F850LP filters were used to identify 1825 high-velocity, outlying ejecta knots through measured proper motions of 0.35 - 0.90 yr(-1), corresponding to V-trans = 5500-14,500 km s(-1) assuming d = 3.4 kpc. The distribution of derived transverse expansion velocities for these ejecta knots shows a striking bipolar asymmetry with the highest velocity knots (V-trans >= 10,500 km s(-1)) confined to nearly opposing northeast and southwest jets'' at P.A. = 45 degrees-70 degrees and 230 degrees-270 degrees, respectively. The jets have about the same maximum expansion velocity of similar or equal to 14,000 km s(-1) and appear kinematically and chemically distinct in that they are the remnant's only S-rich ejecta with expansion velocities above the 10,000-11,000 km s(-1) exhibited by outer nitrogen-rich ejecta, which otherwise represent the remnant's highest velocity debris. In addition, we find significant gaps in the spatial distribution of outlying ejecta in directions that are approximately perpendicular to the jets (P.A. = 145 degrees-200 degrees and 335 degrees-350 degrees). The remnant's central X-ray point source lies some 700 to the southeast of the estimated expansion center ( P.A. = 169 degrees +/- 8.degrees 4) indicating a projected motion toward the middle of the broad southern ejecta knot gap. Extrapolations of measured 9 month proper motions for all 1825 outer ejecta knots and a selected subsample of 72 bright and compact knots suggest explosion dates (assuming no knot deceleration) of 1662 +/- 27 and 1672 +/- 18, respectively. We find some evidence for nonuniform deceleration in different directions around the remnant and find 126 knots located along the northwestern limb among the least decelerated ejecta, suggesting a convergence date of 1681 +/- 19. A remnant age of around 325 yr would imply a +/- 350 km s(-1) transverse velocity for the central X-ray point source.

Multistrange hyperon production in Pb+Pb collisions at 30, 40, 80 and 158 A GeV

A non-monotonic energy dependence of the $K^{+} / \pi^{+}$ ratio with a sharp maximum close to 30 A$\cdot$GeV is observed in central Pb+Pb collisions. Within a statistical model of the early stage, this is interpreted as a sign of the phase transition to a QGP, which causes a sharp change in the energy dependence of the strangeness to entropy ratio. This observation naturally motivates us to study the production of multistrange hyperons ($\Xi$, $\Omega$) as a function of the beam energy. Furthermore it was suggested that the kinematic freeze-out of $\Omega$ takes place directly at QGP hadronization. If this is indeed the case, the transverse momentum spectra of the $\Omega$ directly reflect the transverse expansion velocity of a hadronizing QGP. In this report we show preliminary NA49 results on $\Omega^{-}$ and $\bar{\Omega}^{+}$ production in central Pb+Pb collisions at 40 and 158 A$\cdot$GeV and compare them to measurements of $\Xi^{-}$ and $\bar{\Xi}^{+}$ production in central Pb+Pb collisions at 30, 40, 80 and 158 A$\cdot$GeV.

Two-pion interferometry at small relative momentum for pion sources with transverse and longitudinal expansions in relativistic heavy ion collisions

A two-pion correlation function at small relative momentum for pion sources with transverse and longitudinal expansions in relativistic heavy ion collisions is obtained using two-pion interferometry at small relative momentum and the relations between the real and apparent parameters of the pion source are given. The relations can be used to extract both the temperature and the transverse and longitudinal expansion velocities of pion sources and to verify the correctness of relativistic transformation T' = T(1−v2)1/2 of temperature in relativistic statistical mechanics and thermodynamics.

SN 1979C VLBI: 22 Years of Almost Free Expansion

VLBI measurements of the size of SN 1979C in M100 (NGC 4321) in the Virgo Cluster, from t = 3.7 yr after the explosion, show an expansion ∝tm, which is, with m = 0.95 ± 0.03, almost consistent with being free for 22 years. The last size measurement, at t = 22 yr, may indicate for the first time a change of the expansion of the supernova and suggests, as an alternative, free expansion ∝tm with m = 1.00 ± 0.05 up to tb ~ 17 yr followed by marginally significant deceleration with m = 0.74 ± 0.17. The possible deceleration could be weaker within the errors for tb 17 yr. With the assumption for the density profile of the circumstellar medium (CSM) of ρCSM ∝ r-s, we derive a model-dependent value of s = 1.95 up to a distance from the progenitor, r = rb, that corresponds to tb ~ 17 yr, which changes to s 1.5 for r > rb. For a kinetic energy of the shocked ejecta and CSM shells of Ekin = 1051 ergs, our results require a mass loss to wind velocity ratio for the progenitor of w/w ~ 1 × 10-5 M☉ yr-1 per w = 10 km s-1, an order of magnitude smaller than estimated from radio light-curve fitting. The swept-up mass at t = 22 yr is then Msw = 0.3 M☉ and the inferred mass of the shocked ejecta Mshock-ej = 2 M☉. Our last observations give an image of a barely resolved source with a first hint on the structure of the supernova, consistent with being circular within 9% and possibly center filled. The expanding shock front method (ESM) of combining the transverse radio expansion velocities with the radial optical velocities gives direct distance estimates to M100, with standard errors of D = 16.5 ± 2.5 to 19.8 ± 3.0 Mpc, depending on whether the supernova has a bright center or is a shell without such a center. These estimates are comparable with those from Cepheid observations (e.g., 16.1 and 15.2 Mpc).

Centrality dependence ofthe expansion dynamics in Pb-Pb collisions at 158 A GeV c-1

Two-particle correlation functions of negatively charged hadrons fromPb-Pb collisions at 158 GeV c-1/nucleon have been measured by theWA97 experiment at the CERN SPS. A Coulomb correction procedure thatassumes an expanding source has been implemented. Within the frameworkof an expanding thermalized source model the size and dynamical state ofthe collision fireball at freeze-out have been reconstructed as a functionof the centrality of the collision. Less central collisions exhibit adifferent dynamics than central ones:both transverse and longitudinal expansion velocities are slower, theexpansion duration is shorter and the system freezes out, showing smallerdimensions and higher temperature.

Analyses of transverse momentum spectra in central C+C and C+Ta interactions at4.2A GeV/cbeam momentum within a collective flow model and a boundary model

The transverse momentum spectra of secondary ${\ensuremath{\pi}}^{\ensuremath{-}}$ mesons and participant protons in central C+C and C+Ta interactions at $4.2A$ GeV/c beam momentum have been analyzed within a collective flow model and a boundary model. All the particle spectra within a collective flow model at a given beam energy can be reproduced with a single set of intensive parameters for the initial state of fireball. As typical freeze-out parameters in this beam energy region we find a freeze-out temperature $T=(55--100) \mathrm{MeV},$ and an average transverse expansion velocity at freeze-out $〈\ensuremath{\beta}〉=0.30--0.39.$ The pion spectra within a boundary model including the quantum mechanical effects can be described well. To fit the proton spectra well, within a boundary model, one will need to introduce the transverse collective expansion.

Kinetic freeze-out and radial flow in 11.6 A GeV Au+Au collisions

We study the kinetic freeze-out conditions of hadrons in Au+Au collisions at 11.6 A GeV/ c using different parametrizations of an expanding thermal fireball. We take into account the available double differential momentum spectra of a variety of particle species, covering a large fraction of the total momentum space. The overall fit to the data is very good and indicates a relatively low kinetic freeze-out temperature of about 90 MeV with an average transverse expansion velocity at midrapidity of about 0.5 c .

Hadronic expansion dynamics in central Pb+Pb collisions at 158 GeV per nucleon

Two-particle correlation functions of negative hadrons over wide phase space, and transverse mass spectra of negative hadrons and deuterons near mid-rapidity have been measured in central Pb+Pb collisions at 158 GeV per nucleon by the NA49 experiment at the CERN SPS. A novel Coulomb correction procedure for the negative two-particle correlations is employed making use of the measured oppositely charged particle correlation. Within an expanding source scenario these results are used to ext ract the dynamic characteristics of the hadronic source, resolving the ambiguities between the temperature and transverse expansion velocity of the source, that are unavoidable when single and two particle spectra are analysed separately. The source shape, the total duration of the source expansion, the duration of particle emission, the freeze-out temperature and the longitudinal and transverse expansion velocities are deduced.

Hadronic expansion dynamics in central Pb+Pb collisions at 158 GeV per nucleon

Two-particle correlation functions of negative hadrons over wide phase space, and transverse mass spectra of negative hadrons and deuterons near mid-rapidity have been measured in central Pb+Pb collisions at 158 GeV per nucleon by the NA49 experiment at the CERN SPS. A novel Coulomb correction procedure for the negative two-particle correlations is employed making use of the measured oppositely charged particle correlation. Within an expanding source scenario these results are used to extract the dynamic characteristics of the hadronic source, resolving the ambiguities between the temperature and transverse expansion velocity of the source, that are unavoidable when single and two particle spectra are analysed separately. The source shape, the total duration of the source expansion, the duration of particle emission, the freeze-out temperature and the longitudinal and transverse expansion velocities are deduced.