Violent Collisions Research Articles

Laser ablation mechanism of contamination on surface of single crystal iron

The laser induced damage in high-power laser system has received much attention in the area of laser engineering. Optical components with contaminants, which are installed in the final optical assembly (FOA), can be severely damaged under the action of extremely high laser energy. So the ultra-high cleanliness inside the high-energy laser system is required for both optical and mechanical components. Research shows that a large part of the metal particulate contaminants inside the device come from the mechanical components. The metal particulate contaminants are produced when mechanical structure surface is damaged under the irradiation of stray light. However the research about the cleanliness inside the device is mostly concentrated on the surfaces of optical components currently. The laser ablation of the mechanical components absorbing contaminants is studied little, so it is quite important to investigate the ablation mechanism of mechanical components under laser irradiation. Due to the presence of contaminants on the surfaces of mechanical components, laser ablation of monocrystalline iron absorbing contaminants is investigated by using molecular dynamics simulation. The ablation process of iron material under laser irradiation is presented. The influences of loading mode and energy density of laser as well as contamination on the surface are analyzed in the ablation process of monocrystalline iron. The results indicate that the surface atoms of monocrystalline iron show different motion states under the violent collision of contaminants atoms after laser loading. Ablated iron can be divided into ablation zone, melting zone and crystal zone according to the variation of the temperature and mass density of the atoms in each region of the ablated material. The atoms in each region show macroscopic characteristics of gaseous, liquid and solid atoms respectively. Iron is damaged more easily when laser energy is instantaneously loaded. Contaminants on the surface of iron can be removed, and iron cannot be damaged when laser energy density is below 0.0064 J/cm<sup>2</sup>. The result of the analysis shows that the presence of contaminants makes the ablation of iron easier. Different energy loading modes affect the heat transfer mode directly. Monocrystalline iron materials are more likely to be damaged in the mode of adiabatic laser ablation in the case of short laser pulse. Thermal effect can be thought as a dominant factor for the ablation in the case of long laser pulse. The research results of this paper are helpful for providing the theoretical basis for improving the cleanliness of high-power laser system.

Horrible beauty and (un)easy submission: melodrama and the gothic in Calvary

This article examines Calvary (2014) as a gothic and melodramatic text – as an expression, more specifically, of pathetic melodrama and the Anglo-Irish gothic. As a pathetic melodrama, Calvary presents us with an apparently impassable situation, at the level of both the diegetic narrative and the historical present. It also exhibits considerable suffering and pathos. These melodramatic features are articulated via affect in familiar ways, so that the film reproduces the moral occult and a regressive nationalism. The article argues, though, that Calvary’s excesses are best understood as specific expressions of pathological melodrama, the bog gothic and the Cartesian gothic. In this respect, it is argued that Calvary’s ostensive – dense and allusive – dialogue is a form of speaking suffering – a dark, but potentially productive game. All of Calvary’s affective excesses, it is argued, are critical entities – ghostly witnesses, explosions and violent collisions of mind and body, which nonetheless cohere around particular histories, places and events. However much, then, Calvary may seem to accede to melodramatic redemption or gothic cliché, it is better understood, it is argued, as a form of submission – a necessary giving in and facing up to historical trauma and shame.

Monte Carlo simulation of temporal behavior of surface charging across insulator at flashover initial stage in vacuum

The temporal behaviors of charging on insulator surface at flashover initial stage is studied under dc voltage in vacuum. It suggests that the surface charging process develops synchronously with multipactor phenomena. A saturated region is firstly formed nearby the cathode triple junction (CTJ), and the insulator surface is charged gradually from cathode toward anode as the multipactor develops forward. The surface charge density increases linearly with applied electric field, but a stronger applied electric field will slow down the charging velocity as the results of violent collisions of secondaries with dielectric surface due to the stronger normal field built by surface charges. The charging velocity mainly depends on the secondary electron yield (SEY) for dielectric, i.e., the higher SEY, the greater the charging velocity is. The emitter height from CTJ has great influence on surface charging at areas adjacent to cathode, but little effect on charging behaviors at other areas. Once the multipactor is triggered by adequate initial electrons released from cathode, the surface will be charged spontaneously even if the field emission process at cathode is terminated, implying the generation of secondary electrons turns into a self-sustaining process. This study strongly supports the speculation in our previous publications regarding the mechanisms of stable luminescence preceding flashover, which is attributed to be initialized by Fowler-Nordheim emission of electrons at CTJ and finally generated by the radiative de-excitation of excited atoms/molecules in the self-stabilizing secondary electron emission (SSEE) process.

Orbit Response Recognition During Touchdowns by Instantaneous Frequency in Active Magnetic Bearings

During touchdowns of active magnetic bearings (AMB), the violent collision between rotors and touchdown bearings (TDB) can cause damages to both parts. Orbit response recognition provides a way for the AMB controller to automatically switch the control algorithm to actively suppress the rotor–TDB vibration and promptly relevitate the rotor during touchdowns. A novel method based on Hilbert transform (HT) is proposed to recognize the orbit responses (pendulum vibration, combined rub and bouncing, and full rub) in touchdowns. In this method, the rotor suspension status is monitored by the AMB controller in real-time. When touchdown is detected, the rotor displacement signal during the sampling period is intercepted, and the instantaneous frequency (IF) is calculated by HT. Then, the local variance of IF during the sampling period is calculated, and it is compared with the threshold value. Combined rub and bouncing can be identified for it has the largest local variance. Finally, the mean value of IF during the sampling period is calculated and is compared with the other threshold value. Pendulum vibration can be identified for it has a lower and fixed mean value, while full rub has a larger value. The principle of the recognition method is demonstrated by the simulated results of a thermo-dynamic model. The results reveal that the method is feasible in recognizing the orbit responses and can be implemented in the AMB controller to help switch the control algorithms automatically in case of touchdowns.

FINITE ELEMENT MODELING OF CRASHING BEHAVIOR CONCERNING RECTANGULAR MULTI-LAYERED THIN-WALLED STRUCTURES

<p>Tubular thin-walled structures with different shapes of the cross-sections are widely used in various transportation systems as energy absorbing components to dissipate the kinetic energy during violent collisions and crashes. The aims of this paper are to obtain numerical data on the crashing of thin-walled structures made by multiple layers of steel sheets. A series of finite element calculations was carried out on four different models crashed axially in dynamic conditions by using LS_Dyna V971. The effect of the generated fold depth on the peak load and the mean crashing load of these types of structures were also examined. </p>

Stability analysis of virtual passive actuator with time delay and parameter uncertainties

The purpose of this paper is to present a virtual passive actuator which enables the manipulator joint to behave as a desired spring-damper system. When the manipulator interacts with an unknown environment or human operator, huge contact force will be generated with a high-stiffness trajectory tracking controller: to avoid violent collisions, the virtual passive actuator is proposed to diminish the interaction force, and adjust the system stiffness to adapt to the environment. With the mathematical model of the virtual passive actuator, the system’s stable region is determined based on the Routh–Hurwitz criterion. The influence of operation time delay, filter and model uncertainty is taken into consideration. Finally, the virtual passive actuator is incorporated into a manipulator, and validity is verified through a set of experiments. Experiment results show that, with the proposed controller, virtual passive actuator joint deformation is proportional to the external torque, and the proportional ratio is close to the stiffness setting: it can perform dynamic behavior just like a spring-damper system when subject to step external torque.

Origin of the eclogitic clasts with graphite-bearing and graphite-free lithologies in the Northwest Africa 801 (CR2) chondrite: Possible origin from a Moon-sized planetary body inferred from chemistry, oxygen isotopes and REE abundances

In order to clarify the origin of the eclogitic clasts found in the NWA801 (CR2) chondrite (Kimura et al., 2013), especially, that of the high pressure and temperature (P–T) condition (∼3GPa and ∼1000°C), we conducted ion microprobe analyses of oxygen isotopes and rare earth element (REE) abundances in the clasts. Oxygen isotopic compositions of the graphite-bearing lithology (GBL) and graphite-free lithology (GFL) show a slope ∼0.6 correlation slightly below the CR-CH-CB chondrites field in the O three-isotope-diagram, with a large variation for the former and almost homogeneous composition for the latter. The average REE abundances of the two lithologies show almost unfractionated patterns. Based on these newly obtained data, as well as mineralogical observations, bulk chemistry, and considerations about diffusion timescales for various elements, we discuss in detail the formation history of the clasts. Consistency of the geothermobarometers used by Kimura et al. (2013), suggesting equilibration of various elements among different mineral pairs, provides a strong constraint for the duration of the high P–T condition. We suggest that the high P–T condition lasted 102–103years. This clearly precludes a shock high pressure (HP) model, and hence, strongly supports a static HP model. A static HP model requires a Moon-sized planetary body of ∼1500km in radius. Furthermore, it implies two successive violent collisions, first at the formation of the large planetary body, when the clasts were placed its deep interior, and second, at the disruption of the large planetary body, when the clasts were expelled out of the parent body and later on transported to the accretion region of the CR chondrites. We also discuss possible origin of O isotopic variations in GBL, and presence/absence of graphite in GBL/GFL, respectively, in relation to smelting possibly occurred during the igneous process(es) which formed the two lithologies. Finally we present a possible formation scenario of the eclogitic clasts.

Shock Response and Phase Transitions of MgO at Planetary Impact Conditions.

The moon-forming impact and the subsequent evolution of the proto-Earth is strongly dependent on the properties of materials at the extreme conditions generated by this violent collision. We examine the high pressure behavior of MgO, one of the dominant constituents in Earth's mantle, using high-precision, plate impact shock compression experiments performed on Sandia National Laboratories' Z Machine and extensive quantum calculations using density functional theory (DFT) and quantum MonteCarlo (QMC) methods. The combined data span from ambient conditions to 1.2TPa and 42 000K, showing solid-solid and solid-liquid phase boundaries. Furthermore our results indicate that under impact the solid and liquid phases coexist for more than 100GPa, pushing complete melting to pressures in excess of 600GPa. The high pressure required for complete shock melting has implications for a broad range of planetary collision events.

Two-comet dance gave 67P its duck shape

The rubber-duck comet formed not with a bang, but a whimper. No one actually knows how comet 67P got its distinctive shape. But everyone's favorite comet chaser, the European Space Agency's Rosetta spacecraft, thinks the comets birth wasn't nearly as traumatic as it might have been. Hans Rickmart of Uppsala University in Sweden and colleagues saw two possibilities. One is that two chunks of ice and dirt, the building blocks of comets, merged at slow speeds early in the solar system's history. Alternatively, the two lobes were part of a single larger body that shattered in a violent collision, only for the two chunks to find each other again and stick together

COMETARY NUCLEI. The shape and structure of cometary nuclei as a result of low-velocity accretion.

Cometary nuclei imaged from flyby and rendezvous spacecraft show common evidence of layered structures and bilobed shapes. But how and when these features formed is much debated, with distinct implications for solar system formation, dynamics, and geology. We show that these features could be a direct result of accretionary collisions, based on three-dimensional impact simulations using realistic constitutive properties. We identify two regimes of interest: layer-forming splats and mergers resulting in bilobed shapes. For bodies with low tensile strength, our results can explain key morphologies of cometary nuclei, as well as their low bulk densities. This advances the hypothesis that nuclei formed by collisional coagulation-either out of cometesimals accreting in the early solar system or, alternatively, out of comparable-sized debris clumps paired in the aftermath of major collisions.

MC2: CONSTRAINING THE DARK MATTER DISTRIBUTION OF THE VIOLENT MERGING GALAXY CLUSTER CIZA J2242.8+5301 BY PIERCING THROUGH THE MILKY WAY

CIZA J2242.8+5301 is a merging system with a prominent (~2 Mpc long) radio relic, which together with the morphology of the X-ray emission provides strong evidence for a violent collision along the N-S axis. We present our constraints on the dark matter distribution of this unusual system using Subaru and CFHT imaging data. Measuring a high S/N lensing signal from this cluster is potentially a challenging task because of its proximity to the Milky Way plane (|b|~5 deg). We overcome this challenge with careful observation planning and systematics control, which enables us to successfully map the dark matter distribution of the cluster with high fidelity. The resulting mass map shows that the mass distribution is highly elongated along the N-S merger axis inferred from the orientation of the radio relics. Based on our mass reconstruction, we identify two sub-clusters, which coincide with the cluster galaxy distributions. We determine their masses using MCMC analysis by simultaneously fitting two NFW halos without fixing their centroids. The resulting masses of the northern and southern systems are $M_{200}=11.0_{-3.2}^{+3.7}\times10^{14} M_{\sun}$ and $9.8_{-2.5}^{+3.8}\times10^{14} M_{\sun}$, respectively, indicating that we are witnessing a post-collision of two giant systems of nearly equal mass. When the mass and galaxy centroids are compared in detail, we detect ~ 1' (~190 kpc) offsets in both northern and southern sub-clusters. We find that the galaxy luminosity-mass offset for the northern clump is statistically significant at the ~2 sigma level whereas the detection is only marginal for the southern sub-cluster in part because of a relatively large mass centroid error. We conclude that it is yet premature to uniquely attribute the galaxy-mass misalignment to SIDM and discuss caveats.

Correlation and flow phenomena in pPb and PbPb at CMS

Recent measurements of correlation and flow phenomena in pPb and PbPb collisions from the CMS experiment are presented. Higher-order flow Fourier harmonics (vn) are systematically studied in PbPb collisions using various methods. Flow phenomena in the most violent collisions at the LHC are also studied including the ultra-central PbPb collisions and very high-multiplicity pPb collisions.

Unified description of transverse momentum spectrums contributed by soft and hard processes in high-energy nuclear collisions

The transverse momentum spectrums of π −, π +, K −, K +, $$\bar p$$ , and p produced in p-Pb collisions at √s NN = 502 TeV measured by the CMS Collaboration and in Pb-Pb collisions at √s NN = 2.76 TeV measured by the ALICE Collaboration are described by a two-component Erlang distribution. The first component corresponds to “soft” excitation process and contributes in the low transverse momentum region, which is contributed by 2–5 partons (sea quarks and gluons) with strong interactions. The second component corresponds to “hard” scattering process and contributes in the high transverse momentum region, which is contributed by 2 partons (valent quarks) with violent head-on collision. Each parton source contributes an exponential transverse momentum spectrum. Both the soft and hard processes result in an Erlang distribution. The transverse momentum spectrums of final-state charged particles are then described by the two-component Erlang distribution. The contribution ratio (30–40%) of the hard process extracted from nuclear collisions at the Large Hadron Collider is consistent with that (17–46%) obtained from other methods.

High-resolution simulations of the head-on collision of white dwarfs

The direct impact of white dwarfs has been suggested as a plausible channel for type Ia supernovae. In spite of their (a priori) rareness, in highly populated globular clusters and in galactic centers, where the amount of white dwarfs is considerable, the rate of violent collisions between two of them might be non-negligible. Even more, there are indications that binary white dwarf systems orbited by a third stellar-mass body have an important chance to induce a clean head-on collision. Therefore, this scenario represents a source of contamination for the supernova light-curves sample that it is used as standard candles in cosmology, and it deserves further investigation. Some groups have conducted numerical simulations of this scenario, but their results show several differences. In this paper we address some of the possible sources of these differences, presenting the results of high resolution hydrodynamical simulations jointly with a detailed nuclear post-processing of the nuclear abundances, to check the viability of white dwarf collisions to produce significant amounts of 56Ni. To that purpose, we use a 2D-axial symmetric smoothed particle hydrodynamic code to obtain a resolution considerably higher than in previous studies. In this work, we also study how the initial mass and nuclear composition affect the results. The gravitational wave emission is also calculated, as this is a unique signature of this kind of events. All calculated models produce a significant amount of 56Ni, ranging from 0.1 Msun to 1.1 Msun, compatible not only with normal-Branch type Ia supernova but also with the subluminous and super-Chandrasekhar subset. Nevertheless, the distribution mass-function of white dwarfs favors collisions among 0.6-0.7 Msun objects, leading to subluminous events.

Concussions, Class Actions, and the 2011 Collective Bargaining Agreement: A Look at Concussions and the Response of the National Football League in the 2011 NFL Collective Bargaining Agreement

Violent collisions are an integral part to the success of the NFL, but medical research suggests those same hits have serious health consequences for players long after they leave the league. The dilemma for the NFL becomes ensuring fan support and financial success while protecting the safety of athletes, ultimately limiting liability. In the midst of recent class actions filed by former players against the NFL, which experts say may push the boundaries in both law and science, players and owners agreed to a new collective bargaining agreement. That agreement appears to be a substantial step forward for the NFL in addressing growing concerns over player injuries, concussions in particular, for the future. Although the new guidelines in the collective bargaining agreement may be the first step in protecting players, it is also, not coincidentally, the first step in reducing the liability of the league, and team doctors. While the new collective bargaining agreement may shield the NFL from future liability, the question remains - Will the NFL be held liable for its violent past?

Mediterranean geographies of protest

This special issue of Euro-commentaries tackles the question of what links unprecedented anti-regime uprisings in the MENA (Middle East and North Africa) region, with the largest protests in decades in several European cities. Beyond the specificities of individual cases, uprisings on both sides of the Mediterranean have highlighted strong and often violent collisions between resistance movements and state security. How are these collisions reshaping urban and political geographies in the Mediterranean? The papers presented here explore different aspects of the 2011 protests, and share the view that these are shaped by concerns for social justice, human rights and democracy, which are not a prerogative of the Arab world, but indicate instead more complex geographies.

Compositional evidence for an impact origin of the Moon’s Procellarum basin

The nearside and farside of the Moon are compositionally distinct. The detection of low-calcium pyroxene around large impact basins suggests that the huge Procellarum basin on the nearside may be an ancient impact structure and a relic scar of the violent collision that produced the lunar dichotomy.

GALAXY DISKS DO NOT NEED TO SURVIVE IN THE ΛCDM PARADIGM: THE GALAXY MERGER RATE OUT TOz∼ 1.5 FROM MORPHO-KINEMATIC DATA

About two-thirds of present-day, large galaxies are spirals such as the Milky Way or Andromeda, but the way their thin rotating disks formed remains uncertain. Observations have revealed that half of their progenitors, six billion years ago, had peculiar morphologies and/or kinematics, which exclude them from the Hubble sequence. Major mergers, i.e., fusions between galaxies of similar mass, are found to be the likeliest driver for such strong peculiarities. However, thin disks are fragile and easily destroyed by such violent collisions, which creates a critical tension between the observed fraction of thin disks and their survival within the L-CDM paradigm. Here we show that the observed high occurrence of mergers amongst their progenitors is only apparent and is resolved when using morpho-kinematic observations which are sensitive to all the phases of the merging process. This provides an original way of narrowing down observational estimates of the galaxy merger rate and leads to a perfect match with predictions by state-of-the-art L-CDM semi-empirical models with no particular fine-tuning needed. These results imply that half of local thin disks do not survive but are actually rebuilt after a gas-rich major merger occurring in the past nine billion years, i.e., two-thirds of the lifetime of the Universe. This emphasizes the need to study how thin disks can form in halos with a more active merger history than previously considered, and to investigate what is the origin of the gas reservoir from which local disks would reform.

VARIABILITY OF THE INFRARED EXCESS OF EXTREME DEBRIS DISKS

Debris disks with extremely large infrared excesses (fractional luminosities $> 10^{-2}$) are rare. Those with ages between 30 and 130 Myr are of interest because their evolution has progressed well beyond that of protoplanetary disks (which dissipate with a timescale of order 3 Myr), yet they represent a period when dynamical models suggest that terrestrial planet building may still be progressing through large, violent collisions that could yield large amounts of debris and large infrared excesses. For example, our Moon was formed through a violent collision of two large proto-planets during this age range. We report two disks around the solar-like stars ID8 and HD 23514 in this age range where the 24 {\mu}m infrared excesses vary on timescales of a few years, even though the stars are not variable in the optical. Variations this rapid are difficult to understand if the debris is produced by collisional cascades, as it is for most debris disks. It is possible that the debris in these two systems arises in part from condensates from silicate-rich vapor produced in a series of violent collisions among relatively large bodies. If their evolution is rapid, the rate of detection of extreme excesses would indicate that major collisions may be relatively common in this age range.

The question of Salah Sheekh: Derrida's hospitality and migration law

Hospitality is a self-contradictory concept. Both inclusionary and exclusionary forces are present at the very threshold of hospitality. This article shows how this contradiction works in the practice of migration law. The author takes a Dutch case, the case of Abdirizaq Salah Sheekh, to reveal the force of this contradiction from the moment the asylum seeker arrives on the territory, all the way through to the end of his story at the European Court of Human Rights. The case is, in fact, a symptomatic indictment of how aliens are treated in the Netherlands and, in a more general sense, in Europe as a whole. This article not only demonstrates how the question of hospitality commences with language − the moment we start communicating with the alien − the author also argues that in the violent collision produced by the the two contradictory demands of hospitality, a new subject is created. Not only is the migration procedure questioned, but also the subject of migration law.