Collision Velocity Research Articles

Collision velocity of dust grains in self-gravitating protoplanetary discs.

We have conducted the first comprehensive numerical investigation of the relative velocity distribution of dust particles in self-gravitating protoplanetary discs with a view to assessing the viability of planetesimal formation via direct collapse in such environments. The viability depends crucially on the large sizes that are preferentially collected in pressure maxima produced by transient spiral features (Stokes numbers, St ∼ 1); growth to these size scales requires that collision velocities remain low enough that grain growth is not reversed by fragmentation. We show that, for a single-sized dust population, velocity driving by the disc's gravitational perturbations is only effective for St > 3, while coupling to the gas velocity dominates otherwise. We develop a criterion for understanding this result in terms of the stopping distance being of the order of the disc scaleheight. Nevertheless, the relative velocities induced by differential radial drift in multi-sized dust populations are too high to allow the growth of silicate dust particles beyond St ∼ 10− 2 or 10−1 (10 cm to m sizes at 30 au), such Stokes numbers being insufficient to allow concentration of solids in spiral features. However, for icy solids (which may survive collisions up to several 10 m s−1), growth to St ∼ 1 (10 m size) may be possible beyond 30 au from the star. Such objects would be concentrated in spiral features and could potentially produce larger icy planetesimals/comets by gravitational collapse. These planetesimals would acquire moderate eccentricities and remain unmodified over the remaining lifetime of the disc.

Anchor Collision Analysis of Stone-filled Bags for Submarine Power Cable Protection using a Smoothed Particle Hydrodynamics Method

Woo, J.; Kim, D., and Na, W.B., 2016. Anchor collision analysis of stone-filled bags for submarine power cable protection using a smoothed particle hydrodynamics method. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 720–724. Coconut Creek (Florida), ISSN 0749-0208.We present a method for the safety analysis of stone-filled bags used to protect submarine power cables from anchor collision, using a smoothed particle hydrodynamics (SPH) method capable of dealing with discrete rocks. For this purpose, stone bags were modelled using the SPH method. Using the terminal velocities of five anchors, we simulated the transient dynamic behavior that occurs when the anchors collide with the stone bags. The response was sensitive to the collision velocity, while the displacement responses were also sensitive to the shape of the anchor heads. Stockless anchors provided more sensitive responses, especially stresses, to the larger stone bag size. Our results indicated that the safety performance of the 3.0 m × 3.0 m × 3.0 m stone bag provided sufficient protection for all of the collision cases. It should be noted that the collision velocities corresponded to the maximum (terminal) velocity. For shallower water depths, resulting in anchor velocities less than the terminal velocity, the 1.5 m × 1.5 m × 1.5 m stone bag would also provide suitable protection against anchor collision.

Microstructural characterisation of interfaces in magnetic pulse welded aluminum/aluminum joints

Electromagnetic pulse welding provides novel and useful solutions in the field of multi-material structures. The joining occurs through the high-energy mechanical impact of metal sheets, which leads to a strong, cohesive, metallic bonding between the sheets. The high- velocity collision results in changes of the microstructure in the regions adjacent to the interface. The aim of this work is the microstructural characterization of the interface and the welding zone of Al/Al joints. One typical interface is analysed by optical and electron microscopy and using the electron backscatter diffraction technique. Our results show that the interface exhibits a well-known type of wavy weld geometry. Moreover, the formation of an intermediate layer that is different from the bulk aluminium structure is observed. The microstructure of this intermediate layer consists of ultrafine grains and in some regions exhibits a columnar grain structure. It is likely that this special microstructure (which plays an important role in determining the cohesive, continuous bonding between the metal sheets) is formed through a rapid melting and crystallisation process in the interface region.

GLOBAL MODELING OF NEBULAE WITH PARTICLE GROWTH, DRIFT, AND EVAPORATION FRONTS. I. METHODOLOGY AND TYPICAL RESULTS

ABSTRACT We model particle growth in a turbulent, viscously evolving protoplanetary nebula, incorporating sticking, bouncing, fragmentation, and mass transfer at high speeds. We treat small particles using a moments method and large particles using a traditional histogram binning, including a probability distribution function of collisional velocities. The fragmentation strength of the particles depends on their composition (icy aggregates are stronger than silicate aggregates). The particle opacity, which controls the nebula thermal structure, evolves as particles grow and mass redistributes. While growing, particles drift radially due to nebula headwind drag. Particles of different compositions evaporate at “evaporation fronts” (EFs) where the midplane temperature exceeds their respective evaporation temperatures. We track the vapor and solid phases of each component, accounting for advection and radial and vertical diffusion. We present characteristic results in evolutions lasting 2 × 105 years. In general, (1) mass is transferred from the outer to the inner nebula in significant amounts, creating radial concentrations of solids at EFs; (2) particle sizes are limited by a combination of fragmentation, bouncing, and drift; (3) “lucky” large particles never represent a significant amount of mass; and (4) restricted radial zones just outside each EF become compositionally enriched in the associated volatiles. We point out implications for millimeter to submillimeter SEDs and the inference of nebula mass, radial banding, the role of opacity on new mechanisms for generating turbulence, the enrichment of meteorites in heavy oxygen isotopes, variable and nonsolar redox conditions, the primary accretion of silicate and icy planetesimals, and the makeup of Jupiter’s core.

Nonlinear selective reflection spectroscopy of V‐type atomic system at the gas‐solid interface

We theoretically present the nonlinear selective reflection spectroscopy of V‐type atomic system at gas‐solid interface in a pump‐probe scheme. The saturation and coherence effects are distinguished by solving Liouville equation in the absence and presence of reduced density matrix element between the two excited levels. When the coherence effect exists, two peaks appear in reflection spectroscopy with asymmetry lineshape. We investigate the dependence of reflection spectroscopy on pump field intensity, frequency detuning and coherent decay rate induced by collision between atoms. The lineshape can be explained based on reflection spectroscopy contributed from atoms with negative (before collision) and positive (after collision) velocities, single‐photon and two‐photon processes. This study is helpful for investigating quantum coherence and dynamic processes of atoms at gas‐solid interface.

COLLISIONS OF CO2 ICE GRAINS IN PLANET FORMATION

ABSTRACT In protoplanetary disks, CO2 is solid ice beyond its snow line at ∼ 10 AU ?> . Due to its high abundance, it contributes heavily to the collisional evolution in this region of the disk. For the first time, we carried out laboratory collision experiments with CO2 ice particles and a CO2-covered wall at a temperature of 80 K. Collision velocities varied between 0 and 2.5 m s−1. Particle sizes were on the order of ∼ 100 μm. We find a threshold velocity between the sticking and the bouncing regime at 0.04 m s−1. Particles with greater velocities, but below 1 m s−1, bounce off the wall. For yet greater velocities, fragmentation occurs. We give analytical models for the coefficients of restitution and fragmentation strength consistent with the experimental data. Set in context, our data show that CO2 ice and silicate dust resemble each other in the collisional behavior. Compared to water ice, the sticking velocity is an order of magnitude smaller. One immediate consequence as an example is that water-ice particles mantled by CO2 ice lose any “sticking advantage.” In this case, preferential planetesimal growth attributed to the sticking properties of water ice will be limited to the region between the H2O ice line and the CO2 ice line.

Three-dimensional crush scanning methods for reconstruction of vehicle collision accidents

This paper presents efficient 3-D collision crush modelling methods utilizing photogrammetry and 3-D scanner for reconstruction analysis of vehicle collision accident. Photogrammetry for 3-D collision crush modelling was executed in two methods: Indirect photogrammetry utilizing small amount of preexisting photographic data, and direct photogrammetry utilizing large amount of photographic data collected for the purpose of 3-D modelling. With the 3-D scanner, it was possible to create a more precise 3-D crush model than the photogrammetry method. Using these 3-D crush models constructed with the mentioned methods, several applications in accident reconstruction analysis are presented, based on construction methods and characteristics of the resulting crush model. Indirect photogrammetry produced a 3-D model capable of providing approximate shape of crush with only 2 ~ 3 photos of the damaged vehicle, and made it possible to measure the crush volume for estimating the collision velocity. With direct photogrammetry or 3-D scanner, moving and rotating elaborately built 3-D vehicle deformation models makes it possible to precisely reconstruct the actual collision configuration. Additionally, by elaborately modelling the vehicle and its fragments and then putting them together, it is possible to construct a post-collision model of the whole vehicle.

Experimental study of oblique impact of particles on wet surfaces

Granulation and agglomeration processes are characterized by intense particle–particle and particle–wall contacts. Furthermore, these collisions often happen in the presence of liquid layers due to liquid injection. Therefore the knowledge of micro-mechanics during wet collisions is fundamental for the exact description of such a process. In this work the collision behaviour of dry particles obliquely impacting a target plate covered by liquid layers is characterized by means of restitution coefficients. The coefficient of restitution describes the energy dissipation during an impact and is defined as the ratio of the velocities after and before impact. It is an important parameter for discrete element method (DEM) simulations and depends strongly on the collision parameters (such as collision velocity and angle), particle deformation behaviour (i.e. elastic or plastic) as well as on the properties of the injected liquid (viscosity, layer thickness). To investigate the influence of these parameters on the wet collision behaviour particle–wall impacts were recorded by two synchronized high-speed cameras allowing a three-dimensional analysis of the collision event. The focus of this work is to investigate the influence of liquid layer thickness, viscosity, surface tension and surface roughness on the normal and tangential coefficient of restitution.

Molecular dynamics simulation of impact of palladium clusters on the zirconium substrate

Impact of palladium clusters with different sizes on a zirconium surface at various incident velocities was investigated by molecular dynamics simulation at the room temperature. The impact process was characterized by impacting of the cluster on the surface, intermixing and relaxation of the cluster and substrate atoms. Furthermore, the deformation compression ratio, flattening ratio and embedding depth of the palladium clusters were examined. Additionally, one region of the substrate surface impacting by the cluster was defined to study the evolution of the local surface temperature and stress with time during the impact process. It is found that the penetrating atoms of palladium clusters almost diffuse from the central area to the edge with increasing the collision velocity due to the ultrahigh hardness of the zirconium substrate. Compared to the lower incident velocity, the cluster with a higher incident velocity could generate much stronger shear stress. When the impact velocity reached up to 1100 m/s, the local temperatures was not high enough to melt the zirconium substrate. The local shear stress and heating promoted the combination between the cluster and substrate, but the local melting of the substrate does not emerge.

Potential Risk Quantification Based on Collision Velocity Resulted from Activation of Autonomous Emergency Braking System

Potential Risk Quantification Based on Collision Velocity Resulted from Activation of Autonomous Emergency Braking System

電磁圧接板の接合性に及ぼす衝突速度および衝突時間の影響

電磁圧接板の接合性に及ぼす衝突速度および衝突時間の影響

Viscoelastic finite element analysis of the dynamic behavior of apple under impact loading with regard to its different layers

Annually, large quantities of apple are destroyed and suffer from quality loss because of bruising and other mechanical damages during harvesting, transportation, storage and packaging. Bruising due to impact is one of the most important reasons for the quality loss in the fresh fruit market. Regarding to the accuracy and economic justification, Finite Element Method (FEM) is one of the best ways of investigating the effective factors in bruising produced as a result of impact. In this research, FEM is used to study the dynamic impact to apple. For soft and relatively large objects like apple, the dynamic waves are absorbed by the body of the object through decreasing the kinetic energy during the impact. The amount of energy absorption is dependent upon the viscoelastic properties, geometric size of the object during the collision, and collision velocity. Therefore, in such materials, contact area and the induced stress in contact region are time dependent. In finite element modeling, a nonlinear time-dependent contact is considered. Through the use of finite element models of a typical fruit, the apple, some collision processes of the fruit were studied. The 3D FEM models were created to illustrate, as closely as possible, the typical geometrical structure of an apple fruit, containing the basic inhomogeneous structure of the fruit. In the current study, using Finite element method, the behavior of the various layers of skin, cortex and core of apple was analyzed applying impact test to collision of the fruit with a flat rigid plate as well as the collision of an apple with another apple. The different layers of apple such as skin, cortex and core have not been yet analyzed dynamically so it plays a remarkably important role. The results showed that the parameters of displacement, velocity, acceleration, stress and hydrostatic pressure in the collision of apple to apple were less than that of apple to rigid object at the velocity of 1m/s. Also, changes of these parameters were maximum in a skin and minimum in a core.

A tunnel and a traffic jam: How transition disks maintain a detectable warm dust component despite the presence of a large planet-carved gap

We combined hydrodynamical simulations of planet-disk interactions with dust evolution models that include coagulation and fragmentation of dust grains over a large range of radii and derived observational properties using radiative transfer calculations. We studied the role of the snow line in the survival of the inner disk of transition disks. Inside the snow line, the lack of ice mantles in dust particles decreases the sticking efficiency between grains. As a consequence, particles fragment at lower collision velocities than in regions beyond the snow line. This effect allows small particles to be maintained for up to a few Myrs within the first astronomical unit. These particles are closely coupled to the gas and do not drift significantly with respect to the gas. For lower mass planets (1$M_{\rm{Jup}}$), the pre-transition appearance can be maintained even longer because dust still trickles through the gap created by the planet, moves invisibly and quickly in the form of relatively large grains through the gap, and becomes visible again as it fragments and gets slowed down inside of the snow line. The global study of dust evolution of a disk with an embedded planet, including the changes of the dust aerodynamics near the snow line, can explain the concentration of millimetre-sized particles in the outer disk and the survival of the dust in the inner disk if a large dust trap is present in the outer disk. This behaviour solves the conundrum of the combination of both near-infrared excess and ring-like millimetre emission observed in several transition disks.

Consequences of large impacts on Enceladus’ core shape

The intense activity on Enceladus suggests a differentiated interior consisting of a rocky core, an internal ocean and an icy mantle. However, topography and gravity data suggests large heterogeneity in the interior, possibly including significant core topography. In the present study, we investigated the consequences of collisions with large impactors on the core shape. We performed impact simulations using the code iSALE2D considering large differentiated impactors with radius ranging between 25 and 100km and impact velocities ranging between 0.24 and 2.4km/s. Our simulations showed that the main controlling parameters for the post-impact shape of Enceladus’ rock core are the impactor radius and velocity and to a lesser extent the presence of an internal water ocean and the porosity and strength of the rock core. For low energy impacts, the impactors do not pass completely through the icy mantle. Subsequent sinking and spreading of the impactor rock core lead to a positive core topographic anomaly. For moderately energetic impacts, the impactors completely penetrate through the icy mantle, inducing a negative core topography surrounded by a positive anomaly of smaller amplitude. The depth and lateral extent of the excavated area is mostly determined by the impactor radius and velocity. For highly energetic impacts, the rocky core is strongly deformed, and the full body is likely to be disrupted. Explaining the long-wavelength irregular shape of Enceladus’ core by impacts would imply multiple low velocity (<2.4km/s) collisions with deca-kilometric differentiated impactors, which is possible only after the LHB period.

Rounded boulders on Itokawa as clues to geological processes in the early solar system

Large rounded boulders on Itokawa are a surprising find and may be evidence of forceful inter-boulder collisions occurring over protracted periods of time. Surface textures of some boulders are reminiscent of those on terrestrial aeolian sand grains despite five orders of magnitude difference in scale. Using Hertzian analysis and fracture strength data, we calculate that the maximum collisional velocities involved in the comminution process are ~6–7m/s. We hypothesise that boulder rounding could be a product of collisions in a gravitationally stable orbiting debris field in which boulders acquire collisional energy from YORP spin. Collisional paths may be instigated by Yarkovsky drift and gyroscopic effects of rotation. Collisional energy is dissipated by elastic damping, but rapidly renewed by YORP spinup that takes only hundreds to thousands of years to regenerate comminution-strength collisions. The rounded boulders on Itokawa are found amongst angular, unworn material which suggests a mixed origin for Itokawa׳s regolith.

A probability model of predicting the sand erosion profile in elbows for gas flow

The damage caused by sand erosion is an inevitable problem encountered in the oil and gas transportation system. The elbow is one of most vulnerable fittings of pipelines. For gas flow in elbows, it has been proved that incident solid particles will rebound randomly when they initially impact the wall, and then impact the wall once again with a certain probability, resulting in second collision erosion. In the present work, a probability method is developed to analyze the impact-rebound-impact behavior of sands in elbows for gas flow. First, the Lagrange method is adopted to track motion trajectories of particles, and the motion rules of particles are deduced. First and second collision velocities can then be calculated. Second, by analyzing the probability behavior of a single particle, holistic impact behavior of a large quantity of particles can be characterized. Last, first and second collision-induced erosion rates at specific position are calculated by using erosive correlations, and the three-dimensional erosion profile can then be exhibited over the entire elbow surface. A large number of experimental data are adopted to examine the probability model based on the knowledge of second collision. The erosion profiles along the centerline and on the entire surface of the elbow, which can hardly be measured in experiments, are also calculated and analyzed through the probability model. The present work not only demonstrates the accuracy of the probability model, but also confirms that second collision of solid particles is an important behavior in elbows for gas flow, and plays a significant role in forming the erosion profile.

GMC COLLISIONS AS TRIGGERS OF STAR FORMATION. I. PARAMETER SPACE EXPLORATION WITH 2D SIMULATIONS

We utilize magnetohydrodynamic (MHD) simulations to develop a numerical model for GMC-GMC collisions between nearly magnetically critical clouds. The goal is to determine if, and under what circumstances, cloud collisions can cause pre-existing magnetically subcritical clumps to become supercritical and undergo gravitational collapse. We first develop and implement new photodissociation region (PDR) based heating and cooling functions that span the atomic to molecular transition, creating a multiphase ISM and allowing modeling of non-equilibrium temperature structures. Then in 2D and with ideal MHD, we explore a wide parameter space of magnetic field strength, magnetic field geometry, collision velocity, and impact parameter, and compare isolated versus colliding clouds. We find factors of ~2-3 increase in mean clump density from typical collisions, with strong dependence on collision velocity and magnetic field strength, but ultimately limited by flux-freezing in 2D geometries. For geometries enabling flow along magnetic field lines, greater degrees of collapse are seen. We discuss observational diagnostics of cloud collisions, focussing on 13CO(J=2-1), 13CO(J=3-2), and 12CO(J=8-7) integrated intensity maps and spectra, which we synthesize from our simulation outputs. We find the ratio of J=8-7 to lower-J emission is a powerful diagnostic probe of GMC collisions.

Numerical simulation of particle–particle adhesion by dynamic liquid bridge

In powder handling processes treating wet powders (e.g. wet granulation, coating, and drying), a liquid bridge formed between particles is not static but dynamic due to continuous motion of the particles. Therefore, understanding of a particle–particle adhesion by such a dynamic liquid bridge is an important issue. We here conducted a numerical simulation of the particle–particle adhesion by the dynamic pendular liquid bridge. A computational fluid dynamics (CFD) approach with a constrained interpolation profile (CIP) method was applied to simulate a gas–liquid two phase flow, while a particle motion was calculated by taking into account the dynamic liquid bridge. To verify the simulation results, the following verifications were conducted; (i) wetting behavior of a droplet on a curved surface, (ii) static liquid bridge force between two particles, and (iii) rupture behavior of a liquid bridge with free falling of a particle. As a result, validity of the simulation results was confirmed. The particle–particle adhesion through a droplet on a particle surface was then simulated. In particular, effect of a collision velocity on a wet restitution coefficient was investigated. A velocity criterion for the particle adhesion was determined from the simulation results and compared with that estimated from an analytical model proposed by Ennis et al (1991).

Safety analysis of rock berms that protect submarine power cables in the event of an anchor collision

A procedure for obtaining the dynamic characteristics of rock berms in the event of an anchor collision is developed, a safety analysis is performed, and associated recommendations are made. The drag coefficients of five target anchors were obtained through finite element flow analysis, and their terminal velocities calculated to maximise colliding kinetic energy. The dynamic behaviours of the rock berms were modelled by smoothed particle hydrodynamics (SPH). Six parameters were considered in the collision analysis: anchor type, collision velocity, SPH particle size, ground state, rock berm size, and collision point. The results were verified by an existing field test and a safety analysis was carried out with respect to parameter variation; the associated recommendations are discussed. It was found that rock berm height is the most critical factor in ensuring the safety of power cables. Rock size and installation depth are important to satisfy the safety criteria established in this study.

A study on the critical wall thickness of the inner tube for magnetic pulse welding of tubular Al–Fe parts

Microstructure and mechanical properties of the welding zone highly depend on the thickness of inner tube for jointing two tubes using magnetic pulse welding (MPW). In this study, the critical thickness of inner 20Fe tube for the Al–Fe welding tubes is investigated by combining the theoretical calculation and experiments. Theoretical analysis demonstrates that plastic deformation zone of the inner tube expands from its inner surface to its outer surface. With equilibrium equations and modified Tresca yield criterion, the critical thicknesses of inner tube under various collision velocities are derived so as to assure the high quality of the welding. The results show that the critical thickness of inner tube increases linearly with the discharge voltage. With the discharge voltage increases by every 2kV, the critical thickness increases by 0.5mm accordingly. The theoretical predictions agree well experiments with an error of less than 2.5%. Furthermore, effects of thickness of inner tube on the mechanical properties of welding zone are also investigated.