Collision Velocity Research Articles

Clusters of tribocharged dust aggregates as pebbles in protoplanetary disks

In recent years, the tribocharging of colliding and bouncing submillimeter (submm) particles has been studied as a possible mechanism promoting the formation of large pebbles on centimeter (cm) to decimeter (dm) scales in protoplanetary disks. Here, we observe, for the first time, that it is not only monolithic, spherical particles, but also real dust aggregates, that become tribocharged and end up forming large clusters. For aggregates of ~0.4 mm consisting of ~1 micrometer (µm) sized dust, we determined net charge densities up to 10−7 C/m2 during our drop tower experiments. These charged aggregates form compact clusters up to 2 cm in size via collisions with other clusters and aggregates at collision velocities on the order of 1 cm/s. Size and speed are the only lower limits for growth, currently set by the limits of the experiment. However, these clusters already form under conditions that are well beyond the expected transition to bouncing for uncharged aggregates and clusters. Our findings further support the idea that collisional charging can leapfrog the traditional bouncing barrier and form larger clusters that then serve as large pebbles. These cm-sized clusters are more susceptible to further evolutionary steps via particle trapping, concentration, and planetesimal formation.

Deformation and fragmentation of a water drop in collision with a thin cylinder

We experimentally study the collision of a falling water drop with a thin horizontal cylinder, focusing on phenomena that play a crucial role in the hydrodynamics of the collision but have not been studied previously. We address two different collision scenarios. In the first case, the drop is split into two separate parts connected by a liquid sheet oriented transversely to the cylinder. The surface tension of the sheet causes the parts to approach and merge, or the sheet breaks up and each part continues to move independently. The second scenario is realized at low collision velocities. In this case, the sheet is not formed, and the separation or merging of the drop parts is controlled by their surface tension driven deformations. We analyze the factors (drop velocity, cylinder diameter, and cylinder wettability) that govern the realizability of these scenarios. We also propose a simple criterion that predicts the transition from one scenario to the other.

HCG 57: Evidence for Shock-heated Intergalactic Gas from X-Rays and Optical Emission Line Spectroscopy

Abstract We present Chandra and XMM-Newton X-ray observations of the compact group HCG 57, and optical integral field spectroscopy of the interacting galaxy pair HCG 57A/D. These two spiral galaxies recently suffered an off-axis collision with HCG 57D passing through the disk of A. We find evidence of a gas bridge linking the galaxies, containing ∼108 M ⊙ of hot, ∼1 keV thermal plasma and warm ionized gas radiating in Hα, Hβ, [O iii] and [N ii] lines. The optical emission lines in the central regions of HCG 57D show excitation properties consistent with H ii-regions, while the outer rim of HCG 57D parts of the bridge and the outer regions of HCG 57A show evidence of shocked gas consistent with shock velocities of 200–300 km s−1. In contrast, the X-ray emitting gas requires a collision velocity of 650–750 km s−1 to explain the observed temperatures. These different shock velocities can be reconciled by considering the contributions of rotation to collision velocity in different parts of the disks, and the clumpy nature of the preshock medium in the galaxies, which likely lead to different shock velocities in different components of the turbulent postshocked gas. We examine the diffuse X-ray emission in the group members and their associated point sources, identifying X-ray active galactic nuclei in HCG 57A, B, and D. We also confirm the previously reported ∼1 keV intra-group medium and find it to be relaxed with a low central entropy (18.0 ± 1.7 keV cm2 within 20 kpc) but a long cooling time (5.9 ± 0.8 Gyr).

Topologies of Nanoscale Droplets upon Head-On Collision from Large Molecular Dynamics Simulations.

The binary collision of nanoscale droplets is studied with molecular dynamics simulation for droplets consisting of up to 2 × 107 molecules interacting via a truncated and shifted form of the Lennard-Jones potential. Considering head-on collisions of droplets with a temperature near the triple point that occur in a saturated vapor of the same fluid, this work explores a range of collision topologies. Four droplet sizes, with a radius ranging from 30 to 120 molecule diameters, are simulated with a varying initial relative collision velocity, covering 36 cases in total. Due to the relatively large size of the droplets, this study aims to resolve the differences in the collision behavior between droplets on the micro- and on the macroscale. By analyzing various metrics of the impact, four distinct collision regimes are found: coalescence, stable collision, holes and shattering. Coalescence, observed at low Weber and Reynolds numbers, is the formation of a stable droplet without significant deformations of the merging objects. Stable collisions, characterized by the formation of one stable droplet with notable deformations during collision, occur within a Weber number range between 10 and 505. The holes regime is only observed for droplet radii greater than 30 molecule diameters and a Weber number between 505 to 750, while collision cases surpassing this Weber number fall into the shattering regime, resulting in the breakup into satellite structures.

Structural Parameter Design of Magnetic Pulse Welding Coil for Dissimilar Metal Joints: Numerical Simulation, Parameter Optimization, and Experiments

As a main component of the magnetic pulse welding (MPW) system, the working coil exerts a great influence on the electromagnetic force and its distribution, which, in turn, affects the quality of the MPW joints. This study proposes a structural parameter optimization of the MPW coil, with the objective of achieving a higher induced current density on the flyer plate. The optimal Latin hypercube sampling technique (OLHS), Kriging approximate model, and the Non-Linear Programming by Quadratic Lagrangian (NLPQL) algorithm were employed in the optimization procedure, based on the finite element model built in LS-DYNA. The results of the sensitivity analysis indicated that all the selected parameters of the coil had a specific influence on the induced current density in the flyer plate. The optimized coil structure serves to refine the pulse current flowing path within the coil, effectively reducing the current loss within the coil. Additionally, the structure reduces the adverse effect of the current within the coil on the induced current within the flyer plate. Numerical results show the peak-induced current of the flyer plate increasing by 25.72% and the maximum Lorentz force rising by 58.10% at 25 kJ with the optimized coil structure. The experimental results show that with the same 25 kJ discharge energy, the optimized coil could increase the collision velocity from 359.92 m/s to 458.93 m/s. Moreover, 30 kJ of discharge energy should be needed to achieve the failure mode of base material failure with the original coil, while only 15 kJ should be applied to the optimized coil. These findings verify the optimization model and give some outline for coil design.

Modeling and verification of an improved contact force in multibody systems

In many engineering applications, contact and collision phenomena are commonly observed in multibody systems, which can lead to issues such as dynamic output oscillations, reduced motion accuracy, decreased reliability and lifespan, and even functional failures in mechanical systems. To more accurately describe common collision phenomena and their impact on the dynamic characteristics of multibody systems, this paper introduces a contact force model with improved nonlinear stiffness and damping coefficients. This model is based on Hertz theory and considering the relationship between indentation and velocity during the collision process by incorporating a nonlinear index factor, m. Additionally, the Newton restitution coefficient is used as an evaluation criterion to verify the effectiveness of the improved model. Numerical analyses were conducted on single collisions between joints at different initial collision velocities and restitution coefficients, using both the proposed improved contact force model and existing classical models. Further numerical simulations of steel ball collisions were performed using the improved model, and the results were compared with experimental test data. Ultimately, the study results demonstrate that the improved contact force model is more accurate and effective than existing classical models under various restitution coefficients and collision velocity conditions, and has a wider range of applicability.

Particle fragmentation inside planet-induced spiral waves

ABSTRACT Growing planets interact with their surrounding protoplanetary disc, generating feedback effects that may promote or suppress nearby planet formation. We study how spiral waves launched by planets affect the motion and collisional evolution of particles in the disc. To this end, we perform local 2D hydrodynamical simulations that include a gap-opening planet and integrate particle trajectories within the gas field. Our results show that particle trajectories bend at the location of the spiral wave, and collisions occurring within the spiral exhibit significantly enhanced collisional velocities compared to elsewhere. To quantify this effect, we ran simulations with varying planetary masses and particle sizes. The resulting collisional velocities within the spiral far exceed the typical fragmentation threshold, even for collisions between particles of relatively similar sizes and for planetary masses below the pebble isolation mass. If collisions within the spiral are frequent, this effect could lead to progressively smaller particle sizes as the radial distance from the planet decreases, impacting processes such as gap filtering, pebble accretion, and planetesimal formation.

Collisional damping in debris discs: Only significant if collision velocities are low

Context. Dusty debris discs around main sequence stars are observed to vary widely in terms of their vertical thickness. Their vertical structure may be affected by damping in inelastic collisions. Although kinetic models have often been used to study the collisional evolution of debris discs, these models have not yet been used to study the evolution of their vertical structure. Aims. We extend an existing implementation of a kinetic model of collisional evolution to include the evolution of orbital inclinations and we use this model to study the effects of collisional damping in pre-stirred discs. Methods. We evolved the number of particles of different masses, eccentricities, and inclinations using the kinetic model and used Monte Carlo simulations to calculate collision rates between particles in the disc. We considered all relevant collisional outcomes including fragmentation, cratering, and growth. Results. Collisional damping is inefficient if particles can be destroyed by projectiles that are of much lower mass. If that is the case, catastrophic disruptions shape the distributions of eccentricities and inclinations, and their average values evolve slowly and at the same rate for all particle sizes. Conclusions. The critical projectile-to-target mass ratio (Yc) and the collisional timescale jointly determine the level of collisional damping in debris discs. If Yc is much smaller than unity, a debris disc retains the inclination distribution that it is born with for much longer than the collisional timescale of the largest bodies in the disc. Such a disc should exhibit a vertical thickness that is independent of wavelength even in the absence of other physical processes. Collisional damping is efficient if Yc is of order unity or larger. For millimetre-sized dust grains and common material strength assumptions, this requires collision velocities of lower than ~40 m s−1. We discuss the implications of our findings for exo-Kuiper belts, discs around white dwarfs, and planetary rings.

The Mechanical Properties and the Threshing Damage and Its Effects on Nutritional Quality of Buckwheat Grains

Buckwheat grains will suffer varying degrees of damage during the threshing process. Damaged grains are prone to changes in nutritional quality during storage. To explore the relationship between the threshing damage mechanism and nutritional quality, the mechanical properties of buckwheat with different moisture contents were determined through compression and friction tests, obtaining some conventional mechanical property indicators. On this basis, a 3D collision model of buckwheat–nail tooth was established, and the dynamic process of collision was simulated with LS-DYNA. During collision, the changes in energy, von Mises stress, and critical damage velocity of grains with different moisture contents were analyzed. After the threshing test, the grains were observed and classified according to the degree of damage. The differences in nutritional components of different types of grains were analyzed through physicochemical experiments. The experimental results showed that the failure force, elastic modulus, and ultimate strength of buckwheat grains were negatively correlated with moisture content, while the deformation and friction coefficient were positively correlated with moisture content. During collision, the von Mises stress of the grains showed a pattern of increase and then decrease. The maximum stress value occurred at the contact area center, spreading along the periphery and gradually decreasing. The maximum von Mises stress decreased with increasing moisture content and increased with increasing collision velocity. The critical damage velocities of grains at moisture contents of 11.98%, 15.77%, 18.04%, 20.82%, and 25.22% were 13.07, 11.72, 10.94, 10.55, and 10.15 m/s, respectively. After threshing, grains were divided into three types: undamaged, surface cracked, and shell damaged. The nutritional quality of the last two damaged grains decreased during the storage process. These results are of great significance for optimizing buckwheat threshing parameters, reducing buckwheat damage, and improving the economic performance of the buckwheat industry.

Modeling on shock wave collision between asymmetric clouds driven by powerful laser

The cloud–cloud collision is one of the primary mechanisms proposed for forming massive stars. In addition to astronomical observations, plenty of numerical simulations have been conducted. However, relevant laboratory astrophysical studies remain relatively lacking. Using a magnetohydrodynamic simulation code, we simulate the collision of asymmetric plasma shock waves driven by a laser to model the cloud–cloud collision. We investigate the evolution of the collision region with external magnetic fields in different directions. The results indicate that when a strong magnetic field is perpendicular to the collision velocity (referred to as the collision plane), the development of turbulence within the collision region is effectively suppressed, and the magnetic field component in this direction is significantly amplified, the magnetic field in the collision region exhibits a coherent structure. Such coherent magnetic structures may contribute to the formation of coherent interstellar magnetic fields. Additionally, the probability density function of mass density shifts toward high-density regions. This shift could result in the formation of more massive cores from cloud–cloud collisions in the presence of strong magnetic fields.

Numerical Simulation of Droplet Coalescence Using Meshless Radial Basis Function and Domain Decomposition Method

The present investigation of the dynamic two-binary droplet interactions has gained attention since its use to expand and improve several numerical methods. Generally, its interactions are classified into coalescence, bouncing, reflective, and stretching separation. This study simulated droplet coalescence using the meshless radial basis function (RBF) method. These methods are used to solve the Navier-Stokes equations combined with the Cahn-Hilliard equations to track the interface between two fluids. This work uses the fractional step method to calculate the pressure-velocity coupling in the Navier-Stokes equations. The numerical results were compared with the available data in the literature to validate the proposed method. Based on the validation, the proposed method conforms well with the literature. To identify further coalescence characteristics, the model considered different values in viscosity (2, 4, and 8 cP), collision velocity (1.5 m/s and 3 m/s), and surface tension (0.014, 0.028, and 0.056 N/m) parameters. The increasing viscosity was linearly proportional to the collision time, whereas increased surface tension and collision velocity shortened the collision time.

Dynamics of U(1) gauged Q -balls in three spatial dimensions

We investigate the dynamics of U(1) gauged Q-balls using fully three-dimensional numerical simulations. We consider two different scenarios: first, the classical stability of gauged Q-balls with respect to generic three-dimensional perturbations, and second, the behavior of gauged Q-balls during head-on and off-axis collisions at relativistic velocities. With regard to stability, we find that there exist gauged Q-ball configurations which are classically stable in both logarithmic and polynomial scalar field models. With regard to relativistic collisions, we find that the dynamics can depend on many different parameters such as the collision velocity, relative phase, relative charge, and impact parameter of the colliding Q-balls. Published by the American Physical Society 2024

Kink-antikink collisions in hyper-massive models

We study topological kinks and their interactions in a family of scalar field models with a double well potential parametrized by the mass of small perturbations around the vacua, ranging from the mass of the ϕ4 Klein-Gordon model all the way to the limit of infinite mass, which is identified with a non-analytic potential. In particular, we look at the problem of kink-antikink collisions and analyze the windows of capture and escape of the soliton pair as a function of collision velocity and model mass. We observe a disappearance of the capture cases for intermediary masses between the ϕ4 and non-analytic cases. The main features of the kink-antikink scattering are reproduced in a collective coordinates model, including the disappearance of the capture cases.

The study of the calculation method of the triboelectric rate of particles and surface materials at different collision velocities

The study of the calculation method of the triboelectric rate of particles and surface materials at different collision velocities

Acoustic emission and machine learning algorithms for particle size analysis in gas-solid fluidized bed reactors

Acoustic emission and machine learning algorithms for particle size analysis in gas-solid fluidized bed reactors

Application of Flores model considering variable recovery coefficient in dynamics analysis of ball bearings

PurposeThis paper aims to select an appropriate contact force model and apply it to the interaction model between the balls and the cage in the rolling bearings to describe the elastic–plastic collision phenomena between the two.Design/methodology/approachTaking the ball–disk collision mode as an example, several main contact force models were compared and analyzed through simulation and experiment. In addition, based on the consideration of yield strength of materials and initial collision velocity, a variable recovery coefficient model was proposed, and its validity and accuracy were verified by the ball–disk collision experiments. Then, respectively, the Flores model and the Hertz model were applied to the interaction between the balls and the cage, and the dynamics simulation results were compared.FindingsThe results indicate that the Flores model has good regression of recovery coefficient, indicating good applicability for both elastic and elastic–plastic contacts and can be applied to the contact collision situations of various materials. Under certain working conditions, there are significant differences in the dynamics results of rolling bearings simulated using the Flores model and Hertz model, respectively.Originality/valueThis paper applies the Flores model with variable recovery coefficients to the dynamics simulation analysis of ball bearings to solve the elastic–plastic collision problem between the rolling elements and the cage that cannot be reasonably handled by the Hertz model.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-04-2024-0138/

Dust Growth and Evolution in Protoplanetary Disks

Over the past decade, advancement of observational capabilities, specifically the Atacama Large Millimeter/submillimeter Array (ALMA) and Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instruments, alongside theoretical innovations like pebble accretion, have reshaped our understanding of planet formation and the physics of protoplanetary disks. Despite this progress, mysteries persist along the winded path of micrometer-sized dust, from the interstellar medium, through transport and growth in the protoplanetary disk, to becoming gravitationally bound bodies. This review outlines our current knowledge of dust evolution in circumstellar disks, yielding the following insights: ▪Theoretical and laboratory studies have accurately predicted the growth of dust particles to sizes that are susceptible to accumulation through transport processes like radial drift and settling.▪Critical uncertainties in that process remain the level of turbulence, the threshold collision velocities at which dust growth stalls, and the evolution of dust porosity.▪Symmetric and asymmetric substructures are widespread. Dust traps appear to be solving several long-standing issues in planet formation models, and they are observationally consistent with being sites of active planetesimal formation.▪In some instances, planets have been identified as the causes behind substructures. This underlines the need to study earlier stages of disks to understand how planets can form so rapidly. In the future, better probes of the physical conditions in optically thick regions, including densities, turbulence strength, kinematics, and particle properties, will be essential for unraveling the physical processes at play.

On the elastoplastic behavior in collisional compression of spherical dust aggregates

Aggregates consisting of submicron-sized cohesive dust grains are ubiquitous, and understanding the collisional behavior of dust aggregates is essential. It is known that low-speed collisions of dust aggregates result in either sticking or bouncing, and local and permanent compaction occurs near the contact area upon collision. In this study, we perform numerical simulations of collisions between two aggregates and investigate their compressive behavior. We find that the maximum compression length is proportional to the radius of aggregates and increases with the collision velocity. We also reveal that a theoretical model of contact between two elastoplastic spheres successfully reproduces the size- and velocity-dependence of the maximum compression length observed in our numerical simulations. Our findings on the plastic deformation of aggregates during collisional compression provide a clue to understanding the collisional growth process of aggregates.Graphic abstract

Effect of welding gap on electromagnetic pulse welding of a copper-aluminum alloy joint

ABSTRACT Electromagnetic pulse welding (EMPW) is commonly used for welding copper and aluminum alloy. The welding gap is important in the EMPW process. The research focused on the effect of welding gap on the welding process and the mechanical property of joints. During the EMPW experiments, the deformation, collision, and metal jet were recorded with a high-speed camera. The joint mechanical property was tested. SEM and EDS were employed to examine the bonding interface. Results indicated that the welding gap length did not significantly influence the aluminum alloy plate motion behavior when it was long enough. With the welding gap width increasing, the collision velocity initially increased and subsequently decreased, and the angle between the two plates when they first collided increased continuously. The intensity and duration of the metal jet and the joint mechanical properties also increased first and then decreased. The welding marks widened first and then narrowed.

Study on interface diffusion and welding strength of Cu/Sn dissimilar metal electromagnetic pulse welding based on molecular dynamics simulation

Study on interface diffusion and welding strength of Cu/Sn dissimilar metal electromagnetic pulse welding based on molecular dynamics simulation