Binary Collision Simulations Research Articles

Numerical investigation of non-Newtonian droplet collisions: Comparison of volume of fluid and the local front reconstruction method with experimental data

Droplet-droplet collisions of non-Newtonian fluids are often encountered in industrial applications such as spray drying, fluid catalytic cracking, coating and granulation. However, there has been limited focus on a comprehensive analysis of the dynamics of these non-Newtonian binary droplet collisions. In this work, the non-Newtonian binary droplet collisions of 500 ppm xanthan gum solutions are simulated using the Volume of Fluid method and the Local Front Reconstruction Method to determine the applicability of these methods for the simulations of binary non-Newtonian droplet collisions. After verification, the simulations are compared to experimental data from literature. The outcomes for off-center collisions in the simulations resemble the experimental observations, including the oscillation of the ligament. For head-on collisions, the initial interactions and collision dynamics can be predicted with both methods at low Weber numbers, while LFRM shows improved performance at higher Weber numbers. Further, the use of an effective viscosity for the simplified representation of the droplet collision has been proven unsuccessful due to the absence of a universal effective viscosity for all situations.

Effects of Iron Addition on the Collision of Polycyclic Aromatic Hydrocarbon Clusters: A Molecular Dynamics Study

In this work, soot particle size distributions in iron-doped premixed ethylene flames are examined using scanning mobility particle sizer measurements. It is found that iron addition promotes the growth in soot particle size, and the enhanced particle coagulation is inferred to be an important reason. To support that, the influence of iron addition on the coagulation of polycyclic aromatic hydrocarbon (PAH) clusters, the analogue of incipient soot particles, is further investigated using molecular dynamics simulations. Based on the results of hundreds of binary head-on collision simulations, the collision between two coronene-Fe-coronene dimers is found to have a significantly higher coagulation efficiency than that between two coronene dimers. However, this enhancement effect weakens as the size of the PAH monomer increases. Although the coagulation efficiency can be increased by iron addition, the collision frequency is almost unaffected, as revealed from the binary off-central collision simulations. Moreover, the simulation results of coronene cluster growth via coagulation show that iron addition promotes coronene cluster growth, leading to larger cluster size, which may explain the larger soot particle size observed in iron-doped flames.

Efficient SAV–Hermite methods for the nonlinear Dirac equation

We construct two SAV/CN–Hermite schemes for one and two dimensional nonlinear Dirac equation in this paper. Both SAV/CN methods preserve the energy and mass and an efficient and accurate scheme is proposed by Hermite approximation. The numerical analysis of error estimates is established on SAV/CN scheme. Numerical experiments confirm the expected convergence order. In addition, they are given to show the simulations of binary and ternary collisions.

Signatures of linear Breit-Wheeler pair production in polarized γγ collisions

The polarization characteristics of the linear Breit-Wheeler (LBW) pair-production process in polarized $\ensuremath{\gamma}\ensuremath{\gamma}$ colliders have been investigated via our developed spin-resolved binary collision simulation method. We find that the polarization of $\ensuremath{\gamma}$-photons modifies the kinematics of scattering particles and induces the correlated energy-angle shift of LBW pairs, and the latter's polarization characteristic depends on the helicity configures of scattering particles. We confirm that the polarized $\ensuremath{\gamma}\ensuremath{\gamma}$ collider with an asymmetric setup can be performed with currently achievable laser-driven high-density x rays and high-brilliance $\ensuremath{\gamma}$-photon beams to produce abundant polarized LBW pairs, fulfilling the detection power of polarimetries. Our method and results on the polarized LBW process have plenty of significant applications in strong-field physics, high-energy physics and astrophysics, such as calibrating and monitoring the polarized $\ensuremath{\gamma}\ensuremath{\gamma}$ collider and challenging the current understanding of astrophysical objects through enhancing the opacity of $\ensuremath{\gamma}$-photons to exacerbate the inconsistency between some observations and standard models.

Nanoscale W/Be multilayers: Intermixing during magnetron sputtering deposition and effect of heat treatment

An atomistic study of the W and Be mixing during magnetron sputtering deposition using the SRIM software based on the binary collision simulation by Monte Carlo method is presented. The calculations performed clearly show a strong correlation between the sort of sputtered atoms, their kinetic energies and the asymmetry of interfaces in the model Si/W(40 nm)/Be(4 nm) and Si/Be(40 nm)/W(1.2 nm) systems. The carried out analysis indicates that the stoichiometry of a beryllide (WBex) at an interface during magnetron sputtering is mainly determined by the kinetic energy of incident atoms. The results of the simulations are in good agreement with experimental data. Analysis of the Si/[W(1.1 nm)/Be(1.4 nm)]200 multilayer structure reveals a complete mixing of the layers with the formation of only beryllides. A pure beryllium was detected in the structure Si/[Be(2.62 nm)/W(0.62 nm)]100 where the period of the system and the thickness of the Be layer are more extended, however pure tungsten was not observed. Annealing of the model samples indicates that heating at temperature 350 °C leads to increasing in the WBe12 beryllide at the Be-on-W interface. Annealing of the multilayer structures yields an increase of WBe2 and a decrease of WBe12 with the temperature growth.

二元海水液滴对心碰撞过程数值模拟

Numerical simulations of head-on binary collisions between equal-size seawater droplets were conducted with the volume of fluid (VOF) method and the adaptive grid technique, to investigate the collision physics and mechanics of seawater droplets in shower cooling towers. Trial simulations of head-on collisions of tetradecane droplets in nitrogen medium were performed firstly to give results in good agreement with those of the previous experiments. The binary collisions of equal-size seawater droplets were simulated under room temperature and normal pressure conditions. The stream field and flow mechanism of seawater droplets collision were analyzed, and the effects of droplet diameters and concentrations on the collision process were studied. Two different types of collision outcomes were identified: the coalescence and the reflexive separation, for which the critical Weber numbers were given. The schematic diagrams for various head-on collision regimes of seawater droplets at various Ohnesorge numbers were also obtained.

Application of multi-angle scattering maps to stepped surfaces

This study examines channeling, multiple scattering, and neutralization/re-ionization of ions scattered along the stepped Al(332) plane. Our experimental approach involves probing the surface with 1–2 keV He+ and Ne+ beams, and then systematically mapping the scattered ion fluxes over a large solid angle. This provides comprehensive ion channeling information over all directions, rather than along a few low-index azimuths, as is common practice in ion scattering spectroscopy. We first probe the surface with 2 keV He+ at near-normal incidence, and then map the backscattered particle flux (both ions and neutrals) via time of flight (TOF) spectrometry. The features contained in these maps can be correlated with axial and inter-planar channeling effects, and are reproduced well via binary collision simulations. Sensitivity to the stepped surface topography is heightened considerably for oblique ion incidence in the forward-scattering direction. In this geometry, we used 2 keV Ne+ to probe the surface and mapped the corresponding scattered fluxes of both single and multiply-charged ions. In both cases, the scattering intensity depends strongly on the precise trajectory taken along the surface, and is particularly sensitive to how extensively the incident ions interact with the step edges. We interpret the information contained in these maps by considering several mechanisms for charge transfer and double ion production. The formation of Ne++ appears to be correlated with a previously observed inelastic mechanism that occurs when the collision apsis, Rmin, is less than 0.65 Å. This contributes to an energy loss of 48 ± 8 eV for Ne+ undergoing single scattering; the Rmin threshold for this inelastic step coincides with the emergence of a distinct Ne++ peak. Using the information gained from the maps, we propose methods for extending this approach to chemisorbed layers.

Concurrent segregation and erosion effects in medium-energy iron beam patterning of silicon surfaces

We have bombarded crystalline silicon targets with a 40 keV Fe+ ion beam at different incidence angles. The resulting surfaces have been characterized by atomic force, current-sensing and magnetic force microscopies, scanning electron microscopy, and x-ray photoelectron spectroscopy. We have found that there is a threshold angle smaller than 40° for the formation of ripple patterns, which is definitely lower than those frequently reported for noble gas ion beams. We compare our observations with estimates of the value of the critical angle and of additional basic properties of the patterning process, which are based on a continuum model whose parameters are obtained from binary collision simulations. We have further studied experimentally the ripple structures and measured how the surface slopes change with the ion incidence angle. We explore in particular detail the fluence dependence of the pattern for an incidence angle value (40°) close to the threshold. Initially, rimmed holes appear randomly scattered on the surface, which evolve into large, bug-like structures. Further increasing the ion fluence induces a smooth, rippled background morphology. By means of microscopy techniques, a correlation between the morphology of these structures and their metal content can be unambiguously established.

Iron NRT- and arc-displacement cross sections and their covariances

Displacement damage cross sections have been calculated for natural iron and its constituent individual isotopes based on the latest versions of the evaluated neutron data libraries ENDF/B-VIII.0 and TENDL-2017 using the conventional NRT and recently introduced athermal recombination-corrected (arc) displacement concepts. Their covariance matrices, which quantify the uncertainties and the associated energy-energy correlations, were assessed for the first time based on TENDL-2017 random data files representing the covariances of the underlying nuclear data. In addition to the nuclear data, the covariances due to the ion energy partition, the primary defect survival efficiency and the lattice threshold energy were also propagated to the damage quantities. The arc-dpa was computed employing all available results from molecular dynamics and binary collision simulations. The comparison of the spectrum-averaged arc-dpa with the few available measurements at fission reactors has demonstrated a reasonable agreement within the experimental and calculation uncertainties. The comparison with the current NRT-dpa standard, provided by ASTM up to 20 MeV but without uncertainty, has indicated differences up to 8% above 0.5 keV and 60% below.

Iron ions distribution profile obtained by irradiating the silicon single crystal

Iron ions with energies of 90 and 250 keV and the irradiation dose of 1016 ions/cm2 and xenon ions with energies of 100 keV and dose 7,7×1014 cm-2, 200 keV and dose 2,6 × 1014 cm-2 were implanted in the single crystal of silicon (110). The distribution profiles of implanted impurity, as well as the distribution profiles of the radiation defects in the crystal lattice, were studied by the Rutherford backscattering method in combination with channeling. The experimental results were compared with the results of simulations of binary collisions of the Monte Carlo method in the TRIM program. It is shown that the difference between the experimental data and the calculation program TRIM is more than 35% in all cases.

Observation of a strong-coupling effect on electron-ion collisions in ultracold plasmas.

Ultracold plasmas (UCPs) provide a well-controlled system for studying multiple aspects in plasma physics that include collisions and strong-coupling effects. By applying a short electric field pulse to an UCP, a plasma electron center-of-mass oscillation can be initiated. For accessible parameter ranges, the damping rate of this oscillation is determined by the electron-ion collision rate. We performed measurements of the oscillation damping rate with such parameters and compared the measured rates to both a molecular dynamics (MD) simulation that includes strong-coupling effects and a Monte Carlo binary collision simulation designed to predict the damping rate including only weak-coupling considerations. We found agreement between the experimentally measured damping rate and the MD result. This agreement did require including the influence of a previously unreported UCP heating mechanism whereby the presence of a dc electric field during ionization increased the electron temperature, but estimations and simulations indicate that such a heating mechanism should be present for our parameters. The measured damping rate at our coldest electron temperature conditions was much faster than the weak-coupling prediction obtained from the Monte Carlo binary collision simulation, which indicates the presence of a significant strong-coupling influence. The density averaged electron strong-coupling parameter Γ measured at our coldest electron temperature conditions was 0.35(8).

1602 二相系格子ボルツマン法による異径液滴の衝突挙動解析(OS7-1 複雑流体の流動現象・流れの制御・混相流の応用技術)

1602 二相系格子ボルツマン法による異径液滴の衝突挙動解析(OS7-1 複雑流体の流動現象・流れの制御・混相流の応用技術)

Combined binary collision and continuum mechanics model applied to focused ion beam milling of a silicon membrane

Many experiments indicate the importance of stress and stress relaxation upon ion implantation. In this paper, a model is proposed that is capable of describing ballistic effects as well as stress relaxation by viscous flow. It combines atomistic binary collision simulation with continuum mechanics. The only parameters that enter the continuum model are the bulk modulus and the radiation-induced viscosity. The shear modulus can also be considered but shows only minor effects. A boundary-fitted grid is proposed that is usable both during the binary collision simulation and for the spatial discretization of the force balance equations. As an application, the milling of a slit into an amorphous silicon membrane with a 30keV focused Ga beam is studied, which demonstrates the relevance of the new model compared to a more heuristic approach used in previous work.

Simulation of an Avalanche in a Fluid with a Soft-Sphere/Immersed Boundary Method Including a Lubrication Force

The present work aims at reproducing the local dynamics of a dense granular media evolving in a viscous fluid from the grain scale to that of thousands of grains, encountered in environmental multiphase flows. To this end a soft-sphere collision/immersed-boundary method is developed. The methods are validated alone through various standard configurations including static and dynamical situations. Then, simulations of binary wall-particle collisions in a fluid are performed for a wide range of Stokes number ranging in [10−1, 104]. Good agreement with available experimental data is found provided that a local lubrication model is used. Finally, three- dimensional simulations of gravity/shear-driven dense granular flows in a viscous fluid are presented. The results open the way for a parametric study in the parameter space initial aspect ratio-initial packing.

Binary droplet collision simulations by a multiphase cascaded lattice Boltzmann method

Three-dimensional binary droplet collisions are studied using a multiphase cascaded lattice Boltzmann method (LBM). With this model it is possible to simulate collisions with a Weber number of up to 100 and a Reynolds number of up to 1000, at a liquid to gas density ratio of over 100. This is made possible by improvements to the collision operator of the LBM. The cascaded LBM in three dimensions is introduced, in which additional relaxation rates for higher order moments, defined in a co-moving reference frame, are incorporated into the collision operator. It is shown that these relaxation rates can be tuned to reduce spurious velocities around curved phase boundaries, without compromising the accuracy of the simulation results. The range of attainable Reynolds numbers is therefore increased. Different outcomes from both head-on and off-centre collisions are simulated, for both equal and unequal size droplets, including coalescence, head-on separation, and off-centre separation. For head-on collisions the critical Weber number between coalescence and separation is shown to decrease with decreasing ambient gas pressure. The variation of critical Weber number with droplet size ratio is also studied. Comparisons are made with the theoretical predictions of Tang et al. [“Bouncing, coalescence, and separation in head-on collision of unequal-size droplets,” Phys. Fluids 24, 022101 (2012)], and the effect of ambient gas pressure is again considered. For off-centre collisions, boundaries between different collision outcomes are accurately defined and quantitative comparisons are made with the theoretical predictions of Rabe et al. [“Experimental investigation of water droplet binary collisions and description of outcomes with a symmetric Weber number,” Phys. Fluids 22, 047101 (2010)]. While general agreement between the simulated and theoretical boundaries is presented, deviations due to varying liquid viscosity are observed. Finally, the prediction of the independence of regime boundaries with varying droplet size ratio, when using the symmetric Weber number as defined by Rabe et al., is discussed. Simulation results showing qualitative agreement are presented, although some discrepancies are reported.

Simulations of the merging galaxy cluster Abell 3376

Observed galaxy clusters often exhibit X-ray morphologies suggestive of recent interaction with an infalling subcluster. A3376 is a nearby (z = 0.046) massive galaxy cluster whose bullet-shaped X-ray emission indicates that it may have undergone a recent collision. It displays a pair of Mpc-scale radio relics and its brightest cluster galaxy is located 970 h− 170 kpc away from the peak of X-ray emission, where the second brightest galaxy lies. We attempt to recover the dynamical history of A3376. We perform a set of N-body adiabatic hydrodynamical simulations using the smoothed particle hydrodynamics (SPH) code gadget-2. These simulations of binary cluster collisions are aimed at exploring the parameter space of possible initial configurations. By attempting to match X-ray morphology, temperature, virial mass and X-ray luminosity, we set approximate constraints on some merger parameters. Our best models suggest a collision of clusters with mass ratio in the range 1/6–1/8, and having a subcluster with central gas density four times higher than that of the major cluster. Models with small impact parameter (b < 150 kpc), if any, are preferred. We estimate that A3376 is observed approximately 0.5 Gyr after core passage, and that the collision axis is inclined by i ≈ 40° with respect to the plane of the sky. The infalling subcluster drives a supersonic shock wave that propagates at almost 2600 km s−1, implying a Mach number as high as |$\mathcal {M}\sim 4$|⁠; but we show how it would have been underestimated as |$\mathcal {M}\sim 3$| due to projection effects.

Sputtering of silicon at glancing incidence

Recent experimental sputtering yield data for Si sputtered by 30keVGa FIB shows a discrepancy with binary collision (BC) simulations for angles greater than 85°. This study investigates the factors of BC simulations that could be responsible for the discrepancy at glancing angles, including surface roughness, Ga implantation, electronic stopping, and surface binding energy. It is concluded that the dominant factor at glancing angles is surface roughness. Two roughness models, sinusoidal and pink noise, are used to demonstrate that very small amplitude (∼2Å) roughness is required to obtain quantitative agreement with experiments at very glancing angles. Furthermore, it is shown that large amplitude roughness should be unstable under glancing angle beams.

Assessment of surface potential models by molecular dynamics simulations of atom ejection from (1 0 0)-Si surfaces

In analytic theories as well as binary collision simulations the attractive forces between the surface atoms and a recoil leaving the target are described by a surface potential. While in an isotropic model the recoil only suffers energy loss, in the more commonly used planar model refraction or reflection of the recoil also occur. In this work I discuss the limitations of these models, including a generalized model containing the isotropic and planar models as special cases, by comparing molecular dynamics (MD) and binary collision (BC) simulations of atom ejection from (100)-Si surfaces. It is shown that a surface potential close to planar best describes the MD results. However, in the BC simulations (i) emission yields are systematically too high at low ejection energies, (ii) discrepancies with respect to MD are larger for the ejection of atoms from sub-surface layers, and (iii) the emission yield for top-layer atoms does not increase during erosion of the top layer as suggested by MD. Possible reasons for these deficiencies are discussed. It is concluded that BC results are seriously in question when the effect under investigation depends on the fate of low-energy recoils at the surface.

ZERO IMPACT PARAMETER WHITE DWARF COLLISIONS IN FLASH

We systematically explore zero impact parameter collisions of white dwarfs with the Eulerian adaptive grid code FLASH for 0.64+0.64 M$_{\odot}$ and 0.81+0.81 M$_{\odot}$ mass pairings. Our models span a range of effective linear spatial resolutions from 5.2$\times10^{7}$ to 1.2$\times10^{7}$ cm. However, even the highest resolution models do not quite achieve strict numerical convergence, due to the challenge of properly resolving small-scale burning and energy transport. The lack of strict numerical convergence from these idealized configurations suggest that quantitative predictions of the ejected elemental abundances that are generated by binary white dwarf collision and merger simulations should be viewed with caution. Nevertheless, the convergence trends do allow some patterns to be discerned. We find that the 0.64+0.64 M$_{\odot}$ head-on collision model produces 0.32 M$_{\odot}$ of \nickel[56] and 0.38 M$_{\odot}$ of \silicon[28], while the 0.81+0.81 M$_{\odot}$ head-on collision model produces 0.39 M$_{\odot}$ of \nickel[56] and 0.55 M$_{\odot}$ of \silicon[28] at the highest spatial resolutions. Both mass pairings produce $\sim$0.2 M$_{\odot}$ of unburned \carbon[12]+\oxygen[16]. We also find the 0.64+0.64 M$_{\odot}$ head-on collision begins carbon burning in the central region of the stalled shock between the two white dwarfs, while the more energetic \hbox{0.81+0.81 M$_{\odot}$} head-on collision raises the initial post-shock temperature enough to burn the entire stalled shock region to nuclear statistical equilibrium.

Dynamic binary collision simulation of focused ion beam milling of deep trenches

A new model is presented for the simulation of focused ion beam milling in two-dimensional targets. It combines dynamic Monte Carlo binary collision simulation with cell-based topography simulation, and includes relaxation of cell densities. In the Monte Carlo model advanced algorithms are used for surface treatment, trajectory termination, and collision partner selection. Cell relaxation is performed by minimization of an energy function written in terms of the grid point coordinates, taking the deviation of the cell area from its relaxed value into account, but neglecting viscous forces. Relaxation is limited to areas where nuclear collisions have occurred during cascade simulation. The code is applied to deep trench formation in Si targets by 50keV Ga focused ion beams. The simulation results indicate the importance of material flow and compare well with experiments.