Merging Criteria for Planetesimal Collisions

Material deformation and removal due to single particle impacts on ductile materials using smoothed particle hydrodynamics

The influence of a steel rear barrier on the detonation response of a steel covered explosive struck by a steel projectile

Experimental study and visualization of impacting spherical hydrophobic particles on an air – Liquid interface: Newtonian and Boger liquid analysis

H-section steel members, as a commonly used load-bearing receiving member in building structures, may be subjected to the impact of accidental loads during their service life, and therefore, the impact loads need to be considered when carrying out the design. In this paper, based on experimental testing, the particle swarm optimization algorithm (PSO) is used to optimize the hyperparameters of the multilayer perceptron (MLP), and a combined prediction model PSO-MLP for H-section steel members subjected to lateral impact loads is proposed to predict the damage of the H-section steel members after impact. The results show that the prediction model based on PSO-MLP can predict the damage of the H-beam columns more accurately, and compared to the random forest model (RF) and the support vector machine (SVM), the PSO-MLP model has better prediction accuracy and robustness. In addition, the effects of different features on the impact performance of the members were analyzed, in which the weakest impact location is 0.57 L away from the fixed end and the effects of axial compression ratio, flange, and web thickness were similar to the results of previous studies; the impact angle showed a strong nonlinear relationship with the critical impact velocity, which the weakest impact angle is around 50° from the strong axle; and the height and width of the cross-section showed a linear enhancement of the impact performance.

Impingement on blade surface by fine particles with high velocity is commonly seen in steam turbines, gas turbines and compressors, which affect the service life and reliability of the equipment. Study on particles’ rebound characteristics is of great significance to reduce the blade erosion and to control particle trajectory. Based on the nonlinear explicit dynamics analysis software ANSYS/LS-DYNA, the impacts of fine spherical particles with different diameters (20 to 500μm) on a typical martensitic stainless steel (AISI 403) target with high velocity (50 to 250m/s) have been systematically studied. The influences of incident velocities, impact angles, particles sizes on its rebound characteristics, relative impact depth, and relative dissipated energy have been analyzed. Results show that velocity restitution coefficient e decreased with the impact angle β1, the incident velocity V1, and the particle size dp. However, the role of particle size on the velocity restitution coefficient seemed to be far less than that of the other two factors. Both of particle’s tangential and normal velocity coefficient of restitution declined with the increasing impact angle in most cases. However, when the incident velocity V1 = 200m / s and the impact angle β1 > 45°, the tangential velocity restitution coefficient et of 100 μm and 200 μm particles increased with the increase in the impact angle β1. The reason might be that the relative impact depth drel was located a zone ranged from 0.1515 to 0.1677, where the tangential rebound behavior could be enhanced. Most of the variation of the tangential and normal velocity restitution coefficient along β1 decreased with the increase in the particle diameter. However, when V1 = 200m/s and β1 > 15°, the tangential reflected velocity of the larger particles was enhanced gradually. In addition, the values of the relative impact depth drel increased with the increasing impact angle and incident velocity, and it increased with the increasing particle diameter in most cases. The relative dissipated energy of particles steadily increased with the impact angle and incident velocity, respectively. Particle diameter had little effect on energy dissipation in comparison with the impact angle and incident velocity.

An experimental and numerical study has been performed, where initiation of a high explosive target by high velocity debris from a tungsten sphere was investigated. The target consisted of two spaced plates with the high explosive mounted directly behind the inner plate. Both the impact velocity and target obliquity have been varied to find impact conditions that lead to initiation of the high explosive. Numerical simulations were used to estimate critical impact velocities for a series of impact obliquities. The critical impact velocity was found to increase with increasing obliquity, presumably due to the reduction in residual velocity of the debris cloud, and with the reduction of the effective diameter of the debris cloud as the obliquity increases above 30°. • Presents experiments showing the critical impact conditions leading to initiation. • Complements experimental results with numerical simulations. • Suggests an expression relating the critical impact velocity to obliquity.

Numerical study of shear deformation in Ti–6Al–4V at medium and high strain rates, critical impact velocity in shear

In order to investigate the interaction between double spherical tungsten fragments and Composition B (Comp B) covered by tantalum coverplates, the numerical simulation was carried out by using AUTODYN-3D software. The influence on the shock initiation characteristics of double fragments at various distance separation b1, time separation h1, impact angle α with the same or different velocities were analyzed, and the critical initiation velocity of covered Comp B was obtained. The obtained simulation results show that, under certain circumstances, the superposition effect of shock waves caused by double fragments impact on covered Comp B is obvious. The critical initiation velocity increases with the increase of b1 when fragments impacting on covered charge at the same time with same or different velocities, with α=0, and approaches critical velocity of a single fragment. The critical initiation velocity decreases initially and then increases with the increase of h1 when fragments impacting asynchronously on covered charge with different velocities and α=0. When fragments impacting synchronously on covered Comp B with different velocities, the critical velocity decreases initially and then increases with the increase of α. When fragments impacting asynchronously on covered Comp B with different velocities, the critical velocity increases with the increase of h1. Therefore, a shock initiation of covered Comp B rather be achieved by multiple fragments at lower velocity than by a single fragment at higher.

The formation of planets depends on the underlying protoplanetary disc structure, which influences both the accretion and migration rates of embedded planets. The disc itself evolves on time-scales of several Myr during which both temperature and density profiles change as matter accretes onto the central star. Here we use a detailed model of an evolving disc to determine the growth of planets by pebble accretion and their migration through the disc. Cores that reach their pebble isolation mass accrete gas to finally form giant planets with extensive gas envelopes, while planets that do not reach pebble isolation mass are stranded as ice giants and ice planets containing only minor amounts of gas in their envelopes. Unlike earlier population synthesis models, our model works without any artificial reductions in migration speed and for protoplanetary discs with gas and dust column densities similar to those inferred from observations. We find that in our nominal disc model the emergence of planetary embryos preferably occurs after approximately 2 Myr in order to not exclusively form gas giants, but also ice giants and smaller planets. The high pebble accretion rates ensure that critical core masses for gas accretion can be reached at all orbital distances. Gas giant planets nevertheless experience significant reduction in semi-major axes by migration. Considering instead planetesimal accretion for planetary growth, we show that formation time-scales are too long to compete with the migration time-scales and the dissipation time of the protoplanetary disc. Altogether, we find that pebble accretion overcomes many of the challenges in the formation of ice and gas giants in evolving protoplanetary discs.

The formation of planets depends on the underlying protoplanetary disc structure, which influences both the accretion and migration rates of embedded planets. The disc itself evolves on time-scales of several Myr during which both temperature and density profiles change as matter accretes onto the central star. Here we use a detailed model of an evolving disc to determine the growth of planets by pebble accretion and their migration through the disc. Cores that reach their pebble isolation mass accrete gas to finally form giant planets with extensive gas envelopes, while planets that do not reach pebble isolation mass are stranded as ice giants and ice planets containing only minor amounts of gas in their envelopes. Unlike earlier population synthesis models, our model works without any artificial reductions in migration speed and for protoplanetary discs with gas and dust column densities similar to those inferred from observations. We find that in our nominal disc model the emergence of planetary embryos preferably occurs after approximately 2 Myr in order to not exclusively form gas giants, but also ice giants and smaller planets. The high pebble accretion rates ensure that critical core masses for gas accretion can be reached at all orbital distances. Gas giant planets nevertheless experience significant reduction in semi-major axes by migration. Considering instead planetesimal accretion for planetary growth, we show that formation time-scales are too long to compete with the migration time-scales and the dissipation time of the protoplanetary disc. Altogether, we find that pebble accretion overcomes many of the challenges in the formation of ice and gas giants in evolving protoplanetary discs.

Observations of the disk of dust and gas around a nascent star reveal that the distance from the star at which water in the disk forms ice is variable. This variation might hinder the formation of planets. See Letter p.258 The snow-line in a protoplanetary disk is the distance from the protostar at which a particular volatile gas condenses. Of particular importance in disk evolution is the snow-line for water, because water-ice determines the efficiency of dust and planetesimal coagulation and the formation of comets, ice giants and the cores of gas giants. Lucas Cieza et al. report long-baseline observations of the disk around V883 Ori, a protostar of 1.3 solar masses that is undergoing an accretion outburst. The data provide the first direct image of the water snow-line of a protoplanetary disk and suggest that it migrated outwards during a protostellar outburst, with implications for our understanding of the formation of planetary systems such as our own.

Study on the motion law and force chain characteristics of buffer cushion layer of rolling stone impacting shed hole

The flow and heat transfer characteristics of a droplet obliquely impact on a stationary liquid film is numerically studied by using a coupled level set and volume of fluid method (CLSVOF). The effects of impact angle, Weber number, and film thickness are analyzed. As compared with the normal impact, the flow features and surface heat flux distribution for oblique impact are asymmetric and where the secondary droplets only can be observed on one side. Corresponding mechanisms have also been interpreted. The maximum value of the local surface heat flux decreases and moves to the right with the increase in impact angle, while the minimum local surface heat flux on both sides increases as the impact angle increases. With a greater Weber number or a thinner film thickness, the maximum local surface heat flux is higher, while the minimum local surface heat flux on both sides appears to be an opposite trend. In addition, the impact angle has little effect on the average surface heat flux.

In recent years, light sport aircraft, which not only serve the purpose of personal recreation but also act as a means of transportation for medium and short distance travel, have rapidly gained popularity in the general aviation industry worldwide. The FAA established regulations for this new category of airplanes in 2004. However, the crashworthiness requirements for this type of airplane have not been clearly specified. This study used the finite element method to investigate the effect of the impact angle and speed of the LSA fuselage structure on passenger safety during a crash event. We used sink speed defined by NASA AGATE, ASTM and FAR as parameters. The passenger compartment reducing rate defined by MIL-STD-1290A was used for a safety boundary condition. The results show that the maximum cockpit reducing rate of the airplane impact angle is 30o. When the impact angle increases, owing to the engine mount and fire wall’s reinforced structure, this type of airplane can sustain a greater vertical drop speed. When the impact angle is about 80°~90°, the maximum impact speed the fuselage that can be sustained is 33 m/s. This work also completed a simulation of safe and unsafe ranges for light sport aircraft at various impact angles and vertical drop speeds during impact.

The impingement zone created by the impingement of two free, thin sheets of liquids was studied using photographic techniques. The experimental parameters studied were the impingement angle, single-sheet velocity, single-sheet thickness at impingement, liquid surface tension, and liquid viscosity. Momentum balances of the impingement are in good agreement with observations that a critical mixed sheet velocity must be achieved in order for the liquid to flow in the backward direction of the mixed sheet. The critical mixed sheet velocity for backflow was found to correlate well with the Taylor–Culick velocity. However, if all of the kinetic energy associated with the single-sheet component velocity that is destroyed upon impingement is dissipated into the mixed sheet, then that critical velocity cannot be achieved, and there will be no backflow from the backward part of the mixed sheet. An unusual result of the study is that the projection of the single sheets onto the mixed sheet at impingement presents as a “skin” that can be clearly seen in photographs or a video. This “skin” feature potentially allows for the use of macro photography to measure the thickness of the single sheet at impingement.