Granular Target Research Articles

Ejecta splashing and scaling of projectile oblique impact on granular media

Ejecta splashing is accompanied by the formation of impact craters in oblique impact of a sphere onto a granular target. We investigated the morphology and scaling of the ejection, together with the evolution and final size of crater by performing a series of experiments, varying the impact angle θ and impact speed V0. The experiment categorized the crater shapes in the space parameters Fr and θ and revealed that the maximum ejecta height exhibits two regimes related to Froude number, while the crater length, width, and depth are all collapsed to a master line. Then, the evolution characteristics of the corolla dimensions (top diameter, neck size, bottom diameter, and height) are determined. Moreover, a simple ballistic model taking into account the air drag force acting on the ejecta has been proposed to predict the dynamic processes of the corolla in oblique impacts. Furthermore, the opening of the crater formation deduced by the dynamics of the corolla formed and the collapsing process (i.e., the splashed sand avalanching down along the wall of the crater) have been investigated in detail using a simplified Bouchaud–Cates–Ravi–Edwards model. Our theoretical model demonstrated high accuracy in reproducing the evolution of a crater during impacting and in predicting the final crater scaling after avalanching.

IMPACT OF ONLINE ADVERTISING IN MARKETING OF THE PRODUCT

In recent years, the landscape of marketing has undergone a profound transformation, largely propelled by the exponential growth of the internet and the emergence of various digital platforms and technologies. Online advertising, encompassing a diverse array of formats such as display ads, social media ads, search engine marketing, video ads, and influencer partnerships, has emerged as a cornerstone of modern marketing strategies. One of the primary drivers behind the rise of online advertising is its unparalleled ability to reach highly targeted audiences with precision and efficiency. Unlike traditional advertising mediums like print or television, online advertising offers sophisticated targeting capabilities based on factors such as demographics, interests, behavior, and geographic location. This granular targeting allows advertisers to tailor their messages to specific audience segments, thereby maximizing relevance and resonance. Moreover, the real-time nature of online advertising enables businesses to measure and track the performance of their campaigns with unprecedented granularity. Through metrics such as click-through rates, conversion rates, impressions, and engagement metrics, advertisers can gain valuable insights into the effectiveness of their campaigns and make data-driven optimizations in real-time. This iterative approach to advertising not only improves campaign performance but also enhances return on investment (ROI) by ensuring that marketing budgets are allocated efficiently. Furthermore, online advertising offers unparalleled flexibility and scalability, allowing businesses of all sizes to launch and manage campaigns with varying budgets and objectives. Whether it's a small startup looking to build brand awareness or a multinational corporation aiming to drive sales and conversions, online advertising offers a range of solutions to suit diverse marketing goals and budgets. In addition to its targeting capabilities and measurability, online advertising also plays a crucial role in enhancing brand visibility and awareness in today's crowded marketplace.

Cold granular targets slow the bulk freezing of an impacting droplet.

When making contact with an undercooled target, a drop freezes. The colder the target is, the more rapid the freezing is supposed to be. In this research, we explore the impact of droplets on cold granular material. As the undercooling degree increases, the bulk freezing of the droplet is delayed by at least an order of magnitude. The postponement of the overall solidification is accompanied by substantial changes in dynamics, including the spreading-retraction process, satellite drop generation, and cratering in the target. The solidification of the wetted pores in the granular target primarily causes these effects. The freezing process over the pore dimension occurs rapidly enough to match the characteristic timescales of impact dynamics at moderate undercooling degrees. As a result, the hydrophilic impact appears "hydrophobic," and the dimension of the solidified droplet shrinks. A monolayer of cold grains on a surface can reproduce these consequences. Our research presents a potential approach to regulate solidified morphology for subfreezing drop impacts. It additionally sheds light on the impact scenario of strong coupling between the dynamics and solidification.

Advantages of hybrid positron sources with granular converters

The interest in crystal effects and more precisely for channeling radiation in axially oriented crystals is underlined. An association of a photon radiator using a crystal followed by an amorphous converter may have advantages concerning heating considerations. Such a set of targets, known as a hybrid source has been already investigated and experimental results gathered at KEK are presented. As the heating load with the accompanying thermal stresses have to be studied, variants for the amorphous converter have been considered and also realized: these variants are using small spheres of high Z material, as tungsten: they are providing a better heat dissipation and lower thermo-mechanical effects, leading to better lifespan. These converters, granular, are described and compared to the bulk converters generally used. Their characteristics concerning the yield and the thermal load are analyzed. Experiments carried out at KEK with such converters provided results which are also related. Such granular targets can also be used in conventional sources.

Roles of packing fraction, microscopic friction, and projectile spin in cratering by impact.

From small seeds falling from trees to asteroids colliding with planets and moons, the impact of projectiles onto granular targets occurs in nature at different scales. In this paper, we investigate open questions in the mechanics of granular cratering, in particular, the forces acting on the projectile and the roles of granular packing, grain-grain friction, and projectile spin. For that, we carried out discrete element method computations of the impact of solid projectiles on a cohesionless granular medium, where we varied the projectile and grain properties (diameter, density, friction, and packing fraction) for different available energies (within relatively small values). We found that a denser region forms below the projectile, pushing it back and causing its rebound by the end of its motion, and that solid friction affects considerably the crater morphology. Besides, we show that the penetration length increases with the initial spin of the projectile, and that differences in initial packing fractions can engender the diversity of scaling laws found in the literature. Finally, we propose an ad hoc scaling that collapsed our data for the penetration length and can perhaps unify existing correlations. Our results provide new insights into the formation of craters in granular matter.

Pattern in ejecta curtain generated by the impact into granular targets of various sized particles and application to the ejecta curtain observed in the Hayabusa2 impact experiment

We conducted impact experiments using targets composed of particles with size distributions and projectiles with a size larger than or comparable with the maximum size of particles in targets. The pattern and particle concentration in the ejecta curtain were investigated. The results show three types of ejecta curtain features: (i) filament pattern extending throughout the entire curtain and high concentration, (ii) filament pattern and low concentration, and (iii) mesh-like pattern with a structure on smaller scales than the entire curtain and low concentration. When the target consists of particles using a bimodal size distribution with size differences of more than one order of magnitude, the filament pattern appears, exhibiting case (i). If the target consists of particles with various sizes with size differences of more than one order of magnitude, the filament pattern appears, but the concentration decreases, appearing the features of case (ii). Case (iii) occurs when the target consists of particles with a single size or when the mass of particles with a certain size is dominant. Thus, the size distribution of the particles in the targets determines the pattern and particle concentration in the ejecta curtain. Based on these results, we confirm that the pattern in the ejecta curtain caused by the impact of the Small Carry-on Impactor (SCI) in the Hayabusa2 mission showing case (i) is consistent with the evaluated sizes and masses of grains and boulders in the ejecta curtain.Graphical

Impact drag force exerted on a projectile penetrating into a hierarchical granular bed

Context. The impact of a solid object onto a small-body surface can be modeled by the solid impact onto a hierarchically structured granular target. Aims. We develop an impact drag force model for the hierarchically structured granular target based on the experiment. Methods. We performed a set of granular impact experiments in which the mechanical strength and porosity of target grains were systematically varied. Tiny glass beads (5 μm in diameter) were agglomerated to form porous grains of 2–4 mm in diameter. Then, the grains were sintered to control their strength. A polyethylene sphere (12.7 mm in diameter) was dropped onto a hierarchical granular target consisting of these porous grains. Motion of the penetrating sphere was captured by a high-speed camera and analyzed. Results. We find that the impact drag force produced by the hierarchically structured granular target can be modeled by the sum of inertial drag and depth-proportional drag. The depth-proportional drag in a hierarchical granular impact is much greater than that of the usual granular target consisting of rigid grains. The ratio between the grain strength and the impact dynamic pressure is a key dimensionless parameter for characterizing this extraordinary large depth-proportional drag. Conclusions. Grain fracturing plays an important role in the impact dynamics when the impact dynamic pressure is sufficiently larger than the grain strength. This implies that the effect of grain fracturing should be considered also for the impact on a small body. It may be that the effective strength of the surface grains can be estimated based on kinematic observations of the intrusion or touchdown of a planetary explorator.

Cratering Experiments on Granular Targets With a Variety of Particle Sizes: Implications for Craters on Rubble‐Pile Asteroids

AbstractWe performed cratering experiments on targets composed of glass beads with a power‐law size distribution that simulated the surface of rubble‐pile asteroids, and we improved the previously studied reduction factor on the crater size scaling relationship including the armoring effect using the momentum transfer efficiency. Cratering experiments were conducted using light gas guns at Kobe University and ISAS/JAXA to control the impact velocity, , from 50 to 4,400 m s−1. Two kinds of mixed targets were prepared by mixing glass beads with diameters of 0.1, 1, 3, and 10 mm—one with the smallest diameter beads, and one without. The size ratio of the target bead to the projectile (diameter of 1–3 mm), , changed from 0.03 to 10. The crater size scaling relationships for the mixed targets were found to depend on the first contacted bead size. Notably, first contact with a 10 mm‐sized bead reduced the crater radius by 35% in maximum. The reduction factor due to this armoring effect on the crater size scaling relationship is written as follows: ; it decreased with the increase of the size ratio of the target bead to the projectile, while it increased with the increase of the impact velocity and approached unity. Our improved crater size scaling relationship that includes the reduction factor could be used to reconstruct the crater size frequency distribution on rubble‐pile asteroids such as Ryugu, and it may lead to a revision of the crater chronology of asteroids.

Boulder exhumation and segregation by impacts on rubble-pile asteroids

Small asteroids are often considered to be rubble-pile objects, and such asteroids may be the most likely type of Near Earth Objects (NEOs) to pose a threat to Earth. However, impact cratering on such bodies is complex and not yet understood. We perform three low-velocity (≈ 400 m/s) impact experiments in granular targets with and without projectile-size boulders. We conducted SPH simulations that closely reproduced the impact experiments.Our results suggest that cratering on heterogeneous targets displaces and ejects boulders, rather than fragmenting them, unless directly hit. We also see indications that as long as the energy required to disrupt the boulder is small compared to the kinetic energy of the impact, the disruption of boulders directly hit by the projectile may have minimal effect on the crater size.The presence of boulders within the target causes ejecta curtains with higher ejection angles compared to homogeneous targets. At the same time, there is a segregation of the fine ejecta from the boulders, resulting in boulders landing at larger distances than the surrounding fine grained material. However, boulders located in the target near the maximum extent of the expanding excavation cavity are merely exhumed and distributed radially around the crater rim, forming ring patterns similar to the ones observed on asteroids Itokawa, Ryugu and Bennu. Altogether, on rubble-pile asteroids this process will redistribute boulders and finer-grained material heterogeneously, both areally around the crater and vertically in the regolith. In the context of a kinetic impactor on a rubble-pile asteroid and the DART mission, our results indicate that the presence of boulders will reduce the momentum transfer compared to a homogeneous, fine-grained target.

Cratering experiments on granular targets with a variety of particle sizes: Implications for craters on rubble-pile asteroids

Cratering experiments on granular targets with a variety of particle sizes: Implications for craters on rubble-pile asteroids

Thermal-Mechanical Analysis of Beam Window and Beam Tube for ADS Granular Flow Target

A granular flow target coupled with a beam window was studied. The beam window isolates the accelerator from the target, making the system more secure and flexible. Preliminary analyses for the beam window and beam tube, including neutronics, thermal hydraulics, and structural mechanics were performed by Geant4 and ANSYS. The effects of geometry and coolant flow direction on the temperature field and the stress distribution of the beam window are studied. The results show that the maximum temperature can be reduced by 13% through optimization. Comparing the thermal deposition distribution of the beam tube with and without the beam window, we find that there is an extra peak due to the beam window. In addition, the effect of the cooling pattern on the temperature distribution of the beam tube is also studied. The results show that it is reasonable to arrange six U-shaped cooling channels. Detailed analyses show that the material temperature and the mechanical property of the beam window and beam tube meet the design standards, which confirm the possibility of granular flow target with a beam window for engineering application.

Beam-target configurations and robustness performance of the tungsten granular flow spallation target for an Accelerator-Driven Sub-critical system

The dense granular flow spallation target is a new target concept proposed for an Accelerator-Driven Sub-critical (ADS) system. In this paper, the beam-target configurations of a tungsten granular flow target for the ADS with a thermal power of 1 GW is explored. The beam profile options using different scanning methods are discussed. The critical geometry parameters are adjusted to investigate the performance of the granular target from the aspects of neutron efficiency, stability and temperature distribution in target medium. To figure out how the target under accident conditions would behave, different clogging conditions are induced in the simulation. The dynamic processes are analyzed and some important parameters such as abnormal temperature rise and beam cutoff time window are obtained. The response of the sub-critical reactor to a clogging accident is also investigated. It is indicated that the monitoring of the granular flow by the neutron detectors in the sub-critical core will be effective.

Force and flow characteristics of an intruder immersed in 3D dense granular matter

The motion of a projectile impact onto a granular target results in both the resistance force exerted on the projectile and rheology of granular media. A horizontal arrangement of cylinder quasistatically and dynamically intruding into granular media under different velocities and angles is simulated using discrete element method. Three distinguished drag force regimes are exhibited, including hydrostatic-like force independent of velocity, viscous force related to velocity, and inertial drag force proportional to the square of velocity. Meanwhile, the influence of penetration angles on drag force is examined for these three regimes, and a force model, which is related to penetration depth and angle, is proposed for quasi-static penetration. Then, flow characteristics of the granular media, such as velocity field, pressure field, packing fraction etc., are traced, and a rheology model of packing fraction and inertial number is established.

Influence of grain size on dynamic characterizations of laser-driven grain ejection

When intense laser pulse irradiates a target surface, the energetic processes of generation and expansion of laser-induced plasma will affect a localized pressure impulse around the irradiation zone. As a result, pulsed laser ablating granular target can drive a physical phenomenon of grain ejection. In this work, taking dry glass beads with different grain sizes as an example of granular targets and using high-speed video camera, we experimentally investigate the grain-size dependent dynamics of grain ejection driven by nanosecond laser pulses. The measured video sequences clearly exhibit that the laser-driven grain ejection process can be separated into two regimes: early-stage fast ejecting process and later-stage slow ejecting process. We find that there exists an obvious grain size effect on the kinetic energy of grains in the early-stage ejecting process. In addition, the temporal evolution of transient ejection of a curtain diameter <inline-formula><tex-math id="M6">\begin{document}$ D\left(t\right) $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220243_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220243_M6.png"/></alternatives></inline-formula> corresponding to the later-stage ejecting process obeys the well-known “point source” law, <inline-formula><tex-math id="M7">\begin{document}$ {D\left(t\right)=\alpha t}^{\beta } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220243_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220243_M7.png"/></alternatives></inline-formula>, where both parameters <inline-formula><tex-math id="M8">\begin{document}$ \alpha $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220243_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220243_M8.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M9">\begin{document}$ \beta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220243_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220243_M9.png"/></alternatives></inline-formula> depend on grain size. The observations mentioned above can be reasonably explained by considering the grain size dependent efficiency of impulse coupling between grain and plasma flow and plasma features generated by interaction of laser pulse with granular targets. These experimental results improve the understanding of the mechanism of laser-driven grain ejection.

Modeling Early Clustering of Impact-induced Ejecta Particles Based on Laboratory and Numerical Experiments

Abstract A projectile impact onto a granular target produces an ejecta curtain with heterogeneous material distribution. Understanding how the heterogeneous pattern forms is potentially important for understanding how crater rays form. Previous studies predicted that the pattern formation is induced by inelastic collisions of ejecta particles in early stages of crater formation and terminated by the ejecta’s expanding motion. In this study, we test this prediction based on a hypervelocity impact experiment together with N-body simulations where the trajectories of inelastically colliding granular particles are calculated. Our laboratory experiment suggests that pattern formation is already completed on a timescale comparable to the geometrical expansion of the ejecta curtain, which is ∼10 μs in our experiment. Our simulations confirm the previous prediction that the heterogeneous pattern grows through initial inelastic collisions of particle clusters and subsequent geometric expansion with no further cluster collisions. Furthermore, to better understand the two-stage evolution of the mesh pattern, we construct a simple analytical model that assumes perfect coalescence of particle clusters upon collision. The model shows that the pattern formation is completed on the timescale of the system’s expansion independently of the initial conditions. The model also reproduces the final size of the clusters observed in our simulations as a function of the initial conditions. It is known that particles in the target are ejected at lower speeds with increased distance to the impact point. The difference in the ejection speed of the particles may result in the evolution of the mesh pattern into rays.

Hydrogel sphere impact cratering, spreading and bouncing on granular media

The impact of a hydrogel sphere onto a granular target results in both the deformation of the sphere and the formation of a prominent topographic feature known as an impact crater on the granular surface. We investigate the crater formation and scaling, together with the spreading diameter and post-impact dynamics of spheres by performing a series of experiments, varying the Young's modulus $Y$ and impact speed $U_{0}$ of the hydrogel spheres, and the packing fraction and grain size of the granular target. We determine how the crater diameter and depth depend on $Y$ and show the data to be consistent with those from earlier experiments using droplets and hard spheres. Most specifically, we find that the crater diameter data are consistent with a power law, where the power exponent changes more sharply when $Y$ becomes less than 200 Pa. Next, we introduce an estimate for the portion of the impact kinetic energy that is stored as elastic energy during impact, and thus correct the energy that remains available for crater formation. Subsequently, we determine the deformation of the hydrogel spheres and find that the normalized spreading diameter data are well collapsed introducing an equivalent velocity from an energy balance of the initial kinetic energy against surface and elastic energy. Finally, we observe that under certain intermediate values for the Young's modulus and impact velocities, the particles rebound from the impact crater. We determine the phase diagram and explain our findings from a comparison of the elastocapillary spreading time and the impact duration.

Analytical and numerical approach to convective heat transfer through a granular target with gas cooling

The paper presents an analytical and a numerical approach to studying the effectiveness of the cooling of a granular target by gaseous helium. The increase in the target temperature is caused by an impinging beam which deposits a very high power of 138kW inside the limited volume of the target. A porous domain approach is used to model the flow of helium through the packed-bed target. An efficient one-dimensional analytical model is proposed to describe the transverse flow of gas helium, which accounts for its compressibility and the heat exchange between the target spheres and the cooling gas. The predictions of this model are shown to be in good agreement with more complex numerical studies done with Fluent.

Impact cratering in sand: comparing solid and liquid intruders.

How does the impact of a deformable droplet on a granular bed differ from that caused by a solid impactor of similar size and density? Here, we experimentally study this question and focus on the effect of intruder deformability on the crater shape. For comparable impact energies, we show that the crater diameter is larger for droplets than for solid intruders but that the impact of the latter results in deeper craters. Interestingly, for initially dense beds of packing fractions larger than 0.58, we find that the resultant excavated crater volume is independent of the intruder deformability, suggesting an impactor-independent dissipation mechanism within the sand for these dense beds.

Targeting and Privacy in Mobile Advertising

Mobile in-app advertising is now the dominant form of digital advertising. Although these ads have excellent user-tracking properties, they have raised concerns among privacy advocates. This has resulted in an ongoing debate on the value of different types of targeting information, the incentives of ad networks to engage in behavioral targeting, and the role of regulation. To answer these questions, we propose a unified modeling framework that consists of two components—a machine learning framework for targeting and an analytical auction model for examining market outcomes under counterfactual targeting regimes. We apply our framework to large-scale data from the leading in-app ad network of an Asian country. We find that an efficient targeting policy based on our machine learning framework improves the average click-through rate by 66.80% over the current system. These gains mainly stem from behavioral information compared with contextual information. Theoretical and empirical counterfactuals show that although total surplus grows with more granular targeting, the ad network’s revenues are nonmonotonic; that is, the most efficient targeting does not maximize ad network revenues. Rather, it is maximized when the ad network does not allow advertisers to engage in behavioral targeting. Our results suggest that ad networks may have economic incentives to preserve users’ privacy without external regulation.

Measurement of tungsten granular target worth on VENUS-II light water reactor and validation of the granular target model

In this paper, a benchmark experiment of tungsten granular target worth has been performed on the VENUS-II light water zero-power facility of CIAE in Beijing. The reactivity worth of the tungsten granular target was measured and processed as (-1.007±0.096) mk by a period method. In MCNP simulations, three modeling methods of tungsten grains, i.e., the homogeneous, body center and face center models, were used to calculate the tungsten granular target reactivity worth. Through the comparison, the simulated tungsten granular target reactivity worth values with three modeling methods are in good agreement with the experimental result within errors and the maximum C/E and (C-E) deviations are respectively 3.7% and 0.037 mk. By the perturbation analysis in MCNP and the comparison on the cross sections of tungsten, iron and nickel, the negative reactivity worth of tungsten granular target is mainly caused by the tungsten radiation capture reaction channel. It is recommended to adopt the efficient homogeneous modeling method to simulate the tungsten granular target, which establishes a foundation for the systematic integration design of CIADS.