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In order to investigate the effect of the carrier's initial velocity on the jet's damage power, this paper conducted the numerical simulation of a jet penetrating thick‐walled targets at various following velocities (the carrier's initial velocity). For large stand‐off distance (D), the influence on jet formation parameters was revealed under different following velocities (ranging from 0 to 1000 m/s), and the jet's penetration performances were analyzed at various stand‐off distances (ranging from 3D to 10D). Then, taking 3D as an example, the study investigated the influence mechanism of the coupling between following velocity and impact angle (ranging from −60° to 60°) on jet penetration performance. The results show that an increase in the following velocity causes the jet to bend and break at an earlier time. The following velocity has a minimal effect on jet tip velocity. Additionally, the lateral displacement is linearly correlated with stand‐off distance. For normal penetration, the jet penetration depth decreases exponentially as the following velocity increases at the same stand‐off distance. Especially, the penetration depth decreases by over 80% when the following velocity exceeds 600 m/s, and a further increase in the following velocity has a minimal effect on jet penetration depth. A greater jet penetration depth was achieved for the climbing‐oblique penetration (COP) than the diving‐oblique penetration (DOP) due to an increase in dynamic stand‐off distance. Finally, the ratio of oblique to normal penetration depth for a jet with varying following velocities was derived based on the theory of steady‐jet and hole expansion, and its accuracy was verified. [ABSTRACT FROM AUTHOR]

In order to improve the research and development efficiency and quality of low-density liners in production and scientific research development, PLA and PLA-Cu composite liners were prepared based on 3D-printing technology. In this paper, the relationship between the shock wave velocity D and the particle velocity u of PLA and PLA-Cu materials was tested by a one-stage light gas gun experiment device, and then the Grüneisen equation of state parameters of the two materials was obtained by fitting. The forming process of the two jets was numerically simulated by using the equation of state. When combined with the pulsed X-ray shooting results of the jets, it was found that the jets of the two materials showed obvious characteristics of "expansion particle flow", and the head of the PLA jet had a gasification phenomenon. The length of the PLA jet at 20 μs in the numerical simulation was 127.2 mm, and the average length of the PLA jet at 20 μs in the pulsed X-ray shooting experiment was 100.45 mm. The length of the PLA jet gasification part accounted for about 21% of the total length of the jet. The average velocity of the head of the PLA jet is 7798.35 m/s, and the average velocity of the head of the PLA-Cu jet is 8104.25 m/s. In this paper, 3D-printing technology is used to prepare the liner for the first time, aiming to open up a new preparation technology and provide a new material selection for low-density material liners. [ABSTRACT FROM AUTHOR]

In this paper, the reaction characteristic and its application in shaped charge warhead of a novel reactive material, which introduced copper (Cu) and plumbum (Pb) into traditional polytetrafluoroethylene/aluminum (PTFE/Al), are studied. The thermal analysis and chemical reaction behavior of the PTFE/Al/Cu/Pb mixture are investigated by Differential Scanning Calorimetry (DSC),Thermo-gravimetry (TG), and X-ray Diffraction (XRD) techniques. Then, the shaped charge liners with PTFE/Al/Cu/Pb reactive materials are fabricated, and the X-ray experiments show that they could form reactive jets with excellent performance under the detonation effects of the shaped charge. Based on that, the penetration experiments of shaped charge with PTFE/Al/Cu/Pb reactive liner against steel plates are carried out, and the results demonstrate that the PTFE/Al/Cu/Pb reactive jets could produce a deeper penetration depth compared to the traditional PTFE/Al reactive jets. Meanwhile, the PTFE/Al/Cu/Pb reactive jets also show significant inner-blast effects, leading to dramatically cracking or fragmentation behavior of the penetrated steel plates. This new PTFE/Al/Cu/Pb reactive liner shaped charge presents enhanced penetration behavior for steel targets that incorporates the penetration capability of a high-density and ductility jet, and the chemical energy release of PTFE-matrix reactive materials.

In melt electrospinning, accurate control of jet evolution and Taylor cone process parameters helps to control the final fiber properties. High-speed photography was employed to observe the jet’s formation process and the Taylor cone’s morphology. A multi-physics model of nonisothermal heat transfer was used to predict the fluid flow direction and velocity change. In addition, we investigated the relationships between the process parameters of the Taylor cone and the diameter of melt electrospun fiber. The results show an excellent linear relationship between the process parameters (cone angle, curvature, and Taylor cone height) and fiber diameter. According to the simulation results, the axial fluid flow keeps the maximum and accelerates continuously until the collector captures it. In addition, with the increase of voltage, the fiber strength is lower, and the crystallinity is improved. This work analyzes diameter and process parameters and offers a fresh perspective on electrostatic-fluid interaction and a method to forecast fiber diameter. [ABSTRACT FROM AUTHOR]

In head waves, water jet impacts due to run up can occur as a result of the structural configuration of some floating structures, reducing workability. Wave attenuation near the floater may reduce the risk of water jet impacts. This paper presents a numerical study of the performance and attenuation mechanisms of various plate type fixed free surface breakwaters and their ability to prevent water jet impacts on adjacent structures. Simulations are performed in two dimensions with a numerical method based on the Navier–Stokes equations in the presence of a free surface. The breakwater models are evaluated in two irregular sea states in terms of wave transmission, reflection and energy dissipation and by their ability to reduce water jets impacts on adjacent structures. A 60 degree inclined plate is found to induce a large wave energy reduction, little wave transmission and reflection and to experience little wave loading while effectively reducing water jet impacts. [ABSTRACT FROM AUTHOR]

Mrk 501 is one of the most prominent TeV-emitting blazars and belongs to the class of high synchrotron peaked (HSP) blazars. The Doppler factors derived from the jet kinematics are much too low to provide sufficient beaming for the detected high-energy emission (the so-called Lorentz factor crisis). This BL Lac object is also a prime example of a misaligned AGN with an approximately 90 ∘ difference in orientation between the inner parsec-scale jet and the kpc-scale jet structure. We have performed a detailed analysis of the pc-scale jet kinematics, based on 23 years of VLBA observations (at 15 GHz) and find, in addition to robustly consolidating the already claimed stationary jet features and a hinted absence of component ejections, a significant drift of the outer nuclear jet. The two outermost jet features move with somewhat higher but still subluminal speeds. Albeit, they move orthogonally to the inner jet, which itself does not partake in the drifting motion. The effect of this intriguing kinematics is that the jet appears strongly curved at first (1995) but then appears to straighten out (2018). To our knowledge, this is the first time that the orthogonal swing of just the outer part of a nuclear jet has been observed. We discuss the possible physical nature of this turning maneuver. In addition, we report evidence for jet emission, which most likely originates in a spine–sheath structure. [ABSTRACT FROM AUTHOR]

The axially symmetric, swirl-free gas dynamics and interlinked motion of a cork stopper provoked by the opening of a champagne bottle are modelled rigorously and studied numerically. The experimental study by Liger-Belair et al. (Science Advances, 5(9), 2019) animated the present investigation. Inspection analysis justifies the inviscid treatment of the expanding jet of air enriched with dissolved carbonic acid gas initially pressurised in the bottle. Solving of the resulting Euler equations is facilitated by the open-source software Clawpack. Specific enhancements allow for resolving of the emerging supersonic pockets, associated with surprisingly complex shock structures, as well as the gas-stopper interaction with due accuracy. Our experimental effort provided modelling of the frictional behaviour, constitutive law and reversible (de-)compression of the cork material. Initially, the gas expands inside the bottleneck yet sealed by the stopper, and is hence accelerated by the gas but decelerated by dry sliding friction. Once the stopper has passed the bottle opening, the jet rapidly assumes locally supersonic speed, where a complex shock pattern is detected. Special attention is paid to the formation and dissolution of one or even two Mach discs between the opening and the released stopper. This simulated dynamics is found to be in fairly good agreement with recent experimental findings. It also provides a first insight into the generation of the typical popping sound. [ABSTRACT FROM AUTHOR]

In order to improve the research and development efficiency and quality of low-density liners in production and scientific research development, PLA and PLA-Cu composite liners were prepared based on 3D-printing technology. In this paper, the relationship between the shock wave velocity D and the particle velocity u of PLA and PLA-Cu materials was tested by a one-stage light gas gun experiment device, and then the Grüneisen equation of state parameters of the two materials was obtained by fitting. The forming process of the two jets was numerically simulated by using the equation of state. When combined with the pulsed X-ray shooting results of the jets, it was found that the jets of the two materials showed obvious characteristics of “expansion particle flow”, and the head of the PLA jet had a gasification phenomenon. The length of the PLA jet at 20 μs in the numerical simulation was 127.2 mm, and the average length of the PLA jet at 20 μs in the pulsed X-ray shooting experiment was 100.45 mm. The length of the PLA jet gasification part accounted for about 21% of the total length of the jet. The average velocity of the head of the PLA jet is 7798.35 m/s, and the average velocity of the head of the PLA-Cu jet is 8104.25 m/s. In this paper, 3D-printing technology is used to prepare the liner for the first time, aiming to open up a new preparation technology and provide a new material selection for low-density material liners.

The spreading and wetting behavior of metal droplets impacting substrate surface has an important influence on the quality of jet forming, additive manufacturing, and other products. Nonetheless, due to the high speed and complexity of the impact process, there are few studies on the impact behavior of metal droplet impacting metal substrate. In this paper, the dynamic wetting behavior of Sn droplet impinging on Cu substrate is studied by simulation and experiment. Numerical simulation uses CLSVOF method to establish a single droplet of tin impact on cold copper substrate (temperature below 230 ℃) model. The effects of substrate temperature (50–200 ℃), droplet impact velocity (1–6 m/s) and substrate average roughness (0.05 μm, 0.1 μm, 0.5 μm, 1 μm, and smooth surface) on droplet spreading behavior were investigated. The spreading and wetting behavior of the single droplet liquid impinging on a cold copper substrate was studied by a high-speed camera. The results show that the substrate temperature has a great influence on the droplet spreading process and the final spreading factor but has little influence on the final contact angle. The droplet impact velocity has a great influence on the droplet spreading process, the final spreading factor, and the final contact angle. The average surface roughness of the substrate has little effect on the whole spreading process, but the final contact angle decreases first and then increases with the increase of surface roughness. The results reflect the morphology change of the droplet spreading solidification process, and explain the dynamic wetting state of the droplet spreading more directly. [ABSTRACT FROM AUTHOR]

Mrk 501 is one of the most prominent TeV-emitting blazars and belongs to the class of high synchrotron peaked (HSP) blazars. The Doppler factors derived from the jet kinematics are much too low to provide sufficient beaming for the detected high-energy emission (the so-called Lorentz factor crisis). This BL Lac object is also a prime example of a misaligned AGN with an approximately 90∘ difference in orientation between the inner parsec-scale jet and the kpc-scale jet structure. We have performed a detailed analysis of the pc-scale jet kinematics, based on 23 years of VLBA observations (at 15 GHz) and find, in addition to robustly consolidating the already claimed stationary jet features and a hinted absence of component ejections, a significant drift of the outer nuclear jet. The two outermost jet features move with somewhat higher but still subluminal speeds. Albeit, they move orthogonally to the inner jet, which itself does not partake in the drifting motion. The effect of this intriguing kinematics is that the jet appears strongly curved at first (1995) but then appears to straighten out (2018). To our knowledge, this is the first time that the orthogonal swing of just the outer part of a nuclear jet has been observed. We discuss the possible physical nature of this turning maneuver. In addition, we report evidence for jet emission, which most likely originates in a spine–sheath structure.

The jet morphology of square and circular cross-section shaped charge with same inscribed circle diameter were observed by conducting an X-ray experiment. Depth-of-penetration tests at standoffs 80 and 160 mm were also carried out. The jet formation was studied by using AUTODYN software. The results showed that the jet of square cross-section shaped charge consists of a condensed part and a non-condensed part. The condensed part has similar characteristics to the corresponding part of circular cross-section shaped charge. And the non-condensed part is distributed at the jet tail. The non-condensed part consists of a main fluid moving along the axis and four off-axis fluids with off-axis velocity along the symmetry plane. The noncondensing of jet reduces the penetration depth of square cross-section shaped charge, the decreasing rate increased from 2.67% to 17.3% when the standoff is increased from 80 mm to 160 mm. When the inscribed circle diameter of the charge cross-section increases to 67.2 mm (1.2 times of 56 mm) and the liner structure remains the same, the square cross-section shaped charge will form a cohesive jet. [ABSTRACT FROM AUTHOR]

The article focuses on the development of the new calculation method of jet penetration into the target using ANSYS AUTODYN simulation. The jet formation and its penetration capability dependence on properties of targets are studied. Two kinds of steel armours - Armox 440T and S235JR - in standardized plates of thickness 20 mm are used as materials of the target. The paper contains an analysis of the penetration depth in the targets containing the pure material plates set, combined material plates set and sets of Armox 440T plates with gaps 10 mm and 20 mm among individual plates. The theoretical results of simulation are compared with the results of experiments. [ABSTRACT FROM AUTHOR]

The article presents the influence of the wave shaper position on the jet formation and penetration depth of a shaped charge, which uses the liner of the PG-7VM warhead by numerical method and experimental method. The results show that, when moving the plexiglass wave shaper position towards the liner apex, the jet tip velocity and the jet velocity gradient increase. In addition, the results of the study also pointed out the existence of the wave shaper position at which the penetration depth is greatest. They indicate a match between the simulation and experimental results. The differences between simulated and experimental penetration depth are less than 15%. The results of the study are the basis for the evaluation of the influence of the wave shaper position on the liner collapse and the reasonable use of the wave shaper in the structure of the shaped charge. [ABSTRACT FROM AUTHOR]

On the basis of the RCL circuit model and Pugh–Eichelberger–Rostoker (PER) theory of jet formation and combining with Maxwell equations, the calculation method for the electromagnetic force and the collapse velocity of the liner is obtained through the infinitesimal method. The theoretical model of jet formation of a liner driven by a strong magnetic force is established. The model can calculate the collapse process and jet formation of the liner, jet and tail velocity, jet length, diameter, and other important parameters at any time. Simultaneously, the jet and tail velocity parameters and the shape of the jet under different voltages are calculated using the theoretical model established in this research. To verify the correctness of the theoretical model, a liner has been calculated and compared with the experimental results. The relative error of the jet tip velocity is found to be 3.7%. The results show that the theoretical model established in this research can accurately calculate the jet forming process and the related parameters driven by strong electromagnetic energy. [ABSTRACT FROM AUTHOR]

Abstract This work aims to provide a better understanding of the cumulative effect of successive nanosecond repetitively pulsed (NRP) discharges on the ignition process. Fast chemiluminescence imaging of both the post-discharge and the following flame was used to analyze the ignition of a lean propane/air mixture (ϕ = 0.7) by a train of 82 NRP discharges at a pulse repetition frequency (PRF) of 30 kHz, in comparison with the traditional spark ignition. The transition from the non-equilibrium plasma to the ignition kernel has been imaged. It has been found that, differently from traditional spark ignition and despite similar experimental conditions, each NRP ignition develops in a unique way. NRP discharges generate both highly reactive species and thermal instabilities in the gap: the multi-pulse strategy produce a gas motion resulting in a jetting phenomenon, reported here for the first time. An algorithm within ImageJ software was used to quantitatively describe the observed jetting phenomenon. The results presented in this work suggest that jetting is the driver of the ignition initiated by NRP discharges, at least in the experimental conditions investigated here. [ABSTRACT FROM AUTHOR]

The aim of this paper is to investigate the effect of geometrical parameters on the performance of jet penetration in the process of shaped charge. To this end, the finite element analysis was used to simulate the process. The simulated process was validated by experimental tests and the effect of some parameters including stand-off distance and the liner thickness on the jet penetration depth was studied. The results indicated that choosing the optimal distance between the liner and the target (stand-off distance) can significantly affect the performance of jet penetration in the target. In addition, examining the effect of liner thickness on the penetration depth efficiency revealed that by decreasing the liner thickness, the jet penetration depth on the target increases. It should be noted that ABAQUS finite element software was used in this simulation to analyze the process of shaped charge. [ABSTRACT FROM AUTHOR]

This work investigates the application of an annular multi‐point initiation explosive network to a jet‐shaped charge. The Whitham method was employed to calculate the pressure and region of the Mach wave zone at different initiation diameters. LS‐DYNA software was used to investigate the effects of the control parameters for the annular multi‐point initiation (i. e., number of initiation points, initiation diameter and initiation synchronicity error) on the performance of the jet (i. e., formation and tip velocity). Results indicated that when there are eight or more initiation points, the jet formed under the multi‐point initiation is similar to the result for a peripherally initiated charge. As the initiation diameter increased, the range of the Mach wave zone decreased in size, but the pressure of the Mach wave and the length and the tip velocity of the jet increased, aiding the formation of the jet, increasing the jet penetration capability and improving the utilization rate of the shaped charge. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of the interaction between a planar incident shock wave with a cylindrical gas bubble. Simulations are performed using an inviscid compressible one-fluid solver based upon three conservation laws for the mixture variables, namely mass, momentum, and total energy along with a supplementary transport equation for the volume fraction of the gas phase. The study focuses on the maximum pressure generated by the bubble collapse. The influence of the strength of the incident shock is investigated. A law for the maximum pressure function of the Mach number of the incident shock is proposed. [ABSTRACT FROM AUTHOR]

The aim of this paper is simulation of shaped charge process using Eulerian analysis. To this end, the finite element analysis was used to simulate the process of shaped charge. In this simulation, whole of the model has been considered from Eulerian elements. Verification of finite element method has been confirmed by comparing simulation with experimental tests and Birkhoff model. Comparison of penetration depth in finite element analysis with experimental samples has shown that the results are in good agreement with each other. Also comparing parameters such as liner collapse velocity, velocity distribution in jet length and jets profile has indicated that the simulation results are close to Birkhoff model. It should be noted that ABAQUS finite element software is used in this simulation to analyze the process of shaped charge. [ABSTRACT FROM AUTHOR]

In analytical and numerical studies on bubbles in liquids, often the Rayleigh initial condition of a spherical bubble at maximum radius is used: the Rayleigh case. This condition cannot be realized in practice, instead the bubbles need first to be generated and expanded. The energy-deposit case with its initial condition of a small, spherical bubble of high internal pressure that expands into water at atmospheric pressure is studied for comparison with the Rayleigh case. From the many possible configurations, a single bubble near a flat solid boundary is chosen as this is a basic configuration to study erosion and cleaning phenomena. The bubble contains a small amount of non-condensable gas obeying an adiabatic law. The water is compressible according to the Tait equation. The Euler equations in axial symmetry are solved with the help of the open source software package OpenFOAM, based on the finite volume method. The volume of fluid method is used for interface capturing. Rayleigh bubbles of $$R_\mathrm{max} = 500\,\mu $$ m and energy-deposit bubbles that reach $$R_\mathrm{max} = 500\,\mu $$ m after expansion in an unbounded liquid are compared with respect to microjet velocity, microjet impact pressure and microjet impact times, when placed or being generated near a flat solid boundary. Velocity and pressure fields from the impact zone are given to demonstrate the sequence of phenomena from axial liquid microjet impact via annular gas-jet and annular liquid-nanojet formation to the Blake splash and the first torus-bubble splitting. Normalized distances $$D^{\ast } = D/R_\mathrm{max}$$ (D = initial distance of the bubble centre from the boundary) between 1.02 and 1.5 are studied. Rayleigh bubbles show a stronger collapse with about 50% higher microjet impact velocities and also significantly higher microjet impact pressures. [ABSTRACT FROM AUTHOR]

The centrifugal filtration process in a centrifuge with a conic filter surface has been analyzed under the assumption of a one-dimensional flow. The problem of calculating the process parameters, such as the speed, the filtrate production capacity, and the liquid pressure, has been solved. The solution is illustrated based on an example of calculations for these parameters and presented in the form of their plots. The obtained plots reflect the specific process features and confirm the validity of the one-dimensional problem formulation. [ABSTRACT FROM AUTHOR]

The traditional polytetrafluoroethylene (PTFE)/Al reactive material liner shaped charge generally produces insufficient penetration depth, although it enlarges the penetration hole diameter by chemical energy release inside the penetration crater. As such, a novel high-density reactive material liner based on the PTFE matrix was fabricated, and the corresponding penetration performance was investigated. Firstly, the PTFE/W/Cu/Pb high-density reactive material liner was fabricated via a cold pressing/sintering process. Then, jet formation and penetration behaviors at different standoffs were studied by pulse X-ray and static experiments, respectively. The X-ray results showed that the PTFE/W/Cu/Pb high-density reactive material liner forms an excellent reactive jet penetrator, and the static experimental results demonstrated that the penetration depth of this high-density reactive jet increased firstly and then decreased by increasing the standoff. When the standoff was 1.5 CD (charge diameter), the penetration depth of this reactive jet reached 2.82 CD, which was significantly higher than that of the traditional PTFE/Al reactive jet. Moreover, compared with the conventional metal copper jet penetrating steel plates, the entrance hole diameter caused by this high-density reactive jet improved 29.2% at the same standoff. Lastly, the chemical reaction characteristics of PTFE/W/Cu/Pb reactive materials were analyzed, and a semi-empirical penetration model of the high-density reactive jet was established based on the quasi-steady ideal incompressible fluid dynamics theory.

Shaped charges are used to pierce hard targets in all three versions of warfare land, air and naval. High explosives compositions fillings produce a thin high velocity metal jet which is used for target damage. Shaped charges can efficiently damage tanks possessing thick armour protection, bunkers and aircraft and are also useful for attacking ships and submarines. Shaped charges have a very long history since the Second World War Theoretical modeling started with the steady state theory of Birkhoff in 1948, which was modified by the non-steady state theory known as PER theory of shaped charges. To review the development in the shaped charges three stages are defined. In the first stage development until 1990 is presented when shaped charge theory was fully developed and penetration predictions with fairly good accuracy were possible. In the second stage, review of the work carried out in the last decade of the 20th century is discussed. During this period experimental verification of different parameters was established. The third stage deals with all the work carried out in the 21st century (2000-2010), including tools for advanced diagnostics, new fabrication and inspection, as well as new liner materials were included. The anomalies occurred were resolved by further refinements in the theoretical models. [ABSTRACT FROM AUTHOR]

In the framework of a model based on the gravitational field of the Kerr black hole, we turn to investigate the kinematic behavior of extragalactic jets. We analytically calculate the observable velocities and accelerations along any geodesic. Then, by numerical calculations, we apply our results to a geodesic, typical of the M87 jet, and probe our results by confrontation to recent observations. A transition from non-relativistic to ultrarelativistic speeds at subparsec scale is highlighted. This transition comes sooner and more abruptly than in models based on magnetic paradigm, which indicates that we need a weaker magnetic field to explain observed synchrotron radiation. We attribute the ejection phenomenon to the repulsive effect of the gravitomagnetic Kerr field. [ABSTRACT FROM AUTHOR]

• The jet formation of shaped charges with six different liner materials, i.e. copper, Cu-Zn alloy, steel1006, high nitrogen steel, and two titanium alloys were examined using both experimental and numerical method. • Under the same charge structure, the cohesiveness of the six material jets is different. Among them, copper, Cu-Zn alloy and steel1006 formed cohesive jets, while the other three materials formed non-cohesive jets. The straightness and dispersion of the jet have a significant impact on its penetration ability, and the jet with too large dispersion can hardly cause significant damage to the steel target. • Under the Euler algorithm and the Johnson-Cook model, the shape of the jet head obtained from the simulation can predict the discreteness of the actual jet. Arrowheads or oval heads usually represent cohesive jets, while blunt heads indicate a certain discreteness of jets. The degree of blunting of the jet head may also characterize the degree of dispersion. • Compared with Shock EOS, copper and Cu-Zn alloy are suitable for adopting Linear EOS to ensure the calculation accuracy of speed in simulation. Shock EOS can guarantee the calculation accuracy of steel1006, high nitrogen steel, TC21 and TB6 jets. Both Shock and Linear EOS can guarantee the calculation accuracy such as jet length, while there is a large error in diameter, especially for scattered jets. • Materials with higher strength, stronger strain effects, and lower shear modulus are more prone to form scatter jets. The threshold related to these three material parameters is proposed as a reference for predicting the discreteness of the jet, which provides convenience for judging the cohesion of the jet when the collapse angle is difficult to obtain under the traditional sound speed criterion. • The viewpoint of predicting the jet cohesion by using the macroscopic parameters of the material is put forward, which provides a preliminary reference for the design of shaped charge and the selection of the material of the liner. The Zr-based amorphous alloy material verifies the relevant conclusions, which also provides a reference for its application as a shaped charge liner. A comparative study of the shaped jets of six liner materials, namely, copper, Cu–Zn alloy, Steel1006, high nitrogen steel, and two titanium alloys, was conducted to investigate the influence of the macroscopic mechanical properties of the materials on the jet's performance. The X-ray experiment of the jet was performed, and the numerical calculation of the jet formation was carried out by means of simulation based on the Johnson–Cook model. Copper, Cu–Zn alloy, and Steel1006 formed cohesive jets, whereas the jets of the three other materials had different degrees of radial dispersion. The penetration depth of the jet has a significant negative correlation with its radial dispersion degree. In the numerical calculation, the head shape of the jet under the Euler algorithm can be used to judge its cohesion. Arrowheads or rounded oval heads symbolize cohesive jets, whereas blunt or even mouth-forward "bowl-shaped" heads indicate jets with radial dispersion. The degree of dispersion is positively correlated with the degree of dullness of the jet head. Simulation results show that the influence of material melting temperature and thermal softening effect on jet velocity, length, and diameter is particularly evident, whereas the influence ability of yield strength and strain rate effect is always at a very low level. Materials with small shear modulus, high yield strength, and significant strain effects are more prone to forming non-cohesive jets, which was verified by the jet forming experiments of amorphous alloy with these material characteristics. [ABSTRACT FROM AUTHOR]

In this paper a novel laser impact spot welding (LISW) method is described, in which a hump was formed on the flyer plate on the intended welding spot location by local pre-forming. When the flyer and base plates were placed together to perform welding, the two plates kept in contact over their entire surfaces except at the hump, where a local air gap was enough to guarantee the impact velocity and collision angle to achieve spot welding using laser pulse energy. The presented approach was implemented to join thin titanium foils to copper foils under low laser energy system. Joints with regular shapes were obtained. The microstructure in the weld interface was studied with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was found that the jetting occurred at the central region of the weld spots due to oblique impact. Wave features were observed in the weld interfaces. The impact energy was found to have significant influence on the wave's characteristics. Moreover, SEM images and EDS analysis did not show apparent element diffusion across the weld interface. Besides, the lap shearing test was used to characterize the mechanical properties of the spot welded joints. [ABSTRACT FROM AUTHOR]

Reactive material lining is an efficient damage enhancement technology that incorporates the defeat mechanisms of kinetic energy and chemical energy. The liners are fabricated by cold isostatically pressing at a pressure of 250 MPa. In this paper, the formation behaviors of jet with the polymer-based reactive material liner are investigated by numerical simulation and X-ray photographs. The corresponding simulations of jet formation are presented, and the results agree well with experimental ones. They show that the reactive material liner can shape almost continuous and straight jet. Compared with the conventional copper liner, the reactive material liner can shape jet in a shorter time, but the jet break easily and lose stability due to its poor ductility. Although the penetration depth is sacrificed slightly when penetrating steel target, the reactive material liner produce a significantly enlarged hole-diameter. [ABSTRACT FROM AUTHOR]

We present here the general expressions for the acceleration of massive test particles along the symmetry axis of the Kerr metric, and then study the main properties of this acceleration in different regions of the spacetime. In particular, we show that there exists a region near the black hole in which the gravitational field is repulsive. We provide possible physical interpretations about the role of this effect in terms of the different conserved parameters. The studies of these geodesics are important not only to understand better the structure of the Kerr spacetime but also to its use as a possible mechanism for the production of extragalactic jets. Our results are obtained with the help of expressing the geodesics of the Kerr spacetime in terms of the Weyl coordinates. [ABSTRACT FROM AUTHOR]

Recently, Rimasauskiene et al. ( [1] Mech. Syst. Signal Process. (2015) 50–51) presented an experimental study focusing on synthetic jets. Their paper presented a parameter map based on the Reynolds number. However, the map was shown without an adequate explanation, without definition of the horizontal axis, and without cross-connections with previous literature. This commentary attempts (A) to elucidate the unnecessary misunderstanding for readers and (B) to reveal that the parameter map, based on the Reynolds and Stokes numbers, can be a useful tool in the study of synthetic jets. [ABSTRACT FROM AUTHOR]

The process of formation, flying, penetration of explosively-formed projectiles (EFP) and the effect of water on performance of the charge for underwater applications is simulated by Ansysis Autodyn 2D-Hydro code. The main objective of an explosively formed projectile designed for underwater applications is to disintegrate the target at longer standoff distances. In this paper we have simulated the explosively formed projectile from OFHC-Copper liner for 1200 conical angle. The Affect of water on the penetration of EFP is determined by simulations from Ansysis Autodyn 2-D Hydrocode and by varying depth of water from 1CD-5CD. The depth of penetration against steel target is measured experimentally. Flash X-Ray Radiography (FXR) is used to capture EFP jet formation and its penetration against target is measured by depth of penetration experiments. Simulation results are compared with experimental results. The difference in simulated and experimental results for depth of penetration is about 7 mm, which lies within favorable range of error. The jet formation captured from FXR is quite clear and jet velocity determined from Flash X-ray radiography is the same as the ones obtained by using other high explosives. Therefore, it is indicated that Insensitive Munition (8701) can be utilized instead of Polymer Bonded Explosives (PBX) for air and underwater environments with great reliability and without any hazard. [ABSTRACT FROM AUTHOR]

Jet flows have been applied in numerous fields to control flow separation. Over the last decade, several studies on the production of synthetic jets have been performed. However, little information is available about a number of aspects concerning synthetic jets, including details of the structure and the formation mechanism of such jets. The present study attempts to clarify some of the fundamental flow characteristics of free synthetic jets on the basis of experiments and numerical simulations. Experimental velocity measurements and flow visualizations are performed using the hot-wire anemometer and the smoke wire method, respectively. It is found that both the temporal change in the flow pattern and the time-averaged velocity distribution at the centerline depend on K = ReU/S2 (the ratio of the Reynolds number to the square of the Stokes number). The unsteady downstream flow characteristics are discussed in addition to the relation between the formative point of the synthetic jet and the value of K. Furthermore, the flow pattern and the unsteady flow characteristics of the synthetic jet are compared with those of a continuous jet.

The dynamics of detonation initiated by the shock-induced collapse of a gas-filled ellipsoidal cavity embedded in a condensed-phase explosive is examined computationally. Attention is focused on the details of the evolutionary process following the collapse. The reaction rate is assumed to be pressure-dependent, switching on when the pressure exceeds an ignition threshold. The strength of the incident shock is taken to be such that the reaction would not be initiated without the interaction of the shock with the cavity. The system is modeled as a multi-material mixture, and a high-resolution, Godunov-type scheme is employed to solve the governing equations numerically. The computations are carried out in parallel, and adaptive mesh refinement is used to obtain accurate and well-resolved solutions. It is found that collapse of the cavity produces a detonation provided that the cavity is large enough, or the rate of reaction strong enough; otherwise any reaction initiated by the collapse fizzles out. Details of how the detonation is established are found to depend strongly upon the shape of the cavity. [ABSTRACT FROM AUTHOR]

In this paper, the flow patterns of circular synthetic jets issuing into a quiescent flow at low Reynolds numbers are studied numerically. The results confirm the presence of the three jet flow regimes, i.e. no jet formation, jet flow without rollup and jet flow with rollup reported in the literature. The boundaries of the different jet flow regimes are determined by tracking the structures produced by the synthetic jets in the near field of the jet orifice over several actuation cycles and examining the cycle-averaged streamwise velocity profiles along the jet central axis. When the Stokes number is above a certain threshold value appropriate for the corresponding flow regime, a good correlation between the flow patterns and the jet Reynolds number defined using the jet orifice diameter, Re Do , is also found. Furthermore, the flow structures of synthetic jets with different suction duty cycle factors are compared. The use of a high suction duty cycle factor strengthens the synthetic jet resulting in a greater penetration depth into the surrounding fluid. Overall, the finding from this study enables the flow regimes, in which a synthetic jet actuator with a circular orifice operates, to be determined. It also provides a way of designing more effective synthetic jet actuators for enhancing mass and momentum transfer at very low Reynolds numbers. [ABSTRACT FROM AUTHOR]

Nonlinear shallow water equations and the method of matched asymptotic expansions are used to solve the problem of the impact of a box-type body with a flat bottom on a thin elastic liquid layer at a small angle in the plane formulation. It is established that, at certain values of the input parameters of the problem, the liquid pressure near the body edges becomes less than atmospheric pressure, and the liquid separates from the bottom of the box. Calculations demonstrating the influence of elastic bottom and liquid separation on the body motion are performed. It is shown that the presence of an elastic bottom significantly changes the hydrodynamic pressure distribution and can cause loads higher than in the case of a rigid body. [ABSTRACT FROM AUTHOR]

Shaped charges are very convenient in the formation of very intensive local forces required for penetration and demolition of high strength targets, important in military area. It is generally used to penetrate armored vehicles such as tanks and to demolish concrete fortifications, fuel vessels and bridge constructions, in mining and geophysics, e.g., petroleum research for hard rock penetration and cutting, also in welding and demolition works in industry. In this study, effects of wall thickness variation of the conical copper (CU-OFHC) liner in model group T1 and T2 are investigated. Wall thickness variation is taken into account from the center of the apex to the circular base edge in model group T1 and from the circular base edge to the center of apex in model group T2. In this sense, the relation between the variable thickness, and the geometry and pressure of the jet is investigated. Obtained results are evaluated to determine the stand-off distance between the shaped charge and target which is most important in jet performance. Thickness variation along the liner length changes the liner wall cross-sectional area and increases the liner total mass at both the model groups. Hence, the similar behavior is expected in the two model groups. Hence, the results do not show such a similar behavior between the two model groups. Consequently, It can be said that the dominant parameter is geometrical change, not increase in total mass. This also means that the mass distribution in the liner is determinative parameter in jet formation. [ABSTRACT FROM AUTHOR]

The problem of the impact of a body with a plane bottom (of the type of a box) on a thin liquid layer at a small angle is solved in the two-dimensional formulation. The nonlinear shallow water equations are used, together with the method of matched asymptotic expansions. It is found that at certain values of the input parameters of the problem the liquid pressure diminishes near the lower end of the body and becomes smaller than the atmospheric pressure, which results in liquid separation from the box bottom. The numerical results show that all input parameters of the problem have a considerable effect on the nature of body motion. The liquid separation effect on body motion is analyzed. [ABSTRACT FROM AUTHOR]

Abstract: A computational study based on a front tracking method was conducted by solving an unsteady, axisymmetric Navier–Stokes equation to investigate the mixing efficiency between two colliding droplets subsequent to their merging. Results show that, for droplets of sufficiently dissimilar sizes, the smaller droplet would transform into a mushroom-shaped jet that penetrates into the interior of the larger droplet, and hence greatly facilitates mixing. The jet acquires its energy from the surface energy of the colliding interfaces that are eliminated upon merging. For droplets of similar sizes, jet formation is not favored due to symmetry, and the eliminated surface energy is rapidly dissipated through viscous action of the droplet internal motion. The results qualitatively agree well with previous experimental observations. [Copyright &y& Elsevier]

In the work presented in this article, the behaviour of circular synthetic jets issuing into quiescent surrounding fluid at low Reynolds numbers is experimentally studied for potential mixing applications of synthetic jets at micro-scales or in highly viscous fluids. Sugar solutions and silicone oil are used as the flow media in order to achieve the required low Reynolds numbers. The conditions for jet instability, vortex rollup and synthetic jet formation are investigated using both flow visualisation techniques and particle image velocimetry, and the typical behaviour of synthetic jets at a Reynolds number around unity is also illustrated. The roles of Reynolds number, dimensionless stroke length and Stokes number in determining the characteristics of synthetic jets are examined and found to be largely consistent with the finding obtained at higher Reynolds numbers. Finally, a parameter map of synthetic jet flow patterns is produced based on the results from this study, which can be used to aid the choice of synthetic jet operating conditions for specific applications or anticipate if a desired vortex structure can be obtained at a given synthetic jet operating condition. [ABSTRACT FROM AUTHOR]

A comprehensive numerical simulation has been performed to inter-relate the fluid mechanics of the formation of co-axial jets and their development downstream of the plane of jet emergence. The fluid flow was modeled as being turbulent, with corresponding Reynolds numbers in the jet-formation section of 10000, 20000, and 50000. The model was based on a fully developed pipe flow encountering a double-pipe arrangement and splitting between the two pipes. Subsequent to flow development in the central pipe and in its enveloping annulus, the flow exited into a large free space and became a co-axial free jet. The simulations were performed for geometric parameters which varied the relative cross sections of the central pipe and of the annulus and also varied the length of the jet-formation section. The overall pressure drop responsible for both the jet formation and for the subsequent free-jet development was found to be due to friction in contradistinction to inertial losses. At the jet exit, the velocity profile is discontinuous because of the intrusive presence of the walls of the pipes which bound the jet-formation section. These intrusions cause a double-humped velocity profile which disappeared with decreasing downstream distance from the jet origin. At sufficient downstream distances, the width of the jet was found to be independent of both the geometrical parameters and the Reynolds number. [ABSTRACT FROM AUTHOR]

Results of numerical and experimental modeling of jet formation during a high-velocity oblique impact of metal plates accelerated by an explosion are presented. Numerical simulations are performed by the method of molecular dynamics. The impact of metal plates is directly studied in experiments with flash radiography of phenomena ahead of and behind the impact point. A metallographic study of the samples is performed. It is demonstrated that numerical simulations by the method of molecular dynamics ensure a qualitatively and quantitatively adequate description of jet formation and evolution. [ABSTRACT FROM AUTHOR]

The flow field resulting from the interaction between a planar incident shock in a solid and an embedded ellipsoidal gas cavity is examined computationally. The study is motivated by the need for improved understanding of the role of embedded cavities in the initiation of reaction in a heterogeneous explosive following the application of a shock. The system is modeled as a compressible multi-fluid flow with a sufficiently strong shock in the solid. A high-resolution, Godunov-type capturing scheme is employed to solve the governing equations numerically. The calculations are performed in parallel and use adaptive mesh refinement to obtain well-resolved solutions. The goal is to identify regions in which the shock-cavity interaction results in pressures that are substantially higher than the post-shock pressure that existed prior to the beginning of the interaction. Also of interest are the ways in which the magnitude of the elevated pressure, the extent and location of the regions where it develops, and the mechanisms that underlie such a development are influenced by the strength of the shock and the geometry of the cavity. [ABSTRACT FROM AUTHOR]