Jet Formation Process Research Articles

Effects of Temperature on Melt Electrospinning: Experiment and Simulation Study

In this study, the jet formation progress, jet motion, resultant fiber diameter, fiber mat morphology, inner structures and mechanical properties of the fibers prepared at different heating temperatures via melt electrospinning system were studied, in a comprehensive and systematic manner. The temperature distribution of the melt electrospinning configuration was simulated, in order to provide a good deal of insight into the experimental results. High-speed photography was adopted to capture the images of jet formation process and jet motion during melt electrospinning. The experimental results showed that higher heating temperature at the spinneret results in shorter jet formation time, smaller fiber diameter, more disordered fiber mat, lower degree of crystallinity and strength. The simulation works was carried out, aimed at facilitating to deeply understand the experimental results.

Numerical Study on the Electrohydrodynamic Jet Printing

AbstractElectrohydrodynamic (EHD) printing is an alternative method to fabricate high-resolution micro- and nanostructures with high efficiency, low cost, and low pollution. Numerical simulation is an effective approach to systematically investigate the formation process of EHD jet. However, there are a few articles performing this work. In this study, a finite element model was established. The jet formation process and jetting modes were analyzed. The influence of applied voltage and printing distance on the maximum electric field near the nozzle tip was investigated. The effect of flow rate on the jet diameters was studied. Comparison between numerical and experimental results demonstrated that the proposed simulation model had a high potential for EHD jet analysis. According to the optimized printing conditions (printing distance of 200–300 μm, applied voltage of ∼1100 V, and flow rate of 0.1–0.3 ml/h), stable EHD jet can generate and polyvinyl pyrrolidone (PVP) lines with minimum line-width of 0.9 μm can be printed onto the glass slide.

КУМУЛЯТИВНЫЕ ЗАРЯДЫ С ТОНКИМИ И СВЕРХТОНКИМИ ОБЛИЦОВКАМИ

The aim of the research is to study the formation of cumulative jets in charges with cumulative facings with a wall thickness of the order of the thickness of the jet-forming layer in classical charges. Based on mathematical modeling and experiments, it is shown that in such charges, the detonation products of explosives can play the role of an additional body that affects the axial velocity of the lin-ing throwing and leads to a collapse angle of more than 180 degrees. In this process, the mass of the jet is greater than the mass of the pestle. For the first time, corrugations were experimentally detected on the surface of the lining during its explosive compression. Corrugations may occur on the surface of the lining, leading to instability of the cumulative jet formation process. As a result of the study, it was found that the minimum wall thickness of the cladding is mainly determined by the instability of its surface (the appearance of corrugations on its surface).

Droplet impacting a superhydrophobic mesh array: Effect of liquid properties

For numerous applications in microfluidic engineering, material science, and drug production, droplets of desired size and distribution are generated through liquid impingement on a perforated substrate. The dynamic process of droplet impact, spread, retraction, penetration, jet formation, and fragmentation into satellite droplets is simulated in a droplet impacting a mesh array configuration using an advanced lattice-Boltzmann method. The Weber number and Ohnesorge number are found to significantly influence the dynamics and outcomes, which can be explained by a qualitative and quantitative analysis based on the force and energy balance.

Kinematics of the shaped charge jet formation process

Based on the review of scientific and technical literature, an analysis is made of the problems of the modern theory of formation, extension and destruction of the shaped jet. The focus of the research is on the kinematics of the formation of a shaped jet based on the approach of operating with the characteristics of a real metal when the velocity vector of the cumulative jet is the result of three vectors: strain, strain and displacement of the shaped jet’s material. The numerical experiment of the functioning of the shaped charge is carried out, and the procedure for evaluating the calculation results is proposed. The obtained data correlate with existing knowledge about the physics of the cumulation process.

Study of Overlapping Adjacent Jets for Effective Laser-Induced Forward Transfer Printing

Abstract Laser-induced forward transfer (LIFT) is a well-established, versatile additive manufacturing technology for orifice-free printing of highly viscous solutions and suspensions. In order to improve the efficiency of point-wise LIFT printing, an optical scanner is integrated into the laser printing system to enable the formation of overlapping adjacent jets used for deposition. The objective of this study is to evaluate the ejection behavior and deposition performance under such conditions during LIFT printing for further improvement. The effects of the overlap of adjacent jets are investigated in terms of jet formation and material deposition processes, capturing the jet tilting phenomenon caused by the perturbance induced by previously formed jet(s). The feasibility of optical scanner-assisted LIFT printing of viscous metal-based ink suspension has been successfully demonstrated during conductive line printing with induced overlapping jets. Investigation of various overlap ratios of adjacent jets found that a 30% jet overlap and a 133 µs time interval between laser pulses are optimal, in terms of deposition quality and ejection stability, even when a tilted jet ejection is present for the laser and material system in this study. Furthermore, multilayer polygonal and interdigitated structures are successfully deposited under these identified printing conditions. With the inclusion of an optical scanner, LIFT printing efficiency for viscous inks can be improved as the usage of higher laser frequencies is enabled, providing a faster orifice-free laser printing methodology.

Study on the influencing factors of underwater unexploded ordnance destruction based on shaped charged jet

In order to study the influence of the stand-off distance and the waterproofness on the penetration effect of liner shaped charge (LSC) jet to destroy underwater unexploded ordnance, the LS-DYNA finite element program was applied to simulate the process of the metal jet formation and penetration into steel plate under different stand-off distances and waterproof condition. On the basis, the model experiment was carried out. And combing the experimental and numerical result, it can be found that, if no waterproof measures is adopted the metal jet will directly get contact with the water, the unexploded ordnance will not be successfully destructed, moreover, with the increase of the stand-off distance, the shaped charge jet works more ineffectively.

Numerical investigation of synthetic jets driven by thermoacoustic standing waves

We have carried out a preliminary study of the physical processes leading to the formation of a jet into external quiescent surroundings driven by thermoacoustic standing waves. The standing waves are initiated in a thermoacoustic engine (TAE) utilizing the thermoacoustic effect, and the synthetic jet is produced via a jet ejector where a sudden change in the cross-section is employed. We investigate the characteristics of the proposed synthetic jet actuator using both reduced-order network model and computational fluid dynamics (CFD) simulations. The network technique, which is based on linear thermoacoustic theory, can predict the onset of thermoacoustic instability in the frequency domain. The CFD code solves the fully coupled nonlinear compressible flow equations and enables the time-domain analysis of complex flow patterns, which facilitates comprehension of the jet formation process. Both theoretical analysis and numerical simulations reveal that spontaneous, self-excited oscillations inside the TAE will happen when the temperature ratio is greater than the onset temperature ratio for thermoacoustic instability. CFD simulations further identified the transition from no jet to a clear synthetic jet, which determines the onset temperature ratio for jet formation and the threshold value of a non-dimensional parameter. Finally, we carried out a parametric study to investigate the influence of resonator length and orifice diameter on the onset characteristics as well as other performance parameters including the acoustic intensity, space-averaged mean momentum flux, space-averaged velocity and the jet effectiveness. The proposed thermoacoustically-driven synthetic jet actuator may outperform conventional actuators driven by a piston, loudspeaker or piezoelectric transducers on occasions where the surface temperature is high, and therefore has the potential to be utilized for self-cooling purposes.

Instability of a Conical Liner during Shaped-Charge Jet Formation

Instability of a conical liner during shaped-charge jet formation has been studied. To establish the metal flow pattern during shaped-charge jet formation, we fabricated three-layer copper-copper-Constantan liners consisting of a solid copper conical cone with a cone angle of 45°, a wall thickness 1.5 mm, and a cone pressed into it and rolled from a copper sheet 1.0 mm thick clad on the inside with Constantan 0.2 mm thick by explosive welding. Liner compression was carried out by an RDX charge 20 mm thick. The jet formation process was recorded by flash radiography, and the metal flow pattern was determined by micro sections of recovered slugs. Since the interface between the explosively welded metals is clearly distinguishable in the sections, the residual deformation in different sections of the slug shows the occurrence of liner instability during the compression, which manifested itself as folds oriented along the generatrix of the cone. At the same time, the general flow pattern described in terms of the theory of an ideal incompressible fluid is not disturbed, which follows from radiographs of the shaped-charge jet formation process.

Destructive mechanisms in laser induced forward transfer

Laser Induced Forward Transfer (LIFT) is an additive direct-writing technique, in which a piece of material (ink) is transferred from a donor to a receiver surface, utilizing a laser impulse. In practice, the process of jet formation can suffer from irreproducibility. We identify two possible destructive mechanisms due to multiple optical breakdowns (originating from imperfections of the optical system) and rarefaction waves (originating from impurities), both with harmful consequences caused by cavitation. Based on experiments in a model system that allows for visualization and numerical simulations employing the boundary integral method, we reveal the underlying fluid dynamics of both mechanisms. Finally, to overcome the irreproducibility, we provide recommendations for the industrial use of LIFT.

SPH-BEM simulation of underwater explosion and bubble dynamics near rigid wall

A process of underwater explosion of a charge near a rigid wall includes three main stages: charge detonation, bubble pulsation and jet formation. A smoothed particle hydrodynamics (SPH) method has natural advantages in solving problems with large deformations and is suitable for simulation of processes of charge detonation and jet formation. On the other hand, a boundary element method (BEM) is highly efficient for modelling of the bubble pulsation process. In this paper, a hybrid algorithm, fully utilizing advantages of both SPH and BEM, was applied to simulate the entire process of free and near-field underwater explosions. First, a numerical model of the free-field underwater explosion was developed, and the entire explosion process–from the charge detonation to the jet formation–was analysed. Second, the obtained numerical results were compared with the original experimental data in order to verify the validity of the presented method. Third, a SPH model of underwater explosion for a column charge near a rigid wall was developed to simulate the detonation process. The results for propagation of a shock wave are in good accordance with the physical observations. After that, the SPH results were employed as initial conditions for the BEM to simulate the bubble pulsation. The obtained numerical results show that the bubble expanded at first and then shrunk due to a differences of pressure levels inside and outside it. Here, a good agreement between the numerical and experimental results for the shapes, the maximum radius and the movement of the bubble proved the effectiveness of the developed numerical model. Finally, the BEM results for a stage when an initial jet was formed were used as initial conditions for the SPH method to simulate the process of jet formation and its impact on the rigid wall. The numerical results agreed well with the experimental data, verifying the feasibility and suitability of the hybrid algorithm. Besides, the results show that, due to the effect of the Bjerknes force, a jet with a high speed was formed that may cause local damage to underwater structures.

Numerical study on the jet formation of simple-geometry heavy gas inhomogeneities

The jet formation of gas inhomogeneities under the impact of a planar shock wave is an interesting phenomenon that is closely related to shock convergence. In this study, a series of heavy gas inhomogeneities with very simple geometries (square, rectangle, circle, and triangle) are numerically reproduced to trace the source of the jet formation. Special attention is given to the wave patterns that lead to the formation of specific jet forms. The shock-accelerated multicomponent flow is simulated by solving inviscid compressible Euler equations. An up-wind characteristic space-time conservation element and solution element scheme is adopted, and a five-equation model is used to treat the gas interface. The jet types that emerge in the experimental images are explained based on the numerical results, and a typical shock pattern that ensures the jet formation is uncovered. It is found that, physically, the jet is initiated by the impact of the internal Mach stem, and the jet growth is nourished by the high speed gas flow induced by this Mach stem. The width of the jet is determined by the height of the internal Mach stem. Geometrically, a focal wedge enveloped by slip lines emerges in the gas inhomogeneity, in which the gas is accelerated. It is found that the existence of the focal wedge and the coordinates of the wedge tip can be used as qualitative criteria to illustrate the mechanism of the jet formation. These criteria provide a more intuitive basis for understanding the direction, scale, and process of jet formation.

Splashing conditions for a liquid metal in vacuum arcs: Cyclic processes in a cathode spot

Molten metal extrusion from craters that form on the cathode during vacuum arc burning is considered. Using the hydrodynamic similarity principle, this process is compared with the well-studied splashing process that can develop within the impact of drops impinging one by one on a solid surface (the arc cycle duration is identified with the inverse frequency of the drop train). Based on this analogy, the conditions under which the regime of spreading of a liquid over the cathode surface will change into the splashing regime (accompanied by the formation of microjets and droplets) are analyzed. As it turns out, the conditions realized in vacuum arc cathode spot at near-threshold currents are close to the splashing threshold of liquid metal. This gives grounds to relate the existence of a threshold arc current to the existence of a threshold for the process of liquid metal jet formation.

Влияние микроструктуры металла на процесс струеобразования

Based on a review of the scientific and technical literature, an analysis is made of the influence of the material’s microstructure of the cumulative lining on the breakdown ability of cumulative charges: depending on the average diameter, strength of the boundaries, uniformity of shape, size and orientation of the grains of the metal. A model of the ideal metal structure of the cumulative cladding is advanced and substantiated. An assessment of the current level of development of technologies for the production of cumulative linings is carried out, promising technologies are considered, the possibility of creating cumulative linings with the required microstructure is evaluated. Based on the assumption of the indivisibility and integrity of the metal’s grain, based on the postulates of the mesomechanical phenomenon of grain-boundary sliding, the idea is put forward that it is possible to conduct numerical experiments on the formation of cumulative jets with a different set step of the computational grid. A numerical calculation of the functioning of the cumulative charges of a conical and complex conical-annular shape of the lining for the side of the grain cell of 500, 250 and 125 μm is carried out. The efficiency of the jet formation process is visually assessed depending on the grain size according to the parameters distinguished: the directions of the velocity vectors of the material grain cells, and also the parameter γ is introduced – the angle between the reduced velocity vector of the head of the jet and the axis of symmetry of the jet formation. The results correlate with existing knowledge about the physics of the cumulation process: as the size of the side of the cell-grain decreases, the direction of the metal flow increases, the shape of the head of the jet “stretches”. The proposed method for numerical studies of the relationship between the microstructure of the cumulative lining and the breakdown ability of the cumulative charge opens up wide opportunities for further research, including when modeling cumulative facings with finer grains and for the various modifications of the mathematical formulation of the problem.

Acoustically induced bubble jets

A detailed experimental study on acoustically induced bubble jets produced far from any solid wall is presented. This particular type of bubble jet is observed when a laser-induced gas cavity is seeded in a liquid by an optical dielectric breakdown while applying a strong ultrasonic field. This generation method allows a high degree of control over jet dynamics, achievable by changing critical experimental parameters such as the laser pulse energy (i.e., the size of the bubbles), the driving signal frequency, the seeding phase (relative to the sound field), the acoustic pressure amplitude, and, in particular, the pressure gradient at the bubble inception location. The temporal evolution of the characteristic dimensions of the bubble jets is analyzed by means of numerical simulations of spherical bubble dynamics and their dependence on the local acoustic field. The jet strength is presented in terms of the jet elongation ratio, defined as the axial length over the maximum width. The most relevant parameters related to the jet strength are the maximum radius reached by the bubble prior to the jet formation and the pressure gradient during the bubble collapse phase. The existence of optimum driving parameters and the bubble seeding position to obtain the strongest jets achievable in a given acoustic pressure distribution is discussed. This study is a first step to shed some light on the jet formation process due to known time-variable pressure gradients. The work can be extended to further understanding the role of bubble jets in sonochemical reactions and biological or medical applications.

Jet formation in salt-finger convection: a modified Rayleigh–Bénard problem

Large-scale coherent structures such as jets in Rayleigh–Bénard convection and related systems are receiving increasing attention. This paper studies, both numerically and theoretically, the process of jet formation in two-dimensional salt-finger convection. The approach utilizes an asymptotically derived system of equations referred to as the modified Rayleigh–Bénard convection (MRBC) model, valid in the geophysically and astrophysically relevant limit in which the solute diffuses much more slowly than heat. In these equations, convection is driven by a destabilizing salinity gradient while the effects of the stabilizing temperature gradient manifest themselves as an additional anisotropic dissipation acting on large scales. The MRBC system is specified by two external parameters: the Schmidt number $\mathit{Sc}$ (ratio of viscosity to solutal diffusivity) and the Rayleigh ratio $\mathit{Ra}$ (ratio between the Rayleigh numbers of the destabilizing solutal stratification and the stabilizing thermal stratification). Two distinct $\mathit{Ra}$ regimes are explored for fixed $\mathit{Sc}=1$. In all cases studied the system develops a horizontal jet structure that is maintained self-consistently by turbulent fluctuations, but coarsens over time. For intermediate Rayleigh ratios (e.g. $\mathit{Ra}=6$), the MRBC model captures the relaxation oscillations superposed on the jet structure observed at similar parameter values in direct numerical simulations of the primitive equations. For smaller Rayleigh ratios (e.g. $\mathit{Ra}=2$), a regime for which direct numerical simulation of the primitive equations is difficult because of the presence of fast gravity waves, the MRBC model reveals the existence of statistically steady jets whose properties are studied in detail. Three hierarchical models, the MRBC and further reductions in the form of quasilinear and single-mode approximations, are used to confirm that jets form and are sustained as a result of the interaction between fluctuations (salt fingers) and large-scale horizontally averaged horizontal flows (jets). Even though the small-scale structures exhibited by the three models exhibit clear differences, all three produce the same power-law spectrum of the mean fields at large vertical scales: in all, the spectrum of the mean streamfunction scales as $m^{-3}$ and the mean salinity field scales as $m^{-1}$, with $m$ the vertical wavenumber. A theoretical explanation of these observations based on the dominant balances in the mean and fluctuation equations is provided. As a consequence, the jets have a zigzag profile, a conclusion that is consistent with numerical simulations. Based on numerical observations, a three-component phenomenological model consisting of a linearly growing mode, a linearly damped mode and a mean mode is proposed to explain the observed transition from statistically steady jet structure to jets with superposed oscillations that takes place with increasing Rayleigh ratio.

Design of a Novel Linear Shaped Charge and Factors Influencing its Penetration Performance

In this paper, a novel linear shaped charge (LSC), called a bi-apex-angle linear shaped charge (BLSC), has been designed to investigate the improvement of penetration performance. Compared with a traditional single-apex-angle LSC, a BLSC, which consists of a small-apex-angle liner and a large-apex-angle liner, has been investigated by depth-of-penetration (DOP) test. The results show that the penetration depth of BLSC is 29.72% better than that of an ordinary LSC. An Eulerian method is applied to simulate the entire process of jet formation, as well as penetration on a #45 steel target. The effectiveness of the Eulerian model is demonstrated by the good agreement of the computational results with experimental observations. Furthermore, the numerical simulation is developed to investigate the influence of liner thickness, explosive type, combination of small and large apex angle, ratio of small to all apex angle liner, and standoff distance on the penetration performance of BLSCs. The suggested work and results can provide a guide and reference for the structural design of BLSC.

Supercritical steam outflow through a thin nozzle: forming a hollow jet

The process of hollow jet formation during steam outflow through a thin nozzle was studied; the water steam was initially at high pressure and in the supercritical state. The numerical solution for this problem was obtained with the sonicFoam solver library from the open-source CFD software package OpenFOAM in 2D axisymmetric formula-tion. The reliability of results is estimated by comparing two approaches for simulation of the dynamics of unloading wave propagating in the high-pressure nozzle using the OpenFOAM package with Peng−Robinson equation of state and numerical solution of a similar problem by method of through computation in the case of 1D planar approximation for perfect gas equation of state.

Analysis of the formation mechanism of the slug and jet center hole of axisymmetric shaped charges

Abstract In the jet formation process of axisymmetric shaped charges, the slug is also formed. There is always a central hole in the symmetry axis of the jet and slug. The phenomenon was rarely mentioned and analyzed by the classical theory of shaped charges. For this problem, this paper attempts to explain the existence of the central hole in the jet and slug. Based on the analysis of recovery slug, we know that the jet and slug are in solid state in the process of formation. Through the analysis of X-flash radiographs of the stretching jet and particulation fracture, it is confirmed that the center holes in the jet are also present. Meanwhile, through the analysis of the microstructure of the recovered slug, it is found that there is a wave disturbance near the surface of the central hole. It can be speculated that the wave disturbance also exist in the jet. This effect may be one of the reasons for jet breakup. Due to the presence of the central hole in the jet, the density deficit of the jet obtained by other tests is very reasonable.

Computational and experimental investigation of plasma deflagration jets and detonation shocks in coaxial plasma accelerators

We present a magnetohydrodynamic (MHD) numerical simulation to study the physical mechanisms underlying plasma acceleration in a coaxial plasma gun. Coaxial plasma accelerators are known to exhibit two distinct modes of operation depending on the delay between gas loading and capacitor discharging. Shorter delays lead to a high velocity plasma deflagration jet and longer delays produce detonation shocks. During a single operational cycle that typically consists of two discharge events, the plasma acceleration exhibits a behavior characterized by a mode transition from deflagration to detonation. The first of the discharge events, a deflagration that occurs when the discharge expands into an initially evacuated domain, requires a modification of the standard MHD algorithm to account for rarefied regions of the simulation domain. The conventional approach of using a low background density gas to mimic the vacuum background results in the formation of an artificial shock, inconsistent with the physics of free expansion. To this end, we present a plasma-vacuum interface tracking framework with the objective of predicting a physically consistent free expansion, devoid of the spurious shock obtained with the low background density approach. The interface tracking formulation is integrated within the MHD framework to simulate the plasma deflagration and the second discharge event, a plasma detonation, formed due to its initiation in a background prefilled with gas remnant from the deflagration. The mode transition behavior obtained in the simulations is qualitatively compared to that observed in the experiments using high framing rate Schlieren videography. The deflagration mode is further investigated to understand the jet formation process and the axial velocities obtained are compared against experimentally obtained deflagration plasma front velocities. The simulations are also used to provide insight into the conditions responsible for the generation and sustenance of the magnetic pinch. The pinch width and number density distribution are compared to experimentally obtained data to calibrate the inlet boundary conditions used to set up the plasma acceleration problem.