Penetration of a cumulative jet into a barrier

A solution is given to the problem of the penetration of a cumulative jet with an arbitrary distribution of the velocity along it, taking account of the strength properties of the barrier. Using the example of a jet with a linear distribution of the velocity, the article demonstrates the possibility of obtaining a large puncturing capacity due to a change in the gradient along the jet as a function of the physicomechanical properties of the barrier and the jet. In addition, a distribution of the velocity along the jet is obtained which assures a maximal penetration in a barrier, arranged at a distance where a limiting elongation is not attained either partially or completely over the whole jet.     

爆轰波波形与药型罩结构匹配对杆式射流成形的影响

为提高杆式射流对钢靶的侵彻能力,设计了一种偏心亚半球药型罩,通过爆轰波碰撞理论推导出药型罩压垮速度,并结合改进的PER理论建立了杆式射流成形的模型。分析了药型罩结构参数对爆轰波碰撞压力的影响规律,获得了等质量变壁厚药型罩射流质量及速度分布的变化规律。结果表明:马赫反射压力随偏心距的增大而增大,随外壁曲率半径的增大而减小,而正规斜反射压力与马赫反射压力变化规律相反,且马赫反射压力受药型罩结构影响较大;通过对比不同方案,罩顶与罩口部厚、中间薄形状药型罩形成的射流质量提高了29.5%,头部速度提高了21.3%,且速度梯度最大,相同炸高条件下侵彻深度提高了约2倍装药直径。针对优化结构进行了数值模拟和实验验证,通过对爆轰波波形与药型罩结构合理的匹配设计,使形成的杆式射流成形及侵彻性能得到显著提升。     

Characterization of plunging liquid jets: A combined experimental and numerical investigation

This paper presents a combined experimental and numerical study of the flow characteristics of round vertical liquid jets plunging into a cylindrical liquid bath. The main objective of the experimental work consists in determining the plunging jet flow patterns, entrained air bubble sizes and the influence of the jet velocity and variations of jet falling lengths on the jet penetration depth. The instability of the jet influenced by the jet velocity and falling length is also probed. On the numerical side, two different approaches were used, namely the mixture model approach and interface-tracking approach using the level-set technique with the standard two-equation turbulence model. The numerical results are contrasted with the experimental data. Good agreements were found between experiments and the two modelling approaches on the jet penetration depth and entraining flow characteristics, with interface tracking rendering better predictions. However, visible differences are observed as to the jet instability, free surface deformation and subsequent air bubble entrainment, where interface tracking is seen to be more accurate. The CFD results support the notion that the jet with the higher flow rate thus more susceptible to surface instabilities, entrains more bubbles, reflecting in turn a smaller penetration depth as a result of momentum diffusion due to bubble concentration and generated fluctuations. The liquid average velocity field and air concentration under tank water surface were compared to existing semi-analytical correlations. Noticeable differences were revealed as to the maximum velocity at the jet centreline and associated bubble concentration. The mixture model predicts a higher velocity than the level-set and the theory at the early stage of jet penetration, due to a higher concentration of air that cannot rise to the surface and remain trapped around the jet head. The location of the maximum air content and the peak value of air holdup are also predicted differently.     

Round turbulent jet in a crossf low

We examine the discharge of a turbulent jet from a round opening into an infinite uniform crossflow and find the form of the jet centerline and the distribution of the maximum velocities in the jet along the axis. It is shown that the calculated jet axes and velocity distributions agree well with the experimental values for different ratios of the velocity at the source exit to the crossflow velocity and for different angles of entry of the jet into the crossflow. The study [1] formed the basis for the proposed semiempirical theory.     

Experimental and numerical analysis of the jet dispersion from a bent chimney around an obstacle

An experimental study and a numerical modelling analysis were carried out simultaneously to study the flow field structure issuing from a chimney around an obstacle. The main purpose of this study is to evaluate the impact of the jet emitted from a chimney (bent or straight) on the dynamics and the turbulent features of the surrounding flow. The consideration of these features is particularly pertinent to the understanding of mixing between the interacting flows which may be very important in controlling pollutant dispersion in the atmosphere. The experimental data are depicted by means of a PIV technique; whereas the numerical three-dimensional model is simulated through the resolution of the different governing Navier–Stokes equations. The volume finite method, together with the second order turbulent closure model (RSM), was adopted. Variations in obstacle form (cylindrical or parallelepiped) and chimney configuration (bent or straight) were tested and features studied were: the global jet plume, the windward and leeward jet spread; the size, location and magnitude of the reverse flow region; the penetration and the deflection of the jet trajectory around the obstacle. All these considerations allowed us to characterize well the impact of the injection of the jet emitted from the chimney within the crossflow, and its spreading around the obstacle and within the whole domain. Such characterization is very important with regard to pollutant dispersion and consequently to the environmental impact. Indeed, the different species contained within the emitted fumes are mainly directed by the velocity components and their mixing and progression within the domain and around the obstacle are closely related.     

Simulations of vertical jet penetration using a filtered two-fluid model in a gas–solid fluidized bed

The influence of a vertical jet located at the distributor in a cylindrical fluidized bed on the flow behavior of gas and particles was predicted using a filtered two-fluid model proposed by Sundaresan and coworkers. The distributions of volume fraction and the velocity of particles along the lateral direction were investigated for different jet velocities by analyzing the simulated results. The vertical jet penetration lengths at the different gas jet velocities have been obtained and compared with predictions derived from empirical correlations; the predicted air jet penetration length is discussed. Agreement between the numerical simulations and experimental results has been achieved.     

A numerical study of long rod impact onto a large target

The normal impact of a long rod onto a large target is studied using an Eulerian finite difference scheme. The impact velocity of 1.5 km s^?1 is chosen to be low enough for metal strength to be an important parameter characterising the impact. It is also sufficiently high for the rod to flow as a jet, which is consumed as it penetrates the target.The first numerical study neglects the material strength representation, so that the flow is inviscid. On impact, the flat face of the rod strikes the flat face of the target, and one dimensional analysis is used to check numerical predictions for the initial impact pressure and velocity. A steady state penetration is quickly achieved, at a velocity which is in agreement with theoretical predictions of jet flow. In the second numerical study, an elastic-perfectly plastic representation of material strength is included within the calculation. It is then found that the rod has to travel several rod diameters into the target before the penetration velocity falls from the one dimensional impact value to a steady state value. This implies that the resistance to flow increases with the depth into the target, and consequently the penetration achieved by a rod will be dependent on its diameter, as well as its length.     

聚能射流侵彻页岩储层损伤裂隙形成机制

为研究药型罩对聚能射孔弹侵彻页岩储层的射孔和损伤致裂效果的影响机理,建立了射孔弹-空气-页岩三维模型,设置药型罩的锥角分别为50°、60°、70°和80°,壁厚分别为0.5、1.0和1.5 mm,材料分别为铜、钢、钛和钨。利用ANSYS/LS-DYNA软件进行数值计算,分别从射流速度与形态、页岩射孔效果及页岩孔裂隙形成规律特征等进行系统性分析。研究结果表明：在射孔弹结构中,随着药型罩锥角的减小,射流速度提高、杵体速度降低、侵彻深度增大同时开孔孔径减小。在一定范围内,适当减小药型罩的壁厚,可以提高射流速度、减小杵体质量、增大侵彻深度和开孔倾斜度。药型罩材料对射流速度、杵体结构和页岩射孔效果均有显著影响,其中钨药型罩射孔弹的侵彻深度最大但开孔孔径最小,钛药型罩射孔弹的侵彻深度最小但开孔倾斜度最大,铜比钢药型罩射孔弹的侵彻深度略大但开孔孔径略小。通过研究不同对照组的页岩孔裂隙形成规律特征发现,页岩孔裂隙发育主要发生在杵体对页岩的再扩孔阶段,减小射流初始扩孔孔径、增大杵体直径、提高杵体速度,可以促进页岩孔裂隙发育程度。     

A CORRELATION EQUATION FOR CALCULATING INCLINED JET PENETRATION LENGTH IN A GAS-SOLID FLUIDIZED BED

Numerical simulation of gas-solid flow in a two-dimensional fluidized bed with an inclined jet was performed. The numerical model is based on the two-fluid model of gas and solids phase in which the solids constitutive equations are based on the kinetic theory of granular flow. The improved ICE algorithm, which can be used for both low and high-velocity fluid flow, were used to solve the model equations. The mechanism of jet formation was analyzed using both numerical simulations and experiments. The emergence and movement of gas bubbles were captured numerically and experimentally. The influences of jet velocity, nozzle diameter, nozzle inclination and jet position on jet penetration length were obtained. A semi-empirical expression was derived and the parameters were correlated from experimental data. The correlation equation, which can be easily used to obtain the inclined jet penetration length, was compared with our experimental data and published correlation equations.     

楔形体入水时域解的复边界元数值分析

基于速度势理论,利用复数变量边界元法对二维楔形体常速入水冲击的时域解进行了数值研究。 以相似解作为数值计算的初始条件,采用时域解射流线性近似处理方法,利用复数变量边界元法进行求解,以 减少计算量和数值误差。深入讨论了扩展坐标系求时域解、射流处理、网格划分和网格更新等关键技术。最 后数值计算了不同斜升角楔形体入水时的自由液面隆起、射流飞溅和压力分布,经与相似解结果作比较,自由 液面隆起轮廓基本吻合,而压力分布更符合实际情况,从而证明了模型及分析方法的正确性。     

饱含液体的单胞结构对射流的干扰特性分析

基于虚拟原点理论、冲击波反射理论和射流干扰理论,考虑液体的喷散特性和径向汇聚特性,改进并完善了射流侵彻饱含液体的单胞结构的理论模型,得到了精确的射流受干扰速度区间的表达式、液体喷散速度的表达式和液体喷散流量的表达式。通过对比理论和试验得到的射流受干扰的速度区间,证明了本文理论模型的可靠性。研究结果表明:液体的喷散和径向汇聚都会影响射流的稳定性,使射流出现颈缩和提前断裂,降低射流的剩余头部速度从而降低射流的剩余侵彻能力。     

聚能射流水中侵彻行为的实验研究

对三种不同药型罩形成的射流在水中的侵彻行为进行了实验研究,采用电铜箔测速法及高速摄影法分别获取了聚能射流在水中的行进过程及图像。实验结果表明,水体的边界效应及聚能射流在水中的开坑行为对电铜箔的测量结果影响很大,且聚能射流的准定常侵彻行为在平均速度的变化上表现明显,在准定常侵彻阶段,聚能射流的速度可近似为线性递减,且药型罩构型对射流速度的衰减率影响较大,药型罩材料的影响较小。光测结果给出了聚能射流在水中的运动图像,且据此计算出的射流速度与电测结果吻合较好。     

Conditional sampling of velocity and scalars in turbulent flames using simultaneous LDV-Raman scattering

The laser Doppler velocimeter (LDV) measures the velocity distribution of particles which is often an acceptable representation of the distribution of gas velocities. However, in turbulent two stream mixing flows, the particle velocity distribution will differ from the gas velocity distribution when the particle densities in the two streams are unequal. This bias is explored in a reacting and nonreacting turbulent jet which is surrounded by coflowing air. By adding seed particles to only the coflow air and then to only the jet fluid, the limits of this bias are established. Additional measurements with an LDV triggered laser Raman scattering system demonstrate that the bias in the LDV sampling is propagated to the Raman measurements. An analytical equation is presented which will generate unbiased velocity and scalar distributions from measurements obtained from seeding only one stream at a time.     

射流对工业炸药的引爆及侵彻机制

本文用脉冲X光摄影技术及电探极-记录仪系统,对射流引爆及侵彻工业炸药的过程进行了观测,探讨了的流引爆及侵彻工业炸药的机制,给出了射流与工业炸药作用过程的物理图象。     

Reynolds number effects on the behavior of a non-buoyant round jet

The behavior of a non-buoyant circular water jet discharged from a contraction nozzle was experimentally investigated. In this experiment, the Reynolds number of the jet, based on the mean velocity results obtained by particle image velocimetry (PIV), ranged from 177 to 5,142. From the experimental results, we found that the cross-sectional profile of the axial velocity for a laminar flow near the nozzle did not show a top-hat distribution, whereas the profiles with Reynolds number higher than 437 were almost top-hat. The length of the zone of flow establishment (ZFE) was found to decrease with increasing Reynolds number. The measured centerline velocity decayed more rapidly and, consequently, approached the theoretical equation earlier near the nozzle as the Reynolds number increased. The decay constant for the centerline velocity of the turbulent cases was relatively lower than that discovered in theory. It is assumed that this probably resulted from the use of the contraction nozzle. Verifying the similarity of the lateral velocity profiles demonstrated that the Gaussian curve was properly approximated only for the turbulent jets and not for the laminar or transitional flows. The jet half width seldom grew for the laminar or transitional flows, whereas it grew with increasing axial distance for the turbulent flows. The spreading rates for the turbulent flows gradually decreased with increasing Reynolds number. The normalized turbulence intensity along the jet centerline increased more rapidly with the axial distance as the Reynolds number increased, and tended to the constant values proposed by previous investigators. The Reynolds shear stress levels were also found to increase as the Reynolds number increased for the turbulent jets.     

Numerical analysis of critical jet penetration

In this paper, a computational program for two dimensional unsteady compressible, elastic-plastic flow is used to study low speed jet penetration into a steel target. Based on the computational results, the dynamic process of penetration can be distinguished into two stages, cratering and steady penetrating. This is in agreement with the jet penetration mechanism developed by Cheng Che Min in 1972^[1,2]. The consumption of the jet energy is discussed and the value of the critical jet velocity, which represents the strength resistance of the target, is estimated.     

On longitudinal impact III

Summary The graphodynamical method which was introduced and applied to elastic longitudinal impact in the first two articles of this series is here used to study the transmission of a rectangular compressive pulse along a system consisting of a bar which is joined by friction to a stiff anvil or to a co-axial elastic tube. Some of these cases will in the next article be used as models of pile driving. The influence of the frictional force, of the dimensions of the pulse (the dimensions and impact velocity of the hammer) and of the area and length of the tube on the sign and magnitude of the slide at a concentrated friction joint and on the associated energy loss is determined, and the results are illustrated by diagrams. The results of a few simple experiments agree reasonably well with the theory. The theory is extended to a model of a pile with constant friction per length unit, and it is found that a light hammer with great impact velocity gives better penetration than a heavy hammer with the same energy and thus smaller velocity.     

A novel formula to describe the velocity profile of free jet flow

Having a potential core, the velocity profile in initial zone of incompressible submerged jet flow is different from that in fully developed region. In the former researches, the two regions were studied separately, even a short part between the two regions being considered as a transition region. The velocity profile in fully developed region looked as a Gaussian distribution, which is valid when jet initial region is comparatively short. But when the size of initial zone is long enough not to be able to be neglected, especially for large-size exit, this kind of assumption is not acceptable. Based on the analysis of flow structure of jet flow, a new velocity profile formula of submerged jet flow was proposed, which unites the initial, transition, fully developed regions of jet flow via modifying Gaussian distribution with a radial adjusting coefficient. For the round jet with the medium or high range of Reynolds number, the radial adjusting coefficient is a power function of reciprocal of jet distance. And then some literature experimental data were applied in verification, and the new formula exhibited a good calculation result. This work opened that the jet flow velocity profile at any site along the flow distance can be described via a same formula.     

Experimental investigation flow structure and hydraulic resistance of a cylindrical duct with injection a fan slot jet

The paper describes results of an experimental study of pressure and velocity fields arising during normal injection of a radial slot jet into ducted flow. The experiments were carried out for slots of two different widths and for injection parameters varying in a broad range. The pressure profile along the duct length plotted in generalized coordinates was found to be quite a universal distribution. Experimental correlations for the minimum rarefaction in the separation region behind the injected jet were obtained, and comparison was made with the results of simplest numerical analysis. Results of measurements of local hydraulic losses are presented for the duct section where the normal injection of the slot jet was organized. The experimental data are shown to be underestimated compared with the results predicted by the theory of perfect mixing for a ducted flow with mass supply. The possible reasons of hydraulic losses coefficient behavior are discussed.     

Velocity measurements in a two-phase turbulent jet

The motion of oil droplets in a round turbulent air jet is investigated experimentally. Direct information on the droplets' average velocity is obtained by means of a Laser Doppler velocimeter. Average velocity profiles of the droplets are measured along the axis of the jet and transverse to it. The results are compared to the free jet expansion.The jet Reynolds number is in the range 10⁴–10⁵, the droplets' diameters are 50 μm and below and the volume concentration of the oil in the air is 10^?6.At the jet exit, the air velocity is higher than the droplets' velocity, at the developed region of the jet the droplets' velocity is found to be higher than the free air jet velocity at the same location. In the radial direction, the velocity profiles of the droplets are self similar and the droplets' velocity is lower than the free air jet velocity at the same location. The droplets' velocity decay along the axis of the jet is slower than the air velocity in the free jet and the two-phase jet is narrower than the submerged free air jet at the same exit velocity.