Shock wave structure in simple liquids

The shock wave structure in a liquid is studied by a molecular dynamics simulation method. The interaction between atoms is described by the Lennard-Jones (6–12) potential. In contrast to earlier works, the simulation is performed in a reference frame tied to the shock wave front. This approach reduces non-physical fluctuations and makes it possible to calculate the distribution functions of the kinetic and potential energy for several cross sections within the shock layer. The profiles of flow variables and their fluctuations are found. The surface tension connected with pressure anisotropy within the shock front is calculated. It is shown that the main contribution to the surface tension coefficient comes from the mean virial. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 722–727 (10 May 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Reverse undercompressive shock structures in driven thin film flow

We show experimental and computational evidence of a new structure: an undercompressive and reverse undercompressive shock for draining films driven by a surface tension gradient against gravity. The reverse undercompressive shock is unstable to transverse perturbations while the leading undercompressive shock is stable. Depending on the pinch-off film thickness, as controlled by the meniscus, either a trailing rarefaction wave or a compressive shock separates from the reverse undercompressive shock.     

Shock wave emission upon spherical bubble collapse during cavitation-induced megasonic surface cleaning

When a gas bubble in a liquid interacts with an acoustic wave near a solid surface, the bubble first expands and then collapses. In this paper, a mathematical framework combining the Gilmore model and the method of characteristics is presented to model the shock wave emitted at the end of the bubble collapse. It allows to describe the liquid velocity at the shock front as a function of the radial distance to the bubble center in the case of spherical bubble collapse. Numerical calculations of the liquid velocity at the shock front have shown that this velocity increases with the acoustic amplitude and goes through a maximum as a function of the initial bubble radius. Calculations for different gas state equations inside the bubble show that the Van der Waals law predicts a slightly higher liquid velocity at the shock front than when considering a perfect gas law. Finally, decreasing the value of the surface tension at the bubble/liquid interface results in an increase of the liquid velocity at the shock front. Our calculations indicate that the strength of the shock waves emitted upon spherical bubble collapse can cause delamination of typical device structures used in microelectronics.     

Dynamics of the deformation and destruction of a heterogeneous body (granite) under the influence of an electric discharge

The device is constructed, which makes it possible to simultaneously detect, with a time resolution of 10 ns, fractoluminescence as well as electromagnetic emission and surface deformation observed in a solid during its destruction under the effect of a shock wave. Using this device, time dependences of deformation and destruction of the granite plate caused by an electric breakdown with an energy of 0.2 J in air near its surface are investigated. It is found that the breakdown causes the appearance of a shock wave in granite, the velocity of which is ～5 km/s. The shock wave stimulates emission of a plasma consisting of atoms and ions, which enter into the graphite composition, from the granite surface. It is assumed that the appearance of the plasma is caused by cumulation of the shock wave energy in micropores contained in graphite.     

激光诱导2024铝合金表面动态应变检测

采用PVDF贴片传感器对脉冲激光作用下2024铝合金表面的动态应变进行了测量,分析了动态应变曲线的特性。结果表明,PVDF贴片传感器在动态应变测量中动态响应快,灵敏度高,可有效应用于脉冲激光诱导材料表面动态应变的实时测量。脉冲激光作用过程中,2024铝合金冲击光斑周围材料先受挤压,后压应变减小。脉冲激光作用结束后,2024铝合金冲击光斑周围材料表面粒子在卸载稀疏波和表面稀疏波的作用下不断往复运动,冲击光斑周围材料甚至受到了拉应变的作用。最后随着时间的推移,材料表面粒子的动态响应经反复震荡后逐渐衰弱形成最终的稳定状态。     

Shock wave structure in dense gases

The internal structure of a shock wave front in a gas is studied by molecular dynamics (MD) simulation. A new approach to MD shock simulation is used, which enables one to consider a stationary shock front at rest and radically improves the quality of simulation. The profiles of flow variables and their fluctuations are calculated. The evolution of the velocity distribution function across the shock layer is calculated and compared with the bimodal distribution. The pair distribution function in the shock layer is determined. The surface tension associated with the shock wave is estimated. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 2, 91–96 (25 July 1997)     

强脉冲激光在AZ31B镁合金中诱导冲击波的实验研究

为了研究强脉冲激光在镁合金中诱导冲击波的衰减,采用Nd:Glass脉冲激光(1054 nm,23 ns)对AZ31B变形镁合金试样表面进行冲击,并利用响应快、测量范围大的PVDF压电膜传感器以及示波器实时测量了强脉冲激光在镁合金靶中诱导激光冲击波的相对压力.根据冲击波每次在靶材背面反射时,所经过距离的不同得到激光冲击波在镁合金中的衰减规律.结果表明,在激光能量为5J的强脉冲激光作用下,镁合金中冲击波衰减的平均速度为5.83×10³ m/s,与镁合金中应力波纵波的传播速度相符;强脉冲激光诱导冲击波在镁合金中是以指数规律衰减的.试验所得分析结果对激光冲击强化镁合金的应用具有重要意义. 关键词： 激光 镁合金 压电膜传感器 衰减规律     

Time-resolved shock-wave photography above 193-nm excimer laser-ablated graphite surface

Highly oriented pyrolytic graphite (HOPG) was ablated by a 193-nm ArF excimer laser in air. The fluence was varied in the range 1-25 J/cm². Every laser shot hit a pristine graphite surface. The emerging shock wave was recorded by a nanosecond-resolution photographic arrangement. The velocity of the shock wave as a function of time and laser fluence was measured. The amount of energy that generates the shock wave was determined and found to be about 5-7% of the incident laser energy. The shock wave is already present 10-15 ns after the maximum of the incident laser pulse. These facts imply that, even if high-energy (10-100 eV) ions, atoms, or clusters leave the surface, a layer several 10 nm thick has to be removed during this short period. The temperature of the shock front is ~2500-4000 K, as derived from the measured velocities. Measuring the ablation depth by atomic force microscopy as a function of fluence revealed that the single-shot ablation threshold is 1.4ǂ.2 J/cm², and the effective absorption coefficient is ~1.5᎒⁵ cm^-1.     

Pinch-off transition in Marangoni-driven thin films

We present a mathematical model and analysis for the pinch-off transition as observed in dip-coating experiments at the base of thin liquid films driven up a vertical plate by a thermally induced surface tension gradient with a counteracting gravitational force. Our results show that this transition gives rise to a complex new wave structure involving a nonclassical, reverse undercompressive shock wave.     

面向高超声速飞行器的激波智能预测方法

高超声速飞行器激波位置的准确预测能够有效提升数值模拟的精度和效率。一方面,对高超声速飞行器激波附近网格进行正交和加密处理,可有效提升数值计算精度;另一方面,使用高超声速飞行器激波位置对计算网格进行修正,能够加速CFD计算收敛过程。提出了一种基于机器学习的高超声速飞行器激波智能预测方法,对典型高超声速飞行器外形进行激波位置的高效准确预测。首先,针对典型高超声速飞行器外形和典型飞行状态,使用数值模拟方法获得收敛的流场,并采用基于Mach数等值线的激波提取方法,从流场中判别激波面并提取构成激波面的关键点位置,形成训练数据;然后采用有监督学习算法,学习关键点位置,并利用二次曲线沿流向拟合关键点形成初步的激波线族;最后,基于剖面压力云图,构造基于投影压力图像的智能预测神经网络,对初步形成的激波线族进行修正,并获得三维激波面。大量的实验结果表明,激波预测模型能够对高超声速飞行器激波位置做出准确预测,预测的激波面与CFD数值计算结果中提取的激波面误差在10^-4量级。     

Generation and detection of superstrong shock waves during ablation of an aluminum surface by intense femtosecond laser pulses

Superstrong shock waves of multimegabar level generated during ablation of an aluminum surface by intense (<1 PW/cm²) femtosecond laser pulses have been detected by observing the propagation of a shock wave in air from the ablated surface to a broadband piezoelectric receiver. The estimated initial pressure and velocity of the shock wave (ablation plume) agree well with data obtained earlier by various methods for shock waves propagating inside ablated targets.     

Shadowgraph investigation of plasma shock wave evolution from Al target under 355-nm laser ablation

The propagation of a plasma shock wave generated from an Al target surface ablated by a nanosecond Nd:YAG laser operating at 355 nm in air is investigated at the different focusing positions of the laser beam by using a time-resolved shadowgraph imaging technique. The results show that in the case of a target surface set at the off-focus position, the condition of the focal point behind or in front of the target surface greatly influences the evolution of an Al plasma shock wave, and an ionization channel forms in the case of the focal point set in front of the target surface. Moreover, it is found that the shadowgraph with the evolution time around 100 ns shows that a protrusion appears at the front tip of the shock wave if the focal point is at the target surface. In addition, the calculated results of the expanding velocity of the shock wave front, the mass density, and pressure just behind the shock wave front are presented based on the shadowgraphs.     

平面定常超音速绕流问题的新的数值解法

用特征线法解平面定常超音速绕流问题虽然有效,但当激波很弱、几乎与特征线平行时则很难处理。用有限差分法计算此问题也比较复杂。本文把作者在文章中^[1]提出的新的数值解法,发展并应用到平面定常超音速绕流的问题。仍然采用了许为厚教授提出的新拉格朗日变量^[2],这使边界条件的提法大为简化。此新的数值解法按变量指标之和,一排排地往下计算,方法简单,可以处理各种形状物体的超音速绕流。本文对向上弯曲的抛物形固壁绕流向题的实例进行了具体计算。算出了激波的形状。当激波没有形成以前,相应的普朗特一迈耶气流是有准确分析解的,把数值解与准确解进行了l比较结果是满意的。     

二维晶格中激波的传播和自由面反射的分子动力学数值研究

本文用分子动力学方法研究和计算了面心晶体中沿(1、0、0)面或休心晶体中沿(1、1、0)面上激波的传播和自由面反射现象。在所研完的范围内(u_p=10~75×10⁴cm/sec)粒子速度沿平均值的振动是不衰减的,激皮的传播速度和波后粒子速度的平均值呈线性关系。激波在自由面反射后,自由表面附近的粒子速度为活塞速度的二倍,这和宏观力学结果相同。此外,还出现最外层分子不断地以更高的速度被抛离的现象,这和宏观测量的微物质喷射现象亦类似。在稍后的阶段,还显示出内部粒子间被拉开的"微断裂"现象。     

Estimation of the dynamic cavitation tension of water by a shock tube method

A review is conducted into existing work related to the role bulk cavitation plays during the interaction between a submerged structure and an underwater shock wave. This survey, dealing both with the classical works of Temperley and Cushing as well as recently published studies by the writer, is focused on two important variations from the assumptions made in earlier works. One of these variations, the possibility that water has a non-zero cavitation tension, is studied further by developing a theoretical model to predict the fluid dynamics following a free surface reflection. By introducing experimental data for comparative purposes, it is concluded that, at laboratory scale, the dynamic cavitation tension may be regarded as zero.     

激光驱动高压冲击波的实验观察

本文介绍能量约500J,脉宽约1.2ns的高功率激光脉冲聚焦辐照铝箔靶或铝台阶靶,用可见光超快速扫描相机观察靶背面冲击加热发光的情况。实验表明,采用特别的组合透镜聚焦,使激光冲击波的平面性得到明显的改善。实验测得铝台阶中的冲击波速度为17.6μm/ns,并推算出相应的冲击波压力达4.4Mbar。 关键词：     

Cavitation bubble behavior and bubble–shock wave interaction near a gelatin surface as a study of in vivo bubble dynamics

The collapse of a single cavitation bubble near a gelatin surface, and the interaction of an air bubble attached to a gelatin surface with a shock wave, were investigated. These events permitted the study of the behavior of in vivo cavitation bubbles and the subsequent tissue damage mechanism during intraocular surgery, intracorporeal and extracorporeal shock wave lithotripsy. Results were obtained with high-speed framing photography. The cavitation bubbles near the gelatin surface did not produce significant liquid jets directed at the surface, and tended to migrate away from it. The period of the motion of a cavitation bubble near the gelatin surface was longer than that of twice the Rayleigh's collapse time for a wide range of relative distance, L/R_max, excepting for very small L/R_max values (L was the stand-off distance between the gelatin surface and the laser focus position, and R_max was the maximum bubble radius). The interaction of an air bubble with a shock wave yielded a liquid jet inside the bubble, penetrating into the gelatin surface. The liquid jet had the potential to damage the gelatin. The results predicted that cavitation-bubble-induced tissue damage was closely related to the oscillatory bubble motion, the subsequent mechanical tissue displacement, and the liquid jet penetration generated by the interaction of the remaining gas bubbles with subsequent shock waves. The characteristic bubble motion and liquid jet formation depended on the tissue's mechanical properties, resulting in different damage mechanisms from those observed on hard materials.     

Superspherical cumulation. Converging shock waves with amplitudes increasing faster than in spherical cumulation

Experimental, numerical, and theoretical investigations are made of a gas flow generated by a pulsed high-current discharge in an axisymmetric cavity bounded by a spherical lens adjacent to a flat plate. It is shown that the shock wave forming in the discharge and converging toward the axis is accelerated and amplified as it converges. The amplitude of the shock wave increases faster than does that of a spherical converging shock wave. Zh. Tekh. Fiz. 69, 10–18 (March 1999)     

Effect of debris fragments on direct initiation of spherical detonation waves in stoichiometric oxygen/hydrogen mixtures

Paper reports a result of experiments of spherical shock waves generated by explosions of micro-explosives weighing from 1 to 10 mg ignited by the irradiation of Q-switched laser beam and direct initiation to a spherical detonation wave in stoichiometric oxygen/hydrogen mixtures at 10–200 kPa. We visualized the interaction of debris particles ejected micro-explosives’ surface with shock waves by using double exposure holographic interferometry and high-speed video recording. Upon explosion, minute inert debris launched supersonically from micro-charge surface precursory to shock waves initiated spherical detonation waves. To examine this effect we attached 0.5–2.0 μm diameter SiO₂ particles densely on micro-explosive surfaces and observed that the supersonic particles, significantly promoted the direct initiation of spherical detonation waves. The domain and boundary of detonation wave initiations were experimentally obtained at various initial pressures and the amount of micro-charges.