水下近距爆炸作用下弹性钢板处的空化特性研究

弹性钢板在水下近距爆炸作用下,冲击波会使其附近流体形成局部空化,脉动气泡会使流体形成锥形空化。利用平面冲击波理论对局部空化的形成特性进行了研究,理论分析了结构目标尺度的变化对空化区域形成的影响,并通过具体试验对局部空化理论进行了验证,两者符合较好;通过试验和数值仿真方法研究了气泡脉动引起的锥形空化的形成特点,初步分析了锥形空化的形成原因。结果表明,锥形空化对结构具有较大的冲击作用。     

基于B超成像技术监测和评估超声空化

提出了基于B超成像的技术检测手段,利用B超成像技术监测和定量脉冲冲击波治疗过程中空化气泡活动。进一步基于气泡溶解方程和高斯气泡分布,对测得的散射截面进行数值模拟。结果表明,B超成像技术是监测冲击波脉冲作用下空化气泡时间演化过程的有效工具。通过适当地模拟气泡溶解过程可以估计气泡性质,例如气泡尺寸分布和气体扩散特性。这项技术对评估和控制冲击波治疗期间残留子气泡的空化活动有重要意义,通过调节两个冲击波脉冲之间残留子气泡的溶解过程,使得组织损伤最小,同时不减弱治疗效果。为实现最佳的冲击波治疗提供了可行方法。     

低温流体经过弯管时的空化现象分析

采用完全空化模型,对液氧流经弯管中时的空化现象进行了数值模拟.研究表明:由于弯管内侧流体压力低于外侧,当最低压力降低到空化核不稳定的临界压力时,空化首先在弯管内侧产生;入口速度增加、出口压力降低或流体温度升高,均会导致空化区域增大、强度增加.当空化区域扩展到一定范围时,管内大部分区域的流动状态达到基本稳定,空化现象对入口速度、出口压力等操作参数的变化不再敏感.此外,弯管的几何结构如曲率变化对空化现象也有相当大的影响.     

混流泵启动过程非稳态空化的试验研究

为了分析混流泵启动过程瞬态扬程理论模型的适用性及非稳态空化过程,通过高速摄影和压力测量试验,探讨不同空化数、不同流量工况下启动过程中叶顶区空化状态特性。试验研究结果表明,已有理论模型可应用于混流泵启动过程,同时验证试验结果准确性。在启动过程中,空化最先发生在叶顶区域头部。当启动完成时,叶顶空化区域可分为两部分,一部分为出现在叶顶区的三角形的空化云团;另一部分为刮起涡空化。在同一流量工况下,随着空化数降低,垂直空化涡会在启动过程中出现并且发展。在同一空化数下,随着流量减小,垂直空化涡延迟出现,并且空化区域减小。     

考虑空化效应的舰艇水下爆炸仿真研究

为了分析空化效应对舰艇水下爆炸冲击响应的影响。对一维Bleich—Sandler模型进行了数值仿真计算,验证了计算方法的正确性,分析了空化闭合二次加载产生的速度截断现象,并应用于二维中截面舰艇模型水下爆炸数值仿真。结果表明空化在冲击波到达结构表面后就会发生,空化闭合会产生显著的二次冲击加载作用。这在舰艇水下爆炸仿真计算中必须加以考虑。     

仿体中超声空化效应的B超图像处理

0引言使用高强度聚焦超声(HIFU)照射组织,将产生声空化现象。在声空化过程中,声场能量于空化气泡内高度集中,当能量积累到一定阈值后,空化气泡瞬间崩溃。在气泡崩溃的瞬间能量会释放出来,形成局部的高温、高压、强冲击波等极端物理现象[1]。这将使周围的组织细胞遭到损伤。因此,对HIFU空化的监控显得尤其重要[2]。本文主要研究不同脉冲宽度的高强度聚焦超声(HIFU)在体外仿体中引起的空化气泡群面积随时间的变化。从而实现对HIFU空化的实时监控。     

多孔覆盖层水下爆炸流固耦合分析

针对芯层为塑性多孔材料的抗冲覆盖层在水下非接触爆炸载荷作用时的流固耦合问题开展了理论研究。依据塑性冲击波理论求解覆盖层的动态压缩;采用改进的Taylor板理论求解流体与覆盖层的耦合作用;并以空化理论求解流体空化的传播及空化溃灭的二次加载过程,建立了完整的考虑覆盖层大变形、流固耦合效应及空化效应的理论模型。研究结果揭示了塑性多孔覆盖层在水下爆炸作用时的流固耦合及空化效应的影响。     

由爆炸引起的隧道衬砌与土体相互作用机制的数值模拟分析

为探究爆炸引起的隧道衬砌与土体相互作用机制,基于埋深15 m、含有全饱和颗粒土的单个圆形混凝土线弹性管片组成的隧道模型,对隧道内的管片衬砌与土体在中等幅值的爆破压力脉冲引起的动态变形进行了数值模拟分析.利用有限元程序Abaqus/Standard并结合二维平面应变理论进行建模,同时考虑绝对孔隙压力为零时孔隙水空化,通过精细化单元空间离散,分析了管片衬砌和土体中的应力和变形.研究结果表明:孔隙水空化引起的强非线性对土体性能具有重要影响.具体表现为:孔隙水空化能够显著降低土体的体积模量,并导致大空化带的形成,使得孔隙水从非空化状态向空化状态转变,或从空化状态向非空化状态转变,导致压缩-拉伸过程中土体的应变-应力关系呈现高度非线性,在气蚀区收缩和最终关闭期间,该非线性会在水气混合物溶液中产生高梯度和奇点.     

贮箱及输送管路流场空化发展机理研究

基于Pumplinx仿真分析软件开展了非稳态两相流仿真模拟分析,针对贮箱及下游输送管路,研究空化现象发展机理,并给出了试验验证。结果表明,贮箱出口边界流量增大后输送管内各截面会依次发生空化,截面压力降低至饱和蒸气压。空化现象根据空化程度不同,可以分为局部微量空化和深度空化两种状态,不同状态下空化发展过程不同。在微量空化状态下,空化区域存在于出流口局部,不会向下游发展;在深度空化状态下,空化区域不断发展,贮箱出流口会发生限流,空化夹气逐步发展至出口,从而不满足发动机入口要求。     

冲击波在泡沫铝中的传播和衰减特性

用平面冲击波实验对相对密度为0.396的开孔泡沫铝的冲击波压缩特性进行了研究,采用PVDF传感器技术测量并分析了泡沫铝中的冲击波应力波形及其传播速度.在泡沫铝样品中不同位置处记录的应力波形显示随着在泡沫铝中传播距离的增加,冲击波应力峰值迅速衰减,归一化的冲击波应力峰值随传播距离呈指数关系变化,另外,冲击波的传播速度也随距离的增加而呈线性关系减慢.     

Optical distortion in the field of a lithotripter shock wave

The schlieren observation of cavitation phenomena produced in the tail of a lithotripter shock wave has indicated the presence of some interesting features. The images produced appear to indicate that cavitation transients in the field of a shock wave propagate nonsymmetrically; this is not the case. The apparent lack of symmetry exhibited by the primary cavitation transients is due to a complex optical lensing effect, which is brought about by the change in refractive index associated with the pressure profile of the shock wave. Objects seen through or immersed in the shock-wave field of an electromagnetic acoustic transducer, such as cavitation, appear highly distorted because of the strong positive and negative lensing effects of the compression and rarefaction cycles of the shock wave. A modification of the schlieren technique called the scale method has been used to model the distortion introduced by the shock wave and consequently explain the cavitation distortion. The technique has also been used to quantitatively analyze and partially reconstruct the lithotripter shock wave. The combination of schlieren and scale imaging gives more information about the refractive index field and therefore the shock-wave structure itself.     

叶轮空蚀状态下离心泵振动特性分析

空蚀是指空化过程中产生的空泡溃灭引起过流表面材料损坏的现象。为研究离心泵叶轮空蚀后的振动信号特征,选用IS-50-160-00单级单吸离心泵为试验对象,基于虚拟仪器技术搭建试验泵系统。测得离心泵空蚀条件下的振动信号,采用均方根(RMS)分析、峭度(K)分析两种统计方法对发生空蚀后的离心泵振动信号的平均能量、冲击波能量进行分析,采用短时傅里叶变换(STFT)分析了振动信号的时频域特性。分析结果表明:空蚀条件下整体来看基座方向和轴向方向振动幅值较大且都是无规则振动,而横向方向和纵向方向上的振动信号振幅相对较小;振动信号的能量随着流量的增大呈现先减小后平稳再增加的趋势,空蚀增加了振动信号的能量值;空蚀加剧了液体对离心泵的冲击使得振动信号峭度值增加,且基座方向峭度值大于3可作为空蚀故障的诊断参考标准;通过时频谱分析可知空蚀发生后流体可能对离心泵存在冲击波及冲击波导致的脉冲信号,且空蚀后产生了高频振动信号,高频带的振动信号可为离心泵空蚀故障诊断提供参考。研究叶轮空蚀后离心泵振动信号的特征有助于及时发现离心泵空蚀故障的发生,从而调整运行参数,以免造成严重后果。     

脉冲激光击穿水介质特性的实验研究

针对激光击穿水介质过程中的微观及宏观特性研究,利用调QNd：YAG激光聚焦击穿水介质形成激光声源,采用高速摄像机、高频测量水听器对激光击穿水介质过程中的等离子腔体闪光、空泡脉动、近／远场声波特性等综合效应进行了实验测量。实验表明：激光空泡的特征与水动力空化空泡相似;激光声信号强度在光击穿条件下与入射激光能量具有一定的线性关系;声脉冲高频段占声能的主要部分。研究结果可为水下激光加工、激光医学、激光声的研究提供一定的理论和实验支持。     

Shock-wave propagation and cavitation bubble oscillation by Nd:YAG laser ablation of a metal in water

A highly sensitive fiber-optic sensor based on optical beam deflection is applied for investigating the propagation of a laser-induced plasma shock wave, the oscillation of a cavitation bubble diameter, and the development of a bubble-collapse-induced shock wave when a Nd:YAG laser pulse is focused upon an aluminum surface in water. By the sequence of experimental waveforms detected at different distances, the attenuation properties of the plasma shock wave and of the bubble-collapse-induced shock wave are obtained. Besides, based on characteristic signals, both the maximum and the minimum bubble radii at each oscillation cycle are determined, as are the corresponding oscillating periods.     

Comparison between the effects of cavitation induced by two different pressure-time shock waveform pulses

Acoustic cavitation generates very large localized pressures and temperatures, and thus provides a mechanism whereby physical and biological effects are produced in a high-intensity acoustic field. In this work, we studied the influence of the temporal form of a pressure pulse waveform on the destructive effects of transient cavitation. Two different shock pressure-time waveforms with nearly the same acoustic energy content were used. The first pressure waveform starts with a tensile wave followed by a compressive one, and the second pressure waveform starts with a compressive wave followed by a tensile one. These two pressure waveforms are called direct and inverse-mode pulses respectively. Based on the measurements presented in this work, we can state that, between the two types of shock pressure pulses studied, the direct-mode pulse amplifies systematically tile cavitation effect. This conclusion was achieved from a series of several quantitative and qualitative experiments: cavitation bubble collapse time, disintegration efficacy of plaster balls (a kidney stone-mimicking material), macroscopic study of lesions in agar gel and in vitro isolated rabbit liver tissue destruction. Considering these results and those obtained by other research groups, we can express that the temporal form of a shock pressure pulse has a major role on the cavitation effects.     

Bubble-induced cavitation effect upon solid surfaces

The dynamic damage during cavitation erosion is of a spall nature and results from the interference of rarefaction waves. Spherical shock waves, which arise due to the collapse of cavitation bubbles, generate shock waves in the target. The rarefaction waves appear when the velocity of the contact boundary decreases below the sound velocity in the target material and the shock waves emerge at the free surface. The coordinate of the rarefaction wave formation site determines the zone of potential damage. The interference of rarefaction waves creates the spall channel crack, near which the coaxial cylindrical cracks are formed-as a rule, in the dynamic fatigue regime-in the course of subsequent loading.     

Intraocular laser surgical probe for membrane disruption by laser-induced breakdown

A fiber probe has been designed as a surgical aid to cut intraocular membranes with laser-induced breakdown as the mechanism. The design of the intraocular laser surgical probe is discussed. A preliminary retinal damage distance has been calculated with breakdown threshold, spot size, and shielding measurements. Collateral mechanical-damage effects caused by shock wave and cavitation are discussed.     

多次水下爆炸钢制圆板应变与挠度增长规律分析

在海上战争中,水面舰艇以及潜艇可能会受到多枚水中兵器的攻击,产生的累计损伤问题影响着水面舰艇以及潜艇的持续作战能力和生命力。对膨胀直径为390 mm,厚度为5 mm空气背衬钢制圆板,在50 g药量下,进行持续多次水下爆炸加载试验,对板中心应变、挠度以及厚度减薄率进行测量,试验结果分析表明:第一次爆炸加载时,局部空化使钢板产生的应变是冲击波加载的1倍,当钢板被炸弯后,再次爆炸加载局部空化的二次加载现象不明显。此外,气泡脉动及气泡射流会使钢板发生明显的弹性应变,应变值是初始冲击波作用的10%。对中心挠度和厚度减薄率的数据分析发现,钢板厚度减薄率是随着中心挠度呈线性增加的。该研究对舰船持续作战能力及生命力的评估提供了一定的试验支撑。