大气呼吸模式激光推进的机理分析及数值模拟

就大气呼吸模式的推进机理作了较为系统的分析和研究,导出了考虑反射面几何约束的冲量及冲量耦合系数的近似解析表达式。在数值模拟方面,采用前期点爆炸自模拟解和后期高分辨率PPM格式相结合的新方法,计算了固定的抛物面反射镜聚焦入射平行激光击穿空气形成的高温等离子体流场及其对反射面产生的推动作用。结果表明,推进效应(飞行器获得的推力、总冲量、冲量耦合系数等)与反射面的几何参数和入射激光强度密切相关。83J的激光单脉冲能量算例得到的冲量耦合系数为246Ns/J,略高于同种工况下W.O.Schall等实验得到的耦合系数。     

飞秒激光辐照铝材料的分子动力学数值模拟

采用大尺度分子动力学方法对飞秒激光辐照金属铝材料的效应进行了数值模拟。利用分子动力学方法给出了飞秒激光辐照后,材料表面发生熔化和喷溅,冷凝后形成一层很薄的多孔介质的物理图像,及产生的应力波传播过程等。数值模拟结果表明分子动力学方法可以用于飞秒激光对材料辐照效应的研究。     

吸气式激光推进推力产生机理的数值模

采用二阶精度的Roe格式，分别采用完全气体模型和高温平衡气体模型，对激光引致的激波在流场中的传播以及轴对称激光推力器的推力产生过程进行了数值模拟，揭示了大气吸气模式激光推进的推力产生机理。结果表明，采用高温平衡气体模型可以更精确地模拟等离子体流场的演化过程，计算所得的冲量耦合系数与已有实验结果吻合较好。     

激光推进的基本原理和实验进展综述

激光推进是一种新型的推进模式,按作用机理可分为大气模式激光推进和火箭烧蚀模式激光推进两种。分析了两种推进模式的作用原理,论述了激光推进的最新研究进展。     

激光对涂层玻璃钢材料的烧蚀阈值实验研究

介绍了玻璃钢材料的特性。用连续ＹＡＧ激光辐照不同防火涂料层的玻璃钢材料,进行烧蚀阈值的实验研究。在实验中,用激光光斑仪确定激光辐照玻璃钢材料靶表面的光斑尺寸和光场分布。用红外热像仪测量了激光辐照玻璃钢材料生产的温度响应,并给出了损伤和起燃阈值。     

激光辐照下充内压柱壳动态爆裂的数值模拟

采用动力有限元显式积分程序,对承受内压、在侧面受到激光辐照的圆柱壳的热 力学响应进行了数值模拟,展现了圆柱壳热软化、鼓包和爆裂的动态失效过程。获得的柱壳失效方式与实验现象基本吻合;还给出了不同内压作用下柱壳的破坏时间,获得一些定性的认识。     

飞秒激光对金属材料表面烧蚀区的XRD分析

在激光脉冲宽度为30fs,重复频率为10Hz,靶面处激光功率密度为1013W/cm2条件下,开展了真空中飞秒激光对铝、铜材料的烧蚀效应研究。对材料表面烧蚀区进行X射线衍射分析的结果表明,材料的表面烧蚀区没有发生相变,X射线的反射强度发生变化,但该区晶面对X射线的反射强度随激光功率密度的增加发生了变化,其衍射峰处出现了明显的展宽并出现分叉,说明该区域的不同晶面对飞秒激光的吸收具有明显的取向作用。     

预应力加载下LY12-CZ激光烧蚀特性的实验与模拟研究

飞行器是激光武器的主要打击目标,对常用航空材料激光辐照响应的研究具有重要价值.本文通过实验和有限元模拟的手段,研究了预应力加载下,1080 nm连续激光烧蚀LY12-CZ铝合金的失效机理.实验结果表明,激光烧蚀沿光斑径向存在三种组织,分别为近光斑的枝晶组织、中部的等轴晶组织及远离光斑的原始组织;其中,枝晶、等轴晶组织中存在严重的Cu元素偏析,且等轴晶组织还存在较高的内应力.有限元模拟的结果表明,温度场对所施加的预应力并不敏感;烧穿孔洞形成的一个原因是局部因热软化和高水平热应力造成的屈服;预应力不能显著改变激光的烧蚀特性.     

纳秒脉冲激光烧蚀非晶合金的研究进展

非晶合金是一类原子排列处于长程无序状态的新型结构材料,具有一系列优异的力学和物理性能,从而有望应用于国防、空天等关键领域.在这些领域的应用中,非晶合金易受到强激光或空间等离子体的作用而失效.同时,非晶合金在高能激光辐照下的结构响应本身也极具科学意义.因此,近年来这方面的研究得到了越来越多的关注.论文将重点关注纳秒脉冲激光对非晶合金在大气和水环境下的烧蚀,针对熔化、流体动力学失稳、爆炸沸腾、气泡动力学等烧蚀过程中的几种典型现象,简要介绍相关的实验和理论研究进展.最后,对未来值得进一步研究的方向进行了概述.     

激光烧蚀下不同颗粒度Al-teflon的反应行为

为研究不同颗粒度对Al-teflon反应行为的影响,以颗粒度25μm、1μm和20～200 nm的Al粉和微米级Teflon粉混合制备的反应材料为研究对象,基于脉冲激光烧蚀实验,结合ICCD相机和光谱仪对反应过程中的自发光成像和发射光谱进行瞬态观测。研究结果表明,Al-teflon反应材料在激光烧蚀下的反应行为体现出典型的二次反应特征,具有持续燃烧特征和明显的后燃效应,也具有较长的能量释放时间;同时,其反应行为与Al粉颗粒度密切相关:初始阶段,反应随Al粉颗粒度的降低加剧,随着反应的进行,纳米级Al粉对应的反应材料后续反应能力逐渐下降,反应强度和反应时间都小于1μm铝粉对应的反应材料。     

PVDF压力传感器标定及在激光推进实验中的应用

详细介绍了PVDF压力传感器的设计方法和2个等效测量电路.利用SHPB系统对PVDF压力传感器的动态灵敏度进行了标定,结果显示,不同并联电阻对于PVDF压力传感器动态灵敏度的影响不大,但对脉冲推力上升时间的测量结果有影响,选择较小的并联电阻可以获得更逼真的测试信号.     

抛物面型激光推力器的热力冲击响应

通过实验、机理分析和数值模拟系统地分析了大气模式激光推进中抛物面型推力器的热力冲击问题。在分析激光推进中存在的4种热载荷（入射、辐射、透射和运流）的基础上,建立了相应的热力耦合动态计算方法。多脉冲推进的计算温升与实验结果吻合。计算表明,入射吸收和高温辐射是造成抛物面型激光推力器温升的主要原因,并预测推力器在熔化前首先发生拉伸破坏,揭示了激光推进中热力冲击破坏的机理和严重性。     

Investigation of possibility of flow field diagnostics by laser induced biacetyl phosphorescence

Experimental results of laser induced phosphorescence of biacetyl triplet³A _u in a mixture of biacetyl and N₂ show that: the lifetime of phosphorescence is a function of temperature and independent of density and concentration; the initial phosphorescence intensity is a linear function of density and insensitive to the variation of temperature. The temperature and density distribution of gas flow could be measured by observing the phosphorescence lifetime and initial intensity of biacetyl mixed with N₂ respectively. The velocity distribution could be measured by observing the time-of-flight of the gaseous phosphorescent spot under pulsed laser excitation. Compared with Doppler anemometer, it, to a great extent, avoids the particle lag problem in flow field with large velocity gradient. The phosphorescence decay mechanism is also analysed and the analytical results agree with experimental ones. The project supported by National Natural Science Foundation of China and The Third World Academy.     

平面碰撞与强激光加载下金属铝的层裂行为

在轻气炮和神光Ⅱ强激光装置上开展了金属铝的层裂实验。针对激光打靶层裂实验中样品自由面速度剖面后期振荡容易丢失问题,改进靶设计,获得很好效果。利用轻气炮加载和强激光加载层裂实验应变率的显著差异,并通过数值模拟,讨论了在建立具有预测能力的理论建模中需要关注的损伤成核、演化与汇合问题中的材料特性与应变率相关特性因素。结果表明,对于我们以前建立的动态损伤与断裂模型,微孔洞成核的平均半径、阈值压力、成核速率相关参数以及微孔洞长大的阈值压力等具有材料特性属性,但微孔洞的表面能以及决定材料发生完全层裂的临界损伤度等具有明显的应变率效应。另外,分析还发现,虽然层裂强度具有明显的应变率效应,但是在样品层裂当地,样品由持续拉伸向收缩转变的临界行为,取决于一个很小的临界损伤,这个临界值很可能是材料常数,与应变率无关。     

Numerical simulation of vortex ring formation in the presence of background flow with implications for squid propulsion

Numerical simulations are used to study laminar vortex ring formation under the influence of background flow. The numerical setup includes a round-headed axisymmetric body with an opening at the posterior end from which a column of fluid is pushed out by a piston. The piston motion is explicitly included into the simulations by using a deforming mesh. A well-developed wake flow behind the body together with a finite-thickness boundary layer outside the opening is taken as the initial flow condition. As the jet is initiated, different vortex evolution behavior is observed depending on the combination of background flow velocity to mean piston velocity () ratio and piston stroke to opening diameter () ratio. For low background flow () with a short jet (), a leading vortex ring pinches off from the generating jet, with an increased formation number. For intermediate background flow () with a short jet (), a leading vortex ring also pinches off but with a reduced formation number. For intermediate background flow () with a long jet (), no vortex ring pinch-off is observed. For high background flow () with both a short () and a long () jet, the leading vortex structure is highly deformed with no single central axis of fluid rotation (when viewed in cross-section) as would be expected for a roll-up vortex ring. For , the vortex structure becomes isolated as the trailing jet is destroyed by the opposite-signed vorticity of the background flow. For , the vortex structure never pinches off from the trailing jet. The underlying mechanism is the interaction between the vorticity layer of the jet and the opposite-signed vorticity layer from the initial wake. This interaction depends on both and . A comparison is also made between the thrust generated by long, continuous jets and jet events constructed from a periodic series of short pulses having the same total mass flux. Force calculations suggest that long, continuous jets maximize thrust generation for a given amount of energy expended in creating the jet flow. The implications of the numerical results are discussed as they pertain to adult squid propulsion, which have been observed to generate long jets without a prominent leading vortex ring. PACS 02.60.Cb, 47.32.cf, 47.32.cb, 47.20.Ft, 47.63.M-     

固体发射药等离子体点火过程及弹道效应分析

总结了常规火炮点火的现状,分析了固体药电热化学炮（ＳＰＥＴＣ）点火的潜在优势,并对等离子体点火过程中存在的燃速增强机理进行了讨论。作为等离子体点火的弹道效应分析,利用电热增强固体药内弹道、等离子体发生器、脉冲功率源三者的耦合编码,计算了不同点火功率、不同毛细管径对ＳＰＥＴＣ炮弹道性能的影响。     

Pulse laser ablation of ground glass

We have studied pulse laser ablation of ground glass surface. Ablation process in air and in vacuum has been observed by high speed framing camera. Burst of small fragments of glass has been observed in the present experiment, when ground glass surface is laser ablated through glass plate from rear side. Production of macro particles by laser ablation is an inherent characteristic of ground glass, and no similar phenomena have been observed in case of ablation of other transparent materials. By using ns-duration Nd:YAG laser of 100–400 mJ/pulse, observed maximum particle velocity ranges from 0.5 to 1.5 km/s. In order to understand the particle generation process, scanning electron micrograph observation of the ground surface in the region of no laser irradiation and also in the damaged region, has been made. Cleavage surface structure has been evidenced in the damaged surface area, which stems from plenty of micro cracks covering the virgin glass surface. Effects of surface roughness on the particle generation were studied by using ground glass of quartz with different surface roughness. Produced glass particles were captured in vacuum by a polymethylmetacrylate (PMMA) plate. Size distribution was obtained by analysing the trapped particles on the PMMA plate and revealed that most probable particles size has almost no dependence on the initial surface roughness. Finally, we applied the phenomena to ignite pentaerithritoltetranitrate (PETN) powder explosive, and succeeded in igniting (PETN) powder only by laser ablation of ground glass.