Coherent underwater communication using passive time reversal over multipath channels

Passive time reversal exploits underwater acoustic channels’ spatial and temporal diversity. It can refocus multipath propagated signal at the receiver and can be realized simply by the passive phase conjugation (PPC) method. By the temporal focusing, time delay spread caused by multipath propagation is mitigated for spectral efficient coherent communications. However, the PPC method is unable to eliminate multipath and is limited by channel variations. An adaptive equalizer is therefore needed to compensate residual multipath after refocusing and to track channel temporal variations. Spatial diversity is obtained by using a vertical receiving array. In this paper we used 4-hydrophone array and demonstrated that the adaptive decision feedback equalization in conjunction with PPC significantly decreased the bit error rate.     

双破片同时撞击对B炸药冲击起爆的数值模拟研究

 采用Lee-Tarver点火增长模型对单钨珠撞击带壳B炸药的过程进行了计算,所得到的不同尺寸钨珠引爆炸药的阈值速度与实验结果吻合良好。数值模拟了双钨珠同时撞击带壳B炸药的过程,计算了破片引爆炸药的阈值速度,分析了炸药的点火增长过程。结果表明：双破片同时撞击炸药时,引爆阈值速度随着破片间距的增大呈抛物线规律增大;当破片间距较大且撞击速度略高于阈值速度、双破片同时撞击时,炸药内部爆轰波的初始形态随破片速度而变化。     

微波射频场在微波管中产生爆炸电子发射特性初步分析

 对微波射频场在微波管内引起的场致发射和爆炸电子发射及等离子体的产生进行了分析,推导了等离子体产生强度与微波振幅、材料的电阻率、热传导系数、质量密度和比热容之间的关系,得到了晶须温度分布的表达式,通过数值解析的方式总结出在远大于微波周期的时间尺度上晶须温度提高随时间线性上升。在模型所述材料特性下,温度的上升率达到了3.22×10¹⁰ ℃/s,在100 ns量级就可以使晶须发生气化形成等离子体。     

Effects of phase explosion in pulsed laser deposition of nickel thin film and sub-micron droplets

Nickel (Ni) thin films were deposited on glass substrates in high vacuum and at room temperature with third-harmonic or 355-nm output from a nanosecond Nd:YAG laser. At low laser fluence of 1 J/cm², the deposition rate was about 0.0016 nm/shot which increased linearly until 4 J/cm². Above 4 J/cm², the onset of phase explosion in the ablation abruptly increased the optical emission intensity from laser-produced Ni plume as well as thin-film deposition rate by about 6×. The phase explosion also shifted the size distribution and number density of Ni droplets on its thin-film surface. On the other hand, the surface structures of the ablated Ni targets were compared between the scan-mode and the fixed-mode ablations, which may suggest that droplets observed on the thin-film surface were caused by direct laser-induced splashing of molten Ni rather than vapour-to-cluster condensation during the plume propagation.     

The mechanism of formation of copper aluminide in the thermal explosion mode

The mechanism of formation of copper aluminides in the thermal explosion mode was studied. A molded mixture of copper and aluminum powders was heated to the self-ignition temperature by the radiation of a tungsten heater. The phase transformations were detected by time-resolved X-ray diffraction (TRXRD). The kinetics of heat release due to the chemical reaction was studied by measuring the temperature of the sample. The macrokinetic stages of the process were revealed, and the apparent activation energy of each stage was estimated.     

飞秒激光氘团簇库仑爆炸引发核聚变的机理研究

在超强飞秒激光与氘团簇的相互作用中，利用库仑爆炸模型，分析了可以引发核聚变的高能氘核产生的机理，提出了氘离子团簇膨胀尺寸与时间的关系式，计算了多种尺寸的氘团簇库仑爆炸时氘核的动能以及氘团簇的解体时间． 关键词： 飞秒强激光 氘团簇 库仑爆炸 核聚变     

爆炸力学数值模拟中本构建模问题的讨论

就爆炸力学数值模拟中的本构建模问题的有关内容作了讨论,包括:波传播研究与材料的动态特性研究的相互依赖、容变律与畸变律的解耦与耦合、率型本构关系与失效准则、应变率效应与温度效应的等效性、加载本构关系与卸载本构关系以及流变过程与损伤演化过程的耦合等,并展望了今后的有关发展动向。     

高分子梯度溶液吸声机理的研究

制备了高分子均匀溶液和梯度溶液,并在声管中测试其声衰减性能.实验结果表明,高分子梯度溶液的声衰减效果明显优于(即大于)相应的均匀溶液的声衰减.根据连续分层介质中声波传播理论建立了计算高分子梯度溶液声衰减的数学模型.计算结果与实验结果一致.由实验和理论分析结果得出了高分子梯度溶液的梯度吸声机理,即多次反射、多次吸收,最终将声能转化为热能.     

Investigation of the local structure variance of water molecules in laser-induced thermal desorption process

This paper presents the use of molecular dynamics simulation in the study of laser-induced thermal desorption (LITD) of water molecules adjacent to a laser-heated Au substrate. The local structure of the water molecules is investigated by considering the densities of the oxygen and hydrogen atoms, the average number of neighbors, n_NN, and the average number of H-bonds, n_HB. At an equilibrium temperature of 300 K, the simulation results show that three adsorption water layers are formed in the immediate vicinity of the Au surface, and that each four-fold hollow site on the uppermost Au(0 0 1) surface is occupied by a single water molecule. Following laser-induced heating of the Au substrate with a sub-picosecond laser pulse of 350 fs, the substrate temperature increases to 1000 K. This causes a gradual heating of the adjacent water film, which is accompanied by a decrease in the values of n_NN and n_HB. Hence, it can be concluded that an increase in the water film temperature destroys the hydrogen-bonding network throughout the water film. Although the maximum local temperature of the water film occurs in the region immediately adjacent to the Au substrate, it is determined that the attractive energy between the Au atoms and the water molecules in this region causes the water molecules to aggregate together to form three-dimensional water clusters. Furthermore, this energy prevents the hydrogen bonds in this region from breaking apart as violently as those within the phase explosion region. Finally, it is observed that the phase explosion phenomenon occurs in the region of the water film where the values of n_NN and n_HB are at a minimum.     

Thermal explosion in multiple phase media

Thermal explosion (or self-ignition) is one of the mainly interesting and dangerous phenomena in the combustion. Homogeneous combustible gas mixtures with highly exothermic reaction reveal only two main types of the behaviour: explosive and slow. Transient regime between these main types of behaviour is traditionally called the thermal explosion limit. Addition of another phase (solid or liquid) into combustible gas mixture can lead to essential changes in the dynamical situation. This paper deals with an interesting phenomenon of thermal explosion in two-phase media, so-called thermal explosion with delay. Existence and type of temperature delay before the self-ignition depends on mass and thermal properties of the second phase. The analysis is based on qualitative theory of singular perturbed systems and duck techniques.