液相放电等离子体特性与激波特性的数值分析

以能量平衡方程为基础,采用不同的电导率唯象模型描述了液相放电等离子体圆柱形通道特性,得到了通道内半径、温度、电阻、电流和耗散能量随时间的变化关系,还给出了距离放电间隙中心一定距离处的冲击波压力变化,并与前人利用等离子体通道球状模型计算得到的结果进行了比较。结果表明:把等离子体通道看成球状和看成圆柱状在描述通道压力和通道半径时差异显著,而在描述其他物理特性时差别不大;三种电导率模型在描述等离子体通道物理特性时,变化趋势大体相同,而在描述激波特性时,电导率模型σ₂更符合实际;通过对比电学参数与压力参数的变化,就可以在实验中根据实验数据以及具体的研究问题进行模型的适用性选择。     

水中针-板结构小能量脉冲火花放电特性北大核心CSCD

开展了J量级系统储能下电脉冲参数对水中火花放电特性影响研究。驱动源采用参数可调的固态重频纳秒脉冲电源,放电负载为水中针-板结构(间距1 mm),在低重频条件(约5 Hz)下进行实验。通过调节放电参数、拍摄高速阴影图像、光谱诊断以及声信号测量,研究水中脉冲放电的物理特性,得到不同放电参数下放电演化规律及其对声学、光谱特性影响。实验发现:在J量级储能下,放电通道连通两极后,回路电流在几百ns内快速上升至10 A左右,随后缓慢下降,持续50~60μs。发现预设脉宽对放电影响较大,短脉宽条件下放电会被电源固态开关强制截断出现反向放电,而长脉宽条件下放电通道在后期变得不稳定甚至熄弧中断,出现气泡中二次放电现象。辐射光谱揭示了更多等离子体信息,推断通道电子密度在1018 cm-3量级,随着脉宽增加,特征谱线强度增加,表明活性粒子数密度增加,但粒子种类不变。短脉冲(<150μs)作用下产生的脉冲声波的特征宽度在110~150μs,而当脉宽继续增大,声波脉宽并不继续增加而是保持不变,保持在150μs左右。研究结果对水中小能量火花放电的机理研究有一定参考价值,为水声学、液相等离子体等领域的应用提供思路。     

水中脉冲放电等离子体通道特性及气泡破裂过程

利用高速摄影法对水中放电等离子体通道的边界层进行了研究,结果表明,当输入到单位长度通道内的功率密度较小时,通道边界存在较厚的过渡层,其亮度明显小于通道中间部分.根据等离子体通道的高速阴影照片指出该通道沿轴向具有非均匀性.另外,还给出了等离子体气泡最后破裂时的高速阴影照片,并对气泡破裂过程作了初步的探讨 关键词： 水中放电 等离子体     

光谱法研究闪电回击放电通道的电阻及电流的热效应

闪电放电通道的电阻及电流产生的热效应对雷电灾害研究以及防护设计都具有重要意义,放电通道的热力学特性与其等离子体辐射光谱密切相关。利用无狭缝摄谱仪获得的两次云对地多回击闪电放电的等离子体辐射光谱,依据谱线波长、强度等信息,结合同步地面电场变化资料,应用空气等离子体传输理论,计算了闪电回击放电通道的电导率、峰值电流、核心通道半径,进而得到了闪电回击等离子体通道单位长度的电阻、峰值电流时的热功率及在回击初始前5 μs内通道储存的热能。并与常规金属导体进行比较,分析了闪电回击放电在峰值电流时等离子体通道的热功率与电阻、电流平方之间的相关性关系。结果表明：利用光谱研究得到的闪电放电通道的电阻为0.04～8.41 Ω·m-1、峰值电流时的热功率为0.88×108～2.20×108 W·m-1、回击初始前5 μs内通道储存的热能为1.47×102～3.66×102 J·m-1,以上结果与文献报道的利用其他方法得到的结果相比,在合理的范围内;与常规金属导体相比,闪电回击放电等离子体通道在峰值电流时的热功率与电阻成正比,但与电流的平方呈指数减小的关系;由于闪电等离子体通道的电阻与温度的3/2次方成反比,通常回击放电通道中峰值电流越大,通道温度越高,而电阻会迅速降低,因此热功率也会急剧减小。此结论进一步验证了采用欧姆加热方法加热等离子体的致命缺点。     

空间微小碎片超高速撞击诱发的等离子体特性研究

空间微小碎片超高速撞击航天器表面可产生稠密的等离子体,随着等离子体的扩散可导致静电放电及电磁干扰脉冲的发生,近而威胁航天器在轨安全.本文利用等离子体驱动微小碎片加速器研究了质量为10 5g的空间碎片撞击产生的等离子体基本特性,给出了等离子体总电荷与微小碎片速度之间的关系,获得了等离子体扩散速度参数及等离子体电子密度随时间和空间的演化关系,研究结果对于揭示空间微小碎片撞击诱发放电和电磁干扰脉冲形成的机理具有重要意义.     

三探针方法脉冲放电等离子体特性

基于三探针方法开展了脉冲放电等离子体特性研究,实现了单次脉冲放电等离子体参数的时变特性诊断。采用金属罩屏蔽、示波器锂电池供电等方法降低了电磁信号干扰,利用Labview编制了特定的程序进行三探针诊断数据处理。根据脉冲放电等离子体具有多电荷态离子成分、离子超声速运动等特点,对三探针理论进行相应修正。诊断结果表明,整个放电脉冲内高压引出界面电子温度Te处于2~4eV之间,离子密度ni处于1017~1018 m-3量级之间,与Langmuir单探针诊断结果吻合。     

脉冲放电产生螺旋流注的等离子体特性研究

通过脉冲放电方式产生三维螺旋形的等离子体放电通道,在高速摄像机拍摄下观察到放电通道中的发光电离体以流注形式沿螺旋轨迹快速传播.建立电磁模型解释螺旋放电的形成机制,对造成对称性破缺及影响其手性性质的极向电场进行分析.研究表明,螺旋放电产生的放电通道存在两种不同的手性特征,而脉冲重复频率等放电参数及边界条件对螺旋流注的传播特性存在影响.脉冲电源驱动的电磁场在介质管内所形成的波模是极向电场形成的一个重要来源,当极向电场与轴向电场强度相近时则形成螺旋流注放电.     

脉冲放电破岩等离子体通道长度预测方法

针对高压脉冲放电破岩电弧等离子体通道长度难以预测的问题,构建了高压脉冲放电破岩综合试验平台,测量了花岗岩-自来水组合介质下电弧等离子体通道发展特性及典型电流、电压参数,提取了不同电极间距和脉冲放电次数下岩石表面形成的破碎区域。基于能量平衡方程建立了岩石中电弧等离子体通道的阻抗模型,采用迭代优化算法获取阻抗模型参数的近似最优解,模型计算结果与试验结果的相对误差小于7%。基于优化参数,利用实测电流电压数据预测了等离子体通道的长度。模型预测的等离子体通道长度与实测值的绝对误差均处于毫米量级,且两者的相对误差小于10%,为高压脉冲放电破岩系统电源-电极负载的匹配设计提供了理论支撑。     

纳秒脉冲电压下同轴场畸变开关多通道放电特性

使气体开关形成多通道放电是减小开关电感、通道电阻、电极熔蚀,提高开关寿命和稳定性的有效措施。设计了一种同轴场畸变气体火花开关,研究了开关在纳秒脉冲电压下的多通道放电现象。研究了脉冲电压上升速率与多通道放电特性的关系,比较了两种体积比的SF6/Ar混合气体多通道放电特性。实验结果表明:一定气压下,平均通道数随着脉冲电压上升率增加而增多,电流分布趋向均匀;相同脉冲峰值与气压比值,不同气压下,高气压下的通道数较多;SF6/Ar混合气体中,氩气含量较高情况下多通道放电特性较好。最后,结合J.C.Martin的多通道放电理论对实验结果作出了初步解释。     

气体脉冲放电等离子体阻抗的参量研究

在纯净氦气条件下，对高频纵向脉冲放电的等离子体动态阻抗进行了实验研究.测量了放电等离子体阻抗随放电电压、频率以及气体压强之间的变化关系，表明等离子体阻抗随频率和电压的增加而下降，随气压的升高而增大.定性分析了这些宏观参量与等离子体阻抗变化之间的关系，拟合出等离子体阻抗随电压、频率、气压变化的经验公式，并对数值模拟与实验结果进行了比较研究. 关键词： 等离子体阻抗 气体脉冲放电     

Excitation of the acoustic and leaky waves in the atmosphere by a sub-surface seismic source

We study excitation of acoustic, leaky, and surface waves by a time-harmonic force source located in a homogeneous isotropic elastic half-space contacting a homogeneous gas. The force acts in the normal direction to the interface between the media. We consider the case where the sound velocity in the gas is less than the velocity of the Rayleigh wave propagating along the surface of the solid. An expression is derived for the period-averaged radiation power of the surface Stoneley wave. The total radiation power is calculated for the acoustic wave in the gas and for the leaky pseudo-Rayleigh wave. Variations in the radiation powers of the surface and leaky waves are analyzed as functions of the source depth. If the velocities of compressional and shear waves in the elastic medium significantly exceed the sound velocity in the gas, then the radiation power of the Stoneley wave turns out to be a factor of 10⁶–10⁸ smaller than the radiation powers of other waves. The radiation power of the Stoneley wave decreases monotonically with increasing source depth, and the decrease becomes more pronounced with the increase in the difference between the acoustic impedances of the contacting media. If the shear-wave velocity in the solid is close to the sound velocity in the gas, then the radiation power of the Stoneley wave is comparable with the radiation powers of other waves and exhibits maximum at a certain source depth. For some parameters of the gas and the solid, and for certain source depths, the Stoneley wave carries away more than a half of the total radiation power. It is shown that, for certain relations between the parameters of the media, the radiation power of the Stoneley wave increases due to redistribution of the radiated power from the pseudo-Rayleigh leaky wave. The total power of these waves remains approximatly constant and, with accuracy of the order of 10⁻³, is equal to the radiation power of the Rayleigh wave at the vacuum-solid interface. It is shown that the acoustic-wave power which can be transmitted to the upper layers of the atmosphere during an earthquake does not exceed 0.01% of the total power radiated at a given frequency. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 7, pp. 577–592, July 2006.     

Modeling of pulsed spark discharge in water and its application to well cleaning

This paper presents a simulation model for the generation of strong pressure wave by means of pulsed spark discharge in water and its application to well cleaning. In the simulation model, one-dimensional time-dependent magnetohydrodynamic equations are coupled to a capacitive discharge circuit equation. A cylindrical conducting spark channel formed by electrical breakdown of water gap between a pair of electrodes is treated as a load of which resistance and inductance are allowed to change with time. For describing the spark channel properties accurately, precise calculations on thermodynamic properties and electrical conductivity are included in the simulation model. The simulation results show an excellent agreement with the experimentally measured shock pressure as well as the current and voltage waveforms. The simulation reveals that Joule heating of the spark channel during the very early phase of electrical discharge plays a key role in the formation of shock wave in water. The voltage on a capacitor at breakdown, the circuit inductance, and the resistance of the spark channel are found to be the most important parameters for the shock wave formation. With this technique, a pilot test for the cleaning of a clogged well has been performed in a water well which was constructed as a test-bed for riverbank filtration near the Anseong-cheon (river) in Korea. Well treatments have been carried out with an electrical energy of 510 J stored on a pulsed power system, at which the maximum shock pressure is measured to be around 7 MPa at the position of the well screen, i.e. 0.1 m away from the spark gap. A slug test shows 2.9 times improvement in the hydraulic conductivity of the well, which, combined with a visual inspection inside the well using an underwater camera, clearly demonstrates that the strong pressure wave generated by underwater spark discharge can effectively remove almost all incrustations formed in the well screen and thus improve well performance. Operating parameters for controlling the strength of shock pressure are discussed using the simulation model for extensive applications of the present technique to various types of water wells.     

水下复杂壳体结构在多源激励下的振动及声辐射特性研究

为了综合评估水下复杂壳体结构在多源激励下的振动与声辐射特性,专门设计了一个多舱段复杂壳体模型,分别进行了单台设备激励、多台设备激励的壳体振动与声辐射试验。试验结果表明,多源激励下的结构响应和辐射声场可以近似为单源激励下结构响应和辐射声场的非相干叠加。通过对激励源与结构响应、辐射声场之间传递关系的理论分析,讨论了以单源激励响应非相干叠加合成得到多源激励响应的合理性和误差。研究结果对复杂壳体结构在实际激励下的振动与噪声预报具有重要价值。      

基于四水听器的充水弹性管声速测量方法*

实验室中水声材料声学参数的测量主要在水声声管中进行。管内平面波声速是正确测量这些参数的基础。该文提出一种基于四水听器结合不同边界的测量充水弹性管中声速的新方法。该方法利用4个固定位置处的水听器,采用最小二乘的方法,使得两组水听器分别得到的声管末端入射波声压差值的平方最小的声速即为管内平面波声速。该方法利用单频信号,在每一频率点均可测得声速,可以在任一种声管末端边界下进行测量,同时无需知道各水听器到边界的精确距离,在文中的3种边界下声速测量结果具有很好的一致性,实验操作简单、误差很小。该方法的仿真结果与管内声速的理论值吻合得很好,同时实验测量结果与仿真值之间的误差很小,证明了方法的准确性以及鲁棒性,为声管声速测量提供一个很好的思路。     

水下等离子体声源的冲击波负压特性

基于修正的Rayleigh气泡脉动方程对水下等离子体声源放电产生的 强声冲击波的传播过程进行了分析; 利用Euler方程作为控制方程组, 建立了水下等离子体声源的聚束声场模型, 通过仿真计算获得的传播云图对冲击波负压的形成机理进行了直观的理论分析. 结果表明: 经过聚能反射罩反射汇聚得到的聚束波在反射稀疏波和水的惯性作用下, 聚束波周围水域产生了拉伸, 形成负压区, 如果拉伸力大于水的抗拉上限, 就会使得水中形成不连续现象, 即出现空化气泡; 此外聚能罩边缘处产生的衍射波进一步加剧了负压的产生, 边缘衍射波最终与拉伸波叠加, 使冲击波负压达到最大值; 通过对比仿真波形和实验波形, 从而验证和进一步揭示了冲击波负压的形成原因. 研究结果对认识水下冲击波的传播规律和进一步改进等离子体声源的设计具有指导意义. 关键词： 等离子体声源 冲击波负压 聚束声场模型 气泡     

经典局域共振型水声超材料的吸隔声性能研究*

基于水声超材料吸声机理和多层平行介质平面波理论,建立局域共振型水声超材料结构,通过COMSOL进行建模计算,研究该结构的吸声性能机理,此外为了验证钢背衬的隔声性能,在该水声超材料结构基础上添加一层0.005m厚的钢背衬进行仿真对比。研究结果表明,在频段为200Hz-4000Hz时,水声超材料声学性能较好,吸声性能整体较优,且添加钢背衬的水声超材料隔声性能较优,甚至在某频率点达到15dB的隔声差值;此外通过位移场图进一步揭示水声超材料的吸声机理,发现水声超材料结构的位移场和钢背衬都对吸声性能会产生影响,钢背衬通过影响共振吸收来影响吸声性能,而位移场则通过位移幅度大小影响吸声性能。     

单声线水声MIMO信道容量的研究

本文对水声MIMO(Multiple-Input-Multiple-Output)信道容量受收发阵元数目、间距,收发阵位置、方向,平均接收信噪比以及声速剖面的斜率的影响,在收发阵元之间只存一条声线的情况下,通过WKB近似进行了初步的研究。从计算结果可以看出,当收发阵元对之间只存在一条声线时,阵元间距会对MIMO信道容量产生重要影响:当收发阵元间距足够大时, MIMO系统的信道容量将随着接收信噪比和收发阵元数线性增加,一个m×m的MIMO系统的信道容量将为相应的 SISO(Single-Input-Single-Output)系统的m倍,收发阵的方向也会对MIMO系统信道容量产生较大的影响,另外,收发阵的深度、距离也会对水声MIMO信道容量产生影响,声速剖面的斜率在一般水声信道的声速变化范围内,对信道容量的影响不大。     

水下强声频率对巴马小型猪肺脏病理损伤效应的观察

为探索大功率水声信号对入水哺乳动物肺脏的生物效应,以佩戴专用水下呼吸装置的巴马小型猪为实验对象,采用大功率连续声波为暴露手段,对水下正常体征状态的小型猪进行强声辐射,通过大体解剖、HE染色显微病理图像观察等方法,分析水下强声对哺乳动物肺脏器官的损伤效应。在300 Hz^10 kHz的正弦频段中,当声压级大于190 dB时,出现了不同程度的肺脏损伤。大功率水声信号对入水哺乳动物肺脏的生物效应显著,在相同声压级条件下,3 kHz水下连续波对实验动物肺脏病理损伤效应最大。      

Interfering modes and an axial wave in an underwater sound channel

Experiments on long-range propagation of low-frequency sound that were conducted starting from the mid-1980s indicate a complex character of propagation in an underwater sound channel, in which a source and a receiver are located close to the channel axis. A burst of energy propagating along the axis follows early arrivals, which are well described by the formulas of geometrical acoustics, in plots of acoustic intensity as a function of propagation time and hydrophone depth. This energy burst cannot be described using geometrical acoustics because of caustics with caustic beaks located near the channel axis. Very complex interference processes occur near these caustics. As the distance from the source grows, the dimensions of the interference vicinity increase and start to overlap producing a peculiar “axial wave.” For an arbitrary two-dimensional underwater sound channel, the axial wave can be represented as a sum of the first normal modes and a residue. This conclusion is based on the use of two representations for an acoustic field. The first of them includes the sum of ray components and an axial wave. The second representation consists of ray addends, the sum of the first normal modes, and a residue. Numerical results are obtained for a canonical profile of sound velocity at the frequency of 200 Hz for the distances of 1600–1650 km.