掺杂对CH样品Rayleigh-Taylor不稳定性增长的影响

在神光II激光装置上进行了辐射驱动不同掺杂样品的单模Rayleigh-Taylor(RT)不稳定性实验.结果显示:与纯碳氢(CH)样品相比,掺Br的CH样品的扰动更早、更快地进入非线性区,产生二次谐波,并且掺Br比例越高,CH样品扰动进入非线性区的时间越早,相同时刻扰动的二次谐波的幅度越高.这是因为密度梯度效应抑制了二次谐波的产生,掺Br比例越高,密度梯度标长越小;同时密度梯度效应还抑制三次谐波对基模增长的负反馈,造成基模具有更大的线性增长,导致线性饱和幅值大于经典值0.1λ.     

界面张力对Rayleigh-Taylor不稳定性的影响

将具有简单速度势的Layzer模型和Zufiria模型推广至非理想流体情况, 并分别利用这两种模型研究了界面张力对Rayleigh-Taylor不稳定性的影响. 首先得到了两种模型下气泡的渐近速度和渐近曲率的解析表达式; 其次系统研究了界面张力对气泡的渐近速度和渐近曲率的影响; 最后将两种模型进行了比较, 并将气泡的渐近速度和数值模拟进行了比较. 研究表明: 界面张力压低了气泡的速度, 但对曲率没有影响; 利用简单速度势的Layzer模型所得的气泡的渐近速度比复杂速度势的Layzer模型的值小, 但是比Zufiria模型的值大; 当阿特伍德数等于1时, 简单速度势的Layzer模型和复杂速度势的Layzer模型给出的结果一致. 关键词： Rayleigh-Taylor不稳定性 界面张力 Layzer模型 Zufiria模型     

辐射烧蚀界面的单模Rayleigh-Taylor不稳定性分析

首先对惯性约束聚变(ICF)遇到的流体力学界面不稳定性作了简要的分析;其次讨论了Rayleigh-Taylor(RT)不稳定性在ICF中所起的作用;第三,根据一维数值计算结果,按照考虑密度梯度和烧蚀致稳的RT不稳定性增长的定标规律对不稳定性增长作了估计。最后在计算结果的讨论中,对有关实验测量提出某些倾向性的建议。     

斜界面Rayleigh-Taylor不稳定性混合实验研究

 实验研究了不相溶流体斜界面Rayleigh-Taylor（R-T）不稳定性湍流混合区的混合不对称性特征。利用高压气体加速装有不同液体的箱体,加速度方向由轻液体指向重液体,此时界面是R-T不稳定性的。利用阴影测试技术,研究了初始倾角9°的ZnCl₂溶液/正己烷斜界面的演化规律,得出混合区内混合不对称、斜界面倾角渐增的规律。     

磁场对激光驱动Rayleigh-Taylor不稳定性影响的数值研究

Rayleigh-Taylor不稳定性(RTI)作为流体和等离子体中基础的物理现象,在天体物理、空间物理以及工程领域扮演着重要角色.尤其在惯性约束核聚变(ICF)研究中, RTI等宏观流体不稳定性是不可回避的物理问题.本文利用开源的辐射磁流体模拟程序FLASH对激光驱动调制靶产生的RTI进行了二维的数值模拟,系统地考察和比较了RTI在无磁场、Biermann自生磁场、不同外加磁场情况下的演化.模拟结果表明, Biermann自生磁场和平行流向的外加磁场在RTI演化过程中基本不会改变RTI的界面动力学,而垂直流向的外加磁场对RTI以及RTI尖钉尾部的Kelvin-Helmholtz涡旋有致稳作用,其中磁压力起主导作用.研究结果为后续开展和ICF相关的靶物理研究提供借鉴,也有助于加深对流体混合过程的理解.     

二维不可压流体Kelvin-Helmholtz不稳定性的弱非线性研究

通过将扰动速度势展至三阶,提出了Kelvin-Helmholtz（KH）不稳定性的弱非线性理论.在模耦合过程中观察到一个重要的共振现象,共振使得模耦合过程变得相当复杂,单模扰动很快进入非线性区,产生大量高次谐波,共振加强了非线性作用.分析了单模扰动中二次和三次谐波产生效应,以及对基模指数增长的非线性校正.模拟结果支持了解析理论.利用该理论,分析了KH不稳定的非线性阈值问题. 关键词： Kelvin-Helmholtz不稳定性 弱非线性理论 非线性阈值     

黏性、表面张力和磁场对Rayleigh-Taylor不稳定性气泡演化影响的理论分析

采用理论分析的方法考察了磁场中非理想流体中Rayleigh-Taylor(RT)不稳定性气泡的演化过程,在与磁场垂直的平面中,综合考虑流体黏性和表面张力的影响,推导了二维非理想磁流体RT不稳定性气泡运动的控制方程组,给出了不同情况下气泡速度的渐近解和数值解,分析了流体黏性、表面张力和磁场对气泡发展的影响,分析结果表明:流体黏性和表面张力能够降低气泡速度和振幅,即能够抑制RT不稳定性;而磁场对RT不稳定性的影响是由非线性部分引起的,并且磁场非线性部分的方向决定了磁场是促进还是抑制RT不稳定性的发展,     

可压缩流体的Rayleigh-Taylor和Kelvin-Helmholtz不稳定性

可压缩流体界面不稳定性的小扰动研究中，往往首先规定具体的状态方程形式，然后给出色散关系。这使得所给出的结果仅适用于该状态方程的具体形式，具有特殊性。由于这种特殊性，使得某些机理的讨论变得复杂，且结论相互矛盾。     

表面张力对高雷诺数Rayleigh-Taylor不稳定性后期增长的影响

采用多相流的相场格子Boltzmann方法数值研究了微通道内高雷诺数单模Rayleigh-Taylor(RT)不稳定性的后期演化规律,重点分析表面张力对相界面动力学行为以及气泡与尖钉增长的影响.数值实验表明,随着界面张力的增大,可以有效降低演化过程中相界面结构的复杂程度,并抑制不稳定性后期相界面破裂形成离散液滴.另外,...     

烧蚀RT不稳定性非线性过程模拟

重点研究了不同预热对烧蚀RT弱非线性模耦合的影响。首先不同预热情况烧蚀RT不稳定性的线性增长率曲线明显不同。不同预热情况的模拟结果都表明：烧蚀RT的模耦合系数是k／kc的函数，二次和三次谐波的产生系数C（k）和D（k）遵从定标关系C（k）：c1（1-c2k/kc），D（k）=d1-d2k/kc＋d3（k/kc）^2。弱预热和中等预热情况，二次谐波的产生系数约只有经典RT的50％，三次谐波的产生系数约只有经典RT的20％。两体模耦合起主要作用，n次高次谐波可近似看作是由（”一1）次两体模耦合产生的。由于烧蚀RT不稳定性两体模耦合系数小于1，所以高次谐波的产生系数很快衰减。由于非线性作用变弱，单模扰动的非线性饱和阈值明显增大，意味着基模有较长时间的线性增长，另外，向长波长方向的模耦合系数明显大于经典尺丁不稳定性数值，因此，烧蚀RT非线性模耦合容易形成长的尖顶，从而对点火构成严重威胁。     

Weakly Nonlinear Rayleigh-Taylor Instability in Incompressible Fluids with Surface Tension

A weakly nonlinear model is established for incompressible Rayleigh—Taylor instability with surface tension. The temporal evolution of a perturbed interface is explored analytically via the third-order solution. The dependence of the first three harmonics on the surface tension is discussed. The amplitudes of bubble and spike are greatly affected by surface tension. The saturation amplitude of the fundamental mode versus the Atwood number A is investigated with surface tension into consideration. The saturation amplitude decreases with increasing A. Surface tension exhibits a stabilizing phenomenon. It is shown that the asymmetrical development of the perturbed interface occurs much later for large surface tension effect.     

表面张力对Rayleigh-Taylor不稳定性谐波的影响

Using the method of the parameter expansion up to the third order, explicitly investigates surface tension effect on harmonics at weakly nonlinear stage in Rayleigh-Taylor instability (RTI) for arbitrary Atwood numbers and compares the results with those of classical RTI within the framework of the third-order weakly nonlinear theory. It is found that surface tension strongly reduces the linear growth rate of time, resulting in mild growth of the amplitude of the fundamental mode, and changes amplitudes of the second and third harmonics, as is expressed as a tension factor coupling in amplitudes of the harmonics. On the one hand, surface tension can either decrease or increase the space amplitude; on the other hand, surface tension can also change their phases for some conditions which are explicitly determined.     

Effects of Compressibility on the Rayleigh-Taylor Instability in Z-pinch Implosions

In this paper we shall consider the effect of compressibility on the RT instability in Z-pinch implosions, importance is the comparing growth rates of the RT instability for two systems of the compressible and incompressible MHD plasma. For which reason, we shall use as simple model as possible. Obviously, slab geometry is the most simple. For example, in the case of annular plasma implosion, during the linear growth phase of the RT instability there are vacuums at both sides of the annular plasma shell and its thickness is sufficiently smaller than the pinch radius, allowing us to use slab geometry instead of the annular one. For simplicity, we do not consider the effects of the finite Larmor radius and the sheared axial flow which are the important physical mechanisms to compress the RT instabilities.     

Jet-Like Long Spike in Nonlinear Evolution of Ablative Rayleigh–Taylor Instability

We report the formation of jet-like long spike in the nonlinear evolution of the ablative Rayleigh-Taylor instability （ARTI） experiments by numerical simulations. A preheating model k（T） = KSH[1＋f（T）], where KSH is the Spitzer Harm （SH） electron conductivity and f（T） interprets the preheating tongue effect in the cold plasma ahead of the ablative front [Phys. Rev. E 65 （2002） 57401], is introduced in simulations. The simulation results of the nonlinear evolution of the ARTI are in general agreement with the experiment results. It is found that two factors, i.e., the suppressing of ablative Kelvin Helmholtz instability （AKHI） and the heat flow cone in the spike tips, contribute to the formation of jet-like long spike in the nonlinear evolution of the ARTI.     

A Weakly Nonlinear Model for Kelvin-Helmholtz Instability in Incompressible Fluids

A weakly nonlinear model is proposed for the Kelvin-Helmholtz instability in two-dimensional incompressible fluids by expanding the perturbation velocity potential to third order. The third-order harmonic generation effects of single-mode perturbation are analyzed, as well as the nonlinear correction to the exponential growth of the fundamental modulation. The weakly nonlinear results are supported by numerical simulations. Density and resonance effects exist in the development of mode coupling.     

Quantum Effects on Rayleigh–Taylor Instability of Incompressible Plasma in a Vertical Magnetic Field

Quantum effects on Rayleigh-Taylor instability of a stratified incompressible plasmas layer under the influence of vertical magnetic field are investigated. The solutions of the linearized equations of motion together with the boundary conditions lead to deriving the relation between square normalized growth rate and square normalized wave number in two algebraic equations and are numerically analyzed. In the case of the real solution of these two equations, they can be combined to generate a single equation. The results show that the presence of vertical magnetic field beside the quantum effect will bring about more stability on the growth rate of unstable configuration.     

Weakly nonlinear theory for the ablative Rayleigh-Taylor instability

A weakly nonlinear model is proposed for the Rayleigh-Taylor instability in the presence of ablation and thermal transport. The second harmonic generation efficiency of a single-mode disturbance is computed, as well as the nonlinear correction to the exponential growth of the fundamental modulation. Mode coupling in the spectrum of a multimode disturbance is thoroughly analyzed. The ablative stabilization can be clearly discussed because the derived formulas for the evanescent ablation rate are in agreement with previously known results for incompressible, inviscid, irrotational, and immiscible fluids [S. W. Haan, Phys. Fluids B 3, 2349 (1991)]; M. Berning and A. M. Rubenchik, Phys. Fluids 10, 1564 (1998)]].     

Numerical Simulation of Nonlinear Effects in the Weibel Instability

Radiophysics and Quantum Electronics - We analyze in detail the nonlinear effects taking place in an anisotropic plasma at different stages of the Weibel instability using a new approach to the...