强爆炸火球数值模拟中的算子分裂方法

采用算子分裂方法将辐射流体力学方程组分裂为对流项和刚性源项,设计一种高效求解刚性源项方程组的数值方法.数值实验表明:该方法对时间步长不敏感,计算精度能够满足工程计算要求.在不对方程组做任何近似的情况下,数值给出了较长时间内火球冲击波阵面压力峰值及阵面位置的变化,结果与实际强爆炸中的经验公式吻合较好.     

介质初始密度对波阵面辐射特性的影响

 通过一维辐射流体动力学数值模拟,仔细研究了工作介质的初始密度对在氖中传播的强冲击波的波阵面辐射特性、波阵面亮度温度谱分布的作用。证明对于不同强度的冲击波,工作介质选用不同的初始密度,对其中传播的强冲击波波阵面辐射能力的提高是比较有效的。     

聚能装药水下爆炸冲击波载荷规律

对聚能装药水下爆炸近区压力分布进行了实验研究,将其结果与数值计算结果进行比较,验证了计算模型及方法的可靠性;利用数值计算方法研究了聚能装药水下爆炸冲击波的分布规律、不同装药形式冲击波的传播以及能量的分布。结果表明:水中近场冲击波具有较强的不均匀性,侵彻体前端的水介质中有较强的压力扰动,对正向冲击波有增强作用,等压线呈不规则形状,近场冲击波阵面呈"梨形"演化。     

强爆炸火球辐射流体自适应网格高精度数值模拟

提出一种自适应网格方法,应用于基于Euler方法的强爆炸辐射流体高精度数值求解.通过与Zinn数值结果对比,验证该方法的正确性.研究自适应网格对冲击波和光辐射输出模拟精度的影响,对比不同网格尺度下的计算耗时.在相同的条件下,使用自适应网格与均匀网格加密3倍得到的冲击波超压分布、光辐射输出演化接近,计算效率提升约8.5倍...     

柱形装药水中爆炸近场径向压力测试初探

 针对端面起爆柱形装药水中爆炸近场冲击波阵面呈现非球形传播的特点,采用分幅相机记录了炸药起爆36 μs内近场冲击波的传播情况,分析了冲击波在不同时间和不同位置的形状;采用两台高速扫描相机测量了侧边任意一点的冲击波速度,结合Rankine-Hugoniot关系,得到了冲击波阵面的压力;同时用LS-DYNA方法对侧边任意一点的压力进行了数值模拟,两者得到的结果基本一致。     

Z箍缩动态黑腔形成过程MULTI程序一维数值模拟

Z箍缩动态黑腔能够高效地将Z箍缩丝阵等离子体动能转换为黑腔辐射能,为驱动惯性约束靶丸聚变提供高品质的X射线辐射场.利用一维双温多群辐射磁流体力学程序MULTI-IFE,研究了"聚龙一号"装置驱动电流条件下的Z箍缩动态黑腔形成基本物理过程.数值模拟结果表明,在动态黑腔形成过程中,辐射热波的传播速度比冲击波的传播速度更快,比冲击波更早到达泡沫中心,使中心区域的泡沫在冲击波到达前就已具有较高的辐射温度.对于"聚龙一号"装置动态黑腔实验0180发次采用的负载参数,辐射热波和冲击波在泡沫中的传播速度分别约为36.1 cm/μs和17.6 cm/μs,黑腔辐射温度在黑腔形成初期约80 eV,在冲击波到达泡沫中心前可达100 eV以上,丝阵等离子体外表面发射的X射线能量集中在1000 eV以下.本文给出了程序采用的计算模型、美国"土星"装置丝阵内爆计算结果和"聚龙一号"装置动态黑腔实验0180发次模拟结果.     

地下强爆炸辐射流体动力学过程的二维数值模拟

为评估强爆炸的毁伤效应,利用流固耦合界面处理方法,考虑辐射输运引起的能量沉积过程,建立同时求解辐射流体力学方程和流体弹塑性方程的二维耦合计算方法,给出地下强爆炸时,爆室内辐射波传播、辐射能量沉积到岩土表层、岩土表层膨胀、冲击波在岩土介质中传播、爆室内高压压缩爆室壁、爆室壁振荡增大的完整物理过程.     

水下爆炸强冲击波与平板结构相互作用的理论分析方法

针对水下爆炸一维强冲击波与平板结构的瞬态流固问题开展研究,综合考虑流体和结构材料的可压缩性,引入状态方程建立强冲击波在板表面反射后的波阵面参数关系,得到板表面的反射系数。然后依据动量守恒定律建立平板的运动方程,求解得到板表面的壁压及板的速度时程,形成了水下爆炸强冲击波与平板结构相互作用的理论分析方法。在此基础上给出了强冲击波与平板相互作用的冲量传递比近似估算公式。最后开展平板结构的近距及接触水下爆炸实验,并结合数值计算对理论方法进行验证。结果表明,建立的理论方法与实验及数值模拟结果吻合良好,为水下近距爆炸强冲击波与结构的相互作用分析提供了理论基础。     

一维平面冲击波的移动网格算法

移动网格方法可以有效提高冲击波阵面计算的分辨率,多年来一直受到数值计算研究领域的关注。针对冲击波在凝聚介质中传播的数值模拟,利用变分原理构建了基于移动网格的自适应算法。移动网格的生成依赖包含控制函数的欧拉方程的迭代计算,通过控制方程从物理域到计算域的映射来求解物理量,并研究了不同移动网格迭代方法对计算效率的影响。数值结果显示了该算法的有效性。     

Z箍缩动态黑腔冲击波辐射图像诊断

在“聚龙一号”装置上开展了单层钨丝阵加载重泡沫的动态黑腔实验, 初步研究了Z 箍缩动态黑腔中冲击波传播和黑腔形成的物理过程. 获得了冲击波辐射环的演化图像, 分析了丝阵等离子体与泡沫的作用过程及动态黑腔内的辐射特性. 测得冲击波的向心传播速度为(14.2±1.7) cm/μs, 冲击波平均宽度为0.8-0.9 mm. 冲击波辐射环的发光强度沿角向分布的标准偏差约为±10%, 中心黑腔区的标准偏差约为±4.2%.     

Numerical Study on the Whole Process of Fireball Evolution in Strong Explosion *

With the actual mean free path for radiation in air and a simplified nuclear device, a one-dimension simulation research on fireball phenomenon in a sea level atmosphere is carried out based on the multi-group radiation hydrodynamic codes (RDMG). It is shown that our theoretical calculations can describe the whole process of the fireball evolution for strong explosions from the early X-ray expansion stage to the shock wave propagation stage. The radius of the shock wave and the brightness of the fireball are in good agreement with the experimental results. The whole thermal radiation power curve of the fireball evolution for strong atmospheric explosion at sea level is plotted for the first time. The impact of radiation opacity of the nuclear device material on the early fireball phenomenon is also studied. It is found that trajectories of the radiation fronts and case shocks change with the opacity of equivalent device material. From our simulations, we find that only the early fireball depends on the details of the nuclear device, and after the formation of main shock, the evolution is determined by the properties of hot air for strong atmospheric explosion.     

高空核爆炸火球的二维辐射流体力学计算

本文介绍了高空核爆炸火球的二维辐射流体力学计算的主要计算方法和部分计算结果,用柱对称最大熵变Eddington因子P-1近似方程描述了高空核爆炸的辐射输运过程,讨论了P-1近似的边界条件,对能量方程和P-1近似方程的耦合,给出了一个新的差分格式,提高了耦合计算的精度,显著地减少了计算量,同时有效地克服了辐射输运计算中出现的非物理振荡现象。     

Fully implicit 1D radiation hydrodynamics: Validation and verification

A fully implicit finite difference scheme has been developed to solve the hydrodynamic equations coupled with radiation transport. Solution of the time-dependent radiation transport equation is obtained using the discrete ordinates method and the energy flow into the Lagrangian meshes as a result of radiation interaction is fully accounted for. A tridiagonal matrix system is solved at each time step to determine the hydrodynamic variables implicitly. The results obtained from this fully implicit radiation hydrodynamics code in the planar geometry agrees well with the scaling law for radiation driven strong shock propagation in aluminium. For the point explosion problem the self similar solutions are compared with results for pure hydrodynamic case in spherical geometry. Results obtained when radiation interaction is also accounted agree with those of point explosion with heat conduction for lower input energies. Having, thus, benchmarked the code, self convergence of the method w.r.t. time step is studied in detail for both the planar and spherical problems. Spatial as well as temporal convergence rates are ?1 as expected from the difference forms of mass, momentum and energy conservation equations. This shows that the asymptotic convergence rate of the code is realized properly.     

Experimental investigation and numerical validation of explosion suppression by inert particles in large-scale duct

A large-scale duct with an explosion suppressor was designed to investigate experimentally the explosion suppression by inert particles for a CH₄/O₂/N₂ mixture. The duct is 25 m long and has an internal diameter of 700 mm. Pressure and flame signals were recorded some distance away from ignitor in the duct. Pressure tracking lines of the shock front for the different inert particle cloud densities and the inert particle diameters were made. The measured results indicate that the shock front is decoupled from the flame front in the inert particle cloud, which leads to a suppression of explosion. Also, the experiments suggest that increasing the inert particle cloud density or decreasing the inert particle diameter can enhance the ability to suppress explosion. For the purpose of validation, a two-dimensional numerical model coupled with the element chemical reaction mechanism for the simulations of the CH₄/O₂/N₂ mixture explosion suppression by the inert particles has been developed. This model makes use of the second-order TVD scheme and the MacCormack scheme to calculate gas-phase and particle-phase equations, respectively. The Strang splitting technique is used to treat the stiffness due to the coupling of the governing equations, while the implicit Gear algorithm is used to treat the stiffness due to the chemical reactions. The effect of inert particle cloud density on explosion suppression was investigated using the model. The calculated results indicate that the accumulation of inert particles slows the propagation of the gas-phase shock front and results in explosion suppression. With increased inert particle cloud density, the explosion suppression is more prominent. The calculated results show a qualitative agreement with the measured results in the large-scale duct experiment.     

Gamma-ray bursts and bursters

Major advances have been made in the field of gamma-ray bursts in the last two years. The successful discovery of X-ray, optical and radio afterglows, which were predicted by theory, has made possible the identification of host galaxies at cosmological distances. The energy release inferred in these outbursts place them among the most energetic and violent events in the Universe. Current models envisage this to be the outcome of a cataclysmic event leading to a relativistically expanding fireball, in which particles are accelerated at shocks and produce nonthermal radiation. The substantial agreement between observations and the theoretical predictions of the standard fireball shock model provide confirmation of the basic aspects of this scenario. The continued observations show a diversity of behavior, providing valuable constraints for more detailed, post-standard models which incorporate more realistic physical features. Crucial questions being now addressed are the beaming at different energies and its implications for the energetics, the time structure of the afterglow, its dependence on the central engine or progenitor system behavior, and the role of the environment on the evolution of the afterglow.     

Parametric studies on hydrocarbon fireball using large eddy simulations

Occurrences of fireball close to plant buildings due to the release of flammable hydrocarbon fuel caused by the formation of fuel vapour cloud poses severe safety concerns. On the availability of potential ignition source, the induced fireball would cause the damage to the structures of nuclear power plant by direct contact, radiation and/or convection of hot combustion products through the opening of air intakes and ducts. In the present paper, the accidental/ experimental observations and theoretical studies of fireball are summarised. Computational fluid dynamics (CFD) analyses have been carried out to study the behaviour of fireball using OpenFOAM CFD software. The parametric studies are conducted by varying the mass of fuel, inlet velocity and inlet diameter. The new correlations for fireball diameter and duration have been proposed based on the parametric studies using CFD simulations. The fireball with a larger amount of fuel releases the heat slower and for a longer duration. The high heat released rate (HRR) is observed in case of a larger inlet diameter used for the same mass. The incident radiation from the fireball is calculated at different locations to assess thermal hazard. Analysis performed show that various parameters like fireball diameter, duration and the radiative flux falling at different locations can be predicted well using CFD code.     

Microwave radiation generated by a high-power air explosion

The special features of the generation of microwave radiation by a high-power air explosion accompanied by ionizing radiation are discussed. Coherent pulse caused by asymmetry of the ejection of gamma quanta from a source, incoherent bremsstrahlung of a partially ionized plasma, incoherent radiation of the shock wave front, and radiation in spectral lines lying inside air transparency windows are considered.     

Bending of a strong-explosion-induced shock wave around a density singularity (as applied to the remnants of hypernovae)

The remnants of hypernovae, which can correspond to cosmological gamma-ray bursts, are analyzed on the basis of the Kompaneets equation in the strong explosion approximation. Exact solutions to the Kompaneets equation are obtained, and the shape of shock-wave fronts from a noncentral point explosion in a medium whose density decreases quadratically with the distance from the density singularity and tends to a constant at large distances. The bending of the shock-wave front around a density singularity is discussed. The results are compared with data on X-ray sources that can correspond to hypernovae.