激光腔靶耦合二维辐射输运数值模拟

:间接驱动激光聚变过程中,黑腔内物质处于非局域热动平衡(non-LTE)状态,而且辐射传输具有非平衡、各向异性等特点。为了精确描述黑腔辐射场的演化及其与物质的相互作用,最新研制的激光聚变二维总体LARED集成程序,基于non-LTE的多群辐射输运建模,首次实现了激光黑腔靶实验的全过程数值模拟。数值结果表明,辐射输运计算较好地反映了黑腔辐射场均匀性变化,腔壁光斑区与非光斑区X光发射强度比与实验测量值接近,靶丸压缩形状与实验图像定性一致。     

球形黑腔辐射输运问题的蒙特卡罗模拟

利用蒙特卡罗方法模拟六孔球形黑腔中的辐射输运, 研究靶球辐照均匀性问题. 对于几何结构简单的解析模型, 研究了不同黑腔靶球半径比的靶球辐照均匀性变化规律, 得出的结论与解析的“视因子”方法给出的一致. 对于几何结构复杂的黑腔模型, 如放置有挡板的模型, 解析方法计算困难, 但利用蒙特卡罗方法仍然能够准确模拟计算. 不同挡板大小的理论模型计算结果表明, 挡板对X光输运到靶球表面的分布状况有明显的影响, 如果设置得当则可以提高X光利用效率并显著改善靶球辐照均匀性, 否则可能严重破坏靶球辐照均匀性. 因此, 黑腔中的挡板位置及大小需要精心设计. 应用表明, 蒙特卡罗方法对于具有复杂结构的黑腔辐射输运问题具有很好的适应性.     

点火内爆靶辐射驱动不对称性全过程数值模拟

为满足分层掺杂点火内爆靶辐射驱动不对称性全过程物理分析的需求,在激光聚变二维总体程序LARED集成上发展了辐射输运建模下的多介质ALE方法-RTALE(Radiative Transfer Arbitrary-Lagrangian-Eulerian)。为提高多介质ALE方法的健壮性,发展了驰豫网格重构算法,该重构算法生成的新网格能自适应流场的变化。数值模拟了激波与气柱相互作用的RM不稳定性实验,模拟的气泡变形程度与试验结果基本一致,其中驰豫网格重构算法中的驰豫因子能够很好地反映流场密度梯度。基于辐射多群输运建模的LARED集成程序能够完整模拟辐射驱动不对称性条件下掺杂点火靶二维内爆过程,克服了传统ALE方法计算不下去和算不好的困难,界面变形程度也符合物理分析。     

点火内爆靶辐射驱动不对称性全过程数值模拟

为满足分层掺杂点火内爆靶辐射驱动不对称性全过程物理分析的需求,在激光聚变二维总体程序LARED集成上发展了辐射输运建模下的多介质ALE方法-RTALE(Radiative Transfer Arbitrary-Lagrangian-Eulerian)。为提高多介质ALE方法的健壮性,发展了驰豫网格重构算法,该重构算法生成的新网格能自适应流场的变化。数值模拟了激波与气柱相互作用的RM不稳定性实验,模拟的气泡变形程度与试验结果基本一致,其中驰豫网格重构算法中的驰豫因子能够很好地反映流场密度梯度。基于辐射多群输运建模的LARED集成程序能够完整模拟辐射驱动不对称性条件下掺杂点火靶二维内爆过程,克服了传统ALE方法计算不下去和算不好的困难,界面变形程度也符合物理分析。     

LARED-H程序的激光黑腔靶耦合数值模拟

LARED-H程序是一个可用于激光黑腔靶耦合数值模拟研究的二维辐射流体力学程序。黑腔等离子体所形成的复杂流场使单纯的拉格朗日网格在计算中产生严重扭曲，影响计算精度，并导致计算中断。拉氏加网格重分是计算激光黑腔靶耦合常用的算法。对LARED-H程序的积分网格重分方法在网格重构和物理量重映方面作了较大改进，并利用改进后的LARED-H程序模拟了“神光”-Ⅱ和“神光”-Ⅲ条件下的激光空腔靶耦合物理全过程。     

不同辐射建模下球腔的整体数值模拟

不同辐射建模对于腔内辐射场描述的精确程度不同,需要分析不同建模对腔内辐射温度的影响。开展了三温建模与辐射多群输运建模下LARED集成程序数值模拟两孔球型黑腔模型,实现了球腔的完整数值模拟。数值模拟结果表明,三温与辐射多群输运模拟的等离子体状态接近,辐射温度存在差异。物理分析显示辐射温度差异的主要原因是使用的辐射不透明度,修改辐射不透明度参数后的三温计算结果与输运计算符合更好,从而可以用三温建模更快更准确地估计出所需的激光能量和功率。     

不同辐射建模下球腔的整体数值模拟

不同辐射建模对于腔内辐射场描述的精确程度不同,需要分析不同建模对腔内辐射温度的影响。开展了三温建模与辐射多群输运建模下LARED集成程序数值模拟两孔球型黑腔模型,实现了球腔的完整数值模拟。数值模拟结果表明,三温与辐射多群输运模拟的等离子体状态接近,辐射温度存在差异。物理分析显示辐射温度差异的主要原因是使用的辐射不透明度,修改辐射不透明度参数后的三温计算结果与输运计算符合更好,从而可以用三温建模更快更准确地估计出所需的激光能量和功率。     

靶丸变形实验的多群扩散整体数值模拟

神光Ⅱ装置上靶丸压缩实验中,黑腔长度的变化会影响靶丸赤道和两极受到的辐照强度,从而导致燃料区被压缩成不同的形状。介绍了辐射多群扩散建模下的辐射流体力学方程组及其数值方法。采用最近研制的二维总体LARED集成程序,对神光Ⅱ不同长度黑腔靶丸压缩变形实验进行了整体数值模拟。结果表明辐射多群扩散建模可以反映腔内辐照均匀性变化情况,靶丸压缩形状与神光Ⅱ实验定性一致。     

靶丸变形实验的多群扩散整体数值模拟

神光Ⅱ装置上靶丸压缩实验中,黑腔长度的变化会影响靶丸赤道和两极受到的辐照强度,从而导致燃料区被压缩成不同的形状。介绍了辐射多群扩散建模下的辐射流体力学方程组及其数值方法。采用最近研制的二维总体LARED集成程序,对神光Ⅱ不同长度黑腔靶丸压缩变形实验进行了整体数值模拟。结果表明辐射多群扩散建模可以反映腔内辐照均匀性变化情况,靶丸压缩形状与神光Ⅱ实验定性一致。     

LARED集成程序辐射输运模拟的性能优化

分析定位高置信度辐射传输数值模拟效率瓶颈,针对物理模型和数值离散格式的特点,优化求解算法和程序设计,改进激光聚变二维总体LARED集成程序,提高输运建模模拟黑腔靶实验模型的计算效率：在相同的并行计算条件下,辐射输运方程离散求解计算效率提高2倍以上,全过程整体模拟计算时间减半.     

On the use of principal component analysis to speed up radiative transfer calculations

Radiative transfer is computationally expensive. However, it is essential to many applications, in particular remote sensing retrievals. Principal component analysis of the optical depth and single scattering albedo profiles has been proposed as a possible method to help ease the computational burden. Here we show how the technique could be applied to a practical problem of CO₂ retrievals from high spectral resolution measurements of reflected sunlight in three near infrared bands. We obtain a speed improvement of more than 50 fold (compared to monochromatic computations), while reproducing the radiances to better than 0.1% accuracy.     

A fast infrared radiative transfer model for overlapping clouds

A fast infrared radiative transfer model (FIRTM2) appropriate for application to both single-layered and overlapping cloud situations is developed for simulating the outgoing infrared spectral radiance at the top of the atmosphere (TOA). In FIRTM2 a pre-computed library of cloud reflectance and transmittance values is employed to account for one or two cloud layers, whereas the background atmospheric optical thickness due to gaseous absorption can be computed from a clear-sky radiative transfer model. FIRTM2 is applicable to three atmospheric conditions: (1) clear-sky, (2) single-layered ice or water cloud, and (3) two simultaneous cloud layers in a column (e.g., ice cloud overlying water cloud). Moreover, FIRTM2 outputs the derivatives (i.e., Jacobians) of the TOA brightness temperature with respect to cloud optical thickness and effective particle size. Sensitivity analyses have been carried out to assess the performance of FIRTM2 for two spectral regions, namely the longwave (LW) band (587.3-1179.5 cm⁻¹) and the short-to-medium wave (SMW) band (1180.1-2228.9 cm⁻¹). The assessment is carried out in terms of brightness temperature differences (BTD) between FIRTM2 and the well-known discrete ordinates radiative transfer model (DISORT), henceforth referred to as BTD (F−D). The BTD (F−D) values for single-layered clouds are generally less than 0.8 K. For the case of two cloud layers (specifically ice cloud over water cloud), the BTD (F−D) values are also generally less than 0.8 K except for the SMW band for the case of a very high altitude (>15 km) cloud comprised of small ice particles. Note that for clear-sky atmospheres, FIRTM2 reduces to the clear-sky radiative transfer model that is incorporated into FIRTM2, and the errors in this case are essentially those of the clear-sky radiative transfer model.     

Finite element method for the two-dimensional atmospheric radiative transfer

The finite element method is applied to the solution of the two-dimensional atmospheric radiative transfer. The analysis is mainly focussed on the derivation of the cell or element equation. The Galerkin method and several hybrid methods using the integral and finite difference form of the radiative transfer equation are employed to obtain the cell equation. The assembled system of equations relating the radiances at the lower and upper boundary of the domain is solved by a direct method.     

Effect of the improvement of the HITRAN database on the radiative transfer calculation

The line parameters of the HITRAN 2004 have been updated, as compared with the older editions (the 2000 edition and the 1996 edition). In order to know the effect of the modifications on radiative transfer calculation with high spectral resolution, comparison in optical depth and radiance spectrum have been given between different editions. Four infrared spectral regions are selected, and they cover the three bands of atmospheric infrared sounder (AIRS) and one of geosynchronous imaging fourier transform spectrometer (GIFTS). The comparison has shown that the relative difference between HITRAN 2000 and 2004 and that between HITRAN 1996 and 2004 is decreasing. But the maximal discrepancy between the latest two editions in some spectral intervals is over 1%. It is important to estimate the error of calculation with the line parameters correctly or one has to use the new edition of HITRAN.     

A quasi-analytic solution of the radiative transfer equation for three-dimensional atmospheres

In this study, we present a new solution of the three-dimensional (3-D) radiation transfer equation (RTE). The solution employs a discretization technique to separate the independent variables involved in the 3-D RTE, and the doubling-adding method to solve the RTE explicitly and quasi-analytically. The remarkable feature of the present solution is the application of scaling-function expansion to those terms that are dependent on horizontal coordinates. Scaling-function expansion is suitable for representing irregular horizontal inhomogeneities with small-scale variations. By applying scaling-function expansion, the 3-D RTE can be formulated in the form of a vector-matrix differential equation; matrices involved in the equation are generally sparse and dominantly diagonal matrices, and this considerably reduces the labor involved in matrix calculations. We tested the performance of the present solution via radiative transfer calculations of solar radiation in horizontally inhomogeneous two-dimensional cloud models. The calculated results indicate that even if the resolution of the scaling-function expansion is too coarse in regions around small-scale variations, the influence does not spread problematically to other regions far from the variations; this illustrates the advantage of the scaling-function expansion. The present solution can be used to investigate quantitatively and to estimate the effects of cloud spatial inhomogeneity on the corresponding radiation field.     

Three-dimensional transient radiative transfer modeling using the finite-volume method

This article presents a finite-volume method to calculate transient radiative transfer in a three-dimensional enclosure. The fully implicit scheme is used to discretize the transient term. The step and CLAM spatial differencing schemes are used in this study. The procedure is validated using available published results. The ability of the present formulation in modeling an absorbing, emitting and isotropically scattering medium is examined using heat fluxes and incident radiation. Ray effect and false scattering are discussed.