蒙特卡洛方法数值研究大气颗粒物动力学效应和辐射传输性质

爆发性增强的雾天,空气污染严重能见度低,这与大气边界层湍流性质、悬浮颗粒的动力学及散射性质密切相关.文中基于颗粒群平衡方程和Mie理论,采取加权蒙特卡洛方法,自行开发了Fortran程序.文中计算所得的颗粒尺度分布函数、颗粒散射性质与实验值、理论解一致,验证了数值模型和方法的正确性.此外,数值研究了雾爆发性增强阶段雾滴谱拓宽、能见度降低的机理,讨论湍流输运和颗粒局部聚集效应下颗粒间的碰并过程,并耦合颗粒散射性质,数值分析雾发展中湍流耗散率对颗粒对径向相对速度、系统透过率的影响;以及颗粒对径向相对速度与系统透过率、颗粒尺度的关系.研究结果表明:随着湍流耗散率的增大,颗粒的径向相对速度呈现先缓慢而后快速增大的变化趋势.1 000 s时刻,湍流的耗散率为1.0×10~(-2)m~2/s~3,颗粒径向相对速度(无量纲)为0.096 9;对于0.6μm的可见光,雾环境颗粒系统的透过率为0.47.此外,雾发展中雾滴易与气溶胶碰并,系统的散射性质与水组成的雾滴系统不同,天气的能见度明显降低.

NUMERICAL STUDY ON DYNAMICS EFFECT AND RADIATION TRANSFER CHARACTERISTICS OF ATMOSPHERIC PARTICLE BY MONTE CARLO METHOD

During the burst reinforcement period of fog, air pollution and low visibility are very serious, which is closely related to the turbulence characteristics of the atmospheric boundary layer, the dynamics and scattering properties of suspended particles. Based on the particle population balance equation and Mie theory, a program is self-developed. The computed particle size distribution function and particle scattering property are consistent with the experimental and theoretical data, which verify the correctness of models and numerical method. Numerical study on the mechanism of droplet spectrum broadening, visibility reducing during the fog burst-enhanced phase is conducted, and the e ects of turbulent transport and particle local aggregation on the coagulation of particles are discussed. Combining with particles scattering nature, the influence of particle turbulent dissipation rates on the radial relative velocity and the transmissivity of system in the fog development are analyzed numerically. Relation between the radial relative velocity, the transmissivity of system and the particle size are discussed. The computed results suggest that the radial relative velocity of particles increases slowly and then increases rapidly with the rise of turbulent dissipation rate. At 1 000 s, the turbulent dissipation rate is 1.0×10^-2m²/s³, and the dimensionless radial relative velocity of particle is 0.096 9. For 0.6 μm wavelength of visible light, the transmissivity of fog is 0.47. Furthermore, aerosols are coagulated with fog droplets in the development region of fog to decrease atmosphere visibility, which radiation properties are di erent from pure droplets.