Файл: Doicu A., Wriedt T., Eremin Y.A. Light scattering by systems of particles (OS 124, Springer, 2006.pdf

Fig. 3.58. Monodisperse aggregate with Df = 1.8, N = 130 and rp = 10 nm

Fig. 3.59. Polydisperse aggregate with Df = 1.8, N = 203 and rp between 10 and 20 nm

a soot particle from a combustion processes is shown in Fig. 3.58, while a polydisperse aggregate is shown in Fig. 3.59. For the monodisperse aggregate, the scattering characteristics are shown in Fig. 3.60, and the essential parameters controlling the convergence process are Nrank = 8 and Mrank = 6 for the system T -matrix, and Nrank = 4 and Mrank = 3 for the primary spherules. The di erential scattering cross-sections of the polydisperse aggregate are plotted in Fig. 3.61, and the parameters of calculation for the system T -matrix increase to Nrank = 12 and Mrank = 10. The program described above has been used by Riefler et al. [204] to characterize soot from a flame by analyzing the measured scattering patterns.

238 3 Simulation Results

Fig. 3.60. Normalized di erential scattering cross-sections of a monodisperse aggregate

Fig. 3.61. Normalized di erential scattering cross-sections of a polydisperse aggregate

3.7 Composite Particles

Electromagnetic scattering by axisymmetric, composite particles can be computed with the TCOMP routine. An axisymmetric, composite particle consists of several nonenclosing, rotationally symmetric regions with a common axis of symmetry. In contrast to the TMULT routine, TCOMP is based on a formalism which avoid the use of any local origin translation. The scattering

Fig. 3.62. Geometry of a composite particle consisting of three identical cylinders

characteristics can be computed with localized or distributed sources, and the program supports calculations for dielectric particles. Particle geometries included in the library are half-spheroids with o set origins and three cylinders.

In Fig. 3.62, we show a composite particle consisting of three identical cylinders of radius ksr = 2 and length ksl = 4. The axial positions of the first and third cylinder are given by ksz1 = 4 and ksz3 = −4, and the relative refractive indices are chosen as mr1 = mr3 = 1.5 and mr2 = 1.0. Because mr2 = 1.0, the scattering problem is equivalent to the multiple scattering problem of two identical cylinders. On the other hand, the composite particle can be regarded as an inhomogeneous cylinder with a cylindrical inclusion. Therefore, the scattering characteristics are computed with three independent routines: TCOMP, TMULT, and TLAY. Localized and distributed sources are used for T -matrix calculations with the TCOMP routine, while distributed sources are used for calculations with the TLAY routine. Figure 3.63 shows the di erential scattering cross-sections for a fixed orientation of the composite particle (αp = βp = 45◦), while Fig. 3.64 illustrates the numerical results for a random orientation. The behavior of the far-field patterns are quite similar.

In order to demonstrate the capability of the code to compute the electromagnetic scattering by composite particles with elongated and flattened regions, we consider the geometry depicted in Fig. 3.65. For this application, the sources are distributed on the real and imaginary axes. The parameters specifying the particle shape are: ksr = 1, ksR = 3, ksl = 6 and ksL = 7, while the relative refractive indices are identical mr1 = mr2 = mr3 = 1.5. For the prolate cylinders, Nrank = 10 sources are distributed along the axis of symmetry, and for the oblate cylinder, Nrank = 10 sources are distributed