Numerical Research On The Transient/Polarized Radiative Transfer In Participating Media Exposed To A Laser Irradiation

Radiative transfer in participating media exposed to a laser irradiation is widely found in our daily life and engineering applications,such as the sunlight propagation in the atmosphere,the laser interactions with materials,the medical nondestructive diagnosis and the active optical imaging,etc.The basic governing equation for the propagation of radiation light in participating media is the radiative transfer equation.At present,various numerical and experimental investigations of radiative transfer in participating media have been done by many researcher s.Many key problems and techniques,however,still need to be urgently solved for specific applications such as the radiative transfer in optical complex system s and the radiative transfer considering the time and polarization effects.In this thesis,the optical complex media,transient and polarization effects of the radiation light during its propagation are considered,the transient and polarized radiative transfer problems in media exposed to a laser irradiation are numerically investigated.The time effect should be taken into account for the radiative transfer in a medium exposed to a pulsed laser with a duration time of 10-12~10-15 s,as the propagation time of the radiation light in the medium is at the same level with the duration time of the incident laser.Most of the previous work on transient radiative transfer is limited to homogeneous media or simple graded-index media.Transient radiative transfer in optical complex media has significant applications and needs further investigations.In this thesis,the reflection/transmission effects of radiation light on the interface where the refractive index changes discontinuously are considered and a Monte Carlo method is applied to simulate the radiative transfer process in complex refractive index media.Based on the linearity characteristics of transient radiative transfer in non-emitting media exposed to a laser,a modified Monte Carlo model is proposed by introducing the time shift and superposition principle to the traditional Monte Carlo model.The transient radiative transfer in complex refractive index media exposed to a short-pulsed collimated laser is numerically solved and effects of medium properties on time-resolved signals of reflectance and transmittance are analyzed.Results show that the refractive index distribution and interface reflection mode have important influences on the time-resolved radiation signals.For one-dimensional media with monotonously changing and symmetric refractive index distributions,the transmittance curves coincide exactly with each other while the reflectance curves have obvious differences;the specular reflection at interfaces enhance the transmittance,while the diffuse reflection enhance the reflectance.Polarization is a natural characteristic of the radiation light propagating in a participating medium,as the radiation is essentially a transverse electromagnetic wave.The polarization state of a radiation light can be described by Stokes vector I =(I,Q,U,V)T and the governing equation of the polarized radiative transfer process is the vector radiative transfer equation.Multi-dimensional and transient polarized radiative transfer problems need urgent investigations as most of the previous work on polarized radiative transfer is limited to one-dimensional steady problems.Based on solving the discretized form of the vector radiative transfer equation,the discontinuous finite element method(DFEM)is applied for solving polarized radiative transfer problems.By adopting the separate solution theory,the Stokes vector within the medium is divided into a collimated part and a diffused part which are solved respectively.The collimated Stokes vector can be mathematically obtained by the Beer's law and the diffuse part is obtained by solving the vector radiative transfer equation using the DFEM.A local refined angular discretization scheme is introduced to obtain the refined results in certain directions where the Stokes components have step changes.The DFEM is applied for solving polarized radiative transfer problems in a Mie scattering medium,a Rayleigh scattering medium,an aerosol medium with extremely complex scattering characteristics,and a two-layer inhomogeneous medium.Results show that the DFEM is efficient and accurate for polarized radiative transfer problems in various scattering media.Polarized radiative transfer in multi-dimensional media is solved by using the DFEM.The polarized radiative transfer in rectangular media with different boundary reflection modes and exposed to a collimated beam,the polarized radiative transfer in a two-dimensional emitting and scattering medium are first studied.The polarized radiative transfer in two-dimensional media with complex geometries is further investigated,the Stokes vector components are presented and the shielding effects of the inner obstacles to Stokes vector distributions are discussed.The polarized radiative transfer in three-dimensional media exposed to collimated or diffuse irradiation is investigated,the distributions of Stokes vector components are discussed.Several notable conclusions are drawn from our results.For the medium with the whole left surface covered by an external irradiation,it is found that the zenith Stokes component distribution varies significantly for different viewing azimuth angles.For the medium exposed to a cylindrical irradiation on the left surface center,the Stokes components on the surface are limited to certain solid angles,i.e.,the Stokes component values approach zero for most directions.For the medium with an emitting bottom surface,as the simulation domain and the boundary condition are symmetric,the angular distributions of Stokes components have obvious symmetric features.The DFEM is further extended to the transient vector radiative transferproblems where the time effect and the polarization effect of the radiation light are considered simultaneously.In the DFEM application,the discontinuous on the inner element boundaries is considered and the field variable within the computational domain is solved element by element timely,which makes DFEM can accurately capture the wave front of the radiation energy in the transient radiative transfer problems.In this thesis,the DFEM discretization of the transient vector radiative transfer are presented,the accuracy of the DFEM for transient polarized radiative transfer problems is verified by comparing the DFEM solutions with publish data,and the transient vector radiative transfer in participating media is studied.The time-resolved distributions of Stokes vector components are anal yzed,effects of the boundary reflection mode and the polarization state of the incident beam to the time-resolved Stokes components are investigated.Results show that the boundary reflection mode has the most significant influence on the Stokes vector component V.It is also found that it takes different time for Stokes vector components to reach the steady state for polarized radiative transfer in media with a continuous source term.