Transient Radiative Transport Research Articles

Analysis of Ablation Characteristics of Absorbing Dielectrics Caused by Short Laser Pulses

The effect of ablation of absorbing dielectrics by single and two successive ultra short pulses from an excimer laser source is investigated. The numerical model is based on two photon absorption by the molecular chromophores followed by thermal degradation and diffusion, whereas ablation occurs through sublimation of the material from the surface. The numerical analysis involves solving a set of coupled three- or two-level chromophore rate equations, heat diffusion equation, and the transient radiative transport equation, using two techniques: a fully implicit iterative scheme and a predictor corrector technique (Fromm's scheme). Results for the temperature distribution and ablation depth are obtained for different laser parameters such as the delay time between two successive pulses, laser pulse width, laser fluence, and various material properties such as activation energy, relaxation time, thermal conductivity, and absorption cross section of molecules. The results obtained by consideration of the transient radiative transfer equation are compared with the steady-state formulation, and significant differences are observed in the temperature profiles and the ablation depth.

Equivalent isotropic scattering formulation for transient short-pulse radiative transfer in anisotropic scattering planar media

An isotropic scaling formulation is evaluated for transient radiative transfer in a one-dimensional planar slab subject to collimated and/or diffuse irradiation. The Monte Carlo method is used to implement the equivalent scattering and exact simulations of the transient short-pulse radiation transport through forward and backward anisotropic scattering planar media. The scaled equivalent isotropic scattering results are compared with predictions of anisotropic scattering in various problems. It is found that the equivalent isotropic scaling law is not appropriate for backward-scattering media in transient radiative transfer. Even for an optically diffuse medium, the differences in temporal transmittance and reflectance profiles between predictions of backward anisotropic scattering and equivalent isotropic scattering are large. Additionally, for both forward and backward anisotropic scattering media, the transient equivalent isotropic results are strongly affected by the change of photon flight time, owing to the change of flight direction associated with the isotropic scaling technique.

Transient Radiation Transport in Participating Media Within a Rectangular Enclosure

This paper outlines the formulation of the two-dimensional transient radiation transport through a scattering‐ absorbing medium. The P1 approximation in a Cartesian coordinate system is invoked to simplify the transient radiative transfer equation because it is very complicated to solve in its general form. A boundary-driven radiative problem is considered in which the radiation intensities at some areas on a surface are modeled as boundary conditions and maintained at constant values in all angular directions.

Transient radiation transport in participating media within a rectangular enclosure

Transient radiation transport in participating media within a rectangular enclosure

Cumulative variable formulation for transient conductive and radiative transport in participating media

A new mathematical formulation is proposed for transient conductive and radiative transport in a participating gray, isotropically scattering plane-parallel medium. The methodology can be easily extended to include numerous additional effects. A systematic and unified treatment is presented using cumulative variables that allows for high-order integration using standard initial-value methods in the temporal variable while allowing for an effective orthogonal collocation method to be implemented in the spatial variable. A spectral approach is incorporated in the present context where Chebyshev polynomials of the first kind are used as the basis functions. This article illustrates the methodology and presents some comparisons with previously reported works.