[en] It is not often that I have the opportunity to write something that is not a scientific study, and I am thankful for this opportunity to express my thoughts with regard to pursuits that are greater than those of science. In this van de Hulst assay, to honor Hendrik van de Hulst, I briefly summarize a few points from Hendrik's life that I find especially interesting, including his interests in spiritual (or religious) aspects of life, and his decision to avoid involvement in nucleation problems, a critical and basic uncertainty of current climate models. Finally, after that, I present briefly a few episodes from my own experiences as an apprentice of science and life.

[en] In this paper, we report experimental measurements of spectral line shape parameters (air-broadened width, shift, and line mixing coefficients) for several transitions in the ν₃ Q branch of methane in the 3000–3023 cm^-1 region. 13 high-resolution, room temperature laboratory spectra of pure methane and air-broadened methane recorded with two different Fourier transform spectrometers are fitted. 12 of these spectra were acquired at 0.01 cm^-1 resolution with the McMath-Pierce FTS at the National Solar Observatory on Kitt Peak, and one higher-resolution (~0.0011 cm-1) low pressure methane spectrum was obtained with the Bruker IFS-120HR FTS at the Pacific Northwest National Laboratory, in Richland, Washington. All the spectra were obtained using high purity natural samples of CH₄ and lean mixtures of the same natural CH₄ in dry air. For the 12 spectra recorded at Kitt Peak, three different absorption cells (L= 5, 25 and 150 cm) were used while the methane spectrum at PNNL was obtained using a 19.95 cm long absorption cell. For the analysis, an interactive multispectrum nonlinear least squares fitting software was employed where all the 13 spectra were fitted simultaneously. An accurate and self-consistent set of line parameters were determined by constraining a few of those for severely blended transitions. Line mixing was measured for fourteen transition pairs for the CH₄-air collision system. Lastly, a constant speed dependence parameter, consistent with measured speed dependence values obtained in other methane bands, was applied to all the transitions included in the fitted region. The present measurements are compared to values reported in the literature.

[en] Physics based forward models are the basis on which many experimental diagnostics are interpreted. For some diagnostics, models can be computationally expensive which precludes their use in real time analysis. Reduced models have the potential to capture sufficient physics thereby enabling the desired real time analysis. Using statistical inference and machine learning techniques the application of reduced models for inversion of atomic spectral data used to diagnose magnetic fields in a plasma will be examined. Two approaches are considered, (a) a reduction of the forward model where traditional inversion can be performed on the proxy model, and (b) a reduction of the direct inverse where parameters are a function of measured signal. The resulting inversion is sufficiently fast to be utilized in an online context for digital twinning, and ultimately real-time prediction, design, and control of plasma systems, such as tokamaks. Furthermore, these methods will be demonstrated on both simulated and experimentally measured data.

[en] Linear particle transport in stochastic media is key to such relevant applications as neutron diffusion in randomly mixed immiscible materials, light propagation through engineered optical materials, and inertial confinement fusion, only to name a few. We extend the pioneering work by Adams, Larsen and Pomraning (recently revisited by Brantley) by considering a series of benchmark configurations for mono-energetic and isotropic transport through Markov binary mixtures in dimension d. The stochastic media are generated by resorting to Poisson random tessellations in 1d slab, 2d extruded, and full 3d geometry. For each realization, particle transport is performed by resorting to the Monte Carlo simulation. The distributions of the transmission and reflection coefficients on the free surfaces of the geometry are subsequently estimated, and the average values over the ensemble of realizations are computed. Reference solutions for the benchmark have never been provided before for two- and three-dimensional Poisson tessellations, and the results presented in this paper might thus be useful in order to validate fast but approximated models for particle transport in Markov stochastic media, such as the celebrated Chord Length Sampling algorithm. (authors)

[en] Highlights: • Published path-integral formulations of radiative transfer are revised. • Incorrect mathematics is revealed in some of theme. • Plausibility of these methods should be investigated carefully. • New formulation for turbid media is derived using probabilistic approachapproach. During the last decades, a number of publications have appeared, which dealt with applications of path-integral methods in modeling of radiative/light transfer through various turbid media. This approach seems to be an elegant way how to incorporate multiple scattering effects into calculations of radiometric quantities and thus provides an alternative to time-consuming numerical simulations or to too simplifying approximations. However, some of the published works are based on incorrect mathematical assumptions and the functions derived there actually yield infinite value results in particular calculations. They appear to be unusable without some additional ad hoc normalization. It is therefore questionable, how plausible the conclusions drawn in those works are. This paper revises the derivations of a path-integral form of the Green’s function in radiative transfer problems, reveals their critical points, and compares the resulting formulas with the formulas obtained by an alternative probabilistic approach. Moreover, it provides at least an approximate expressions usable for radiance calculations in turbid media.

[en] Highlights: • Band gap influences on hemispherical properties of complex gratings are studied. • Each property is consistent from measurements and modeling. • Property spectra exhibit wavelength-selective and polarization-sensitive features. • Features are elaborated using electromagnetic patterns in the near field. This work both numerically and experimentally investigates hemispherical radiative properties (absorptance, reflectance, and transmittance) of complex gratings. Crystal silicon is employed to generate samples. One is a double-sided polished 520-μm-thick substrate, and the other is a substrate with complex gratings on its top. The incidence wavelength λ ranges from 0.5 μm to 1.7 μm, containing the intrinsic band gap wavelength λ_bg = 1.09 μm. Wavelength-selective and polarization-sensitive features are exhibited in spectra of properties for complex gratings. The wavelength-selectivity and polarization-sensitivity are elaborated using near-field electromagnetic patterns at three representative wavelengths (λ < λ_bg, λ ≈ λ_bg, and λ > λ_bg).

[en] Highlights: • A fast GPU Monte Carlo implementation for radiative heat transfer in graded-index media. • Optimizations for the performance of the Monte Carlo implementation based on the architecture of NVIDIA GPUs. • Significant speedups compared with the equivalent CPU implementations using single-core/multi-core. • The implementation has a high flexibility and can be easily modified to simulate radiative heat transfer in a variety of cases with different geometries, boundary conditions or media. • The optimization methods for the GPU implementations based on the architecture of NVIDIA GPUs can be used as references for other researchers when building GPU applications. Simulating radiative heat transfer in a graded-index (GRIN) medium is particularly challenging because of curve ray propagation trajectories. As an effective method, the Monte Carlo method is easy to implement with high precision. However, the Monte Carlo method is time consuming, and the computing time increased substantially when combined with the Runge-Kutta ray tracing technique to obtain the ray trajectories in the GRIN medium. Because the Monte Carlo method is ideally suited for parallel processing architectures and acceleration with graphics processing units (GPUs), we have developed a fast GPU Monte Carlo implementation for radiative heat transfer in GRIN media. The performance of the GPU implementation has been improved by combining the ray tracing process with the binary search and optimizing the code based on the architecture of GPUs. In particular, the utilization of the GPU hardware has been maximized, and the warp inactivity has been substantially reduced. Two- and three-dimensional GRIN medium models were evaluated to assess the accuracy and performance of the GPU implementations. Compared with the equivalent central processing unit (CPU) implementations, the GPU implementations provided in this paper show a great capability for producing physically accurate results with substantial speedups. The speedup of the GPU implementation on a single GPU for the two-dimensional case reaches 43.13 × against the equivalent CPU implementation using a single CPU core and 5.65 × against the equivalent CPU implementation using 6 CPU cores (12 threads). The speedup of the GPU implementation on a single GPU for the three-dimensional case reaches 35.61 × against the equivalent CPU implementation using a single CPU core and 2.07 × against the equivalent CPU implementation using 14 CPU cores (28 threads).

[en] Highlights: • Soil is considered as plane parallel two-layered media with constant or spatial variation of the refractive index. • The refractive index mismatch and Fresnel reflection at the boundaries and interfaces are assumed. • The computational approach is in agreement with existing results on two-layered media. • Radiation emerging from the soil is greatly influenced by the radiative properties of O and A-horizon. The knowledge of soil optical properties is very important for agriculture production and decision making in selecting and managing land to cultivate. These properties can be retrieved from the radiative transfer technique if appropriate method is used to solve the direct problem of the radiative transfer equation. The radiative transfer equation describes the propagation of the radiation in the soil. Thus, understanding the interactions of the radiation with agricultural soil is an active research area. In this paper, the reflectance characteristics of typical agricultural soil are retrieved. The soil, considered as a semitransparent plane parallel two-layered media with the refractive index mismatch, is illuminated at the top by a collimated radiation. This refractive index mismatch causes reflection and refraction to occur at the soil boundaries and interfaces. The two layered agricultural soil radiative properties and the scattering phase function are determined from the Mie theory assuming that the soil layers consist of only clay particles. With these radiative properties, the equation of radiative transfer is solved using the discrete spherical harmonics method (DSHM) under Marshak boundary conditions. The computational results are in agreement with existing literature results on two-layered media. The two-layered agricultural soil is then studied with a constant or spatial variation of the refractive index. The effects of soil radiative properties and the scattering phase function on the radiation characteristics are discussed. The anisotropy of the soil reflectance is analyzed and results demonstrate that a predominantly backscattering soil reflects more radiation than the predominantly forward scattering and Rayleigh scattering soil. These findings are applicable in the context of agricultural soil as it provides insight for developing optical equipment for quick soil properties measurement before crop planting.

[en] Highlights: • Optical forces exerted on-axis axisymmetric beams are studied. • The beams are non-dark of the second kind and dark beams. • The framework used is the Rayleigh limit of the generalized Lorenz-Mie theory. • The relationship with the traditional dipole theory of forces is discussed. • A mini-review of optical forces in the Rayleigh limit of GLMT is provided. A recent work has been devoted to the study of the Rayleigh limit of generalized Lorenz-Mie theory for on-axis beams in the case of non dark axisymmetric beams of the first kind. The present work complements this previous work by studying the case of non-dark axisymmetric beams of the second kind and of dark axisymmetric beams. This paper being presumably the last one of a series devoted to the Rayleigh limit of the generalized Lorenz-Mie theory in the case of lossless particles, it is complemented by a mini-review allowing one to gain an overview of the issue.

[en] Highlights: • Can imaging transform Fourier spectrometers quantify flare combustion efficiency?. • Species column densities found from spectroscopic model. • Velocities found from optical flow algorithm. • Inferring column densities and velocity fields are both mathematically ill-posed. • Technique works but is susceptible to scene change artifacts. Flaring plays a critical role in reducing the environmental impact of upstream oil and gas processing by converting methane and other gaseous hydrocarbons into CO₂, which has a lower global warming potential. This process is highly efficient under ideal conditions but efficiency may be significantly lower under certain scenarios such as fuel stripping under crosswind and emission of volatile organic compounds and unburned fuels due to over-aeration or over-steaming in assisted flares. This study assesses the potential of using imaging Fourier transform spectrometers (IFTSs) to directly measure combustion efficiency by combining species column densities estimated from a spectroscopic model with intensity-weighted velocities found using an optical flow model. Simulated measurements using a computational fluid dynamics (CFD)-large eddy simulation of a flare in a crosswind are used to establish the technique's viability, followed by experimental measurements on a heated gas vent to validate the optical flow model. Finally, preliminary measurements are carried out on a laboratory-scale steam- and air-assisted flare. While the simulated measurements and heated vent experiments support the feasibility of this approach, experimentally-derived spectra from the lab-scale flare were contaminated with artifacts attributed to turbulent fluctuations, which complicates the quantitative interpretation of the IFTS data.