The majority of stars form in clusters which are themselves birthed in Giant Molecular Clouds (GMCs). The radiation produced by clusters during their formative phase heats and ionizes the surrounding gas and drives outflows via radiation pressure. The combination of these processes, referred to as radiative feedback, is a proposed mechanism for limiting the star formation efficiency (SFE) in molecular clouds. In this thesis, we use 3D numerical simulations of turbulent GMCs, completed using the code FLASH and a sophisticated radiative transfer scheme, to explore the effects of radiative feedback on cluster formation and the larger scale interstellar medium (ISM). We present suites of simulations that vary the initial GMC mass from 10^4 to 10^6 M$_{\odot}$ and consider both gravitationally bound and unbound clouds. We find that clusters form within the highly filamentary clouds where they can undergo subsequent merging. Radiative feedback only plays a minor role in lowering the SFE of 10^6 M$_{\odot}$ GMCs. However, it completely disrupts intermediate mass clouds (~10^5 M$_{\odot}$), reducing the SFE by a factor of two. We then examine the escape fraction of UV photons from GMCs --- a quantity relevant to the structure of the ISM and cosmic reionization. We show that the escape fraction is dynamic and can vary by factors of two over short timescales because of the rapid growth and collapse of HII regions. The escape fractions from massive GMCs are typically low (~5%) while intermediate mass models are characterized by escape fractions nearing 100%. We combine our GMC models to represent the escape fraction from a population of clouds in dwarf starburst and spiral-type galaxies. We successfully reproduce the star formation rates in these galaxies and find typical escape fractions of 8% in all cases. These results place important constraints on galactic-scale models studying the ISM and cosmic reionization. / Thesis / Doctor of Philosophy (PhD)

In this thesis we present TREVR (Tree-based Reverse Ray Tracing), a general algo- rithm for computing the radiation field, including absorption, in astrophysical sim- ulations. TREVR is designed to handle large numbers of sources and absorbers; it is based on a tree data structure and is thus suited to codes that use trees for their gravity or hydrodynamics solvers (e.g. Adaptive Mesh Refinement). It achieves com- putational speed while maintaining a specified accuracy via controlled lowering of resolution of both sources and rays from each source. TREVR computes the radiation field in O(N log(N)) time without absorption and O (Nlog(N)log(N)) time with absorption. These claims are substantiated by mathematically predicting and testing the algorithm’s general scaling. The scalings arise from merging sources of radiation according to an opening angle criterion and walking the tree structure to trace a ray to a depth that gives the chosen accuracy for absorption. The absorption-depth refinement criterion is unique to TREVR and is presented here for the first time. We provide a suite of tests demonstrating the algorithm’s ability to accurately compute fluxes, ionization fronts and shadows. Two novel test cases are presented here for the first time as part of this suite. / Thesis / Master of Science (MSc) / In this thesis we present TREVR (Tree-based Reverse Ray Tracing), a general method for computing the effects of of radiation in astrophysical simulations.

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The fundamental definitions of radiative transfer theory are given and the two-dimensional equation of transfer is derived, density of radiation is defined, and two-dimensional two-intensity transfer model is presented. An operational interpretation of the latter model is given interms of military truck transport supply and the functional dependencies of the terms in the transfer equations are evaluated. For this interpretation the density equations are given and the study state and time dependent solutions of the density equations are discussed in polar coordinates. This work was conducted for the U. S. Army Transportation Research Command, Fort Eustis, Virginia, 1961, Task 9R38-11-009-02. / Master of Science

Computational thermo-fluid models of a free surface flow under the dominant radiative transfer have been developed for the design and control of a modern optic fiber drawing process. Although modeling of the fiber drawing process has been of interest for the past three decades, most of the previous studies were limited to low draw speeds and small preforms. Large preforms drawn at high speeds have been used in modern fiber drawing systems to improve production efficiency and reduce cost. Several assumptions commonly made in previous studies have to be relaxed to address the new challenges. In this study, instead of using the Rosseland approximation, the radiative transfer equation (RTE) was solved directly for the radiation fluxes using the finite volume method (FVM). The complete two-dimensional free surface flow was simulated along with the coupling of the radiative transfer. Unlike most of the previous studies that only considered the furnace domain and that assumed the glass velocity at the exit, we included the post-chamber in the computation domain and predicted the fiber solidification location. Furthermore, the mixed convection of the air in the post-chamber was also considered, and was shown to have significant effects on the fiber solidification. On the basis of the computational model, a reduced order model (ROM) was developed for a mixed HŁ /LQG controller designed to regulate the fiber diameter under the effects of disturbances. The ROM was derived on the basis of the computational model. Optimal numerical eigenfunctions were obtained through the Karhunen-Loeve expansion using the computational model. The GalerkinŁ s method was then applied to obtain the state space ROM. The numerical model was shown to be efficient and was verified experimentally. The ROM characterizes the dynamics of the system accurately as compared with the computational model. The simulations using the full computational model showed that the closed-loop system is robust and superior to the open-loop system in the regulation of fiber diameter. The modeling and control methods can be applied to the design optimization and parameter regulation of the high-speed large-preform draw processes as well as other manufacturing processes that involve similar thermal-fluid transports.

The atmospheres of exoplanets are being characterised in increasing detail by observational facilities and will be examined with even greater clarity with upcoming space based missions such as the James Webb Space Telescope (JWST) and the Wide Field InfraRed Survey Telescope (WFIRST). A major component of exoplanet atmospheres is the presence of cloud particles which produce characteristic observational signatures in transit spectra and influence the geometric albedo of exoplanets. Despite a decade of observational evidence, the formation, dynamics and radiative-transport of exoplanet atmospheric cloud particles remains an open question in the exoplanet community. In this thesis, we investigate the kinetic chemistry of cloud formation in hot Jupiter exoplanets, their effect on the atmospheric dynamics and observable properties. We use a static 1D cloud formation code to investigate the cloud formation properties of the hot Jupiter HD 189733b. We couple a time-dependent kinetic cloud formation to a 3D radiative-hydrodynamic simulation of the atmosphere of HD 189733b and investigate the dynamical properties of cloud particles in the atmosphere. We develop a 3D multiple-scattering Monte Carlo radiative-transfer code to post-process the results of the cloudy HD 189733b RHD simulation and compare the results to observational results. We find that the cloud structures of the hot Jupiter HD 189733b are likely to be highly inhomogeneous, with differences in cloud particle sizes, number density and composition with longitude, latitude and depth. Cloud structures are most divergent between the dayside and nightside faces of the planet due to the instability of silicate materials on the hotter dayside. We find that the HD 189733b simulation in post-processing is consistent with geometric albedo observations of the planet. Due to the scattering properties of the cloud particles we predict that HD 189733b will be brighter in the upcoming space missions CHaracterising ExOPlanet Satellite (CHEOPS) bandpass compared to the Transiting Exoplanet Space Survey (TESS) bandpass.

In this thesis a new three–dimensional (3D) vector radiative transfer model, the Solver for Polarized Atmospheric Radiative Transfer Applications (SPARTA) is introduced, validated against benchmark results, and applied to scientific problems. SPARTA employs the statistical forward Monte Carlo technique for efficient column–response pixel–based radiance calculations comprising polarization for 3D inhomogeneous cloudless and cloudy atmospheres. By means of SPARTA, two scientific issues in the field of radiative transfer are investigated. A sensitivity study has been conducted to illustrate the errors introduced by neglecting the effects of polarization in radiation simulations. Two atmospheric scenarios have been considered: a pure one–dimensional (1D) Rayleigh atmosphere and two–dimensional (2D) realistic inhomogeneous dust fields. In case of a purely molecular atmosphere, these errors strongly depend on molecular anisotropy, isotropic reflection, and more importantly, on single scattering albedo and optical thickness (saturation occurs for τ close to 1). Overall errors in the reflected field range up to about 10.5%. On the other hand, for rather high optical thickness, the bias induced by ignoring polarization for realistic inhomogeneous atmospheres is negligible (less than 1%). In addition, solar radiative transfer simulations for LIDAR–measured fields of optical properties of Saharan dust have been performed in order to quantify the effects induced by neglecting the horizontal photon transport and internal inhomogeneities (3D radiative effects) in radiance simulations including polarization. Results are presented for two exemplary mineral dust fields constructed from LIDAR observations. For each case, three radiative calculations are investigated: a 1D calculation according to the plane–parallel (1D mode); an Independent Pixel Approximation (IPA mode); and the 2D mode. The differences in domain–averaged normalized radiances of reflection and transmission are insignificant between the 1D or IPA and 2D calculation modes. However, local differences were observed since extinction is hinge on horizontal spatial variability. In the areas with large spatial variability in optical thickness, the radiance fields of the 2D mode differ about ±20% for the first and second Stokes elements (I, Q) from the fields of the 1D mode. This work points to a brand–new field: the quantification of the sensitivity of polarization to 3D radiative effects.

Ionising radiation is present in a variety of astrophysical problems, and it is particularly important for shaping the process of star formation in molecular clouds, containing hot, high-mass stars. In order to account for the effects of ionising radiation within numerical models of star formation, we need to combine a hydrodynamics method with a radiative transfer method and obtain a radiation hydrodynamics scheme (RHD). In this thesis I achieve live radiation hydrodynamics by coupling the Smoothed Particle Hydrodynamics (SPH) code Phantom with the Monte Carlo Radiative Transfer (MCRT) code CMacIonize. Since SPH is particle-based and MCRT is grid-based, I construct an unstructured, Voronoi grid in order to establish a link between the two codes. In areas with large density gradients, a Voronoi grid based purely on the SPH particle positions achieves insufficient resolution, and therefore I propose a novel algorithm for inserting a small number of additional grid cells to improve the local resolution. Furthermore, the MCRT calculations require the knowledge of an average density for each Voronoi cell. To address this, I develop an analytic density mapping from SPH to a Voronoi grid, by deriving an expression for the integrals of a series of kernel functions over the volume of a random polyhedron. Finally, I demonstrate the validity of the live RHD through the benchmark test of D-type expansion of an H II region, where good agreement is shown with the existing literature. The RHD implementation is then used to perform a proof-of-concept simulation of a collapsing cloud, which produces high-mass stars and is subsequently partially ionised by them. The presented code is a valuable tool for future star formation studies, and it can be used for modelling a broad range of additional astronomical problems involving ionising radiation and hydrodynamics.

A 'model-to-radiance' comparison of simulated brightness temperatures and radiances from the Hadley Centre Global Environmental Model 2 (HadGEM2-A) with longwave measurements from the High Resolution Infrared Radiation Sounder/4 (HIRS/4) and the Infrared Atmospheric Sounding Interfermeter (IASI) onboard the MetOp-A satellite is presented for all-sky and clear-sky global means. The fast Radiative Transfer model for TOVS 10 (RTTOV-10) is applied to HadGEM2 output to simulate observational-equivalent data. The results are compared with corresponding broadband analyses. A method is developed to extend hyperspectral IASI radiances to cover the whole outgoing terrestrial spectrum, in order to identify any compensating biases, and explore wavebands in the unobserved Far Infrared (FIR) region. For the all-sky HIRS analysis, the model overestimates brightness temperatures in the atmospheric window region with the greatest biases over areas associated with deep convective cloud. In contrast to many global climate models, much smaller clear-sky biases are found indicating that model clouds are the dominating source of error. Simulated values in upper atmospheric CO2 channels approximate observations better as a result of compensating cold biases at the poles and warm biases at lower latitudes, due to a poor representation of the Brewer Dobson circulation in the 38 level 'low-top' configuration of the model. Simulated all and clear-sky outgoing longwave radiation evaluated against the Clouds and the Earth's Radiant Energy System (CERES) and HIRS OLR products reveal good agreement, in part due to cancellation of positive and negative biases. Through physical arguments relating to the spectral energy balance within a cloud, it is suggested that broadband agreement could be the result of a balance between positive window biases and unseen negative biases originating from the water vapour rotational band in the FIR (not sampled by HIRS). Simple sensitivity tests show that dramatically altering existing cloud properties has little effect on the prominent window biases, however raising clouds a maximum of 5 atmospheric levels minimises the error in cloud contaminated channels, due to the introduction of spatially compensating errors. Sensitivities to the way ice clouds are parameterised in RTTOV-10 display a range of up to 2.5 K in window channels but absolute biases still exceed 3 K for all choices. Because of the lack of satellite based FIR observations due to a technological gap in the spectral region, an algorithm is created to 'fill in' the available data. Correlations between selected IASI channels and simulated unobserved wavelengths in the far infrared are used to estimate radiances between 25.25 - 644.75 cm-1 at 0.5 cm-1 intervals. The same method is used in the 2760 - 3000 cm-1 region. The spectrum is validated by comparing the Integrated Nadir Longwave Radiance (INLR) product (spanning the whole 25.25 - 3000 cm-1 range) with the corresponding broadband measurements from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Terra and Aqua satellites at simultaneous nadir overpasses, revealing mean differences of 0.3 Wm-2sr-1 (0.5% relative difference) lower for IASI relative to CERES and significantly lower biases in nighttime only scenes. Averaged global data over a single month produces mean differences of about 1 Wm-2sr-1 in both the all and the clear-sky (1.2% relative difference). The new high resolution spectrum is presented for global mean clear and total skies where the far infrared is shown to contribute 44% and 47% to the total OLR respectively, which is consistent with previous estimates. In terms of spectral cloud radiative forcing, the FIR contributes 19% and in some subtropical instances appears to be negative, results that would go un-observed with a traditional broadband analysis. The equivalent complete IASI OLR model product is simulated from GCM data using RTTOV-10. The same process of applying predictors to the satellite measurements is applied to the model simulated radiances, with appropriate modifications, to produce a directly comparable model product. Annual mean all-sky radiances are still greatly overestimated at all wavenumbers with a total radiance bias of 4.52 Wm-2 across the whole range. Compensating negative biases outside of the HIRS coverage that were hypothesised are absent, with the far infrared contributing to the overall bias rather than cancelling it. Equivalent clear-sky biases are much lower overall at 0.39 Wm-2, in part due to spectral and spatial cancellation of errors. A flux-to-flux comparison is enabled by estimating the spatial distribution of anisotropic factors, using collated HIRS OLR fluxes and IASI OLR radiances, which yields global mean model fluxes in excess of 12 Wm-2 higher than observations in the all-sky. The difference between this and the fluxes calculated using the climate model's broadband radiation code (Edward-Slingo) are around 10 Wm-2 which is outside the range of uncertainty in the method used to estimate the flux. However, it is discussed that tuning of the climate model's broadband code to known flux values is a required practice to ensure global energy budgets balance but can produce inaccurate parameterised variables. An equivalent analysis adjusting the ice cloud parametrisation to reflect the radiances that have the biggest differences to the original configuration selected showed a bias reduction of 4.5 Wm-2, which is still not enough to completely explain its size, suggesting the existence of residual cloud problems. Finally, it is suggested that the way forward in separating and constraining cloud errors, in both radiative transfer codes, is a rigorous process of testing them with observation cloud properties and reanalysis data as inputs.

Nano-manufacturing is receiving significant attention in industry due to the ever-growing interest in nanotechnology in research institutions. It is hypothesized that single-step or direct-write nano-scale machining might be achieved by coupling nano-probe field emission with radiation transfer. A laser may be used to heat a workpiece within a microscopic region that encloses an even smaller nanoscopic region subjected to a focused electron beam. The electron-beam supplies marginal heat sufficient to remove a minute volume of material by evaporation or sublimation. Experimentally investigating this hypothesis requires an estimate of the power needed in the electron-beam. To this end, a detailed numerical study is conducted to study the possibility of using the nano-probe field emission for nano-machining. The modeling effort in this case is divided into two parts. The first part deals with the electron-beam propagation inside a target workpiece. The second part considers the temperature increase due to the energy transfer between the electron-beam and the workpiece itself. A Monte Carlo/Ray Tracing technique is used in modeling the electron-beam propagation. This approach is identical to that of a typical Monte Carlo simulation in radiative transfer, except that proper electron scattering properties are employed. The temperature distribution inside a gold film is predicted using the heat conduction equations. Details of the various numerical models employed in the simulation and a series of representative results will be presented in this dissertation.