Browsing by Author "Mahan, James R."

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    Advances in Radiation Heat Transfer and Applied Optics, Including Application of Machine Learning
    Yarahmadi, Mehran (Virginia Tech, 2021-01-14)
    Artificial neural networks (ANNs) have been widely used in many engineering applications. This dissertation applies ANNs in the field of radiation heat transfer and applied optics. The topics of interest in this dissertation include both forward and inverse problems. Forward problems involve applications in which numerical simulation is expensive in terms of time consummation and resource utilization. Artificial neural networks can be applied in these problems for speeding up the process and reducing the required resources. The Monte Carlo ray-trace (MCRT) method is the state-of-the-art approach for modeling radiation heat transfer. It has the disadvantage of being a complex and computationally expensive process. In this dissertation, after first identifying the uncertainties associated with the MCRT method, artificial neural networks are proposed as an alternative whose computational cost is greatly reduced compared to traditional MCRT method. Inverse problems are concerned with situations in which the effects of a phenomenon are known but the cause is unknown. In such problems, available data in conjunction with ANNs provide an effective tool to derive an inverse model for recovering the cause of the phenomenon. Two problems are studied in this context. The first is concerned with an imager for which the readout power distribution is available and the viewed scene is of interest. Absorbed power distributions on a microbolometer array making up the imager is produced by discretized scenes using a high-fidelity Monte Carlo ray-trace model. The resulting readout array/scene pairs are then used to train an inverse ANN. It is demonstrated that a properly trained ANN can be utilized to convert the readout power distribution into an accurate image of the corresponding discretized scene. The recovered scene of the imager is helpful for monitoring the Earth's radiant energy budget. In the second problem, the collection of scattered radiation by a sun-photometer, or aureolemeter, is simulated using the MCRT method. The angular distribution of this radiation is summarized using the probability density function (PDF) of the incident angles on a detector. Atmospheric water cloud droplets are known to play an important role in determining the Earth's radiant energy budget and, by extension, the evolution of its climate. An extensive dataset is produced using an improved atmospheric scattering model. This dataset is then used to train and test an inverse ANN capable of recovering water cloud droplets properties from solar aureole observations.
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    Appréciation thermodynamique des Rougon-Macquart d'Émile Zola
    Mahan, James R. (Virginia Tech, 2024-08-13)
    In 1975 the eminent French philosopher and historian of science, Michel Serres, published Feux et signaux de brume: Zola, in which he postulates that the well-known Rougon-Macquart series by Émile Zola was an "epic of entropy." His seminal book stirred up interest in the Zola criticism community, with several literary scholars seeking to interpret the work of the founder of the naturalism movement in terms of thermodynamics. This thesis was partially informed by our reading of Serres' book, enlightened by our own expertise as an engineer specializing in the field of thermodynamics. We hypothesize that the difference between Zola's somber naturalism and the more optimistic literary realism movement from which it emerged in the latter half of the 19th century reflects the timely appearance and vulgarization of thermodynamic principles in the French popular press. We begin by presenting an overview of this then-new branch of science, whose foundations were established by Nicolas Sadi Carnot in 1824 and whose maturation was assured, principally by Rudolf Clausius, between 1850 and 1865. We then present the thermodynamic principles which we postulate arguably could have played a determinant role in the emergence of Zola's naturalism in 1870, as outlined in his Roman experimental (1880). Foremost among these is the celebrated Second Law and its distressing consequence to many, the inevitable entropy death of the universe. Having established the fundamental concepts of thermodynamics, we undertake an analysis of Serres' book aimed at exposing and critically commenting on the theoretical basis of his thermodynamic reading of Zola. We then follow the same approach in our analysis of several critical articles from the archival literature that owe their genesis to Serres' work. Finally, we undertake a close reading of three novels from the Rougon-Macquart series that we have either not considered elsewhere in the thesis or whose thermodynamic implications we have not yet exhausted: Nana, La Bête humaine, and L'Assommoir. According to the hypothesis of this thesis, Zola consistently followed the same disastrous paradigm, informed by thermodynamic principles, when he imagined the disastrous life trajectories of his principal characters. We have applied a version of this paradigm in interpreting our own close reading of a representative sample of works from the Rougon-Macquart series. Michel Serres sought to point out the obvious parallels that exist between the catastrophic nature of the series and the Second Law of Thermodynamics. We wish to go a bit further in promoting the idea that the transformation of literary realism into Zola's brand of naturalism may have been prompted by the vulgarization of thermodynamics in the contemporary popular press. Lastly, the content of this master's thesis provides a new perspective to and vocabulary for naturalist criticism.
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    Development of a Water Cloud Radiance Model for Use in Training an Artificial Neural Network to Recover Cloud Properties from Sun Photometer Observations
    Meehan, Patrick James (Virginia Tech, 2021-06-09)
    As the planetary climate continues to evolve, it is important to build an accurate long-term climate record. State-of-the-art atmospheric science requires a variety of approaches to the measurement of the atmospheric structure and composition. This thesis supports the possibility of inferring cloud properties from sun photometer observations of the cloud solar aureole using an artificial neural network (ANN). Training of an ANN requires a large number of input and output parameter sets. A cloud radiance model is derived that takes into consideration the cloud depth, the mean size of the cloud water particles, and the cloud liquid water content. The cloud radiance model derived here is capable of considering the wavelength of the incident sunlight and the cloud lateral dimensions as parameters; however, here we consider only one wavelength—550 nm—and one lateral dimension—500 m—to demonstrate its performance. The cloud radiance model is then used to generate solar aureole profiles corresponding to the cloud parameters as they would be observed using a sun photometer. Coefficients representative of the solar aureole profiles may then be used as inputs to a trained ANN to infer the parameters used to generate the profile. This process is demonstrated through examples. A manuscript submitted for possible publication based on an early version of the cloud radiance model was deemed naïve by reviewers, ultimately leading to improvements documented here.
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    Effects on Heat Transfer Coefficient and Adiabatic Effectiveness in Combined Backside and Film Cooling with Short-Hole Geometry
    La Rosa Rivero, Renzo Josue (Virginia Tech, 2018-08-30)
    Heat transfer experiments were done on a flat plate to study the effect of internal counter-flow backside cooling on adiabatic film cooling effectiveness and heat transfer coefficient. In addition, the effects of density ratio (DR), blowing ratio (BR), diagonal length over diameter (L/D) ratio, and Reynolds number were studied using this new configuration. The results are compared to a conventional plenum fed case. Data were collected up to X/D =23 where X=0 at the holes, an S/D = 1.65 and L/D=1,2. Testing was done at low L/D ratios since short holes are normally found in double wall cooling applications in turbine components. A DR of 2 was used in order to simulate engine-like conditions and this was compared to a DR of 0.92 since relevant research is done at similar low DR. The BR range of 0.5 to 1.5 was chosen to simulate turbine conditions as well. In addition, previous research shows that peak effectiveness is found within this range. Infrared (IR) thermography was used to capture temperature contours on the surface of interest and the images were calibrated using a thermocouple and data analyzed through MATLAB software. A heated secondary fluid was used as 'coolant' in the present study. A steady state heat transfer model was used to perform the data reduction procedure. Results show that backside cooling configuration has a higher adiabatic film cooling effectiveness when compared to plenum fed configurations at the same conditions. In addition, the trend for effectiveness with varying BR is reversed when compared with traditional plenum fed cases. Yarn flow visualization tests show that flow exiting the holes in the backside cooling configuration is significantly different when compared to flow exiting the plenum fed holes. We hypothesize that backside cooling configuration has flow exiting the holes in various directions, including laterally, and behaving similar to slot film cooling, explaining the differences in trends. Increasing DR at constant BR shows an increase in adiabatic effectiveness and HTC in both backside cooling and plenum fed configurations due to the decreased momentum of the coolant, making film attachment to the surface more probable. The effects of L/D ratio in this study were negligible since both ratios used were small. This shows that the coolant flow is still underdeveloped at both L/D ratios. The study also showed that increasing turbulence through increasing Reynolds number decreased adiabatic effectiveness.
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    High Performance Broadband Photodetectors Based on Graphene/Semiconductor Heterostructures
    Wang, Yifei (Virginia Tech, 2022-04-15)
    Graphene, a monolayer of carbon atoms, has gained prominence to augment existing chip-scale photonic and optoelectronic applications, especially for sensing in optical radiation, owing to its distinctive electrical properties and bandgap as well as its atomically thin profile. As a building block of photodetection, graphene has been co-integrated with mature silicon technology to realize the on-chip, high-performance photo-detecting platforms with broad spectral response from the deep-ultraviolet (UV) to the mid-infrared (MIR) regime. The recent state-of-the-art graphene-based photodetectors utilizing the combination of colloidal quantum dots (QDs) and graphene have been intensively studied, where QDs function as the absorber and the role of graphene is as a fast carrier recirculating channel. With such a configuration, an ultrahigh sensitivity can be achieved on account of the photogating mechanism; however, the response time is slow and limited to the millisecond-to-second range. To achieve balance between high sensitivity and fast response time, we have demonstrated a new photodetector that is based on graphene/two-dimensional heterostructures. The homogeneous thickness and the large contact of the heterostructure give rise to fast carrier transporting between the thin absorber layer and the graphene, leading to a fast response time. This thesis carefully investigates the optimization of fabrication as well as optoelectronic characterization of photodetectors based on graphene/semiconductor heterostructures field-effect transistors (GFETs). GFETs with different architectures were demonstrated and systematically studied under optical illumination ranging from deep-UV to MIR at varying optical powers. Noise behaviors have been studied under different device parameters such as device structure, area and gate-bias. Results show that the flicker noise of graphene-based devices can be explained by the McWhorter model in which the fluctuation of carrier numbers is the dominant process of noise in low frequencies; thus, it can be scaled down by reducing the number of introduced charged carriers with optimized fabrication. Besides, the impact of absorber on top of graphene and the bottom substrate has been comprehensively explored through various experimental techniques including current-voltage (IV), photo-response dynamics, and noise characterization measurements. With our configuration, the high sensitivity and fast response time of photodetectors can be obtained at the same time. In addition to this, the study of the bottom substrate with different doping levels suggests a concept of dual-photogating effect which is induced by the top absorbent material and the photoionization of the doped silicon substrate. In summary, this thesis showcases novel device architecture and fabrication procedures of GFETs photodetectors, optimizes device structure, quantifies the performance and evaluates the effect of various absorbent materials and substrate. It provides insight into the improvement of possible routes to achieve a broadband photo-detecting system with higher sensitivity, faster response time and lower noise level.
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    Investigating Hydration and Dynamics of Biomolecules in Solutions using High Precision Terahertz Spectroscopy
    Doan, Luan Cong (Virginia Tech, 2022-04-21)
    Biomolecules function only in aqueous environments and their dynamics are strongly influenced by physiological conditions including the temperature and the presence of co-solutes. The presence of biomolecules in aqueous solutions will change the dynamics and structure of water, and as a response, water will form hydration layers around biomolecules. The dynamics of hydration water, as well as hydrated proteins, lead to translation, rotation, and oscillating dipoles that, in turn, give rise to absorption in the megahertz-to-terahertz frequencies. However, the strong absorption of water in this frequency range leads to a significant challenge in obtaining terahertz dielectric spectra of aqueous biomolecular solutions. In response, I have employed a high sensitivity terahertz frequency-domain spectroscopy to overcome these issues on a large range of frequencies from 10 MHz to 1.12 THz. The high dynamical range of the system combined with a variable-path-length cell allows precise measurement of the complex dielectric response of the solutions. Employing Debye and Lorentzian approximations, I have decomposed contributions of the dielectric response of the solutions. The structure and dynamics of hydration shells and hydrated biomolecules have been identified. Performing experiments on a number of biomolecules have verified the certainty of the methods, thus, enriching the knowledge of the biological science of dynamics and functions of biomolecules.
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    Manufacturing and Characterization of Gold-Black and Prediction and Measurement of its Directional Spectral Absorptivity
    Munir, Nazia Binte (Virginia Tech, 2021-01-26)
    Gold-black has emerged as a popular absorptive coating for thermal radiation detectors in aerospace applications. The performance and accuracy of thermal radiation detectors largely depends on the surface optical properties of the absorptive coating. If the absorptivity of the layer is directional or wavelength dependent, then so will be the detector gain itself. This motivates our interest in the manufacture, physical characterization, and study of the wavelength and polarization sensitivity of the directional spectral absorptivity of gold-black. A first-principle model based on lossy antenna theory is presented to predict the polarization dependent directional spectral absorptivity of gold-black in the visible and near infrared. Results for normal spectral absorptivity are in good agreement with measurements reported in the literature. However, suitable experimental data were not available to validate the theory for directional spectral absorptivity. Therefore, an experimental campaign to fabricate and measure the directional spectral behavior of gold-black had to be undertaken to validate the first-principle model. New in-plane bidirectional reflectance distribution function (BRDF) measurements for two thicknesses (~4 μm and ~8 μm) of gold-black laid down on a gold mirror substrate are reported in the visible (532 nm) and near-infrared (800 and 850 nm) for p- and s-polarizations. The investigation is then extended to a three-layer sample, which is shown to exhibit off-specular reflectivity. Described are processes for laying down gold-black coatings and for measuring their in-plane BRDF as a function of thickness, wavelength, and polarization state. A novel method for retrieving the directional absorptivity from in-plane BRDF measurements is presented. The influence of polarization on directional absorptivity is shown to follow our earlier theory except at large incident zenith angles, where an unanticipated mirage effect is observed.
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    Nanostructures for Coherent Light Sources and Photodetectors
    Ho, Vinh Xuan (Virginia Tech, 2020-05-14)
    Large-scale optoelectronic integration is limited by the lack of efficient light sources and broadband photodetectors, which could be integrated with the silicon complementary metal-oxide-semiconductor (CMOS) technology. Persistent efforts continue to achieve efficient light emission as well as broadband photodetection from silicon in extending the silicon technology into fully integrated optoelectronic circuits. Recent breakthroughs, including the demonstration of high-speed optical modulators, photodetectors, and waveguides in silicon, have brought the concept of transition from electrical to optical interconnects closer to realization. The on-chip light sources based on silicon are still a key challenge due to the indirect bandgap of silicon that impedes coherent light sources. To overcome this issue, we have studied, fabricated, and characterized nanostructures including single semiconductor epilayers, multiple quantum wells, and graphene-semiconductor heterostructures to develop coherent light sources and photodetectors in silicon. To develop coherent light sources, we reported the demonstration of room-temperature lasing at the technologically crucial 1.5 m wavelength range from Er-doped GaN epilayers and Er-doped GaN multiple-quantum wells grown on silicon and sapphire. The realization of room-temperature lasing at the minimum loss window of optical fiber and in the eye-safe wavelength region of 1.5 m is highly sought-after for use in many applications in various fields including defense, industrial processing, communication, medicine, spectroscopy and imaging. The results laid the foundation for achieving hybrid GaN-Si lasers providing a new pathway towards full photonic integration for silicon optoelectronics. Silicon photodiodes contribute a large portion in the photodetector market. However, silicon photodetectors are sensitive in the UV to near infrared region. Photodetection in the mid-infrared is based on thermal radiation detectors, narrow bandgap materials (InGaAs, HgCdTe) semiconductors, photo-ionization of shallow impurities in semiconductors (Si:As, Ge:Ga), and quantum well structures. Such technology requires complicated fabrication processes or cryogenic operation, resulting in manufacturing costs and severe integration issues. To develop broadband photodetectors, we focus on graphene photodetectors on silicon. Graphene generates photocarriers by absorbing photons in a broadband spectrum from the deep-ultraviolet to the terahertz region. Graphene can be realized as the next generation broadband photodetection material, especially in the infrared to terahertz region. Here, we have demonstrated high-performance hybrid photodetectors operating from the deep-ultraviolet to the mid-infrared region with high sensitivity and ultrafast response by coupling graphene with a p-type semiconductor photosensitizer, nitrogen-doped Ta2O5 thin film.
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    A New Paradigm for End-to-End Modeling of Radiometric Instrumentation Systems
    Ashraf, Anum Rauf Barki (Virginia Tech, 2020-04-14)
    Earth observing instruments, such as those embarked on the Earth Radiation Budget Experiment (ERBE) and Clouds and the Earth's Radiant Energy System (CERES), have been used to monitor the arriving solar and the upwelling solar reflected and longwave emitted radiation from low Earth orbit for the past three decades. These instruments have played a crucial role in studying the Earth's radiation budget and developing a decadal climate data record. Prior to launch, these instruments go through several robust design phases followed by rigorous ground calibration campaigns to establish their baseline characterization spectrally, spatially, temporally, and radiometrically. The knowledge gained from building and calibrating these instruments has aided in technology advancements as the need for developing more accurate instruments has increased. In order to understand the prelaunch performance of these instruments, NASA's Langley Research Center (LaRC) has partnered with the Thermal Radiation Group at Virginia Tech to develop first-principle, dynamic electrothermal, numerical models of scanning radiometers that can be used to enhance the understanding of such instruments. The body of research presented here documents the construction of these models by highlighting their development and results and possible applications to the next generation of Earth radiation budget instrument. Much of the effort reported here is based on the author's contribution to NASA's now-deselected Radiation Budget Instrument (RBI) project.
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    Optical and Thermal Radiative Simulation of an Earth Radiation Budget Instrument
    Fronk, Joel Seth (Virginia Tech, 2021-06-08)
    Researchers at the NASA Langley Research Center (LaRC) are developing a next-generation instrument for monitoring the Earth radiation budget (ERB) from low Earth orbit. This instrument is called the DEMonstrating the Emerging Technology for measuring the Earth's Radiation (DEMETER) instrument. DEMETER is a candidate to replace the Clouds and Earth's Radiant Energy System (CERES) instruments which currently monitor the ERB. LaRC has partnered with the Thermal Radiation Group at Virginia Tech to model and evaluate the thermal and optical design of the DEMETER instrument. The effort reported here deals with the numerical modeling of the optical and thermal radiative performance the DEMETER instrument. The numerical model is based on the Monte Carlo Ray-Trace (MCRT) method. The major optical components of the instrument are incorporated into the ray-trace model using 3-D surface equations. A CAD model of the instrument baffle is imported directly into the ray-trace environment using an STL triangular mesh. The instrument uses a single freeform mirror to focus radiation on the detector. A method for incorporating freeform surfaces into a ray-trace model is described. The development and capabilities of the model are reported. The model is used to run several ray-traces to compare two different quasi-black surface coatings for the DEMETER telescope baffle. Included is a list of future tests the Thermal Radiation Group will use the model to accomplish.
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    Thermal Analysis of the Detector in the Radiation Budget Instrument (RBI)
    Pfab, Jonathan Francis (Virginia Tech, 2018-02-06)
    Earth radiation budget instruments are devices designed to study global climate change. These instruments use telescopes embarked on low-earth-orbit satellites to measure Earth emitted and reflected solar radiation. Radiation is sensed as temperature changes caused by radiation absorbed during scans of the earth on a delicate gold-black coated detector. This work is part of a larger effort to develop an end-to-end dynamic electro-thermal model, based on first-principles, for the next generation of earth radiation budget instruments, the Radiation Budget Instrument (RBI). A primary objective of this effort is to develop a numerical model of the detector to be used on RBI. Specifically, the sensor model converts radiation arriving at the detector, collimated and focused through telescopes, into sensible heat; thereby producing a voltage. A mathematical model characterizing this sensor is developed. Using a MATLAB algorithm, an implicit finite-volume scheme is implemented to determine the model solution. Model parameters are tuned to replicate experimental data using a robust parameter estimation scheme. With these model parameters defined, the electro-thermal sensor model can be used, in conjunction with the remaining components of the end-to-end model, to provide insight for future interpretation of data produced by the RBI.
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    Vat Photopolymerization of High-Performance Materials through Investigation of Crosslinked Network Design and Light Scattering Modeling
    Feller, Keyton D. (Virginia Tech, 2023-06-08)
    The reliance on low-viscosity and photoactive resins limits the accessible properties for vat photopolymerization (VP) materials required for engineering applications. This has limited the adoption of VP for producing end-use parts, which typically require high MW polymers and/or more stable chemical functionality. Decoupling the viscosity and molecular weight relationship for VP resins has been completed recently for polyimides and highperformance elastomers by photocuring a scaffold around polymer precursors or polymer nanoparticles, respectively. Both of these materials are first shaped by printing a green part followed by thermal post-processing to achieve the final part properties. This dissertation focuses on improving the processability of these material systems by (i) investigating the impact of scaffold architecture and polysalt monomer composition on photocuring, thermal post-processing, and resulting thermomechanical properties and (ii) developing a Monte Carlo ray-tracing (MCRT) simulation to predict light scattering and photocuring behavior in particle-filled resins, specifically zinc oxide nanoparticles in a rigid polyester resin and styrene butadiene rubber latex resin. The first portion of the dissertation introduces VP of a tetra-acid and half-ester-based polysalt resin derived from 4,4'-oxydiphthalic anhydride and 4,4-oxydianiline (ODPA-ODA), a fully aromatic polyimide with high glass transition temperature and thermal stability. This polyimide, and polyimides like this, find use in demanding industries such as aerospace, automotive and electronic applications. The author evaluated the hypothesis that a non-bound triethylene glycol dimethacrylate (TEGDMA) scaffold would facilitate more efficient scaffold burnout and thus achieve parts with reduced off-gassing potential at elevated temperatures. Both resins demonstrated photocuring and were able to print solid and complex latticed parts. When thermally processed to 400 oC, only 3% of the TEGDMA scaffold remained within the final parts. The half-ester resin exhibits higher char yield, resulting from partial degradation of the polyimide backbone, potentially caused by lack of solvent retention limiting the imidization conversion. The tetra-acid exhibits a Tg of 260oC, while the half-ester displays a higher Tg of 380 oC caused by the degradation of the polymer backbone, forming residual char, restricting chain mobility. Solid parts displayed a phase-separated morphology while the half-ester latticed parts appear solid, indicating solvent removal occurs faster in the half-ester composition, presumably due to reduced polar acid functionality. This platform and scaffold architecture enables a modular approach to produce novel and easily customizable UV-curable polyimides to easily increase the variety of polyimides and the accessible properties of printed polyimides through VP. The second section of this dissertation describes the creation and validation of a MCRT simulation to predict light scattering and the resulting photocured shape of a ZnO-filled resin nanocomposite. Relative to prior MCRT simulations in the literature, this approach requires only simple, easily acquired inputs gathered from dynamic light scattering, refractometry, UV-vis spectroscopy, beam profilometry, and VP working curves to produce 2D exposure distributions. The concentration of 20 nm ZnO varied from 1 to 5 vol% and was exposed to a 7X7 pixel square ( 250 µm) from 5 to 11 s. Compared to experimentally produced cure profiles, the MCRT simulation is shown to predict cure depth within 10% (15 µm) and cure widths within 30% (20 µm), below the controllable resolution of the printer. Despite this success, this study was limited to small particles and low loadings to avoid polycrystalline particles and maintain dispersion stability for the duration of the experiments. Expanding the MCRT simulation to latex-based resins which are comprised of polymer nanoparticles that are amorphous, homogeneous, and colloidally stable. This allows for validating the MCRT with larger particles (100 nm) at higher loadings. Simulated cure profiles of styrene-butadiene rubber (SBR) loadings from 5 vol% to 25 vol% predicted cure depths within 20% ( µm) and cure widths within 50% ( µm) of experimental values. The error observed within the latex-based resin is significantly higher than in the ZnO resin and potentially caused by the green part shrinking due to evaporation of the resin's water, which leads to errors when trying to experimentally measure the cure profiles. This dissertation demonstrates the development of novel and functional materials and creation process-related improvements. Specifically, this dissertation presents a materials platform for the future development of unique photocurable engineering polymers and a corresponding physics-based model to aid in processing.