Browsing by Author "Ma, Lin"

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    4D combustion and flow diagnostics based on tomographic chemiluminescence (TC) and volumetric laser-induced fluorescence (VLIF)
    Wu, Yue (Virginia Tech, 2016-12-02)
    Optical diagnostics have become indispensable tools for the study of turbulent flows and flames. However, optical diagnostics developed in the past have been primarily limited to measurements at a point, along a line, or across a two-dimensional (2D) plane; while turbulent flows and flames are inherently four-dimensional (three-dimensional in space and transient in time). As a result, diagnostic techniques which can provide 4D measurement have been long desired. The purpose of this dissertation is to investigate two of such 4D diagnostics both for the fundamental study of turbulent flow and combustion processes and also for the applied research of practical devices. These two diagnostics are respectively code named tomographic chemiluminescence (TC) and volumetric laser induced fluorescence (VLIF). For the TC technique, the emission of light as the result of combustion (i.e. chemiluminescence) is firstly recorded by multiple cameras placed at different orientations. A numerical algorithm is then applied on the data recorded to reconstruct the 4D flame structure. For the VLIF technique, a laser is used to excite a specific species in the flow or flame. The excited species then de-excite to emit light at a wavelength longer than the laser wavelength. The emitted light is then captured by optical sensors and again, the numerical algorithm is applied to reconstruct the flow or flame structure. This dissertation describes the numerical and experimental validation of these two techniques, and explores their capabilities and limitations. It is expected that the results obtained in this dissertation lay the groundwork for further development and expanded application of 4D diagnostics for the study of turbulent flows and combustion processes.
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    50-kHz-rate 2D imaging of temperature and H2O concentration at the exhaust plane of a J85 engine using hyperspectral tomography
    Ma, Lin; Li, Xuesong; Sanders, Scott T.; Caswell, Andrew W.; Roy, Sukesh; Plemmons, David H.; Gord, James R. (Optical Society of America, 2013-01-01)
    This paper describes a novel laser diagnostic and its demonstration in a practical aero-propulsion engine (General Electric J85). The diagnostic technique, named hyperspectral tomography (HT), enables simultaneous 2-dimensional (2D) imaging of temperature and water-vapor concentration at 225 spatial grid points with a temporal response up to 50 kHz. To our knowledge, this is the first time that such sensing capabilities have been reported. This paper introduces the principles of the HT techniques, reports its operation and application in a J85 engine, and discusses its perspective for the study of high-speed reactive flows. (C) 2013 Optical Society of America
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    Aerodynamics of a Transonic Turbine Vane with a 3D Contoured Endwall, Upstream Purge Flow, and a Backward-Facing Step
    Gillespie, John Lawrie (Virginia Tech, 2017-08-09)
    This experiment investigated the effects of a non-axisymmetric endwall contour and upstream purge flow on the secondary flow of an inlet guide vane. Three cases were tested in a transonic wind tunnel with an exit Mach number of 0.93-a flat endwall with no upstream purge flow, the same flat endwall with upstream purge flow, and a 3D contoured endwall with upstream purge flow. All cases had a backward-facing step upstream of the vanes. Five-hole probe measurements were taken 0.2, 0.4, and 0.6 Cx downstream of the vane row trailing edge, and were used to calculate loss coefficient, secondary velocity, and secondary kinetic energy. Additionally, surface static pressure measurements were taken to determine the vane loading at 4% spanwise position. Surface oil flow visualizations were performed to analyze the flow qualitatively. No statistically significant differences were found between the three cases in mass averaged downstream measurements. The contoured endwall redistributed losses, rather than making an improvement distinguishable beyond experimental uncertainty. Flow visualization found that the passage vortex penetrated further in the spanwise direction into the passage for the contoured endwall (compared to the flat endwall), and stayed closer to the endwall with a blowing ratio of 1.5 with a flat endwall (compared to no blowing with flat endwall). This was corroborated by the five hole probe results.
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    A closed-form method for calculating the angular distribution of multiply scattered photons through isotropic turbid slabs
    Sun, Xueqiang; Li, Xuesong; Ma, Lin (Optical Society of America, 2011-11-01)
    This paper develops a method for calculating the angular distribution (AD) of multiply scattered photons through isotropic turbid slabs. Extension to anisotropic scattering is also discussed. Previous studies have recognized that the AD of multiply scattered photons is critical for many applications, such as the design of imaging optics and estimation of image quality. This paper therefore develops a closed-from method that can accurately calculate the AD over a wide range of conditions. Other virtues of the method include its simplicity in implementation and its prospective for extension to anisotropic scattering. (C) 2011 Optical Society of America
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    Combined Experimental and Numerical Study of Active Thermal Control of Battery Modules
    He, Fan (Virginia Tech, 2015-04-16)
    Lithium ion (Li-ion) batteries have been identified as a promising solution to meet the increasing demands for alternative energy in electric vehicles (EVs) and hybrid electric vehicle (HEVs). This work describes experimental and numerical study of thermal management of battery module consisting of cylindrical Li-ion cells, with an emphasis on the use of active control to achieve optimal cooling performance with minimal parasitic power consumption. The major contribution from this work is the first experimental demonstration (based on our review of archival journal and conference literature) and the corresponding analysis of active thermal control of battery modules. The results suggest that the active control strategy, when combined with reciprocating cooling flow, can reduce the parasitic energy consumption and cooling flow amount substantially. Compared with results using passive control with unidirectional cooling flow, the parasitic energy consumption was reduced by about 80%. This contribution was achieved in three steps, which was detailed in this dissertation in chapters 2, 3, and 4, respectively. In the first step, an experimental facility and a corresponding CFD model were developed to capture the thermal behavior of multiple battery cells. Based on the experimental and CFD results, a reduced-order model (ROM) was then developed for active monitoring and control purposes. In the second step, the ROM was parameterized and an observer-based control strategy was developed to control the core temperature of battery cells. Finally, based on the experimental facility and the ROM model, the active control of a battery module was demonstrated. Each of these steps represents an important facet of the thermal management problem, and it is expected that the results and specifics documented in this dissertation lay the groundwork to facilitate further study.
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    Development and validation of a reconstruction algorithm for three-dimensional nonlinear tomography problems
    Lei, Qingchun; Wu, Yue; Xu, Wenjiang; Ma, Lin (Optical Society of America, 2016-07-06)
    This work reports the development and experimental validation of a reconstruction algorithm for three-dimensional (3D) nonlinear tomography problems. Many optical tomography problems encountered in practice are nonlinear, for example, due to significant absorption, multiple-scattering, or radiation trapping. Past research efforts have predominately focused on reconstruction algorithms for linear problems, and these algorithms are not readily extendable to nonlinear problems due to several challenges. These challenges include the computational cost caused by the nonlinearity (which was compounded by the large scale of the problems when they are 3D), the limited view angles available in many practical applications, and the measurement uncertainty. A new algorithm was therefore developed to overcome these challenges. The algorithm was validated both numerically and experimentally, and was demonstrated to be able to solve a range of nonlinear tomography problems with significantly enhanced efficiency and accuracy compared to existing algorithms.
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    Development and Validation of Reconstruction Algorithms for 3D Tomography Diagnostics
    Lei, Qingchun (Virginia Tech, 2017-01-10)
    This work reports three reconstruction algorithms developed to address the practical issues encountered in 3D tomography diagnostics, such as the limited view angles available in many practical applications, the large scale and nonlinearity of the problems when they are in 3D, and the measurement uncertainty. These algorithms are: an algebraic reconstruction technique (ART) screening algorithm, a nonlinear iterative reconstruction technique (NIRT), and an iterative reconstruction technique integrating view registration optimization (IRT-VRO) algorithm. The ART screening algorithm was developed to enhance the performance of the traditional ART algorithm to solve linear tomography problems, the NIRT was to solve nonlinear tomography problems, and the IRT-VRO was to address the issue of view registration uncertainty in both linear and nonlinear problems. This dissertation describes the mathematical formulations, and the experimental and numerical validations for these algorithms. It is expected that the results obtained in this dissertation to lay the groundwork for their further development and expanded adaption in the deployment of tomography diagnostics in various practical applications.
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    Development of Multi-perspective Diagnostics and Analysis Algorithms with Applications to Subsonic and Supersonic Combustors
    Wickersham, Andrew Joseph (Virginia Tech, 2014-12-16)
    There are two critical research needs for the study of hydrocarbon combustion in high speed flows: 1) combustion diagnostics with adequate temporal and spatial resolution, and 2) mathematical techniques that can extract key information from large datasets. The goal of this work is to address these needs, respectively, by the use of high speed and multi-perspective chemiluminescence and advanced mathematical algorithms. To obtain the measurements, this work explored the application of high speed chemiluminescence diagnostics and the use of fiber-based endoscopes (FBEs) for non-intrusive and multi-perspective chemiluminescence imaging up to 20 kHz. Non-intrusive and full-field imaging measurements provide a wealth of information for model validation and design optimization of propulsion systems. However, it is challenging to obtain such measurements due to various implementation difficulties such as optical access, thermal management, and equipment cost. This work therefore explores the application of FBEs for non-intrusive imaging to supersonic propulsion systems. The FBEs used in this work are demonstrated to overcome many of the aforementioned difficulties and provided datasets from multiple angular positions up to 20 kHz in a supersonic combustor. The combustor operated on ethylene fuel at Mach 2 with an inlet stagnation temperature and pressure of approximately 640 degrees Fahrenheit and 70 psia, respectively. The imaging measurements were obtained from eight perspectives simultaneously, providing full-field datasets under such flow conditions for the first time, allowing the possibility of inferring multi-dimensional measurements. Due to the high speed and multi-perspective nature, such new diagnostic capability generates a large volume of data and calls for analysis algorithms that can process the data and extract key physics effectively. To extract the key combustion dynamics from the measurements, three mathematical methods were investigated in this work: Fourier analysis, proper orthogonal decomposition (POD), and wavelet analysis (WA). These algorithms were first demonstrated and tested on imaging measurements obtained from one perspective in a sub-sonic combustor (up to Mach 0.2). The results show that these algorithms are effective in extracting the key physics from large datasets, including the characteristic frequencies of flow—flame interactions especially during transient processes such as lean blow off and ignition. After these relatively simple tests and demonstrations, these algorithms were applied to process the measurements obtained from multi-perspective in the supersonic combustor. compared to past analyses (which have been limited to data obtained from one perspective only), the availability of data at multiple perspective provide further insights into the flame and flow structures in high speed flows. In summary, this work shows that high speed chemiluminescence is a simple yet powerful combustion diagnostic. Especially when combined with FBEs and the analyses algorithms described in this work, such diagnostics provide full-field imaging at high repetition rate in challenging flows. Based on such measurements, a wealth of information can be obtained from proper analysis algorithms, including characteristic frequency, dominating flame modes, and even multi-dimensional flame and flow structures.
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    Experimental and Modeling Study of the Thermal Management of Li-ion Battery Packs
    Wang, Haoting (Virginia Tech, 2017-10-13)
    This work reports the experimental and numerical study of the thermal management of Li-ion battery packs under the context of electric vehicle (EV) or hybrid EV (HEV) applications. Li-ion batteries have been extensively demonstrated as an important power source for EVs or HEVs. However, thermal management is a critical challenge for their widespread deployment, due to their highly dynamic operation and the wide range of environments under which they operate. To address these challenges, this work developed several experimental platforms to study adaptive thermal management strategies. Parallel to the experimental effort, multi-disciplinary models integrating heat transfer, fluid mechanics, and electro-thermal dynamics have been developed and validated, including detailed CFD models and lumped parameter models. The major contributions are twofold. First, this work developed actively controlled strategies and experimentally demonstrated their effectiveness on a practical sized battery pack and dynamic thermal loads. The results show that these strategies effectively reduced both the parasitic energy consumption and the temperature non-uniformity while maintaining the maximum temperature rise in the pack. Second, this work established a new two dimensional lumped parameter thermal model to overcome the limitations of existing thermal models and extend their applicable range. This new model provides accurate surface and core temperatures simulations comparable to detailed CFD models with a fraction of the computational cost.
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    Experimental Investigation of Dimples as a Heat Transfer Enhancement Feature in Narrow Diverging and Converging Channels
    Srinivasan, Shreyas (Virginia Tech, 2013-08-22)
    Detailed heat transfer coefficient distributions have been obtained for narrow converging and diverging channels with and without enhancement features. The enhancement feature considered for this study is dimples (inline and staggered) on the main heat transfer surfaces. All the measurements are presented at Reynolds numbers of 3500, 8900, 18000, and 7000, 14000, 28000 for converging and diverging channels respectively. Pressure drop measurements for the overall channel are also presented to evaluate the heat transfer enhancement geometry with respect to pumping power requirements. The test models were studied for wall heat transfer coefficient measurements using the transient liquid crystal technique. The modeled wall inner surfaces were sprayed with thermochromic liquid crystals, and a transient test was used to obtain the local heat transfer coefficients from the measured color change. Analysis of results shows that dimples, in general, have very good enhancement capabilities and staggered dimpled surfaces provide considerably higher heat transfer coefficients and a reasonable pressure drop compared to inline dimpled configuration. Additionally, this study was extended to understand the effect of strategic placement of dimples (staggered) at various locations along the channel to understand regions that contribute significantly to the overall enhancement.
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    Flame Surface Density Measurements and Curvature Statistics for Turbulent Premixed Bunsen Flames
    Capil, Tyler George (Virginia Tech, 2017-02-21)
    In this work, turbulent premixed combustion was analyzed through CH (methylidyne) planar laser induced fluorescence (PLIF). Flame topography measurements in terms of flame surface density and curvature were calculated based on the flame front detected by the CH PLIF signal. The goal of this work was to investigate turbulent flames with extremely high turbulence intensity using a recently developed HiPilot burner (a Bunsen-type burner). The studies were first conducted on a series of piloted jet flames to validate the methodology, and then conducted on the highly turbulent flames generated by the HiPilot burner. All flames were controlled by combusting methane and air under a fuel to air equivalence ratio of Φ=1.05, and the Reynolds number varied from 7,385 to 28,360. Flame surface density fields and profiles for the HiPilot burner are presented. These flame surface density measurements showed an overall decrease with height above the burner. In addition, curvature statistics for the HiPilot flames were calculated and probability density functions of the curvature samples were determined. The probability density functions of curvature for the flames showed Gaussian-shaped distributions centered near zero curvature. To conclude, flame topography measurements were verified on jet flames and were demonstrated on the new HiPilot flames.
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    Investigation of Endoscopic Techniques for Flow and Combustion Measurements
    Kang, Min Wook (Virginia Tech, 2014-07-18)
    This work investigated the application of fiber-based endoscopes (FBEs) in combustion and flow measurements, especially for multidimensional and quantitative measurements. The use of FBEs offers several unique advantages to greatly reduce the implementation difficulty and cost of optical diagnostics. However, the use of FBEs requires registering the locations and orientations of the FBEs carefully for quantitative measurements, and degrades the spatial resolution of the images transmitted. Hence this work conducted a series of controlled tests to quantify the accuracy of the view registration process and the spatial resolution degradation for FBEs. The results show that, under the conditions tested in this work, the view registration process can be accurate within ±0.5 degree and the FBEs can resolve spatial features on the order of 0.25 mm. The combined effects of such view registration uncertainty and spatial resolution degradation are reflected in the re-projection error, which was shown to be within ±0.5 pixels under typical conditions used in this work. Finally, based on these understanding, experiments were conducted to obtain instantaneous measurements of flame structures at kHz temporal resolution using FBEs, demonstrating the capability of resolving flame features on the order of 0.2~0.3 mm in three-dimensional.
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    A Low Order Aerodynamic Model of Embedded Total Temperature Probes
    Heersema, Nicole Amanda (Virginia Tech, 2014-11-25)
    Measurement of the total conditions downstream of fans is of primary importance to aeroengine development. Historically, these measurements have been acquired with the use of traditional total condition probes mounted to the guidevanes or engine cowling; however, such a setup can have significant impact on the flow. Difficulties in obtaining direct measurements with traditional total conditions probes have led to the development of an embedded shielded probe. In order to support this development, a model was desired to be developed that accurately modelled the recovery using a low-order analysis that could be implemented quickly. The creation and validation of such a model is the primary focus of the present research. Of secondary interest is to prove the hypothesis that aerodynamics will dominate the recovery of such a sensor. Based around the calculations for recovery used by Moffat, the model uses a linear vortex panel method to calculate the aerodynamics of the sensor. Higher order corrections were also suggested to improve the accuracy of the model. Several of these corrections, which take into account compressibility and variance of individual recovery factors, were included in the final model. Other corrections, such as improved paneling for the panel method and the inclusion of pitch angle have not been incorporated at this time but are part of an ongoing effort to improve and expand the capabilities of the model. Model validation was performed in three steps, starting with comparing the calculations for the recovery without aerodynamics to values present in literature for traditional Shielded probes. The aerodynamics and the panel method used to generate them were validated separately using the widely available program Xfoil. Validation of the combined model could only be accomplished via experimental testing. Several sensors, based on the predictions of the model, were 3D printed for use in experimental testing. Three key geometric parameters were identified and varied within the limits of interest to create the set of sensors tested. The purpose of this was two-fold. One: validate the model or identify key missing aerodynamic effects for inclusion. Two: prove the secondary hypothesis that aerodynamics will dominate the recovery. Testing was performed at a range of Mach numbers, yaw angles, and pitch angles commonly present in aeroengines. The data collected for model validation were simultaneously used to prove the hypothesis that aerodynamic effects dominated the recovery. This hypothesis was concluded to be true for the range of parameters tested. The model was determined to be valid for the range of parameters tested, although with the caveat that not all aerodynamic effects are fully accounted for and physical testing or CFD analysis is advised to verify results once design parameters have been narrowed down sufficiently. Further refinement of the experimental data and investigation of the aerodynamic effects are the subject of further study.
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    Method to correct the distortion caused by amplified stimulated emission as motivated by LIF-based flow diagnostics
    Li, Xuesong; Zhao, Yan; Ma, Lin (Optical Society of America, 2012-04-01)
    Amplified stimulated emission (ASE) represents a significant issue in two-photon laser-induced fluorescence (TPLIF). The ASE effects are nonlinear and nonlocal, i.e., the ASE effects distort the LIF signal nonlinearly, and the distortion at one location depends on conditions at other locations. In this sense, the ASE effects pose a greater challenge to quantitative TPLIF than quenching and ionization. This work therefore seeks a method to correct such distortion. The method uses two LIF measurements, one with low signal-to-noise ratio (SNR) and negligible ASE distortion and another with high SNR but significant distortion, to generate a faithful measurement with high SNR. Extensive simulations were performed to evaluate the performance of this method for practical applications. c 2012 Optical Society of America OCIS codes: 300.2530, 300.6420, 120.1740.
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    Multi-dimensional Flow and Combustion Diagnostics
    Li, Xuesong (Virginia Tech, 2014-06-10)
    Turbulent flows and turbulent flames are inherently multi-dimensional in space and transient in time. Therefore, multidimensional diagnostics that are capable of resolving such spatial and temporal dynamics have long been desired; and the purpose of this dissertation is to investigate three such diagnostics both for the fundamental study of flow and combustion processes and also for the applied research of practical devices. These multidimensional optical diagnostics are a 2D (two dimensional) two-photon laser-induced fluorescence (TPLIF) technique, a 3D hyperspectral tomography (HT) technique, and a 4D tomographic chemiluminescence (TC) technique. The first TPLIF technique is targeted at measuring temporally-resolved 2D distribution of fluorescent radicals, the second HT technique is targeted at measuring temperature and chemical species concentration at high speed, and the third TC technique is targeted at measuring turbulent flame properties. This dissertation describes the numerical and experimental evaluation of these techniques to demonstrate their capabilities and understand their limitations. The specific aspects investigated include spatial resolution, temporal resolution, and tomographic inversion algorithms. It is expected that the results obtained in this dissertation to lay the groundwork for their further development and expanded application in the study of turbulent flow and combustion processes.
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    Numerical and experimental validation of a three-dimensional combustion diagnostic based on tomographic chemiluminescence
    Cai, Weiwei; Li, Xuesong; Li, Fei; Ma, Lin (Optical Society of America, 2013-03-25)
    Three-dimensional (3D) measurements are highly desirable both for fundamental combustion research and practical monitoring and control of combustion systems. This work discusses a 3D diagnostic based on tomographic chemiluminescence (TC) to address this measurement need. The major contributions of this work are threefold. First, a hybrid algorithm is developed to solve the 3D TC problem. The algorithm was demonstrated in extensive tests, both numerical and experimental, to yield 3D reconstruction with high fidelity. Second, an experimental approach was designed to enable quantifiable metrics for examining key aspects of the 3D TC technique, including its spatial resolution and reconstruction accuracy. Third, based on the reconstruction algorithm and experimental results, we investigated the effects of the view orientations. The results suggested that for an unknown flame, it is better to use projections measured from random orientations than restricted orientations (e.g., coplanar orientations). These findings are expected to provide insights to the fundamental capabilities of the TC technique, and also to facilitate its practical application.
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    Numerical Loss Prediction of high Pressure Steam Turbine airfoils
    Nunes, Bonaventure R. (Virginia Tech, 2013-10-24)
    Steam turbines are widely used in various industrial applications, primarily for power extraction. However, deviation for operating design conditions is a frequent occurrence for such machines, and therefore, understanding their performance at off design conditions is critical to ensure that the needs of the power demanding systems are met as well as ensuring safe operation of the steam turbines. In this thesis, the aerodynamic performance of three different turbine airfoil sections ( baseline, mid radius and tip profile) as a function of angle of incidence and exit Mach numbers, is numerically computed at 0.3 axial chords downstream of the trailing edge. It was found that the average loss coefficient was low, owing to the fact that the flow over the airfoils was well behaved. The loss coefficient also showed a slight decrease with exit Mach number for all three profiles. The mid radius and tip profiles showed near identical performance due to similarity in their geometries. It was also found out that the baseline profile showed a trend of substantial increase in losses at positive incidences, due to the development of an adverse pressure zone on the blade suction side surface. The mid radius profile showed high insensitivity to angle of incidence as well as low exit flow angle deviation in comparison to the baseline blade.
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    Particle Sensing in Gas Turbine Inlets Using Optical Measurements and Machine Learning
    Moon, Chi Young (Virginia Tech, 2021-01-19)
    Propulsion systems are exposed to a variety of foreign objects that can significantly damage or impact their performance. These threats can range from severe dangers such as sandstorms and volcanic eruptions, which can induce engine failure in minutes, to condensation and moisture during ground tests that can negatively impact the engine's fuel efficiency. While numerous computational and experimental studies have investigated the effects of particle ingestion on the component level, an accurate in-situ measurement technique is needed for a systematic understanding of the effects and real-time engine health monitoring. Optical measurement techniques are attractive for this application due to their non-intrusive nature. However, conventional optical particle measurement methods assume the particle to be spherical, which introduces large errors for measuring particles with complex and irregular shapes, such as sand, volcanic ash, and ice crystals. The light-particle interaction contains information on the desired parameters, such as particle shape and size. The research presented in this dissertation uses this idea for a novel particle sensor concept. Scattering and extinction of light by particles are chosen as crucial features that can identify the particle as its unique signature. Numerical tools are used to simulate the scattering and extinction for particles the sensor is expected to encounter. Machine learning models are trained using the data to use scattering and extinction as inputs and estimate the particle parameters. Different types and applications of supervised machine learning models were investigated, including a layered approach with numerous models and a generalized approach with a single neural network. The particle sensor is first demonstrated using data found in the literature. This study confirmed the importance of non-spherical particles in the library to guide the machine learning models. Further demonstrations are made at a full engine and wind tunnel scale to measure injected condensation and sand sprays, respectively. The mass flow rates of the ingested material were calculated using the model outputs and validated.
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    Physics-Informed, Data-Driven Framework for Model-Form Uncertainty Estimation and Reduction in RANS Simulations
    Wang, Jianxun (Virginia Tech, 2017-04-05)
    Computational fluid dynamics (CFD) has been widely used to simulate turbulent flows. Although an increased availability of computational resources has enabled high-fidelity simulations (e.g. large eddy simulation and direct numerical simulation) of turbulent flows, the Reynolds-Averaged Navier-Stokes (RANS) equations based models are still the dominant tools for industrial applications. However, the predictive capability of RANS models is limited by potential inaccuracies driven by hypotheses in the Reynolds stress closure. With the ever-increasing use of RANS simulations in mission-critical applications, the estimation and reduction of model-form uncertainties in RANS models have attracted attention in the turbulence modeling community. In this work, I focus on estimating uncertainties stemming from the RANS turbulence closure and calibrating discrepancies in the modeled Reynolds stresses to improve the predictive capability of RANS models. Both on-line and off-line data are utilized to achieve this goal. The main contributions of this dissertation can be summarized as follows: First, a physics-based, data-driven Bayesian framework is developed for estimating and reducing model-form uncertainties in RANS simulations. An iterative ensemble Kalman method is employed to assimilate sparse on-line measurement data and empirical prior knowledge for a full-field inversion. The merits of incorporating prior knowledge and physical constraints in calibrating RANS model discrepancies are demonstrated and discussed. Second, a random matrix theoretic framework is proposed for estimating model-form uncertainties in RANS simulations. Maximum entropy principle is employed to identify the probability distribution that satisfies given constraints but without introducing artificial information. Objective prior perturbations of RANS-predicted Reynolds stresses in physical projections are provided based on comparisons between physics-based and random matrix theoretic approaches. Finally, a physics-informed, machine learning framework towards predictive RANS turbulence modeling is proposed. The functional forms of model discrepancies with respect to mean flow features are extracted from the off-line database of closely related flows based on machine learning algorithms. The RANS-modeled Reynolds stresses of prediction flows can be significantly improved by the trained discrepancy function, which is an important step towards the predictive turbulence modeling.
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    Practical aspects of implementing three-dimensional tomography inversion for volumetric flame imaging
    Cai, Weiwei; Li, Xuesong; Ma, Lin (Optical Society of America, 2013-11-01)
    Instantaneous three-dimensional (3D) measurements have been long desired to resolve the spatial structures of turbulent flows and flame. Previous efforts have demonstrated tomography as a promising technique to enable such measurements. To facilitate the practical application, this work investigated four practical aspects for implementing 3D tomographic under the context of volumetric combustion diagnostics. Both numerical simulations and controlled experiments were performed to study: (1) the termination criteria of the inversion algorithm; (2) the effects of regularization and the determination of the optimal regularization factor; (3) the effects of a number of views; and (4) the impact of the resolution of the projection measurements. The results obtained have illustrated the effects of these practical aspects on the accuracy and spatial resolution of volumetric tomography. Furthermore, all these aspects are related to the complexity and implementing cost (both hardware cost and computational cost). Therefore, the results obtained in this work are expected to be valuable for the design and implementation of practical 3D diagnostics. (C) 2013 Optical Society of America