I have published multiple journal articles, conference papers, and abstracts which are listed below. PDFs of the articles (open access articles, accepted manuscripts or preprints) are provided where possible, and links are provided to the version of record. An asterisk (*) denotes equal contribution.

Journal Articles

  1. Joshua M. Herzog, Gianna Agosta, and Volker Sick. On the impact of radiative transfer in fluorescence imaging of bacterial films and suspensions. Journal of Quantitative Spectroscopy and Radiative Transfer 324 (2024): 109063.
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    Abstract
    Fluorescence imaging and spectroscopy are convenient, rapid, and simple methods to analyze chemical samples including biological materials such as bacterial biofilms and suspensions. In principle, these techniques could be used to diagnose or discriminate between infectious bacteria in infections of the skin or ocular surface (microbial keratitis, MK). However, the extension of these techniques to macroscopic turbid media that strongly absorb and scatter light is difficult. Radiative transfer effects obscure the relationship between microscopic scattering and absorption properties and macroscopically observable quantities such as fluorescence intensity, transmission, and reflection. A combination of experimental measurements of aqueous bacteria cell suspensions and Monte Carlo radiation transfer simulations are performed to better understand these effects. Several general observations, e.g., that fluorescence intensity is maximized in scattering-dominated media, are discussed in detail. It was found that wavelength-dependent radiative transfer effects are observable even at moderate optical densities (OD ~1; well below the diffusion limit). Careful consideration of radiative transfer effects using physically rigorous models is needed to determine single-cell scattering and absorption properties and interpret quantitative fluorescence measurements accurately in most cases of interest. A detailed discussion of radiative transfer effects and analytical models is provided. In the context of surface infections and MK, it was found that radiative transfer effects may be negligible for the model bacteria E. coli in some cases. However, more accurate measurements of microbe optical properties are needed to confirm and extend this conclusion to other species. Overall this work demonstrates that quantitative fluorescence imaging and spectroscopy of bacterial films and suspensions is feasible, but requires detailed sample characterization and careful consideration of radiative transfer effects.
  2. Joshua M. Herzog, and Volker Sick. MCRAD: A Monte Carlo photon transport code for analysis of fluorescence and elastic scattering diagnostics. SoftwareX 27 (2024): 101672.
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    Abstract
    Radiative transfer, or the propagation of radiation (light) through a system, is an important problem in imaging and optical diagnostics in complex media including turbulent gas flows, biological tissues, and particle suspensions. Monte Carlo (MC) methods are the de facto standard for radiative transfer simulation in complex media. While many sophisticated tools exist within this domain, few are well-suited to investigating laser- and fluorescence-imaging in turbid media. Here, a fast MC code for investigating light propagation in turbid media is presented. This tool is intended to enable detailed, physics-based analysis of laser and imaging techniques of simple model systems for use in experimental design and interpretation.
  3. Maria A. Woodward, Joshua M. Herzog, and Angela Verkade. UV Light is Not a Toy-Recreational Use and Eye Harm. JAMA Ophthalmology (2024).
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  4. Joshua M. Herzog, and Volker Sick. Fluorescence imaging for the anterior segment of the eye. Frontiers in Photonics 4 (2024).
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    Abstract
    Diagnostic technologies for the anterior segment of the eye, especially for hard-to-diagnose diseases such as microbial keratitis, are still lacking. Although in vivo confocal microscopy and optical coherence tomography are becoming more widely applicable to a variety of conditions, they are often prohibitively expensive, require specialized training and equipment, and are intrinsically insensitive to chemical changes. Here, ultraviolet-fluorescence imaging is proposed as a new technique to aid in investigation of the anterior segment. In this work, a novel two-color line-of-sight fluorescence imaging technique is described for imaging of the anterior segment. The technique is applied to seven ex vivo porcine eyes to illustrate the utility of the technique. The image data was used to estimate an effective fluorescence quantum yield of each eye at 370 nm. The eyes were then inoculated with bacteria to simulate microbial keratitis, a common sight-threatening infection, and the measurement was repeated. A simplified fluorescence-extinction model was developed to describe and analyze the relative intensities of the eye and biofilm fluorescence. Overall, the technique appears to have utility in clinical practice and with proper development may be suitable for detecting chemical changes in the eye, or the presence of foreign matter; however, further investigation is needed to develop the technique and analysis procedures into a quantitative diagnostic tool.
  5. Haobo Xu*, Joshua M. Herzog*, Yimin Zhou*, Yashar Bashirzadeh, Allen Liu, and Solomon Adera. Visualization and Experimental Characterization of Wrapping Layer Using Planar Laser-Induced Fluorescence. ACS nano 18 (2024): 4068-4076.
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  6. Joshua M. Herzog, and Volker Sick. Design of a line-of-sight fluorescence-based imaging diagnostic for classification of microbe species. Measurement Science and Technology 34.9 (2023): 095703.
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    Abstract
    Fluorescence imaging of certain biochemicals, including flavins and pyridine nucleotides, has utility in characterizing the metabolic state of tissue and in discriminating between microbial species. There is significant clinical utility in this class of imaging techniques but most measurements reported to date require specialized training and equipment rendering most implementations unsuitable for routine medical imaging. Here, a low-cost and robust imaging technique is designed using ultraviolet-induced fluorescence of pyridine nucleotides (primarily NADH) and flavins (primarily FAD) in microbial samples. The diagnostic is optimized to distinguish between different microbial species based on previously reported spectral data using a ratiometric imaging approach. A detailed performance analysis is provided that relates the measured fluorescence intensity ratio (FIR) to the relative concentration ratio of NADH to FAD using a simplified spectroscopic model. Analysis suggests the technique is sensitive to changes in the NADH/FAD concentration ratio over several orders of magnitude, with better than 10% FIR precision on a per-pixel basis for microbial smears as thin as 10 s of microns at a resolution of 30 mm−1 and exposures of 20 ms. Representative microbe samples from eight species were imaged to demonstrate the proposed technique. Results show that the FIR varies by an order of magnitude across different species but the intra-species variation is only ∼5% for the conditions used here. An additional imaging band may be necessary to classify species that contain red pigments or bacteriochlorophyll. Radiative trapping was discussed as a possible limitation of the technique, but no clear evidence for radiative trapping was observed here. Overall, the results suggest that the proposed approach is feasible for rapid, low-cost, and robust characterization of microbial samples.
  7. Joshua M. Herzog, and Volker Sick. Quantitative Spectroscopic Characterization of Near-UV/visible E. coli (pYAC4), B. subtilis (PY79), and Green Bread Mold Fungus Fluorescence for Diagnostic Applications. Journal of Fluorescence (2023): 1-13.
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    Abstract
    Ultraviolet (UV)-excited visible fluorescence is an attractive option for low-cost, low-complexity, rapid imaging of bacterial and fungal samples for imaging diagnostics in the biomedical community. While several studies have shown there is potential for identification of microbial samples, very little quantitative information is available in the literature for the purposes of diagnostic design. In this work, two non-pathogenic bacteria samples (E. coli pYAC4, and B. subtilis PY79) and a wild-cultivated green bread mold fungus sample are characterized spectroscopically for the purpose of diagnostic design. For each sample, fluorescence spectra excited with low-power near-UV continuous wave (CW) sources, and extinction and elastic scattering spectra are captured and compared. Absolute fluorescence intensity per cell excited at 340 nm is estimated from imaging measurements of aqueous samples. The results are used to estimate detection limits for a prototypical imaging experiment. It was found that fluorescence imaging is feasible for as few as 35 bacteria cells (or ∼30 µm3 of bacteria) per pixel, and that the fluorescence intensity per unit volume is similar for the three samples tested here. A discussion and model of the mechanism of bacterial fluorescence in E. coli is provided.
  8. Haobo Xu, Yimin Zhou, Dan Daniel, Joshua Herzog, Xiaoguang Wang, Volker Sick, and Solomon Adera. Droplet attraction and coalescence mechanism on textured oil-impregnated surfaces. Nature Communications 14 (2023): 4901.
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    Abstract
    TDroplets residing on textured oil-impregnated surfaces form a wetting ridge due to the imbalance of interfacial forces at the contact line, leading to a wealth of phenomena not seen on traditional lotus-leaf-inspired non-wetting surfaces. Here, we show that the wetting ridge leads to long-range attraction between millimeter-sized droplets, which coalesce in three distinct stages: droplet attraction, lubricant draining, and droplet merging. Our experiments and model show that the magnitude of the velocity and acceleration at which droplets approach each other horizontally is the same as the vertical oil rise velocity and acceleration in the wetting ridge. Moreover, the droplet coalescence mechanism can be modeled using the classical mass-spring system. The insights gained from this work will inform future fundamental studies on remote droplet interaction on textured oil-impregnated surfaces for optimizing water harvesting and condensation heat transfer.
  9. Mitchell Reisetter, Joshua Herzog, Eri Amezcua, Kenneth Kim, Chol-Bum Kweon, and David Rothamer. Non-Intrusive Accelerometer-Based Sensing of Start-Of-Combustion in Compression-Ignition Engines. SAE International Journal of Advances and Current Practices in Mobility 5 (2023): 2330-2343.
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    Abstract
    A non-intrusive sensing technique to determine start of combustion for mixing-controlled compression-ignition engines was developed based on an accelerometer mounted to the engine block of a 4-cylinder automotive turbo-diesel engine. The sensing approach is based on a physics-based conceptual model for the signal generation process that relates engine block acceleration to the time derivative of heat release rate. The frequency content of the acceleration and pressure signals was analyzed using the magnitude-squared coherence, and a suitable filtering technique for the acceleration signal was selected based on the result. A method to determine start of combustion (SOC) from the acceleration measurements is presented and validated. In-cylinder pressure (used to calculate heat release rate) and accelerometer data were collected on a 1.9-L compression-ignition direct-injection engine (Z19DTH) over a wide range of speeds (1000-3250 RPM) and loads (2-8 bar IMEPg), for single- and double-injection strategies, and for fuels of several different cetane numbers (35.5 and 48.5). The relationship between engine block acceleration and the time derivative of heat release rate was verified using the experimental results and indicates that unsteadiness in the rate of heat release is the driving factor for the engine block acceleration signal. The accelerometer-based method developed allows detection of SOC for the main injection with a root mean square error less than 0.75 CAD for the range of conditions tested in this work.
  10. Joshua M. Herzog, Dustin Witkowski, and David A. Rothamer. Combined scattering-referenced and co-doped aerosol phosphor thermometry using the Ce,Pr:LuAG phosphor. Applied Physics B 127.7 (2021): 103.
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    Abstract
    As a means of increasing the temperature range of high-precision measurements for aerosol phosphor thermometry (APT), the co-doped Ce,Pr:LuAG phosphor was investigated for use with simultaneous Pr3+-scattering-referenced APT (SRAPT), Ce3+-SRAPT, and co-doped APT techniques. Phosphor characterization was performed in a heated air jet at atmospheric pressure, and APT performance was estimated for the three simultaneous imaging techniques. A method for combining temperature measurements using a weighted average approach is discussed in detail and demonstrated via heated air jet imaging experiments. The combination of techniques results in a usable temperature range for the Ce,Pr:LuAG phosphor of 300 to at least 900 K, with better than 35 K estimated temperature precision from 500 to 900 K at a seeding density of 120 mm-3, improving diagnostic precision and temperature range over that of any single APT technique using the Ce,Pr:LuAG phosphor. The combination of SRAPT and co-doped measurements may improve the temperature range and performance of other similar phosphors as well.
  11. Joshua M. Herzog, Dustin Witkowski, and David A. Rothamer. Characterization of Ce:CSSO, Pr:CSSO, and co-doped Ce,Pr:CSSO phosphors for aerosol phosphor thermometry. Measurement Science and Technology 32.5 (2021): 054008.
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    Abstract
    The phosphor Ce:Ca3Sc2Si3O12(Ce:CSSO) was recently investigated for aerosol phosphor thermometry (APT) and shown to be capable of temperature imaging up to at least 1400 K. To date, no thorough characterization of the temperature dependent emission properties of the phosphor has been performed up to 1400 K. Additionally, due to limited sensitivity over certain temperature ranges continuous temperature imaging from 300 to 1400 K was not possible. Here, cerium and praseodymium doped into CSSO are investigated to address these limitations. Singly-doped Ce:CSSO and Pr:CSSO, and co-doped Ce,Pr:CSSO phosphor samples were studied. Emission lifetimes and quenching behavior were characterized in a tube furnace for each phosphor. Results from the singly-doped phosphors were used to interpret the co-doped phosphor results. The phosphor characterization data was used to estimate thermometry performance at temperatures relevant to low-temperature ignition studies in engines. The proposed diagnostic uses a combination of co-doped APT, host-referenced APT, and scattering-referenced APT approaches to increase the range over which high-precision temperature measurements can be made. Characterization results include the first reported measurements of luminescence lifetimes for Ce:CSSO and Pr:CSSO at temperatures up to 1400 K, and the highest reported quenching temperature for a phosphor using 4f5d emission (1230 K for Ce:CSSO). Furnace measurements showed a normalized temperature-sensitivity of > 0.25%/K from room temperature to 1400 K for at least one diagnostic approach using the co-doped Ce,Pr:CSSO phosphor. Estimates suggest that the co-doped Ce,Pr:CSSO phosphor is feasible for APT at temperatures ranging from 300 to 925 K and 1030 to at least 1400 K, with better than 40-K single-shot precision. The performance of Ce:CSSO and Ce,Pr:CSSO are similar throughout the investigated temperature range.
  12. Dustin Witkowski, Joshua M. Herzog, and David A. Rothamer. Combustion-relevant temperature imaging with scattering referenced aerosol phosphor thermometry applied to Eu:BAM. Combustion and Flame 224 (2021): 233-238.
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    Abstract
    This work demonstrates precise aerosol phosphor thermometry (APT) measurements over a large temperature range by applying scattering-referenced APT (SRAPT) with the phosphor divalent Europium doped in Barium Magnesium Aluminate (Eu:BAM). Measurements were taken using Eu:BAM seeded into an air jet mixing with the products of a concentric methane-air flat flame. In-situ calibration of the SRAPT ratio versus temperature was performed from 500 to 1400 K. Peak temperature sensitivities of 1–1.5 %/K were observed, approximately 4 times higher than the spectral luminescence intensity ratio method typically used for Eu:BAM. Despite its moderate quenching temperature (700 K), the high initial signal intensities before the onset of thermal quenching for Eu:BAM allowed reliable single-shot measurements to be made continuously from 500 K to greater than 1200 K. These results highlight the importance of high signal levels for extending the measurement range of SRAPT.
  13. Joshua M. Herzog, Dustin Witkowski, and David A. Rothamer. Combustion-relevant aerosol phosphor thermometry imaging using Ce,Pr:LuAG, Ce:GdPO4, and Ce:CSSO. Proceedings of the Combustion Institute 38.1 (2021): 1617-1625.
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    Abstract
    Aerosol phosphor thermometry (APT) is a promising temperature-imaging diagnostic that is currently being developed for combustion applications. To date, gas-phase APT measurements have been limited to temperatures below 1000 K due to thermal quenching and poor sensitivity at high temperatures. In this work, three phosphor compositions are investigated for application at flame relevant temperatures: Ce,Pr:LuAG, Ce:GdPO4, and Ce:CSSO. The phosphors were characterized in a temperature-controlled furnace, and measurements of gas temperature were performed in a seeded air jet after mixing with the products of an atmospheric methane-air flame. Furnace and flame measurements demonstrate that two of the phosphors are capable of temperature imaging at over 1000 K, with an upper temperature limit of at least 1400 K. Temperature precision estimates indicate 20-K or better single-shot precision from 500 to 1300 K for a seeding density corresponding to an added heat capacity of less than 1% of that of air at 1200 K at a spatial resolution of 1.12 line pairs per millimeter. This work represents the highest reported temperature measurements made using any APT technique for gas temperature measurements, and represents the highest measured quenching temperature of any phosphor exhibiting fast allowed emission for APT. These results extend the capabilities of APT for single-shot gas-phase temperature imaging up to at least 1400 K. This new capability will allow APT to be applied in combustion environments to study problems such as low-temperature ignition in engines.
  14. Joshua M. Herzog, Dustin Witkowski, and David A. Rothamer. Characterization of the Ce,Pr:LuAG phosphor for co-doped aerosol phosphor thermometry. Journal of Luminescence 229 (2021): 117665.
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    Abstract
    The Ce,Pr:LuAG phosphor is investigated for use in co-doped aerosol phosphor thermometry (APT). The phosphor was characterized by measuring the emission intensity for each ion as a function of excitation laser fluence and temperature. A simplified three-level model was used to interpret and fit the data. The phosphor was then used for co-doped APT temperature-imaging experiments performed in a heated air jet up to a maximum temperature of 694 K. The three-level model fit was used to analyze sources of bias in the imaging measurements. Both ions’ emission intensities show a nonlinear dependence on excitation fluence that can potentially be explained by temperature-dependent ground- and excited-state absorption processes included in the three-level model. Single-shot temperature precision from the temperature-imaging experiments was measured to be 22, 23, and 32 K for mean temperatures of 497, 603, and 694 K, respectively, at a spatial resolution of 1.12 lp/mm and average fluence of 33 mJ/cm2. The results extend the measurement range for co-doped APT to at least 700 K. The findings also provide a simple model for phosphor signal non-linearity with excitation fluence for low activator ion concentrations, and provide a method for estimating temperature bias from several sources including laser fluence.
  15. Christopher D. Noble, Joshua M. Herzog, David A. Rothamer, Alex M. Ames, Jason Oakley, and Riccardo Bonazza. Scalar Power Spectra and Scalar Structure Function Evolution in the Richtmyer-Meshkov Instability Upon Re-Shock. Journal of Fluids Engineering 142.12 (2020): 121102.
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    Abstract
    The Richtmyer–Meshkov instability of a twice-shocked gas interface is studied using high-speed planar laser-induced fluorescence in the Wisconsin Shock Tube Laboratory's vertical shock tube. The initial condition is a shear layer with broadband diffuse perturbations at the interface between a helium–acetone mixture and argon. This initial condition is accelerated by a shock of nominal strength M = 1.9, and then accelerated again by the transmitted shock that reflects off the end wall of the tube. Three individual experiments are analyzed, the energy spectrum and the structure functions of the light gas mole fraction field are calculated and compared.
  16. Christopher D. Noble, Joshua M. Herzog, Alex M. Ames, Jason Oakley, David A. Rothamer, and Riccardo Bonazza. High speed PLIF study of the Richtmyer-Meshkov instability upon re-shock. Physica D: Nonlinear Phenomena 410 (2020): 132519.
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    Abstract
    The Richtmyer–Meshkov instability (RMI) of a twice-shocked gas interface is studied using high-speed planar-laser induced fluorescence (PLIF) in the Wisconsin Shock Tube Laboratory’s vertical shock tube. The initial condition (IC) is a shear layer with broadband diffuse perturbations at the interface between a helium-acetone mixture and argon. This IC is accelerated by a shock of nominal strength, and then accelerated again by the transmitted shock that reflects off the end wall of the tube. An estimate of the light gas mole fraction is extracted from high-speed imaging using an iterative process that accounts for the nonlinear temperature dependence of the acetone’s fluorescence quantum yield (FQY) and absorption cross-section. A vorticity deposition model for the initial growth rate after re-shock is compared with the Mikaelian model for re-shock. Previously used in literature, the number of generations is shown to naturally arise from a normalisation of the scalar transport equation. A self-similar analysis is then performed using the mole fraction data to explore the evolution of the RMI after reshock and the higher order moments of the light gas mole fraction are compared with a proposed model.

Conference Papers

  1. Joshua M. Herzog and Volker Sick. Towards Two-Color Fluorescence Imaging for Diagnosis of Microbial Keratitis. in Biophotonics Congress: Optics in the Life Sciences 2023 (2023): DTu1A.3.
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    Abstract
    Novel methods are needed for diagnosis of microbial keratitis. Two-color fluorescence imaging is proposed and tested on ex vivo porcine eyes. Results show the technique may be feasible but further quantitative characterization is needed.
  2. Joshua M. Herzog and Volker Sick. Classification of Microbial Samples Using Two-Color Line-of-Sight Fluorescence Imaging. in Biophotonics Congress: Optics in the Life Sciences 2023 (2023): DTu1A.7.
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    Abstract
    Rapid, low-cost diagnosis of infections is challenging and requires innovation. Two-color fluorescence imaging is proposed to distinguish between microbial species. Microbial smear images show the technique is promising for classifying species in vitro.
  3. Joshua M. Herzog, Alex Ames, Christopher Noble, Jason Oakley, Riccardo Bonazza, and David A. Rothamer. Iterative correction of shocked acetone high-speed PLIF measurements in the Richtmyer-Meshkov instability. Proceedings of the International Symposium on Shock Waves 32 (2019): 993-1006.
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    Abstract
    Experimental investigations of shock-induced turbulent mixing often employ spatially and temporally resolved optical diagnostics such as planar laser-induced fluorescence (PLIF) to observe the mixing process. However, due to the complex dependence of the PLIF signal on local conditions, quantitative interpretation of the PLIF data is difficult. This work investigates the possibility for rigorous interpretation of PLIF imaging data from Richtmyer-Meshkov instability (RMI) experiments by taking local temperature and composition into account to provide a more quantitative estimate of local tracer mole fraction and temperature. A correction strategy is outlined using an adiabatic mixing assumption. The mixing assumption was validated against high-fidelity simulation data from the Miranda hydrodynamics code, and the correction strategy was applied to high-speed PLIF data taken following reshock of a perturbed helium-argon interface at Mach 1.8. Comparison to simulation suggests that the adiabatic mixing approximation is valid to within approximately 30 K for the current conditions, and more typically within 20 K, but requires a good estimate of the temperatures of the unmixed helium and argon regions. When applied to experimental data, the correction strategy resulted in significant differences in the span-wise averaged light-gas mole fraction (on the order of 10% absolute throughout much of the profile) compared to a constant-property correction, demonstrating the magnitude of the errors introduced without correcting for these PLIF dependencies.

Archived Abstracts

  1. Christopher Noble, Alexander Ames, Joshua Herzog, Jason Oakley, David Rothamer, Raymond McConnell, and Riccardo Bonazza. High-Speed Simultaneous Velocity and Concentration measurements of the Richtmyer-Meshkov Instability Upon Re-Shock. APS Division of Fluid Dynamics Fall 2021 Meeting (2021): P10.007.
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    Abstract
    The Richtmyer-Meshkov instability of a twice-shocked gas interface is investigated in the vertical shock tube of the Wisconsin Shock Tube Laboratory at the University of Wisconsin- Madison. The initial condition is a shear layer, containing broadband perturbations, formed at the interface between a helium-acetone mixture and argon. The interface is accelerated with a shock of nominal strength M=1.76 with an initial Atwood number A=0.5. Concurrent, high-speed planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) are performed using a pulse-burst laser operating at 20,000 Hz. The 266 nm and 532 nm laser outputs are co-propagated into the shock tube, forming a dual wavelength light sheet. Acetone is added to the helium to act as a fluorophore; 200 nm titanium dioxide particles are added to both the helium and the argon to perform PIV. Images are collected with two high-speed CMOS cameras. Presented preliminary results will include integral measures, such as thickness and mixedness; spectral quantities, including scalar and kinetic energy spectra; combined moments and structure functions.
  2. Alexander Ames, Christopher Noble, Joshua Herzog, David Rothamer, Jason Oakley, and Riccardo Bonazza. High-speed simultaneous velocity & density measurement of compressible multifluid shock-vortex interaction. APS Division of Fluid Dynamics Fall 2021 Meeting (2021): P10.011.
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    Abstract
    In the intermediate stages of Richtmyer-Meshkov/Rayleigh-Taylor instability development, prior to the development of turbulence, vortical flow structures are commonly observed at the outer extents of the mixing region. The evolution of these structures upon reshock may play an important role in the tails of the mixedness distribution, but they are difficult to repeatably study because of the inherently unstable mixing process. To capture these vortex-shock interaction events in isolation, an ensemble of simultaneous PLIF/PIV measurements were acquired at 20 kHz following the firing of a small, open-ended, argon-filled shock tube (D= 2.2 cm) upwards into ambient nitrogen. The measurements capture the initial propagation of the argon vortex ring as well as its perturbation upon M= 1.75 shock and subsequent reshock.
  3. Christopher Noble, Joshua Herzog, Alex Ames, Jason Oakley, David Rothamer, and Riccardo Bonazza. Scalar Power spectrum and Structure Function analysis of the Richtmyer-Meshkov Instability Upon Re-Shock. APS Division of Fluid Dynamics Fall 2019 Meeting (2019): A33.006.
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    Abstract
    The Richtmyer-Meshkov instability of a twice-shocked gas interface is investigated in the vertical shock tube of the Wisconsin Shock Tube Laboratory at the University of Wisconsin- Madison. The initial condition is a shear layer, containing broadband perturbations, formed at the interface between a helium-acetone mixture and argon. The interface is accelerated with a shock of nominal strength M =1.9 with an initial Atwood number of A =0.43. Acetone is used as a molecular tracer for PLIF, allowing the extraction of concentration data by using a pulse burst laser system at 20kHz to excite acetone to fluoresce. The resulting fluorescence signal is measured using a high-speed Phantom camera. The evolution of the scalar power spectrum is investigated. As seen in previous single shock experiments a region of -5/3 slope is seen at late post-shock times, however at late re-shock times a larger region of -8/3 spectrum is observed. The measurement limit of the present experiments is estimated to be within the inertial range that may exist thus the measured slope is not expected to be a dissipation effect but the slope of the inertial range. Scalar structure functions are calculated, with the anomalous exponent being plotted against the structure function order also showing a non-KOC scaling. The terms in the scalar power spectrum evolution equation are calculated showing an asymmetry about the centre of the mixing layer and suggesting the emergence of an inertial range.

Ph.D. Thesis

  1. Joshua M. Herzog. Quantitative Temperature & Formaldehyde Concentration Imaging for High-Pressure Turbulent Fuel Jet Ignition. The University of Wisconsin-Madison (2020): 28260435.
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    Abstract
    Understanding the process of turbulent fuel jet ignition in engines is critical to improve engine designs. Unfortunately, there is much we do not understand about the ignition process, and current diagnostic methods are insufficient to improve our understanding of the coupling between temperature, velocity, and chemistry in high-pressure turbulent jet ignition. Here, a diagnostic approach is designed that can simultaneously measure temperature, velocity, and formaldehyde concentration in a turbulent fuel jet during low-temperature ignition in an optically-accessible engine. Particle image velocimetry (PIV), aerosol phosphor thermometry (APT), and formaldehyde planar laser-induced fluorescence (PLIF) are used in combination. A detailed characterization of formaldehyde photophysics is performed using spectral simulations and experimental data. Several thermographic phosphors are characterized in detail (including physical and luminescence properties), models are developed for phosphor signal intensity and APT performance, and a method is outlined and demonstrated to combine simultaneous APT techniques to increase the temperature measurement range. The APT methods are applied to atmospheric pressureheated jets to validate the performance estimates, and identify issues in their application. A thorough analysis of design considerations for particle-based techniques is also provided, and performance estimates are made for the combined diagnostic. It was found that the Ce:LuAG phosphor with 355-nm excitation can be most easily integrated with PIV and formaldehyde PLIF measurements, and has good performance characteristics over the 700-1000 K temperature range of interest, with ∼20 K estimated precision on average. Multiple scattering was found to impose a significant limitation on particle seeding density. Several phosphor materials and techniques were also found to be viable for temperature imaging well above 1000 K (Ce:GdPO4, Eu:BAM, and Ce:CSSO). Performance predictions for formaldehyde suggest that detection limits are on the order of 100 ppm throughout much of the temperature and pressure range expected during ignition, and a ratiometric background correction approach was discussed to avoid interference from phosphor luminescence, or other broadband background sources. PIV is readily integrated into the APT measurement. The proposed approach is capable of simultaneous temperature, velocity, and formaldehyde concentration imaging of low-temperature ignition processes, and provides a significant step towards improving our understanding of high-pressure turbulent jet ignition.