Samuel Grauer

Pennsylvania State University | Penn State · Department of Mechanical Engineering

During the formation of soot, the particles undergo a fine structural transformation, akin to graphitization, from “young” to “mature” soot. This aging process is accompanied by a change in optical properties, which manifests in the soot optical band gap and dispersion exponent, Eg and ζ. Although both quantities are regularly determined via broadb...

Particle tracking velocimetry (PTV) is widely used to measure time-resolved, three-dimensional velocity and pressure fields in fluid dynamics research. Inaccurate localization and tracking of particles is a key source of error in PTV, especially for single camera defocusing, plenoptic imaging, and digital in-line holography (DIH) measurements. To a...

Background-oriented schlieren (BOS) is an imaging technique that can be used to characterize the density field in a compressible flow. This information is often employed to assess shock formations and predict aerodynamic performance in high-speed ground tests. However, measurements in ground test facilities are subject to large uncertainties due to...

We report a new workflow for background-oriented schlieren (BOS), termed “physics-informed BOS,” to extract density, velocity, and pressure fields from a pair of reference and distorted images. Our method uses a physics-informed neural network (PINN) to produce flow fields that simultaneously satisfy the measurement data and governing equations. Fo...

This is a comprehensive, critical, and pedagogical review of volumetric emission tomography for combustion processes. Many flames that are of interest to scientists and engineers are turbulent and thus inherently three-dimensional, especially in practical combustors, which often contain multiple interacting flames. Fortunately, combustion leads to...

Forward error propagation is an established technique for uncertainty quantification (UQ). This article covers practical applications of forward error propagation in the context of particle counting measurements. We begin by presenting pertinent error models, including the Poisson noise model, and assess their role in UQ. Next, we describe several...

Particle tracking velocimetry (PTV) is widely used to measure time-resolved, three-dimensional, three-component velocity and pressure fields in fluid dynamics research. Inaccurate localization and tracking of particles is a key source of error in PTV, especially for single camera defocusing, plenoptic imaging, and digital in-line holography (DIH) s...

We report a new workflow for background-oriented schlieren (BOS), termed "physics-informed BOS," to extract density, velocity, and pressure fields from a pair of reference and distorted images. Our method uses a physics-informed neural network (PINN) to produce flow fields that simultaneously satisfy the measurement data and governing equations. Fo...

Multibeam tunable diode laser absorption spectroscopy (TDLAS) sensors are used to measure mass flow in a complex, inhomogeneous flowfield with limited optical access. The cost and complexity of a TDLAS array increases with the number of beams, but adding beams does not necessarily improve accuracy. A technique to optimize the location and orientati...

This document presents additional results from the numerical simulations of the single-camera background-oriented Schlieren tomography (BOST) technique. Ray-tracing numerical assessments were performed to mimic the experimental configuration.

We report a new approach to flow field tomography that uses the Navier–Stokes and advection–diffusion equations to regularize reconstructions. Tomography is increasingly employed to infer 2D or 3D fluid flow and combustion structures from a series of line-of-sight (LoS) integrated measurements using a wide array of imaging modalities. The high-dime...

The understanding and predictive modeling of explosive blasts require advanced experimental diagnostics that can provide information on local state variables with high spatiotemporal resolution. Current datasets are predominantly based on idealized spherically symmetric explosive charges and point-probe measurements, although practical charges typi...

The tomographic imaging of chemical species distributions has undergone rapid development in the last 20 years, driven by the combination of key scientific and technological challenges, the emergence of new optoelectronic hardware, and fundamental progress in inverse problems. We describe here the state of the art in Chemical Species Tomography usi...

Axisymmetric tomography is used to extract quantitative information from line-of-sight measurements of gas flow and combustion fields. For instance, background oriented schlieren (BOS) measurements are typically inverted by tomographic reconstruction to estimate the density field of a high-speed or high-temperature flow. Conventional reconstruction...

We report a new approach to flow field tomography that uses the Navier-Stokes and advection-diffusion equations to regularize reconstructions. Tomography is increasingly employed to infer 2D or 3D fluid flow and combustion structures from a series of line-of-sight (LoS) integrated measurements using a wide array of imaging modalities. The high-dime...

Terahertz time-domain spectroscopy (THz-TDS) is an optical diagnostic used to noninvasively measure plasma electron density and collision frequency. Conventional methods for analyzing THz-TDS plasma diagnostic data often do not account for measurement artifacts and do not quantify parameter uncertainties. We introduce a novel Bayesian framework tha...

Multi-beam tunable diode laser absorption spectroscopy (TDLAS) can be used to estimate the mass flow rate in a complex, inhomogeneous flowfield. The cost and complexity of a TDLAS array increases with the number of beams, but additional beams do not necessarily improve the accuracy of estimates. To-date, the arrangement of beams in TDLAS mass captu...

We present a linear model for absorption tomography with velocimetry (LATV) to reconstruct 2D distributions of partial pressure, temperature, and streamwise velocity in a high-speed flow. Synthetic measurements are generated by multi-beam tunable diode laser absorption spectroscopy (TDLAS). The measurement plane is tilted relative to the streamwise...

We propose a novel approach to background-oriented schlieren (BOS) tomography (BOST) that unifies the deflection sensing and reconstruction algorithms. BOS is a 2D flow visualization technique that renders light deflections due to refraction in the fluid. Simultaneous BOS measurements from unique views can be reconstructed by tomography to estimate...

A pulsed gas Jet is imaged using the background-oriented schlieren technique to probe sensitivity to operating conditions, such as pressure ratio. Multiple inversion schemes will be considered to improve flow property estimates.

High-resolution absorption spectroscopy is a promising method for non-invasive process monitoring, but the computational effort required to evaluate the data can be prohibitive in high-speed, real-time applications. This study presents a fast method to estimate absorbance spectra from transmitted intensity signals. We employ Bayesian statistics to...

Hyperspectral absorption tomography (HAT) reconstructs the distribution of key gas parameters, including composition, pressure, and temperature, from multi-beam absorbance data with numerous spectral resolution elements. There is a nonlinear relationship between the parameters of interest and the spectral absorption coefficient, which must be incor...

We apply background-oriented schlieren (BOS) imaging with computed tomography to reconstruct the instantaneous refractive index field of a turbulent flame in 3D. In BOS tomography, a network of cameras are focused through a variable index medium (such as a flame) onto a background of patterned images. BOS data consist of pixel-wise "deflections" be...

Gas-phase tomography refers to a set of techniques that determine the 2D or 3D distribution of a target species in a jet, plume, or flame using measurements of light, made around the boundary of a flow area. Reconstructed quantities may include the concentration of one or more species, temperature, pressure, and optical density, among others. Tomog...

Presentation slides for the defense of my doctoral dissertation, "Bayesian Methods for Gas-Phase Tomography."

We present the first use of background-oriented schlieren (BOS) tomography as a combustion diagnostic. BOS imaging captures deflections of light due to gradients in the optical density of a variable index medium, such as a flame. Multiple cameras are combined to tomographically-reconstruct the refractive index, which can be related to key combustio...

Over the last two decades, considerable focus has been placed on improving the fidelity of TiRe-LII heating and cooling models, with the objective of improving the robustness of inferred aerosol parameters. While simple models fail to capture all the salient features of experimental cooling curves, more elaborate models that account for such featur...

Mid-wavelength infrared (MWIR) cameras are increasingly used to obtain quantitative estimates of single-species hydrocarbon concentrations. However, in principle, it is also possible to quantify each component of a mixture using multiple MWIR cameras equipped with different filters. This paper presents the first attempt to do this, via MWIR images...

Multiple models of varying complexity have been developed to analyze TiRe-LII data, but often with little regard as to how model complexity affects the robustness of inferred results. This study applies Bayesian model selection to evaluate three competing soot sublimation models for time-resolved laser-induced incandescence (TiRe-LII), and demonstr...

Competing theories have been proposed to account for how the latent heat of vaporization of liquid iron varies with temperature, but experimental confirmation remains elusive, particularly at high temperatures. We propose time-resolved laser-induced incandescence measurements on iron nanoparticles combined with Bayesian model plausibility, as a nov...

This paper presents a novel model for measuring the emission rate of a heated gas plume using hyperspectral data from an FTIR imaging spectrometer. The radiative transfer equation (RTE) is used to relate the spectral intensity of a pixel to presumed Gaussian distributions of volume fraction and temperature within the plume, along a line-of-sight th...

This paper presents a novel error model for TiRe-LII signals and illustrates how the model can be used to diagnose a detection system, quantify uncertainties in TiRe-LII, and characterize fluctuations in the measured process. Noise in a single TiRe-LII measurement shot obeys a Poisson-Gaussian noise model. Variation in the aerosol results in shot-t...

Gas distributions imaged by chemical species tomography (CST) vary in quality due to the discretization scheme, arrangement of optical paths, errors in the measurement model, and prior information included in reconstruction. There is currently no mathematically-rigorous framework for comparing the finite bases available to discretize a CST domain....

This work introduces broadband-absorption based chemical species tomography (CST) as a novel approach to reconstruct hydrocarbon concentrations from open-path attenuation measurements. In contrast to monochromatic CST, which usually involves solving a mathematically ill-posed linear problem, the measurement equations in broadband CST are nonlinear...

This software is an implementation of the general error model described in the work entitled "General error model for analysis of laser-induced incandescence signals" by T. A. Sipkens and coworkers, which simulates signals according to the described error model (sufficient for generating Figure 1 in the associated work) and demonstrates a fitting p...

Measurements of time-resolved laser-induced incandescence (TiRe-LII) are used to infer the size and concentration of nanoparticles, among other quantities-of-interest. Reliable models of nanoparticle heating and cooling are required to obtain accurate estimates from TiRe-LII data. Fundamental research into nanoscale heat transfer has resulted in a...

Chemical species tomography (CST) experiments can be divided into limited-data and full-rank cases. Both require solving ill-posed inverse problems, and thus the measurement data must be supplemented with prior information to carry out reconstructions. The Bayesian framework formalizes the role of additive information, expressed as the mean and cov...

Reconstruction accuracy in chemical species tomography depends strongly on the arrangement of optical paths transecting the imaging domain. Optimizing the path arrangement requires a scheme that can predict the quality of a proposed arrangement prior to measurement. This paper presents a new Bayesian method for scoring path arrangements based on th...

Due to the inherent ill-posed nature of chemical species tomography (CST) problems, additional information based on the presumed species distribution must be introduced into the reconstruction procedure. The role that this prior information plays in tomographic reconstruction differs depending on whether the CST problem is discrete ill-posed or ran...

Line-of-sight-attenuation chemical species tomography is a diagnostic in which the spatial distribution of a gaseous species is reconstructed from line-of-sight light-absorption measurements. In this approach, the measurement field is discretized into pixels wherein the species concentration of interest is presumed constant. Because the number of p...