Dr. Samuel Grauer, Georgia Institute of Technology

Fast and Robust Volumetric Refractive Index Measurement by Unified Background-oriented Schlieren Tomography
January 15, 2020


Turbulence is inherently 3D and quantitative, instantaneous, volumetric measurements of turbulent flows are required to resolve turbulent phenomena, validate numerical models, and inform the design of vehicles, projectiles, mixing vessels, combustors, and other devices. Background-oriented schlieren (BOS) tomography (BOST) is an increasingly popular strategy for 3D fluid and combustion measurement that can be used to measure the density field of supersonic and hypersonic flows, using high-speed cameras, as well as facilitate low-cost 3D density, temperature, or mixing measurements, using commercial cameras. This talk gives an overview of BOST and introduces a novel reconstruction algorithm that unifies the deflection sensing and tomography 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 the fluid’s 3D refractive index field. The cameras are focused through the fluid on textured background patterns. Deflections between an undistorted reference image and a distorted image are typically determined by gradient-based optical flow (OF), which is a complex inverse problem and a potential source of error in BOST. I present an alternative approach to BOST that unifies the OF equations and deflection model. The new operator simultaneously calculates the image distortions seen by each camera for a discrete refractive index distribution. Unified BOST (UBOST) thus reconstructs observed gradients instead of inferred deflections, which are influenced by user-selected OF parameters. The UBOST operator has one-third as many equations as the classical BOST operator. I show that this formulation reduces the effects of model error and the computational cost of reconstruction. These advantages are demonstrated with a numerical experiment using phantoms of varied complexity. Best practice UBOST reconstructions were more accurate than classical reconstructions of the exact deflections for each phantom. Moreover, UBOST estimates converged substantially faster, resulting in a ≥62.5% speedup with my solver.


Dr. Samuel Grauer

Postdoctoral Fellow in the Ben T. Zinn Combustion Laboratory at the Georgia Institute of Technology. He completed a B.Sc. in Mechanical Engineering at the University of Manitoba in 2014 and a Ph.D. at the University of Waterloo in September 2018. Dr. Grauer develops advanced 2D and 3D optical diagnostics, augmented by statistical post-processing techniques, to conduct basic research on fluid, mass, and heat transfer, especially in reacting flows. He is currently using time-resolved emission tomography to locate the onset of aeroacoustic instabilities in complex interacting flames, hyperspectral absorption tomography to identify the presence of backscatter in swirl flames, and fast absorbance inference for real-time process control.