Tomographic Plenoptic Background Oriented Schlieren Imaging

Abstract

Within the last decade, tomographic background oriented schlieren has become an advanced flow diagnostic used to reconstruct three-dimensional density and/or refractive index fields. These three-dimensional measurements have been used to characterize a wide range of applications that contain flow features varying in both shape and length scale. This dissertation presents the development of the tomographic plenoptic BOS implementation, which was used to: (1) perform a systematic study assessing the reconstruction of multiple flow features and how their interaction affects the final solution and (2) highlight the implications of using plenoptic cameras in a tomographic BOS setting. The experiments outlined in this work used four plenoptic cameras surrounding an octagonal tank facility. Solid, transparent objects were submerged in a nearly refractive index matched solution in order to create a small refractive index difference. These semi-rigid objects provided the ability to systematically change an array of desired variables.

The tomographic plenoptic BOS implementation was successfully developed and tested using both ad hoc phantoms and experimental data sets. The use of such phantoms in implementation testing not only provided validation of implementation performance, but it also provided insight on achievable resolution of the experimental measurement system. The experimental data sets showed that: (1) two features will be individually resolved as long as measurements from a single camera are able to observe feature separation, (2) the error in the solution increases as the size of the feature decreases as a result of spatial resolution, and (3) the use of volumetric masking in the implementation is critical in order to achieve an accurate solution. It was also determined that the limited angular information collected by a single plenoptic camera does not replace the need to acquire measurements across a large angular range. The benefit to using plenoptic cameras stems from the ability to generate multiple views from a single camera, where there is potential for hardware reduction in future tomographic experiments.