Synthetic aperture imaging for three dimensional resolution of fluid flows

Abstract

Fluid mechanics and instrumentation have a long history together, as experimental fluids studies play an important role in describing a more complete physical picture in a variety of problems. Presently. state-of-the-art instruments for fluid flows aim to resolve various quantities in three-dimensions. This thesis describes a novel three dimensional imaging system intended to extend laboratory measurement capabilities in complicated flows where knowledge is incomplete. In particular, the imaging system is designed to perform three-dimensional velocimetry in densely seeded flows where object geometry may partially occlude the field as well as to measure and locate bubbles, droplets and particles in three-dimensions in multiphase flows. An instrument of this kind has ramifications in a variety of engineering applications from air-sea interaction to Naval hydrodynamics to turbulence and beyond. The imaging system is based upon synthetic aperture (SA) imaging, which has received much attention in the computer vision community recently. In focus images from an array of synchronized cameras are recombined in software post-capture using a refocusing algorithm to generate a focal stack of synthetic images. Each synthetic image has a narrow depth of field, and objects residing at this depth appear sharp while off-plane objects appear blurred. The refocusing algorithm not only allows for 3D reconstruction of a scene, but also enables "see-through" effects, whereby an object occluded in some of the camera views will be seen in the synthetic images. In this thesis, considerations for development of a three-dimensional measurement system for fluid flows based on the SA imaging field are made. A high-performance three-dimensional particle image velocimetry technique is described and validated. Also, a method for auto-calibration of mutli-camera setups for fluids experiments is derived and developed. Finally, algorithms are generated for application to multiphase flows and the technique is applied to a circular plunging jet with results showing excellent agreement to prior literature and yielding new insight into the problem.