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Description
This thesis reports on developments towards an experimental visualization technique for the extraction of finite time strains and associated Lagrangian flow structures based on the time evolution of quadrilateral clouds of hydrogen bubbles that passively trace unsteady flows in water channels. To this end, the thesis seeks to generate distinct clouds of hydrogen bubbles with straight edges in a uniform flow and digitize the time-evolution of the clouds for quantitative analysis. Experiments are conducted in the open water channel experimental setup designed by GUNT Hamburg. The setup includes a hydrogen bubble probe with a platinum wire with a diameter of 0.25 mm with 2 mm wide supports. A settling chamber with glass beads and a single honeycomb flow straightener with porosity of 0.92 are intended to reduce turbulence levels and increase flow uniformity in the test section. The GUNT setup, however, produces highly non-uniform flows in the test section. To improve uniformity and reduce turbulence levels, a probe with a wire diameter of 0.025 mm and supports of diameter 0.81 mm are designed and manufactured, a rubber gasket was designed for the settling chamber, and several honeycombs are additively manufactured with porosities ranging from 0.83 to 0.91. A Go-Pro records the hydrogen bubble motion at a resolution of 1920x1080 pixels and a frame rate of 120 fps. A Canny 2D edge detection algorithm is programmed to extract edges from images and post-process metrics that determine the root mean square deviation of the cloud edges from a straight line. The new probe increases the uniformity by an average of 66% as measured by the root mean square deviation. Consecutive added placement of three honeycombs with a porosity of 0.83, 0.91 and 0.91, increases the flow uniformity by 3, 14 and 28%, respectively. The gasket improves the uniformity by 45%. The addition of probe designed in house, along with the rubber gasket and four honeycombs, leads to optimal improvement of uniformity by 85% as compared to the original setup. This uniformity is sufficient to observe strains in a converging channel.
