Measurements in the Wake of a Ventilated Hydrofoil

The purpose of this study is to develop the necessary algorithms to determine the bubble size distribution and velocity in the wake of a ventilated or cavitating hydrofoil utilizing background illumination. A simplified experiment was carried out to validate the automatic bubble detection algorithm at Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota. The experiment was conducted in the high-speed water tunnel. First, particle shadow velocimetry (PSV) images of a bubbly flow were collected. All parts of the image that are above the global threshold are segmented by an edge detection method based on the Canny algorithm. The utilized algorithm was made to detect partly overlapping bubbles and reconstruct missing parts. After all images have been analyzed, the bubble velocity can be determined by applying a tracking algorithm. This study has shown that the algorithm enables reliable analysis of irregularly shaped bubbles even when bubbles are highly overlapped in the wake of the ventilated hydrofoil. It is expected that this technique can be used to determine the bubble velocity field as well as the bubble size distributions.