Measurements in the wake of a ventilated hydrofoil: a step towards improved turbine aeration techniques

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 the Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota. The experiment was conducted in the SAFL high-speed water tunnel. First, particle shadow velocimetry (PSV) images of a bubbly flow were collected. Bubbles were identified in the images using an edge detection method based on the Canny algorithm. The utilized algorithm was designed to detect partly overlapping bubbles and reconstruct missing parts. After all images were analyzed, the bubble velocity was 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