Application of a shadow image velocimetry to a ventilated hydrofoil wake

Abstract

A study is being carried out at the Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota to develop the necessary algorithms to determine the velocity deficit and bubble size distribution in the bubbly wake behind a ventilated or cavitating hydrofoil. This is done by utilizing background illumination of the bubbly wake. Shadows of the bubbles, cast on a bright background, were collected by a high-speed video camera. For detecting bubble shadows, the image was segmented using an edge detection technique such as Canny algorithm. Then, a curvature profile method was employed to distinguish individual bubbles within a cluster of highly overlapping bubbles. The utilized algorithm was made to detect partly overlapping bubbles and reconstruct the missing parts. The movement of recognized individual bubbles was tracked on a two-dimensional plane within a flow volume. To suppress only high frequency background variations generated by out-of-focus bubbles, a Gaussian low-pass filter was applied in this study. To obtain quantitative results, the wake of a ventilated NACA0015 hydrofoil was next investigated by applying a shadowgraphy technique and the described bubble detection algorithm. Results regarding bubbles’ appearance, their velocity, and void fraction are presented. It was found that most of the bubbles were shaped like a circle and with a radius of 0.22 mm and the bubble’s velocity was dependent of its size. Local void fraction was further analyzed and a possible problem was discussed in the paper.

Graphical abstract