Abstract: Velocity measurement of highly aerated flows is a key issue in air-water two-phase flow research and hydraulic engineering. Bubbly flows, self-aerated or artificially-aerated, are very complicated in evolution mechanism and rapid turbulent fluctuations; currently-used techniques are limited in accurate measurements of their flow velocity and turbulences. Optical flow-based flow visualization technologies attract a great interest in the field of image recognition due to its unique advantages. The optical flow method enables calculation of the high-resolution optical fields for the meso-scale movement of air bubbles by using the brightness of pixels on the image, thereby making up for the shortcomings of traditional particle image velocimetry. This paper discusses the principle of classical optical flow methods and reviews the existing optical flow technologies applicable to highly aerated flows, so as to provide a comprehensive picture of the advances and the state of the art in the aerated flow visualization applications of these technologies. Then, we sum up the key processing techniques and parameter selections for applications to the complicated cases of highly aerated flows. Finally, the challenges and frontiers of related theoretical and practical issues faced in highly aerated flow measurements are discussed, and their future development directions are prospected.

Key words: highly aerated flow, optical flow method, flow visualization, review, key parameters, frontier issues