Application of local optical flow methods to high-velocity free-surface flows: Validation and application to stepped chutes

The entrainment of air on a high-velocity spillway leads to a rapid bulking in flow depth and developments of complex flow patterns downstream of the inception point of aeration. This study examines the feasibility of two local optical flow techniques the Lucas-Kanade method and the Farneback method applied to high-velocity air-water skimming flows above a stepped chute. Such methods are not widely known to the multiphase flow community. Experimental studies were undertaken in a relatively large-size stepped spillway model. Validation test cases were performed using a synchronised ultra-high-speed camera and phase-detection probe setup. The optical flow technique detected changes in brightness due to reflectance difference associated with passages of air-water interfaces. The standard deviation of luminance correlates with void fraction and bubble count rate, and may be used as a predictor for uncertainties in optical flow estimation. The streamwise optical flow properties were in close agreement with those determined by the phase-detection probe next to the sidewall, with increasing differences for void fraction greater than 50% (C > 0.5). In the sidewall region, however the bubble count rate and interfacial velocity distributions were underestimated compared to the channel centreline's interfacial properties. The tests demonstrated that the optical flow methods can provide useful qualitative and quantitative information on complex air-water flow patterns.