Aligning free surface properties in time-varying hydraulic jumps

Abstract

Hydraulic jumps occur commonly in natural channels and energy dissipation systems of hydraulic structures in the violent transition from supercritical to subcritical flows. They are characterised by large flow aeration, high turbulence and strong fluctuations of the free surface and the jump toe. For free surface measurements, fast-sampling, fixed-point instruments such as acoustic displacement meters (ADMs) and wire gauges (WGs) are commonly used, while LIDAR technology is a relatively new method for recording instantaneous free surface motions of aerated flows. While each of these instruments has been shown previously to provide reasonable results for basic and advanced free surface properties, differences between instruments and experiments remain unexplained. To systematically analyse these differences, simultaneous laboratory experiments of aerated hydraulic jumps were conducted. Good agreement between the three instruments was obtained for basic free surface properties including elevations, fluctuations, skewness, kurtosis, and frequencies, as well as advanced free surface properties such as integral time and length scales. These new results indicate that any of these instruments can be used for the recording of free surface properties albeit the free surface integral scales are sensitive to the spacing of instruments. A key finding of this research was that differences between repeated experiments as well as previous studies were observed when using the visual jump toe for alignment. However, this bias could be resolved by using the mean jump toe location recorded with the LIDAR. Therefore, future research of the fundamental flow processes should simultaneously measure the instantaneous jump toe to provide more consistent results across studies.