Opportunities of LIDAR Measurements in Air-water Flows

Air-water flows are found in fast-flowing natural waters and in human-made water infrastructure such as flow conveyance and energy dissipation systems. Typical examples of air-water flows include hydraulic jumps downstream of supercritical flows in stilling basins and high-velocity flows in spillways. Air-water flows are characterised by interactions between air and water phases at various scales as well as strong fluctuations of the free-surface at the air-water interface above the flows. The complexity of air-water flows limits the accuracy and use of numerical models and laboratory experiments are typically conducted to provide information about the air-water flow processes and design guidelines.

The free-surface flow features of air-water flows have been extensively investigated providing important information about the free-surface profiles, free-surface time and length scales and characteristic frequencies. Previous studies used pointer gauges, resistance or capacitance probes and acoustic displacement sensors to record the free-surface features in air-water flows. Although these instruments provided important information on the air-water free-surface properties, only one spatial location can be recorded by each individual instrument leading to significant loss in spatial information of these complex flows.

In the present laboratory study, a novel remote sensing technology, LIDAR, is applied to measure the free-surface profiles of a range of highly vigorous and aerated hydraulic phenomena. The time-varying free-surface elevations of a longitudinal section were continuously recorded with a 2D industrial LIDAR sampling at frequencies of 35 Hz and spatial resolution of up to 0.25 degrees. The spatial resolution of LIDAR demonstrated significant advantages of LIDAR measurements compared to other instruments. LIDAR was able to provide a continuous free-surface profile and capture small scale features of the free-surface, while statistical comparisons of point source measurements were comparable between LIDAR and other instrument types. Overall, LIDAR measurements give more insights into the temporally and spatially nature of complex aerated flows. The successful implementation of LIDAR confirmed its reliability and encourages the future LIDAR application in hydraulic engineering. Opportunities to use LIDAR technology in prototype scale air-water flows are currently explored to provide the missing information on scalability of laboratory scale experiments to the real-world.