On the effect of aeration on laser ranging measurements of free water surface

Aerated flows pose both challenges and opportunities in the measurement of free water surface. In this study, an amplitude-modulated continuous wave scanning LIDAR device was used to measure a distance from the device sensor to the water column surface subjected to different degrees of aeration, while reference measurements were performed by high-speed imaging. Different aeration conditions were generated by variation of the air flow rate supplied to the liquid and by using perforated plates with different hole arrangements. The LIDAR device was shown to produce level readings consistently below visible water levels. The measurement error of the LIDAR method is largely determined by the volume fraction of air in water and was lowest at about 0.1 air volume fraction. The error increases linearly until a very high air volume fraction (i.e., above 0.55), while the LIDAR method also performs poorly in unaerated water. Although the measurement uncertainty of LIDAR in the range of aerated water is by at least an order of magnitude higher than in the case of measuring dry solid surfaces, the method is viable for free surface measurement under low to moderate aeration conditions that don't produce excessive foaming.