Evaluation of high-speed laser triangulation and LIDAR for turbulent water surface ranging

Abstract

This study presents a comparison of high-speed laser triangulation and laser scanning, two advanced measurement methods for capturing the free water surface of complex hydraulic phenomena characterized by turbulent, non-stationary, and non-homogeneous flows. The methods were applied to measure the turbulent, aerated water surface of a flow under supercritical flow conditions, defined by high Reynolds and Froude numbers. Measurements were conducted using a high-speed camera operating on the principle of laser triangulation, and a laser scanner serving as a light source. Data evaluation was performed for three cross-sections along the confluence, indicating a very good overall agreement between both measuring systems with mean discrepancies of 0.26 mm horizontally and 2.7 mm vertically, which fall within the reported systematic and statistical errors of the light detection and ranging (LIDAR) device. With most relative errors within ±10 %, the mean differences between the two measurement systems were an order of magnitude smaller than the temporal and spatial fluctuations of the observed free surfaces, and the instantaneous errors remained within the physical range of these fluctuations. Further error analysis revealed that high-speed triangulation underestimates free surface levels in flows with steadier, well-defined water surfaces, and overestimates in regions with highly transient and poorly defined surfaces compared to LIDAR. These discrepancies were attributed to differing system sensitivities. The high-speed camera sensor exhibits higher sensitivity, capturing more reflections from water droplets and successive reflections from submerged air bubbles compared to the LIDAR device. This highlights the need for further investigation into these error sources and the development of more robust data filtering techniques. Overall, both methods effectively captured the complex dynamics of turbulent aerated flows, showing promising capabilities for measuring turbulent water surfaces in laboratory environments.