Measuring turbulent water surfaces using laser scanning

Abstract: Water surface measurements of turbulent free surface flows are an important part of hydraulic measurements. Such flows are encountered in a wide range of applications in civil, chemical, environmental, mechanical, mining and nuclear engineering. Our presentation will provide an overview of application of the laser scanning as a measurement method for acquiring the non-homogeneous and nonstationary topography of free surface flows. Laser ranging is a measurement method, applied in a wide range of applications. In comparison with conventional methods (resistance probes, U-manometers, ultrasonic sensors, point gauges etc.) LIDAR offers an advantage particularly in cases with complex water surface topography like in those found in highly turbulent two-phase flows, where other methods do not provide sufficiently accurate results. Due to the specificity of the interaction between light emitted from LIDAR and turbulent water surface, the number of successfully measured points is often low, while distance measurements have high measurement uncertainty. The exact light reflection from the measured water is often not known. It can appear on bubbles, water surface, droplets and/or foam, while also being single reflection, multiple reflections or even multiple reflections including reflection from the wall of the measuring station. This may lead to over or underestimation of the measured distance to the water surface. LIDAR measurements of transversal water profile along the confluence of two supercritical flows were compared with measurements with a high-speed camera. The high-speed camera was operated on the principle of laser triangulation, using only the illumination from the LIDAR laser beam. Since, no other state-of-the-art method for measuring instantaneous water surface profiles exists, LIDAR and triangulation methods could only be compared with each other. The results show good agreement between both methods for the average turbulent water profile and fair agreement for instantaneous profiles.