Development of an optical measuring method for measurement of the turbulent two-phase flow with free surface

Hydraulic engineering will face several challenges in the future in the field of flood risk management, water supply, environmental hydraulic engineering, ecohydraulics, energy production etc. Water surface measurements of turbulent free surface flows are an important part of hydraulic measurements. It is crucial to perform precise and reliable measurements of hydraulic phenomena. Conventional measurement methods (piezometers, ultrasonic gauges, etc.) fail in the case of two-phase flow and do not allow for the reliable dynamic measurements with high spatial resolution. Current commercial LIDAR devices require high thresholds to trigger single distance acquisitions. The mechanism responsible for triggering a commercial LIDAR on a water surface is specular reflection. The laser beam is reflected from the water surface in several consecutive reflections. A reflection capable of triggering the LIDAR must be reflected precisely in direction of LIDAR receiving optics with sufficient intensity. However, the point from where such a reflection originates depends on the previous reflection or series of previous reflections. Within this project, a comprehensive analysis of LIDAR, laser triangulation with high-speed camera, 3D imaging localisation with two or more fast cameras and CFD flow modelling will be applied to measure and model properties of turbulent free surface water surfaces. We will focus on highly-aerated flows. Among selected measurement methods laser triangulation and 3D visualization with two or more fast camera imaging are limited to laboratory usage, while LIDAR is a very reliable and robust measurement method for outdoor use. Interpretation of LIDAR measurements will be performed by optical flow modelling of successive LIDAR laser beam reflections. This will make it possible to recognize the water surface properties. Optical flow algorithms provide an estimation of a sequence of specular reflections within the aerated flow surface. Exact representation is, however, impossible due to the highly turbulent nature of the fluid flow. In addition to the measurements of aerated water surface properties, we are going to measure a raw signal from LIDAR using waveform digitizing system or fast oscilloscope. The signals will be then processed on personal computer by using signal processing algorithms in time and frequency domains developed in WP4. In such a way by the LIDAR measured depth profile will be extracted and correlated with the one measured with the reference optical systems. The proposed project aims to develop knowledge and improve LIDAR operation for highly aerated turbulent free surface flow. We will develop a model that relates the state of the water surface to the optical flow of LIDAR beam at successive reflections from bubbles´ surfaces at the water surface. A model of successive specular reflections will be provided, which will enable later implementation within the newly developed prototype WS-LIDAR (Water Surface LIDAR). As a result, prototype WS-LIDAR devices will have a capability to measure solid surfaces and aerated turbulent water surfaces, greatly surpassing current LIDAR capabilities. With the laser scanner measurements adapted for measurements of turbulent, aerated water flows, we will perform a calibration of 3D numerical CFD models, which currently cannot simulate such flows with satisfactory accuracy without the available calibration data. Past research projects show the research capability of the research group and enables exchange of information, knowledge and skills among researchers and research topics. The research group has thus all available human, organizational and material resources that guarantee successful implementation of proposed project. Proven cooperation with industry enables promotion of its results to Slovenian industry after the project is finished.