Turbulent free-surface flows are encountered in several engineering applications and are typically characterized by the entrainment of air bubbles due to intense mixing and surface deformation. The resulting complex multiphase structure of the air–water interface presents a challenge in precise and reliable measurements of the free-water-surface topography. Conventional methods by manometers, wave probes, point gauges or electromagnetic/ultrasonic devices are proven and reliable, but also time-consuming, with limited accuracy and are mostly intrusive. Accurate spatial and temporal measurements of complex three-dimensional free-surface flows in natural and man-made hydraulic structures are only viable by high-resolution non-contact methods, namely, LIDAR-based laser scanning, photogrammetric reconstruction from cameras with overlapping field of view, or laser triangulation that combines laser ranging with high-speed imaging data. In the absence of seeding particles and optical calibration targets, sufficient flow aeration is essential for the operation of both laser- and photogrammetry-based methods, with local aeration properties significantly affecting the measurement uncertainty of laser-based methods.

The issue of ever-increasing amounts of waste activated sludge (WAS) produced from biological wastewater treatment plants (WWTPs) is pointed out. WAS can be effectively reduced in the anaerobic digestion (AD) process, where methanogens break down organic matter and simultaneously produce biogas in the absence of oxygen, mainly methane and CO₂. Biomethane can then be effectively used in gas turbines to produce electricity and power a part of WWTPs. Hydrodynamic cavitation (HC) has been identified as a potential technique that can improve the AD process and enhance biogas yield. Rotary generators of hydrodynamic cavitation (RGHCs) that have gained considerable popularity due to their promising results and scalability are presented. Operation, their underlying mechanisms, parameters for performance evaluation, and their division based on geometry of cavitation generation units (CGUs) are presented. Their current use in the field of wastewater treatment is presented, with the focus on WAS pre/treatment. In addition, comparison of achieved results with RGHCs relevant to the enhancement of AD process is presented.

The continuous monitoring of tall industrial buildings is necessary to ensure safe operation. With technological advances in terrestrial laser scanning and other non-contact measurement methods, the methods and techniques for assessing the stability of tall industrial chimneys are evolving. This paper presents a method for determining the non-verticality and straightness of chimneys that offers significant advantages over existing methods. Narrow bands of scanned point clouds are processed at selected height intervals. Using the RANSAC method, points that do not belong to the chimney shell are filtered and the centre of the circle or ellipse is adjusted using the least squares method. The proposed method enables the efficient filtering of point clouds due to frequent obstructions on the chimney shell, the determination of the regularity of the chimney shell shape, a mathematical analysis of the chimney axis curvature, and an intuitive graphical representation of chimney non-verticality. The comparison of the results with other studies confirms the efficiency of the method.

For measuring free-water surfaces conventional methods, such as resistance-type probes, U-manometers, point gauges, ultrasonic sensors, etc are still most commonly used in modern hydro engineering. These methods give accurate results at suitable flow conditions, but are insufficient when the water surface is characterized by turbulence and two-phase flow, with fast dynamics. This paper presents the use of laser scanning as a measurement method for the acquisition of free-water-surface profiles of hydraulic phenomena with turbulent, non-stationary, and non-homogeneous free-surface flows. Results shows that laser scanning can provide accurate measurements of free-water-surface profiles with high spatial and temporal resolution, even in cases of turbulent flows with high vertical fluctuations on the water surface. The comparison with the reference values determined by analysing the images taken with the high-speed camera showed that measurement uncertainty ranges from ±5 mm to ±10 mm, which is more than an order of magnitude smaller than the range of local vertical water-surface fluctuations. The average profiles, envelopes of vertical water surface fluctuation around the mean value, as well as the construction of complex water surface topography of intensive waving can be determined by processing laser scanning data. The results contribute to the wider use of this non-contact measurement method, which provides important information on water flow properties to many fields of hydro-engineering. The application of 2-D laser scanning for free-water-surface acquisition was conducted in the model of supercritical junction flow, where the development of standing waves leads to the phenomenon of self-aerated flow.