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It has been found, for the first time, that rotating polygons (m = 1, 2) can exist in a system of two immiscible fluids, which can be in three different steady states: stable funnel, cycling switching, and rotating twin funnel. These states are achieved due to different disk acceleration values at fixed aspect ratio (h/R) = 1 and Reynolds numbers. The acceleration time of rotating disk is found to have a significant effect on instability development. [ABSTRACT FROM AUTHOR]

A study of cavitating smooth hydrofoil and foil with a structural surface was carried out. The structural surface morphology was made by laser ablation technology with a step of 0.01 mm and wavelength of 1064 nm. High-speed visualization of cavitating hydrofoils at various of flow bulk velocity was performed. In the paper is presented a comparison of cavitation flow regimes for the smooth and the roughness hydrofoils at equal cavitation numbers. The maximal lengths of the attached cavities for different flow regimes are compared. The structural surface of hydrofoil allows delaying the nucleation of cavitation and reducing the attached cavity size at higher velocities of the incoming flow was obtained.

This review addresses the current state of research into active control and suppression of vortex rope in hydroturbines under off-design operating conditions. Only active control methods that can be "switched on" when required under off-design operating conditions are considered in this work. The review focuses on air addition into the flow, as well as various auxiliary fluid jets. It includes all the best practices for vortex rope suppression in numerical and experimental studies. It can be inferred from the review that a modern flow control system should be comprehensive, designed for a specific hydroturbine geometry, and obtain feedback from the flow. Injecting ~2% of air from the impeller fairing cone appears optimal for suppressing pressure pulsations without significant efficiency loss. The cost of air injection is rarely estimated, but the use of an automatic venting system can minimize overheads and potentially improve efficiencies at low gas contents. Fluid jets ranging from 3% to 12% of the main flow rate can efficiently suppress pressure pulsations, but their high energy requirements limit their use. Azimuthal perturbation of the flow appears promising as it does not require significant energy loss, but practical implementation remains challenging as one needs to accurately know the system dynamics and be capable of real-time manipulation of the flow. [ABSTRACT FROM AUTHOR]

An experimental study of the cavitation flow around the National Advisory Committee for Aeronautics 0012 hydrofoil with different surface morphology was carried out in this work. The surface morphology was set by modern laser ablation technology. The average values and intensity of vapor–gas cavities were determined. It has been revealed that laser texturing delays the emerging cavitation and somewhat decreases its intensity at higher cavitation numbers. A decrease in the cavitation number leads to an increase in its intensity for a smooth hydrofoil in comparison with a rough one, which is also expressed in an increase in the frequency of cavities. The paper presents a comparison of the flow regime with equal cavitation numbers, which clearly describes the features of the development of a vapor–gas cavity on the suction side of the foil with different surface morphologies. The paper provides an explanation of the reasons for the influence of surface morphology on the development of cavities. [ABSTRACT FROM AUTHOR]

The paper reports the results of experimental and numerical studies of vortex motion in an industrial-scale glass bioreactor (volume, 8.5 L; reactor vessel diameter D, 190 mm) filled 50–80%. The model culture medium was a 65% aqueous glycerol solution with the density ρg = 1150 kg/m3 and kinematic viscosity νg = 15 mm2/s. The methods of particle image velocimetry and adaptive track visualization allow one to observe and measure the vortex motion of the culture medium. In this work, the vortex flow investigation was performed in a practical bioreactor at the operation regimes. Our research determines not only the optimal flow structure, but also the optimal activator rotation speed, which is especially important in the opaque biological culture. The main result is that, similar to the case of two rotating immiscible liquids, a strongly swirling jet is formed near the axis, and the entire flow acquires the pattern of a miniature gas–liquid tornado. The aerating gas interacts with the liquid only through the free surface, without any mixing. This intensifies the interphase mass transfer due to the high-speed motion of the aerating gas. [ABSTRACT FROM AUTHOR]

Operation of hydraulic turbines beyond optimal conditions leads to formation of precessing vortex core in a draft tube that generates powerful pressure pulsations in a hydraulic system. In case of resonance it leads to stability decreasing of hydraulic unit and electrical grid on the whole. In present work, such regimes are explored in a conical part of simplified turbine model. Studies are performed at constant flowrate Q = 70 m3/h and varying the runner rotational speed to explore different loads of the hydroturbine unit. The experiments involve pressure measurements, high speed-visualization and velocity measurements by means of laser Doppler anemometer technique. Interesting finding is related with abrupt increasing precession frequency at low swirl parameter of flow near optimal regime.

We report on experimental studies of the formation of vortex ropes in a laboratory simplified model of hydroturbine draft tube. Work is focused on the observation of various flow patterns at the different rotational speed of turbine runner at fixed flow rate. The measurements involve high-speed visualization and pressure pulsations recordings. Draft tube wall pressure pulsations are registered by pressure transducer for different flow regimes. Vortex rope precession frequency were calculated using FFT transform. The experiments showed interesting features of precessing vortex rope like twin spiral and formation of vortex ring.