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.

This work aims to advance methods for the active control of vortex phenomena in hydraulic turbines. The investigation was conducted on a Francis turbine model operating under partial load conditions, where large-scale vortex structures develop, inducing high levels of pressure pulsations detrimental to turbine structural integrity. Control of the vortex core characteristics is based on the injection of supplementary jets through the central streamline body of the runner. Experimental results are presented for velocity fields under various flow control strategies, differing in the spatial orientation of the jets and their total flow rate. An analysis is presented of the integral flow characteristics and the evolving vortex structures downstream of the Francis turbine model runner. The mechanisms governing vortex core control under different supplementary jet injection methods are described. It is found that, irrespective of jet direction, a total injected flow rate of 3% of the main flow significantly suppresses pressure pulsations. However, radial jet injection (parallel to the runner plane) proves more effective than axial injection or a combination of both directions. Alongside the injected flow fraction, the dimensionless momentum flux coefficient is identified as a key parameter governing the effectiveness of control on the precession vortex core.

The application of machine learning to solve engineering problems is in extremely high demand. This article proposes a tool that employs machine learning algorithms for predicting the frequency response of an unsteady vortex phenomenon, the precessing vortex core (PVC), occurring in a conical diffuser behind a radial swirler. The model input parameters are the two components of the time-averaged velocity profile at the cone diffuser inlet. An empirical database was obtained using a fully automated experiment. The database associates multiple inlet velocity profiles with pressure pulsations measured in the cone diffuser, which are caused by the PVC in the swirling flow. In total, over 10³ different flow regimes were measured by varying the swirl number and the cone angle of the diffuser. Pressure pulsations induced by the PVC were detected using two pressure fluctuations sensors residing on opposite sides of the conical diffuser. A classifier was constructed using the Linear Support Vector Classification (Linear SVC) model and the experimental data. The classifier based on the average velocity profiles at the cone diffuser inlet allows one to predict the emergence of the PVC with high accuracy (99%). By training a regression artificial neural network, the frequency response of the flow was predicted with an error of no more than 1.01 and 5.4% for the frequency and power of pressure pulsations, respectively.

This paper describes the results of experimental studies of a flow using the prototype of a propeller-type microhydraulic turbine. The tests are carried out on a testbed in which atmospheric air is used as a working medium. The measurements carried out with the help of a two-component laser-Doppler anemometer are used to obtain velocity distributions behind a runner in the case where the operating modes of the device change in a wide range. It is shown that the created model microhydraulic turbine has optimal parameters for the conditions set during the design, and a change in the operating mode of the device from nominal parameters to underload or overload increases the residual swirl of the flow and the generation of strong hydrodynamic instability in the form of a precessing vortex rope. In this case, axial velocity over the cross section is distributed unevenly and the flow pulsation level is increased.

This paper presents the development of active control methods for vortex phenomena in hydro turbines. The flow pattern downstream of a simplified turbine runner was studied under conditions typical of a hydro turbine operating at partial load, which are prone to generating large-scale vortex structures and inducing powerful pressure pulsations. Active control was achieved through the injection of additional air jets into the center of the runner cone. The results of experiments covering velocity distributions, velocity pulsations, and pressure pulsations following the injection of jets are presented. Control jets, regardless of their orientation, successfully suppress pressure pulsations. However, jets oriented radially provide the most effective suppression of vortices and reduce the total flow swirl in the draft tube. The pattern of jet supply directly affects the formation of a recirculation zone downstream of the runner. Experimental data on optimal injection align with previous theoretical estimates based on flow linear stability analysis.

This article is devoted to detailed experimental studies of the flow behind the impeller of an air model of a propeller-type microhydroturbine in a wide range of operating parameters. The measurements of two component distributions of averaged velocities and pulsations for conditions from part load to strong overload are conducted. It is shown that the flow at the impeller outlet becomes swirled when the hydraulic turbine operating mode shifts from the optimum one. The character of the behavior of the integral swirl number, which determines the state of the swirled flow, is revealed. Information about the flow peculiarities can be used when adjusting the hydraulic unit mode to optimal conditions and developing recommendations to expand the hydraulic turbine operation control range with preservation of high efficiency. This stage will significantly save time at the stage of equipment design for specific field conditions of water resource.