Owing to the complexity of the load rejection process, the accompanied complex low frequency pressure fluctuations and their sources have not been determined. Herein, the load rejection transient process of a pump-turbine was simulated with a three-dimensional (3-D) large eddy simulation method and a dynamic mesh technology. The simulation results were validated against the experimental data. Through the joint time-frequency analysis of simulated pressure, the complex low frequency pressure fluctuation components were captured, which are generally lower than the rated rotational frequency of the runner. A comprehensive formation mechanism of these complex low frequency pressure fluctuation components was attributed to three transient hydraulic phenomena and their interactions: the water hammer in the pump-turbine, local reverse flow near the runner inlet, and 3-D blocking water ring in the vaneless space. These findings facilitate the elucidation and elimination of complex low frequency high amplitude pressure fluctuations during the load rejection transient process.

Cavitation limits the efficient and stable operation of rotating machinery. The exploration of control methods for hydrofoil cavitation is important for improving the performance of hydraulic machinery. The leading-edge protuberances of the humpback flipper and the spine structure of the tail fin of sailfish are two common bionic structures for cavitation control; however, the control effects of both have limitations. Accordingly, in this study, a passive control method for hydrofoil cavitation was developed by combining the two bionic structures. With the large eddy simulation method, the cavitation processes of wavy leading-edge hydrofoil, bionic fin spine structure hydrofoil, and novel bionic combined structure hydrofoil were studied under a cavitation number of σ = 0.8. The control mechanisms of the three bionic structures for the hydrofoil cavitation were investigated. The results indicated that the novel bionic combined hydrofoil realised the superposition and complementation of the control effects of the two single bionic structures and achieved a better cavitation inhibition effect, reducing the total volume of cavitation by 43 %. In addition, it enhanced the stability of the flow field and reduced the standard deviation of the pressure coefficient on the suction surface by up to 46.55 %. This research provides theoretical support for the optimisation and modification of the blades of hydraulic machinery, such as propellers and pump turbines.

Cavitation is a challenging and complex phenomenon in hydraulic machineries such as pumps, turbines, and marine propellers. The shedding and collapse of the cavitation cloud are responsible for hydraulic instability. Recently, the modified wavy leading-edge hydrofoil has attracted considerable attention for passive cavitation control. To reduce hydrofoil cavitation, the leading-edge protuberances of humpback whale flippers were introduced in hydrofoil modification, and its effect on the cavitation flow was investigated in the present study. First, a sinusoidal modification method for the leading-edge of the hydrofoil was adopted. The lift and drag coefficients, cavitation morphology, and flow characteristics of the basic and bionic hydrofoils were numerically calculated using the large eddy simulation method, and the accuracy of the numerical model was verified via comparison with available experimental results. Then, the wavelengths (0.25C, 0.5C) and amplitudes (0.05C, 0.12C) of the leading-edge protuberances were varied to study the effects of the structural parameters on the cavitation. The results indicated that the wavy leading-edge can improve the lift–drag characteristics, restrain the hydrofoil cavitation, and reduce the cavitation volume by approximately 30%. Reducing the wavelength and increasing the amplitude help restrain the instability of cavitation shedding. Increasing the amplitude can reduce the pressure amplitude by approximately 60%, significantly reduce the cavitation volume, and enhance the cavitation suppression.

In this study, the aerodynamic characteristics of multi-protuberance airfoils modeled on the fin of a humpback whale were investigated by numerically simulating the National Advisory Committee for Aeronautics 63₄-021 airfoils. For a Reynolds number of 1.8 × 10⁵, the numerical simulation results obtained with no spacing between protuberances were compared with experimental values to verify the accuracy of the numerical method. The effects of different spacings on the aerodynamic performance of the airfoil at various angles of attack were investigated by setting the spacing between the two protuberances as 0.25, 0.5, and 0.75c. The results show that the lift coefficient was increased by 5%–15%, and the drag coefficient remained constant under different conditions when the spacing between the protuberances was 0.25c. The twin-protuberance airfoils with different spacings exhibited similar two-step stall characteristics. However, only the unspaced twin-protuberance airfoil had a one-sided stall. Simulations of the multi-protuberance airfoil showed that airfoils featuring spacings between protuberances exhibited good lift characteristics at large angles of attack. With an increase in the angle of attack, the lift coefficient steadily changed. Compared with the full-protuberance airfoil, the drag coefficient of the airfoil with spacing was significantly reduced.

Stall is a complex phenomenon in aircraft that must be suppressed during flight. As a novel passive control method, bionic leading-edge protuberances (LEPs) have attracted widespread interest, particularly for delaying stall. Bionic protuberances at the leading edge of airfoils have been designed to control stall and increase the stability of unmanned aerial vehicles during operation, and it is the flow control mechanism associated with this application that is investigated in this study. First, numerical simulations are conducted to obtain the aerodynamic characteristics of original and bionic airfoils based on the S1223 large-lift airfoil. Next, the impact of the LEP amplitude is investigated. Finally, a vortex definition parameter, the Liutex vector, is utilized to analyze the influence of LEPs on vortices. The results show that bionic LEPs inspired by those on humpback whale flippers can improve the aerodynamic performance of airfoils under the extreme conditions that exist after stall, resulting in an ∼22% increase in the lift–drag ratio. LEPs are found to segment the flow field near the wing surface. The flow becomes bounded between adjacent protuberance structures, significantly inhibiting the development of flow separation and providing a drag reduction effect. This study thus provides a new approach for improving aircraft performance.

Vertical axis wind turbines (VAWTs) have garnered significant attention due to their omnidirectionality, low noise, suitability for turbulent conditions, and high efficiency in offshore cluster deployments. VAWTs are more sensitive to dynamic stall, which can lead to reduced performance and reliability. Selecting appropriate blade airfoils can enhance both performance and reliability. This study employs a parametric method to design 12 input parameters for controlling the blade airfoil, with the average power coefficient of a single vertical wind turbine blade and the coefficient of variation of the main shaft as output parameters. The Non-dominated Sorting Genetic Algorithm II multi-objective genetic algorithm is used for blade airfoil optimization, selecting the two best-performing airfoils from the Pareto front. The average power coefficient of a single blade increased by 8.6% and 4.3%, respectively, while the coefficient of variation decreased by up to 6.7% and 5.3%. The analysis of the blades at different azimuth angles indicates that optimized blades can suppress flow separation at high angles of attack, enable faster wake recovery, and reduce energy loss.