Changliang Ye

Hydraulic machinery has accompanied the development of human civilization from early waterwheels and Archimedean screws to modern pumps, hydraulic turbines, and reversible pump-turbines

This study aims to elucidate the rotor–stator interaction (RSI) mechanism in a centrifugal pump by employing high-fidelity stress-blended eddy simulation (SBES) turbulence modeling combined with reduced-order techniques [dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD)]. The core objective extends beyond confirming the dominant frequency characteristics, to quantifying the energy hierarchy of RSI-induced flow and resolving the energy cascade in a coupled space–time–frequency domain. The present work investigates the RSI in a pump based on the European Research Community on Flow, Turbulence and Combustion standard through three-dimensional, unsteady computational fluid dynamics simulations. Initially, the SBES and scale-adaptive simulation turbulence models were compared. It was observed that the SBES model had good predictive accuracy compared to the experimental data for the pressure coefficient, velocity profiles, and pressure fluctuations in the vaneless region. Further transient study using SBES showed that blade passing frequency (fBPF) and its second harmonic are the predominant frequencies in terms of pressure pulsations. Also, the energy variability of frequencies was distinct across the flow path. Reduced-order analysis showed low-dimensional features of the flow. DMD's isolation of the first four dominant modes, corresponding to fBPF and its harmonics, was successful, with their spatial structures visualizing the large-scale periodic wakes and associated small-scale vortex shedding from RSI. A reconstruction based only on the first 5 DMD modes is able to capture the essential dynamics of the actual flow field in the study. POD analysis confirmed previous results for the energetic behavior. It clearly indicates that the first four modes contain more than 66% of the total energy. Moreover, the corresponding spectra of modal coefficients perfectly align with the DMD frequencies. This synergy of results confirms that periodic wakes from RSI are the primary energy-driving mechanism. This work seeks to reveal how the transient vortex structures influence pressure and Reynolds stress fluctuations, thereby providing physical insights beyond conventional spectral analysis for flow-induced vibration and noise control, and offers a basis for optimizing fluid machinery design.

The development of renewable energy and the increasing demand for electricity underscore the importance of pumped storage for grid stability. Under low-flow pump operating conditions, pump-turbines frequently exhibit hump characteristics, causing severe hydraulic instability and strong pressure pulsations. This study investigates the formation of a hump using full-channel numerical simulations based on the Scale-Adaptive Simulation turbulence model. The numerical flow–head characteristics were validated against the available experimental H–Q data, while the pressure pulsation results were used for qualitative mechanism analysis. The results reveal three major mechanisms: pre-swirl and spiral backflow in the draft tube, non-uniform runner inflow, and vortex flow-induced separation in the wicket gates. An analysis of entropy production reveals that vortex dissipation is responsible for as much as 71% of hydraulic losses in the hump region. In order to mitigate these effects, four stabilizing fins were installed inside the draft tube. The simulations indicate that the fins possess the capability to inhibit swirl and backflow, confine the vortices within the fin–runner interface, improve inflow uniformity and reduce overall hydraulic losses. As a result, the structural modification significantly attenuates the pressure pulsation amplitudes at key monitoring points and visibly shortens the recovery periods. The region of the hump and positive slope of the performance curve are considerably reduced while the head near the region of the hump is increased. Although the intrinsic hump characteristic is still present, the fin-based flow-control strategy can effectively improve the performance and stability of the pump-turbine, which can guide the design and optimization of high-efficiency pumped-storage plants.

The blade outlet angle is a critical design parameter of vortex pump impellers, exerting a significant influence on the pump’s hydraulic performance and internal flow characteristics. In this study, numerical simulations combined with experimental validation were conducted to investigate a vortex pump, with three impellers featuring blade outlet angles of 50°, 60°, and 65° analyzed based on the SST k–ω turbulence model. To quantify irreversible energy losses, entropy production theory was adopted, while the Liutex method was utilized to characterize rigid-body vorticity. The results demonstrate that increasing the blade outlet angle leads to a reduction in head under both small-flow-rate and design-flow-rate conditions, impairs flow uniformity, strengthens vortex structures, and elevates total entropy production—with turbulent dissipation being the dominant contributor to energy losses. Additionally, larger outlet angles enhance the sensitivity of internal flow structures to off-design operating conditions. These findings offer valuable guidance for the optimization of impeller design and the development of energy-efficient vortex pumps.

Counter-rotating marine current turbines demonstrate significant potential for tidal power extraction. However, existing studies neglect the critical interaction between upstream (λ1) and downstream (λ2) tip-speed ratios (TSRs) within rotating wake environments. This work systematically investigates TSR balancing strategy via coupled analytical (momentum theory with wake rotation) and numerical methods (ANSYS 2022 R1, SST k-ω turbulence model) across a broad TSR design range (λ1, λ2: 0.5–9.7). Results reveal that the rear rotor exhibits greater sensitivity to TSR changes. Contrary to conventional design paradigms, novel geometries with λ2/λ1 ≤ 1.3 can still achieve a competitive power coefficient, indicating the potential for efficient operation under such conditions and exceeding the performance of a single-rotor configuration. Analytical solutions show that momentum theory underpredicts power performance at high TSRs due to tip losses, while numerical simulations identify optimal TSR intervals, yielding a 9.3% efficiency gain over a single-rotor system. Furthermore, increasing λ1 accelerates the onset of negative torque in the rear rotor, as λ2 approaches its operational limit. This study challenges prevailing assumptions by demonstrating that rear rotor can efficiently operate at elevated TSRs (λ2/λ1) under stabilized axial induction factors, offering actionable insights for optimizing high-output marine energy systems.

The increasing integration of renewable energy into new-type power systems accentuates the critical role of hydroelectric power in grid stabilization and baseload support. Nevertheless, inherent flow instabilities in hydraulic turbines operating under off-design conditions induce power oscillations. These oscillations constrain the flexible regulation capabilities of hydroelectric power systems. This study investigates a large Francis turbine, employing a multiphase flow model to numerically simulate flow characteristics across a comprehensive range of operating conditions. Pressure distribution on runner blades is quantitatively analyzed utilizing a streamline coordinate system. A third-generation vortex method identifies the complex vortex structures within the flow channel. Building upon these analyses, this study delineates conditions predisposing the turbine to power oscillations, elucidates the mechanism of these vortex-induced oscillations, and proposes an optimal air admission strategy for their mitigation. Results show that the turbine is susceptible to power oscillations in below-rated head and overload conditions, with a characteristic frequency of 4.93 Hz, corresponding to flow separation vortices. These vortices generate negative torque on the blades, with the resulting torque fluctuations directly causing power oscillations. An optimal air admission strategy—admission volume of 1 m3/s—significantly reduces the amplitude of power oscillations by up to 81.3%. The conclusions presented herein offer valuable recommendations for optimizing the stable operating range of hydraulic turbines, enabling timely mitigation of output power fluctuations, and enhancing the flexible control capability of hydraulic systems.

As the central component of a pumped storage power station, the pump-turbine frequently enters the hump region during operation, which impacts the safe functioning of the unit. This study investigates the flow patterns at various guide vane openings within the hump area, employing the Omega vortex identification method. The results reveal that transitioning into the hump region from design conditions results in an unsteady flow state in the vaneless space, seriously threatening the stability of the pump-turbine. Notably, at the lowest point in the hump zone, the vaneless space generates high-speed circulation characterized by large velocity gradients and severe flow separation. The region of high turbulent kinetic energy intensifies with increasing guide vane opening. Additionally, low-frequency pressure pulsations become the predominant frequency in the hump zone, and the relative amplitude of these pulsations reach higher values, which is caused by rotating stall. The Omega vortex identification technique illustrates that the vortex structure is minimized during optimal efficiency in the vaneless space. As the flow rate in the hump region decreases, persistent high-energy vortices accumulate in the vaneless space, resulting in a significant increase in vorticity intensity and causing unsteady flow in the pump turbine.

Mixed-flow pumps, optimized for marine engineering, provide a balance of high efficiency and adaptability, accommodating varied flow and head demands across challenging oceanic settings and are essential for reliable operations in tidal energy and subsea applications. The primary purpose of this paper is to perform a numerical analysis of the cavitation flow characteristics of the mixed-flow pump under differing operational circumstances. The cavitation simulation was implemented to explore the cavitation bubbles evolution and the pressure pulsation characteristics in the impeller region under diverse flow rates, utilizing the Shear Stress Transport (SST) turbulence model and the Zwart-Gerber-Belamri cavitation model as a foundation. The findings indicate that cavitation bubbles initially distribute at the leading edge of blade suction surfaces at the cavitation growth stage. The bubbles spread gradually with the decline of the available net positive suction head (NPSH_a). At the same time, many bubbles appear in the area below the blade and extend to the rim of the suction side of blades. As the flow rate decreases, the critical net positive suction head (NPSH_c) gradually declines. The dominant pressure pulsation frequency at the impeller inlet is the blade passing frequency, and the vibration at the impeller shroud inlet is more intense than that at the hub. The dominant frequency at the impeller outlet is mainly the blade passing frequency. With the development of cavitation, it changes to impeller rotation frequency at low flow rates, while the dominant frequency remains unchanged at high flow rates.