To improve knowledge of the unsteady hydrodynamic characteristics of a floating horizontal-axis tidal turbine (HATT) in complex marine environments, a computational fluid dynamics (CFD) numerical method is established to analyze the functioning of a HATT under free surface and pitch conditions: an amplitude of 2.5°–12.5° and a period of 3–15 s. The appropriateness of the CFD method is validated against piled turbine tests in a circulating flume, which turned out to be only 1.43% off at the rated tip speed ratio. The power and thrust coefficients exhibit periodic variation with a frequency twice the pitch's motion. From the ecological perspective, the wake velocity field presents radial warps accompanied by several ellipsoidal low-velocity core regions, which corresponds to the fused vortex rings in the vortex zone. Notably, the wake is more influenced with large rather than small amplitudes and periods; meanwhile, the wake is more sensitive to frequency changes than to amplitude changes. The results could lay a foundation for future research into more complex motion response of floating tidal turbines.

Developing reliable tidal-energy turbines of a large size and capacity links to preservation of the structural safety and stability of the blades. In this study, a bidirectional fluid–structure coupling method was applied to analyze the hydrodynamic performance and structural characteristics of the blade of a tidal-stream turbine. Analyses were conducted on the transient and stable structural stresses, fatigue, and deformations under the influence of water depth and turbine rotational speed. The performance predictions with and without fluid–structure coupling are similar to measurements. The water-depth change has little effect on the stress and deformation change of the blade, while the turbine-speed change has the most significant effect on it. When the turbine just starts, the blade will be subject to a sudden change load. This is due to the increase in turbine speed, resulting in the sudden load. Similar to the trend of blade stress, the blade safety factor is lower near the root of the blade, and the turbine-speed change has a more significant impact on the blade structure’s safety. However, the number of stress cycles in the blade at different rotational speeds is within the safety range.

Under non-design situations, the Francis pump-turbine, a frequently employed core energy conversion device in pumped storage power plants, plays a significant role in ensuring their stable operation. This article examines the unstable flow inside a Francis pump-turbine using energy gradient theory and numerical computations and model experiments. Research has indicated that forced impacts and flow separation between fluids and blades are significant causes of deteriorating mechanical energy gradients, whereas non-optimal angles between stay and guide vanes can readily cause flow separation. The strong helical flow at the blade inlet is the primary cause of the unstable flow, and the hub and blade outlet are the primary locations for the high shear force work area brought on by large-scale vortex structures, and the middle portion of the impeller blade is where the unstable flow field begins and needs more attention.

In floating tidal turbine farms, the global wake interaction affects the ecological footprint, hydrodynamic performance, and the project's investment return. In this paper, a computational fluid dynamics model with advanced overset mesh techniques investigates the response of a side-by-side tidal turbine array under roll and current. The predicted power coefficients are confirmed against experiments, showing a relative error of 2.37% at the rated TSR = 3.9. The variations of the hydrodynamic load and wake of the turbines are investigated in terms of the roll period and amplitude. The results show that the roll results in the interaction between the two parallel turbines, and the load and wake are simultaneously affected by the adjacent turbine and the roll motion. Specifically, platform roll introduces dual-frequency fluctuations arising from rotor rotation and platform motion, while the mean power (CP ≈ 0.34) and thrust (CT ≈ 0.81) coefficients remain nearly constant. Complex interactions occur between the blade tip vortices in the wakes. The vortex instabilities relate to the roll motion, increasing the recovery of momentum in the wake, while also creating a more unstable inflow for potential downstream turbines.

To elucidate the hydraulic instability mechanism during the trip transition process of axial-flow pumps, this study conducts unsteady numerical simulations using computational fluid dynamics (CFD) with the shear stress transport (SST) turbulence model and the Rortex method. The transient variations in external performance parameters, pressure pulsations, and internal vortex dynamics are investigated throughout the trip sequence. Results reveal that the rapid deceleration of the rotating components leads to unsteady responses in flow rate, torque, and axial force. The transition to turbine mode is characterized by periodic fluctuations in impeller loading. Vortex evolution exhibits distinct stages: tip leakage and hub separation vortices intensify following a trip, while the zero-flow stage triggers the regeneration of leading and trailing edge vortices, accompanied by vortex-induced blockage in the impeller region. A Reynolds-averaged enstrophy transport equation based on rigid vorticity is introduced to analyze the energy transfer and dissipation associated with vortex evolution. The analysis demonstrates that flow reversal, pressure gradient redistribution, and enstrophy transport collectively govern the development and breakdown of vortices. This study provides novel insights into the unsteady flow physics during pump runaway events and offers a theoretical basis for improving hydraulic stability and control strategies in axial-flow pump stations.

Tidal current turbines operating on floating platforms are subject to surge motion, which can strongly influence wake evolution and turbine performance. To address this, this paper investigates the transient wake characteristics of tidal current turbines under surge motion by implementing the actuator line model within large eddy simulation. Dynamic mode decomposition (DMD) is applied to extract coherent flow structures and dominant dynamic modes under both fixed and surge conditions. The results indicate that the surge motion increases turbulence intensity, leading to a greater energy loss and faster wake recovery. The DMD method effectively identifies the primary dynamic modes in the wake. Under fixed conditions, modal energy is highly concentrated at the rotational frequency, blade passage frequency, and their harmonics, with the 1-2fn modes linked to support-structure vortex shedding. In contrast, surge motion excites additional modes at the first and second multiples of the surge frequency, as well as coupled modes with blade rotation, amplifying unsteady flow interactions. These findings highlight the critical role of surge-induced dynamics in wake evolution and provide quantitative insights for improving tidal turbine performance and array optimization.