To address the low efficiency and unclear internal loss mechanisms of azimuth waterjet propulsion (AWP) systems operating under shallow and complex flow conditions, this study investigates an AWP unit at 950 rpm with a thrust of 1.63 kN. Steady numerical simulations are conducted under mooring and low-speed conditions, focusing on thrust coefficient, impeller efficiency, pump efficiency, and diffuser flow characteristics, with comparisons to a conventional mixed-flow pump. The results show that the propeller hydraulic efficiency at the design condition is approximately 52%, significantly lower than the 80–93% typical of mixed-flow pumps. The diffuser contributes nearly 80% of the total hydraulic loss, dominated by secondary flow effects. From the perspective of radial equilibrium in the guide vanes, secondary flow development is closely linked to spanwise momentum non-uniformity and deviation from equilibrium. The inclined outflow from the impeller induces strong spanwise imbalance, while the nearly 180° turning in the diffuser suppresses conventional force terms and establishes a pressure-gradient-dominated inertial balance associated with streamline curvature. This mechanism drives transverse migration and entrainment, promoting the formation of counter-rotating vortex pairs and secondary flows. Four major vortex concentration regions are identified, where interactions between secondary flow and recirculation generate complex three-dimensional vortex structures, including induced and spiral separation vortices. These vortices locally block the flow passage, causing pressure fluctuations and energy dissipation. The mid-span region of the guide vanes is identified as the primary location of loss accumulation. These findings provide theoretical and engineering guidance for diffuser optimization in AWP systems. It should be noted that the present study is based solely on numerical simulations, and no experimental validation for the investigated AWP configuration is currently available. Future experimental studies are needed to further verify the predicted hydraulic performance and flow structures.
This study numerically investigates the effect of low-momentum blowing applied to a toroidal propeller on aerodynamic performance and aeroacoustic characteristics. A Delayed Detached Eddy Simulation (DDES) approach coupled with the Ffowcs–Williams and Hawkings (FW–H) acoustic analogy is employed to resolve unsteady flow structures and predict far-field noise signatures. Two configurations are examined at 3000 revolutions per minute (RPM): a baseline propeller and one equipped with active flow control (AFC). The results show that the introduction of AFC produces negligible changes in thrust, torque, and figure of merit (FM), indicating that equivalent aerodynamic performance is maintained. In contrast, noticeable acoustic benefits are achieved. Significant reductions in primary tonal noise and overall sound pressure level (OASPL) are observed along the propeller axis direction, with a maximum tonal reduction of approximately 6 dB and an OASPL decrease of up to 1.8 dB. Flow-field analysis using Q-criterion iso-surfaces and z-vorticity contours reveals that blowing weakens coherent vortex structures near the upper surface of the back blades around 0.75
R
. The weakened vortical activity reduces unsteady surface pressure fluctuations, thereby attenuating dipole-type acoustic sources responsible for dominant axial radiation.
M. Chen, Omar D. Lopez Mejia, Jielong Cai et al.· International Journal of Aer...· 0 citations
Oscillating water column wave energy converters require self-rectifying turbines capable of maintaining stable performance under bidirectional airflow. This study numerically investigates the aerodynamic performance and internal flow characteristics of an axial-flow impulse turbine using OpenFOAM under both uniform and reciprocating airflow conditions. Rotor motion was modeled using the Multiple Reference Frame approach, and the numerical model was validated against experimental data for one-way flow at an inlet velocity of 8.71 m s−1 and rotational speeds ranging from 300 to 1300 rpm. The CFD results successfully reproduced the experimental efficiency trend, yielding a peak efficiency of η = 0.4269 at 700 rpm and ϕ ≈ 0.95, which closely aligns with the experimental peak efficiency of η = 0.4425. Validation metrics demonstrated a high degree of accuracy, with an RMSE of 0.0219, a mean absolute error of 0.0197, a maximum absolute error of 0.0399, a squared Pearson correlation coefficient of 0.826, and a peak-efficiency difference of 3.5%. Flow-field analysis revealed that low rotational speeds resulted in high outlet velocities and incomplete energy extraction, whereas excessive rotational speeds caused flow misalignment, downstream vortex formation, and additional aerodynamic losses. Under reciprocating flow conditions, characterized by a sinusoidal velocity amplitude of 8.71 m s−1 and periods of 0.5–2.0 s at 700 rpm, both input and torque coefficients exhibited hysteresis, with the strongest loops observed at the shortest periods. Examinations of streamline, pressure, and velocity distributions indicated that residual flow during flow reversal alters the effective inlet direction in the subsequent half-cycle, resulting in flow memory and a phase-dependent turbine response. As the present computational domain excludes the OWC chamber, these findings characterize turbine-level aerodynamic performance rather than the complete system power coefficient.
Muhamad Aiman Jalani, Hiroto Shinohara, Y. Imai· Energies· 0 citations
The propulsion performance and propeller design of ships have traditionally been evaluated mainly under calm-water conditions. However, under actual sea conditions, waves can increase added resistance, change the stern wake distribution, reduce propulsive efficiency, and affect cavitation behavior. This study evaluates the propulsion performance and designs a propeller for an 1800 TEU container ship under regular wave conditions using computational fluid dynamics. Resistance and self-propulsion simulations are conducted for eleven wavelength ratios in the range of 0.5≤λ/LPP≤2.0, with a fixed wave steepness of H/λ=0.01. The results show that the required power increases significantly in the resonance wavelength range because of the combined effects of added resistance, wake variation, and reduced propulsive efficiency. The Brake Horsepower (BHP) transfer function obtained from the regular wave simulations is combined with representative sea-state spectra using the spectral method to estimate the Daily Fuel Oil Consumption (DFOC) under actual operating sea states. The total long term DFOC is estimated as 37.084 t/day. For the propeller design, the wake distribution at the propeller plane is analyzed at λ/LPP=1.1, as a representative wave condition where the ship motion and propulsion performance variation become significant. The wake analysis shows that the instantaneous inflow changes considerably according to the wave phase, which can affect blade loading and cavitation. Based on this analysis, a new propeller geometry is designed with the cavitation performance as the primary consideration while also improving the propulsion performance. The designed propeller reduces the cavity volume over the selected wave phases and decreases the delivered power by approximately 2.1% in calm water and 3.4% in wave conditions. These results demonstrate the importance of considering wake variation and cavitation characteristics in practical propeller design under actual operating conditions.
S. Lee, K. Paik, S. Jeong et al.· Journal of Marine Science an...· 0 citations
Distributed propulsion (DP) systems offer increased lift, decreased drag, higher propulsion efficiency, and reduced fuel consumption. However, complex aeropropulsive interactions can significantly affect the aerodynamic performance of distributed propulsion systems, particularly under near-stall conditions. To examine the aeropropulsive coupling effects of distributed ducted fans under near-stall conditions, an overwing configuration with an array of five ducted fans was numerically simulated based on a newly developed body force model (BFM). It is found that the suction capacity of the middle fan is markedly weakened as the wing approaches the stall angle of attack (AoA), resulting in markedly lower aerodynamic performance. However, the suction capacity of the edge fans is relatively strong, with only a slight reduction in the aerodynamic performance. Relative to the edge fan, the middle fan exhibits reductions in mass flow rate and thrust of up to 26% and 46%, respectively. As the stall AoA of the wing approached, the middle fan experiences the highest total pressure distortion but the lowest swirl distortion, whereas the edge fans exhibit the opposite trend. The largest discrepancy in total pressure distortion between the middle and edge fans amounts to 26%, while it raises to 66% when discussing the swirl distortion coefficient. At the ducted fan inlet, swirl below 0.8 span of the blade forms in local symmetric vortices, whereas swirl near the blade tip develops into local asymmetric vortices with an increase in the AoA. These findings provide valuable insight into the integrated design of distributed propulsion systems under off-design conditions.
Wei Jia, Qingguo Kong, Tao Liu et al.· Journal of Aerospace Enginee...· 0 citations
Under high-flow operating conditions, axial-flow pumps often experience a pronounced efficiency deterioration caused by intensified tip leakage flow, adverse pressure gradients, and flow separation, leading to enlarged high entropy production regions. This study investigates the role of blade loading redistribution in regulating these loss-generating flow mechanisms. An inverse design method is employed to modify the blade velocity moment distribution, while a surrogate-assisted optimization framework serves as a tool to identify an improved loading configuration. The optimized model maintains a nearly identical head under high-flow conditions and improved the hydraulic efficiency. Comprehensive analyses of entropy production, pressure field, and internal flow structures reveal that the optimized blade loading effectively mitigates the adverse pressure gradient near the blade tip, suppresses tip leakage flow development, and alleviates trailing-edge separation. Consequently, the high entropy production regions contract markedly, and the trailing-edge entropy production peak is reduced. These results indicate that the efficiency improvement originates from the controlled evolution of leakage and separation losses, providing new physical insights into loss mitigation mechanisms in axial-flow pumps.
Xinyu Li, Xiangfei Chen, Haichao Sun et al.· The Physics of Fluids· 0 citations
Pump-turbines often experience performance deterioration under high-load conditions beyond their best efficiency point, while the underlying flow mechanisms remain insufficiently understood. In this study, we investigate the relationship between internal flow structures and energy performance in a pump-turbine operating at a rated head of 202 m over a range of guide vane openings. Energy losses are evaluated using an average kinetic energy-based method and compared with an entropy production approach. A threshold-independent rigid vorticity method is adopted for vortex identification, and a streamline-based coordinate system is introduced for spatial quantification of energy loss and blade loading. The results show that hydraulic losses are mainly concentrated in the draft tube (66–75%) and runner (25–30%) under high-load conditions. A coupled vortex system formed by separation vortices and horseshoe vortices governs localized dissipation in the runner. In the draft tube, a columnar vortex rope generates strong shear layers that dominate energy loss in the cone and elbow regions. At high flow rates, negative incidence induces pressure-side separation, forming negative torque regions that reduce net runner torque and lead to output power deterioration. These findings highlight the dominant role of coupled vortex structures and pressure redistribution in performance degradation under high-load operation.
Lingkai Zhu, Kai Liang, Yunkuan Yu et al.· Applied Sciences· 0 citations