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Swimmers propel their bodies forward by generating vortices around themselves, which produce fluid force during underwater undulatory swimming (UUS). This study aimed to investigate the propulsive and braking contributions of the vortices of the lower limbs, trunk, and upper limbs during UUS. The kinematic data and three-dimensional digital model were collected from nine male swimmers. Vortex generation was obtained using computational fluid dynamics, and the fluid force of six vortices was determined from the vortex circulation, swimmers' segment velocity, and length. Foot vortices contributed 96.7 % to producing braking fluid force during the first half of the downward kick. Vortices of the feet and the ventral side of the trunk contributed 69.3 % and 58.8 % to producing the propulsive fluid force during the last half of the downward kick, respectively. During the first half of the upward kick, the vortices of the feet and ventral side of the trunk contributed to producing the 87.3 % of propulsive and 93.3 % of braking fluid force, respectively. During the last half of the upward kick, 63.1 % of propulsive and 86.9% of braking fluid forces were produced by vortices on the ventral side of the trunk and feet, respectively. Small fluid forces and contributions were detected for vortices of the arms, lower legs, dorsal sides of the shoulders, and waist. These results indicate that the vortices of the feet and ventral side of the trunk mainly contribute to the increase and decrease in the horizontal UUS velocity.

The main goal of this paper is to explore the mechanism of vortex-induced vibration (VIV) of a streamlined box girder from the perspective of flow field and pressure distribution. In this paper, using the computational fluid dynamic method, the VIV performance of fluid under specific working conditions is simulated and analyzed, especially the distribution and evolution laws of vortex structures in the whole process of VIV are studied in depth. Based on the analysis of the flow field distribution, the corresponding relationship between vortex structures and vortex-induced pressures (VIPs) is discussed. The results demonstrate that the primary cause of VIV for streamlined box girders at large attack angles is the circulation process of the massive vortex structures production and dissipation on the upper surface, rather than the alternate shedding of symmetrical vortex pairs. When vortex structures remain stable, negative VIPs rise in absolute value, negative VIPs occur when vortex structures move backward, and positive VIPs increase when vortex structures fall off.

In multi-energy complementary systems, the inherent randomness and volatility of renewable energy generation necessitate hydropower units with rapid start-up and flexible regulation capabilities to operate as energy-regulation units, ensuring grid stability and effective renewable energy integration. Consequently, hydraulic turbines are compelled to operate for prolonged periods in low-load regions characterized by low efficiency, severe cavitation, and intense vibrations, which significantly jeopardize operational safety and stability. To address these issues, this study develops a multi-condition, multi-objective optimization platform for a Francis turbine runner based on the Analytic Hierarchy Process-Entropy Weight (AHP) method and the Multi-Objective Lichtenberg Algorithm (MOLA). Bézier curves parameterize the runner blades, whereas the AHP-Entropy Weight method determines the optimal weight coefficients for operating conditions across the full load range (20%-100%Pr). The optimization objectives combine weighted efficiency across all conditions and weighted minimum pressure values with MOLA implementing multi-objective design optimization. The results demonstrate that the optimized runner reduces the high-entropy production zones in both the runner and draft tube, thereby lowering the energy losses and enhancing the efficiency throughout the full load range. Specifically, the turbine efficiency increased by 5.4% at 20% Pr and by 2.83% at 50% Pr. The optimized blade geometry significantly shrinks the low-pressure regions, thus improving cavitation resistance. Furthermore, passage vortices, flow separation vortices, and draft tube vortex rope under low-load conditions are effectively suppressed, reducing the pressure pulsation amplitudes by 85% at 0.20fn (20% Pr) and 32% at 0.20fn (50% Pr) while maintaining the rated-load performance. These findings provide critical insights for optimizing the turbine stability and runner design in multi-energy complementary systems.

Imitating Dolphin fish-like movement is productive method for enhancing their hydrodynamic capabilities. This work aims to analyze and understand the oscillations of tail fluke of Dolphin, which can be used as a propulsive mechanism for underwater fish robots and vehicles. The objective of the work is to achieve the desired oscillating amplitude by simulating the NACA 0012 profile using computational models and Set up the swimming movement of the dolphin, imitating a fish like model. Computational techniques were employed to examine the propulsive capabilities of the oscillating hydrofoil, inspired by the dolphin's biological propulsion. The evolutionary of fluid pattern in the field surrounding both Dolphin fish model and the NACA0012 hydrofoil, from initial motion to cruising, was established, and the hydrodynamic impact was subsequently studied. An user-defined function (UDF) was developed to create a dynamic mesh interface with CFD code ANSYS FLUENT for establishing the oscillations of Dolphin tail across the flow field. Influencing hydrodynamic coefficients such as lift and drag coefficients at different frequencies were also obtained. The findings shown that when the acceleration of the Dolphin fish model increases, the time averaged drag force coefficient drops because The wake field's vortex disperses to have some beneficial effects and pressure of water surrounding the fish head intensifies to produce a large resistance force. Simulation results show a 98% agreement at lower frequency and speed levels but a 5% deviation at higher frequency and speed due to turbulence effects in both models. It was established that the vortex superposition enhances the Dolphin fish like model rather than lowering its positive impacts. The Strouhal number, which is obtained by the fluid field's evolution rule, can be linked to the Kármán vortex street span with reverse.

Tight formation flight, as a significant way for fixed-wing unmanned aerial vehicle (UAV) to execute missions, generates synergistic aerodynamic effects that significantly influence the motion decision-making and control of UAVs. In aerial refueling missions, this is manifested as complex aerodynamic effects such as vortices affecting the path planning of the refueling UAV. This paper proposes a path-planning method for fixed-wing UAVs to conduct aerial refueling under the constraints of synergistic aerodynamics. Firstly, an environment constraint model for vortex distribution is obtained from aerodynamic experimental data of the refueling formation. Subsequently, by utilizing the differential flatness property of fixed-wing UAVs, the nonlinear system states and control variables are mapped to linear functions of flat outputs. This allows the establishment of segment constraints for the path, enabling the use of a key-point heuristic algorithm in the flat output space to generate the aerial refueling flight path. Furthermore, a flat output minimum snap algorithm is applied for multi-constraint optimization of the flight path, resulting in a smooth and feasible optimal path. Simulation experiments demonstrate the effectiveness and advancement of the proposed path-planning method under the influence of vortices.

The pump turbine, as the core equipment of a pumped storage power plant, is most likely to operate in the hump zone between condition changes, which has a great impact on the stable operation of the power plant, and the high sedimentation of a natural river will lead to wear and tear in the overflow components of the equipment. Therefore, this paper is based on the Euler–Lagrange model, and seeks to investigate the distribution of vortices in the hump zone of the pump turbine and its effect on the movement of sand particles. The study shows that as the flow rate increases, the strip vortex in the straight cone section of the draft tube becomes elongated, and the cluster vortex in the elbow tube section gradually decreases. The strip vortex encourages the sand particles to move along its surface, while the cluster vortex hinders the movement of the sand particles. The accumulation areas of the sand particles in the straight cone section and the elbow tube section increase axially and laterally, respectively. The blade vortex in the runner gradually occupies the flow channel as the flow rate increases, and the blade vortex near the pressure surface encourages the sand particles to move towards the suction surface, resulting in the serious accumulation of sand particles on the suction surface. As the flow rate increases, the number of blades where sand particles accumulate increases and the accumulation area moves towards the cover plate and the outlet. The flow separation vortex in the double-row cascade decreases as the flow rate increases, which drives the sand movement in the middle and lower sections of the vanes. The area of sand accumulation in the stay vane decreases with increasing flow rate, but the area of sand accumulation between the guide vanes increases and then decreases. The vortex on the wall surface of the volute gradually decreases with the flow rate, and the vortex zone at the outlet first decreases, then disappears, and finally reappears. The vortex at the wall surface suppresses the sand movement, and its sand accumulation area changes from elongated to lumpy and finally to elongated due to the increase in flow. The results of the study provide an important theoretical reference for reducing the wear of pump turbine overflow components.

In the field of ocean engineering, the variation of flow field during ship-to-ship (STS) interaction has been a hot topic. Noteworthy, the effect of vortex distribution on flow field characteristic variations during STS interaction remains insufficiently researched. This study modifies the RNG k-ε model using the OpenFOAM platform and verifies its reliability by comparing it with literature data. Subsequently, extended research is conducted to investigate the flow field characteristics of two different ship hull sections under different Reynolds numbers (Re=68,000 and Re=6800), analyzing velocity components, vortex distribution, and trends in pressure and turbulent kinetic energy fields relative to the vortex field. The research reveals that Re primarily governs changes in upstream and downstream flow fields, while in the gap field, the variation in flow field characteristics is more constrained by geometry and boundary conditions. This research provides a valuable reference for assessing flow field characteristics in STS interactions.

Vortex drop shaft (VDS) spillways are eco-friendly hydraulic structures used for safely releasing flood. However, due to the complexity of the three-dimensional rotational flow and the lack of suitable measuring devices, current experimental work cannot interpret the flow behavior reliably inside the VDS spillway, consequently experimental and CFD study on a VDS spillway with an elliptical tangential inlet was conducted to further discern the interior three-dimensional flow behavior. Hydraulic characteristics such as wall pressure, swirl angle, annular hydraulic height and Froude number of the tapering section are experimentally obtained and acceptably agreed with the numerical prediction. Results indicated that the relative dimensionless maximum height of the standing wave falls off nearly linearly with the increasing Froude number. Nonlinear regression was established to give an estimation of the minimum air-core rate. The normalized height of the hydraulic jump depends on the flow phenomena of pressure slope. Simulated results sufficiently reveal the three-dimensional velocity field (resultant velocity, axial velocity, tangential velocity and radial velocity) with obvious regional and cross-sectional variations inside the vortex drop shaft. It is found that cross-sectional tangential velocity varies, resembling the near-cavity forced vortex and near-wall free vortex behavior. Analytic calculations for the cross-sectional pressure were developed and correlated well with simulated results.

Underwater vehicles are widely used in underwater detection, and bionic design, e.g. imitating fish, is an important way to improve their hydrodynamic performance. Most research is on the mechanical structure and kinematics of fish-like models, while evolvement rules and hydrodynamic effects of fluid field around fish have not been studied adequately. In this paper, a kinematic model to simulate the swimming motion of fish is established according to the actual movement of tuna, and the model is optimized to achieve convenient adjustment in the initial oscillating position and maximum oscillating amplitude. A numerical simulation model is established and the boundary condition is verified by experiments with a physical fish-like prototype and a set of force measuring devices. The evolvement rule of the fluid field around the fish-like model from starting to cruising is determined and the hydrodynamic effect is analyzed. The issue of the effect of fluid fields with various averaged fish-like model densities in the buoyancy regulating process is discussed. Finally, a novel way to calculate the Strouhal number is proposed. The results show that the averaged drag force coefficient decreases as the fish-like model speeds up, because the water pressure near the fish head strengthens to generate a larger resistance force and the vortex in the wake field disperses to produce a less positive effect. The viewpoint that the superposition of vortices will benefit the fish-like model instead of weakening the positive effect is confirmed. The width value of the reversed Kármán vortex street can be connected with the Strouhal number, and the Strouhal number can be calculated by the evolvement rule of the fluid field. This research will contribute to bionic autonomous underwater vehicle design.

In the current study, the transient flow characteristics on the top surface of a steel slab continuous casting strand were numerically investigated using a large eddy simulation combined with volume of fluid (LES + VOF) model. The validation of numerical simulation was verified via nail board measurement in the industrial continuous casting mold. The effects of casting speed on the top surface level profile and the instantaneous distribution of vortex were discussed. The level variation profile migrated after a period of time, moving from one side of the wide face of the mold to the other. The wave height and transient variation degree of the standing wave increased with an increase in the casting speed. The region near the SEN was more likely to promote the formation of vortices. The vortex generation became easier when the vorticity peaks were concentrated on the outer edge of the low-speed confluence area near the submerged entry nozzle. In addition, the effect of surface velocity on the instantaneous level fluctuation was analyzed. The frequency of level fluctuations was highest at 3~4 mm, and the high-frequency range of velocity fluctuation was 20~60 mm/s at 0.9 m/min casting speed for a 1500 mm × 200 mm caster section. The linear relationship between the level fluctuation and surface velocity magnitude was obtained. The present work aimed at evaluating the dynamic problem of the standing wave at the liquid powder–molten steel interface on the top surface of the mold, which is helpful in optimizing the casting parameters for regular casting practice and improving the quality of the steel slabs.