Journal of Hydrodynamics最新文献

Superhydrophobic drag reduction technique has garnered significant attention in recent years due to its outstanding performance in reducing frictional drag, presenting promising application potential in areas such as marine vessels and pipeline transportation. This paper provides a comprehensive review of the wetting theory and slip theory associated with superhydrophobic surfaces, and systematically summarizes the mechanisms of turbulence drag reduction from the perspective of coherent turbulent structures. The research on the combination of superhydrophobic and other drag reduction techniques to improve the drag reduction effect is summarized, focusing on the combined drag reduction of superhydrophobic-riblet. The stability and recoverability of the gas layers on superhydrophobic surfaces are critical for advancing this technology toward practical engineering applications. Consequently, this review places particular emphasis on recent research progress concerning the enhancement and restoration of the underwater gas-liquid interface stability on superhydrophobic surfaces.

Rogue waves pose a significant threat to the safety of ships and offshore structures, making it crucial to understand their physical mechanisms, such as spatial-temporal focusing, which can lead to their formation. This study investigates three-dimensional focused waves using the newly developed deep-water high-level Green-Naghdi (HLGN) model. Through numerical simulations, we evaluate the selection of the involved wave numbers within the HLGN model and present the algorithm for the three-dimensional implementation. Validation of the model is conducted through numerical reproduction of the three-dimensional focused waves considered in other’s laboratory measurements. The simulated wave profiles and velocity fields are compared with experimental data, demonstrating strong agreement. Discussion is provided about the robustness and accuracy of the HLGN model in simulating three-dimensional focused waves under deep-water conditions.

To clarify the internal flow field characteristics of cavity vortex in the sediment transport pipe (STP) of the desilting channel with a swirling flow generator (DCSFG), this study adopted a method combining model test, numerical simulation, and theoretical analysis to investigate flow field characteristics such as water flow regime, cavity morphology, pressure, flow velocity and vorticity, analyze the distribution of combined vortex indexes and radial pressure difference of cavity vortex, and discuss the motion feature differences between the combined vortex in the cavity vortex and the ideal combined vortex. The results show that large eddy simulation (LES) exhibits higher accuracy than the Realizable k -ε model, the distribution of combined vortex n values along typical cross-sections inside the STP ranges from −0.901 to 0.913 radially, indicating quasi-forced vortex motion on the inner side of the vortex area and quasi-free vortex motion on the outer side, the theoretical values of radial pressure difference align well with the simulation results, with a maximum relative error of 15%, confirming that the flow characteristics of the vortex are in accordance with the motion features of combined vortex, the distribution of radial pressure, tangential velocity, and vorticity in the cavity vortex conform to the distribution pattern of ideal combined vortex, whereas significant differences exist in terms of fluid force conditions, structural composition, and generation mechanism. The research findings may provide reference for further analyzing the sediment transport mechanism in the cavity vortex and for the practical engineering design and application of the DCSFG.

To enhance the gas-liquid mixed transport performance of the first-stage centrifugal impeller of the multistage side-channel pump, a diagonal perforation oriented towards the exit is fabricated in the front shroud of the impeller. Based on the Euler-Euler non-homogeneous model and the SST k -ω turbulence model, the gas-liquid two-phase unsteady numerical simulation of the internal flow under various inlet gas volume fraction (IGVF) is conducted, the reliability of the simulation is verified through comparison with experiments. The results indicate that under the circumstances of high flowrate and high IGVF, the perforation design of the front shroud can increase the head of the centrifugal impeller by 4%–7% while the efficiency is slightly decreased under gas-liquid two phase flow. According to the internal flow analysis and Liutex vortex identification, the high-pressure and high-speed fluid in the front pump chamber is introduced into the impeller through the front shroud perforation, smashing and dispersing the originally aggregated bubble groups in the flow channel, causing the average pressure in the impeller to rise after the perforation, increasing the number and intensity of vortexes, significantly reducing the number and the accumulation area of bubbles, greatly reducing the air volume fraction of the impeller. The bubble blockage phenomenon in the flow channel is observably improved, and the gas-liquid mixed transport capacity of the centrifugal impeller is significantly enhanced, providing a theoretical basis for the optimization design of the gas-liquid two-phase flow of vane pumps.

With the advancements in computer technology, the simulation-based design (SBD) technology has emerged as a highly effective method for hull form optimization. The SBD approach often employs various methods to evaluate the hydrodynamic performance of the sample ships. Although the surrogate model is applied to SBD method to replace time-consuming evaluation, many high-fidelity data are typically required to guarantee the accuracy of the surrogate model, resulting in significant computational costs. To improve the optimization efficiency and reduce computational burdens, we propose a novel hull form optimization framework utilizing the multi-fidelity deep neural network (MFDNN), leveraging multi-source data fusion and transfer learning. This framework constructs an accurate multi-fidelity surrogate model which correlates design parameters with hydrodynamic performance of the hull by blending data with different fidelity. Besides, computational fluid dynamics (CFD) evaluations based on viscous flow are served as the high-fidelity model, while potential-theory evaluations represent the low-fidelity model. Then, this framework is validated using mathematical functions to prove its practicability in optimization. Finally, the optimization design of the resistance of the DTMB-5415 ship is carried out. Our findings demonstrate that this framework can take into account both efficiency and accuracy, which is preferable in optimization tasks. The optimized hull form obtained by the framework has better resistance performance.

This paper aims to elucidate the vortex evolution characteristics generated by the tongue of the semi-spiral suction chamber and its influence on the cavitation of the pump. Based on the turbulent viscosity correction model, the internal flow of a centrifugal pump with a specific speed of 160 was simulated, and experimental data verified the simulation. This study focuses on analyzing the conditions of large flow rate, high-efficiency, and partial flow rate. The results show that the tongue will induce a tongue-induced vortex. The tongue-induced vortex extends from the tongue region to the impeller region, and its shape is curved and slender. The shape and volume of the tongue-induced vortex are related to the flow rate. The vortex’s shape is blurred and small in the partial flow rate. There is a complete and obvious curved slender vortex in the high-efficiency zone. In large flow conditions, the vortex’s shape is consistent with the high-efficiency zone and the volume is larger. The vortex’s strength is positively correlated with the circulation of the inlet, which is in the suction chamber. The tongue-induced vortex affects the distribution position of the low-pressure zone on the blade, thereby promoting the leading edge cavitation.

An intelligent flow control on the flow separation over an airfoil under weak turbulent conditions is investigated and solved by deep reinforcement learning (DRL) method. Both single and synthetic jet control at the airfoil angles of attack of 10°, 13°, 15° are compared by training a neural network for closed-loop active flow control strategy based on the soft actor-critic (SAC) algorithm. The training results demonstrate the effectiveness of the deep reinforcement learning-based active flow control method in suppressing the flow separation at high angles of attack, validating its potential in complex flow environments. To improve the stability of the shedding vortex alley over airfoil, a novel reward function considering the vorticity statistics in terms of both vortex and asymmetric shear intensity is first proposed in this work. This vorticity driven reward is demonstrated to perform better in suppressing the rotation and shear intensity and the aerodynamic optimization than the traditional one. Moreover, it can accelerate the convergence speed during the exploration phase. Moreover, it can accelerate the convergence speed during the exploration phase. This study provides valuable insights for future applications of DRL in active flow control under more complex flow conditions.

The KCS Container Ship executing zigzag and turning circle maneuvers in shallow water is studied numerically with CFD simulation. Different factors affecting the accuracy of the numerical prediction such as grid density, overset discretization, propeller model, turbulence modeling and setup condition have been carefully investigated. Numerical predictions for port side and starboard side turning circle maneuvers and 3 different zigzag maneuvers are compared with experimental data obtained by Flanders Hydraulics (FH) for the KCS ship model at scale 1/52.66 with Froude number Fr = 0.095 and water depth to draft ratio h/T = 1.2 in tank conditions. To better assess the accuracy of the numerical prediction, simulations have been performed both with a non-confined and confined configuration.

To illustrate the influence of shear-thinning polymer solution on hydrodynamic and flow structure of element assistance and its mechanism of action, this article conducted a numerical investigation for flow past a cylinder in typical shearing-thinning polymer solutions at Re = 60 based on the finite volume method (FVM). One flexible polymer (PEO) and two rigid polymers (XG, DG), whose rheological properties were experimental fitted using the Carreau-Yasuda model by other literature, were chosen to perform the numerical simulation. The vortex size and the back-flow region length both became smaller in polymer flows, and this inhibition effect was most significant in XG flow. On the contrary, except for the 50 ppm PEO flow, the shedding frequency was promoted in polymer flow. Meanwhile, the three polymers exhibited enhancement effects on the lift coefficient fluctuation, and inhibition effects on the drag coefficient; only XG flow significantly promoted the drag coefficient fluctuation. The dynamic model decomposition (DMD) analysis further indicated that the vortex intensity of each mode in polymer flow was stronger than that in water flow. New structures appeared in PEO, DG flows, while only quantitative difference was found in XG flow compared to water flow. The polymers also affected the mode growth rate and thus the flow stability. The rigid polymer only induced the dispersion degree of the growth rates, while flexible polymer may trigger positive values. In summary, the above hydrodynamic characteristics of shear-thinning polymer flow past a cylinder could provide theoretical support for further understanding the flow characteristics of non-Newtonian fluids.

Underwater structures near the water surface can induce disturbances, leading to wave breaking, which involves complex physical mechanisms. Building on previous studies, this paper conducts a 3-D simulation of breaking waves generated by a NACA0012 hydrofoil at different angles of attack. In contrast to earlier studies, which mostly concentrated on the hydrofoil’s macro-physical parameters like lift and drag, this study focuses on the dynamics of entrained bubbles after free surface breaking. The results indicate that at higher angles of attack, bubbles are swept to greater depths, and both the number and volume of the bubbles increase. By analyzing the bubble velocity data, it is found that the underwater bubble motion is primarily dominated by longitudinal movement, while transverse and vertical bubble velocities are symmetrically distributed. Additionally, vortex structures in the flow field are investigated using the third-generation vortex identification method, Liutex-Omega. It is observed that the vortex structures in the hydrofoil’s wake interact with those generated by free surface breaking downstream, reducing the survival time of large bubbles and increasing the number of small bubbles. Consequently, the bubble number density power law exponent shifts from −10/3–−9/2 as bubble radius increases.