Journal of Hydrodynamics最新文献

The biomimetic hydrofoils are frequently employed to enhance cavitation performance, although the underlying mechanisms remain to be fully elucidated. This study utilizes a cavitation visualization experimental system and mechanical characterization to experimentally investigate the transient cavitation features of a NACA0015 hydrofoil and its biomimetic counterparts with modified lending-edge. The findings demonstrate that, in comparison with the flat hydrofoil, the biomimetic hydrofoil experiences a cavitation morphology transition at a lower cavitation number, with a reduction of up to 0.38. Moreover, the maximum cavity length and the maximum cavitation area are reduced by 17.11%, 17.32%, signifying a reduction in cavitation intensity. Proper orthogonal decomposition (POD) analysis revealed that the primary mechanism for the enhanced cavitation performance of the leading-edge wave structured biomimetic hydrofoil is the suppression of cloud cavitation shedding. At an attack angle of 6°, the biomimetic hydrofoil exhibited the highest lift coefficient increase of 18.56%, corresponding to a lift-to-drag ratio improvement of 9.56%. By analyzing the cavitation patterns of the two hydrofoils, it is evident that the rate of change in the maximum cavity length isolines for the biomimetic hydrofoil is lower than that of the flat hydrofoil. For an equivalent level of cavitation intensity, the biomimetic hydrofoil exhibits a lower cavitation number compared with the flat hydrofoil. These demonstrate that the wavy leading-edge design of the biomimetic hydrofoil effectively reduces the severity of cavitation, thereby confirming the efficacy of the biomimetic hydrofoil in enhancing cavitation performance.

Very short-term prediction of ship motion is critically important in many scenarios such as carrier aircraft landings and marine engineering operations. This paper introduces the newly developed functional deep learning model, named as Deep Operator networks neural network (DeepOnet) to predict very short-term ship motion in waves. It takes wave height as input and predicts ship motion as output, employing a cause-to-effect prediction approach. The modeling data for this study is derived from publicly available experimental data at the Iowa Institute of Hydraulic Research. Initially, the tuning of the hyperparameters within the neural network system was conducted to identify the optimal parameter combination. Subsequently, the DeepOnet model for wave height and multi-degree-of-freedom motion was established, and the impact of increasing time steps on prediction accuracy was analyzed. Lastly, a comparative analysis was performed between the DeepOnet model and the classical time series model, long short-term memory (LSTM). It was observed that the DeepOnet model exhibited a tenfold improvement in accuracy for roll and heave motions. Furthermore, as the forecast duration increased, the advantage of the DeepOnet model showed a trend of strengthening. As a functional prediction model, DeepOnet offers a novel and promising tool for very short-term ship motion prediction.

This paper investigates the bubble collapse characteristics near dual cylinders within confined spaces. Firstly, the impacts on the bubble morphology, with respect to the bubble positions and the cylinder spacings, are explored using high-speed photography experiments. Subsequently, based on the circle theorem, the liquid velocity field is qualitatively analyzed and compared with the experimental bubble interface motion. Finally, employing the Kelvin impulse theory, an analysis of the variation in Kelvin impulse at various cylinder spacings is conducted, which shows good consistency with the bubble centroid movement. The main conclusions are summarized as follows: (1) High-velocity regions are observed on both sides of the bubble. Low-velocity regions are observed between the bubble and cylinders. As the cylinder spacing and the bubble abscissa increase, the liquid velocity in the high-velocity regions decreases, and the low-velocity regions expands. (2) The characteristics of the bubble cross-sectional roundness, interface displacement, and cross-sectional area are significantly affected by the cylinder spacing and the bubble abscissa. (3) As the bubble abscissa increases, the Kelvin impulse intensity initially rises rapidly and subsequently declines gradually to a fixed value. As the cylinder spacings increases, the Kelvin impulse intensity decreases.

In this paper, the collapse dynamic properties of the cylindrical bubble near an arched cylinder bulge are researched relying on the conformal transformation and Kelvin impulse model. The properties of the liquid velocity distribution, Kelvin impulse distribution and the attraction zone of the jet are analyzed when the bubble and the bulge are arranged symmetrically and asymmetrically. The results show that, firstly, on the side of the bubble close to the bulge, there is a minimum collapse velocity of the bubble surface, which decreases as the bulge angle increases. In addition, the bulge’s effects on the Kelvin impulse strength and direction become larger as the bulge angle increases. When the bubble is incepted at the joint of the flat wall and the bulge, the impulse strength reaches its maximum. Finally, as the bulge angle increases from 45°–120°, the area of the jet attraction zone is gradually expanding, with its maximum width gradually increasing from 1.1–1.8 times the chord length of the bulge.

This study investigates the interactions between cavitation bubbles and the interfaces of two immiscible liquids, with practical implications and potential applications in the fields such as ultrasonic emulsification and wastewater treatment. To explore the influence of liquid viscosity on the interaction between the cavitation bubble and flat liquid-liquid interface, visualization experiments were performed on the laser-induced cavitation bubbles near two liquid-liquid interfaces composed of deionized water and two silicone oils with different viscosities (50 mPa·s, 500 mPa·s) by using high-speed photography. Three different positions were employed for the generation of cavitation bubbles, i.e., at the interface, in the water, and in the silicone oil. The evolutions of cavitation bubbles and the corresponding interface deformations at different dimensionless standoff distances γ between the cavitation bubble and the interface were observed. The results show that the difference in the viscosity of silicone oil significantly affects the physical phenomena occurred during the interaction between the millimeter-scale cavitation bubble and the interface. On this basis, the qualitative and quantitative analyses for the cavitation bubble jet dynamics indicate that the critical value of γ for jet penetration through the interface between the water and the higher-viscosity silicone oil (interface 2, γ = 0.33) is lower than that for the interface between the water and the lower-viscosity silicone oil (interface 1, γ = 0.69). Besides, the jet generated by the cavitation bubble near interface 1 possesses a higher maximum velocity. These indicate that increased viscosity inhibits the development of the jet. The cavitation bubbles that initiate in the water near Interface 1 consistently migrate away from the interface and do not split, while those near interface 2 would migrate towards the interface at intermediate γ and would split at γ <0.91. In addition, the jet behaviours of cavitation bubbles near interface 2 at different γ are examined and classified into four types.

The characteristics of water and sand two-phase flow and their wear features in a rotating jet wear device at various impact angles are investigated by the wear weight loss test, spraying paint abrasion distribution experiment and numerically multiphase simulation. The results reveal that the weight loss of specimen abrasion initially increases and then decreases as the impact angle rises, peaking at about 40°. The annular abrasion distribution on the test disk can be obtained by the simulation model which adopts the slip grid method to handle the rotation of disk, aligning well with experimental results. Furthermore, the abrasion distribution and weight loss predicted by the Oka abrasion model and the Grant and Tabakoff (G&T) collision rebound model closely match the experimental data. At lower impact angles (15°–45°), the jet velocity is low while the rotational speed is high, and the two-phase jet flow spreads towards the specimen’s outer edge due to centrifugal force, which results in the increased wear on the specimens with the disk’s radius. At the impact angle of 60°, high abrasion rate strip is observed near the specimen’s centerline in both the paint spray test and numerical simulation. At this angle, the jet collides with the rotating wall and generates a spiral trajectory along the circumferential position of the disc, forming vortices at the downstream of the nozzle. The particle aggregate inside the vortices, forming high sediment concentration distribution and high wear rate strip on the specimen. This work will establish a foundation for the simulation and testing of sediment wear in hydraulic machineries.

To enhance understanding of the flow characteristics around a sphere in both stratified and unstratified (UNS) fluids, large eddy simulations (LES) were conducted using a temperature-dependent density model at Re = 3 700. The simulations were performed for flow around a sphere under UNS and stratified conditions (Fr = 3). Horizontal and vertical vorticity, velocity, and streamline distributions were compared, and the evolution of vortex structures in the wake was analyzed. Furthermore, we quantified the velocity deficit, the root mean square (rms) of velocity components in all directions, and the turbulent kinetic energy (TKE) distribution. Additionally, the horizontal and vertical wake lengths were examined. The results demonstrate that the employed numerical simulation method accurately captures the behavior of stratified fluids, with outcomes in close agreement with experimental and numerical findings from previous studies. In the case of homogeneous fluid, a lower density value results in a faster decay of the velocity deficit. In stratified fluids, the vortex structures in the wake evolve through three distinct stages: 3-D, non-equilibrium (NEQ), quasi-two-dimensional (Q2D). For x / D > 2, the rms velocity in the vertical direction exceeds that in the other two directions. In UNS fluid, the TKE distribution forms a vertically elongated spindle shape, while in stratified fluid, it assumes an elliptical shape, being vertically compressed and horizontally expanded. The vertical extent of the density and density gradient distributions surpasses that of the wake.

Navigating efficiently across vortical flow fields presents a significant challenge in various robotic applications. The dynamic and unsteady nature of vortical flows often disturbs the control of underwater robots, complicating their operation in hydrodynamic environments. Conventional control methods, which depend on accurate modeling, fail in these settings due to the complexity of fluid-structure interactions (FSI) caused by unsteady hydrodynamics. This study proposes a deep reinforcement learning (DRL) algorithm, trained in a data-driven manner, to enable efficient navigation of a robotic fish swimming across vortical flows. Our proposed algorithm incorporates the LSTM architecture and uses several recent consecutive observations as the state to address the issue of partial observation, often due to sensor limitations. We present a numerical study of navigation within a Kármán vortex street created by placing a stationary cylinder in a uniform flow, utilizing the immersed boundary-lattice Boltzmann method (IB-LBM). The aim is to train the robotic fish to discover efficient navigation policies, enabling it to reach a designated target point across the Kármán vortex street from various initial positions. After training, the fish demonstrates the ability to rapidly reach the target from different initial positions, showcasing the effectiveness and robustness of our proposed algorithm. Analysis of the results reveals that the robotic fish can leverage velocity gains and pressure differences induced by the vortices to reach the target, underscoring the potential of our proposed algorithm in enhancing navigation in complex hydrodynamic environments.

This paper considers local scour around a pipeline under turbulent flow. The Navier-Stokes equations are solved with a shear stress turbulence model. The original bed deformation equation based on an analytical sediment transport model is used to describe the changes in the bottom surface. The proposed sediment transport equation is based on Coulomb’s friction law for granular flow, Prandtl’s friction law for turbulent flow, and agrees with a large number of phenomenological formulas by other authors. A numerical algorithm for solving the mathematical model of bed surface erosion is implemented in OpenFOAM. Numerical simulations of the problem show that under the influence of turbulent flow generated at the pipeline streamline, a characteristic bottom wave of low steepness appears, the parameters of which asymptotically agree with the experimental data. Based on the analysis of experimental and numerical studies of the considered case, an assumption about the self-similar behavior of the bed surface evolution is made. Based on this assumption, a new method of constructing the self-similar dependence of the bed surface on time and space coordinates is proposed. In the proposed approach, the average values of tangential bottom stresses are determined for a number of self-similar bottom surface shapes, and then the rates of change of bottom wave lengths and amplitudes are calculated using the proposed analytical model. A comparison with experimental data and numerical calculations shows that the solution error does not exceed a few percent and the computational time is reduced by up to 30 times.

The placement of baffles in culvert structures has a significant impact on local hydrodynamics and sediment transport, which are essential for improving fish passage and enhancing aquatic habitats. Large eddy simulations (LESs) are performed to study the influence of triangular baffle designs on flow structure and sediment transport in standard box culverts with varying baffle sizes and spacings. The Reynolds numbers based on the baffle size for these conditions are 37 778 or 75 012. The LES method is validated using experimental data of mean streamwise velocities, and a good agreement is achieved. The simulations demonstrated that the presence of baffles with various designs significantly altered the flow field in their vicinity. Various distinct flow features, such as separation zones, recirculation zones, and resting zones (RZs), are captured. The RZ and low positive velocity zone (LPVZ) show a positive correlation with the baffle size in the culvert, while the negative velocity zone (NVZ) exhibits a negative correlation. The flow field is primarily characterized by a large recirculation zone between the baffles, with the baffle size being the predominant influencing factor. The spacing between the baffles partially constrains the extension of the recirculation zone. The range of turbulence parameters, including the turbulent kinetic energy (TKE) and Reynolds shear stress (RSS), expands as the baffle size increases and the baffle spacing decreases. The presence of baffles suppresses ejections and sweeps interaction turbulence events downstream of the baffles. The near-bed coherent structures are responsible for the erosion zone between the baffles.