Journal of Hydrodynamics最新文献

In this paper, a multi-objective optimization strategy for a volute mixed flow pump is implemented based on inverse design theory. The controllable velocity moment (CVM) design parameters are defined based on the impeller outlet velocity moment and its partial derivative in the streamline direction, and flow control is further realized by directly adjusting the velocity moment distribution inside the impeller. The significance order of the effect of the CVM parameters on the efficiency and NPHS_c at the best operation points is first investigated, and the elliptical basis function (EBF) approximation model and MOPS optimization algorithm are combined to carry out multi-objective optimization. Compared with the baseline model, an improvement in the efficiency and head at the best operation points of 0.48%, 1.07 m is obtained for the optimal model, with a widened and efficient operation range, and NPHS_c is reduced from 4.545 m–4.235 m, with a slowed cavitation development. The proposed CVM method effectively realizes flow control under multiple conditions, optimizing the pressure distribution on the blade pressure side and suppressing the reverse jet in the cavity closure area.

In the present paper, the simultaneous resonance of a cylindrical bubble under triple-frequency acoustic excitation is investigated theoretically. Specifically, based on the multi-scale method, the dimensionless oscillation equations and the second-order analytical solutions of the primary-subharmonic-subharmonic (PRI-SUB-SUB), primary-superharmonic-superharmonic (PRI-SUPER-SUPER) simultaneous resonances are obtained. Based on the analysis of the frequency response, the nonlinear dynamic behavior of the cylindrical bubble and its influencing factors are analyzed. The primary conclusions include: (1) Under triple-frequency acoustic excitation, the frequency response of PRI-SUB-SUB presents a single peak, and that of PRI-SUPER-SUPER presents two peaks. (2) The polytropic exponent affects both the peak value and position of the resonance peak in the frequency response. (3) The unstable region in frequency response curve of the simultaneous resonance is significantly affected by the total amplitude and equilibrium radius, presenting a positive correlation.

The misoperation of hydraulic components such as pumps and valves in pressurized pipelines triggers water hammer phenomena and seriously threats the safe operation of hydraulic systems. At present, the main water hammer simulation methods are method of characteristics (MOC), and further investigation of new algorithms is needed. Therefore, a new method for simulating the water hammer using the finite volume method (FVM), semi-implicit method for pressure linked equations (SIMPLE) algorithm is proposed in the present work. Compared with the experimental data, the accuracy and reliability of the proposed algorithm are verified. Results show that the IAB, MIAB friction models not only predict the first pressure peak but also accurately predict the pressure attenuation. From the comparison of the MOC, SIMPLE algorithms, the results of the two algorithms are almost the same in front of the valve, while near the upstream tank, when using the same friction model, the pressure attenuation predicted by the SIMPLE algorithm is slightly greater than that of the MOC method and closer to the experimental data. Therefore, the newly proposed algorithm can serve as an alternative to the MOC method in simulating water hammer. The investigation enriches the numerical methods of hydraulic transients and lays the foundation for subsequent program development.

The cavitation bubble precipitation refers to the formation process of the spherical cavities, known as cavitation bubbles, as the ambient pressure of water decreases. In the fields of hydraulic machinery, the saturated vapor pressure of clean water is often used as the pressure threshold for cavitation occurrence. However, the engineering practice has demonstrated that, the incipient cavitation pressure may be significantly higher than the saturated vapor pressure, especially in sand-laden water conditions. Therefore, to determine a reasonable cavitation pressure threshold and ensure the accurate cavitation flow simulations and effective assessment of cavitation risks for hydraulic machinery operating in sand-laden water conditions, an experimental investigation is conducted. First, a high-precision experimental setup based on the vacuum pump, high-frequency pressure sensor and high-speed camera is constructed. This setup allows for the continuous pressure reduction in water, acquisition of high-precision pressure data and tracking of the entire cavitation bubble precipitation process. Second, based on the experiments in clean water conditions, the relationship between the cavitation bubble precipitation degree and pressure is established, and two key states of incipient cavitation and boiling cavitation are defined. Third, based on the experiments in sand-laden water conditions, it is observed that the numerous cavitation nuclei on sand surfaces make both the incipient and boiling cavitation pressure in sand-laden water higher than those in clean water. The quantitative relationship between the sand concentration and diameter, and the cavitation pressure is established, providing a more reasonable cavitation pressure threshold. This investigation enhances the understanding of cavitation bubble precipitation in sand-laden water and supports the development of more accurate cavitation models for hydraulic machinery operating in sand-laden water conditions.

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.